On the effect of surface topography and humidity on lubricated running-in of a carbon based coating

In air of 50-60% relative humidity, no transition from boundary to EHL or mixed lubrication could be observed. For all oils and all surface topographies, the friction was always constant at levels typical for boundary lubrication. In dry air a thick tribo-layer composed of oxide was formed on the steel counter body but not in humid air. This oxide, along with a fine polishing of the surfaces and a macroscopic pressure reduction due to wear of the counter body, is believed to be important for the running-in ability.