Grain Boundary Induced Stresses in Nanocrystalline Ceramic Coatings and Thin Films

非技术描述：内应力是薄膜和涂层性能和失效的主要因素，这些薄膜和涂层广泛应用于微电子器件，航空航天和能源系统的保护涂层，微机电系统（MEMS），化学传感器和燃料电池。布朗大学最近的研究发现了在纳米晶陶瓷中控制这些应力的新方法，在纳米晶陶瓷中，小晶粒尺寸直接导致相邻晶体之间的大量界面（即晶界）。这些晶界上原子键合的微小变化可以在纳米颗粒薄膜和涂层中产生很大的应力（相反，在晶界少得多的传统大颗粒材料中，相同的过程产生的应力微不足道）。这项研究探索了在几种不同的、技术上重要的材料中控制这些应力的新方法。纳米晶金刚石（NCD）的工作包括与通用汽车公司的科学家进行实质性合作，他们对干式加工的低摩擦涂层感兴趣。该项目还包括与几个制造MEMS器件的团队合作，其中压力管理至关重要。另一个焦点是氧化陶瓷，其晶界诱导应力与关键的电化学性能有关。除了支持布朗大学的学生外，这项研究还雇佣了三一学院的本科生（与三一学院的一名教员Walden合作）。布朗大学在教育方面的努力包括为研究生设立的年度项目，旨在培养研究和指导技能，以及为K-12教师设立的国家认可的专业发展研讨会。技术细节：各种材料的纳米晶薄膜和涂层是广泛新兴应用的候选材料。在这个项目中选择研究的陶瓷包括纳米晶金刚石（NCD）薄膜，通用汽车公司希望将其用于铝合金的干式加工，以及几种氧化物。这些薄膜的晶界结构与残余应力之间的关系尚未很好地确定，本研究有望导致晶界工程策略，从而显著推进这些材料的应用。在NCD中，晶界诱导应力相对较大，因此可以操纵晶界诱导应力对材料的总应力状态产生重大影响。在这里，与氢的反应似乎特别重要，其他化学效应也在研究中。在氧化物陶瓷中，晶界诱导应力通常较小。然而，这些薄膜组成的适度变化会引起与重要电化学现象相关的应力（例如，固体氧化物燃料电池电解质中的离子和电子电导率）。因此，精确的应力测量提供了与这些材料的关键电化学性能直接相关的晶界现象的信息。这些应力研究提供了有价值的信息，补充了其他更成熟的技术（如阻抗谱）获得的数据。从事这些不同材料的晶界诱导应力研究的学生正在接受各种不同实验和建模技术的积极训练。在NCD方面的努力包括等离子体化学气相沉积制备薄膜，用电子显微镜和拉曼光谱详细表征薄膜，以及用连续体有限元和原子方法建模。氧化物的研究工作包括金属有机化学气相沉积、溶胶凝胶合成、电子显微镜和详细的电化学建模。

NON-TECHNICAL DESCRIPTION:Internal stresses are a major factor in the performance and failure of thin films and coatings that are used for a wide range of applications, including microelectronic devices, protective coatings for aerospace and energy systems, microelectromechanical systems (MEMS), chemical sensors, and fuel cells. Recent research at Brown University has discovered new methods for controlling these stresses in nanocrystalline ceramics, where the small grain sizes lead directly to an extremely large number of interfaces between neighboring crystals (i.e., grain boundaries). Small changes in the atomic bonding at these grain boundaries can produce large stresses in nanograined films and coatings (in contrast, the same processes produce insignificant stresses in conventional large grained materials where there are far fewer grain boundaries). This research explores new methods of controlling these stresses in several different, technologically important materials. Work on nanocrystalline diamond (NCD) includes a substantial collaboration with scientists at General Motors who are interested in low friction coatings for dry machining. This project also includes collaborations with several groups making MEMS devices, where stress management is crucial. Another focus is oxide ceramics where grain boundary induced stresses are related to key electrochemical properties. In addition to supporting students at Brown University, this research employs undergraduates from Trinity College (in conjunction with co-PI Walden, a Trinity faculty member). Educational efforts at Brown include an annual program for graduate students which foster research and mentoring skills and a state-accredited professional development workshop for K-12 teachers.TECHNICAL DETAILS:Nanocrystalline films and coatings of various materials are candidates for a wide range of emerging applications. The ceramics chosen for study in this project include nanocrystalline diamond (NCD) films which General Motors hopes to employ for dry machining of Al alloys, and several oxides. Relationships between grain boundary structure and residual stresses in these films are not well established, and this research is expected to lead to grain boundary engineering strategies that will significantly advance the application of these materials. In NCD, the grain boundary induced stresses are relatively large, and can thus be manipulated to have a significant impact on the total stress state of the material. Here, reactions with hydrogen appear to be particularly important, and other chemical effects are also being investigated. In oxide ceramics, the grain boundary induced stresses are generally smaller. However, modest changes in the composition of these films can induce stresses that are related to important electrochemical phenomena (e.g., ionic and electronic conductivity in solid oxide fuel cell electrolytes). Thus, precise stress measurements provide information about grain boundary phenomena, which are directly related to key electrochemical properties of these materials. These stress studies provide valuable information that compliments data obtained with other, more established techniques such as impedance spectroscopy. The students conducting the research on grain boundary induced stresses in these different materials are being actively trained in a variety of different experimental and modeling techniques. The efforts on NCD include film fabrication by plasma chemical vapor deposition, detailed film characterization with electron microscopy and Raman spectroscopy, and modeling with both continuum finite element and atomistic methods. The work on oxides includes film fabrication by metal organic chemical vapor deposition, sol gel synthesis, electron microscopy, and detailed electrochemical modeling.