Control of Intrinsic Stresses in Ceramic Thin Films and Coatings Produced by Chemical Vapor Deposition

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The primary motivation for this research project is the need to control residual stresses in ceramic thin films and coatings, which encompasses a wide range of materials used for both electronic and structural applications. Residual stresses arise from the thermal expansion mismatch between film and substrate, and from various intrinsic mechanisms during film growth. The viability of ceramic films and coatings are typically evaluated by calculating thermal stresses, however, this calculation is often inaccurate because it ignores intrinsic stresses. Previous work at Brown demonstrates that tensile stresses caused by island coalescence during film growth often overshadow thermal stresses. This work also shows that these intrinsic stresses can be controlled by altering processing during island coalescence. These efforts are focused on CVD diamond, which is an excellent system for these studies since diamond has large intrinsic stresses that are problematic for a number of applications and because the extensive literature on CVD diamond provides a strong basis for understanding growth stresses. Building on the previous success with CVD diamond, the following activities will be carried out during the project: 1) Establish that intrinsic tensile stresses can be controlled by altering processing during grain coalescence in a wide range of ceramic thin films and coatings; 2) Develop better models for describing the evolution of intrinsic stresses; 3) Investigate relationships between residual stress and film properties (fracture, etc.), which will be based on the processing methods developed at Brown that provide a unique approach for varying stress without changing the substrate, grain size, or film composition; 4) Control intrinsic stresses to modify the total residual stress in novel ways; 5) Investigate discrepancies between Raman spectroscopy and other methods that are used to measure residual stress in CVD diamond; 6) Provide direct input to ongoing work on oxide coatings at Oak Ridge National Laboratory; and 8) Continue a variety of educational efforts, including the ExSEL program at Brown (co-PI Prof. Rankin), and an accredited Materials Science workshop for local middle and high school teachers that was developed by the PI. Research activities will continue to involve undergraduates from both Brown and Trinity (an undergraduate teaching institution). %%%Ceramic and diamond thin films are used in a variety of technologically important applications such as computers and telecommunications. During processing these thin films are develop stress that limit their effectiveness in the application. This research project, which will be carried out by two investigators at Brown University and one from Trinity College (an undergraduate institution), will investigated methods of controlling the stress that form. The results from this work will lead to better understanding and improved control of stresses in a variety of thin films and coatings, thus enhancing their usage in the application.***