Hard Coatings: Toughness Enhancement through Responsive Phase Change

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Hard coatings are used in a wide range of industries and prolong the life and increase the performance of components that experience sliding contact. Coatings on cutting tools represent an annual world market of $20-70 billion, and are essential for manufacturing in aerospace, automotive, and energy industries. Research over the last two decades has resulted in a considerable increase in the hardness of these coatings. However, despite these improvements, a major shortcoming of these ceramic coatings remains: they are brittle, leading to crack formation and premature failure in many applications. This award supports fundamental research in materials science and engineering to address this shortcoming. New coating materials are explored which are not only hard, but also tough. The new coating materials are designed such that atoms rearrange when exposed to large loads. This process stops small cracks from growing into large cracks, which can prevent failure in current hard coating materials, and in turn, failure of the component. This research has the potential to transform industrial coating materials design and facilitate the development of wear-resistant hard coatings for emerging applications ranging from environment-friendly lubricant-free cutting tools to high-performance bearings for fuel-efficient jet engines and high-temperature turbines for gas, wind, or concentrating solar power plants.This integrated experimental and theoretical research effort will explore phase-change toughening mechanisms for hard coating materials. The key idea is to create a materials system where the stress concentration near crack tips induces a transformation from a metastable cubic to a stable hexagonal phase, facilitating plasticity and/or local expansion which, in turn, suppresses crack propagation and results in a dramatic increase in the fracture toughness. The research explores if the ductility (and therefore the toughness) in wear-resistant nitrides and carbides can be increased through stress-induced phase change, without sacrificing their hardness. Such transformation toughening is well known for bulk ceramic materials like zirconia and, if successfully applied to coating materials, has the potential to dramatically increase the wear-resistance of protective coatings. The study involves (i) first-principles calculations to predict most promising compositions, (ii) coating synthesis by reactive sputtering including control of microstructure through ion-irradiation, epitaxial constraints, and temperature, (iii) high pressure phase transition studies using a diamond anvil cell, and (iv) materials characterization by nanoindentation to demonstrate enhanced toughness.