Micro pitting

Micro pitting is a fatigue failure of the surface of a material commonly seen in rolling bearings and gears.<ref name=":0">Template:Cite journal</ref> It is also known as grey staining, micro spalling or frosting.

Pitting and micropitting

The difference between pitting and micropitting is the size of the pits after surface fatigue. Pits formed by micropitting are approximately 10–20 μm in depth, and to the unaided eye, micropitting appears dull, etched or stained, with patches of gray.<ref>Template:Cite journal</ref> Normal pitting creates larger and more visible pits. Micropits are originated from the local contact of asperities produced by improper lubrication.

Causes

In a normal bearing the surfaces are separated by a layer of oil, this is known 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 occurs only under poor EHD lubrication conditions and although it can affect all types of gears, it can be particularly troublesome in heavily loaded gears with hardened teeth.<ref>Template:Cite journal</ref><ref name=":0" />

A surface with a deep scratch might break exactly at the scratch if stress is applied. One can imagine that the surface roughness is a composite of many very small scratches. So high surface roughness decreases the stability on heavy stressed parts. To get a good overview of the surface an areal scan (Surface metrology) gives more information that a measurement along a single profile (profileometer). To quantify the surface roughness the ISO 25178 can be used.

Calculation

ISO/TS 6336-22 contains a method for a calculation of risk of micropitting in gear sets.<ref>Template:Cite book</ref><ref name=":0" />

See also

• Pitting corrosion
• Corrosion

References

<references /> Template:Corrosion-stub Template:Materials-sci-stub