Splat-deposition Behavior and Coating Properties of Low-pressure plasma-sprayed CoNiCrAlY Powder
Low-pressure plasma-sprayed MCrAlY coatings exhibit excellent high-temperature oxidation resistance and hot corrosion resistance and can be used as high-temperature sealing coating materials,which are widely used to protect the hot-end components in turbine engines.Meanwhile,thermally sprayed coatings are formed on a substrate by the impingement of fully or partly melted feedstock particles,where the next layer is deposited on top of the previously deposited layer until the desired coating thickness is achieved.Therefore,the flattening nature of the molten particles on the substrate or previously deposited coating during thermal spraying is vital to the coating process.However,the relationship between the flattening behavior of individual molten MCrAlY particles and the stacking behavior of subsequently deposited particles with the coated structure,as well as the performance during low-pressure plasma spraying remain unclear.In this study,commercially available CoNiCrAlY powders manufactured via gas atomization are thermally sprayed onto nickel-based superalloy GH3536 substrates via low-pressure plasma spraying.Individual splat depositions are realized on the mirror-polished superalloy substrate and previously deposited CoNiCrAlY coating specimens.A steel slit featuring a hole with a diameter of approximately 10 mm is installed between the plasma torch and substrate to obtain particles with homogeneous thermal and velocity,as well as to avoid the effects of substrate-temperature increase and the accompanying change in substrate topography induced by the heating flow of the plasma jet.Following splat acquisition,a CoNiCrAlY coating with a thickness of several hundred micrometers is sequentially deposited on a blast-treated superalloy substrate.The top and bottom surface morphologies of individual splats and stacked deposits,as well as the microstructure and properties of the coatings,are evaluated systematically.The results show that when spraying is conducted under a reduced ambient pressure,the individual molten CoNiCrAlY particle spread rapidly on the surface of the substrate and solidify into a disk-shaped splat.During the spreading of the molten droplets,the desorption of the adsorbed gas condensation due to temperature increase at the collision zone between the molten droplet and substrate surface,together with the entrapped gas during the in-flight process of the molten particle,aggregated at the bottom of the flattening droplet.Subsequently,the gathered gas escaped from the edge of the flowing deposition on the substrate surface and formed a few short splashing fingers connected to the central core.Owing to the good wetting performance of the similar materials,the subsequent molten particles spread more extensively on the surface of the previously deposited coating.Numerous fully melted and sufficiently flattened splat deposits combined with each other via mechanical interlocking and stacked layerwise to form a dense coating.The porosity of the coating obtained under reduced ambient pressure can be controlled to less than 1％,the oxygen content of the coating is about 0.32％,the adhesion strength exceeds 75 MPa,and the vacuum-heat-treated coating exhibits favorable antioxidation resistance at 1 050 ℃.Vacuum heat treatment can improve the microstructural uniformity and cohesive strength of the coating as well as enable the formation of an interdiffusion layer with a thickness of several tens of micrometers at the interface between the coating and substrate,thus further improving the bonding strength between the coating and substrate.The performance of the coating satisfies the technical requirements of protective coatings used for hot-end components in turbine engines.In summary,the low-pressure plasma-sprayed thick CoNiCrAlY coating exhibits a uniform and dense structure and excellent comprehensive properties,which are closely related to the flattening behavior of single molten particles on the substrate and the stacking process of the previously deposited coating.The results provide a basis for the optimization of coating structures and performances by controlling the flattening and stacking behaviors of thermally sprayed particles.
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