The Lubricity of Gases

A sealed reciprocating tribometer has been used to study the influence of different gaseous environments on the friction and wear properties of AISI52100 bearing steel at atmospheric pressure and 25 °C. Helium, argon, hydrogen, carbon dioxide and nitrogen all give high friction and wear, suggestive of very little, if any tribofilm formation under the conditions studied. Dry air and oxygen also give high friction, slightly lower than the inert gases, but produce extremely high wear, much higher than the inert gases. This is characteristic of the phenomenon of “oxidational wear”. The two gases ammonia and carbon monoxide give relatively low friction and wear, and XPS analysis indicates that this is due to the formation of adsorbed ammonia/nitride and carbonate films respectively. For the hydrocarbon gases studied, two factors appear to control friction and wear, degree of unsaturation and molecular weight. For the saturated hydrocarbons, methane and ethane give high friction and wear but propane and butane give low friction after a period of rubbing that decreases with molecular weight. The unsaturated hydrocarbons all give an immediate reduction in friction with correspondingly low wear. Raman analysis shows that all the hydrocarbons that reduce friction and wear form a carbonaceous tribofilm on the rubbed surfaces.