Grain Boundary Induced Stresses in Nanocrystalline Ceramic Coatings and Thin Films

Brown University, Providence RI

Investigators

Abstract

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.