《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_j.1151-2916.2003.tb03464.x[1]

High-Temperature Water Vapor Effects JAm. Cera Soc,86|8l128-9102003) ournal Consequence of Intermittent Exposure to Moisture and Salt Fog on the High-Temperature Fatigue Durability of Several Ceramic-Matrix Composites Larry P, Zawada, James Staehler, *and Steve Steel Materials and Manufacturing Directorate, Air Force Research Laboratory, AFRL/MLLN Wright-Patterson AFB. Ohio 45433-7817 Fatigue behavior of four ceramic-matrix composites(CMCs) degradation is accelerated whenever moisture is was documented at 1000 C, and a fifth composite was docu- a humid day contains from 0.003 to 0.025 atm( mented at 1200 C. Additional fatigue specimens were cycle 1×10°Pa)ofH2O. For co ustors. the com for set blocks of cycles, removed from the fatigue machine, and from burning hydrocarbon fuel contain -10%H,O exposed in a cyclie corrosion tester for 24 h with a fog of I atm). For combustors operating at 10 atm, the hot gas deionized water and a fog of deionized water containing 0.05 contains "10-15 vol% of H,O. The degradation in SiC/SiC wt% NaCL. BN-fiber-coated NicalonMisinc and Nicalon/ CMCs under turbine engine combustor conditions has been thor- AlO3 experienced a pronounced decrease in fatigue life oughly documented. A thorough discussion about CMC degrada- (86%)with salt fog exposure. Nicalon/C experienced rapid tion mechanisms in combustors is given by More and co- loss of the SiC exterior seal coat and a 30% decrease in life workers 2 and by Ferber et al. 3 with salt fog exposure Nextel610/AS and Nextel720/AL,O3 The components at the back of the turbine engine(divergent demonstrated no loss in fatigue performance or retain nozzle or exhaust-washed structures) are exposed to high-velocity strength with water or salt fog exposure. Changes to the hot exhaust gases containing a high moisture content and are periodically exposed to moisture whenever there rain or con- they influenced moisture sensitivity. Bn fiber coatings, BN densation of moisture. Fighter aircraft, such as the F-18 Hornet, the or BN/SiC, alternate matrix prepreg, and matrix filler type F-16 Falcon, and the F-15 Eagle, are considered all-weather had no influence on improving moisture resistance. Direct fighters, which means they fly in all types of weather, including exposure to moisture fog produced accelerated rates of rain. While parked on the ground, the exhaust nozzles and degradation in the BN fiber coating and greatly decreased structures are normally covered by a waterproof tarp, but the tarps atigue durability need to be removed before engine start Direct impingement of rain on these structures periodically occurs as a normal part of L Introduction operations during the service life of military aerospace platforms Periodic washing of the aircraft also results in exposure to water. lE class of materials known as ceramic-matrix composites Although CMC parts occasionally are exposed to rain, they also CMCs)continues to be an attractive choice for many aero- are exposed to salt fog from ocean mists and exhaust de space turbine engi plications, CMCs retain strength to much temperature alloys used in jet engines. Salt fog (h,o t nacl) than conventional nickel-based superal However. it is the exposure also has a strong effect on Sic-fiber strength."In GKOspect of higher material operating temperatures and decreased addition, salts are associated with the dissolution(fluxing) of the ooling air that is most attractive to the aerospace design commu- normally protective oxide scales, such as SiO2. as well as other nity. Higher gas temperatures and less cooling air translate directly oxides, such as B,O3. Richardson and Kowalik have shown that hot corrosion of Nicalon M/C with Na SO, occurs above 900%C. with less cooling air for the combustor has the important effect of extensive degradation of the protective Sic surface coating and decreasing the production of nitrous oxides (NO,), which are loss of the carbon-matrix material. Thus. water and salt fog are an detrimental to earth's protective ozone layer. Currently, CMCs are important factor for long-term mechanical performance at high eing demonstrated in turbine components, including combustor temperature. Environmental testing early in the evaluation process liners, turbine nozzles, shrouds, transition ducts, diffusers, and can help determine if these environmental effects are a problem. exhaust structures, such as divergent flaps and seals, as well as There is extensive literature on the subject of moisture and how structures that have hot exhaust gases flowing over them(exhaust- it affects the oxidation of SiC. 7-2Opila and Hann and Pila. 10 washed structures have shown that the presence of water vapor increases the rate of dence of accelerated degradation after only a few hours of hot SiO, growth on SiC at high temperature and that it leads to accelerated rates of SiC recession. There is also extensive literature dation of fibers and fiber on the subject of moisture and how it affects the degradation of BN fiber coatings. -20 Sensitivity of Bn to moisture has been shown to be strongly dependent on the quality of the BN, impurities present in the BN and moisture, levels of moisture, temperature, E.J. Oplia-contributing editor and amount of matrix cracking. most of the work to date has involved exposures and oxidative studies of coupons without the application of stress. The goal of the work reported in this article is to assess the overall extent that moisture affects the high- Manuscript No 186663. Received September 13, 2002; approved April 1, 2003 mperature fatigue durability of CMCs. Initial results on two Materials Society(TMS), Seattle, WA, February tn-o, z2 A c- idation of High- CMCs from two earlier investigations by Lee et al. and Steel et Temperature Materials at the 131st Annual Meet al.2 are reviewed and discussed along with the results for three 1282

Consequence of Intermittent Exposure to Moisture and Salt Fog on the High-Temperature Fatigue Durability of Several Ceramic-Matrix Composites Larry P. Zawada, James Staehler,* and Steve Steel Materials and Manufacturing Directorate, Air Force Research Laboratory, AFRL/MLLN, Wright-Patterson AFB, Ohio 45433-7817 Fatigue behavior of four ceramic-matrix composites (CMCs) was documented at 1000°C, and a fifth composite was docu￾mented at 1200°C. Additional fatigue specimens were cycled for set blocks of cycles, removed from the fatigue machine, and exposed in a cyclic corrosion tester for 24 h with a fog of deionized water and a fog of deionized water containing 0.05 wt% NaCl. BN-fiber-coated NicalonTM/SiNC and Nicalon/ Al2O3 experienced a pronounced decrease in fatigue life (
86%) with salt fog exposure. Nicalon/C experienced rapid loss of the SiC exterior seal coat and a 30% decrease in life with salt fog exposure. Nextel610/AS and Nextel720/Al2O3 demonstrated no loss in fatigue performance or retained strength with water or salt fog exposure. Changes to the constituents of Nicalon/SiNC were evaluated to determine if they influenced moisture sensitivity. BN fiber coatings, BN or BN/SiC, alternate matrix prepreg, and matrix filler type had no influence on improving moisture resistance. Direct exposure to moisture fog produced accelerated rates of degradation in the BN fiber coating and greatly decreased fatigue durability. I. Introduction THE class of materials known as ceramic-matrix composites (CMCs) continues to be an attractive choice for many aero￾space turbine engine applications. CMCs retain strength to much higher temperatures than do metals, and they offer lower density than conventional nickel-based superalloys. However, it is the prospect of higher material operating temperatures and decreased cooling air that is most attractive to the aerospace design commu￾nity. Higher gas temperatures and less cooling air translate directly to increased thrust and decreased fuel consumption. In addition, less cooling air for the combustor has the important effect of decreasing the production of nitrous oxides (NOx), which are detrimental to earth’s protective ozone layer. Currently, CMCs are being demonstrated in turbine components, including combustor liners, turbine nozzles, shrouds, transition ducts, diffusers, and exhaust structures, such as divergent flaps and seals, as well as structures that have hot exhaust gases flowing over them (exhaust￾washed structures). Many of these demonstration components have exhibited evi￾dence of accelerated degradation after only a few hours of hot time. The degradation involves oxidation of fibers and fiber coatings, and the degradation is accelerated whenever moisture is present. Air on a humid day contains from 0.003 to 0.025 atm (1 atm 1 105 Pa) of H2O. For combustors, the combustion products from burning hydrocarbon fuel contain
10% H2O (PH2O 0.1 atm). For combustors operating at 10 atm, the hot gas stream contains
10–15 vol% of H2O. The degradation in SiC/SiC CMCs under turbine engine combustor conditions has been thor￾oughly documented. A thorough discussion about CMC degrada￾tion mechanisms in combustors is given by More and co￾workers1,2 and by Ferber et al.3 The components at the back of the turbine engine (divergent nozzle or exhaust-washed structures) are exposed to high-velocity hot exhaust gases containing a high moisture content and are periodically exposed to moisture whenever there is rain or con￾densation of moisture. Fighter aircraft, such as the F-18 Hornet, the F-16 Falcon, and the F-15 Eagle, are considered all-weather fighters, which means they fly in all types of weather, including rain. While parked on the ground, the exhaust nozzles and structures are normally covered by a waterproof tarp, but the tarps need to be removed before engine start. Direct impingement of rain on these structures periodically occurs as a normal part of operations during the service life of military aerospace platforms. Periodic washing of the aircraft also results in exposure to water. Although CMC parts occasionally are exposed to rain, they also are exposed to salt fog from ocean mists and exhaust deposits. Fused salt deposits accelerate the hot corrosion of most high￾temperature alloys used in jet engines. Salt fog (H2O  NaCl) exposure also has a strong effect on SiC-fiber strength.4,5 In addition, salts are associated with the dissolution (fluxing) of the normally protective oxide scales, such as SiO2, as well as other oxides, such as B2O3. Richardson and Kowalik6 have shown that hot corrosion of NicalonTM/C with Na2SO4 occurs above 900°C, with extensive degradation of the protective SiC surface coating and loss of the carbon-matrix material. Thus, water and salt fog are an important factor for long-term mechanical performance at high temperature. Environmental testing early in the evaluation process can help determine if these environmental effects are a problem. There is extensive literature on the subject of moisture and how it affects the oxidation of SiC.7–12 Opila and Hann8 and Opila9,10 have shown that the presence of water vapor increases the rate of SiO2 growth on SiC at high temperature and that it leads to accelerated rates of SiC recession. There is also extensive literature on the subject of moisture and how it affects the degradation of BN fiber coatings.13–20 Sensitivity of BN to moisture has been shown to be strongly dependent on the quality of the BN, impurities present in the BN and moisture, levels of moisture, temperature, and amount of matrix cracking. Most of the work to date has involved exposures and oxidative studies of coupons without the application of stress. The goal of the work reported in this article is to assess the overall extent that moisture affects the high￾temperature fatigue durability of CMCs. Initial results on two CMCs from two earlier investigations by Lee et al.21 and Steel et al.22 are reviewed and discussed along with the results for three other CMCs. In addition, results from two specially designed E. J. Oplia—contributing editor Manuscript No. 186663. Received September 13, 2002; approved April 1, 2003. Presented at the Symposium on Water Vapor Effects on Oxidation of High￾Temperature Materials at the 131st Annual Meeting of the Minerals, Metals, & Materials Society (TMS), Seattle, WA, February 18–20, 2002. *Member, American Ceramic Society. High-Temperature Water Vapor Effects J. Am. Ceram. Soc., 86 [8] 1282–91 (2003) 1282 journal

Consequence of Moisture and Salt Fog on High-Temperature Fatigue of Ceramic-Matrix Composites 283 moisture investigations are used to qualify to what extent the at 1200%C to investigate the potential of this CMC. At 1200C, the high-temperature fatigue behavior of certain types of CMC is Al,O3 matrix should show no sign of softening or weakening, and affected by the direct exposure to moisture the n720 fiber should be relatively stable for the 27.7 h anticipated for reaching run out Fatigue testing was conducted in laboratory air using a servo- I. Materials hydraulic fatigue machine. The fatigue cycle used a load rat of 0.05 and a frequency of I Hz. Run out was defined as 100 000 To thoroughly explore the role of moisture on the fatigue cycles. Results of the standard fatigue experiments were used to durability of CMCs, five different CMCs were purchased from enerate a fatigue diagram of stress versus cycles to failure for their manufacturer and evaluated. all the cmcs have been used as each CMC. These results were also used as the base line for the an eight-harness-satin-weave(8HSW) fiber architecture, and each fatigue plus moisture experime of these material systems has been described in detail else- where -14 Therefore, only a brief description is given here for An environmental chamber with temperature control, humidity control(not used), and fog- generating capability was used to each CMC. Three of the CMCs contain ceramIc-grade Nicalon investigate the influence of water fog and salt fog exposu fibers. The first is Nicalon/Al,O3 from Lanxide Corp. The CMC contains an Al,O, matrix, and the fibers are coated with a dual high-temperature fatigue performance. Chamber temperature was layer of BN and then SiC. The bn is there for debonding, whereas controlled at 35.C and atmospheric pressure. During operati the SiC is applied over the BN to protect it during matrix synthesis. compressed air(gauge pressure of 0. 1 MPa) was humidified by A unique process involving metal oxidation is used to create the passing through a bubble tower, after which it was mixed with the Al,O, matrix. The second CMC is Nicalon/SINC(Nicalon/silicon desired solution in a nozzle. The nozzle atomized the solution and nitrocarbide)from Dow Corning Corporation, and the fibers are air to a corrosive fog. The chamber used -0.76 L of solution/h coated with BN and then Si, N, Multiple polymer infiltration and Operation of the chamber in the fog mode resulted in air that was pyrolysis(PIP) cycles are used to synthesize the amorphous fully saturated with water(PHo s 0.055 atm) and a thick fog of matrix. The third CMC is Nicalon/C from Hitco Technologies water particles. Test specimens were suspended near the center of This Cmc has no fiber coating. but does have b, c filler added to the chamber by racks. During the exposures, the specime the matrix to serve as a glass former and a Sic exterior coating to became fully saturated as solution condensed on them, but were protect the inhibited carbon matrix from oxidation. The matrix is never immersed in water. Specimens were arranged with sufficient thesized using pip followed by chemical infiltration space between test pieces for adequate fog circulation, and in such (CVI) of carbon. The fourth and fifth CMCs are oxide/oxide a way that did not allow condensation from one specimen to fall on CMCs that use a porous matrix to impart toug into the CmC another specimen. During the exposure, the test specimens were in One CMc is a Nextel 610/aluminosilicate(N610/AS)from Gen- continual contact with water and air eral Electric, and the other is a Nextel 720/Al,O3(N720/A)from ionized water was used for the water fog exposures, while COI Ceramics, Neither of these all-oxide CMCs has a coating deionized water mixed with salt(Nacl)was used for the salt fog applied to the fibers. There is no evidence of an interphase between exposures. A Nacl concentration of 0.05 wt% was used. This the fibers and matrix after processing. The matrix of both CMCs is concentration was selected to be 2 orders of magnitude above nthesized using a sol-gel method. the concentrations at typical flying altitudes for most aircraft An additional aspect of this investigation was to determine what and 2 orders of magnitude below what is normally found in the onstituents in the Cmc contributed to moisture-induced degra- saltwater oceans. Salt concentration in the water of the earth's dation. The Nicalon/SiNC system( 8HS W) was selected for further oceans is 3. 4 wt% near the shore and 3. 6% at the center of the study, because it demonstrated accelerated degradation when oceans. The salt concentration of ocean saltwater was not used exposed to moisture. Factors that were changed included the this investigation, because it already had been shown to have ber-matrix interphase, fiber orientation of polymer cross- a very pronounced effect on SiC fibers" and produced failure link chemistry, and filler. The variations of the CMC were during the first few thousand cycles of testing for the Nicalon/ BN-Si3Na interphase, 0/90 lay-up, basic prevtesdvarepPerepreg, tal salt fog exposure were applied alternately to the specimens.The BN-Si3N4 interphase, 0/90 lay-up, optional prepreg, Si3Na filler In performing each experiment, cyclic loading and environmen- 11BN-Si3Na interphase, quasi-isotropic lay-up. Sis Na filler; (iv)BN interphase, 0/90 lay-up, basic prepreg, SiC run-out condition for the fatigue plus moisture experiments filler; and(v)BN interphase, 0/90 lay-up, optional prepreg, SiC filler cluded a total of eight blocks of fatigue cycling at 1000C and The fiber coating changes and prepreg changes are a direct seven 24 h salt fog exposures. Stress levels were selected to be the attempt at identifying those compositions that are more resistant to same as for the standard isothermal fatigue experiments. The moisture-induced degradation. Cloth lay-up, prepreg, and filler measured life at each stress level was divided into separate blocks hanges should have an impact on the structure and porosity of the The increments were 5%. 10%.15%. 20%. 25%.50%.75%. and matrix. The goal is to alter geometry and distribution of the 00%. Each test specimen was first fatigued at 1000C at the elected stress level for 5% of the expected fatigue life. At the en internal oxidation. This has been well documented for carbon/ of the block of fatigue cycles, the specimen was held at zero carbon composites, and, more recently, it has been demonstrated in applied load and rapidly cooled to room temperature. The test specimen was then removed from the fatigue test frame and placed hanges should also influence matrix crack density and crac in the environmental chamber for a 24 h exposure. After 24 h, the distributions within the CMC. It is also speculated that these test specimen was placed in a drying oven under slight vacuum at hanges might affect the residual stress state, thus influencing the 37C for-12 h. This step was required to remove pockets of water stress level at which matrix cracks open up under tensile load that would remain in the large pores of the specimen. Initial trial runs of the test procedure revealed that, if the water remained in the large pores, it produced small localized delaminations during Il. Experiments heating. After the specimen was dried, it was placed back into the fatigue test frame, ramped to temperature in -5-10 min, held for (PL) stress and for selection of appropriate stress levels for the procedure was repeated five times. After the first five blocks, the atigue tests. Tensile and fatigue specimens were 150 mm long cycle increment was then increased to 25% for the remaining three with a dogbone shape and a ection width of 10 mm blocks of fatigue testing. Testing was terminated when either the Tension tests were performed in ory air at room temperature specimen failed or 100% estimated fatigue life was reached d at 1000oc for four CMcs wi N720/A system was tested Several specimens that reached the run-out condition were tension

moisture investigations are used to qualify to what extent the high-temperature fatigue behavior of certain types of CMC is affected by the direct exposure to moisture. II. Materials To thoroughly explore the role of moisture on the fatigue durability of CMCs, five different CMCs were purchased from their manufacturer and evaluated. All the CMCs have been used as an eight-harness-satin-weave (8HSW) fiber architecture, and each of these material systems has been described in detail else￾where.21,22 Therefore, only a brief description is given here for each CMC. Three of the CMCs contain ceramic-grade Nicalon fibers. The first is Nicalon/Al2O3 from Lanxide Corp. The CMC contains an Al2O3 matrix, and the fibers are coated with a dual layer of BN and then SiC. The BN is there for debonding, whereas the SiC is applied over the BN to protect it during matrix synthesis. A unique process involving metal oxidation is used to create the Al2O3 matrix. The second CMC is Nicalon/SiNC (Nicalon/silicon nitrocarbide) from Dow Corning Corporation, and the fibers are coated with BN and then Si3N4. Multiple polymer infiltration and pyrolysis (PIP) cycles are used to synthesize the amorphous matrix. The third CMC is Nicalon/C from Hitco Technologies. This CMC has no fiber coating, but does have B4C filler added to the matrix to serve as a glass former and a SiC exterior coating to protect the inhibited carbon matrix from oxidation. The matrix is synthesized using PIP followed by chemical vapor infiltration (CVI) of carbon. The fourth and fifth CMCs are oxide/oxide CMCs that use a porous matrix to impart toughness into the CMC. One CMC is a Nextel 610/aluminosilicate (N610/AS) from Gen￾eral Electric, and the other is a Nextel 720/Al2O3 (N720/A) from COI Ceramics. Neither of these all-oxide CMCs has a coating applied to the fibers. There is no evidence of an interphase between the fibers and matrix after processing. The matrix of both CMCs is synthesized using a sol–gel method. An additional aspect of this investigation was to determine what constituents in the CMC contributed to moisture-induced degra￾dation. The Nicalon/SiNC system (8HSW) was selected for further study, because it demonstrated accelerated degradation when exposed to moisture. Factors that were changed included the fiber–matrix interphase, fiber orientation, type of polymer cross￾link chemistry, and filler. The variations of the CMC were (i) BN–Si3N4 interphase, 0/90 lay-up, basic prepreg, Si3N4 filler; (ii) BN–Si3N4 interphase, 0/90 lay-up, optional prepreg, Si3N4 filler; (iii) BN–Si3N4 interphase, quasi-isotropic lay-up, basic prepreg, Si3N4 filler; (iv) BN interphase, 0/90 lay-up, basic prepreg, SiC filler; and (v) BN interphase, 0/90 lay-up, optional prepreg, SiC filler. The fiber coating changes and prepreg changes are a direct attempt at identifying those compositions that are more resistant to moisture-induced degradation. Cloth lay-up, prepreg, and filler changes should have an impact on the structure and porosity of the matrix. The goal is to alter geometry and distribution of the porosity. It is known that extensive porosity promotes rapid internal oxidation. This has been well documented for carbon/ carbon composites, and, more recently, it has been demonstrated in combustor tests of SiC/SiC composites.1–3 In addition, such changes should also influence matrix crack density and crack distributions within the CMC. It is also speculated that these changes might affect the residual stress state, thus influencing the stress level at which matrix cracks open up under tensile load. III. Experiments Tension tests were conducted to identify the proportional limit (PL) stress and for selection of appropriate stress levels for the fatigue tests. Tensile and fatigue specimens were 150 mm long with a dogbone shape and a gauge section width of 10 mm. Tension tests were performed in laboratory air at room temperature and at 1000°C for four CMCs, while the N720/A system was tested at 1200°C to investigate the potential of this CMC. At 1200°C, the Al2O3 matrix should show no sign of softening or weakening, and the N720 fiber should be relatively stable for the 27.7 h anticipated for reaching run out. Fatigue testing was conducted in laboratory air using a servo￾hydraulic fatigue machine.21,22 The fatigue cycle used a load ratio of 0.05 and a frequency of 1 Hz. Run out was defined as 100 000 cycles. Results of the standard fatigue experiments were used to generate a fatigue diagram of stress versus cycles to failure for each CMC. These results were also used as the base line for the fatigue plus moisture experiments. An environmental chamber with temperature control, humidity control (not used), and fog-generating capability was used to investigate the influence of water fog and salt fog exposure on high-temperature fatigue performance. Chamber temperature was controlled at 35°C and atmospheric pressure. During operation, compressed air (gauge pressure of 0.1 MPa) was humidified by passing through a bubble tower, after which it was mixed with the desired solution in a nozzle. The nozzle atomized the solution and air to a corrosive fog. The chamber used
0.76 L of solution/h. Operation of the chamber in the fog mode resulted in air that was fully saturated with water (PH2O  0.055 atm) and a thick fog of water particles. Test specimens were suspended near the center of the chamber by racks. During the exposures, the specimens became fully saturated as solution condensed on them, but were never immersed in water. Specimens were arranged with sufficient space between test pieces for adequate fog circulation, and in such a way that did not allow condensation from one specimen to fall on another specimen. During the exposure, the test specimens were in continual contact with water and air. Deionized water was used for the water fog exposures, while deionized water mixed with salt (NaCl) was used for the salt fog exposures. A NaCl concentration of 0.05 wt% was used. This concentration was selected to be 2 orders of magnitude above the concentrations at typical flying altitudes for most aircraft and 2 orders of magnitude below what is normally found in the saltwater oceans. Salt concentration in the water of the earth’s oceans is
3.4 wt% near the shore and 3.6% at the center of the oceans. The salt concentration of ocean saltwater was not used in this investigation, because it already had been shown to have a very pronounced effect on SiC fibers4 and produced failure during the first few thousand cycles of testing for the Nicalon/ SiNC composite. In performing each experiment, cyclic loading and environmen￾tal salt fog exposure were applied alternately to the specimens. The run-out condition for the fatigue plus moisture experiments in￾cluded a total of eight blocks of fatigue cycling at 1000°C and seven 24 h salt fog exposures. Stress levels were selected to be the same as for the standard isothermal fatigue experiments. The measured life at each stress level was divided into separate blocks. The increments were 5%, 10%, 15%, 20%, 25%, 50%, 75%, and 100%. Each test specimen was first fatigued at 1000°C at the selected stress level for 5% of the expected fatigue life. At the end of the block of fatigue cycles, the specimen was held at zero applied load and rapidly cooled to room temperature. The test specimen was then removed from the fatigue test frame and placed in the environmental chamber for a 24 h exposure. After 24 h, the test specimen was placed in a drying oven under slight vacuum at 37°C for
12 h. This step was required to remove pockets of water that would remain in the large pores of the specimen. Initial trial runs of the test procedure revealed that, if the water remained in the large pores, it produced small localized delaminations during heating. After the specimen was dried, it was placed back into the fatigue test frame, ramped to temperature in
5–10 min, held for 5 min at temperature, and fatigue cycling was initiated. This procedure was repeated five times. After the first five blocks, the cycle increment was then increased to 25% for the remaining three blocks of fatigue testing. Testing was terminated when either the specimen failed or 100% estimated fatigue life was reached. Several specimens that reached the run-out condition were tension August 2003 Consequence of Moisture and Salt Fog on High-Temperature Fatigue of Ceramic-Matrix Composites 1283

1284 Journal of the American Ceramic Sociery-Zawada et al. Vol 86. No. 8 tested to measure residual tensile strength. The specimen fracture urfaces were observed using Sem to determine damage and failure modes The effect of interrupting the fatigue test, but without environ- Fiber Form: 8HSW mental fog exposure, was also investigated to determine what effect interrupting the fatigue tests had on fatigue study, first-generation Nicalon/SINC samples with a PL of 75 MPa were fatigued at 100 MPa and 1000oC for blocks of 5000 cycles and then thermally cycled using the same cooling and heating rates as for the fog experiments. After 25 000 cycles, the increment w changed to 25 000 cycle intervals. Results from these interrupted t and thermally cycled tests were compared with identical speci mens that were isothermally fatigued. This gave a one-to-on comparison and documented the effect of the thermal cycles Results (1 Tensile Strain(%) Multiple tension tests(typically three)were performed at room emperature and elevated temperature for the five CMCs. Average Fig. 1. Tensile stress versus strain behavior for Nicalon/Al,o mechanical behavior values from the tension tests are shown in Table I The stress-strain traces for Nicalon/Al,O, are shown in Fig toom-temperature behavior exhibits a well-defined PL. Increasing to failure. It is very difficult to identify a traditional Pl for this the temperature to 1000.C has a distinct effect on the stress versus CMC. Temperature has no effect on the stress-strain response, but rain behavior. At temperature, the PL increases slightly, the there is a slight decrease in the uts strain to failure decreases, and the ultimate tensile strength ( UTs) Behavior of the N720/A system(Fig. 5)is also essentially linear is essentially unchanged. Significant changes in the stress-strain to failure Tests at 1200%C result in a lower modulus value and behavior as a function of temperature make designing components significantly more strain to failure. However, there is little de crease in the uts very difficult and is an early indicator that this CMC may not be a good candidate material system for operation at 1000C Tensile behavior of the Nicalon/SINC system is shown in Fig (2) Fatigue, Fatigue plus Salt Fog, and Fatigue plus 2. This CMC also demonstrates a well-defined PL. Above the Pl, Water Fog the response remains essentially linear up to the UTS. The tensile The results of the fatigue plus salt fog experiment for Nicalon/ AL,O, are shown in Fig. 6. Run out is not achieved during the tress-strain traces decreasing within a tight range. There is no fatigue tests until the stress level decreases to 75 MPa. This stress bservable effect of temperature on fast-fracture behavior The tensile behavior of the Nicalon/C is noticeably different controlling feature for long service life. Fatigue plus salt fog tests om the first two CMCs(Fig. 3). There is a small initial linear at 75 MPa ran out, but at 100 MPa, the life of the test specimen is region up to 55 MPa, followed by a subtle transition to slightl significantly decreased. In addition, the retained strength after run out at 75 MPa decreases to 1 14 MPa for the isothermal fatigue linear behavior out to failure. Picking a PL stress level for this specimen and 69 MPa for the salt fog specimen. These results material is very subjective. Such behavior is likely the result of the during fatigue testing pronounced matrix cracking that occurs during synthesis of the and that severe degradation occurs during the salt fog experiment. matrix. At very low loads, these matrix cracks start to open and to This CMC demonstrates poor durability during fatigue, and th extend, but, with the low-fracture-energy carbon matrix, this is not retained strength values are low. Therefore, no further fatigue plus a sudden or rapid event, as shown for the first two CMCs. The salt fog experiments have been conducted ress-strain traces at both temperatures are essentially identical The UTS of this CMC is 250 MPa, and the uts at 1000.C is slightly higher than at 23C. Overall, temperature has little effect on fast-fracture behavior The behavior of the N610/As system is shown in Fig 4. This Load Rate =0.05 m CMC exhibits stress versus strain behavior that is essentially linear Fiber Form: 8HSW Table 1. Average mechanical Behavior Values from Room- and Elevated-Temperature Tension Tests on Five Different cmcs CMC system UTS (MPa) (GPa) (o) PL (MPa) Nicalon/Al 031000 Nicalon/SiNC Nicalon/SiNC Nicalon/ 1000 670.60 Nextel610/AS 700.36100 0.00.1020.3040.50.6 Nextel6IO/AS Strain(%) Nextel720/A 690.2480 Nextel720/A 0.30 Fig. 2. Tensile stress versus strain behavior for Nicalon/SiNC

tested to measure residual tensile strength. The specimen fracture surfaces were observed using SEM to determine damage and failure modes. The effect of interrupting the fatigue test, but without environ￾mental fog exposure, was also investigated to determine what effect interrupting the fatigue tests had on fatigue life. In this study, first-generation Nicalon/SiNC samples with a PL of 75 MPa were fatigued at 100 MPa and 1000°C for blocks of 5000 cycles and then thermally cycled using the same cooling and heating rates as for the fog experiments. After 25 000 cycles, the increment was changed to 25 000 cycle intervals. Results from these interrupted and thermally cycled tests were compared with identical speci￾mens that were isothermally fatigued. This gave a one-to-one comparison and documented the effect of the thermal cycles on fatigue live. IV. Results (1) Tensile Multiple tension tests (typically three) were performed at room temperature and elevated temperature for the five CMCs. Average mechanical behavior values from the tension tests are shown in Table I. The stress–strain traces for Nicalon/Al2O3 are shown in Fig. 1. Room-temperature behavior exhibits a well-defined PL. Increasing the temperature to 1000°C has a distinct effect on the stress versus strain behavior. At temperature, the PL increases slightly, the strain to failure decreases, and the ultimate tensile strength (UTS) is essentially unchanged. Significant changes in the stress–strain behavior as a function of temperature make designing components very difficult and is an early indicator that this CMC may not be a good candidate material system for operation at 1000°C. Tensile behavior of the Nicalon/SiNC system is shown in Fig. 2. This CMC also demonstrates a well-defined PL. Above the PL, the response remains essentially linear up to the UTS. The tensile behavior at 23° and 1000°C is essentially identical, with all the stress–strain traces decreasing within a tight range. There is no observable effect of temperature on fast-fracture behavior. The tensile behavior of the Nicalon/C is noticeably different from the first two CMCs (Fig. 3). There is a small initial linear region up to 55 MPa, followed by a subtle transition to slightly nonlinear increase in strain with increasing stress, followed by linear behavior out to failure. Picking a PL stress level for this material is very subjective. Such behavior is likely the result of the pronounced matrix cracking that occurs during synthesis of the matrix. At very low loads, these matrix cracks start to open and to extend, but, with the low-fracture-energy carbon matrix, this is not a sudden or rapid event, as shown for the first two CMCS. The stress–strain traces at both temperatures are essentially identical. The UTS of this CMC is 250 MPa, and the UTS at 1000°C is slightly higher than at 23°C. Overall, temperature has little effect on fast-fracture behavior. The behavior of the N610/AS system is shown in Fig. 4. This CMC exhibits stress versus strain behavior that is essentially linear to failure. It is very difficult to identify a traditional PL for this CMC. Temperature has no effect on the stress–strain response, but there is a slight decrease in the UTS. Behavior of the N720/A system (Fig. 5) is also essentially linear to failure. Tests at 1200°C result in a lower modulus value and significantly more strain to failure. However, there is little de￾crease in the UTS. (2) Fatigue, Fatigue plus Salt Fog, and Fatigue plus Water Fog The results of the fatigue plus salt fog experiment for Nicalon/ Al2O3 are shown in Fig. 6. Run out is not achieved during the fatigue tests until the stress level decreases to 75 MPa. This stress level is below the PL and clearly identifies the PL as the controlling feature for long service life. Fatigue plus salt fog tests at 75 MPa ran out, but at 100 MPa, the life of the test specimen is significantly decreased. In addition, the retained strength after run out at 75 MPa decreases to 114 MPa for the isothermal fatigue specimen and 69 MPa for the salt fog specimen. These results identify that substantial degradation occurs during fatigue testing and that severe degradation occurs during the salt fog experiment. This CMC demonstrates poor durability during fatigue, and the retained strength values are low. Therefore, no further fatigue plus salt fog experiments have been conducted. Fig. 1. Tensile stress versus strain behavior for Nicalon/Al2O3. Fig. 2. Tensile stress versus strain behavior for Nicalon/SiNC. Table I. Average Mechanical Behavior Values from Room￾and Elevated-Temperature Tension Tests on Five Different CMCs CMC system Temperature (°C) UTS (MPa) Modulus (GPa) Strain (%) PL (MPa) Nicalon/Al2O3 23 196 187 0.56 60 Nicalon/Al2O3 1000 193 153 0.39 72 Nicalon/SiNC 23 197 107 0.32 85 Nicalon/SiNC 1000 214 101 0.42 75 Nicalon/C 23 221 73 0.47 54 Nicalon/C 1000 254 67 0.60 56 Nextel610/AS 23 205 70 0.36 100 Nextel610/AS 1000 173 77 0.26 83 Nextel720/A 23 144 69 0.24 80 Nextel720/A 1200 140 55 0.30 35 1284 Journal of the American Ceramic Society—Zawada et al. Vol. 86, No. 8

August 2003 Consequence of Moisture and Salt Fog on High-Temperature Fatigue of Ceramic-Matrix Composites 285 250F Load Rate=0.05 mr Loading Rate: 0.05 mm/s Fiber Form: 8HSW Fiber Form: 8HS 200 150 23°c 5 1000c 020 0.00.1 050.6 0.0 010 Strain (%) Fig. 5. Tensile stress versus strain behavior for N720/A Fig. 3. Tensile stress versus strain behavior for Nicalon/C The second material investigated was the Nicalon/SINC, results BN-coated fibers experienced greatly accelerated degradation out during standard fatigue testing is achieved at 100 MPa. The selected for further study using only deionized water. The ol are shown in Fig. 7 and are discussed in detail by Lee et al.- Run Therefore, the Nicalon/SiNC and Nicalon/Al,O, systems w stress-strain traces in Fig 3 show that 100 MPa is above the Pl of tive was to determine if the salt was causing the rapid degradation 75 MPa. Thus, during fatigue testing, this CMC reaches run out or if it was primarily the moisture. Results for the fatigue plus sa even with extensive matrix cracking. However, once fatigue fog are compared with the results of fatigue plus water fog in Table ombined with salt fog, there is a significant decrease in life at 100 Il. Presented in Table ll are the maximum fatigue stress levels and MPa. Fatigue life is decreased by I order of magnitude for each of corresponding decrease in life as a result of the the three stress levels studied and clearly identifies that the salt fog For Nicalon/SINC. the decrease in life as a result of water fo exposures result in an additional aggressive degradation mecha- exposure is the same as for the salt fog exposures. Three specimens nism that dominates the failure process. tested at 125 MPa are all within -1000 cycles of each other, and The fatigue results for Nicalon/C are shown in Run out their lives are extremely short. This clearly identifies that th is achieved at 75 MPa for the isothermal fatigue te 的能 primary culprit is moisture and that any contribution to degrada- g tests, and the 75 MPa stress level is actually tion from the small concentration of salt is overwhelmed by the ress of 55 MPa. The salt fog experiments primary degradation caused by moisture. However, once the stress life of 30% over the standard fatigue experiments level is decreased to below the PL, the two specimens tested at 75 The last system salt fog tested was the N610/AS ite,and MPa run out, thus highlighting the contribution of matrix cracking the fatigue results are shown in Fig. 9. This CMC exhibits to accelerated degradation excellent fatigue resistance at 1000C, with run out very near the Decreased life with water fog wa UTS of the CMC. The introduction of salt fog has no effect on the alonAl O3 system. Two specimens stress of fatigue life or retained strength values. In fact, the tensile strength 75 MPa failed before reaching run out. failed at after fatigue testing is typically 5%10% higher than the as- a stress level where specimens exposed to salt fog ran out. Such processed value findings indicate that there is some variability in the Pl value and The three CMCs with Nicalon fibers experienced decreased lives with the introduction of salt fog, and the two Cmcs with Tension Test T=1000°c 10o0c 0.05 mm/s Fiber 8HSW 0.00.1 Cycles(N Strain(%) Fig. 6. Fatigue diagram of stress versus cycles to failure for Nicalon/ Al,, Specimens were subjected to fatigue and interrupted fatigue plus Fig 4. Tensile stress versus strain behavior for N610/AS

The second material investigated was the Nicalon/SiNC; results are shown in Fig. 7 and are discussed in detail by Lee et al.21 Run out during standard fatigue testing is achieved at 100 MPa. The stress–strain traces in Fig. 3 show that 100 MPa is above the PL of 75 MPa. Thus, during fatigue testing, this CMC reaches run out even with extensive matrix cracking. However, once fatigue is combined with salt fog, there is a significant decrease in life at 100 MPa. Fatigue life is decreased by 1 order of magnitude for each of the three stress levels studied and clearly identifies that the salt fog exposures result in an additional aggressive degradation mecha￾nism that dominates the failure process. The fatigue results for Nicalon/C are shown in Fig. 8. Run out is achieved at 75 MPa for the isothermal fatigue tests and the salt fog tests, and the 75 MPa stress level is actually above the PL stress of 55 MPa. The salt fog experiments result in a decrease in life of 30% over the standard fatigue experiments. The last system salt fog tested was the N610/AS composite, and the fatigue results are shown in Fig. 9. This CMC exhibits excellent fatigue resistance at 1000°C, with run out very near the UTS of the CMC. The introduction of salt fog has no effect on the fatigue life or retained strength values. In fact, the tensile strength after fatigue testing is typically 5%–10% higher than the as￾processed value. The three CMCs with Nicalon fibers experienced decreased lives with the introduction of salt fog, and the two CMCs with BN-coated fibers experienced greatly accelerated degradation. Therefore, the Nicalon/SiNC and Nicalon/Al2O3 systems were selected for further study using only deionized water. The objec￾tive was to determine if the salt was causing the rapid degradation, or if it was primarily the moisture. Results for the fatigue plus salt fog are compared with the results of fatigue plus water fog in Table II. Presented in Table II are the maximum fatigue stress levels and corresponding decrease in life as a result of the exposures. For Nicalon/SiNC, the decrease in life as a result of water fog exposure is the same as for the salt fog exposures. Three specimens tested at 125 MPa are all within
1000 cycles of each other, and their lives are extremely short. This clearly identifies that the primary culprit is moisture and that any contribution to degrada￾tion from the small concentration of salt is overwhelmed by the primary degradation caused by moisture. However, once the stress level is decreased to below the PL, the two specimens tested at 75 MPa run out, thus highlighting the contribution of matrix cracking to accelerated degradation. Decreased life with water fog was also observed for the Nicalon/Al2O3 system. Two specimens tested at a fatigue stress of 75 MPa failed before reaching run out. These specimens failed at a stress level where specimens exposed to salt fog ran out. Such findings indicate that there is some variability in the PL value and, Fig. 3. Tensile stress versus strain behavior for Nicalon/C. Fig. 4. Tensile stress versus strain behavior for N610/AS. Fig. 5. Tensile stress versus strain behavior for N720/A. Fig. 6. Fatigue diagram of stress versus cycles to failure for Nicalon/ Al2O3. Specimens were subjected to fatigue and interrupted fatigue plus salt fog exposure. August 2003 Consequence of Moisture and Salt Fog on High-Temperature Fatigue of Ceramic-Matrix Composites 1285

1286 Journal of the American Ceramic Sociery-Zawada et al. Vol 86. No. 8 T=1000°c T=1000°c 0o0°c atique 150 口 Fatigue ▲ Fatigue+ Salt Fog ue t Salt Fo 10010110210310410510° Cycles(N) ycles(N) 7. Fatigue diagram of stress versus cycles to failure for Nicalon/ diagram of stress cles to failure for n610/A SINC. Specimens were subjected to fatigue and interrupted fatigue plus salt subjected to fatigue and interrupted fatigue plus salt fog ubsequently, in fatigue resistance of this CMC. However, it does Al,O3 CMC has excellent strength retention after air exposure at document that H,O is responsible for the accelerated de 1200° for 1000h The n720/A nts were performed only using deionized It was speculated that, if changes were made to the constituents water, because the N61O/As system showed no influence of the in the Nicalon/SiNC CMC, there might be a change in the moisture alt. This CMC was tested at 1200C, and the results are shown in sensitivity. Results from these experiments are shown in Table Ill Fig. 10. These results have been discussed in more detail else- None of these chang pacted moisture resistance. Neither the where by Steel et al. 2 but are also included here for completeness fiber coating nor the alternate prepreg material improved resis In summary, the behavior mirrored that observed for the N61oAs tance to moisture. It was hoped that the changes would produce a composite, in that the fatigue limit was near the UTS and that the more uniform microstructure, with a change in the morphology of specimens either failed at low cycle counts or ran out. The run-out the porosity. It has been observed that porosity contributes to the stress levels were repeated with the fatigue plus water fo experiments, and all these ran out as well. Thus, this CMC system mlos adation of the material by providing paths for oxygen and ure to penetrate into the interior of the test specimen experienced no degradation from the fatigue process or the fatigue However, optical observations of the cut edges of test specimens plus water fog exposure. Residual strength tests showed the fatigue revealed that the geometry and shape of the porosity was not specimens to be stronger than the as-received specimens. Clearly, altered significantly. These observations suggested that significant this was exceptional fatigue behavior, especially for a test tempe mprovements in processing of this CMC may some day result in ature of 1200C. Excellent retained strength was not unexpecte decreased sensitivity to moisture because Campbell et al. have shown that this CMC looses only Results of the fatigue and interrupted fatigue experiments are for 1000 h. Carelli et al. 2 have also shown that a N720/mullite- number of times the interrupted experiments were cooled 0o and -7% of its load-carrying capability after air exposure at 1200C shown in Table IV. Table Iv shows the cycles to failure temperature and then heated back to test temperature. The rupted fatigue specimens demonstrated a decrease in life of 72406 ue exper 250 °c T=1000°c V. Discussion re fatigue life of CMCs is generally associated with the onset of no gation, run out of 100 000 cycles is observed to be at stress levels equal to or slightly above the Pl. However, exposure to water fog lowers the run-out stress level to below the PL. Even though the Nicalon/SiNc and Nicalon/c systems have extensive matrix cracks in 5% ufficiently sealed, at least at the surfaces, so that oxidation species do not have a path to penetrate into the Cmc. For Nicalon/SINC the matrix is in residual compression, and this should also aid in keeping cracks closed. All three Nicalon-fiber CMCs are appa ently dense enough to limit any substantial oxygen diffusion into the CMC as long as the existing matrix cracks remain closed. As the existing cracks open, they provide an easy path for oxygen Cycles(n) transport throughout the matrix, promoting rapid oxidation, Kallur et al. studied the high-temperature fatigue behavior of Nicalon/ Fig. 8. Fatigue diagram of stress versus cycles to failure for Nicalon/C. SiNC using humidity exposures as well as salt fog, similar to this pecimens were subjected to fatigue and interrupted fatigue plus salt fo investigation. The test specimens were unique in that they con- tained many machined holes with a nominal diameter of 1. 8 mm

subsequently, in fatigue resistance of this CMC. However, it does document that H2O is responsible for the accelerated degradation. The N720/A experiments were performed only using deionized water, because the N610/AS system showed no influence of the salt. This CMC was tested at 1200°C, and the results are shown in Fig. 10. These results have been discussed in more detail else￾where by Steel et al.22 but are also included here for completeness. In summary, the behavior mirrored that observed for the N610/AS composite, in that the fatigue limit was near the UTS and that the specimens either failed at low cycle counts or ran out. The run-out stress levels were repeated with the fatigue plus water fog experiments, and all these ran out as well. Thus, this CMC system experienced no degradation from the fatigue process or the fatigue plus water fog exposure. Residual strength tests showed the fatigue specimens to be stronger than the as-received specimens. Clearly, this was exceptional fatigue behavior, especially for a test temper￾ature of 1200°C. Excellent retained strength was not unexpected, because Campbell et al.23 have shown that this CMC looses only
7% of its load-carrying capability after air exposure at 1200°C for 1000 h. Carelli et al.24 have also shown that a N720/mullite– Al2O3 CMC has excellent strength retention after air exposure at 1200°C for 1000 h. It was speculated that, if changes were made to the constituents in the Nicalon/SiNC CMC, there might be a change in the moisture sensitivity. Results from these experiments are shown in Table III. None of these changes impacted moisture resistance. Neither the fiber coating nor the alternate prepreg material improved resis￾tance to moisture. It was hoped that the changes would produce a more uniform microstructure, with a change in the morphology of the porosity. It has been observed that porosity contributes to the degradation of the material by providing paths for oxygen and moisture to penetrate into the interior of the test specimen. However, optical observations of the cut edges of test specimens revealed that the geometry and shape of the porosity was not altered significantly. These observations suggested that significant improvements in processing of this CMC may some day result in decreased sensitivity to moisture. Results of the fatigue and interrupted fatigue experiments are shown in Table IV. Table IV shows the cycles to failure and number of times the interrupted experiments were cooled to room temperature and then heated back to test temperature. The inter￾rupted fatigue specimens demonstrated a decrease in life of
24% as compared with the isothermal fatigue experiments. V. Discussion High-temperature fatigue life of CMCs is generally associated with the onset of nonlinear stress–strain behavior. In this investi￾gation, run out of 100 000 cycles is observed to be at stress levels equal to or slightly above the PL. However, exposure to water fog lowers the run-out stress level to below the PL. Even though the Nicalon/SiNC and Nicalon/C systems have extensive matrix cracks in them from processing and
5% porosity, they are sufficiently sealed, at least at the surfaces, so that oxidation species do not have a path to penetrate into the CMC. For Nicalon/SiNC, the matrix is in residual compression, and this should also aid in keeping cracks closed. All three Nicalon-fiber CMCs are appar￾ently dense enough to limit any substantial oxygen diffusion into the CMC as long as the existing matrix cracks remain closed. As the existing cracks open, they provide an easy path for oxygen transport throughout the matrix, promoting rapid oxidation. Kalluri et al.25 studied the high-temperature fatigue behavior of Nicalon/ SiNC using humidity exposures as well as salt fog, similar to this investigation. The test specimens were unique in that they con￾tained many machined holes with a nominal diameter of 1.8 mm. Fig. 7. Fatigue diagram of stress versus cycles to failure for Nicalon/ SiNC. Specimens were subjected to fatigue and interrupted fatigue plus salt fog exposure. Fig. 8. Fatigue diagram of stress versus cycles to failure for Nicalon/C. Specimens were subjected to fatigue and interrupted fatigue plus salt fog exposure. Fig. 9. Fatigue diagram of stress versus cycles to failure for N610/AS. Specimens were subjected to fatigue and interrupted fatigue plus salt fog exposure. 1286 Journal of the American Ceramic Society—Zawada et al. Vol. 86, No. 8

August 2003 Consequence of Moisture and Salt Fog on High-Temperature Fatigue of Ceramic-Matrix Composites 287 Table I. Test Results for the Fatigue Plus Salt Fog and Fatigue plus Water Fog Studies MC systems investigated (fatigue stress: fatigue life reduction(MPa: %))5 Test Dow Corning BP Chemicals Hitco Dupont/Lanxide General Electric CoI Ceramics Type of test conducted Nicalon/SiNC Nicalon/C Nicalon/Al,O3 N6IO/AS N720/A5 terrupt fatigue salt fog exposure 1000 150:85 1000 135:0 125:3 100:25 100:73 100:0 75:0 nterrupt fatigue water fog 1000 125:86 1000 125:85 1000 125:86 75:0 75:5 1000 Ich CMC is listed with the stress level tested and the corresponding decrease in life for that particular stress level. "Listed are the fatig 1200°C Test specimens had a percent open area(POA)of 25 and 35, and have shown that there are acoustic emission(AE) events below the this represented a very high density of holes For a POA of 35, the well-defined PL in SiC/SIC CMCs Matrix cracking always starts distance between holes was smaller than the diameter of the holes to occur at the same strain level. and often this strain level occurs The salt fog exposures produced the shortest fatigue lives, and the at a corresponding stress level that is well below the PL. The decrease in life was -80%. Such small amounts of material results from this investigation suggest that small, dispersed cracks between the holes meant that oxidative species did not have to have an important impact on fatigue life whenever high levels of netrate distances into the specimens. Results from the moisture are presen present investigation and those from Kalluri et al. demonstrate It appears that the dual coating of BN/SiC or BN/Si3 N4 applied the extensive decrease in high-temperature durability as a result of to the Nicalon fibers does little to protect the CMC, fractograph alt fog exposure. The present investigation shows that, for the studies have shown that, in many instances, the cracks in the Nicalon/SINC system, introducing thermal cycles result in a 24 matrix typically penetrate the dual coatings, and debonding occurs decrease in fatigue life. However, each thermal cycle results in between the fiber and the BN, suggesting the coating applied over <15 min of additional hot time. when this hot time is taken into the bn does not exhibit sufficient strain tolerance. Matrix cracks account, the resulting decrease in fatigue life is 15%.The that penetrate the fiber coatings appear to propagate between the thermal cycles do decrease the fatigue lives, but the effect is small BN and fiber. Such cracking allows ingress of oxidative speci compared with the 85%decrease in fatigue life for the fatigue plus Carbon at the fiber surface reacts, followed by the formation of a ter fog experiments. This data supports the observation that it is thin (0. 15 um)SiO, layer on the surface of the fiber. At 1000C the excessive moisture that produces the large decrease in fatigue the BN oxidizes to form B, O, At elevated temperatures, the B,O lives, and not the thermal cycle reacts with SiC and Sio, present to form borosilicate glass. In Both CMCs with a BN-fiber coating demonstrate a pronounced addition, BN with an interplanar [d(002)) spacing of 3.67 A has PL, and the maximum stress level to reach run out during standard shown extensive weight loss at 700C in a H-O/N, environment. 4 isothermal fatigue is higher than for the moisture exposure This is important, because it identifies that BN at 700 C exhibits experiments. This is an important issue and clearly indicates that significant weight loss with only moisture present even small, unconnected matrix cracks that occur naturally in the It is well-known that moisture promotes degradation in BN- CMC during synthesis of the matrix have an influence on fatigue containing CMCs2, 13, 6-8, and that increasing the life as the level of moisture present increases. Morscher et ai accelerates the rate at which degradation occurs. At elevated temperatures, even small amounts of moisture(PHo 0.003 atm result in gradual removal of the boron from the glass. However igh amounts of moisture result in rapid volatilization of B2O3 as Tension Test HBO2(g), H3 BO3(g), and H3 B3O6(g), with the primary gaseous 1200°c T=1200°c species being HBO,(g). These reactions are very sensitive to the amounts of moisture present, structure of the BN, impurities, and The moisture causes boron to be volatized from the borosilicate Fatique glass, resulting in silica glass formation that is relatively free of boron. It is this low viscosity silica glass that causes strong bonding to occur between the fibers and the matrix, resulting in Table I. Impact of Processing Changes on the Moisture Sensitivity for Nicalon/SiNC ▲ Fatigue+ Water Matrix Fiber/matrix Filler Cloth Process polymer 10°101102103104105105 BN/SIN Si Cycles(N) 2 Option-1 BN/SI3N4 Si,N4 0/90 74, 75 3 Standard BN/Si3N4 0/90 Fig. 10. Fatigue diagram of stress versus cycles to failure for N720/A Specimens were subjected to fatigue and interrupted fatigue plus water fog specimens were tested for each condition

Test specimens had a percent open area (POA) of 25 and 35, and this represented a very high density of holes. For a POA of 35, the distance between holes was smaller than the diameter of the holes. The salt fog exposures produced the shortest fatigue lives, and the decrease in life was
80%. Such small amounts of material between the holes meant that oxidative species did not have to penetrate long distances into the specimens. Results from the present investigation and those from Kalluri et al.25 demonstrate the extensive decrease in high-temperature durability as a result of salt fog exposure. The present investigation shows that, for the Nicalon/SiNC system, introducing thermal cycles result in a 24% decrease in fatigue life. However, each thermal cycle results in
15 min of additional hot time. When this hot time is taken into account, the resulting decrease in fatigue life is
15%. The thermal cycles do decrease the fatigue lives, but the effect is small compared with the 85% decrease in fatigue life for the fatigue plus water fog experiments. This data supports the observation that it is the excessive moisture that produces the large decrease in fatigue lives, and not the thermal cycles. Both CMCs with a BN-fiber coating demonstrate a pronounced PL, and the maximum stress level to reach run out during standard isothermal fatigue is higher than for the moisture exposure experiments. This is an important issue and clearly indicates that even small, unconnected matrix cracks that occur naturally in the CMC during synthesis of the matrix have an influence on fatigue life as the level of moisture present increases. Morscher et al.26 have shown that there are acoustic emission (AE) events below the well-defined PL in SiC/SiC CMCs. Matrix cracking always starts to occur at the same strain level, and often this strain level occurs at a corresponding stress level that is well below the PL. The results from this investigation suggest that small, dispersed cracks have an important impact on fatigue life whenever high levels of moisture are present. It appears that the dual coating of BN/SiC or BN/Si3N4 applied to the Nicalon fibers does little to protect the CMC. Fractographic studies have shown that, in many instances, the cracks in the matrix typically penetrate the dual coatings, and debonding occurs between the fiber and the BN, suggesting the coating applied over the BN does not exhibit sufficient strain tolerance. Matrix cracks that penetrate the fiber coatings appear to propagate between the BN and fiber. Such cracking allows ingress of oxidative species. Carbon at the fiber surface reacts, followed by the formation of a thin (
0.15 m) SiO2 layer on the surface of the fiber. At 1000°C, the BN oxidizes to form B2O3. At elevated temperatures, the B2O3 reacts with SiC and SiO2 present to form borosilicate glass. In addition, BN with an interplanar [d(002)] spacing of 3.67 Å has shown extensive weight loss at 700°C in a H2O/N2 environment.14 This is important, because it identifies that BN at 700°C exhibits significant weight loss with only moisture present. It is well-known that moisture promotes degradation in BN￾containing CMCs12,13,16–18,20,21,25–30 and that increasing the PH2O accelerates the rate at which degradation occurs. At elevated temperatures, even small amounts of moisture (PH2O  0.003 atm) result in gradual removal of the boron from the glass. However, high amounts of moisture result in rapid volatilization of B2O3 as HBO2(g), H3BO3(g), and H3B3O6(g), with the primary gaseous species being HBO2(g). These reactions are very sensitive to the amounts of moisture present, structure of the BN, impurities, and temperature. The moisture causes boron to be volatized from the borosilicate glass, resulting in silica glass formation that is relatively free of boron. It is this low viscosity silica glass that causes strong bonding to occur between the fibers and the matrix, resulting in Fig. 10. Fatigue diagram of stress versus cycles to failure for N720/A. Specimens were subjected to fatigue and interrupted fatigue plus water fog exposure. Table II. Test Results for the Fatigue Plus Salt Fog and Fatigue Plus Water Fog Studies† Type of test conducted Test temperature (°C) CMC systems investigated (fatigue stress:fatigue life reduction (MPa:%))‡ Dow Corning BP Chemicals Hitco Dupont/Lanxide General Electric COI Ceramics Nicalon/SiNC Nicalon/C Nicalon/Al2O3 N610/AS N720/A§ Interrupt fatigue  salt fog exposure 1000 150:85 1000 135:0 1000 125:74 125:33 1000 100:85 100:25 100:73 100:0 1000 75:0 75:0 Interrupt fatigue  water fog 1000 125:86 122:0 exposure 1000 125:85 108:0 1000 125:86 1000 75:0 75:50 1000 75:0 75:75 † Each CMC is listed with the stress level tested and the corresponding decrease in life for that particular stress level. ‡ Listed are the fatigue stress (MPa) and the reduction in life (%). § Conducted at 1200°C. Table III. Impact of Processing Changes on the Moisture Sensitivity for Nicalon/SiNC† Process Matrix polymer Fiber/matrix interphase Filler material Cloth lay-up Reduction in life (%) 1 Standard BN/Si3 N4 Si3N4 0/90 85 2 Option-1 BN/Si3N4 Si3N4 0/90 74, 75 3 Standard BN/Si3N4 Si3N4 Quasi 80, 82 4 Standard BN SiC 0/90 66, 73 5 Option-1 BN SiC 0/90 77, 79 † Each process history is listed with the corresponding decrease in life. Two specimens were tested for each condition. August 2003 Consequence of Moisture and Salt Fog on High-Temperature Fatigue of Ceramic-Matrix Composites 1287

1288 Journal of the American Ceramic Sociery-Zawada et al. Vol 86. No. 8 Table IV. Cycles to Failure for Nicalon/SiNC Tested in Fatigue at 1000.C and 100 MPa Nicalon/SINC Stress(MPa) Cycles (Nd Stress(MPa) Cycles(Na) Thermal cycles 63 Average 39 653 Average 30 102 Shown are the lives for isothermal fatigue and fatigue tests that were periodically errupted and cooled to room temperature. significant strength degradation in the CMC. The rate of degrada- 20 um tion may also depend on the size of the test specimens and how far the reactive species must penetrate into the CMC. Results from the present investigation indicate that direct exposure to moisture Fig. 12. High-magnification SEM image of the fracture surface of appears to significantly accelerate the rate of degradation com- Nicalon/SiNC tested in fatigue plus water fog(1000 C, 100 MPa, N pared with the amount of moisture normally found in laboratory air 23 289 cycles). Substantial portion of the fracture surface was very fla (Puo 0.003 atm). It is not currently known how much moisture with extensive glass formation. is required to maximize the rate of degradation for each CMC cx Direct exposure of the CMc to moisture also has an added oxidation of the bn interphase and extensive glass formation. It ffect at room temperature. At room temperature, direct moisture was observed that all fibers that were originally touching each Asposure with BN results in the formation of additional B2O3 and other had been bonded together by a neck of glass and that all the as. The standard free energy for his reaction is-22 089 cal bers in the image fractured in the same fracture plane. In many 92.3 k)at 298 K, making it thermodynamically favorable(but cases. fracture initiated at the location where the fibers had been slow). In addition, direct contact with moisture causes B2O3 to be bonded together. Evidence of how strongly the glassy phase dissolved out of the CMC to form boric acid. Warm water can bonded the fiber to the matrix is also demonstrated in Fig. 13 dissolve up to 15.7 g/(100 cm) of B2O3. Thus, even at room Shown are the fibers that were orientated 90 to the loading emperature, direct contact to water results in accelerated loss of the BN-fiber coating An important implication is that a CMC with bonded that it cleaved in two pieces during failure. A very-high- matrix cracks may slowly degrade over a long period of time if left magnification image of the fibers and oxidation region is shown in n a humid environment Fig. 14. In many locations, the glass has the appearance that it Nicalon/SiNC CMC specimens tested in water fog were sub- flowed or oozed out from the region between the fibers and matrix ected to fractographic examination using SEM. A typical fracture There were also significant stretches between the fiber and matrix urface is shown in Fig. Il. The right side of Fig. I l is near the where there was a substantial gap where the Bn layer had been machined edge of the test specimen, and the fracture appearance is completely removed, as well as round holes in the glass that were extremely flat and can be described as embrittled. A study of the likely the channels that formed to allow evolving gas species to features on the fracture surface showed that embrittlement was not scape. These channels are the result of rapid volatilization o likely from diffusion of oxygen into the specimen from the surface, B,O3 as a result of the high moisture levels. The three fibers shown as had been observed on earlier glass-ceramic CMCs. There was in the image w likely touching before testing and are now not a well-defined embrittled zone all the way around the speci strongly bonded together by SiO, necks that are several microme men. Instead, there was ample evidence that there were multip ters in length. Standard fatigue testing produced embrittled zones, racks that grew into the specimen and that the majority originated with some of the features that were observed on the at the machined edges. Figure 12 is from a region of flat fracture specimens. However, the moisture studies produced sig that was in the interior of the specimen near the transition from flat nore glass formation and extensive removal of the BN fracture to fiber pullout. Even at this region, there was substantial The fractographic analysis documents showed qualitatively that 1 mn 20 Fig. 11. Low-magnification SEM image of the fracture surface Fig. 13. High-magnification SEM image of the fracture surface of Nicalon/SiNC tested in fatigue plus water fog(1000oC, 100 MPa, Ne Nicalon/SINC tested in fatigue plus water fog(1000C, 100 MPa, Ne 23 289 cycles). Notice the contrast of the embrittled zone near the right 23 289 cycles). Fiber in the middle of the image was 90 to the loading dge direction and was so strongly bonded to the matrix that it was cleaved

significant strength degradation in the CMC. The rate of degrada￾tion may also depend on the size of the test specimens and how far the reactive species must penetrate into the CMC. Results from the present investigation indicate that direct exposure to moisture appears to significantly accelerate the rate of degradation com￾pared with the amount of moisture normally found in laboratory air (PH2O  0.003 atm). It is not currently known how much moisture is required to maximize the rate of degradation for each CMC system. Direct exposure of the CMC to moisture also has an added effect at room temperature. At room temperature, direct moisture exposure with BN results in the formation of additional B2O3 and NH3-gas. The standard free energy for his reaction is 22 089 cal (–92.3 kJ) at 298 K, making it thermodynamically favorable (but slow). In addition, direct contact with moisture causes B2O3 to be dissolved out of the CMC to form boric acid. Warm water can dissolve up to 15.7 g/(100 cm3 ) of B2O3. Thus, even at room temperature, direct contact to water results in accelerated loss of the BN-fiber coating. An important implication is that a CMC with matrix cracks may slowly degrade over a long period of time if left in a humid environment. Nicalon/SiNC CMC specimens tested in water fog were sub￾jected to fractographic examination using SEM. A typical fracture surface is shown in Fig. 11. The right side of Fig. 11 is near the machined edge of the test specimen, and the fracture appearance is extremely flat and can be described as embrittled. A study of the features on the fracture surface showed that embrittlement was not likely from diffusion of oxygen into the specimen from the surface, as had been observed on earlier glass-ceramic CMCs. There was not a well-defined embrittled zone all the way around the speci￾men. Instead, there was ample evidence that there were multiple cracks that grew into the specimen and that the majority originated at the machined edges. Figure 12 is from a region of flat fracture that was in the interior of the specimen near the transition from flat fracture to fiber pullout. Even at this region, there was substantial oxidation of the BN interphase and extensive glass formation. It was observed that all fibers that were originally touching each other had been bonded together by a neck of glass and that all the fibers in the image fractured in the same fracture plane. In many cases, fracture initiated at the location where the fibers had been bonded together. Evidence of how strongly the glassy phase bonded the fiber to the matrix is also demonstrated in Fig. 13. Shown are the fibers that were orientated 90° to the loading direction. The fiber in the middle of the image was so strongly bonded that it cleaved in two pieces during failure. A very-high￾magnification image of the fibers and oxidation region is shown in Fig. 14. In many locations, the glass has the appearance that it flowed or oozed out from the region between the fibers and matrix. There were also significant stretches between the fiber and matrix where there was a substantial gap where the BN layer had been completely removed, as well as round holes in the glass that were likely the channels that formed to allow evolving gas species to escape. These channels are the result of rapid volatilization of B2O3 as a result of the high moisture levels. The three fibers shown in the image were likely touching before testing and are now strongly bonded together by SiO2 necks that are several microme￾ters in length. Standard fatigue testing produced embrittled zones, with some of the features that were observed on the water fog specimens. However, the moisture studies produced significantly more glass formation and extensive removal of the BN interface. The fractographic analysis documents showed qualitatively that Fig. 11. Low-magnification SEM image of the fracture surface of Nicalon/SiNC tested in fatigue plus water fog (1000°C, 100 MPa, Nf 23 289 cycles). Notice the contrast of the embrittled zone near the right edge. Fig. 12. High-magnification SEM image of the fracture surface of Nicalon/SiNC tested in fatigue plus water fog (1000°C, 100 MPa, Nf 23 289 cycles). Substantial portion of the fracture surface was very flat with extensive glass formation. Fig. 13. High-magnification SEM image of the fracture surface of Nicalon/SiNC tested in fatigue plus water fog (1000°C, 100 MPa, Nf 23 289 cycles). Fiber in the middle of the image was 90° to the loading direction and was so strongly bonded to the matrix that it was cleaved. Table IV. Cycles to Failure for Nicalon/SiNC Tested in Fatigue at 1000°C and 100 MPa† Isothermal fatigue Nicalon/SiNC Interrupted fatigue Nicalon/SiNC Stress (MPa) Cycles (Nf) Stress (MPa) Cycles (Nf) Thermal cycles 100 22 113 100 63 172 7 100 16 634 100 12 068 2 100 80 213 100 15 065 3 Average 39 653 Average 30 102 4 † Shown are the lives for isothermal fatigue and fatigue tests that were periodically interrupted and cooled to room temperature. 1288 Journal of the American Ceramic Society—Zawada et al. Vol. 86, No. 8

August 2003 Consequence of Moisture and Salt Fog on High-Temperature Fatigue of Ceramic-Matrix Composites 78春 q4-378 salt fog exposure(1000C, 100 MPa, N= 66002 cycles).(B) Combina tion of high-temperature fatigue and environmental exposure resulted in 10 um apid and extensive spallation of the exterior SiC coating(A)Isothermal ig. 14. High-magnification SEM image of the fracture surface of Nicalon/SINC tested in fatigue plus water fog(1000C, 100 MPa, N= when the exterior coating remains intact. Pierce and Zawada 23 289 cycles). Extensive glass formation bonds the fibers ported that there was no observable loss in retained strength after 4500 thermal cycles to temperatures as high as 1.C. During these rapid thermal cycles, and for all the fatigue experiments significantly more degradation occurred at the fiber-matrix inter- conducted in this investigation, there was no evidence of coating phase region for the fatigue plus moisture experiments spallation. In a separate study, 'coated Nicalon/C test specimens In studying the fracture surfaces, we could easily follow the were indented to compromise the exterior coating and then owth of the cracks from the surface into the interior of the subjected to alternation moisture exposure and thermal treatments specimen. Development of the glassy phases was very pronounced at 1 100C. The moisture exposures consisted of either a 5 wt% nearer the surface, where the cracks formed, and was less prevalent NaCl concentration or a 3.5 wt%(NH4hSO4 0.5 wt% Nacl closer to the crack front. None of the tests ran for more than -28 h concentration. None of the exposed test specimens experienced Therefore, there was no evidence of pronounced matrix oxidation loss of the exterior coating Staehler and Zawada"observed very d subsequent matrix recession as was seen in the combustor tests little coating loss after extensive use in the Nicalon/C divergent that ran for thousands of hours. - Most of the environmental flaps on a ground-tested F-110 turbine engine. No appreciable attack was limited to the fiber-matrix interphase re coating loss was observed by Pierce et al. for Nicalon/C At high temperature, a borosilicate glass is likely to be soft and divergent flaps on a ground-tested F-4 14 aerospace turbine engine should not promote fracture. However, at room temperature, it is In each of these studies, a flap from each engine ground test was extremely brittle and promotes fracture of the fibers and signifi tested for retained strength, and no loss of strength was measured cantly lowers the load-carrying capability of the composite. It has These results show that thermal cycling alone does not generall been shown that the retained strength after burner-rig testing only produce significant thermally induced stress levels, thus, the reaches the PL, which is the strain limit for the matrix. As soon thermal cycling does not degrade the material. Degradation does as the matrix starts to crack, the composite fails. Similar behavior not start to occur until there is loading to stress levels that produce also has been observed in this investigation for the Nicalon/SiNc strains of sufficient magnitude to open coating and matrix cracks ystem, because it is extremely susceptible to this rapid embrittle- Stress levels that crack the exterior coating combined with mois ment. These results clearly identify the continuing problem of ture exposure promote an additional degradation process of rapid bn to promote debonding between the fiber and matrix loss of the exterior SiC coating during high-temperature fatigue The Nicalon/C composite experienced only a modest decrease Salt fog exposure combined with fatigue testing at 1000C was in fatigue life with moisture exposure. This decrease was substan- ound to have no effect on n610/AS. No effect of moisture was tially less than for the Nicalon/Al,O, composite or the Nicalon/ observed for N720/A as well. All specimens maintained the SINC composite. Several observations could be made from this as-received tensile strength and elastic prop Moisture and result. Not having a BN-coated fiber appeared to have resulted alt exposure significantly degraded the long-term hi slower environmental degradation. However, the salt fog expo- temperature durability of CMCs reinforced with Nicalon fibers sures resulted in a 30% decrease in fatigue life. This pointed to the The first was decrease of Nicalon fiber strength from hot corrosion effect of oxidation of the fiber to form SiO, and reaction of the of the salt at high temperatures. The second was pronounced SiO, with moisture as a suspected degradation mechanism. The reactions of the Bn coating the fibers. Use of N610-Al2O3 fibers filler of B.C likely oxidized to form a glass, and this glass also was and lack of a fiber-matrix interface may have accounted for the volatilized by moisture. In addition, introduction of moisture had a insensitivity of N610/As to salt fog. There was 13% SiO, in the pronounced effect on the test coupon. After the first moisture matrix. In addition, the n720 fiber contained mullite and Al,O exposure and subsequent block of fatigue cycles at temperature, Wannaparhun et al. showed that, under simulated combustor the exterior coating was observed to have completely blistered an conditions. SiO could be leached out of the n720 fiber. Water Such pronounced spallation suggested extensive volatilization. An the loss was confined to the surface of the specime d there was bservation such as this has serious implications. This material has no resulting loss in tensile strength. Campbell et al. exposed a relatively low PL, with extensive matrix cracks resulting from N720/A(same CMC as this study) to a water-vapor environment the processing of the CMC. All the fatigue tests were abo at 1200%C for 1000 h. Greater than 85% strength retention wa PL. Early spalling of the exterior protective coating allowed rapid observed after exposure, and no weight change was observed ingress of oxygen into the CMc along all the matrix cracks. This These results suggested that the rate at which hydrolysis occurred allowed for oxidation of the Nicalon fibers as well as rapid was strongly dependent on the combination of temperature, atmos- oxidation of the carbon matrix. Actual component stress level may phere, and pressure. However, for the conditions tested in this e low, but if the coating spalls off quickly, then there is nvestigation and by Campbell et al, there was no evidence of accelerated degradation of the matrix as well as the fibers There is substantial information demonstrating that the nica- Changes to the constituents in nicalon/sinc did not alter lon/C composite exhibits excellent high-temperature durability moisture sensitivity. Although no base line fatigue tests were

significantly more degradation occurred at the fiber–matrix inter￾phase region for the fatigue plus moisture experiments. In studying the fracture surfaces, we could easily follow the growth of the cracks from the surface into the interior of the specimen. Development of the glassy phases was very pronounced nearer the surface, where the cracks formed, and was less prevalent closer to the crack front. None of the tests ran for more than
28 h. Therefore, there was no evidence of pronounced matrix oxidation and subsequent matrix recession as was seen in the combustor tests that ran for thousands of hours.1–3 Most of the environmental attack was limited to the fiber–matrix interphase region. At high temperature, a borosilicate glass is likely to be soft and should not promote fracture. However, at room temperature, it is extremely brittle and promotes fracture of the fibers and signifi￾cantly lowers the load-carrying capability of the composite. It has been shown that the retained strength after burner-rig testing only reaches the PL,17 which is the strain limit for the matrix. As soon as the matrix starts to crack, the composite fails. Similar behavior also has been observed in this investigation for the Nicalon/SiNC system, because it is extremely susceptible to this rapid embrittle￾ment. These results clearly identify the continuing problem of using BN to promote debonding between the fiber and matrix. The Nicalon/C composite experienced only a modest decrease in fatigue life with moisture exposure. This decrease was substan￾tially less than for the Nicalon/Al2O3 composite or the Nicalon/ SiNC composite. Several observations could be made from this result. Not having a BN-coated fiber appeared to have resulted in slower environmental degradation. However, the salt fog expo￾sures resulted in a 30% decrease in fatigue life. This pointed to the effect of oxidation of the fiber to form SiO2 and reaction of the SiO2 with moisture as a suspected degradation mechanism. The filler of B4C likely oxidized to form a glass, and this glass also was volatilized by moisture. In addition, introduction of moisture had a pronounced effect on the test coupon. After the first moisture exposure and subsequent block of fatigue cycles at temperature, the exterior coating was observed to have completely blistered and essentially spalled off the entire gauge section. Very little coating remained on the gauge section, and this is clearly shown in Fig. 15. Such pronounced spallation suggested extensive volatilization. An observation such as this has serious implications. This material has a relatively low PL, with extensive matrix cracks resulting from the processing of the CMC. All the fatigue tests were above the PL. Early spalling of the exterior protective coating allowed rapid ingress of oxygen into the CMC along all the matrix cracks. This allowed for oxidation of the Nicalon fibers as well as rapid oxidation of the carbon matrix. Actual component stress level may be low, but if the coating spalls off quickly, then there is accelerated degradation of the matrix as well as the fibers. There is substantial information demonstrating that the Nica￾lon/C composite exhibits excellent high-temperature durability when the exterior coating remains intact. Pierce and Zawada31 reported that there was no observable loss in retained strength after 4500 thermal cycles to temperatures as high as 1100°C. During these rapid thermal cycles, and for all the fatigue experiments conducted in this investigation, there was no evidence of coating spallation. In a separate study,32 coated Nicalon/C test specimens were indented to compromise the exterior coating and then subjected to alternation moisture exposure and thermal treatments at 1100°C. The moisture exposures consisted of either a 5 wt% NaCl concentration or a 3.5 wt% (NH4)2SO4  0.5 wt% NaCl concentration. None of the exposed test specimens experienced loss of the exterior coating. Staehler and Zawada33 observed very little coating loss after extensive use in the Nicalon/C divergent flaps on a ground-tested F-110 turbine engine. No appreciable coating loss was observed by Pierce et al.34 for Nicalon/C divergent flaps on a ground-tested F-414 aerospace turbine engine. In each of these studies, a flap from each engine ground test was tested for retained strength, and no loss of strength was measured. These results show that thermal cycling alone does not generally produce significant thermally induced stress levels; thus, the thermal cycling does not degrade the material. Degradation does not start to occur until there is loading to stress levels that produce strains of sufficient magnitude to open coating and matrix cracks. Stress levels that crack the exterior coating combined with mois￾ture exposure promote an additional degradation process of rapid loss of the exterior SiC coating during high-temperature fatigue. Salt fog exposure combined with fatigue testing at 1000°C was found to have no effect on N610/AS. No effect of moisture was observed for N720/A as well. All specimens maintained the as-received tensile strength and elastic properties. Moisture and salt exposure significantly degraded the long-term high￾temperature durability of CMCs reinforced with Nicalon fibers. The first was decrease of Nicalon fiber strength from hot corrosion of the salt at high temperatures. The second was pronounced reactions of the BN coating the fibers. Use of N610-Al2O3 fibers and lack of a fiber–matrix interface may have accounted for the insensitivity of N610/AS to salt fog. There was 13% SiO2 in the matrix. In addition, the N720 fiber contained mullite and Al2O3. Wannaparhun et al.35 showed that, under simulated combustor conditions, SiO2 could be leached out of the N720 fiber. Water vapor resulted in the formation of volatile Si(OH)4 and was responsible for the loss of the mullite phase in the fiber. However, the loss was confined to the surface of the specimen, and there was no resulting loss in tensile strength. Campbell et al.23 exposed N720/A (same CMC as this study) to a water-vapor environment at 1200°C for 1000 h. Greater than 85% strength retention was observed after exposure, and no weight change was observed. These results suggested that the rate at which hydrolysis occurred was strongly dependent on the combination of temperature, atmos￾phere, and pressure. However, for the conditions tested in this investigation and by Campbell et al., there was no evidence of degradation. Changes to the constituents in Nicalon/SiNC did not alter moisture sensitivity. Although no base line fatigue tests were Fig. 14. High-magnification SEM image of the fracture surface of Nicalon/SiNC tested in fatigue plus water fog (1000°C, 100 MPa, Nf 23 289 cycles). Extensive glass formation bonds the fibers. Fig. 15. Photograph of a Nicalon/C test specimen tested in fatigue plus salt fog exposure (1000°C, 100 MPa, Nf 66 002 cycles). (B) Combina￾tion of high-temperature fatigue and environmental exposure resulted in rapid and extensive spallation of the exterior SiC coating. (A) Isothermal fatigue specimen is shown for comparison. August 2003 Consequence of Moisture and Salt Fog on High-Temperature Fatigue of Ceramic-Matrix Composites 1289

Journal of the American Ceramic Sociery-Zawada et al. Vol 86. No. 8 onducted to establish run-out stress levels for each of the variants attributed to the excessive moisture and not the low salt concen- the measured tensile behavior and UTSs at 23 and 1000.C were tration or the thermal cycles. Changes to the constituents of the very similar to those of the base line material. Furthermore, the Nicalon/SINC system did not influence moisture resistance and shapes of the stress-strain traces were almost identical. Therefore, suggested that future work to improve fatigue durability should be it was assumed that the fatigue behavior would be similar. During focused on the fiber coatings standard fatigue testing, the maximum stress level to reach run out For CMCs with a Sic exterior coating that serves as an was 100 MPa, and 100 MPa was used as the base line for all environmental barrier coating, the combination of high- comparisons. The susceptibility to moisture was measured to be temperature fatigue loading and moisture exposure initiated accel the same as that of the base line material. This finding has erated coating loss and decreased life important ramifications for future development of this CMC and None of the oxide/oxide CMCs investigated in this study other composites. Changes to the constituents may alter the experienced degradation from the fatigue plus moisture experi- structure of the microstructure and the macroporosity levels, and ments. At the temperatures investigated, the oxide CMCs were this should have a positive effect on transthickness strength, but stable, and there was no evidence of hydrolysis when moisture was matrix cracks still allow ingress of moisture into the interior of the present. Evidence in the literature suggested that, under specific CMC. All the combinations in the present study have used a BN conditions, reactions between the SiO in the fiber and moisture coating on the fiber and highlights the limitations of using may be a problem. Processing improvements continue to yield BN-fiber coatings applied at relatively low temperatures oxide/oxide CMCs with higher tensile strengths. However, much (1000C). Exposure of BN and SiC to oxygen at high tempera- more work remains to be done to understand the envelope of ire results in oxide formation, but the introduction of moisture temperatures, stresses, and atmospheric conditions where oxide/ ccelerates this process and serves to flux the resulting oxides and oxide CMCs remain stable accelerate the rate of degradation Improvements have been made in moisture resistance for CMCs with a Bn interphase. Cofer and Economy have shown that References keeping the Bn interlayer spacing of the hexagonal planes at 3.3 A results in no weight loss in H, O/air at 700 C. Processing at K. L. More, P. F. Tortorelli, M. K. Ferber, and J.R. Keiser, "Observations of higher temperatures also produces a more stable BN, and doping Accelerated Silicon Carbide Recession by Oxidation at High Water-Vapor Pressures, the bn with silicon improves moisture resistance. o,/It has 2K. L. More. P. F. Tortorelli R. Walker. J. R. Keiser. W. D been reported recently that forcing the debonding to occur between the bn and the matrix instead of between the bn and the fiber also opposites in Simulated and Ac environments", Paper No, 99-GT- these solutions are effective only in slowing down the rate of New York, 199g pal Gas Turbin mproves the durability of melt-infiltrated SiC/SiC. However Congress. ASME International Ferber, H. T. Lin, and J. Keiser, "Oxidation Behavior of degradation. As shown in this study, combining matrix cracks and moisture exposure rapidly decreases high-temperature durability Mechanical. Thermal and Er To date. none of the current generation BN-coated SiC-fiber Composites and Components, ASTM STP 1392. Edited by M. G CMCs have demonstrated lol es once the matrix has been Lara-Curzio, and S. T. Gonczy. American Society for Testing and Materials, West cracked. The consequence is that CMOs with bN fiber coatings thy, T. Mah, C. A. Folsom, and A. P. Katz, "Microstructural have to be used at stress levels that do not produce matrix cracks 1000C in Air after Salt(Nac Given the current state of manufacturing and design, this is ST. A. Parthasarathy, C.A. Folsom, and L. P. Zawada, "Combined Effects of proving to be challenging. gth of uncoated ar Over the past several years, there have been substantial efforts BN-Coated Nicalon Fibers, J Am Ceram. Soc., 81 [7 1812-18(1998) focused on increasing the tensile strength of CMCs through bG. Richardson and R. w. Kowalik. "Oxidation and Hot Corrosion of Dupont Lanxide Enhanced SiC/SiC and Hitco SiC/C Composites, " Ceram. Eng. Sci. Proc., I improved process control. There has been less damage to the fibers B3.A Haynes, M J. Lance, K M. Cooley, M K Ferber, R.A. Lowden, and D P as a result of better handli chniques. Better processing control during application of the fiber-matrix interphase has resulted in tinton."CVD Mullite Coatin less fiber being exposed, which results in less fiber-to-fiber 5 E. J. Opila and R. E, Hann Jr, "Paralinear Oxidation of SiC in Water Vapor, touching, less carbon on the surface of the fibers, and less oxygen J Am. Ceram Soc, 80 (0) 197-205(1997). J. Opala,“ Oxida etics of Chemically Vapor Deposited Silicon Carbide lents have resulted in some SiC-fiber-reinforced CMCs that have Wet Oxygen, J. Am Ceram Soc., 77[3]730-36(1994). tensile strengths that range from 300 to 400 MPa. However. the ate of Silicon Carbide with Water- maximum usable stress level under high-temperature fatigue load- c,823]625-56(199 ing conditions remains tied to the pl for long life. However. for oson,Corrosion of Silicon-Based Ceramics in Combustion Environ- ments,"J.Am. Ceram. Soc., 76[1]3-28(1993). function of the UTS. Using the rule of mixtures for fiber volume Composite, "J.Am. Ceram. Soc., 821112777-84(1999) gradation in a SiC-Sic oxide/oxide CMCs, long life during mechanical fatigue is more a fractions (-30%0-35%) ngle-fiber strength. we know that K. L. More, P. F. Tortorelli, H. T. Lin, E. Lara-Curzio, and R. A. Lowden ignificant gains can be made in the Uts of oxide/oxide Cmcs "Degradation mechanisms of bn Interfaces in Water-Containing Environments, Electrochem. Soc. Proc., 98-99, 382 The fatigue limit, as well as the fatigue plus moisture exposure C. G. Cofer and J. Economy, "Oxidative and Hydrolytic Stabil limit, should result in proportion to the increase in tensile strengt Nitrides" carbon, 33 [4] 389-95(1995). ach to Improving the Oxidation Resistance of Carbonaceous strength values at 1200@C of 219 MPa and a fatigue run-out stress Nitride. "J. Mater. Sci, 24, 2353-57(1989) are for Chemically Vapour-Deposited Boron a1200170 MPa. Given the results in this investigation, the nG.N Mater,>的△C) in H,0-Containing Atm曰m and R. E. Tressler, "Environmental Durability o fatigue plus moisture run out stress level is speculated to be -170 diate Temperatures, Ceram. Eng. Sci. Proc., 18[3]525-33(1997) MPa. In terms of moisture resistance, the oxide-fiber-containing CMCs currently demonstrate superior high-temperature durability Thomas-Ogbuji, "Degradation of SiC/BN/SiC Composite in the Burner Rig, Ceram. Eng. Sci Proc., 19(4]257-64(1998 over SiC-fiber CMCs because of fiber strength degradation and L. U.J. T Ogbuji, "A Pervasive Mode of Oxidation Degradation in a SiC-Sic stability of BN fiber coatings. IN Jacobson, S. Farmer, A. Moore, and H. Sayir, "High-Temperature Oxid of Boron Nitride: I, Monolithic Boron Nitride, J. Am. Ceram. Soc., 82[2] 393-9 (1999) VI. Conclusions S Jacobson, G. N. Morscher, D. R. Bryant, and R. E. Tressler," High Layers in Composites, Those CMCs with Nicalon fibers coated with BN experienced a vere decrease in fatigue durability atigue was combined a Woven Nicalon/Si- with salt and moisture exposure. The accelerated degradation was Composite ,"J Am Ceram Soc, 81[7]1797-811(1998)

conducted to establish run-out stress levels for each of the variants, the measured tensile behavior and UTSs at 23° and 1000°C were very similar to those of the base line material. Furthermore, the shapes of the stress–strain traces were almost identical. Therefore, it was assumed that the fatigue behavior would be similar. During standard fatigue testing, the maximum stress level to reach run out was 100 MPa, and 100 MPa was used as the base line for all comparisons. The susceptibility to moisture was measured to be the same as that of the base line material. This finding has important ramifications for future development of this CMC and other composites. Changes to the constituents may alter the structure of the microstructure and the macroporosity levels, and this should have a positive effect on transthickness strength, but matrix cracks still allow ingress of moisture into the interior of the CMC. All the combinations in the present study have used a BN coating on the fiber and highlights the limitations of using BN-fiber coatings applied at relatively low temperatures (
1000°C). Exposure of BN and SiC to oxygen at high tempera￾ture results in oxide formation, but the introduction of moisture accelerates this process and serves to flux the resulting oxides and accelerate the rate of degradation. Improvements have been made in moisture resistance for CMCs with a BN interphase. Cofer and Economy14 have shown that keeping the BN interlayer spacing of the hexagonal planes at 3.33 Å results in no weight loss in H2O/air at 700°C. Processing at higher temperatures also produces a more stable BN, and doping the BN with silicon improves moisture resistance.16,36,37 It has been reported recently that forcing the debonding to occur between the BN and the matrix instead of between the BN and the fiber also improves the durability of melt-infiltrated SiC/SiC.38 However, these solutions are effective only in slowing down the rate of degradation. As shown in this study, combining matrix cracks and moisture exposure rapidly decreases high-temperature durability. To date, none of the current generation BN-coated SiC-fiber CMCs have demonstrated long lives once the matrix has been cracked. The consequence is that CMCs with BN fiber coatings have to be used at stress levels that do not produce matrix cracks. Given the current state of manufacturing and design, this is proving to be challenging. Over the past several years, there have been substantial efforts focused on increasing the tensile strength of CMCs through improved process control. There has been less damage to the fibers as a result of better handling techniques. Better processing control during application of the fiber–matrix interphase has resulted in less fiber being exposed, which results in less fiber-to-fiber touching, less carbon on the surface of the fibers, and less oxygen and other impurities present in the interphase. All these improve￾ments have resulted in some SiC-fiber-reinforced CMCs that have tensile strengths that range from 300 to 400 MPa. However, the maximum usable stress level under high-temperature fatigue load￾ing conditions remains tied to the PL for long life. However, for oxide/oxide CMCs, long life during mechanical fatigue is more a function of the UTS. Using the rule of mixtures for fiber volume fractions (
30%–35%) and single-fiber strength, we know that significant gains can be made in the UTS of oxide/oxide CMCs. The fatigue limit, as well as the fatigue plus moisture exposure limit, should result in proportion to the increase in tensile strength. Recent strength gains in the N720/A CMC have resulted in strength values at 1200°C of 219 MPa and a fatigue run-out stress at 1200°C of 170 MPa. Given the results in this investigation, the fatigue plus moisture run out stress level is speculated to be
170 MPa. In terms of moisture resistance, the oxide-fiber-containing CMCs currently demonstrate superior high-temperature durability over SiC-fiber CMCs because of fiber strength degradation and stability of BN fiber coatings. VI. Conclusions Those CMCs with Nicalon fibers coated with BN experienced a severe decrease in fatigue durability when fatigue was combined with salt and moisture exposure. The accelerated degradation was attributed to the excessive moisture and not the low salt concen￾tration or the thermal cycles. Changes to the constituents of the Nicalon/SiNC system did not influence moisture resistance and suggested that future work to improve fatigue durability should be focused on the fiber coatings. For CMCs with a SiC exterior coating that serves as an environmental barrier coating, the combination of high￾temperature fatigue loading and moisture exposure initiated accel￾erated coating loss and decreased life. None of the oxide/oxide CMCs investigated in this study experienced degradation from the fatigue plus moisture experi￾ments. At the temperatures investigated, the oxide CMCs were stable, and there was no evidence of hydrolysis when moisture was present. Evidence in the literature suggested that, under specific conditions, reactions between the SiO2 in the fiber and moisture may be a problem. Processing improvements continue to yield oxide/oxide CMCs with higher tensile strengths. However, much more work remains to be done to understand the envelope of temperatures, stresses, and atmospheric conditions where oxide/ oxide CMCs remain stable. References 1 K. L. More, P. F. Tortorelli, M. K. Ferber, and J. R. Keiser, “Observations of Accelerated Silicon Carbide Recession by Oxidation at High Water-Vapor Pressures,” J. Am. Ceram. Soc., 83 [1] 211–13 (2000). 2 K. L. More, P. F. Tortorelli, M. K. Ferber, L. R. Walker, J. R. Keiser, W. D. Brentnall, N. Miralya, and J. B. Price, “Exposure of Ceramic and Ceramic-Matrix Composites in Simulated and Actual Combustor Environments”; Paper No. 99-GT- 292 in International Gas Turbine and Aeroengine Congress. ASME International, New York, 1999. 3 M. K. Ferber, H. T. Lin, and J. Keiser, “Oxidation Behavior of Non-Oxide Ceramics in a High-Pressure, High-Temperature Steam Environment”; pp. 201–15 in Mechanical, Thermal, and Environmental Testing and Performance of Ceramic Composites and Components, ASTM STP 1392. Edited by M. G. Jenkins, E. Lara-Curzio, and S. T. Gonczy. American Society for Testing and Materials, West Conshohocken, PA, 2000. 4 T. A. Parthasarathy, T. Mah, C. A. Folsom, and A. P. Katz, “Microstructural Stability of Nicalon Fibers at 1000°C in Air after Salt (NaCl) Exposure,” J. Am. Ceram. Soc., 78 [7] 1992–96 (1995). 5 T. A. Parthasarathy, C. A. Folsom, and L. P. Zawada, “Combined Effects of Exposure to Salt (NaCl) Water and Oxidation on the Strength of Uncoated and BN-Coated Nicalon Fibers,” J. Am. Ceram. Soc., 81 [7] 1812–18 (1998). 6 G. Richardson and R. W. Kowalik, “Oxidation and Hot Corrosion of DuPont Lanxide Enhanced SiC/SiC and Hitco SiC/C Composites,” Ceram. Eng. Sci. Proc., 18 [3] 599–606 (1997). 7 J. A. Haynes, M. J. Lance, K. M. Cooley, M. K. Ferber, R. A. Lowden, and D. P. Stinton, “CVD Mullite Coatings in High-Temperature, High-Pressure Air–H2O,” J. Am. Ceram. Soc., 83 [3] 657–59 (2000). 8 E. J. Opila and R. E. Hann Jr., “Paralinear Oxidation of SiC in Water Vapor,” J. Am. Ceram. Soc., 80 [1] 197–205 (1997). 9 E. J. Opila, “Oxidation Kinetics of Chemically Vapor Deposited Silicon Carbide in Wet Oxygen,” J. Am. Ceram. Soc., 77 [3] 730–36 (1994). 10E. J. Opila, “Variation of the Oxidation Rate of Silicon Carbide with Water￾Vapor Pressure,” J. Am. Ceram. Soc., 82 [3] 625–36 (1999). 11N. S. Jacobson, “Corrosion of Silicon-Based Ceramics in Combustion Environ￾ments,” J. Am. Ceram. Soc., 76 [1] 3–28 (1993). 12L. Thomas-Ogbuji, “A Pervasive Model of Oxidation Degradation in a SiC–SiC Composite,” J. Am. Ceram. Soc., 82 [11] 2777–84 (1998). 13K. L. More, P. F. Tortorelli, H. T. Lin, E. Lara-Curzio, and R. A. Lowden, “Degradation Mechanisms of BN Interfaces in SiC/SiC Composites in Oxygen- and Water-Containing Environments,” Electrochem. Soc. Proc., 98–99, 382–95 (1998). 14C. G. Cofer and J. Economy, “Oxidative and Hydrolytic Stability of Boron Nitride—A New Approach to Improving the Oxidation Resistance of Carbonaceous Structures,” Carbon, 33 [4] 389–95 (1995). 15T. Matsuda, “Stability to Moisture for Chemically Vapour-Deposited Boron Nitride,” J. Mater. Sci., 24, 2353–57 (1989). 16G. N. Morscher, D. Bryant, and R. E. Tressler, “Environmental Durability of Different BN Interphases (for SiC/SiC) in H2O-Containing Atmospheres at Interme￾diate Temperatures,” Ceram. Eng. Sci. Proc., 18 [3] 525–33 (1997). 17L. Thomas-Ogbuji, “Degradation of SiC/BN/SiC Composite in the Burner Rig,” Ceram. Eng. Sci. Proc., 19 [4] 257–64 (1998). 18L. U. J. T. Ogbuji, “A Pervasive Mode of Oxidation Degradation in a SiC–SiC Composite,” J. Am. Ceram. Soc., 81 [11] 2777–84 (1998). 19N. Jacobson, S. Farmer, A. Moore, and H. Sayir, “High-Temperature Oxidation of Boron Nitride: I, Monolithic Boron Nitride,” J. Am. Ceram. Soc., 82 [2] 393–98 (1999). 20N. S. Jacobson, G. N. Morscher, D. R. Bryant, and R. E. Tressler, “High￾Temperature Oxidation of Boron Nitride: II, Boron Nitride Layers in Composites,” J. Am. Ceram. Soc., 82 [6] 1473–82 (1999). 21S. S. Lee, L. P. Zawada, J. M. Staehler, and C. A. Folsom, “Mechanical Behavior and High-Temperature Performance of a Woven Nicalon/Si-N-C Ceramic-Matrix Composite,” J. Am. Ceram. Soc., 81 [7] 1797–811 (1998). 1290 Journal of the American Ceramic Society—Zawada et al. Vol. 86, No. 8

August 2003 Consequence of Moisture and Salt Fog on High-Temperature Fatigue of Ceramic-Matrix Composites 4S. Steel, L P. Zawada, and S Mall, "Fatigue Behavior of a Nextel 720/Alumina 33. L Pierce and L. P. Zawada, "Tensile Behavior of SiC/ C and Rene'41 Following osite at Room and Elevated Temperature, Ceram. Eng. Sci. Proc 2213]695-702(2001) -L. P. Zawada and A Katz, "Evaluation of Exterior Coatings in Nicalon/Carbon X. Campbell, E. V Carelli, K. L. More, P Varghese, S Seal, and V. H Desai, Using Environmental Exposure and Tension Testing"; pp. 30-32 in Environmental Matrix CMC, Siemens Westinghouse Power Corporation Technical Document TR 2-MAT-PTP7-4-2000, April, 2002. Edited by L. P. Zawada and A. Katz. Air Ford TP-02076, Siemens Westinghouse Library in Orlando, Mail Code 00707-23 Research Laboratory, Wright-Patterson AFB, OH, 2002. 2E.A. V. Carelli, H Fujita, J. Y. Yang, and F W. Zok, "Effects of Thermal Aging J. M. Stachler and L. P. Zawada,"Performance of Four Ceramic-Matrix n the Mechanical Properties of a Porous-Matrix Ceramic Composite, "J.A. Ceram Composite Divergent Flap Inserts Following Ground Testing on an F-110 Turbofan Engine,J. Am. Ceram Soc., 83 [7 1727-32(2000 Kalluri, M. Verrilli, and G. Ojard, "Effect of Environment on Behavior 3J. L. Pierce, L. P. Zawada, and R. Srinivasan, "Tensile Properties of Nicalon- /SiNC Ceramic-Matrix Composite, Ceram. Eng. So c,23 Fiber-Reinforced Following Aerospace Turbine Engine Testing, J.Mater. 589-98(2002) Eng Perforn,123]354-62(2003) 2G. N. Morscher, J. Z Gyekenyesi, and R. T. Bhatt,"Damage Accumulation in 2D naparhun, S. Seal, V. H. Desai, P. Varghese, and C. Campbell,"A Woven SiC/SiC Ceramic-Matrix Composites"; see Ref 3, pp. 306-19 troscopic and Thermodynamic Investigation of Nextel-720/Alumina 27G. N. Morscher, J. Hurst, and D. Brewer, "Intermediate-Temperature Stress eramic-Matrix Composite in Air and Water Vapor at 1 100oC,JAm Ceram Soc. ven BN-Interphase, SiC-Matrix J.Am. Ceran.Soc,8361441-49(2000 A. w. Moore, H. Sayir, S. C. Farmer, and G. N. Morscher, "Improved Interfac ee and N. S. Jacobson, "Chemical Stability of the Fiber Coating/Matrix Coatings for SiC Fibers in Ceramic Composites, Ceram. Eng. Sci. Proc., 16 [41 nterface in Silicon-Based Ceramic-Matrix Composites, J. A. Ceram Soc., 78[3] 711-15(1995) G N. Morscher, H. M. Yun, and F. Hurwitz, " High-Temperature Si-Doped BN W. Sheldon and E. Y. Sun. ""Oxidation of BN-Coated SiC Fibers in Interphases for Woven SiC/SiC Composites, "Ceram. Eng. Sci. Proc., 23[3]295-302 sites,J. Am. Ceram Soc., 79[21539-43(1996) 3G N. Morscher, H M. Yun, J.A. DiCarlo, and L. Thomas-Ogbuji, "Effect of BN Fiber-Reinforced Silicon Carbide Composites in Air, " Mater. Sci. Eng. A, 231 Interphase that Debonds between the Interface and the Matrix in SiC/SiC Com 143-50(1997) ites,J.Am. Ceram. Soc.,(2003), in press

22S. Steel, L. P. Zawada, and S. Mall, “Fatigue Behavior of a Nextel720/Alumina (N720/A) Composite at Room and Elevated Temperature,” Ceram. Eng. Sci. Proc., 22 [3] 695–702 (2001). 23C. X. Campbell, E. V. Carelli, K. L. More, P. Varghese, S. Seal, and V. H. Desai, “Effect of High-Temperature Water-Vapor Exposure on Nextel 720 in an Alumina￾Matrix CMC,” Siemens Westinghouse Power Corporation Technical Document TP-02076, Siemens Westinghouse Library in Orlando, Mail Code 00707–235. 24E. A. V. Carelli, H. Fujita, J. Y. Yang, and F. W. Zok, “Effects of Thermal Aging on the Mechanical Properties of a Porous-Matrix Ceramic Composite,” J. Am. Ceram. Soc., 85 [3] 595–602 (2002). 25S. Kalluri, M. Verrilli, and G. Ojard, “Effect of Environment on Fatigue Behavior of a NicalonTM/SiNC Ceramic-Matrix Composite,” Ceram. Eng. Sci. Proc., 23 [3] 589–98 (2002). 26G. N. Morscher, J. Z. Gyekenyesi, and R. T. Bhatt, “Damage Accumulation in 2D Woven SiC/SiC Ceramic-Matrix Composites”; see Ref. 3., pp. 306–19. 27G. N. Morscher, J. Hurst, and D. Brewer, “Intermediate-Temperature Stress Rupture of a Woven Hi-Nicalon, BN-Interphase, SiC-Matrix Composite in Air,” J. Am. Ceram. Soc., 83 [6] 1441–49 (2000). 28K. N. Lee and N. S. Jacobson, “Chemical Stability of the Fiber Coating/Matrix Interface in Silicon-Based Ceramic-Matrix Composites,” J. Am. Ceram. Soc., 78 [3] 711–15 (1995). 29B. W. Sheldon and E. Y. Sun, “Oxidation of BN-Coated SiC Fibers in Ceramic-Matrix Composites,” J. Am. Ceram. Soc., 79 [2] 539–43 (1996). 30H. T. Lin and P. F. Becher, “Effect of Fiber Coating on Lifetime of Nicalon Fiber-Reinforced Silicon Carbide Composites in Air,” Mater. Sci. Eng. A, 231, 143–50 (1997). 31J. L. Pierce and L. P. Zawada, “Tensile Behavior of SiC/C and Rene’ 41 Following Isothermal Exposure and Thermal Fatigue,” J. Mater. Sci., 35, 2973–84 (2000). 32L. P. Zawada and A. Katz, “Evaluation of Exterior Coatings in Nicalon/Carbon Using Environmental Exposure and Tension Testing”; pp. 30–32 in Environmental Performance of Coated CMC, The Technical Cooperation Program, Report No. TR-MAT-PTP7–4-2000, April, 2002. Edited by L. P. Zawada and A. Katz. Air Force Research Laboratory, Wright-Patterson AFB, OH, 2002. 33J. M. Staehler and L. P. Zawada, “Performance of Four Ceramic-Matrix Composite Divergent Flap Inserts Following Ground Testing on an F-110 Turbofan Engine,” J. Am. Ceram. Soc., 83 [7] 1727–32 (2000). 34J. L. Pierce, L. P. Zawada, and R. Srinivasan, “Tensile Properties of Nicalon￾Fiber-Reinforced Carbon Following Aerospace Turbine Engine Testing,” J. Mater. Eng. Perform., 12 [3] 354–62 (2003). 35S. Wannaparhun, S. Seal, V. H. Desai, P. Varghese, and C. Campbell, “A Combined Spectroscopic and Thermodynamic Investigation of Nextel-720/Alumina Ceramic-Matrix Composite in Air and Water Vapor at 1100°C,” J. Am. Ceram. Soc., (2003), in press. 36A. W. Moore, H. Sayir, S. C. Farmer, and G. N. Morscher, “Improved Interface Coatings for SiC Fibers in Ceramic Composites,” Ceram. Eng. Sci. Proc., 16 [4] 409–16 (1995). 37G. N. Morscher, H. M. Yun, and F. Hurwitz, “High-Temperature Si-Doped BN Interphases for Woven SiC/SiC Composites,” Ceram. Eng. Sci. Proc., 23 [3] 295–302 (2002). 38G. N. Morscher, H. M. Yun, J. A. DiCarlo, and L. Thomas-Ogbuji, “Effect of BN Interphase that Debonds between the Interface and the Matrix in SiC/SiC Compos￾ites,” J. Am. Ceram. Soc., (2003), in press.
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