THE EFFECT OF HIGH-TEMPERATURE FLOW ON THE MORPHOLOGY AND CHEMICAL COMPOSITION OF NICKEL–GRAPHITE COMPOSITE POWDER IN THE PLASMA SPRAYING PROCESS

Abstract

Changes in the structure of clad NPG–75 graphite powder in the application of plasma coatings were examined. The starting NPG–75 powder consisted of 25 wt.% graphite and 75 wt.% nickel. During plasma spraying of the NPG–75 powder, the graphite content in the coatings significantly reduced and did not exceed 4 wt.%. Several factors contributing to the reduced graphite content in the coatings were considered, specifically: effect of high temperature on the integrity of the nickel shell and graphite burnout as the powder passed through the plasma flow, blowing of the nickel shell by a vortex plasma flow and subsequent graphite burnout, nickel delamination upon collision of clad particles with the substrate, and influence of plasma spraying parameters. To assess the effect of high temperatures on the NPG–75 powder structure, the starting powders were oxidized in air at 600–1000 ºС for 60 min. The influence of plasma flow on the nickel shell integrity was determined by spraying the NPG–75 powder into water, followed by metallographic analysis to evaluate the nickel content. A mechanism for nickel delamination upon collision of NPG–75 powder particles with the substrate was proposed. The influence of plasma spraying parameters on the structure and graphite content of the resulting coatings was studied in detail. The research enabled the identification of methods for retaining graphite content in Ni–C composite coatings.