Silicon nitride balls: Effects of surface finish and geometry
There are several important advantages to bearing performance that are due to the improved surface characteristics of silicon nitride balls. Because the microstructure of silicon nitride can be uniformly engineered to sub-micron grain size, the surface of a silicon nitride ball can be finished to <0.15 micro-inch Ra and the roundness is often better than 0.00001 tolerance. This precision results in a coefficient of friction on steel 70% lower than steel-on-steel which translates to a 30% lower internal bearing friction, lower cage wear, lower internal temperatures, and reduced raceway wear. The high precision geometry of a silicon nitride ball minimizes the difference in variation between the balls within a bearing. This effect lowers vibration and noise that occur as the balls spin within the raceway and cage. Bearing vibration can become a contributing source of failure because high vibration tends to result in raceway "false brinelling" damage. Additionally, the lower vibration and ball variation reduce the non-repeatable run-out of hybrid bearings can be significantly compared to steel bearings.
