Influence of HIPing Post-Treatment on the Cavitation Erosion in HVOF Thermally Sprayed WC-NiCrBSi Coatings

Cavitation erosion caused by the energy released from collapsing bubbles is a major failure mode of engineering components in hydraulic and marine environments. Thermal spray coatings provide a cost-effective and environmentally friendly solution to improve the cavitation erosion resistance of components. Functionally graded WC-NiCrBSi coatings were deposited by the high-velocity oxy-fuel (HVOF) process and post-retreated using hot isostatic pressing (HIPing) to improve the interlamellar bonding at two different temperatures of 850 °C and 1200 °C. ASTM G32 cavitation erosion tests were conducted in seawater for a total test time of 24 hours. Microstructural and post-cavitation test investigations were conducted on the coating surface and cross sections using scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS). Microstructural phases were investigated using x-ray diffraction (XRD). Changes in microstructure, hardness, fracture toughness, and porosity due to HIPing post-treatment are discussed. The results of this investigation show that the cavitation erosion resistance of coatings improves after HIPing post-treatment. This improvement was five times the cavitation wear resistance of as-sprayed coating at the HIPing temperature of 1200 °C, surpassing the cavitation erosion resistance of the AISI 440 C steel substrate. Microstructural changes leading to the strengthening of interlamellar and carbide-matrix boundaries, which reduce intergranular and transgranular crack propagation, are attributed to this improvement. A combination of the improvement in hardness, elastic modulus, porosity, and fracture toughness due to microstructural changes resulted in the superior cavitation erosion resistance of HIPed coatings.