Q: What are the definitions of wear categories? A: ABRASION Hard particles or protuberances (e.g., grind marks) pressed against a surface. They only have to be harder than the surface receiving wear damage. Produces cutting or shearing actions. EROSION Loss of material from a surface to impinging fluid stream, or a fluid stream with solid particles If fluids react with surface, erosion/corrosion = accelerated process. Can be intentional erosion = sand blasting, grit blast cleaning. Can be unintentional erosion = ash and flue gas exiting a combustor or passing over heat exchanger tubes. Cavitation - imploding bubbles. Low stress, high speed particles impinging on a surface create wear chips. Can be minimized by increasing hardness of the surface through a wear resistant coating. ADHESION Loss of material from surface asperities in relative motion, started by localized bonding due to very high local stresses (and possibly temperatures). Transfer of material may be permanent. Asperities may fracture, forming wear debris -- which may oxidize -- changing the wear category back to abrasion. SURFACE FATIGUE Fracture of material from a surface caused by cyclic stresses produced by repeated rolling or sliding over that surface. Examples of surface fatigue would be bearing tracks and reciprocating shafts in sleeves.