Ni electrodeposition is a suitable process for producing thick deposits and thick metallic microstructures, especially for producing relatively deep micromoulds in Microsystems industry. Ni-P deposits, due to their better properties compared to Ni deposits – particularly high mechanical properties (hardness, wear resistivity), corrosion resistance, magnetic properties, a higher fatigue limit and lower macroscopic deformation – can be a very good alternative for producing Microsystems, especially for MEMS or Microengines. According to few limited literature and our primary investigations, dispersion coating and adding particle into the electrolyte can be considered as an approach in order to decrease the stress and ease the deposition of Ni-P galvanically. Although in the last decades the influence of particles embedment in the matrix by electroplating techniques have attracted the scientific interest, there are still contradictions among the research community concerning the influence of codeposited particles on the microstructure and strengthening properties of Ni-based composites coatings. In many cases, it is believed that the enhanced mechanical performance of the coatings is mainly caused by a change in the microstructure of the metal matrix and not so much by the presence of the particles themselves. In other words, the role of particle characteristics - like their nature, size and concentration - on the layer features and strengthening mechanism of electrodeposited Ni-based composite coating with different matrix is not cleared well. Furthermore, the incorporation of particles into the deposit is mainly considered as a key factor for determining the composite coatings properties. Despite of existing models of ECD, the mechanism of particle incorporation into the film under influence of different particle characteristics has not been well understood yet. Therefore, the main aim of this study is to shed light on the effect of particle characteristics (size, concentration, type) on the codeposition process, microstructure and strengthening mechanisms in Ni and Ni-P electrodeposited composite coatings.

Functional coatings produced using scalable and cost-effective processes such as electrodeposition and etching lead to the creation of random roughness at multiple length scales on the surface. The first part of thesis work aims at developing a fundamental mathematical understanding of multiscale coatings by presenting a fractal model to describe wettability on such surfaces. These surfaces are described with a fractal asperity model based on the Weierstrass-Mandelbrot function. Using this description, a model is presented to evaluate the apparent contact angle in different wetting regimes. Experimental validation of the model predictions is presented on various hydrophobic and superhydrophobic surfaces generated on several materials under different processing conditions. Superhydrophobic surfaces have myriad industrial applications, yet their practical utilization has been severely limited by their poor mechanical durability and longevity. Toward addressing this gap, the second and third parts of this thesis work present low cost, facile processes to fabricate superhydrophobic copper and zinc-based coatings via electrodeposition. Additionally, systematic studies are presented on coatings fabricated under different processing conditions to demonstrate excellent durability, mechanical and underwater stability, and corrosion resistance. The presented processes can be scaled to larger, durable coatings with controllable wettability for diverse applications. Apart from their use as superhydrophobic surfaces, the application of multiscale coatings in photo-thermal conversion systems as solar selective coatings is explored in the final part of this thesis. The effects of scale-independent fractal parameters of the coating surfaces and heat treatment are systematically explored with respect to their optical properties of absorptance, emittance, and figure of merit (FOM). / Master of Science

This study focuses on the design, fabrication, and characterization of deterministically engineered, three-dimensional architectures to be used as high-performance electrodes in energy storage applications. These high-surface-area architectures are created by the robotically-assisted sequential electrodeposition of structural and sacrificial layers in an alternating fashion, followed by the removal of the sacrificial layers. The primary goal of this study is the incorporation of these highly laminated architectures into the battery electrodes to improve their power density without compromising their energy density. MEMS technologies, as well as electrochemical techniques, are utilized for the realization of these high-power electrodes with precisely controlled characteristic dimensions. Diffusion-limited models are adopted for the determination of the optimum characteristic dimensions of the electrodes, including the surface area, the thickness of the active material film, and the distance between the adjacent layers of the multilayer structure. The contribution of the resultant structures to the power performance is first demonstrated by a proof-of-concept Zn-air microbattery which is based on a multilayer Ni backbone coated with a conformal Zn film serving as the anode. This primary battery system demonstrates superior performance to its thin-film counterpart in terms of the energy density at high discharge rates. Another demonstration involves secondary battery chemistries, including Ni(OH)2 and Li-ion systems, both of which exhibit significant cycling stability and remarkable power capability by delivering more than 50% of their capacities after ultra-fast charge rates of 60 C. Areal capacities as high as 5.1 mAh cm-2 are reported. This multilayer fabrication approach is also proven successful for realizing high-performance electrochemical capacitors. Ni(OH)2-based electrochemical capacitors feature a relatively high areal capacitance of 1319 mF cm-2 and an outstanding cycling stability with a 94% capacity retention after more than 1000 cycles. The improved power performance of the electrodes is realized by the simultaneous minimization of the internal resistances encountered during the transport of the ionic and electronic species at high charge and discharge rates. The high surface area provided by the highly laminated backbone structures enables an increased number of active sites for the redox reactions. The formation of a thin and conformal active material film on this high surface area structure renders a reduced ionic diffusion and electronic conduction path length, mitigating the power-limiting effect of the active materials with low conductivities. Also, the highly conductive backbone serving as a mechanically stable and electrochemically inert current collector features minimized transport resistance for the electrons. Finally, the highly scalable nature of the multilayer structures enables the realization of high-performance electrodes for a wide range of applications from autonomous microsystems to macroscale portable electronic devices.

Conventional tin-lead solder alloys have been widely used in electronics interconnection owing to their properties such as low melting temperature, good ductility and excellent wettability on copper and other substrates. However, due to the worldwide legislation addressing the concern over the toxicity of lead, the usage of lead-containing solders has been phased out, thus stimulating substantial efforts on lead-free alternatives, amongst which eutectic Sn-Ag and Sn-Cu, and particularly Sn-Ag-Cu alloys, are promising candidates as recommended by international parties. To meet the increasing demands of advanced electronic products, high levels of integration of electronic devices are being developed and employed, which is leading to a reduction in package size, but with more and more input/output connections. Flip chip technology is therefore seen as a promising technique for chip interconnection compared with wire bonding, enabling higher density, better heat dissipation and a smaller footprint. This thesis is intended to investigate lead-free (eutectic Sn-Ag, Sn-Cu and Sn-Ag-Cu) wafer level solder bumping through electrodeposition for flip chip interconnection, as well as electroplating lead-free solderable finishes on electronic components. The existing knowledge gap in the electrochemical processes as well as the fundamental understanding of the resultant tin-based lead-free alloys electrodeposits are also addressed. For the electrodeposition of the Sn-Cu solder alloys, a methanesulphonate based electrolyte was established, from which near-eutectic Sn-Cu alloys were achieved over a relatively wide process window of current density. The effects of methanesulphonic acid, thiourea and OPPE (iso-octyl phenoxy polyethoxy ethanol) as additives were investigated respectively by cathodic potentiodynamic polarisation curves, which illustrated the resultant electrochemical changes to the electrolyte. Phase identification by X-ray diffraction showed the electrodeposits had a biphasic structure (β-Sn and Cu6Sn5). Microstructures of the Sn-Cu electrodeposits were comprehensively characterised, which revealed a compact and crystalline surface morphology under the effects of additives, with cross-sectional observations showing a uniform distribution of Cu6Sn5 particles predominantly along β-Sn grain boundaries. The electrodeposition of Sn-Ag solder alloys was explored in another pyrophosphate based system, which was further extended to the application for Sn-Ag-Cu solder alloys. Cathodic potentiodynamic polarisation demonstrated the deposition of noble metals, Ag or Ag-Cu, commenced before the deposition potential of tin was reached. The co-deposition of Sn-Ag or Sn-Ag-Cu alloy was achieved with the noble metals electrodepositing at their limiting current densities. The synergetic effects of polyethylene glycol (PEG) 600 and formaldehyde, dependent on reaching the cathodic potential required, helped to achieve a bright surface, which consisted of fine tin grains (~200 nm) and uniformly distributed Ag3Sn particles for Sn-Ag alloys and Ag3Sn and Cu6Sn5 for Sn-Ag-Cu alloys, as characterised by microstructural observations. Near-eutectic Sn-Ag and Sn-Ag-Cu alloys were realised as confirmed by compositional analysis and thermal measurements. Near-eutectic lead-free solder bumps of 25 μm in diameter and 50 μm in pitch, consisting of Sn-Ag, Sn-Cu or Sn-Ag-Cu solder alloys depending on the process and electrolyte employed, were demonstrated on wafers through the electrolytic systems developed. Lead-free solder bumps were further characterised by material analytical techniques to justify the feasibility of the processes developed for lead-free wafer level solder bumping.

Traditionally, lead-based bearing overlays dominate the commercial automotive market and it has been proven that an excellent combination of properties can be attained through their use. However, lead is a toxic metal and a cumulative poison in humans. According to the European Union End-of-Life Vehicle (ELV) Directive proposed in 1997, vehicles that registered in'all the member states after 1st July 2003 should contain no lead, mercury, cadmium and hexavalent chromium. In this study, a new sulphate-gluconate electrolyte was used to produce multilayer SnCo coatings, aimed at a lead-free overlay for future market use. Tin-cobalt compositionally modulated alloy (CMA) coatings produced from sulphategluconate electrolytes have been previously examined as a potential replacement for lead-free bearing overlays [1]. However, some obstacles may exist which limit their potential use on an industrial scale. For example, long electroplating times are required to produce a thick coating which is very undesirable from an industrial viewpoint, and also the possible elemental interdiffusion occurring in the coating system under engine operating temperatures could rapidly deteriorate the coating properties. In addition, there is an increasing demand from automotive industry to further improve bearing overlay properties, for example for high performance and high compression ratio engines.

This thesis investigates the effect of microstructural strengthening mechanisms on the overall mechanical performance of micro-truss periodic cellular metals (PCMs). Prior to the author’s work, the primary design considerations of micro-truss PCMs had been topological issues, i.e. the architectural arrangement of the load-supporting ligaments. Very little attention had been given to investigate the influence of microstructural effects within the cellular ligaments. Of the four broad categories of strengthening mechanisms in metals, only solute and second phase strengthening had previously been used in micro-trusses; the potential for strengthening micro-truss materials by work-hardening or grain size reduction had not been addressed. In order to utilize these strengthening mechanisms in micro-truss PCMs, two issues needed to be addressed. First, the deformation-forming method used to produce the micro-trusses was analyzed in order to map the fabrication-induced (in-situ) strain as well as the range of architectures that could be reached. Second, a new compression testing method was developed to simulate the properties of the micro-truss as part of a common functional form, i.e. as the core of a light-weight sandwich panel, and test the effectiveness of microstructural strengthening mechanisms without the influence of typical high-temperature sandwich panel joining processes, such as brazing. The first strengthening mechanism was achieved by controlling the distribution of plastic strain imparted to the micro-truss struts during fabrication. It was shown that this strain energy can lead to a factor of three increase in compressive strength without an associated weight penalty. An analytical model for the critical inelastic buckling stress of the micro-truss struts during uniaxial compression was developed in terms of the axial flow stress during stretch forming fabrication. The second mechanism was achieved by electrodeposition of a high-strength nanocrystalline metal sleeve around the cellular ligaments, producing new types of hybrid nanocrystalline cellular metals. It was shown that despite the added mass, the nanocrystalline sleeves could increase the weight-specific strength of micro-truss hybrids. An isostrain model was developed based on the theoretical behaviour of a nanocrystalline metal tube network in order to predict the compressive strength of the hybrid materials.

Zinc and its alloys are among the most widely utilised metallic coatings for the sacrificial protection of steel. Although excellent in this mode of protection, these coatings are often less durable when subjected to environments of combined wear and corrosion due to their intrinsic relative softness and ductility. A possible and fast growing way of improving the durability of these coating is by the codeposition of inert particles into the zinc and zinc-alloy matrix. The main aim of this research was therefore to improve the durability of zinc and zinc-nickel coatings by the incorporation of inert particles via electrolytic codeposition methods. The first five chapters of this thesis comprise literature review on the electrodeposition of zinc, its alloys and composite electrodeposition in general. A major part of which was dedicated to the review of various conventional methods and parameters such as current density, agitation, temperature, solution composition, bath additives and pH usually investigated in electrodeposition. The experimental work was principally based on DC electrodeposition and was aimed at understanding the deposition behaviour of zinc and zinc-nickel electrodeposition baths, conditions which influence them and solution compatibility to the introduction of silica particles. A systematic study on the deposition behaviour of both zinc/silica and zinc-nickel/silica composite baths was carried out with particular interest on the rate of particle incorporation and the influence of particles on zinc-nickel alloy deposition. The complimentary codeposition behaviour of the nickel and silica particles was observed. The influence of bath additives such as N,N Dimethyldodecylamine (NND) and sodium nitrate on the rate of silica incorporation was also studied. Both additives were found to improve the rate of particle incorporation for the zinc/silica. The morphologies and compositions of the coatings were analysed with the use of SEM and FEGSEM. Corrosion performance studies were carried out in a neutral salt spray chamber and linear polarisation resistance methods used to determine barrier corrosion properties of the coatings. Anodic polarisation studies were also carried out. The results show an improvement in the corrosion performance of these coatings with the addition of silica particles Reciprocating wear tests were used to determine the wear behaviour of the coatings in terms of weight loss. Improvement in wear resistance was not observed in the zinc/silica coatings probably due to the high content of silica in the coatings. Lower silica contents may be required for the desired improvements. However, there were obvious improvements in the wear behaviour of the zinc-nickel/silica coatings due to the presence of the silica particles.

Growth mechanism of electroplated copper columns has been systematically studied by simulations and characterizations. A two-dimensional cross-sectional kinetic Monte Carlo (2DCS-KMC) model has been developed to simulate the electrodeposition of single crystal copper. The evolution of the microstructure has been visualized. The cluster density, average cluster size, variance of the cluster size and average aspect ratio were obtained from the simulations. The growth history of the deposition from the first atom to an equivalent of 100 monolayers was reconstructed. Following the single-lattice 2DCS-KMC model for a single crystal, a two-dimensional cross-sectional poly-lattice kinetic Monte Carlo (2DCSP-KMC) model has been developed for simulation of the electrodeposition of polycrystalline copper on both a copper and a gold substrate. With this model, the early-stage nucleation and the grain growth after impingement of nuclei can be simulated; as such the entire growth history is reconstructed in terms of the evolution of microstructure, grain statistics and grain boundary misorientation. The model is capable of capturing some key aspects of nucleation and growth mechanisms including the nucleation type (e.g. homogeneous or heterogeneous), texture development, the growth of grains and higher energetic state of grain boundaries. The model has also proven capable of capturing the effects of deposition parameters including applied electrode potential, concentration of cupric ions and temperature. Their effects are largely dependent on the substrates. The early-stage electrocrystallization of Cu on polycrystalline Au has been studied by ex-situ AFM observations. The evolution of surface morphology of the electrodeposited copper on a sputtered Au seed layer from 16ms to 1000s was observed and their formation mechanism discussed. The heterogeneous nucleation phenomenon, the competitive growth both longitudinally and laterally, and the dominant growth of some nuclei were experimentally observed, which are also visualized by the relevant KMC simulation results at a smaller size scale and a shorter time scale. A heuristic model is therefore proposed to describe the mechanism of the early-stage electrocrystallization of Cu on a polycrystalline Au seed layer. Electroplated copper columns plated for different times have been characterized in terms of the evolution of their external morphology, cross-sectional microstructure and crystal structure. The microstructure of electroplated copper columns is characteristic of bi-modal or tri-modal grain size distribution. The results indicate that recrystallization has occurred during or after the plating, top-down and laterally. Slight changes of the crystal structure were observed by in-situ XRD and it was found that the changes of the (111) and (200) planes occurred at different stages of self-annealing. Finally, the results indicate the presence of organic additives is not essential for self-annealing of a copper column to occur.

Electrodeposition of diamond-like carbon (DLC) films was studied on different substrates using two different electrochemical methods. The first electrochemical method using a three-electrode system was studied to successfully deposit hydrogenated DLC films on Nickel, Copper and Brass substrates. The as-deposited films were characterized by scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), fourier transform infrared spectroscopy (FTIR) and cyclic voltammetry (CV). A variety of experimental parameters were shown to affect the deposition process. The second electrochemical method was developed for the first time to deposit hydrogen free DLC films on Ni substrates through a two-electrode system. The as-deposited films were characterized by Raman spectroscopy and FTIR. According to Raman spectra, a high fraction of diamond nanocrystals were found to form in the films. Several possible mechanisms were discussed for each deposition method. An electrochemical method was proposed to deposit boron-doped diamond films for future work.

Tin electrodeposits produced from aqueous electrolytes are frequently used within the electronics industry due to their high solderability and corrosion protection. One limitation to using these deposits is their spontaneous formation of long conductive filament whiskers. These whiskers grow post-electrodeposition and increase the risk of unwanted electrical shorts within electronic devices. In this thesis, tin electrodeposits produced from a proprietary bright acid Tinmac electrolyte, currently used in industry, were studied. Electrodeposits were produced using a range of current densities with and without agitation and were characterised with respect to crystallographic orientation, topography and surface finish. Moreover, the intermetallic compound (IMC) growth produced at the copper substrate-tin coating interface was assessed over a period of time as its growth is considered to be a significant driving force behind whisker formation. In addition, a technique for the electrochemical anodic oxidation of tin electrodeposits on copper substrates was developed. This technique was used throughout this project for the study of IMC growth from tin electrodeposits as it was able to effectively remove the tin whilst leaving the IMCs and substrate unaffected. Ionic liquids exhibit promising electrochemical characteristics for electrodeposition but are still not widely utilised in industry. Their ability to deposit tin coatings has been studied in the present investigation. Trials concentrated on process optimisation to produce uniform electrodeposits by varying current density, SnCl2.2H2O concentration, and electrolyte composition. These deposits were then characterised and compared to tin coatings of similar thickness produced from Tinmac with respect to topography, surface finish, crystallographic orientation, IMC growth, and whisker propensity. Electrodeposits produced from the ionic liquid electrolyte exhibited a different crystallographic texture, topography, and IMC growth compared to those produced from Tinmac. Moreover, the deposit produced from the ionic liquid featured increased whisker growth compared to those produced from Tinmac, but in a wider context, far less growth than conventional tin electrodeposits in the literature. In addition, by exploiting other electrochemical characteristics of ionic liquids, such as their large potential window, future work may be able to produce novel tin or tin alloy electrodeposits which may further reduce the whisker propensity of deposits produced in this investigation.