Comparison of various C2Hx for high‐temperature lubrication by in situ pyrolysis

The ability of directed streams of three representative hydrocarbon gases ‐ acetylene C2H4, ethylene C2H4, and ethane C2H6 ‐ to provide extended‐duration lubrication to high‐temperature sliding contacts via surface deposition of pyrolytic carbon has been demonstrated. One order‐ and two order‐of‐magnitude reductions in friction coefficient and wear rate of self‐mated silicon nitride sliding contacts can be realised by this technique. The ability of these gases to provide ‘adequate’ lubrication at high temperature is illustrated through mapping the normal load/temperature/precursor flow rate space over which reduced friction may be maintained. Acetylene was the most effective precursor for pyrolytic carbon deposition, providing adequate lubrication over the broadest range of normal load/temperature/flow rate combinations, while ethane was the least effective. The boundary of the regions of adequate lubrication represents the locus of contact conditions with equal rates of lubricious carbon deposition and removal by wear. The shape of this boundary, as explored in the mapping study, supports a proposed model in which the removal rate is proportional to the product of normal load and sliding speed, while the deposition rate is proportional to the product of precursor flow rate and an Arrhenius temperature dependence.