There's a machine that uses a type of plastic material (reinforced Teflon) as a wear ring. The plastic rides on a highly polished metal surface and eventually wears out at which point the machine must be rebuilt. If the plastic and metal sliding surfaces are exposed to bone dry (ie: no water vapor whatsoever) hydrogen, the plastic will wear out after roughly 10,000 hours of use. If the plastic and metal sliding surfaces are exposed to bone dry helium, the plastic will wear out after only 40 hours of use. Obviously, this material is no good for this service and another material must be used. So wear rate seems to be highly dependant on the gas involved. Note that all other parameters are identical, including pressure, temperature, contact stress, velocity, etc… I've also heard circumstantial evidence that coefficient of friction may also depend on the gas involved. I don't believe the conventional view (of friction coefficient or wear rate) acknowledges any sort of difference based on what gas materials are exposed to, but at least in the case of wear rate, I have overwhelming evidence that wear rate is strongly related to which gas is involved. Now I'm wondering if coefficient of friction might also be related. Does anyone know if or how coefficient of friction might vary depending on the gas involved?