Studies On The Processing Parameters On Electroless Deposition Of W Fiber Surface Modification

Tungsten fibers are ideal reinforcing material of high temperature alloys and refractory metal materials due to high density,high melting point,high strength,good tensile strength and specifice strength.However,it cannot reach the ideal interface bonding state with the CuW matrix in high temperature due to its surface which is easy to be oxidized.Therefore,it is necessary to deposit activating elements on W fiber to improve binding properties of fiber and CuW matrix.In this paper,the nickel coatings on pretreated tungsten fibers were fabricated by chemical deposition method,in order to improve the surface activity of tungsten fibers.In this paper,we studied the pretreatment of W fibers,nickel deposition rate and nickel coatings microstructure in acidic and alkaline conditions systematically;meanwhile,we discussed the influence of the complexing agent and Cu²⁺ in solution on nickel coatings microstructure and composition.The results of this study are as follows:?1?In the early stages of chemical deposition,W fibers were cleaned,getted rid of oil and roughened to impro've the activity of the fiber surface,which is conduci.ve for the formation of active nickel on its surface.However,the roughening time should not be too long.Ultra-roughening process would damage the internal structure of W fibers and cause a massive drop in performance.Experimental results showed that the best roughening time was 15min.In the chemical deposition process,the dehydrogenation reaction of hypophosphite near the W fibers is easy to happen with self catalysis of nickel pieces and reduce nickel ions in solution.Active atoms of nickel were formed on the surface of the W fibers,providing as the catalytic structure of the nickel coatings will change from the cellular structure to the smooth surface with the increase of the pH value of the solution,uniform transition.When pH is 9,nickel coatings on W fibers has best surface morphology,and most small grain.When the pH value decreased from 9 to 5,phosphorus content of the nickel coatings would increased gradually.It is found that the sharp diffraction peaks nickel coatings change for the diffuse diffraction peaks by XRD.The structure of nickel coatings gradually shifts to be amorphous.?3?From the point of electroless bath,the type and concentration of complexing agent control the number of free nickel ions and impurity particles in solution.With the increase of the concentration of complexing agent,the binding ability of complexing agent and nickel ion is enhanced.The number of free nickel ions and nickel hydroxide particles decreases.Experiments were made by citric acid,malonic acid and lactic acid as complexing agent.It found that the stability of solution increased gradually with the increase of concentration of complexing agent.When the concentration of citric acid was lg/L,nickel coatings had fine particles,and its surface roughness was the largest.?4?Cu²⁺ was putted into electroless bath.The nickel deposition is blocked.The formation of the morphology were completely different from the original.Nickel is no longer deposited on W fibers in the form of layered structure uniformly.However,the nickel initiated at the original tungsten fiber grooves and formed a net structure.If the reduction potential of nickel and copper ions was changed,the morphology of the nickel coatings will change correspondingly.?5?Performance testing was carried out for nickel coatings prepared by different complexing agents,It found nickel coatings had low hardness and a good plastic deformation capacity when citric acid was used as complexing agent in solution.and when nickel coatings were existed as granular structure on W fiber,nickel activation effect is the best.