Tribological and electrochemical performances of HVOF sprayed NbC-NiCr coatings

A wide range of mechanical parts are protected against wear and corrosion by thermally sprayed WC- or Cr3C2-based hardmetals. However, little is known about the potential of other hardmetal coating formulations. Since promising results were obtained with bulk NbC-based hardmetal cutting tools, in this work we investigated NbC-based coatings, which have not been reported yet in the literature. NbC particles were homogeneously dispersed with volume fractions of 25 % and 40 % in a Ni-20wt.%Cr matrix by mechanical alloying, and the resulting powders were sprayed onto stainless steel substrates by the High-Velocity Oxygen Fuel (HVOF) process. Irrespective of the process parameters, the coatings exhibit low porosity (<1 vol%) and hardness values of about 900 HV0.3 (NbC-25NiCr) and 1000 HV0.3 (NbC-40NiCr). The composition with higher amount of NbC phase is less hard because its greater brittleness results in cracking during indentation. The sliding wear resistance of the NbC-based coatings, tested by ball-on-disc tribometry from room temperature up to 600 degrees C, is generally intermediate between that of WC-CoCr and Cr3C2-NiCr references, and it is usually comparable to that of TiC-NiCr, another type of alternative hardmetal coating, tested in our previous work. Though the NbC-NiCr coatings exhibited limited three-body abrasion resistance due to the occurrence of some brittle fracture together with ductile grooving, they offer a particularly interesting performance at 400 degrees C, with lower wear rates than Cr3C2-NiCr and TiC-NiCr. Additionally, the NbC-NiCr coatings tested in this work, especially the one with 40 vol% matrix phase, exhibit excellent corrosion resistance. Corrosion current densities (similar to 0.1 mu A/cm(2)) and passive current densities (<1 mu A/cm(2)), both obtained by electrochemical polarization tests, were lower than the corresponding values found for all reference coatings (WC-CoCr, Cr3C2-NiCr and TiC-NiCr), Thus, the NbC-NiCr coatings are promising for applications where a good balance between corrosion resistance and wear resistance over a range of temperatures is required.