adhesive bonding of hot-dipped galvanized steel: use of tof-sims for forensic analysis of failed...

SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 27, 705–715 (1999)

Adhesive Bonding of Hot-dipped GalvanizedSteel: Use of ToF-SIMS for Forensic Analysisof Failed Joints

M. F. Fitzpatrick and J. F. Watts*School of Mechanical and Materials Engineering, University of Surrey, Guildford, Surrey GU2 5XH, UK

The interfacial chemistry of environmental failure of adhesively bonded hot-dipped galvanized steel joints,fabricated using a structural epoxy in a lap configuration, has been investigated by time-of-flight (ToF) SIMS.The failed lap shear joints show areas of apparent interfacial failure but these regions are limited to thinstrips at the ends of the overlap, termed initiation zones. An initial study highlighted the importance ofsmall area surface analysis, using XPS, to demonstrate that electrochemical activity was responsible for theinitial bond degradation and the formation of the initiation zones at the ends of the overlap. Despite havingbeen employed successfully in a number of adhesion studies, XPS is unable to give the molecular level ofspecificity that is required for a full understanding of the mechanism of such an adhesively bonded system.

A method has been developed, using ToF-SIMS, for mapping the initiation zone of the adhesive joint.The images obtained support earlier evidence of electrochemical activity at the initiation zone showing thepresence of cations (Mg2Y), indicating that cathodic behaviour played a role in the formation of the initiationzone. The ToF-SIMS line scans indicate residual polymer in the initiation zone, which supports the hypothesisof a dual effect of electrochemical behaviour and ingress of water being responsible for the formation ofthe initiation zone. This suggests weakening rather than the clear separation, prior to mechanical testing,observed in the case of classical cathodic delamination. The ToF-SIMS images extend this model by showingcorresponding cation-rich and adhesive-rich areas within the initiation zone, possibly demonstrating that thedifferent processes dominate in different regions (a result of localized electrochemical activity), and enablescathode size to be estimated. Copyright 1999 John Wiley & Sons, Ltd.

KEYWORDS: ToF-SIMS; adhesive bonding; adhesion; environmental failure; hot dipped galvanized steel; epoxy structural adhesive

INTRODUCTION

One of the most important requirements of an adhe-sive joint is the retention of strength during its ser-vice life. Durability is a key issue in the widening useof adhesive bonding technology and is a problem thathas been discussed by many authors.1,2 In the automo-tive industry, adhesives are used successfully for non-structural parts (bonnet, boot, etc.), but adhesive bondinghas yet to be used for the primary structure of a steelhigh-volume-production automobile. At present, the steelindustry is concentrating on reducing weight with no sac-rifice to performance, affordability, safety, durability andcost; another benefit would be reduced fuel consumption.3

These aims have led to design changes, allowing a reduc-tion in thickness and the steady growth of materials suchas galvanized steels. These factors, however, have high-lighted potential problems, including in-service vibrationsand reduced stiffness when using discontinuous joiningtechniques such as spot welding. Adhesively bonded gal-vanized steel flanges have the potential to overcomesuch shortcomings with no weight penalty.4 Dickie and

* Correspondence to: J. F. Watts, School of Mechanical and Materi-als Engineering, University of Surrey, Guildford, Surrey GU25XH, UK.E-mail: [email protected]

Contract/grant sponsor: British Steel plc

co-workers have studied in detail the interfacial chemistryof corrosion-induced and mechanical-load-induced bonddegradation of a hot-dipped galvanized steel (HDGS)system.5,6 Dickie discovered that electrochemical activ-ity was responsible for interfacial failure regions, withanodic sites identified on the unloaded samples and bothanodic and cathodic sites identified on the loaded sam-ples. Electrochemical activity at an interfacial site in aniron substrate system has also been recognized by Davisand Watts, who reported that cathodic delamination wasresponsible for a true interfacial failure.7

This paper reports a surface analysis investigation,predominantly by time-of-flight (ToF) SIMS but instigatedusing preliminary work carried out by small-area XPS, ofthe failure mechanism of adhesively bonded hot-dippedgalvanized steel with a commercial epoxy. Lap shearjoints were exposed to a damp atmosphere for extendedperiods of time prior to mechanical testing. The failedjoints show areas of apparent interfacial failure but theseregions are limited to thin strips at the ends of the overlap;for the purposes of this study, these thin strips have beentermed initiation zones. These initiation zones seem tobe a result of environmental exposure and appear to actas initiation sites for crack propagation on mechanicaltesting, acting as ‘notch-like’ features. It is the studyof these areas of the failed surfaces that is reportedin this paper, with a view to establishing the role ofelectrochemical activity at the crevice tip and its role insubsequent joint failure.

CCC 0142–2421/99/080705–11 $17.50 Received 27 October 1998Copyright 1999 John Wiley & Sons, Ltd. Revised 8 February 1999; Accepted 12 February 1999

706 M. F. FITZPATRICK AND J. F. WATTS

The definition of the exact locus of failure and hencefull understanding of failure interfaces is vital whenconsidering the mechanism of bond failure, because it isthe chemistry at the interface that determines the adhe-sion of the system, which in turn has an effect on theperformance of the system in an aggressive environment.Surface analysis techniques such as XPS and SIMS offerthe potential of interfacial study, and have been widelyused in previous work from this laboratory in the studyof both adhesion and subsequent failure mechanisms.7 – 9

Imaging ToF-SIMS, if used in the correct manner, couldaid the study further by creating a two-dimensional visu-alization of the failure surface in a much shorter periodof time than could be achieved using line scan analysisinvolving sample positioning and tilting for each analysis.The current study highlights the ability of ToF-SIMS toprovide not only mass-selected images of diagnostic elec-trochemical ions but also to map characteristic moleculesand mass fragments from the adhesive, and sets out todevelop a rapid imaging technique for the analysis ofadhesively bonded lap joints of an HDGS/epoxy system.

EXPERIMENTAL

Materials

In this study, single lap shear specimens were fabricatedfrom 110 mmð20 mm coupons with an overlap of 10 mm.The substrate used in the study was 1.2 mm thick HDGSthat had been pretreated with a phosphate conversion coat-ing followed by a chrome rinse. A commercial epoxyadhesive (3M 7823) was used and a glue line thicknesswas set at 250µm using glass ballotini. Whilst the exactformulation of the adhesive is not revealed by the manu-facturer, the safety data sheet indicates that the adhesive isan epichlorohydrin–bisphenol A epoxy with fillers, amor-phous silica and strontium chromate(VI), presumably as acorrosion inhibitor; more detail is given in Table 1. Thejoints were aged for 12 months at 35°C in a 95% relativehumidity chamber and then tested mechanically using a1195 Instron machine with a 5 kN load cell at a cross-headspeed of 5 mm min�1.

Table 1. Composition of the 3M commercial epoxy7823 as given by the safety data sheet

CompositionComponent of adhesive (%)

Epichlorohydrin bisphenol A epoxy 40 45Resin fillers 20 25Methacrylate butadiene styrene Resin 9 11Aliphatic glycidal ether 5 10Epoxy resin ester adduct 1 10Amorphous silica 2 5Strontium chromate(VI) 2 3Phenolic resin 1 23-(4-Chlorophenyl)-1,1-dimethylurea 1 2

Surface analysis

Small-area XPS (250µm) and ToF-SIMS were usedto study the failure surfaces of the lap shear joints.Analysis by XPS was performed using a VG Scien-tific EsCALAB 220iXL instrument. The excitation sourceused was monochromatic Al K˛ radiation. ToF-SIMS wascarried out using a VG Scientific Type 23 instrumentequipped with a pulsed metal ion source, a two-stagereflectron mass spectrometer and a 30 kV gallium liquid-metal ion gun. The data system used was VGX 7000software that runs on a DEC PDP11/73 computer under amicro-RSX operating system. ToF-SIMS line scans wereconstructed from spectra acquired across the initiationzone of the respective failed surfaces. All the SIMS datawere normalized by dividing the intensity of the ion ofinterest by that of the total ion yield in the mass rangem/z D 5–400. After carefully establishing the static limitfor the adhesive system, mass-selected images were col-lected to determine if certain ions were concentrated inparticular areas, allowing their role in the mode of failureto be determined.

RESULTS AND DISCUSSION

Visual assessment of failed surfaces

The visual characteristics of the failure surfaces are shownin Plate 1 and a schematic indicating the areas analysed isgiven in Fig. 1. It can be seen that the fracture path passes

Figure 1. A schematic representation of the failure of a lap shear joint, showing the position of the ToF-SIMS line scan.

Surf. Interface Anal. 27, 705–715 (1999) Copyright 1999 John Wiley & Sons, Ltd.

Plate 1. Photograph of a typical joint used in the XPSand ToF-SIMS.

Copyright © 1999 John Wiley & Sons, Ltd Surf. Interface Anal. 27 (1999)

Plate 2. ToF-SIMS image highlighting cationrich areas, possibly indicating that localcathodic cells are set up within the initiationzone. (Map shows total counts, m/z = 40, 52and 24).

Plate 3. ToF-SIMS image showing thatcharactersitic polymer ions, also, form inconcentrated regions within the initiationzone. This may give evidence to the ingressof water playing a role in initial bonddegradation as well as electrochemicalactivity. (Map shows total counts, m/z = 88,77 + 91, 60 + 43).

Plate 4. Showing that cationic rich areas within the initiation zone do not coincide with polymer adhesiverich areas. (a) m/z = 88 on 24; (b) m/z = 77 + 91 on 24.

ADHESIVE BONDING OF HOT-DIPPED GALVANIZED STEEL 707

apparently at random through the overlap, with differentroutes being taken by the crack. With the joint shown, itcan be seen that fracture has passed from close proximityof one substrate to the other, approximately half-wayalong the overlap. Joints of this particular adhesive systemall display this characteristic strip of interfacial failure atthe ends of the overlaps. By studying the formation of thisso called ‘initiation’ zone it is hoped that an understandingof initial bond degradation may be acquired, so surfaceanalysis was concentrated in this region during the study.An HDGS/adhesive system may have the potential forextensive electrochemical activity to occur, with exposediron acting as a cathode and protective zinc acting asthe anode; the penetration of interfacial failure is not asgreat as with Davis and Watts’ true cathodic delaminationmodel experienced using pure iron substrates.7

Small area XPS

The XPS survey spectra were recorded from the loca-tions identified in Fig. 1 and are shown in Fig. 2. Table 2indicates the quantified results of the elemental surfaceanalysis from areas of interest on the failed interfaces. Thespectra for the two initiation zones show carbon at 47.4%

for the metal side and 66.2% for the polymer side. Morenotably there were cations (Mg2C and Ca2C) and anions(Cl�) present in the initiation zones but not in those adja-cent to them. The presence of these cations was alwaysassociated with the low assay of carbon, which is indica-tive of the ‘metal’ failure surface. This indicates that theyare not markers for the inorganic components of the adhe-sive, although this is probably where they have originatedfrom. It is clear that such ions have leached from theadhesive during the exposure to high levels of humidityand are subsequently deposited on the area of cathodicactivity. If they were merely indicative of a failure withinthe adhesive, such ions would always be observed whena bulk of the adhesive is examined; this is not the casefor either XPS or ToF-SIMS analysis. Zinc peaks wereobserved on both sides of the failed joint, suggesting thatdegradation of the substrate may have occurred.

The XPS analysis indicates that electrochemical activ-ity has occurred within the initiation zone, with evi-dent cathodic and anodic sites existing at the interface.The carbon level, however, is higher than one wouldexpect for pure cathodic delamination. This may sug-gest that the initiation was the product of a conjointeffect of aggressive hydroxyl ions and water ingress. Thismay explain the high carbon concentration and possibly

Figure 2. Small area XPS analysis taken from areas defined in Table 2.

Table 2. Quantification of small-area XPS analysistaken fr om the areasshown in Fig. 1

Area analysedOverlap 1 Overlap 2

Metal initiation Adjacent Polymer Adjacent metalzone polymer zone initiation zone zone

Elements (A) (B) (C) (D)

C ls 47.5 69.9 66.2 55.6O ls 33.3 23.2 22.3 33.0Zn 2p3/2 5.1 1.9 1.5 4.1N ls 4.5 3.0 8.0 4.9Si 2p 2.1 1.0 2.4P 2p 6.2 2.03Cl 2p 1.2 1.1

Copyright 1999JohnWiley & Sons,Ltd. Surf. InterfaceAnal. 27, 705–715 (1999)


leads only to weakening of the interfacial bond andnot actual separation. Small-area XPS analysis of theinitiation zones at the ends of the overlap was able todemonstrate that electrochemical activity was responsiblefor initial bond degradation. X-ray photoelectron spec-troscopy offers elemental and chemical state informa-tion, together with the ability to yield quantitative surfacechemical analyses and indications of overlayer thickness;however, it inherently lacks the specificity to provide themolecular information essential for a full understandingof the adhesive and interfacial chemistry of a polymer.ToF-SIMS, in contrast, demonstrates the potential to fulfilthis requirement with high spatial resolution via the useof a focused ion beam and a high degree of molecularsensitivity, which hopefully is capable of determining therole of the polymer in failure.

Time-of-flight SIMS analysis of adhesive and failuresurfaces

In order to achieve the intended aim of this work—the useof mass-selected ToF-SIMS images as a diagnostic tool forthe analysis of adhesive joint failure surfaces—a series ofpreliminary experiments were undertaken to ensure that allthe potential limitations encountered in static SIMS of acomplex polymer system were considered and, if possible,circumvented. A series of thin-film analyses were carriedout to identify the characteristic peaks of the virgin adhe-sive. This was followed by a damage study to determinethe static limit for the polymer system under consid-eration, which would ensure that mass-selected imagescould be obtained within the static limit, thus ensuringthat the analysis was truly representative of the polymersurface. Point analyses were used to construct line scansacross the initiation zone and to establish a protocol forthe removal of surface topography in the presentation of

mass-selected data. Once these objectives had been met,ToF-SIMS images could be acquired from the initiationzone of an aged lap shear joint.

Characterization of the adhesive.This was achieved byfirst studying the uncured adhesive. A controlled volumeof a 1%(w/w) solution of the adhesive in acetone wasdropped onto aluminium foil and the solvent was allowedto evaporate. This yielded a thin polymer film that was notsusceptible to electrostatic charging during analysis. Theresultant positive SIMS spectra are presented in Fig. 3 andthe expected ions from epoxy groups (atm/z D 91, 252and 269) and bisphenol groups (m/z D 135 and 231) areseen; the relevant fragment patterns are shown in Table 3.These observations are in agreement with previous workon commercial materials of this type.10

Whilst these ions characterize the uncured adhesive,they may not be characteristic of the cured adhesive thatwill be present at the locus of failure of an adhesivejoint. A comparison of the adhesive in its uncured andcured state was made by preparing specimens in whichthin layers of polymer were smeared onto aluminiumsubstrates; one specimen was then heated at 180°C for30 min in air to simulate cure. Spectra from the uncuredsample showed only traces of siloxane present, with peaksatm/z D 73 and 147, whereas for the cured material thesepeaks were dominant in the spectrum, as shown in Fig. 4.

This observation prompted the question of whether thenew peaks atm/z D 73 and 147 were present as aresult of polymerization of siloxanes in the formulationas adhesion promoters or due to poly(dimethyl siloxane)(PDMS) to which SIMS is particularly sensitive and oftenappears in spectra of contaminated samples. In an effortto rule out contamination during cure, the experiment wasrepeated using an exceptionally clean oven. Similar spec-tra were found for the two different curing environments,

Figure 3. Positive ToF-SIMS spectrum of a 1% (w/w) solution of Adhesive in acetone, characterizing the adhesive and showing typicalepoxy peaks at m/z 269, 252 and 191.

Surf. InterfaceAnal. 27, 705–715 (1999) Copyright 1999JohnWiley & Sons,Ltd.


Table 3. Typical characteristic positive fragment ions of the epoxy-type adhesive

m/z Fragment/ion

57 (epoxy)

191 (epoxy)

252 (epoxy)

269 (epoxy)

135 (bisphenol A)

213 (bisphenol A)

Figure 4. Positive ToF-SIMS spectrum of the cured adhesive.

indicatingthatthesourceof thesiloxanewastheadhesiveitself.

Poly(dimethylsiloxane)is aknownmould-releaseagentthat,oncuring,wouldhaveapropensityto segregateto thesurfacein an attemptto reducethe surfacefreeenergy of

the system.If this is the casewith our adhesivesystem,we may not actually encounterthe m/z D 73 and 147peaksin the final analysisof the fracturedlap shearjoint.It wasdecided,therefore,to producea simulatedfracturesurfaceto analysethe bulk polymer/adhesive.This was



done by applying a thick smear of the adhesive onto ametallic sample backing plate, usually used for mount-ing specimens. A circular cover plate was placed over theadhesive, forcing a bulb of adhesive to rise; the adhesivewas cured and a simulated fracture was created by slicingthrough the base of the adhesive bulb (Fig. 5). Analy-sis of the simulated fracture surface showed a dramaticchange in the relative intensities of the suspected PDMSpeaks, bisphenol and other characteristic adhesive peaks(m/z D 91, 77 and inorganic88SrC), as shown in Fig. 6.Comparison of these data with those from a dry joint thathad failed in a cohesive fracture surface showed goodagreement, confirming that peaks observed atm/z D 73and 147 did not play a major role in the characterizationof an adhesive fracture surface.

With the adhesive fully characterized by fingerprintspectra, the next step was to determine the static limitfor the adhesive.

Establishment of the static limit for ToF-SIMS analysis ofthe adhesive.For any ToF-SIMS study of a polymer, astatic limit for the system must be established in orderto ensure analysis of the unmodified polymer. Havingdetermined the static limit, it is important to discover ifuseful images are capable of being achieved under the saidstatic limit. Consecutive analyses were carried out at onemagnification, thus incrementally increasing the ion dose

for each analysis. This procedure was repeated in differentareas at different magnifications, enabling a static limit tobe obtained via plots of relative counts versus selectedmasses (at a constant magnification) or relative countsversus magnification (for individual selected masses).

An accurate calculation of the primary ion dose isessential so that the limitations of static conditions can beunderstood clearly. With the initial analysis complete, thetotal primary ion dose rates for the possible experimentalvariations were calculated using

Total primary ion doseD NfPIconttp ð 6.25

A.1/

whereA D analysis area (cm2), Nf D number of frames,P D pulses per pixel (256ð 256 pixels per frame),Icont D continuous primary beam current (nA) andtp Dprimary ion pulse lengthD 50 ns (1 ampD 6.25ð1018 ions s�1). In the case of the analytical conditionsused in the current work, the following values apply,directly selectable from acquisition software:A and Icont

are determined as described below;N D 50; P D 32767;and tp D 50 ns.

The primary ion current was measured using a Faradaycup positioned in the path of the beam and a KeithleyModel 485 autoranging picoammeter; the analysis areawas interpolated from unpublished work from this labo-ratory, in which the rastered analysis areas for different

Figure 5. Schematic representation showing the steps involved in fabricating a simulated fracture surface: (a) smear excess of adhesiveonto the metal backing plate; (b) place cover plate over the top, forcing a bulb of adhesive to rise; (c) after curing.

Figure 6. Positive ToF-SIMS spectrum of simulated fracture surface, showing the removal of siloxane peaks.



beam energies were determined by carrying out imagingSIMS on a precision-ruled Si wafer that was etched at dif-ferent beam energies and four selected magnifications.11

With all the data required for total primary ion dose peranalysis collated, graphs showing relative peak intensityversus total ion doses were plotted (Figs 7 and 8). All peakintensities were normalized to total ions.m/z D 5–400/.An alternative route to normalization was carried out usingthe m/z D 27 ion, representative of C2H3

C. This pro-duced good results; indeed, the profiles were the sameshape as those presented below but, because of the pos-sible presence of27AlC in the spectrum (perhaps froman alumanosilicate filler in the adhesive, or residual alu-minium on the HDGS surface), the normalization schemeusing the total ion intensity was preferred. Normalizingdata was considered necessary to compensate for the vary-ing sensitivity of different masses, due to their differentcross-sections. Characteristic masses selected for moni-toring damage were as follows: aromatics (m/z D 67, 91,115, 128); other fingerprinting organic masses (m/z D 105,135 and 141); and inorganic SrC (m/z D 88/, which waschosen for reasons that will become clear later in the dis-cussion (Table 4).

Figure 7 shows data from the damage study carried outatð500 magnification and, apart from an initial increasein all masses, which may be considered to be a resultof removal of general hydrocarbon contaminant from thesurface, no decrease in the intensity of any of the peaks isnoted. This suggests that the ‘static’ limit for the polymersystem has not been reached by at least 2ð 1013 ionscm�2. With magnification increased toð1000, Fig. 8(a)and 8(b) show a continuous decrease in the lower massaromatics (m/z D 67 and 91) after the first exposure;this is accompanied by a complementary increase ofinorganic mass 88. Such a result is surprising becauseit is normally the larger ions that would have been moresensitive to damage. In this instance the emergence of theSrC is critically associated with the loss in intensity ofm/z D 67 and 91. Often, inorganic components such asfillers or pigments are covered by a thin layer of organicsin adhesives. These organic molecules may, therefore,be more susceptible to damage than the other organicmolecules. Also, because these organic ions reduce inintensity, a matching rise in intensity would be expectedfrom the attenuated inorganic. Taking these considerationsinto account and examining the data shown in Fig. 8, it

Table 4. Characteristic positive fragments/ionsused in the damage study

m/z Fragment/ion

67

91

115

128

105

141

appearsthat thereare two ‘static’ limits for the system:onefor the lower massorganics(2ð1013 ionic cm�2) andone for the higher massorganics(4ð 1013 ions cm�2).Despitethe apparentambiguityof the definition of staticconditionsfor the adhesivesystem,both valuesgiven arewithin thegeneralstaticlimit of 1013 ionscm�2 asdefinedby Briggs,12 andimagingshouldbeachievedsuccessfullywithin the set experimentalconstraintsto allow imagingof a damage-freesurface

Line scan across the initiation zone. With the adhesivecharacterizedand the experimentalconditionsverified, aToF-SIMS line scanwascarriedout acrossthe initiationzoneof an agedjoint; Fig. 1 showsthe areasanalysed.Peaksof interestwerechosento conducta profile of peakintensity versusposition.For example,the cationic peakof Mg2C (m/z D 24) increaseddramaticallyin intensityat

Figure 7. Graph showing relative peak intensities (normalized to total ions) versus total ion dose at ð500 magnification.



Figure 8. (a) Graph showing relative peak intensity (normalized to total ions) versus total ion dose at ð1000 magnification. Note therapid decrease of intensity of medium mass peaks at m/z D 67 and 91, matched with the increase at m/z D 88. (b) Graph showingrelative peak intensity (normalized to total ions) versus total ion dose at ð1000 magnification (medium to higher mass).

the crack tip of the initiation zone.The original profileof element intensity versus relative position (Fig. 9a)displayshow the line scanwasdominatedby topography;by once again normalizing intensities to total ions, ithasbeenpossibleto resolvetopographicalissuesandseehow relative intensitiesof selectedmassesvary acrossthe initiation zone. Here, the sensitivity of SIMS washighlighted,with tracesof polymer being found for thefirst time in the initiation zone.

Theimportanceof theline scanbecomesapparentwhenanalysingthe normalizedintensities(Fig. 9b) and it iseasyto seedistinctive surfacecharacteristicsassociatedwith different areasacrossthe initiation zone and thethick polymersection.The line scan,therefore,highlightsions and fragmentsthat would be relevantin subsequentimaging. The line scan also offers a one-dimensionalview of theelectrochemicalactivity in the initiation zone,and visual assessmentof the spectraindicatesthat there

may be a concentrationof cationsat the crack tip. Thisis confirmedwhen looking at normalizedline scansofm/z D 24 and the characteristicepoxy massm/z D 135.The line scanalso reinforcesthe importanceof viewingtheinitiation zonein theform of atwo-dimensionalimage,becausethe imagesshown in the resultsdisplay cation-rich areasnot just confined to the initiation crack tip.It also reinforces the view that the Mg2C ion can beusedconfidently as a marker for cathodicactivity. Theinclusion of the 88SrC profile in Fig. 9 (SrC is indicativeof the strontiumchromatepigment) is shownto vary inconcentrationwith theepoxyfragmentatm/z D 135ratherthanthe 24MgC ion.

Onceall thepreliminarystepsfor thedevelopmentof arapid imagingtechniquefor the forensicanalysisof failedjoints have beencompletedand relevantand character-istic peakshavebeenidentified, it becomespossibleforsuitableimagesof the initiation zoneto be acquiredand



Figure 9. Positive ToF-SIMS line scan across the initiation zone as described in Fig. 1: (a) selected masses demonstrating dominanceof topography; (b) normalized line scans of m/z 24 (MgC) and m/z 135 (C9H11OC) markers for cation and adhesive, respectively bothnormalized to total ion counts.

socontributeto the understandingof the formationof theinitiation zone.

Time-of-flight SIMS images of a single lap joint initiationzone.Peaksof interestfor mappingof the initiation zonewereconsideredto be asfollows:

40Ca2C and 24Mg2C—cationic markers that seemtoplay a key role in failure;Sr—inorganicmarker;m/z D 77 and91, 105 and115—organicmarkers;m/z D 60and43—productsof aminecuringagent;and52Cr2C and64Zn2C—substratemarkers.

The imagesobtaineddisplaywell-definedcationic-richzones(Plate 2), possibly suggestingthat local cathodiccells aresetup in within the initiation zone.The imagescharacteristicof adhesivepeaks(m/z D 77 and 91 andm/z D 88) also show tracesof polymer within the initi-ation zoneto be ‘patchy’ (Plate3), indicating that wateringressmayweakenareaswithin theadhesiveandsoleadto very thin cohesivefailure throughthe polymer withinthe initiation zone.The obviousquestionof whetherthecationicMg2C patchescoincidedwith theadhesivepatcheswassimply answeredby overlayingthe two setsof part-nerimages.Plate4 confirmsthatthereareareaswithin theinitiation zonethat arecationrich andadhesivedeficient



and, conversely, adhesively rich areas that are deficient incations.

The data acquired in this manner could be improvedclearly by the use of an appropriate algorithm to correctfor sample topography. The data of Fig. 9 show that thiscan be achieved readily by the use of the total ion yieldor, indeed, a suitable non-specific ion such as27C2H3

C.This is not possible using the VG Scientific data system,although the problem has been circumvented recently onour Poshenreider ToF-SIMS by the installation of a KoreScientific TDC and data system that is PC based.

FAILURE MECHANISM: THE ROLE OFELECTROCHEMICAL ACTIVITY

As mentioned in the introduction, cathodic behaviour hasbeen found to be responsible for initial degradation inother lap shear systems. Cathodic activity was definedby the presence of cations at the interface. The idea ofcathodic markers identifying electrochemical activity wasfirst introduced by Castle and Epler in the mid-1970s.13

Cathodic delamination can be defined as the detachment ofstrongly adhering organic coatings under the influence ofa cathodic potential and has been recognized as a problemfor electrochemical active surfaces since 1936.

Consider a metal discontinuously covered with anorganic coating. The resulting system is vulnerable tocathodic delamination when subsequently exposed to ahostile atmosphere. The coated metal may behave as acathode (electron accepting) and the exposed metal as ananode (electron donating). The ensuing electrochemicalreaction produces cathodic products (specifically hydroxylproducts) under the coating that may have deleteriouseffects on the bonding of the metal to the coating andso cause the coating to ‘delaminate’. Cathodic reactionsin the presence of oxygen involve an increase in pH, eitherby the consumption of hydrogen ions or by the reductionof oxygen to hydroxyl ions

O2 C 2H2OC 4c� ) 4OH� .2/

The above explanation is widely accepted and hasbeen reported extensively by Watts in previouscommunications,14,15 but is particularly applicable toan HDGS substrate. A freshly cut edge of HDGSis a natural galvanic cell, with the zinc coatingcorroding preferentially to the protected iron under anodicdissolution

Zn) Zn2C C 2e� .3/

As a result, the cathodic reaction will take place, gen-erating hydroxyl ions due to the reduction of water andoxygen via the consumption of electrons. These aggressivehydroxyl ions are able to penetrate between the polymeradhesive and the metal substrate, thus weakening the inter-facial bond and allowing anodic dissolution to continue,as proposed by Dickieet al.6

The images obtained support earlier evidence of elec-trochemical activity at the initiation zone showing thepresence of cations.Mg2C/, indicating the role of cathodicbehaviour in the formation of the initiation zone. It shouldbe noted that this concentration of cations is ‘patchy’ andnot, as suggested by the line-scan data, concentrated at the

crack tip. A major advantage of the imaging technique isthat it descibes a two-dimensional profile and so allows anestimation of the size of the cathodic cells, which in thecase shown appears of the order of 100µm across. Thesensitivity of SIMS is demonstrated by the clear poly-mer images obtained. These patches of polymer withinthe initiation zone, along with the corresponding images,suggest that a conjoint effect seems to be responsible forthe formation of the initiation zone. The traces of polymerseem to indicate that water ingress plays a role, along withcathodic behaviour, creating the cathodic weakening effectproposed earlier rather than the pure cathodic delamina-tion model proposed by Davis and Watts.7 This theory wassupported further by the earlier small-area XPS data thatshowed 47% C 1s at the initiation zone: a level not highenough for a cohesive polymer surface but too high for apure interfacial cathodic delamination failure surface.

The mapping technique was able to take the work astep further by analysing a large area relatively quickly,demonstrating that there seems to be corresponding cation-and polymer-rich areas within the initiation zone. This isthe principal advantage of the imaging technique; pro-vided that vital preliminary work has been carried out tounderstand the spectra of the polymer, a sample with aknown zone of interest can be analysed quickly to yieldspatial information on a two-dimensional plane. This rapidimaging technique confirmed the earlier conjoint effecttheories of the formation of an initiation zone and pro-gressed the research by observing that electrochemicalbehaviour and water ingress dominate in different regions.

CONCLUSIONS

A thin strip of interfacial failure has been identified atthe ends of overlaps on a Hots/epoxy lap shear sys-tem: such strips are called initiation zones. Small-areaXPS established electrochemical activity in the initiationzone. Following this preliminary result a method has beendeveloped for mapping the initiation zone of an adhe-sive joint successfully, with images obtained supportingearlier evidence of cathodic behaviour playing a role inthe formation of the initiation zone highlighted by thepresence of cations.Mg2C/. Line scan analysis showingpolymer in the initiation zone also supports the claim ofthe previous studies of a possible dual effect of electro-chemical behaviour and ingress of water being responsiblefor the formation of the initiation zone, suggesting weak-ening rather than separation prior to mechanical testing,as observed by Davis and Watts’ cathodic delaminationmodel.7 The ToF-SIMS imaging approach is able to takethe work a step further by showing corresponding cation-rich and adhesive-rich areas within the initiation zone,showing local cathodic cells of¾100 µm across, demon-strating that the different processes dominate in differentregions.

Acknowledgements

This work is part of a project funded by British Steel plc. The authorsthank Dr John Ling (Welsh Technology Centre) and Drs Alan Seedsand Anthony Cronin (Automotive Engineering Group) for their advice,guidance and encouragement. The authors also wish to thank Dr TimCarney (VG Scientific), who carried out the small area XPS, andMr Andy Brown, who assisted with the ToF-SIMS data acquisition.

Surf. Interface Anal. 27, 705 715 (1999) Copyright 1999 John Wiley & Sons, Ltd.


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