Exploratory Testing and Analysis of Full Ceramic Ball Bearings

A small sample lot of Conrad type ball bearings fabricated of silicon nitride have been fatigue tested under full film liquid lubrication conditions. Analysis of the failed bearings points to a failure mechanism which is different than that of steel bearing fatigue. A failure model has been developed based on the static strength of the material, manufacturing process effects and basic contact mechanics. The failure mode found identifies one limitation of the use of ceramic bearings. The limitation is dictated by the tensile stresses produced in operation and the strength of the finished component. It was demonstrated that silicon nitride fails by mechanical fatigue when subject to high Hertzian contact stress. It must be clearly understood, however, that the high Hertzian contact stresses (above 350 k.s.i.) used in this test program are greater than those of recommended design practice. The bearing manufacturing, testing and failure analysis are discussed.