EFFECTS OF SINTERING TEMPERATURE ON THE MICROSTRUCTURES AND ADHESION STRENGTH OF SLURRY SPRAYED YSZ/NiCoCrAlYTa FUNCTIONALLY GRADED-THERMAL BARRIER COATING ON INCONEL 625

Fadzlun Nadrah Mohd Sharuddin; Muhamad Azizi Mat Yajid

Fadzlun Nadrah Mohd Sharuddin Department of Materials, Manufacturing and Industrial Engineering, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia.
Muhamad Azizi Mat Yajid Materials Research and Consultancy Group (MRCG), Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310, Skudai, Johor, Malaysia.

Keywords: Thermal barrier coating, functionally graded coating, yttria stabilized zirconia, sintering temperature

Abstract

Thermal barrier coating (TBC) has been widely used in protecting turbine components exposed to extreme temperature conditions. The conventional TBC system comprises of a metallic bond coat layer deposited on the substrate and yttria stabilized zirconia (YSZ) top layer. However, the thermal expansion mismatch between the bond coat and top coat layer has become a problem that leads to coating delamination. To mitigate this issue, the functionally graded-thermal barrier coating (FG-TBC) method is adopted, where a homogenous coating structure consists of composite materials layered on the substrate by gradually changing the compositional ratios over the coating thickness. This smooth transition from the substrate to the top layer minimizes the thermal expansion mismatch between the coating layers, reduces the internal residual stresses, and increases the bonding strength. On the other hand, plasma spray has been the commonly used method for depositing TBC. However, in this study, YSZ/NiCoCrAlYTa FG-TBC was fabricated by using the slurry spray technique (SST), considering the lower cost and simplicity of the method. In determining the optimum sintering temperature, effects of various sintering temperatures (i.e., 950, 1000, 1050, 1100, and 1150 ℃) on the microstructures and adhesion strength were investigated. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were applied to characterize the microstructures, and a pull-off adhesion test was conducted to measure the adhesion strength between the coating layers and the substrate. The findings showed that samples sintered at 1100 ℃ revealed dense FG-TBC layers with a continuous reaction layer at the substrate interface, minimal porosity, and no presence of cracks, with the highest adhesion strength, which is 22.87 MPa. Therefore, the effective sintering temperature for slurry-sprayed YSZ/NiCoCrAlYTa FG-TBC was considerably achieved at 1100 ℃.