Different manufacturing routes for ceramic balls and rollers used in hybrid bearings can lead to different microstructures and mechanical properties of a silicon nitride grade, even being delivered by one supplier. This fact, experienced by the authors, makes it necessary to do testing on the different balls or rollers themselves. This will make sure that the minimum properties on strength, fracture toughness, and microstructure requirements according to ASTM F2094/F2094M-14 and ISO 26602 are fulfilled on the bearing component and not only on special bar samples prepared for bending tests. A first approach done in the bearing industry was a simple crush test of two or three balls on top of each other. This gave a very rough estimate of a “static load rating.” The material resistance to cracks and Hertzian contact damage was tested by indentations with tungsten carbide spheres or doing impact strength testing. A first approach for “real” strength measurement of ceramic spheres was introduced by using a diametrally compressed “C-Sphere” specimen. This was further developed to be more easily machinable and was simply called “Notched ball test.” A similar test was also developed in recent times for ceramic cylindrical roller rollers, called the “Notched roller test.” The goal of this paper was to discuss and summarize test methods for strength and combinations of strength with other properties or effects like toughness and/or residual stresses for ceramic rolling elements that have been established so far for hybrid bearings. This supported the understanding for the need to establish the notched ball test, which was already established as a National Standard ÖNORM M6341:2013 in Austria, and also in ISO and ASTM.
