Two-Step Sintering Improved Compaction of Electrophoretic-Deposited YSZ Coatings

Two-step sintering is proposed to enhance the compactness and phase stability of yttria-stabilized zirconia (YSZ) coating on Inconel 625. The sintering treatment is denoted as low step (LS), low-first step (LFS), high-first step (HFS), and high step (HS). Low and high refer to 750 and 1200 °C, respectively. The YSZ layer was investigated using a scanning electron microscope, x-ray diffractometer, and electron backscattered diffractometer. The results revealed that three relatively compact and uniform layers consisting of Y-ZrO₂ (~20 μm) and Fe₂O₃ (~2 μm) and Cr₂O₃ (~10 μm) were developed as a result of LFS treatment. The LFS treatment triggered optimum ZrO₂ grain growth and the formation of the thermally grown oxides. The LS treatment was not sufficient to induce grain recrystallization and coating compaction. HFS and HS treatment resulted in a higher coating porosity and induced microcracks in the coatings. A nearly balanced fraction of 58:42 for the monoclinic and tetragonal ZrO₂ phase was obtained in the LFS coating. The coating exhibited the highest hardness of 1027.10 HV, the best adhesion strength, and the lowest thermal conductivity of 4.73 W/mK. The LFS treatment is beneficial in obtaining a compact YSZ layer with high phase stability and low thermal conductivity.