A Hypothetical Mechanism for Rubber Abrasion

Abstract

Abstract A hypothetical mechanism for creating subsurface cracks during frictional sliding has been proposed as part of the process of abrasive wear of rubber. It consists of the unbounded elastic expansion of microscopic precursor voids until they burst open as cracks, under the action of internal pressure or of a triaxial tension in the surrounding rubber. This conjecture accounts for enhanced resistance to abrasion for compounds reinforced with carbon black, in terms of increased stiffness without much loss of extensibility, and for the lack of correlation of abrasion resistance with other measures of strength. It. should be noted that it is specific to soft, extensible materials, and thus it also accounts for marked differences in the nature of the wear process in rubbery materials compared to plastics and metals. Only rubbery materials appear to abrade away by a linking up of microcracks at right angles to the sliding direction to produce characteristic wear ridges known as the Schallamach abrasion pattern. Three mechanisms of generating a sufficiently large inflation pressure or triaxial tension are discussed. The most probable one seems to be thermal decomposition of rubber, generating volatile decomposition products—a microscale blowout process. This would be aggravated by a simultaneous softening of the rubber on heating. Although strictly conjectural, the proposed wear mechanism suggests a number of interesting experiments. For example, it would be helpful to know whether the character of wear changes when abrasion is carried out under a large superimposed hydrostatic pressure. Also, not enough is known about abrasion in vacuo. Is the process altered in kind, as well as in rate? And if thermal decomposition is, indeed, an important mechanism of initating subsurface cracks, what is the effect of changes in thermal conductivity of the abrading material on the rate of wear?