Tribological behavior of hafnium diboride thin films

Transition metal diborides and their coatings offer an excellent combination of high hardness, high chemical stability and high thermal conductivity, thus they are excellent candidates for a wide range of tribological applications. In this work, stoichiometric hafnium diboride films were grown by chemical vapor deposition from a single-source, heteroatom-free precursor Hf(BH₄)₄ under conditions that afford highly conformal and smooth films. HfB₂ films of thickness ∼ 0.6 μm deposited on steel substrates were subjected to pin-on-disk wear testing against a counter face disc of AISI 440C martensitic stainless steel. Based on wear measurements, both as-deposited (X-ray amorphous) and annealed (nanocrystalline) samples showed very high wear resistance compared to uncoated samples. For the annealed samples, SEM analysis indicated the formation of a wear resistant composite body in the wear scar, even at depths far exceeding the film thickness, which appears to dramatically improve wear resistance. No mild-to-severe wear transition was observed which indicates that mild wear occurred throughout the wear test. This ensemble of results, when considered in the light of high contact pressures (∼ 700 MPa) used in the study, makes the HfB₂ material potentially attractive for wear-resistant applications.