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.

Composite coatings have great potential due to the possibility to combine properties of two different materials in one coating. This way, new surface properties can be tailored and applied to any material's surface. Among different manufacturing routes, electrodeposition has the biggest potential in creating composite metal matrix coatings, especially nanocomposites. Nevertheless, there is a knowledge gap between the deposition of composite coatings in laboratory conditions, described in the literature, and those that are now in place on an industrial level. While micro-composites have been industrialised for about ten years, the production of Ni/SiC nanocomposite coatings by electroplating is still far from an industrial manufacturing floor. This is due to the lack of understanding of the mechanisms of nanoparticles codeposition leading to scattering results.

The production of nanocomposite coatings is much more sensitive to the process parameters compared to microcomposite. The correlation between parameters and their influence on the codeposition are still not fully identified and understood. The codeposition models proposed in the literature are only valid in specific conditions, but composite depositions behave differently, or even opposite if some of the variables are modified.

The main objective of this work is to identify the particle-current-electrocrystallization interactions in the production of Ni/SiC nanocomposites. A series of experiments are designed to isolate single variables and identify the controlling parameters of these interactions and their impact on the final properties.

In this thesis, the effect of current density, type of current and particles size are identified as primary variables controlling the metal crystallisation and coatings properties.

Among many parameters, a specific current waveform in pulse reverse mode proved to increase the codeposition rate effectively, doubling the content of nanoparticles compared to other techniques. Ultrasound assistance is also considered as stirring method when particles are suspended in the deposition bath to increase their stability and dispersion. The effect of Ultrasound on the particles codeposition and metal crystallisation is studied and compared to silent condition.

Moreover, a surface treatment for the particle has been proven successful in making any particle to behave similarly in the Ni deposition bath. Furthermore, the codeposition rate doubled or tripled compared to untreated ones thanks to this treatment. Both ultrasonic agitation and surface treatment reduce the formation of aggregates, improving the particle dispersion and metal microstructure thus increasing the final hardness.

The work proved the synergistic effect between particle and metal microstructure which affected the final properties of the coating. Therefore, when tailoring the composite coating to improve hardness, it is not only the amount of the particles that should be considered but also their influence on the electrocrystallisation process.

This study analysed the influence of the codeposition of SiC particles with different sizes: 50 nm, 500 nm and 5 μm, and the type of bath agitation (stirring or ultrasonic) on the electrocrystallisation of nickel coatings. The composites matrix microstructure was analysed by means of SEM, EBSD and XRD, to evaluate the grain size, crystal orientation, and internal stresses and was benchmarked against pure nickel samples electrodeposited in equivalent conditions. The codeposition of nano- and microsize particles with an approximate content of 0.8 and 4 vol.%, respectively, caused only a minor grain refinement and did not vary the dominant < 100 > crystal orientation observed in pure Ni. The internal stress was, however, increased by particles codeposition, up to 104 MPa by nanoparticles and 57 MPa by microparticles, compared to the values observed in pure nickel (41 MPa). The higher codeposition rate (11 vol.%) obtained by the addition of submicron-size particles caused a change in the grain growth from columnar to equiaxial, resulting in deposits with a fully random crystal orientation and pronounced grain refinement. The internal stress was also increased by 800% compared to pure nickel. The ultrasound (US) agitation during the deposition caused grain refinement and a selective particle inclusion prompting a decrease in the content of the particles with the larger particles. The deposits produced under US agitation showed an increase in the internal stresses, with double values compared to stirring. The increase in the deposits microhardness, from 280 HV in pure Ni to 560 HV in Ni/SiC submicron-US, was linked to the microstructural changes and particles content.

The present paper describes the study of the synergism between the matrix microstructure and reinforcement phase in electrodeposited nanocomposite coatings. Adding hard nanoparticles into the metallic matrix leads to hardening of the coating. The effects of particle load, size and dispersion on hardening as well as their influence on metal microstructure refinement were studied. The relative contributions of strengthening factors in Ni/nano-SiC composites, namely, Hall–Petch strengthening, Orowan strengthening, enhanced dislocation density and particles incorporation, were evaluated. The production of various coatings under different stirring conditions and powders resulted in dissimilarities in the incorporation of particles. The Hall–Petch relationship for pure nickel was determined using samples produced under different current densities. Additionally, the grain refinement resulting from the particle codeposition and agitation mode were identified as influential factors in grain-size strengthening. Dislocation density strengthening was significant in electrodeposits produced using ultrasonic agitation, while it was negligible in layers produced under other conditions. Particles codeposition affected the magnitude of Orowan strengthening, resulting in cases where strengthening was negligible despite the presence of particles. The sum of contributions and the modified Clyne methods were used to calculate the hardness of the composites based on the contribution of each strengthening factor, and the calculation results were in good agreement with experimental data. The wear behavior of the composites was analyzed by pin-on-disk measurements, and the results correlated with the strengthening mechanisms. Particle size, dispersion and content increased the strengthening effects as well as the hardness and wear resistance of the coatings.

This work has explored the potential of using pulse reverse (PR) plating for increasing the deposited fraction of SiC nanoparticles. Two PR waveforms were selected, a short pulse (500 Hz) waveform and a newly modified and adapted pulsed sequence that equals the plating thickness to the particles’ diameter (50 nm) for the on-time and half-diameter during the anodic time. The pulse waveforms were designed with 4 and 10 A⋅dm⁻² as the average current density and cathodic peak current density, respectively. Direct current (DC) deposits at the same values were also produced as reference. In all cases, the codeposition of nano-SiC particles influenced the microstructure. The electroplating under DC 10 A⋅dm⁻² showed the strongest grain refinement and increased the content of the particles (up to 2% vol.) PR using high-frequency achieved a similar codeposition. The maximum particle incorporation was achieved by the proposed adapted pulse waveform, doubling the SiC content produced by other set-ups (up to 4% vol.); increasing the microhardness of the deposits to 400 HV, despite no grain refinement compared to the pure metal. From these results, it was observed a relationship between the influence of the plating method on the microstructure, the particle content, and the material's hardness.

Ni-based mixed particles composite coatings were designed to achieve superior wear resistance by combining hard carbides and solid lubricants as a reinforcing particles mix. Pure nickel and single-particles composites were electrodeposited in the same conditions for benchmarking. A pre-study was carried out to optimise the current density to avoid loss of process efficiency due to hydrogen evolution. The production process was also improved by employing ultrasounds to avoid porosity and dendritic growth in the metal caused by conductive MoS₂ particles. The presence of MoS₂ particles led to nanocrystallinity in the nickel matrix, confirmed by electron backscatter diffraction (EBSD) maps and transmission electron microscopy (TEM). The microstructural changes and codeposition in the different composites were correlated to microhardness and pin-on-disc tests. An extremely high hardness was observed in the mixed particles composite (≈1110 HV) due to the combined effect of the nanocrystalline matrix and high codeposition rate (≈15 vol% SiC and ≈8 vol% MoS2). The codeposition of MoS₂ particles provided a self-lubrication capability to the coating, reducing the friction coefficient compared to pure Ni from 0.15 to 0.07. The wear rate was reduced more than 12 times by the mixed reinforcement compared to pure Ni and more than 6 times compared to Ni-SiC.

This work focuses on the significance of the particle's residence time in composite electrodeposition. Ni-SiC were produced using different pulse-reverse (PR) waveforms based on the average particle diameter (60 nm). The waveform was designed to deposit a metal thickness equivalent to the average diameter during the cathodic pulse and strip half during the anodic pulse. The shape of the waveform was modified to study its effect on particle codeposition by changing the cathodic and anodic pulse current density peaks and cycle time while maintaining the same charge. For benchmark, samples were also produced under direct current (DC), matching the cathodic pulse current density peaks and average. The electrodeposition was done in an additive-free and SDS-Watts bath. ζ-potential measurements were employed to determine the interaction between particles and electrolyte. A relation was established between particle incorporation rate and residence time at the electrode interface, examining. The microhardness of the deposits was also studied. The SiC content was more than doubled by PR compared to DC, from 2 vol.% to 5.5 vol.% even after SDS addition. The increase in codeposition rate was related to the anodic pulse time, supported by the existing models on the mechanisms of particle codeposition.

This work explores the surface protection against wear provided by electroplated metal matrix composite coatings containing hard and lubricant particles. The second phase mix was selected to provide wear resistance by hardening the material and decreasing the friction coefficient. In this study, the capacity of providing wear protection by nano-SiC and self-lubrication by submicron graphite was addressed. Nickel-based composites with a dual powder mix of SiC 60 nm and graphite 400 nm, combined on a 10:10 g L&minus;1 ratio, were produced by electrocodeposition. In addition, to better understand their synergy, mono-composites with SiC 60 nm or Graphite 400 nm with a powder load of 10 g L&minus;1 were also produced. Pure nickel was also electrodeposited under the same conditions as a benchmark. Electron backscatter diffraction (EBSD) maps and chemical composition analysis were used to correlate the results from microhardness, wear resistance, and friction to the microstructure and particle incorporation rate. The wear rate tested by pin-on-disc decreased when the codeposition fraction and microhardness increased. Three main factors were determined to contribute to the coating hardness: Intrinsic hardness of the particle type, strengthening by grain refinement, and dispersion strengthening. The composites containing SiC provided the best wear protection due to the highest microhardness and grain refinement.