7.2 Composite Deposition from Particle Suspensions
249
can usually be seen due to the Ag incorporation, it does not alter the functionality
of the major metallic component of the coating; moreover, the Ag content leads to
the ennoblement of Ni coatings and the reduced corrosion rate is retained also after
the exposure to the bacteria colony [199]. Similar results were obtained for Zn/Ag
coatings where the bio-fouling effect was not studied [200]. Here, the optimum Ag
content was reported to be around 2 vol.%.
Various other goals led to the idea that composite coatings with Ag particles
may exhibit beneficial properties. The enhanced catalytic activity of Ag-doped Ni–
P coating was taken advantage of in the hydrogen evolution reaction in alkaline
media [201]. With an Ag content of as low as 4 wt.%, a drastic decrease in the
overvoltage of the hydrogen evolution reaction could be achieved. The onset potential
of the hydrogen evolution was 340 mV less negative for the Ag-containing coating
as observed in the potentiodynamic curves, and the hydrogen evolution rate at a
constant potential was nearly doubled as compared to the Ag-free Ni–P coatings.
(Similar results were obtained for Ni(TiO 2 ) coatings [202], although the gain both
in the onset potential and in the hydrogen evolution rate was smaller than for the
Ag-doped Ni–P deposits.)
Ag particle incorporation was also used as a methodical simplification of obtaining
near-eutectic Sn–Ag–Cu alloys [93]. The bath optimization of the ternary Sn–Ag–
Cu deposition is very cumbersome, partly due to the large difference in the onset
potential of the reduction of the components. To overcome this difficulty, the Ag
particles were added to a relatively simple Sn–Cu bath. The desired metal content of
0.7 wt.% (Cu) and 3.0–3.9 wt.% (Ag) could be achieved by the appropriate choice
of Ag particle and Cu
2+ ion concentrations and by tuning the current density.
In contrast to the diversity of the application opportunity of coatings with dispersed
Ag particles, the motivation of plating Ni and Ni-containing alloys together with Al
and/or Cr particles was nearly exclusively the optimization of the oxidation behaviour
of the coatings obtained. The advantage of the nanostructured composite materials
over the alloys of identical bulk composition stems from the difference of the oxide
layer formation on the surface [203]. Upon the high-temperature oxidation of the
nanostructured composites, a scale composed of mainly Al 2 O 3 or Cr 2 O 3 is produced
instead of NiO whose growth is suppressed. The Al and Cr particles also serve as
nucleation centres for their pure oxide scale, which is missing for the bulk alloys
of the same composition. Also, the nanoparticles and oxide-forming centres are
readily available at the surface of the composite coatings, while the diffusion of
metal forming the protective oxide to the sample surface is needed for the alloys.
There is a difference also in the oxide composition, leading to the reduction of the
formation of a mixed oxide containing the matrix component when the composite is
annealed [204].
For both Al and Cr particle loading, there is a threshold particle content above
which the retardation effect of the matrix oxidation can be seen. This limit was
found for Ni-based composites to be around 28 and 11 wt.% per cent for the two
metals, respectively [205]. The beneficial effect of the codeposited particles is sizedependent. For nanoparticles, the blocking of the matrix oxidation is effective at
249
can usually be seen due to the Ag incorporation, it does not alter the functionality
of the major metallic component of the coating; moreover, the Ag content leads to
the ennoblement of Ni coatings and the reduced corrosion rate is retained also after
the exposure to the bacteria colony [199]. Similar results were obtained for Zn/Ag
coatings where the bio-fouling effect was not studied [200]. Here, the optimum Ag
content was reported to be around 2 vol.%.
Various other goals led to the idea that composite coatings with Ag particles
may exhibit beneficial properties. The enhanced catalytic activity of Ag-doped Ni–
P coating was taken advantage of in the hydrogen evolution reaction in alkaline
media [201]. With an Ag content of as low as 4 wt.%, a drastic decrease in the
overvoltage of the hydrogen evolution reaction could be achieved. The onset potential
of the hydrogen evolution was 340 mV less negative for the Ag-containing coating
as observed in the potentiodynamic curves, and the hydrogen evolution rate at a
constant potential was nearly doubled as compared to the Ag-free Ni–P coatings.
(Similar results were obtained for Ni(TiO 2 ) coatings [202], although the gain both
in the onset potential and in the hydrogen evolution rate was smaller than for the
Ag-doped Ni–P deposits.)
Ag particle incorporation was also used as a methodical simplification of obtaining
near-eutectic Sn–Ag–Cu alloys [93]. The bath optimization of the ternary Sn–Ag–
Cu deposition is very cumbersome, partly due to the large difference in the onset
potential of the reduction of the components. To overcome this difficulty, the Ag
particles were added to a relatively simple Sn–Cu bath. The desired metal content of
0.7 wt.% (Cu) and 3.0–3.9 wt.% (Ag) could be achieved by the appropriate choice
of Ag particle and Cu
2+ ion concentrations and by tuning the current density.
In contrast to the diversity of the application opportunity of coatings with dispersed
Ag particles, the motivation of plating Ni and Ni-containing alloys together with Al
and/or Cr particles was nearly exclusively the optimization of the oxidation behaviour
of the coatings obtained. The advantage of the nanostructured composite materials
over the alloys of identical bulk composition stems from the difference of the oxide
layer formation on the surface [203]. Upon the high-temperature oxidation of the
nanostructured composites, a scale composed of mainly Al 2 O 3 or Cr 2 O 3 is produced
instead of NiO whose growth is suppressed. The Al and Cr particles also serve as
nucleation centres for their pure oxide scale, which is missing for the bulk alloys
of the same composition. Also, the nanoparticles and oxide-forming centres are
readily available at the surface of the composite coatings, while the diffusion of
metal forming the protective oxide to the sample surface is needed for the alloys.
There is a difference also in the oxide composition, leading to the reduction of the
formation of a mixed oxide containing the matrix component when the composite is
annealed [204].
For both Al and Cr particle loading, there is a threshold particle content above
which the retardation effect of the matrix oxidation can be seen. This limit was
found for Ni-based composites to be around 28 and 11 wt.% per cent for the two
metals, respectively [205]. The beneficial effect of the codeposited particles is sizedependent. For nanoparticles, the blocking of the matrix oxidation is effective at