JAm. Ceram.Soc.83[8l1999-2005(2000 urna Stress-Corrosion Cracking of Silicon Carbide Fiber/Silicon Carbide Composites Russell H. Jones. Charles H. Henager Jr * Charles A. Lewinsohn, and Charles F. Windisch Jr Pacific Northwest National Laboratory, Richland, Washington 99352 Ceramic-matrix composites are being developed to operate at specimens are tested at room temperature after high-temperature elevated temperatures and in oxidizing environments. Consid- exposure without the application of stress. /This environmental erable improvements have been made in the creep resistance of process results in bulk embrittlement and loss of fracture strength SiC fibers and, hence in the high-temperature properties of A time-dependent, environmentally induced crack growth pro- SiC fiber/SiC (SiC SiC) composites; however, more must be cess that results from removal of the fiber-matrix interphase has known about the stability of these materials in oxidizing been demonstrated and modeled by Henager and Jones, Jones et environments before they are widely accepted. Experimental al, and Windisch et al. o This process shares many features with weight change and crack growth data support the conclusion the OEM, except that crack growth is controlled by the time- that the oxygen-enhanced crack growth of SiC/Sic occurs by dependent reduction of the fiber-bridging stress instead of the ore than one mechanism, depending on the experimental formation of a brittle glass layer. An interphase removal mecha- conditions. These data suggest an oxidation embrittlement nism(IRM) results in a K and in stage Il growth, but the mechanism(OEM) at temperatures <1373 K and high oxygen characteristics of stage II for the IRM differ from those for the pressures and an interphase removal mechanism (IRM) at OEM. Local embrittlement does not necessarily occur and, in fact, temperatures of 2700 K and low oxygen pressures. The OEM the composite strength typically is unaffected by cracking, results from the reaction of oxygen with sic to form a glass demonstrated by Jones er al layer on the fiber or within the fiber-matrix interphase region. The purpose of this paper is to summarize the evidence for the The fracture stress of the fiber is decreased if this layer is existence of the OEM and the IRM in SiC/Sic and to define their thicker than a critical value (d>dd) and the temperature characteristics. Brief descriptions of the important features of each below a critical value(T< Ty), such that a sharp crack can be mechanism are provided. Key microstructural parameters control ustained in the laver. The IRM results from the oxidation of ng( nechanism are emphasize ecrease of stress that is arried by the bridging fibers. Interphase removal contribute to suberitical crack growth by decreasing the fiber-bridging stresses and, hence, increasing the crack-tip stress. The IRM Il. Oxidation of Sic/sic Composites occurs over a wide range of temperatures for d d and ma Oxidation of an SiC/ Sic composite with a carbon interphase occur at T> TR for d> d. This paper summarizes the can result in both removal of the interphase and formation of Sio vidence for the existence of these two mechanisms and from reaction with the fiber or the matrix. For composites attempts to define the conditions for their operation. containing BN interphases, oxidation also can result in removal of the interphase and formation of a borosilicate-glass phase. a key feature of the oxidation behavior observed by Windisch et al. for L. Introduction SiC/SiC with a I um thick interphase, in oxygen at 2.4 X 10Pa NVIRONMENTALLY induced crack growth of ceramic-matrix of pressure(atmospheric pressure of oxygen)and 1373 K, for nay result from several mechanisms. An oxidation mes up to 10- s, was weight loss alone. Little or no SiO2 embrittlement mechanism(OEM), as proposed by Evans et al l formation occurred in any of the materials with carbon interphases, and observed by Heredia et al.,Lin and Becher, and Raghuraman although some boron-containing glass phase was observed for the et al, results from the reaction of the environment with the fiber material with a BN interphase or fiber-matrix interphase to cause local embrittlement. This The thermogravimetric analysis (TGA)results for the kinetics mechanism requires porosity or microcracks produced by applica- of mass loss are shown as a function of pressure and temperature tion of a stress before or during exposure to the environment in Figs. I and 2 for material with a carbon interphase. Complete allow ingress of the environment and formation of a brittle glass small test samples at a pressure of 2.4 X 10" Pa and a temperature stress-intensity threshold, Kth, below which crack growth does not of 1373 K. Clearly, the reaction rate increased with increasing occur, and of two growth stages, consistent with the stress- temperature. An activation energy of -50 kJ/mol was reported by Windisch et al, which could be explained as diffusion con- K-dependent regime for K values greater than the threshold value, nterphase recession rate(RR)was determined from the weight. trolled, through a boundary layer, or reaction-rate controlled. An stage II is the K-independent regime following stage I. In porous loss measurements and by optical microscopy. Both methods gave vapor infiltration, this embrittlement has been observed when recession-rate equations very similar to the physically measured log(rr)=0.9 log(Po)-99 (1) F. Zok-contributing editor where RR is the interphase recession rate, in m pascals. Greater recession rates were measured by by optical microscopy, which could suggest weig Manuscript No. 190054. Received July 2, 1998; approved January 18, 2000 bers, as well as from oxidation of the carbon interphase material Member, American Ceramic Society However, the forms derived for the equation by both methods wer 1999Stress-Corrosion Cracking of Silicon Carbide Fiber/Silicon Carbide Composites Russell H. Jones,* Charles H. Henager Jr.,* Charles A. Lewinsohn,* and Charles F. Windisch Jr. Pacific Northwest National Laboratory, Richland, Washington 99352 Ceramic-matrix composites are being developed to operate at elevated temperatures and in oxidizing environments. Consid￾erable improvements have been made in the creep resistance of SiC fibers and, hence, in the high-temperature properties of SiC fiber/SiC (SiCf /SiC) composites; however, more must be known about the stability of these materials in oxidizing environments before they are widely accepted. Experimental weight change and crack growth data support the conclusion that the oxygen-enhanced crack growth of SiCf /SiC occurs by more than one mechanism, depending on the experimental conditions. These data suggest an oxidation embrittlement mechanism (OEM) at temperatures <1373 K and high oxygen pressures and an interphase removal mechanism (IRM) at temperatures of *700 K and low oxygen pressures. The OEM results from the reaction of oxygen with SiC to form a glass layer on the fiber or within the fiber–matrix interphase region. The fracture stress of the fiber is decreased if this layer is thicker than a critical value (d > dc) and the temperature below a critical value (T < Tg), such that a sharp crack can be sustained in the layer. The IRM results from the oxidation of the interfacial layer and the resulting decrease of stress that is carried by the bridging fibers. Interphase removal contributes to subcritical crack growth by decreasing the fiber-bridging stresses and, hence, increasing the crack-tip stress. The IRM occurs over a wide range of temperatures for d < dc and may occur at T > Tg for d > dc. This paper summarizes the evidence for the existence of these two mechanisms and attempts to define the conditions for their operation. I. Introduction ENVIRONMENTALLY induced crack growth of ceramic-matrix composites may result from several mechanisms. An oxidation embrittlement mechanism (OEM), as proposed by Evans et al. 1 and observed by Heredia et al.,2 Lin and Becher,3 and Raghuraman et al.,4 results from the reaction of the environment with the fiber or fiber–matrix interphase to cause local embrittlement. This mechanism requires porosity or microcracks produced by applica￾tion of a stress before or during exposure to the environment to allow ingress of the environment and formation of a brittle glass layer.5 Modeling of this mechanism suggests the presence of a stress-intensity threshold, Kth, below which crack growth does not occur, and of two growth stages, consistent with the stress￾corrosion cracking of ceramics.1 Stage I is the strongly K-dependent regime for K values greater than the threshold value; stage II is the K-independent regime following stage I. In porous materials, such as SiC fiber/SiC (SiCf /SiC), produced by chemical vapor infiltration, this embrittlement has been observed when specimens are tested at room temperature after high-temperature exposure without the application of stress.6,7 This environmental process results in bulk embrittlement and loss of fracture strength. A time-dependent, environmentally induced crack growth pro￾cess that results from removal of the fiber–matrix interphase has been demonstrated and modeled by Henager and Jones,8 Jones et al.,9 and Windisch et al. 10 This process shares many features with the OEM, except that crack growth is controlled by the time￾dependent reduction of the fiber-bridging stress instead of the formation of a brittle glass layer. An interphase removal mecha￾nism (IRM) results in a Kth and in stage II growth, but the characteristics of stage II for the IRM differ from those for the OEM. Local embrittlement does not necessarily occur and, in fact, the composite strength typically is unaffected by cracking, as demonstrated by Jones et al. 11 The purpose of this paper is to summarize the evidence for the existence of the OEM and the IRM in SiCf /SiC and to define their characteristics. Brief descriptions of the important features of each mechanism are provided. Key microstructural parameters control￾ling each mechanism are emphasized. II. Oxidation of SiCf /SiC Composites Oxidation of an SiCf /SiC composite with a carbon interphase can result in both removal of the interphase and formation of SiO2 from reaction with the fiber or the matrix. For composites containing BN interphases, oxidation also can result in removal of the interphase and formation of a borosilicate-glass phase. A key feature of the oxidation behavior observed by Windisch et al. 10 for SiCf /SiC with a 1 mm thick interphase, in oxygen at 2.4 3 104 Pa of pressure (atmospheric pressure of oxygen) and 1373 K, for times up to 104 s, was weight loss alone. Little or no SiO2 formation occurred in any of the materials with carbon interphases, although some boron-containing glass phase was observed for the material with a BN interphase. The thermogravimetric analysis (TGA) results for the kinetics of mass loss are shown as a function of pressure and temperature in Figs. 1 and 2 for material with a carbon interphase. Complete burnout of the carbon interphase occurred within ,104 s in the small test samples at a pressure of 2.4 3 104 Pa and a temperature of 1373 K. Clearly, the reaction rate increased with increasing temperature. An activation energy of ;50 kJ/mol was reported by Windisch et al.,10 which could be explained as diffusion con￾trolled, through a boundary layer, or reaction-rate controlled. An interphase recession rate (RR) was determined from the weight￾loss measurements and by optical microscopy. Both methods gave recession-rate equations very similar to the physically measured equation, as follows: log ~RR! 5 0.9 log ~ pO2! 2 9.9 (1) where RR is the interphase recession rate, in m/s, and pO2 is in pascals. Greater recession rates were measured by weight loss than by optical microscopy, which could suggest weight loss from the fibers, as well as from oxidation of the carbon interphase material. However, the forms derived for the equation by both methods were F. Zok—contributing editor Manuscript No. 190054. Received July 2, 1998; approved January 18, 2000. *Member, American Ceramic Society. J. Am. Ceram. Soc., 83 [8] 1999–2005 (2000) 1999 journal