Micro pitting

Micro pitting is a fatigue failure of the surface of a material commonly seen in rolling bearings and gears. It is also known as grey staining, micro spalling or frosting

Mechanism of micro pitting

Surface asperities on the rolling element interact with those on the raceway (in rolling bearings, in gears the gear tooth faces come together) causing plastic deformation. The cyclic deformation (Fatigue) causes cracks to be initiated and grow into the bulk of the material. The cracks branch and some return to the surface allowing small pieces of material to break away. The broken material can be smaller than 1 micrometre across.

Causes of micro pitting

In a normal bearing the surfaces are separated by a layer of oil, this is know as Elastohydrodynamic (EHD) lubrication. If the thickness of the EHD film is of the same order of magnitude as the surface roughness, the surface topography is able to interact and cause micro pitting. A thin EHD film may be caused by excess load or temperature, a lower oil viscosity than is required, low speed or water in the oil. Water in the oil can make micro pitting worse by causing hydrogen embrittlement of the surface. Micro pitting only occurs under poor EHD lubrication conditions.