7.1 Composite Preparation by Codeposition of Metals
227
Finally, it is worth of mentioning a study in which cathodic electrodeposition and
anodization were combined in a reverse pulse deposition method to obtain a metal–
metal oxide composite [52]. The bath contained Ag, W and Co, which resulted in
the formation of a deposit containing Ag and W during the forwards (cathodic)
current pulse. The tungsten content conserved the deposit during the anodic pulse
by preventing its dissolution when the cobalt was deposited by the oxidation of the
dissolved Co
2+ ions, leading to the formation of a cobalt oxide whose composition was not defined. The typical deposit composition under optimized deposition
conditions was about 3 wt% O, 6 wt% Co, 80 wt% Ag and 2 wt% W. The grain
size of the metal was less than 100 nm for the above composition, while a further
grain refinement to 20 nm was observed as the W content was increased up to 14
wt.%. The pulse parameters were optimized to obtain a dispersion-type deposit and
to avoid a multilayer formation. The cobalt oxide in the deposit served as a dispersed
component that offered self-lubrication properties for the coating.
7.2 Composite Deposition from Particle Suspensions
7.2.1 Preliminary Remarks on the Importance of Composite
Plating
In Sect. 7.2, we deal with the cases when the particles to be incorporated into the
coatings are present in the bath. This is often termed as either suspension plating
or dispersion plating. Since the forces that lead to the particle incorporation are
essentially not gravitational, suspension plating takes place at cathodes of nearly
arbitrary position, and the cathode orientation effect is usually quite small. Composites can be synthesized also by sediment plating on horizontal and upward-facing
cathodes if particles are collected by sedimentation before or during the plating
process. Here, the driving force of the incorporation is related to gravity, and the
interaction between the growing cathode and the particles is of secondary importance. Accumulation of particles on a conducting surface can be triggered also by
electrophoresis, which forms a sediment-like quasi-continuous template. The latter
two cases will be dealt with in Chap. 11 dealing with templated method, restricting
here only to cases when the codeposited particles are suspended in the bulk solution
until the moment of their incorporation to the solid deposit. Nevertheless, the term
metal matrix composite (MMC) is widely used for all kinds of such phase mixtures
regardless of the preparation method.
The significance of electroplating of composites is obvious if we consider that
the resulting coatings cannot be synthesized in the bulk form by using a molten
metal mixed with nanoparticles to be incorporated. Beside the density difference
of the metals and the filling particles, the wetting of the particles is weak by the
melt, and the two components can also react at elevated temperatures. The large
curvature of the nanoparticle surface also prevents their wetting by molten metals.
227
Finally, it is worth of mentioning a study in which cathodic electrodeposition and
anodization were combined in a reverse pulse deposition method to obtain a metal–
metal oxide composite [52]. The bath contained Ag, W and Co, which resulted in
the formation of a deposit containing Ag and W during the forwards (cathodic)
current pulse. The tungsten content conserved the deposit during the anodic pulse
by preventing its dissolution when the cobalt was deposited by the oxidation of the
dissolved Co
2+ ions, leading to the formation of a cobalt oxide whose composition was not defined. The typical deposit composition under optimized deposition
conditions was about 3 wt% O, 6 wt% Co, 80 wt% Ag and 2 wt% W. The grain
size of the metal was less than 100 nm for the above composition, while a further
grain refinement to 20 nm was observed as the W content was increased up to 14
wt.%. The pulse parameters were optimized to obtain a dispersion-type deposit and
to avoid a multilayer formation. The cobalt oxide in the deposit served as a dispersed
component that offered self-lubrication properties for the coating.
7.2 Composite Deposition from Particle Suspensions
7.2.1 Preliminary Remarks on the Importance of Composite
Plating
In Sect. 7.2, we deal with the cases when the particles to be incorporated into the
coatings are present in the bath. This is often termed as either suspension plating
or dispersion plating. Since the forces that lead to the particle incorporation are
essentially not gravitational, suspension plating takes place at cathodes of nearly
arbitrary position, and the cathode orientation effect is usually quite small. Composites can be synthesized also by sediment plating on horizontal and upward-facing
cathodes if particles are collected by sedimentation before or during the plating
process. Here, the driving force of the incorporation is related to gravity, and the
interaction between the growing cathode and the particles is of secondary importance. Accumulation of particles on a conducting surface can be triggered also by
electrophoresis, which forms a sediment-like quasi-continuous template. The latter
two cases will be dealt with in Chap. 11 dealing with templated method, restricting
here only to cases when the codeposited particles are suspended in the bulk solution
until the moment of their incorporation to the solid deposit. Nevertheless, the term
metal matrix composite (MMC) is widely used for all kinds of such phase mixtures
regardless of the preparation method.
The significance of electroplating of composites is obvious if we consider that
the resulting coatings cannot be synthesized in the bulk form by using a molten
metal mixed with nanoparticles to be incorporated. Beside the density difference
of the metals and the filling particles, the wetting of the particles is weak by the
melt, and the two components can also react at elevated temperatures. The large
curvature of the nanoparticle surface also prevents their wetting by molten metals.