The effect of temperature and environment on the friction of some well characterized refractory surfaces

Abstract

This paper describes a study of the frictional behaviour of a hard slider traversing the clean surface of molybdenum, tungsten and vanadium carbide crystals in ultra high vacuum. The effect of adding very low pressures of oxygen or hydrogen sulphide was investigated. The surfaces involved were characterized by means of a high energy electron gun incorporated in the system to allow glancing incidence diffraction studies throughout the friction experiments. At all temperatures the coefficient of friction (μ) of molybdenum and tungsten surfaces is isotropic. At high temperatures molybdenum surfaces show frictional behaviour which is heavily dependent on the nature of the slider. Thus with polycrystalline molybdenum sliders the friction rises with temperature but with sapphire sliders it falls. This effect is also seen on tungsten surfaces but is less pronounced. In the presence of an active gas a number of unexpected phenomena are observed. The (110) surface of tungsten gives low values of μ in high pressures of oxygen and at room temperature but the (100) surface does not. At higher temperatures both surfaces of tungsten are protected by oxygen. On molybdenum surfaces hysteresis effects are seen. Thus cooling a (100) molybdenum surface in oxygen from 1000°C leads to a higher friction than that shown while heating. The same effect is seen on both tungsten and molybdenum surfaces in the presence of traces of H 2S. This leads to the singular observation that over a range of conditions adding traces of oxygen or hydrogen sulphide causes an increase in μ. On vanadium carbide surfaces the frictional behaviour is typical of a very brittle solid. It is shown that the fall in friction on adding oxygen is due to the chemisorbed oxygen rather than any crystalline oxide formed.