Microstructural characterization and evolution under high temperature oxidation of CoCrAlYTa-10%Al2O3 coating deposited by high-velocity oxygen fuel thermal spraying

Abstract

The CoCrAlYTa-10%Al2O3 coating was fabricated on H13 steel substrate using high-velocity oxygen fuel (HVOF) thermal spraying. A detail characterization on the microstructures, element distribution and phase composition of the HVOF sprayed CoCrAlYTa-10%Al2O3 coating was conducted using scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS), x-ray diffraction (XRD) and transmission electron microscopy (TEM). The microstructural evolution and microhardness of the coating at different temperatures were investigated. The results showed that the coating had a dense and uniform microstructure with a low porosity of 0.44%. The primary phases for the CoCrAlYTa-10%Al2O3 coating were identified as Co-based solid solution, Cr-based solid solution, Al2O3 phase and TaC particles. Compounds such as Cr7C3, Al13Co4 and aluminum-yttrium oxides Al5Y3O12 were also confirmed by TEM observation results. Owing to the rapidly cooling rates of the molten droplets, nano-crystalline phase was existed in the coating. The average microhardness of the coating was 640 HV0.1. Its high microhardness derived from the presence of certain volume fraction of hard Al2O3, Cr7C3 and TaC grains within the coating. After 1h of oxidation, oxides like CrO2 and Ta2O5 were formed when the coating exposed at the temperature up to 800 °C. At the temperature above 600 °C, the hardness of the coating remained at a high level (600 HV0.1). It possessed better high temperature softening resistance than H13 steel, indicated that the coating exhibited good mechanical properties at high temperature.