Galling

This article is about adhesion between metal surfaces. For the botanical concept, see bark-galling.
Galled threads on an NPT fitting. (zoom in on threads for better view)
An electron microscope image shows transferred sheet-material accumulated on a tool surface during sliding contact under controlled laboratory conditions. The outgrowth of material or localized, roughening and creation of protrusions on the tool surface is commonly referred to as a lump.
The damage on the metal sheet, wear mode, or characteristic pattern shows no breakthrough of the oxide surface layer, which indicates a small amount of adhesive material transfer and flattening damage of the sheet's surface. This is the first stage of material transfer and galling build-up.
The damage on the metal sheet illustrates continuous lines or stripes, indicating a breakthrough of the oxide surface-layer.
The damage on the metal sheet or characteristic pattern illustrates an "uneven surface," a change in the sheet material's plastic behavior and involves a larger deformed volume compared to mere flattening of the surface oxides.

Galling is a form of wear caused by adhesion between sliding surfaces. When a material galls, some of it is pulled with the contacting surface, especially if there is a large amount of force compressing the surfaces together.[1] Galling is caused by a combination of friction and adhesion between the surfaces, followed by slipping and tearing of crystal structure beneath the surface.[2] This will generally leave some material stuck or even friction welded to the adjacent surface, whereas the galled material may appear gouged with balled-up or torn lumps of material stuck to its surface.

Galling is most commonly found in metal surfaces that are in sliding contact with each other. It is especially common where there is inadequate lubrication between the surfaces. However, certain metals will generally be more prone to galling, due to the atomic structure of their crystals. For example, aluminium is a metal that will gall very easily, whereas annealed (softened) steel is slightly more resistant to galling. Steel that is fully hardened is very resistant to galling.

Galling is a common problem in most applications where metals slide in contact with other metals. This can happen regardless of whether the metals are the same or different. Alloys such as brass and bronze are often chosen for bearings, bushings, and other sliding applications because of their resistance to galling, as well as other forms of mechanical abrasion.

  1. ^ Budinksi, Kenneth; Budinski, Steven (2015). "Interpretation of galling tests". Wear. 332 (1): 1185–1192. doi:10.1016/j.wear.2015.01.022.
  2. ^ Dohda, Kuniaki (2021). "Galling phenomena in metal forming". Friction. 9 (4): 665–685. doi:10.1007/s40544-020-0430-z. S2CID 228815215.