Sand erosion performance of detonation gun applied tungsten carbide/cobalt-chromium coatings

Sand erosion studies of thermally sprayed WC-Co-Cr (Denotation-Gun LW45) have been undertaken using a sand/waterjet impingement rig. Results are presented which show that the erosion rate of sprayed compared to sintered tungsten carbide-cobalt-chrome is similar for low energy imparts but the sintered material outperforms by 4 times the sprayed material for high energy impacts. This reflects the anisotropic microstructure of the thermally sprayed coating with a preferred crack propagation parallel to the coating surface followed by crack interlinking and spalling. This is the dominant erosion mechanism present. A minor erosion mechanism consists of micro-cutting and ploughing at low angles of particle impact. The coatings have a relatively high density of defects including thermal stress induced transverse cracks, voids, oxides, and grit blasting remnants. Such defects are shown to accelerate the erosion process considerably because they aid crack initiation and growth leading to partial, mono or multi-splat spalling of loose material. The influence of slurry jet angle was found to be more pronounced under low energy conditions where maximum erosion occurred at 90 degrees and the minimum at 30 degrees in contrast to the high energy erosion rates which were independent of jet angle. This is a result of the lower levels of fluctuating stresses imparted to the coating during low energy impacts leading to the impact angle having a greater effect on sub critical growth rate than for the high energy conditions. (C) 1997 Elsevier Science S.A.