Slurry erosion of thermal spray coatings and stainless steels for hydraulic machinery

The slurry erosion of two coatings applied by oxy fuel powder (OFP) and wire arc spraying (WAS) processes onto sand-blasted AISI 304 steel was studied, and the results were compared to those obtained with AISI 431 and ASTM A743 grade CA6NM stainless steels, which are commonly used for hydraulic turbines and accessories. The adherence of the coatings to the substrate was measured according to ASTM C 633 standard, while the microstructure and worn surfaces were characterized by optical and scanning electron microscopy. Slurry erosion tests were carried out in a modified centrifugal pump, in which the samples were placed conveniently to ensure grazing incidence of the particles. The slurry was composed of distilled water and quartz sand particles with an average diameter between 212 and 300 μm (AFS 50/70) and the solids content was 10 wt% in all the tests. The mean impact velocity of the slurry was 5.5 m/s and the erosion resistance was determined from the volume loss results. The coated surfaces showed higher erosion resistance than the uncoated stainless steels, with the lower volume losses measured for the E-C 29123 deposit. SEM analysis of the worn surfaces revealed intense plastic deformation in both coated and bare stainless steels, with little evidence of brittle fracture in the microstructure. The measured adhesive strength of the coatings was considered acceptable for the processes employed.