ssims, xps and microstructural studies of ac-phosphoric acid anodic films on aluminium

SURFACE AND INTERFACE ANALYSIS, VOL. 15, 36!2-376 (1990)

SSIMS, XPS and Microstructural Studies of ac-Phosphoric Acid Anodic Films on Aluminium''

J. A. Treverton,' J. Ball,' D. Johnson,' J. C. Vickerman' and R. H. West' ' Centre for Surface and Materials Research, UMIST, P O Box 88, Manchester, M60 lQD, UK * Alcan International, Banbury Laboratories, Banbury, Oxon, OX16 7SP, UK

ac-Phosphoric acid anodizing of aluminium results in the initial formation of a featureless barrier film on the metal surface, followed by nucleation and growth of a filament structure. The rate of filament growth and filament diameter are dependent upon the anodizing temperature. Static secondary ion maw spectrometry (SSIMS) and XPS indicate that organic contamination levels on all anodized surfaces are low regardless of anodizing temperature and that contaminant levels tend to decrease as anodizing prooeeds. Apart from the very intense Al' in the positive ion spectra and the 0- and OH- peaks in the negative ion spectra, SSIMS spectra were characterized by the appearance of a series of peaks from AI,O,H, fragments. Appearance of AIO' or AIOH' clusters in the positive ion spectra depended on the anodizing conditions. Both XPS and SSIMS confirm the presence of pbos- phates on the anodized surfaces but differences in results from the two types of analyses imply that unknown factors affect the concentration present.

Low contaminant levels, corrosion inhibition from surface phosphates and the filamented topography developed will aU affect the adhesive properties of the surface.

INTRODUCTION

Increasing international environmental concern over chromate effluent from conventional lacquer and paint pretreatment lines' is enhancing the general interest in no-rinse and chromate-free types of pretreatment. Amongst the many forms of chromate-free processes that have been used successfully on a commercial scale are ac-sulphuric' and ac-phosphoric acid3 anodizing. With regard to lacquer and paint adhesion, however, ac-phosphoric anodizing is of particular interest. Under suitable experimental conditions, it rapidly results in a filamented surface topography4 with a marked simi- larity to that produced by the much slower dc- phosphoric acid anodising process,' widely used as an adhesive bonding pretreatment in the aircraft industry.

Some investigations have been made of the develop- ment of structure during ac-phosphoric acid anodi~ing.~ However, the studies were made of films formed at room temperature and not of films formed at the ele- vated temperatures used on typical commercial anodizing lines. In addition, the relationship between surface chemistry and the adhesive properties of the film has not been explored.

Of the techniques available for surface chemical studies, XPS is of particular value for quantitative analysis and chemical state studies and static secondary ion mass spectrometry (SSIMS) is of value for trace

Paper presented at ECASIA 89, Antibes, France, 23-27 October 1989.

0142-2421/90/06036%03 WS.00 0 1990 by John Wiley & Sons, Ltd.

element analysis and detailed evaluation of the chemical structure of surfaces.6 As a result, both techniques were used to study the surface chemistry of the ac-phosphoric acid anodizing process. High-resolution scanning electron microscopy (SEM) was used to investigate the structure of the films formed.

EXPERIMENTAL ~~~~~~~ ~

All samples were prepared using a laboratory anodizing cell designed to simulate the type of conditions likely to be encountered on commercial pretreatment lines. A range of anodizing times and temperatures were used. Anodizing was carried out in 10% (w/v) Analar phosphoric acid in high-purity deionized water. The anodizing voltage was 20 V (r.m.s.) and the frequency was 50 cps. Samples used in the SIMS analyses and SEM studies were anodized for computer-controlled periods of time, rinsed in high-purity water and dried in a jet of cold air. Dwell times in the electrolyte were estimated as 1 s prior to anodizing and between 0.3 and 0.5 s after completion of the process. The area anodised was 10 mm x 10 mm and the remainder of the surface of the sample was masked off using PVC tape. Care was taken to ensure that the mask material did not touch the areas of interest. Owing to the masking, the edges of the samples experienced a greater voltage than elsewhere, so were avoided in the subsequent analyses. For trans- portation and storage, samples were wrapped in aluminium foil. Samples for XPS analysis were prepared in a similar manner. However, timing of anodizing was

Received 9 November 1989 Accepted 5 February 1990

370 J. A. TREVERTON ET AL.

manual and use of a large-capacity power supply meant that the required current density could be maintained without masking of the sample, so the full 20 mm x 50 mm immersed area of the sample was anodized. The aluminium alloy used throughout the investigation contained 0.5% magnesium and 0.5% iron as the principle alloying elements.

SSIMS analysis

A VG UHV SIMSLAB instrument equipped with a VG MM 12-12 quadrupole mass spectrometer was used for the SSIMS analyses. Samples were bombarded with a 2 keV beam of argon atoms (UMIST-designed ion/atom primary source) generating a target current density of 1 nA into an area of 27 mm', i.e. 4 nA cmW2. Secondary ions were energy filtered using a Wittmaack-type energy analyser prior to mass separation. On all of the samples studied, positive and negative secondary ion mass spectra were recorded over a mass range m/z = 0-300. A 500 eV beam of electrons was used for charge com- pensation during the acquisition of negative ion spectra.

A 0 Barrier layer

a) 1.9 Seconds

A DEC PDPll computer was used for instrument control, data acquisition and processing.

XPS analysis

A VG ESCALAB Mk 1 XPS spectrometer equipped with a 500 mm spherical-sector electron analyser and a VGS lo00 Apple computer-based data-acquisition system was used for the XPS analyses.

Randomly selected areas of each sample (-15 mm x 10 mm) were analysed in the spectrometer. On all surfaces wide-scan survey spectra and high- resolution spectra of the A1 2s, 0 Is, P 2s and C 1s peaks were recorded. Aluminium Ka x-rays were used throughout and the analyser was used in the FAT mode with the pass energy set to 20 eV for high-resolution spectra and 50 eV for wide-scan spectra.

The C 1s peak from adventitious carbon was set to a binding energy of 285 eV to calibrate the binding energy scale.

For quantification, integrated counts above linear backgrounds were taken as a measure of peak intensity. With the exception of aluminium, peak intensities were converted to atomic percentages using the quantifica-

b) 3 Seconds

c) 5 Seconds d) 10 Seconds Figure 1. SEM micrographs of alumina barrier films on aluminium. Conditions: anodizing voltage 10 V; temperature 55 "C.

STUDIES OF ac-PHOSPHORIC ACID ANODIC FILMS ON ALUMINIUM 371

microscope. To allow cross-sections of the films to be observed in the microscope, samples were cooled to - 196 "C in liquid nitrogen and bent through 180" to produce brittle fractures extending throughout the film to the metal substrate. Surface charging was prevented by platinum coating the samples in an Ion Tech sputter-coating unit. Conditions of coating were selected to give particle sizes in the deposited platinum that were less than the 3 nm resolution of the microscope.'

RESULTS

Figure 2. SEM micrographs of alumina barrier films formed at different temperatures: (a) 45°C; (b) 70 "C. Current density = 300 Am-=.

tion factors given in Ref. 7. In the case of aluminium oxides and hydroxides, however, use of the factors resulted in aluminium/oxygen ratios that were constant- ly higher than expected from the formulae of the com- pounds, so it was considered necessary to calculate new factors. The factors (f,,.,) were determined from the 0 1s value given in Ref. 7 and the intensities (Ipeak) of the Al 2s, A1 2p and 0 1s peaks from the XPS spectrum of an alumina surface, prepared by argon etching a boric acid barrier layer anodic film until all contaminant elements and boron from the anodizing solution had been removed. Assuming a 1 : 1.5 aluminium/oxygen ratio, a factor of 0.195 for the Al 2s peak was calculated using the following expression

I A , 2s f A l 2 s = - x WO ls

I 0 Is

SEM studies

SEM studies were carried out in the upper high- resolution stage of an IS1 DS130 scanning electron

SEM micrographs taken from the edges of the cracks formed by bending clearly show the cross-sections and the surface topographies of the films, although the surface topographies are foreshortened because of the acute angles of the surfaces to the detection system of the microscope. From the micrographs, it can be seen that anodizing for increasing periods of time in solu- tions of constant temperature and at constant current density resulted in an initial growth of a featureless 'barrier film' of uniform cross-section (Fig. I), followed by nucleation and growth of a filamented surface structure that continues to increase in thickness for up to 10 s of anodizing. Barrier layer thickness and filament diameter appeared to change little as anodizing proceeded. However, an increase in the temperature of anodizing resulted in a decrease in the diameter of the filaments, consistent with the greater dissolution of the film in sol- utions of higher tempera t~re .~ An increase in the temperature of the anodizing solution also results in an increase in overall film thickness (Fig. 2).

All positive ion spectra were dominated by a peak at m/z = 27 due to the Al+ ion. The spectra did not contain the typical clusters of hydrocarbon peaks observed on untreated aluminium surfaces that are associated with the presence of residual rolling lubri- cants and additivesg Representative positive ion SSIMS spectra (Fig. 3) of m/z 30-100 highlight the principal changes in surface chemistry of the anodic films occurring as anodizing time increased. In both spectra, peaks at m/z = 39, 41, 43, 44, 45, 61, 70 and 86, were observed. The m/z = 39 peak can be derived from pot- assium, but observation of the m/z = 41, 43 and 45 suggests that all four peaks can be attributed to aliphatic hydrocarbons. Some contribution to the m/z = 43 peak was associated with the A10+ fragment, and peaks at m/z = 44 and 61 were associated with AlOH+ and AlO,H, + fragments, respectively. Similarly, peaks at m/z = 70 and 86 were associated with AlzO+ and A1,0,+ fragments, respectively. The main differences in the two spectra were the loss in intensity of the hydrocarbon-derived fragments, the decrease in intensity of the m/z = 43 peak and significant increases in the intensities of the AlOH', AIO,H, + and AI,O+ peaks. The increase in intensity of the m/z = 44 AlOH+ peak was particularly large.

Although the same general trends were observed on all of the anodized surfaces, the rate at which anodizing resulted in loss of hydrocarbon peaks and the appearance of the AlOH' fragment varied depending on the anodizing temperature. On the surface anodized at


(a 1 8000

6000 m u = 11000 J 0 u

2 0 0 0

0 30 40 50 60 70 80 90 100

mss: charge (rn/z)

( b )

8000 A ~ O H + a+ intensity: 99,774

J 4000

30 40 50 60 70 80 90 100

Figure 3. SSIMS positive ion spectra of films formed at 55 "C (m/z = 30-1 00) : (a) 3 s pretreatment time; (b) 5 s pretreatment time.

mass: charge (dz)

55"C, both effects occurred between 3 and 5 s of anodizing and little further change occurred between 5 and 10 s of anodizing (Table 1). At 60 and 70"C, however, increases in organic fragments occurred between 3 and 5 s of anodizing (Table 1) and it was only when the anodizing times were increased to 10 s that the m/z = 44 peak increased in intensity and a reduction in hydrocarbon peaks occurred.

Above m/z = 100, positive ion spectra only contained very-low-intensity peaks at m/z values characteristic of traces of aromatic hydrocarbons.

Table 1. Positive ion SIMS intensities

Temperature Time mlz ("C) ( 5 ) 27 43 44

55 3 55 5 55 10 60 3 60 5 60 10 70 3 70 5 70 10

59 580 99 774 91 771 90 705 53 175 54 680 45 536 64 932 50 244

6907 1047 1298 5579 2852 421

5539 221 3 463

2394 61 59 61 33 1672 81 9

3206 1 503 531

351 2

61 70 a7

493 282 593 1117 847 443 827 587 352 524 397 406 412 169 370 457 288 186 367 141 286 283 157 422 483 300 176

Negative ion spectra contained the intense 0- and OH- peaks, together with the low-intensity C-, CH-, C,- and C2H- peaks (Fig. 4) that again indicated low levels of organic contamination. Peaks at m/z = 32 and 33 corresponding to 0,- and OzH-, respectively, and the more intense peaks at m/z = 43,59,63 and 79 corresponded to the AlO-, A10,-, PO,- and PO3- fragments, respectively. Above m/z = 100, the peaks at m/z = 102 and 103 corresponded to A1203- and A1203H- clusters, respectively, and the large fragment at m/z = 119 corresponded to the Al,O,H- cluster. A small fragment at m/z = 161 corresponded to the A130,- cluster, and to date this is believed to be the highest mass aluminium oxide cluster observed in SSIMS spectra of aluminium surfaces.

Intensities of the C,H- fragments in the negative ion spectra varied with anodizing conditions in a similar manner to the hydrocarbon clusters in the positive ion spectra. On surfaces anodized at 55 "C, the hydrocarbon intensities decreased with increased anodizing time (Fig. 4, Table 2), with the majority of the decrease occurring between 3 and 5 s of anodizing. Corresponding increases in the AlO,H, and PO, clusters were also observed (Fig. 4, Table 2). On surfaces anodized at 60 and 70 "C (Table 2), hydrocarbon contaminant peaks increased in intensity on surfaces anodized for between 3 and 5 s.

STUDIES OF ac-PHOSPHORIC ACID ANODIC FILMS ON ALUMINIUM 313

6000

ffl

4000 C 7 0

2000

0 20 30 40 50 60 70 80 90 100

miss: charge ( m / z )

Y

10

0

6

4

2

0 20 30 40 50 60 70 80 90 100

mss: charge (m/z)

65

50

35

20 100 110 120 130 140 150 160 170 180

mss: charge (m/z)

100

80

ul u 60

0 40

c J

u 20

0 100 110 120 130 140 150 160 170 180

mss charye ( 4 2 )

Figure 4. SSIMS negative ion spectra of films formed at 55 "C with pretreatment times of 3 s (top) and 5 s (bottom): (a) m/z = 20-100; (b) m/z = 1 00-1 80.


Table 2. Negative ion SIMS intensities

Temperature Time ("C) (S) 16 17

55 55 55 60 60 60 70 70 70

3 5 10 3 5 10 3 5 10

50 206 49 728 67 028 40 598 59 546 32 397 38 598 56 288 54 901

43 798 28 186 57 044 26 980 48 870 18 002 31 641 47 592 34 750

25

3924 501 1062 4540

22 061 802 3608

16 022 1147

43

4849 7400 5808 3661 4743 4040 3267 41 02 321 2

mlz 59 63 79 102 103 119 161

3453 4108 904 15 15 12 5 3708 2366 1076 44 38 109 14 5512 3088 2048 64 38 121 25 2246 2529 406 20 16 51 4 1184 1505 436 28 96 153 4 1821 2650 1242 26 2 95 5 1838 2323 490 17 22 47 3 994 1231 335 24 79 93 2 2805 2227 666 39 36 154 1 1

On the surfaces anodized at 55 "C, PO,- and AlOH- and A10,- intensities (Fig. 4, Table 2) indicated a marked decrease in the surface phosphate concentration with increased anodizing time. A similar but less marked variation in the phosphate levels was implied from the spectra of the samples anodized at 70°C (Table 2), but on the sample anodized at 60°C no significant variation was observed (Table 2).

XPS analysis

Wide-scan XPS spectra from the sample contained peaks from phosphorus, aluminium, oxygen and carbon (Fig. 5). Change of anodizing time (Fig. 5 ) and temperature had little effect on the survey spectra. High- resolution spectra of aluminium and phosphorus peaks also varied little with increases in anodizing time (Fig.

T

5 a

(C) -

10 secs. pretreatment I 1700 counts -

1 3 secs. pretreatment (a)

I la 1 I

V

100 300 500 700 900

BINDING ENERGY (ev)

Figure 5. XPS survey spectra of alumina films. Effect of increasing pretreatment times: (a) 3 s; (b) 5 s; (c) 10 s; all in 10% phosphoric acid at 70 "C and 20 V.

6) or temperature. The mean binding energy of the A1 2s peak in the spectra from all surfaces (Table 1) was typical of the oxidized state of the metal" and the P 2p value was close to the value expected from phosphates compared.'' Standard deviations of the binding energy measurements (Table 3) of kO.2 eV can be accounted for by a +0.1 eV error in the assessment of peak maxima and scale calibration.

A I f2d (c)

10 secs. pretreatment 300 counts

I \ P (2~71

I \ 5 secs. pretreatment

1'1 h

3 secs. pretreatment

%+lL 110 120 130 140 150

BINDING ENERGY (eV1

Figure 6. XPS Al 2s and P 2p region of alumina films. Effect of increasing pretreatment times: (a) 3 s; (b) 5 s; (c) 10 s; all in 100% phosphoric acid at 70°C and 20 V.

~~~~~~~~~ ~

Table 3. Summary of binding energies from all samples

Peak

Al 2s fJ 2P 0 1s

Binding energy

119.5i0.2 134.5 * 0.1 531.4 * 0.2

STUDIES OF aoPHOSPHORIC ACID ANODIC FILMS ON ALUMINIUM 375

Variations with anodizing conditions also had little effect on the concentrations of the elements calculated from the XPS spectra of the surfaces (Table 4). Again, however, the low concentrations of carbon were consistent with the presence of low levels of organic contaminants on all of the anodized surfaces. Like the SSIMS analysis, variations of carbon on the surfaces anodized at 55 "C indicated that removal of organic contaminants continued with an increase in anodizing time (Table 4), and on samples anodized at high temperatures, increases in contamination levels occurred between 3 and 5 s of anodizing. However, the variations recorded may simply reflect random variations in contaminant levels from the instrument or from the environment.

Unlike SSIMS analysis, where the intensities of peaks from phosphates and aluminium indicated significant differences in surface phosphate levels, phosphorus/ aluminium ratios from XPS analysis showed that within the larger analytical depth of XPS analysis, phosphate levels were constant and independent of anodizing time or temperature (Table 4). It should be noted that the absolute values of the element concentrations may be in error as a result of the use of sensitivity factors. However, the errors are the same for all of the surfaces studied so they will not affect the comparative studies described here.

DISCUSSION

Barrier layer film formation followed by the formation of a porous film is the method of growth of sulphuric and phosphoric acid anodic films on aluminium. It has been suggestedI2 that the onset of pore growth is initi- ated by localized thickening of the barrier film, which subsequently leads to the formation of an array of randomly distributed fine pores in the film. Concentration of electrical fields at the bases of the larger fine pores and the resultant field-assisted dissolution of material from the pore base leads to the formation of the regular array cylindrical (0.02-0.05 p) pores that extend throughout the film." In phosphoric acid anodizing, extensive dissolution of the pore structure in the outermost regions of the film leads to the filamented structures' that are effective in promoting good adhesive properties of the film. Anodizing sequences and dissolution rates of the outermost regions of the films are the same for both ac- and dc-phosphoric acid anod-

izing? although continued removal of material during the cathodic cycle means that in ac anodizing the overall rate of film dissolution is greater.4 Rates of film dissolution also increase with increased anodizing temperature. The composition of anodic films immediately surrounding the pores is believed to be amorphous and it is this region that dissolves within the anodizing solution to leave the filament structure formed by the crys- talline framework of the film.

The regularity of the filament structures coupled with the presence of the barrier film, irrespective of the anodizing conditions (Figs 1 and 2), suggests that in high- temperature ac anodizing, dissolution rates are sufficient to replace completely the porous region of the film by the filamented ~tructure.~ Other than this, however, the micrographs are consistent with the initial growth of the barrier layer film followed by growth of the porous film. Higher dissolution rates in the solu- tions formed at higher temperatures are also consistent with the presence of finer filaments and thinner barrier layer films on the surfaces produced at higher temperatures (Fig. 2).

The SSIMS analysis has shown a number of features that may be related to adhesive, lacquer and paint adhesion. Most importantly, the spectra show that the organic contaminant levels were very low and that longer anodizing times resulted in further decreases in contaminant levels. Low levels of contamination means that organic coatings will wet the surface effectively and allow intimate contact between the coating and the substrate, which is a prerequisite for good adhe~ion. '~ The transition from the m/z = 43 A10+ fragment to the m/z = 44 AlOH' fragment observed in SSIMS as anodizing proceeded also implies that changes in the surface chemistry of the aluminium may be occurring; clearly, this could also affect the adhesive properties of the surface. Such changes may be related to removal of the amorphous regions around the pores to reveal the crys- talline zone beneath. However, appearance of the AlOH + component was accompanied by decreases in all fragments associated with organic contaminants, so it may be a peak with a particularly high sensitivity to contamination levels. Again, therefore, the presence of the peak may simply reflect the low levels of contamination present on ac-anodized surfaces. XPS analysis confirmed the presence of low levels of

contaminants on the surface, but the binding energies of the A1 2s peaks were not sufficiently sensitive to changes in the chemical environment to reflect the chemical

Table 4. Effect of anodizing times and temperatures on surface composition

Anodizing Temperature Time Atomic % Atomic ratio

("C) (min) C Al 0 P P/Al C/AI

70 3 11.2 32.4 53.1 3.3 0.10 0.35 5 12.1 31.3 53.6 2.9 0.09 0.39

10 11.3 31.6 53.7 3.3 0.10 0.36 60 3 12.4 31.6 52.9 2.8 0.09 0.39

5 13.0 30.5 53.2 3.3 0.12 0.43 10 10.5 32.5 54.3 2.6 0.08 0.32

55 3 13.0 30.2 53.7 3.2 0.11 0.43 5 10.3 32.2 54.4 3.0 0.09 0.32

10 9.8 32.8 54.0 3.4 0.10 0.30


changes implied by the AlO+-AlOH+ transitions in the SIMS spectra. Measurement of the A1 2p or A1 Auger peaks, or the Auger ~arameter, '~ may give a better correlation. However, changes in surface phosphate levels implied by the changes in the P02/A102- peak intensity ratios in the SIMS spectra were not observed in the XPS analyses. Differences of this kind may reflect slight changes in the experimental conditions used to prepare the sets of samples for the XPS and SSIMS studies. For example, if phosphates do not absorb on the growing anodic film during anodizing but only when the current is switched off, the variation in phosphate levels may simply reflect differences in the time between anodizing and rinsing. Alternatively, the differences in the two sets of analyses may be derived from the higher degree of surface sensitivity of the SSIMS. For this to be the case, however, variation of the composition of the surface layer sampled by SSIMS must be accompanied by corresponding composition variations beneath the surface, such that the total concentration in the 2.5 nm layer analysed by XPS appears constant. Regardless as to the cause of the differences, however, the fact that phosphates provide corrosion inhibition means that it is important that factors affecting the total concentrations and distribution within the surface layer should be studied further.

Onset of filament growth similar to the kind observed in the current study has been correlated with the improvement of the adhesive properties of dc- phosphoric acid anodic filmsi6 and boehmite surfaces." Such filaments either deflect peel stresses into the bulk of the organic coating or result in the formation of a fibre-reinforced interfacial layer. Variations in durability of the coated surfaces in corrosive environments,

however, are likely to be derived also from the variations in surface chemistry noted.I8 In future, therefore, it will be of interest to correlate the results obtained with adhesion and corrosion resistance studies so that relative contributions of surface chemical and topo- graphic changes can be assessed.

CONCLUSIONS

ac-Phosphoric acid anodizing of aluminium alloys results in the formation of featureless barrier films or filamented structures on the surface, depending upon the anodizing time. The process is also a very effective method of removing organic contaminants from the metal surface. SSIMS analyses show that changes in the surface chemical state of aluminium occur during anodizing but these cannot be detected by XPS analysis. Phosphates remain on the anodized surfaces but the factors that determine the concentrations present are not known.

The production of clean filamented surface structures confirms that the ac-phosphoric acid anodizing pro- vides an effective base for adhesives, paints and lac- quers. In addition, the presence of surface phosphates and changes in the chemical state of the aluminium will affect the durability of the coated material.

Acknowledgement

The authors wish to thank Mr M. P. Amor of A l a n International, Banbury, for the use of the SEM images. The SEM method of exami- nation described was pioneered by Mr Amor.

REFERENCES

1. T. S. Sehmbhi, C. Barnes and J. J. B. Ward, Trans. Inst. Met.

2. E. A. Wootton, sheet Met. Ind. 53,297 (1 976). Finish. 62, 55 (1984).

3. N. C. Davies and P. G. Sheasby, European Patent No. 01 181 173 (November 1 985).

4. D. J. Arrowsmith, D. A. Moth and A. Maddison, Trans. Inst. Met. Finish. 65, 38 (1987).

5. J. D. Venables, D. K. McNamara, J. M. Chen, T. S. Sun and R. L. Hopping, Appl. Surf. Sci. 3,88 (1 979).

6. D. Briggs, A. Brown and J. C. Vickerman, Handbook of Sec- ondary Ion Mass Spectrometry. Wiley, Chichester (1 989).

7. C. D. Wagner, L. E. Davies. M. V. Zeller, J. A. Taylor, R. H. Raymond and L. H. Gale, Surf. Interface Anal. 3, 21 1 (1981).

8. A. C. Evans and J. Franks, Scanning 4,169 (1 984). 9. J. A. Treverton and M. P. Thomas, Int. J. Adhes. Adhes. 9,

10. T. E. Madey, C. D. Wagner and A. Joshi, J. Electron. Spec- 211 (1989).

trosc. flelat. Phenom. 10, 359 (1 977).

11.

12.

13.

14. 15. 16.

17.

18.

C. D. Wagner, W. M. Riggs, L. E. Davies, J. F. Moulder and G. E. Muiliberg (eds), Handbook of X-ray Photoelectron Spec- troscopy. Perkin-Elmer, Eden Prairie, M N (1 979). J. P. O'Sullivan and G. C. Wood, Proc. R. Soc. London, Ser. A 317,511 (1970). J. S. Ahearn, T. S. Sun, C. Froede, J. D. Venables and R. L. Hopping, SAMPEO. 12,39 (1980). A. J. Kinloch, J. Mat8f. Sci. 15, 21 41 (1 981 ). R. H. West and J. E. Castle, Surf. Interface Anal. 4,68 (1 982). J. S. Ahearn, T. S. Sun, J. D. Venables, C. Froede, D. K. McNamara, J. M. Chen and R. L. Hopping, Met. Soc. A 125 (1 982). R. Bainbridge, P. Lewis and J. M. Sykes, Int. J. Adhes. Adhes. 2,175 (1982). G . D. Davies, J. S. Ahearn, L. J. Matzieno and J. D. Venables, J. Mater. Sci. 20. 925 (1 985).

ssims, xps and microstructural studies of ac-phosphoric acid anodic films on aluminium

Documents