Adhesion properties and fracture mechanism of plasma sprayed NiCrAl/Al2O3-13wt.%TiO2 coatings by post annealing

Plasma-sprayed NiCrAl/Al2O3-13wt.%TiO2 coatings (AT13) deposited on mild steel substrate were annealed with varying temperatures in air. The adhesion of the coating was evaluated by tensile adhesive strength test. The microstructure and the fracture mechanism were studied using optical microscopy, X-ray diffraction, and scanning electron spectroscopy/energy dispersive spectroscopy. It was found that the tensile bond strength of the coatings increased with increasing of annealing temperature at first and then decreased with increasing of annealing temperature further. The as-sprayed coating fractured at the interfaces of substrate/bond layer and bond layer/ceramic coating with a brittle-ductile mixed fracture. The measured strength expressed the adhesive strength and internal adhesive strength of the coating. The failure of the coating annealed at 300, 400, and 500 degrees C took place at the interface of substrate/bond layer and had a mixed fracture surface of transgranular cleavage fracture and localized ductile fracture. The strength obtained is the adhesive strength between the coating and the steel substrate. The coating annealed at 400 degrees C had a maximum strength of 42.9MPa. When the temperature is above 600 degrees C, the bonding strength would be damaged. Therefore, there is a proper annealing temperature which can significantly improve the bond strength of the coating.