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 approximate to 1.2 %-1.4 % porosity and approximate to 1000 HV 0.3 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 approximate to 2 x 10 - 6 mm3/(N center dot 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 degrees C and then notably dropped to less than 1 x 10-7mm3/(N center dot m) at 400 degrees C. Up to 300 degrees 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 degrees 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 degrees 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 degrees C.