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%Al _2 O _3 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%Al _2 O _3 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%Al _2 O _3 coating were identified as Co-based solid solution, Cr-based solid solution, Al _2 O _3 phase and TaC particles. Compounds such as Cr _7 C _3 , Al _13 Co _4 and aluminum-yttrium oxides Al _5 Y _3 O _12 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 HV _0.1 . Its high microhardness derived from the presence of certain volume fraction of hard Al _2 O _3 , Cr _7 C _3 and TaC grains within the coating. After 1h of oxidation, oxides like CrO _2 and Ta _2 O _5 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 HV _0.1 ). It possessed better high temperature softening resistance than H13 steel, indicated that the coating exhibited good mechanical properties at high temperature.

Keywords

• HVOF thermal spraying
• CoCrAlYTa-10%Al2O3 coating
• microstructure
• oxidation
• microhardness