Most of us are familiar with the concepts of wear and tear, but the mechanisms of action underlying these phenomena can be surprisingly complex. Wear is defined as the loss of material from a solid’s surface due to mechanical action exerted by some other solid. It is such a universal process that only organic materials are immune since they can self-replicate and essentially restore surface fatigue. Therefore, every branch of materials science must contend with one or more different wear mechanisms.
Industry applications of wear testing cover a wide range of materials, from engineering plastics through to advanced ceramic materials like silicon carbide (SiC) and silicon nitride (Si3N4). Ceramic lined components, and systems designed for aggressive material handling are also routinely assessed for performance under dynamic wear conditions (i.e., for excellent wear resistance at high temperatures).
But what exactly is wear and how are the different modes defined?
Understanding the Physics of Wearing
Though there are four overarching wear mechanisms, each is linked to frictional processes. Generally, frictional processes occur between contact surfaces which are in relative motion, causing the gradual loss of substance due to mechanical interactions and local deterioration at the interface. The main physical phenomena resulting in friction forces are shearing and ploughing.
- Shearing is defined as the opposing directional forces acting along the surface of a material; occurring when two contacting surfaces slide across one another.
- Ploughing, or plowing, occurs when the asperities of a harder surface cut into the surface along a softer one.
These are interlinked but separate phenomena which are both governed by the coefficient of friction (µ); a measure of the amount of friction between two surfaces.
What are the Different Wear Mechanisms?
There are numerous factors that contribute to the wear resistance of bulk materials, and numerous different modes of wearing. However, these can largely be categorised as one of three main mechanisms:
- Abrasive wear: The main mechanism of action used in industrial wear applications, abrasion occurs when hard asperities or protuberances on one surface are forced against another, and a velocity is applied. Estimates suggest abrasive wear is encountered in 36–58% of cases in industrial wear applications.
- Adhesive wear: Caused by localised molecular bonding, adhesive wear is characterised by particle transfer mechanisms between surfaces, and often occurs between poorly lubricated surfaces. Adhesive wear is less common than abrasion; being found in 23–45% of cases.
- Fatigue wear: Also described as surface fatigue, fatigue wear occurs when subsurface cracks appear due to fracture. Surface fatigue is encountered less frequently than the other two primary mechanisms of wearing: in approximately 14–15% of cases.
- Corrosive wear: Uniquely influenced by environmental factors, corrosion wear is the least common of the four main wear mechanisms; observed in just 4–5% of cases.
Numerous sub-categories exist, which may be described as wear modes. However, the distinction between a wear mode and a wear mechanism is poorly defined. The Encyclopedia Britannica agrees with the four basic types of wear outlined above, while ISO classifications suggest there are six main categories with fifteen subcategories. Yet it is easiest to differentiate the two classes mechanistically. Thus, there are four main mechanisms and various underlying modes, which include:
- Adhesion and transfer
- Plastic deformation
- Surface fracture
- Electrochemical reactions
How to Measure Different Forms of Wear
There are various interlinked factors determining the wear rate of materials. Custom friction testing equipment is typically used to determine the amount of material worn away relative to the duration and extent of contact between the workpiece and a tribological component. Essentially, a load is applied to a workpiece under set conditions to determine the wear resistance properties of the material in question. High-strength engineering materials are often assessed for abrasion resistance using scratch tests with extremely hard indenters.
Regardless of the test apparatus used to assess wear resistance, measuring surface deterioration from any mode is usually carried out via observation with microscopy, metallography, or radioactive isotope techniques.
If you have found this article interesting and want to read more, refer to our industrial wear applications page for more details.