Influence of graphite content on the tribological properties of plasma sprayed alumina-graphite coatings

The main aim of this research was to investigate the influence of graphite as an additive to a plasma-sprayed alumina coating matrix for tribological trends. For the deposition process, pure alumina and alumina with graphite (5%, 10% and 15%) weight percentage powders were used. The graphite-modified alumina coatings were formed on steel substrates by atmospheric plasma spraying using an air-hydrogen plasma. The influence of graphite content on the elemental composition, phase structure, friction coefficient, and specific wear rate was investigated. It was obtained that the addition of graphite enhanced the gamma-Al2O3 phase content in the as-sprayed coatings. The highest gamma-Al2O3 phase fraction was obtained when the amount of graphite in the feedstock powders was 10 wt.%. The elemental composition results indicated that the concentration of carbon in the alumina -graphite coatings was up to six times lower in comparison to the initial feedstock, and had varied from-1.7 to 2.45 wt.%. The incorporation of graphite slightly reduced the friction coefficient of the composite coatings due to the formation of a self-lubricating layer under dry-sliding conditions. The specific wear rates were in the range of-10-6 mm3/(Nm) for the alumina-graphite composite coatings, which were similar to the alumina coating, and were an order lower compared to the wear rate of steel.