Quantitative Methods for Analysis of Fracture Surfaces in Polycrystalline Ceramics

The variation of fracture mirror radius with fracture stress was investigated in flint glass, steatite, alumina, silicon carbide and silicon nitride. The mirror radius depends on local stress at the fracture origin. Therefore, the difference between observed mirror size and that expected based upon the nominal fracture stress was used to estimate the magnitude of large scale residual stresses in flint glass, steatite and silicon carbide. Fracture mirrors formed in impact and delayed fracture specimens were studied. The remaining flexural strength of alumina scratched in various normal alcohol environments was measured. The highest remaining strength was observed for specimens scratched in butyl alcohol which is also the environment in which the zeta potential of alumina is nearest to zero. The effect of surface treatments on the impact resistance of ceramics was investigated. Compressive surface layers and low elastic modulus coatings were used to obtain substantial improvements in impact resistance. (Author)