Behavior of Cr3C2-containing Hardmetal Coatings on Different Substrates during High Temperature Treatment in Nitrogen Atmosphere

For high-temperature service hardmetal coatings, such as Cr₃C₂-NiCr are occasionally applied in a protective nitrogen or nitrogen-containing atmosphere. It is known, that Cr₃C₂ can interact with nitrogen and form Cr₂N or CrN, but the mechanism and consequences of interaction with the substrate material, suitable for high-temperature application as well, are not studied so far. In this work the interaction of 73%WC-20%”Cr₃C₂”-7%Ni, 42%WC-42%Cr₃C₂-16%Ni, 70%Cr₃C₂-10%WC-20%Ni (experimental composition) and Cr₃C₂-NiCr coatings deposited on specimen of 25 mm diameter of 1.4828 (AISI 309) or 2.4856 (Alloy 625) with nitrogen was studied. Heat treatment was performed in a tube furnace with a small nitrogen flow (2 l/h) in the temperature range 700°C-900°C for 2-32 days. The as-sprayed and heat treated coatings were investigated by XRD, after conventional metallographic preparation and by FESEM and EDS mappings. Besides the changes in the coatings focus was made on the coating-substrate interface. Formation of nitrides was observed in the coating down to the interface. High nitrogen concentrations were inhomogeneously distributed. It is assumed that the nitrogen formation leads to the formation of carbon, which is able to diffuse into the substrate, leading there to precipitations close to the interface.