Nanoscratch and nanofriction behavior of hafnium diboride thin films

Abstract

Hafnium diboride (HfB 2) offers an excellent combination of high bulk hardness (29 GPa), high melting point (3295 °C) and high wear resistance, thus making it an attractive material for wear resistant coatings. In this work, the nanoscale friction response of as-deposited and annealed HfB 2 films is reported. The films were subjected to nanoscratch experimentation and the material response was investigated by measuring lateral forces to obtain friction coefficient, and by calculating Hertzian contact pressures. Both as-deposited and annealed films show favorable friction behavior with respect to TiN. While as-deposited HfB 2 and TiN deform plastically at all normal loads used in the experiments, annealed HfB 2, which is nanocrystalline in contrast to the X-ray amorphous as-deposited coatings, responds elastically at lower loads. The friction behavior of the films shows no significant dependence on sliding velocity indicating negligible effect of frictional heating. The films, however, exhibit a prolonged effect of running-in which is present over the entire duration of the experiments and leads to increase in the friction coefficient with number of passes. Annealed films display narrower and shallower scratch grooves, leading to a higher nanoscratch resistance. The excellent overall response of annealed HfB 2 films to nanoscratch testing confirms the potential of these films for wear resistant coatings.