Tribological performance of NbC-based hardmetal HVOF coatings with Fe-Cr-Mo matrix

NbC-based hardmetals are technologically employed as metalworking tools in bulk form but have scarcely been investigated as coatings. Following a previous report on High Velocity Oxy-Fuel (HVOF) sprayed NbC-NiCr coatings, this work investigates NbC-40 vol%(Fe-20 wt%Cr-15 wt%Mo) HVOF-sprayed coatings, where the Fe-based matrix is meant to reduce the cost and hazardousness of the material. Quite dense coatings with ≈1.2 %–1.4 % porosity and ≈1000 HV0.3 hardness were obtained, irrespective of the gas flow rates employed during the HVOF deposition. In ball-on-disc unidirectional dry sliding wear tests, the coatings exhibited specific wear rates of ≈2 × 10−6 mm3/(N·m). These were higher than the wear rates obtained with the NbC-NiCr coatings and with Cr3C2-NiCr and WC-CoCr reference samples. The performance of the NbC-FeCrMo coatings improved progressively, compared with the above references, as the temperature increased. Their specific wear rate remained constant up to 300 °C and then notably dropped to less than 1 × 10−7 mm3/(N·m) at 400 °C. Up to 300 °C the dominant wear mechanisms were abrasive grooving and surface fatigue (micro-pitting). The incidence of micro-pitting decreased and that of abrasion increased as the temperature increased and/or when the counterpart was changed from WC-6%Co to Al2O3. At 400 °C, tribo-oxidation prevailed as a uniform mixed-oxide tribofilm covered the wear track and provided protection against wear by the counterpart. The tribofilm retained its protective effect up to 600 °C, although its tendency to spallation increased progressively. Overall, the NbC-40FeCrMo coating composition seems well-suited for sliding wear applications in a range of temperatures between 400 and 600 °C.