A study of high temperature oxidation and hot corrosion of advanced multi layer thermal barrier coatings

Turbine blades used in industries are operated at high temperatures for maximum efficiency and power output. Due to the elevated operating temperatures the blade materials are prone to damage. Thermal barrier coatings (TBCs) are thin film coatings used for the protection of turbine blade materials used in industries. The failure of TBCs is due to several factors like severe oxidation, hot corrosion, creep, wear and spallation. The study of elevated temperature damage of TBCs has become major research area and the improvement towards increased life of the coatings is the objective of this study. Oxidation is mainly seen in the bond coat in which aluminum is present. Aluminum is highly reactive and forms alumina easily when reacted with oxygen present in air. This oxidation leads to the growth of oxide layer in between the bond coat and ceramic top coat which ultimately causes the delamination. In this study, the oxidation of bond coat is measured using Thermo Gravimetric Analysis (TGA) experiments and the growth kinetics is determined. Two different types of bond coats NiCoCrAlY and NiCoCrAlY+0.25%Hf were oxidized isothermally and the growth kinetics are evaluated. Another important cause for the failure of TBCs can be due to hot corrosion. Type I (705°C) and Type II (900°C) hot corrosion condition experiments are performed using Dean Rig testing equipment and the results are analyzed. This hot corrosion caused during in-service conditions can be due to the many impurities present in the operating gases and the fuels. In this study, different compositions of alumina are added to the ceramic Yttria Stabilized Zirconia (YSZ) top layer and the hot corrosion of the samples is evaluated. The samples are prepared by two processes of Air Plasma Spray (APS) and Electron Beam Physical Vapor Deposition (EB-PVD) with composition change of 85%YSZ-15%Alumina and 70%YSZ-30%Alumina. After the hot corrosion of TBC samples the mechanical property changes are analyzed using the nano indentation technique. Indentation is performed using the Berkovich indenter on the cross section of the multi layer thermal barrier coating samples and the hardness and elastic behavior are evaluated.;The oxidation results showed the growth process follows parabolic rate and activation energies for NiCoCrAlY and NiCoCrAlY+0.25%Hf bond coats are 270.98 kJ/mol and 126.50 kJ/mol respectively. The presence of hafnium showed resistance in oxidation during isothermal process with the extension of time. The increase of alumina in the multi TBCs showed better performance in both Type I and Type II hot corrosion resistance with EBPVD processed multi layer thermal barrier coatings. Study of Nano Indentation results demonstrated Young's modulus and hardness decreased which indicates the effect of hot corrosion.