J. Oberste Berghaus, Industrial Materials Institute / -National Research Council Canada, Boucherville, QC, Canada; B. Marple, National Research Council of Canada, Boucherville, QC, Canada; C. Moreau, National Research Council Canada (CNRC-NRC), Boucherville,, QC, Canada
Nanostructured WC-Co coatings can potentially exhibit improved hardness, toughness and wear resistance over conventional tungsten carbide materials when processed under optimized conditions. The potential for such improvements is generating considerable interest in developing techniques for producing these coatings. In the present study, 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 plasma operating conditions of varying torch current, gas flow rates and composition, and related to the resulting microstructure, phase composition (EDS, SEM, XRD) and mechanical properties of the coatings, such as hardness (Vickers) and abrasion/erosion resistance (ASTM G-65 & G-76). It was observed that the coating quality was generally compromised by the high reactivity of the small particles in the surrounding plasma, as well as by the reduced particle impact velocities. To compensate for these shortcomings, high in-flight particle velocities, in excess of 800 m/sec, were generated. Furthermore, adjusting the suspension feed rate, thereby varying the thermal load on the plasma, allowed an independent control of the in-flight particle temperatures. Results showed that short dwell times of the particles in the heating plasma flame, in conjunction with low particle jet temperatures, could limit the decomposition of WC into brittle W2C/W3C and amorphous phases. The roles of substrate cooling conditions and surface oxide formation are discussed.
Summary: Nanostructured WC-Co coatings can potentially exhibit improved hardness, toughness and wear resistance over conventional tungsten carbide materials when processed under optimized conditions. The potential for such improvements is generating considerable interest in developing techniques for producing these coatings. In the present study, 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 plasma operating conditions of varying torch current, gas flow rates and composition, and related to the resulting microstructure, phase composition (EDS, SEM, XRD) and hardness (Vickers) of the coatings. It was observed that the coating quality was generally compromised by the high reactivity of the small particles in the surrounding plasma, as well as by the reduced particle impact velocities. To compensate for these shortcomings, high in-flight particle velocities, in excess of 800 m/sec, were generated. Furthermore, adjusting the suspension feed rate, thereby varying the thermal load on the plasma, allowed an independent control of the in-flight particle temperatures. Results showed that short dwell times of the particles in the heating plasma flame, in conjunction with low particle jet temperatures, could limit the decomposition of WC into brittle W2C/W3C and amorphous phases. The roles of substrate cooling conditions and surface oxide formation are discussed.
