Hard Coatings: Toughness Enhancement through Responsive Phase Change

硬质涂层广泛应用于各种行业，可延长滑动接触部件的使用寿命并提高其性能。刀具涂层代表着每年200 - 700亿美元的世界市场，对航空航天、汽车和能源工业的制造至关重要。过去二十年的研究已经使这些涂层的硬度有了相当大的提高。然而，尽管有了这些改进，这些陶瓷涂层的一个主要缺点仍然存在：它们很脆，在许多应用中导致裂纹形成和过早失效。该奖项支持材料科学和工程的基础研究，以解决这一缺点。开发了一种既硬又有韧性的新型涂层材料。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。这一过程可以防止小裂纹成长为大裂纹，从而防止当前硬涂层材料的失效，从而防止组件的失效。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发展，用于新兴应用，从环保型无润滑油切削工具到节能喷气发动机的高性能轴承，以及燃气、风能或聚光太阳能发电厂的高温涡轮机。这项综合实验和理论研究将探索硬质涂层材料相变增韧机理。关键思想是创建一个材料系统，在该系统中，裂纹尖端附近的应力集中诱导从亚稳立方相转变为稳定的六方相，促进塑性和/或局部扩展，从而抑制裂纹扩展并导致断裂韧性的显着增加。该研究探讨了在不牺牲硬度的情况下，是否可以通过应力诱导的相变来提高耐磨氮化物和碳化物的延展性（以及韧性）。这种转变增韧是众所周知的大块陶瓷材料，如氧化锆，如果成功地应用于涂层材料，有可能显著提高保护涂层的耐磨性。该研究包括(i)第一性原理计算，以预测最有前途的成分；（ii）反应溅射合成涂层，包括通过离子辐照、外延约束和温度控制微观结构；（iii）使用金刚石砧细胞进行高压相变研究；（iv）通过纳米压痕表征材料，以证明增强的韧性。

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.