Comparative study of the dry sliding wear behaviour of HVOF-sprayed WC–(W,Cr)2C–Ni and WC–CoCr hardmetal coatings

Recent studies of HVOF-sprayed WC–(W,Cr)2C–Ni coatings indicated an applicability at temperatures above 600 °C. Therefore, in the present paper the dry sliding wear behaviour of coatings prepared from one selected feedstock powder was studied up to 750 °C and compared to a WC–10%Co–4%Cr coating as reference. Coatings were prepared onto stainless steel substrates by a liquid-fueled high velocity oxygen-fuel (HVOF) spray process. A Cr–W–Ni–C “mixed” matrix phase and chromium-rich large (W,Cr)2C grains were formed as a result of metallurgical reactions between the components WC, Cr3C2 and Ni during spraying; therefore, the resulting coating can be better designated as WC–(W,Cr)2C–Ni. The dry unidirectional sliding wear behaviour of the coatings, studied according to ASTM G99 against Al2O3 counterbodies at four different temperatures (25 °C, 400 °C, 600 °C, 750 °C) and at two sliding speeds, was interpreted on the basis of their microstructure, micromechanical properties and oxidation behaviour. The morphology and composition inside the wear scars were also studied by SEM and micro-Raman spectroscopy. The results show that the WC–(W,Cr)2C–Ni coating exhibits good wear resistance up to 750 °C, whereas WC–CoCr, in spite of its very mild wear at low temperature, suffers from catastrophic oxidation at 750 °C