Machining technology for high performance ceramics

A strength evaluation technology is required for high performance ceramics (silicon nitride, silicon carbide and alumina) especially because these materials are used as structural materials and are subjected to mechanical and thermal external forces. The machining of these materials is basically characterized by brittle microfractures of the surface layer. Therefore, surface flaws and residual stresses are created in the worked surface layer. This so affected layer often degrades the strength of machined ceramics. The strength degradation is observed not only in the machined structural parts but also in the ground test piece to be evaluated. Therefore, the reliability of machined parts and the evaluation of the strength for high performance ceramics is affected by the machining process. In this work, the main features of grinding process for ceramics and the relationship between the strength properties and the grinding conditions are shown. A proper grinding condition by a diamond wheel gives a worked surface without degradation but under non suitable grinding conditions the strength degrades remarkably. The damaged worked surface can be healed to some degree by means of a heat treatment although the healing is not perfect. Therefore, in the calculation (estimation) of the fracture probability of ceramics, the machining condition must be considered.