Mechanical properties and oxidation resistance of Mo-Si-B and Mo-Hf-Si-B coatings obtained by magnetron sputtering in DC and pulsed DC modes

Mo-Si-B and Mo-Hf-Si-B coatings are deposited by magnetron sputtering in two regimes (direct and pulsed current) using composite ceramic cathodes. The structure of the coatings is investigated by XRD, SEM, EDS, and GDOES. Mechanical and tribological properties are measured by nanoindentation and tribological pin-on-disc testing. The short-term oxidation resistance of the coatings in the air is evaluated at 1500 degrees C. Results show that pulsed current deposition decreases the coating's growth rate and the grains size of coating's main constituent h-MoSi2 phase while promoting the formation of MoB2. The introduction of hafnium increased the coating's density and suppressed the formation of unwanted columnar structure. The highest hardness of 28 GPa, Young modulus of 366 GPa, and elastic recovery of 64% were achieved for the Mo-Si-B coating deposited at direct current. Optimal tribological performance, including a stable friction coefficient and low wear rate of 6.9x10(-6) mm(3)/(Nm), was determined for Mo-Hf-Si-B coating also obtained using direct current. All coatings were oxidation-resistant at 1500 degrees C, but the Mo-Hf-Si-B composition demonstrated oxidation resistance superior to that of Mo-Si-B coatings because of the formation of two-layered HfO2/SiO2 protective film. The thickness of the oxide layer was higher for the coatings deposited using pulsed current.