Abstract
Nanostructured WC-12%Co coatings were deposited by suspension plasma spraying of submicron feedstock powders, using an internal injection plasma torch. The liquid carrier employed in this approach allows for controlled injection of much finer particles than in conventional thermal spraying, leading to thin coatings with a fine surface finish. A PEI (polyethylene-imine) dispersant was used to stabilize the colloidal suspension in an ethanol carrier. In-flight particle states were measured for a number of operating conditions of varying plasma gas flow rates, feed rates and standoff distances, and related to the resulting microstructure, phase composition (EDS, SEM, XRD) and Vickers hardness. High in-flight particle velocities, in excess of 800 m/sec, were generated, leading to dense coatings. It was observed that the coating quality was generally compromised by the high temperature and reactivity of the small particles. To compensate for this shortcoming, the suspension feed rate was adjusted, thereby varying the thermal load on the plasma. Results showed that a slightly larger agglomerate size, in conjunction with low particle jet temperatures, could somewhat limit the decomposition of WC into brittle W2C/W3C and amorphous cobalt containing binder phases.
