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Eprints ID
: 3879
To link to this article: DOI 10.1007/s11249-009-9471-1
URL: http://dx.doi.org/10.1007/s11249-009-9471-1
To cite this version: Malfatti, C.F and Veit, H.M. and Santos, C.B and
Metzner, M. and Hololeczek, H. and Bonino, Jean-Pierre ( 2009) Heat
Treated NiP–SiC Composite Coatings: Elaboration and Tribocorrosion
Behaviour in NaCl Solution. Tribology Letters, vol.36 (2). pp. 165-173.
ISSN 1573-2711
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Heat Treated NiP–SiC Composite Coatings: Elaboration
and Tribocorrosion Behaviour in NaCl Solution
C. F. Malfatti
Æ
H. M. Veit
Æ
C. B. Santos
Æ
M. Metzner
Æ
H. Hololeczek
Æ
J.-P. Bonino
Abstract Tribocor
rosion behaviour of heat-treated NiP
and NiP–SiC composite coatings was investigated in a
0.6 M NaCl solution. The tribocorrosion tests were per-
formed in a linear sliding tribometer with an electro-
chemical cell interface. It was analyzed the influence of
SiC particles dispers ion in the NiP matrix on current den-
sity developed, on coefficient of friction and on wear
volume loss. The results showed that NiP–SiC composite
coatings had a lower wear volume loss compared to NiP
coatings. However, the incorporation of SiC particles into
the metallic matrix affects the current density developed by
the system during the tribocorrosion test. It was verified
that not only the volume of co-deposited particles (SiC
vol.%) but also the number of SiC particles per coating
area unit (and consequently the SiC particles size) have
made influence on the tribocorrosion behaviour of NiP–SiC
composite coatings.
Keywords Coatings Wear-resistant Corrosion
Surface modification
1 Introduction
Many coatings have been applied to increase the wear
resistance of surfaces. Electrolytic codeposition is a low-
cost and low-temperature method suitable for producing
metal matrix composite coatings.
Ni–P alloys have attracted the attention of researchers in
the last few decades due to its characteristics concerning
the effect of phosphorous content on its crystalline struc-
ture [1]. A transition from a crystalline to amorphous
structure takes place progressively with phosphorous con-
tent in electrodeposited Ni–P coatings, resulting in amor-
phous structures when it exceeds 15 at.% [2]. However,
Ni–P alloys, present low hardness and consequently low
wear resistance, restraining its use by industry. Crystalline
Ni–P structures can be obtained, though, by heat treatment
above 350 °C, when crystallization of nickel and precipi-
tation of nickel phosphide, Ni
3
P, take place [1]. The heat
treatment of Ni– P alloys has resulted in hardness of the
order of 1000 Hv, the same as obtained for hard chromium
[3]. Moreover, studies to improve the propert ies of NiP
coatings have been performed and structures with better
wear resistance have been successfully achieved by the
incorporation of ceramic particles in the metallic matrix,
such as SiC, Al
2
O
3
and Cr
2
O
3
[4–7].
The composite coatings electrodepo sition process con-
sists of SiC particles co-deposition in a NiP metallic
matrix. The particles are intentionally added to the elec-
trolyte during the electrochemical deposition. Several
mechanisms are proposed to explain it [8–13]. Electro-
plated composite coatings with metallic matrix are used to
obtain new layers with different properties under the tech-
nological point of view for industrial applications. The
incorporated particles in a metallic matrix increase the
wear resistance of some coatings and the tribological
C. F. Malfatti (&) H. M. Veit
Universidade Federal do Rio Grande do Sul
(UFRGS)—PPGEM/UFRGS, Av. Bento Gonc¸alves,
9500 Setor IV—Pre
´
dio 75, sala 212,
Porto Alegre 91501-970, Brazil
e-mail: celia.malfatti@ufrgs.br
C. B. Santos M. Metzner H. Hololeczek
Fraunhofer Institut IPA Stuttgart, Abteilung Oberfla
¨
chtechnick,
Stuttgart, Germany
J.-P. Bonino
Universite
´
Paul Sabatier—CIRIMAT—UMR CNRS 5085,
Toulouse, France
properties can be higher than those of hard chrome coat-
ings. An equivalent mass loss was observed for NiP–SiC
(17 at.% P) and hard chrome layers [14]. With the
increasing availability of nanoparticles, the interest for
electrolytic and electroless composite coatings containing
nanoparticles is growing [15–17]. The major challenges
with the codeposition of nanoparticles are the achievement
of a high level of codeposition, and the agglomeration of
particles suspended in the electrolytes.
Most of the research on NiP composites is focused on
mechanical and wear properties [18–21] and corrosion
resistance of these coatings [6–13, 22, 23]. The results
from the electrochemical and tribological characterization
showed a very complex particle/matrix system. Previous
investigations on composite NiP and NiP–SiC coatings
(mean size of SiC particles values about 600 nm) revealed
that heat-treated NiP coating has a lower wear volume
loss compared to composite NiP–SiC coatings in
bi-directional ball-on-disc sliding tests [19]. On the other
hand, some studies [22] concerning the corrosion resis-
tance of Ni–P–SiC revealed that the corrosion resistance
has a dir ect dependence of matrix/particles interface
characteristics. The results showed that the densities of
current develop ed by the heat-treated NiP–SiC composite
coatings increased with the amount of particles incorpo-
rated, probably due to the voids produced by discontin-
uous interface around the particle. This effect, however, is
more important for heat-treated composite coatings than
as-plated ones.
Nevertheless, NiP–SiC composite coatings have been
investigated to replace the hard chromium in conditions
where a continuous sliding of the metal components is
combined with the presence of an aggressive environment
and in this case, when a metal or alloy is subjected to
sliding wear in a corrosive environment, the total material
removal rate differs from that predicted by simply adding
the wear rate measured in the absence of corrosion and the
corrosion rate observed in absence of wear [24, 25]. Hence,
through tribo-electrochemical techniques it is possible to
obtain combined effects during a wear test in an electrolyte
under controlled conditions [26–31]. In this way, the tribo-
electrochemical techniques will contribute to the funda-
mental understanding of chemical and mechanical effects
in a total material degradation process.
The aim of this work is to study the tribocorrosion
behaviour of heat-treated NiP–SiC (17 at.% P) composite
coatings with different particles incorporated concentra-
tions (number of SiC particles/lm
2
and volume (%) of SiC
particles). The electrochemical and tribological perfor-
mances of the composite coatings were characterized by
current density developed, coefficient of friction and wear
volume loss in a 0.6 M NaCl solution.
2 Experimental
NiP–SiC (17 at.% P) composite coatings with different
amount of SiC particles incorporated were prepared on
steel substrates (A = 1.76 cm
2
) by electrodeposition from
a plating bath containing NiSO
4
50 g L
-1
, NiCl
2
6H
2
O
60 g L
-1
,H
3
PO
3
20 g L
-1
,H
3
PO
4
50gL
-1
,Na
2
SO
4
50 g L
-1
and different SiC particles concentration (with a
mean diameter of 600 nm) in suspension. The electrolyte
temperature was 80 °C and pH 2.
The electrodeposition was carried out at 0.1 A cm
-2
current density for 45 min (what resulted in a 50 lm
thickness composi te coating measured by optical micros-
copy cros s-sections) in a thermostatic cell (140 ml), where
both static vertical electrodes were immersed. A stirring
system was used keeping the particles in suspension and
moving them towards the cathode.
After electrodeposition, the specimens were cleaned by
ultrasound for 2 min, heat treated at 420 °C for 1 h in an
inert atmosphere and polished before the tribocorrosion
tests.
The particles inco rporated per unit area were evaluated
by image analysis (Imagetools software) of micrographs
obtained by Scanning Electron Microscopy. The SiC par-
ticles volume (SiC vol.%) was evaluated through chemical
analysis of composites and Energy Dispersive Spectros-
copy (EDS).
The electrochemical behavi our of obtained composite
coatings was investigated by cyclic voltammetry. Mea-
surements were performed using a computer-controlled
potentiostat EGG PAR model 273 in a conventional three-
electrode cell. Platinum was used as a counter electrode
and saturated calomel as a reference electrode. Potentio-
dynamic polarization was measured with a scan rate of
0.5 mV s
-1
, from -400 to ?500 mV (SCE). The electro-
lyte was a 0.6 M NaCl solution (corrosive electrolyte) and
the exposed area of working electrode was 0.64 cm
2
.
Vickers microhard ness measurements were carried out
at a load of 50 g along the metallographic cross-section of
the samples.
Surface morphology and surface modifications of com-
posite coatings were characterized by Optical Microscopy
and Scanning Electron Microscopy.
During the tests wear samples were kept under poten-
tiostatic control and the current density was monitored
(tribocorrosion system). With this system it was possible to
control mechanical and electrochemical parameters in the
interface cell. The experiments were performed at room
temperature (25 °C) in a 0.6 M NaCl solution. The same
three-electrode setup employed for the cyclic voltammetry
was used to impose an electrochemical potential on the
exposed surface of the samples and to measure the current
density. The given values of the electrochemical potential
are referred to the SCE saturated in KCl electrode.
Tribocorrosion tests were conducted by reciprocating
tribometer with a ball-on-plate configuration and properly
interfaced to an electrochemical cell (Fig. 1); both were
developed at Fraunhofer Institut (IPA). The tests were
performed at a normal load of 8 N, an oscillation frequency
of 2 Hz, a stroke length of 1 mm and an anodic potential
(?400 mV
SCE
). The number of fretting cycles was 3,500.
All the tribocorrosion tests were performed against 5 mm
diameter corundum balls. Corundum balls have high wear
resistance, good chemical inertness and high electrical
insulating properties.
The volumetric material losses after the wear tests were
determined by white light interferometry.
3 Results and Discussion
3.1 Elaboration of NiP–SiC Composite Coatings
The tribocorrosion behaviour of NiP–SiC (17 at.% P)
composite coatings, with different particles incorporated
concentrations, was investigated in this work (Table 1).
The SiC particles were embedded as individual particles
in the NiP matrix (Figs. 2 and 3). And they were homo-
geneously distributed in the matrix through all the coating
thickness (Fig. 3).
It was verified that, as it is shown in Fig. 4, the amount
of SiC incorporated particle (SiC vol.%), evaluated by
chemical analysis, increased rapidly, starting from low
concentration of particles in suspension reaching a level of
high concentration, which shows the saturation of the
incorporation phenomenon, agreeing with the results
reported in literature [7]. However, the SiC incorporated
particle number by coating area unit, evaluated by images
analysis (SEM micrographs), continued increasing with the
quantity of particles in suspension. This effect was fol-
lowed by the size reduction of the incorporated particle
(Fig. 2 and Table 1) and demonstrated a selective phe-
nomenon of incorporation that was discussed in previous
works published [10, 22, 32, 33]. This selective phenom-
enon of incorporation can be explained based on the
probability of mechanical interaction between the cathode
and the particles in suspension. It should be considered that
the particle is completely incorporated in the metallic
matrix after a certain critical thickness of the deposit, when
its ejection caused by arriving particles is not possible. The
necessary time for the definitive incorporation of one par-
ticle is, therefore, a funct ion of the particle size, i.e., the
bigger the particle size, the larger the time required to its
definite incorporation into the metallic matrix [13]. Gros-
jean [34] studying the SiC incorporation (average size of
1 lm, and granulometric distribution between 0.3 and
1.5 lm to 80% of the particles) in a NiP matrix obtained by
autocatalytic (nickel electroless) reduction, observed a
higher amount of incorporated particles with size between
0.3 and 0.9 lm, and the particles with size (over 3.9 lm)
were not incorporated. Nevertheless, differently from the
results presented in this work, this characteristic does not
change as a function of the amount of particles in sus-
pension; the author worked with low concentrations of SiC
in suspension (from 5 to 30 g L
-1
) and used experimental
conditions different from those used in the present work.
The increase of particles concentration in suspension,
increases the probability of ejection of the higher particles
during the incorporation process, as a result of the impacts
caused by particles in suspension. This effect reduces,
therefore, the probability of incorporation of bigger parti-
cles comparatively to smaller ones, which are more rapidly
incorporated and over which the impact is less effective.
Because of that, the incorporation of smaller particles is
more favourable [10]. For the codeposition of Ni–SiC with
Fig. 1 Linear sliding tribometer with a interface cell, both were
developed in Fraunhofer Institut IPA (Germany)
Table 1 Tested samples by tribocorrosion
Sample SiC concentration
in the electrolyte
(g L
-1
)
SiC particles
incorporated
Wear rate
(mm
3
/Nm)
9 10
-2
Number of
particles/
lm
2
Volume
(%)
NiP 0 0 0 2.81
NiPSiC10 10 0.21 3 2.78
NiPSiC80 80 0.84 17 0.68
NiPSiC200 200 1.44 17 0.63
particle sizes between 0.3 and 5 lm, it was found that for a
given number density of particles in the plating solution,
the number density of codeposited SiC-particles increases
with decreasing particle size. Hence, small SiC particles
codeposit easier than large particles. Based on a relation
between the number density of codeposited particles and
the number density of particles in the plating solution, it
was shown that the size and number density of SiC parti-
cles in the plating solution are important parameters in the
codeposition process [32].
3.2 Electrochemical Behaviour of Heat Treated
NiP–SiC Composite Coatings
In previous investigations, it was shown [22] that not only
the volume of co-deposited particles (SiC vol.%) but also
the number of SiC particles per coating area unit (and
consequently the SiC particles size) have an influence on
the electrochemical behaviour of NiP–SiC composite
coatings. The measurements of the open circuit potential
showed no correlation between the measured pote ntial and
the amount of particles incorporated in the composite
coatings. All the samples presented the mean values of
open circuit potential by -270 mV
SCE
. However, the elec-
trochemical measurements without load (Fig. 5) showed
that NiP–SiC200 composite coatings developed higher
current density than NiP–SiC80 and both had the same
quantity of incorporated SiC600, in volumetric fraction
(SiC vol.%). On the other hand, for the NiP/SiC200 com-
posite coatings, the particle number by area units is about
50% higher than in the case of NiP/SiC80, due to size
reduction of the incorporated particle. This behaviour
indicates that the composite coatings containing smaller
particles developed higher current density. The size
reduction of the incorporated particle increased the metallic
matrix/particle interface area; hence, the composite coating
has a tendency to diminish its resistance against local
corrosion, because of the voids produced by discontinuo us
interface around the particles. It was reported in the liter-
ature that there are voids in the composite coatings, pro-
ducing discontinuous particle/matrix interfaces. Verelst
[35] observed the formation of an interface between the
Fig. 2 SEM micrographs of composite NiP–SiC coating obtained from a bath with different concentration of SiC particles: a 10 g L
-1
SiC,
b 80 g L
-1
SiC, c 200 g L
-1
SiC
