Laser surface sealing of plasma sprayed yttria stabilized zirconia ceramics

Abstract

Zirconia-based ceramics alloyed with different oxides such as calcia, magnesia, ceria and yttria have been used as thermal barrier coatings for gas turbines and other high temperature applications; such coatings have usually been applied by a surface welding technique which involves bonding the oxide mixtures to the metal substrate by plasma spraying. However, several problems have arisen during service due to penetration and diffusion of corrosive fuel through the resultant porosity which affects the life of the ceramic layer. Because of this problem, therefore, a new technique for the coating process is being investigated using laser technology.The present study investigates the possibility of using a high power CO2 laser to seal a thin layer of as-sprayed partially yttria stabilized zirconia (8wt%).A 2 kW CO2 laser has been used at different powers and traverse speeds (0.8 to 1.7 kW and 5 to 370 mm/s respectively) with a 3 mm beam diameter. Two plasma sprayed ceramic layers, (0.15 and 0.4 mm) thick consisting of 8wt% yttria stabilized zirconia ceramic respectively coated onto a 0.1 mm Ni-23Cr-6Al-0.4Y bond layer, were investigated.Microstructural studies of single tracks and partially overlapping tracks have been made using optical microscopy, scanning electron microscopy and X-ray diffractometry. Studies were made of surface topography, porosity, cracking and compositional analysis and proportions of phases formed.Zirconia-based ceramics alloyed with different oxides such as calcia, magnesia, ceria and yttria have been used as thermal barrier coatings for gas turbines and other high temperature applications; such coatings have usually been applied by a surface welding technique which involves bonding the oxide mixtures to the metal substrate by plasma spraying. However, several problems have arisen during service due to penetration and diffusion of corrosive fuel through the resultant porosity which affects the life of the ceramic layer. Because of this problem, therefore, a new technique for the coating process is being investigated using laser technology.The present study investigates the possibility of using a high power CO2 laser to seal a thin layer of as-sprayed partially yttria stabilized zirconia (8wt%).A 2 kW CO2 laser has been used at different powers and traverse speeds (0.8 to 1.7 kW and 5 to 370 mm/s respectively) with a 3 mm beam diameter. Two plasma sprayed ceramic layers, (0.15 and 0.4 mm) thick con...