There are several forms of  metallic wear. In each case the process can be defined generally as the unwanted removal of material from a surface due to mechanical action. This is in contrast to desirable manufacturing processes that occur by similar mechanisms such as machining, grinding or shot blasting.  The most common harmful forms of wear are abrasion, erosion, adhesion and fretting. 

Abrasive wear entails cutting or gouging of the surface of a solid metal when in contact with partially constrained but slowly moving, hard non-metallic or metallic particles from an adjacent, moving surface. The particles are confined between two surfaces being pressed together. Generally no lubricants are used. Heat due to friction is always created. Wear debris from the solid, softer metal is destroyed in the process. Abrasion often occurs in ore or earth moving equipment.

Erosion is similar to abrasive wear but the shearing force that degrades the metal surface is provided by free-moving solid particles that impact the wearing surface as they are conveyed by a flowing fluid – either liquid or gas. Larger, harder particles cause more damage than smaller, softer particles. If the fluid is corrosive to the wear surface then another mechanism – erosion-corrosion (E-C) – occurs that combines both wear and corrosion so that the result is worse than either acting alone. Erosion and/or E-C often occur on pump impellers, marine vessel propellers and in certain pressure let down valves. 

Abrasion, erosion and E-C are typically controlled by selection of a sufficiently resistant material for the given application. This material’ s key parameter is its high overall hardness or its surface hardness. Options include high hardness steels, steels with tungsten carbide particles added, high manganese steels, certain austenitic stainless steels and cobalt-based alloys. A case hardened surface layer can also be effective. In the case of E-C, the corrosion resistance of the selected material for the given service conditions is usually more important than its abrasion resistance although both affect results. 

Adhesive wear is the result of micro-welding of the microscopic high points, called asperities, on two metal surfaces sliding on one another without a sufficient lubricant.  As this continues asperities break off and are transferred to the softer metal while frictional heat causes interfacial temperature to increase. Failure is relatively common due to the many dynamic machine applications where a proper lubrication system is essential.  Thus the primary control measures are selecting a proper lube, assuring it is cooled to control interfacial temperature and providing a filter for most wear debris generated. 

Fretting is a form of adhesion that occurs due to micro-motion between two contacting metal surfaces that are not expected to move during a given period. Generally, no lubricant is used. Powered wear debris is generated that stays at the interface. This process often occurs while mechanical devices are being stored or transported and subject to external vibrations that create very small relative movements. Surface damage creates stress risers that act when the equipment is put into service. Premature failure by fatigue can result from this assumed, “static” exposure. Control can be difficult but some actions include temporarily inserting non-metallic materials between contacting surfaces or increasing the hardness or shot peening both surfaces to minimize the chance of in-service fatigue crack initiation. It is important not to overlook this potential threat.

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