Microstructural and Tribological Investigation of High-Velocity Suspension Flame Sprayed (HVSFS) Al2O3 Coatings

Abstract

Al 2 O 3 coatings were manufactured by the high-velocity suspension flame spraying (HVSFS) technique using a nanopowder suspension. Their structural and microstructural characteristics, micromechanical behavior, and tribological properties were studied and compared to conventional atmospheric plasma sprayed and high-velocity oxygen-fuel-sprayed Al 2 O 3 coatings manufactured using commercially available feedstock. The HVSFS process enables near full melting of the nanopowder particles, resulting in very small and well flattened lamellae (thickness range 100 nm to 1 lm), almost free of transverse microcracking, with very few unmelted inclusions. Thus, porosity is much lower and pores are smaller than in conventional coatings. Moreover, few interlamellar or intralamellar cracks exist, resulting in reduced pore interconnectivity (evaluated by electrochemical impedance spectroscopy). Such strong interlamellar cohesion favors much better dry sliding wear resistance at room temperature and at 400°C.

Suspension High Velocity Oxy-Fuel (SHVOF)-Sprayed Alumina Coatings: Microstructure, Nanoindentation and Wear

“…The rapid decrease in friction after the initial rise to ~0.7 is consistent with removal of the coating, and then the following gradual decrease in friction may be caused by formation of a film on counterbody and/or the coating wear track created by the reattachment of wear debris. Tribofilm formation in atmospheric plasma spraying and HVOF alumina coatings was also reported by Bolelli et al in the dry sliding wear of alumina coatings against alumina counterbodies [4,23].…”

“…Using a higher flame power of 101 kW permitted almost complete melting of the feedstock material and nucleation of the metastable gamma form of alumina. Therefore, the SHVOF process does not inherently promote full coating melting and phase transformation, as was indicated by the work of Bolleli et al [4], but the preservation of the alpha phase is dependent on the careful selection of spray parameters. It is evident that the use of the higher combustion flame power, as in the 101 kW, yielded a denser coating, with the lower degree of porosity being only present in regions matching the inter-pass spacing.…”

“…Alumina is an engineering ceramic often used as a feedstock for thermally sprayed coatings, because of its low cost as well as high hardness, good wear resistance and chemical stability [4]. In its sintered form, alumina is normally comprised of the stable alpha phase (corundum) which exhibits the best mechanical and tribological properties among the alumina phase types.…”

“…Bolelli et al [4] showed that a dense SHVOF alumina coating yielded the lowest alpha:gamma ratio compared to regular HVOF and atmospheric plasma spraying, due to more extensive melting of the feedstock powder and almost complete avoidance of the alpha phase. Toma et al [10] produced alumina coatings using SHVOF, in which the coating was comprised of a majority (60 wt%) alpha alumina phase with remainder gamma, in contrast to a majority gamma phase coating with a minority of alpha (30 wt%) produced using standard HVOF.…”

“…It however must be noted that processing conditions were not the same for both tests and hence it is difficult to make a direct comparison. Importantly, it indicates that the retention of a large amount of alpha alumina and avoidance of the gamma phase is possible, when the SHVOF spray parameters are not too severe [4,11]. Tribological testing of SHVOF in comparison to regular HVOF sprayed alumina coatings has been performed by Rauch et al [11], with an order of magnitude reduction in wear rate for the SHVOF coating containing ~3 wt% alpha alumina.…”