Wear is the expulsion or debase of the material from the surface of a strong body therefore of mechanical activity or rubbing of the body.Wear of Engine heading is the debase of the bearing material from its surface because of sliding grinding between the crankshaft and bearing.
Wear may consolidate impacts of different concoction and physical procedures continuing amid the erosion between two checking materials: small scale cutting, Plastic distortion, liquefying, splitting, smaller scale furrowing, crack, welding, substance communication.
The mechanisms of wear:
- Abrasive wear
- Erosive wear
- Adhesive wear
- Fatigue wear
- Corrosive wear
Abrasive wear occurs when a harder material is rubbing or machined against a softer material.If there are only two rubbing parts contacted in the friction process the wear is called two body wear.In this case the wear of the softer material is caused by the roughness on the harder surface.
If the wear is caused by a hard particle (grit) trapped inside the rubbing surfaces it is called three body wear. The particle may be either free or partially attached into any one of the mating materials.
In the micro-level abrasive action results in one of the following wear modes:
Ploughing. The material is shifted to the sides of the wear groove. The material will not remove from the surface.
Cutting. A chip forms in front of the cutting grit. The material is removed (lost) from the surface in the volume equal to the volume of the groove.
Cracking (brittle fracture). The material cracks in the subsurface regions surrounded by the wear groove. The volume of the lost material is higher than the volume of the wear track.
Adhesion wear is a result of micro-junctions produced by welding between the opposing grits on the rubbing surfaces of the counterbodies. The load applied to the contacting asperities is so high that they deform to each other forming micro-joints.Adhesion wear is a result of micro-junctions produced by welding between the opposing grits on the rubbing surfaces of the counterbodies. The load applied to the contacting asperities is so high that they deform to each other forming micro-joints.The motion of the rubbing counterbodies result in crack of the micro-joints. The welded grit ruptures in the non-deformed (non-cold worked) regions.Thus some of the material is moved by its counterbody. This effect is called galling or scuffing.When areas of the rubbing surfaces are joined during the friction a compatibility seizure of one of the bodies by the counterbody may occur.
The factors decreasing adhesive wear:
- Harder rubbing materials
- Lower load.
- Contaminated rubbing surfaces.
- Presence of solid lubricants.
- Anti-wear additives in oil.
- Presence of a lubrication oil.
Fatigue wear of a material is caused by a cycle loading during friction. Fatigue occurs if the applied load is more than the fatigue strength of the material.Fatigue cracks start at the material surface and spread to the near surface regions. The cracks may connect to each other resulting in separation of the material pieces.One of the types of fatigue wear is fretting wear caused by cycling sliding of two surfaces each other with a small oscillation. The friction force produces alternating compression or tension stresses, which result in surface fatigue.Fatigue of an engine bearing may result in the propagation of the cracks up to the layer and total removal of the overlay.
Wear may be increased by corrosion (oxidation) of the rubbing surfaces.Increase in temperature and removal of the protecting oxide films from the surface during the friction increase the rate the oxidation process. Friction provides continuous removal of the oxide film followed by continuous formation of new film oxide.Hard oxide particles removed from the surface and trapped inside the sliding and rolling surfaces rationally increase the wear rate by three body abrasive wear mechanism.
Erosive wear is caused by degradation of particles (solid, liquid or gaseous), which remove fragments of materials from the surface due to effect momentum.Erosive wear of Engine bearings may be caused due to cavitation in the lubrication oil. The cavitation bubbles may form when the oil exits from the convergent gap between the journal and bearing surfaces. The oil pressure rapidly reduce providing conditions for voids formation (the pressure is lower than the oil vapor pressure). The bubbles (voids) then collapse producing a shock wave, which removes particles of the bearing material from the bearing.