preadhesion laser surface treatment of carbon fiber reinforced peek composite

J. Adhesion, 1995, Vol. 55, pp. 77-97 Reprints available directly from the publisher Photocopying permitted by license only

© 1995 OPA (Overseas Publishers Association) Amsterdam B.V. Published in The Netherlands

under license by Gordon and Breach Science Publishers SA

Printed in Malaysia

Preadhesion Laser Surface Treatment of Carbon Fiber Reinforced PEEK Connposite*

M. ROTEL and J. ZAHAVI

Israel Institute of Metals, Technion, Israel Institute of Technology, Haifa 32000, Israel

A. BUCHMAN and H. DODIUK**

Materials and Processes Department, RAFAEL, P.O. Box 2250, Haifa 31021, Israel

(Received July 28.1994: in final form March 14.1995)

An excimer UV laser (193 nm) was used for preadhesion surface treatment of PEEK (polyetheretherketone) composite. This method presented an allemative to other l imited and pollut ing conventional surface treatment methods. Experimental results indicated that laser preadhesion treatment significantly improved the shear and tensile adhesion strength of structural epoxy F M 300 2K bonded PEEK composite adherends compared with untreated and SiC blasted substrates. Best results were obtained with laser energies of 0.18 or IJ/Pcm.^ Shear strength of laser-treated joints was improved by 450% compared wi th that of untreated PEEK composite and by 200% compared wi th SiC-blasled pretreatment at ambient and at extreme temperatures. An order of magnitude of improvement was found in the tensile strength-of laser-treated PEEK composite in a sandwich structure compared wi th non-treated or abraded sandwich joints. The mode of failure changed froni adhesive to cohesive as the number of pulses or laser energy increased during treatment. The latter phenomenon was correlated wi th surface cleaning as revealed by XPS, wi th morphology changes as revealed by scanning electron microscopy, and by chemical modification as indicated by FTIR and XPS. The bulk of the PEEK composite adherend was not damaged by the laser irradiat ion during treatment as indicated by the identical flexural strength before and after laser treatment. I t can be concluded that the excimer laser has a potential as a precise, clean and simple preadhesion surface treatment for PEEK composite.

K E Y WORDS Excimer laser, PEEK composite; surface treatment; FTIR; XPS; adhesion

INTRODUCTION

Fiber-reinforced thermoplast ic composi te mater ials are increasingly being considered

for a wide var iety o f appl icat ions. Adhesives are often requi red for j o i n i n g these

mater ia ls for s t ruc tu ra l and other purposes. I n order to achieve a s t rong and durable

adhesive bond there is a need to establish an effective surface p rebond ing t reatment and

to develop a scientific understanding o f the mechanism o f surface mod i f i ca t ion and its

effect on adhesion.

Thermoplast ic composi te mater ials such as ca rbon reinforced polyetheretherketone

( P E E K ) are used for engineering appl icat ions where toughness, fatigue, impact and

* One ofa Collection of papers honoring James P. Wightman, who received the 13th Adhesive and Sealant Council Award at the ASC's 1993 Fall Convention in St. Louis, Missouri, USA, in October 1993.

** Corresponding author

77

78 M . ROTEL et al.

abrasion resistance and therma l stabi l i ty^ are needed. I n aerospace manufacture,

especially for w i n g and ta i l components, i n electrical and electronic au tomot i ve ,

bearing, medical,^ nuclear, and chemical industr ies this composi te is found . I n a l l o f

these appl icat ions adhesive bond ing is the preferred me thod of j o i n i n g .

Surface energies of thermoplast ic a romat ic composites tend to be low, causing

di f f icu l ty i n we t t ing o f the surface by an adhesive. Conven t iona l surface t reatments such

as sand blast ing, mechanical abrasion, chemical etching or swel l ing, as wel l as fusion o r

we ld ing j o i n i n g methods,^ may cause de lamina t ion defects and damage the fibers and

the bu l k composite. These methods also present occupat iona l heal th and safety and

env i ronmenta l r isks.

C o r o n a and plasma surface t reatment do not damage the po lymer bu t require

special env i ronments for the act ion. Thus, as each o f these treatments have l im i ta t i ons ,

an al ternat ive t reatment should be prov ided.

U V lasers may offer an add i t i ona l surface t reatment for po lymer composi te

adherends.** Lasers have been used for cleaning po lymer surfaces, for m ic ro fabr i ca t ing

o f polymers by ablat ion,^ precise cu t t i ng , etc. M o s t organic mater ials adsorb U V

rad ia t i on , creat ing photochemica l reactions such as scissioning, b ranch ing and cross-

l i n k i n g o n the surface of the po lymer on l y several molecular layers deep (300-500 A),

w i t h o u t damag ing the b u l k polymer. The effect of laser i r r a d i a t i o n on P E E K is

especially interest ing since P E E K is a model po lymer w i t h o u t any a l iphat ic groups.

In te rac t ion of a laser beam w i t h mat ter gives rise to m u l t i p h o t o n exc i ta t ion o f the

po lymer ic bonds, wh i ch is then fo l lowed by therma l decompos i t ion resul t ing in

ab la t ion . F o r ab la t ion to take place there exists a threshold laser energy density beyond

wh i ch irreversible damage of the sample surface occurs.* I n the case of P E E K

composi te, no signif icant p r o d u c t i o n o f gaseous species occurs below 0.1J/P. A t higher

fluences, aboveO.2 J/P, f ragmentat ion and volat i le species were detected. Above 0.4 J/P,

a complete convers ion of the oxygen i n the P E E K po lymer to C O occurred, o r i g i na t i ng

f rom the ca rbony l and the ether l inkages.

F r o m va r ia t i on of etch rate w i t h fluence an effective threshold for the composi te

P E E K was determined wh i ch is 0.42 J /Pcm^ . Below this value removal of the ma t r i x i n

the surface region occurs and over l ong exposure weak etching o f fibers takes place.

Once the ma t r i x is removed, the bare fibers are immune to etching at l ow fluences. Near

the threshold for ab la t ion , cone-l ike structures appear. These structures p robab ly

result f r om the semicrystal l ine character of the po lymer , since the crystal l ines have

different U V absorpt ions and etch more s lowly than the amorphous s u r r o u n d i n g

mater ia l .

Above the ab la t ion threshold the composi te fibers are observed to etch smooth ly ,

and the crystal l i tes also ablate w i t h the bu l k mater ia l and the etched region becomes

smooth . Mic ropar t i c les and debris redeposit on to the surface leaving a dust -hke

texture. W h e n the ab la t ion threshold was great ly exceeded, fibers were th inned and

buckled.

The ma in effects caused by the laser i r r ad ia t i on , such as surface chemical modi f ica

t i o n (oxygen deplet ion), morpho log ica l a l te ra t ion and f o rma t i on of cone-l ike struc

tures,''^ ^ were s imi la r to those found in previous invest igat ions on polyether imide,

polycarbonate,^- a l u m i n i u m a l l o y a n d a l u m i n i u m oxide. '^ The U V laser etch ing

was, thus, used as a preadhesion surface t reatment w i t h the advantages of chemical

PREADHESION LASER SURFACE T R E A T M E N T 79

and morpho log i ca l mod i f i ca t ion and cleaning o f the po lymer surface w i t h m i n i m a l

fiber damage.

I n the present invest igat ion, the app l i ca t ion of an excimer U V laser for preadhesion

surface t reatment of P E E K composi te adherend has been studied. The objective o f the

research was to establish the effect of the excimer laser on the surface st ructure o f the

composi te and to correlate the mic ros t ruc tu re w i t h the macrobehavior , as reflected i n

shear and tensile load ing and fai lure loca t ion o f bonded j o i n t s and sandwich structures

bonded w i t h a s t ruc tu ra l epoxy adhesive.

EXPERIMENTAL

Adherends and Adhesives

Compos i te plates 100 x 100 x 1.6 m m were prepared f r o m commerc ia l P E E K re in

forced w i t h un id i rec t iona l carbon fibers (APC-2/AS-4), a prepreg p roduc t of I C I L t d . ,

USA. The plates were produced f rom 14 prepreg layers i n a un id i rec t iona l sequence

(0/0)^ la id u p and consol idated under pressure o f 110 Psi (0.76 M P a ) at 385°C. Var ious

degrees o f crystaf l izat ion were achieved by coohng the p roduc t at different rates (1,7

and 33°C/min).

B o n d i n g of treated and nont reated P E E K composi te adherends was carr ied ou t w i t h

a s t ruc tu ra l epoxy adhesive F M 300 2 K, a p roduc t o f Amer ican Cyanamid , USA. The

adhesive was po lymer ized at 120°C under a pressure o f 35 Psi (0.24 M P a ) for 1.5 hrs. N o

pr imer was used.

Surface Treatment

A l l samples were w iped clean w i t h acetone before t reatment. T w o references were used:

a non-t reated adherend and a SiC (36 mesh) abraded adherend for a l l the experiments

conducted and described. Laser t reatment was appl ied using a U V A r F (193 nm)

excimer laser model 201 M S C , a p roduc t of L a m b d a Physik, Germany, w i t h the

fo l l ow ing parameters: repet i t ion rate of 5 Hz , in tens i ty o f 0.18-6.0 J /Pcm^ , number

o f pulses rang ing between 1 to 500 P, and a beam area between 2 x 0.5 cm^ and

0.3 X 0.1 cm^ accord ing to the fluence needed. The area o f the beam was changed using

a focusing lens. A l l experiments were conducted at ambient temperature and i n a i r

env i ronment . The parameters tested are summar ized i n Table I . The specimens were

moved under the beam by means o f a con t ro l led X - Y table.

TABLE I Laser parameters for APC2/AS4 reinforced PEEK composite samples

Laser energy J/P-cm^ Repetition rate Hz Pulse No. Beam area cm^

ao9 a i 8

5 5

10,52,100 1,5,10

53,106.502 1.10,102 1,10,101

2 x 0 . 5 2 x 0 . 5

LOO 6.00

5 5

0.8 X 0.25 0.3 X 0.1

80 M . ROTEL et al.

Testing

O p t i m a l laser parameters were chosen using several techniques by wh ich t reated and

untreated samples were compared:

a. The morpho logy o f the surface after laser t reatment was observed using a

Scanning E lec t ron Microscope (SEM) model JSM-840, Jeol, Japan.

b. The chemical changes i n the surface were studied i n an external specular reflec

t i o n mode using a Four ie r T rans fo rm In f ra Red ( F T I R ) Nico le t 5 D X spec

t ropho tomete r equipped w i t h a hor i zon ta l stage in a near - to -normal incidence

and a gold-coated m i r r o r as a reference.

c. Chemica l depth prof i le and surface compos i t i on were analysed using X-Ray

Photoe lect ron Spectroscopy (XPS), w i t h a model PH I555 spectrometer w i t h an

A l Koc X- ray source at 10 K V , 40 m A and 3 x 1 0 " ^ to r r .

d. Adhesive j o i n t propert ies were determined using single lap shear j o i n t s (SLS)

accord ing to A S T M D-1002-72. The SLS specimens were tested i n an I n s t r o n

machine, model 1185, at a rate of 2mm/min at - 30°C, R T and 120°C to fai lure.

The mode o f fai lure was determined visual ly to be either adhesive ( in lerfacial ,

100% coverage of the adherends) or cohesive ( 2 0 0 % coverage o f the adherends,

b o t h adherends covered), or mixed.

The effect o f laser t reatment on the po lymer ma t r i x is most ly ablat ive pho todecom-

pos i t ion .^ ' As most organic mater ials exh ib i t very h igh absorp t i on i n the U V range,

most of the energy is absorbed in a t h i n surface layer (0.1 — 0.5 \x) wh ich is the on l y par t

o f the mater ia l that is affected o r changed.^** I n order to prove this assumpt ion and t o

measure the effect o f laser i r r a d i a t i o n on the b u l k propert ies o f the laminate, flexural

tests were conducted on treated and untreated samples.

I n a flexural test, tension occurs on one side and compression o n the other side o f the

sample. The tensional side is very sensitive to changes on the surface. A simple beam

2.2 X 21.7 X 100 m m (span 50 m m ) was tested accord ing to A S T M D-790 (3-po in t

flexural test). The beam was loaded at a rate of 2 mm/m in u n t i l fai lure. The side loaded

by tension was laser treated at var ious cond i t ions and compared w i t h non-t reated o r

SiC-abrasion-treated specimens. Stress, s t ra in and flexural modu lus were calculated

and the mode of fai lure was determined visual ly.

P E E K is a par t ly -crysta l l ine mater ia l w i t h an e q u i h b r i u m concent ra t ion o f 4 8 %

crystal l ine phase. The rma l h is tory of the P E E K processing affects its degree o f

c rys ta l l in i ty as wel l as its mechanical and morpho log i ca l propert ies. The efficiency o f

laser t reatment may also be affected by this factor. I n order to determine the o p t i m a l

c rys ta l l in i ty for achieving the best effect of laser t reatment , several P E E K composites

w i t h different crystal l ine concentrat ions were laser treated, bonded and tested.

Var ious crystal l ine concentrat ions were achieved by app ly ing different coo l ing rates

d u r i n g laminate processing (1 , 7 and 33°C/min). The var ious laminates were bonded

(SLS jo in ts ) w i t h F M 300 2 K after laser or abrasion t reatment and compared w i t h

non-treated ones. The level of c rys ta l l in i ty was determined by Phi l ips M o d e l PN-2000

X- ray apparatus equipped w i t h a C u l a m p at the range of 15-35 degrees.

After o p t i m a l laser and crystalhne cond i t ions were determined and no damage was

revealed to the composi te adherends, a sandwich s t ructure typ ica l for aerospace


appl icat ions was prepared. The st ructure conta ined t w o P E E K composi te skins

37 X 37 X 2 m m and a Nomex^ honeycomb. The skins were treated w i t h the o p t i m a l

laser cond i t ions (compared w i t h abraded and non-t reated ones) and bonded to the

honeycomb w i t h F M 300 2K .

The whole st ructure was tensile loaded to fai lure in an I ns t ron machine at a rate o f 2

mm/min . The tensile strength was calculated and the mode o f fai lure was visual ly

assessed.

RESULTS AND DISCUSSION

Surface Morphology

S E M micrographs of the nontreated and SiC-abraded P E E K composi te adherend are

shown i n F igure l a and b. The surface of the abraded adherend is marked ly damaged,

cracked and the exposed fibers are b roken .

S E M micrographs of the P E E K composi te adherend after U V laser t reatment at

different cond i t ions are presented in Figures I c - f A t lower energies (less than the

ab la t ion t h r e s h o l d * - 4 2 0 m J / P c m ^ ) and at a low number of pulses, etching o f the

surface is observed w i t h shal low cracks. A t a higher number o f pulses, rounded granules

are formed on the surface ^ 2\i i n diameter. These granules g row in size (2 -6 |a) as the

number o f pulses increases. The fibers close to the surface are being exposed bu t seem

undamaged.

The phenomenon of the granules formed on the surface as a result of laser t reatment

is typ ica l to var ious other materials. ' ' '^- A t higher laser energies (above threshold)

the phenomena at the surface are to ta l l y different. A t a low number of pulses the etching

of the surface is homogeneous w i t h o u t granules. A t a higher number o f pulses the fibers

are exposed and fine powder collects on the surface, p robab ly due to condensat ion of

ablated m a t e r i a l . ' *

A t very h igh energy (6J/Pcm^), deformed fibers are exposed and the ma t r i x between

them is ablated (Fig. 10- I t can also be seen that below threshold the effect o f h igh energy

w i t h a low number o f pulses is equivalent to low energy w i t h a h igh number of pulses.

The f o rma t i on of granules extending f rom the treated surface due to laser t reatment

s igni f icant ly enlarges the surface area and cont r ibutes to better mechanical in te r lock

ing of the adhesive to the adherend.

FTIR

Signif icant chemical changes i n the P E E K composi te surface were detected by

F T I R after laser t reatment. Typ ica l F T I R spectra o f the surface of laser-treated samples

at the same energy (0.18 J/P cm^) but at different number o f pulses are presented i n

Figure 2.

The chemical mechanism at low laser energies is different f rom that at h igh energies.

A t l ow energies, C = 0 groups are formed (1653 and 1161 c m " ' ) , ma in cha in bonds

( C — O — C ) break ( 1 2 5 0 c m " ' ) , C H end groups are formed (685 c m " ') and new

crossl inkings ( 0 - 0 ) appear at 951 and 1111 cm " ' . A t higher energies, the chemical

82 M . ROTEL et al.

changes are less p ronounced. N o C = 0 groups are formed, on ly new </> - H groups and

</) — 0 cross l ink ing can be observed.

A t 6 J / P c m ^ a vast ab la t ion , ca rbon iza t ion o f the surface and a to ta l decrease i n a l l

the absorpt ions o f the organic groups occurs.

F IGURE 1 SEM micrographs of PEEK composite surface after treatment at various parameters, (a) non-treated, (b) SiC-abraded. (c) laser-treated: 0.I8J/P cm^ SOP, (d) 0.18J/P c m ^ lOOP, (e) l J / P c m \0 6J/P c m ^ 10 P


c

F IGURE 1 iContinueiH

XPS

XPS surface spectra and surface a tomic concent ra t ion of laser-treated and untreated

composi te P E E K specimens completed the i n fo rma t i on gained f rom S E M and F T I R

results. The or ig ina l C j^ spectrum o f P E E K ' ^ is composed of a m a i n peak o f ary l ic C at

84 M . ROTEL et al.

e

F I G U R E I (Continued)

285.0 ev (C - H , C - C), and smaller C peaks at 286.5 ev ( C - O , etheric), 287.6ev (C = O,

carbony l ) and 291.6ev ( C = C, phenyl). The O s p e c t r u m o f P E E K is composed of 2

peaks at 533.6ev (C - O) and 532.2ev (C = O) and 532.7 (O - C = O)'** appeared after

extended ox ida t ion .


/ REFERENCE , 8 0 m J / P c m 2 / 5 PULSES /

W A V E N U M B E R { C M " ' )

F I G U R E 2 FTIR spectra of PEEK composite surface laser treated at 0.18 J/P at various no. of pulses.

The results of the laser-treated samples compared w i t h the o r ig ina l P E E K spectrum

are shown i n Figures 3a and b and i n Table I I .

After laser t reatment w i t h low energy (0.18 J/100 P) the C^^ spectrum shows that the

etheric peak (286.4 ev) decreased, the ketonic peak increased and new groups appeared

(O - C - C - CH3, 285.8 ev and O - C - O, 290.0 ev).'^ A t higher energy on ly m i n o r

O O

chemical changes were indicated (Fig. 3a).

The oxygen peaks after laser t reatment show increase of the etheric peak and

decrease of the ca rbony l peak. The to ta l intensi ty o f the oxygen peaks was reduced by

ha l f (Fig. 3b).

The to ta l ra t io o f O/C decreases, p robab ly due to new crossUnking bonds {(p — <p) as

ind icated by F T I R . The same phenomenon (increase of C/O rat io) was also observed i n

References 7 and 8 due to bond break ing and evo lu t ion o f C O derivatives. A s imi la r

mechanism o f c ross l ink ing was also observed as a result of P E E K bombardmen t w i t h

an electron beam.^*' Table I I summarizes the XPS results.

The surface XPS spectra (Fig. 4) clearly show a cleaning process imposed by the laser

i r r ad i a t i on . Var ious contaminants such as M g and Si wh i ch were present at the

non-t reated APC-2/AS-4 surface are absent after t reatment at 0.18 J/100 P. Th is

phenomenon is typ ica l o f the laser t reatments o f a l l the mater ials tested.^ " ' ^

86 M. ROTEL et al.

^' 4000. c

" ;oon.

Is

NO TREATMENT

iPi sp t is

B i n d i n g E n i r y i . ! ,' f',•

•tooo.

::ooo_

0.18 J/P-cm' loop

6000_( m 5p f i s

IJ/P-cm^ lOP

e i n J - r t i E n e r q . , ,/

^ sec .

•-• 7 CO.

j - I f a i-iii 0 l i

!

J

// A .

\

•\

0.18 J/P-cm' lOOP

1

—

:.' e

? 1 • : • r 5 i l s i : 2 T = i . ^ . w

•_ : :•

- :. IJ/P-cm'

L , k : . 1 / lOP

y — --^ -

5 3 8 SSr ? 1 ' I ' l ' r o I * r I ' :

5 ^ 5 1 s i : 5;- c ^ < : . - _ < • 5 ^

F IGURE 3 Comparison of XPS spectra of C „ (a) and 0 „ (b) of PEEK composite non-treated and laser-treated.


TABLE I I XPS results for laser treated and non-treated APC-2/AS-4 reinforced PEEK composite

Treatment None 0.18J/100P IJ/lOP

Peaks C i ,

c — o — c c = o C = C(<I>) o - c o - c

+ +(284.7 ev) + {286.4 ev) + (289. lev) + (291.3ev)

+ +(284.5 ev) -(287.4ev) + (290.0ev) + (291.4ev) + (285.8 ev)

+ +(284.6 ev) + (286. lev) + (287.8 ev) + (291.0ev)

Peaks O i , C — o — C c = o o — c = o

+ +(535.3 ev)

" ++ (531 .7 ev)

+ (533.3ev) + +(532.0ev)

+ +(533.3 ev)

+ (531.7ev)

Ratio O/C 0.197 0.05 0.091

+ + Strong peak + Weak peak — Poor peak

Shear Strength and Failure Mode

Based on the S E M , F T I R and XPS results, laser parameters were chosen for t rea t ing

the P E E K composi te samples. A wide range o f cond i t ions was tested at R T and the

o p t i m a l ones were tested at extreme temperatures. The results of SLS strength of

APC-2/AS-4 j o i n t s bonded w i t h F M 3 0 0 2 K at var ious laser parameters and tested at

different temperatures are summar ized i n Table I I I , compared w i t h those for untreated

and abraded adherends.

I t is evident that U V laser i r r ad ia t i on is effective as a preadhesion t reatment on the

APC-2/AS-4 reinforced P E E K adherends at al l the chosen cond i t ions tested. There are

two o p t i m a l laser cond i t ions for pretreatment (0.18J/100P and IJ / lOP) . A t higher

energies and greater number o f pulses the effectiveness of the t reatment is reduced,

p robab ly due to ab la t ion and carbon iza t ion of the surface.

A t the o p t i m a l laser t reatment condi t ions, the single lap shear strength is increased

by 2 5 0 % compared w i t h SiC abrasion t reatment and by 4 5 0 % compared w i t h

non-treated adherend.

Fu r the rmore , SLS strengths o f laser-treated adherends d i d not s igni f icant ly deter io

rate at lower test temperatures whi le a signif icant decrease was observed for n o n -

treated ( - 3 7 % ) and abraded ( - 2 3 % ) j o i n t s at the same test ing temperatures. A t

higher temperatures, this effect is less pronounced due to the therma l propert ies of the

adhesive.

V isua l inspections of the fai lure surfaces shows clearly tha t laser t reatment caused a

d is t inc t improvement o f fai lure mode f r o m adhesive (interfacial) i n non-laser treated

adherend to most ly cohesive (mixed) fo l l ow ing laser t reatment at al l temperatures

tested. Th is indicates tha t the interfacial adhesion was signi f icant ly improved .

S E M micrographs of the f ractured adhesive surfaces exh ib i t a smooth adhesive

fai lure i n the non-t reated adherend (Fig. 5a) and a mixed fai lure i n the SiC-abraded and

88 M . ROTEL et al.

LU

UNTREATED

Mg

J i .

180 m J / P , l O O P

0

ATOMIC CONC %

C 661 0 2 8 9 Mg 2 8 SI 2 2

ATOMIC CONC %

C 92 7

0 7 3

J L

I J / P lOP

J I I I L

ATOMIC CONC %

C 7 4 6 0 21.5 SI 3 9

S. Si

5 4 0 4 2 0 3 0 0 180 6 0

B I N D I N G E N E R G Y , ( e V )

F I G U R E 4 XPS spectra of surface atomic concentration of PEEK composite nontreated and laser-treated.

laser-treated adherends (Fig. 5b, c). A t higher energy, the fai lure is most ly cohesive

(Fig. 5d). F igure 5e and f reveal the effectiveness o f the fine mo rpho logy formed on

the adherend due to laser t reatment wh i ch causes in te r l ock ing o f the adhesive on the

granules formed on the adherend. These granules are t o r n ou t d u r i n g fai lure and are

visible on b o t h sides of the failed surfaces. They are not visible at higher laser energies.

Flexural Test

The f lexural fracture st rength (a,,), f racture s t ra in (e ,), and modu lus (E) data determined

for P E E K composi te samples subjected to different surface treatments are presented i n

Tab le IV .

N o change i n the mechanical propert ies of the adherend due to surface t reatment is

observed; the values are a l l w i t h i n the tolerance of the reference (untreated) results.


T A B L E I I I The effect of laser pretreatment of APC-2/AS-4 reinforced PEEK composite adherends on SLS strength at

various temperatures

Sample Laser Treatment Lap Shear Strength (MPa)

-WC RT + 120°C

Untreated _ 3.8+0.3 (a)* 6.0 ± 0.6 (a) 3.8 + 0.4 (a) SiC Abraded - 11.3±0.S(a) 14.7 ±1.3 (m) 12.3+0.6 (a) Laser treated 0.1J/P•cm^ 50P - 26.6 + 0.7 (m) -O. lJ /Pcm^ lOOp - 27.0 ± 0.6 (m) -0.18J/Pcm\P - 24.S + 3.6 (m) -0.t8J/P•cm^ lOOP 23.2 ± 1.1 (m) 27.2 ± 0.3 (m) 21.4 ± Km)

1J/P•cm^ 10 P 2S.4±0.S(m) 27.8 + 0.6{m) 21.2± 1.8(m) 1J/P•cm^ SOP - 22.4 +4.3 (cc) -

•a-adhesive failure m-mixed failure (adhesive/cohesive in adhesive) c-cohesive failure (in adhesive) cc-cohesive failure (in adherend)

These results prove that m ic ros t ruc tu ra l changes resul t ing f rom laser t reatment occur

on ly i n the t h i n outer layers (few molecular layers o f the specimen)^ ^ and do not affect

the ent ire b u l k of the adherend. S E M micrographs of the fracture showed dis t inct

s t ruc tu ra l differences between the tensile and the compression loaded sides of the

specimen and the fracture was s imi lar for treated and untreated specimens.

Effect of Crystallinity

The X-ray spectrum of P E E K composi te is composed o f three con t r i bu t i ons : amor

phous phase, Q , crystal l ine phase, Q , and carbon fibers, Q^.

The crystal l ine phase has 4 m a i n peaks at 26 = 18.9°, 20.9°, 23", 29^ The amorphous

phase has a b road peak at 26 = 19-20° and the carbon fibers have a peak at 26 = 25"}^

C o m p a r i n g the X- ray spectra o f the P E E K composi te consol idated at var ious coo l ing

rates (Fig. 6a) shows that at slower rates (1 °C/min) the area o f the crystal l ine peaks was

the largest when compared w i t h the amorphous peak, meaning tha t the degree o f

crys ta l l in i ty increases w i t h reducing coo l ing rate. The degree of c rys ta l l in i ty was

calculated and is presented i n Table V.

Laser t reatment causes shi f t ing o f the crystal l ine peaks to lower 26, p robab ly due to a

change i n the crystal l ine st ructure caused by the laser i r r a d i a t i o n (Fig. 6b). C o m p a r i n g

the lower energy laser t reatment (0.I8J/100P) w i t h the higher energy laser t reatment

( l J /10P)on the X- ray spectra indicates tha t at higher energies the crystal l ine peaks are

sharper and larger than the carbon fiber peaks (a l though more fibers were exposed at

this energy). Increasing laser energy causes crysta l l iza t ion at higher temperatures

leading to more perfect crystals. '^ Th is is another p roo f tha t at h igh energies the laser

effect is most ly the rma l .

90 M . ROTEL et al.

The effect o f degree of crystalHzat ion of the adherend on the effectiveness o f the laser

t reatment was tested mechanical ly by SLS j o i n t s w i t h F M 300 2 K s t ruc tu ra l adhesive

(Table V).

The results show tha t for a l l the degrees of c rys ta l l in i ty laser t reatment marked ly

improves the lapshear strength compared w i t h untreated or abrasion-t reated


c

d

F I G U R E 5 (Continued)

adherends. The degree of crystalHzat ion has no effect on the SiC-abraded adherend as

the adhesion mechanism for this t reatment consists of mechanical i n te r lock ing , wh i ch

is no t affected by crysta l l izat ion.

The lower-energy laser t reatment imposes a photochemica l mechanism wh i ch

chemical ly modif ies the adherend's surface and is, thus, affected by the degree o f

crystaUini ty. The lowest coo l ing rate shows the highest lap shear slrength.^^ The

high-energy laser t reatment, the effect of wh ich is most ly therma l , vast ly affects the

92 M. ROTEL ef al.

. F I G U R E S {Continued) . ,r<,

mat r i x c rys ta l l in i ty , compared w i t h the low-energy t reatment , possibly increasing the

degree of c rys ta l l in i t y due to anneal ing wh ich results in higher lapshear strength t han

for the low-energy laser t reatment. I n b o t h cases, the lowest coo l ing rate shows the

highest strength as ind icated in Table V. These results suppor t the F T I R , XPS, S E M


TABLE IV Effect of surface treatment on flexural properties of APC-2/AS-4 reinforced PEEK composite

Treatment fT,(MPa) E(GPa)

None 1.55 + 0.04 1.94 + 0.03 121.4 + 4.2 SiC 1.52+0.05 1.88+0.08 120.3 + 3.0 0.1J/50P 1.48 + 0.06 2.05 + 0.14 133.8+4.7 0.18 J/50 P 1.40 + 0.1 1.91+0.1 130.9+0.5 0.18J/100P 1.52 + 0.06 1.97 + 0.2 130.0 + 2.2 I.OJ/lOP 1.55+0.02 2.07 + 0.06 130.2 + 2.6 i.0J/50P 1.54 + 0.04 2.06 + 0.07 " ' ^ 130.6 + 3.9

TABLE V Single lap shear strength (MPa) of joints bonded with F M 300 2K adhesive using APC-2/AS4 reinforced

PEEK composite adherends wi th different degrees of crystallinity

Cooling rate °C/min Laser treatment SiC No treatment (Crystallinity) (%) 0.18J/P•cm^ lOOP 1J/P•cm^ 10 P

1 (32.7) 29.5+1.3 (m)* 35.5 + 0.5(m) 14.3 ± 1.2 (m) 4.6 ± 1.5(a) 7 (30.8) 27.2 +0.8 (m) 27.8+0.7 (m) 14.7 +2 .0 (m) 6.1 +0.8 (a)

33(21.2) 26.4 + 4.0 (m) 30.3+ 1.6(m) 14.8 + 0.5 (m) 5.3 + 0.1 (a)

*a-adhesive failure m-mixed failure

and mechanical behavior results, i nd ica t ing t w o different mechanisms: below the

ab la t ion c r i t i ca l po in t where the mechanism is most ly chemical ab la t ion , and above the

c r i t i ca l po in t where the mechanism is a c o m b i n a t i o n of chemical and therma l ab la t ion .

S E M fractographs (F ig. 7) of laser-treated adherends after SLS fai lure indicate tha t the

inter facia l par t of the mixed fai lure was actual ly pa r t l y cohesive i n the adherend, and

tha t the P E E K m a t r i x was t o r n ou t f rom between the ca rbon fibers, i nd i ca t i ng excellent

adherence.

SANDWICH STRUCTURE ^

The skins of the s t ructure were treated using t w o o p t i m a l laser parameters (IJ/P/lOP

and 0.18 J/IOOP) and compared w i t h non-t reated and abrasive SiC-treated skins. The

tensile results o f the skins bonded to N o m e x ^ honeycomb w i t h F M 300 2 K adhesive

are summar ized i n Tab le V L

The results show tha t laser t reatment is preferable for the composi te sandwich

structure. The tensile strength is one order of magn i tude higher for the treated

compared w i t h the non-t reated adherend. The fai lure shifts f rom adhesive i n the

non-t reated P E E K , to d iv ided adhesive for the SiC-abraded, and to cohesive i n the

honeycomb and in the adhesive in the laser-treated ones (Fig. 8).

94 100

A P C - 2 / A S - 4

R E F

I 00

0 64

0 36

0 16

0 0 4

35 0

A P C - 2 / A S - 4

NO TREATMENT^^ ^ r ^ C / m i n

_ 0 18 J/P lOOP

1 J/P lOP ^

1 1 1 I I I I I , 1 15.0 2 0 0 25 0 30 0

(b) 35.0

2 G

F IGURE 6 X-ray diffraction of PEEK composite, (a) non-treated PEEK composite at various cooling rates during consolidation; (b) the effect of laser treatment at cooling rate of T^C/min.

T A B L E V I Tensile strength of composite sandwich structure

Treatment (T,<MPa) Fracture mode

None 4.9 + 0 Adhesive from skin SiC 49.5+0.2 Adhesively divided from honeycomb and skin IJ/lOP 55.6 ±6.4 ] cohesive in adhesive and

0.18J/100P 60.7 + 5.5 J in honeycomb


F I G U R E 7 SEM fractograph of PEEK composite bonded wi th F M 300 2K (21.2% crystaIIinity)-cohesive failure.

F I G U R E 8 Failure mode after tensile loading of a sandwich structure for various treatments of the PEEK composite skin. See Color Plate I .

CONCLUSIONS ^

The use of an A r F excimer laser as a preadhesion surface t reatment for a composi te

thermoplast ic substrate ( P E E K ma t r i x reinforced w i t h AS-4 carbon fibers) has been

investigated. S t ruc tu ra l epoxy bonded single lap j o i n t s w i t h laser-treated adherends

96 M.KOTEL etal. i

had signi f icant ly higher lap shear st rength (450% and 250%) t h a n j o i n t s w i t h untreated

or abras ion- treated adherends (respectively).

Scanning electron microscopy revealed morpho log ica l changes i nc l ud ing increase i n

surface roughness and pa r t i a l exposure of carbon fibers. F T I R and XPS spectroscopy

indicated chemical changes o f the surface fo l l ow ing laser t reatment , i nc lud ing increase

i n ca rbony l groups as wel l as f o r m a t i o n o f new crosshnking bonds and removal o f

con tam ina t i on (Si, M g ) f rom the surface. Us ing o p t i m a l laser t reatment parameters

resulted i n a cohesive o r mixed mode of fai lure i n ambient and extreme test tempera

tures.

S E M , F T I R and XPS analysis as wel l as a crys ta lhn i ty effect ind icated the presence o f

two different mechanisms at higher o r lower laser energy treatments. A t l o w energy

(0.18 J/P) the effect was most ly photochemica l and morpho log ica l . A t h igh energy

(1 J/P) the effect was most ly the rma l . Thus, on ly the h igh energy t reatment was affected

by the degree o f c rys ta l l in i t y o f the adherend.

The b u l k of the adherend was not damaged by the laser t reatment as indicated by the

short beam flexural test of treated compared w i t h non - or abras ion- treated specimens.

The c o m b i n a t i o n of increased surface roughness, surface cleaning and chemical

mod i f i ca t ion by the laser t reatment of P E E K composi te adherends is responsible for

the h igh strength of these jo in t s , best i l lus t ra ted in the s t ruc tu ra l sandwich j o i n t .

I t can be concluded f r o m the present s tudy tha t the A r F excimer laser is an effective,

clean and accurate me thod for surface preadhesion t reatment o f P E E K composi te

compared w i t h convent iona l abras ion methods. Th is conf i rms the po ten t ia l of using

U V lasers for preadhesion surface t reatment of many other substrates as was demon

strated i n ou r previous researchers.^" '^

D u r a b i l i t y tests are being conducted and their results w i l l be publ ished i n Par t I I of

this research.

Acknowledgements

The authors wish to thank Dr. Y. Gefen for supporting this research and Mr . S. Ashkenasy for his technical assistance.

References

1. RAPRA Review Reports 2 (2) 1986 {Report 16). 2. J. C. Roberts. J. A. Ecker and P. J. Biermann. NTIS Rep. No. N93-25569/3/XAD (1993). 3. D. M. Maguire,S/lMP£ J . ,25 ( l ) , 11 (1989). 4. J. R. J. Wingfield, Int. J. Adhesion and Adhesives, 13 (3), 151(1993). 5. H. M . Masuhara and H. Fukumura, Poiym. News, 17, 5 (1991). 6. P E . Dyer, S.T. Lau, G. A. Oldershaw and D. SchudeU. Mater. Res., 7 (5), 1152(1992). 7. E. Occhiello, F. Garbassi and V. Malatesta, Die Angewandte Makromolekulare Chemie, 169 (2780), 143

(1988). 8. P. E. Dyer. G. A. Oldeshaw and D. Schudel, Appl. Phys.. B51. 314 (1990). 9. E. Wiirzberg, A. Buchman, E. Zylberstein, Y. Holdengraber and H. Dodiuk. Int. J. Adhesion and

Adhesives, 10 (4), 254 (1990). 10. A. Buchman, H. Dodiuk, M. Rotel and J. Zahavi, Int. J. Adhesion and Adhesives, I I (3), 144 (1991). 11. H. Dodiuk. A. Buchman. S. Kenig, M. Rotel. J. Zahavi and T. J. Reinhart. J . Adhesion, 41,93 (1993). 12. Z. Gendler, M. Rotel. A. Buchman, A. Rosen, J. Zahavi, M . Bramberger and H. Dodiuk, J. Mater. Sci.

29,1521(1994). 13. P. Davis, C. Courty, N. Xanthopoulos and H. J. Mathieu, J. Mater. Sci. Utters, 10, 335 (1991).


14. Z. KoIIia and E. Hontzopulos, SPIE, 1503, 215, Excimer lasers applications I I I (1991). 15. S.J. Fang and R. Salovey, Po/>'m. Eng. and Sci, 29 1241 (1989). 16. R. Srinivasan, J. Appl. Phys., 73 (6), 2743 (1993). 17. T. Sugama, N. R. Carciello and M. Muira, Int. J. Adhesion and Adhesives, 12(1), 27 (1992). 18. D. J. Pawson, A. P. Ameen, R. D. Short, P. Denison and F. R. Jones, Surf. Interface Anal. 18, 13 (1992). 19. G. Beamson and D. Briggs. "H igh Resolution XPS of Organic Polymers," in The Scienta ESCA300

Database (John Wiley & Sons, U.K.. 1992). 20. A. B. Strong, S. R. Blad, G. R. Bryce and D. D, Olcott, SAMPE Quarterly, 22,45 (1991). 21. S. L. Kaplan, R. W. Rose, Int. J. Adhesion and Adhesives, I I (2). 109 (1991). 22. D. E. Sphar and J. M . Schultz, Polymer Composites, 11 (4), 201 (1990). 23. F. M . Sonnenschien and C. M . Ronald, Appl. Phys. Utters, 57 (5), 425 (1990).

T H E J O U R N A L OF A D H E S I O N , Volume 55, Numbers 1-2.

C O L O R PLATE 1. See M . Rotel et al, Figure 8.

preadhesion laser surface treatment of carbon fiber reinforced peek composite

Documents