Effect of spraying power on microstructure and property of nanostructured YSZ thermal barrier coatings

Nanostructured yttria stabilized zirconia (n-YSZ) thermal barrier coatings were fabricated by atmospheric plasma spraying (APS) at different spraying powers. The microstructures, phase stability and mechanical properties of the n-YSZ coatings were examined by using scanning electron microscopy (SEM), X-ray diffraction (XRD) and Vickers indentation, respectively. The adhesion strength of coatings was evaluated according to ASTM C633-01 standard. Thermal cyclic oxidation method was carried out to study the effect of spraying power on thermal shock resistance of the coatings. Results showed that the n-YSZ coatings had a bimodal microstructure consisting of well melted splats and partially melted nanostructured areas. Both the porosity and the content of nanostructure in n-YSZ coatings decreased with increasing of the spraying power. After heat treatment at 1573 K, the porosity of the coating decreased and the hardness increased noticeably compared with that of the as-sprayed coating, indicating the sintering effect of the nanostructured coating. However, porosity of the coating deposited by the lowest spraying power (22 kW) still retain above 10% after annealing at 1573 K for 24 h, showing higher sintering resistant ability. Although phase structures certainly changed during heat treatment, phase compositions were more stable for the coatings deposited by the relatively low spraying powers. The average bond strength and thermal cycling life of the coating prepared by the lowest spraying power (22 kW) were about 29 MPa and 292 cycles, respectively, which were approximately twice as much as those of the coating deposited by the highest spraying power (42 kW). The best thermal shock resistance obtained for the n-YSZ coatings prepared at 22 kW can be partially attributed to its high content of nanozones, which play an important role in reducing the oxygen diffusion, releasing the thermal stress and interrupting crack propagation.