Surface Engineering

 

 Wear and Wear Mechanism

 Author: C. X. Li, The University of Birmingham, UK

What is wear?

Wear is the progressive loss of materials from contacting surfaces relative in motion.

Along with fatigue and corrosion, wear has been known as one of the three major factors limiting the life and performance of an engineering component and an engineering system, whether the system is as big as a heavy machine, or as small as a tiny electronic device.

Damages of wear are twofold.

Firstly, the loss of materials from the contacting surface reduces the dimension of the component. This often leads to the increased clearance between the moving parts, and consequently results in high vibration, high noise, reduced efficiency and system malfunction. If dynamic loading is involved, the reduced component dimension could promote fatigue fracture, leading to a catastrophic failure.

Secondly, the material detached from worn surface, known as wear debris, is similarly harmful. It may cause contamination, for example, when a machine for food or beverage processing has problems with wear. It may act as abrasive when trapped inside the contacting surface, causing further increased wear rate. It may also block a valve, a critical pipeline, an oil filter or accumulated in an electrical contacting point, preventing the normal function of a system. The cost of wear is enormous, and thus great efforts have been made ever since the early ages of industry, with aims to reduce or eliminate wear.

Question:

– Thinking what will happen when the piston rings in a car engine are worn.

 

Classification of wear

Various forms of wear exist in industry and in our daily life. Different methods can be used to categorise a wear process. For examples, wear can be divided as

lubricated wear and unlubricated wear;

severe wear and mild wear;

sliding wear, rolling contact wear and impact wear.


However, all wear process involve one or a combination of wear mechanisms including abrasion, adhesion, fatigue and oxidation or other tribo-chemical actions.

In order to solve a wear problem, it is necessary to understand the underlying wear mechanism. In the following section, we will introduce two most frequently encountered wear mechanisms in industry, i.e. wear by abrasion and wear by adhesion. Oxidation wear will also be addressed since it is a mild form of wear. If wear can not be avoided at all, changing its nature from abrasion or adhesion to oxidation (e.g. by surface engineering the contacting parts) can considerably reduce the wear rate. We shall mainly discuss wear of metallic materials, although all solid material, e.g. metal, ceramic, polymer, can suffer from wear damage.

 

Abrasive wear

What is abrasive wear?

Abrasive wear (wear by abrasion) is the most frequently encountered wear mechanism in industry. There are two types of situation where abrasive wear may occur.

– The first is referred to as three-body abrasion which involves foreign hard particles, either trapped between two sliding surfaces and abrading one or both surfaces, or embedded in a softer surface and abrading the opposing one. Examples of three body abrasion can readily be found in mining industry and in machines working in desert.

 

a) b)

        Fig. 1: a) three body abrasion and b) two body abrasion

– The second type of abrasive wear is named as two-body abrasion which occurs in metal-on-metal contact when the protuberances (asperities) on a hard surface plough or cut through the other surface. Since asperities exist in every engineering surface and they never can be eliminated even by very sophisticated polishing, thus, the possibility of two-body abrasion will always exist. Frequently, two-body and three-body abrasions are combined in service as an application that was originally metal against metal (two-body) may evolve into three body abrasion, such as the generation of work hardened wear debris or the introduction of abrasive particles through contaminated lubricant.

 

    

        Fig. 2: Steel surface worn by abrasion

 

Mechanism of abrasive wear

The mechanism of material removal in abrasive wear is basically the same as machining and grinding during a manufacturing process.

At the onset of wear, the hard asperities or particles penetrate into the softer surface under the normal contact pressure. When a tangential motion is imposed, the materials in the softer surface is removed by combined effects of 'micro-ploughing', 'micro-cutting' and 'micro-cracking'. As a result, the worn surface is generally characterised by grooves and scratches as an example shown in Fig. 2. The wear debris often has a form of micro-cutting chips.

 

Prediction of abrasive wear rate

Several models have been proposed to predict the volume loss in abrasive wear. A simplest one involves the scratching of materials by a conical hard particle (indenter). Under a applied load of P, the hard particle penetrates the material surface to a depth of h which is linearly proportional to the applied load (P) and inversely proportional to the hardness (H) of the surface being abraded. As sliding occurs, the particle will plough (cut) the surface producing a groove, with the material originally in the groove being removed as wear debris. If the sliding distance is L, the wear volume (V) can be expressed as:

(1)

In which, k is wear coefficient partly reflecting the influences of geometries and properties of the particles (or asperities), and partly reflecting the influences of many other factors such as sliding speed and lubrication conditions.

 

How to improve abrasive wear resistance?

Equation indicates that wear loss can be reduced by

– decreasing contact load,

– reducing sliding distance,

– reducing k value by eliminating the presence of hard particles and reducing surface roughness of the counterface.

However, these changes are frequently impossible in practice if the function of a machine is to be maintained. Then, increasing hardness (H) by means of material selection, heat treatment and surface engineering will be the most effective and relatively easy way to solve an abrasive wear problem.

 

Adhesive wear (Wear by adhesion)

Engineering surface is never perfectly flat. The surface of a most highly polished engineering component show irregularities or asperities. When two such surfaces are brought into contact, the real contact actually occurs only at some high asperities which is a small fraction, e.g. 1/100 of the apparent contacting area. As a result, plastic deformation and intermetallic adhesion will occur, forming cold weld junctions between the contacting asperities. The strength of junction is determined by the surface structure and by the mutual solubility of two contact metals. The tendency of adhesion is the lowest for a pair of metals with almost zero mutual solubility, but this is limited to very few metals. Most metallic materials show appreciable tendency of adhesion.

 

Fig. 3 Adhesive wear occurs by material transfer

 

When two contact surfaces undergo relative movement, tearing must take place either at the (cold weld) junction or inside the original materials depending on which is weaker. If the strength of the adhesion junction is relatively low, as in the case of a contact pairs with low mutual solubility, or metallic surfaces separated by oxide film, tearing will take place at the junction and material loss during wear will be minimal. However, when tearing occurs inside the softer material, a fragment of the softer material will be dragged away and adhering to the harder body, as a schematic shown in Fig. 3. This process is known as material transfer.

The transferred fragment is plastically deformed during continued action of sliding so that the transferred materials frequently has a plate-like morphology. Multi-material transfer and plastic deformation of the transferred material result in a layered surface morphology on the counterface as shown in Fig. 4. The next stage is detachment of the transferred layer, probably when the bond between the layer and the underlying material has been locally weakened by cyclic loading.

 

    

        Fig.4 Transferred layers of titanium alloy on a steel surface

 

Prediction of wear rate

Adhesive wear is dominated by material transfer and removal of the transferred material. The former is determined by the material properties and the strength of adhesion junction while the latter by the sliding conditions. Archard proposed that adhesive wear rate, W, can be expressed by the following equation

(2)

– P is the applied normal load,

– H is the hardness of the softer material of the two contact bodies,

– K is referred to as wear coefficient.

K is related to the possibility of generation of wear debris from each contact. K would be higher if two contacting materials have higher mutual solubility. K would also be higher when wear occurs in a vacuum or an inert atmosphere, such that adhesive wear is one of the major problems for contacting components, e.g. bearing, in a spacecraft or satellite.

 

How to improve adhesive wear resistance?

– Improving mechanical properties (hardness and strength) of the contacting material because tearing is more likely to happen in adhesion junction.

– Material selection or changing the chemical nature of the surfaces, e.g. by surface engineering, will fundamentally reduces the possibilities of adhesion and reduce adhesive wear rate.

A tailored surface engineering process will increase the surface hardness of a contacting surface, and reduce the possibility of adhesion between contacting surfaces, therefore, surface engineering is the most effect way to improve the adhesive wear resistance.

 

Oxidation wear

Metal surface is normally covered with a layer of oxide, which could prevent metal-to-metal contact, and thus avoiding the formation of adhesion and reducing the tendency of adhesive wear. In this connection, oxide is a favourable factor in reducing wear rate of metallic materials. However, whether such beneficial effect can be realised or not is strongly dependent on the material properties and on contact conditions.

When the hardness of the metal underlying an oxide layer is low, or when the contact load is relatively higher, the metal beneath the oxide layer will plastically deformed, and asperities in the hard surface will penetrate through the thin oxide layer, leading to the normal metal-to-metal contact. In such case, wear by abrasion or adhesion will occur depending on the mechanical properties and chemical properties of the contacting metals. The beneficial effect of oxide is minimal and wear rate is generally high.

On the other hand, when the underlying metal is hard enough to support the oxide film, such as on a surface engineered hard surface, a process known as oxidation wear will occur.

The mechanism of oxidation wear is schematically shown in Fig. 5. At the beginning of a wear process, the original oxide film on the metal surface was removed when hard asperities rub across the high point of the oxide layer, leaving the underlying metal uncovered. The fresh metal will quickly react with oxygen in air to form a fresh oxide layer, which will then be scraped off again by asperities in the following cycle. Such an "oxidation - scrape", or "chemical - mechanical" cycle repeats during the oxidation wear process, producing wear debris of finely powdered oxide. The worn surface is smooth and frequently covered with a layer or patches of oxide which sometimes can be visually seen. For steel, the worn surface becomes dark brown (colour of iron oxide). The morphologies of a surface worn by oxidation wear is shown in Fig. 6.

 

Fig. 5 Schematic of oxidation wear process

 

    

        Fig. 6: A surface engineered cobalt alloy worn by oxidition ware

 

It needs to be mentioned that during sliding, the high surface temperature induced by frictional heating, and the reduced activation energy of oxide formation caused by plastic deformation, can increase the oxidation rate. Thus, rapid oxidation can be achieved, and the oxide layer can grow thicker during sliding than that under static conditions. This ensures the fresh metal is rapidly covered with a new layer of oxide after the original oxide film was worn away.

Oxidation wear will not happen in vacuum or in inert atmosphere, since re-oxidation is not possible.

Oxidation wear is a mild form of wear. When the predominant wear mechanism is changed from abrasive or adhesive to oxidation wear, wear rate can be decreased by several orders of magnitude.

 

Questions:

– Thinking which material property is most important in determining the wear resistance of a component.

– Different methods are used in practice to improve the wear resistance, can you name some? As far as you believe, which method is most effective?