Controlling Stress Evolution in Ceramic Thin Films and Coatings: Investigations of Mechanical and Chemical Responses

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This research focuses on controlling intrinsic stresses in several different polycrystalline ceramic films and coatings where residual stresses have an important effect on key materials properties. The intrinsic stresses are produced by the film growth process, and can thus be controlled during processing. While previous work has examined intrinsic stresses in metal films, ceramics have been investigated in far less detail. Four inter-related efforts will lead to a significant increase in both knowledge and our ability to control stress-related effects. First, in MEMS devices, residual stress effects at small length scales are critical. By controlling stress gradients in polycrystalline SiC, we expect to eliminate bending problems in free standing SiC films for MEMS applications. Secondly, non-stoichiometric oxide films are important for a variety of electrochemical applications. Initial work with TiO2-X shows that stresses due to compositional variations can be accurately measured, and that the defect chemistry of the material determines the type of stress changes observed. Additional work on oxides will explore yittria stabilized zirconia and CeO2-X, both of which are used as electrolytes in solid oxide fuel cells. As well, new models of tensile and compressive intrinsic stresses have been developed with previous NSF funding. These will be integrated into a unified model, applicable to a broad range of vapor-deposited polycrystalline films. Finally, the fracture resistance and adhesion of SiC coatings will be measured as a function of residual stresses and stress gradients. Our demonstrated ability to vary the film microstructure and stress independently during deposition will make it possible to isolate the effects of residual stresses. All three investigators are engaged in significant educational activities. Profs. Rankin and Sheldon will continue to promote several different K-12 outreach activities. Prof. Rankin is also the Associate Director of the Sheridan Center for Teaching and Learning at Brown, where she is regularly engaged in advancing the teaching skills of faculty and graduate students from a wide range of different disciplines. During the academic year, Prof. Walden is on the faculty at Trinity College, an undergraduate teaching institution. We will employ Trinity undergraduates in research efforts, providing them with experience in a research setting. This research will provide new knowledge and materials fabrication capabilities for controlling the mechanical stresses that invariably exist in coatings and thin-films of ceramic materials. These stresses can be either detrimental to the materials performance (e.g., by promoting failure) or beneficial (e.g., by improving toughness). Thus, proper control of these stresses is critical. Work is focused on two types of materials, silicon carbide that is of interest in miniature machines (so-called micro-electro-mechanical systems or MEMS), and oxide ceramics that are useful for electrochemical applications such as fuel cells. Much of the knowledge that is obtained from these investigations will also be broadly applicable to thin-films and coatings that are used for a wide variety of other applications, including microelectronic devices, jet engines, and hard coatings that are used to extend the life of different types of machinery.