Concepts for novel oxidation resistant thin film materials

Abstract

This thesis explores different strategies to improve the oxidation resistance of novel protective ceramic thin film materials. Material degradation in extreme environments poses a critical challenge in aerospace, energy, and other high-temperature applications. The durability and operational efficiency of components exposed to oxidative environments/conditions can be significantly improved through advanced deposition techniques, optimized coating architectures, and tailored material compositions. This work systematically investigates three key approaches: MoSi2-based coatings as stand-alone protective coating materials, architectural coating concepts incorporating protective Al-Si-O and Si-O top coatings, and alloying strategies to improve the oxidation behavior of transition metal carbides and borides. The study of MoSi2-based coatings focused on their phase formation and oxidation resistance, using both direct current magnetron sputtering (DCMS) and high-power pulsed magnetron sputtering (HPPMS). The microstructural evolution of these coatings has been studied based on their deposition parameters, revealing that HPPMS synthesized thin films exhibiting improved density and oxidation resistance at high temperatures (above 1200 °C). These results highlight the formation of a continuous protective silica scale that effectively limits oxygen diffusion, preventing accelerated oxidation and oxygen inward diffusion. However, oxidation performance at lower temperatures (around 600 °C) remains a challenge due to the so-called pesting phenomenon, where competing MoOx and SiOx oxide formation results in porous, non-protective oxide scales. Despite this minor limitation, HPPMS deposited coatings exhibit superior performance (only 650 nm oxide scale after 100 h at 1200 °C), highlighting the critical influence of deposition parameters and microstructure in optimizing the oxidation resistance of MoSi2-based coatings.To further improve the oxidation resistance of oxidation-prone thin film materials, architectural coating concepts using Al-Si-N, Al-Si-O, and Si-O protective top coatings on transition metal diborides (TM = W, Ti, Hf) were explored. A Si interlayer was introduced between the oxide-based top coatings and the diborides to prevent in-situ oxidation during growth by reactive (plasma enhanced) cathodic arc evaporation. Experimental results show that Si-O and Al-Si-O top coatings provide exceptional oxidation resistance for TiB2.7 and HfB2.4 diborides, maintaining stability up to 1200 °C. Transmission electron microscopy (TEM) analysis confirmed the stability of the dense oxide scales that effectively block oxygen diffusion, highlighting the superior resistance against oxygen inward diffusion of these coatings. In contrast, Al-Si-N top coatings exhibited accelerated oxidation above 1000 °C. However, the results underscore that Al-Si-O and Si-O protective top coatings provide excellent long term oxidation resistance (up to 30 h at 1200 °C) for transition metal diborides that are otherwise highly susceptible to oxidation.The final part of this study focused on alloying strategies for transition metal carbides (TMCs) and borides (TMB2) to enhance their oxidation resistance. As a first step, a high-throughput combinatorial approach was used to investigate ternary TM-X-C (TM = Ti, Zr, Hf, Ta, W and X = Al, Si) thin films. Experimental results, supported by density functional theory (DFT) calculations, confirmed that metastable face-centered cubic (fcc) solid solutions with Al and Si alloying up to 25/30 at.% could be synthesized. The oxidation behavior of these coatings was subsequently investigated, revealing that TMCs alloyed with Al and Si demonstrated significantly improved oxidation resistance at 750 °C. A two-layer scale consisting of a metal oxycarbide layer followed by an outer mixed Ti-(Al/Si) scale was exemplarily proven on top of the unoxidized Ti-Al/Si-C films. In comparison, their binary counterparts oxidized at temperatures below 600 °C. A similar alloying strategy was applied to TMB2, where ternary Cr-Al-B, Cr-Si-B, and quaternary Cr-Al-Si-B thin films were synthesized by combinatorial co-sputtering. Si showed a limited solubility limit of about 4 at.% in the AlB2-structured thin films. Beyond this threshold, the formation of a secondary CrSi2 phase was observed in the ternary and quaternary coatings. Isothermal oxidation at 1200 °C for 1 h proved the crucial role of Si in improving the oxidation resistance. While binary CrB2-based systems oxidize at around 600 °C, Cr-Al-B was unable to withstand these conditions. However, in the quaternary Cr-Al-Si-B system, Si also acts as a key element, facilitating the formation of protective alumina scales. TEM analysis confirmed a layered oxide structure in the quaternary system, consisting of Si-O, Cr-O, and Al-O, which contributed to the improved oxidation resistance of TMB2. Overall, this thesis provides a comprehensive evaluation of novel coating materials, architectural approaches and alloying concepts to enhance oxidation resistance of protective ceramic coating materials. These findings contribute to the development of advanced high-temperature materials for applications in extreme environments, offering pathways to increase component lifetimes and improve operational efficiency in diverse high-temperature applications.