Wear is defined as the removal of material from a solid surface as the result of a force from another solid. Wear primarially occurs as the result of two surfaces sliding over each other under a load, transferring material from one surface to the other. It is often thought that if the transfer rate is high, the wear rate will be high. While this may be true in most situations, it becomes difficult to quantify the wear of the materials in that manner because there may or may not be a net loss of material. It is much more accurate to think that less transfer means less wear.
friction occurs when to surfaces, in contact at a junction, slide over one another. If the junction is stronger than the structure of teh surface, wear occurs. If the junction is less resistive to a shearing force than the surfaces, wear will not occur. This is where lubricants come into the picture. A lubricant works by creating a layer of easily deformable medium between the two surfaces. This lubricant forms a junction with each face that yeilds easily to shearing. the images below show two sufaces in contact under a load, the interloccking of teh surface features is visible. When this system experiences a shear force, F, the surfaces are forced to yield to the stress because the juction will not yield. When a lubricant is applied, a thin film of a constant hydrodynamic pressure separates the two surfaces. When this system is sheared, the lubricant yields as its layers slip over each other.