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

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.