Microstructural-Mechanical Property Relationship in CVD Deposited alpha-Aluminum Oxide and Titanium Carbonitride Coatings

While chemical vapor deposited coatings have been widely used in many different industries, the influence of the microstructural features and the thermal stress on the macroscopic properties is not well understood. In this thesis, a comprehensive study of the grain size, aspect ratio, texture, hardness, and residual stresses of Al2O3 and TiCxN 1-x coatings on four tool inserts is described. To do this, methods were developed to measure microstructural features and calculate residual stresses. A correlation was then established between the residual stress calculated in the observed microstructures and the experimentally measured hardness. Assuming that the observed correlation holds in all cases, the thermal stresses in a number of synthetic microstructures were calculated to isolate the influence of individual microstructural characteristics on the hardness.;Two-dimensional finite element analysis (FEM) is used for the residual stress calculation in these materials. The thermal strains and stored elastic energy in the alpha-Al2O3 layer are larger than those in the TiCxN1-x layer. Furthermore, the mean value and distribution of stored elastic energy are influenced by the texture in the alumina layer. Coatings with weaker texture have a broader distribution of thermal stresses. Coatings with alumina oriented so that the [0001] direction is parallel to the film growth direction have less stored elastic energy. This is because the thermal expansion perpendicular to [0001] is less than the thermal expansion parallel to [0001] and, therefore, the thermal expansion mismatch between the alumina coating and the substrate is minimized when grains are oriented with [0001] perpendicular to the substrate.;The thermal stresses in hypothetical coatings with synthetic microstructures were also computed. These calculations tested the effects of coating thickness, channel crack spacing, composition of the TiCxN1-x layer, grain aspect ratio, and cobalt enrichment of the substrate on the thermal stresses. In one set of calculation, AlON was substituted for Al2O 3. Based on the thermal stresses, it is concluded that the composition of the TiCxN1-x layer and the cobalt enrichment of the substrate have the greatest impact on coating hardness. The calculation also suggested that the substitution of AlON for Al2O3 is a promising potential route for improving hardness.;Electron backscatter diffraction mapping has been used to study the microstructural features and texture of Al2O3 and TiCxN 1-x. The Al2O3 layers have [0001] or [101¯4] texture in the growth direction that are 4.2 to 8.8 times random. The TiC xN1-x layers have weak [112] or [101] textures in the growth direction and are highly twinned, with coherent twins making up 13%--20% of the grain boundary length. Nanoindentation was used to measure the hardness of the TiCxN1-x and alpha-Al2O 3 layers in each coating. The hardest coatings consist of highly twinned TiCxN1-x layers with weak [112] texture and alpha-Al 2O3 with strong [101¯4] texture.