A framework for modeling the nanomechanical and nanotribological properties of high temperature HfBxCy coatings

Abstract

Highly conformal HfBxCy coatings were synthesized using low-temperature chemical vapor deposition (CVD) using a mixture of the precursors hafnium borohydride and dimethylbutene at a substrate temperature of 300 or 600 °C. The film composition was a function of the relative gas injection rates and the substrate temperature, which resulted in variable mechanical properties. Nanoscratch experiments revealed that the coefficient of friction (μ) can attain a superlubricity level of 0.05 for a C content of ~35 at% while retaining high hardness and modulus as evaluated by nanoindentation. This value of µ is comparable to diamond-like carbon, and underlines the potential of HfBxCy hard thin film coatings for tribological applications where conformality and high temperature are required. On the equilibrium phase diagram, the composition of the present HfBxCy films falls within a three-phase region consisting of the phases HfB2, HfC, and a-C. However, XPS data do not indicate the presence of C precipitates, and the B peak position is consistent with the possible presence of B4C. At such low growth temperature, the material is far from equilibrium and potentially might not consist of equilibrium phases. However, as a framework within which to view the measured mechanical properties, we calculate the properties that would be expected if films consisted of a mixture of HfB2, HfC, B4C, Hf and nanovoids. This heuristic approach predicts trends in general agreement with experiment.