Influence of PTFE as Thin Top Layer on Hydrophobicity and Wear Behaviour of HVOF Sprayed TiC+50%NiCr Coating on SS410 Steel

Hydro turbines are the key features of the developing globe for utilizing one of the important renewable energies, namely hydro power. The longevity of these hydro machineries links directly to the surface properties of critical component like impellers, as these components are exposed to slurry and cavitation erosion during their use and should be addressed by the properly designed material surfaces for consistent efficiency. The surface properties can be altered and improved by developing coating in conventional steels that are used for impellers. A recommended hard coating for such applications involves titanium carbide (TiC) with Ni and Cr binders. Along with hardness, an increase in surface hydrophobicity can also reduce wear. High velocity oxygen fuel (HVOF) spraying process comes under the umbrella of thermal spray process that utilizes melting of powders via burning of fuel and these molten powders were bombarded on the surface with supersonic velocity to generate a coating. In this paper, HVOF thermal sprayed coating consisting of TiC and NiCr has been utilized for SS410 steel. In addition, a thin layer of polytetrafluoroethylene (PTFE) has also been investigated on the HVOF sprayed surface to enhance its hydrophobicity. The developed surface has been characterized by static contact angle, hardness, porosity, surface roughness, and coating thickness. Variable impingement parameters, namely sand concentration (20000 & 40000 ppm), angle of impingement (45 & 90°) for slurry testing, stand-off-distance (4 & 8 cm), and flow velocity (15 and 30 m/sec) for cavitation testing were opted for analysis the wear resistance of candidate specimens. From the results, it has been observed that the implementation of a PTFE layer leads to super hydrophobicity. However, the hardness and surface roughness have been reduced with the assistance of PTFE layer. Meanwhile, the slurry and cavitation erosion resistance were also found to be improved by the PTFE layer due to the increase in static contact angle. In the case of cavitation erosion, maximum jet velocity and stand-off distance contribute to more wear, owing to sufficient bubble generation. Next, in case of slurry erosion testing, mixed aspect of erosion wear with respect of influence of parameters was observed for coated and uncoated samples. Ductile mode of failure was observed for SS410, and PTFE sprayed specimens, on the other side, HVOF sprayed TiC+50%NiCr coating showed mixed mode of erosion.