Phase evolution, densification behaviour and thermal and mechanical properties of Gd2O3 co-doped Y2O3 stabilised ZrO2

Abstract

This study reports an interesting effect of doping of 6-15 mol.% Gd2O3 in Y2O3 stabilized ZrO2 (YSZ) in enhancing thermal insulation properties at high temperature for developing improved thermal barrier coating (TBC). The powder blends were mechanically milled followed by cold compaction and sintering in air at 1400-1600 degrees C. Gd2O3 co-doped YSZ reveals a significantly reduced sintering rate compared to YSZ. The higher the dopant concentration, the lower the sintering rate. This retarded densification after the incorporation of Gd2O3 in YSZ can be attributed to the interdiffusion of Gd3 + ions into the lattice of partially stabilised ZrO2 comprising a mix of monoclinic and tetragonal phases. X-ray diffraction, Raman scattering, and transmission electron microscopy confirm that diffusion of Gd3+ introduces lattice distortion and transforms monoclinic and tetragonal phases into tetragonal and cubic phases, respectively. Incorporation of Gd2O3 in YSZ removes the tendency of reversion, improves the coefficient of thermal expansion (CTE), and reduces the thermal diffusivity by about 46 % at room temperature and 26 % at 400 degrees C, which in turn reduces the thermal conductivity proportionately. In addition, 15 mol.% Gd2O3 co-doped YSZ after sintering records a nano-hardness of 15.94 GPa, compared to 7.39 GPa of YSZ, but a reduced compressive strength of 456.44 MPa, compared to 1004.84 MPa of YSZ, due to higher degree of retained porosity. Hence, the doping of YSZ with Gd2O3 is effective in restricting densification, reducing thermal conductivity, and improving CTE at elevated temperatures, all necessary attributes for more effective TBC.