J.Am. Ceram.Soc,891373-1379(2006 DOl:10.ll1551-2916.2005.00906.x o 2006 The American Ceramic Society urna Combined Effect of Salt Water and high-Temperature Exposure on the Strength Retention of NextelM720 Fibers and Nextel M720 Aluminosilicate Composites Triplicane A. Parthasarathy, + Michael K. Cinibulk, and Larry P. Zawada Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/MLLN, Wright-Patterson AFB, Ohio45433-7817 The relative contribution of fiber strength loss to reported deg- considered the key mechanism for degradation in mechanical radation in the mechanical behavior of Nextel 720-aluminosil- properties. In addition, the porosity levels in the matrix are icate composites after exposure to salt fog(AsTM b1l7) was such that the fibers are also exposed to environment and fiber explored. Single filament tension tests were performed on Nex- strengths are now known to be very sensitive to environmental telm720(3M, Inc,Minneapolis, MN) fibers after immersion in attack.3. 6, 10 Thus, the composite degradation could arise from NaCl solutions followed by high-temperature exposure in air. either fiber degradation, or matrix degradation. The results were compared with the behavior of control spec The effect of sea-salt on degradation of Si-based materials mens which received high-temperature exposure but were not well known. The first comprehensive study on commercially immersed in NaCl solution. There was no degradation in fiber available composites was conducted by Zawada et al. This trengths for NaCl solutions below I wt%. However, significant ork showed that SiC-based composites were affe antly by exposure to salt-fog before a thermal excursion in air temperatures between 900 and 1150.C, while no degradation at 1000C, while oxide-based composites were relatively inert to was observed upon an exposure to 1200"C. The relative contri such exposures. Salt-fog exposure involves exposing the spec bution of fiber strength loss to composite degradation was esti men in a chamber that has the ability to generate a fog from a mated as nearly 50%, indicating that both fibers and matrix lution of water and salt of a fixed concentration(for details see interface degrade from exposure to salt water. X-ray diffraction ASTM Bl17 or Zawada et al. ) This first study explored Nex and transmission electron microscopy of the exposed fibers and tel 610/Aluminosilicate composites under salt-fog exposure and omposites were conducted to help rationalize the observations. Nextel"720/ Alumina composites under moisture exposure and Microstructure of degraded fibers showed presence of Na at found no significant degradation(Nextel is the trademark of 3M s near the surface, without any evidence of a Inc, Minneapolis, MN). However, the later work by richard- from segregation or fo on et al. under burner rig testing that included an intermittent amorphous phase. The degraded composites t -fog exposure, showed that Nextel720/ Aluminsilicate com- atrix and fiber/matrix interfaces had Na rich sites suffered significant strength loss after ex It must be noted that the study of Zawada et al. was limited to 1000 for the salt-fog exposure effects, while the work of Richardson et al. reached 1093c during the rig test. From these studies it . Introduction s that either oxide composites are prone to under salt-fog exposure if they are exposed above 1000., or S UCCESSFUL transition of ceramic composites to high-temper that silica-containing Nextel"720 fibers are prone to degrad ature structural applications is in the early stages. An im- tion under salt-fog exposure. As the matrix is porous, the fibers portant factor in such transition is the knowledge and extension in these composites are typically exposed to the environment of the life of these materials under environmental conditions Thus, the composite degradation mechanism could be because that exist in the actual application. These vary significantly from of that of the fiber, the matrix or because of a complex combi one application to another. In the application of nozzles for nation of fiber and matrix degradation(through changes in size landing and the resultant effects during operation is a serious D). To differentiate the diffe concern.In such applications, the material is often exposed to separately nechanisms it is useful to study the degradation of the fibers salt-containing sea water at ambient conditions, immediately followed by high-temperature excursions typical of the use. The strength and microstructure of Nextel 720 fibers was studied conversion of water to steam during rapid heating is the fir and compared with strength results on composites from earlier concern. Many composites appear to survive this well. The sec- work. to determine the relative contribution of fiber degrad ond concern is the chemical effect of the nacl left on the surface tion to that of the composites. In addition, to determine the of the composite in affecting the microstructure and thus me- contribution from matrix degradation, the composites of Ney chanical behavior of the composite. The oxide composites rely el 720/Aluminosilicate exposed to burner rig cycles after sal on matrix porosity for their toughness, and the matrix den- fog exposure by the earlier work?were studied for microstruc- sification from reaction or impurity-induced faster kinetics is ural degradation. Nextel720 fibers are shown to lose strength under certain conditions, which correlates with presence of Na at grain boundaries of surface grains. The fiber degradation is shown to be a function of both salt water concentration and the xposure temperature after the salt water immersion. It wa found that the extent of fiber degradation alone is insufficient Manuscript No 20989. Received September 12, 2005: approved November 23, 2005. laboratory under USAF explain the composite strength loss. Further, microstructural 'Author to whom correspondence should be addressed. e-maiE: Triplicane Parthasarathy observations of rig-tested composites support the conclusion that both matrix and fiber degrade when exposed to salt water ES Inc, Dayton OH 454532. prior to high-temperature exposure in air. 1373
Combined Effect of Salt Water and High-Temperature Exposure on the Strength Retention of NextelTM720 Fibers and NextelTM720- Aluminosilicate Composites Triplicane A. Parthasarathy,w,z Michael K. Cinibulk, and Larry P. Zawada Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/MLLN, Wright-Patterson AFB, Ohio 45433-7817 The relative contribution of fiber strength loss to reported degradation in the mechanical behavior of Nextelt720-aluminosilicate composites after exposure to salt fog (ASTM B117) was explored. Single filament tension tests were performed on Nextelt720 (3M, Inc., Minneapolis, MN) fibers after immersion in NaCl solutions followed by high-temperature exposure in air. The results were compared with the behavior of control specimens which received high-temperature exposure but were not immersed in NaCl solution. There was no degradation in fiber strengths for NaCl solutions below 1 wt%. However, significant degradation was observed at 5 wt% NaCl upon exposure to temperatures between 9001 and 11501C, while no degradation was observed upon an exposure to 12001C. The relative contribution of fiber strength loss to composite degradation was estimated as nearly 50%, indicating that both fibers and matrix/ interface degrade from exposure to salt water. X-ray diffraction and transmission electron microscopy of the exposed fibers and composites were conducted to help rationalize the observations. Microstructure of degraded fibers showed presence of Na at grain boundaries near the surface, without any evidence of a crystalline phase, indicating weakening from segregation or formation of an amorphous phase. The degraded composites showed that matrix and fiber/matrix interfaces had Na rich regions/phases. I. Introduction SUCCESSFUL transition of ceramic composites to high-temperature structural applications is in the early stages.1–3 An important factor in such transition is the knowledge and extension of the life of these materials under environmental conditions that exist in the actual application. These vary significantly from one application to another. In the application of nozzles for Navy aircraft engines, exposure to salt water during takeoff and landing and the resultant effects during operation is a serious concern.3–7 In such applications, the material is often exposed to salt-containing sea water at ambient conditions, immediately followed by high-temperature excursions typical of the use. The conversion of water to steam during rapid heating is the first concern. Many composites appear to survive this well. The second concern is the chemical effect of the NaCl left on the surface of the composite in affecting the microstructure and thus mechanical behavior of the composite. The oxide composites rely on matrix porosity for their toughness,8 and the matrix densification from reaction or impurity-induced faster kinetics is considered the key mechanism for degradation in mechanical properties.9 In addition, the porosity levels in the matrix are such that the fibers are also exposed to environment and fiber strengths are now known to be very sensitive to environmental attack.5,6,10 Thus, the composite degradation could arise from either fiber degradation, or matrix degradation. The effect of sea-salt on degradation of Si-based materials is well known.11–14 The first comprehensive study on commercially available composites was conducted by Zawada et al. 4 This work showed that SiC-based composites were affected signifi- cantly by exposure to salt-fog before a thermal excursion in air at 10001C, while oxide-based composites were relatively inert to such exposures. Salt-fog exposure involves exposing the specimen in a chamber that has the ability to generate a fog from a solution of water and salt of a fixed concentration (for details see ASTM B117 or Zawada et al. 4 ). This first study explored Nextelt610/Aluminosilicate composites under salt-fog exposure and Nextelt720/Alumina composites under moisture exposure and found no significant degradation (Nextel is the trademark of 3M Inc., Minneapolis, MN). However, the later work by Richardson et al. 7 under burner rig testing that included an intermittent salt-fog exposure, showed that Nextelt720/Aluminsilicate composites suffered significant strength loss after exposure. It must be noted that the study of Zawada et al. was limited to 10001C for the salt-fog exposure effects, while the work of Richardson et al. reached 10931C during the rig test. From these studies it appears that either oxide composites are prone to degradation under salt-fog exposure if they are exposed above 10001C, or that silica-containing Nextelt720 fibers are prone to degradation under salt-fog exposure. As the matrix is porous, the fibers in these composites are typically exposed to the environment. Thus, the composite degradation mechanism could be because of that of the fiber, the matrix or because of a complex combination of fiber and matrix degradation (through changes in size and volume fraction of porosity). To differentiate the different mechanisms it is useful to study the degradation of the fibers separately. In the present work, the effect of salt-water exposure on the strength and microstructure of Nextelt720 fibers was studied, and compared with strength results on composites from earlier work,7 to determine the relative contribution of fiber degradation to that of the composites. In addition, to determine the contribution from matrix degradation, the composites of Nextelt720/Aluminosilicate exposed to burner rig cycles after saltfog exposure by the earlier work7 were studied for microstructural degradation. Nextelt720 fibers are shown to lose strength under certain conditions, which correlates with presence of Na at grain boundaries of surface grains. The fiber degradation is shown to be a function of both salt water concentration and the exposure temperature after the salt water immersion. It was found that the extent of fiber degradation alone is insufficient to explain the composite strength loss. Further, microstructural observations of rig-tested composites support the conclusion that both matrix and fiber degrade when exposed to salt water prior to high-temperature exposure in air. 1373 Journal J. Am. Ceram. Soc., 89 [4] 1373–1379 (2006) DOI: 10.1111/j.1551-2916.2005.00906.x r 2006 The American Ceramic Society N. Jacobson—contributing editor This work was funded in part by the Air Force Research laboratory under USAF Contract #-F33615-01-C-5214. w Author to whom correspondence should be addressed. e-mail: Triplicane.Parthasarathy @wpafb.af.mil z UES Inc., Dayton OH 45432. Manuscript No. 20989. Received September 12, 2005; approved November 23, 2005
1374 Journal of the American Ceramic Society-Parthasarathy et al Vol. 89. No. 4 Experimental Procedures lated using the parameter, (i-0 which has been shown to nts in this work involved immersing fibers in salt be appropriate in previous work. Th stress and e ex water at room temperature(RT), followed by heating them in Weibull modulus were obtained for each In addi- air and holding at the temperature of interest. Prior work by tion the mean standard deviation and sta ror were cal Zawada et al. has shown that during salt-fog exposure(ASTM culated for each sample, each having 50 dat The results 117) of these composites, the solution condenses on the sa ples and saturates them fully, much like a specimen that wa The control fiber strength results are shown in Fig. 1(a), along immersed in the solution. Thus immersing the fibers in salt water th data in the literature. Fiber strengths measured after 1-h was taken to simulate the salt-fog exposures. The fibers were exposure in air were not available in the literature: instead tested for strength using single filament tension tests examined data on Nextel"720 fibers after 100 and 1000 h exposure in for phases present using X-ray diffraction(XRD) and for mi- air, obtained from prior reports, are shown. All but one of crostructural effects using scanning and transmission micros the data are from the fiber manufacturer. Given that fiber copy (sEM and TEM, respectively). Samples from the burner strengths are known to vary from batch to batch, the strengths e test after salt-fog exposure conducted by earlier work of Ri- are in reasonable agreement with prior results except at 1200oC et al. were obtained for TEM studies The higher strength of the data from Wilson could possibly ens were used because fiber strengths are known to vary from be due to flaw healing that may occur during long duration batch to batch anneals The thermal exposure in air in this work was limited to tem- The data on control specimens are shown compared with peratures between 900 and 1200.C in static ambient air inside lata on samples exposed to I wt% NaCl in Fig. 1(b). The data the furnace, the humidity was not measured but is typically are plotted as a function of the exposure temperature before ting. The plot shows the mean and the standard error. The 50%. Tows of Nextel 720 fibers(3M Inc, batch 299302), solid line corresponds to the control and the dotted line corre- nominally 60 wt al and 40 wt% mullite were desized at 700C for 15 min in air. The control specimens were held in air sponds to the I wt% Nacl samples In both cases, the strength for I h at temperature. The test samples of the fibers were im of the fiber was retained after a 1000oC exposure; it decreased mersed in freshly prepared aqueous NaCl solutions using com- slightly above 1000@C and significantly above 1150.C. The data mercial grade NaCl and de- ionized water. Two different are seen to be virtually indistinguishable; it is clear that at I wt% oncentrations. I wt% NaCl and 5 wt% NaCl were used. NaCl concentration, salt water exposure has no significant effect The samples were taken out of solution after I min and sus- on fiber strength. XRD was used to verify that a 1 wt%NaCI pended between two alumina pins and held down by two other solution was not too weak to deposit NaCl on the fibers; sig nificant amount (i.e. easily detectable by X-ray) of NaCl was alumina pins, all of them in an alumina boat with serrations to present on the fibers prior to the thermal exposure The Weibull parameters extracted from the raw data, along a larger alumina boat with an alumina cover that enclosed the with the mean and standard deviation, are shown in Table l For samples, except for small gaps between the cover and the boat The samples were first dried in an oven at 125 C. They were then the control samples, the Weibull modulus increased slightly as subjected to a heat treatment in air in a high-temperature air degraded less than the stronger fibers. The Weibull modulus was raised and lowered gradually at 10C/min. The hold at tempe ature was I h for all the samples. ples, but not significantly different at higher temperatures Figure 2 shows the results of fiber strengths after exposure to samples were all tested for mechanical strength using a synergie 5 wt% Nacl solution before the air anneal. The results are The individual filaments were separated from the tow and then clear that at this concentration the strength is decreased signif- gripped directly using MTS-supplied spring-loaded plastic grips icantly even at 900C, but the effect gradually decreases to zero lined with aluminum foil. Fifty filaments with a gauge length of (actually slightly stronger)at 1200.C 2.54 cm were tested for each exposure condition. An average filament diameter was assumed and the fiber strength was cal- ulated from the applied load and the assumed diameter. All tests w onducted at Rt Tablel. The statistics of the fiber strength data measured Selected samples were examined under sem to study the ef- Under Different Exposure Conditions fect of salt water concentration and anneal temperature on the Standard Standard surface coverage. XRD of selected samples were conducted to 。 Mean deviation n verify the presence of NaCl in as-dried samples, and to look for Control amples prepared from fibers that showed the most significan 900°,1h6.92.172030.351000.035 1000°C,1h6.92.172030.351000.03 oss in strength, to study microstructural changes and phase 1100C, I h 5.6 2.05 1.89 0.38 50 0.054 formations/reactions. TEM was also performed on composites subjected to burner rig testing after salt-fog exposure. These l150°C,lh7.51.921.800.29500.041 composites were prepared by Hexcel via a 200°C.1h7.71.501410.22500.031 eneral Electric Aircraft Engines Inc (GEAE, Cincinnati, OH) 1% NaCl using a woven cloth(balanced 8HS weave)of Nextel 720 fibers 900%C1h 7.8 2.15 2.02 0.31 50 0.044 l000°C 1102.132030.22500.032 by mixing alumina powder with silica derived from a polymeric 1100%C.I h 6.42.031.89 0.048 precursor used as a binder 7 l150°C,1h5.61.971.820.38500.053 1200°C,1 6.21.451.350.26500.036 (1) Fiber Strengths 00°C,Ih5.51.661.530.31 500.04 1000°C,1h6.21.501.400.29500.040 All the results on measured strengths were examined using a Weibull plot, which uses the natural logarithm of fracture stress 1100°C,1h4.31.461.330.37500.052 along the x-coordinate and the natural logarithm of the prob- l150°C,lh491451.380.35 ability of survival along the y-axis. The probability was calcu 0°C,1h491.701.560.38 500.054
II. Experimental Procedures The experiments in this work involved immersing fibers in salt water at room temperature (RT), followed by heating them in air and holding at the temperature of interest. Prior work by Zawada et al. 4 has shown that during salt-fog exposure (ASTM B117) of these composites, the solution condenses on the samples and saturates them fully, much like a specimen that was immersed in the solution. Thus immersing the fibers in salt water was taken to simulate the salt-fog exposures. The fibers were tested for strength using single filament tension tests, examined for phases present using X-ray diffraction (XRD) and for microstructural effects using scanning and transmission microscopy (SEM and TEM, respectively). Samples from the burner rig test after salt-fog exposure conducted by earlier work of Richardson et al. were obtained for TEM studies. Control specimens were used because fiber strengths are known to vary from batch to batch. The thermal exposure in air in this work was limited to temperatures between 9001 and 12001C in static ambient air inside the furnace; the humidity was not measured but is typically 50%. Tows of Nextelt720 fibers (3M Inc., batch # 299302), nominally 60 wt% alumina and 40 wt% mullite, were desized at 7001C for 15 min in air.7 The control specimens were held in air for 1 h at temperature. The test samples of the fibers were immersed in freshly prepared aqueous NaCl solutions using commercial grade NaCl and de-ionized water. Two different concentrations, 1 wt% NaCl and 5 wt% NaCl, were used. The samples were taken out of solution after 1 min and suspended between two alumina pins and held down by two other alumina pins, all of them in an alumina boat with serrations to support the alumina pins. The entire assemblage was kept within a larger alumina boat with an alumina cover that enclosed the samples, except for small gaps between the cover and the boat. The samples were first dried in an oven at 1251C. They were then subjected to a heat treatment in air in a high-temperature air furnace with MOSi2 heating elements. The temperature was raised and lowered gradually at 101C/min. The hold at temperature was 1 h for all the samples. The desized samples, control samples, and salt-water exposed samples were all tested for mechanical strength using a Synergie 400 test unit (MTS Systems Corporation, Eden Prairie, MN). The individual filaments were separated from the tow and then gripped directly using MTS-supplied spring-loaded plastic grips lined with aluminum foil. Fifty filaments with a gauge length of 2.54 cm were tested for each exposure condition. An average filament diameter was assumed and the fiber strength was calculated from the applied load and the assumed diameter. All tests were conducted at RT. Selected samples were examined under SEM to study the effect of salt water concentration and anneal temperature on the surface coverage. XRD of selected samples were conducted to verify the presence of NaCl in as-dried samples, and to look for gross reaction in heat-treated samples. TEM was performed on samples prepared from fibers that showed the most significant loss in strength, to study microstructural changes and phase formations/reactions. TEM was also performed on composites subjected to burner rig testing after salt-fog exposure. These composites were prepared by Hexcel via a process licensed from General Electric Aircraft Engines Inc. (GEAE, Cincinnati, OH), using a woven cloth (balanced 8HS weave) of Nextelt720 fibers of 3M Inc. with an aluminosilicate matrix, which was fabricated by mixing alumina powder with silica derived from a polymeric precursor used as a binder.7 III. Results (1) Fiber Strengths All the results on measured strengths were examined using a Weibull plot, which uses the natural logarithm of fracture stress along the x-coordinate and the natural logarithm of the probability of survival along the y-axis. The probability was calculated using the parameter, (i0.5)/N, which has been shown to be appropriate in previous work.15 The reference stress and Weibull modulus were obtained for each Weibull plot. In addition the mean, standard deviation, and standard error were calculated for each sample, each having 50 data points. The results are shown in Table I. The control fiber strength results are shown in Fig. 1(a), along with data in the literature. Fiber strengths measured after 1-h exposure in air were not available in the literature; instead data on Nextelt720 fibers after 100 and 1000 h exposure in air, obtained from prior reports, are shown.16–18 All but one of the data are from the fiber manufacturer. Given that fiber strengths are known to vary from batch to batch, the strengths are in reasonable agreement with prior results except at 12001C. The higher strength of the data from Wilson16 could possibly be due to flaw healing that may occur during long duration anneals. The data on control specimens are shown compared with data on samples exposed to 1 wt% NaCl in Fig. 1(b). The data are plotted as a function of the exposure temperature before testing. The plot shows the mean and the standard error. The solid line corresponds to the control and the dotted line corresponds to the 1 wt% NaCl samples. In both cases, the strength of the fiber was retained after a 10001C exposure; it decreased slightly above 10001C and significantly above 11501C. The data are seen to be virtually indistinguishable; it is clear that at 1 wt% NaCl concentration, salt water exposure has no significant effect on fiber strength. XRD was used to verify that a 1 wt% NaCl solution was not too weak to deposit NaCl on the fibers; significant amount (i.e., easily detectable by X-ray) of NaCl was present on the fibers prior to the thermal exposure. The Weibull parameters extracted from the raw data, along with the mean and standard deviation, are shown in Table I. For the control samples, the Weibull modulus increased slightly as the fiber strengths decreased, indicating that the weaker fibers degraded less than the stronger fibers. The Weibull modulus was higher for the NaCl treated fibers in the 9001 and 10001C samples, but not significantly different at higher temperatures. Figure 2 shows the results of fiber strengths after exposure to 5 wt% NaCl solution before the air anneal. The results are shown compared with the control set data shown in Fig. 1. It is clear that at this concentration the strength is decreased significantly even at 9001C, but the effect gradually decreases to zero (actually slightly stronger) at 12001C. Table I. The Statistics of the Fiber Strength Data Measured Under Different Exposure Conditions m so Mean Standard deviation n Standard error Control 9001C, 1 h 6.9 2.17 2.03 0.35 100 0.035 10001C, 1 h 6.9 2.17 2.03 0.35 100 0.035 11001C, 1 h 5.6 2.05 1.89 0.38 50 0.054 11501C, 1 h 7.5 1.92 1.80 0.29 50 0.041 12001C, 1 h 7.7 1.50 1.41 0.22 50 0.031 1% NaCl 9001C, 1 h 7.8 2.15 2.02 0.31 50 0.044 10001C, 1 h 11.0 2.13 2.03 0.22 50 0.032 11001C, 1 h 6.4 2.03 1.89 0.34 50 0.048 11501C, 1 h 5.6 1.97 1.82 0.38 50 0.053 12001C, 1 h 6.2 1.45 1.35 0.26 50 0.036 5% NaCl 9001C, 1 h 5.5 1.66 1.53 0.31 50 0.044 10001C, 1 h 6.2 1.50 1.40 0.29 50 0.040 11001C, 1 h 4.3 1.46 1.33 0.37 50 0.052 11501C, 1 h 4.9 1.45 1.38 0.35 50 0.049 12001C, 1 h 4.9 1.70 1.56 0.38 50 0.054 1374 Journal of the American Ceramic Society—Parthasarathy et al. Vol. 89, No. 4
pril 2006 Strength Retention of Nextel720 Fibers and Nextel720-Aluminosilicate Composite 1375 Comparison with prior work 5w%oNaCl-T1h Vs Control control 5w%NaCl Std Err h-Control this work o 100hr-Wilson-3M 14 -A1000hr-3M-Fiber Guide D 100hr-Hay Boakye, 2001 D Std Dev 800 900 1000 110 1200 Anneal Temperature 1w%oNaCl-T 1h Vs Control Fig. 2. Effect of anneal temperature after exposure to 5 wt% NaCl so- shown an commatisop with the results obtained on control specimens is Std Err shows that grain boundaries near the fiber surface were enriched Na. Note that Fig 4(b)shows two EDS results superposed, tained from locations shown marked in Fig 4(a). Note also that the eds does not show any peak at 2.62 keV(Kz peak of CD Figure 5 compares TEM images of the control composite(as- received) at the fiber/matrix interface with a composite exposed Std Dev 1hr-1woNaCl to salt-fog and burner rig testing. Note that the matrix pores and grains have coarsened significantly. Figure 6 shows higher mag- nification images of the burner rig exposed composite along with EDS spectra. There is a Na-rich phase present at the fiber-ma- trix interface. Figure 6(b) shows an alumina grain in the matrix, 1100 1200 that shows a Na-rich phase at its surface, presumably a sodium Anneal Temperature,C aluminosilicate. The EDS spectra obtained from the alumina Fig 1.(a) Fiber strengths shown compared with data in the litera- grain interior is superposed on that obtained from the surface ure. 6-s(b)The effect of anneal temperature after exposure to I wt% region in Fig. 6(b). Clearly the salt water has penetrated to the NaCl, on the strength of Nextel 720 fibers, is shown compared with fiber during the salt-fog exposure. control specimen data are virtually indistinguishable IV. Discussion (2) Microstructure The results of this work show that nextel m720 fibers do not Figure 3 shows SEM images of the as-annealed fiber tows for suffer any degradation in strength when subjected to I wt% salt three conditions. Figure 3(a)shows fibers exposed to I wt% water before anneal in air at temperatures of up to 1200.C Nacl after an air anneal at 1 150C, I h. Figure 3(b) shows fibers XRd confirmed the presence of Nacl on the fiber surface after exposed to 5 wt% NaCl after air anneal at the same temperature drying, but not after the air anneal exposure. Thus, the lack of as Fig 3(a). Figure 3(c)shows fibers exposed to same salt con- an effect at I wt% NaCl could be because of loss by evaporation centration as in Fig 3(b), but after anneal in air at 1200C for 1 of the Nacl during the heat up or during the anneal, as dis- h. It is clear that ficant remnant of the molten salt is ussed again later with regard to lack of effect at 1200.C present on the fiber in Fig. 3(b). in contrast to Figs. 3(a) This lack of degradation at I wt% is consistent with the re- and(c) sults obtained by Zawada et al. who used 0.05 wt% salt solu- A comparison of XRD patterns obtained on fiber samples tion to study the effect of intermittent salt- fog exposure on the exposed to l wt% NaCl and annealed in air at 1200C, I h with static and fatigue strength of a variety of composites. They that obtained on a control specimen(no exposure to salt)su found no significant effect on oxide-oxide composites(Nex- jected to the same heat treatment showed no significant differ tel610-Aluminosilicate), while the Sic-based composites ence. A similar profile comparison for the case of a sample showed significant loss of fatigue strength. exposed to 5 wt% NaCl and annealed at 1150oC, I h showed no This work also shows that Nextel 720 fibers do suffer a ignificant differences between the control and salt water-ex nificant degradation in strength when exposed to 5 wt% salt osed specimens. XRD patterns of the fibers(both I wt% NaCl water before an anneal in air at temperature between 900 and nd 5 wt% Nacl)in the as-dried condition(before high-ten 1.C. We are una ble to claim a full understanding for the perature air anneal) showed significant presence of NaCl on the degradation; however segregation-induced weakening or glassy fiber surface phase-induced weakening are suggested for the following rea- Figure 4 shows a TEM image and energy-d sons. The TEM analysis of the specimen that suffered the most copy(eDs) spectra obtained on a fiber exposed to 5 wt% Nacl trength loss(Fig 4)shows that no microstructural change oc- ind heat treated at 1 100C. I h. This condition resulted in the urred within the fiber. There is neither grain growth nor any most degradation in fiber strength. The TEM image(Fig 4(a)) gn of reaction. The presence of Na at the grain boundaries of owed no gross reaction or microstructural changes in the fiber, grains near the surface was the only finding No Cl K peak at ompared with as-received fiber. The EDS spectra(Fig. 4(b)) 2.62 keV is present in the EDs Na exists at the grain boundary
(2) Microstructures Figure 3 shows SEM images of the as-annealed fiber tows for three conditions. Figure 3(a) shows fibers exposed to 1 wt% NaCl after an air anneal at 11501C, 1 h. Figure 3(b) shows fibers exposed to 5 wt% NaCl after air anneal at the same temperature as Fig. 3(a). Figure 3(c) shows fibers exposed to same salt concentration as in Fig. 3(b), but after anneal in air at 12001C for 1 h. It is clear that a significant remnant of the molten salt is present on the fiber surface in Fig. 3(b), in contrast to Figs. 3(a) and (c). A comparison of XRD patterns obtained on fiber samples exposed to 1 wt% NaCl and annealed in air at 12001C, 1 h with that obtained on a control specimen (no exposure to salt) subjected to the same heat treatment showed no significant difference. A similar profile comparison for the case of a sample exposed to 5 wt% NaCl and annealed at 11501C, 1 h showed no significant differences between the control and salt water-exposed specimens. XRD patterns of the fibers (both 1 wt% NaCl and 5 wt% NaCl) in the as-dried condition (before high-temperature air anneal) showed significant presence of NaCl on the fiber surface. Figure 4 shows a TEM image and energy-dispersive spectroscopy (EDS) spectra obtained on a fiber exposed to 5 wt% NaCl and heat treated at 11001C, 1 h. This condition resulted in the most degradation in fiber strength. The TEM image (Fig. 4(a)) showed no gross reaction or microstructural changes in the fiber, compared with as-received fiber. The EDS spectra (Fig. 4(b)) shows that grain boundaries near the fiber surface were enriched in Na. Note that Fig. 4(b) shows two EDS results superposed, one for the grain boundary and another for grain interior, obtained from locations shown marked in Fig. 4(a). Note also that the EDS does not show any peak at 2.62 keV (Ka peak of Cl). Figure 5 compares TEM images of the control composite (asreceived) at the fiber/matrix interface with a composite exposed to salt-fog and burner rig testing. Note that the matrix pores and grains have coarsened significantly. Figure 6 shows higher magnification images of the burner rig exposed composite along with EDS spectra. There is a Na-rich phase present at the fiber-matrix interface. Figure 6(b) shows an alumina grain in the matrix, that shows a Na-rich phase at its surface, presumably a sodium aluminosilicate. The EDS spectra obtained from the alumina grain interior is superposed on that obtained from the surface region in Fig. 6(b). Clearly the salt water has penetrated to the fiber during the salt-fog exposure. IV. Discussion The results of this work show that Nextelt720 fibers do not suffer any degradation in strength when subjected to 1 wt% salt water before anneal in air at temperatures of up to 12001C. XRD confirmed the presence of NaCl on the fiber surface after drying, but not after the air anneal exposure. Thus, the lack of an effect at 1 wt% NaCl could be because of loss by evaporation of the NaCl during the heat up or during the anneal, as discussed again later with regard to lack of effect at 12001C. This lack of degradation at 1 wt% is consistent with the results obtained by Zawada et al. 4 who used 0.05 wt% salt solution to study the effect of intermittent salt-fog exposure on the static and fatigue strength of a variety of composites. They found no significant effect on oxide–oxide composites (Nextelt610-Aluminosilicate), while the SiC-based composites showed significant loss of fatigue strength. This work also shows that Nextelt720 fibers do suffer a significant degradation in strength when exposed to 5 wt% salt water before an anneal in air at temperature between 9001 and 11501C. We are unable to claim a full understanding for the degradation; however segregation-induced weakening or glassy phase-induced weakening are suggested for the following reasons. The TEM analysis of the specimen that suffered the most strength loss (Fig. 4) shows that no microstructural change occurred within the fiber. There is neither grain growth nor any sign of reaction. The presence of Na at the grain boundaries of grains near the surface was the only finding. No Cl Ka peak at 2.62 keV is present in the EDS. Na exists at the grain boundary Comparison with prior work 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 800 1200 Anneal Temperature, °C Anneal Temperature, °C Strength, GPa 1.2 1.4 1.6 1.8 2.0 2.2 Strength, GPa 1h- Control - this work 100hr-Wilson-3M 1000hr-3M-FiberGuide 100hr-Hay & Boakye, 2001 1w%NaCl -T,1h Vs Control 800 900 1000 1100 1200 1hr-Control 1hr-1w%NaCl Std. Dev Std.Err. (a) (b) 0 400 Fig. 1. (a) Fiber strengths shown compared with data in the literature.16–18 (b) The effect of anneal temperature after exposure to 1 wt% NaCl, on the strength of Nextelt720 fibers, is shown compared with control specimens. The data are virtually indistinguishable. 5w%NaCl -T,1h Vs Control 1 1.2 1.4 1.6 1.8 2 2.2 800 900 1000 1100 1200 Anneal Temperature, °C Strength, GPa Std.Dev Std.Err control 5w%NaCl Fig. 2. Effect of anneal temperature after exposure to 5 wt% NaCl solution at room temperature, on the strength of Nextelt720 fibers, is shown in comparison with the results obtained on control specimens. April 2006 Strength Retention of Nextelt720 Fibers and Nextelt720-Aluminosilicate Composite 1375
1376 Journal of the American Ceramic Society-Parthasarathy et al ol.89,No.4 1%Nac-1150C.1h 5%NaC|1150C.1h 5%NaC-1200C,1h 40 40 um 40μ 1%Vs.5% 1150C 1200°C Fig 3. The effect of salt concentration and the effect of anneal temperature on the macrostructure of the Nextel 720 fiber tows after the heat treatment which is a bsent after a 1200C heat treatment as either a segregated solute cation or in a thin amorphous To examine the relative effect of fiber strength loss on the sodium aluminosilicate film. No crystalline phases other than composite degradation, the degradation extents of the two were alumina and mullite were observed. Loss in strength from either compared. The maximum in fiber strength loss occurs at around segregation or from the presence of a glassy film is suggested as a 1100.C, and it is of the order of 30%. This is low compared with plausible mechanism for the strength degradation. the loss in strength observed by Richardson et al. after burner rig testing, during which the composite reached a maximum in temperature of 1093 C(Fig. 3 in Richardson et al.).These composites were exposed to a 5% salt fog at rt before the rig test. The strength loss with number of rig cycles is shown in Fig. 7. The results from control specimens of the composite are included. The 1500 cycle run lasted 50 h, as shown on the top of the plot. It is seen that a loss in strength of 60% occurs very early followed by no further loss. Comparing this with the fiber strength loss at 1100C, the fiber degradation accounts for only half the degradation of the composite, suggesting that changes in the matrix and interface must also contribute to degradation of this composite during salt-fog tests. The TEM results support this conclusion. Figure 6 shows that O. there was significant coarsening of the grains and pores in the matrix. In addition sodium-rich phases were present at the fiber matrix interface. At higher magnifications, a phase rich in Na (b)1000 was found to form on the surface of alumina grains. It is sus- pected that these might be sodium aluminosilicates, which could reduce the strength of the matrix 800 One of the striking findings of this work is that after exposure to 5 wt% NaCl, an anneal in air at 1200C does not cause any significant strength loss of the fibers, although significant strength loss occurs at lower temperatures. Given that the deg- radation at lower temperatures is linked to the of Na at the grain boundaries. the lack of an effect at 1200C implies that either Nacl evaporates faster than it can diffuse into the fiber, or that fiaw healing takes place at higher temperatures vapor pressure of molten NaCl as a function of temperature obtained from Barin 9 is shown plotted in Fig. 8(a), along 00 with the variation in strength loss of the fibers exposed to 5 wt% NaCl, as function of the anneal temperature. The trends are consistent with the suggestion of NaCl evaporation, al though the strength loss at 1200.C being positive remains un Energy (kev xplained unless flaw healing occurs. In addition, from these vapor pressures, PNacl, a simple calculation was made to esti- ransmission electron microscopy image of mate the flux of Nacl and the time taken to evaporate the Nacl, gy using Fick's law for diffusion. The interdiffusion coefficient for spectroscopy analysis shows the presence Nacl(g)in nitrogen was estimated using the formula given in oundaries near the surface, but no Na in the grain Bird et al
as either a segregated solute cation or in a thin amorphous sodium aluminosilicate film. No crystalline phases other than alumina and mullite were observed. Loss in strength from either segregation or from the presence of a glassy film is suggested as a plausible mechanism for the strength degradation. To examine the relative effect of fiber strength loss on the composite degradation, the degradation extents of the two were compared. The maximum in fiber strength loss occurs at around 11001C, and it is of the order of 30%. This is low compared with the loss in strength observed by Richardson et al. 7 after burner rig testing, during which the composite reached a maximum in temperature of 10931C (Fig. 3 in Richardson et al.7 ). These composites were exposed to a 5% salt fog at RT before the rig test. The strength loss with number of rig cycles is shown in Fig. 7. The results from control specimens of the composite are included. The 1500 cycle run lasted 50 h, as shown on the top of the plot. It is seen that a loss in strength of 60% occurs very early followed by no further loss. Comparing this with the fiber strength loss at 11001C, the fiber degradation accounts for only half the degradation of the composite, suggesting that changes in the matrix and interface must also contribute to degradation of this composite during salt-fog tests. The TEM results support this conclusion. Figure 6 shows that there was significant coarsening of the grains and pores in the matrix. In addition sodium-rich phases were present at the fibermatrix interface. At higher magnifications, a phase rich in Na was found to form on the surface of alumina grains. It is suspected that these might be sodium aluminosilicates, which could reduce the strength of the matrix. One of the striking findings of this work is that after exposure to 5 wt% NaCl, an anneal in air at 12001C does not cause any significant strength loss of the fibers, although significant strength loss occurs at lower temperatures. Given that the degradation at lower temperatures is linked to the presence of Na at the grain boundaries, the lack of an effect at 12001C implies that either NaCl evaporates faster than it can diffuse into the fiber, or that flaw healing takes place at higher temperatures. Considering the kinetics of evaporation of NaCl, the vapor pressure of molten NaCl as a function of temperature obtained from Barin19 is shown plotted in Fig. 8(a), along with the variation in strength loss of the fibers exposed to 5 wt% NaCl, as function of the anneal temperature. The trends are consistent with the suggestion of NaCl evaporation, although the strength loss at 12001C being positive remains unexplained unless flaw healing occurs. In addition, from these vapor pressures, PNaCl, a simple calculation was made to estimate the flux of NaCl and the time taken to evaporate the NaCl, using Fick’s law for diffusion. The interdiffusion coefficient for NaCl (g) in nitrogen was estimated using the formula given in Bird et al. 20 1%NaCl-1150C,1 h 5%NaCl-1150C,1 h 5%NaCl-1200C,1 h 1% vs. 5% 1150°C vs. 1200°C 40 µm (a) (b) (c) 40 µm 40 µm Fig. 3. The effect of salt concentration and the effect of anneal temperature on the macrostructure of the Nextelt720 fiber tows after the heat treatment as observed under SEI mode in an scanning electron microscopy is shown. Note that the residue of molten NaCl is observed only with 5 wt% NaCl, which is absent after a 12001C heat treatment. 0 200 400 600 800 1000 0.5 1 1.5 2 2.5 3 Grain Boundary Interior Grain Counts Energy [keV] O Na Cu Al Si Ar Eproxy (a) (b) Fig. 4. Transmission electron microscopy image of Nextelt720 fiber after exposure to 5 wt% NaCl and heat treatment at 1100C, 1 h. Energy dispersive spectroscopy analysis shows the presence of Na at the grain boundaries near the surface, but no Na in the grain interior. 1376 Journal of the American Ceramic Society—Parthasarathy et al. Vol. 89, No. 4
April 2006 Strength Retention of Nextel 720 Fibers and Nextel720-Aluminosilicate Composite CONTROL Coarsening (b) 5%Sat:1100c Pores Pores A comparison of the transmission electron microscopy images of Nextel720-aluminosilicate composite specimens(a exposed to 5 wt% NaCI fog and burner rig tested by richardson et al. shows that the matrix has coarsened significantly both with regard grains and with regard to pores. tained from tables given by hirschfelder et al.2 JNaCI= DNaC N2 RT L (1) and those for collision integral, Ip. from Bird et al. The fiber tow was taken to be filled with nacl solution in the voids be tween fibers and the fibers were taken to have a 0. 1 um Nacl DNaC-N 0008583732(1/MNx2)+(1/MNa) solution coating. The distance, L, for diffusion was taken to be 0.5 m, the approximate width of the furnace. The results are shown in Fig. &(b). These estimates show that Nacl will eva away completely in I h at 1200C. The loss of NaCl here DNaC-Nz is the gas diffusivity in cm/s: MN2, MNacI the the supported by the sEM results shown in Fig. 3. An alternate molecular weights of N2, NaCl; and P the absolute pressure. in atmospheres. Values for the collision diameter, /12, wer-e, in ossibility is that flaw healing takes place at the higher temper es, as has been suggested by Petry and Mah. 100mm 20 nm 2000 Fiber-Matrix Interface Alumina Interior 1500 25 Fig. 6. Tral energy dispersive spectroscopy(EDS) analysis performed on a Nextel720/aluminosilicate mposite after 5 wt% salt-fog esting, revealed(a) Na-rich phase at interface and(b) Na-rich phase on the surface of alumina grains. Note that the eds has two patterns showing the absence of na in the alumina interior
JNaCl ¼ DNaCl;N2 PNaCl RT 1 L (1) DNaClN2 ¼ 0:0018583T3=2 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1=MN2 Þþð1=MNaClÞ p Pr2 12 ID where DNaClN2 is the gas diffusivity in cm2 /s; MN2 , MNaCl the molecular weights of N2, NaCl; and P the absolute pressure, in atmospheres. Values for the collision diameter, r12, were obtained from tables given by Hirschfelder et al. 21 and Svehla22 and those for collision integral, ID, from Bird et al. 20 The fiber tow was taken to be filled with NaCl solution in the voids between fibers and the fibers were taken to have a 0.1 mm NaCl solution coating. The distance, L, for diffusion was taken to be 0.5 m, the approximate width of the furnace. The results are shown in Fig. 8(b). These estimates show that NaCl will evaporate away completely in 1 h at 12001C. The loss of NaCl is supported by the SEM results shown in Fig. 3. An alternate possibility is that flaw healing takes place at the higher temperatures, as has been suggested by Petry and Mah.23 CONTROL 5% Salt : 1100C Open Pores Fine Pores Coarsening Of Matrix 100 µM 100 µM (a) (b) Fig. 5. A comparison of the transmission electron microscopy images of Nextelt720-aluminosilicate composite specimens (a) control specimen with (b) specimen exposed to 5 wt% NaCl fog and burner rig tested by Richardson et al. 7 shows that the matrix has coarsened significantly both with regard to alumina grains and with regard to pores. 0 0.5 1 1.5 2 2.5 3 200 400 600 800 1000 Fiber - Matrix Interface Counts Energy [keV] 0.5 1 1.5 2 2.5 3 Energy [keV] O Al Si Ar Na 0 500 1000 1500 2000 Alumina Interior Alumina Surface Counts O Al Na Si Ni (a) (b) Fig. 6. Transmission electron microscopy images and energy dispersive spectroscopy (EDS) analysis performed on a Nextelt720/aluminosilicate composite after 5 wt% salt-fog exposure and burner rig testing, revealed (a) Na-rich phase at interface and (b) Na-rich phase on the surface of alumina grains. Note that the EDS has two patterns superposed showing the absence of Na in the alumina interior. April 2006 Strength Retention of Nextelt720 Fibers and Nextelt720-Aluminosilicate Composite 1377
1378 Journ Society-Parthasarathv et al ol.89,No.4 50 hrs 150 The effect of exposure to salt water at rt prior to ar Control peratures of90°-1200° C on the strength of Nex tel M720 fibers was studied SEM. XRD. and tem were used to characterize the fibers and composites of Nextel"720/Alumino- 100 ilicate exposed to burner rig cycles after a 5 wt% salt-fog ex- osure. The findings show that Nextel 720 fibers do not suffer ◆ 5w%nAcl fiber strength degradation when exposed to I wt% NaCl solu tion prior to air anneal at all temperatures. However, streng loss of up to 30% was observed when exposed to 5 wt% NaCl solution. TEM results showed the presence of Na at surface Nextel 720/Aluminosilicate CMC grain boundaries, but no grain growth or reactions were ob- Data from Richardson et aL. ( 7) served. Segregation-induced weakening or glassy-phase induced weakening is suggested as possible mechanism of degradation 1000 1500 However, even at 5 wt% Nacl concentration annealing at a Rig Cycles perature of 1200C appears to have no significant effect. The ffect of burner rig cycles on the retained strength of Nex- composite degradation seen in burner rig tests is partly attrib- silicate composites as reported by Richardson et al. ted to the fiber degradation, but tem observations also point ss in strength, while fiber degradation is only about 30% to additional degradation from significant coarsening in the as determined in this study(see Fig 3) The matrix and interface must matrix and formation of Na-rich phases at the fiber-matrix in have caused the remaining degradation in agreement with findings terface shown in Figs. 7 and 8. Acknowledgments It is a pleasure to acknowledge the help of Ms. Tara Podnar(undergraduate measurements, and Dr. Geoff E. Fair(AFRL/MLLN) for critical comn ged. In particular the estimates of diffusion suggested by the associate editor is appreciated. Samples of composites exposed to burner rig conditions were kindly 0.1 provided by the late Dr George Y. Richardson, NAVAIR. References 0.05 L P. Zawada and S. S. Lee."Evaluation of Four CMCs for aeros Trubine Engine Divergent Flaps and Seals. "Cera. Eng. Sci. Proc., 16 14337- -60 P. Zawada and S. S. Lee. ""Evaluation of the Fatigue Performance of Five CMCs for Aerospace Applications", pp. 1669-74 in Sixth International fatigue Congress, Edited by G. Lutjering and H. Nowack. Pergamon, Berlin, 1996. M. Staehler and L. P. Zawada. "Performance of Four Ceramic-Matrix 850 Composite Divergent Flap Inserts Following Ground Testing on an F110 Anneal Temperature, C nce of Intermitten Exposure to Moisture and Salt Fog on the High-Temperature Fatigue Durabil- ity of Several Ceramic-Matrix Composites. "J. Am. Ceram. Soc, 86[8]1282-91 A Parthasarathy, T Mah, C. A Folsom, and A P. Katz, ""Microstructural PNac 1 Stability of Nicalon(TM) at 1000 C in Air After Exposure to Salt(Naci) Water, J.Am. Ceran.So,781992-6019959 T. A. Parthasarathy,C.A m. and L. P. Zawada."Combined Effects of Exposure to Salt (Nacl)Water and Oxidation on the Strength of 1.00 Uncoated and BN-Coated Nicalon Fibers, J. Am. Cera. Soc., 81[7 1812-8 5% NaCl within 400 fibers per tow Environment on the Mechanical Properties of Ceramic Matrix Composites, SAMPE Tech Conf. Proc. 2002. R.J. Kerans R. S. Hay, T A. Parthasarathy, and M. K. Cinibulk ""It diffusing to furnace wall 0. M. A. Mattoni, J. Y. Yang. C. G. Levi, and F. w. Zok, " "Effects of Matrix 1% NaCl Porosity on the Mechanical Prop ll12s a Poro (001) J. Am. tel 720TM, 610TM. and Tyranno-SA Fiber Tows: Effects of Precursors on Fiber Strength, "Ceram. Eng. Sci. Proc., 21 [4]229-35(2000) 1100 1200 and SiN4"; pp. 99-136 in Handbook of Ceramics and Composites, Vol 1, Edited Temperature,C D. S. Fox and J I. Smialek "Burner Rig Hot Corrosion of silicon Carb Fig8. The vapor pressure of NaCl(from Barin%)is shown plotted and Silicon Nitride,"J Am Ceran. Soc, 73.303-11(1990) along with loss in fiber strength after air anneal (with 5 wt% NaCl pric exposure)showing that the improvement in fiber strength correlates with ronments, J. Am. Ceram Soc, 76[1] 3-28(1993) the trend of vapor pressure increase with temperature. (b)shows a p D. S. Fox. M. Q. N. Nguyen. "Sea-Salt Corrosion and Strength of a Sintered a-Silicon Carbide, J. Am. Ceran. Soc. 81 6] 1565-70 Ficks law, as a function of temperature. At 1200C. it is reasonable to K. Kromp, " Statistical Properties of Weibull Estimators. expect all NaCl to have evaporated before doing any damage. J. Mater.Sa26,6741-52(1991)
V. Summary The effect of exposure to salt water at RT prior to an anneal in air at temperatures of 9001–12001C on the strength of Nextelt720 fibers was studied. SEM, XRD, and TEM were used to characterize the fibers and composites of Nextelt720/Aluminosilicate exposed to burner rig cycles after a 5 wt% salt-fog exposure. The findings show that Nextelt720 fibers do not suffer fiber strength degradation when exposed to 1 wt% NaCl solution prior to air anneal at all temperatures. However, strength loss of up to 30% was observed when exposed to 5 wt% NaCl solution. TEM results showed the presence of Na at surface grain boundaries, but no grain growth or reactions were observed. Segregation-induced weakening or glassy-phase induced weakening is suggested as possible mechanism of degradation. However, even at 5 wt% NaCl concentration, annealing at a temperature of 12001C appears to have no significant effect. The composite degradation seen in burner rig tests is partly attributed to the fiber degradation, but TEM observations also point to additional degradation from significant coarsening in the matrix and formation of Na-rich phases at the fiber–matrix interface. Acknowledgments It is a pleasure to acknowledge the help of Ms. Tara Podnar (undergraduate student at The Ohio State University) for help with fiber strength measurements, and Dr. Geoff E. Fair (AFRL/MLLN) for critical comments on the manuscript. The sincere reading of the manuscript and useful comments/suggestions by the associate editor and the reviewers of this Journal are acknowledged. In particular the estimates of diffusion suggested by the associate editor is appreciated. Samples of composites exposed to burner rig conditions were kindly provided by the late Dr. George Y. Richardson, NAVAIR. References 1 L. P. Zawada and S. S. Lee, ‘‘Evaluation of Four CMCs for Aerospace Trubine Engine Divergent Flaps and Seals,’’ Ceram. Eng. Sci. Proc., 16 [4] 337– 9 (1995). 2 L. P. Zawada and S. S. Lee, ‘‘Evaluation of the Fatigue Performance of Five CMCs for Aerospace Applications’’; pp. 1669–74 in Sixth International Fatigue Congress, Edited by G. Lutjering and H. Nowack. Pergamon, Berlin, 1996. 3 J. M. Staehler and L. P. Zawada, ‘‘Performance of Four Ceramic-Matrix Composite Divergent Flap Inserts Following Ground Testing on an F110 Turbofan Engine,’’ J. Am. Ceram. Soc, 83 [7] 1727–38 (2000). 4 L. P. Zawada, J. Staehler, and S. Steel, ‘‘Consequence of Intermittent Exposure to Moisture and Salt Fog on the High-Temperature Fatigue Durability of Several Ceramic–Matrix Composites,’’ J. Am. Ceram. Soc, 86 [8] 1282–91 (2003). 5 T. A. Parthasarathy, T. Mah, C. A. Folsom, and A. P. Katz, ‘‘Microstructural Stability of Nicalon(TM) at 1000 C in Air After Exposure to Salt (NaCl) Water,’’ J. Am. Ceram. Soc., 78 [7] 1992–6 (1995). 6 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] 1812–8 (1998). 7 G. Y. Richardson, C. S. Lei, and R. N. Singh. ‘‘Influence of Turbine Engine Environment on the Mechanical Properties of Ceramic Matrix Composites,’’ SAMPE Tech. Conf. Proc., 2002. 8 R. J. Kerans, R. S. Hay, T. A. Parthasarathy, and M. K. Cinibulk, ‘‘Interface Design for Oxidation Resistant Ceramic Composites,’’ J. Am. Ceram. Soc., 85 [11] 2599–632 (2002). 9 M. A. Mattoni, J. Y. Yang, C. G. Levi, and F. W. Zok, ‘‘Effects of Matrix Porosity on the Mechanical Properties of a Porous-Matrix, All-Oxide Ceramic Composite,’’ J. Am. Ceram. Soc, 84 [11] 2594–602 (2001). 10E. E. Boakye, M. D. Petry, R. S. Hay, and L. M. Douglas, ‘‘Monazite Coatings on Nextel 720TM, 610TM, and Tyranno-SA Fiber Tows: Effects of Precursors on Fiber Strength,’’ Ceram. Eng. Sci. Proc., 21 [4] 229–35 (2000). 11N. S. Jacobson, J. L. Smialek, and D. S. Fox, ‘‘Molten Salt Corrosion of SiC and Si3N4’’; pp. 99–136 in Handbook of Ceramics and Composites, Vol. 1, Edited by N. P. Cheremisinoff. Marcel Dekker, New York, 1990. 12D. S. Fox and J. I. Smialek, ‘‘Burner Rig Hot Corrosion of Silicon Carbide and Silicon Nitride,’’ J. Am. Ceram. Soc, 73, 303–11 (1990). 13N. S. Jacobson, ‘‘Corrosion of Silicon-Based Ceramics in Combustion Environments,’’ J. Am. Ceram. Soc, 76 [1] 3–28 (1993). 14D. S. Fox, M. D. Cuy, and Q. N. Nguyen, ‘‘Sea-Salt Corrosion and Strength of a Sintered a-Silicon Carbide,’’ J. Am. Ceram. Soc, 81 [6] 1565–70 (1998). 15A. Khalili and K. Kromp, ‘‘Statistical Properties of Weibull Estimators,’’ J. Mater. Sci., 26, 6741–52 (1991). 0 50 100 150 0 500 1000 1500 Rig Cycles # Residual RT Strength, MPa 50 hrs Control 5w%NaCl Nextel 720/Aluminosilicate CMC Data from Richardson et al. (7) Fig. 7. The effect of burner rig cycles on the retained strength of Nextelt720-aluminosilicate composites as reported by Richardson et al. 7 shows a 60% loss in strength, while fiber degradation is only about 30% as determined in this study (see Fig. 3) The matrix and interface must have caused the remaining degradation in agreement with findings shown in Figs. 7 and 8. 0 0.05 0.1 0.15 850 950 1050 1150 1250 Anneal Temperature, °C Vapor Pressure, atm –30 –60 –90 0 30 0.01 0.10 1.00 10.00 800 900 1000 1100 1200 Temperature, °C Time, for evapration hr Strength Loss, % 1% NaCl 5% NaCl RT L PNaCl 1 JNaCl =DNaCl,N2, NaCl from trapped solution within 400 fibers per tow diffusing to furnace wall Fig. 8. The vapor pressure of NaCl (from Barin19) is shown plotted along with loss in fiber strength after air anneal (with 5 wt% NaCl prior exposure) showing that the improvement in fiber strength correlates with the trend of vapor pressure increase with temperature, (b) shows a plot of the time to evaporate all of NaCl in the fiber tows, estimated using Fick’s law, as a function of temperature. At 12001C, it is reasonable to expect all NaCl to have evaporated before doing any damage. 1378 Journal of the American Ceramic Society—Parthasarathy et al. Vol. 89, No. 4
April 2006 Strength Retention of Nextel 720 Fibers and Nextel720-Aluminosilicate Composite 137 D.Wilson"newHighTemperatureOxideFibers"http://www.3m.com 2R. B. Bird, W.E. Stewart, and L. E N, Transport Pheno market//ceram _ High_ Temp_Oxide_ Fibers pdf (3M Jw Fiber Selection Guide. 3M In 23. 0. Hirschfelder, C F Curtiss, and R B. Bird. Molecular theory of gases and IR. S. Hay I. Effect of Temperatu in-Doping on Coated Fiber Ten A. SwehLa"Estimated Viscosities and Thermal Conductivities of gases at 32.1962 I. Barin. Thermochemical Data of Pure Substances dition. vch D. Petry and T. Mah, Effect of Thermal Exp Verlagsgesellschaft. New York, 1995 Nextel"550 and 720 Filaments, "J.Am. Ceram. Soc. 82[10]2801-7(1999). D
16D. Wilson ‘‘New High Temperature Oxide Fibers,’’ http://www.3m.com/ market/industrial/ceramics/pdfs/New_High_Temp_Oxide_Fibers.pdf, (3M Inc). 17 Fiber Selection Guide, 3M Inc., St. Paul, MN. 18R. S. Hay and E. E. Boakye, ‘‘Monazite Coatings on Fibers: I, Effect of Temperature and Alumina-Doping on Coated Fiber Tensile Strength,’’ J. Am. Ceram. Soc., 84 [12] 2783–92 (2001). 19I. Barin, Thermochemical Data of Pure Substances I, 3rd edition, VCH Verlagsgesellschaft, New York, 1995. 20R. B. Bird, W. E. Stewart, and L. E. N., Transport Phenomena, 2nd edition, J Wiley & Sons, New York, 2002. 21J. O. Hirschfelder, C. F. Curtiss, and R. B. Bird, Molecular theory of gases and liquids. Wiley & Sons Inc., New York, 1954. 22R. A. Svehla ‘‘Estimated Viscosities and Thermal Conductivities of Gases at High Temperatures’’ (NASA Tech Report R-132, 1962). 23D. Petry and T. Mah, ‘‘Effect of Thermal Exposures on the Strengths of Nextelt 550 and 720 Filaments,’’ J. Am. Ceram. Soc, 82 [10] 2801–7 (1999). & April 2006 Strength Retention of Nextelt720 Fibers and Nextelt720-Aluminosilicate Composite 1379
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