Structural Studies of Surface Deformation in MgO, SiC and Si3N4.

Abstract : A programme of research has been initiated to study the detailed mechanical behaviour of silicon nitride and silicon carbide materials when subjected to surface loading in air at ambient temperatures. The overall objective is to develop a quantitative understanding of the parameters controlling microhardness, friction and wear behaviour. The effect of crystallographic orientation and load on the Knoop microhardness of silicon carbide single crystals and hot-pressed silicon nitride and silicon carbide has been investigated. Although no significant correlation of microhardness with general microstructure (or alpha/beta ratio in Si3N4) has yet been established, an empirical equation was fitted for each material which allowed confirmed identification of unknown material samples. Attempts to reveal slip behaviour by dislocation etch-pitting techniques on one of the polar (0001) surfaces have proved successful for undeformed crystals of low dislocation density. Microstructural and crystallographic techniques are currently being developed to allow transmission electron microscopy investigations to be made of the deformation substructure at carefully controlled depths beneath the tested surfaces.