Slurry and cavitation erosion resistance of thermal spray coatings

Abstract

The slurry and cavitation erosion resistance of six thermal spray coatings were studied in laboratory and compared to that of an uncoated martensitic stainless steel. Nickel, chromium oxide and tungsten carbide coatings were applied by oxy fuel powder (OFP) process and chromium and tungsten carbide coatings were obtained by high velocity oxy fuel (HVOF) process. The microstructure of the coatings was analyzed by light optical microscopy (LOM) and scanning electron microscopy (SEM), as well as by X-ray diffraction (XRD). The cavitation erosion resistance of the coatings was measured in a vibratory apparatus according to ASTM G32 standard and the slurry erosion tests were carried out in a modified centrifugal pump in which the samples were conveniently placed to guarantee grazing incidence conditions, as well as in a high velocity jet erosion testing machine. The results showed that the slurry erosion resistance of the steel can be improved up to 16 times by the application of the thermally sprayed coatings. On the other hand, none of the coated specimens showed better cavitation resistance than the uncoated steel in the experiments. The main mass removal mechanisms observed in all the coatings submitted to slurry erosion were micro-cutting and micro-ploughing as well as detachment of hard particles. In cavitation erosion, OFP coatings showed brittle fracture and microcracking, and in nickel-based coatings some ductile deformation was also observed. In HVOF coatings, detachment of small particles led to coalescence of pores in WC/Co coatings while in CrC coatings the main wear mechanism was brittle fracture of particles.