148
5 Compositionally Modulated and Multilayered Deposits
and Co and left Ni nearly unaffected [52]. The key point of the mechanism test is
usually the comparison of the deposit composition with calculations either allowing
or excluding the exchange process. The diffusion-limited displacement mechanism
was found to be valid also for reverse pulse plating of Ni–Cu CMA coating [50].
In such a case, the rate of the dissolution of the LN metal is the sum of the reverse
current density applied and the diffusion-limited current density of the MN metal.
The displacement-based multilayer deposition method was applied for the
following systems (indicating the metals in the order of the displaced/displacing
ones): Cu/Ag [47, 51], Ni/Cu [44, 48, 50, 53–56], Co/Cu [49, 54, 57, 58] and Ni–
Co–Fe/Cu [52]. The Ni/Cu and the Co/Cu systems are worthwhile of comparison
because they show distinct differences. The maximum amount of Ni displaced by
Cu was found to be 4–5 atomic layers [44, 53] or even below the detection limit
of the method applied [54], which is attributed to the relatively passive nature of
Ni and its good alloying ability with Cu, hence leading to a conformal surface Cu
layer. However, Co displacement does not lead to a self-limited Cu thickness, which
may cause a significant porosity and the formation of a three-dimensional structure
instead of well-ordered layer system [49, 54, 57, 58].
In spite of the fact that some physical properties of the deposits (like magnetoresistance of the Co/Cu and Ni/Cu samples) show the sign of the composition modulation
at the nanometre scale, the occurrence of the true layer structure was evidenced with
some direct imaging method only for layer thicknesses larger than 50 nm [57, 58].
Although the composition change of the deposit is a clear evidence of the displacement and the reaction can be followed with a quartz crystal microbalance experiment
[55], we have no clear picture on the morphology of many displacement-processed
deposits.
5.3.3 Multilayer Formation with Various Electrical
Waveforms
If the single-bath method is used with a metal pair with regular codeposition, the
concentration ratio of the LN and MN metals in usually high varying between 7 and
300. After finishing the high-current pulse, the deposition of the MN metal is certainly
mass transport limited. If the mass transport-limited deposition is maintained during
the low-current pulse, it may unfavourably impact the deposit morphology. When
the relative position of the polarization curves is like 1a and 2a in Fig. 5.3, there is
obviously no opportunity to deviate from the mass transport-limited deposition in
the pulse leading to the deposition of the MN metal since the LN layer will be at
least partly etched away. However, if the relative position of the partial polarization
curves is like 1a and 2b, there is more freedom to choose the deposition conditions
of the MN metal.
This deposition condition adjustment opportunity was exploited for the deposition
of Ni in Ni/Cu multilayers produced for various purposes [44, 59, 60]. The common
5 Compositionally Modulated and Multilayered Deposits
and Co and left Ni nearly unaffected [52]. The key point of the mechanism test is
usually the comparison of the deposit composition with calculations either allowing
or excluding the exchange process. The diffusion-limited displacement mechanism
was found to be valid also for reverse pulse plating of Ni–Cu CMA coating [50].
In such a case, the rate of the dissolution of the LN metal is the sum of the reverse
current density applied and the diffusion-limited current density of the MN metal.
The displacement-based multilayer deposition method was applied for the
following systems (indicating the metals in the order of the displaced/displacing
ones): Cu/Ag [47, 51], Ni/Cu [44, 48, 50, 53–56], Co/Cu [49, 54, 57, 58] and Ni–
Co–Fe/Cu [52]. The Ni/Cu and the Co/Cu systems are worthwhile of comparison
because they show distinct differences. The maximum amount of Ni displaced by
Cu was found to be 4–5 atomic layers [44, 53] or even below the detection limit
of the method applied [54], which is attributed to the relatively passive nature of
Ni and its good alloying ability with Cu, hence leading to a conformal surface Cu
layer. However, Co displacement does not lead to a self-limited Cu thickness, which
may cause a significant porosity and the formation of a three-dimensional structure
instead of well-ordered layer system [49, 54, 57, 58].
In spite of the fact that some physical properties of the deposits (like magnetoresistance of the Co/Cu and Ni/Cu samples) show the sign of the composition modulation
at the nanometre scale, the occurrence of the true layer structure was evidenced with
some direct imaging method only for layer thicknesses larger than 50 nm [57, 58].
Although the composition change of the deposit is a clear evidence of the displacement and the reaction can be followed with a quartz crystal microbalance experiment
[55], we have no clear picture on the morphology of many displacement-processed
deposits.
5.3.3 Multilayer Formation with Various Electrical
Waveforms
If the single-bath method is used with a metal pair with regular codeposition, the
concentration ratio of the LN and MN metals in usually high varying between 7 and
300. After finishing the high-current pulse, the deposition of the MN metal is certainly
mass transport limited. If the mass transport-limited deposition is maintained during
the low-current pulse, it may unfavourably impact the deposit morphology. When
the relative position of the polarization curves is like 1a and 2a in Fig. 5.3, there is
obviously no opportunity to deviate from the mass transport-limited deposition in
the pulse leading to the deposition of the MN metal since the LN layer will be at
least partly etched away. However, if the relative position of the partial polarization
curves is like 1a and 2b, there is more freedom to choose the deposition conditions
of the MN metal.
This deposition condition adjustment opportunity was exploited for the deposition
of Ni in Ni/Cu multilayers produced for various purposes [44, 59, 60]. The common