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

Highly conformal HfB_xC_y 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 HfB_xC_y hard thin film coatings for tribological applications where conformality and high temperature are required. On the equilibrium phase diagram, the composition of the present HfB_xC_y 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.