Improving interfacial, mechanical and tribological properties of alumina coatings on Al alloy by plasma arc heat-treatment of substrate

Highlights: • Columnar δ-Al₂O₃ induces epitaxial growth of γ-Al₂O₃ grains in coating after PA-HT. • Epitaxial growth greatly enhances interfacial bonding of Al₂O₃ coating on Al alloy. • Penetration of Al₂O₃ droplets into Al alloy increases interfacial anchorage force. • Crystal structure of the alumina coatings can be refined after PA-HT of substrate. • Mechanical and tribological properties of the coatings are improved after PA-HT. - Abstract: Plasma sprayed ceramic coatings can be used to improve the mechanical properties and wear resistance of aluminum alloys, but there are still some challenges to effectively increase their interfacial adhesion. Thus we conducted plasma arc-heat treatment (PA-HT) of Al alloy substrate before plasma spraying, hoping to tune the microstructure of Al₂O₃ coatings and improve their interfacial strength as well as mechanical and tribological properties. The influences of PA-HT on the microstructure of alumina coatings were analyzed by X-ray diffraction, transmission electron microscopy and scanning electron microscopy, while its effect on mechanical and tribological properties were evaluated by a nano-indentation tester and a friction and wear tester. Results demonstrate that a few columnar δ-Al₂O₃ generated on substrate surface after PA-HT at 200–250 °C can induce the epitaxial growth of γ-Al₂O₃ grains in Al₂O₃ coatings, thereby enhancing their interfacial bonding. Besides, elevating substrate temperature can help alumina droplets to melt into the interior of substrate and eliminate holes at the interface, finally increasing the interfacial anchorage force. More importantly, no interfacial holes can allow the heat of droplets to be rapidly transmitted to substrate, which is beneficial to yield smaller crystals in coatings and greatly enhance their strength, hardness and wear resistance.