ANNEALING OF PLASMA-SPRAYED WC-CO COATING

The effects of vacuum annealing on the physical soundness and resultan t tensile bond strength and wear properties of plasma-sprayed WC-17%Co coatings on a ductile cast iron substrate have been investigated. The as-sprayed specimens were annealed at temperatures from 500 to 1000 d egrees C for up to 12 h. The results showed that, due to differential sintering within the coating, vertical cracks were formed at the inter face between darker cobalt-rich and lighter cobalt-deficient splats. T he average length and density of cracks increased with increasing temp erature at intermediate annealing temperatures from 500 to 700 degrees C, but decreased with increasing annealing temperature and time at hi gher annealing temperatures due to crack sintering and ovulation effec ts. Significant precipitation of WC crystallites and interdiffusion at the coating-substrate interface also occurred at the higher temperatu res. The annealing-induced cracks produced a detrimental effect on the bond strength of plasma-sprayed WC-17%Co coatings. The decrease in te nsile bond strength was especially pronounced for coatings annealed at intermediate temperatures from 500 to 700 degrees C. For coatings ann ealed at 800 degrees C and above, the tensile bond strength improved s teadily over that of coatings annealed at the lower temperatures, thei r magnitudes increasing with increasing annealing temperature and time . Despite the above, bond strengths of the as-sprayed coating and thos e annealed at and above 900 degrees C were undetermined because failur e occurred in the adhesive. Cylinder-on-cylinder wear tests showed tha t the wear behaviours of the as-sprayed coating and of coatings anneal ed under various conditions were comparable. Two wear mechanisms were identified: plastic deformation of the gamma-Co matrix and spallation of splats. The wear test results were consistent with spallation being the dominant wear mechanism, both in as-sprayed and annealed conditio ns.