Pii: S1359-6462(99)00427-3

Barsoum and El-Raghy [1] have recently developed reactive hot-pressing techniques to form bulk, polycrystalline Ti3SiC2 from powders of Ti, SiC, and graphite. This ternary carbide, first synthesized by Jeitschko and Nowotny [2] in the late 1960s, exhibits a rather surprising combination of properties. Not only is the ratio of hardness (;4 GPa) to elastic modulus (;320 GPa) more typical of ductile metals [3,4], but recent experimental studies have revealed a range of inelastic deformation modes not typically observed in ceramics [5–9]. These include grain bending, grain buckling, and significant amounts of basal slip at ambient temperatures [9,10]. It has also been shown that Ti3SiC2 [5,6,10], and related ternaries [11,12], are exceptionally damage tolerant materials. A dislocation-based model has recently been proposed to explain this exceptional damage tolerance [9]. The basic elements of the model are shear deformation by dislocation arrays, cavitation, creation of dislocation walls and kink boundaries, buckling and delamination. The delaminations and associated damage appear to be contained by the kink boundaries. It is this containment of damage that is believed to play a major role in endowing Ti3SiC2 (and by extension related ternary carbides and nitrides) with their damage tolerant properties. Studies have also revealed the presence of grain bridging and sliding [5,6], but to a much larger degree than observed in well-studied systems such as Al2O3, Si3N4, and SiC [e.g., 14–16]. Indeed, the deformation processes observed in individual grains of Ti3SiC2 seem to enhance grain bridging by increasing pullout distances and suppressing grain rupture. This plasticity, however, also suggests that Ti3SiC2 may be susceptible to cyclic-fatigue failures [6]. Damage associated with cyclic loading in many ceramics is generally attributed to cycle-dependent frictional wear at grain bridging sites. As such, ceramic microstructures designed for damage tolerance are generally more prone to cyclic fatigue degradation; this has been well documented in a range of monolithic and composite ceramics [e.g., 16–19]. To date, however, neither the fatigue-crack propagation nor the resistance-curve (R-curve) properties of monolithic Ti3SiC2 have been characterized. Therefore, the aim of this work is to investigate damage processes associated with grain bridging under both cyclic and static loading. We specifically compare coarse and fine-grained microstructures, with the ultimate aim of developing an understanding of mechanisms controlling fracture and cyclic fatigue properties in this unusual class of monolithic ceramics. Scripta mater. 42 (2000) 761–767