New advances on the thermo-mechanical properties of protective ceramic coatings

Abstract

The energy and aerospace sectors are increasingly demanding greater sustainability and efficiency, and hence, novel material concepts that can withstand extreme conditions and last longer. This demand drives the development of protective ceramic coatings to enhance and tailor mechanical, chemical, and thermal stability.The present work focuses on two prominent coating material classes: transition metal diborides and transition metal nitrides, exemplified here by physical vapor deposited (PVD) hexagonal TiB2-based coatings and face-centered cubic Ti1-xAlxN-based ones. In relation to an application-driven development approach, an essential aspect of this thesis was to balance fundamental innovation with practical applicability during coating characterization and development, enabling both the exploration of new concepts and the retention of a clear pathway toward immediate implementation. Therefore, within the first part of the thesis, a systematic investigation of TiB2-based thin films is conducted, focusing on their thermomechanical properties. In particular, the influence of coating architecture and alloying on the mechanical properties and fracture behavior at room temperature and especially elevated temperatures are the key aspects. The findings of Publication I demonstrate that the hardness anisotropy of hexagonal TiB2±z thin films grown by direct current magnetron sputtering can be employed to intentionally modify the mechanical properties across the coating cross-section. Structural gradients have been induced by varying the deposition pressure, which in turn controls the crystallographic orientation of hex-TiB2+-z. A direct correlation between texture and hardness was confirmed through a combination of synchrotron nano-diffraction and small-angle nanoindentation experiments. An alternative approach to enhance the mechanical properties is the incorporation of an additional element, as investigated in Publication II. This study is based on the ternary Ti1-xMoxB2+-z system. Through using two different synthesis routes, DCMS and HiPIMS, the energy input provided during growth strongly effects the microstructure and mechanical properties of Ti1-xMoxB2±z coatings. In detail, hardness, fracture toughness and fracture strength were improved by the addition of Mo to TiB2±z when using HiPIMS, whereas DCMS-deposited coatings exhibit a decline in mechanical performance. Building on these findings, Publication III addresses the fracture toughness of binary, ternary and quaternary Ti-TM-Si-B2+-z (TM= Ta, Mo) thin films at elevated, temperatures. Extending the in-situ cantilever bending tests from RT to 850 °C provides significant insight into the coating material behavior under service relevant conditions. The binary and the quaternary Ta-containing samples demonstrate a fully linear-elastic response over the entire temperature range. In contrast, the ternary Ti-Si- B2+-z and quaternary Ti-Mo-Si-B2+-z coatings exhibit (pronounced) ductility exceeding 600 °C. This transition in fracture behavior is primarily dominated by the phase constitution and spatial distribution of Si, where segregation – either as nanoclusters or along grain boundaries – enables plastic deformation. The second part of this thesis addresses the resistance against thermal degradation via oxidation and hot corrosion, with a particular focus on long-term experiments. Within Publication IV the oxidation resistance of Ti1-xAlxN-based coatings was increased through a novel alloy concept, Ti-Al-Ta-Si-N. To address the application relevance of the thesis, Ti-Al-Ta-Si-N was synthesized by cathodic arc evaporation, serving as a link to application-relevant techniques and facilitating practical implementation. By adding simultaneously Ta and Si into Ti1-xAlxN the oxidation resistance is drastically enhanced, while mechanical properties are not altered compared to unalloyed Ti1-xAlxN. Exposure to oxygen containing atmosphere at 850 °C for up to 500 h leads to the formation of a multi-layered oxide scale composed of an Al2O3 top-layer and a rutile (Ti,Ta,Si)O2 layer beneath – separated into two sublayers – being responsible for the excellent oxidation resistance. Even after 500 h oxidation the scale thickness is below 600 nm, clearly outperforming so far reported results. Furthermore, the Ti-Al-Ta-Si-N alloy concept also proved excellent resistance in hot gas corrosion atmospheres, clearly surpassing unalloyed concepts.In summary, both concepts for TiB2 and Ti1-xAlxN coating material families represent innovative ways to enhance mechanical and fracture characteristics, as well as oxidation and corrosion stability. This research is crucial for paving the way for novel protective coatings that enable next steps in high-temperature aerospace and energy production applications, both in the laboratory and in real-world conditions.