Wear is an erosion process that occurs through surface contact. There are many environmental factors that can affect the wear rate of a material in operation such as stress loading, temperature, the type of surface contact, as well as the type of material.
Wear is an erosion process that occurs through surface contact. Material is either removed or deformed from one surface as a result of the other surface’s mechanical action. There are many environmental factors that can affect the wear rate of a material in operation such as stress loading, temperature, the type of surface contact, as well as the type of material. Tribology is the study of the wear process, including adhesive wear, abrasive wear, surface fatigue, fretting wear, erosive wear, and corrosion and oxidation wear.
Categories of Wear
The wear process can be categorized into three stages: primary, secondary, and tertiary. The primary stage is also known as the early run-in period and during that stage the surfaces adapt to each other and the rate of wear can vary widely between low and high extremes. The secondary stage is also known as the mid-age process and during this stage the rate of wear is steady. Most of the operational life of the material occurs during this stage, but this stage can be shortened in severe environments with higher strain rates, temperature, or sliding velocities. The tertiary stage is also known as the old-age period and during this stage, the material undergoes rapid failure due to a high rate of wear.
Adhesive wear occurs when surfaces experience frictional contact. Generally, as two surfaces move against one another, wear debris will displace from one surface and reattach to the other. The degree of surface roughness determines the severity of the material movement in the plastic zone that forms between the two surfaces during wear. Because the energy density of the system must increase in order for the material to flow, the process requires a high contact pressure to occur.
In contrast, abrasive wear occurs when a harder surface is moving relative to a softer surface. In this case, only the softer surface will lost material due to roughness of the harder surface. Abrasive wear can be categorized according to its mode: either two-body or three-body abrasive wear. Two-body wear happens when particles on the harder surface, or grits, remove material from the softer surface. Three-body wear happens when the particles are not constrained and roll or slide down a surface freely.
Wear Fatigue and Wear Failure
Although adhesive and abrasive wear are the most common wear mechanisms, there are a variety of other wear mechanisms seen in certain situations. In surface fatigue, the material is weakened by cyclic loading, and particles are detached from the surface by the growth of superficial or subsurface microcracks. These particles can then engage in wear as the surfaces move relative to one another. Similarly, fretting wear is caused by the cyclical rubbing of two surfaces. Over time, the rubbing, or fretting, removes particles from the surfaces which can then contribute further to wear. In contrast, erosive wear involves a short sliding motion and develops within a very short time interval. During erosive wear, the particles impact the material’s surface and gradually remove material through repeated deformations. When abrasive particles are transported, this type of wear is especially likely to be found in piping systems. Finally, corrosion and oxidation wear occurs when a chemical reaction between the worn material and corroding medium leads to a loss of material on the worn surface.
Wear can occur anytime two surfaces are in contact and moving relative to one another. Wear can be reduced through the use of lubricants that protect both surfaces. It is important to identify the types of wear that can be present in any given situation and then to identify the type of lubricant that will be the most effective against that wear mechanism.