Temperature Dependence of Friction Coefficient in Ultrahigh Vacuum for Hydrogenated Amorphous Carbon Coating

Among diamond-like carbon (DLC) coatings, hydrogenated amorphous carbon (a-C:H) coatings are of great interest since some of them may exhibit coefficients of friction in the millirange, so-called “superlow friction” in ultrahigh vacuum. However, there are still many points to be clarified and improved to employ them as solid lubricant for actual vacuum applications. For example, in space environment solid lubricants are required to function at both low and high temperature ranging from −150 to 100°C. To apply them as solid lubricant in such an extreme environment, it is necessary to know the evolution of the tribological behavior in temperature, leading to their application limit. Furthermore, tribological behavior of a-C:H coatings is known to depend on tribochemistry and on mechanical properties like viscoplasticity. Since both could be affected by temperature, a better understanding of superlow friction mechanisms is expected from experiments at various temperatures. In this present work, pin-on-disk reciprocating friction tests were conducted at various temperature conditions ranging from −130 to 300°C under ultrahigh vacuum (10−7Pa) to study the effect of temperature on the coefficient of friction of an a C:H coated flat mated against steel (AISI 52100) pins. For all temperatures, superlow friction regime could be reached, as it was observed usually at room temperature for this sample. However, an effect of temperature is evidenced on the duration of “running-in” phase, i.e. the number of cycles required to reach a superlow friction regime. The duration becomes shorter at higher temperatures and longer at lower temperatures. Also, the application limit in temperature is found between 200 and 300°C, at which the friction coefficient slowly increases after running-in, to reach values above 0.01. In light of these results, the mechanisms of superlow friction are discussed in terms of tribochemistry and mechanical properties of the coating.