A comparative study of WC-CoCr and CrC-NiCr coatings deposited by HVOF and HVAF for mitigating combined cavitation and silt erosion in hydraulic machinery
Hydraulic turbines often suffer material damage from combined cavitation and silt erosion (CSE), particularly in silt-laden rivers. This leads to frequent maintenance and significant revenue loss, even when using industry-standard High-Velocity Oxy-Fuel (HVOF) sprayed WC-CoCr coatings. To address these limitations, this study presents a systematic evaluation of four cermet coating systems to identify a more durable alternative. Two material systems (WC-CoCr and CrC-NiCr) are deposited by two thermal spray processes (HVOF, and High-Velocity Air-Fuel, HVAF) and subjected to comprehensive laboratory CSE testing and in-service field validation on Francis turbine sealing rings at Nathpa Jhakri hydropower station. The HVAF-sprayed WC-CoCr coating demonstrated unequivocally superior performance, exhibiting the lowest cumulative mass loss, the longest time to substrate exposure (8 minutes), and the smallest crater dimensions in CSE wear tests. This performance is directly attributed to its superior as-sprayed properties, including the highest average hardness (1236±82 HV0.5) and fracture toughness (5.7±0.55 MPa √m), a dense (<1% porosity) microstructure, and the suppression of brittle decarburised phases. Field tests in a Francis turbine further confirm that HVAF WC-CoCr is the most effective coating, consistent with laboratory findings. Coatings failed by binder plastic deformation, crack growth and carbide fracture. This work validates the HVAF WCCoCr coating as a high-performance solution capable of extending the operational lifetime of hydroturbine components in severe CSE environments.