Thermophysical and Microstructural Studies on Thermally Sprayed Tungsten Carbide-Cobalt Coatings

Abstract

The development of new hardmetal coating applications such as fatigue-loaded parts, structural components, and tools for metal forming is connected with improvement of their performance and reliability. For modelling purposes, the knowledge of thermophysical, mechanical, and other material data is required. However, this information is still missing today. In this study, the thermophysical data of a WC-17Co coating sprayed with a liquid-fuelled HVOF-process from a commercial agglomerated and sintered feedstock powder from room temperature up to 700°C was determined as an example. The dependence of the heat conductivity on temperature was obtained through measurement of the coefficient of thermal expansion, the specific heat capacity, and the thermal diffusivity. Heat conductivities ranging from 29.2 W/(mK) at 50°C to 35.4 W/(mK) at 700°C were determined. All measurements were performed twice (as-sprayed and after the first thermal cycle) to take into account the structural and compositional changes. Extensive XRD and FESEM studies were performed to characterize the phase compositions and microstructures in the as-sprayed and heat-treated states. Bulk samples obtained by spark plasma sintering from the feedstock powder were studied for comparison.

Sliding wear investigation of suspension sprayed WC–Co nanocomposite coatings

“…The M 6 C (Co 3 W 3 C) and M 12 C (Co 6 W 6 C), the so-called -phases, are possible equilibrium phases in the W-C-Co system, after loss of carbon in the spray process. Sometimes, their existence in as-sprayed coatings is also reported, as summarized in earlier studies [8,46]. After the heat treatment at 920 °C WC, metallic tungsten and two -phases M 6 C (Co 2 W 4 C fit the peaks better than those of Co 3 W 3 C) and M 12 C (Co 6 W 6 C) were observed in the coating.…”

“…Both W 2 C and metallic tungsten are often detected in the as-sprayed WC-Co coatings [46], including earlier studies of Verdon et al [47] and Stewart et al [48]. Both studies [47,48] investigated the formation of WC-Co coatings with HVOF spray processes which were identical or comparable with the spray process in the current study, using conventional and nanostructured agglomerated and sintered feedstock powders.…”

“…Heat treatment of WC-Co in an inert atmosphere above 600 °C can lead to structural changes, bringing the phase composition closer to the equilibrium state, as discussed earlier [8,46]. The M 6 C (Co 3 W 3 C) and M 12 C (Co 6 W 6 C), the so-called -phases, are possible equilibrium phases in the W-C-Co system, after loss of carbon in the spray process.…”

Influence of Post-treatment on the Microstructural and Tribomechanical Properties of Suspension Thermally Sprayed WC–12 wt%Co Nanocomposite Coatings

“…In addition, the XRD diffraction pattern of suspension sprayed WC-12Co coating from powder 1 (P1C) in this work resembles those of Detonation Gun Spray (DGS) coatings, sprayed with excess of oxygen [18], or from coatings prepared from strongly carbon-deficient powders [41]. Both W 2 C and metallic tungsten are often detected in as-sprayed WC-Co coatings [25], including earlier studies of Verdon et al [42] and Stewart et al [43]. Both studies [42,43] investigated the formation of WC-Co coatings with HVOF spray processes which are comparable with the spray process in the current study, using conventional and nanostructured agglomerated and sintered feedstock powders.…”

“…Due to the high oxidation rate above 600°C, the thermal treatment of WCCo coatings has to be carried out in inert atmosphere or vacuum. Microstructural effects appearing in a WC-17%Co coating during thermal analyses for determination of the thermophysical properties have been described previously [25]. These microstructural changes were also studied by mechanical spectroscopy [26].…”