5.3 The Single-Bath Method
141
5.3 The Single-Bath Method
5.3.1 Impact of the Codeposition Mode of the Components
on the Composition Modulation During Two-Pulse
Plating
The single-bath method is a very widespread tool for obtaining compositionally
modulated coatings. The large number of reports on this method does not allow a
complete overview of each single-layer combination and bath classification. Instead,
the principles of the method will be discussed in depth, and representative examples
will be presented.
The basic tool of the multilayer deposition is the so-called two-pulse plating, in
which either the potential or the current is varied between two fixed values. The
current is cathodic during both pulses. The features of the codeposition modes
discussed here are elucidated with most attention on two-pulse plating, but their
consequence on many other deposition modes is straightforward.
When the goal is the deposition of an alloy, it is generally desired that the deposition potential of the alloy constituents is brought close enough, which is mostly done
by the application of a selective complexing agent that is preferably attached to the
ions of the more noble (MN) metal, hence decreasing its deposition potential. For
the single-bath deposition of a layered system where a pure MN layer is required,
however, it is necessary that the components can be deposited in distinct potential
intervals. This means that from the viewpoint of the difference between the onset
potentials of the deposition of the components, the chemistry of the solutions used
for plating alloyed and layered systems must fulfil different criteria.
If all components of a CMA coating are present in a single solution, the prerequisite of obtaining a composition modulation is that the deposit composition must
vary with one of the plating parameters. In principle, the dependence of the composition on the temperature or on the solution agitation intensity could also be used for
composition modulation. While the change of temperature cannot be instantaneous,
the modulation of either the flow rate or the electrode rotation was seldom applied
for obtaining CMA deposits. However, the transient width between the subsequent
layers in flow-modulated deposits is weakly predictable. Therefore, the nearly exclusive manner of the electrochemical composition modulation is based on the variation of the electrical parameter of the deposition, which can be either the current
density or the electrode potential. It has to be emphasized that the regulation of
these two parameters is not always interchangeable even if their interrelation is
mutually unambiguous (which means that the polarization curve of the system is
monotonous). Hence, the choice of the appropriate regulation conditions requires a
deep understanding of the electrochemical background.
The composition of an alloy usually varies with the current density, regardless of
the codeposition mode of the components. The mole fraction of the preferentially
deposited component(s) increases when the current density decreases. This is true
141
5.3 The Single-Bath Method
5.3.1 Impact of the Codeposition Mode of the Components
on the Composition Modulation During Two-Pulse
Plating
The single-bath method is a very widespread tool for obtaining compositionally
modulated coatings. The large number of reports on this method does not allow a
complete overview of each single-layer combination and bath classification. Instead,
the principles of the method will be discussed in depth, and representative examples
will be presented.
The basic tool of the multilayer deposition is the so-called two-pulse plating, in
which either the potential or the current is varied between two fixed values. The
current is cathodic during both pulses. The features of the codeposition modes
discussed here are elucidated with most attention on two-pulse plating, but their
consequence on many other deposition modes is straightforward.
When the goal is the deposition of an alloy, it is generally desired that the deposition potential of the alloy constituents is brought close enough, which is mostly done
by the application of a selective complexing agent that is preferably attached to the
ions of the more noble (MN) metal, hence decreasing its deposition potential. For
the single-bath deposition of a layered system where a pure MN layer is required,
however, it is necessary that the components can be deposited in distinct potential
intervals. This means that from the viewpoint of the difference between the onset
potentials of the deposition of the components, the chemistry of the solutions used
for plating alloyed and layered systems must fulfil different criteria.
If all components of a CMA coating are present in a single solution, the prerequisite of obtaining a composition modulation is that the deposit composition must
vary with one of the plating parameters. In principle, the dependence of the composition on the temperature or on the solution agitation intensity could also be used for
composition modulation. While the change of temperature cannot be instantaneous,
the modulation of either the flow rate or the electrode rotation was seldom applied
for obtaining CMA deposits. However, the transient width between the subsequent
layers in flow-modulated deposits is weakly predictable. Therefore, the nearly exclusive manner of the electrochemical composition modulation is based on the variation of the electrical parameter of the deposition, which can be either the current
density or the electrode potential. It has to be emphasized that the regulation of
these two parameters is not always interchangeable even if their interrelation is
mutually unambiguous (which means that the polarization curve of the system is
monotonous). Hence, the choice of the appropriate regulation conditions requires a
deep understanding of the electrochemical background.
The composition of an alloy usually varies with the current density, regardless of
the codeposition mode of the components. The mole fraction of the preferentially
deposited component(s) increases when the current density decreases. This is true