In unlubricated sliding contact, essentially all the mechanical work done to overcome friction is converted into heat produced in the vicinity of real contacts. The amount of frictional heat flux q is proportional to the friction coefficient ü, the normal force F, and the sliding velocity ν, but is inversely proportional to the nominal contact area An (e.g., q = (ü × F × ν)/An). The real areas of contact, being much smaller than the nominal contact area, give rise to much higher local heat fluxes in the vicinity of asperity contacts. Because the frictional heat flux enters the contacting bodies through these regions (or locations known as “hot spots”), their local temperatures (referred to as “flash temperature”) can be much higher than the overall or “bulk” surface temperature, as discussed in References 1-3.
Previous studies have demonstrated that frictional heat can profoundly affect the friction and wear behavior of both metallic and ceramic materials. In most steels and nonoxide ceramics, frictional heat was found to foster oxidation. The occurrence of phase transformations on or near the sliding surfaces was also cited in the literature for certain steels and ZrO2-based ceramics.
Except for SiC, BeO, and AlN, most ceramics have significantly lower thermal conductivity than do metals. When in sliding contact, ceramics cannot dissipate frictional heat generated at sliding interfaces as effectively as most metallic alloys. Large temperature gradients can often develop between areas of real contact and surrounding regions, thus creating high thermal stresses.