Tribology of NiCrAlY+Al2O3 composite coatings by plasma spraying with hybrid feeding of dry powder+suspension

Abstract

NiCrAlY layers containing different amounts of Al2O3 (0, 3, 6, 12, 18wt%) were deposited onto stainless steel substrates by a “hybrid” plasma spray process whereby the NiCrAlY powder was fed in dry form whilst fine Al2O3 powder, dispersed in ethanol, was injected through a suspension feeding system.The Al2O3 reinforcement, consisting of fine, rounded particles interspersed within larger NiCrAlY lamellae, only causes marginal changes in hardness, due to the limited particles-matrix cohesion. Nonetheless, at room temperature, ball-on-disk dry sliding wear rates against sintered Al2O3 counterparts decrease from ≈5×10−4mm3/(Nm) for pure NiCrAlY to ≈5×10−6mm3/(Nm) with 18wt% Al2O3 addition. Pure NiCrAlY indeed suffers adhesive wear, whereas, on the composite coatings, the pull-out of some Al2O3 particles triggers the formation of a tribo-layer of smeared oxide fragments, which mediates the contact with the counterbody.At 400°C and at 700°C, all wear rates are levelled to ≈8×10−5mm3/(Nm) and ≈2×10−5mm3/(Nm), respectively. An oxide layer grows on the NiCrAlY matrix upon high-temperature exposure, resulting in a tribo-oxidation wear mechanism, which makes the addition of Al2O3 irrelevant. At 700°C, coatings are further strengthened by partial healing of interlamellar defects and by fine-grained β-NiAl precipitating within the metal matrix.