The role of NZP additions in plasma-sprayed YSZ: microstructure, thermal conductivity and phase stability effects

A concept for lowering the thermal conductivity of plasma-sprayed yttria-stabilized zirconia (YSZ) was evaluated by adding a low thermal conductivity secondary phase. Ca0.5Sr0.5Zr4P6O24, a member of the zirconium phosphate or NZP family, was co-sprayed with YSZ using the recently patented small-particle plasma-spray process. The amount of NZP phase in the YSZ varied from 4 to 18 vol.%. Transmission electron microscopy revealed that the plasma-sprayed NZP existed as 30–50 nm size grains when sprayed with YSZ. Between 100 and 1200°C, reductions in the thermal conductivity of NZP/YSZ composite coatings compared to YSZ-only coatings were traced to porosity effects. The NZP phase destabilized the t′-ZrO2 phase of the YSZ, favoring the formation of t-ZrO2, then m-ZrO2, a phase with a high thermal conductivity. Due to this phase change, a large hysteresis was observed in thermal conductivity during the initial heat-up and cool down cycles of the NZP containing samples.