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，这些薄膜和涂层用于广泛的应用，包括微电子器件，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控制纳米晶体陶瓷中这些应力的新方法，其中小晶粒尺寸直接导致相邻晶体之间的极大量界面（即，晶界）。 这些晶界处的原子键合的微小变化可以在纳米颗粒膜和涂层中产生大的应力（相比之下，相同的工艺在晶界少得多的常规大颗粒材料中产生微不足道的应力）。 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。 该项目还包括与制造MEMS器件的几个小组的合作，其中压力管理至关重要。 另一个焦点是氧化物陶瓷，其中晶界诱导的应力与关键的电化学性能相关。 除了支持布朗大学的学生外，这项研究还雇用了Trinity学院的本科生（与Trinity教职员工共同PI瓦尔登合作）。 布朗大学的教育工作包括为研究生提供的年度计划，培养研究和指导技能，以及为K-12教师提供的国家认可的专业发展研讨会。技术支持：各种材料的纳米晶体薄膜和涂层是广泛的新兴应用的候选者。 在这个项目中选择用于研究的陶瓷包括通用汽车公司希望用于铝合金干加工的纳米金刚石（NCD）薄膜和几种氧化物。 这些薄膜中的晶界结构和残余应力之间的关系还没有很好地建立起来，这项研究预计将导致晶界工程策略，这将显着推进这些材料的应用。 在NCD中，晶界引起的应力相对较大，因此可以被操纵以对材料的总应力状态具有显著影响。 在这里，与氢的反应似乎特别重要，其他化学效应也正在研究中。 在氧化物陶瓷中，晶界诱导应力通常较小。 然而，这些膜的组成的适度变化可诱发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传导性）。因此，精确的应力测量提供了有关晶界现象的信息，这与这些材料的关键电化学性能直接相关。 这些应力研究提供了有价值的信息，补充了用其他更成熟的技术（如阻抗谱）获得的数据。 进行这些不同材料中晶界诱导应力研究的学生正在积极接受各种不同实验和建模技术的培训。 NCD的努力包括薄膜制造等离子体化学气相沉积，详细的薄膜表征与电子显微镜和拉曼光谱，和建模与连续有限元和原子的方法。 氧化物方面的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模型等。

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.