Characterization of machining-induced sub-surface damage of a high strength silicon nitride

Surface machining was performed on a high strength silicon nitride (Si₃N₄) using a statistical experimental design in which the down feed, table speed, wheel surface speed and the grit size were systematically varied. The material's resulting strength was related to the machining forces, which in turn were correlated to the various machining parameters. Several different characterization techniques were utilized to evaluate the sub-surface damage induced by grinding of the high strength silicon nitride. The sub-surface damage is comprised of micro-cracking and plastic deformation which leads to residual stresses. The micro-cracking was characterized with optical and scanning electron microscopy, while the depth of the plastic deformation was determined by grazing incidence x-ray analysis and transmission electron microscopy. Results from this study consistently showed that the machining-induced plastic deformation was limited to depths of approximately one grain thickness for this Si₃N₄, independent of the grinding conditions investigated.