august 2000 Tablel similarities and differenees between oem and IrM M Solid reaction produc Gaseous reaction product Brittle glass layer Reduced interface strength arrow temperature range ncreasing Local strength reduction No or little effect on local strength and, on engineering strength dependent behavior hold stress intensity for crack growth itical crack growth in stages i and nl these two mechanisms result primarily from the fundamentally and both 0.15 and 1.0 um thick interphases.2, 19, The fiber different crack growth mechanisms. The proposed formation of a relaxation-mechanism(FRM) regime includes that area where brittle glass phase that locally decreases the fiber strength accounts crack growth occurs in the absence of a significant environmental for the OEM characteristics occurring within a narrow temperature effect and is controlled by fiber creep, as described in Section range, the decreased local and engineering strength, and the IV(B). The fiber-debond model described in Fig. 6 also describes absence of a need for a dynamic stress. On the other hand, the the conditions controlling the FRM, except that the debond length removal of the interphase and the resulting decrease in crack- in the latter case is controlled by the stress in the fiber. and there closure forces by the bridging fibers during the IRM accounts for is no time-dependent change in the interface caused by oxidation the temperature dependence and the lack of an effect on engineer- The effect of coatings, glass-forming additions such as BC, and ing strength up to the time for total burnout of the interphase environmental species other than oxygen were not considered in developing the failure- mechanism map in Fig. 7. The map can be both the toughness and the dynamic crack velocity of a material sed to identify the type of crack growth process leading to failure and higher velocities. The OEM has the potential to affect both needed to consider the effects of coatings, glass-forming addition processes, whereas the IRM appears to affect only the subcritical crack growth process. Hydrog and other environmental species, such as H,O; alkali elements duced crack growth of metals and alloys can affect both the fracture toughness and the subcritical uch as sodium or potassium; or combustion gases. The devel crack velocity, as does the OEM whereas aqueous stress corrosion present work, because the boundaries shown in Fig. 7 most likely implications of this phenomenon are that a process that both will shift, and some regimes may not exist, depending on the decreases the K and ses da/dt in stage lI can have a much conditions greater impact on material performance than one that affects only The SCC mechanism map shows the T and Po, values at which the subcritical da/dr the oEM and the IRM have been observed. In all cases. for Two primary variables that define the operative OEM or IRM samples exposed to Po, values equal to or greater than those of air, environmental parameter space are(1)temperature and(2) Po, as but tested at <1073 K, the OEM type of mechanism occurred.For map given in Fig. 7. The IRM section was determined with at 1073 K and 2 x 10 Pa at 1373 K, the IRM type of mechanism composite materi c-grade Nicalon fibers occurred. Other variables that may be involved include time, the fiber-matrix interphase thickness, and the composition of the glas phase that forms on the fiber, the presence of fluxing agents, such as boron in BN interphases, alter glass-flow properties It is possible that the IRM operates at short times and the OEm at longer times. For instance, weight change tests of specimens with a I um thick arbon interphase show that all of the carbon air oxidizes before significant SiO, forms, therefore, the IRM could be operative until the SiO, formed is thick enough to embrittle the fiber or to bond the fiber to the matrix, leading to the oEm. Of course, this 1423 and 1773 K, based on measurement Gf thich perature liley iso embrittlement or bonding would only occur below a critical ten G ature, T, where the oxide is brittle. The critical ten related to the glass-transition temperature, T. nges between an amorphous scale formed between silicon and SiC, obtained by Futakawa and Stein- 14731673 brech, and those for an amorphous SiO2, reported by Bansal and The observations by Lara-Curzio et al. o and Becher et al.of 999):C, 0.15 micron the IRM in air at 698 K, where the growth of the glass layer would 04 1-0izr m B-enhanced matrix be very slow, support the idea of the IRM operating at short times, th the potential for the oEm at longer times, i.e., at lower Lara-Curzio and Ferber (1997): icons d Heredia et al.(1995): C.0.1-3.0 tresses or slower failure mechanisms. At 1073-1373K and low o Ishikawa(1994): C, 0.3 microns ition from irm to oem ailure. however at high stresses. it has been demonstrated Failure-mechanism map for continuous-fiber ceramic composites (O)data obtained from posttreatment, room-temperature experi- point is further supported by the oxidation-kinetic data reported by er symbols designate OEM observations, closed symbols des- Costello and Tressler for Sic in pure, dry FRM) show the time required to form an SiO2these two mechanisms result primarily from the fundamentally different crack growth mechanisms. The proposed1 formation of a brittle glass phase that locally decreases the fiber strength accounts for the OEM characteristics occurring within a narrow temperature range, the decreased local and engineering strength, and the absence of a need for a dynamic stress. On the other hand, the removal of the interphase and the resulting decrease in crack￾closure forces by the bridging fibers during the IRM accounts for the temperature dependence and the lack of an effect on engineer￾ing strength up to the time for total burnout of the interphase. Many environmentally induced cracking processes can affect both the toughness and the dynamic crack velocity of a material, which in effect shifts the da/dt–K curve to lower stress intensities and higher velocities. The OEM has the potential to affect both processes, whereas the IRM appears to affect only the subcritical crack growth process. Hydrogen-induced crack growth of metals and alloys can affect both the fracture toughness and the subcritical crack velocity, as does the OEM, whereas aqueous stress corrosion normally affects only the subcritical crack growth behavior.32 The implications of this phenomenon are that a process that both decreases the KIc and increases da/dt in stage II can have a much greater impact on material performance than one that affects only the subcritical da/dt. Two primary variables that define the operative OEM or IRM environmental parameter space are (1) temperature and (2) pO2 , as summarized in the stress-corrosion cracking (SCC) mechanism map given in Fig. 7. The IRM section was determined with composite material reinforced with ceramic-grade Nicalon fibers and both 0.15 and 1.0 mm thick interphases.2,19,33–37 The fiber￾relaxation-mechanism (FRM) regime includes that area where crack growth occurs in the absence of a significant environmental effect and is controlled by fiber creep, as described in Section IV(B). The fiber-debond model described in Fig. 6 also describes the conditions controlling the FRM, except that the debond length in the latter case is controlled by the stress in the fiber, and there is no time-dependent change in the interface caused by oxidation. The effect of coatings, glass-forming additions such as B4C, and environmental species other than oxygen were not considered in developing the failure-mechanism map in Fig. 7. The map can be used to identify the type of crack growth process leading to failure of a material, assuming coating failure. Separate maps would be needed to consider the effects of coatings, glass-forming additions, and other environmental species, such as H2O; alkali elements, such as sodium or potassium; or combustion gases. The develop￾ment of maps that consider these factors is beyond the scope of the present work, because the boundaries shown in Fig. 7 most likely will shift, and some regimes may not exist, depending on the conditions. The SCC mechanism map shows the T and pO2 values at which the OEM and the IRM have been observed. In all cases, for samples exposed to pO2 values equal to or greater than those of air, but tested at ,1073 K, the OEM type of mechanism occurred. For tests conducted at 1073–1373 K, but at pO2 values of #19.5 3 103 Pa at 1073 K and 2 3 103 Pa at 1373 K, the IRM type of mechanism occurred. Other variables that may be involved include time, the fiber–matrix interphase thickness, and the composition of the glass phase that forms on the fiber; the presence of fluxing agents, such as boron in BN interphases, alter glass-flow properties. It is possible that the IRM operates at short times and the OEM at longer times. For instance, weight change tests10 of specimens with a 1 mm thick carbon interphase show that all of the carbon in the interphase oxidizes before significant SiO2 forms; therefore, the IRM could be operative until the SiO2 formed is thick enough to embrittle the fiber or to bond the fiber to the matrix, leading to the OEM. Of course, this embrittlement or bonding would only occur below a critical temper￾ature, Tc, where the oxide is brittle. The critical temperature likely is related to the glass-transition temperature, Tg, which ranges between 1423 and 1773 K, based on measurements for an amorphous scale formed between silicon and SiC, obtained by Futakawa and Stein￾brech,38 and those for an amorphous SiO2, reported by Bansal and Doremus.39 The observations by Lara-Curzio et al. 40 and Becher et al. 41 of the IRM in air at 698 K, where the growth of the glass layer would be very slow, support the idea of the IRM operating at short times, with the potential for the OEM at longer times, i.e., at lower stresses or slower failure mechanisms. At 1073–1373 K and low stresses, a transition from IRM to OEM may occur before sample failure; however, at high stresses, it has been demonstrated10,42,43 that failure occurs by the IRM before a transition to the OEM. The point is further supported by the oxidation-kinetic data reported by Costello and Tressler44 for SiC in pure, dry oxygen. Those data show the time required to form an SiO2 layer thick enough to Table I. Similarities and Differences between OEM and IRM OEM IRM Differences Solid reaction product Gaseous reaction product Brittle glass layer Reduced interface strength Narrow temperature range Local strength reduction Increasing da/dt with increasing temperature No or little effect on local strength and, on engineering strength Similarities Time-dependent behavior Threshold stress intensity for crack growth Subcritical crack growth in stages I and II Fig. 7. Failure-mechanism map for continuous-fiber ceramic composites ((ƒ) and (E) data obtained from posttreatment, room-temperature experi￾ments; other symbols designate OEM observations; closed symbols des￾ignate IRM or FRM). August 2000 Stress-Corrosion Cracking of Silicon Carbide Fiber/Silicon Carbide Composites 2003