Microstructure and mechanical properties of ceramic and metallic thermal spray coatings

Within the present work microstructure and mechanical properties of thermal spray coatings will be discussed with an emphasis on thermal barrier coating applications. Yttria stabilized zirconia is a commonly used material for thermal barrier coatings. The potential of a novel nanostructured feedstock material was investigated and the properties compared to a conventional feedstock material.; Thermal spray coatings consist of a complex structure of interlocked particles, pores and cracks and the coating microstructure directly influences the mechanical properties. It is necessary for the coating to be tolerant to strain that is evolving, for example, from thermal expansion mismatch. Porous microstructures lower coating stiffness and also increase the ability to endure irreversible deformation by microcracking. The novel nanostructured feedstock microstructure consists of partially molten particles that form agglomerates where a large fraction of the pores are of size less than 1 mum in diameter.; The effect of the deposition process and its parameters on the microstructure and hardness was studied. The porosity was determined by image analysis and correlated to the measured hardness. Selected samples were extensively studied using a depth sensitive indentation technique. Elastic modulus and hardness were primarily investigated; however, also fracture behavior is reported and discussed.; Coating mechanical properties are directly associated with the resistance to damage caused by mechanical forces. The processes that primarily damage the coating surface include abrasion, erosion, and sliding wear as well as cavitation. However, mechanical interaction with the surface also affects the material to a depth that is related to the extent of the stress field. Mechanical processes, such as plastic deformation associated with Hertzian spherical contact, may initiate failure under the surface. Mechanical stresses that act within the coating originate from external mechanical loading, thermal expansion mismatch or thermal gradient, and residual stress.; As well, other non-mechanical effects are related to mechanical properties. A significant part of the residual stress is a result of oxidation growth and can cause cracking and delamination. Cracking, in turn, enhances penetration of corrosive substances, thereby influencing high temperature stability and overall corrosion resistance. Chemical processes are usually enhanced by high temperature and include oxidation, phase reaction and formation of new phases, which is controlled primarily by diffusion. Diffusion processes at high temperature cause grain boundary movement and grain growth. These processes lead to high temperature, time-dependent deformation and material densification via sintering. (Abstract shortened by UMI.)...