Annealing of plasma-sprayed WC-Co coating

Abstract

The effects of vacuum annealing on the physical soundness and resultant 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 °C for up to 12h. The results showed that, due to differential sintering within the coating, vertical cracks were formed at the interface between darker cobalt-rich and lighter cobalt-deficient splats. The average length and density of cracks increased with increasing temperature at intermediate annealing temperatures from 500 to 700 °C, but decreased with increasing annealing temperature and time at higher annealing temperatures due to crack sintering and ovulation effects. Significant precipitation of WC crystallites and interdiffusion at the coating-substrate interface also occurred at the higher temperatures. The annealing-induced cracks produced a detrimental effect on the bond strength of plasma-sprayed WC-17%Co coatings. The decrease in tensile bond strength was especially pronounced for coatings annealed at intermediate temperatures from 500 to 700 °C. For coatings annealed at 800 °C and above, the tensile bond strength improved steadily over that of coatings annealed at the lower temperatures, their magnitudes increasing with increasing annealing temperature and time. Despite the above, bond strengths of the as-sprayed coating and those annealed at and above 900 °C were undetermined because failure occurred in the adhesive. Cylinder-on-cylinder wear tests showed that the wear behaviours of the as-sprayed coating and of coatings annealed under various conditions were comparable. Two wear mechanisms were identified: plastic deformation of the γ-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 conditions.