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

Abstract

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.