The effect of sintering temperature on the microstructure and phase transformation in tetragonal YSZ and LZ/YSZ composites

In this paper, the effect of sintering temperature on the microstructure and phase transformation in lanthanum zirconate/yttria stabilized zirconia (LZ/YSZ) composites was studied. The phase stability of tetragonal YSZ was found to be decreased with the increase in sintering temperature along with the increase in volume fraction of LZ in LZ/YSZ composites. The addition of LZ in YSZ resulted in the phase transformation from tetragonal YSZ to monoclinic YSZ at high temperatures. Minor fraction of monoclinic phase was found in YSZ matrix of 30LZ and 50LZ composites after sintering at 1473 K for 200 h. The fraction of cubic phase was also found to increase with the increase in sintering time and temperature. However, no correlation of cubic phase was found to exist with the addition of LZ content. Coherent growth between LZ/YSZ interface was observed using transmission electron microscopy. Due to different expansion coefficients of LZ and YSZ, the largest thermal stress appeared (during cooling process) across the LZ/YSZ interface. Thermal stress promoted the phase transformation (tetragonal to monoclinic) in YSZ and distorted the grains. In LZ/YSZ composites, the average tensile stress increased with the increase in LZ content and temperature. At all sintering temperatures, the YSZ exhibited tensile stress while a compressive stress exited in LZ matrix. In all three (30LZ, 50LZ and 70LZ) composites, the maximum average stress was present in the LZ/YSZ matrix at sintering temperature of 1675 K.