Evaluation of the wear resistance behavior of zn ni and zn-ni sio2 composite coatings

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
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Volume: 03 Issue: 11 | Nov-2014, Available @ http://www.ijret.org 466
EVALUATION OF THE WEAR RESISTANCE BEHAVIOR OF ZN-NI
AND ZN-NI/SIO2 COMPOSITE COATINGS
Tolumoye J Tuaweri1
, Olala M Olali2
, Emmanuel E Jumbo3
1
Department of Mechanical/Marine Engineering, Niger Delta University, Wilberforce Island, Bayelsa State, Nigeria
2
3
Abstract
The sliding wear resistance behavior of Zn-Ni and Zn-Ni/SiO2 composite coatings were investigated using a sliding wear rig.
Wear behavior was mainly evaluated based on weight loss measurements. Effect of applied load and no. of cycles on the weight
loss of the coatings were given particular attention. Field Emission Gun Scanning Electron Microscopy (FEGSEM) was used to
investigate the wear tracks to determine the prevalent wear mechanisms. It was found that weight loss of Zn-11%Ni/1%SiO2 was
lower than those of Zn-11%Ni and Zn-11%Ni/11%SiO2 with a constant load of 10N and increasing number of cycles. Their
weight loss took the following trend; Zn-11%Ni/1%SiO2<Zn-11%Ni<Zn-11%Ni/11%SiO2. However, with increasing normal load,
all three systems of coatings exhibited a mixed response at a constant cycle of 100. At a higher load, a reverse trend to that of a
lower load was observed. At 10N the weight loss of Zn-11%Ni<Zn-11%Ni1%SiO2<Zn-11%Ni/11%SiO2 while at 25N it took a
reverse trend where weight loss of Zn-11%Ni/11%SiO2<Zn-11%1%SiO2<Zn-11%Ni. Morphology of the wear tracks revealed
that the wear mechanism of Zn-Ni/SiO2 coatings were mainly a function of particle content. Optimum wear resistance behavior
was observed for coatings of Zn-Ni containing 1wt% SiO2
Keywords: Zn-Ni electrodeposition, composites, wear resistance, SiO2 nanoparticles
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1. INTRODUCTION
The effective use of zinc and its alloys as sacrificial coatings
is widely reported [1-3]. Research on these coatings has
evolved tremendously over the years from the traditional
zinc coatings to its alloys and recently their composites. The
systematic development of these coatings is due to the
increasing demand for more durable coatings needed for use
in more aggressive working conditions [4,5] such as wear
which is a major factor responsible for the breakdown of
most simple and complex machines. During contact of two
surfaces in relative motion, wear is the damage of a solid
surface involving progressive loss of materials. Wear
between two unlubricated surfaces in relative motion one
against the other presents several interactions both on the
macroscale and nanoscale [4]. Zinc and its alloys are
relatively soft and ductile; and are highly prone to wear and
degradation when in contact with, and in sliding motion
relative to another surface. For instance high strength steel
fasteners used in automotive and aerospace industries are
often coated with Zn- or Cd-based coatings for anodic
protection [7]. In addition to corrosion protection, it is
important for the coatings to have sufficient wear resistance
to withstand the wear and abrasion during handling and
torquing of the fasteners [2]. One way to improve the
corrosion or wear resistance of these coating is to co-deposit
the relevant second phase particles such as Al2O3 [8,9], SiO2
[4,10,11], or solid lubricants such as transition-metal (TM)
dichalcogenides (MoS2,WS2, NbS2, etc.) [12] via composite
electrodeposition. In this study, the wear resistance
behaviour of Zn-Ni and Zn-Ni/SiO2 composite coatings
were investigated and its findings reported.
2. MATERIALS AND METHODS
2.1 Production of Coatings
The composite coatings were produced from acid sulphate
Zn-Ni/SiO2 baths containing ZnSO4. 7H2O 57.5 g/l, NiSO4.
6H2O 131 g/l, Na2SO4.10H2O 162 g/l, SiO2 13-52 g/l. Silica
particles of approximately 20 nm in aqueous suspension
were used (both from Alfa Aesar). Agitation was effected
using vibro-agitation with a vibromixer [13]. The diameter
of the perforated plate attached to the vibromixer, which
effected the agitation was 4.5 cm. A Zenith Variac supplied
power to the vibromixer. Coatings were electroplated onto
mild steel. Prior to electrodeposition, the mild steel panels
were first cathodically cleaned in an alkaline bath containing
25.0 g/l of NaOH, 25.0 g/l of Na2CO3 and 50.0 g/l of
Na3PO4 and etched in 50 vol.% (S.G 1.18) hydrochloric acid
for approximately 20 seconds, washed in running tap water
and then in deionised water. They were then transferred
immediately into the bath for electroplating to avoid re-
oxidation of the surface. All the electrodeposition
experiments were carried out galvanostatically using DC
currents. The anode material in all cases was 99% zinc foil.
Coating compositions and morphologies were analysed
using Field Emission Gun Scanning Electron microscope
(FEGSEM) fitted with an energy dispersive analysis (EDX)
facility. Weight percent of silicon in the deposits was
analysed with the EDX analyser and converted to the weight
percentage of silica.

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2.2 Wear Studies
The test method utilised was based on a linear reciprocating
ball-on-flat plane geometry described in ASTM Standards,
designation; G 133-95. The direction of the relative motion
between the sliding surfaces reverses in a periodic fashion
such that sliding occurs back and forth and in a straight line.
The wear rig used in the investigations was custom made.
The entire wear procedure involved the reciprocatory
motion of a compartment of lubricating bath (without
lubricants) with the test sample rigidly fitted to its base
against a stationary steel ball bearing with a normal load
applied to it. Normal loads of 10N and 25N were used. The
relative humidity was 40% and the lengths of stroke
approximately 2.5 cm. Each friction pair (ball and flat
specimen) was ultrasonically cleaned in acetone before and
after each test to remove any possible surface contaminants
and wear debris. The spherical steel ball bearing already
clamped to its holder was lowered onto the flat specimen
ensuring that the reciprocating drive shaft motion was
horizontal and parallel to the surface of the flat specimen.
The desired load was then applied and the wear process
initiated. The degree of wear resistance was deduced based
on weight loss using electrical balance with 0.01mg
accuracy. Three replicate tests were carried out for each
specimen condition, so as to minimize data scattering, and
their average reported in this work. Morphologies of the
wear tracks were studied using a Field emission gun
scanning electron microscope (FEGSEM).
Fig. 1 Schematic diagram of the reciprocating wear test
(Adapted from [14] with slight modifications)
3. RESULTS AND DISCUSSION
3.1 Effect of Increasing Cycles
Fig 2 shows the wear resistance behavior in terms of weight
loss versus number of wear cycles on Zn-Ni and Zn-Ni/SiO2
coatings containing 1 wt % and 11 wt % of 20 nm SiO2
particles respectively. Weight loss of the coatings without
SiO2 and those with 11 wt% SiO2 appears to take an
increasing trend whilst coatings with only 1wt% SiO2 shows
a mixed response with increase in the number of cycles.
However, a unique trend in this figure is the magnitude of
weight loss for each of the coatings. For all the number of
wear cycles investigated, the weight loss values for the
coating with 1 wt % SiO2 were lower than those of the Zn-
Ni alloy and the alloy with 11 wt% SiO2 while weight loss
values for the Zn-Ni alloy were lower than coatings with 11
wt% SiO2. This implies that the wear resistance of Zn-Ni
alloy is improved in the presence of approximately 1 wt%
SiO2 but reduced at 11 wt% SiO2. Friction coefficient and
wear rate of alloys decreases with increasing silicon content
up to 2 wt% Si, but increases as the silicon content increases
above this level due to particles coarsening and gathering in
certain places leading to silicon free area and thus reduction
of its reinforcement effect [15]. Cracking behavior of such
alloys was attributed to the uneven distribution of silicon
particles in the alloy [15]. This behavior can be related to
that of agglomeration in composite electrodeposition which
often appears to be detrimental to the integrity of the
coatings. Also, the addition of 1% silicon into zinc-based
alloys increases their tensile strength, hardness and wear
resistance but reduces ductility [16].
Fig. 2 Wear behaviour of Zn-Ni/SiO2 electrodeposits
containing 11%Ni and different amounts of 20 nm SiO2
particles. (A) 0% SiO2 (B) 1% SiO2 (C) 11% SiO2. Load 10
N
The results presented in Fig 2 are in agreement with the
other investigators [17]. For both powder processed methods
and electrodeposited methods, coatings with the highest
amount of particles exhibited the highest erosion rates [17].
Similarly, Fig 3(a-c) shows the surface morphology of Zn-
11%Ni, Zn-11%Ni-1%SiO2 and Zn-11%Ni-11%SiO2
coatings respectively. The presence of cracks is quite
evident in the coatings containing the highest amount of
SiO2 particles (see Fig 3c). Generally, cracks are points of
weakness in coatings and could lead to brittle fragmentation
caused by increased microcracking tendency due to the
reinforcement phase [18]. It is therefore not surprising that a
coating with such cracks showed the weakest resistance to
wear

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(a)
(b)
(c)
Fig 3. SEM micrographs showing morphologies of (A) Zn-
11%Ni (B) Zn-11%Ni 1%SiO2 (C) Zn-11%Ni-11%SiO2.[5]
(a)
(b)
Fig. 4. SEM micrographs of wear surfaces of (a) Zn-11%Ni
and (b) Zn-11%Ni/11%SiO2 Load 20 N and 100 cycles. [5]
Fig 4 shows the surface morphologies of typical worn
grooves of (a) Zn-11% Ni and (b) Zn-11% Ni-11%SiO2. It is
evident from this figure that the severity of the damage and
the size of the worn surface of the coating with 11% SiO2
were greater than that without particles. This appears to
corroborate results in Fig 2 and suggest that the wear
resistance of Zn-11%Ni-11%SiO2 is much lower than the
same alloy without particles in the coating. Also, the
morphologies of the worn surfaces show the presence of
material waves [19,20] for the coatings with 11% SiO2,
while such features were less obvious for the coatings
without particles. Material waves tend to occur when
coatings with cracks are subjected to wear (see Fig. 3c).
Reasons for the improvement in wear behavior of the
coatings with 1 wt%SiO2 and reduction with 11 wt% SiO2
as shown in Fig 2 are not very clear. However, a possible
explanation is that with lower particle contents of these
coatings which were produced from baths with relatively
lower bath particle loading of 26g/l SiO2, the dispersion of
the particles in the alloy matrix may have been relatively
homogeneous since lower levels of agglomeration were

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expected, but at excessively high contents of particles in the
bath, agglomeration occurs leading to the incorporation of
agglomerates in the deposit. Incorporation of such pockets
of agglomerates could lead to localised stress, which
possibly contributed to the crack propagations obvious in
Fig 3 (c) and hence the high rate of wear observed in Fig 4
for coatings containing 11% SiO2, possibly due to
‘ploughing’ effect of agglomerated particles.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
10 15 20 25
WidthofGroove,mm
Load, N
A
B
Fig. 5 Width of worn groove versus applied load for (A) Zn-
11 %Ni and (B) Zn-11%Ni-11%SiO2 coatings
Fig. 6 Weight loss versus increasing load for (A) Zn-11%Ni
(B) Zn-11%Ni-1%SiO2 (C) Zn-11%Ni-11%SiO2. 100 cycles
As further evidence of the poorer wear resistance behavior
of coatings with the highest amount of SiO2 particles, Fig 5
shows the width of each worn groove as a function of
increasing load for Zn-11%Ni and Zn-11%Ni-11%SiO2. For
all the loads applied, width of the wear track for the coatings
with 11%SiO2 was much larger than those without particles
in the coating. This is an indication that coatings with
11%SiO2 were less resistant to wear than those without
particles for the prevalent experimental conditions.
3.2 Effect of Increasing Load
Fig 6 shows the influence of increasing load on the wear
behavior of Zn-11%Ni, Zn-11%Ni-1%SiO2 and Zn-11%Ni-
11%SiO2 at 100 cycles. For the range of loads investigated,
the weight loss tends to show a mixed response with
increasing normal load. At a lower load of 10 N, the weight
loss is lower for the coating without particles and increases
with increase in amount of particles in the coatings. At
slightly higher loads of 15 and 20 N, weight loss of all three
types of coatings tends to normalize. However, at the
highest load of 25 N, reverse is the case where the weight
loss becomes higher for the coatings without particles and
decreases with increasing amount of particles. Zinc-based
matrix alloy exhibited lower wear rate than that of the
composite prior to seizure and the trend reversed during
seizure [21]. This behavior has been attributed [21] to the
difference in lubricity, thermal stability and load bearing
characteristics of microconstituents of the individual
coatings. The low melting characteristics of various
microconstituents in the (zinc-based) matrix alloy enables it
to perform better than the composite at low operating
temperatures (generated at low speed/pressure) only. On the
contrary, higher thermal stability imparted by the particles
could cause the composite to exhibit better wear properties
at higher operating temperatures generated at high
speed/pressure [21]. This was probably the case with the
present work as evident in Fig 6 where the weight loss for
the coating without SiO2 particle was lower than those of the
composites with SiO2 and increased with increasing amount
of particles in the coatings at a load of 10 N. On the
contrary, the weight loss was highest for the coating without
particles and decreases with increase in particle content of
the coating at 25 N. A possible reason for this behavior is
that at a lower load of 10 N, the major microconstituent (i.e
zinc-rich phase η) of the zinc-alloy matrix, with a
lubricating nature [22] probably acted as a solid lubricant,
leading to lower coefficients of friction, and hence better
wear resistance. However, the microconstituent of the (zinc-
based) alloy system being a low melting material is
restricted in its positive role to lower operating temperatures
[23-24]. Generally frictional heating is likely to be higher at
more severe conditions (i.e higher loads). Increasing
frictional heating causes larger adhesion of the sample
material to the disc making the occurrence of wear more
probable [25]. The presence of particles could reduce such
frictional heating in case of the composite than the alloy by
reducing the effective area fraction of the matrix part of the
sample in contact with the counterface [21]. Accordingly,
(excessive) adhering tendency of the composite with the
counterface reduces over the matrix alloy under identical
test conditions, thereby decreasing the extent of frictional
heating [21]. This behavior probably accounted for the
observed higher wear rate of the alloy without SiO2 particles
at a higher load of 25 N as compared to its composite
counterparts in Fig 6. It is evident that at 25 N the weight
loss decreases with increase in SiO2 content of the coatings.
Although Fig 6 generally tends to exhibit a mixed response
to increasing load, there is a general tendency for the weight
loss of Zn-11%Ni and Zn-11%Ni-1%SiO2 to increase with
increasing Load while that of Zn-11%Ni-11%SiO2 shows an
inconsistent trend.

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4. CONCLUSIONS
 Wear resistance behaviour of Zn-Ni and Zn-Ni/SiO2
coatings with different amounts of SiO2 particles
shows that weight loss of Zn-11%Ni/1%SiO2 was
lower than those of both Zn-11%Ni and Zn-
11%Ni/11% SiO2. The wear resistance of these
coatings all with 11%Ni took the following order;
coatings with 1 wt %SiO2> 0 wt %SiO2> 11 wt %SiO2
with increasing number of cycles.
 With increasing normal load, the coatings tend to
exhibit a mixed response at a constant cycle of 100. At
a higher load, a reversed trend to a lower load was
observed. The difference appears to be due to
differences in lubricity as a function of thermal
stability of the individual coatings as a result of
frictional heating. Also, possible compaction of porous
sites which may have occurred due to possible
codeposition of agglomerates and strain hardening
with the higher load appears to play key role in the
differences in their wear behavior.
 Morphologies of the worn grooves were dissimilar
with changes in SiO2 content of the coatings. Worn
grooves of Zn-11%Ni/11% SiO2 exhibited re-current
wave forms in contrast to that without particles and
with 1% SiO2, indicating the presence of a high
density of cracks due to the large amount of particles
incorporated.
 Although, 1wt%SiO2 content appears to be the
optimum value for best wear resistance in the set of
coating systems studied, the range of difference in
particle content between each of them is quite wide.
For a better understanding, optimization of coatings
with a closer range of particle content and
determination of their coefficient of friction may be
necessary.
ACKNOWLEDGEMENTS
The authors would like to thank Dr. G.D. Wilcox for his
invaluable guidance throughout this project.
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