Journal J Am Ceram Soc, 80 [8]2029-12 (1997) Tensile Stress Rupture of Sic/sicm Minicomposites with Carbon and Boron Nitride Interphases at elevated temperatures in Air Gregory N Morscher" T Case Western Reserve University, Cleveland, Ohio 4413 The stress-rupture properties of precracked minicompos- sealing"the cracked surface of the composite, prohibiting ites were determined in air at temperatures in the range of apid oxidation of the interphase from occurring 7000-1200oC. The minicomposite systems consisted of a At intermediate temperatures, the interphase may be con single tow of Nicalon or Hi-Nicalon fibers with carbon sumed if oxidation in the matrix crack is too slow to promote boron nitride (BN) interphases and a chemical-vapor crack sealing. -8 If the temperature is low enough to volatilize infiltrated silicon carbide (CVI-SiC)matrix. The stress- the interphase and not form stable solid- or liquid-oxidation rupture results were compared to single-fiber stress- products, the fiber-matrix bonding is destroyed, with a loss of rupture data and composite data in the literature. Severe load transfer and possible fiber degradation. If oxidation prod- embrittlement occurred for carbon interphase minicon ucts do form in place of the original interphase material, the posites. However, BN interphase minicomposites showed composite is embrittled, because of the strong bond that is only mild degradation in the rupture properties. This was formed between the fiber and the matrix and/or fiber degrada- true even though the BN interphase reacted and vaporized tion. This intermediate temperature regime poses the greates because of water vapor in the atmosphere at intermediate threat to composite embrittlement, because oxygen and other temperatures(7000-950C)and glass formation occurred vapor species have easy access to the interphase at higher temperatures(950%-12000C). The severe degra At higher temperatures, whether beneficial sealing can occur dation in rupture properties that occurred for carbon in for SiC/SiC under thermomechanical cycling must be deter- terphase composites at intermediate temperatures was due mined. a type of sealing phenomena has been shown to occur to degradation of the Nicalon-fiber properties from the en- for a Nic /BN/BMASm matrix composite. In this system, the vironment. The rupture properties of the BN-interphas outer skin(50 um) of the cracked matrix undergoes a phase minicomposites were controlled by the fib transformation during stress-rupture testing at 1100C, which erties at temperatures of less than -900oC and greater seals the cracks at the surface of the composite, prohibiting -1100C. In the range of -90011000C, most fibers embrittlement. However, this is different than what is hypoth to the matrix because of a glass layer that formed between esized for SiC, because the BMAs matrix itself transforms to the fiber and matrix, resulting in fiber stress concentrations cause a seal, whereas, with SiC, the oxidation product must that led to the mild embrittlement of the BN-interphase seal the cracks incomposite The other factor that may or may not be related to the en vironment is the mechanical properties of the fibers. Ulti L. Introduction mately, the composite mechanical properties can only be as HE success of a silicon carbide(SiC)-fiber-reinforced Sic good as the strength, rupture, and creep properties of the fibers matrix(SiC/SiC)composite as a high-temperature mate The fiber mechanical properties can degrade, because rial will be dependent on the ability of the composite to main- chemical reactions between the fiber, interphase, matrix, and/or tain desirable mechanical properties in an oxidizing environ- environment. The fiber strength also can degrade, because ment. Several factors can lead to deleterious composite intrinsic chemical instability, slow crack growth, or creep The objective of this study was to determine and explain the The most vulnerable component of the SiC/SiC system is stressed-oxidative stability over a wide range of temperatures the interphase that separates the fiber and the matrix. For a and times in air for carbon and BN interphase Sic/Sic ceramic- cracked composite in an oxidizing environment at an elevated matrix composites(CMCs). A minicomposite(single-tow com- temperature, the interphase is exposed to the environment. To posite"approach was used that enabled many specimens with maintain the debonding and sliding properties that are required different fiber/interphase combinations to be fabricated, The for good composite mechanical properties, either the interphase use of minicomposites also enabled several simple stress must be resistant to oxidation or the composite system must pture rigs to be built so that long-term(1000 h)experiment protect the interphase from oxidation. It is most desirable for could readily be performed. The results are discussed in rela- the interphase to be an oxidation-resistant material; however, to tion to the known fiber stress-rupture properties. The micro- date uch interphase has been proven effective at high structural features are described and related to the thermo temperatures for the SiC/SiCm system. The current interphase chemical reactions that occurred over the time/temperature of choice is boron nitride(BN), 2 which reacts with oxygen at for ndings were then related to the ob- intermediate temperatures(.) to form boron oxide served mechanical behavior and discussed in regard to their (B2O3 )liquid. It is hoped that the B2O, liquid will react with relevance to SiC/SiC composites the Sic matrix to form a borosilicate glass that is capable of Il. Experimental R. Naslain--contributing editor was pursued because it re- Several different variations were processed Two different commercial fibers(Nicalon 202 and Hi- Resident Research Associate at NASA Lewis Research Center, Cleveland, OH er, Nippon Carbon Co., Tokyo, Japan
Tensile Stress Rupture of SiCf/SiCm Minicomposites with Carbon and Boron Nitride Interphases at Elevated Temperatures in Air Gregory N. Morscher*,† Case Western Reserve University, Cleveland, Ohio 44135 The stress-rupture properties of precracked minicomposites were determined in air at temperatures in the range of 700°–1200°C. The minicomposite systems consisted of a single tow of Nicalon or Hi-Nicalon fibers with carbon or boron nitride (BN) interphases and a chemical-vaporinfiltrated silicon carbide (CVI-SiC) matrix. The stressrupture results were compared to single-fiber stressrupture data and composite data in the literature. Severe embrittlement occurred for carbon interphase minicomposites. However, BN interphase minicomposites showed only mild degradation in the rupture properties. This was true even though the BN interphase reacted and vaporized because of water vapor in the atmosphere at intermediate temperatures (700°–950°C) and glass formation occurred at higher temperatures (950°–1200°C). The severe degradation in rupture properties that occurred for carbon interphase composites at intermediate temperatures was due to degradation of the Nicalon-fiber properties from the environment. The rupture properties of the BN-interphase minicomposites were controlled by the fiber rupture properties at temperatures of less than ∼900°C and greater than ∼1100°C. In the range of ∼900°–1100°C, most fibers fused to the matrix because of a glass layer that formed between the fiber and matrix, resulting in fiber stress concentrations that led to the mild embrittlement of the BN-interphase minicomposites. I. Introduction THE success of a silicon carbide (SiC)-fiber-reinforced SiC matrix (SiCf /SiCm) composite as a high-temperature material will be dependent on the ability of the composite to maintain desirable mechanical properties in an oxidizing environment. Several factors can lead to deleterious composite properties at elevated temperatures in oxidizing environments. The most vulnerable component of the SiCf /SiCm system is the interphase that separates the fiber and the matrix. For a cracked composite in an oxidizing environment at an elevated temperature, the interphase is exposed to the environment. To maintain the debonding and sliding properties that are required for good composite mechanical properties, either the interphase must be resistant to oxidation or the composite system must protect the interphase from oxidation. It is most desirable for the interphase to be an oxidation-resistant material; however, to date, no such interphase has been proven effective at high temperatures for the SiCf /SiCm system. The current interphase of choice is boron nitride (BN),1,2 which reacts with oxygen at intermediate temperatures (∼450°C) to form boron oxide (B2O3) liquid. It is hoped that the B2O3 liquid will react with the SiC matrix to form a borosilicate glass that is capable of ‘‘sealing’’ the cracked surface of the composite, prohibiting rapid oxidation of the interphase from occurring. At intermediate temperatures, the interphase may be consumed if oxidation in the matrix crack is too slow to promote crack sealing.3–8 If the temperature is low enough to volatilize the interphase and not form stable solid- or liquid-oxidation products, the fiber–matrix bonding is destroyed, with a loss of load transfer and possible fiber degradation. If oxidation products do form in place of the original interphase material, the composite is embrittled, because of the strong bond that is formed between the fiber and the matrix and/or fiber degradation. This intermediate temperature regime poses the greatest threat to composite embrittlement, because oxygen and other vapor species have easy access to the interphase. At higher temperatures, whether beneficial sealing can occur for SiC/SiC under thermomechanical cycling must be determined. A type of sealing phenomena has been shown to occur for a Nicf ‡ /BN/BMASm matrix composite.2 In this system, the outer skin (∼50 mm) of the cracked matrix undergoes a phase transformation during stress-rupture testing at 1100°C, which seals the cracks at the surface of the composite, prohibiting embrittlement. However, this is different than what is hypothesized for SiC, because the BMAS matrix itself transforms to cause a seal, whereas, with SiC, the oxidation product must seal the cracks. The other factor that may or may not be related to the environment is the mechanical properties of the fibers. Ultimately, the composite mechanical properties can only be as good as the strength, rupture, and creep properties of the fibers. The fiber mechanical properties can degrade, because of chemical reactions between the fiber, interphase, matrix, and/or environment. The fiber strength also can degrade, because of intrinsic chemical instability, slow crack growth, or creep. The objective of this study was to determine and explain the stressed-oxidative stability over a wide range of temperatures and times in air for carbon and BN interphase SiC/SiC ceramicmatrix composites (CMCs). A minicomposite (single-tow composite9 ) approach was used that enabled many specimens with different fiber/interphase combinations to be fabricated. The use of minicomposites also enabled several simple stressrupture rigs to be built so that long-term (1000 h) experiments could readily be performed. The results are discussed in relation to the known fiber stress-rupture properties. The microstructural features are described and related to the thermochemical reactions that occurred over the time/temperature range for testing. These findings were then related to the observed mechanical behavior and discussed in regard to their relevance to SiC/SiC composites. II. Experimental Procedure (1) Minicomposites Tested The minicomposite approach was pursued because it required relatively short lengths (tens of feet) of coated tow. Several different minicomposite variations were processed. Two different commercial fibers (Nicalon 202 and HiR. Naslain—contributing editor Manuscript No. 191885. Received April 15, 1996; approved December 26, 1996. Supported by the NASA HITEMP Program. *Member, American Ceramic Society. † Resident Research Associate at NASA Lewis Research Center, Cleveland, OH. ‡ Ceramic-grade Nicalon™ fiber, Nippon Carbon Co., Tokyo, Japan. J. Am. Ceram. Soc., 80 [8] 2029–42 (1997) Journal 2029
Journal of the American Ceramic Sociery--Morscher Vol. 80. No 8 Nicalon), three different interphases(one carbon and two was performed on single-fiber microcomposites that is de types of BN), and two different chemical-vapor-infiltrated- scribed in detail elsewhere. 3 The fibers were mounted with matrix(CVI-matrix) vendors were used in this study, as de- epoxy onto cardboard tabs. The epoxy was gripped, and acous- cribed in Table 1. Examples of minicomposite polished cross tic emission(AE) sensors were attached to the epoxy so that sections are shown in Figs. 1((aHe)) for each minicomposite cracking could be monitored. The ultimate failure load had been determined for each minicomposite type (Table I)from The general minicomposite processing approach is identical monotonic tensile tests, and the load for significant micro to common CVI-SiC composites. For cracking was fairly well characterized by the AE activity. A minicomposites(C-Nic), the tows wer precracking load was chosen based on the AE data. The crack ardon. an pacing was then determined from polished longitudinal sec- onsecutive steps by the same vendor tions of each minicomposite(Fig. 2). The average crack spac- shown in Table I. This is not a saturation crack spacing, b a was very uniform, and the interphase thickness was -0.4 um ing for each minicomposite for the applied precracking load is (Fg.1(a) For the bn interphase minicomposites(3MBN-Nic, PBN cause saturation had not occurred at or below the precracking HN, 3MBN-HN), the tows were first coated by a vendor and oad for any of the minicomposites that were tested in this then the coated tows were mounted on graphite racks and com- study. Twenty five to fifty unload/reload cycles were per ited with Sic by a different vendor. The BN interphase formed during this precracking step were produced at different temperatures and differ in com The stress-rupture test was either performed in the univers sition and microstructure. The BN for the 3MBN-Nic(Fig esting machine or in simple dead-weight load stress-rupture (b)and 3MBN-HN (Fig. l(e))minicomposites was deposited rigs. The distance from the top to the bottom of the outside using triethylborane at 1050 C. o This bn has been well char- walls of the furnace was 35 mm. The hot zone was -12 mm acterized on tows and in composites by Brennan. It is uniform The temperatures that were chosen for stress-rupture testing and has >5 at. %oxygen and -15 at carbon. It can be de- were in the range of 700-1200.C. For all the stress-rupture scribed as being composed of small turbostratic crystals that tests, a constant load was applied at room temperature, the temperature was increased(100%C/min) to the desired value (PBN-HN) was processed with boron trichloride(BCl,)and and the time was monitored for-100 h or until failure. If no ammonia(NH,OH)at 1400@C. This BN is more pure with failure occurred after-100 h, the temperature was decreased to ittle oxygen or carbon. The BN is turbostratic with the c-axis room temperature, so that the minicomposite surface could be well aligned normal to the fiber axis. 2 There also is significant observed. After examination, the minicomposite was then re- oating nonuniformity across the tow. The outer fibers of the mounted and reloaded to the original stress-rupture load for ow can have a bn thickness of up to 3 um, whereas the another 100+ h, after which the same procedure was followed interior fibers of the tow are relatively uniformly coated with nless failure had occurred. This type of very-low-cycle ther- BN.5 um thick(Figs. I(c)and(d)) mal fatigue was followed until failure or for times at tempera he minicomposites also differed in physical appearance. ture of up to 1000 h. For some experiments, if the samples did The PBN-HN, 3MBN-Nic, and C-Nic minicomposites were not fail after several hundred hours, the temperature was in- very compact( before and after SiC infiltration), with the fibers creased or the retained room-temperature tensile strength was undled tightly together. The overall diameter of the minicom- determined as noted below osites was <l mm(Fig. 1). The appearance of the 3MBN-HN minicomposites were very fibrous(before and after Sic infil. (3) Scanning Electron Microscopy tration), with the fibers loosely bundled. This specific tow must Minicomposite fracture surfaces and some longitudinal have been spread during the Bn coating step. The result was a tions were examined using a conventional minicomposite that effectively had many single-fiber com microscopy(SEM) microscope(Model JEOL 840A, tes that were loosely connected to each other. The overall Tokyo, Japan)and a field-emission SEM(FESEM) mici diameter of these minicomposites was 2-3 mm. Although the (Model $4500, Hitachi, Tokyo, Japan). The FESEM was used minicomposites were nonuniform in many respects, the test at 5 kv, which enabled energy-dispersive spectroscopy (EDS) nethodology that is described below should have minimized spectra to be obtained for relatively small areas. Under these the importance of these nonuniformities conditions, boron was easily detected for BN (2) Mechanical Testing IlL. Results Tensile testing consisted of, first, precracking the minicom- posites and, second, subjecting them to a constant-load stress (1 Minicomposite Tensile Stress-Rupture Results rupture test. The precracking was performed using a universal The load, time, temperature, and ae data from a typical testing machine(Model 4502, Instron, Canton, MA). The test precrack and stress-rupture test for a PBN-HN minicomposite set-up is almost identical to one that was used in a study that tested at 950"C are shown in Fig. 3. The precracking step consisted of 50 unload/reload cycles with loads of 11-55 kg Both fibers are produced by Nippon Carbon; in this paper, the fibers have been The AE energy per event is shown as a function of time, the majority of which occurred on the first loading cycle Table I. Properties of Minicomposites Tested diameter(um) per tow Interphase ad(kg) (cracks/mm) 50 Carbon19±2[2.311.3[14 Nicalon 202 14.4 10±1[24]647.7[6 0.59 PBN-HN Hi-Nicalon 13.3 PBN14.5±1[2.]11.3[16 3MBN-HN Hi-Nicalon 13.9 4905 BN14.5±1[20]11.3[5y were cach sup plied by B: E: Goodrich aerospace. nIcation 202 a. d Hi-Nicalon were ma ufrcturecd by Nippon carbon tol Nic. PBN-HN. and 3MBN-HN the mber stress, thrich is the smes on th t fibers m the bers were s ll loaded she he oss d divided by the cross-sectionitle area of the fiber. "i he numbe brous, an accurate fiber therefore, the failure and precrack stress are only approximations. It was too difficult to obtain a crack density. Longitudinal polished sections were obtained which showed cracks; however, this minicomposite type was too fibrous to quantify the average crack spacin
Nicalon),§ three different interphases (one carbon and two types of BN), and two different chemical-vapor-infiltratedmatrix (CVI-matrix) vendors were used in this study, as described in Table I. Examples of minicomposite polished cross sections are shown in Figs. 1((a)–(e)) for each minicomposite type. The general minicomposite processing approach is identical to common CVI-SiC composites. For the carbon interphase minicomposites (C-Nic), the tows were mounted on graphite racks, coated with carbon, and then composited with SiC in consecutive steps by the same vendor. The carbon interphase was very uniform, and the interphase thickness was ∼0.4 mm (Fig. 1(a)). For the BN interphase minicomposites (3MBN-Nic, PBNHN, 3MBN-HN), the tows were first coated by a vendor and then the coated tows were mounted on graphite racks and composited with SiC by a different vendor. The BN interphases were produced at different temperatures and differ in composition and microstructure. The BN for the 3MBN-Nic (Fig. 1(b)) and 3MBN-HN (Fig. 1(e)) minicomposites was deposited using triethylborane at 1050°C.10 This BN has been well characterized on tows and in composites by Brennan.2 It is uniform and has >5 at.% oxygen and ∼15 at.% carbon. It can be described as being composed of small turbostratic crystals that are not well aligned with the fiber axis. The PBN material (PBN-HN) was processed with boron trichloride (BCl3) and ammonia (NH4OH) at 1400°C.11 This BN is more pure with little oxygen or carbon. The BN is turbostratic with the c-axis well aligned normal to the fiber axis.12 There also is significant coating nonuniformity across the tow. The outer fibers of the tow can have a BN thickness of up to 3 mm, whereas the interior fibers of the tow are relatively uniformly coated with BN ∼0.5 mm thick (Figs. 1(c) and (d)). The minicomposites also differed in physical appearance. The PBN-HN, 3MBN-Nic, and C-Nic minicomposites were very compact (before and after SiC infiltration), with the fibers bundled tightly together. The overall diameter of the minicomposites was #1 mm (Fig. 1). The appearance of the 3MBN-HN minicomposites were very fibrous (before and after SiC infiltration), with the fibers loosely bundled. This specific tow must have been spread during the BN coating step. The result was a minicomposite that effectively had many single-fiber composites that were loosely connected to each other. The overall diameter of these minicomposites was 2–3 mm. Although the minicomposites were nonuniform in many respects, the test methodology that is described below should have minimized the importance of these nonuniformities. (2) Mechanical Testing Tensile testing consisted of, first, precracking the minicomposites and, second, subjecting them to a constant-load stressrupture test. The precracking was performed using a universal testing machine (Model 4502, Instron, Canton, MA). The test set-up is almost identical to one that was used in a study that was performed on single-fiber microcomposites that is described in detail elsewhere.13 The fibers were mounted with epoxy onto cardboard tabs. The epoxy was gripped, and acoustic emission (AE) sensors were attached to the epoxy so that cracking could be monitored. The ultimate failure load had been determined for each minicomposite type (Table I) from monotonic tensile tests, and the load for significant microcracking was fairly well characterized by the AE activity. A precracking load was chosen based on the AE data. The crack spacing was then determined from polished longitudinal sections of each minicomposite (Fig. 2). The average crack spacing for each minicomposite for the applied precracking load is shown in Table I. This is not a saturation crack spacing, because saturation had not occurred at or below the precracking load for any of the minicomposites that were tested in this study. Twenty five to fifty unload/reload cycles were performed during this precracking step. The stress-rupture test was either performed in the universal testing machine or in simple dead-weight load stress-rupture rigs. The distance from the top to the bottom of the outside walls of the furnace was 35 mm. The hot zone was ∼12 mm. The temperatures that were chosen for stress-rupture testing were in the range of 700°–1200°C. For all the stress-rupture tests, a constant load was applied at room temperature, the temperature was increased (100°C/min) to the desired value, and the time was monitored for ∼100 h or until failure. If no failure occurred after ∼100 h, the temperature was decreased to room temperature, so that the minicomposite surface could be observed. After examination, the minicomposite was then remounted and reloaded to the original stress-rupture load for another 100+ h, after which the same procedure was followed, unless failure had occurred. This type of very-low-cycle thermal fatigue was followed until failure or for times at temperature of up to 1000 h. For some experiments, if the samples did not fail after several hundred hours, the temperature was increased or the retained room-temperature tensile strength was determined as noted below. (3) Scanning Electron Microscopy Minicomposite fracture surfaces and some longitudinal sections were examined using a conventional scanning electron microscopy (SEM) microscope (Model JEOL 840A, JEOL, Tokyo, Japan) and a field-emission SEM (FESEM) microscope (Model S4500, Hitachi, Tokyo, Japan). The FESEM was used at 5 kV, which enabled energy-dispersive spectroscopy (EDS) spectra to be obtained for relatively small areas. Under these conditions, boron was easily detected for BN. III. Results (1) Minicomposite Tensile Stress-Rupture Results The load, time, temperature, and AE data from a typical precrack and stress-rupture test for a PBN-HN minicomposite tested at 950°C are shown in Fig. 3. The precracking step consisted of 50 unload/reload cycles with loads of 11–55 kg. The AE energy per event is shown as a function of time, the majority of which occurred on the first loading cycle. § Both fibers are produced by Nippon Carbon; in this paper, the fibers have been designated ‘‘Nic’’ and ‘‘HN,’’ respectively. Table I. Properties of Minicomposites Tested Name of minicomposite† Fiber type‡ Average fiber diameter§ (mm) Number of fibers per tow Interphase Failure load¶ (kg) Precracking load¶ (kg) Average crack density along length of minicomposite (cracks/mm) C-Nic Nicalon 202 15.3 450 Carbon 19 ± 2 [2.3] 11.3 [1.4] 0.44 BN-Nic Nicalon 202 14.4 250 BN 10 ± 1 [2.4] 6.4–7.7 [1.6] 0.59 PBN-HN Hi-Nicalon 13.3 490 PBN 14.5 ± 1 [2.1] 11.3 [1.6] 1.1 3MBN-HN Hi-Nicalon 13.9 490§ BN 14.5 ± 1 [2.0]†† 11.3 [1.5]†† ?‡‡ † C-Nic was supplied by Hyper-Therm Corp., Huntington Beach, CA, or B. F. Goodrich Aerospace, Brecksville, OH; BN-Nic, PBN-HN, and 3MBN-HN were each supplied by B. F. Goodrich Aerospace. ‡ Nicalon 202 and Hi-Nicalon were manufactured by Nippon Carbon, Tokyo, Japan. § The average fiber diameter was determined by measuring the diameters of 50 fibers from a polished cross section for each minicomposite type. ¶ The value in brackets is the fiber stress, which is the stress on the fibers if the fibers were fully loaded, i.e., the load divided by the cross-sectional area of the fiber. ††The number of fibers per tow is assumed to be the same as that for PBN-HN. Because the minicomposite was so fibrous, an accurate fiber count was impossible; therefore, the failure and precrack stress are only approximations. ‡‡It was too difficult to obtain a crack density. Longitudinal polished sections were obtained which showed cracks; however, this minicomposite type was too fibrous to quantify the average crack spacing. 2030 Journal of the American Ceramic Society—Morscher Vol. 80, No. 8
August 1997 Tensile Stress Rupture of sic/siCm Minicomposites with C and BN Interphases at Elevated Temperatures in Air 2031 100 (c) 10 Fig. 1. Optical micrographs of the polished cross sections for(a)C-Nic, (b) 3MBN-Nic, (c and d) PBN-HN, and(e)3MBN-HN minicomposites
Fig. 1. Optical micrographs of the polished cross sections for (a) C-Nic, (b) 3MBN-Nic, (c and d) PBN-HN, and (e) 3MBN-HN minicomposites. August 1997 Tensile Stress Rupture of SiCf/SiCm Minicomposites with C and BN Interphases at Elevated Temperatures in Air 2031
203 Journal of the American Ceramic Sociery-Morscher Vol. 80. No 8 cracked, the fibers were carrying the load at the matrix crack For the Nicalon minicomposites(Fig. 4(a)), a definite dif- nd Bn interphase minicomposites. The C-Nic minicomposite life was far more sensitive to stress than that of 3MBN-N minicomposites, where life was only mildly dependent on stress. For the Hi-Nicalon minicomposites(Fig. 4(b), no real difference was apparent between the two BN-coated systems ss dependence ior stress rupture at 750 and 950C, with a slightly more signifi- cant stress dependence at 1200C A few samples that did not fail were tested at room tem- rature to determine the retained strength after stressed oxi- dation. The 3MBN-Nic microcomposites were very difficult to handle after the experiment and could not be tested. The BN Hn composites could be handled and tested. The retained strength at room temperature of two minicomposites that were originally tested at 1200C for 882 h(328 MPa)and 965 h(385 Fig. 2. Optical micrograph of a polished longitudinal section of a MPa)was 525 and 792 MPa, respectively (-25% and 38% respectively, of the ultimate strength at room temperature) tion after the minicomposite was cycled 50 times to a load of 55 kg at (2)Microstructural Observations room temperature (A) C-Nic Minicomposites: The fracture surfaces of C- Nic minicomposites that have been tested at 700@C are shown The data for all the stress-rupture results are shown in in Fig. 5. Figure 5(a)shows a typical fracture surface for short graphical form on log scales in Figs. 4(a) and(b). The mini- time(<l h) failure at 700C. There is moderate fiber pullout, composites always failed in the hottest region of the furnace. and the fracture surface appears to be essentially the same as The data is plotted as the stress on the fibers, assuming that the room-temperature fast-fracture surfaces. For longer rupture fibers carried the full load. The number of fibers were counted times(2-10 h), the fracture surfaces are characterized by long for each minicomposite (Table I) from several (at least five) fiber pullout lengths(Fig. 5(b). The pullout length is almost 2 shed cross sections. The number of fibers for a given mini mm for this broomlike fracture surface. The fracture surfaces type was found to differ from sample to sample by for the longest rupture times(106 h) are planar(Fig. 5(c) less than ten fibers. The highest number of fibers that were Some fibers appeared to be in contact with the matrix; this counted from the observed cross sections was used as the fiber was true even of samples that have been tested at higher loads tow count. The average fiber diameter was determined from the for shorter times( Fig. 5(d)). a glass or reaction layer could not average of fifty fibers(Table I). The cross-sectional area was be discerned between the fiber and matrix layer. However, the then determined predominant feature was fibers in contact with the matrix. For The data were plotted as stress applied to the fibers because longer rupture times, the planar fracture surface was charac he minicomposite area varies along the length, and the con- terized by most fibers contacting the matrix(Fig. 5(e)).How- sistent attribute of the minicomposites was the number of fiber ever, fracture of these fibers usually did not occur at the point per sample. Also, because the minicomposites were pre- of fiber/matrix contact(Figs. 5(d)and(e)). For all he C-Nic fracture surfaces, the interphase area that was originally carbon as vacant The volume fraction of fibers was determined from or C-Nic and pB minicomposites. The other minicomposites were not ana- cal fracture surfaces for HN-PBN ruptured ne 6 shows typi- the same for the other two bl 1000r15 I< Temperature 4500 E00 Load 3500 G00 2.3to14 09 failed in 2500 40 4 5 AE Energy per event1000 0100020003000400050006000700080009000 Time Fig. 3. Typical tensile data for a room-temperature precrack experiment and a high-temperature stress-rupture experiment, including load, temperature, and AE activity versus tim
The data for all the stress-rupture results are shown in graphical form on log scales in Figs. 4(a) and (b). The minicomposites always failed in the hottest region of the furnace. The data is plotted as the stress on the fibers, assuming that the fibers carried the full load. The number of fibers were counted for each minicomposite (Table I) from several (at least five) polished cross sections. The number of fibers for a given minicomposite type was found to differ from sample to sample by less than ten fibers. The highest number of fibers that were counted from the observed cross sections was used as the fiber tow count. The average fiber diameter was determined from the average of fifty fibers (Table I). The cross-sectional area was then determined. The data were plotted as stress applied to the fibers because the minicomposite area varies along the length,¶ and the consistent attribute of the minicomposites was the number of fibers per sample. Also, because the minicomposites were precracked, the fibers were carrying the load at the matrix crack and minicomposite failure always occurred at a matrix crack. For the Nicalon minicomposites (Fig. 4(a)), a definite difference in stress-rupture behavior exists between the carbon and BN interphase minicomposites. The C-Nic minicomposite life was far more sensitive to stress than that of 3MBN-Nic minicomposites, where life was only mildly dependent on stress. For the Hi-Nicalon minicomposites (Fig. 4(b)), no real difference was apparent between the two BN-coated systems. Both minicomposite types showed mild stress dependence for stress rupture at 750° and 950°C, with a slightly more significant stress dependence at 1200°C. A few samples that did not fail were tested at room temperature to determine the retained strength after stressed oxidation. The 3MBN-Nic microcomposites were very difficult to handle after the experiment and could not be tested. The BN– HN composites could be handled and tested. The retained strength at room temperature of two minicomposites that were originally tested at 1200°C for 882 h (328 MPa) and 965 h (385 MPa) was 525 and 792 MPa, respectively (∼25% and 38%, respectively, of the ultimate strength at room temperature). (2) Microstructural Observations (A) C-Nic Minicomposites: The fracture surfaces of CNic minicomposites that have been tested at 700°C are shown in Fig. 5. Figure 5(a) shows a typical fracture surface for short time (<1 h) failure at 700°C. There is moderate fiber pullout, and the fracture surface appears to be essentially the same as room-temperature fast-fracture surfaces. For longer rupture times (2–10 h), the fracture surfaces are characterized by long fiber pullout lengths (Fig. 5(b)). The pullout length is almost 2 mm for this broomlike fracture surface. The fracture surfaces for the longest rupture times (106 h) are planar (Fig. 5(c)). Some fibers appeared to be in contact with the matrix; this was true even of samples that have been tested at higher loads for shorter times (Fig. 5(d)). A glass or reaction layer could not be discerned between the fiber and matrix layer. However, the predominant feature was fibers in contact with the matrix. For longer rupture times, the planar fracture surface was characterized by most fibers contacting the matrix (Fig. 5(e)). However, fracture of these fibers usually did not occur at the point of fiber/matrix contact (Figs. 5(d) and (e)). For all the C-Nic fracture surfaces, the interphase area that was originally carbon was vacant. (B) BN Interphase Minicomposites: Figure 6 shows typical fracture surfaces for HN-PBN ruptured minicomposites; however, the observations are the same for the other two BN- ¶ The volume fraction of fibers was determined from several digitized images of minicomposite polished cross sections and found to vary over the range of 0.2–0.25 for C-Nic and PBN-HN minicomposites. The other minicomposites were not analyzed. Fig. 3. Typical tensile data for a room-temperature precrack experiment and a high-temperature stress-rupture experiment, including load, temperature, and AE activity versus time. Fig. 2. Optical micrograph of a polished longitudinal section of a PBN-HN minicomposite showing a relatively uniform crack distribution after the minicomposite was cycled 50 times to a load of 55 kg at room temperature. 2032 Journal of the American Ceramic Society—Morscher Vol. 80, No. 8
August 1997 Tensile Stress Rupture of Sic/siCm Minicomposites with C and BN Interphases at Elevated Temperatures in Air 2033 L1000 5v Time at 700oC, hr. ◇3 MBN Hi-Nic ○PBNH|Nc 700c 1200c 0.1 Time at 700oC, hr. Fig. 4. Minicomposite stress-rupture data for(a) Nicalon-fiber minicomposites and(b) Hi-Nicalon-fiber minicomposites. temperature conditions(700oC), the fracture surfaces appear to for an outside fiber of the PBn minicomposite, where the BN be the same as room-temperature fracture surfaces, with some thickness could be 3 um. The thinner BN interphase regions pullout and a jagged matrix contour(Fig. 6(a)). For more- also were depleted of BN. For most of the lower-temperature severe conditions(950oC after >20 h), the fracture surface is (700 and 950oC for times of dition(1200oC), a planar fracture surface was again evident; 98 h), glass is clearly evident between the fiber and matrix and however, the fibers had failed at slightly above and slightl spreads out from the fiber/matrix interphase region into the below the matrix fracture surface(Fig. 6(c) matrix crack(Fig. 7(a)). EDS was performed on this glass, and The most surprising observation is shown in Fig. 7(b). Frac- only silicon and oxygen could be detected--no boron. There is ture had occurred in the hot zone after 98 h at 950C ( Fig. 7(a)) little glass formation on the SiC at this temperature, as is evi- The sample was subsequently broken at a distance of-20 mm denced by the texture of the SiC matrix from the fracture surface while handling the minicomposite For the most-severe test conditions(1200 C), glass is evi- after it was removed from the test rig. The temperature that ent everywhere on the matrix surface and between the fiber orresponded to this region of the furnace(as determined from and the matrix( Fig. 9(a). Again, EDS analysis indicates only the furnace le), where this handling fracture occurred silicon and oxygen in the glass. Fiber fracture occurs very near an 500C. The interphase area that was previously the matrix crack plane; however, there is the appearance of
interphase minicomposite types. Again, for the mildest time/ temperature conditions (700°C), the fracture surfaces appear to be the same as room-temperature fracture surfaces, with some pullout and a jagged matrix contour (Fig. 6(a)). For moresevere conditions (950°C after >20 h), the fracture surface is planar (Fig. 6(b)), with significant oxide formation between the fiber and matrix (Fig. 7(a)), where most of the fibers appear to be fused to the matrix. For this sample, there are at least 15 fibers that are pulled out and appear to fracture independent of the fiber-fused-to-matrix region. For the most severe test condition (1200°C), a planar fracture surface was again evident; however, the fibers had failed at slightly above and slightly below the matrix fracture surface (Fig. 6(c)). The most surprising observation is shown in Fig. 7(b). Fracture had occurred in the hot zone after 98 h at 950°C (Fig. 7(a)). The sample was subsequently broken at a distance of ∼20 mm from the fracture surface while handling the minicomposite after it was removed from the test rig. The temperature that corresponded to this region of the furnace (as determined from the furnace profile), where this handling fracture occurred, was no more than 500°C. The interphase area that was previously filled by BN was vacant. The fiber that is shown in Fig. 7(b) is for an outside fiber of the PBN minicomposite, where the BN thickness could be 3 mm. The thinner BN interphase regions also were depleted of BN. For most of the lower-temperature (700° and 950°C for times of <20 h) minicomposites that were tested, the interphase region was vacant or had thin silica (SiO2) layers on the fiber and/or matrix that were only 0.1–0.2 mm thick (Fig. 8). BN was present between the fiber and the matrix of one PBN-HN sample (700°C for 50 h). For slightly more-severe test conditions (950°C for time t $ 98 h), glass is clearly evident between the fiber and matrix and spreads out from the fiber/matrix interphase region into the matrix crack (Fig. 7(a)). EDS was performed on this glass, and only silicon and oxygen could be detected—no boron. There is little glass formation on the SiC at this temperature, as is evidenced by the texture of the SiC matrix. For the most-severe test conditions (1200°C), glass is evident everywhere on the matrix surface and between the fiber and the matrix (Fig. 9(a)). Again, EDS analysis indicates only silicon and oxygen in the glass. Fiber fracture occurs very near the matrix crack plane; however, there is the appearance of Fig. 4. Minicomposite stress-rupture data for (a) Nicalon-fiber minicomposites and (b) Hi-Nicalon-fiber minicomposites. August 1997 Tensile Stress Rupture of SiCf/SiCm Minicomposites with C and BN Interphases at Elevated Temperatures in Air 2033
2034 Journal of the American Ceramic Sociery-Morscher Vol. 80. No 8 (b) (a) E88128KU 5818NmHD34 8169108 5.8kv ak 0 Fig. 5. Typical fracture surfaces of carbon- interphase minicomposites for(a) shorter-term rupture time and higher-rupture-load conditions(0.8 h, 55 kg), (b) intermediate rupture-time and rupture-load conditions(2.3 h, 33 kg), and (c) longer-term rupture time and lower-rupture-load conditions (115 h, 22 kg at 700C. Higher-magnification micrographs of Figs. 5(a) and(c)are shown in Figs. 5(d)and(e), respectivel minor pullout. Figure 9(b) shows a fiber sticking above the at 1200.C. The hot-zone region is marked on the figure, as well matrix crack plane with adherent matrix material. The textured as the colder region, which resulted in a multicolored appea region closest to the fiber is SiC (as indicated by EDS)and the ance, because of differing glass thickness, as a function smoother region surrounding the SiC is Sio2. Failure occurred temperature along the furnace profile. The cracks are clearly visible in the hot zone and are more difficult to see(even wrenched the matrix and glass from the opposite matrix sur- though they are present) as the temperature of exposure de- face. i.e. in a brittle manner creased. An example of an k is shown in Fig. 10(b) An example of the degree of crack opening that has occurred Crack openings were on the order of 5-15 um. For the Hi- at temperature is shown in Fig. 10. The optical micrograph of Nicalon minicomposites, crack openings either were not ob- the longitudinal section is from a sample that failed after 39 h served or were barely visible for lower-temperature rupture
minor pullout. Figure 9(b) shows a fiber sticking above the matrix crack plane with adherent matrix material. The textured region closest to the fiber is SiC (as indicated by EDS) and the smoother region surrounding the SiC is SiO2. Failure occurred at or between the two matrix crack surfaces and, in some cases, wrenched the matrix and glass from the opposite matrix surface, i.e., in a brittle manner. An example of the degree of crack opening that has occurred at temperature is shown in Fig. 10. The optical micrograph of the longitudinal section is from a sample that failed after 39 h at 1200°C. The hot-zone region is marked on the figure, as well as the colder region, which resulted in a multicolored appearance, because of differing glass thickness, as a function of temperature along the furnace profile. The cracks are clearly visible in the hot zone and are more difficult to see (even though they are present) as the temperature of exposure decreased. An example of an open crack is shown in Fig. 10(b). Crack openings were on the order of 5–15 mm. For the HiNicalon minicomposites, crack openings either were not observed or were barely visible for lower-temperature rupture Fig. 5. Typical fracture surfaces of carbon-interphase minicomposites for (a) shorter-term rupture time and higher-rupture-load conditions (0.8 h, 55 kg), (b) intermediate rupture-time and rupture-load conditions (2.3 h, 33 kg), and (c) longer-term rupture time and lower-rupture-load conditions (115 h, 22 kg at 700°C). Higher-magnification micrographs of Figs. 5(a) and (c) are shown in Figs. 5(d) and (e), respectively. 2034 Journal of the American Ceramic Society—Morscher Vol. 80, No. 8
August 1997 Tensile Stress Rupture of sic/siCm Minicomposites with C and BN Interphases at Elevated Temperatures in Air 2035 RBNd2 s e kv xe.88k"is (b) 5199PWD14 Fig. 7. Higher-magnification FESEM micrographs of the PBN-HN (c) of the hot-zone fracture surface and (b) a portion of a fracture surface caused intentionally -22 mm from the hot-zone fracture surface after the experiment). Furnace temperature at the location in Fig. 7(b)was 500°C The Weibull plots are shown for C-Nic and PBN-HN mini- composites in Figs. 11(a)and(b), respectively. The fiber strength, or, was determined from the relationship 0=-12 (1) where A is the fracture mirror constant and rm is the fracture MPar-min in the literature for Nicalon fibers. %/ ange of2.0-3.5 mirror radius. The value of a varies over An a value of 2.5 MPa/2 was for Nicalon and Hi-Nicalon in Fig. 11.tt However. the absolute values of the fiber strengths are not 8159=109mM012 considered to be important for this study. Instead, Fig. 12 shows the rupture strength, which has been normalized by room-temperature strength for the C-Nic and PBN-HN mini- Fig. 6. Typical fracture surfaces of PBN-HN minicomposites for composites, so that any inaccuracy in A was avoided stress ruptures at(a)700C, 50 h,(b)950C, 98 h, and(c) 1200oC, The rupture strengths, as measured by the loads that are normalized stresses of the average strengths that are dete mined from the fiber fracture mirrors. Fracture mirrors that conditions, even for experiments that lasted almost 1000 h at 7) Fiber fracture Mirror Analysis The radii of a number of individual fiber fracture mirrors PBN-HN minicomposites. The smallest ounted in the populations for C-Nie and (e.g, Figs. 5(c)and(d)were measured for several samples orresponds to a fracture mirror radius of 9.5 um(approaching the diamet
conditions, even for experiments that lasted almost 1000 h at 950°C. (3) Fiber Fracture Mirror Analysis The radii of a number of individual fiber fracture mirrors (e.g., Figs. 5(c) and (d)) were measured for several samples. The Weibull plots are shown for C-Nic and PBN-HN minicomposites in Figs. 11(a) and (b), respectively. The fiber strength, sf , was determined from the relationship sf 4 Arm −1/2 (1) where A is the fracture mirror constant and rm is the fracture mirror radius. The value of A varies over the range of 2.0–3.5 MPazm1/2 in the literature for Nicalon fibers.9,14–16 An A value of 2.5 MPazm1/2 was for Nicalon and Hi-Nicalon in Fig. 11.†† However, the absolute values of the fiber strengths are not considered to be important for this study. Instead, Fig. 12 shows the rupture strength, which has been normalized by room-temperature strength for the C-Nic and PBN-HN minicomposites, so that any inaccuracy in A was avoided. The rupture strengths, as measured by the loads that are applied to the minicomposites, are in good agreement with the normalized stresses of the average strengths that are determined from the fiber fracture mirrors. Fracture mirrors that ††Smooth fiber fracture surfaces were counted in the populations for C-Nic and PBN-HN minicomposites. The smallest value of ln strength 4 −0.2 in Fig. 11 corresponds to a fracture mirror radius of 9.5 mm (approaching the diameter of the fiber). Fig. 6. Typical fracture surfaces of PBN-HN minicomposites for stress ruptures at (a) 700°C, 50 h, (b) 950°C, 98 h, and (c) 1200°C, 36 h. Fig. 7. Higher-magnification FESEM micrographs of the PBN-HN minicomposite that failed at 950°C after 98 h (Fig. 5(b)) ((a) a portion of the hot-zone fracture surface and (b) a portion of a fracture surface caused intentionally ∼22 mm from the hot-zone fracture surface after the experiment). Furnace temperature at the location in Fig. 7(b) was ∼500°C. August 1997 Tensile Stress Rupture of SiCf/SiCm Minicomposites with C and BN Interphases at Elevated Temperatures in Air 2035
Journal of the American Ceramic Sociery-Morscher Vol. 80. No 8 CVI SiC Nicalon Fiber Sio2 5.0 kV: 38. 0k1. 00pm eek::e CVI Sic Sio2 Hi-Nicalon Fiber 4 5.0kV X30. 0K 1a8pm mediate temperatures for BN-interphase minicomposites(a) usually on the fiber or(b) sometimes on the matrix surrounding the fiber. Figure 8(a)is a 3MBN-Nic minicomposite that ruptured after 42 h at 700C, Fig. 8(b)is a 3MBN-HN minicomposite that ruptured after 12.5 h at 700C. correspond to individual fiber failure were easily observed for evident that Hi-Nicalon surface flaws grow and dominate under all the C-Nic samples(room temperature and 700C tested), these stressed-oxidation conditions vith the exception of the sample that was tested at 700C for For the PBN-HN sample that ruptured after 98 h at 950C 2.3 h(Fig. 5(b)). Only 16 fiber fracture surfaces could be individual fiber failure did not occur, because of the strong obtained for this sample, because the long pulled-out fibers bonding that occurred from the glass formation between the were moving in the SEM microscope and many of the fibers fiber and matrix. Therefore, the fiber strengths were not deter during the fracture event. There were a few internal flaws. which caused fiber failure for room-temperature fracture(Fig 11(a); however, surface flaws usually caused fiber failure at IV. Discussion room temperature and always caused fiber failure at 700C Fracture mirrors that corresponded to individual fiber failure fr 1) Comparisons to Known Fiber and Composite posites that were tested at room temperature lower temperature Stress-Rupture Data (700C), and higher temperature(1200oC). Internal flaws were Yun and DiCarlo& have shown that the fast-fracture strength detected 70% of the time for individual fiber tensile tests on and stress-rupture data of Nicalon 9,20 and Hi-Nicalon%, 20 fi- as-produced Hi-Nicalon fibers. This is similar to what was bers can be related using a Larson-Miller21 2 approach. The found in this study for room-temperature fracture of PBN-HN Larson-Miller approach is empirical; yet, it has been used suc- minicomposites(50%, see Fig. 11(b). However, there were cessfully to quantify stress-rupture properties of metals and fewer internal fiber flaws for the minicomposite that was tested ceramics. The Larson-Miller parameter, q, relates the time- at 700C for 50 h(22%)and no internal flaws for the mini- temperature condition, where time-dependent rupture occurs composite that was tested at 1200@C for 3 h(Fig. 11(b)). It is for a given stress
correspond to individual fiber failure were easily observed for all the C-Nic samples (room temperature and 700°C tested), with the exception of the sample that was tested at 700°C for 2.3 h (Fig. 5(b)). Only 16 fiber fracture surfaces could be obtained for this sample, because the long pulled-out fibers were moving in the SEM microscope and many of the fibers had fracture surfaces with a ‘‘lip;’’ i.e., they failed in bending during the fracture event. There were a few internal flaws, which caused fiber failure for room-temperature fracture (Fig. 11(a)); however, surface flaws usually caused fiber failure at room temperature and always caused fiber failure at 700°C. Fracture mirrors that corresponded to individual fiber failure from PBN-HN minicomposites were observed for minicomposites that were tested at room temperature, lower temperature (700°C), and higher temperature (1200°C). Internal flaws were detected 70% of the time for individual fiber tensile tests on as-produced Hi-Nicalon fibers.17 This is similar to what was found in this study for room-temperature fracture of PBN-HN minicomposites (50%, see Fig. 11(b)). However, there were fewer internal fiber flaws for the minicomposite that was tested at 700°C for 50 h (22%) and no internal flaws for the minicomposite that was tested at 1200°C for 3 h (Fig. 11(b)). It is evident that Hi-Nicalon surface flaws grow and dominate under these stressed-oxidation conditions. For the PBN-HN sample that ruptured after 98 h at 950°C, individual fiber failure did not occur, because of the strong bonding that occurred from the glass formation between the fiber and matrix. Therefore, the fiber strengths were not determined for this sample. IV. Discussion (1) Comparisons to Known Fiber and Composite Stress-Rupture Data Yun and DiCarlo18 have shown that the fast-fracture strength and stress-rupture data of Nicalon19,20 and Hi-Nicalon19,20 fibers can be related using a Larson–Miller21,22 approach. The Larson–Miller approach is empirical; yet, it has been used successfully to quantify stress-rupture properties of metals and ceramics. The Larson–Miller parameter, q, relates the time– temperature condition, where time-dependent rupture occurs for a given stress: Fig. 8. Typical examples of a glass layer formed at intermediate temperatures for BN-interphase minicomposites (a) usually on the fiber or (b) sometimes on the matrix surrounding the fiber. Figure 8(a) is a 3MBN-Nic minicomposite that ruptured after 42 h at 700°C; Fig. 8(b) is a 3MBN-HN minicomposite that ruptured after 12.5 h at 700°C. 2036 Journal of the American Ceramic Society—Morscher Vol. 80, No. 8
August 1997 Tensile Stress Rupture of sic/siCm Minicomposites with C and B Interphases at Elevated Temperatures in Air 2037 with carbon interphases. The minicomposites seem to have better rupture properties than the 0/90 Nic-SiC composites The average fiber volume fraction was determined to be% in the loading direction for these composites. 4 If the volume fraction of fibers was less at the fracture surface. the data for the 0/90 Nic-SiC would improve in Fig. 13(a). Also, because these composites were 0/90, other factors, such as the mor phology of the porosity and crossply cracks, could lead to greater embrittlement compared to the"simple" minicompos- ites of this stud The BN-interphase minicomposite data mimics the fiber rupture data, although the BN-minie is lower at room temperature. To compare the rupture behavior of Hi-Nicalon fibers to that of the Hn minicomposites, the fiber data was normalized to the room-temperature strength of the bn minicomposites (Fig. 13(b)). Two regions represente the different mechanisms that were considered to cause fiber PBN7 2 5.0 kv x1 0a Weiderhorn et al.25). The lower-temperature re gion(mild dependence on q) is due to slow crack growth, and the higher-temperature region(strong dependence on g) is due to creep rupture(Fig t lower temperatures(≤950°C b data), the minicomposite rupture data follows the fiber data (the same slope), indicating that the rupture strength decreases because of a slow-crack-growth mechanism(in air) that is in- (1200C), the minicomposite rupture strength was within the scatter of the single-fiber rupture data and appeared to have the same dependence on q. These two regions are designated"Fi- ber Slow-Crack Growth Controlled Degradation"andFiber Creep-Rupture Controlled Degradation, respectively, in Fig The BN-HN minicomposite rupture strengths between he low-and high-temperature regions fall below the fiber rupture strength. Thi region is designated""Compos Embrittlement due to Stressed-Oxidation'in Fig. 13(b)and must be caused by the interaction of the precracked mini composite with the environment. The 3MBN-Nic mini- PBN8 x4.00k7. 5eu however,more tests would need to be performed&. 13(a): composites seem to have the same three regions(Fi this observation. The significant features of these obse Fig 9. Higher. cation micrographs of PBN-HN mi ite that failed at for carbon- and BN-interphase minicomposites are of the bright fibers in the middle of are oxidized; i.e, the fibers had failed prior to minicomposit (2) C-Nic Mimicomposite Time-Dependent Failure The observation that the carbon interphase is lost via oxida T(log tR+C) ion and that the fibers eventually contact the matrix was as xpected5-7 Because the interphase was vacated and SiO, for mation was negligible at 700%C. embrittlement that is caused C the Larson-Miller constant, which was 22 for both Nicalon bers is considered unlikely. The fact that fiber fracture the fi where T is temperature(in K), Ig the time to rupture (in h), and by strong bonding and severe stress concentrations on and Hi-Nicalon fibers. 8 The minicomposite stress-rupture data do not occur most often at the point of fiber-matrix can then be related to the fiber stress-rupture data via the Lar- indicates that the fibers either are not bonded or son-Miller relationship, using the same C value for the mini- strongly bonded to the matrix. In addition, the fiber surface mirror regions increase(the fiber strength decreases) F13)0)dp如的 ther flaws are growing or new flaws are created od q. The individual fiber data from the literature are from stress- fiber sur rupture experiments(1180%-1400C). 9,20 The scatter in the rupture must be due to fiber degradation. data is large for the stress-rupture data, as represented in Fig Also shown in Fig. 13(a)are composite stress-rupture data Time-Dependent Failure composite 13(b) 3) BN-Interphase Min for Nicalon/C/CVI-SiC composites, 23 and Nicalon/BN/BMAS The BN-interphase minicomposites showed significantly glass-ceramic composites. Both of the composites were pre- better time-dependent failure properties, in comparison to the cracked; however, the exterior skin of the BMAS composite carbon-interphase minicomposites. This was observed sealed"the interior of the composite from the environment, though significant microstructural changes had occurred as described earlier, enabling this composite to survive for (A) Effect of Humidity on BN: The BN volatilization was more than 1.5 years surprising, especially at such low temperatures($500C). It is The rupture-strength decrease of C-interphas I known that BN may react with water vapor, even at room es(ig. 13(a))with q is significantly greater than that of mperature, depending on the purity and crystallinity of the BN-interphase minicomposites(Figs. 13(a)and(b). The rup- BN 26,27 However. there is recent evidence that bN can be ture strengths of the carbon-interphase minicomposites are very consumed at intermediate temperatures in humid environ- similar to the data in the literature for Nic SiCm composites ments, depending on the crystallinity
q 4 T(log tR + C) (2) where T is temperature (in K), tR the time to rupture (in h), and C the Larson–Miller constant, which was 22 for both Nicalon and Hi-Nicalon fibers.18 The minicomposite stress-rupture data can then be related to the fiber stress-rupture data via the Larson–Miller relationship, using the same C value for the minicomposites. Figures 13(a) and (b) show plots of the Nicalon and HiNicalon minicomposite and fiber data on applied stress versus q. The individual fiber data from the literature are from stressrupture experiments (1180°–1400°C).19,20 The scatter in the data is large for the stress-rupture data, as represented in Fig. 13(b). Also shown in Fig. 13(a) are composite stress-rupture data for Nicalon/C/CVI-SiC composites8,23 and Nicalon/BN/BMAS glass-ceramic composites.2 Both of the composites were precracked; however, the exterior skin of the BMAS composite ‘‘sealed’’ the interior of the composite from the environment, as described earlier, enabling this composite to survive for more than 1.5 years. The rupture-strength decrease of C-interphase minicomposites (Fig. 13(a)) with q is significantly greater than that of BN-interphase minicomposites (Figs. 13(a) and (b)). The rupture strengths of the carbon-interphase minicomposites are very similar to the data in the literature for Nicf /SiCm composites with carbon interphases. The minicomposites seem to have better rupture properties than the 0/90 Nic–SiC composites. The average fiber volume fraction was determined to be ∼22% in the loading direction for these composites.24 If the volume fraction of fibers was less at the fracture surface, the data for the 0/90 Nic–SiC would improve in Fig. 13(a). Also, because these composites were 0/90, other factors, such as the morphology of the porosity and crossply cracks, could lead to greater embrittlement compared to the ‘‘simple’’ minicomposites of this study. The BN-interphase minicomposite data mimics the fiberrupture data, although the BN-minicomposite ultimate strength is lower at room temperature. To compare the rupture behavior of Hi-Nicalon fibers to that of the HN minicomposites, the fiber data was normalized to the room-temperature strength of the BN minicomposites (Fig. 13(b)). Two regions represented the different mechanisms that were considered to cause fiber rupture (after Weiderhorn et al.25). The lower-temperature region (mild dependence on q) is due to slow crack growth, and the higher-temperature region (strong dependence on q) is due to creep rupture (Fig. 13). At lower temperatures (#950°C data), the minicomposite rupture data follows the fiber data (the same slope), indicating that the rupture strength decreases because of a slow-crack-growth mechanism (in air) that is inherent to the fibers themselves. At the higher temperatures (1200°C), the minicomposite rupture strength was within the scatter of the single-fiber rupture data and appeared to have the same dependence on q. These two regions are designated ‘‘Fiber Slow-Crack Growth Controlled Degradation’’ and ‘‘Fiber Creep-Rupture Controlled Degradation,’’ respectively, in Fig. 13(b). The BN-HN minicomposite rupture strengths between the low- and high-temperature regions fall below the fiber rupture strength. This region is designated ‘‘Composite Embrittlement due to Stressed-Oxidation’’ in Fig. 13(b) and must be caused by the interaction of the precracked minicomposite with the environment. The 3MBN-Nic minicomposites seem to have the same three regions (Fig. 13(a)); however, more tests would need to be performed to confirm this observation. The significant features of these observations for carbon- and BN-interphase minicomposites are discussed below. (2) C-Nic Minicomposite Time-Dependent Failure The observation that the carbon interphase is lost via oxidation and that the fibers eventually contact the matrix was as expected.5–7 Because the interphase was vacated and SiO2 formation was negligible at 700°C, embrittlement that is caused by strong bonding and severe stress concentrations on the fibers is considered unlikely. The fact that fiber fracture origins do not occur most often at the point of fiber–matrix contact indicates that the fibers either are not bonded or are not strongly bonded to the matrix. In addition, the fiber fracture surface mirror regions increase (the fiber strength decreases) with longer rupture times (Fig. 12), from which one can infer that either flaws are growing or new flaws are created on the fiber surfaces. Therefore, the cause of C-Nic minicomposite rupture must be due to fiber degradation. (3) BN-Interphase Minicomposite Time-Dependent Failure The BN-interphase minicomposites showed significantly better time-dependent failure properties, in comparison to the carbon-interphase minicomposites. This was observed even though significant microstructural changes had occurred. (A) Effect of Humidity on BN: The BN volatilization was surprising, especially at such low temperatures (#500°C). It is well known that BN may react with water vapor, even at room temperature, depending on the purity and crystallinity of the BN.26,27 However, there is recent evidence that BN can be consumed at intermediate temperatures in humid environments, depending on the crystallinity.28 Fig. 9. Higher-magnification micrographs of PBN-HN minicomposite that failed at 1200°C after 36 h (Fig. 5(c)). EDS analysis indicates that the fracture surface of the bright fibers in the middle of Fig. 9(a) are oxidized; i.e., the fibers had failed prior to minicomposite failure. August 1997 Tensile Stress Rupture of SiCf/SiCm Minicomposites with C and BN Interphases at Elevated Temperatures in Air 2037
Journal of the American Ceramic Sociery-Morscher Vol. 80. No 8 1200c Fracture Surface PBNS 5.8kV×1.egk3e 700c 1 mm Fig. 10. (a) Longitudinal length of minicomposite that ruptured after 36 h at 1200oC, where the hot-zone section of the minicomposite is indicated by the 1200C" marking,(b)a high ication FESEM micrograph of an opened matrix crack. The experiments on Hi-Nicalon minicomposites were per- formed in the summer months(June-September), which meant .The National Weather Service in Cleveland, OH is at Hopkins International that the atmosphere was very hu The experiments that were performed on the Nic-BN minicomposites were in No- outside air and is often at negative pressure. For the air-conditioning system that wa vember and December, i.e with lower ambient humidity. Dat lat outsId that was obtained from the National weather Service2 enabled an approximation of the water content in the air. The water estimates of hur that were used to determine the water pressure of the atmo- sphere in the laboratory; they are considered to be conservative
The experiments on Hi-Nicalon minicomposites were performed in the summer months (June–September), which meant that the atmosphere was very humid. The experiments that were performed on the Nic-BN minicomposites were in November and December, i.e., with lower ambient humidity. Data that was obtained from the National Weather Service29 enabled an approximation of the water content in the air.‡‡ The water ‡‡The National Weather Service in Cleveland, OH is at Hopkins International Airport, which is <0.5 mi. from where the experiments were performed. The airhandling system in the building where the experiments were performed takes in all outside air and is often at negative pressure. For the air-conditioning system that was in place at the time of the experiments, the humidity in the building was estimated to be the same as that outside for a relative humidity of #60%. For relative outside humidities of 60%–100%, the inside humidity would be 60%–80%. These are the estimates of humidity that were used to determine the water pressure of the atmosphere in the laboratory; they are considered to be conservative. Fig. 10. (a) Longitudinal length of minicomposite that ruptured after 36 h at 1200°C, where the hot-zone section of the minicomposite is indicated by the ‘‘1200C’’ marking; (b) a higher-magnification FESEM micrograph of an opened matrix crack. 2038 Journal of the American Ceramic Society—Morscher Vol. 80, No. 8