A soft metal or plastic layer on one of the surfaces. Rolling bearings Use rollers as the intermediate rolling element for very high radial loads. Use balls for radial and some axial load. Use needle rollers for rapidly fluctuating radial loads. Link to INA Bearing Company Ltd site which contains extensive information about their products, applications and technical papers.
Friction is very low.
Have to be manufactured to very close tolerances - can be expensive. For operation at higher temperatures, more exotic materials must be used, which is likely to be very expensive. Usually fail by fatigue - hence some short lives can occur. Bearing lives may be determined by the ISO equation for the basic rating life used in conjunction with manufacturers charts and data for basic load ratings, static or dynamic. While the vast majority of rolling bearings use hardened steel for the rolling elements and tracks, some applications require other materials, such as plastics or ceramics.