Performance, Process And Principle Study Of Electroplating Ni-W Alloy Coating On ABS Plastic

ABS plastic is light, nontoxic and smooth-shined. Can be made in different shapes and electroplated easily. However, the application of ABS plastic is limited due to its low-hardness and wear-resistance. Compare to ordinary coating, Ni-W alloy coating owns high hardness and wear-resistanc, good appearance shape and lustre. Therefore, electroplating Ni-W alloy coating on ABS surface, can raise wear-resistanc and the hardness of film greatly , and expand the application of ABS.In this paper, a Ni-W alloy coating was plating on ABS plastics. Several methods including alkaline degreasing, chemical coarsing, metal ion absorption, activating process and copper electroless plating, thick copper electroplating were taken in order to make ABS surface metalizing. Through several experiments, definited the best process of ABS surface metalizing. Then the electroplated Ni-W alloy coating on the ABS that was surface metalized. With the methods as microscopical observation, surface hardness test, surface shape observation, adhesion test, studied the influence of changing for electrical deposition rate and surface hardness of technology parameters. Applied EDS and X-ray diffraction the structure and the composition of the coating were studied, the structure of the Ni-W alloy of different tungsten's content was analyzed.Through investigation and analysis, the following about ABS surface metalzed could be concluded:1) Metalizing ABS plastic surface by alkaline degreasing, chemical coarsing, metal ion absorption, activating process and copper electroless plating. Then the copper coating was thickened by electroplating in order to improve adhesion between following coating and plastic matrix. After that Ni-W alloy coating was plated.2) By tested various coasing and activating processes, high chromic anhydride coasing solution and silver nitrate ionic activating solution was chosen for its low cost and convenience.3) The experiments of electroless copper plating were taken, the electroless-plating effect on ABS plastic and the process parameter effect on the plating solution's stability and the matrix adhesion was studied. As the result, the process and parameter of electroless copper plating were confirmed.Through study and analysis, the following about Ni-W alloy eletroplating could be concluded:1) When the nickel sulfate's content in bath increased, the nickel ion in bath also increased. On the surface of cathode, the nickel ion gathered much more than the hydrogen ion, hydrogen liberation decreasing, plating speed and the surface hardness were improved, surface look also changed fine. And along with the raising of sodium tungstate, WO₄^2- ion in bath grew, the tungsten atom that could be deposited increased, but the Ni²⁺ ion density convenient decreased, so, affected the nickel atom of the surface of cathode to separate out, also reduce tungsten atom to deposit, that made plating speed reduced first after increasing. At the same time as sodium tungstate increased, the lattice distortion of Ni-W solid solution increased, then the hardness rised, but when Ni-W alloy changed its structure from nano-crystalline state into amorphous state, the crystals were easy to slid, surface hardness reduced. When the sodium tungstate in bath was too much, it would make against to form compact appearance and make the specimen's surface look bad. Citric acid actioned as complexing agent, when its content was low, it couldn't satisfy the metal ion in bath to form more complex compound, that affected the plating speed, but too much citric acid wouldn't bring larger raising for electroplating effect.2) The cathode current density had direct influence for surface hardness. This was the influence of high plating speed that allowed the growth of alloy crystal grain of coating, and the crystal would like to change in small size. So, the surface hardness increased. Along with the increase of current density, much more substances were liberated on the cathode surface, and much more alloy was deposited on the cathode surface, so the plating speed increased. But hydrogen liberating along with the increase of current density become more violent, on the contrary, the plating speed reduced. At the same time, metal deposition rate had a upper limit, so the plating speed tend to stabilized. Additionally exceptional current density made the consuming of depositing ions neat the cathode more than theirs supplying, as the result, the specimen's appearance quality dropped.3) The pH of bath owns direct influence on specimen's surface look. Under the acid condition, the coating appearance was glossy and compact. And along with pH increasing, hydrogen liberation decreased, plating speed and specimen's hardness increased. When the plating speed was too high, it obstructed the Ni-W crystal grain becoming small, the specimen's hardness increased no longer.4) When plating bath temperature increased, the metal ion proliferation ability would strengthen, the density of metal ion in the diffusion layer of cathode would increase. So that, not only the plating speed was accelerated, also the Ni-W crystal grain became smaller and the surface became more compact. Then the surface hardness and the appearance were improved. When temperature was too high, the complexing and additive agents would decompose and volatilize, the bath became instability, and caused bad influence for plating speed, specimen surface hardness and shape look.5) The XRD results prove that the Ni-W alloy was a substitutional solid solution with nickel as solvent and tungsten as solute. And the size of crystal grain is between 20～40 nm. Along with the increase of tungsten's content, the Ni-W alloy structure gradually changed into amorphous state, when the tungsten's content is at 45.56% , Ni-W alloy coating was amorphous structure.6) Through orthogonal experiment results, it showed that: The content of nickel sulfate/sodium tungstate owned the most influence on specimen surface hardness, had fairly important influence for wear-resistance and surface appearance. Plating bath temperature owned the minimum influence for surface hardness, wear-resistance and surface appearance. PH value was notable for the influence of wear-resistance, had some influence of surface hardness, it also had influence for surface appearance. The current density had some influence of surface hardness and wear-resistance, but not quite sticked out for the influence of surface appearance. 7) Through orthogonal test optimization, the best parameters of Ni-W alloy plating was given: NiSO₄·6H₂O=60g/L, Na₂WO₄·2H₂O =150g/L, C₆H₈O₇=130g/L, T=65℃, pH=5, current density=20 A/dm², plating time=30 min. The surface hardness could reach 668.25 HV, which was 2.94 times of pure nickel coating, wear-resistance was 4.2 times of pure nickel coating. The adhesion test result showed that the combination between coating and the matrix was fine, the coating's appearance was compact and glossy.