Thermo-mechanical properties of sputter deposited TMSi₂ coatings (TM = Mo, Ta, Nb)

Abstract

Transition-metal disilicides (TMSi₂) based thin films are suggested as promising candidates for novel protective coating materials used in various high temperature applications. In this study, we investigate the phase formation, microstructure, mechanical properties (H, E and KIC) as well as the oxidation kinetics of sputter deposited TMSiₓ (TM = Mo, Ta, Nb) films. TaSiₓ and NbSiₓ coatings are stabilized in their single hexagonal structure, whereas MoSi2 coatings exhibit small fractions of T1-Mo5Si3 in addition to the dominant metastable hexagonal β-phase. The oxidation behavior of the coatings was examined up to 1400 °C. The analyses indicate an outstanding behavior for MoSi₂ coatings by forming a dense protective silica scale of only 650 nm after 100 h at 1200 °C obtaining an extremely high interfacial stability. In contrast, TaSi₂ suffers accelerated oxidation due to the formation of mixed oxide scales consisting of Ta₂O₅ and SiO₂, being not effective in the oxidation protection. Moreover, the NbSi₂ coating shows retarded oxidation kinetics up to 30 h at 1200 °C by forming a dense, protective SiO₂ scale of only 628 ± 77 nm with no detected Nb-based oxides. Micro-cantilever bending experiments reveal that TaSi₂.₈ coating exhibit highest fracture toughness, KIC, of 2.7 ± 0.2 MPa.m₁/₂ compared to 2.3 ± 0.1 and 1.7 ± 0.1 MPa.m₁/₂ for NbSi₂.₁ and MoSi₁.₉, respectively.