On the Possibility of Chemo-Mechanical Action in Magnetic Float Polishing of Silicon Nitride

Abstract

Chromium oxide abrasive has been reported in the literature to provide efficient chemo-mechanical polishing action for silicon nitride ceramic. Since aluminum oxide and chromium oxide abrasives are nearly of the same hardness, magnetic float polishing tests were conducted on silicon nitride balls with these two abrasives to investigate mechanical versus chemo-mechanical aspects of polishing. Tests results show higher removal rates and smoother surface texture (with fewer pits) with chromium oxide abrasive compared to aluminum oxide abrasive. Formation of pits due to brittle fracture seems to be the more predominant mode of material removal with aluminum oxide abrasive than with chromium oxide abrasive. While there may be some mechanical action (abrasion) with chromium oxide abrasive initially, subsequent removal is believed to be due to chemo-mechanical action. This could be due to degeneration of the chromium oxide abrasive (both mechanical and chemical) during polishing. Various hypotheses for the material removal mechanism (both mechanical and chemo-mechanical) were considered. Based on that, the higher removal rates and smoother surface texture on the silicon nitride balls with chromium oxide abrasive in semifinish polishing is interpreted here as possibly due to chemo-mechanical action. Higher chemical stability of aluminum oxide abrasive (compared to chromium oxide abrasive) and the known role of chromium oxide as a catalyst for the oxidation of silicon nitride are some of the reasons attributed for this action.