Wear
Wear is the damaging, gradual removal or deformation of material at solid surfaces.
[4] The wear rate is affected by factors such as type of loading (e.g., impact, static, dynamic), type of motion (e.g., sliding, rolling), temperature, and lubrication, in particular by the process of deposition and wearing out of the boundary lubrication layer.
Types of wear are identified by relative motion, the nature of disturbance at the worn surface or "mechanism", and whether it effects a self regenerative or base layer.
[citation needed] The asperities or microscopic high points (surface roughness) found on each surface affect the severity of how fragments of oxides are pulled off and added to the other surface, partly due to strong adhesive forces between atoms,[9] but also due to accumulation of energy in the plastic zone between the asperities during relative motion.
However, oxidation films, lubricants and contaminants naturally occurring generally suppress adhesion,[10] and spontaneous exothermic chemical reactions between surfaces generally produce a substance with low energy status in the absorbed species.
[11] Adhesive wear can lead to an increase in roughness and the creation of protrusions (i.e., lumps) above the original surface.
In industrial manufacturing, this is referred to as galling, which eventually breaches the oxidized surface layer and connects to the underlying bulk material, enhancing the possibility for a stronger adhesion[11] and plastic flow around the lump.
Two-body wear occurs when the grits or hard particles remove material from the opposite surface.
The common analogy is that of material being removed or displaced by a cutting or plowing operation.
Three-body wear occurs when the particles are not constrained, and are free to roll and slide down a surface.
An open contact environment occurs when the surfaces are sufficiently displaced to be independent of one another There are a number of factors which influence abrasive wear and hence the manner of material removal.
Three commonly identified mechanisms of abrasive wear are:[citation needed] Plowing occurs when material is displaced to the side, away from the wear particles, resulting in the formation of grooves that do not involve direct material removal.
The displaced material forms ridges adjacent to grooves, which may be removed by subsequent passage of abrasive particles.
These cracks then freely propagate locally around the wear groove, resulting in additional material removal by spalling.
An associated problem occurs when the small particles removed by wear are oxidized in air.
Erosive wear is caused by the impact of particles of solid or liquid against the surface of an object.
[10][16] The impacting particles gradually remove material from the surface through repeated deformations and cutting actions.
Due to the nature of the conveying process, piping systems are prone to wear when abrasive particles have to be transported.
The material characteristics of the particles, such as their shape, hardness, impact velocity and impingement angle are primary factors along with the properties of the surface being eroded.
[19] For ductile materials, the maximum wear rate is found when the impingement angle is approximately 30°, whilst for non-ductile materials the maximum wear rate occurs when the impingement angle is normal to the surface.
[19] A detailed theoretical analysis of dependency of the erosive wear on the inclination angle and material properties is provided in.
The fundamental cause are chemical reactions between the worn material and the corroding medium.
[21] Wear caused by a synergistic action of tribological stresses and corrosion is also called tribocorrosion.
[6] Under nominal operation conditions, the wear rate normally changes in three different stages:[citation needed] The wear rate is strongly influenced by the operating conditions and the formation of tribofilms.
The secondary stage is shortened with increasing severity of environmental conditions, such as high temperatures, strain rates and stresses.
[citation needed] In explicit wear tests simulating industrial conditions between metallic surfaces, there are no clear chronological distinction between different wear-stages due to big overlaps and symbiotic relations between various friction mechanisms.
Surface engineering and treatments are used to minimize wear and extend the components working life.
[1][22] Several standard test methods exist for different types of wear to determine the amount of material removal during a specified time period under well-defined conditions.
ASTM International Committee G-2 standardizes wear testing for specific applications, which are periodically updated.
To gain further insights into the nature of the particles, chemical (such as XRF, ICP-OES), structural (such as ferrography) or optical analysis (such as light microscopy) can be performed.
