R-Curve and Cyclic Fatigue Behavior in Alumina-Particles- Reinforced Silicon Carbide

A silicon carbide ceramic composite material toughened by the incorporation of 20 vol% spherical alumina particles (SiC-P) and a monolithic reference material (SiC-M) were prepared by hot-pressing at 2000° C for 6 hours. SiC-P exhibited a fracture toughness as high as 6 MPa√m, whereas SiC-M showed a fracture toughness of 2.8 MPa√m. R-curve- and cyclic fatigue behavior were investigated for both materials. Flaw tolerance and R-curve behavior were evaluated using the indentation-strength technique. SiC-P exhibited enhanced flaw tolerance and rising crack resistance with crack extension, properties that were attributed to the presence of internal residual stresses from thermal expansion mismatch and crack wake bridging by both second phase particles and interlocking matrix grains. Because of the lack of toughening mechanisms, SiC-M exhibited a flat R-curve. Cyclic bending tests with beam shaped specimens and cyclic crack propagation tests using compact-tension specimens were performed at a load ratio of 0.1 in order to investigate the fatigue behavior. Strength degradation with increasing number of cycles and true cyclic fatigue-crack growth were observed for SiC-P, whereas SiC-M as a typically brittle ceramic was not susceptible to cyclic fatigue degradation. Fractographical results suggest that the mechanisms that enhance the R-curve of SiC-P are degraded when subjected to cyclic loads, resulting in amplified susceptibility to cyclic fatigue.