Electroplating of Black Nickel Composite Coatings

Abstract

Black nickel coatings have specific properties which make them a suitable choice for different applications. Different processes are used to produce black nickel coatings which alter coating properties accordingly. These coatings are often electroplated over a sublayer like bright nickel to enhance their properties. Despite high industrial demand, not many works have been reported in improving black nickel coatings properties other than optical properties. Therefore, the electrodeposition of black nickel composite coatings with a main focus on mechanical properties improvement for jewellery and decorative applications has been systematically studied in this thesis. These articles need to keep good appearance in terms of blackness and shininess, and not degrade easily. The composite coating electrodeposition method was selected for mechanical properties enhancement as many studies on electroplating nano-composite coatings with both matrix and fine particle dispersion, have shown superior properties. The traditional method to obtain nano-composite coatings is direct addition of solid nanoparticles into the electroplating solution. However this method increases the tendency for particle agglomeration in the solution. To overcome this issue a sol technology has been applied to minimise agglomeration of nanoparticles. By adding transparent sol solution to the electrodeposition solution, sol solution can in-situ transform to solid nano-particles and immediately be surrounded by an ionic cloud. These ionic clouds transport particles to the cathode, resulting in deposition of a highly dispersive nano-composite coating. In this study, sol technology has been applied to black nickel electrolyte to produce black nickel composite coatings. This research is divided into two major phases. Phase (I) is a feasibility study of two novel sol-enhanced black nickel composite coatings by electroplating on nickel plated brass substrate. The electroplating was done from black nickel solutions after 12.5 ml/l TiO2 sol and ZrO2 sol were added individually. Their properties were compared with traditional black nickel coatings. Based on phase (I) results, TiO2 sol addition was chosen for a more detailed study in phase (II). In this part, process parameters were optimised for electroplating black nickel composite coatings with enhanced properties. Different concentrations of TiO2 sol were added to the electroplating solution at different pHs to evaluate the properties of the black nickel coatings. The solution pH was lowered to break down particles and to decrease agglomeration. Due to the lower deposition rate at low pH, black nickel coating thickness was low, and consequently the substrate interfered with the coating analysis. Therefore, thicker coatings were electroplated by prolonging the electroplating time in order to compare the coatings at a similar condition in terms of the substrate effect. Nickel plated brass substrate was also replaced by copper for phase (II) experiments as copper did not interfere with the coating analyses using XRD and EDS. Black nickel solutions and coatings characteristics were evaluated by different analysis instruments. Mechanical properties including hardness, scratch resistance and wear were studied using two different nanoindentation machines and a wear tester. Surface imaging was also performed using AFM and Hysitron nanoindentation instruments. Colour and shininess of the black nickel coatings were studied using Machine Vision. The EDS analysis revealed that nickel, zinc and sulphur are the main constituents of black nickel coatings. Substrate and metallic nickel in the coating are the main factors affecting the coating reflectance properties. Blackness is also affected by zinc and sulphur contents. In phase (I), all three coatings showed similar blackness and shininess visually. Later on a more detailed study of colour and reflectance confirmed that the black nickel coatings’ appearance did not change by TiO2 sol addition at pH = 5.5. Corrosion resistance evaluation by potentio-dynamic polarisation analysis showed a great improvement for both black nickel-TiO2 and black nickel-ZrO2 composite coatings by 12.5 ml/l sol additions. Black nickel coating electroplated from TiO2 sol added solution is a composite coating as ICP-MS results confirmed the consumption of titanium during electrodeposition of the black nickel coatings. Particle size measurement confirmed that TiO2 sol mainly contained ultra-fine particles (< 4 nm). TiO2 sol showed different interactions with different electroplating solutions based on bath chemistry and properties. TiO2 sol addition to the traditional black nickel solution and Watts nickel bath resulted in partial agglomeration of white TiO2 particles, which was more severe for the black nickel solution at the same sol concentration addition. Experiments revealed that zinc chloride is the main solution ingredient encouraging agglomeration. EDS analysis also supports the hypothesis that zinc ions are the preferred metal ions surrounding TiO2 particles. Nanohardness and scratch resistance results showed improved mechanical properties for both black nickel-TiO2 and black nickel-ZrO2 composite coatings. Black nickel-TiO2 showed better mechanical properties in comparison with traditional black nickel and black nickel-ZrO2 composite coatings. A large variation in mechanical property measurements was observed for the black nickel-ZrO2 composite coating. The pop-in events observed in the Continuous Stiffness Measurement (CSM) of the three coatings affirms a laminar structure in the black nickel coatings. Depending on the applications of the black nickel coatings, different combinations of mechanical/optical properties are achievable by changing process parameters. Best hardness improvement was obtained by 12.5 ml/l TiO2 sol addition, lowering pH to 4, and prolonging electroplating time to 20 minutes. At this condition, hardness was improved by 18% whereas wear resistance remained almost unchanged, but the blackness decreased and reflectance increased. To have both hardness and wear resistance improvement with no optical property changes in comparison with traditional black nickel coating, 12.5 ml/l TiO2 sol should be added to electroplating solution of pH = 5.5 for 10 minutes electroplating. At this condition hardness and wear resistance improved about 8% and 33% respectively, in comparison with the traditional black nickel coating. Different wear mechanisms have been observed for black nickel coatings, including abrasion wear, adhesive wear and delamination. The wear and scratch resistance tests showed similar results. By adding 12.5 ml/l TiO2 and ZrO2 particles to the coating, scratch resistance showed a great improvement of ~ 40%. Lowering pH to 4 decreased the deposition rate, consequently titanium incorporation was low. It is hypothesised that at lower concentrations of TiO2 sol addition at pH = 4, less agglomeration of particles compensated for lower zinc deposition. As the TiO2 sol concentration increased, TiO2 become more prone to agglomeration. Together with the lower zinc deposition rate, this caused less TiO2 incorporation into the coating. Morphology analysis of the black nickel coatings revealed a spherical nodular microstructure. Black nickel and black nickel-TiO2 composite coatings showed an ultra-fine microstructure and very smooth surface with nano-structured topography. TiO2 and ZrO2 sol-enhanced composite coatings showed more refined, uniform and compact microstructure and smoother surface in comparison with the traditional black nickel coating. The Black nickel-TiO2 composite coating had the finest microstructure in comparison with the black nickel-ZrO2 composite coating and the traditional black nickel coating. Solution pH change had a great influence on optical properties of the coatings. Increasing pH resulted in blacker and less shiny coatings.