Editing Micro pitting

{{Short description|Fatigue failure of the surface of a material insufficiently lubricated}}

{{More citations needed|date=January 2025}}

'''Micro pitting''' is a [[fatigue (material)|fatigue failure]] of the surface of a material commonly seen in rolling [[Bearing (mechanical)|bearings]] and [[gears]].<ref name=":0">{{Cite journal |last1=Olson |first1=Robin |last2=Michaud |first2=Mark |last3=Keller |first3=Jonathan |date=2021 |title=Case Study of ISO/TS 6336-22 Micropitting Calculation |url=https://www.geartechnology.com/ext/resources/issues/0321x/micropitting-calculation.pdf |journal=Gear Technology |issue=March/April 2021}}</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 [[material fatigue|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>{{Cite journal |last=Errichello |first=R. L. |date=2012 |title=Morphology of Micropitting |url=https://www.geartechnology.com/ext/resources/issues/1112x/morphology_of_micropitting.pdf |journal=Gear Technology |pages=74–81 |via=}}</ref> Normal pitting creates larger and more visible pits. Micropits are originated from the local contact of [[Asperity (material science)|asperities]] produced by improper [[lubrication]].

==Causes==
In a normal [[bearing (mechanical)|bearing]] the surfaces are separated by a layer of [[lubricant|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>{{Cite journal |last1=Snidle |first1=R W |last2=Evans |first2=H P |last3=Alanou |first3=M P |last4=Holmes |first4=M J A |date=2003 |title=Understanding Scuffing and Micropitting of Gears |url=https://www.sto.nato.int/publications/STO%20Meeting%20Proceedings/RTO-MP-AVT-109/MP-AVT-109-14.pdf |journal=Rto-Mp-Avt-109 |publisher=NATO}}</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>{{Cite book |url=https://www.iso.org/standard/73528.html |title=ISO/TS 6336-22:2018 Calculation of load capacity of spur and helical gears Part 22: Calculation of micropitting load capacity |date=2018 |publisher=International Standards Organisation}}</ref><ref name=":0" /> 

==See also==
*[[Pitting corrosion]]
*[[Corrosion]]

== References ==
<references />

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[[Category:Corrosion]]
[[Category:Materials degradation]]
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