Modified Thick Thermal Barrier Coatings

Abstract

This thesis studies the microstructures of modified zirconia based thick thermal barrier coatings as well as their properties. Plasma sprayed yttria stabilised zirconia (8Y2O3-ZrO2) was the basic reference coating, but magnesia (MgO) and ceria (CeO2) stabilised zirconia coatings were also studied. Coating microstructures were mainly modified by post treatments, such as phosphate based sealing treatments and laser glazing. These procedures were carried out in order to improve particular coating properties such as erosion resistance, thermal cycling resistance and hot corrosion resistance. The work concentrated mainly on optimising the coating modification procedures, performing detailed coating characterisation, determining the coating mechanical and thermal properties and testing their high temperature properties in hot corrosion and thermal cycling experiments.The modification procedures changed coating microstructures near the surface. Phosphate sealants penetrated approximately 300-400 µm into the coating microcracks and pores reducing the open porosity by 24-48 % depending on the coating material. It was found that the sealant improved the cohesion of the splat boundaries by adhesive binding and chemical bonding mechanisms. In laser glazing it was possible to control the melting of the ceramic coating surface. Optimal thickness of the melted layer was 50-150 µm leading to a dense surface layer with specific vertical macrocrack structure.Modification processes strongly affected on the coating mechanical and wear properties. Microhardness of the phosphate sealed coatings was increased by 15-55 % and as much as 70-100 % in the case on laser-glazed coatings. The strengthening effect of the phosphate sealing was clearly seen in the four-point bending tests, where the modulus of rupture in bending (RB) of the 8Y2O3-ZrO2 coating was increased by more than 200 %. At the same time, the bending modulus (EB) of the phosphate sealed coating was almost eight times higher than the as-sprayed reference coating. In the laser-glazed 8Y2O3-ZrO2 coating the modulus of rupture in bending was one fourth and the bending modulus only one fifth that of the as-sprayed coating. Erosion resistance of the 22MgO-ZrO2 and 8Y2O3-ZrO2 coatings was improved by 65-70 % due to the phosphate based sealing treatment. The average improvement in the laser-glazed 8Y2O3-ZrO2 coating was 35 %.Thermal conductivity (k(T)) of all studied zirconia based coatings at a temperature range of RT-1250°C was more than doubled by the phosphate sealing. Sealing also weakened the high temperature phase stability of the 8Y2O3-ZrO2 coating at temperatures over 1000°C. Laser glazing had only a minor effect on the thermal properties of the coating. Depending on the macrocrack structure and its orientation, laser glazing either slightly raised or slightly lowered thermal conductivity.Modification processes had no clear beneficial effect on coating hot corrosion resistance, when exposed in air to a NaSO4-V2O5 based deposit at 650, 750, and 850°C for 48-1000 hours. The penetration of melt deposit into the phosphate sealed coatings was lowered in some degree if compared to the as-sprayed coatings. However, the phosphate sealed coatings failed in hot corrosion tests mainly because of the strong compressive stresses generated during the test. The compressive stresses were mainly induced when tetragonal and cubic zirconia phases transformed to monoclinic zirconia. The microstructure of the laser-glazed coatings was not optimal considering the hot corrosion test method (melt deposit exposure). The melt deposit penetrated through the vertical cracks in the laserglazed top layer and affected the coating structure much as it did in the case of as-sprayed coatings. The laser-glazed zone itself at the top of the coating was rather unaffected.Thermal cycling resistance of the 8Y2O3-ZrO2 coating was lowered by the phosphate sealing treatment. The reasons for the deterioration of the strain tolerance of the phosphate sealed coating were the increased elastic modulus due to better cohesion of splats and compressive internal stresses. Thermal cycling behaviour of the laser-glazed 8Y2O3-ZrO2 coatings was superior compared to the reference coating. Reduced elastic modulus due to the macrocracks made the laser-glazed coating much more strain tolerant.