On chemomechanical polishing of Si₃N₄ with Cr₂O₃

Abstract

Chromium oxide (Cr₂O₃) is found to be an excellent abrasive for polishing of silicon nitride (Si₃N₄) work material. As the hardness or Cr₂O₃ is nearly the same as that of Si₃N₄, it appears that the mechanism of material removal would be more chemical action than mechanical abrasion. However, no evidence of any compound formation of Si₃N₄ work material with Cr₂O₃ abrasive has been reported in the literature thus far. Instead, the role of Cr₂O₃ was considered to be one of a catalyst, in view of the well known role of Cr₂O₃ as a catalyst, rather than its direct involvement in the chemical reactions with Si₃N₄. In this paper, we report new evidence on the chemomechanical action by examining the wear debris generated in polishing Si₃N₄ work material with Cr₂O₃ abrasive using a scanning electron microscope with an X-ray microanalyser and a small-angle X-ray diffraction apparatus. The analysis shows conclusively that Cr₂O₃ does participate in chemomechanical polishing of Si₃N₄ to form chromium nitride and chromium silicate, thus establishing the role of Cr₂O₃ not as a mere catalyst but as actively taking part in the chemical reactions during chemomechanical polishing. A new model of the chemomechanical polishing of Si₃N₄ work material in air and in water environments with Cr₂O₃ abrasive is presented. This model is based on the formation of such reaction products as chromium silicate and chromium nitride in addition to the formation of a silica layer on Si₃N₄ surface as well as gaseous reaction products, such as ammonia (in water) and nitrogen (in air).