Optimizing the Structure and Properties of Silicon Nitride for Rolling Contact Bearing Performance

Abstract

The total potential for the application of ceramic materials to rolling-element bearings has yet to be defined. There seems to be no doubt, however, that a commercially important segment of technology will benefit from the unique operating capabilities of ceramic and hybrid ceramic/steel bearings. In order to assure reliability and cost effectiveness in these bearings, it is essential that bearing manufacturers have a means to discriminate among the many materials and suppliers that are competing to supply the market with “bearing-quality” silicon nitride. In this program, 11 candidate monolithic and two silicon carbide whisker-reinforced, silicon nitride materials were evaluated with respect to their rolling-contact fatigue performance. When they were tested at a maximum Hertzian contact pressure of 5.930 GPa (860 ksi), with steel loading balls and SAE-20 mineral oil lubricant, the materials exhibited life differences of several orders of magnitude. Attempts to correlate the fatigue performance with such properties as density, hardness and elastic moduli were generally unsuccessful. However, the microstructural features such as porosity, second-phase distribution and, in particular, the crystal size and morphology, showed a good qualitative relationship to the fatigue life and failure mode. Under these relatively high-stress conditions, fatigue durability increased dramatically as the microstructure tended toward finer, more equiaxed grains and a uniform, minimum distribution of second phases.