Tuning microstructural and oxidative characteristics of direct current- and high-power pulsed magnetron sputtered MoSi2-based thin films

Abstract

A comparative study on nonreactively direct current magnetron sputtered (DCMS) and high-power pulsed magnetron sputtered (HPPMS) MoSi2-based coatings has been implemented with the objective of advancing the knowledge on the growth conditions and oxidation resistance of MoSi2 thin films. The energy supplied during the growth process (i.e., deposition temperature and ionization degree) exerts a significant influence on the phase formation and morphology. At 200 °C, highly dense but x-ray amorphous films are prevalent, whereas an increase up to 400 °C leads to dense and fine-columnar structured hexagonal MoSi2 films. Increased growth temperatures (≥500 °C for DCMS) and strongly ionized plasma states result in the formation of dual-phase structures (h-MoSi2 and t-Mo5Si3), accompanied by slightly underdense but strongly columnar grains. The MoSi1.92 HPPMS film (1000 Hz, 10% duty cycle) grown at 500 °C exhibits the maximum hardness of 22.8 GPa and an elastic modulus of approximately 400 GPa. In long-term oxidation tests conducted at 600, 850, and 1200 °C (up to 100 h), all MoSi2-based films exhibited a temperature-dependent scale formation. Up to 850 °C, the formation of a continuous, dense protective scale is disrupted by the competing growth of MoOx and SiOx. At temperatures exceeding 1200 °C, all MoSi2-based coatings analyzed demonstrate exceptional oxidation resistance, resulting in the formation of a continuous, dense SiO2 scale. At 1500 °C for 30 min, the initially slightly underdense and dual-phased MoSi1.92 coating achieved a scale thickness of only 670 nm, thereby demonstrating the exceptional oxidation resistance capabilities of HPPMS-grown MoSi2-based coatings.