The mechanical, tribological, and corrosion properties of two hybrid coating systems were assessed: 1) a tungsten–tungsten carbide (W-WC) top layer and a laser cladded cobalt– chromium (Co-Cr) interlayer (Stellite 6 superalloy) applied to a 316 stainless steel substrate; and 2) the same W-WC top layer and an HVOF spray-and-fused Ni-W-Cr-B interlayer (Colmonoy 88 superalloy) applied to an Inconel 718 substrate. X-ray diffraction, energy dispersive spectroscopy, and scanning electron microscopy were used to analyze the microstructure of the coating layers. Microindentation was used to measure surface hardness and the hardness profile of the coating systems. Rockwell indentation was used to assess coating adhesion according to CEN/TS 1071-8. Surface load-carrying capacity was also assessed by measuring micro- and macrohardness at high loads. Tribological properties were assessed with a linear reciprocating ball-on-flat sliding wear test, and corrosion resistance was measured by potentiodynamic polarization and electrochemical impedance spectroscopy. W-WC layers showed class I adhesion to both the SS 316 and Inconel 718 substrates, with and without an interlayer. Hardness profile measurements on cross section showed hardness of 13.6 GPa and 7.0 GPa for W-WC and Co-Cr, respectively, with average hardness of 9.7 GPa for Ni-W-Cr-B. Furthermore, hardness measurements at different high loads revealed that the addition of an interlayer increases surface hardness by up to 200% compared to the same coating system provided without an interlayer, quantifying the additional load-carrying capacity provided by the supplementary interlayer. The tribological measurements show that, except for the Inconel 718 / Ni-W-Cr-B / W-WC system, the hardest interlayer or substrate leads to the highest wear rates. In addition, the W-WC layer showed excellent corrosion protection, with no pitting observed after potentiodynamic polarization testing.

This case study is the result of an investigation on HVOF Cr 3 C 2 -NiCr coating failure of on-off metal seated ball valve (MSBV) used in supercritical steam lines in a power plant and solution. HVOF Cr 3 C 2 -NiCr coating is used to protect thousands of MSBVs without incident. However, in this case the valves are challenged with exposure to rapid high pressure and temperature variation resulting in a unique situation where the coating experiences cracking and cohesive failure. A detailed investigation was undertaken by Velan, Kennametal Stellite, the National Research Council of Canada and the Ecole Polytechnique to determine possible failure cause and develop solutions in which highlights are presented.