Microstructure and mechanical properties of plasma sprayed Al2O3-YSZ composite coatings

Alumina-yttria stabilized zirconia (Al2O3-YSZ) composite coatings are recognized as a potential alternative material for thermal barrier coatings. In this paper, Al2O3-YSZ composite coatings were deposited using atmosphere plasma spraying. The effects of alumina addition and annealing treatment on the microstructure and mechanical properties of the plasma sprayed Al2O3-YSZ composite coatings were investigated by XRD, SEM and an instrumented indentation testing system. Owing to the existence of YSZ in the composite coatings, the formation of amorphous alumina was suppressed. The quenching stress is high enough to trigger microcracking, these microcracks will further propagate or close under the post deposition thermal mismatch stress. Young's modulus of the coatings keeps an upward tendency from 50.56 GPa to 55.93 GPa and hardness increase from 5.36 GPa to 5.93 GPa with alumina contents varying from 10 wt% to 55 wt%. After an annealing treatment, Young's modulus and hardness increase compared with the as-sprayed ones. Fracture toughness of Al2O3-YSZ composite coatings is higher than the YSZ one, and it increases firstly and then decreases with alumina addition varying from 10 wt% to 55 wt%. The fracture toughness of coatings is related to the amount of heterogeneous specific interface between Al2O3 and YSZ splats and the high microcracks density in the composite coatings.