The running-in of amorphous hydrocarbon tribocoatings: a comparison between experiment and molecular dynamics simulations

Amorphous hydrocarbon (a-C: H) films have enormous potential as low friction, wear resistant coatings. Here, we present a plasma assisted chemical vapour deposition process for a-C: H that exhibits growth rates of 100 nm min and higher. The tribological performance of the resulting a-C: H films has been studied experimentally by reciprocating sliding of an a-C: H-coated Si ball on an a-C: H-coated 100Cr6 steel substrate and by subsequent micro Raman spectroscopy of the wear track. Running-in of the coatings is observed and characterised by a rapid decrease in the friction coefficient accompanied by a significant increase in sp hybridisation in the wear track. In order to gain a deeper understanding of the underlying running-in mechanisms, the sliding of two a-C: H films under a load of 5 GPa has been studied by classical molecular dynamics employing a range-corrected Brenner bond-order potential. The simulations reproduce the experimental trends and explain the running-in by a combination of smoothing and chemical passivation of both tribosurfaces. Consequently, both mechanisms should be controlled in order to produce tribological coatings for applications with optimum energy-efficiency.