htlyst,
A "sliding" bearing may not necessarily be "hydrodynamic" in nature, if the operating conditions (film lambda, relative surface velocities, loads, fluid viscosity, fluid flash point, etc) are not sufficient to produce hydrodynamic contact. And as for rolling element bearings, they are designed to produce hydrodynamic contacts whenever possible, in order to maximize load capacity and fatigue life.
The term hydrodynamic implies a lubrication fluid film condition at the point of contact that is hydrodynamic in nature, as opposed to mixed or boundary conditions. This implies that the bearing elements are fully separated by a load bearing, dynamically generated fluid film.
In theory, any material could be used for a hydrodynamic journal bearing, even SiN. While SiN or SiC materials would have benefits such as high compressive fatigue strength, corrosion resistance, and high temperature capability, these characteristics are normally not required for most oil lubed journal bearing designs. The limiting factor with oil lubed journal bearings usually becomes when the temperature rise within the local oil film exceeds the flash point of the lubricant. For even the best oils, this is usually 450degF or less. And thus most metals are still OK with these temp limits.
SiN is preferred over SiC for rolling element bearings because it seems to have better facture toughness.
Hope that helps.
Terry
