Hybrid bearings-that is, bearings with ceramic rolling elements and steel rings-are often used in applications with reduced (i.e., boundary or mixed) lubrication conditions. The mechanisms by which hybrid bearings perform significantly better than full-steel ones in these cases are so far unclear, although a number of published works have shown experimental results in which appreciable performance benefits were obtained by the use of hybrid bearings under boundary or mixed lubrication. In this article, the reduced lubrication performance of hybrid rolling contacts, versus full-steel ones, is studied in detail by means of rolling bearing fatigue experiments and a theoretical micropitting model. It is found that the large improvement in surface fatigue resistance of hybrid contacts cannot be explained solely on the basis of the unavoidable differences in some of the roughness parameters existing between the full-steel and hybrid contacts. It is also necessary to take into account a considerable reduction in the effective boundary friction coefficient of the hybrid contact. In the numerical micropitting simulations it was found that the boundary friction coefficient of a hybrid contact must be about two times lower than that for the corresponding full-steel contact, in order to be able to predict the experimental observations reasonably well. A similar ratio of the boundary friction coefficients was obtained in a number of dedicated tests, thus confirming the results of the micropitting model. The mechanisms of the strong micropitting resistance of hybrid bearings under reduced lubrication conditions are discussed in detail, shedding new light on the operational tribology and performance capabilities of bearings with rolling elements made of silicon nitride ceramics.

Four material types were considered within an experimental investigation to identify the failure mechanism resulting from cavitation exposure. These materials were zirconia, silicon nitride and alumina with stainless steel as reference. An ultrasonic transducer was utilised to produce cavitation conditions and the configuration was "static specimen method" using a 5mm diameter probe, 20kHz and 50µm of amplitude. The exposure times were periods from 15 seconds to 2 hours.Experimental methods employed to characterise wear mechanisms were light microscopy, scanning light interferometry, scanning electronic microscopy.It was found that zirconia and silicon nitride demonstrated evidence of plastic deformation. Zirconia showed evidence of time delayed for transformation of phase. Alumina showed evidence of fracture type failure mechanism with negligible plastic deformation. All wear mechanisms are discussed and the materials are ranked in terms of cavitation resistance performance.

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