Control of particles codeposition and strengthening mechanisms in nickel based nanocomposite coatings
Abstract: Surface durability is a key factor in the service life of components. Depending on the aggressiveness of the environment, surface deterioration by wear or corrosion leads to failure of the bulk material and eventually to the loss of functionality of the component. Therefore, designing surfaces to withstand service requirements is a critical aspect for industrial product realisation.Electroplating is an attractive technique to mass-produce affordable protective coatings due to its low cost and high performance, easy maintenance of the process, and adjustable production times. Producing nanocomposite coatings by electroplating has received significant attention for decades due to their potential to provide excellent wear and corrosion protection.Nanocomposites provide the possibility of combining different materials to achieve multifunctionality and, due to the nanometer size of the reinforcer phase, promote additional strengthening effects in the matrix not present in microcomposites. Additionally, the reduction in the size of the reinforcer provides advantages in wear protection as the risk of third-body abrasion is reduced. However, the industrial applicability remains limited due to the lack of control in their production process.The present work focuses on the relationship between the input parameters and the codeposition of SiC, MoS2 and graphite particles, identifying critical factors and providing methods to control the process better. Furthermore, a correlation between the nickel matrix microstructure and codeposition is established, linking them to the strengthening effects and final performance of the nanocomposite coating.New methods were developed to provide a reproducible electroplating process. A surface treatment for the reinforcing powder minimised the differences between the particles surface state deriving from different batches, supplier or production routes. Composites produced with surface-treated nanoparticles showed reproducible results displaying similar codeposition rate andhardness values. Additionally, a pulse-reverse plating waveform, adapted to the SiC particles average size, was designed and optimised to deposit a reproducible and improved particles content even in the presence of anionic surfactants, typically used to reduce coatings porosity and defects.The study of the impact of the reinforcer phase on the electrocrystallisation of the nickel matrix showed that the microstructure was significantly affected by the size, chemistry and dispersion of the particles, promoting changes in the preferred crystal orientation, grains morphology and size. The strengthening mechanisms were linked to the microstructural changes resulting from the process parameters, particles codeposition and the agitation mode. Different models were used to predict the hardness of the composites based on the contribution and combination of each strengthening factor: Hall-Petch, Orowan, enhanced dislocation density and particles incorporation, showing a good agreement with the experimental data.Furthermore, the wear behaviour of the composites was analysed and connected to the hardening effects. The analysis highlighted how particles content, dispersion, type and size of the reinforcer contribute to the protection against wear.A novel multifunctional composite coating based on a dual dispersion mix of hard SiC particles and self-lubricant MoS2 particles was designed, resulting in a surface with high hardness, low friction and low wear.
CLICK HERE TO DOWNLOAD THE WHOLE DISSERTATION. (in PDF format)