Understanding Wear and Recognizing Different Wear Modes

Wear is defined by ASTM International as "damage to a solid surface (generally involving progressive loss of material), caused by the relative motion between that surface and a contacting substance or substances".

For a number of products, it is a fairly simple task to identify whether something is worn, however it is often not easy to understand how it reached that condition.

Material removal in most cases takes place gradually due to a repetitive motion, but wear is a complicated process and influenced by several parameters. Among these are the following:

• contact geometry
• length of exposure
• environmental conditions
• material composition and hardness
• interacting material surfaces
• normal force
• sliding speed

The contacting surfaces may seem reasonably smooth at first glance, but in actuality there will be asperities that form contact junctions between the materials. As seen in the valleys and peaks in Figure 1, the load is supported by these asperities and there is a tendency towards deformation during sliding of the surfaces over each other. Repetitive movement causes eventual removal of material from one or both surfaces.

Figure 1.

Wear Vs Abrasion

The terms abrasion and wear are frequently used interchangeably, however there is a difference. Abrasion is the action causing wear and ASTM defines it as “the wearing away of any part of a material by rubbing against another surface". Removal of a portion of the surface by some kind of mechanical action such as sliding back and forth of an object, rubbing, wearing tires on traffic paint, wind erosion and the like is termed wear abrasion.

Mar abrasion is when a surface becomes permanently deformed, but the surface does not break.

Types of Wear

It is the type of relative motion that is used to define the generated wear. The complexity of wear means that a number of modes can be recognized.

The main modes are outlined below:

• Abrasive Wear - Wear because of hard protuberances or hard particles forced against and moving along a solid surface is known as abrasive wear. These hard particles may be common abrasives such as silicon carbide and aluminum oxide, or naturally occurring contaminates such as dust particles and sand [crystalline silica (quartz)]. Rolling abrasion or three-body abrasion occurs in case the abrasive particles are allowed to roll.
• Adhesive wear – Adhesive wear is caused by localized bonding between contacting solid surfaces resulting in material transfer between the two surfaces or loss from either surface. Adhesive wear is not as common as abrasive wear and takes place when materials slide against each other without any lubrication. This type involves local cold welds being formed between surfaces contacting under a load and plowing or tangential shearing of junctions. This type of wear involves the formation of local cold welds between surfaces contacting under a load and plowing or tangential shearing of the junctions.

Figure 2.

• Catastrophic wear – Accelerating or rapidly occurring surface damage, deterioration or change of shape caused by wear to the degree that the component’s service life is considerably shortened or function destroyed.
• Corrosive wear – In this kind of wear, electrochemical or chemical reactions with the environment are significant.
• Crocking – Color transfer from a colored fabric surface to an adjacent area of the same fabric or to another surface by rubbing action.
• Cutting wear – In solid impingement erosion, cutting wear is the erosive wear related to the dissipation of kinetic energy of impact arising from the tangential component of the velocity of the impacting particles.
• Deformation wear – In solid impingement erosion, deformation wear is the erosive wear of a material related to the dissipation of kinetic energy of impact arising from the normal component of the velocity of the impacting particles. It is hence the sole component of wear for particles impacting at a 90° angle of attack.
• Erosion – This is the damage caused by particulate in liquids or gases striking a surface.

Figure 3.

• Erosive wear – Erosive wear involves progressive loss of original material from a solid surface because of mechanical interaction between that surface and a fluid, multi-component fluid, or impinging liquid or solid particles.
• Fatigue wear – This is the wear of a solid surface caused by fracture arising from material fatigue.
• Fretting wear – This is the wear that arises as a result of fretting which, in tribology, involves a small amplitude oscillatory motion, usually tangential, between two solid surfaces in contact
• Frosting – Frosting is a change in color in a restricted area of fabric caused by abrasive wear.
• Impact wear – This is the wear because of collisions between two solid bodies where some component of the motion is perpendicular to the tangential plane of contact.
• Mar abrasion – Mar abrasion includes permanent deformations that have not ruptured the surface of a coating, but change or mar the appearance of its surface.
• Pitting – This is a form of wear characterized by the presence of surface cavities, the formation of which is attributed to processes such as fatigue, local adhesion, or cavitations.
• Rolling wear – This includes wear because of the relative motion between two non-conforming solid bodies whose surface velocities in the nominal contact location are identical in magnitude, direction and sense.
• Rolling abrasion – This is an abrasion form that takes place when debris or abrasive particles or debris are allowed to “roll” between the surface and a contacting substance.
• Scoring – This is a severe wear form that appears as extensive scratches and grooves in the direction of sliding.
• Scratching – The mechanical removal or displacement, or both, of materials from a surface by the action of abrasive particles or protuberances sliding across the surfaces. Typically in the form of a line, caused by the relative movement of an object across and in contact with the surface.
• Scuffing – This is a wear form that occurs in inadequately lubricated tribo-systems that is characterized by macroscopically observed changes in surface texture with features related to the direction of relative motion.
• Sliding wear – This is a kind of wear due to relative motion in the tangential plane of contact between two solid bodies. This is typically recognized by linear grooves that are generated from a reciprocating or unidirectional contact.
• Three-body wear – This is an abrasive wear form in which wear occurs due to loose particles generated of introduced between the contacting surfaces.

Figure 4.

• Two-body abrasive wear – This is an abrasive wear form in which the protuberances or hard particles that produce the wear of one body are fixed on the surface of the opposing body.

Figure 5.

• Wear – Damage to a solid surface, generally involving progressive loss of material, due to relative motion between that surface and a contacting substance or substances.

This information has been sourced, reviewed and adapted from materials provided by Taber Industries.

For more information on this source, please visit Taber Industries.