Metallurgical Factors Influencing Adhesion Wear

The above article Friction and Wear gives a brief overview of how friction occurs and how it may lead to wear in addition to a worked example of an aluminium pin on a steel disc. Through the Archard-Child equation it can be observed that the wear coefficient is a function of, among other things, the hardness of the material. In section in this blog such as Oxidation and High Temperature Corrosion it was explained how some surface treatments which affect a materials composition may increase the hardness of the material through the introduction of carbides and/or nitrides. While it is readily accepted that hardness is a materials ability to resist indentation, as indicated by a hardness measurement being taken by measuring an indent made in a materials surface and the smaller the indent the harder the material, but how is it that one material is harder than another?

There may be a number of factors affecting this such as the unit cell structure. Metals are composed of repeated structures joined together that are known as unit cells and these cells may take one of many forms. The most common forms mentioned, mostly due to their applicability to steels is BCC and FCC, or body centre-cubic and face centre-cubic respectively along with a third, HCP or hexagonal close packed. Representations of each lattice structure can be seen below;

lattice

An FCC material will tend to allow more material deposition when compared with a BCC structured material, this is due to the FCC structure having a higher amount of slip planes whereby it can have atoms move within the repeating crystal lattice structure and therefore deposit them in the form of adhesive wear. HCP offers the least amount of slip planes and is therefore the crystal structure least likely to experience loss of material through adhesive wear. Aluminium is one example of an FCC material while Tungsten is BCC and Titanium is an HCP material and by comparing the hardness of these materials one can see that hardness does increase from FCC to BCC and finally HCP. This increase in hardness is linked to the decreasing amount of slip planes between the three compared lattice structures, indicating that since hardness and adhesive wear resistance are linked, that the crystal lattice structure of a material and its adhesive wear resistance are also linked to the point of the fewer slip planes present in the structure, the less prone to adhesive wear the material should be.

The adhesion between two surface may also be increased in there is interfacial metallic adhesion between the two connecting bodies. This is where one material surface acts to donate electrons and the other surface accepts them. As outer valance electron are considered free moving inside of a metal structure, the sharing of electrons in the instance of interfacial metallic adhesion can be very strong and if this force is overcome can lead to the softer of the two metals suffering wear. This form of adhesion may be increased if the metal surfaces do not possess a metal-oxide layer and the substrate metals are in contact.
Another instance in which the tendency for adhesive wear may increase is if the two metals in contact have a tendency to form intermetallic compounds such as gold and aluminium. As wear occurs in the presence of friction and friction produces heat, the tendency for the metals for form intermetallic compounds is increased with heat and the increased temperature provides the atoms with energy necessary for these compounds to form.

For more information on adhesive and abrasive wear resistance in steels please read P. L. Hurrick’s journal article on the matter at the following website;
http://www.data4design.co.uk/PDF%27s/WRMet.pdf

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