deposition of high wear resistance of ni-composite coatings
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7/29/2019 Deposition of High Wear Resistance of Ni-Composite Coatings
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As new mater ials, composite coatings offer
many attractive proper ties that can b e very
useful for various engineering applications
such as machine construction and aero-
engines. C omposite coatings are produ ced by
direct entrapm ent of solid particles dur ing the
build-up of the m etal matrix by electro- or
electroless deposition[1].
In ord er to reinforce the mechanical or
antifrictional properties of the coating, the
foreign par ticles can be of refractor y[2-4] or
dry-lubr icant nature[5,6]. T he properties of
the coat ings are govern ed by the type and size
of the part icle, its content in the coating and
the mode of distribution. T he addition of
other alloying elements such as phosphoru s to
the metal matr ix (N i) adds to the improve-
ment of properties. Electroless Ni-P coatingsare characterized by their superior hardn ess
and corrosion resistance. G enerally, the elec-
troless plating process is ham pered by its slow
rate of deposition, complicated op eration and
high cost. Instead, electrodeposition of Ni-P
comp osite coatings is faster, th e solution is
more stable and has fewer replenishment
problems.
T he present work was under taken to eluci-
date the mechanism and principal factors
affecting the incorporation of the solid parti-
cles into a growing deposit. T he aim was to
improve the plating process technology and to
modify the coating properties such as wear
resistance. Most of the work was carried ou t
on N i-P-SiC composite coatings.
Experiment al procedure
Plating solution (Watt’s bath)
(1) Composition
NiSO4.6H 2O 300 g/l
NiCl2.6H 2O 60 g/lH 3BO3 40 g/l
NaH 2PO2 5 g/l
Dispersed particles:
• Mater ial SiC , Al2O
3, sand, quar tz
• Content in solution 20-100 g/l
• Particle size < 7 µm
(2) Operating conditions:-
Temperature 60°C
pH 4
Current density 2.5-10 A/dm2
Mechan ical st ir ring 150 rpm
Wear measurements
For m easurement of wear resistance, the
coated samples (20 × 50 × 1 mm ) were rotated
17 8
Ant i-Corrosion M ethods and M ater ials
Volume 44 · Number 3 · 1997 · pp. 178–185
© M CB Un iv ersi ty Press · I SSN 00 03 -5 59 9
Saher Shawki and
Z. A bdel Hamid
The authorsSaher Shaw ki an d Z. Abdel Hamid are at the Central
M etallurgical R&D Institute, Helwan, Cairo, Egypt.
Abstract
Electrodeposited Ni-P composite coatings incorporating a
variety of inorganic particles w ere obtained from Watt ’s
nickel bath containing sodium hypop hosphite. The mecha-
nism of co-deposit ion of various particles (SiC, Al2O3,
quartz and sand) was studied in view of the electro-kinetic
charge characterizing th e solid particles. M eans to
improve the mob ility of the particles in the plating solution
w ere investigated u sing sodium oleate as surface active
agent. The purpose was to increase particle content in t he
coating t o att ain high hardness values. Special attention
w as given to the deposit ion process using SiC particles.
The surface morp holo gy, hardness and w ear resistance of
the com posite coatings w ere determined. Hardness values
w ere maximized by simple heat treatm ent in air atmos-
phere which led to the precipitation of the hard Ni3P
phase. Sound, coherent and h igh w ear resistance coatings
could be produced.
Depo sit ion of highw ear resist ance of Ni-
com posit e coat ings
Contribut ed pape rs
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tangentially in a sand/water (ratio 4:2)[7].
T he rotation speed was kept constant at 150
rpm and the wear effect was measured in
term s of loss in weight expressed as µ /hr.T he
wear measurements were performed to be
within the th ickness of the coating without
reaching to the base metal.
Zeta-potential
T he zeta meter was used to determ ine the
direction and speed of the solid p articles
moving in a dilute plating solution between
two electrodes under an applied electric
potential difference. T he sign and magnitude
of the potent ial (ζ ) of the solid part icles are
related to the mobility of the par ticles in
solution.
Results and d iscussion
Ni-P - particle com posites
Preliminary experiments showed that N i-P
composites containing 4.5 wt. per cent ph os-
phorus could be deposited from Watt’s nickel
bath with the addition of 5 g/l sodium
hypophosphite. T he solid particle inclusion in
the coating (2-4.5 wt per cent for SiC) was
found to depend on t he amount of the parti-
cles in the electrolyte (20-80 g/l).
T he effect of current density on the
amount of particle inclusion is illustrated in
Figure 1, which depicts that th e part icle
content in the coating increases with the
increase of applied current. While deposition
is very slow at cur rent density (c.d) less than 2
A/dm2, deposits with c.d. higher than 7.5
A/dm2 are brittle and can be spalled off at the
edges of the substrate becau se of the bu ilt-in
high internal stress in the coating. T he results
indicated that the mobility of solid par ticles in
the electrolyte is increased by the appliedelectric curren t.
T he growth of the N i-P-SiC coating is
almost regular during th e deposition process,
as shown in F igure 2. T he deposition rate at a
certain current d ensity is not affected by the
amount of solid part icles in plating solution as
indicated b y the results given in Figure 3 for
SiC and Al2O3. T he deposition rate is known
to be controlled by the composition and
operat ing conditions of the plating electrolyte
such as c.d, pH value and temp erature.
Mechanism of deposition
T he precise nature of the process of co-
deposition and the arr ival to the cathode of
the solid par ticle was subject to con troversy.
T hree main mechanisms were previously
suggested[1] to explain the difference in the
ability to deposit various types of solid par ti-
cles: electrophoresis, mechanical entrapment
and physical adsorption
In this research it was initially thought that
the inclusion of solid par ticles depends largely
on their mobility and electrokinetic natu re in
the plating solution. T he work described
below has been designed to elucidate the
aspect of the deposition process on the basis
of the electrokinetic mechanism.
Solid par ticles in solutions are electrolyti-
cally charged by adsorbing ions on their sur-
faces. Th e sign and magnitude of the elec-
trolytic charge is known as zeta potent ial (ζ ).
T he values of zeta potential for different
particles were determ ined in d ilute solution of
pH 4 containing all the plating species. T he
results indicated that the various particles are
negatively charged with a magnitude ranging
from –35 to –60 mV. F igure 4 demon stratesthe dependence of the amount of incorporat-
ed part icles on the magnitude of ζ potential.
T he m echanism of solid par ticle electrode-
position cou ld be suggested as follows: posi-
tively charged N i ions in solution are adsorbed
on negatively charged solid par ticles. T he
particle with adsorbed ions m igrates to the
cathode where metal ions are reduced to N i
atoms forming the coating with th e entrapped
solid par ticle occupying a place in the metal
(or alloy) matr ix. A schemat ic diagram illus-
trating the mechanism is shown in F igure 5.
T he results shown in Figure 4 indicated that
the higher the magnitude of the charge on the
part icle (ζ potential), the lower the am ount
179
Deposit ion of high wear resistance of Ni-composite coatings
Saher Shaw ki and Z. Abdel Ham id
Ant i-Corrosion M ethods and M ater ials
Volume 44 · Number 3 · 1997 · 178–185
Figure 1 Effect of current density on SiC content of t he composite (particle
content in plating solution, 80g/l)
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deposited in the coating. This can be explained
on the basis of the following assumptions:
• Particle/ion mobility: with high ζ potentiala greater number of Ni ions are adsorbed
on the surface of the solid particle. Th e
aggregates of Ni ions surround ing a part i-
cle behave as one large charged body sus-
pended in the electrolyte. T he mob ility of
the later formation is, therefore, expected
to decrease under the heavy weight of the
coupled formation. T he result will be a
smaller number of solid particles reaching
the cathode.
• Relative amount occluded: since Ni ions
surround ing each particle are immediately
reduced to N i atoms at the cathode surface,
the relative amount of par ticles of high ζ
potential (quartz, sand) incorporated in the
18 0
Deposit ion of high wear resistance of Ni-composite coatings
Saher Shaw ki and Z. Abdel Hami d
Ant i-Corrosion M ethods and M ater ials
Volume 44 · Number 3 · 1997 · 178–185
Figure 3 The effect of particle content in the electrolyte on the deposit ion rat e of composite coatings
Figure 4 Effect of Zeta potential on the p ercentage of particle in th e composite
Figure 2 Effect of plating t ime on the coating t hickness of composite coatings
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coating will be less than the amount of
par ticles of lower potential (SiC, Al2O3). In
solution, the later type of part icles are
surrou nded by fewer Ni ions; the result is
high content of particles in the coating.
Although both assumptions may be valid
together, the aut hors are in favour of the
particle/ion m obility. For confirmation of that
view a number of experimen ts were carried
out u sing additives of anionic surfactant
(sodium oleate) in variable concent rations.
T he aim was to overcome the gravitational
effect of the assumed heavy aggregates of ions
entrapping the solid part icle. T he results of
surfactant addition showed an increase in th e
amoun t of part icles deposited in the coating
by the increase of the concentration of sodium
oleate up to an op timum value of 5 × 10 -3
mole/l (F igure 6) . H igher concentrations of
oleate did not yield a correspond ing increase
in the SiC in the coating.
Na oleate is known to be p resent in acidic
solutions of pH 3-7 in a polarized form[8]. In
a plating solution of pH 4 N a oleate is
adsorbed on the surface of SiC par ticles. T he
following mechan ism can be suggested as an
interpretation for the results shown in
Figure 6.
T here exists in solution a competition
between N i and N a ions (both positively
charged) to cover the n egatively charged SiC
par ticle. M eanwhile, the oleate group , with
its -CO O - part, are attached to the adsorbed
Ni ions. T he surface of the solid par ticle is
then par tly covered with th e oleate while
the tail (-R) han gs in t he solution bulk
(Figure 7).
With increased amou nts of Na oleate a
larger area of the particle’s surface is coveredwith N a+ at the expense of the area covered
with N i++. O n reaching the cathode surface,
nickel (and phosphor us) will be deposited
with a higher content of SiC. T he mecha-
nism can thu s justify the ascending par t of
the relation shown in Figure 6. H owever, the
optim um values of SiC con tent are explained
as being du e to adsorption of only one
mon olayer of the oleate, which hard ly com-
petes with N i ions.
To confirm the later idea, SiC part icles
were treated separately with the surfactant
(withou t Ni ions) prior to addition to the
18 1
Deposit ion of high wear resistance of Ni-composite coatings
Saher Shaw ki and Z. Abdel Ham id
Ant i-Corrosion M ethods and M ater ials
Volume 44 · Number 3 · 1997 · 178–185
Figure 5 Schematic diagram of t he mechanism of solid particle mobilit y in
the plating electrolyte
Figure 6 The effect of surfactant concentration on th e content of SiC in the
deposit
Figure 7 Schematic diagram of the mechanism of
adsorption of surfactant (Na-oleate) on a solid particle
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plating solution (cond itioning). T he result
(shown in F igure 6) was the deposition of SiC
in a smaller amoun t than that obtained with
optimum value of the surfactant.
Propert ies of compo site coatingMorphology and structure
T he deposited Ni-P-SiC coatings were dark
grey in colour, rough to the touch an d firm ly
adherent to th e base metal. T he roughness
was affected by the amoun t and size of SiC
particles.
Plates 1-3 gives scanning electron micro-
scopic views of the surface of Ni-P composites
with SiC, sand and Al2O3 inclusions. T he fine
par ticles were dispersed hom ogeneously in
the matr ix.Plate 4 is a photomicrograph of unetched
cross section of the as-plated N i-P-SiC coat-
ing (4.3 wt per cent SiC) which shows that the
SiC part icles (less than seven microns) are
evenly distributed in the deposit.
Hardness and wear
T he properties of the coatings are generally
related to and controlled by the amount of
solid part icles in th e coating. T he microhard-
ness tests on the coatings are affected by the
solid par ticles’ inclusion. T he resulting inden-
tation is not a m easure of the p lastic deforma-
tion of the alloy material but the indenter
force is diverged by the embed ded r igid and
uncom pressible particles. For th is reason the
examination of hardn ess of various coatings
was restricted to the assessment of surface
hardness and n ot to the coating profile struc-
ture (cross-section). T he effect of SiC content
in the electrolyte (20-100 g/l) on the hard ness
of two types of deposits (N i-SiC and Ni-P-
SiC) is shown in F igure 8, cur ves (a) and (b )respectively. T he hardn ess values of the
comp osites increase with the increase of SiC
in the coating.
In th e as-deposited N i-P coating, P exists
as super-saturated solid solution in the n ickel
matr ix. On heat treatment at 400°C for one
hour the coating hardn ess was increased from
400 to 500 VHN as a result of the precipita-
tion of Ni3P phase[9]. T he values, however,
depend on the P content in the alloy.
T he incorporation of SiC into Ni-Pdeposits led to further increases in the hard -
ness values when subjected to the same heat
treatment cycle (Figure 8 cu rve (c)). A value
up to 1,200 VHN could be reached with SiC
182
Deposit ion of high wear resistance of Ni-composite coatings
Saher Shaw ki and Z. Abdel Hami d
Ant i-Corrosion M ethods and M ater ials
Volume 44 · Number 3 · 1997 · 178–185
Plate 1 Surface morpholo gy of composite coatings,
Ni-P-SiC
Plate 2 Surface morpholo gy of composite coatings,
Ni-P-Sand
Plate 3 Surface morpholo gy of composite coatings,
Ni-P-Al2O3
Steelsubstrate
Sampleholdingresin
Composite
coating
Plate 4 The phot omicrograph of cross section of the N i-P-SiC coating
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content in the coating of 4.3 wt per cent. T he
drastic increase is due to formation of N i3P
phase as ident ified by X-ray diffraction analy-
sis. Figure 9 shows a compar ison between the
X-ray pattern for the as-plated and heat t reat-
ed N i-P-SiC coatings.
T he wear reistance and h ardn ess of elec-
trodeposited composite coatings were found
to be related to each ot her as well as to the
particle content in the coating. C oatings with
maximum hardness exhibited highest wear
resistance. A tangential mot ion wear test in
sand/water mixture was used to assess the
resistance to sliding and abrasive wear. Th e
weight loss of the coated sample was deter-
mined and the relation between the test time
and loss in coat ing thickness ( µ /hr) was
drawn. Figure 10 shows a group of the
straight line relationships obt ained with
various coatings. T he abrasion resistance
was estimated by calculating the reciprocal
of the slope of the straight line. T he results
are shown in the collective diagram, F igure
11, which indicates the following generalfeatures:
• Phosphorus deposition: slightly increased the
wear resistance of Ni electrod eposits (coat-
ing B compared to A);
• SiC inclusion; considerably increased the
wear resistance of the as-plated N i-P coat-
ings (coatings D and E ); and
• Heat treatment: further imp roved the wear
resistance of the composite coatings (coat-
ings C and F ).
183
Deposit ion of high wear resistance of Ni-composite coatings
Saher Shaw ki and Z. Abdel Ham id
Ant i-Corrosion M ethods and M ater ials
Volume 44 · Number 3 · 1997 · 178–185
Figure 8 The effect of SiC (wt . per cent) in the coating on th e hardness of
deposits
Figure 9 The X-ray diffraction patt ern for Ni-P-SiC compo site: withou t heat treatm ent and w ith heat treatm ent
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T he highest values of wear resistance could beachieved by heat-treated N i-P-SiC coatings.
Conclusions
• Ni-P composite coatings can be electrode-
posited from Watt’s Ni solution containing
Na hypophosphite and u ltrafine particles of
SiC, Al2O3, sand or quartz.
• T he particle content in the plating solution
and the applied c.d. were found to be
important factors in cont rolling the inclu-
sion of the solid part icles into the deposit.
T he amount was, however, limited by the
volume conten t of the part icles that can be
accommodated in the m etallic matrix of the coating.
• T he co-deposition of solid particles with
Ni-P coatings was explained on the basis of
the m obility and electrokinetic natu re of
the part icles in the electrolyte.
• Under the same operating conditions, the
amou nt of part icles deposited in the coating
could be increased from 4 to 9 wt per cent
by the addition of surface active agents of
anionic type to the plating solution.
• It has been shown that the coating proper-
ties such as hardness and wear resistance
were related and controlled by the am ounts
and distribution of the solid par ticles in the
coating matrix. T he hardn ess of Ni-P-SiC
18 4
Deposit ion of high wear resistance of Ni-composite coatings
Saher Shaw ki and Z. Abdel Hami d
Ant i-Corrosion M ethods and M ater ials
Volume 44 · Number 3 · 1997 · 178–185
Figure 1 0 The relationship betw een the test t im e and loss in coating th ickness
0 50 100 150 200 250
0 250 500 750 1,000 1,250
Wear resistance X10 2 ( m/hr) –1
VHN
µ
Ke y
Wear resistance
VHN
A
B
C
D
E
F
Ni
Ni-P as plated
Ni-P heat treated
Ni-SiC
Ni-P-SiC as plat ed
Ni-P-SiC heat t reated
Figure 1 1 Hardness and w ear resistance of the com posite coatings compared t o ot hers electrodeposited
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coating could be increased by heat treat-
ment t o values over 1,200 VHN . T hese
coatings are coherent and characterized by
high wear resistance which can be of
benefit in several industr ial applications.
Attention should be given to th e internal
stresses and the m eans of releasing these
stresses, which may hamper the coating
performance if they are subjected to post-
mechanical working operations.
References
1 Celis, J.P., Roos, J.R., Buelen s, C. and Fransaer, J.,
“ M echanism of e lectro lyt ic composite p lat ing: survey
and t rends” , Transactions of th e Institu te of M etal
Finishers, Vol. 69 No. 4, 1991, p. 133.
2 Poeton, A.R., “ Composite coat ings for advancedperformance” , M etals & M ater ia ls, No. 702, 1988.
3 Per iene, N. , Cesuniene, A. and M atul ionis, E. , “ Code-
posit ion of mixt ures of dispersed particles wit h nickel-
phosphorus electrodeposits” , Plating & Surface
Finishing , Vol. 81 No. 10, 1994.
4 Roos, J.R., Celis, J.P., Fransaer, J. and Buelens, C., “ The
development of com posite plating for advanced
mater ia ls” , Jouirnal of M etals , No. 11, 1990, p. 60.
5 Cowieson, D.R., Sadowska-Mazur, J. and Warw ick,
M .E., “ Codeposit ion of non-metall ic particles w ith t inand t in-n ickel a l loy” , Proceedings of 3rd Int ernational
Congress for Surface Technolog y , Berlin, 1985.
6 Lipp, L.C.” Sol id lubr icants – their advantages and
l im i tat ions” , Lubrication Engineering , Vol. 32 No. 11,
1976, p. 574.
7 Soror, T.Y., “ Electroless deposit ion of nickel as related
to th e structure, morphology and properties of the
coati ngs” , PhD Thesis, Cairo University, 1995 .
8 Arbi ter, N. and Wil l iams, E.K.C., “ Condit ioning in o le ic
acid flotat ion” , in Fine Particle Processing , SME-AIME,
USA, Vol. 1, 1980 , pp. 802-31.
9 Changgeng, X. , Zonggeng, D. and Li jun, Z. , “ The
properties of electrodeposited Ni-P-SiC compo site
coat ing ” , Pating & Surface Finishing, Vol. 75 No. 10,
1988.
185
Deposit ion of high wear resistance of Ni-composite coatings
Saher Shaw ki and Z. Abdel Ham id
Ant i-Corrosion M ethods and M ater ials
Volume 44 · Number 3 · 1997 · 178–185
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