Progress in Organic Coatings 77 (2014) 72 80

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Progress in Organic Coatings

j our nal homep age: www.elsev ier .com

Effects n sand cor oat

N.W. KhuFontana Corros

a r t i c l

Article history:Received 2 JanReceived in reAccepted 19 AAvailable onlin

Keywords:Epoxy composMWCNTAdhesionWearCorrosion

notubpos

orce mspectWCNr resiMWCt incrhich

1. Introdu

AA2xxx series alloys are widely used for structural componentswithin the spacecraft industry because they have good corrosionresistance, high toughness, good fatigue strength, good formabil-ity and recyclability. However, numerous intermetallic compounds(IMC) decreformation othe surrounIn addition,service in hsion of Al atheir corrosto aggressivto catastropprocess is aaged by screnvironmenthe tribologtion and higthe coated s

Nowadaindustrial ccomponentstrength, to

CorresponE-mail add

er msible with special additives [17,18]. Carbon nanotubes (CNTs) areconsidered to be an ideal reinforcing agent for high-strength poly-mer composites because of their ultrahigh tensile strength, highaspect ratio and high thermal and electrical conductivity [1921].It was reported [2226] that carbon llers in polymer matrices

0300-9440/$ http://dx.doi.oase the corrosion resistance of the Al alloys due to thef complex galvanic couples between different IMCs andding Al matrix during exposure to electrolytes [16].

specic environmental conditions, such as long-termumid and salty environments, may induce the corro-lloys such that polymer coatings are needed to retardion. However, organic coatings can fail upon exposuree environments via underlm corrosion, which can leadhic failure of the coatings in service [716]. The failureccelerated when organic coatings are physically dam-atches or wear, which allows access of the aggressivet to the interface. It is therefore important to improveical performance of organic coatings because low fric-h wear resistance of the coatings can effectively preventystems from wear-induced failure.ys, there is tremendous interest in the scientic andommunities to apply polymer composites for structurals because they can provide a signicant improvement inughness and chemical and wear resistance over the pure

ding author. Tel.: +1 614 688 4128.ress: frankel.10@osu.edu (G.S. Frankel).

could reduce the friction of the polymer composites via their solidlubricating effect. Recently, Hu and co-workers [27] found thatmultiwalled CNT (MWCNT) incorporation improved the corrosionresistance of lead-tin electroplated coatings. Therefore, it is pos-sible that incorporation of MWCNTs in polymer matrices couldimprove the tribological performance and corrosion resistance.There is insufcient data on the tribological and corrosion prop-erties of polymer composite coatings lled with MWCNTs [2226].The understanding of a correlation between the MWCNT incorpo-ration in the epoxy coatings and their tribological and corrosionperformance is essential for their successful application.

In this study, the epoxy coating and epoxy composite coatingslled with different MWCNT contents were prepared on AA2024-T3 substrates and the tribological and corrosion performance of thecoated samples were systematically investigated.

2. Experimental details

Epoxy resin (PD381-94) and hardener (EC-283) both fromAkzoNobel were mixed at a weight ratio of 3:1. AA2024-T3 substrates were coated with epoxy containing 0, 0.1, or0.5 wt% MWCNT, which were designated as Epo, EpoMCT0.1 andEpoMCT0.5, respectively. All the polymer coatings were cured at

see front matter 2013 Elsevier B.V. All rights reserved.rg/10.1016/j.porgcoat.2013.08.003 of carbon nanotube content on adhesiorosion resistance of epoxy composite c

n, B.C. Rincon Troconis, G.S. Frankel

ion Center, The Ohio State University, Columbus, OH 43210, United States

e i n f o

uary 2013vised form 16 July 2013ugust 2013e 14 September 2013

ite coating

a b s t r a c t

The effects of multiwalled carbon nacorrosion resistance of the epoxy comstrates were evaluated using atomic ftest and electrochemical impedance coatings improved with increasing Mtion coefcient and increased the wealubricating and rolling effects of the ite coatings. Finally, EIS indicated thadue to a decreased porosity density, wsamples.

ction polym/ locate /porg coat

trength and wearings on AA2024-T3

e (MWCNT) content on the adhesion strength and wear andite coatings prepared on aluminum alloy (AA) 2024-T3 sub-icroscopy (AFM), blister test, ball-on-disk micro-tribological

roscopy (EIS). The adhesion strength of the epoxy compositeT content. Increased MWCNT content also decreased the fric-stance of the epoxy composite coatings due to improved solidNTs and the improved load bearing capacity of the compos-eased MWCNT content increased the coating pore resistance

resulted in an increase in the total impedance of the coated

2013 Elsevier B.V. All rights reserved.

atrices. In addition, tailoring of their properties is pos-

N.W. Khun et al. / Progress in Organic Coatings 77 (2014) 72 80 73

room temperature (RT 2224 C) for about 4 weeks, which isabout twice the manufacturers specications. Prior to coating, thecommercially available AA2024-T3 panels (nominal compositionfrom the supplier: 3.84.9% Cu, 1.21.8% Mg, 0.30.9% Mn, 0.5% Fe,0.5% Si, 0.25% Zn, 0.1% Cr, 0.05% Ti) were cut into 2 cm 2 cm pieceswith and then abraded with SiC paper to 1200 grit in ethyl alcohol.The substrates were then cleaned ultrasonically in ethyl alcoholfollowed by air drying. The MWCNTs (Baytubes C 150 HP) werepurchased from Bayer Materials Science, Germany. The diameter,length, number of walls, and bulk density of the MWCNTs accord-ing to the manufacturers specications were 520 nm, 1>10 m,315, and 140230 kg m3, respectively. The MWCNTs were dis-persed in the epoxy resin by sonication for about 45 min. Thethickness of the coatings used was about 2 m for the adhesionmeasurement with the AFM scratching and about 50 m for theother experiments.

The adhesion strength of the coated samples was diagnosedusing an AFM (Veeco Instruments Dimensions 3100) in contactmode with a diamond coated Si tip (force constant, k = 42 N/m, tipradius = 20 nm) at scan angle and rate of 0 and about 2 Hz (1024line 1024 line), respectively. For the adhesion measurement, thecoatings were prepared to cover 70% of the total substrate surfaceareas so the AFM scratching could be done at the coating edge. Theuncovered areas of the substrates acted as a z-reference for indica-tion of removal of the coatings during AFM imaging. The adhesionwas assessed in terms of number of scans required at a particularset-point voltage (applied load) to completely remove the coat-ing from the substrate and coating removal rate (m/scan) [28,29].The results were conrmed by two to three measurements persample.

The adhesive strength of the coatings was also diagnosed usingthe blister test [30]. Hexagonal specimens of 5.85 cm2 of area werecut from a 1.2 mm thick AA2024-T3 plate. A hole 7 mm in diameterwas drilled in the center of each hexagonal specimen until of thetotal thickness remained. The undrilled side of each specimen was

abraded using SiC papers to 800 grit in ethyl alcohol. Random abra-sion was performed such that the nal scratches were not aligned.The samples were then cleaned in an ultrasonic bath with ethylalcohol for 20 min followed by air drying, and storage in a desicca-tor for 3 days. The coatings were applied on the specimens with adraw down bar. Prior to the blister test, a through-hole to the backof the coating was made by electrochemical etching in the center ofthe 7 mm hole. This process was performed by applying a potentialof 2.125 V to the AA2024-T3 substrate in 0.5 M NaCl solution with a0.5 mm diameter stainless steel wire as counter electrode until anaverage hole diameter of 3.1 mm was reached.

In the blister test setup, the lower side of the coated sampleswas pressurized with deionized (DI) water using a syringe pumpat a constant infusion rate of about 0.05 mL/h. The blister pres-sure was measured with a pressure transducer and recorded in acomputer with time. At the same time, the top of the blister wasmagnied at 10.5 and recorded with a CCD camera with a reso-lution of 307.2 k pixels attached to a OLYMPUS SZX12 stereoscope,generating pictures with a scale of 52 pixels/mm. The non-planarshape of the blister reected light away from the objective of thestereoscope and appeared as a black circle on the image, whichfacilitated the measurement of its radius. Delamination of the coat-ing can be characterized by the strain energy release rate, G, usingthe following equation [30]:

G =[

(Pr)4

(17.4Et)

]1/3(1)

where P is the pressure in the blister, r is the blister radius, t is thethickness of the lm and E is the lm elastic modulus. The averageYoungs modulus, maximum Pr product and coating thickness wereused for this calculation.

The fracture stress and Youngs modulus of the coatings wereevaluated using a tensile test. A 0.25 mm thick lm of the two-partepoxy mixture was cast into a Teon mold and cured under the

Fig. 1. Surface ip in a4 and (eh) 8 V morphologies of Epo sample after scratching using AFM with a diamond coated Si t for different scans. (Z scale 4.5 m, scan area: 100 m 100 m). scan size of 100 m 100 m at different set-point voltages of (ad)

74 N.W. Khun et al. / Progress in Organic Coatings 77 (2014) 72 80

Fig. 2. Surface using at a set-point v

same condiwere cut fr15). The tenas recommetensile prop

The fricta ball-on-diRT. In each and a 6 mmin a circulaunder a normeasured umeasuremebological pr

The surfastudied usinoptical microbservationa deposition

The thernitrogen (N(TGA, TA 29

The elecout in a 0.5tion (Gamrytested arearated standmesh counspectroscopthe frequenof 10 mV armade at intwere conr

ults

adhcoati morphologies of (ad) EpoMCT0.1 and (eh) EpoMCT0.5 samples after scratching oltage of 4 V for 22 scans. (Z scale 4.5 m, scan area: 100 m 100 m).

tions mentioned above. The dog-bone shaped samplesom a template using a surgical blade (BardParker No.sile tests were performed at a strain rate of 0.1 min1,nded by the ASTM D882-10 standard test method for

3. Res

Theposite erties of thin plastic sheeting.ion and wear of the samples were investigated usingsk micro-tribometer (CSM) operated in rotary mode attest, the sample was rotated at a sliding speed of 2 cm/s

diameter Cr6 steel ball was slid on each sample surfacer path of 3 mm radius for about 12 m sliding distancemal load of 1 N. The wear tracks on the samples weresing optical prolometry (Veeco Contour GT-K). Threents on each sample were conducted to get average tri-operties.ce morphologies and topographies of the samples wereg scanning electron microscopy (SEM, Quanta 200) andoscopy (OM) and optical prolometry (OP). Prior to SEM, a gold layer was deposited on the samples for 1 min at

rate of about 4.7 nm/min to avoid charging.mal stability of the coatings was examined under a2) environment using a thermal gravimetric analyzer50) from RT to 800 C at a heating rate of 10 C/min.trochemical measurements of the samples were carried

M NaCl solution using an electrochemical worksta- Reference 600) with a three-electrode cell at RT. The

on the samples was a 1 cm diameter circle. A satu-ard calomel (SCE) reference electrode and a platinumter electrode were used. Electrochemical impedancey (EIS) was performed at the open circuit potentials incy range of 105102 Hz with an AC excitation signalound the open circuit potential. Measurements wereervals from 5 to 45 h after immersion. The EIS resultsmed by two measurements per sample.

by AFM sctip [28,29,3scratched Eof 4 V (20the edge wplete removstrength ofAFM scratcat the edge indicate thasive failurewere requiAl substratthe removasmaller tharemove thecating that tof the Epo c

The incocomposite cand EpoMCface after 21.76 and

The addiproperties. ings improvthe scratchmance of thadhesion onAFM with a diamond coated Si tip in a scan size of 100 m 100 m

and discussion

esion strengths of the epoxy coating and epoxy com-ngs with different MWCNT contents were investigated

ratching in contact mode with a diamond coated Si1]. Fig. 1ad show the surface morphologies of thepo sample after different scans at a set point voltage.5 N). Progressive removal of the epoxy coating fromith increased scan number was observed, with com-al of the coating at the 22nd scan. When the adhesive

the coating is lower than its cohesive strength, thehing normally causes the delamination of the coatingvia an adhesive failure [28,29,31]. Therefore, the resultst the removal of the Epo coating was caused by adhe-

during the scratching. Fig. 1eh shows that 7 scansred to completely remove the Epo coating from thee at the higher set-point voltage of 8 V (33 N) andl rate was 10.2 m/scan, which is about three timesn the removal rate (about 3.18 m/scan) required to

same coating at the lower set-point voltage of 4 V, indi-he higher set-point voltage results in the easier removaloating due to the higher scratch load.rporation of MWCNTs makes the removal of the epoxyoating more difcult, as shown in Fig. 2. The EpoMCT0.1T0.5 samples both had material remaining on the sur-2 scans, and the average removal rates were smaller,0.77 m/scan, respectively.tion of MWCNTs can have different effects on the epoxyIncreased MWCNT content in the epoxy composite coat-es the resistance to the removal of the coatings duringing probably due to the improved tribological perfor-e epoxy composite coatings along with their promoted

the underlying Al substrates, as discussed below.

N.W. Khun et al. / Progress in Organic Coatings 77 (2014) 72 80 75

Fig. 3. Fracture stress (circles) and Youngs modulus (squares) as a function ofMWCNT concentration.

The interfacial adhesion between the coating and substrate canbe affected by the curing process because of shear stress inducedat the interface by the residual stress of the coating [3234]. Theincreased MWCNT content likely improves the adhesion strengthof the epoxy composite coatings because the incorporation of asecond component in the epoxy matrix, such as MWCNTs, canrelax the residual stress of the coating by shearing the weaklybound carbon aggregates against the matrix [35,36]. The incorpo-ration of MWCNTs can also enhance the load bearing capacity ofMWCNT reinforced composites, which greatly improves its scratch

Fig. 4. (a) BlistEpoMCT0.1 an0.05 mL/h with

Fig. 5. Mean friction coefcients of Al substrate and Epo, EpoMCT0.1 and EpoMCT0.5samples measured using ball-on-disk micro-tribological test by sliding against Cr6steel balls of 6 mm in diameter in a circular path of 3 mm in radius for about 12 min sliding distance at a sliding velocity of 2 cm/s under a normal load of 1 N.

resistance bfore proposscratch reslarger loadmore, the Mserve as a sduring the sMWCNTs csurfaces being force [3reduce the caused by ties on thebecause thethe AFM tipsurface rouings were ma magnica13.75 2.32

T co asph, th

note enlovaMWCNsurfaceFig. 2eing didthat ththe remer pressure and radius, and (b) blister pressure blister radius of Epo,d EpoMCT0.5 samples, measured at a constant infusion rate of about

DI water, as a function of time. Fig. y resisting its plastic deformation [37,38]. It is there-ed that the higher MWCNT content should increase theistance of the epoxy composite coatings through the

bearing capacity of the composite coatings. Further-WCNTs dispersed in the epoxy composite matrices canolid lubricant to reduce abrasive action of the AFM tipcratching [2226]. During contact sliding, the rolling ofan lessen the scratching of the AFM tip on the coatingcause the MWCNTs can roll and slip under lateral slid-9]. As a result, the increased MWCNT content shouldscratch-induced wear of the epoxy composite coatingsthe AFM tip. Finally, the inuence of surface asperi-

AFM scratching should be taken into consideration large surface asperities can disturb the scratching of

over the coating surface. The root mean square (Rq)ghnesses of the Epo, EpoMCT0.1 and EpoMCT0.5 coat-easured using OP in an area of 318 m by 238 m at

tion of 20 and found to be 6.13 0.53, 8.95 1.2 and m, respectively. As seen in Figs. 1 and 2, the increasedntent in the epoxy composite coatings enlarged theirerities via the increased aggregation of the MWCNTs. Ine surface morphology of the scratched EpoMCT0.5 coat-

apparently change with scan number, which indicatesarged asperities should be one of the reasons preventingl of the composite coating during the AFM scratching.6. TGA results of Epo, EpoMCT0.1 and EpoMCT0.5 samples.

76 N.W. Khun et al. / Progress in Organic Coatings 77 (2014) 72 80

Fig. 7. Surfacesliding against1 N, observed

The mecand frictiontensile, blisses proposperformed ofracture strcontent incration of M topographies and morphologies of worn (a and b) Al substrate and (c and d) Epo, (e and Cr6 steel balls of 6 mm in diameter in a circular path of 3 mm in radius for about 12 m inusing OP and SEM, respectively. The inset in (b) shows EDS spectrum of wear debris on th

hanical strength, adhesion strength, wear resistance of the epoxy composite coatings were evaluated usingter and micro-tribological tests to support the hypothe-ed above. Fig. 3 displays the results of tensile testsn the epoxy sample and epoxy composite samples. The

ess decreased by almost a factor of 2 as the MWCNTreased to 0.5 wt%, which indicates that the incorpo-WCNTs does not improve the mechanical properties.

Weak intercan degradOn the othecontent incMWCNT ag

Blister tof the epoxthe blister p f) EpoMCT0.1 and (g and h) EpoMCT0.5 samples, respectively, after sliding distance at a sliding velocity of 2 cm/s under a normal load ofe wear track of the Al substrate.

actions between the carbon nanotubes and the matrixe the cohesive strength of composite samples [40,41].r hand, the Youngs modulus increased as the MWCNTreased due to the enhanced collective behavior of thegregates [4245].ests were conducted to evaluate the adhesive strengthy and composite coatings [30,4648]. Fig. 4 illustratesressure and radius, and the product pressure radius

N.W. Khun et al. / Progress in Organic Coatings 77 (2014) 72 80 77

and (

for the diffsure increaa sharp dropa maximumsure radiucontent. That the beginsure when rate. Normareaches a cduring blistis faster thabefore the mcritical blisthe coatingincreased wadhesion sthigher MWhigher maxblister radia

The adhthe improvwith the inEpo, EpoMC19 J/m2, resup to 29% iincrease in and AA2024

The ball-of MWCNTcomposite cthe Al substdifferent Mballs for abofound that tgenerates aabrasive wecoating the

The frictball decreasthat carbonreleased duthe steel bacles can serv

s analls asite tingsed r

inu be t

frictweeient ess

crease ins in

studse thetweh ana effeed Mf theall anhile

tal/pgradereaseFig. 8. Surface morphologies of worn steel balls slid on (a) Al substrate

erent coatings as a function of time. The blister pres-sed with time until it reached a maximum, after which

occurred. The pressure radius product also exhibited value. The maximum values of pressure and pres-s both increased signicantly with increasing MWCNTe blister radius slowly increased with a constant slopening of the test until a point near the maximum pres-the radius started to increase at a considerably higherlly, the blister growth should occur when the pressureritical (maximum) value [46]. The pressure decreaseser growth owing to an increase in blister volume thatn the water infusion rate. The increase in blister radiusaximum pressure was probably associated with sub-

ter growth caused by the visco-elastic properties of [30]. The time required for rapid blister radial growthith increased MWCNT content due to the promotedrength of the composite coating. It is clear that theCNT containing epoxy composite coating exhibited aimum pressure and a longer time for the onset of rapidl growth.esion strength was calculated using Eq. (1) to conrmed adhesion strength of the epoxy composite coatingscorporation of MWCNTs. The mean Ga values of theT0.1 and EpoMCT0.5 coatings were about 12, 15, andpectively. Addition of MWCNTs to the epoxy resulted inncrease in the adhesion strength, which elucidates the

surfacesteel bcompoite coaincreas

Theshouldhighering betcoefcroughnthe dethat thcoatingin thisdecreaareas bsmoottion viincreasness osteel bthem wthe methe dethe incthe bonding established between the composite epoxy-T3.on-disk micro-tribometer was used to access the effect

incorporation on the friction and wear of the epoxyoatings. Fig. 5 presents the mean friction coefcients ofrate, epoxy coating and epoxy composite coatings withWCNT contents during sliding against 6 mm Cr6 steelut 12 m sliding distance under a normal load of 1 N. It ishe sliding of the steel ball on the rigid Al alloy substrate

high friction coefcient of about 0.49 by inducing thear of the both rubbing surfaces, which is reduced by

alloy with epoxy.ion coefcient of the epoxy coating and a sliding steeled with increasing MWCNT content. It has been shown

particles dispersed in epoxy composite coatings can bering sliding and transferred to the interfaces betweenlls and coatings [2226]. Thus, the released carbon parti-e as a solid lubricant to effectively lubricate the rubbing

to decreaseThe fric

resistance friction [49tance becau[37,38,424epoxy comcontributesreduced we

Generalball on a pothe polymecomposite of the coaticurve were10% weight[37]. The T1in the insetb) EpoMCT0.5 sample, observed using OM.

d as spacers to prevent a direct contact between thend coatings, leading to the reduced friction of the epoxycoatings. The reduced friction of the epoxy compos-

with increased MWCNT content is also related to theolling effect of the MWCNTs [39].ence of surface roughness on the friction of the coatingsaken into account because a rougher surface generatesion during sliding contact via mechanical interlock-n two mating asperities [4952]. Therefore, the frictionof the coatings should increase with increased surfaceassociated with increased MWCNT content. However,ed friction coefcient of the coatings (Fig. 5) indicatesuence of the surface roughness on the friction of the

terms of mechanical interlocking was not signicanty. On the other hand, increased surface roughness cane friction of the coatings by reducing the real contacten the steel balls and coatings. Adhesion between two

d clean surfaces in contact can give rise to high fric-ctive shear strength of the contact interface [52]. WithWCNTs in the coating, the increased surface rough-

coating reduced the interfacial adhesion between thed coating by decreasing the real contact area between

the increased amount of the transferred MWCNTs atolymer interface depressed the interfacial adhesion viad cleanness of the two contacting surfaces. Therefore,d MWCNT content in the epoxy composite coatings led

d friction of the coated samples.tion of a solid material is closely related to its wearsince a higher surface wear can result in a higher52]. A higher Youngs modulus can improve wear resis-se of the lower plastic deformation for a given load5]. It is clear that the increased Youngs modulus of the

posite coatings with increased MWCNT content (Fig. 3) to the decreased friction of the coatings (Fig. 5) via thear of the coatings.ly, frictional heat generated during the sliding of a steellymer surface can induce high friction via softening ofr [5357]. Therefore, the thermal stability of the epoxycoatings was investigated. Fig. 6 shows the TGA resultsngs with different MWCNT contents. Details of the TGA

not analyzed, but the temperature corresponding to loss (T10) was taken as an index of thermal stability0 increased with increased MWCNT content as shown

of Fig. 6. In addition, the weight loss of the coatings

78 N.W. Khun et al. / Progress in Organic Coatings 77 (2014) 72 80

decreased and the remaining weight of the coatings at about 790 Cincreased with increased MWCNT. Because the thermal conductiv-ity of MWCNTs is signicantly higher than that of the epoxy matrix,the incorporation of the MWCNTs improves the thermal stability ofthe epoxy cothe coatingMWCNTs slwhich increimproved treduce the the coatingthermal stacorrelated t

Fig. 7 shdifferent saabout 12 msubstrate gconrmed b(8.3 0.57 sive marks wthe Al substby the slidiTribo-layerdebris undethe wear trmation andwear of theon the weawas signicthe wear trasomewhat hinterface cavia third bo

As showwear so themeasurableand d. The rbecame lesDuring the sfrictional hesurface plascomposite decreased paddition, thcoatings detact area bethe softenincating and rheat duringulus of the during the track was sas shown irelated to thsurface plascoating. Thecoated samis closely reincreased Mof the epox

The worments werethe steel babing is evidcoating, shoMWCNT did

yquist plots of (a) Epo, (b) EpoMCT0.1 and (c) EpoMCT0.5 samples measured NaCl solution using EIS.

s transferred materials, as. No observation of wear wased after cleaning with acetone and ethyl alcohol. The trans-materials isolated the contact between the rubbing surfacesnsequently lowered their wear. At the same time, the lubri-and rolling effects of the MWCNTs also help to prevent thef the steel ball. The results clearly conrm that the incorpora-

MWCNTs reduces the wear of not only the epoxy composites but also the counter surfaces.

experimental results have clearly revealed that the higherT containing epoxy composite coatings have better stiff-d tribological performance along with the higher adhesionmposite coatings by facilitating heat dissipation withins [5862]. Besides, the high thermal conductivity of theowed the degradation of molecular chains around them,ased the decomposition temperature [5862]. Since thehermal stability of the epoxy composite coatings canfriction of the coatings by preventing the softening ofs via the dissipation of the frictional heat, the increasedbility of the epoxy composite coatings (Fig. 6) can beo their decreased friction coefcient (Fig. 5).ows the surface topographies and morphologies of themples after sliding against the 6 mm steel balls for

sliding distance. The sliding of the steel ball on the Alenerated signicant wear of the substrate, which wasy the measurable wear width (684 41 m) and depthm) of the Al substrate, as shown in Fig. 7a. The abra-ith severe plastic ow were found on the wear track of

rate (Fig. 7b), resulting from the abrasive wear causedng of the harder steel ball on the softer Al substrate.s formed by the agglomeration and compaction of wearr the repeated sliding of the steel ball were found onack of the Al substrate [54]. It is supposed that the for-

detachment of the tribo-layers also contributed to the Al substrate [54]. The EDS spectrum of the wear debrisr track (inset of Fig. 7b) indicated an oxygen peak thatantly larger than that found on the surface away fromck (not shown). Since the oxygen-rich wear debris arearder, the irregular shaped wear debris at the Al/steel

n generate the high friction and wear of the Al substratedy abrasive wear [63].n in Fig. 7c and d, the Epo coating exhibited very low

wear width and depth of the Epo coating were not. However, the remaining wear scar is evident in Fig. 7cemaining wear scars on the epoxy composite coatingss evident with increased MWCNT content in Fig. 7eh.liding, the softening of the epoxy coating caused by theating signicantly suppressed the abrasive wear via thetic ow. The increased thermal stability of the epoxycoatings with increased MWCNT content apparentlylastic ow via the reduced softening of the coatings. Ine increased surface roughness of the epoxy compositecreased the frictional heating by reducing the real con-tween the rubbing surfaces, which in turn depressedg of the coatings. Moreover, the enhanced solid lubri-olling effects of the MWCNTs also lowered the frictional

the sliding. Furthermore, the improved Youngs mod-epoxy composite coatings reduced surface plastic owsliding. Therefore, the severe plastic ow on the wearignicantly reduced by the increased MWCNT contentn Fig. 7d, f and h. Such reduced plastic ow can bee decreased friction of the coatings (Fig. 5) because thetic ow promotes contact between the steel ball and

similar friction and wear results of the uncoated andples conrm that the frictional behavior of the sampleslated to their wear resistance. It can be deduced that theWCNT content improves the tribological performance

y composite coatings.n surfaces of the steel balls used in the sliding experi-

studied using optical microscopy. Signicant wear ofll slid on the Al substrate from the metal-on-metal rub-ent in Fig. 8a. The steel ball slid on the EpoMCT0.5wn in Fig. 8b, and on the other coatings containing

not have any signicant wear on its surface and only

Fig. 9. Nin 0.5 M

exhibitobservferred and cocating wear otion ofcoating

TheMWCNness an

N.W. Khun et al. / Progress in Organic Coatings 77 (2014) 72 80 79

Fig. 10. (a) Po nce (Rand Epo, EpoM ction for electroche

strength. HMWCNT coepoxy comp

The corrcoatings ovtigated usinFig. 9a, the plete semicbulk properlow frequenat the Al subthe semicirsion can becoating becthe electrollying Al subthe diameteEpo sampleincorporatiite coating the epoxy mthe epoxy ctent in thediameter ofentire immMWCNT cocomposite c

The EIS dcuit model Fig. 10. Rs isresistance are resistance (Rpo), (b) constant phase element (CPE1) and (c) charge transfer resistaCT0.1 and EpoMCT0.5 samples, measured in 0.5 M NaCl solution using EIS, as a fun

mical reactions on the coated samples.owever, it is still necessary to investigate the effect ofntent on the corrosion protective performance of theosite coatings.osion protective performance of the epoxy compositeer their Al substrates in a 0.5 M NaCl solution was inves-g EIS and the Nyquist plots are presented in Fig. 9. InNyquist plot of the Epo sample contains a nearly com-ircle in the high frequency range corresponding to theties of the coating and an incomplete semicircle in thecy range corresponding to the double-layer propertiesstrate/electrolyte interface. The decreased diameter of

cle in the high frequency range with increased immer- related to the increased ionic conduction inside theause the prolonged immersion allowed permeation ofyte through porosity in the coating to reach the under-strate. The EpoMCT0.1 sample exhibited less change inr of the semicircle in the high frequency range than the

as shown in Fig. 9b. This indicates that 0.1 wt% MWCNTon improved the pore resistance of the epoxy compos-possibly because the 3D dispersion of the MWCNTs inatrix probably reduced the ionic conduction paths of

omposite coating. The further increase in MWCNT con- EpoMCT0.5 sample resulted in a further increase in

the semicircle in the high frequency range during theersion as shown in Fig. 9c, revealing that the increasedntent further improved the pore resistance of the epoxyoating.ata in Fig. 9 were tted to the proposed equivalent cir-in the inset of Fig. 10a and the results are presented in

the solution resistance, Rpo and CPE1 represent the porend constant phase element of the coating, respectively,

and Rct andphase elemtively. In thmore accurtrochemicaand epoxy a function oslowly withdropped at for the restpermeationimmersion EpoMCT0.1in the Rp ocmatrix lesscoating exhthe EpoMCsion (Fig. 1composite

An increally relatedincreased Cas shown ielectrolyte ite coatingsthe epoxy ccontent, whcoatings asings. A corrof the coatiinuenced dl) and (d) total impedance magnitude (|Z|) at 0.01 Hz of Al substrate,of immersion time. The inset in (a) show an equavalent circuit model CPE2 are the charge transfer resistance and constantent at the Al substrate/electrolyte interface, respec-is study, CPEs are used instead of capacitors to provideate tting of the non-ideal characteristics of the elec-l interface. Fig. 10a shows the Rpo of the epoxy coatingcomposite coatings with different MWCNT contents asf immersion time. The Rpo of the Epo coating decreased

increased immersion from 5 to 30 h and then sharplyabout 35 h, becoming almost stable at about 1100 cm2

of the immersion period. This indicates that increased of the electrolyte into the Epo coating with prolongeddecreased the Rpo of the coating. Although the Rpo of the

coating decreased with immersion time, no sharp dropcurred. The 3D dispersion of the MWCNTs in the epoxyened the amount of through-porosity. The EpoMCT0.5ibited a small drop at about 25 h. However, the Rpo ofT0.5 coating were the largest throughout the immer-0a), indicating that the pore resistance of the epoxycoatings increased with increased MWCNT content.ase in the capacitance of an organic coating is gener-

to water uptake in the coating [64,65]. Therefore, thePE1 of the epoxy coating with increased immersion,n Fig. 10b, indicates the increased permeation of theinto the coating. The CPE1 of all the epoxy compos-

increased during immersion. In addition, the CEP1 ofomposite coatings decreased with increasing MWCNTich can be related to the reduced water uptake in the

sociated with the improved pore resistance of the coat-elation between the Rpo (Fig. 10a) and CPE1 (Fig. 10b)ngs clearly indicates that the CPE1 of the coatings wasby their pore density.

80 N.W. Khun et al. / Progress in Organic Coatings 77 (2014) 72 80

As shown in Fig. 10c, Rct of the epoxy composite coatings waslarger than that of the epoxy coating during the entire immersionperiod probably due to the lower permeation of the electrolyteinto the polymer/metal interface although a signicant differencein the Rct with MWCNT content was not found. This suggests thatthe incorporation of MWCNTs improves the corrosion protectiveperformance of the coatings over their underlying substrates inthe 0.5 M NaCl solution since the larger Rct corresponds to a loweranodic dissolution of the underlying Al substrate.

The total impedance, assessed by the magnitude at the lowestmeasured frequency of 0.01 Hz, |Z|0.01, is a measure of the pro-tectiveness of the coating. The larger |Z|0.01 of the epoxy coatedsample relative to the Al substrate conrms that the epoxy coatingeffectively in the 0.5 Mcoated samcoating. Homuch highetent improvcomposite c

4. Conclus

In this scomposite T3 substratremoval ofscratching the adhesivof MWCNTadhesion stblister teststent. The trwere charawear of theof the 6 mmtent probabMWCNTs ancoatings. Thposite coatiby the imprsured usingNaCl solutiotent increasbecause thethe amountcomposite ctent as the in the compthat the hignot only thecorrosion re

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Effects of carbon nanotube content on adhesion strength and wear and corrosion resistance of epoxy composite coatings on A...1 Introduction2 Experimental details3 Results and discussion4 ConclusionsReferences