Microstructure and Tribological Behaviour of NiWCrBSi Coating Produced by Flame Spraying

The application of hard and resistant coatings is a possibility to increase the lifetime of parts exposed to heavy loads and harsh environments. This work is focused on the investigation of mechanical properties of flame sprayed NiWCrBSi coatings, addressing in particular coating microstructure, hardness, friction behaviour and wear mechanisms. Microstructure and phase composition of coatings were characterized by optical microscopy, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and X-ray diffraction analysis. Microhardness of coated and uncoated samples was measured and their tribological properties were evaluated by ball-on-disk tests under dry conditions at room temperature. The morphology of the worn scar at different sliding conditions was investigated using scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and the corresponding wear profile by 3D profilometry. The coatings presented mainly a heterogeneous lamellar structure, containing inhomogeneities, such as unmelted particles, micro-cracks, oxides and various types of pores. The coatings underwent severe wear at low sliding speeds, where the predominating wear mechanism was identified as massive exfoliation. However, when increasing the sliding speed and the sliding distance, a transition in the predominant wear mechanism was observed, passing from the combination of oxidative/soft abrasive mechanism to fatigue delamination. These results indicate the importance of controlling coating microstructure through process parameters to optimize its tribological behaviour.