238
7 Composites
Plating mode: pulse and reverse pulse plating of composites. Existing theories
on particle codeposition in the course of metal growth all refer to the steady-state
d.c. deposition. These theories anticipate that the volumetric ratio of the particles in
the coating is limited, and the codeposition ratio achieves saturation as the colloid
concentration in the bath grows. From an experimental point of view, such saturation
is often observed, or even a decline of the particle codeposition ratio can be seen if the
particle concentration in the bath is too high. In order to overcome these limits, the
reverse pulse codeposition method was suggested by Podlaha and Landolt [97]. Their
systematic study of the impact of duty cycle (η) on the process revealed that the proper
setup of the deposition parameters can provide a significant particle codeposition
ratio increment. In the example studied, the apparent deposition efficiency showed a
plateau down to η ≈ 0.3, while the codeposition ratio increased steadily in the interval
of 0.8 < η < 0.15. This study indicates that the particle incorporation together with
the metal deposition is not fully inverted as the metal dissolution was triggered by the
anodic current, but the particle removal has a hysteresis. Hence, the application of the
reverse pulse method opens a convenient way to increase the particle incorporation
ratio.
It is to be noted that the reasons of the application of a reverse pulse deposition
mode are fundamentally different for metal and composite plating. For metals, the
pulse with anodic current is for surface shaping by dissolving undesired protrusions,
hence leading to smoother deposit than with d.c. plating. In order to achieve a good
selectivity of the dissolution of elevated spots of the deposit (peaks and dendrites),
the anodic current is typically larger than the cathodic current, although the anodic
pulse length is small enough to keep the charge balance in favour of deposition. For
the reverse pulse plating of the composites, the dissolution has no role in the surface
shaping but it merely serves to get rid of a part of the metal that is not needed to hold
the particles and to create new surface sites for the adsorption of the particles in the
next cathodic pulse. Therefore, the anodic pulse in reverse pulse composite plating
is often of lower current density than the cathodic one (see, e.g., [98]). This current
density ratio is also favourable for the retention of the already incorporated particles
since the dissolution is mostly restricted to areas relatively far from the particles.
Obviously, the above discussion is valid for inert (non-conducting) particles that
themselves do not dissolve. For composites containing metallic particles, reverse
pulse plating can be applied when the metal particles are either passive or relatively
noble as compared to the matrix-forming metal.
From the viewpoint of the balance between deposition and dissolution, composite
plating with cyclic voltammetry can be considered as being analogous to reverse pulse
plating, provided that the potential limits allow the regulation of the desired balance
between the cathodic and anodic processes [99].
Instead of reverse pulse plating, simple pulse plating (with zero current during
the off-time) also proved to be appropriate for composite plating. This approach
creates a synergy of composite plating and pulse plating since both lead to grain
refinement. Besides, the off-time allows the replenishment of the particles available
for incorporation, just like that of the metal ion concentration near the cathode.
7 Composites
Plating mode: pulse and reverse pulse plating of composites. Existing theories
on particle codeposition in the course of metal growth all refer to the steady-state
d.c. deposition. These theories anticipate that the volumetric ratio of the particles in
the coating is limited, and the codeposition ratio achieves saturation as the colloid
concentration in the bath grows. From an experimental point of view, such saturation
is often observed, or even a decline of the particle codeposition ratio can be seen if the
particle concentration in the bath is too high. In order to overcome these limits, the
reverse pulse codeposition method was suggested by Podlaha and Landolt [97]. Their
systematic study of the impact of duty cycle (η) on the process revealed that the proper
setup of the deposition parameters can provide a significant particle codeposition
ratio increment. In the example studied, the apparent deposition efficiency showed a
plateau down to η ≈ 0.3, while the codeposition ratio increased steadily in the interval
of 0.8 < η < 0.15. This study indicates that the particle incorporation together with
the metal deposition is not fully inverted as the metal dissolution was triggered by the
anodic current, but the particle removal has a hysteresis. Hence, the application of the
reverse pulse method opens a convenient way to increase the particle incorporation
ratio.
It is to be noted that the reasons of the application of a reverse pulse deposition
mode are fundamentally different for metal and composite plating. For metals, the
pulse with anodic current is for surface shaping by dissolving undesired protrusions,
hence leading to smoother deposit than with d.c. plating. In order to achieve a good
selectivity of the dissolution of elevated spots of the deposit (peaks and dendrites),
the anodic current is typically larger than the cathodic current, although the anodic
pulse length is small enough to keep the charge balance in favour of deposition. For
the reverse pulse plating of the composites, the dissolution has no role in the surface
shaping but it merely serves to get rid of a part of the metal that is not needed to hold
the particles and to create new surface sites for the adsorption of the particles in the
next cathodic pulse. Therefore, the anodic pulse in reverse pulse composite plating
is often of lower current density than the cathodic one (see, e.g., [98]). This current
density ratio is also favourable for the retention of the already incorporated particles
since the dissolution is mostly restricted to areas relatively far from the particles.
Obviously, the above discussion is valid for inert (non-conducting) particles that
themselves do not dissolve. For composites containing metallic particles, reverse
pulse plating can be applied when the metal particles are either passive or relatively
noble as compared to the matrix-forming metal.
From the viewpoint of the balance between deposition and dissolution, composite
plating with cyclic voltammetry can be considered as being analogous to reverse pulse
plating, provided that the potential limits allow the regulation of the desired balance
between the cathodic and anodic processes [99].
Instead of reverse pulse plating, simple pulse plating (with zero current during
the off-time) also proved to be appropriate for composite plating. This approach
creates a synergy of composite plating and pulse plating since both lead to grain
refinement. Besides, the off-time allows the replenishment of the particles available
for incorporation, just like that of the metal ion concentration near the cathode.