Microstructures, properties and high-temperature carburization resistances of HVOF thermal sprayed NiAl intermetallic-based alloy coatings

In this paper, the microstructure, mechanical properties and high-temperature carburization resistance of HVOF thermal sprayed NiAl intermetallic-based coatings were investigated by scanning electron microscopy, X-ray diffraction and micro-indentation testing. It was found that the addition of CeO2 and Cr improves the wetting and bond strength of the coatings to the substrate, which decreases the tendency of brittle peeling during thermal spraying. The addition of CeO2 and Cr is also beneficial in forming coating layers with less cracks and pores. The NiAl intermetallic coatings containing CeO2 and Cr also exhibit higher hardness, improved elastic modulus and increased η ratio. The hardness and elastic modulus of the NiAl intermetallic-based alloy coatings were all decreased after carburizing at high temperature. The η ratio was also decreased after carburization for coatings without the addition of CeO2 but to a minor degree for those containing CeO2. These observations are related to the reduced depletion of Al and inter-diffusion between the coating layer and the substrate due to the presence of CeO2 and/or Cr. Compared with uncoated HP steel, the samples with different NiAl coatings exhibited excellent carburization resistance in 2 vol.% CH4 balance hydrogen gas mixture with a carbon activity of ac=3 at 1100 °C. This may result from the diffusion barrier role of the NiAl coatings and the presence of oxide films such as Al2O3 and CeAlO3.