Fundamental Mechanisms and Models for Electrodeposited Metal Matrix Nano Composites

Metal matrix nanocomposite can be produced by electrodeposition of metals from plating baths containing a dispersion of inert nanoparticles. There is a great body of literature describing the production of new nanomaterials with desirable properties e.g. dispersion strengthened coatings for tribological applications, enhanced resistance in electrical printed circuits. Understanding of bath chemistry/nanoparticle interactions has proved difficult. Empirical models based on that proposed by Guglielmi tend to be system specific and while the model accounts for the concentration of particles and current density it excludes hydrodynamic effects and particle characteristics. Few models attempt to account for the behaviour of surfactants. Only two papers have recognised the effect of surface topography, where more uneven surfaces promote particle adsorption and encapsulation, however in the related field of lubrication, the hydrodynamic effects and behaviour of micro and nanoparticles are well established and surface morphology is recognised as highly influential.

Recent work at Leicester has demonstrated the importance of electrophoresis in encapsulating particles in MMNCs. In Co-WS2 nanocomposites, an anionic surfactant with a low Co content bath could greatly enhance particle encapsulation Figure 1a). The same surfactant with high Co content severely reduced particle incorporation in the matrix, Figure 1b). In the low Co bath, increased encapsulation is believed to be due to a rough surface mechanically trapping particles. As the coating grows, more particles are entrapped and the surface becomes rougher thus trapping more particles. Thus a feedback mechanism which promotes particle adsorption and encapsulation. In the higher Co bath coatings get smoother as they deposit and the feedback mechanism was avoided. The significance of surface morphology in mechanical entrapment has not been fully recognised or exploited. As yet no model has attempted to explain these effects.

The potency of the electrophoretic effect was demonstrated when pulse reverse plating (PRP) as applied with an anionic surfactant. Here, a short pulse of anodic current is employed to attract negatively charged particles to the working electrode, creating an increased particle concentration adsorbed on the surface. The current then switches to cathodic and adsorbed particles are encapsulated. The process is then repeated over a number of cycles and a well dispersed MMNC is created, Figure 1c). By changing the length of time, tc, between anodic pulses it is possible to change control the number of particles encapsulated in a way that has never been achieved before. Similarly, by changing the concentration of anionic surfactant, (SDS) in the bath, the charge on the particles and hence the level of the electrophoretic attraction in the anodic phase may be enhanced. Both these effects can be seen in Figure 1d) which shows the SiC content of Co-SiC MMNC coatings prepared with variable SDS and tc.

Since no model exists which describes this behaviour, separation of particle adsorption during the anodic phase and particle encapsulation in the cathodic phase calls for a new theoretical model supported by extensive experimentation. This model will incorporate the transport of nanoparticles across the hydrodynamic boundary layer and describe how that layer is affected by adsorbed and encapsulated particles. The scope for 4* publications is potentially very large and ground breaking in the field.
Applicants from chemistry, materials, physics and engineering are sought, but this is not restrictive.