Metallic thin film fatigue dominated by interface character

由于在微米和纳米尺度下赋予的独特性质，薄膜被用于各种各样的应用中。例如，金属薄膜是刚性和柔性电子器件和传感器、航天器或微机电系统（MEMS）表面反射镜的理想导电材料。在大多数应用中，薄膜受到循环载荷，并通过特定的疲劳机制失效。然而，亚微米薄金属膜的行为，并因此失败，不同于散装材料。这是由于衬底和自由表面的界面的显著存在，这强烈影响了与体疲劳有关的位错过程。尚未研究的是界面的类型如何控制薄膜的疲劳行为。界面可以被认为是硬的，用刚性陶瓷或金属基底形成，软的，靠近聚合物基底，或者膜可以没有界面并且是独立的。界面类型是控制金属薄膜变形机制和最终失效的主要参数，它是薄膜微观结构、屈服强度和循环应力幅的函数，为了获得薄膜及其特定界面的变形和失效信息，需要对其进行全面系统的研究。计划采用先进的原位微观力学测试方法来研究硬界面、软界面或无界面类型对金属薄膜损伤形成和失效的作用。值得注意的是使用复杂的原位膨胀测试、X射线衍射技术和透射电子显微镜以及循环机械测试来观察膜变形、可能的晶粒生长、挤出形成、膜破裂或分层。直接观测将使界面诱导机制的微观结构响应解耦。这些先进的技术和薄膜力学行为的专业知识是埃尔朗根-纽伦堡弗里德里希-亚历山大大学（Benoit Merle博士）和奥地利科学院Erich Schmid材料科学研究所（Megan Cordill博士）之间拟议合作的基础。两组之间独特的协同作用将使类似样品在独立和不同支撑条件下的疲劳行为首次得到系统比较。新的知识界面的具体故障机制将被用来生成基于机制的模型，薄膜故障，以及耐疲劳薄膜应用提供改进的设计标准。

Thin films are used in a wide variety of applications due to the unique properties which are imparted at the micro- and nanoscale. Metal films, for example, are ideal for electrical conduction in rigid and flexible electronics and sensors, reflectors for surface mirrors on spacecraft or micro-electrical-mechanical systems (MEMS). In most applications, thin films are cyclically loaded and fail through specific fatigue mechanisms. However, sub-micrometric thin metal films behave, and thus fail, differently than bulk materials. This has been attributed to the prominent presence of an interface to a substrate and to the free surface, which strongly affects the dislocation processes pertaining to bulk fatigue. What has not yet been examined is how the type of interface controls the fatigue behavior of thin films. Interfaces can be considered as hard, created with a rigid ceramic or metal substrate, soft, next to a polymer substrate, or films can have no interface and be free-standing. It is believed that the interface type is a dominant parameter that controls the deformation mechanisms and final failure of the thin metal films as a function of the microstructure, yield strength and cyclic stress amplitude.In order to obtain deformation and failure information on thin films and their specific interface, a thorough and systematic investigation is required. It is planned to use advanced in-situ micro-mechanical testing methods to examine the role of the interface type, hard, soft or no interface, on the damage formation and failure of thin metal films. Of note is the use of sophisticated in-situ bulge testing, X-ray diffraction techniques and transmission electron microscopy with cyclic mechanical testing to observe film deformation, possible grain growth, extrusion formation, film cracking or delamination. Direct observations will enable the decoupling of the microstructural response from the interface induced mechanisms. These advanced techniques and the expertise on thin film mechanical behavior is the foundation of the proposed collaboration between the Friedrich-Alexander University Erlangen-Nürnberg (Dr. Benoit Merle) and the Erich Schmid Institute of Materials Science of the Austrian Academy of Sciences (Dr. Megan Cordill). The unique synergies between the groups will notably allow the first systematic comparison of the fatigue behavior of similar samples in free-standing and in different supported conditions. The new knowledge about the interface specific failure mechanisms will be used to generate mechanism based models for thin film failure as well as provide improved design criteria for fatigue resistant thin film applications.