Modification of thermally sprayed ceramic oxide coatings by chemical densification processing

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 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, solvothermal reactions occurring between the solvent and chromium/aluminium from the bond coat, lead to the crystallization of zirconia at the topcoat/bondcoat interface and in the connected pores of the ceramic layer. The porosity of the densified YSZ top coat was reduced by 50% and the adhesion of the top coat was also improved. The tribological properties of the plasma-sprayed and post-treated YSZ coatings were investigated by using tribometer in form of the ball-on-disc test and the three-ball-on-disc test under dry conditions. After the sliding tests the worn surfaces were examined. The formation of micro cracks along the splat boundaries are considered as main wear mechanism, which finally leads to material failure of plasma-sprayed YSZ coatings. The post-treated YSZ coatings presented higher wear resistance and very low friction coefficient after 1350 sliding cycles compared with the as-sprayed coatings. It is caused by the enhanced interfacial strength between the splats of the plasma-sprayed YSZ coatings after the solvothermal treatment. The application of scratch tests and Rockwell tests showed that specimens post-treated at 700 °C for 72 hours have much better mechanical properties compared to as-sprayed YSZ coatings. This observation correlates with the tribological investigation indicating that the tribological properties of the plasma-sprayed ceramic coatings can be improved by the more cohesive structure. A first impression of the wear mechanisms of the coatings was also given. Thermal shock resistance and high temperature oxidation resistance of the plasma-sprayed YSZ thermal barrier coatings (TBCs) were examined. The post-treated two-layer samples showed increased high-temperature oxidation resistance and thermal shock resistance. Various factors contribute to a durability improvement of the plasma-sprayed YSZ thermal barrier coating. The densification of the YSZ top coat leads to a limitation of oxygen penetration towards the bond coat, which controls the growth of the TGOs at the high temperature applications. Moreover, the inter-lamellar bonding and the ceramic top-coat/bond-coat adherence were also enhanced. The two improved properties are considered to be the major factors for the higher protective efficiency of the post-treated YSZ TBCs. The spalling failure mechanisms for the examined TBCs were discussed.