5.4 Properties of Electrodeposited CMAs
163
increase of the tensile stress. Although the actual values of the tensile stress increment
as compared to the components of the multilayer are difficult to compare since the
studies cited applied different thickness ratios (Ni:Cu thickness ratio was 9:1 [59] or
1:1 [60]), an approximately fourfold increment could be seen in both cases. The crosssectional electron microscopic study of the Ni/Cu multilayer deposit as a coating on
a bulk Cu tensile stress test specimen showed that the increased tensile stress was
due to the barrier-type behaviour of the multilayered coating against the dislocation
penetration upon the deformation [60]. The decrease in the tensile stress occurred
at Ni(25 nm)/Cu(2.5 nm) and Ni(5 nm)/Cu(5 nm) for the two sample types. Vickers
hardness was also found to increase with decreasing bilayer thickness, which has the
same origin as for the tensile stress increment. The increment was found to be two to
three times as compared to the pure Cu substrate [13, 60], which is a good agreement
between the works applying different deposition methods (however, another study
mentioned only 25% increment [114]). When it could be studied (i.e., when the
single-bath method was used), the loss in hardness occurred at 10 nm periodicity
[60]. When a gradient composition coating was compared with multilayered ones
obtained under the same deposition conditions, the composition gradient was found
to yield the best hardness values [115]. This was attributed to the unique misfit
dislocation distribution in the gradient samples (where the dislocation distribution is
nearly even, as compared to multilayered ones where they accumulate at the layer
interface). The tribological behaviour of the Ni/Cu multilayers is even more difficult
to compare due to both the lack of the data and the variation in the measurement
protocol [100, 113, 114]. The general conclusion of the sliding wear data is that the
wear resistance also increases with the decrease in periodicity down to an optimum
level. The friction coefficient of the multilayer coating depends on the load, varies
between the values obtained for the two pure components and approaching that of
Cu with decreasing layer thicknesses.
Ni–W coatings are also in the forefront of coating research due to the large hardness of Ni–W alloys. One of the key problems related to Ni–W coatings is the
development of fractures at the surface when the deposition is carried out with a
constant current. This problem can be overcome by layering Ni–W alloys of various
compositions onto each other, while the internal stress decreases and the hardness
improves [116]. The application of a three-pulse method leads to a further improvement of the deposit properties, prominently the wear resistance, without changing
the composition as compared to the two-pulse-plated deposits [62].
The multilayered Ni–W CMAs contain amorphous layers if the W content within a
layer is larger than about 30 at.%; therefore, the multilayer composed of Ni–W layers
of alternating composition show a crystalline/amorphous structural transition beside
the composition modulation. For the Ni–P system, a similar coupled composition–
structure transition can be achieved with the single-bath method [117] (for similar
samples deposited with the double-bath method onto a rotating cathode, see [31]).
At high-current density, the deposit is crystalline (with less than 7 at.% P in the cited
work), while the lower current density was adjusted so that a Ni–P alloy with eutectic
composition could be obtained (19.3 at.% P). With increasing the interface density by
applying two-pulse plating with increasing frequency, the mechanical properties like
163
increase of the tensile stress. Although the actual values of the tensile stress increment
as compared to the components of the multilayer are difficult to compare since the
studies cited applied different thickness ratios (Ni:Cu thickness ratio was 9:1 [59] or
1:1 [60]), an approximately fourfold increment could be seen in both cases. The crosssectional electron microscopic study of the Ni/Cu multilayer deposit as a coating on
a bulk Cu tensile stress test specimen showed that the increased tensile stress was
due to the barrier-type behaviour of the multilayered coating against the dislocation
penetration upon the deformation [60]. The decrease in the tensile stress occurred
at Ni(25 nm)/Cu(2.5 nm) and Ni(5 nm)/Cu(5 nm) for the two sample types. Vickers
hardness was also found to increase with decreasing bilayer thickness, which has the
same origin as for the tensile stress increment. The increment was found to be two to
three times as compared to the pure Cu substrate [13, 60], which is a good agreement
between the works applying different deposition methods (however, another study
mentioned only 25% increment [114]). When it could be studied (i.e., when the
single-bath method was used), the loss in hardness occurred at 10 nm periodicity
[60]. When a gradient composition coating was compared with multilayered ones
obtained under the same deposition conditions, the composition gradient was found
to yield the best hardness values [115]. This was attributed to the unique misfit
dislocation distribution in the gradient samples (where the dislocation distribution is
nearly even, as compared to multilayered ones where they accumulate at the layer
interface). The tribological behaviour of the Ni/Cu multilayers is even more difficult
to compare due to both the lack of the data and the variation in the measurement
protocol [100, 113, 114]. The general conclusion of the sliding wear data is that the
wear resistance also increases with the decrease in periodicity down to an optimum
level. The friction coefficient of the multilayer coating depends on the load, varies
between the values obtained for the two pure components and approaching that of
Cu with decreasing layer thicknesses.
Ni–W coatings are also in the forefront of coating research due to the large hardness of Ni–W alloys. One of the key problems related to Ni–W coatings is the
development of fractures at the surface when the deposition is carried out with a
constant current. This problem can be overcome by layering Ni–W alloys of various
compositions onto each other, while the internal stress decreases and the hardness
improves [116]. The application of a three-pulse method leads to a further improvement of the deposit properties, prominently the wear resistance, without changing
the composition as compared to the two-pulse-plated deposits [62].
The multilayered Ni–W CMAs contain amorphous layers if the W content within a
layer is larger than about 30 at.%; therefore, the multilayer composed of Ni–W layers
of alternating composition show a crystalline/amorphous structural transition beside
the composition modulation. For the Ni–P system, a similar coupled composition–
structure transition can be achieved with the single-bath method [117] (for similar
samples deposited with the double-bath method onto a rotating cathode, see [31]).
At high-current density, the deposit is crystalline (with less than 7 at.% P in the cited
work), while the lower current density was adjusted so that a Ni–P alloy with eutectic
composition could be obtained (19.3 at.% P). With increasing the interface density by
applying two-pulse plating with increasing frequency, the mechanical properties like