In general, wear is mechanically induced surface damage that results in the progressive removal of material due to relative motion between that surface and a contacting substance or substances. A contacting substance may consist of another surface, a fluid, or hard, abrasive particles contained in some form of fluid or suspension, such as a lubricant. As is with friction, the presence of wear can be either good or bad. Productive, controlled wear can be found in processes like machining, cutting, grinding, and polishing. However, in most technological applications, the occurrence of wear is highly undesirable. It is an enormously expensive problem since it leads to the deterioration or failure of components. In terms of safety, it is often not as serious (or as sudden) as a fracture, and this is because the wear is usually anticipated.
Certain material characteristics such as hardness, carbide type, and volume percent can have a decided impact on the wear resistance of a material in a given application. Wear, like corrosion, has multiple types and subtypes that are predictable to some extent and are rather difficult to test reliably and evaluate in the lab or service.
Abrasive Wear
Abrasive wear is defined as the loss of material due to hard particles or hard protuberances that are forced against and move along a solid surface. It occurs when a hard rough surface slides across a softer surface. This mechanism is sometimes referred to as grinding wear. The harder material may be one of the rubbing surfaces or hard particles that have found their way between the mating surfaces. These may be ‘foreign’ particles or particles resulting from adhesive or delamination wear. Abrasion mainly involves microscale cutting and plowing processes. How an asperity slides over a surface determines the nature and intensity of abrasive wear. There are two basic modes of abrasive wear:
- Two-body abrasive wear. Two-body wear occurs when the grits or hard particles remove material from the opposite surface. The common analogy is that material is removed or displaced by a cutting or plowing operation.
- Three-body abrasive wear. Three-body wear occurs when the particles are not constrained and are free to roll and slide down a surface. The contact environment determines whether the wear is classified as open or closed. An open contact environment occurs when the surfaces are sufficiently displaced to be independent of one another.
There are many different strategies for mitigating abrasive wear, but the general rule for materials selection is that the harder, the better. Materials containing a relatively large percentage of hard, wear-resistant alloy carbides, such as selected tools and high-speed steels.
Adhesive Wear
Adhesive wear originated through bonding asperities or microscopic high points (surface roughness) between two sliding materials. When a peak from one surface comes into contact with a peak from the other surface, instantaneous micro welding may take place due to the heat generated by the resulting friction. This results in detachment or material transfer from one surface to the other. For adhesive wear to occur, the surfaces must be in intimate contact. This may cause unwanted displacement and attachment of wear debris and material compounds from one surface to another. Adhesive wear can lead to an increase in roughness and the creation of protrusions (i.e., lumps) above the original surface. Surfaces held apart by lubricating films, oxide films, etc., reduce the tendency for adhesion. In some engineering applications, surfaces slide in the air without lubricant, and the resulting wear is termed dry sliding.
Adhesive wear depends on the materials involved, the degree of lubrication provided, and the environment. Adequate lubrication allows smooth, continuous operation of machine elements, reduces wear rate, and prevents excessive stresses or seizures at bearings. When lubrication breaks down, components can rub destructively against each other, causing heat, local welding, destructive damage, and failure. For instance, austenitic stainless steels (e.g., AISI 304) sliding against themselves are very likely to transfer material and gall, resulting in severe surface damage. Other materials prone to adhesive wear include titanium, nickel, and zirconium. On the other hand, aluminium bronze has found increasing recognition for a wide variety of applications requiring resistance to mechanical wear. Its wear resistance is based on the transfer from the softer metal (aluminium bronze) to the harder metal (steel), forming a thin layer of softer metal on the harder metal.
For example, the main function of motor oil is to reduce friction and wear on moving parts (to reduce adhesive wear) and to clean the engine from sludge. At the same time, a filter is designed to remove contaminants and abrasive particles from engine oil.