Microstructure and wear resistance of environmentally friendly NbC-FeCr coatings: An evaluation of HVOF process parameters

Health risks of Co and classification of W and Co as critical raw materials are currently the driving forces behind the search for WC-Co(Cr) replacement. NbC-based hardmetal compositions have just recently emerged and are now under consideration for use as thermally sprayed wear resistant coatings. NbC is proposed to be particularly suitable for combination with Fe-based binders. In this study an agglomerated and sintered 80NbC-15Fe5Cr (wt.-%) powder was processed by means of liquid-fueled HVOF. The experiments were carried out in two series: The influence of spray distance (305 and 395 mm) and nozzle geometry (lengths of 150 and 200 mm with diameters of 11 to 14 mm) were investigated in the first series. Powder feed rate (77 to 114 g/min) and oxygen-fuel-ratio (λ = 1.0 and λ = 1.2) were varied in the second series. In-flight particle velocity of approximately 700 m/s and temperature of 2150 °C resulted in coatings of the highest quality achieved in this study. SEM and optical microscopy of the coatings showed a higher thickness with higher in-flight particle temperatures and porosities between 2.1 and 4.6 %. XRD and EDS analyses revealed the presence of oxygen as FeNb2O6. Strongly correlated with porosity, Vickers microhardness ranges from approximately 770 to 900 HV0.3. Unidirectional and reciprocating dry sliding wear rates of 1.2 × 10-6 and 2.1 × 10-6 mm3/(N × m), respectively are fully competitive to those of the reference coating systems WC-CoCr and Cr3C2-NiCr. Overall, the shorter spray distance with a smaller nozzle was advantageous when combined with λ = 1.0 and powder feed rate of 102 g/min.