Scratch and Wear Resistance of Transparent Topcoats on Carbon Laminates 1
Transcript of Scratch and Wear Resistance of Transparent Topcoats on Carbon Laminates 1
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Progress in Organic Coatings 67 (2010) 209219
Contents lists available at ScienceDirect
Progress in Organic Coatings
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / p o r g c o a t
Scratch and wear resistance of transparent topcoats on carbon laminates
M. Barletta a,, D. Bellisario b, G. Rubino a, N. Ucciardello a
a Universit degli Studi di Roma Tor Vergata, Dipartimento di Ingegneria Meccanica, Via del Politecnico, 1 - 00133 Roma, Italyb Universit degli Studi di Roma La Sapienza, Dipartimento di Meccanica e Aeronautica, Via Eudossiana, 18 - 00184 Roma, Italy
a r t i c l e i n f o
Article history:
Received 30 June 2009
Received in revised form
17 September 2009Accepted 8 October 2009
Keywords:
Powder paints
Epoxycarbon laminates
Scratch
Wear
a b s t r a c t
Thepresentinvestigationdeals withthe application of low-curable powder paints on epoxycarbon lami-
nates. Carbonlaminates werefirst peened to corrugate theirsurface,hence increasing the wettability and
allowing a better adhesionof the overlying coatings. Powder coatings were thenelectrostatically sprayed
onto peened and unpeened substrates and baked into a convection oven. Their aesthetic and tribologi-
cal performance was comparatively evaluated. Powder coated peened carbon laminates exhibited good
adhesion and visual appearance as well as noteworthy scratch resistance and tribological performance.
2009 Elsevier B.V. All rights reserved.
1. Introduction
Carbon laminates are literally spreading in aeronautic and
aerospace applications, in manufacturing of high performance
components for cars and motorcycles, in assembling of high added
values items, in electronic and medical devices, in sport and fit-
ness equipments as well as in all those market shares in which the
technological challenge and the exclusivity of the design receive
much attention [1,2]. Finishing has always been one of the major
concerns for carbon laminates manufacturers and painters [2].
Nowadays, aesthetic and protective finishing is often provided by
wet paints (mostly, solventborne [3]), which allow good visual
appearance and mechanical properties with a simple deposition
process and a spontaneous drying [3]. Increasing environmental
concerns and the even more stringent regulations are, however,
limiting the emission of volatile organic compoundsduring the fin-
ishing processes, thus demanding alternative technologies, which
make use of dry painting formulations [3,4]. Powder coating is a
well-known viable and eco-friendly alternative to wet painting [5].Yet, the applications of powder coatings are generally restricted
by several drawbacks as poor levelling, reduced adhesion, high
baking temperature and difficult-to-deposit procedure onto com-
plexgeometry, heatsensitiveand/or electricalinsulating substrates
[58]. Accordingly, carbon laminates can be extremely difficult to
powder coat [9].
Corresponding author.
E-mail address: [email protected] (M. Barletta).
They are semi-conductive materials, in which an electrically
conductive carbon fibre is dispersed inside a non-conductive epoxy
matrix with limited wettability to molten painting polymers. Such
issues definitely complicate the deposition process, as powder
coatings involve the usage of powder paints which must be elec-
trostatically sprayed onto the substrate and, then, oven-baked to
allow their melting, levelling and curing. This process requires,
at least, a uniformly semi-conductive material characterized by a
relatively wettable surface to the molten polymer powders [5,9].
Furthermore,dependingon theway inwhichthe carbonlaminate is
manufactured, itcouldbe more orless heat sensitive[2]. This would
push towards an application method requiring a baking procedure
at relatively low temperature and, in any case, at temperature well
below the 170 C mostly used for the customary applications of
the powder coatings [5]. Lastly, carbon laminates are susceptible to
the release of a not negligible amount of volatile compounds (i.e.,
degassing) when baked at moderate or high temperature (>115 C)
[9]. If the gas release takes place during the curing process of an
overlaying coating, thevolatilecompound emitted from thebulk ofthe carbon laminates can remain trapped inside the film, thus giv-
ing rise to the formation of unaesthetic, porous andbrittle topcoats
[9].
The basic idea should be to preventively treat the surface of the
carbon laminates in order to promote the wettability of the sur-
face, induce a corrugated morphology on it and facilitate the rapid
release of the volatile compounds during the baking. This is, there-
fore,the context inwhichthe present work investigatesthe effectof
pre-treating the carbon laminates by a peening process with glass
beads to improve the visual appearance, adhesion strength and
wear resistance of the overlying organic coatings. In this respect,
0300-9440/$ see front matter 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.porgcoat.2009.10.015
http://www.sciencedirect.com/science/journal/03009440http://www.elsevier.com/locate/porgcoatmailto:[email protected]://dx.doi.org/10.1016/j.porgcoat.2009.10.015http://dx.doi.org/10.1016/j.porgcoat.2009.10.015mailto:[email protected]://www.elsevier.com/locate/porgcoathttp://www.sciencedirect.com/science/journal/03009440 -
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210 M. Barletta et al. / Progress in Organic Coatings 67 (2010) 209219
transparent polyester-based powder coatings were first electro-
statically sprayed onto peened and unpeened substrates and then
baked at moderate temperature in a convection oven. Their visual
appearance, scratch resistance and tribological performance were
comparatively evaluated. The experimental findings revealed that
peened and subsequently powder coated carbon laminates exhibit
good adhesion, good visual appearance as well as noteworthy
scratch resistance and wear response.
2. Experimental
Commercially available 2040 mm1050mm1.1 mm carbon
laminates were supplied by Carbon-Composite Technology (Wald-
stetten, Germany). They consist in a standard laminate structure
composed of continuous 3K carbon fabric (205245 g/m2 in plain)
with a fibre alignment bidirectional (50% at 0 and 50% at 90).
The matrix is epoxy-based with a Tg of170C. Custom size cuts
were performed by abrasive water jet up to the final dimension
of 6mm4 mm. Pre-treatments of the carbon laminates prior to
the deposition process were scheduled as follows: (i) peening with
glass beads [3]; (ii) cleaning of the carbon laminates by power-
washing and rinsing; (iii) overnight drying and stabilization in
convection oven at moderate temperature (60 C).Peening of the as-received carbon laminates was performed
by an abrasive jet of glass beads [10]. In this process, the carbon
laminates enter the space into which nearly spherical glass par-
ticles (factor shape 0.95) with average diameters in the range of
100800m are injected by means of an air stream issued at mod-
erate or high pressure (4, 6 and 8 bar) and varying operating time
(1, 2 and 3 min). Powder particles impacting the carbon laminates
release their kinetic energy and are supposed to cause manifold
effects: (i) the micro-grooving of the softer epoxy matrix; (ii) the
selective removal of the surface contaminants; (iii) a stress-release
action beneficial to relax the carbon laminates residual stresses.
Peening wasfollowedby thecleaning processes, which canremove
theresiduals of theimpacts between theglassbeads andthe carbon
laminates as well as some occasional organic contaminants whosepresence could compromise the performance of the whole coating
process. Finally, the oven-drying and stabilization process allows
the further release of the internal stresses of the pre-treated car-
bon laminates and the evacuation of most of the volatile organic
compounds still retained inside the bulk of the material.
The 3D morphology of the carbon laminates before and after
peening was measured by using a Taylor Hobson Surface Topogra-
phy System (TalySurf CLI 2000, Taylor Hobson, Leicester, UK) with
the non-contact 300m Chromatic Aberration Length (CLA) HE
gauge. The absence of contact between the gauge and the coating
was chosen to prevent any damage to the surface being measured.
200profiles(step 100m)20 mmlongwere recordedfor each sam-
ple to cover a wide enough area (400 mm2) of the entire surface
structure. TalyMap software Release 3.1 was then used for ana-lytical examination of the experimental data. Standard amplitude,
spacing and hybrid roughness parameters (Gaussian filter) were
considered to depict the surface morphology of the carbon lami-
nates.
Upon pre-treatments, the carbon laminates were electrostati-
cally sprayed (ESD PC15, Siver Srl, Terni, Italy) with low-curable
outdoor resistant polyester-based transparent painting powders
(20m average diameter, 0.80 factor shape, PPG Industries,
Bellaria,Italy). Applied voltage, feeding pressure and auxiliarypres-
sure were set at 90 kV, 1.5 bar and 1.0 bar, respectively. Deposition
time wassetat 6 s.Afterthedeposition,thecoated carbonlaminates
were submitted to the curing process in a convection oven (Nad-
deo RT11, Naddeo Engineering, Scafati, Italy) at 135C for20 min.
Coating thickness of about 120m (ISO 2178 and ISO 2370) could
be achieved with an error of10%. All the coatings failing to agree
with this specification were disregarded.
The surface roughness of the coated carbon laminateswere mea-
sured using a contact probe surface profiler (TalySurf CLI 2000,
Taylor Hobson, Leicester, UK). Optical (DM IRM, Leika) and stereo-
scopic microscopy (SMZ-1500, Nikon) were used to catch high
resolution images of the surface morphology after the coating
process. The adhesion strength of the coatings were analyzed by
scratch tests (Micro-Combi Tester, C.S.M. Instruments, Peseaux,
Switzerland) equipped with a Rockwell C-type conical indenter
(800m tip radius), and operating in progressive mode (track
3 mm, scratch speed 1 mm/min, load 100mN30N) at about 20 C
and 40% RH. SEM (SEM Leo Supra 35, Oberkochen, Germany) was
used to observe the residual scratch patterns, which were rebuilt
using the contact probe surface profiler with 2 m lateral reso-
lution. Calculated features of the 3D scratch patterns were the
volume of the plastic pile-up formations, VPILE andthe scratch ditch,
VDITCH [11]. Tribological tests with alternative dry-sliding motion
wereperformedby a standardtribometer (Linear Reciprocating Tri-
bometer, C.S.M. Instruments, Peseaux, Switzerland) at about 20 C
and 40% RH. Samples were tested at 3 N load and back-and-forth
sliding (stroke length 10mm, frequency 5 Hz, duration 20, 50, 100,
500 and 2000s) of the upper SAE52100 steel ball (6mm diameter).
Wear rate of the coatings was assessed by contact probe surfaceprofiler, measuring the area involved by the action of the antago-
nist, the wear volume and the minimum and maximum height of
the wear pattern.
3. Results and discussion
3.1. Analysis of the peening process
Peening byglassbeads wasfound tobe successful in pre-treating
the carbon laminates and to make them ready for the painting pro-
cess. The carbon laminates are exposed to the repeated impacts of
glass beads,which,moving at relatively high pressure,can impinge
on the substrate and release their kinetic energy. The impacts canmodify the surface morphology of the carbon laminates and estab-
lish a micro-corrugated topography as already shown by Barletta
and Gisario in a previous study on similar substrates [9].
Fig. 1 shows the trends of the amplitude, spacing and hybrid
roughness parameters before and after the peening process.
Increasing the peening pressure and the exposure time, the surface
of the carbon laminates becomes progressively rougher. Average
roughness Ra and ISO 10 points height Rz can approach high values
as 0.5m and 5m, respectively, which are, at least, one order of
magnitude more than the corresponding values of the untreated
substrates. Corrugation of the surface morphology after peening
process is also stated by the increase in the modulus of spacing
and hybrid roughness parameters. In particular, skewness Rsk tends
to assume negative values. This means that the surface profilesbecomeeven more anti-symmetrical aroundthe mean line andthis
is more likely ascribable to the random impacts of the glass beads
onto the softer polymer matrix of the carbon laminates. Similarly,
Kurtosis Rku increases, thus supporting the basic idea of a rougher
and widely micro-grooved morphology being established after the
peening process.
Peening of thecarbon laminatesby glass beads atlowerpressure
or for shorter time was not accounted for as it would lead to irrel-
evant modification of the substrate morphology. At the same time,
peening of the carbon laminates at higher pressure or for longer
time were excluded as it would lead to over-peening phenomena.
Under such circumstances, the structure of the composite material
would be damaged andsome local delamination phenomena could
occur, thuscompromisingthe overall performance of the laminates.
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M. Barletta et al. / Progress in Organic Coatings 67 (2010) 209219 211
Fig. 1. Evolution of carbon laminates morphology vs. peening process parameters.
A typical consequence of the over-peening is the removal of some
fibres bundles from the polymeric matrix [9], which is extremely
detrimental to the visual appearance and functional properties of
the composite material.
Average roughness of the coated carbon laminates were mea-
sured on both unpeened and peened substrates and, whatever
the peening parameters, average roughness Ra of0.05m was
found. Once heated, the polymer powders melt, their viscosity
drops down and, accordingly, they were able to level and fill the
cavities which cancharacterize themorphology of thepeened sam-
ples,particularlythose peened underthe severest conditions. In any
case, a smooth surface finishing is established and, in this respect,
the starting morphology of the underlying substrates is irrelevant.
Therefore, the build-up of a good surface structure and, conse-quently, a good visualappearancecan also be achieved bypolyester
coatings deposited onto rougher substrates.
3.2. Scratch response of polyester coatings onto unpeened carbon
laminates
Micro-grooving of the carbon laminates is extremely helpful in
improving the scratch and wear resistance of the overlying organic
topcoats. In Fig. 2, the residual deformation response after scratch
(i.e., depth of the residual scratch pattern) is comparatively eval-
uated for peened and unpeened substrates. Unpeened substrates
show a rapid increase in the depth of the residual scratch pattern
with the increase in the applied load (i.e., evaluation length). The
residual depth trend shows first small jumbling at a normal force of
7.5N (the first small saddle) and, again, a bigger jumbling event
at 1011 N (the second saddle). The jumbling events could be
explained as the resultof a sort of stick-slip motion occurring dur-
ing the scratch testof the coating material.Therefore, the scratching
process evolves by recurring jerks instead of a smooth path as
observed by Zhang andValentine forscratching of bulk PMMA [12].
Yet, Zhang et al. observed how in the bulk PMMA the time of the
Fig. 2. Residual depth vs. normal force with the peening process parameters.
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Fig. 5. Friction coefficient vs. normal force with the peening process parameters.
3.3. Scratch response of polyester coatings onto peened carbon
laminates
The polyester coatings deposited onto the peened substrates
show a definitely better scratch response whatever the settings
of the peening parameters. Peened substrates do exhibit smaller
residual scratch patterns if compared with the unpeened one
(Fig. 2), whatever the choice of the peening parameters (time and
pressure). The maximum residual depths of 4550m confirm the
coatings remainwell adhered to theunderlying substrates after the
scratch tests. The residual depth trends run very regular, that is,
according to a very smooth pattern, at low scratch load. First jum-
bling events take place at15N (sample peened at6 bar for 1 min)
and the phenomena intensify at higher load (1820 N), where
even the samples exhibiting the smaller residual depths must face
some irregularities along their pattern.
The better scratch behaviour of the polyester coating onto thepeened carbon laminates can be ascribable to the corrugated sur-
face morphology produced on them by the peening process. The
micro-grooving of the softer epoxy matrix generates a longer
interface between the substrate and the overlying coating which
promotes their adhesion [9,17]. Moreover, the peak-to-valley
topography physically opposes to the lateral propagation of the
surface cracks generated by the action of the scratching indenter
insidethe outermostlayerof thecoating material [18]. The peening
process also allows the selective removal of the organic contami-
nants from the surface of the carbon laminates, thus improving
their surface wettability and, consequently, the adhesion on them
of the overlying organic coatings [10,17]. Finally, the peening pro-
cess causes a stress-release action beneficial to relax the carbon
laminatesresidualstresses[9]. Thisway,whenthecoatingsaresub-mitted to the scratching procedure, the stresses field generated by
theindenterinside theoutermost layer of thematerialis notsuper-
imposed to those already insisting on a highly stressed substrates
and this could be beneficial to the overall scratching behaviour.
However, it is not possible to establish a ranking among the
samples peened under different peening time and pressure. For
example, the carbon laminate peened at 4 bar for 1 min (i.e., the
softer peening program) shows the higher residual depth trend,
whichpotentially meansthe worsescratch behaviour.Nonetheless,
its residual scratch pattern is one of the smoothest with few jum-
bling events occurring up to 25 N scratch load (Fig. 2). The samples
peened under more energetic conditions exhibit less deep residual
scratch pattern butsome jumbling can occur at lower normal loads
(Fig. 2).
Fig. 6. SEM image of the residual scratch pattern of the polyester coating onto a
peened carbon laminate (8 bar for 1 min): (a) the whole scratch pattern; (b) zoom
of the residual scratch pattern at high scratch load.
The trend of friction coefficient in Fig. 5 is helpful in support-
ing the interpretation of the leading mechanism involved in the
scratching of polyester coatings onto peenedcarbon laminates. The
friction force tends to increase first as a result of the rapid increase
in thepenetrationdepth duringthe first momentof thescratch test.
Then, the friction force tends to stabilize with a slight decreasing
branch. The onset of jumbling takes place at15N orlittlemorefor
the most part of the investigated samples and the agreement with
the residual depth data is very good. In fact, the coating exhibiting
Fig.7. 3Ddeformationresponse(ditch andpile-upvolume) vs.peeningparameters.
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Fig. 8. Wear tracks on polyester powder coatings: (a) unpeened carbon laminates; (b) peened (8 bar for 1 min) carbon laminates.
fewer jumbling events is that deposited onto the under-peened
4 barfor 1 mincarbon laminate,as beforestated bythe examination
of its smooth residual depth trend.
The analysis of the typical scratch track of a polyester coating
onto a peened carbon laminate (peening pressure 8 bar, peening
time 1 min) is reported in Fig. 6. The SEM image does not reveal
massive delamination phenomena, but onlyminor damages mostly
located at the bottom of the scratch pattern and along its sides.
The scratch behaviour of the polyester coatings is nearly the same
whatever the settings of the peening parameters. All the coat-
ings deposited onto the peened substrates did not fail. Yet, they
showedlarge groove formations afterscratch.This meansthat, dur-
ing the scratching procedure, the load is essentially applied to the
advancing half front of the indenter, thus determining a significant
increase in the actualstress induced in the coating material located
ahead. This phenomenon also takes to a reduced loading condition
Table 1Wear rate of the peened (8bar for 1 min) and unpeened samples.
Sample Worn surface (mm2) Worn volume (mm3) Maximum height of the wear
pattern (m)
Minimum height of the wear
pattern (m)
Unpeened 1 m 2.37 0.00812 14.3 3.42
Unpeened 2.5 m 3.1 0.0175 16.8 5.65
Unpeened 5 m 3.21 0.0192 15.4 5.97
Unpeened 25 m 4.22 0.0361 20.3 8.54
Unpeened 50 m 4.64 0.0442 23.2 9.53
Unpeened 100 m 5.13 0.0558 26.2 10.9
Peened 1 m 0.507 5.4E4 8.44 1.07
Peened 2.5 m 2.32 0.00706 12.8 3.04
Peened 5 m 2.97 0.0154 18.5 5.23
Peened 25 m 4.11 0.0324 19.1 7.89
Peened 50 m 4.26 0.0344 20.3 8.08
Peened 100 m 4.83 0.0476 23.5 9.86
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M. Barletta et al. / Progress in Organic Coatings 67 (2010) 209219 215
at the rear of the indenter (i.e., at the back half of the indenter).
Accordingly, the coating material ahead of the advancing indenter
tends to significantly deform and a pile-up of material is generated
as confirmed by the SEM image (Fig. 6b) and in agreement with
the mechanism early proposed (Fig. 3). The formation of the plastic
pile-up further increases thestressinduced into the ductile coating
material. In addition, the contact condition between the advancing
indenter and the coating generates a radial tensile stress which is
presumably the highest at the sides of the indenter. Such stress
is further corroborated by a tensile stress developed at the rear
of the contact, that is, whereas the indenter is nearly separated
from the deformed material. Tensile stress will thus occur initially
at the sides of the indenter, thus generating cracks located at the
track edge and nearlyparallel to the scratch direction (Fig. 6a). Par-
tial ring cracks are generated ahead of the indenter, which, passing
over them, tend to push the cracks generated deep into the track.
Cracking of the coatings can also be supplemented by its bend-
ing into the scratch track as a result of the advancing and deep
penetrating indenter. The sum of these failures lead to potentially
through-thickness conformal cracking at the front and sides of the
indenter (Fig. 6b). Cracking also occurs at the rear of the contact
between the indenter and the coating surface due to the tensile
stresses (Fig. 6b). Together with the conformalcracking, the tensile
cracking is by far the much contributing mechanism to the visible
damageproduced in thebottomof thescratch track of thepolyester
coating onto the peened carbon laminates.
3.4. 3D deformation response of polyester coatings onto carbonlaminates
Fig. 7 reports the 3D deformation response and, in particu-
lar, the ditch and pile-up volume trends according to the peening
parameters.No data areavailable forthe polyestercoating onto the
unpeened carbon laminates, as it catastrophically fails (Fig. 4) and
does not allow the measurement of the 3D features of the residual
scratch pattern. All the coatings depositedonto the peenedsamples
did not fail and thus allowed the measurements of the ditch and
pile-up volume. Yet, it is extremely difficult to establish a ranking
Fig. 9. The appearance of first cracks during the wear test of the polyester coatings onto the carbon laminates: (a, c and e) unpeened 1 m sliding distance; (b, d and f) peened
(8bar for 1 min) 1m sliding distance.
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Fig. 10. Wear pattern: (a) unpeened 100m sliding distance; (b) peened (8bar for 1 min) 100 m sliding distance; (c) unpeened 25 m sliding distance; (d) peened (8 bar for
1 min) 25 m sliding distance; (e) unpeened 5m sliding distance; (f) peened (8bar for 1 min) 5 m sliding distance; (g) unpeened 2.5m sliding distance; (h) peened (8bar for
1 min) 2.5m sliding distance; (i) unpeened 1m sliding distance; (j) peened (8bar for 1 min) 1 m sliding distance.
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M. Barletta et al. / Progress in Organic Coatings 67 (2010) 209219 217
between the samples peened under the different conditions. The
sample peened at8 bar for 1 min exhibits the best behaviour with a
moderate pile-up volume and, above all, with the lowest ditch vol-
ume. A slight worsening in the 3D response is observed if, keeping
the peening pressure at 8 bar, the samples peened for longer are
looked into. This result should be related to a sort of over-peening
which could be detrimental to the final behaviour of the polyester
powder coatings. Peening at 6 and 4 bar takes to worse 3D defor-
mation response, with mostly larger pile-up and ditch volume. In
particular, the worst behaviour is exhibited by the under-peened
4 bar for 1 min carbon laminate. Under that circumstance, the sub-
strate surface is poorlycorrugated andthe peening is notso helpful
in improving the adhesion between the coating and the underly-
ing carbon laminate. However, no massive failure phenomena or
delamination occur, thus revealing, once more, the reliability of
the peening process as pre-treatment technique for improving the
adhesion between the carbon laminate and the overlying powder
coatings. Carbon laminates peened at 6 bar and for any peening
time shows intermediate behaviour,with pile-up and ditch volume
of intermediate extent.
The aforementionedresults push towards the definitionof three
differentclasses of samples:(i) theunder-peened (i.e.,those peened
at 4 bar for 1min) or fairly peened samples (those peened at 6 bar
for any timeandat 4 bar for 2 and 3 min), which doexhibitvery highor average pile-up and ditch volume, respectively; (ii) the properly
peened samples (i.e., those peened at 8 bar for 1 min), which do
exhibit theminimumvalues of pile-up andditch volumeamong the
investigated ones; (iii) lastly, the over-peened samples (i.e., those
peened at8 bar for 2 and 3 min), which do exhibit slight larger pile-
up and ditch volume, despite the more energetic peening process.
3.5. Wear response of polyester coatings on carbon laminates
Wear response of the polyester coating onto unpeened carbon
laminates was compared with the coating deposited onto the car-
bon laminates peened at 8 bar for 1 min, even if all the coatings
deposited onto the peened samples tend to behave the same way
whatever the choice of the peening parameters.Wear rate wasaffectedby thepre-treatmentsof thecarbon lam-
inates, with thecoatingsontothe peenedsubstrates being worn out
slowly than the coatings onto the unpeened substrates. Fig. 8a and
b shows a stereoscopic image of the wear tracks on the polyester
coatings deposited onto the unpeened and peened (8 barfor 1 min)
carbon laminates, respectively. Table 1 summarizes the results
of the wear test. After 1 m sliding distance, the wear parameters
in Table 1 and the SEM images at varying magnification (Fig. 9)
show how the peened samples behave definitely better than the
unpeened one. In fact, its worn volume is less than one order of
magnitude smaller. Even the extent of the worn surface as clearly
visible also from the SEM image (particularly, in Fig. 9c and d) and
the minimum and maximum height of the wear pattern are defi-
nitely smaller for the coatings depositedonto the peenedsubstrate.These results could be quite surprising as wear in thick coating is
generally related to the material properties and less to the way in
whichthe overlyingcoating materialand thesubstrateinteract. Yet,
the slower wear phenomena which characterize the coating onto
the peened substrate can be more likely ascribed to the different
way the stresses inside the coating material are distributed during
the wear tests. As said before, when the antagonist (i.e., the steel
ball in the wear test) acts onto the surface of a ductile coating, the
material is submitted to a very peculiar stress distribution (Fig. 3).
This is what probably happens duringthe wear test of thepolyester
coating onto the carbon laminates. Such a stress distribution could
cause the birth of first cracks in very short time, as SEM images
in Fig. 9e and f show. The propagation of the cracks is therefore
accelerated or not depending on the substrate characteristic [9].
Fig.11. SEMimagesof thewearpatternafter (a)1 m sliding distance and (b)100m
sliding distance.
Peenedsamples present a highlycorrugated surface. As said before,
the resulting peak-to-valley topography and the larger interfacial
area between the substrate and the overlying polyester coating
are certainly helpful in withstanding the action of the antago-
nist to spread and propagate the surface cracks, thus slowing the
wear phenomena. To the contrary, a smoother interface between
the carbon laminates and the coating is detrimental to the wear
response. In fact, there is no opposition to the cracks propagation,
which canfreelyspread over thecoating anddetermine fasterwear
phenomena. However, the difference in wear behaviour between
the coatings deposited onto the peened and unpeened samples
tends to decrease by increasing the sliding distance (Table 1 and
Fig. 10). In fact, once the cracks due to the action of the antagonist
are spread over the coating surface and propagated, the counter-action of a rougher interface between the coating and substrate
tend to become even more limited. At higher sliding distance, the
difference between the wear volume of the coatings onto peened
and unpeened substrates is still appreciable but it averages a mere
1022%.
Despite the different kinetic by which the wear track is formed
anddeveloped on thepolyester coatings depositedontothe peened
and unpeened substrates, the mechanism of material removal is
basically the same, as SEM images in Fig. 11 confirm. Fractures of
the outermost layers of material are provoked by the action of the
antagonist during the initial stage of the wear test. By other side,
this is the moment in which the pressure applied by the antago-
nist is the highest as it is concentratedaround its tipand, therefore,
acts on a restricted portion of coating. The high specific load insist-
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218 M. Barletta et al. / Progress in Organic Coatings 67 (2010) 209219
Fig. 12. CLA profilometry of the wear pattern after (a) 1 m sliding distance, (b) 2.5m sliding distance, (c) 5 m sliding, (d) 25 m sliding distance, (e) 50m sliding distance and
(f) 100 m sliding distance.
ing onto the surface of polyester coating generates a severe stress
distribution inside the material according to the model reported
in Fig. 3 and provokes a quick and widespread fracturing of the
outermost layers of the coating (Fig. 11a). The coating material
is therefore torn off as result of the interaction with the antago-
nist.The residual weartrack showsminimummaterial deformation
after the release of the load and, accordingly, minimum is the
displacement of coating material sideways. Such fractures pro-
gressively spread over the surface until some material is detachedfrom the coating, thus forming debris still perceptible around the
wear track (Fig. 10). Increasing the sliding distance, the antago-
nist tends to deeply penetrate inside the coating and, accordingly,
the pressure it is able to apply progressively decreases. The result-
ing wear track changes its physiognomy (Fig. 11b). The wear track
becomes more spread over the coating surface and two different
zones can be distinguished: (i) the zone in the bottom of the wear
track where the fracturing phenomena are still perceptible; (ii) the
outer zone, where fracturing phenomena does not occur. In the
latter zone, it is possible to note a remarkable residual deforma-
tion of the coating after the release of the load with sort of stripes
marking the surface and running parallel to the edge of the wear
track. Some coating material is displaced sideways along the bor-
der of the wear track. Furthermore, fracturing in the bottom of thewear track is less apparent. The change in contact condition from
a more concentrated load during the initial stage of the wear test
to a more dispersed one during the final stage seems to cause a
corresponding change in the response of the coating material to
the antagonist. There is a sort of transition from a brittle-like to a
ductile-like response which was often reported in the pertinent lit-
erature [11,15]. 3D analysisof theweartrackssupportstheprevious
considerations (Fig. 12). The transition to a ductile-like response
of the polyester coating, when higher sliding distances (5m) are
approached, is confirmed by the typical jumbling of the wear pat-
tern(Fig. 12cf). Thejumblingevents areeven more apparentgoing
towards higher sliding distance during the wear test. They start
to be perceptible after 5 m sliding distance (Fig. 12c) and, then,
they progressively increase (Fig. 12df). Theformation of such jum-
bling was previously found during scratch test at moderate or high
load and the phenomenon was ascribed to the typical stick-slip
behaviourof some ductile bulk polymers(like PMMA[12]) or, alter-
natively,to the peculiar stress distribution affecting most of ductile
coating material underprogressive loadscratch test [13,15]. As said
before, it is difficult to discern between the two mechanisms and
establish which one is the most active one. Yet, both mechanisms
are those typical of ductile polymeric materials. Therefore, wear
response of the polyester coatings after higher sliding distancesis typical of a ductile material, while during the first stage of the
wear test, only fracturing phenomena and brittle removal of mate-
rialwithout significant deformation was clearly observed (Fig. 11a).
This supports the hypothesis of a brittle to ductile transition of the
polyester coating response to the antagonist by simply increasing
the sliding distance during the wear test and, thus, changing the
corresponding contact conditionsbetween the antagonistitself and
the surface being investigated.
Lastly, even after 100 m sliding distance, the coating keeps on
being adhered onto the underlying substrates, without the occur-
rence of noticeable delamination phenomena (Figs. 11b and 12f),
thus showing the overall good wear response of thepolyester pow-
der coatings onto carbon laminates whatever their starting surface
conditions.
4. Conclusions
The deposition of environmentally friendly transparent
polyester powder finishing onto carbon laminates with particular
emphasis on the effect of the substrate pre-treatments onto the
scratch and wear response of the overlying coating was the matter
of the present investigation.
The experimental evidences lead to the following conclusions:
The micro-grooved morphology after peening is very promis-
ing for promoting a good adhesion between the electrostatically
sprayed coatings and the carbon laminates.
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M. Barletta et al. / Progress in Organic Coatings 67 (2010) 209219 219
Themorphology of thecarbon laminatesis a function of thepeen-
ing pressure and time. If they increase, a rougher morphology is
reached. Scratch response of the topcoats onto peened and unpeened car-
bon laminates is different, with the former warranting the best
performance as results of the better adhesion strength between
the film and the underling rougher substrates. It is extremelydifficult toestablish a ranking among thepolyester
coatings deposited onto the peened carbon laminates using dif-
ferent parameters. Yet, the analysis of 3D deformation response
of the coatings allows the definition of three different classes: (i)
the under-peened exhibiting the worst scratch response; (ii) the
properly peened samples, exhibiting the best scratch response;
(iii) the over-peened samples, exhibiting a scratch response
worse than the properly peened, despite the more energetic
peening process. Wear response of the polyester coating on peened carbon lami-
nates are better, with the largest difference arising at low sliding
distance whereas the corrugated surface morphology opposes
better to the action of the antagonist. At higher sliding distance, wear of polyester coating onto peened
and unpeened carbon laminates are very similar, with wear vol-
ume differences averaging 1020%. The change in contact condition from a more concentrated load
during the initial stage of the wear test to a more dispersed
one during the final stage is supposed to cause a corresponding
change in the response of the coating material to the antago-
nist, with a transition from a brittle-liketo a ductile-likeresponse
arising. No delamination was found to affect thepolyester coating during
the wear test, even after longer sliding distance.
References
[1] C. Soutis, Fibre reinforced composites in aircraft construction, Progress inAerospace Sciences 41 (February (2)) (2005) 143151.
[2] D. Gay, S.V. Hoa, S.W. Tsai, Composite Materials: Design and Applications, CRCPress, Boca Raton, FL, USA, 2002.
[3] R. Lambourne, T.A. Strivens, Paint and Surface CoatingsTheory and Practice,second edition, Woodhead Publishing Limited, 1999 (Reprinted 2004).
[4] P.G. de Lange, Powder Coatings Chemistry and Technology, second edition,William Andrew Publishing, 2005.
[5] K.D. Weiss, Paint and coatings: a mature industry in transition, Progress inPolymer Science 22 (1997) 203245.
[6] M. Barletta, A. Gisario, S. Guarino, G. Rubino, Development of smooth finishesin electrostatic fluidized bed (EFB) coating process of high-performance ther-moplastic powders (PPA 571 H), Progress in Organic Coatings 57 (4) (2006)
337347.[7] M. Barletta, G. Bolelli,S. Guarino,L. Lusvarghi, Developmentof matte finishesin
electrostatic(EFB) andconventionalhot dipping(CHDFB)fluidizedbed coatingprocess, Progress in Organic Coatings 59 (2007) 5367.
[8] S.S. Lee, H.Z.Y. Han, J.G. Hilborn, J-A.E. Manson, Surface structure build-up inthermosettingpowder coatings during curing, Progressin Organic Coatings 36(1999) 7988.
[9] M. Barletta, A. Gisario, Electrostatic spray painting of carbon fibre-reinforcedepoxy composites, Progress in Organic Coatings 64 (2009) 339349.
[10] M. Barletta, S. Guarino,V. Tagliaferri,Metal cleaning madeeasy: a fluidizedbedsystem is a cost-effective option for degreasing processes, Metal Finishing 102(12) (2004) 2328.
[11] A. Krupicka, M. Johansson, A. Hult, Use and interpretation of scratch tests onductile polymer coatings, Progress in Organic Coatings 46 (2003) 3248.
[12] S.L. Zhang, J.M.Valentine, Stick-slipand temperatureeffect in the scratching ofmaterials, Tribology Letters 12 (4) (2002) 195202.
[13] V. Jardret, P. Morel, Viscoelastic effects on the scratch resistance of polymers:relationship between mechanical properties and scratch properties at varioustemperature, Progress in Organic Coatings 48 (2003) 322331.
[14] S.J. Bull, Failure modes in scratch adhesion testing, Surface and Coatings Tech-nology 50 (1991) 2532.
[15] M. Barletta, A. Gisario, L. Lusvarghi, G. Bolelli, G. Rubino, On the combined useof scratchtests andCLA profilometry forthe characterizationof polyester pow-der coatings: influence of scratch load and speed, Applied Surface Science 254(2008) 71987214.
[16] M. Barletta, L. Lusvarghi, F. Pighetti Mantini, G. Rubino, Epoxy-based ther-mosetting powder coatings: surface appearance, scratch adhesion and wearresistance, Surface and Coatings Technology 201 (1617) (2007) 74797504.
[17] M Barletta, L. Santo, V. Tagliaferri, Surface preparation and coating of metalcoilsby using a fullyintegrated manufacturingsystem,International JournalofComputer Integrated Manufacturing 20 (5) (2007) 452464.
[18] R. Singh, D. Gilbert, J. Fitzgerald, S. Harkness, D. Lee, Science 272 (1996) 396.