Tribology is the science that studies friction, wear, and lubrication of touching surfaces in relative motion. It relies on concepts coming from materials science, physics, and chemistry among others, making it a multidisciplinary domain. From these three points (friction, wear, and lubrication), wear constitutes a large concern for industrial businesses, as it accounts for over 50% of loss of machine usability. Therefore, controlling the conditions that affect wear would not only prevent machine failure but would ultimately allow you to optimise the industrial processes.
But before starting to point out how wear should be prevented, we should ask ourselves… what is wear exactly?
What is Wear?
Wear is 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 lubricant film is inserted between the rubbing surfaces.
Wear can be classified according to how the damaging of the surfaces occurs. Commonly, there is:
- Adhesive wear
- Abrasive wear
- Surface fatigue
- Fretting wear
- Erosive wear
- Corrosive and oxidation wear
Each type of wear is caused by one or more mechanisms. These mechanisms can sometimes overlap, leading to an increased rate of wear.
Types of Wear
In this section, we give more details 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 created 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 the other. 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 encountered in industrial applications. Abrasive wear can be produced following three kinds of mechanisms, namely ploughing (if there is no material removed from the surfaces, but rather grooves are formed from the 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 localised 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 a 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 the surface, among others.
2.6. Corrosive and oxidation wear
Corrosive and oxidation wear occur as a combined effect of chemical and mechanical action. Chemical action increases the 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.
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 change the material properties of the touching surfaces. The latter can be done, by chemical modification or hardening 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 with most common materials.
For more than 70 years, Molykote® has the confidence of engineers, manufacturers and designers around the world thanks to its ability to cope with proven solvency current challenges in lubrication. Molykote® provide lubrication and wear avoidance solutions for various industries and applications that require reliable, long-term lubrication. With varying product technologies, varying lubrication applications are covered, with products that are tailored to specific loads, speeds and viscosities.
Krytox™ oils and greases offer high performance over a wide temperature range and provide superior quality lubrication in extremely challenging conditions. These oils are composed of fluorine, carbon and oxygen’ known as PFPE. Krytox™ lubricants come in both oils and greases that contain additives that are tailored to provide performance over specific high and low load, speed and temperature applications.
KAJO lubricants are tailored to the agricultural, railway and marine industries; covering an array of environmentally demanding applications. As well as providing premium lubrication in a range of applications, KAJO offer biodegradable lubricants that are approved by European EcoLabel and Blue Angel certifications.