Adhesive wear occurs when components are moved against another, causing their surface layer particles to be removed. This phenomenon arises when there’s no material in between components or when the material is worn out due to high contact pressure. Adhesion is derived from the Latin word adhaerere and describes the bonding of dissimilar particles or surfaces to one another. There are a number different theories to the emergence of adhesion which can be roughly divided into mechanical and specific mechanisms. These theories do not necessarily contradict each other, more so than explain different areas of adhesion.
Mechanisms of adhesion
The mechanical mechanism theory of adhesion is the oldest. Adhesive materials fill the pores of the surface, interlocking different particles together. One example is glue and wood.
There are four different specific theories for specific mechanisms of adhesion.
- The first one is chemical adhesion. Materials form chemical compounds at their joining point. They can form covalent, ionic or hydrogen bonds and can range from strong (covalent, ionic) to weak (hydrogen).
- Another theory of adhesion is based on electrostatic mechanisms. Materials pass electrons to form an electrical bond at the joint. It creates a force between two materials as long as there are charge carriers, such as electrons and ions.
- The third theory of adhesion is dispersive. It is based on the van der Wall forces which postulate that there’s attraction between two molecules as they each have regions of slight positive and negative charge. En masse, these molecules generate enough force to form adhesion.
- The diffusive theory occurs when two matters have an affinity to bond. Proper motion of molecules is activated by heat, causing them to be diffused into one another. However, metals cannot merge at the joint by diffusion due to metallic bonding.
Prevention of adhesive wear
Adhesive wear arises when two metals rub together, causing friction and wear. Adding high contact pressure, the touching points of the materials can be deformed and be cold welded together. Cavities can also emerge as bonds are broken and particles rub off. Adhesive wear can occur anyplace where two metals are in contact with each other: tools, valves, fittings and more. It can result in many different problems, including scuffing, tool breakage and freezing/seizing.
To prevent adhesive wear, there are several measures that can be taken. The hardness of the materials can be increased by case-hardening or tempering. One way to achieve this is by using Borocoat® diffusion layers as they increase hardness levels to up to 2800HV, depending on the base material. Borocoat® does not only prevent wear, but also improves adhesion, increasing durability and performance. Another method that can be taken towards preventing adhesion wear is by choosing highly incompatible materials, making cold-welding highly impossible.