Ni/nano-TiO2 composite electrocoatings: Correlation between structural characteristics microhardness and wear resistance

Article abstract of DOI:10.1524/zpch.2011.0052

Nanocomposite coatings were obtained by electrochemical codeposition of TiO₂ nano-particles (mean diameter 21 nm, Degussa P25) with nickel, from an additive-free Watts type bath. Pure Ni and composite Ni-TiO₂ coatings were electrolyt

Full text of DOI:10.1524/zpch.2011.0052

Z. Phys. Chem. 225 (2011) 313–324 / DOI 10.1524/zpch.2011.0052
© by Oldenbourg Wissenschaftsverlag, München
Ni nano-TiO2 Composite Electrocoatings:
Correlation Between Structural Characteristics
Microhardness and Wear Resistance
∗
By Stella Spanou and Evangelia A. Pavlatou
Laboratory of General Chemistry, School of Chemical Engineering, National Technical University of
Athens, 9, Heroon Polytechniou Str., Zografos Campus, Athens 15780, Greece
(Received September 8, 2010, accepted in revised form December 7, 2010)
Electrodeposition / Crystalline Orientation / Microharness / Wear Resistance /
Grain Size
Nanocomposite coatings were obtained by electrochemical codeposition of TiO2 nano-particles
(mean diameter 21 nm, Degussa P25) with nickel, from an additive-free Watts type bath. Pure Ni
and composite Ni-TiO2 coatings were electrolytically deposited under both direct and pulse current
conditions and an extended region of electrolysis conditions (pH, current density, TiO2 loading in
the bath). Pure Ni deposits were produced under the same experimental conditions for comparison.
The aim of this study was to correlate the observed structural characteristics of the coatings
(crystallographic orientation and grain size of nickel matrix,) and incorporation percentage with the
resulting microhardness values and tribological behavior. Overall, the data have demonstrated that
when attributing the observed strengthening effect of composites, not only grain refinement and
dispersion strengthening mechanisms, but also preferred crystalline orientation should be taken into
consideration. The variation of the wear rate is directly associated with the microhardness variations
and consequently, governed by the same parameters that determine the synergistic strengthening
mechanism proposed. Pure and composite coatings with [110] crystalline orientation exhibited the
highest microhardness values and wear resistance.
1. Introduction
TiO2 dispersion-hardened metal electrocoatings show a clear increase of the hardness
and wear resistance compared with the pure matrix ones [1–10]. There are a con-
siderable number of research reports concerning the effects of bath composition and
operation conditions on the amount of inert particles codeposited with the metal matrix
and the resulting properties of the composite coatings [1,2,4,5,8–10]. However, limited
research is carried on the effect of codeposited particles on the microstructure [1,2,10]
and the correlation between structural characteristics and resulting properties [11].
Concerning TiO2 particle-reinforced Ni composite electrocoatings it has been
proven that they exhibit improved mechanical properties [1–4,7–11], accompanied
* Corresponding author. E-mail: pavlatou@chemeng.ntua.gr
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by interesting photocatalytic activity as reported by de Tacconi et al. [12]. Specific-
ally, the incorporation of TiO2 particles into the nickel matrix leads to the formation
of finer structures [13,14], modifies the preferred orientation [10,11,13,14] and lattice
defects [10,15], while decreases the residual tensile stresses [16] and increases the hard-
ness [1–4,7–11], the wear [1–4,7–10] and corrosion resistance [1,4,7,9,10,16] of the
coatings, compared to pure nickel deposits.
The electrocrystallization of Ni is known to be a highly inhibited process as a con-
sequence of hydrogen codeposition. Depending on plating conditions, mainly pH value
of the bath and current density, Amblard et al. observed a predominance of a definite in-
hibitor, which selectively promotes one mode of growth and leads to a deposit exhibiting
specific structural characteristics [17]. It has been established that nickel deposits from
an additive free Watts type bath under direct current conditions exhibit three inhibited
textures [110], [210] and [211] attributed to the presence of atomic, molecular forms of
adsorbed hydrogen and colloidal Ni(OH)2 in the catholyte interface, respectively [17].
Moreover, the observed [100] mode of growth is considered to be the most “free” from
inhibiting chemical species, presenting few structural defects, maximum ductility and
high values of grain size [11,14], low tensile strength, yield strength, and microhard-
ness [11,18–20]. An attempt to formalize a correlation between electrodeposition condi-
tions and the texture of Ni nano-TiO2 coatings under both direct (DC) and pulse current
conditions (PC) has been conducted in our previously published research [11,14].
In this work, extended data of our previously reported study on the microhardness
values of pulse deposited composites [11], as well as new experimental data on nanos-
tructured Ni and Ni nano-TiO2 deposits prepared under direct current conditions are
presented, and overall highlight the effect of crystallographic texture on the relationship
between crystallites grain size, microhardness and wear resistance.
2. Experimental
Pure Ni and composite Ni-TiO2 coatings were electrolytically deposited from an
additive-free nickel Watt’s solution under conditions described in Table 1. The elec-
trodeposition experiments were performed on rotating disk electrode (RDE) under
direct and pulse current conditions. The TiO2 powder (Degussa P25) with a mean diam-
eter of 21 nm was used as received without any pre-treatment. Titania particles were
maintained in suspension by continuous magnetic stirring for at least 24 h before de-
position, as well as during the electrolysis process. The concentration of TiO2 particles
on the surface was evaluated by using fluorescence X-ray spectroscopy (XRF) [14] and
energy dispersive X-ray spectroscopy (EDS) measurements [11]. The preferred orien-
tation of both pure and composite coatings as well as the average Ni grain size was
determined as described elsewhere [14]. The thickness of the produced coatings was at
least 50 μm in order to obtain preferred orientations fully developed and therefore attain
results comparable and reliable.
Measurements of the Vickers microhardness (HV in GPa) of pure nickel and
Ni TiO2 composite deposits were performed on the surface by using a Reichert mi-
crohardness tester under 50 g load for 15 s and the corresponding final values were
determined as the average of 10 measurements.
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Table 1. Experimental parameters for the preparation of pure Ni and composite Ni nano-TiO2 electrocoat-
ings and wear test conditions.
Electrolyte composition
NiSO4· 6H2O
NiCl2· 6H2O
H3BO3
300 g L−1
35 g L−1
40 g L−1
Electrodeposition conditions
Temperature (◦C)
Substrate
50
Brass disc (diameter 25 mm)
Cathode rotation rate (rpm)
Anode
Magnetic stirring (rpm)
600
Ni foil
250–320
Direct current (DC)
TiO2 powder Degussa P25 (dm = 21 nm)
20, 100 g L−1
1, 2, 3, 3.5, 4, 4.4, 5
0.5, 1, 2, 5, 10, 20, 40
pH
Current density (A dm−2
)
Pulse current (PC)
TiO2 powder Degussa P25 (dm = 21 nm)
20 g L−1
2, 3.5, 4
1, 5, 20
50%
pH
Current density (A dm−2
Duty cycle (d.c)
)
Frequency (Hz)
0.1, 1, 10, 100, 1000
Wear test conditions
Load
10N
Pin’s ball
Corundum (d = 6 mm)
Ball’s linear velocity (m s−1
Sliding cycles
)
0.06
5000
Ambient conditions
T = 25 ◦C and humidity 50%
Tribological properties were studied by ball-on-disc measurements using a CSEM
tribometer apparatus. All wear tests were performed in ambient conditions against
corundum balls, without lubrication. The experimental conditions of tribological meas-
urements are also given in Table 1. The coefficient of friction (cof) was recorded
continuously during the sliding tests, while the volumetric wear factor (cw) was calcu-
lated as reported in [21].
3. Results and discussion
It is well established that Ni nano-TiO2 composite coatings microhardness and tribo-
logical behavior are strongly affected by the volume fraction of the particles code-
posited with the metal matrix and, thus a considerable number of efforts have been
made to correlate the amount of codeposited particles with the electrodeposition pa-
rameters, such as current density [10,11,14] and particles loading in the bath [1–4,9].
This however may be unfortunate since the correlation of property data with nonmetal-
lic particulate inclusion percentages may be erroneous due to synergistic effects arising
from other factors such us the modified metal microstructure [22,23]. Therefore, the mi-
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S. Spanou and E. A. Pavlatou
crohardness and wear resistance of the coatings will be assessed from the viewpoint of
crystal orientation, grain size, type of imposed current and nanoparticles codeposition
percentage.
3.1 Correlation between structural characteristics and microhardness
of deposits
Generally, the strengthening mechanisms of polycrystalline metals and composites can
be described as follows: (a) grain refinement strengthening from Hall–Petch relation-
ship; (b) dispersion strengthening due to Orowan mechanism; and (c) crystal orien-
tation [24]. Despite the significant number of investigations on the system Ni nano-
TiO2, less attention has been paid to the possible effect of grain orientation on the
strengthening mechanism [11]. It should be noticed that in order to obtain a dispersion-
strengthening effect, it is crucial to achieve a uniform embedding of non-agglomerated
nano-particles in the matrix. It is worth to be mentioned that, according to previous ex-
perimental findings by applying HRFE-SEM [25], as well as GDOS [14] techniques
the distribution of the TiO2 nano-particles on the surface and within the thickness has
been indicated as uniform, without serious problems of agglomeration. In order to elu-
cidate the hardening mechanism of both direct and pulse plated pure and composite
nickel coatings, the microhardness values of deposits exhibiting three different textures
([110], [100], Random) have been plotted as a function of the codeposition percentage
and the reciprocal square root of average grain size, while the average grain size was
varied within the range 15–500 nm.
Direct current conditions. From Fig. 1a it is apparent that an increase of the particles’
loading in the bath results in definite increment of nanoparticles in the composite, in-
dependently of the applied electrolysis conditions (Fig. 1a). Nevertheless, no explicit
relationship between the incorporation percentage and the resulting microhardness
values is revealed; a definitely more pronounced effect is apparent in the case of the
composites oriented through [100] axis. Unlike to these results, previous studies on
Ni nano-TiO2 electrocoatings have supported a corresponding linear relationship, for
however limited number of samples [2–4,8–10]. Given that the [100] textured deposits
have been found to be “softer” amongst coatings exhibiting the other three preferred
orientations [11,18,19,26], the incorporation of sufficient amount of TiO2 nanoparti-
cles in such a kind of matrix is not expected to efficiently improve the composites’
microhardness. At the same time composites exhibiting [110] preferred orientation
demonstrate a trend of increasing microhardness values with increasing nanoparticles
incorporation rate. This behaviour could be attributed to a dispersion-strengthening
mechanism, in which titania nanoparticles inhabit the grain boundaries of the nanocrys-
talline nickel matrix coatings, and act as obstacles to the grain movement and grain
boundaries migration [2,3,27].
According to our previous report concerning deposits prepared under DC con-
ditions, it has been demonstrated that the titania incorporation in the nickel matrix
preferably induces the predominance of [100] crystalline orientation [14], and thus the
majority of coatings plotted in Fig. 1 is oriented through this axis. The embedding of
TiO2 nanoparticles imputes grain refinement, which is more pronounced in the case of
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Fig. 1. Variation of composites microhardness values with different textures as a function of (a) volume
percentage of codeposited nano-TiO2 particles in nickel matrix and (b) the average grain size (d−1 2).
composites with [100] texture (Fig. 1b). Moreover, deposits exhibiting [110] orientation
present lower values of average grain size and higher microhardness values compared to
the ones oriented through [100] axis, which is consisted with bibliographic data [11,26].
Furthermore, the microhardness of both pure and composite coatings seems to increase
with decreasing average grain size. Nevertheless, in the case of pure Ni deposits ori-
ented through [100] axis, no significant variation is detected and thus no obvious grain
size dependence is revealed. The highest microhardness value is observed for a com-
posite exhibiting [110] orientation, probably due to the synergistic mechanism involved
between dispersion strengthening (highest incorporation value archived i.e. 9 6 vol %)
and grain refinement strengthening (lowest average grain size archived i.e. 15 nm). In
order to explain the origin of composites’ microhardness variations – for both orienta-
tions – that can not be attributed either to grain refinement or dispersion strengthening,
nor to their combination, special attention should be paid on the electrolysis conditions
(mainly pH) and how they influence the internal stresses, grain-boundary sliding, hy-
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S. Spanou and E. A. Pavlatou
drogen embrittlement and quality of preferred orientation [10,18,28,29]. An attempt to
explore the effect of this last parameter will be published elsewhere.
Data from Fig. 1 demonstrated that Ni nano-TiO2 composite coatings exhibited
higher microhardness values compared to pure nickel ones, provided that they exhibit
the same preferred orientation and similar average grain size. The experimental data,
for the composites prepared under DC conditions, reveal that it is crucial to take into
consideration the preferred crystalline orientation in order to define if there is a direct
correlation between codeposition rate and microhardness.
Pulse current conditions. Pulse electrodeposition, under specific electrolysis condi-
tions (pH, Jp), was performed so as to study the perturbations of Ni crystal growth
induced by pulse frequency on Ni nanoTiO2 coatings exhibiting three well developed
textures ([110], [100], random), as described extensively in Ref. [11]. It has been proven
that composites prepared in the PC regime exhibited higher incorporation percentages
than those obtained under DC conditions, and systematically the highest incorporation
rates were achieved at pulse frequencies
100 Hz. In Fig. 2 all experimental data
concerning the variation of microhardness values as function of volume percentage of
codeposited nano-TiO2 particles in nickel matrix (Fig. 2a) and the average grain size
(d−1 2) (Fig. 2b) are presented.
Specifically, the composite deposits with [100] preferred crystalline orientation ex-
hibit the highest average grain size values among the other deposits and show the lowest
values of microhardness, despite the efficient embedment of TiO2 nanoparticles into
the matrix (Fig. 2a,b). These results are consisted with those coatings oriented through
[100] axis and were prepared under DC conditions (Fig. 1). On the other hand, ran-
domly oriented composites present considerable increase of microhardness in a narrow
distribution of mean grain size (Fig. 2b) and codeposition range (Fig. 2a). A similar be-
haviour was reported recently concerning randomly oriented pure nickel deposits [18].
Additionally, our data demonstrate that composites oriented through [110] axis exhibit
enhanced microhardness compared to [100] and randomly oriented ones, as well as the
lowest mean crystallite sizes, characterised by a broad range of titania incorporation
percentage.
Overall, experimental data revealed that the nickel average grain size for both pure
and composite coatings, independently of the type of current imposed, is directly as-
sociated to the induced nickel crystalline orientation, where deposits oriented through
[110] axis exhibited the lowest mean grain size compared to the other observed pre-
ferred orientations. In addition, the dependence of hardness values on the mean grain
size (Figs. 1, 2) indicates that for attributing the strengthening effect of composites both
grain refinement and dispersion strengthening mechanisms should be taken into account
for a given preferred orientation.
3.2 Correlation between structural characteristics and wear resistance of
deposits
Figure 3 represents the typical evolution of friction coefficient for pure and compos-
ite coatings oriented through [110] (Fig. 3a) and [100] axis (Fig. 3a), during sliding
against corundum ball under non-lubricated conditions. Irrespective of the presence or
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Fig. 2. Variation of composites microhardness values with different textures as a function of (a) volume
percentage of codeposited nano-TiO2 particles in nickel matrix and (b) the average grain size (d−1 2) [11].
The coatings were produced under an extended region of pulse electrolysis conditions: ( ) pH = 4 and
Jp = 1 A dm−2, (•) pH = 3 5 and Jp = 5 A dm−2, ( ) pH = 2 and Jp = 20 A dm−2
.
not of TiO2 nano-particles in the metal matrix, deposits oriented through [110] axis
seem to exhibit higher resistance to sliding at the first 1000 cycles, compared to deposits
with the [100] texture. Moreover, it is illustrated that composite coatings demonstrate
lower mean friction coefficient values compared to the pure Ni ones, regardless the
preferred orientation, which is in agreement with relevant literature [3,7,30,31]. These
results indicate that the microstructure design attained by the electrolytic embedding
of fine particles into the metal matrix is a simple and effective way to obtain metallic
lubricating coatings.
Figure 4 shows the dependence of Vickers microhardness values and wear rate on
the preferred orientation and TiO2 nanoparticles content in coatings produced under PC
conditions. The data concerning the variation of microhardness values of Ni nano-TiO2
composites as a function of pulse frequency were published in our recent work [11],
however the description of wear rate behaviour as a result of the applied experimental
conditions consist new data that contribute to the overall understanding of the systems’
intrinsic and non intrinsic properties behaviour.
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S. Spanou and E. A. Pavlatou
Fig. 3. Evolution of friction coefficient for pure Ni and composite Ni nano-TiO2 coatings sliding against
corundum ball (d = 6 mm), exhibiting (a) the [110] and (b) [100] preferred orientation.
All composites prepared under the highest pH and the lowest applied current dens-
ity value (pH = 4, Jp = 1 A dm2) as a function of pulse frequency (0 1–1000 Hz) at
constant duty cycle (dc = 50%) exhibited the same preferred orientation and increasing
values of incorporation percentage, as illustrated in Fig. 4a. The highest microhard-
ness value was obtained at the highest applied frequency, where the highest incor-
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Fig. 4. (a) Variation of microhardness values [11] and (b) volumetric wear factor values of compos-
ite Ni nano-TiO2 coatings prepared under pulse plating conditions as a function of pulse frequency, at
a duty cycle of 50%. Composite coatings prepared under the following conditions pH = 4, 2 and Jp = 1,
20 A dm−2. (Numbers in brackets notify the corresponding nickel preferred orientation of the coating).
poration percentage (∼ 9 8 vol %) was observed (Fig. 4a). Given that all composites
prepared under these experimental exhibit the same preferred orientation accompanied
with small variation of mean grain size (22–31 nm, see also Fig. 2b), it is revealed
that the leading factor for determining microhardness value is the codeposition rate
of nanoparticles in the Ni matrix. On the contrary, the hardness variation as a func-
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S. Spanou and E. A. Pavlatou
tion of increasing pulse frequency values, under the lowest pH and the highest applied
current density (pH = 2, Jp = 20 A dm2), has been proven to be directly associated
with the imposed preferred orientation and the corresponding average grain size values
(Fig. 2). Thus, for applied frequencies 10 Hz the improvement of the quality of “soft”
[100] mode of crystalline growth against random one, and the simultaneous increasing
of grain size from 42–49 nm to ∼ 74 nm, result to a considerable decrease of micro-
hardness, despite the fact that under these conditions the highest codeposition rate was
achieved ∼ 10 7 vol % (Fig. 4a). Thereby, it has been shown that in order to ascribe
the observed strengthening mechanism of Ni nano-TiO2 composites, it is crucial to take
into consideration the preferred crystalline orientation of the coatings in combination
with grain refinement and dispersion strengthening effect of the nanoparticles.
Additionally, it is revealed that the variation of the wear rate under the afore-
mentioned electrodeposition conditions is directly associated with the microhardness
variations and consequently, governed by the same parameters that determine the pro-
posed strengthening mechanism. All coatings exhibiting the highest microhardness
values are characterized by low wear rates. However, the imposition of the “soft” [100]
texture accompanied by increasing values of grain size results to a substantial increase
of the wear rate, which is stimulated by application of increasing values of pulse fre-
quency. The highest wear resistance is observed at the highest applied frequency, where
high codeposition rate was observed ∼ 9 8 vol % in a nanocrystalline nickel matrix
∼ 21 nm (Fig. 4b). Moreover, deposits exhibiting [110] mode of growth demonstrated
increased wear resistance compared to those exhibiting [100] as expected, if the geo-
metrical characteristics of crystals oriented through axis [100] and [110] are taken into
account as proposed by Lampke et al. [32]. Finally, pulse plated Ni nano-TiO2 compos-
ite coatings exhibit much lower wear rates than pure nanocrystalline nickel, indicating
that the wear resistance of nickel coatings is obviously improved by incorporating tita-
nia nanoparticles.
Overall, the experimental data revealed that the application of pulse frequency
affected the microhardness and wear resistance of composites by provoking microstruc-
tural changes in the deposits, expressed through alterations of crystal growth and
corresponding grain size, as well as by varying the amount of codeposited titania
nanoparticles with the nickel matrix.
4. Conclusions
Concluding, experimental data revealed that the nickel average grain size for both pure
and composite coatings, independently of the type of current imposed, is directly asso-
ciated to the induced nickel crystalline orientation.
Deposits oriented through [110] axis exhibited the lowest mean grain size values
compared to the other observed preferred orientations along with higher microhardness
and wear resistance. While, the imposition of [100] texture is accompanied by increas-
ing values of grain size, and systematically results to low microhardness values and
considerable increase of the wear rate, even after high nanoparticles embedment. The
data of this work indicate that the alteration of electrolysis conditions (e.g. particles
loading, pH, current density, type of current) in order to increase the titania nanopar-
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ticles incorportation percentage in the “soft” [100] Ni matrix does not consequently
enhance composites’ microhardness.
Ni nano-TiO2 composite coatings exhibited higher microhardness values compared
to pure nickel ones, provided that they exhibit the same preferred orientation and simi-
lar average grain size. The observed dependence of hardness values on the mean grain
size indicated that for ascribing the strengthening effect of composites both grain refine-
ment and dispersion strengthening mechanisms should be taken into account for a given
preferred orientation.
Finally, the application of pulse frequency affected the microhardness and wear
resistance of composites by stimulating microstructural changes in the deposits, ex-
pressed through alterations of crystal growth and corresponding grain size, as well as
by varying the amount of codeposited titania nanoparticles with the nickel matrix.
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