ournal Am.ceam.So,717-200198 Multilayered Oxide Interphase Concept for Ceramic-Matrix Composites Woo Y Lee, .t Edgar Lara-curzio, and Karren LMore Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 Hi-Nicalon/SiC minicomposite specimens containing three materials at high temperatures. It also seems that one of the oxide interphase layers(amorphous SiO2, monoclinic ZrO2, major complications associated with studying new interface and amorphous SiO,)were prepared by chemical vap concepts is the preparation of sufficiently reproducible and deposition. The minicomposites exhibited graceful comp well-characterized composite specimens for proper mechanical ite failure behavior with reasonable load-carrying capabil d interfacial evaluation ity in room-temperature tensile tests. Much of the compos- The present study explores the possibility of designing a ite behavior and load-carrying capability was retained eve multilayered oxide interphase as a new avenue for accommo- after matrix precracking and subsequent oxidation in air at dating the complex interface ma 60C for 10 h In both the a not be easily satisfied by a single oxide material. In order to mens,crack deflection and fiber pull-out occurred prefer experimentally test this concept, SiC/SiC minicomposite speci- entially within the multilayered interphase region. The po mens containing three oxide interface layers(amorphous SiO2 tential merits and uncertainties associated with this monoclinic ZrO2, and amorphous Sio2) were prepared by multilayered oxide interphase approach were discussed in CVD. One of the key design aspects for this model system the context of designing environmentally durable interfaces was to utilize the significant mismatch between SiO2 and trix ZrO2 in thermoelastic properties to cause preferential crack deflection and debonding at the multilayer interfaces. Another . Introduction important design consideration was that the CVD SiO, layer might protect the Hi-Nicalon fiber surface from long-term composites in high-temperature structural applications may be strength. From a processing point of view, CVD was used limited by the environmental instability exhibited by conven- because SiO2 and ZrO2 could be easily prepared and infiltrated tional interphase materials, namely C and Bn fiber coatings into a fiber tow for preparing relatively uniform minicomposite produced by chemical vapor deposition(CVD). Therefo there has been a strong interest to explore the feasibility of utilizing oxide interphase materials that are chemically stable II. Experimental Procedure s well as mechanically compatible with state-of-the-art com- posite systems. 1 2 It is well-recognized that, before such ar The SiO,/ZrO,/SiO, multilayer interphase was infiltrated oxidation-resistant interphase material can be demonstrated, challenging set of interface materials criteria must be met. The using a small hot-wall CVD reactor Nippon Carbon, Japan) temperatures for a reasonable period of time i phacs tial must SiCla to ZrCl, to SiCl4 with CO 2 and H2 as a source of oxygen first key criterion is obviously that the interphase material mus be stable with respect to the fiber and matri The reactor was operated at a temperature of 1050@C and a The second cri pressure of 10 kPa. The main features of the CVD reactor are terion is that the interphase material must provide a mechanism described elsewhere except for several important modifica tions. An Al,O, tube (5.0 cm inside diameter and 61 cm in terface region. In case of non-oxide fibers, another important length, Vesuvius McDanel, Beaver Falls, PA)was used as the issue to consider is the possibility of protecting the fiber sur- face from oxidation feed SiCl4 through the outer path while the inner path was Kerans' recently reviewed some of the oxide interface con- packed with small Zr wire pieces for chlorination with HCI cepts and interphase materials which have been considered for The chlorination region of the reactor was heated to 600C oxide composite systems: porous oxides, damaged oxides, using a resistance heater. The ZrCl, flow rate was estimated by assuming complete reaction of Zr with the HCl flow through ened interfaces. In general, experimental results are relatively the chlorinator. The flow rate of SiCl,(99.999%,Aldrich,Mil- scarce in terms of validating the usefulness of the above inter- waukee, WI), which is a liquid at room temperature, was con- face concepts For non-oxide composite systems such as Sic/ trolled using a vapor source controller ( Source V, Tylan Gen SiC composites, a single oxide interphase material has not been demonstrated, to date, to exhibit the weak interface behavior was found to be an important processing parameter for con while being sufficiently compatible with the fiber and matrix trolling the microstructural quality of oxide CVD coatings, was typically measured to be on the order of -0 2 Pa/min at a reactor pressure of I Pa Four Hi-Nicalon fiber tows (7.5 cm long) were placed R. J. Kerans--contributing editor in the reactor using an alumina holder in the direction parallel to the gas flow. After the deposition of the multilayered oxide coating, the alumina holder was transferred into another CVD reactor for SiC matrix infiltration withor the fiber tow from the holder. the sic matrix was infiltrated Lockheed at 900-950C for 8 h using SiCl3 CH, and H2 precursors at a Martin Energy Research Corp under Contract No DE-AC05-96OR22464. pressure of 0.7 kPa. The experimental configuration and ials Science and Engineering. Stevens In- rocedures used for the sic infiltration are described else- stitute of Technology, Hoboken, New Jersey 07030. 717
Multilayered Oxide Interphase Concept for Ceramic-Matrix Composites Woo Y. Lee,*,† Edgar Lara-Curzio,* and Karren L. More* Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 Hi-Nicalon/SiC minicomposite specimens containing three oxide interphase layers (amorphous SiO2, monoclinic ZrO2, and amorphous SiO2) were prepared by chemical vapor deposition. The minicomposites exhibited graceful composite failure behavior with reasonable load-carrying capability in room-temperature tensile tests. Much of the composite behavior and load-carrying capability was retained even after matrix precracking and subsequent oxidation in air at 960°C for 10 h. In both the as-prepared and oxidized specimens, crack deflection and fiber pull-out occurred preferentially within the multilayered interphase region. The potential merits and uncertainties associated with this multilayered oxide interphase approach were discussed in the context of designing environmentally durable interfaces for ceramic-matrix composites. I. Introduction THE major motivation for the present study is that the use of toughened and therefore damage-tolerant ceramic-matrix composites in high-temperature structural applications may be limited by the environmental instability exhibited by conventional interphase materials, namely C and BN fiber coatings produced by chemical vapor deposition (CVD). Therefore, there has been a strong interest to explore the feasibility of utilizing oxide interphase materials that are chemically stable as well as mechanically compatible with state-of-the-art composite systems.1,2 It is well-recognized that, before such an oxidation-resistant interphase material can be demonstrated, a challenging set of interface materials criteria must be met. The first key criterion is obviously that the interphase material must be stable with respect to the fiber and matrix phases at high temperatures for a reasonable period of time. The second criterion is that the interphase material must provide a mechanism for crack deflection and fiber pull-out at the fiber–matrix interface region. In case of non-oxide fibers, another important issue to consider is the possibility of protecting the fiber surface from oxidation. Kerans1 recently reviewed some of the oxide interface concepts and interphase materials which have been considered for oxide composite systems: porous oxides, damaged oxides, easy-cleaving oxides, ductile interphases, and segregant weakened interfaces. In general, experimental results are relatively scarce in terms of validating the usefulness of the above interface concepts. For non-oxide composite systems such as SiC/ SiC composites, a single oxide interphase material has not been demonstrated, to date, to exhibit the weak interface behavior while being sufficiently compatible with the fiber and matrix materials at high temperatures. It also seems that one of the major complications associated with studying new interface concepts is the preparation of sufficiently reproducible and well-characterized composite specimens for proper mechanical and interfacial evaluation. The present study explores the possibility of designing a multilayered oxide interphase as a new avenue for accommodating the complex interface materials criteria which may not be easily satisfied by a single oxide material. In order to experimentally test this concept, SiC/SiC minicomposite specimens containing three oxide interface layers (amorphous SiO2, monoclinic ZrO2, and amorphous SiO2) were prepared by CVD. One of the key design aspects for this model system was to utilize the significant mismatch between SiO2 and ZrO2 in thermoelastic properties to cause preferential crack deflection and debonding at the multilayer interfaces. Another important design consideration was that the CVD SiO2 layer might protect the Hi-Nicalon fiber surface from long-term oxidation, although it is realized that this type of protection might come at the expense of some initial reduction in fiber strength. From a processing point of view, CVD was used because SiO2 and ZrO2 could be easily prepared and infiltrated into a fiber tow for preparing relatively uniform minicomposite specimens. II. Experimental Procedure The SiO2/ZrO2/SiO2 multilayer interphase was infiltrated into a SiC fiber tow (Hi-Nicalon™, Nippon Carbon, Japan) using a small hot-wall CVD reactor by gas switching from SiCl4 to ZrCl4 to SiCl4 with CO2 and H2 as a source of oxygen. The reactor was operated at a temperature of 1050°C and a pressure of 10 kPa. The main features of the CVD reactor are described elsewhere3 except for several important modifications. An Al2O3 tube (5.0 cm inside diameter and 61 cm in length, Vesuvius McDanel, Beaver Falls, PA) was used as the reactor chamber. A coaxial, two-path gas injector was used to feed SiCl4 through the outer path while the inner path was packed with small Zr wire pieces for chlorination with HCl. The chlorination region of the reactor was heated to 600°C using a resistance heater. The ZrCl4 flow rate was estimated by assuming complete reaction of Zr with the HCl flow through the chlorinator. The flow rate of SiCl4 (99.999%, Aldrich, Milwaukee, WI), which is a liquid at room temperature, was controlled using a vapor source controller (Source V, Tylan General, Torrance, CA). The rate of air leaks into the reactor, which was found to be an important processing parameter for controlling the microstructural quality of oxide CVD coatings, was typically measured to be on the order of ∼0.2 Pa/min at a reactor pressure of 1 Pa. Four Hi-Nicalon fiber tows (∼7.5 cm long) were placed in the reactor using an alumina holder in the direction parallel to the gas flow. After the deposition of the multilayered oxide coating, the alumina holder was transferred into another CVD reactor for SiC matrix infiltration without removing the fiber tow from the holder. The SiC matrix was infiltrated at 900–950°C for 8 h using SiCl3CH3 and H2 precursors at a pressure of 0.7 kPa. The experimental configuration and procedures used for the SiC infiltration are described elsewhere.4 R. J. Kerans—contributing editor Manuscript No. 190817. Received July 28, 1997; approved December 19, 1997. Supported by the Laboratory-Directed Research Development Program of Oak Ridge National Laboratory, managed for the Department of Energy by Lockheed Martin Energy Research Corp. under Contract No. DE-AC05-96OR22464. *Member, American Ceramic Society. † Present address: Department of Materials Science and Engineering, Stevens Institute of Technology, Hoboken, New Jersey 07030. J. Am. Ceram. Soc., 81 [3] 717–20 (1998) Journal 717
718 Communications of the American Ceramic Sociery Vol 81. No. 3 For tensile tests, the ends of the minicomposite spec, The 14 were imbedded in aluminum blind rivets tensile tests were conducted using an in-house developed ma- hine at a constant loading rate of I um/s. Self-aligning cou plers were incorporated into the load train to minimize spurious nding loading. Fracture surfaces were characterized by scan- ning electron microscopy(SEM, Hitachi S-800). A transmis- sion electron microscope (TEM, Hitachi HF2000) equipped structure and composition of the multilayered oxide interphase region before and after oxidation A TEM image of the multilayered oxide interphase region of 0020.4060.81121.4 an as-prepared minicomposite specimen 1)shows the esence of the am ous SiO,. monoclinic ZrO, and amor Displacement (mm) phous SiO2 layers. Interestingly, another thin layer enriched with Zr(30 nm) was observed adjacent to the fiber surface Fig. 2. Tensile behavior of as-prepared minicomposites The incorporation of this contamination layer was attributed to It appeared that a small amount of residual ZrClg in the chlo- sile tests were stopped after a total displacement of l mm. The rinator might have been transported to the fiber surface during maximum loads observed for these specimens, 90 and IoN first sio, laver took place. The average thickness of the first containing a carbon interphase coating prepared in our lab- CVD SIO, layer was-100 nm, whereas the second CVD SiO oratory. The fracture surface of one of the tensile speci- was-50 nm thick. The Zro, layer was -200 to 400 mens is shown in Fig 3. Significant crack deflection was ob- thick, depending on axial location. The thickness of the mul- served within the multilayer interphase region. Although tilayered oxide interphase was relatively uniform in the radial exact deflection locations were difficult to determine from direction of the fiber tow but was not uniform along the lens these SEM images, it appeared that crack deflection and of the specimens because of reagent depletion along the direc- fiber pull-out occurred preferentially at the inner Sioz/zrOz tion of gas flow. In particular, the thickness of the SiO, layers interface. It was fairly difficult to prepare TEM specimens tended to decrease along the flow direction in the range of -50 showing the entire interphase region near the fracture locations, to 200 nm. The Zro, layer had a rough growth morpholog because the inner SiO,/ZrO, interface tended to become com- Consequently, the inner SiO, /ZrO, interface was relativ ly separated during TEM specimen preparation. The av- smooth while the outer ZrO,/SiO, interface was highly irregu length of fiber pull-out was on the order of tens of mi- lar and interlocked Otherwise the multilayered interphase re- crometers ion in its as-prepared conditions was mostly free of impurities der to assess the effects of oxidation on the or microcracks behavior of the minicomposites two specimens were loaded to Figure 2 shows the load-displacement curves of two as- 80N to induce matrix cracking. The precracked specimens red minicomposite specimens. Both specimens exhibited en remo ed from the tensile machine and exposed to graceful composite behavior with evidence of multiple matrix ambient air at 960 C for l and 10 h using a small furnace. After he oxidation treatments, the specimens were retested As cracking. Ihe specimens remained in one piece when the ten- shown in Fig. 4, the specimens still exhibited composite be- havior after the oxidation treatments although their maximum load capability (80 N) was somewhat decreased in compar sImens It appeared that the curvature of the tail end of the load-displacement curves became steeper with the longer oxidation treatment. The SEM fracture surfaces and TEM image of the specimen exposed to the 10 h oxidation treatment are shown in Figs. 5 and 6. From these images, it was apparent that crack deflection occurred mostly at the inner SiO,/ZrO, interface and to a less extent at the outer ZrO,/SiO interface as well. The preferred location for fiber pull-out mained the inner SiO,/ZrO, interface At least for 8 h of Sic infiltration at 900-950C and 10 h of oxidation treatment at 960C, the SiO /ZrO2/SiO2 interphase exhibited excellent chemical stability as evidenced by the pres ervation of their sharp interface boundaries( Fig. 6). No diffu- sion of Si and Zr within the multilayered structure, as well as no carbon as an impurity phase in the interphase region, was Fiber V. Discussion and conclusions The observed mechanical behavior and oxidation resistance of the minicomposites containing the multilayered oxide inter- Fig. 1. TEM image of the multilayered oxide interphase of an as hase are encouraging. It seems that one of the plausible rea prepared minicomposite specimen sons for the observed composite behavior may be the signif
For tensile tests, the ends of the minicomposite specimens were imbedded in aluminum blind rivets using epoxy. The tensile tests were conducted using an in-house developed machine at a constant loading rate of 1 mm/s. Self-aligning couplers were incorporated into the load train to minimize spurious bending loading. Fracture surfaces were characterized by scanning electron microscopy (SEM, Hitachi S-800). A transmission electron microscope (TEM, Hitachi HF2000) equipped with a field emission gun (<1.5 nm probe size) and an energy dispersive spectrometer (EDS) was used to characterize the structure and composition of the multilayered oxide interphase region before and after oxidation. III. Results A TEM image of the multilayered oxide interphase region of an as-prepared minicomposite specimen (Fig. 1) shows the presence of the amorphous SiO2, monoclinic ZrO2, and amorphous SiO2 layers. Interestingly, another thin layer enriched with Zr (∼30 nm) was observed adjacent to the fiber surface. The incorporation of this contamination layer was attributed to the presence of the internal Zr chlorinator in the CVD chamber. It appeared that a small amount of residual ZrCl4 in the chlorinator might have been transported to the fiber surface during the initial heating sequence, just before the deposition of the first SiO2 layer took place. The average thickness of the first CVD SiO2 layer was ∼100 nm, whereas the second CVD SiO2 layer was ∼50 nm thick. The ZrO2 layer was ∼200 to 400 nm thick, depending on axial location. The thickness of the multilayered oxide interphase was relatively uniform in the radial direction of the fiber tow, but was not uniform along the length of the specimens because of reagent depletion along the direction of gas flow. In particular, the thickness of the SiO2 layers tended to decrease along the flow direction in the range of ∼50 to 200 nm. The ZrO2 layer had a rough growth morphology. Consequently, the inner SiO2/ZrO2 interface was relatively smooth while the outer ZrO2/SiO2 interface was highly irregular and interlocked. Otherwise the multilayered interphase region in its as-prepared conditions was mostly free of impurities or microcracks. Figure 2 shows the load–displacement curves of two asprepared minicomposite specimens. Both specimens exhibited graceful composite behavior with evidence of multiple matrix cracking. The specimens remained in one piece when the tensile tests were stopped after a total displacement of 1 mm. The maximum loads observed for these specimens, 90 and 110 N, compared favorably to those of Nicalon/SiC minicomposites containing a carbon interphase coating prepared in our laboratory.4 The fracture surface of one of the tensile specimens is shown in Fig. 3. Significant crack deflection was observed within the multilayer interphase region. Although exact deflection locations were difficult to determine from these SEM images, it appeared that crack deflection and fiber pull-out occurred preferentially at the inner SiO2/ZrO2 interface. It was fairly difficult to prepare TEM specimens showing the entire interphase region near the fracture locations, because the inner SiO2/ZrO2 interface tended to become completely separated during TEM specimen preparation. The average length of fiber pull-out was on the order of tens of micrometers. In order to assess the effects of oxidation on the mechanical behavior of the minicomposites, two specimens were loaded to 80 N to induce matrix cracking. The precracked specimens were then removed from the tensile machine and exposed to ambient air at 960°C for 1 and 10 h using a small furnace. After the oxidation treatments, the specimens were retested. As shown in Fig. 4, the specimens still exhibited composite behavior after the oxidation treatments although their maximum load capability (∼80 N) was somewhat decreased in comparison to the as-prepared specimens. It appeared that the curvature of the tail end of the load–displacement curves became steeper with the longer oxidation treatment. The SEM fracture surfaces and TEM image of the specimen exposed to the 10 h oxidation treatment are shown in Figs. 5 and 6. From these images, it was apparent that crack deflection occurred mostly at the inner SiO2/ZrO2 interface, and to a less extent at the outer ZrO2/SiO2 interface as well. The preferred location for fiber pull-out remained the inner SiO2/ZrO2 interface. At least for 8 h of SiC infiltration at 900–950°C and 10 h of oxidation treatment at 960°C, the SiO2/ZrO2/SiO2 interphase exhibited excellent chemical stability as evidenced by the preservation of their sharp interface boundaries (Fig. 6). No diffusion of Si and Zr within the multilayered structure, as well as no carbon as an impurity phase in the interphase region, was observed within the detection limits of EDS. IV. Discussion and Conclusions The observed mechanical behavior and oxidation resistance of the minicomposites containing the multilayered oxide interphase are encouraging. It seems that one of the plausible reasons for the observed composite behavior may be the signifiFig. 1. TEM image of the multilayered oxide interphase of an asprepared minicomposite specimen. Fig. 2. Tensile behavior of as-prepared minicomposites. 718 Communications of the American Ceramic Society Vol. 81, No. 3
March 1998 Communications of the American Ceramic Sociery 10m 2pm“t Fig. 3. Fracture surface of the as-prepared minicomposite after the tensile test. present time. However, if some of these hypotheses can be validated for their generality, the multilayered oxide concept 120 may offer a new way of designing and tailoring the interface behavior of ceramic-matrix composites with many possible materials combinations as long as sufficient thermochemical stability and property mismatch exist between the constituent 1 h lavers. The use of the SiO,/ZrO, combination for crack deflec- tion and fiber pull-out, while protecting the Hi-Nicalon fiber surface from long-term oxidation with the SiO, layer, is obvi- ously an excellent example of this multiple interphase func- 40 tionality. From a processing point of view, the simplicity of 10h multilayered coatings based on single component oxides is attractive for manufacturing In view of these practical potentials, the experimental results from this study actually raise many questions relating to the ,60.811.2 basic mechanisms associated with the apparent weak interfacial behavior and fiber pull-out behavior observed with the SiO2/ Displacement(mm) ZrO,/SiO2 interphase. It is obvious that improving the CVD process to precisely control the thickness uniformity of the Tensile behavior of minicomposites which were precracked SiO /ZrO2/SiO2 interphase with minimum impurity contami- temperature and subsequently exposed to oxidation at 960 C nation is the first critical issue. Then the effects of individual laver thickness and relative thickness ratios on the mechanical behavior of the Hi-Nicalon/SiC minicomposite can be studied by tensile tests at room temperature and elevated temperatures cant coefficient of thermal expansion(CTE)mismatch between It is expected that, if the interphase region is irreversibly dam- rO2(10.5 x 10-K- for polycrystalline monoclinic at 1300 aged or weakened by the stresses generated during the initial K)and SiO2(-05 x 10-6K- for fused silica at 1300 K), which cool-down process, its fracture behavior at or near the stress- results in the development of extreme residual strains and con- free temperature still be uentweakening'' of their interfaces. At the deposition It will also be important to quantify the effects of depositing conditions used in this study, amorphous Sio2 is typically de- a SiO2 layer on the Hi-Nicalon fiber surface on the fiber sited with a nodular surface morphology. On the other hand trength and oxidation behavior. The possible influences of ZrO2 is polycrystalline, and tends to grow in a columnar fash- texture, roughness, and crystallinity of the Zro2 layer should ion. Because of their differences in growth characteristics, the also not be ignored. For example, the TEM and SEM images outer ZrO, /SiO, interface was much more irregular and inter- showed that the monoclinic ZrO, layer tended to be textured, as locked than the inner Sio,/ZrO, interface. The fiber pull-out evidenced by its columnar morphology. Since monoclinic ZrO 2 behavior observed preferentially at the inner SiO/ZrO2 inter- is a highly anisotropic material(e.g, linear thermal expansion gace may be attributed to the fact that there is less resistance to in the c-direction is-9 times higher than in the b-direction), bonding and sliding at this smoother interface. It is also he texture of the ZrO, layer may be an important varial ortant to point out that the fiber pull-out behav affecting the stress state of the multilayered interphase ably aided by the presence of the amorphous SiO, layer which The effects of thermochemical changes in the Sio/Zro2 can be thought of as a relatively compliant layer with its rela- on mec hanical behavior and long-term stability tively low elastic modulus of 74 GPa should be studied as a function of time and temperature, as With the limited experimental data and some processing-. ZrSiO4 formation is thermodynamically inevitable for the related uncertainties (i.e, fiber surface contamination), we rec- SiO/ZrO2/SiO2 interface ognize that the above arguments are rather speculative at the rtant issue is the possible contribution of car
cant coefficient of thermal expansion (CTE) mismatch between ZrO2 (∼10.5 × 10−6 K−1 for polycrystalline monoclinic at 1300 K) and SiO2 (∼0.5 × 10−6 K−1 for fused silica at 1300 K), which results in the development of extreme residual strains and consequent ‘‘weakening’’ of their interfaces. At the deposition conditions used in this study, amorphous SiO2 is typically deposited with a nodular surface morphology. On the other hand, ZrO2 is polycrystalline, and tends to grow in a columnar fashion. Because of their differences in growth characteristics, the outer ZrO2/SiO2 interface was much more irregular and interlocked than the inner SiO2/ZrO2 interface. The fiber pull-out behavior observed preferentially at the inner SiO2/ZrO2 interface may be attributed to the fact that there is less resistance to debonding and sliding at this smoother interface. It is also important to point out that the fiber pull-out behavior is probably aided by the presence of the amorphous SiO2 layer which can be thought of as a relatively compliant layer with its relatively low elastic modulus of 74 GPa. With the limited experimental data and some processingrelated uncertainties (i.e., fiber surface contamination), we recognize that the above arguments are rather speculative at the present time. However, if some of these hypotheses can be validated for their generality, the multilayered oxide concept may offer a new way of designing and tailoring the interface behavior of ceramic-matrix composites with many possible materials combinations as long as sufficient thermochemical stability and property mismatch exist between the constituent layers. The use of the SiO2/ZrO2 combination for crack deflection and fiber pull-out, while protecting the Hi-Nicalon fiber surface from long-term oxidation with the SiO2 layer, is obviously an excellent example of this multiple interphase functionality. From a processing point of view, the simplicity of multilayered coatings based on single component oxides is attractive for manufacturing. In view of these practical potentials, the experimental results from this study actually raise many questions relating to the basic mechanisms associated with the apparent weak interfacial behavior and fiber pull-out behavior observed with the SiO2/ ZrO2/SiO2 interphase. It is obvious that improving the CVD process to precisely control the thickness uniformity of the SiO2/ZrO2/SiO2 interphase with minimum impurity contamination is the first critical issue. Then the effects of individual layer thickness and relative thickness ratios on the mechanical behavior of the Hi-Nicalon/SiC minicomposite can be studied by tensile tests at room temperature and elevated temperatures. It is expected that, if the interphase region is irreversibly damaged or weakened by the stresses generated during the initial cool-down process, its fracture behavior at or near the stressfree temperature may still be graceful. It will also be important to quantify the effects of depositing a SiO2 layer on the Hi-Nicalon fiber surface on the fiber strength and oxidation behavior. The possible influences of texture, roughness, and crystallinity of the ZrO2 layer should also not be ignored. For example, the TEM and SEM images showed that the monoclinic ZrO2 layer tended to be textured, as evidenced by its columnar morphology. Since monoclinic ZrO2 is a highly anisotropic material (e.g., linear thermal expansion in the c-direction is ∼9 times higher than in the b-direction5 ), the texture of the ZrO2 layer may be an important variable affecting the stress state of the multilayered interphase region. The effects of thermochemical changes in the SiO2/ZrO2/SiO2 interphase on mechanical behavior and long-term stability should be studied as a function of time and temperature, as ZrSiO4 formation is thermodynamically inevitable for the SiO2/ZrO2/SiO2 interface.6 Another important issue is the possible contribution of carFig. 3. Fracture surface of the as-prepared minicomposite after the tensile test. Fig. 4. Tensile behavior of minicomposites which were precracked at room temperature and subsequently exposed to oxidation at 960°C in air. March 1998 Communications of the American Ceramic Society 719
Communications of the American Ceramic Sociery Vol 81. No. 3 20r Fig. 5. Fracture surface of the minicomposite specimen oxidized for 10 h after the tensile test. not detected during our TEM/EDS characterization. Neverthe- Sic less, it will be of importance to resolve this carbon contami- nation issue by quantifying the exact level of carbon impurity in the interphase region by more composition-sensitive tech- Sio niques such as electron energy loss spectroscopy and secondary lon mass spectroscopy. Acknowledgments: We thank Kevin Cooley and Jerry McLaughlin for performing the Cvd experiments and Dorothy Coffey for the SEM analy preparing the TEM specimens. References Matrix Composites. Edited by R. Naslain, J. Lamon, and D. Doumeingts. Wood- and P K. Liaw. Oxidation-Resistant Interface Coat ites": pp. 151-59 in Proceedings of the 10th Ann Fiber Energy Materials, Report No ORNL/FMP-96/1, Oa M. A. Borst, w. Y Lee, Y. Zhang, and P K. Liaw, "" Preparation and Char- 0.1Im eam.Soe,80间6]1591-94(199 on Chemical Vapor Deposition. The Electrochemical Society, Pennington, NJ S. Touloukian, R. K, Kirby, R E. Taylor, and T. Y.R. Lee, Thermal bon impurities, which may be present in the interphase region, EE.M. Levin, C. R. Robbins, H F. McMurdie; p 110 in Phase ansion: Nonmetallic Solids: p. 451. Plenum, New York, 1977. to the observed weak interface behavior. Minet et al. 7 observed Ceramists. E ited by M. K. Reser. American Ceramic Society, West some carbon contamination by Raman spectroscopy in Zr( coatings prepared with the same precursor chemistry. In con- JMinet, F, Langlais, and R. Naslain, " On the Chemical Vapour Deposition of Zirconia from ZrCl-H-CO -Ar Gas Mixture: IL. An Experimental Ap trast, the presence of carbon in the oxide interphase region was proach, "J. Less-Common Met, 132, 273(1987)
bon impurities, which may be present in the interphase region, to the observed weak interface behavior. Minet et al.7 observed some carbon contamination by Raman spectroscopy in ZrO2 coatings prepared with the same precursor chemistry. In contrast, the presence of carbon in the oxide interphase region was not detected during our TEM/EDS characterization. Nevertheless, it will be of importance to resolve this carbon contamination issue by quantifying the exact level of carbon impurity in the interphase region by more composition-sensitive techniques such as electron energy loss spectroscopy and secondary ion mass spectroscopy. Acknowledgments: We thank Kevin Cooley and Jerry McLaughlin for performing the CVD experiments and Dorothy Coffey for the SEM analysis and preparing the TEM specimens. References 1 R. J. Kerans, ‘‘Control of Fiber–Matrix Interface Properties in Ceramic Matrix Composites’’; pp. 301–12 in Proceedings of High Temperature Ceramic Matrix Composites. Edited by R. Naslain, J. Lamon, and D. Doumeingts. Woodhead Publishing Limited, France, 1994. 2 D. P. Stinton, E. R. Kupp, J. W. Hurley, R. A. Lowden, S. Shanmugham, and P. K. Liaw, ‘‘Oxidation-Resistant Interface Coatings for SiC/SiC Composites’’; pp. 151–59 in Proceedings of the 10th Annual Conference on Fossil Energy Materials, Report No. ORNL/FMP-96/1, Oak Ridge National Laboratory, Oak Ridge, TN, 1996. 3 M. A. Borst, W. Y. Lee, Y. Zhang, and P. K. Liaw, ‘‘Preparation and Characterization of Chemically Vapor Deposited ZrO2 Coating on Nickel and Ceramic Fiber Substrates,’’ J. Am. Ceram. Soc., 80 [6] 1591–94 (1997). 4 E. R. Kupp, E. Lara-Curzio, D. P. Stinton, R. A. Lowden, and T. M. Besmann, ‘‘CVI Processing of Minicomposites for Evaluation of Interface Coating Materials in Composites’’; in Proceedings of the 14th International Conference on Chemical Vapor Deposition. The Electrochemical Society, Pennington, NJ, in press. 5 Y. S. Touloukian, R. K. Kirby, R. E. Taylor, and T. Y. R. Lee, Thermal Expansion: Nonmetallic Solids; p. 451. Plenum, New York, 1977. 6 E. M. Levin, C. R. Robbins, H. F. McMurdie; p. 110 in Phase Diagram for Ceramists. Edited by M. K. Reser. American Ceramic Society, Westerville, OH, 1964. 7 J. Minet, F. Langlais, and R. Naslain, ‘‘On the Chemical Vapour Deposition of Zirconia from ZrCl4–H2–CO2–Ar Gas Mixture: II. An Experimental Approach,’’ J. Less-Common Met., 132, 273 (1987). h Fig. 6. TEM image of the multilayered oxide interphase of the oxidized minicomposite specimen. Fig. 5. Fracture surface of the minicomposite specimen oxidized for 10 h after the tensile test. 720 Communications of the American Ceramic Society Vol. 81, No. 3