Microstructure and tribological properties of plasma sprayed alumina and alumina-graphite coatings

Al2O3 and Al2O3-graphite composite coatings were prepared on stainless steel by atmospheric plasma spraying. The influence of spraying distance and graphite addition on the coating microstructure, phase composition and tribological properties were investigated. The elemental composition results indicated that the increase of spraying distance slightly increased the graphite concentration in composites. The X-ray diffraction results of Al2O3 coatings revealed the presence of α-Al2O3, γ-Al2O3 and β-Al2O3 phases. The friction coefficient of Al2O3 coatings varied in the range of 0.74-0.75, whereas with the addition of graphite into the alumina powders, the friction coefficient of the coatings reduced to 0.34-0.38. It was found that the wear resistance of the Al2O3-graphite composite coating was superior to that of the Al2O3 coating when the spraying distance was 60 mm.