Galling

Galling is a severe form of adhesive wear that occurs when metal surfaces slide against each other under high contact pressures, often with insufficient lubrication. It involves adhesion between microscopic asperities, leading to material transfer, local tearing, and sometimes micro-welding of the contact regions. The result can be roughened surfaces, raised ridges, transfer of material to the counterface, and in extreme cases, seizure or galling of the parts.

The underlying mechanism is adhesive in nature: metals at the contact nodes bond during sliding, particularly when the surfaces are similar in composition or have high chemical affinity. oxide and friction layers can influence adhesion, and diffusion of atoms at elevated temperatures can strengthen the adhesive bonds. Galling is more likely at high normal pressures, low lubricant film effectiveness, and elevated temperatures, which accelerate bonding and material transfer.

Galling commonly occurs in applications involving metal forming, threading, fasteners, and other sliding contacts such as bearings or gears. It is a concern for stainless steels, aluminum alloys, titanium, and other metals that can adhere strongly under load. Symptoms include roughened contact surfaces, visible material transfer, and, if unchecked, seizure or cold welding that prevents relative movement.

Prevention and mitigation focus on reducing adhesion and contact pressure, improving lubrication, and altering material combinations. Strategies include using appropriate lubricants or solid lubricants, selecting harder counterfaces, applying anti-galling coatings or platings, choosing dissimilar metal pairings or coatings, and ensuring proper fit and surface finish. Regular inspection and controlled disassembly procedures also help limit galling-related failures.