ON THE FINISHING OF SI3N4 BALLS FOR BEARING APPLICATIONS

The conventional method of producing of Si3N4 balls for bearing applic ations by grinding and lapping using diamond abrasive at low speeds (< a few hundred rpm) and higher loads (several tens of N/ball) is genera lly an expensive and time-consuming operation (several weeks). It also leads to the formation of scratches, microcracks, and pits on the fin ished 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 concer n. This paper deals with an alternate technology for finishing Si3N4 b alls for hybrid bearing applications using magnetic float polishing (M FP) process that overcomes some of these limitations. A methodology fo r finishing of HIP'ed Si3N4 balls from the as-received condition by MF P is presented. It involves the mechanical removal of material initial ly using harder abrasives with respect to the workmaterial (of differe nt 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 Si3 N4 balls, High material removal rates (1 mu m/min) with minimal subsur face damage is obtained with harder abrasives, such as B4C or SiC (rel ative to Si3N4) due to the use of a flexible support system, small pol ishing loads (approximate to 1 N/ball), and fine abrasives but high po lishing speeds (compared to conventional polishing) by rapid accumulat ion of minute amounts of material removed by microfracture. Final poli shing of the Si3N4 balls using a softer abrasive, such as CeO2 (that c hemo-mechanically react with the Si3N4 workmaterial) results in high q uality 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 C MP is very effective for obtaining excellent surface finish (R-a appro ximate 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 appl ication. (C) 1998 Elsevier Science S.A.