Types Of Wear In Tribology

Wear can be defined as the damage or removal of material that a solid surface has undergone due to sliding, rolling, or impacting against another solid surface. It is not a material property, but rather a system response. Typically, wear is undesirable as it can lead to increased friction, and ultimately to material failure or loss of functionality. Thus, in order to reduce wear (and consequently friction) a thin film of lubricant is inserted between the rubbing surfaces.

Wear can be classified according to how the damaging of the surfaces occurs. Commonly, there is:

  1. Adhesive wear
  2. Abrasive wear
  3. Surface fatigue
  4. Fretting wear
  5. Erosive wear
  6. Corrosive and oxidation wear

Each type of wear is caused by one or more mechanisms. These mechanisms can synergically overlap, leading to an increased rate of wear.

Types of wear

In this section, more details are given on each one of the types of wear previously listed. The most common types are both adhesive and abrasive wear, but surface fatigue, fretting, erosive, and corrosive wear can also appear in many industrial applications under specific conditions.

2.1. Adhesive wear

Adhesive wear is the type of wear originated by the contact and interaction of asperities between two touching surfaces with strong adhesive force. In other words, opposing asperities bond to each other and shear off as one surface slides over another. Its intensity depends on several physical and chemical factors, but generally will increase with higher values of surface energy density. Oxidation films, the presence of lubricants, contaminants or lower loads, however, will suppress its effect.

2.2. Abrasive wear

Abrasive wear takes place when a rough, hard surface glides across a surface that is relatively softer. It is also the most frequent type of wear mechanism encountered in industry. Abrasive wear can be produced following three kinds of mechanisms, namely plowing (if there is no material removal from the surfaces, but rather grooves are formed from material being displaced), cutting (if there is material removal in the form of debris), or by fragmentation (if there is material removal, and at the same time there is localized fracture of the touching surfaces due to the removed debris).

2.3. Surface fatigue

Surface fatigue is the type of wear in which the surface of a material is weakened by cyclic loading. It is produced when the wear particles are separated from the solid surfaces by the accumulation of microdamage in the material. The wear mechanism in action combines the formation of cracks and voids in the solid surfaces, which are amplified with the running of cycles.

2.4. Fretting wear

Fretting wear occurs between two contacting surfaces in constant cyclical rubbing of small amplitude. It initiates fatigue cracks, which often result in fretting fatigue failure in shafts and other highly stressed components.

2.5. Erosive wear

Erosive wear is the type of wear produced by short sliding motion over a very short interval of time. It is produced when particles gradually remove material from the surface by continuous impact, deforming and damaging it. The impact of erosive wear can be affected by the nature of the impacting particles (their size, shape, and hardness), as well as the velocity and angle with which they impact to the surface, among other.

2.6. Corrosive and oxidation wear

Corrosive and oxidation wear occur as a combined effect of chemical and mechanical action. Chemical action increases porosity of the surface, while mechanical action leads to wear out. There are also a series of factors that affect wear in this case, such as an improper design of the mechanism, or the inability to remove sufficient heat or moister from the contact region.

  1. How can wear be prevented?

There is a wide array of methods to prevent wear from happening. Most commonly, the most trivial thing to do is to use wear resistant materials, or to use products (such as sugru no drill glue) that can prevent wear in the first place itself. You could also change the material properties of the touching surfaces. The latter can be done, for example, by hardening or by chemical modification of the surfaces. When this is not possible, modern oils and greases present an easy and quick solution to wear. At present, a very large selection of lubricants is available in the market, each one specifically designed for a particular need or industrial application, and compatible to most common materials.

Wear Reduction Methods:

We all understand that wear can only be mitigated, not prevented. We can minimize wear, but we won’t be able to eradicate it. Many strategies have been devised to minimize wear, as detailed below. [16]

4.1 Prevention of Overloading:

Overloading should be avoided since it causes lubricants to break down and puts an excess force on the worn surface.

4.2 Maintain a Proper Clearance:

When the clearance between the surfaces is too small, a lubricating oil layer cannot be applied to the worn surface, resulting in metal-on-metal contact. If there is more space between the surfaces, motion is lost. Due to the sheer absence of lubrication, the machine’s parts wear down quickly, making it loud and vibrating.

4.3 Better Lubrication:

Lubrication produces a lubricant film in the space between the contacting surfaces, which improves its smoothness and avoids material contact. Improper lubrication leads to wear of surfaces.

4.4 Improving the Surface Finishing:

Various sorts of straight or circular lays are formed when parts are passed through the machining process, that cannot be seen with the human eye. Because of the good surface, a line contact rather than a point contact is created, which is advantageous in processes. Good surface finish uniformly distributes load rather than maintaining asperity contacts leading to reduction of wear.

4.5 High Surface Hardness:

In compared to soft surfaces, hard surfaces wear down faster. Heat treatment is used to enhance the surface hardness of the shaft, bearing, and guide way, which reduces wear.

4.6 Proper Surface Treatment:

Mechanical wear can be minimized by applying a hard coating of metal, such as chromium or galvanic, on the surface. As a result, it may be argued that if a hard layer is applied to the surface of a wear-resistant metal, the part’s wear can be minimized.

4.7 Protection of Surface Against the Ingress of Dirt, Dust and Metal Particles:

If debris, dirt, or metal particles get inside the bearings, they get crushed much more. If the particles are tougher than the surface of the part, the surface will wear down and gets damaged.

4.8 Proper Atmosphere:

Dust, debris, moisture, dangerous chemical vapour are all present in the atmosphere, affecting machining operations and reducing their service life.

Several other techniques for wear reduction include proper maintenance, varying clearance adjustments over time, good planning, integration of preventive maintenance, controlling the preventive maintenance tasks, selection of appropriate material for the component, reducing sliding pairs with the replacement of rolling pairs, and using a fully automated maintenance facility [17].

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