Hybrid bearings, i.e., bearings with steel rings and bearing grade silicon nitride (Si3N4) rolling elements, have been increasingly used in applications operating in challenging environments [1-4] such as high-speed, oil-free air conditioning and refrigeration compressors, general fluid machinery, electrical machinery and gearboxes. In some of these applications, both the boundary or mixed lubrication regime and solid particle contamination influence the bearing performance. In the case of a boundary or mixed lubrication regime, the bearing raceway surfaces can become susceptible to early surface distress and surface-initiated fatigue cracks, depending on the operating conditions, as well as the boundary friction-coefficient and wear behaviour acting on the contacting surfaces [5,6]. The failure risk on the raceway surfaces follows fatigue damage accumulation. In the case of a contaminated environment, solid particles entrapped in the rolling contact create dents with raised edges that can break through the lubrication film, even if the bearing operates in good full-film conditions. These larger but local surface dents lead to early surface-fatigue [7,8]. Dedicated experiments and modelling results are shown here to compare the physical mechanisms and performance of hybrid bearings with all-steel bearings for typical bearing surfaces in thin-film conditions in the first instance, and for larger denting features in the second instance.
Surface performance in poor lubrication
The experimental evidence shows that prominent performance benefits can be obtained by using hybrid bearings under boundary or mixed lubrication [1-4], including enhanced surface distress performance . The performance of hybrid rolling contacts in poor lubrication  is examined in detail by means of rolling bearing fatigue experiments and a combined surface distress and wear model (described in ). The research has also found that the large improvement in surface-fatigue resistance of hybrid contacts cannot be explained solely by normal differences in some of the roughness parameters existing between the all-steel and the hybrid contacts. It is necessary to take into account a considerable reduction (by a factor of two) in the effective boundary friction-coefficient of the hybrid contact, in comparison with an all-steel one, observed in dedicated tests .