Laser cleaning process of high-pressure turbine blade: Characterization and removal of surface contaminants

Laser cleaning has been introduced as an effective method for removing contaminants from the surface of a turbine blade. In this investigation, a continuous wave fiber laser with a power density of 7.96 x 106 W/cm2, a wavelength of 1064 nm, and a focusing distance of 305 mm under an argon gas was applied. The used laser removed CaO-MgO-Al2O3-SiO2 (CMAS) as contaminants from the Y2O3-stabilized ZrO2 layer of a thermal barrier coating (TBC) on a high-pressure turbine blade made of a nickel-based superalloy substrate. At first, the characterization of the CMAS and TBC properties, such as thickness, roughness, and morphology, as well as phase identifications were performed. CMAS on the high-pressure turbine blade (HPTB) surface had a mixture of granular and plate structures. The major imperfections of the surface of the out-of-service HPTB were infiltration of CMAS into the TBC, chemical interaction between CMAS and the TBC, transverse cracks, delamination of the TBC, structural changes of the TBC, separation of YSZ from TGO, and deformation of the TBC on the pressure side and the leading edge of HPTB. The main aim of this work was to remove non-infiltrated CMAS from the TBC coating through the laser cleaning process. Then, various parameters of the laser cleaning process such as power and scanning speed were investigated. After that, various analyses of laser-cleaned samples such as scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), optical microscopy (OM), X-ray diffraction (XRD), thickness test, and roughness measurement were accomplished for the optimum parameters of the laser cleaning process. Finally, the mechanism involved in the laser cleaning process to remove the contamination was presented in detail. The results demonstrated that the optimum parameters of the laser cleaning process to completely eliminate targeted contaminants were a power density of 7.16 x 106 W/cm2, a scanning speed of 0.01 mm/s, and a focusing distance of 305 mm under an argon gas atmosphere.