Near-nanostructured WC-18 pct Co coatings with low amounts of non-WC carbide phase: Part I. Synthesis and characterization

Abstract

Near-nanostructured WC-18 pct Co coatings, with low amounts of non-WC carbide phases, have been synthesized using high velocity oxygen fuel (HVOF) thermal spraying under spraying conditions of varying fuel chemistry, fuel-oxygen ratio, and powder particle size. The results show that the temperature the particles experience during spraying depends on the preceding parameters. Compared to available published results on WC-Co system coatings, nanostructured WC-18 pct Co coatings, synthesized in these experiments, contain very low amounts of non-WC carbide phase (less than 10 pct vol). This is comparable to that of the conventional WC-12 pct Co coating, prepared in the present study for comparison purposes. Regardless of whether the binder phase in the agglomerated feedstock powder particles melt or not, the WC particles do not appear to experience significant growth as a result of the spraying. The size of WC particles remains in the 200 to 500 nm range, consistent with that present in the feedstock powder. The as-received near-nanostructured WC-18 pct Co feedstock powder exhibits morphological characteristics that lead to low amounts of non-WC carbide phases in the coatings. The microstructure and phase constitution of the coatings depend on particle size of the feedstock powder and flame characteristics of the fuels during spraying. A higher particle temperature causes more decomposition of the WC phase but reduces porosity in the coatings, this occurs with higher flame temperature and smaller particle sizes. Propylene fuel produces less decomposition of the WC phase despite the higher flame temperature and, thus, provides the best combination of dense coating with low amount of non-WC phase.