Investigation on the Mechanism of Electrocodeposition and the Structure-Properties Correlation of Nickel Nanocomposites

There is an increasing interest in nanostructured and nanocomposite surface finishings for automotive and aerospace applications. The widespread applicability of these novel materials is based on their unique mechanical, physical, and chemical properties. An advantageous production method is the electrocodeposition (ECD) process from metal plating baths containing dispersed nanoparticles. By using this technique, a broad range of substrate sizes and shapes can be coated cost-effectively. However, the prediction of the amount as well the distribution of nanoparticles within the metal film fails frequently. There is no complete understanding of the particle incorporation mechanism. The goal of this research was to improve the fundamental understanding of the ECD mechanism. In order to identify the forces affecting the codeposition behavior of nanoparticles in a metal matrix, the effects of a variety of interrelated process parameters on the composite film formation have been investigated systematically. Nanocomposites containing metal and metal oxide nanoparticles in a nickel matrix have been prepared by means of ECD from two different types of nickel plating baths, an acidic sulfamate (pH 4.3) and an alkaline pyrophosphate bath (pH 9.5). The effect of deposition conditions on the ECD process was investigated utilizing two electrode configurations, viz. a parallel plate electrode (PPE) and impinging jet electrode (IJE) and different deposition techniques, viz. direct current (DC) deposition, both pulse plating (PP) and pulse-reverse plating (PRP). The surface charge and sedimentation behavior of the nanoparticles in these electrolytes were characterized by zeta potential and stability measurements. The surface charge, hydrodynamic diameter and colloidal stability of the nanoparticles in the nickel electrolytes were mainly affected by the composition and pH of the bath. The particles tend to form agglomerates in both nickel baths. Smaller agglomerates and an improved colloidal stability occurred in the case of the alkaline bath. Composites with a maximum particle content of either ~3.6 vol-% of 13 nm Al2O3 or ~10.4 vol-% of 21 nm TiO2 were obtained using a parallel plate electrode and DC deposition conditions. Both jet plating as well as pulse plating resulted in a distinct increase of the particle codeposition. A maximum incorporation of ~12 vol-% of 50 nm Al2O3 particles in a nickel matrix was achieved using an unsubmerged IJE system, while PP and PRP resulted in composites with particle contents up to 11 vol-% of 13 nm Al2O3. The particle incorporation increased with the particle content of the electrolyte for all deposition conditions studied. A beneficial effect on the amount of codeposited particles was found with decreasing average current density. The Al2O3 and TiO2 particles were found to be negatively charged in the alkaline pyrophosphate bath, and positively charged in the acidic sulfamate bath. It could be shown that negatively charged particles codeposited preferentially within the nickel matrix. The effect of PP and PRP conditions, e.g. pulse frequency, duty cycle and value of the peak current density, on the ECD of Ni-Al2O3 composites was studied using rectangular current pulses in the order of milliseconds. In general, low duty cycles and high pulse frequencies resulted in an enhanced particle codeposition. Using the unsubmerged IJE system, the effects of jet flow rate, particle loading and current density on the particle incorporation were studied. Referring to the experimental results from the ECD of 50 nm alumina with nickel using an IJE system, a kinetic model was developed. Therefore, the particle flux to the electrode was derived from an analysis of the total force acting on the particle in front of the electrode. The model took into account the convective diffusion of particles to the electrode surface, and the effect of gravitational and buoyancy forces on the particle flux. The gravitational force was found to be important for the ECD of 300 nm particles, but not for 50 nm particles. The effect of an external magnetic field on the ECD of Co or Fe3O4 nanoparticles in a nickel matrix has been studied for different current densities, particle contents of the electrolyte and magnetic flux density. The particle incorporation showed a distinct dependency on the orientation of an externally applied magnetic field. While the particle incorporation increased in a perpendicular field (perpendicular with regard to the electrode surface), it decreased in a parallel orientation. The influence of the magnetic field on the ECD of magnetic nanoparticles with nickel was explained by the interplay of Lorentz force and magnetophoretic force. The structure and the properties of the nickel matrix were significantly altered due to the codeposition of nanoparticles. The pure nickel deposits from the sulfamate bath exhibited a strong <100> texture, and those from the pyrophosphate bath a strong <110> preferred orientation. With increasing plating current density and particle incorporation, a variation in the crystallite size and a loss of texture was observed. High resolution TEM imaging proved a complete embedding of nanoparticles by the nickel matrix without any voids. In the case of both nickel baths, the Vickers microhardness showed a tendency to increase with the amount of particle incorporation. The enhanced hardness of the composite films was associated with modifications in the microstructure of the nickel matrix as well as with the nanoparticle incorporation. The wear resistance as examined by linear abrasion test increased with decreasing current density and due to the particle incorporation. Furthermore, the incorporation of magnetic nanoparticles resulted in a distinct increase of the magnetic hardness of the nickel matrix