EFFECT OF MICROSTRUCTURE ON MATERIAL-REMOVAL MECHANISMS AND DAMAGE TOLERANCE IN ABRASIVE MACHINING OF SILICON-CARBIDE

Effects of microstructural heterogeneity on material-removal mechanism s and damage-formation processes in the abrasive machining of silicon carbide are investigated. It is shown that the process of material rem oval in a conventional silicon carbide material with equiaxed-grain mi crostructure and strong grain boundaries consists of the formation and propagation of transgranular cracks which results in macroscopic chip ping. However, in a silicon carbide material, containing 20 vol% yttri um aluminum garnet (YAG) second phase, with elongated-grain microstruc ture and weak grain boundaries, intergranular microcracks are formed a t the interphase boundaries, leading to dislodgment of individual grai ns. These different mechanisms of material-removal affect the nature o f machining-induced damage. While in the conventional silicon carbide material the machining damage consists of transgranular median/radial cracks, in the heterogeneous silicon carbide material, abrasive machin ing produces interfacial microcracks distributed within a thin surface layer. These two distinct types of machining damage result in a diffe rent strength response in the two forms of silicon carbide materials, In the case of the conventional silicon carbide, grinding damage resul ts in a dramatic decrease in strength relative to the as-polished spec imens. In contrast, the ground heterogeneous silicon carbide specimens show no strength loss at all.