Mechanisms of High-Temperature Fatigue in Silicon Carbide Ceramics

produce a composite, which is typically accomplished by incorporating continuous fibers, whiskers, platelets, or second The high-temperature mechanical properties of an in situ phase particles (3). For monolithic ceramics, in situ toughening toughened silicon carbide, with Al, B and C sintering additives can also be effective with microstructures consisting of elongated (ABC-Sic), have been examined at temperatures from ambient to grains encased with a residual glassy film. Such microstructures 13OO0C with the objective of characterizing the role of the graininduce intergranular fracture and are thus effective in promoting boundary phase. It was found that elevated temperatures up to toughening from the consequent crack bridging, as has been well 130OOC do not severely compromise the strength, toughness and demonstrated in silicon nitride (Si3N4) ceramics (4). The problem fatigue resistance of ABC-SiC, compared to properties at ambient in monolithic ceramics is that although the amorphous graintemperatures. Mechanistically, the damage and shielding boundary film is critical for good low-temperature toughness, its mechanisms governing cyclic fatigue-crack advance are presence at high temperatures provides a preferred site for essentially unchanged between -250 and 13oooc, involving a softening and creep cavitation, which typically limits the highmutual competition between intergranular cracking ahead of the temperature strength, creep and oxidation resistance. crack tip and interlocking grain bridging in the crack wake. The Recently, in an attempt to avoid Such tradeoffs between lowunusually good high-temperature properties of ABC-Sic are temperature toughness and high-temperature Seen&, a attributed to in situ crystallization of grain-bomdary amorphous monolithic S ic with additions of A1 metal as well as B and c phase, which on subsequent cooling also marginally enhances the (termed ABC-SiC) has been developed. At ambient temperatures, ambient-temperature mechanical properties. In comparison to ABC-Sic exhibits fracture toughnesses as high a~ 9 MPadm mth commercial Sic (Hexoloy), the ABC-Sic displays superior strengths of -650 MPa (5), mechanical properties that are among strength, fracture toughness, and fatigue-crack growth resistance the highest reported for s ic . The high toughness has been at all temperatures from 25' to 13OO0C. attributed to various crack-bridging processes in crack wake resulting from the intergranular crack path (6); specifically, cracktip shielding from both elastic bridging and frictional pullout of Introduction the grains provide the major contributions, with the frictional pullout component being the more potent. At elevated As a high-temperature structural material, silicon carbide (Sic) temperatures, however, a critical factor governing properties is the ceramics offer many advantages, including a high melting viscosity of the grain-boundary phase, which results from the temperature, low density, high elastic modulus and strength, and presence of sintering additives that are present as densification good resistance to creep, oxidation and wear. This combination of aids (7). The softening of this phase can severely degrade properties makes it a promising candidate for use in such properties (8); however, in situ crystallization can provide an applications as gas turbines, piston engines and heat exchangers excellent means to increase its viscosity at such high (1,2), although its use to date has been severely limited by its poor temperatures. toughness properties. In the present work, we examine how the elevated temperature The low inherent fracture toughness of conventional S ic ceramics mechanical properties of ABC-Sic are affected by the nature of (Kc is typically -2-3 MPadm) can be improved, however, by the grain-boundary filmlphase, and investigate whether its several processing and reinforcement routes. One approach is to superior room-temperature strength and fracture toughness Fatigue & Fracture Behavior of High Temperature Materials Edited by P.K. Liaw TMS (The Minerals, Metals & Materials Society), 2000