Design of Boride- and Silicide-based coating materials

Abstract

Novel coating materials with advanced compositional designs are imperative to extend the operational ranges of components exposed to severe high-temperature oxidative environments. Under these conditions, the applied materials are susceptible to degradation through oxidation mechanisms and experience internal diffusion-driven processes such as phase transformation and microstructural changes influencing their overall integrity. This thesis provides new pathways for developing ceramic coating materials featuring mechanical stability and remarkable oxidation resistance in high-temperature environments. Detailed insights are given on understanding the fundamental mechanisms governing the oxidation behaviour of physical vapour deposited silicide- and boride-based coatings, as well as on unravelling new possibilities in controlling the structure-mechanical properties of hard carbide-based coatings. The first part of the thesis is focused on binary TM-silicide thin films, with a detailed description of three TMSi2 systems (TM = Nb, Mo, Ta) in terms of high-temperature oxidation behaviour up to 1400 °C. The results indicate the decisive role of the metal partner within these disilicides, especially during the formation of mixed oxide scales. Above 800 °C, TaSi2 coatings exhibit accelerated kinetics with inferior oxidation resistance compared to their counterparts due to the formation of non-protective mixed oxide scales of Ta2O5 and SiO2. In contrast, MoSi2 and NbSi2 yield continuous protective SiO2 scales inhibiting oxygen inward diffusion and providing high-temperature oxidation resistance. The MoSi2 is distinguished by outstanding oxidation resistance at 1200 °C, owing to the selective oxidation of Si and the formation of continuous glassy protective silica scales (650 nm after 100 h) with no competing Mo-based oxides. In the second part of the thesis, new design possibilities are demonstrated to enhance the high-temperature oxidation resistance of physical vapour deposited TM-diborides (TM = Ti and Cr) based on alloying with TMSi2 phases (TM = Ti, Cr, Mo or Ta). For bulk diborides, the addition of Si-based secondary phases (e.g. SiC or TMSi2) is an established strategy yielding multi-phase systems with improved oxidation resistance. However, it is so far an unexplored strategy for PVD-based diborides. In this work, the non-reactive sputter deposition of alloyed TMB2 / TMSi2 targets yielded novel quaternary Ti-TM-Si-B2±z and Cr-TM-Si-B2±z coating materials with single-phased hexagonal AlB2 structures. These quaternary films exhibit a unique combination of high oxidation resistance and good mechanical properties. The Ta-containing Ti-Ta-Si-B2±z exhibits high hardness of 36 GPa and an improved oxidation resistance with retarded oxidation kinetics up to 1000 °C due to formation of protective borosilicate scales. However, best results are obtained for the Mo-containing Ti-Mo-Si-B2±z and Cr-Mo-Si-B2±z coatings, showing reasonable hardness values up to 27 GPa, while outperforming all previously reported TMB2-based coatings by featuring remarkable high-temperature oxidation resistance at 1200 °C due to formation of protective, glassy SiO2 scales effectively inhibiting oxygen inward diffusion. The superior oxidation behaviour is attributed to a phase separation of MoSi2 within the quaternary diboride at elevated temperatures (between 600 and 700 °C). The formed precipitates control the Si mobility within the microstructure and act as reservoirs for supplying Si to the highly protective SiO2 scales while suppressing the formation of volatile B2O3. The results pave a promising path for stabilizing single-phased quaternary diboride coating materials featuring remarkable oxidation resistance and good mechanical stability.The final part of the thesis focuses on synthesizing alternative high-temperature ceramics within the family of transition metal carbides. New possibilities for the non-reactive synthesis of NbC coatings with improved structure-mechanical properties have been explored via high-power impulse magnetron sputtering. Through tuning the pulse parameters (ton, f), a wide variation of target peak power densities between 0.1 and 1.48 kW/cm2 has been achieved, resulting in a significant change in the film growth conditions. Adapting the varied plasma conditions allows for a distinct control over stoichiometry and microstructure within the films, ranging from C-rich films with nano-crystalline/amorphous to crystalline/columnar stoichiometric films. The mechanical properties of the coatings correlate with the enhanced growth behaviour - dominated by ions at higher power density - and an exceptional hardness up to 38 GPa is observed for the nearly stoichiometric films. In addition, the fracture toughness also peaks out for the crystalline morphology obtaining 2.78 MPa·m1/2 at a C/Nb ratio of 1.06.