Microstructure and wear behavior of Fe-based amorphous HVOF coatings produced from commercial precursors

Wear resistant highly amorphous Fe60Cr8Nb8B24 (at.%) coatings of about 280 pm thickness were prepared through high velocity oxygen fuel (HVOF) thermal spray process onto API 5L X80 steel substrate. Feedstock powders were produced by gas atomization with low purity precursors by modifying AISI 430 stainless steel with additions of niobium (Fe-Nb) and boron (Fe-B). It was found that the coatings were mostly amorphous with some embedded FeNbB and Fe2B borides. The formation of a large fraction of amorphous phase was attributed to the high cooling rates of molten droplets combined with a proper powder composition. The average Vickers hardness of the coating (HV0.3 = 838 +/- 23) was about four times higher than that of the API 5L X80 substrate (HV0.3 = 222 +/- 5). The excellent wear resistance of the amorphous coating in the pin-on-disc measurements was attributed to its large fraction of amorphous phase (similar to 66%) with reinforcing hard Fe2B and FeNbB borides, high Vickers hardness, low oxygen content (<0.41%), and relatively low porosity (similar to 5). The wear rates of the amorphous coatings were about two orders of magnitude lower than that of the API 5L X80 steel substrate (1.0 x 10(-5) and 8.5 x 10(-4) mm(3). N-1 . m(-1), respectively). API 5L X80 steel exhibited dominant adhesive wear at low sliding speed and oxidative wear at high sliding speed. HVOF coatings presented oxidative wear regardless of the sliding speed. (C) 2016 Elsevier B.V. All rights reserved.