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

Abstract

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 CeO 2 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 CeO 2 and Cr is also beneficial in forming coating layers with less cracks and pores. The NiAl intermetallic coatings containing CeO 2 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 CeO 2 but to a minor degree for those containing CeO 2. 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 CeO 2 and/or Cr. Compared with uncoated HP steel, the samples with different NiAl coatings exhibited excellent carburization resistance in 2 vol.% CH 4 balance hydrogen gas mixture with a carbon activity of a c=3 at 1100 °C. This may result from the diffusion barrier role of the NiAl coatings and the presence of oxide films such as Al 2O 3 and CeAlO 3.