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

Transition-metal disilicides (TMSi2) based thin films are suggested as promising, novel protective coating materials used in various high-temperature applications. In this study, we investigate the phase formation, microstructure, and mechanical properties (i.e. H, E, and KIC) of sputter-deposited TMSix films (TM = Mo, Ta, Nb) in correlation with the varied bias potential. The as-deposited TaSix and MoSix coatings show Si sub-stoichiometries with Si/Me (x)< 2, while all the NbSix coatings are overstoichiometric in Si. All TaSix and NbSix coatings are stabilized in their preferred hexagonal structure, whereas the MoSix coatings exhibit small fractions of T1-Mo5Si3 next to the dominant metastable hexagonal β-phase. The oxidation behaviour of the coatings was examined up to 1400 °C. MoSix based films are distinguished by an outstanding oxidation resistance, forming dense and protective silica scales of only 650 nm after 100 h at 1200 °C – also obtaining an extremely high interfacial stability. In contrast, TaSix suffers accelerated oxidation at 1200 °C due to the formation of mixed, non-protective scales consisting of Ta2O5 and SiO2. Moreover, NbSix coatings show retarded oxidation kinetics up to 60 h at 1200 °C, forming a dense and uniform SiO2 scale of only 533 ± 131 nm. Micro-cantilever bending experiments reveal that TaSi1.7 coating exhibit the 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 NbSi2.4 and MoSi1.9, respectively.