Enhancing the Incorporation of Particles during Electrocodeposition by Surface Modification (Het verhogen van de inbouw van deeltjes tijdens elektrocodepositie door middel van oppervlaktemodificatie)

Electrocodeposition is a coating technology where particles are added to an electroplating bath in order to deposit composite coatings. Unfortunately, the presence of particles in the plating solution is insufficient to obtain composite coatings. In this work, it is attempted to increase particle codeposition by modification of the particle surface. This should increase the attractive interaction force between particle and electrode, and consequently, the incorporation probability. From literature, three beneficial particle properties could be readily identified, namely hydrophobicity, specific metal affinity and electrical conductivity.An attempt was made to unequivocally determine the influence of a fourth surface property, the surface charge or potential of the particle. By smart selection criteria, BaSO4, CaF2 and K1.33Mn8O16 were identified as having potential determining ions (pdi) that are electrochemically silent. It was shown that in water and in diluted plating baths the article surface potential could be controlled by additions of the pdi. A clear relation between surface potential and codeposition was shown by codeposition tests in three different metals under acid as well as alkaline conditions. Although current step measurements proved that these pdi-additions had an influence on the electroplating process itself, the correlation between current steps and codeposition changes was close to zero. It was concluded that a positive surface potential or surface charge is a promoting surface quality for particle incorporation.In following parts of the thesis, three techniques were explored to apply favorable surface properties to particles that show low incorporation rates. TiO2, Al2O3 covered TiO2 and Al2O3 were chosen as particles to be modified with these techniques. The first proposed method made use of coupling agents, specific molecules that assist in increasing the compatibility of two different surfaces. The surface of TiO2 was grafted using a large selection of commercially available coupling agents, having two types of reactive head groups, silanes and phosphonic acids. Phosphonic acids showed a higher grafting density on TiO2 than silanes for the same functional group. For bulkier functional groups, a limited grafting density was obtained. With respect to codepositon, hydrophobic modifications were the most efficient, followed by mercapto groups. Heterocyclic functional groups proved disappointing. An explorative study was started for using mixtures of hydrophilic and hydrophobic grafting agents.Secondly, surfactants were added to the metal electroplating bath. These compounds could possibly lower the ordering of water molecules close to the electrode and particle surface. Furthermore, an extra attractive depletion force could arise and assist in adsorption on the electrode. An extensive series of surfactants was tested as additives to enhance the codeposition of Al2O3 covered TiO2, resulting in several important observations. In the electroplating bath, the measured critical micelle concentration was one order of magnitude lower than in water. In addition, it was found that anionic surfactants adsorbed well on this particle s surface, while a cationic surfactant did not adsorb. In codeposition experiments no strong influence was found if the surfactant concentration remained below the cmc. However, if the surfactant concentration was above the cmc, codeposition was suppressed due to particle agglomeration. It was concluded that surfactants are unsuitable additives in increasing the codeposition of hydrophilic particles.The third technique consisted of applying a second material on the particle surface with favorable codeposition properties. First, Ag and Au were deposited resulting in TiO2 or Al2O3 covered TiO2 particles decorated with Ag or Au nanoparticles. The modification with Ag resulted in the highest codeposition increase in nickel, but this modification is susceptible to ageing, namely Ag dissolved in the acid Ni plating bath, while Au persisted. In zinc or copper plating baths, little influence of the metal encapsulation was noticed. Possibly, a galvanic couple was formed between the noble metal nano-particle and the deposited metal, when the noble metal touched the electrode. This results in localized dissolution of the less noble metal electrode instead of immediate fixation. This was not observed in nickel, because nickel shows a strong electrochemical irreversibility.Carbon was selected as a second modifying material since it is not only electrically conductive, but also hydrophobic and chemically inert. It was deposited on TiO2 and Al2O3. Carbon was successfully deposited with nanometer thickness using a two-step deposition mechanism: first a carbon precursor was deposited on the particles and subsequently, the particles were pyrolyzed to evacuate contaminants and increase the crystallinity. At pyrolysis temperatures above 1000 °C, only a small amount of contaminants were retained in the carbon coating. Also the electrical conductivity and hydrophobicity were most favorable at these synthesis conditions, although also agglomeration was likely to occur. A strong connection was found between the incorporation probability and modified particle properties, such as hydrophobicity, electrical resistivity and particle size. Increased incorporation rates were observed for the purest carbon coatings with the highest water contact angle and lowest electrical resistivity. The incorporation rate of the particles was compared to the theoretical particle supply, using an extension of the Levich equation.nrpages: 191status: publishe