Guide to Adhesive Wear

On this page you can learn about adhesive wear and how to reduce it.  We have covered the topic under the following headings:-

Definition

Adhesive wear is the second most common form of wear in industry. It is defined as:

'The action of one material sliding over another with surface interaction and welding (adhesion) at localised contact areas'

Adhesive wear may be between metallic materials, ceramics or polymers, or combinations. It is dependent on adhesion between the materials and that in turn depends on surface films like oxides or lubricants, as well as the mutual affinity of one material for another

If loads are light and the natural spontaneous oxidation of a metal can keep up with the rate of its removal by wear, then that wear rate will be relatively low (the oxide acting as a lubricant). It is called: Mild Wear.

If loads are high and the protective oxide is continually disrupted to allow intimate metallic contact and adhesion, then the wear rate will be high. It is called: Severe Wear

With materials which have thin, brittle oxides, notably stainless steel, aluminium alloys and titanium, the protective oxide is easily disrupted and the consequent massive adhesion and wear is called: Galling

The terms Mild wear, Severe wear and Galling are used with specific meanings. They are in relation to unlubricated sliding. Click on lubrication to see the effects on adhesive wear of adding an oil or grease.

Mild wear is characteristic of dry sliding metals where the conditions are such that the naturally protecting oxide can continuously reform at the sliding contact, so acting with a degree of dry lubrication and reducing the wear rate. It also occurs with hardened alloys (usually steels) when, even under high contact loads and speeds, the underlying substrate can support the oxide and prevent its disruption by deformation below it.

Severe wear occurs (generally in soft metals or alloys) when the conditions are such that the oxide is disrupted at a greater rate than which it can reform, so that clean metal is exposed below and massive adhesion occurs between the mating surfaces.

It is not uncommon for soft materials to show sudden transitions between these two wear regimes. With mild steel at low load, mild wear results. As the load is increased, a point is reached when the oxide cannot keep pace and there is a sudden 100 fold increase in wear rate. At even higher loads, the frictional heating is such that the oxidation rate rapidly increases and can again form a protective layer; and mild wear is re-established.

The objective of Surface Engineering a component is often to eliminate this possibility of sever wear by hardening the surface (nitriding, carburising, etc) and supporting the natural oxide.

Galling is a particular form of very sever adhesive wear reserved for materials that have thin, brittle oxides that are easily disrupted under load. It leads to seizure of fasteners and couplings in particular. It is also referred to as 'pick-up' or 'transfer'. Click on anti-galling to see some surface engineering solutions.

 

Typical Adhesive Dry Rubbing Wear Factors for Surface Treatments and Coatings

As determined from a pin-on-disc sliding test using a polished hardened steel pin rubbing against the treated disc surface.

Units are in m³/Nm, that is volume removed/unit distance of sliding/unit loading. Typical values for some base materials are also included.

 

Coatings to reduce dry adhesive wear

Generally, coatings with high hardness will give lower adhesive wear rates in sliding situations. The hardest coatings are found amongst the thermally sprayed ceramics and cermets, with the best of the electroplates being hard chrome.

The selection is application-specific and you should take remember to consult the information on Mild and Severe Wear, on Scuffing and on the effect of Lubrication.

Click below to see more detailed information on specific surface treatments to reduce adhesive wear:

• Apticote 100 Hard Chrome plating
• Apticote 300 Hard Anodising for aluminium alloys
• Apticote 400 Electroless Nickel coatings
• Apticote 800 Thermally Sprayed coatings
• PVD ceramics for cutting and forming tools
• Apticote Keronite 3000 for aluminium alloys

Coatings for resisting galling, particularly with stainless steel, aluminium alloys and titanium alloys, include:

• Apticote 100 Hard Chrome plating
• Apticote 300 Hard Anodising for aluminium alloys
• Apticote 800 Thermally Sprayed coatings
• Apticote Keronite 3000 for aluminium alloys
  

Lubrication and its Effect on Adhesive Wear

A lubricant is usually either a grease or an oil which conforms to the following principles:

A lubricant:

• Separates the moving surfaces
• Has low shear strength
• Has high viscosity (resistance to shear)
• Acts to remove heat from the contacting surfaces
• Is usually a hydrocarbon or fluorocarbon

The viscosity:

• Decreases as temperature rises
• Increases as pressure increases
• Both are exponential relationships

The purpose of the lubricant is to separate the surfaces and to eliminate contact and wear. This is achieved by the generation of a wedge of oil as it is drawn into the contact region by the motion of the parts.

The film thickness generated in this way is dictated by:

h, the film thickness, is proportional to (viscosity)^0.7, (Speed)^0.7, and (Load)^-0.13

In the context of Surface Engineering, the relevance of lubricated contact is primarily related to Boundary Lubrication, where the film thickness h is insufficient to properly separate the surface, so that a coating or treatment is required to complete the protection of the parts. At higher speeds, the lubricant film thickness increases so that separation begins (Mixed Lubrication) and, finally, until there is no contact between the two surfaces (Hydrodynamic Lubrication)

Typical adhesive wear rates with a hardened steel against another hardened steel are:

 

Surface Coatings and Lubrication

In general, if a rubbing surface is well lubricated with an oil or grease, further reductions in friction are difficult to achieve through the application of surface coatings. In fact, polymer coatings like PTFE are likely to perform less well in the presence of an oil; the normal mechanism of polymer film transfer to the mating surface is disrupted by the lubricant. Certain coatings have an affinity for oil and provide extra protection for parts operating under extreme conditions. This is so in arduous Scuffing situations (click on the word for more details) in cams and tappets and in automotive cylinder bores. Coatings for anti-scuffing include:

• Apticote 100 Hard Chrome Plate
• Diamond-Like Carbon coatings