J.Am. Ceran.Soe.,90214050-4054(2007) Dol:10.ll1551-29162007.02060x c 2007 The American Ceramic Society journal Low-Temperature Oxidation Embrittlement of SiC ( Nicalon )/CAS Ceramic Matrix Composites Kevin p. plucknett*, f Materials Engineering Program, Department of Process Engineering and Applied Science, Dalhousie University Halifax, Canada, B3J 1Z1 Hua-Tay Ceramic Science and Technology Group, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6068 The influence of extended duration(up to 1000 h), low ter ture (i.e,>800C)exposure on materials with a carbon-based ture oxidation heat-treatments(375-600.C)has been assess fiber/matrix interlayers in an oxidizing environment, typically tibe g a model ceramic matrix composite system with a graphitic under fast fracture conditions. Subsequent to that work,the r matrix interphase. For this study a Nicalon"fiber effects of extended duration oxidation exposure were exam- reinforced CaO-Al2OxSiOz matrix composite was selected ined, and particular emphasis was placed upon composite ( Nicalon"/CAS), which possesses a thin (20-40 nm) stability in the intermediate temperature range(i.e, 5000-800oC) for both unloaded and static fatigue-loaded conditions. It is now ed under both unloaded and static fatigue-loaded conditions, fe well established that, in the absence of a protective coating. pipe-line "oxidation of the carbon- or boron nitride-based in- 1000 h terlayer occurs, resulting in considerable property degrada exposure, resulting in a transition to a nominally brittle tion. While high temperature pretreatments can inhibit this node (i.e, negligible fiber pull-out). The degree of effect, by"plugging"the exposed fiber ends with SiO2, they mechanical property degradation increases with increasing tem- are only useful for applications at stresses below the onset of perature, such that strength degradation, and a transition to matrix microcracking (i.e, the composite proportional limit) minally brittle failure, is apparent after just 10 h at 600C More recently, these studies of CMc behavior have been ex- Static fatigue loading between 450 and 600C demonstrated tended to assess cyclic fatigue loading at intermediate tempera- generally similar trends, with reduced lifetimes being observed tures. It was noted that a significant decrease in the fatigue with increasing temperature Based upon the unloaded oxidation limit occurs at 800oC, relative to room temperature, with a re- experiments, combined with previously obtained intermediate duction down to the microcracking stress, ome. In a manner nd high-temperature oxidation stability studies, a simple envi- similar to unstressed intermediate temperature aging, this degra- onmental embrittlement failure mechanism map is presen dation behavior was attributed to oxidation of the carbon inter for Nicalon"/CAS. The implications of this study for advanced phase and the subsequent formation of a silica bridge between omposite designs with multiple thin carbon-based interphase the fiber and matrix, which results in a strong fiber-matrix bond vers are also discussed While these previous studies have concentrated upon the effects of high and intermediate temperature oxidation expo- L. Introduction tability of such materials at lower temperatures (i.e, below 700C), where carbon oxidation can still occur. For example, it TINUoUS fiber-reinforced ceramic matrix composites known that carbon/carbon composites exhibit oxidation at (CMCs) are promising candidates for a number of ad- mperatures as low as 400 C, and anced structural applications, including use in gas turbine en- ticipated that CMCs with carbon-based interlayers could show gines, automotive brakes, and heat exchangers. While there ong-term property degradation at similar temperatures. Sim are now several classes of these materials, developed with both larly, there have been several recent studies on the cyclic thermal dense and porous matrices, the majority of prior work has ex- shock degradation of Nicalon"/CAs within comparable tem- plored the development and mechanical assessment of material erature ranges 24 25 In the present work, particular emphasis with compliant interphases between the fiber and matrix. For has been placed upon assessing extended duration environmen- nonoxide composites, these interphase layers are required to tal stability at these lower temperatures, for both unstressed and lower the interfacial fracture energy and allow fiber-matrix stressed conditions debonding, and are typically based on the use of materials ch as carbon or A number of studies have environmental stability of Sic- nforced Cl Il. Experimental Procedures from both experimental and theoretical p es.- Sever. al of these have primarily assessed the effects of high ter Oxidation heat treatments have been performed on a continuous Nicalon fiber-reinforced glass-ceramic composite, with a devitrified Cao-AlO3-SiO2 matrix(Nicalon"/CAS Type Il. R. Naslain--contnbuting editor 10, 90]3s, Corning, NY). The primary crystalline phase in this naterial is anorthite( CaAl,Si,O%), with a small amount of fine (<I um)zircon(ZrO4) precipitates present at the fiber /matrix interface. 6. I4A continuous, in situ formed carbon layer is preser between the fiber and matrix, with a thickness typically between addressed. e-mail: kevin plucknett(a dal. 20 and 40 nm. A summary of the as-received Nicalon"/CASLow-Temperature Oxidation Embrittlement of SiC (Nicalont)/CAS Ceramic Matrix Composites Kevin P. Plucknett* ,w Materials Engineering Program, Department of Process Engineering and Applied Science, Dalhousie University, Halifax, Canada, B3J 1Z1 Hua-Tay Lin* Ceramic Science and Technology Group, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6068 The influence of extended duration (up to 1000 h), low temper￾ature oxidation heat-treatments (3751–6001C) has been assessed using a model ceramic matrix composite system with a graphitic fiber/matrix interphase. For this study a Nicalont fiber reinforced CaO–Al2O3–SiO2 matrix composite was selected (Nicalont/CAS), which possesses a thin (B20–40 nm) carbon-based interphase. Oxidation exposure has been conduct￾ed under both unloaded and static fatigue-loaded conditions. For unstressed oxidation exposure, degradation of the carbon-based interphase is apparent at temperatures as low as 3751C, after 1000 h exposure, resulting in a transition to a nominally brittle failure mode (i.e., negligible fiber pull-out). The degree of mechanical property degradation increases with increasing tem￾perature, such that strength degradation, and a transition to nominally brittle failure, is apparent after just 10 h at 6001C. Static fatigue loading between 4501 and 6001C demonstrated generally similar trends, with reduced lifetimes being observed with increasing temperature. Based upon the unloaded oxidation experiments, combined with previously obtained intermediate and high-temperature oxidation stability studies, a simple envi￾ronmental embrittlement failure mechanism map is presented for Nicalont/CAS. The implications of this study for advanced composite designs with multiple thin carbon-based interphase layers are also discussed. I. Introduction CONTINUOUS fiber-reinforced ceramic matrix composites (CMCs) are promising candidates for a number of ad￾vanced structural applications, including use in gas turbine en￾gines, automotive brakes, and heat exchangers.1–6 While there are now several classes of these materials, developed with both dense and porous matrices, the majority of prior work has ex￾plored the development and mechanical assessment of materials with compliant interphases between the fiber and matrix. For nonoxide composites, these interphase layers are required to lower the interfacial fracture energy and allow fiber–matrix debonding, and are typically based on the use of materials such as carbon or boron nitride. A number of studies have addressed issues related to the environmental stability of SiC-based fiber-reinforced CMCs, from both experimental and theoretical perspectives.7–20 Sever￾al of these have primarily assessed the effects of high tempera￾ture (i.e., 48001C) exposure on materials with a carbon-based fiber/matrix interlayers in an oxidizing environment, typically under fast fracture conditions.7–10 Subsequent to that work, the effects of extended duration oxidation exposure were exam￾ined,11–17 and particular emphasis was placed upon composite stability in the intermediate temperature range (i.e., 5001–8001C) for both unloaded and static fatigue-loaded conditions. It is now well established that, in the absence of a protective coating, ‘‘pipe-line’’ oxidation of the carbon- or boron nitride-based in￾terlayer occurs,10–20 resulting in considerable property degrada￾tion. While high temperature pretreatments can inhibit this effect,21 by ‘‘plugging’’ the exposed fiber ends with SiO2, they are only useful for applications at stresses below the onset of matrix microcracking (i.e., the composite proportional limit). More recently, these studies of CMC behavior have been ex￾tended to assess cyclic fatigue loading at intermediate tempera￾tures.22 It was noted that a significant decrease in the fatigue limit occurs at 8001C, relative to room temperature, with a re￾duction down to the microcracking stress, smc. In a manner similar to unstressed intermediate temperature aging, this degra￾dation behavior was attributed to oxidation of the carbon inter￾phase and the subsequent formation of a silica bridge between the fiber and matrix, which results in a strong fiber–matrix bond. While these previous studies have concentrated upon the effects of high and intermediate temperature oxidation expo￾sure, there is only minimal information regarding the long-term stability of such materials at lower temperatures (i.e., below 7001C), where carbon oxidation can still occur. For example, it is known that carbon/carbon composites exhibit oxidation at temperatures as low as 4001C,23 and consequently it can be an￾ticipated that CMCs with carbon-based interlayers could show long-term property degradation at similar temperatures. Simi￾larly, there have been several recent studies on the cyclic thermal shock degradation of Nicalont/CAS within comparable tem￾perature ranges.24,25 In the present work, particular emphasis has been placed upon assessing extended duration environmen￾tal stability at these lower temperatures, for both unstressed and stressed conditions. II. Experimental Procedures Oxidation heat treatments have been performed on a continuous Nicalont fiber-reinforced glass–ceramic composite, with a devitrified CaO–Al2O3–SiO2 matrix (Nicalont/CAS Type II, [01, 901]3S, Corning, NY). The primary crystalline phase in this material is anorthite (CaAl2Si2O8), with a small amount of fine (o1 mm) zircon (ZrO4) precipitates present at the fiber/matrix interface.6,14 A continuous, in situ formed carbon layer is present between the fiber and matrix, with a thickness typically between 20 and 40 nm. A summary of the as-received Nicalont/CAS R. Naslain—contributing editor *Member, the American Ceramic Society w Author to whom correspondence should be addressed. e-mail: kevin.plucknett@dal.ca Manuscript No. 23158. Received May 2, 2007; approved July 26, 2007. Journal J. Am. Ceram. Soc., 90 [12] 4050–4054 (2007) DOI: 10.1111/j.1551-2916.2007.02060.x r 2007 The American Ceramic Society 4050