Abstract

Thermal barrier coatings have been used for the last half century to protect parts in high-temperature service from premature damage. Thermal barrier coatings are mostly produced by thermal spray techniques, especially plasma spray processes. They are widely used in the aerospace and automotive industries and in power plants, applications in which there is an ever-increasing demand for further improvement of functionality and durability. In the field of material design, use of composite coatings rather than monolithic material, and nano-structure instead of conventional grain sizes, are among the solutions most often considered. Suspension plasma spray is an innovative process for production of nano-structured thermal barrier coatings. This research project was directed toward a superior thermal barrier coating using suspension plasma spray deposition of the alumina-yttria stabilized zirconia composite material with nano-crystalline structure. Crystallization of the amorphous phase is introduced as a new route toward nano-crystallinity in ceramics, as was previously applied in the metals and alloys. The suspension plasma spray process was used in production of coatings with comparatively high amorphous content. The work concentrates on three major aspects of thermal barrier coating improvement. The primary focus of the project is amorphous phase formation and the roles it plays in properties and structure. This phase, which was found beneficial to nano-crystallinity, improves the mechanical properties after heat treatment. The second concentration is the suspension plasma spray process and the versatility of its resulting microstructures, and a comparison of suspension with the conventional plasma spray process. The third aspect is the properties of alumina-YSZ composite material as a choice for thermal barrier coating application. It was found that the material can compete with the present YSZ material in some aspects (e.g. thermal resistivity and hardness), but suffers from some other deficiencies (e.g. brittleness and high erosion rate). This suggests considering some other compositions of the composite, while continuing in microstructural improvement of this material for thermal barrier application.