Surface Temperatures Generated With Ceramic Materials in Oscillating/Fretting Contact

This paper summarizes the results of a study of the tribological behavior of ceramic materials in unlubricated oscillating/fretting contact with particular emphasis on frictionally-generated surface temperatures. The study was carried out using an oscillating contact device coupled to an infrared microscope. The contact geometry consists of a stationary spherical specimen loaded against a thin sapphire optical flat driven by an electromagnetic shaker. With this system, measurements can be made of friction, wear, and surface temperature over a wide range of loads, frequencies, and vibration amplitudes. Four ceramic materials were investigated, namely zirconium oxide (zirconia), tungsten carbide, and two different forms of aluminum oxide (alumina and sapphire). The first important finding was that each material exhibited unique and characteristic tribological behavior. Instantaneous variations in friction and surface temperature occurring within a single cycle of oscillation (e.g., in less than 0.005 seconds) could readily be measured with this technique. By digitizing the surface temperature, friction, and velocity signals, comparisons are made in the time and frequency domains. Frequency content correlations are determined using Fourier transform techniques. In addition, instantaneous frictional heat generation rates are calculated using the digitized friction and velocity data. Based on a series of experiments at constant oscillation frequency and amplitude, a correlation appears to exist between wear and surface temperature for the ceramics studied. Sapphire-on-sapphire and zirconia-on-sapphire produced the highest wear and the highest surface temperature rises (ca. 130–140 K). Tungsten carbide-on-sapphire produced the lowest wear and lowest surface temperature rise, while alumina exhibited intermediate behavior. In all cases, the rate of frictional heat generation was relatively low. It may be that the rapid fluctuations in surface temperature under these conditions—with two major temperature peaks per cycle—could lead to a kind of thermal stress fatigue of the ceramics as a wear mechanism. The use of the IR microscope in the scanning mode, coupled with scanning electron microscopy of the wear scars and theoretical treatment of sub-divided areas, can shed light on the nature and distribution of real areas of contact.