Definition - What does Galling mean?
Galling is a form of wear caused by adhesion between sliding surfaces. It is a severe form of adhesive wear caused by the transfer of material between metallic sliding surfaces under load, especially if there is poor lubrication.
Galling is a common problem in sheet metal formation, bearings and pistons in engines, hydraulic cylinders, air motors, and many other industrial operations.
Unlike other forms of wear, galling is usually not a gradual process, but occurs quickly and spreads rapidly as the raised lumps induce more galling.
Galling also known as scoring.
Corrosionpedia explains Galling
Galling is caused by a combination of friction and adhesion between surfaces, followed by slipping and tearing of the crystal structure beneath the surface. This generally leaves some material stuck or even friction welded to the adjacent surface, while the galled material may appear gouged with balled-up or torn lumps of material stuck to its surface. Abrasive or corrosive media can increase the risk of galling by removing the passive oxide film and making the two metal surfaces more active.
Galling often occurs in high-load, low-speed applications, but it also occurs in high-speed applications with very little load and results in significant increase of friction and damage to the mating surfaces.
Certain metals are more prone to galling, due to the atomic structure of their crystals. For example, aluminum is a metal which galls very easily, whereas annealed (softened) steel is slightly more resistant to galling. Steel that is fully hardened is very resistant to galling.
A common solution for galling is to use two dissimilar materials, as they are less likely to suffer from adhesive wear. Using an anti-galling surface coating on one of the mating parts can help solve galling problems. They also provide wear and corrosion resistance in addition to their anti-galling properties.
Galling is distinctive from gouging or scratching in that galling involves the visible transfer of material as it is adhesively pulled (mechanically spalled) from one surface, leaving it stuck to the other in the form of a raised lump.