Wear behaviour of high velocity suspension flame sprayed (HVSFS) Al2O3 coatings produced using micron- and nano-sized powder suspensions

Three Al2O3-based suspensions were prepared using two nano-sized Al2O3 powders (having analogous size distribution and chemical composition but different agglomeration behaviour) and a micron-sized one. High velocity suspension flame sprayed (HVSFS) coatings were produced using these suspensions as feedstock and adopting two different sets of deposition parameters.The coatings produced by the micrometric powder suspension are denser and more wear resistant (as revealed by ball-on-disk testing) than those deposited using the two nanopowder suspensions, irrespective of the deposition parameters. The tribological behaviour of all coatings is dictated by the formation and subsequent removal of surface tribofilms: these tribofilms are more stable in the coatings obtained by the micrometric powder suspension, although they exhibit the largest tensile residual stress (as determined by X-ray diffraction and hole drilling techniques).Even though the as-deposited coatings are quite smooth (Ra around 1.3–2 μm), polishing to Ra around 0.1 μm further improves the wear resistance in all cases, as it delays the onset of tribofilm delamination phenomena.