On the finishing of Si3N4 balls for bearing applications

The conventional method of producing of Si3N4 balls for bearing applications by grinding and lapping using diamond abrasive at low speeds (<a few hundred rpm) and higher loads (several tens of N/ball) is generally an expensive and time-consuming operation (several weeks). It also leads to the formation of scratches, microcracks, and pits on the finished balls resulting from large radial and circumferential cracks and dislodgement of grains. Since failure of ceramics initiates from such defects, the reliability of Si3N4 balls in service is of prime concern. This paper deals with an alternate technology for finishing Si3N4 balls for hybrid bearing applications using magnetic float polishing (MFP) process that overcomes some of these limitations. A methodology for finishing of HIP'ed Si3N4 balls from the as-received condition by MFP is presented. It involves the mechanical removal of material initially using harder abrasives with respect to the workmaterial (of different materials of progressively lower hardnesses and finer grain sizes) followed by final chemo-mechanical polishing (CMP) using preferably a softer abrasive for obtaining superior finish with minimal surface or subsurface defects, such as scratches, microcracks, or pits on the Si3N4 balls, High material removal rates (1 mu m/min) with minimal subsurface damage is obtained with harder abrasives, such as B4C or SiC (relative to Si3N4) due to the use of a flexible support system, small polishing loads (approximate to 1 N/ball), and fine abrasives but high polishing speeds (compared to conventional polishing) by rapid accumulation of minute amounts of material removed by microfracture. Final polishing of the Si3N4 balls using a softer abrasive, such as CeO2 (that chemo-mechanically react with the Si3N4 workmaterial) results in high quality Si3N4 balls of bearing quality with superior surface finish (R-a < 4 nm, R-t < 0.04 mu m) and damage-free surface. It is found that CMP is very effective for obtaining excellent surface finish (R-a approximate to 4 nm and R-t approximate to 40 nm) on Si3N4 ceramic material and CeO2 in particular is one of most suitable material for this application. (C) 1998 Elsevier Science S.A.