Hard Coatings: Toughness Enhancement through Responsive Phase Change

硬质涂层广泛应用于各行各业，可延长滑动接触部件的使用寿命并提高其性能。切削刀具涂层每年的全球市场规模为200亿至700亿美元，是航空航天、汽车和能源行业制造业必不可少的。过去二十年的研究已经导致这些涂层的硬度显著增加。然而，尽管有这些改进，这些陶瓷涂层的一个主要缺点仍然存在：它们很脆，导致在许多应用中形成裂纹和过早失效。该奖项支持材料科学和工程的基础研究，以解决这一缺陷。探索新的涂层材料，不仅硬，而且坚韧。新的涂层材料被设计成当暴露在大载荷下时原子会重新排列。该过程阻止小裂纹生长成大裂纹，这可以防止当前硬涂层材料的失效，进而防止部件的失效。这项研究有可能改变工业涂层材料的设计，并促进耐磨硬涂层的开发，用于新兴应用，从环保的无润滑剂切削工具到高性能轴承，用于燃油效率的喷气发动机和高温涡轮机，用于燃气，风力，或聚光太阳能发电厂。这种综合实验和理论研究工作将探索阶段-改变硬涂层材料的增韧机制。关键的想法是创建一个材料系统，其中裂纹尖端附近的应力集中诱导从亚稳立方相到稳定的六方相的转变，促进塑性和/或局部膨胀，这反过来又抑制裂纹扩展，并导致断裂韧性的急剧增加。这项研究探讨了耐磨氮化物和碳化物的延展性（以及韧性）是否可以通过应力诱导相变来增加，而不会牺牲它们的硬度。这种相变增韧对于氧化锆等块体陶瓷材料是众所周知的，如果成功地应用于涂层材料，则有可能显著提高保护涂层的耐磨性。该研究涉及（i）第一原理计算，以预测最有前途的组合物，（ii）涂层合成反应溅射，包括通过离子辐照，外延约束和温度的微观结构的控制，（iii）高压相变研究使用金刚石砧细胞，和（iv）材料表征纳米压痕，以证明增强韧性。

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.