Failure behavior of alumina and alumina/silicon carbide nanocomposites with natural and artificial flaws

Alumina/silicon carbide nanocomposites with 5 vol.% SiC nanoparticles were produced by slip casting, pressureless sintering, and hot isostatic pressing. The grain size dependence of both the bend strength and fracture toughness have been investigated. The strength exceeds 1 GPa at a grain size of 1.7 μm. Crack opening displacements (COD) were measured, revealing that crack tip toughness is considerably lower than in pure alumina and an R-curve behavior is unlikely to occur. By introducing artificial pores with a size of 60 μm, the micromechanical fracture process has been studied in both pure alumina and nanocomposites. In contrast to alumina, where an annular precrack forms prior to fracture, it is suggested that precrack formation is strongly impeded in the nanocomposites and failure is controlled by microcrack initiation. The high strength of Al₂O₃/SiC nanocomposites seems to be the result of an unusually high crack initiation stress.