The Influence of Spraying Parameters and Production Features of Nickel–Graphite Composite Powders on the Structure and Chemical Composition of Plasma Coatings

Abstract

The paper examines the effect of spraying parameters and production features of the nickel-clad NPG-75 graphite powder on the structure and chemical composition of plasma coatings. The starting NPG-75 powder consisted of graphite (25 wt.%) and a nickel shell (75 wt.%). During plasma spraying of the NPG-75 powder, the graphite content of the resulting coatings significantly reduced and did not exceed 4 wt.%. Several possible reasons for the reduction in graphite content of the coatings were considered. The influence of spraying parameters was studied using powder produced by the rolling process. This powder consisted of conglomerated graphite particles coated with nickel. In the spraying of coatings from the clad conglomerated powder, variations in plasma torch power from 20 to 29 kW and spraying distance from 80 to 300 mm did not have a significant effect and did not increase the graphite content of the coating. At powers below 20.4 kW, NPG-75KP coatings could not be formed. The optimal spraying parameters were found to be a plasma torch power of 24–25 kW and a spraying distance of 160 mm. Under these parameters, the graphite content of the coating reached a maximum of 5.3 wt.%, which was still much lower than in the starting powder (25 wt.%). The best results were shown by plasma coatings from the NPG-75 composite powder material produced without rolling but by direct cladding of solid graphite grains. Even though the quality of the nickel shell produced via autoclave cladding was lower, the graphite content of the resulting coating reached approximately 24 wt.%. The optimal spraying parameters remained the same as for the conglomerated NPG-75 powder. The research confirmed the critical importance of the methods employed to prepare the starting material for preserving the graphite content of plasma-sprayed Ni–C composite coatings.