**6.1 Advanced bearing steels and ceramic bearings**

The need for more efficient and more powerful jet engines for military and commercial applications has caused a need for lighter, more durable bearing materials. Harder metal alloys and ceramic bearings are approaches to serve these needs. Changes in bearing materials, however may not be completely compatible with current lubricant basestocks and additive packages.

Many advanced bearing materials are made from carburized stainless steels. The materials begin with a stainless steel which can be formed into the desired shape. The part is then heat treated in the presence of a carbon source resulting in the formation of surface carbides [37]. The surface carbides increase the hardness of the surface significantly. Phosphate esters have been shown to interact with the stainless steels in the absence of carburization [38], but in the presence of all three components, metal carbides, phosphate esters and polyol esters the decomposition is much more rapid [39]. When carburized bearings were tested with polyol ester based lubricants formulated with phosphate esters, an increase in fatigue life and wear performance was observed [40].

Ceramic bearings have good potential for high temperature use in turbine engines. Unlubricated ceramic bearings performed poorly, however when an appropriate lubricant was added they performed better [41]. Typical lubricant additives, however did not perform well under conditions typically seen in steel bearings. At very high temperatures, a film was formed but it did not decrease friction or increase bearing life [42]. To form a lubricious anti-wear coating, the ceramics were pretreated to introduce a thin film of iron which allowed the phosphates to from an anti-wear coating [43].
