The general term wear can also be understood as consumption and is the continuing loss of material from the surface of a solid body. The progressive consumption is caused by grinding, rolling, pounding, scratching, as well as chemical and thermal stress on the material.
The combination of hardness and strength of a material plays a significant role in its wear. To prevent wear it is not enough that a work piece is particularly hard. If the material is very hard it is often also quite brittle. This results in added difficulties when forming the material. So if the core of the steel does not contain any meaningful toughness, the material will break at a certain point. If the material is too tough, the wear will occur especially on the surface more quickly than with harder metals. A good ratio between hardness on the exterior and toughness of the core of the sheet metal is therefore very important, also in terms of its wear.
There are a number of different types and forms of wear. These differ in the mechanisms which cause the wear. If forces are created by load stress or friction, then abrasions or surface ruin will take place. The outer layer of a material body is modified as a result of a chemical reaction, then adhesions and tribo-oxidation are the active mechanisms.
Abrasion = scraping off
Abrasion will always occur where a solid body with a rough surface meets a mating part with a soft surface. Small particles are pressed into the softer material or scraped off it through friction, shock, flow or rotating oscillation. Abrasion can also be caused by a liquid that contains rough, hard or angular particles such as sand. Moving particles rub against the mating part as a result of the flowing motion and scrape particles out of the surface. This is also called hydro abrasion.
Adhesion = stickiness
Wear as a result of adhesion only occurs if the parts are very similar with regard to their composition in the metal structure. The sliding and swaying movement of parts that are tightly pushed together can cause abrasion on the bordering surfaces. Holes, warping or material flaking can happen. The shaved off material combines and sticks to the surface. This is also called cold bonding. Sufficient lubrication may be able to prevent or at least minimise this type of wear.
The wear mechanisms are further supported by roller movements, pushing, vibration, dragging or flowing, which can occur in any variation of sequences. These movements cause the material to tire and thereby the decay of the formerly intact surface. Small grooves, also called pitting, are created and small tears occur that will certainly expand with time and can result in breakage. Sufficient lubrication prevents wear in this case as well.
Tribo-chemical reaction – Tribo-oxidation
The sliding, vibrating or flowing of two bodies on and at each other can cause chemical reactions on the surfaces that build oxide layers. The use of precious metals, plastics or ceramics can affect this tear mechanism in a positive way.
Types of wear
They identify the form of stress that has been caused. Examples of types of stress are wear from sliding, wear from thrust, impact wear, wear from rolls or rollers, wear from jets, wear from vibration and erosion.
Machine parts are constantly exposed to impact from materials like stones, gravel, crushed stone. This type of wear is called impact wear. Impact plates are installed to protect these components. They are made of especially wear resistant steels like Hardox, Creusabro and the like.
Wear from sliding
Components rub against each other as a result of sliding motions. This kind of wear from sliding can be found in engines for example. The piston movement rubs on the cylinder and causes wear and tear. Deformation of the surface will be the result and so-called tongues are created. Even the smallest particles could rub on a surface so that cracks, scratches or ablation of the material can occur.
Wear from shocks
The movement of components thrusting against each other or abutting materials of a part, as may be found in shredding machines, will cause wear by shocks. This process deforms the surfaces and tears or material abrasion can be the result.
Wear from rolls or rollers
Rollers or rolls transfer great force onto an object while they move. Pressure and friction forces act on the material surfaces, which can result in surface ruin. Surface cracks or also cracks on the inside of the material may be the result, which then in turn will facilitate the formation of dimples.