Microstructural and mechanical characterization of Ni-base thermal spray coatings deposited by HVOF

The present work has been conducted in order to determine the microstructural features, hardness and elastic modulus of two different Ni-base coatings deposited by means of HVOF thermal spray, onto a SAE 1045 plain carbon steel substrate. The morphology and chemical composition of the phases that are present in the coatings were characterized by means of SEM, EDS and XRD techniques. Image analysis was used for the evaluation of the coatings porosity. Both conventional and instrumented indentation tests were also carried out on the surface and cross section of the coatings, in order to evaluate the effect of coating microstructure on hardness and elastic modulus. Conventional indentation tests were conducted using a Knoop indenter and a maximum load of 9.8 N. Instrumented indentation tests, in which the indenter depth and applied load were recorded continuously, were carried out employing a Vickers indenter and maximum loads of 0.49, 0.98, 1.96, 4.9 and 9.8 N. Instrumented nanoindentation tests (in a continuous stiffness measurement mode) were also conducted employing a Berkovich indenter with a maximum load of 9.8 N. The elastic modulus was computed by means of the Oliver and Pharr method and compared with the values determined by means of the method earlier advanced by Marshall et al. The results obtained indicate that the elastic modulus values determined on the cross section of the coatings are higher than those obtained on the surface, clearly indicating the anisotropy of the structure. Also, the values found employing a Berkovich indenter are very similar to those derived by means of the Vickers indenter. In addition, the these values are in agreement with those determined by taking into consideration the elastic recovery of the short Knoop diagonal after removal of the load.