Thomas Industrial Library

12.3 ADHESIVE WEAR

When (clean) surfaces such as those shown in Figure 12-1 (p. 338) are pressed against one another under load, some of the asperities in contact will tend to adhere to one another due to the attractive forces between the surface atoms of the two materials.[4] As sliding between the surfaces is introduced, these adhesions are broken, either along the original interface or along a new plane through the material of the asperity peak. In the latter case, a piece of part A is transferred to part B, causing surface disruption and damage. Sometimes, a particle of one material will be broken free and become debris in the interface, which can then scratch the surface and plough furrows in both parts. This damage is sometimes called scoring or scuffing * of the surface. Figure 12-5 shows an example of a shaft failed by adhesive wear in the absence of adequate lubricant.[5]

* Note that scuffing is often associated with gear teeth, which typically experience a combination of rolling and sliding.

The original adhesion theory postulated that all asperity contacts would result in yielding and adhesion due to the high stresses present. It is now believed that in most cases of contact, especially with repeated rubbing, only a small fraction of the asperity contacts actually result in yielding and adhesion; elastic deformations of the asperities also play a significant role in the tractive forces (friction) developed at the interface.[6]

CONTAMINANTS Adhesive bonding at the asperities can only occur if the material is clean and free of contaminants. Contaminants can take the form of oxides, skin oils from human handling, atmospheric moisture, etc. Contaminants in this context also include materials deliberately introduced to the interface such as coatings or lubricants. In fact, one of the chief functions of a lubricant is to prevent these adhesions and thus reduce friction and surface damage. A lubricant film effectively isolates the two materials and can prevent adhesion even between identical materials.

SURFACE FINISH It is not necessary for the surfaces to be “rough” for this adhesive-wear mechanism to operate. The fine-ground finish of the part in Figure 12-1 a is seen to have as many asperities available for this process as the rougher milled surface in Figure 12-1 b (p. 338).

COLD-WELDING If the mating materials are metals, and are extremely clean, the adhesive forces will be high and the sliding friction can generate enough localized heat to weld the asperities together. If the clean metal surfaces are also finished to a low roughness value (i.e., polished), and then rubbed together (with sufficient force), they can cold-weld (seize) with a bond virtually as strong as the parent metal. This process is enhanced if done in a vacuum, as the absence of air eliminates contamination from surface oxidation.

GALLING This describes a situation of incomplete cold-welding where, for whatever reason (usually contamination), the parts do not completely weld together. But, portions of the surfaces do adhere, causing material to be transferred from one part to the other in large streaks visible to the naked eye. Galling generally ruins the surfaces in one pass.

These factors explain the reasons for what is common knowledge among machinists and experienced engineers: the same material should generally not be run against itself . There are some exceptions to this rule, notably for hardened steel on hardened steel, but other combinations such as aluminum on aluminum must be avoided.