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Oxide treatments of Al 1100 for adhesive bonding-surface characterization
538
Applications
of Surface
OXIDE TREATMENTS OF Al 1100 FOR ADHESIVE SURFACE CHARACTERIZATION A.E. YANIV,
N. FIN, H. DODIUK
14 May 1984; accepted
for publication
BONDING
-
and I.E. KLEIN
Materials und Processes Depcrrtmen~, M~nrs~ry oJ DeJmce. Armwneni P.O. Box 2250, H&i 31021, lsrael Received
Science 20 (1985) 538-548 North-Holland. Amsterdam
27 November
Drr~eloptnmt Aurhorrrc,
1984
A study was made to characterize by spectroscopic methods the surface of oxides formed on Al 1100 by chromate conversion coating, chromic acid anodi& and sulfuric acid anodizing. Adhesive bond strength to silicone rubber was determined using single lap joint specimens. and correlation between the microscopic structure and macroscopic values was found. It was observed that the layer formed in chromate conversion coating was essentially hydrated chromia. and it yielded the lowest bond strength. Sulfuric acid anodizing produced hydrated alumina, incorporating sulfate ions in its matrix; an intermediate bond strength was found with this pretreatment. Chromic acid anodizmg gave rise to a compact, almost anhydrous alumina film, and the highest bond strength was attained.
1. Introduction Various pretreatments of metallic surfaces are usually carried out prior to application of adhesive, mainly to enhance the bond strength, durability, and to improve its corrosion resistance. Understanding why a certain pretreatment is more compatible with a given adhesive system than another depends on the physical and chemical character of the adherent surface. Information concerning the chemical composition of the surface is provided mainly by Auger Electron Spectroscopy (AES), Secondary Ion Mass Spectroscopy (SIMS), Ion Scattering Specroscopy (ISS), and Electron Spectroscopy for Chemical Analysis (ESCA). The nature of the chemical bonding at the surface layers is clarified by IR Spectroscopy (and ESCA to a limited extent). Complete characterization combines a number of the above techniques. A number of articles have been published during the last decade on the study of aluminum surfaces after various conventional pretreatments. Wyatt et al. [l] utilized ESCA to study very thin layers of polymer on aluminum. Baun and coworkers [2,3] tested aluminum surfaces prior to adhesive bonding by means of AES, ISS and SIMS; they found that absorption of water or the presence of Mg or Cu impair the durability of the joint. Sun et al. [4] characterized the nature of the surface of Al 2024-T3 after FPL treatment 03’78-5963/85/$03.30 Q Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
A.E. Yank et al. / Oxrde treatments
of Al 1100
539
using AES techniques; the presence of Mg and Cu had an adverse effect on its adhesion properties. A similar finding was obtained by Kinloch and Smart [8,9]. The mechanism of growth of the anodic film formed in phosphoric acid was followed [5] by AES and SEM. Changes introduced in surface pretreatments of aluminum after exposure to humidity were studied [6] by ESCA; aluminum oxide (cellular structure) was converted to aluminum hydroxide (cornflake structure), and thus the bond strength decreased. FPL was least susceptible to humidity because of an outer protective phosphate layer. Chromate conversion coatings on aluminum were analyzed by a number of researchers. Stupian and coworkers [10,11] found in AES and ESCA that the layer on Al 1100 is composed of hydrated chromium oxide on top of a spine1 type aluminum chromium oxide (which should be thick enough to provide protection against corrosion), The presence of Cr+6 and OH- or HF in the chromate solution was mandatory to ensure the integrity of the conversion coating. Venables and coworkers [12] used STEM and ESCA to study adhesive joints on chromate-treated aluminum surfaces. They found that roughening the surface improved the durability of the coating probably due to mechanical interlocking. Following our previous studies [13,14], the present work deals with a microscopic chemical characterization of aluminum surfaces prior to the application of adhesives [15]. Al 1100 was chosen as a substrate because of its low content of alloying elements. Five pretreatments which are commonly used as corrosion resistant coatings were studied: chromate conversion coating, chromic acid anodizing (with or whithout sealing), and sulfuric acid anodizing (with or without sealing). Silicone adhesives used mainly in the electronic industry were applied in order to correlate the microscopic properties of the surfaces to the macroscopic bond strength. Spectroscopic techniques used were AES, ESCA and IR.
2. Experimental 2. I. Materials
and procedures
(i) Al 1100; commercially pure aluminum 1 mm thick, of nominal composition (99% Al, 1% Si + Fe) was used throughout this work. Five different surface treatments were used prior to adhesive application: (1) Commercial chromate conversion coating, using commercial solution No. 720 manufactured by Chemotas under license of Metallgeselschaft, in accordance with MIL-C-5541. (2) Chromic acid anodizing without dichromate sealing. (3) Chromic acid anodizing with dichromate sealing *. (4) Sulfuric acid anodizing without water sealing.
540
A.E.
Yank
el al. / Oxtde treatments
ofAl
1100
(5) Sulfuric acid anodizing with water sealing *. Before the application of adhesive, the substrates were vapor degreased in a TP-35 solution (manufactured by DuPont) for 5-10 min. (ii) RTV 108 (one component RTV silicone rubber) manufactured by General Electric, USA. (iii) Sylgard 186 (two components silicone rubber) manufactured by DowCorning, USA, which is normally applied over a primer DC-1200. RTV 108 was applied by brushing, and polymerized for 6 days at RT. DC-1200 primer was applied by dipping to a uniform coating and allowed to dry for 1 h at RT. Syl 186 was applied by brushing and cured for 4 h at 65°C. The bond line thickness was 0.2 t_ 0.1 mm. Single Lap Joints were prepared in accordance to ASTM D-1002-72 [17]. Peel and wedge test will be dealt with in one of our next papers. No deformation of the aluminum samples was observed. 2.2. Analyticul
methods
AES spectra were obtained in a Physical Electronics Industries Auger-SIMS Spectrometer Model 545A, operated at 3 keV, 0.5 PA. Before analysis, the outermost layer (approximately 100 nm) was removed with an argon sputtering gun to remove carbon impurities in the adsorbed layer. IR spectra were run in a Beckman IR 4250 reflectance spectrometer, equipped with a specular reflectance attachment (angle of incidence 20”). ESCA spectra were obtained in a Physical Electronics Industries model 550 spectrometer using a Mg anode at 10 keV and 40 mA. The bonding strength was meaured in a 5 ton Instron (bridgehead speed of 0.2 cm/min) at 25°C. The mode of tailure - whether adhesive or cohesive ~ was evaluated by visual inspection.
3. Results Typical AES curves are given in figs. 1 and 2, and typical ESCA spectra in figs. 3-5. The AES-ESCA results are compiled in table 1. It should be noted that during sputtering the non-conductive surfaces became electrostatically charged, causing an energy shift in the ESCA peaks. The difference between the experimental peak of carbon 1s and the literature-recorded value of 284.6 eV served to correct the energy data for oxygen, aluminum, chromium and sulfur at each depth. Fig. 3 is a chromium 2p ESCA absorption peak for chromate conversion coating. It can be seen in table 1 that the concentration of Al increases and Cr and 0 decreased with penetrating depth into the film. * Both processes tively.
were carried
out in accordance
with MIL-B-8625
1161, types I and II, respec-
Fig, 1. AES spectrum
dE
ofAl
400
600
300
200
100
A!!
A. E. Yanio er al. / Oxrde treatments
of outermost
’ Al
45-75ev
00
496-513
oCr
516-529eV
500
layer of chromic
conversion
I 100
541
700
600
E(eV)
coating.
CV
___,-.-x-n
Al
Sputtering tme ( nil” 100H/min
Fig. 2. AES depth profile
604
of chromate
592 BINDING
conversion
580 ENERGY,
Fig. 3. ESCA peak of chromium
coating.
566 EV
(2~) for chromate
conversion
coating.
)
Al Cr
0 Al Cr
Chromic acid; anodizing; unsealed
Chromic acid; anodizing; sealed
0
Al S
530.6 74.1 576.6
531.6 74.6 517.6
531.6 74.6 169.6
0
Sulfuric acid; anodizing; sealed
69.2 26.6 4.2
70.4 28.7 0.9
76.8 22.1 0.5
531.6 74.6 576.6
531.6 74.6 576.6
531.1 74.1 169.1 161.6
531.6 74.6 169.6 162.6
531.1 74.1 169.1
0 Al S
Sulfuric acid; anodizing; unsealed
65.1 33.7 1.1
511
Al Cr
Chromate; conversion coating
65.5 32.2 2.3
70.8 2X.3 0.9
68.7 30.5 0.8
64.0 34.5 1.5
18.2 28.1
(eV) 530.5
(at%) 62.5
(eV) 531
0 118 576
(at%) 53.7
Energy
4.2 33.3
Cont.
100 nm Cont.
Element
10 nm
Treatment Energy
Table 1 ESCA results for Al 1100 oxide coatings of peaks
530.6-531.6 eV AI,O, (531.5)+Cr,O, 74.6 eV Al 2OJ (74-74.5) 575.66576.6 eV Cr,O, (576.5)
531.6 eV AI,O, (531.5) 74.6 eV AI,O, (74474.5) 576.5-577.6 eV Cr,Ol (576.5)+CrO,
530.6-531.6 eV Al,O, (531.5) 74.1-74.6 eV Al *O, (74-74.5) 169.1-169.6 eV SO:- (168.5-169) 161.6 eV S in Al *O, matrix
531.1-532.1 eV AI,O, (531.5) 74.1-74.6 eV Al >O, (74-74.5) 169.1-169.6 eV SOi- (168.5-169) 161.6-162.6 eV S in AI,O, matrix
530.55531 eV AI,O, (531,5)+Cr,O, 118-119 eV oxidized 576-577 eV Cr,O, (576.5)
Identificatton
(530.5)
(578)
(530.5)
h -1 Y
A. E. Yaniu et al. / Oxide treatments
180 BINDING
168 ENERGY
o/Al 1100
543
156 (eV
)
Fig. 4. ESCA peak of sulfur (2~) for unsealed sulfuric acid anodic film.
Figs. 4 and 5 present sulfur 2p ESCA peaks for unsealed and sealed sulfuric acid anodic films, respectively, at various depths. In both cases sulfur appears as sulphate on the surface, but an additional structure is revealed with deeper penetration. The concentration of sulfur increases with depth (up to 200 nm). Table 1 also shows the results for chromic acid anodic films. Only one ESCA peak for chromium was found, and its concentration was higher for sealed anodizing (performed in dichromate solution), than for unsealed.
180
168 BINDING ENERGY
156
( eV 1
Fig. 5. ESCA peak of sulfur (2~) for sealed sulfuric acid anodic film.
Table 2 IR absorption Surface treatment
of oxide coatings IR absorption AIO-H stretch
on Al 1100
(cm-‘)
+ H *O AI=O. H ,O stretch
AI=0 stretch
Chromic acid; 3450 anodizing; (weak) unsealed
1650 (broad)
to 1450 (weak
Chromic acid; 3450 (medium) anodizing; sealed
1630 (medium)
> 1450 (weak)
Conversion coating
3300 to 3500
_
Sulfuric acid; anodizing; unsealed
3000 to 3600 (strong)
1650 (medium)
Sulfuric acid; anodizing; sealed
3000 to 3600 (strong)
1630 (strong)
Al=0 + S + SO: - AI-0 * H. AI-O-AI stretch bend, stretch
_
930
i 750
950 (broad)
to750
_
_
> 1550 (weak)
1350 (broad)
to
920 (strong)
> 1500 (medium)
1350 (broad)
to
920 (strong)
3.1. IR The main absorptions for the various surface treatments on Al 1100 are summarized in table 2. The identification of the various absorptions was based on Dorsey’s results [18]. In the chromate conversion there is no absorption above 700 cm ‘, except for absorbed water. The main IR absorptions of the anodized Al are the absorbed and free water, Al = 0, Al = 0. H,O, barrier layer, Al-O --) H and Al-O-Al stretches. In the sulfuric anodized specimens, SOispecies and Al = 0 4 S absorption appears at 1130-1180 and 1325 cm--‘, respectively. 3.2. Tensile shear test The results of the tensile shear strength of the two rubbers are given in fig. 6. The results are an average of 5 specimens for each combination of surface treatments and adhesive with and without primer. The highest value of lap shear strength and partially cohesive failure was found for RTV 108 on chromic anodized Al. The same pattern appears for SYL 186 without primer. The addition of DC-1200 primer prior to application of SYL 186 changes the trend with some advantage to sulfuric anodizing treatment. Adhesive type failure occurred at the primer-adhesive interfase. In all cases, chromate conversion treatment
545
A. E. Yank et al. / Oxide treatments of AI I100 Unprimed
Syi
II36
=
Primed
Syl
166
w
RTV
106 Max.
Av
hlin
:anvers~on :oating
‘chromic ac anodizing sealed
‘chromic ac anodizing unsealed
’ sulfuric
ac anodizing unsealed
’ sulfuric
1
ac
anadtzing sealed
Fig. 6. Schematic representation of lap shear strength results (SYL 186 without primer; adhesive failure; SYL 186 with primer; adhesive failure at the primer-adhesive interfase. RTV 108; adhesive failure except for the sealed and unsealed chromic acid anodic coating, which is cohesive).
resulted in the lowest lap shear strength the aluminum-polymer interface.
and exhibited
an adhesive
failure
at
4. Discussion 4.1. The structure and composition
of the conversion coating
It can be seen from Auger and ESCA depth profiles that the amount of Al’- increases with the depth of penetration into the coating which is a few tens of rim’s thick. The concentrations of oxygen and chromium diminish with depth, reaching a steady value with an O/Cr ratio of 1.8 indicating a Cr,O, structure (O/Cr = 1.5). This is in accordance with the work of Katzman et al. [lo], who found that the coating is composed of a layer of chromium oxide, superimposed on a CrAl spine1 layer. Our ESCA analysis began after sputtering to 10 nm to remove carbon impurities and only Cr3+ was established.
546
A.E. Yrrniv et crl. / Oxide treatments
of Al 1 It30
IR spectra taken to 700 cm-’ have revealed absorbed H,O, but no Cr oxide could be detected in this region, since their absorption occurs at lower frequencies [19]. The amount of absorbed water increases with the increase of the thickness of the deposited coating, the color of which darkens with an increase in thickness. 4.2. Unsealed and sealed sulfuric ucid anodizing It is now well established that the anodic coating is basically aluminum oxide containing free or absorbed water and incorporating some anions from the acid used. The coating is composed of two layers: a barrier layer of approximately 30 nm, and a porous layer of lo-25 pm. According to Dorsey [18], the Al = 0 bond is dominant for the barrier layer, whereas in the porous layer Al-O-Al appears. Although the authors assume that these bonds are relevant to the present work, it could not be proved owing to the fact that the IR absorption in the 900-1300 cm-’ is too broad. The presence of sulfur anions in the anodic coating is well established, but in the present work two separate species were found in IR and ESCA spectra. On the surface the SO:- species appears, and a second species, Al = 0 --, S, is incorporated in the deeper layer of the oxide lattice [18]. The concentration of sulfur anions increases with the depth, suggesting a mechanism of outward movement of Al cations and an inward movement of sulfur anions. The water sealed coating is similar in composition to the unsealed film, but due to the added water some new species are formed from the oxide-water interaction. This can be concluded from the noticeable asymmetry of the ESCA peaks. The IR spectra show that water addition causes an increase in a disordered structure. The absorption intensity related to water increases, and a shift of absorption peak occurs as a result of stretching of Al = 0. H,O to lower frequencies due to the change of the H,O-A120, spatial structure. Similar to the sealed coating in the unsealed film, sulfur anions appear as two species, but its overall concentration is lower in the latter case. This is mainly due to the larger amount of absorbed water, but is most likely also affected by some sulfur extraction during sealing. 4.3. Unsealed and sealed chromic acid anodizing The coating obtained from this solution is more compact than that from sulfuric acid. The film contains only a small amount of absorbed water, as shown by symmetrical peaks in ESCA and low intensity of water in the IR spectra. The aluminum oxide incorporates a small amount of chromium (approximately 1%) as Cr,O, and/or Cr-Al spine1 and some CrO, in the unsealed film.
A. E. Yaniu
et al. /
Oxrde
treatments ofAl 1100
In the sealed film a cornflake-like layer is formed which, in accordance reported results [6], is brittle and unsuitable for adhesion bonding. 4.4. Suitability
547
with
of surface treatment for bonding
Silicone rubbers were chosen since they are known to be “weak” adhesives and adhesive failure occurs mainly in the boundary, thus allowing the study of pretreatment/adhesive interface. In the case of RTV 108, the highest bond strength was obtained, as expected, with the chromic acid anodizing. The lower strength obtained with sulfuric acid anodizing could be explained by interference of absorbed water. Chromate conversion treatment was found to be unsuitable for adhesive bonding and gave the lowest results. In the case of SYL 186 without primer, there is also an advantage to chromic anodizing. SYL 186 is viscous and adequate wettability of the surface could not be obtained unless a primer was used. The primer has a low molecular weight and a low viscosity, which makes it possible to wet the substrate and penetrate the treatment coating. The strength of the bonding increased considerably when a primer was used. In the case of unsealed sulfuric acid, a high bond strength was obtained which (as explained above) is most likely due to penetration of the primer into the porous layer. The influence of humidity will be the subject of future studies.
5. Summary and conclusion In the present study the structure of oxides formed on Al 1100 during chromic conversion treatment and sulfuric or chromic acid anodizing was evaluated using Auger, ESCA and IR spectroscopy. Each technique gave partial information and only the combined data allowed us to clarify the structure. A partial correlation between the aluminum oxide structures and shear strength of adhesive bonding with single or two pack silicon rubber was found. Chromate conversion coating is unsuitable as a pretreatment for adhesive bonding of aluminum to silicone rubber. In the case of one pack silicone rubber (RTV 108) the highest bond strength was obtained with chromic anodizing pretreatment which gives a compact almost anhydrous film. In the case of the two pack silicone rubber (Sylgard 186) which was applied over a primer coating the highest value of the adhesive bond strength was obtained with sulfuric acid anodizing. In this process a porous layer of the oxide formed allowing penetration of the primer into the pores and good adhesion to SYL 186.
548
A. E. Yank
et al. / Oxtde treatments
of Al I IO0
In future work peel and wedge tests should be included to supplement data on bond strength. It is recommended to study the influence of adverse environments on the character of the oxides formed in various treatments and on the macroproperties, i.e. the nature and strength of the adhesive bonding.
Acknowledgements
The authors wish to thank Ms. S. Uziel for technical Drori for many useful discussions.
assistance,
and Mr. L.
References [l] D.M. Wyatt, R.C. Gray, J.C. Carver and D.M. Hercules, Appl. Spectrosc. 28 (1974) 439. [2] N.T. DcDevitt, W.L. Baun and J.S. Solomon, J. Electrochem. Sot. 123 (1976) 1058. [3] W.L. Baun and J.S. Solomon. Technical Report, AFML-TR-79-4165, Final Report for Period 1.78-9.79. [4] T.S. Sun, J.M. Chen, J.D. Venables and R. Hopping, Appl. Surface Sci. 1 (1978) 202. [5] J.S. Ahearn, T.S. Sun, C. Froede, J.D. Benables and R. Hopping, SAMPE Quart. 12 (1980) 39. [6] J.D. Venables, D.W. McNamara, J.M. Chen, B.M. Ditchek, T.1. Morgenthaler, T.S. Sun and R.L. Hopping, in: Proc. 12th National SAMPE Technical Conf., October 1980, p. 202. [7] T.S. Sun, G.D. Davis, J.D. Venables and J.M. Chen, Report No. AD/A 107664, NTIS October 1981. [8] A.J. Kinloch and N.R. Smart, J. Adhesion 12 (1981) 23. [9] A.J. Kinloch, K.E. Bishop and N.R. Smart, J. Adhesion 14 (1982) 105. (lo] H.A. Katzman, G.M. Malouf, R. Bauer and G.W. Stupian. Appl. Surface Sci. 2 (1979) 416. [ll] G.W. Stupian and P.D. Fleishauer, Appl. Surface Sci. 9 (1981) 250. [12] E.A. Poboda, S.P. Kodali, R.C. Curley. D. McNamara and J.D. Venables, Appl. Surface Sci. 9 (1981) 359. [13] A.E. Yaniv, I.E. KLein, J. Sharon and H. Dodiuk, Surface Interface Anal. 5 (1983) 93. [14] I.E. Klein, J. Sharon, A.E. Yaniv, H. Dodiuk and D. Katz. Intern. J. Adhesion Adhesives 3 (1983) 159. [15] A.C. Maloney, in: Surface Analysis and Pretreatment of Plastics and Metals, Ed. D.M. Brewis (Applied Science Publishers, 1982) p. 175. [16] US MIL-B-8625. [17] ASTM D-1002-72, 1916 Race Street, Philadelphia, Pennsylvania 19103, USA. [18] G.A. Dorsey, Jr., Plating 156 (1969) 177, 180; 157 (1970) 1117. [19] D.K. Ottesen and A.S. Nagelberg, Thin Solid Films 73 (1980) 347.
Applications
of Surface
OXIDE TREATMENTS OF Al 1100 FOR ADHESIVE SURFACE CHARACTERIZATION A.E. YANIV,
N. FIN, H. DODIUK
14 May 1984; accepted
for publication
BONDING
-
and I.E. KLEIN
Materials und Processes Depcrrtmen~, M~nrs~ry oJ DeJmce. Armwneni P.O. Box 2250, H&i 31021, lsrael Received
Science 20 (1985) 538-548 North-Holland. Amsterdam
27 November
Drr~eloptnmt Aurhorrrc,
1984
A study was made to characterize by spectroscopic methods the surface of oxides formed on Al 1100 by chromate conversion coating, chromic acid anodi& and sulfuric acid anodizing. Adhesive bond strength to silicone rubber was determined using single lap joint specimens. and correlation between the microscopic structure and macroscopic values was found. It was observed that the layer formed in chromate conversion coating was essentially hydrated chromia. and it yielded the lowest bond strength. Sulfuric acid anodizing produced hydrated alumina, incorporating sulfate ions in its matrix; an intermediate bond strength was found with this pretreatment. Chromic acid anodizmg gave rise to a compact, almost anhydrous alumina film, and the highest bond strength was attained.
1. Introduction Various pretreatments of metallic surfaces are usually carried out prior to application of adhesive, mainly to enhance the bond strength, durability, and to improve its corrosion resistance. Understanding why a certain pretreatment is more compatible with a given adhesive system than another depends on the physical and chemical character of the adherent surface. Information concerning the chemical composition of the surface is provided mainly by Auger Electron Spectroscopy (AES), Secondary Ion Mass Spectroscopy (SIMS), Ion Scattering Specroscopy (ISS), and Electron Spectroscopy for Chemical Analysis (ESCA). The nature of the chemical bonding at the surface layers is clarified by IR Spectroscopy (and ESCA to a limited extent). Complete characterization combines a number of the above techniques. A number of articles have been published during the last decade on the study of aluminum surfaces after various conventional pretreatments. Wyatt et al. [l] utilized ESCA to study very thin layers of polymer on aluminum. Baun and coworkers [2,3] tested aluminum surfaces prior to adhesive bonding by means of AES, ISS and SIMS; they found that absorption of water or the presence of Mg or Cu impair the durability of the joint. Sun et al. [4] characterized the nature of the surface of Al 2024-T3 after FPL treatment 03’78-5963/85/$03.30 Q Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
A.E. Yank et al. / Oxrde treatments
of Al 1100
539
using AES techniques; the presence of Mg and Cu had an adverse effect on its adhesion properties. A similar finding was obtained by Kinloch and Smart [8,9]. The mechanism of growth of the anodic film formed in phosphoric acid was followed [5] by AES and SEM. Changes introduced in surface pretreatments of aluminum after exposure to humidity were studied [6] by ESCA; aluminum oxide (cellular structure) was converted to aluminum hydroxide (cornflake structure), and thus the bond strength decreased. FPL was least susceptible to humidity because of an outer protective phosphate layer. Chromate conversion coatings on aluminum were analyzed by a number of researchers. Stupian and coworkers [10,11] found in AES and ESCA that the layer on Al 1100 is composed of hydrated chromium oxide on top of a spine1 type aluminum chromium oxide (which should be thick enough to provide protection against corrosion), The presence of Cr+6 and OH- or HF in the chromate solution was mandatory to ensure the integrity of the conversion coating. Venables and coworkers [12] used STEM and ESCA to study adhesive joints on chromate-treated aluminum surfaces. They found that roughening the surface improved the durability of the coating probably due to mechanical interlocking. Following our previous studies [13,14], the present work deals with a microscopic chemical characterization of aluminum surfaces prior to the application of adhesives [15]. Al 1100 was chosen as a substrate because of its low content of alloying elements. Five pretreatments which are commonly used as corrosion resistant coatings were studied: chromate conversion coating, chromic acid anodizing (with or whithout sealing), and sulfuric acid anodizing (with or without sealing). Silicone adhesives used mainly in the electronic industry were applied in order to correlate the microscopic properties of the surfaces to the macroscopic bond strength. Spectroscopic techniques used were AES, ESCA and IR.
2. Experimental 2. I. Materials
and procedures
(i) Al 1100; commercially pure aluminum 1 mm thick, of nominal composition (99% Al, 1% Si + Fe) was used throughout this work. Five different surface treatments were used prior to adhesive application: (1) Commercial chromate conversion coating, using commercial solution No. 720 manufactured by Chemotas under license of Metallgeselschaft, in accordance with MIL-C-5541. (2) Chromic acid anodizing without dichromate sealing. (3) Chromic acid anodizing with dichromate sealing *. (4) Sulfuric acid anodizing without water sealing.
540
A.E.
Yank
el al. / Oxtde treatments
ofAl
1100
(5) Sulfuric acid anodizing with water sealing *. Before the application of adhesive, the substrates were vapor degreased in a TP-35 solution (manufactured by DuPont) for 5-10 min. (ii) RTV 108 (one component RTV silicone rubber) manufactured by General Electric, USA. (iii) Sylgard 186 (two components silicone rubber) manufactured by DowCorning, USA, which is normally applied over a primer DC-1200. RTV 108 was applied by brushing, and polymerized for 6 days at RT. DC-1200 primer was applied by dipping to a uniform coating and allowed to dry for 1 h at RT. Syl 186 was applied by brushing and cured for 4 h at 65°C. The bond line thickness was 0.2 t_ 0.1 mm. Single Lap Joints were prepared in accordance to ASTM D-1002-72 [17]. Peel and wedge test will be dealt with in one of our next papers. No deformation of the aluminum samples was observed. 2.2. Analyticul
methods
AES spectra were obtained in a Physical Electronics Industries Auger-SIMS Spectrometer Model 545A, operated at 3 keV, 0.5 PA. Before analysis, the outermost layer (approximately 100 nm) was removed with an argon sputtering gun to remove carbon impurities in the adsorbed layer. IR spectra were run in a Beckman IR 4250 reflectance spectrometer, equipped with a specular reflectance attachment (angle of incidence 20”). ESCA spectra were obtained in a Physical Electronics Industries model 550 spectrometer using a Mg anode at 10 keV and 40 mA. The bonding strength was meaured in a 5 ton Instron (bridgehead speed of 0.2 cm/min) at 25°C. The mode of tailure - whether adhesive or cohesive ~ was evaluated by visual inspection.
3. Results Typical AES curves are given in figs. 1 and 2, and typical ESCA spectra in figs. 3-5. The AES-ESCA results are compiled in table 1. It should be noted that during sputtering the non-conductive surfaces became electrostatically charged, causing an energy shift in the ESCA peaks. The difference between the experimental peak of carbon 1s and the literature-recorded value of 284.6 eV served to correct the energy data for oxygen, aluminum, chromium and sulfur at each depth. Fig. 3 is a chromium 2p ESCA absorption peak for chromate conversion coating. It can be seen in table 1 that the concentration of Al increases and Cr and 0 decreased with penetrating depth into the film. * Both processes tively.
were carried
out in accordance
with MIL-B-8625
1161, types I and II, respec-
Fig, 1. AES spectrum
dE
ofAl
400
600
300
200
100
A!!
A. E. Yanio er al. / Oxrde treatments
of outermost
’ Al
45-75ev
00
496-513
oCr
516-529eV
500
layer of chromic
conversion
I 100
541
700
600
E(eV)
coating.
CV
___,-.-x-n
Al
Sputtering tme ( nil” 100H/min
Fig. 2. AES depth profile
604
of chromate
592 BINDING
conversion
580 ENERGY,
Fig. 3. ESCA peak of chromium
coating.
566 EV
(2~) for chromate
conversion
coating.
)
Al Cr
0 Al Cr
Chromic acid; anodizing; unsealed
Chromic acid; anodizing; sealed
0
Al S
530.6 74.1 576.6
531.6 74.6 517.6
531.6 74.6 169.6
0
Sulfuric acid; anodizing; sealed
69.2 26.6 4.2
70.4 28.7 0.9
76.8 22.1 0.5
531.6 74.6 576.6
531.6 74.6 576.6
531.1 74.1 169.1 161.6
531.6 74.6 169.6 162.6
531.1 74.1 169.1
0 Al S
Sulfuric acid; anodizing; unsealed
65.1 33.7 1.1
511
Al Cr
Chromate; conversion coating
65.5 32.2 2.3
70.8 2X.3 0.9
68.7 30.5 0.8
64.0 34.5 1.5
18.2 28.1
(eV) 530.5
(at%) 62.5
(eV) 531
0 118 576
(at%) 53.7
Energy
4.2 33.3
Cont.
100 nm Cont.
Element
10 nm
Treatment Energy
Table 1 ESCA results for Al 1100 oxide coatings of peaks
530.6-531.6 eV AI,O, (531.5)+Cr,O, 74.6 eV Al 2OJ (74-74.5) 575.66576.6 eV Cr,O, (576.5)
531.6 eV AI,O, (531.5) 74.6 eV AI,O, (74474.5) 576.5-577.6 eV Cr,Ol (576.5)+CrO,
530.6-531.6 eV Al,O, (531.5) 74.1-74.6 eV Al *O, (74-74.5) 169.1-169.6 eV SO:- (168.5-169) 161.6 eV S in Al *O, matrix
531.1-532.1 eV AI,O, (531.5) 74.1-74.6 eV Al >O, (74-74.5) 169.1-169.6 eV SOi- (168.5-169) 161.6-162.6 eV S in AI,O, matrix
530.55531 eV AI,O, (531,5)+Cr,O, 118-119 eV oxidized 576-577 eV Cr,O, (576.5)
Identificatton
(530.5)
(578)
(530.5)
h -1 Y
A. E. Yaniu et al. / Oxide treatments
180 BINDING
168 ENERGY
o/Al 1100
543
156 (eV
)
Fig. 4. ESCA peak of sulfur (2~) for unsealed sulfuric acid anodic film.
Figs. 4 and 5 present sulfur 2p ESCA peaks for unsealed and sealed sulfuric acid anodic films, respectively, at various depths. In both cases sulfur appears as sulphate on the surface, but an additional structure is revealed with deeper penetration. The concentration of sulfur increases with depth (up to 200 nm). Table 1 also shows the results for chromic acid anodic films. Only one ESCA peak for chromium was found, and its concentration was higher for sealed anodizing (performed in dichromate solution), than for unsealed.
180
168 BINDING ENERGY
156
( eV 1
Fig. 5. ESCA peak of sulfur (2~) for sealed sulfuric acid anodic film.
Table 2 IR absorption Surface treatment
of oxide coatings IR absorption AIO-H stretch
on Al 1100
(cm-‘)
+ H *O AI=O. H ,O stretch
AI=0 stretch
Chromic acid; 3450 anodizing; (weak) unsealed
1650 (broad)
to 1450 (weak
Chromic acid; 3450 (medium) anodizing; sealed
1630 (medium)
> 1450 (weak)
Conversion coating
3300 to 3500
_
Sulfuric acid; anodizing; unsealed
3000 to 3600 (strong)
1650 (medium)
Sulfuric acid; anodizing; sealed
3000 to 3600 (strong)
1630 (strong)
Al=0 + S + SO: - AI-0 * H. AI-O-AI stretch bend, stretch
_
930
i 750
950 (broad)
to750
_
_
> 1550 (weak)
1350 (broad)
to
920 (strong)
> 1500 (medium)
1350 (broad)
to
920 (strong)
3.1. IR The main absorptions for the various surface treatments on Al 1100 are summarized in table 2. The identification of the various absorptions was based on Dorsey’s results [18]. In the chromate conversion there is no absorption above 700 cm ‘, except for absorbed water. The main IR absorptions of the anodized Al are the absorbed and free water, Al = 0, Al = 0. H,O, barrier layer, Al-O --) H and Al-O-Al stretches. In the sulfuric anodized specimens, SOispecies and Al = 0 4 S absorption appears at 1130-1180 and 1325 cm--‘, respectively. 3.2. Tensile shear test The results of the tensile shear strength of the two rubbers are given in fig. 6. The results are an average of 5 specimens for each combination of surface treatments and adhesive with and without primer. The highest value of lap shear strength and partially cohesive failure was found for RTV 108 on chromic anodized Al. The same pattern appears for SYL 186 without primer. The addition of DC-1200 primer prior to application of SYL 186 changes the trend with some advantage to sulfuric anodizing treatment. Adhesive type failure occurred at the primer-adhesive interfase. In all cases, chromate conversion treatment
545
A. E. Yank et al. / Oxide treatments of AI I100 Unprimed
Syi
II36
=
Primed
Syl
166
w
RTV
106 Max.
Av
hlin
:anvers~on :oating
‘chromic ac anodizing sealed
‘chromic ac anodizing unsealed
’ sulfuric
ac anodizing unsealed
’ sulfuric
1
ac
anadtzing sealed
Fig. 6. Schematic representation of lap shear strength results (SYL 186 without primer; adhesive failure; SYL 186 with primer; adhesive failure at the primer-adhesive interfase. RTV 108; adhesive failure except for the sealed and unsealed chromic acid anodic coating, which is cohesive).
resulted in the lowest lap shear strength the aluminum-polymer interface.
and exhibited
an adhesive
failure
at
4. Discussion 4.1. The structure and composition
of the conversion coating
It can be seen from Auger and ESCA depth profiles that the amount of Al’- increases with the depth of penetration into the coating which is a few tens of rim’s thick. The concentrations of oxygen and chromium diminish with depth, reaching a steady value with an O/Cr ratio of 1.8 indicating a Cr,O, structure (O/Cr = 1.5). This is in accordance with the work of Katzman et al. [lo], who found that the coating is composed of a layer of chromium oxide, superimposed on a CrAl spine1 layer. Our ESCA analysis began after sputtering to 10 nm to remove carbon impurities and only Cr3+ was established.
546
A.E. Yrrniv et crl. / Oxide treatments
of Al 1 It30
IR spectra taken to 700 cm-’ have revealed absorbed H,O, but no Cr oxide could be detected in this region, since their absorption occurs at lower frequencies [19]. The amount of absorbed water increases with the increase of the thickness of the deposited coating, the color of which darkens with an increase in thickness. 4.2. Unsealed and sealed sulfuric ucid anodizing It is now well established that the anodic coating is basically aluminum oxide containing free or absorbed water and incorporating some anions from the acid used. The coating is composed of two layers: a barrier layer of approximately 30 nm, and a porous layer of lo-25 pm. According to Dorsey [18], the Al = 0 bond is dominant for the barrier layer, whereas in the porous layer Al-O-Al appears. Although the authors assume that these bonds are relevant to the present work, it could not be proved owing to the fact that the IR absorption in the 900-1300 cm-’ is too broad. The presence of sulfur anions in the anodic coating is well established, but in the present work two separate species were found in IR and ESCA spectra. On the surface the SO:- species appears, and a second species, Al = 0 --, S, is incorporated in the deeper layer of the oxide lattice [18]. The concentration of sulfur anions increases with the depth, suggesting a mechanism of outward movement of Al cations and an inward movement of sulfur anions. The water sealed coating is similar in composition to the unsealed film, but due to the added water some new species are formed from the oxide-water interaction. This can be concluded from the noticeable asymmetry of the ESCA peaks. The IR spectra show that water addition causes an increase in a disordered structure. The absorption intensity related to water increases, and a shift of absorption peak occurs as a result of stretching of Al = 0. H,O to lower frequencies due to the change of the H,O-A120, spatial structure. Similar to the sealed coating in the unsealed film, sulfur anions appear as two species, but its overall concentration is lower in the latter case. This is mainly due to the larger amount of absorbed water, but is most likely also affected by some sulfur extraction during sealing. 4.3. Unsealed and sealed chromic acid anodizing The coating obtained from this solution is more compact than that from sulfuric acid. The film contains only a small amount of absorbed water, as shown by symmetrical peaks in ESCA and low intensity of water in the IR spectra. The aluminum oxide incorporates a small amount of chromium (approximately 1%) as Cr,O, and/or Cr-Al spine1 and some CrO, in the unsealed film.
A. E. Yaniu
et al. /
Oxrde
treatments ofAl 1100
In the sealed film a cornflake-like layer is formed which, in accordance reported results [6], is brittle and unsuitable for adhesion bonding. 4.4. Suitability
547
with
of surface treatment for bonding
Silicone rubbers were chosen since they are known to be “weak” adhesives and adhesive failure occurs mainly in the boundary, thus allowing the study of pretreatment/adhesive interface. In the case of RTV 108, the highest bond strength was obtained, as expected, with the chromic acid anodizing. The lower strength obtained with sulfuric acid anodizing could be explained by interference of absorbed water. Chromate conversion treatment was found to be unsuitable for adhesive bonding and gave the lowest results. In the case of SYL 186 without primer, there is also an advantage to chromic anodizing. SYL 186 is viscous and adequate wettability of the surface could not be obtained unless a primer was used. The primer has a low molecular weight and a low viscosity, which makes it possible to wet the substrate and penetrate the treatment coating. The strength of the bonding increased considerably when a primer was used. In the case of unsealed sulfuric acid, a high bond strength was obtained which (as explained above) is most likely due to penetration of the primer into the porous layer. The influence of humidity will be the subject of future studies.
5. Summary and conclusion In the present study the structure of oxides formed on Al 1100 during chromic conversion treatment and sulfuric or chromic acid anodizing was evaluated using Auger, ESCA and IR spectroscopy. Each technique gave partial information and only the combined data allowed us to clarify the structure. A partial correlation between the aluminum oxide structures and shear strength of adhesive bonding with single or two pack silicon rubber was found. Chromate conversion coating is unsuitable as a pretreatment for adhesive bonding of aluminum to silicone rubber. In the case of one pack silicone rubber (RTV 108) the highest bond strength was obtained with chromic anodizing pretreatment which gives a compact almost anhydrous film. In the case of the two pack silicone rubber (Sylgard 186) which was applied over a primer coating the highest value of the adhesive bond strength was obtained with sulfuric acid anodizing. In this process a porous layer of the oxide formed allowing penetration of the primer into the pores and good adhesion to SYL 186.
548
A. E. Yank
et al. / Oxtde treatments
of Al I IO0
In future work peel and wedge tests should be included to supplement data on bond strength. It is recommended to study the influence of adverse environments on the character of the oxides formed in various treatments and on the macroproperties, i.e. the nature and strength of the adhesive bonding.
Acknowledgements
The authors wish to thank Ms. S. Uziel for technical Drori for many useful discussions.
assistance,
and Mr. L.
References [l] D.M. Wyatt, R.C. Gray, J.C. Carver and D.M. Hercules, Appl. Spectrosc. 28 (1974) 439. [2] N.T. DcDevitt, W.L. Baun and J.S. Solomon, J. Electrochem. Sot. 123 (1976) 1058. [3] W.L. Baun and J.S. Solomon. Technical Report, AFML-TR-79-4165, Final Report for Period 1.78-9.79. [4] T.S. Sun, J.M. Chen, J.D. Venables and R. Hopping, Appl. Surface Sci. 1 (1978) 202. [5] J.S. Ahearn, T.S. Sun, C. Froede, J.D. Benables and R. Hopping, SAMPE Quart. 12 (1980) 39. [6] J.D. Venables, D.W. McNamara, J.M. Chen, B.M. Ditchek, T.1. Morgenthaler, T.S. Sun and R.L. Hopping, in: Proc. 12th National SAMPE Technical Conf., October 1980, p. 202. [7] T.S. Sun, G.D. Davis, J.D. Venables and J.M. Chen, Report No. AD/A 107664, NTIS October 1981. [8] A.J. Kinloch and N.R. Smart, J. Adhesion 12 (1981) 23. [9] A.J. Kinloch, K.E. Bishop and N.R. Smart, J. Adhesion 14 (1982) 105. (lo] H.A. Katzman, G.M. Malouf, R. Bauer and G.W. Stupian. Appl. Surface Sci. 2 (1979) 416. [ll] G.W. Stupian and P.D. Fleishauer, Appl. Surface Sci. 9 (1981) 250. [12] E.A. Poboda, S.P. Kodali, R.C. Curley. D. McNamara and J.D. Venables, Appl. Surface Sci. 9 (1981) 359. [13] A.E. Yaniv, I.E. KLein, J. Sharon and H. Dodiuk, Surface Interface Anal. 5 (1983) 93. [14] I.E. Klein, J. Sharon, A.E. Yaniv, H. Dodiuk and D. Katz. Intern. J. Adhesion Adhesives 3 (1983) 159. [15] A.C. Maloney, in: Surface Analysis and Pretreatment of Plastics and Metals, Ed. D.M. Brewis (Applied Science Publishers, 1982) p. 175. [16] US MIL-B-8625. [17] ASTM D-1002-72, 1916 Race Street, Philadelphia, Pennsylvania 19103, USA. [18] G.A. Dorsey, Jr., Plating 156 (1969) 177, 180; 157 (1970) 1117. [19] D.K. Ottesen and A.S. Nagelberg, Thin Solid Films 73 (1980) 347.