Factors Influencing Wear

Lubrication is a significant contributing factor to wear resistance, specifically in adhesive wear. In “thick-film” lubrication, adequately thick lubricating film completely eliminates metallic contact, and a metallic wear is decrease to negligible amount. This is however, the ideal condition, and more frequently “boundary lubrication” occurs. It is the condition of intermittent metallic contact that occurs while the oil film might not be continuously maintained. The amount of wear based upon speed, pressure, nature of the mating surfaces and viscosity of the residual oil film. Though, in several cases, lubrication is impractical or is not required, as in braking.

Although real melting of the metal occurs only in rare instances, the influence of heat produced by dry wear can reduce wear resistance in many ways. It might temper hardened structures, cause phase changes that enhance hardness and brittleness, and decrease mechanical properties. It might accelerate corrosion reactions also.

The dominant frictional factor for metallic materials is welding while two clean surfaces of the same metal actually touch each other, they shall weld together because of atomic attraction. If by friction, enough pressure is applied to breakthrough any residual separating material such like dirt, oil or absorbed moisture and the surfaces are in enough contact to have elastic or plastic deformation occur, after that seizing or welding takes place. The softening of metals by high temperature enhances the ease of plastic deformation and facilitates welding. Seizing might cause complete stoppage, or if relative motion is not prevented, pieces of the opposite face might be pulled out. The resultant projection then might cause scoring, galling, & excessive local wear.
Many methods might be used to minimise the danger of seizing. One is to utilize thin layers of hard surfacing material. The utilization of at least one metal that forms some sort of lubricating film or thin, sulfide , tightly adherent oxide, or phosphate coating is frequently helpful. Aluminium oxide is extremely effective in preventing welding. For parts that operate under such high pressures that elastic deformation permits intimate contact, the best preventive method is a lubricant that combines along with the metal surface to form a “corrosion” product of sufficient strength to keep the surfaces separated. The use of materials of high elastic limit shall minimise seizure because of intimate contact by avoiding plastic deformation.

Impact is a factor in wear, since the suddenly applied load may cause plastic flow and a change in shape. Proper design must provide for higher compressive yield stress than the compressive stress produced by impact. The design should also provide adequate support in order that subsurface flow does not occur.

Fatigue failure is necessarily included in a discussion of wear since it is a gradual deterioration due to use. It should be remembered that fatigue cracks initiate on surface almost in all cases except some very particular situations. The cracks initiate onto the surface because of following cause:

1.    The surface is weaker inherently than inside material.
2.    All shaping, forming and cutting processes are carrying out on the surface.
3.    The surface is exposed to hostile atmosphere that causing corrosion.
4.    Material is handled through surface.
5.    Some of kind of stress concentration like sharp radiused shoulders, notch, and fillets etc. are present on surface to fulfill some design requirement.
6.    Almost all types of loads (axial, bending, torsion or combination thereof) induce maximum stress on the surface. All these factors combined with wear will eventually result in failure.

Proper design to eliminate stress concentrations at notches and sharp angles will increase fatigue strength. As fatigue failures are always because of tensile stress, residual compressive stress at the surface shall provide additional protection. This might be accomplished by case hardening, such like carburizing & by shot peening.