Preparation And Properties Of Co-Ni-Graphene (G/Go) Composite Coatings On Continuous Casting Mould Surface

The development of high-speed continuous casting puts forward higher requirements on the surface performance of the mold.The surface modification technology such as electroplating,electroless plating,thermal spraying and laser cladding can effectively solve the problems of easy wear and high temperature corrosion of the copper plate on the continuous casting mold.Based on the preparation of Co-Ni alloy coating and Ni-graphene composite coating,the preparation conditions of Co-Ni-graphene composite coating were optimized.The morphology and composition of the coating were analyzed by field emission scanning electron microscope combined with energy spectrometer.The microstructure of the coating was analyzed by XRD and Laman spectroscopy.The friction coefficient of the coating is measured by Material surface performance comprehensive tester,and the thermal conductivity is measured by a laser heat conduction analysis meter.Evaluate the corrosion resistance of the coatings by using an electrochemical comprehensive tester to measure the electrochemical impedance spectrum?EIS?and the Tafel polarization curve of the coatings.The influence mechnism of the composition and microstructure of the coatings on the performance of the coating was analyzed.Co-Ni alloy is the preferred material for surface protection of continuous casting mold.The effect of Co content on the microstructure and properties of Co-Ni alloy coating on Cu substrate under as-plated and 300°C heat treatment conditions was studied.At experimental conditions,the abnormal deposition of Co in the coating is23.03²4.70mol%.The surface of the coating is a cellular structure,the surface and section microhardness of coatings have different trends.The coating is mainly face-centred cubic?fcc?structure in the form of Co-Ni replacement solid solution.A small amount hexagonal closed-packed?hcp?structure of Co appears in the coating with high Co content?83.12wt%?.300°C heat treatment promotes the growth of the coating grains.When the Co content in the as-plated coating exceeds 49.84wt%,the corrosion resistance of the coating is deteriorated.the corrosion resistance of the coating with 66.90wt%Co content is the best,and the hardness of the coating with high Co content is greater after 300°C heat treatment.The Ni-graphene composite coatings were prepared on the copper plate of the steelmaking continuous casting mold.The friction coefficient of the coatings decrease and the microhardness significantly increase with the amount of grapheme oxide?GO?added increases.When the concentration of GO in the plating solution is 0.12g·L^-1,the average microhardness of the coating is 1142 HV_1N,and the friction coefficient is0.548.The wear resistance is superior to the conventional Ni-Co alloy coating currently produced,which can effectively ensure the quality of the slab and improve the service life of the mold.The electroplating process of Ni-Co-GO composite coating was optimized by orthogonal test method.The analysis results show that the addition of GO can reduce the grains in?111?,?200?,?220?crystal orientation in the coatings.Higher the coatings friction coefficient,more oxides were formed at the friction interface.GO is reduced in the electroplating process and accumulated on the cathode and anode.The smooth and less porous of coating is beneficial to enhance the lubrication of graphene and reduce the friction coefficient.The optimium process of Ni-Co-graphene composite coatings as follows.The bath temperature is 30?,the graphene concentration is 0.3g/L,the cathode current density is 4.8A·dm^-2,pH is 4,and CoSO₄ concentration is120 g/L.The effects of different salt solutions and G/GO on the coating of Co-Ni alloy coating were further studied under the optimized process parameters.Co-Ni-GO composite coatings are mainly face-centered cubic?fcc?structure,while Co-Ni-G?sulfamate?and other coatings coexist with fcc and hexagonal closed-packed?hcp?structures,and the coating grains are all oriented in the?111?direction.Graphene is the most complete and ordered in the Co-Ni-G?sulfamate?composite coating.Co-Ni?sulfamate?coating has the highest thermal conductivity,and Co-Ni-G?sulfamate?composite platings take second level of 341.829 W/?m·K?.The composite coating porous structure hinders the diffusion of heat,completely ordered graphene contributes to heat transfer.The Co-Ni-G?sulfamate?composite coatings have the lowest friction coefficient of 0.37,which is mainly sliding friction and has good wear resistance.The alloy coatings are mainly adhesive friction and have poor wear resistance.Co-Ni?sulfamate?coating has the best corrosion resistance,and graphene oxide is difficult to shield well in the coatings due to many defects,so the GO composite coatings have poor corrosion resistance.The corrosion resistance of Co-Ni-G?sulfamate?composite coating is better than that of Co-Ni alloy coating.The complete and orderly graphene increases the resistance of electron transfer and contributes to the improvement of corrosion resistance of the composite coating.