Abstract
Thermally sprayed Stellite 6 coatings offer high wear and corrosion-erosion resistance at high temperature but the creep behaviour of such coatings is not well understood. In this paper, the microstructure and creep behaviour of a HVOF Stellite 6 coating was investigated. The coating was tested in the as-sprayed condition and after an isothermal heat-treatment of 1 h at 1050 °C. The as-sprayed coating comprised FCC Co-Cr-W solid solution with 2–3 wt% HCP phase, whereas the heat-treated coating comprised FCC Co-Cr-W solid solution, 11 wt% M23C6 carbides and 7 wt% M7C3 carbides. Small punch creep tests were conducted at 700 °C on the as-sprayed and heat-treated coatings. The heat-treated coating exhibited minimum steady-state strain rates approximately 2 orders of magnitude lower than the as-sprayed coating when tested at similar loads, leading to times to failure approximately two orders of magnitude longer for the heat-treated coating within the selected load range. Fracture analysis showed cracking along powder particle boundaries was the main mode of cracking in the as-sprayed coating whereas for the heat-treated coating, fracture along the carbide / matrix interface was the main fracture mechanism.
Highlights
Sprayed coatings are widely used to extend the lifetime of substrate components in power generation [1,2,3]
The phases present in the assprayed Stellite 6 were reported by Pala et al [22] as almost exclusively FCC Co-Cr-W solid solution with 2–3 wt% HCP Co-based phase, as measured by quantitative Rietveld analysis of X-ray diffraction (XRD) data
Considering the Small Punch Creep (SPC) behaviour of the coatings first, the minimum steady-state strain rates observed for the heat-treated coating were approximately 2 orders of magnitude lower than those observed for the as-sprayed coating
Summary
Sprayed coatings are widely used to extend the lifetime of substrate components in power generation [1,2,3]. Hard-facing alloys, such as Cobalt-based Stellite 6, offer high wear and corrosion-erosion resistance at high temperature, which makes them attractive candidates for use as thermally sprayed coatings in power generation [4,5,6]. The durability of thermally sprayed coatings is significantly influenced by creep: stress induced cracking, buckling and delamination have all been shown to be influenced by creep [7,8,9]. Understanding the creep properties of thermally sprayed coatings is important for high temperature applications. Thermally sprayed coatings are typically too thin for traditional creep test methods such as uni-axial creep testing. The small punch test can be used in a constant load (small punch creep) or constant displacement (small punch tensile) configuration and has been used to evaluate the mechanical properties of thermally sprayed MCrAlY coatings [10,11,12,13,14] and a nickel aluminide diffusion coatings [15]
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