Microhardness variation in heat-treated conventional and nanostructured NiCrC coatings prepared by HVAF spraying

A nanostructured NiCrC coating for high temperature erosion–corrosion protection was prepared by high velocity air-fuel (HVAF) spraying. While previous studies on nanostructured materials were most focused on the short-term development in microstructure and properties during heat treatment, no systematic investigation of the microhardness variation as a function of microstructural development during long-term heat treatment has been presented. In this work, HVAF-sprayed nanostructured NiCrC coating was heat treated at 650 °C for up to 200 h in air. The coating microstructures have been characterized by scanning electron microscopy, transmission electron microscopy, high resolution scanning electron microscopy and X-ray diffraction analysis. A Vickers microhardness tester was employed to determine the hardness variation of the coatings. In addition, conventional coarse-grained NiCrC coating produced by HVAF technique with the identical composition was also studied for comparison. The results indicated that the nanostructured NiCrC coating possessed a very compact and uniform microstructure, and exhibited good thermal stability during long-term heat treatment. The grain growth during thermal exposure caused the softening of the coating; however the carbide precipitation and content increasing resulted from phase transformation compensated the decrease by grain coarsening, and further led to the increase in the overall coating hardness.