6.2 Nanocrystalline Deposits of Metallic Elements
199
Pulse plating is an adequate tool for reducing the grain size of nc-Ni below the
level achievable with d.c. plating [59, 61, 64, 78, 88, 90, 92]. Studies concerning
the impact of the pulse parameters show [59, 93] that a sufficiently small duty cycle
is required for the grain size reduction. While the theory of pulse plating defines a
practical limit for the pulse length where capacitive damping is not substantial (with
t ON > ~0.5 ms), several studies used ultranarrow pulses with a length of 10–200 μs
[68, 77, 98, 99]. The preparation of nc-Ni with these parameters was also successful,
although the background of the process is not fully understood. Some current density
values applied in pulse plating of nc-Ni are also beyond the usual working range of
electrodeposition, ranging up to a few A cm
−2 , and these attempts are either related
[68, 99] or unrelated [64, 71, 100, 101] to the application of ultranarrow pulses.
The properties of the deposits obtained in the above-mentioned works are in line
with other studies, but the impact of the application of pulses with ultrahigh current
density on the current efficiency has never been mentioned. Preparation of nc-Ni
with potential pulses was also exemplified [86].
The structure of the deposit is an essential issue in nearly all works dealing with
nc-Ni electrodeposits. While the determination of the grain size with XRD is indispensable, the comparison of the XRD results with TEM observation is also common.
Data concerning the comparison of the substrate-side and the solution-side grain sizes
show that sometimes a grain refinement takes place as the deposition proceeds [63,
66]. However, in some other works, a uniform structure was found throughout the
nanocrystalline deposits [17, 18, 64], and the mechanical properties measured for
both sides of the deposits were in accord with the structural data.
Annealing of nc-Ni always leads to grain coarsening, hence offering a tool to
produce materials with a desired grain size. However, annealing of electrodeposited
nc-Ni is a complex phenomenon that may involve either segregation or abnormal
grain growth. Segregation takes place when non-metallic impurities such as C and
S are homogeneously distributed in the as-received specimen, and annealing leads
to their accumulation at the grain boundary [69, 89, 90, 92]. This segregation has a
strong impact of the mechanical properties of annealed samples. Abnormal growth
[19, 76, 88, 96] is also strongly related to the grain boundary segregation. In such
cases, a few grains can grow only, but the growth of many other grains is significantly
hindered due to the segregation of the impurities to the grain boundary, leading to a
bimodal grain size distribution.
Abundant literature data are available for the physical properties of electrodeposited nc-Ni. The study of the mechanical properties of nc-Ni has long been a core
topic of the field [63, 69, 71, 72, 99, 102], including also hardness [54, 55, 81],
tensile properties [87, 90, 91, 94] and wear behaviour [98, 103]. The validity of the
Hall–Petch relationship is particularly often studied, for which a representative result
is shown in Fig. 6.6. A few works dealt with the electrical resistivity [23, 57, 59, 60]
and magnetism [56, 58, 104] of nc-Ni.
Concerning the chemical properties of nc-Ni, corrosion resistance is a key issue.
It was found that the corrosion resistance of the nc-Ni specimen increase with
decreasing grain size in either alkaline [84, 100, 105] or neutral chloride [84, 101]
media. For sulphuric acid solutions, there is no agreement in the literature because
199
Pulse plating is an adequate tool for reducing the grain size of nc-Ni below the
level achievable with d.c. plating [59, 61, 64, 78, 88, 90, 92]. Studies concerning
the impact of the pulse parameters show [59, 93] that a sufficiently small duty cycle
is required for the grain size reduction. While the theory of pulse plating defines a
practical limit for the pulse length where capacitive damping is not substantial (with
t ON > ~0.5 ms), several studies used ultranarrow pulses with a length of 10–200 μs
[68, 77, 98, 99]. The preparation of nc-Ni with these parameters was also successful,
although the background of the process is not fully understood. Some current density
values applied in pulse plating of nc-Ni are also beyond the usual working range of
electrodeposition, ranging up to a few A cm
−2 , and these attempts are either related
[68, 99] or unrelated [64, 71, 100, 101] to the application of ultranarrow pulses.
The properties of the deposits obtained in the above-mentioned works are in line
with other studies, but the impact of the application of pulses with ultrahigh current
density on the current efficiency has never been mentioned. Preparation of nc-Ni
with potential pulses was also exemplified [86].
The structure of the deposit is an essential issue in nearly all works dealing with
nc-Ni electrodeposits. While the determination of the grain size with XRD is indispensable, the comparison of the XRD results with TEM observation is also common.
Data concerning the comparison of the substrate-side and the solution-side grain sizes
show that sometimes a grain refinement takes place as the deposition proceeds [63,
66]. However, in some other works, a uniform structure was found throughout the
nanocrystalline deposits [17, 18, 64], and the mechanical properties measured for
both sides of the deposits were in accord with the structural data.
Annealing of nc-Ni always leads to grain coarsening, hence offering a tool to
produce materials with a desired grain size. However, annealing of electrodeposited
nc-Ni is a complex phenomenon that may involve either segregation or abnormal
grain growth. Segregation takes place when non-metallic impurities such as C and
S are homogeneously distributed in the as-received specimen, and annealing leads
to their accumulation at the grain boundary [69, 89, 90, 92]. This segregation has a
strong impact of the mechanical properties of annealed samples. Abnormal growth
[19, 76, 88, 96] is also strongly related to the grain boundary segregation. In such
cases, a few grains can grow only, but the growth of many other grains is significantly
hindered due to the segregation of the impurities to the grain boundary, leading to a
bimodal grain size distribution.
Abundant literature data are available for the physical properties of electrodeposited nc-Ni. The study of the mechanical properties of nc-Ni has long been a core
topic of the field [63, 69, 71, 72, 99, 102], including also hardness [54, 55, 81],
tensile properties [87, 90, 91, 94] and wear behaviour [98, 103]. The validity of the
Hall–Petch relationship is particularly often studied, for which a representative result
is shown in Fig. 6.6. A few works dealt with the electrical resistivity [23, 57, 59, 60]
and magnetism [56, 58, 104] of nc-Ni.
Concerning the chemical properties of nc-Ni, corrosion resistance is a key issue.
It was found that the corrosion resistance of the nc-Ni specimen increase with
decreasing grain size in either alkaline [84, 100, 105] or neutral chloride [84, 101]
media. For sulphuric acid solutions, there is no agreement in the literature because