What Is Abrasive Wear?
Abrasive wear is a wear mechanism that results in a loss of material from a surface due to hard particles. This type of wear is also known as abrasion. Abrasive wear can be categorised into two, two-body abrasive wear and three-body abrasive wear.
Two body abrasive wear occurs when one of the contacting surfaces is harder than the other or when hard particles are attached to the opposing material.
Three-body abrasive wear results from the presence of hard particle between the two sliding surfaces, free to roll and move. Although not all abrasion mechanisms directly remove surface material, they all have the common effect of increasing the surface roughness due to displacing material.
In two-body abrasive wear, it is clear that the harder particles will slide across the contacting surface. However, in three-body abrasive wear, the free particles can either roll or slide. From this, it is best to describe the abrasion mechanisms rolling and sliding abrasion.
The hardness of the materials or free particles influences the wear rate of the abrasion mechanism. Particles with a higher hardness produce a higher wear rate compared to their softer counterparts. In turn, this results in a greater loss of material. Therefore, the ratio between the hardness of the two contacting surfaces or particles is a good determinant for the wear rate.
Hard particles can be generated by local oxidation or from sliding wear. Upon the generation of these particles, further damage via abrasion will occur. Sliding abrasion can simply occur due to a difference in surface roughness between the contacting materials.
Abrasive Wear Mechanisms
A model commonly used to help visualise abrasion is the removal of material via plastic deformation, which is ploughing. The volume of material removed is related to the normal load, the sliding distance, and the shape of the particle. During ploughing, the material is displaced to the side of the grooves and below the contact, although only a little amount or no material is directly removed.
Another abrasive wear mechanism is cutting. During cutting, direct material removal occurs, generating wear particles and wear debris. As the name suggests, this type of wear mechanism is related to machining. However, not all material displaced is directly removed as wear debris as some material will be displaced to the sides of the ridges.
Fragmentation, as the name suggests, leads to fragments of materials separated from the surface. The cause of fragmentation is by cutting and local fractures in the material. Development of local fractures and lead to other mechanisms such as spalling. Abrasive wear by brittle fracture is responsible for this. At sufficient loads, a blunt contacting surface under high stress can result in subsurface cracking. At the local points of contact, high loading causes local plastic deformation, resulting in the generation of cracks with a different geometry, leading to wear.
For two-body (sliding) abrasion of metals, μ (coefficient of friction) lies typically between 0.4 and 1, and for three-body (rolling), it is commonly lower: 0.2–0.5.
Effects of Lubricant on Abrasive Wear
The effects of lubricants on abrasion is not as significant as the effects seen on sliding wear. Using a lubricant during sliding wear will reduce the coefficient of friction and hence wear rate to a large extend. However, abrasive particles are frequently larger than the lubricant film meaning the lubricant will not prevent contact between the materials. Therefore, lubricants effects on abrasion will not be as significant as those seen on sliding wear.
Typical Abrasion Wear Rates
The wear rate between sliding materials is defined by the dimensionless coefficient K. The magnitude of K represents the significance of the severity of the wear rate. In two-body (sliding) abrasion, the value K will typically be within the range 5×10-3 – 50×10-3. On the other hand, for three-body abrasion, K will lie between 0.5×10-3 – 5×10-3.
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