Metal diborides offer an excellent combination of high hardness, high chemical stability and thermal conductivity. In particular, hafnium diboride (HfB2) has a melting temperature of 3250 °C, a bulk hardness of 29 GPa and good corrosion resistance, all of which make it an excellent candidate for wear resistant coatings. In spite of such attractive combination of properties, relatively few reports of transition metal boride thin films compared to transition metal nitrides could be found. This can be traced to the processing difficulties associated with depositing these films. Magnetron sputtering is a straightforward method to deposit such films, but the control of stoichiometry is a non-trivial issue. Using a single source precursor, Jayaraman et al. have reported that dense, stoichiometric, and conformal HfB2 coatings can be grown by chemical vapor deposition (CVD) at low temperature. Upon annealing for 1 h at 700 °C, these X-ray amorphous coatings transformed to nanocrystalline ones, resulting in hardness of 40 GPa and elastic modulus of 430 GPa. An important issue for the potential use of these coatings is their wear resistance. Wear of hard coatings is generally not abrasive as long as the counterface material does not contain any hard phases. However, if the work material adheres strongly to the coating it could result in severe cohesive delamination or detachment of the coating.
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