Investigations on cavitation erosion of high velocity oxy fuel (HVOF) sprayed nickel based cermet coatings on Monel K-500 alloy

Abstract

Monel K-500 is a nickel-based alloy having excellent mechanical and anti-corrosive properties. It is widely used in power generation turbines, aircraft, springs and fasteners for the marine industry, components for chemical processing units. In marine hydraulic applications, various wear phenomena such as cavitation erosion (CE), corrosion and slurry erosion (SE) contribute to its degradation. Among these degradation modes, cavitation erosion is a key challenge faced by components exposed to marine environments. It is pertinent to mention that, in addition to cavitation erosion, slurry erosion and corrosion also contribute to the cumulative wear loss of the target components. Wear largely depends on the surface properties of the target components. It is concluded from the literature that enhancement of components’ mechanical and tribological properties by surface modification techniques can be a possible way to control this loss. In this regard, High-Velocity Oxy-Fuel (HVOF) process has been considered very promising to deposit coatings on several substrate materials. In the current work, two Nickel-based WC reinforced coating powders; WC-10Ni5Cr and WC-18Hastelloy C have been deposited on Monel K-500 by HVOF-spraying. The developed coated Monel alloys were analyzed in detail under scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD). The coated and uncoated alloys were mechanically analyzed in the light of density calculation, microhardness measurement and fracture toughness calculation. The wear performances of the HVOF-sprayed coatings and uncoated Monel alloy were studied under cavitation erosion, cavitation-corrosion and slurry erosion environments. The application of the HVOF-sprayed coatings resulted in a relatively denser and harder coated Monel surface in comparison to the uncoated Monel surface. The average density and average microhardness of WC-10Ni5Cr coating were found to be higher than WC-18Hastelloy C coating and uncoated Monel alloy. The fracture toughness of WC-10Ni5Cr coating was found to be higher than WC-18Hastelloy C coating. The corrosion studies also revealed that the corrosion resistance of HVOF-sprayed WC-10Ni5Cr coating was superior to HVOF-sprayed WC-18Hastelloy C coating, owing to higher Cr content in the former. The cavitation tests were performed for 10 hours following the ASTM G32-10 standard. Both WC-10Ni5Cr, as well as WC-18Hastelloy C coatings, successfully reduced the erosion volume loss of the Monel alloy. The relatively superior performance of WC-10Ni5Cr coating could be attributed to a better combination of its microhardness and fracture toughness. The formation of craters, cavities, and debonding of splats were found to be the signatures of cavitation erosion in the coatings. Whereas, microplastic tearing and microcracks were observed as the primary erosion mechanism in the Monel alloy. CE tests were conducted for 15 hours in a corrosive (3.5% NaCl) environment. It was observed that due to the effect of the corrosive medium, several pits and pores were generated over the coating surface. These defects provided additional nucleation sites for CE in the coatings resulting in a high level of erosion losses in the coatings in the given corrosive environment than in a pure cavitation environment. Both the coatings and Monel alloy were subjected to slurry erosion tests for 90 minutes at normal (90°) and oblique (30°) impingement angles. Both the coatings significantly reduced the erosive wear in Monel alloy in both the impingement conditions with WC-10Ni5Cr coating, as a better candidate. Ploughing and micro-cutting were found as the primary erosion mechanisms in Monel alloy whereas, coating spallation, micro-cutting and crater formation were the primary erosion mechanisms in the coatings. The combined result of the CE and SE studies established that HVOF-sprayed WC-10Ni5Cr coating can be a potential alternative coating material to reduce the overall wear losses of the Monel K-500 alloy in marine and hydraulic environments.