| The SiC fibers derived from the precursor PCS have an irreplaceable role in application of aerospace engine because of the excellent properties of low density,high strength,high modulus,high temperature resistance,oxidation resistance and fine diameter.For now,the fiber is mainly used to reinforce ceramic matrix composites.However,it has been reported that the SiC fibers can be used to enhance metals such as aluminum to increase the strength and durability of the matrix.However,due to the sensitivity of aeronautical strategic materials,the specific preparation process has not been reported.The poor wettability between SiC fibers and liquid metal can be enhanced by preparing metal coating on the SiC fiber surface.On the other hand,the coating can inhibit the interfacial reaction between the fibers and the metallic matrix at high temperatures.In this paper,uniform nickel coatings with controllable thickness are prepared on the PCS-SiC fibers by electroplating technology.The paper focused on the influence rule of electroplating parameters on the morphology and thickness of the nickel coating.The concerned parameters are plating time,cathodic current density,temperature and pH value for DC plating,and pulse current density,pulse frequency,duty ratio for the pulse plating.The continuous plating of nickel on the surface of SiC fibers was achieved through the self-developed device.The structural stability of nickel-plated continuous SiC fibers after heat treatment was evaluated.The main conclusions are as follows:(1)Using normal DC plating technology,the SiC fiber can be coated by nickel through adjusting the plating time,pH value,temperature,and cathode current density.The thickness of the nickel layer increases significantly with the increase of plating time,and the surface of the fiber gradually becomes coarse.The cathode current density has great influence on the surface morphology of nickel coating.The small current density will cause the coating to be incomplete.When the current density is too high,large nickel agglomerates are formed on the surface of SiC fibers.Bath temperature mainly affects the growth morphology of crystal in nickel layer.As the temperature rises,the nickel crystal changes from spherical to pyramidal.The pH value of the bath mainly affects the compactness of the coating.When the pH is too low,hydrogen evolution reaction governs the electroplating process,results in the coating of loose and incomplete.When the pH is too high,the solution is prone to precipitate.According to the above rules,the optimal conditions for continuous nickel plating on the surface of continuous SiC are presented.(2)SiC fibers can also be coated with nickel under pulse plating through adjusting pulse current density,pulse frequency and occupation ratio.The pulse current density has great influence on the surface morphology of nickel coating.Low current density leads to formation of incomplete coating.As the current density is too high,large nickel agglomerates are formed on the fiber surface.The thickness of the coating increases with the increase of the duty ratio,and decreases with the increase of the pulse frequency.The nickel coating obtained by pulse plating has finer crystalline structure than that prepared by DC plating.(3)When the continuous SiC fibers with nickel coating are exposed to insert gases at 500 ℃ for 6h,no interfacial reaction between nickel coating and SiC fibers is detectable.While under 700 ℃,the nickel coating on the surface of the SiC fiber reacted with the SiC fibers.The longer the thermal insulation time is,the greater the interfacical reaction proceeds.After 30 minutes reaction,layer of Ni2Si-C with a thickness of 100-200nm appeared.In addition,grain coarsening of the nickel coating is observed after heat treatment at 700℃. |
