Which phenomenon is caused by mating tooth surfaces welding together and ripping apart?

• A. Pitting
• B. Galling
• C. Wear
• D. Spalling

Answer: (D)

Galling is the phenomenon at play here. It’s a form of adhesion wear that happens when two metal surfaces slide against each other under high pressure and not enough lubricant. Tiny peaks on the mating surfaces can weld together temporarily (a cold weld) due to the intense contact and friction. When the parts continue to move, those welded areas are torn apart, pulling material away and often transferring metal to the opposite surface. That combination of brief bonding and subsequent tearing is what defines galling.

This differs from pitting, which is tiny pits from surface fatigue; wear, a broad term for material loss due to friction; and spalling, which is chunks breaking off from a surface due to fatigue. Spalling involves fragmentation from fatigue, not the adhesive bonding and tearing characteristic of galling.