Scratch and Wear Resistance of Transparent Topcoats on Carbon Laminates 1

8/3/2019 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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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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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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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.


11/11


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.

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Documents

Scratch and Wear Resistance of Transparent Topcoats on Carbon Laminates 1