Influence of Vacuum Heat Treatment on the Residual Stress of Thermal Spray Cermet Coatings.

The aim of this investigation was to ascertain the changes in residual stress profile after the post-treatment of functionally graded thermal spray WC-NiCrBSi coatings, and relate these changes to the microstructural transformations. Through thickness residual stress measurements were performed using the neutron diffraction and x-ray diffraction techniques. Functionally graded HVOF (JP5000) WC-NiCrBSi coatings were thermally sprayed on AISI 440C steel substrate discs. These coatings were then heat-treated in an inert atmosphere at 1200°C for one hour. Microstructural investigations using SEM, XRD, and light microscopy were used to identify the changes after the heat-treatment. Microhardness and indentation modulus values were also investigated to analyse the changes in the mechanical properties after the heat-treatment. Results of this investigation indicated that the residual stress gradient within the coating material decreases after the heat-treatment, and the stress field in the substrate, near the coating substrate interface, changes from tensile to compressive. Residual stress measurements via x-ray diffraction provided average compressive values which were similar to those evaluated via the neutron diffraction technique, however the definition of stress gradient as a function of coating thickness was much better resolved via the neutron diffraction technique. The changes in the stress gradient after the heat-treatment were related to the microstructural changes which resulted in the formation of diffusion zones at the coating substrate interface, and also at the interface of functionally graded coating, improvement in coating elasticity due to the changes in the bonding mechanism from mechanical interlock to metallurgical bonding, and the formation of secondary phases within the coating microstructure. [ABSTRACT FROM AUTHOR]