Modification of thermally sprayed ceramic oxide coatings by chemical densification processing [thesis]

Yaping Ye
2016
Plasma spraying is a very efficient deposition technology for ceramic coatings. Plasma-sprayed yttria stabilized zirconia (YSZ) ceramic coatings have been widely used as thermal barrier coatings and wear- and corrosion-resistant coatings in high temperature applications and aggressive environments due to their high hardness, wear resistance, heat and chemical resistance, and low thermal conductivity. However, the highly porous structure and moderate adhesion of the plasma-sprayed YSZ coatings
more » ... ayed YSZ coatings are the most often encountered problems for their high temperature applications involving wear and corrosion. Therefore, the main aim of this work was to reduce the porosity and improve the adhesive strength of the plasma-sprayed YSZ coatings by a novel chemical densification process. In this study, a two-layer system - consisting of atmospheric plasma-sprayed 8 wt% yttria-stabilized zirconia (8YSZ) as top coat and Ni-based alloy coatings as a bond coat - was used as a model system. In order to conduct the chemical densification post-treatment on the two-layer system at atmospheric pressure, a simple experimental apparatus was created. It contains an oven and a gas system. The temperature may go up to 1100 °C and the concentration of hydrogen chloride in nitrogen gas can be computer-regulated and -controlled. First of all, the solubility of zirconia in a mixture of molten salts in various gas atmospheres in dependence of temperatures was determined. The maximum solubility of zirconia was achieved in a mixture of equimolar molten salts of KCl, ZnCl2, K2SO4 and ZnSO4 in a nitrogen atmosphere containing 2% hydrogen chloride and at temperatures ranging between 600 °C and 800 °C. This was selected as optimal condition for the chemical densification process und used for the post-treatment of the two-layer samples. Investigations of the microstructure of post-treated YSZ coatings showed that the densification of the ceramic coatings could be achieved at 800 °C for a shorter time. During the chemical densification process, solvother [...]
doi:10.5282/edoc.19702 fatcat:qertrrzq7rf75pja62bj5w3rwm