A pragmatic approach and quantitative assessment of silt erosion characteristics of HVOF and HVAF processed WC-CoCr coatings and 16Cr5Ni steel for hydro turbine applications

In the present study, WC-CoCr hard composite coatings were deposited on the hydro turbine steels using two different thermal spray techniques such as High Velocity Air Fuel (HVAF) and High Velocity Oxy Fuel (HVOF). The slurry erosion tests were performed with three different jet impingement kinetic energies and at five different impact angles. The influence of impact angle on the erosion wear behavior of the coatings was analyzed using CFD simulation technique. Erosion mechanisms were investigated in detail using SEM and laser scanning confocal microscopy. Among all the WC–CoCr coated samples, the HVAF coating sprayed at highest spray velocity has shown superior erosion resistance compared to the HVOF coatings. It could be attributed to the better adhesion, higher density, enhanced hardness and surface compressive residual stresses coupled with lower oxidation of the coated samples. The erosion wear mechanisms were found to be nearly micro-cutting and plowing with layered material removal at a lower impact angle and a low cycle fatigue linked to repeated impact leading to rounding of the edges and brittle cracking of the WC grains at normal impact conditions.