Grain Boundary Induced Stresses in Nanocrystalline Ceramic Coatings and Thin Films

非技术说明：内应力是影响薄膜和涂层性能和失效的主要因素，这些薄膜和涂层应用广泛，包括微电子设备、航空航天和能源系统的保护涂层、微电子机械系统(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陶瓷中这些应力的新方法，在纳米晶陶瓷中，细小的颗粒尺寸直接导致相邻晶体之间大量的界面(即晶界)。这些晶界的原子键的微小变化可能会在纳米颗粒薄膜和涂层中产生很大的应力(相比之下，相同的工艺在传统的大颗粒材料中产生的应力微不足道，因为传统的大颗粒材料的晶界要少得多)。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。在纳米金刚石(NCD)方面的工作包括与通用汽车公司的科学家进行大量合作，后者对用于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，在这些小组中，压力管理至关重要。另一个焦点是氧化物陶瓷，其中晶界诱导应力与关键的电化学性质有关。除了支持布朗大学的学生外，这项研究还聘请了三一学院的本科生(与三一学院的教职员工皮伊·沃尔登共同参与)。布朗大学的教育工作包括为研究生举办的培养研究和指导技能的年度计划，以及为K-12教师举办的经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适合广泛的新兴应用。在这个项目中选择的陶瓷包括通用汽车公司希望用来干式加工铝合金的纳米晶金刚石(NCD)薄膜，以及几种氧化物。这些薄膜中的晶界结构和残余应力之间的关系还没有得到很好的确定，这项研究有望导致晶界工程策略的提出，从而显著促进这些材料的应用。在NCD中，晶界诱导应力相对较大，因此可以操纵以对材料的总应力状态产生显著影响。在这里，与氢的反应似乎特别重要，其他化学效应也在研究中。在氧化物陶瓷中，晶界感生应力一般较小。然而，这些薄膜成分的轻微变化可能会引起与重要的电化学现象(例如，固体氧化物燃料电池电解液中的离子和电子传导性)相关的应力。因此，精确的应力测量提供了有关晶界现象的信息，这些现象与这些材料的关键电化学性质直接相关。这些压力研究提供了有价值的信息，补充了用其他更成熟的技术获得的数据，如阻抗谱。在这些不同材料中进行晶界诱导应力研究的学生正在接受各种不同的实验和建模技术的积极培训。对NCD的研究工作包括：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对薄膜进行详细的表征，以及用连续介质有限元和原子学方法进行建模。氧化物方面的工作包括金属有机化学气相沉积制备薄膜、溶胶凝胶合成、电子显微镜和详细的电化学模拟。

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.