Nanocomposite coating materials can afford an excellent combination of chemical, physical and mechanical properties, including high or super-hardness along with toughness. There is an extensive literature concerning the growth and properties of transition metal nitride, carbide and boride materials, as well as ternary (pseudo-binary) combinations. Our group previously reported the conformal growth and favorable mechanical properties of HfB2 and Hf-B-N hard coatings by chemical vapor deposition (CVD) using the high vapor pressure precursor hafnium borohydride, Hf(BH4)4, at substrate temperatures below 300°C. Our objective is to extend the use of the HfB2 system for tribological applications, for which a low coefficient of sliding friction is desirable. A useful analogue is C-alloyed TiB2, which exhibits super-hardness and good thermal stability. However, there have been no previous reports of the growth and properties of Hf-B-C alloys.

We report the CVD of Hf-B-C nanocomposite coatings on Si and on steel discs using hafnium borohydride precursor with a co-flow of dimethylbutene (DMB), (CH3)3CCH=CH2, as the carbon source. Depositions are performed in a high vacuum chamber with 0.1-0.5 mTorr of hafnium borohydride and 0.1-0.4 mTorr of DMB at a substrate temperature of 250-700 C. The resulting carbon contents are 10-33 at. %. DMB acts as growth inhibitor which reduces the film growth rate by a factor of 2-6 compared to growth using the precursor alone; for high temperature depositions, DMB also enhances the film density and decreases the surface roughness. XPS analysis indicates that Hf-B-C films consist of a mixture of HfB2, HfC and B4C phases. As-deposited films are XRD amorphous with hardness values of 8-10 GPa and reduced modulus of 92-120 GPa. Upon annealing at 700°C for 3 hrs, the films transform partially to a nanocrystalline structure, which increases the hardness and modulus. Multilayer films of (HfB2 / Hf-B-C)n afford a means to engineer the hardness and modulus to desirable values. The tribological properties of Hf-B-C films are superior to those of HfB2 films. This system affords conformal coating at low growth temperature, suitable for complex structures such as MEMS.