Studies of Air Plasma Sprayed Thermal Barrier Coatings and Their Performance for Applications in Siemens Medium Size Gas Turbines

Siemens medium size gas turbines are widely used in various industrial applications, for example, power generations including reserve power supply, mechanical drives for oil and gas industries, ships, and paper industries, and, etc. Such different applications lead to various types of operation conditions in the turbines, such as both base and peak loadings, varied turbine inlet temperatures, and, different grades of fuel. Air plasma sprayed thermal barrier coatings (TBCs) that effectively increase gas turbine efficiency and prolong life of hot components have been extensively introduced in the gas turbines. Due to the unique constitution of the ceramic coating on metallic materials, it is more critical for TBC to survive from all the different operation conditions. In case of loading types and cycle types, a study of cycle effects on TBC adhesion has been made since the TBC adhesion is strongly related to TBC life. In this study, isothermal, thermal cycle, and, thermal shock exposures have been performed prior to the adhesion tests and the cyclic tests have obviously affected the TBC life. In the Siemens turbines, ≥1mm thick 8% Yttria stabilized Zirconia coatings have been used on some hot components to not only reduce metal temperature but also minimize thermal fatigue damages on the components. In order to further increase life of such components, a low conductivity TBC with a lifetime comparable to conventional thin TBCs is intended to be found and to be used as a thin TBC to replace the thick TBC applications. For this purpose, a group of ceramic coatings have been investigated by various life tests, thermal conductivity measurement, and microstructure examination. In addition to the studies of TBC properties and low conductivity TBC, impacts of TBC surface roughness on turbine performance have also been evaluated and found to be important.