7.2 Composite Deposition from Particle Suspensions
235
was promoted near the bottom of the particle where the particle was fixed at the
beginning of its incorporation into the growing coating.
7.2.5 Key Experimental Parameters in Suspension Plating
Effect of the surfactants and other solution components on the stability of the dispersions. The effect of the surfactants on the stability of suspensions as well as on
the codeposition characteristics of the suspended particle is one of the most studied
aspects of dispersion plating. Especially in the case of particles made of intrinsically
hydrophobic materials (like PTFE, MoS 2 ), the application of surfactants is indispensable. In general, cationic surfactants are considered to be a better option than
anionic surfactants since the electrophoresis of the positively charged particles helps
their transport towards the cathode surface. However, since electrophoresis is by far
not the sole means of the particle transport, the application of anionic surfactants is
often feasible, too.
The study of Walsh et al. [70] is one of the most detailed works about the impact
of various surfactants on one particular metal (particle) codeposition system (which
was Ni(SiC) in their case). This work ranged to 24 surfactants of various types,
including cationic, anionic and neutral ones. The conclusion of the work tells that
“no simple correlation was found between surfactant type or structure and the particle
content of the deposits or the performance of coatings, showing the complexity of
surfactant action in this system” [70]. Since the adsorption of the surfactant on either
a specific metal surface or on a particular type of particle depends on the chemical
nature of all components involved, the surfactant studied show a great variety in
works dealing with various metal/particle systems (e.g., Zn(SiC) [71], Ni(SiC) [72],
Cu(MWCNT) [73], Ni–Zn(MWCNT) [74], etc.). While some trials are highly based
on preliminary experience on the deposition of the same metal without nanoparticles,
the chemical similarity of the particles and the surfactant may also be of importance.
This trend prevails for cases like suspension of hydrophobic fluorocarbon particles
with surfactants having high fluorine content [75] or stabilizing carbon nanotubes in
hydrophobic ionic liquids [76].
It is to be noted that complexing agents and additives may also act as surfactant
that adsorbs on the surface of the nanoparticles, although they are used primarily
for the sake of optimizing the metal deposition. This was well shown for carbamide
in the Ni(Al 2 O 3 ) deposition process [77] and sodium glutamate for the Cu(Al 2 O 3 )
system [78]. For the latter case, a detailed analysis was made for the adsorption of Cucontaining complexes on the nanoparticles by anchoring the complex through various
functional groups of the glutamate ion on the surface of the alumina particle. The
modification of pH can also fundamentally change the surface state of the particles
and their coverage with metal ions, especially for oxide particles where the surface
oxygen atoms can bind either hydrogen or metal ions, depending on the acidity of
the medium.
235
was promoted near the bottom of the particle where the particle was fixed at the
beginning of its incorporation into the growing coating.
7.2.5 Key Experimental Parameters in Suspension Plating
Effect of the surfactants and other solution components on the stability of the dispersions. The effect of the surfactants on the stability of suspensions as well as on
the codeposition characteristics of the suspended particle is one of the most studied
aspects of dispersion plating. Especially in the case of particles made of intrinsically
hydrophobic materials (like PTFE, MoS 2 ), the application of surfactants is indispensable. In general, cationic surfactants are considered to be a better option than
anionic surfactants since the electrophoresis of the positively charged particles helps
their transport towards the cathode surface. However, since electrophoresis is by far
not the sole means of the particle transport, the application of anionic surfactants is
often feasible, too.
The study of Walsh et al. [70] is one of the most detailed works about the impact
of various surfactants on one particular metal (particle) codeposition system (which
was Ni(SiC) in their case). This work ranged to 24 surfactants of various types,
including cationic, anionic and neutral ones. The conclusion of the work tells that
“no simple correlation was found between surfactant type or structure and the particle
content of the deposits or the performance of coatings, showing the complexity of
surfactant action in this system” [70]. Since the adsorption of the surfactant on either
a specific metal surface or on a particular type of particle depends on the chemical
nature of all components involved, the surfactant studied show a great variety in
works dealing with various metal/particle systems (e.g., Zn(SiC) [71], Ni(SiC) [72],
Cu(MWCNT) [73], Ni–Zn(MWCNT) [74], etc.). While some trials are highly based
on preliminary experience on the deposition of the same metal without nanoparticles,
the chemical similarity of the particles and the surfactant may also be of importance.
This trend prevails for cases like suspension of hydrophobic fluorocarbon particles
with surfactants having high fluorine content [75] or stabilizing carbon nanotubes in
hydrophobic ionic liquids [76].
It is to be noted that complexing agents and additives may also act as surfactant
that adsorbs on the surface of the nanoparticles, although they are used primarily
for the sake of optimizing the metal deposition. This was well shown for carbamide
in the Ni(Al 2 O 3 ) deposition process [77] and sodium glutamate for the Cu(Al 2 O 3 )
system [78]. For the latter case, a detailed analysis was made for the adsorption of Cucontaining complexes on the nanoparticles by anchoring the complex through various
functional groups of the glutamate ion on the surface of the alumina particle. The
modification of pH can also fundamentally change the surface state of the particles
and their coverage with metal ions, especially for oxide particles where the surface
oxygen atoms can bind either hydrogen or metal ions, depending on the acidity of
the medium.