MICROSTRUCTURE AND STRENGTH OF ADVANCED CERAMICS AFTER MACHINING

Transmission electron microscopy preparation techniques were developed to investigate surface samples and cross-section specimens of Al2O3, ZrO2-Al2O3 and Si3N4 ceramics after machining. Besides the well-known brittle fracture processes, all these materials undergo microplastification during ultrasonic machining, diamond grinding and diamond polishing. These plastic deformations give rise to the high measured values of residual stresses in machined ceramic surfaces. Enhanced bending strength as a result of compressive residual stresses could be observed in only the ZrO2-Al2O3 ceramic. Although Al2O3 and Si3N4 also showed compressive residual stresses after grinding and ultrasonic machining, the bending strength was lowered by up to 35% compared with samples with polished surfaces. An explanation was found by scanning electron microscopy (SEM)-fractography, by quantification of machining defects and by roughness measurements.