ON THE POSSIBILITY OF CHEMOMECHANICAL ACTION IN MAGNETIC FLOAT POLISHING OF SILICON-NITRIDE

Chromium oxide abrasive has been reported in the literature to provide efficient chemo-mechanical polishing action for silicon nitride ceram ic. Since aluminum oxide and chromium oxide abrasives are nearly of th e same hardness, magnetic float polishing tests were conducted on sili con nitride balls with these Two abrasives to investigate mechanical v ersus chemo-mechanical aspects of polishing. Tests results show higher removal rates and smoother surface texture (with fewer pits) with chr omium 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 thin with chromium oxid e abrasive. While there may be some mechanical action (abrasion) with chromium oxide abrasive initially, subsequent removal is believed to b e due to chemo-mechanical action. This could be due to degeneration of the chromium oxide abrasive (both mechanical and chemical) during pol ishing. Various hypotheses for the material removal mechanism (both me chanical and chemo-mechanical) were considered. Based on that, the hig her removal rates and smoother surface texture on the silicon nitride balls with chromium oxide abrasive in semifinish polishing is interpre ted here as possibly due to chemomechanical action. Higher chemical st ability of aluminum oxide abrasive (compared to chromium oxide abrasiv e) and the known role of chromium oxide as a catalyst for the oxidatio n of silicon nitride are some of the reasons attributed for this actio n.