2098
Langmuir 1992,8, 2098-2100
Mechanical Instability of Anodic Films on Aluminum 0. Teschke,. M. U. Kleinke, and F. Galembeck Instituto de Fisica and Instituto de Quimica, UNICAMP, 13081 Campinus, SP, B r a d Received April 9,1992. In Final Form: June 15, 1992
The mechanical instability of anodic oxide films formed on aluminum rod surfaces was analyzed using Euler's method applied to thin cylindricalshells under compressive forces. The ratio A y / E between the aluminum oxidelaluminumwork of adhesion per unit area Ahy and the bulk modulusE of the film-forming aluminum (hydrous) oxide was determined for coating fiis formed under various anodic currents. Building up anodic films formed on metallic surfaces involvesmay steps according to the literature:lp2(a) metal dissolution and hydrous oxide precipitation; (b) oxide particle nucleation and growth1 (c) high-field ion injection. Nucleation followed by particle growth produces colloidal particles of hydrous metal oxides, whose tendency to gelation is well kn0wn.3 Further morphological changes are related to individual particle change (e.g., crystallization and recrystallization) and film changes (due to the evolution of interparticle forces and to geometrical constraints) throughout aging, both in a wet environment and during the drying process. Particle and film aging, water removal, and oxide condensation reactions all introduce some degree of stress within the film and in the filmsubstrate interfa~e.~ Consequently, stresses in growing oxide films arise from the volume change which occurs when a specificquantity of a metal is converted into metal oxide. The ratio of the volume of oxide to the volume of the metal (Pilling-Bedworthratio) is 1.28: and this may c a w buckling of the anodic film especially when the oxide forms on surfaces with large curvatures. In this work, oxide film formation and buckling were observed, during anodization of aluminum metal. The observations made and the quantitative results thus obtained are interpreted by using Euler's method applied to thin cylindrical shells. The electrodes were prepared from a 2 mm diameter aluminumwire supplied by Alcan, 99.2 7% pure. They were abraded with emery paper (grit 600), then polished with diamond paste (through 0.5 pm) and annealed at 300 "C for 2 h in 10-l mbar of air. The specimens were immersed in chromic-phosphoric acid solution for 5 min at 80 "C, washed with distilled water, and anodized. The counter electrode is a cylindrical platinum mesh and the radial distance between the concentric electrodes is 1.5 cm. The cell was made completely from PTFE and was thermostated at 25 1 "C. The electrolyte solution (prepared with double distilled water) was 15% H2S04 (Merck analytical grade). The galvanostat is a PAR Model 273A galvanostatlpotentiostat. After removal from the electrolyte, the electrode surfaces were examined in a JEOL TS-300scanning electron microscope. Observation of the samples shows that aluminum rods have a smooth surfacebefore anodization,and as the anodic film is grown, the appearance of a bulge in the oxide film is observed, as is shown in Figure la. The oxide pores are
*
(1)Diggle, J. W.;Downie, T. C.; Goulding, C. M. Chem. Rev. 1969,69, 365. (2) Campbell, D. S. Handbook of the Thin Film Technology;Maissel, L. I., Glang, R.; Eds.; McGraw-HF: New York 1970; p 12. (3)Voyutaki, S. Colloid Chemrstry; Mir: Moscow, 1978; p 343-ff. (4)Vermilyea, D.A. J. Electrochem. SOC.1963, 110, 345. ( 5 ) Bradhurst, D. H.; Leach, J. S. L. J. Electrochem. SOC.1966,113, 124.5.
enlarged in this area, exactly as is observed in protrusions of blocks of flexible foams under compression. Figure l b shows a later stage of film formation. At this point, longitudinal striae are seen in the outer periphery of the oxide film. These striae correspond to the maxima of outward, radial displacements of the compressed A 1 2 0 3 film. Figure 2c shows a crack formed as the result of bending of the oxide layer at its cusps. Film bulging and striae formation shown in Figure l b indicate that the strained oxide layer becomes unstable as it thickens, whenever the electrical charge used in building up the oxide film exceeds a critical value. The value of this critical coating thickness is determined by using an argument based on Euler's theory of mechanical instability of thin cylindrical shells under compressive stresses.6 This theory predicts that striae will be formed periodically, at wavelengths Am = 2uR/m, where R is the rod radius and m is the number of oxide striae, whenever the shell is under compressivestress. For a thin Cylindrical shell the critical value of the compressive strain cc is given by Euler's expression6 h2(m2- 1) 2(1- v2)R2 where h is the film thickness and Y is Poisson's ratio. Consequently the elastic layer under compression and adhering to an elastic substrate undergoes an elastic instability corresponding to the well-known Euler instability as soon as the compressive stress reaches a critical value, which depends on the modulus of the layer as well as on its thickness. At the compression limit for buckling, the stress within the film is given by eq 1and consequentlythe stored energy per unit length in the coatingis 2uR h ec2 El2. The energy dissipated in the buckling process is equal to the bonding surface energy per unit length7 Ayl2uR(,where Ay is ( h y d + yol- ymo) and ymois the oxidemetal interfacial tension, y,,,~is the metal-liquid tension, and hyol is the liquid-oxide tension; consequently cc
=
2uR h :e E12 = IAhyl2uR (2) To test the ideas presented above, we have carried out several experiments on anodization of aluminum rods of various sizes with two additives added to the electrolyte media. Ethylene glycol and glycerine were added to aqueous sulfuric acid to change the characteristics of the oxide film, in particular the AhyIE values. Both glycerine and ethylene glycol addition to the electrolyte results in a large change in the AhylE values compared with the plain ~
(6) Timoshenko,S. P.;Cere, J. M. Theory ofElastic Stability;2nded.; McGraw-Hill-Kogakusha: Tokyo, 1961. (7)Cherepanov, G. P.Mechanics of Brittle Fracture; McGraw-Hill: New York, 1979; p 640.
0743-7463/92/2408-2098$03.00/0 0 1992 American Chemical Society
Letters
Langmuir, VoZ.8, No. 9, 1992 2099 Table I. Effect of Electrolyte Additives on the Number of Striae and A?/E Values Formed in the Anodic Oxide Films, for Sulfuric Acid at 7.5% and Z = 50 mA*cm-2 A?/E,
solution
critical thickness, Pm
no. of striae
10-10m
Pure 2% glycerine + 10% ethylene glycol
18.5 18.5 25.5
16 28 18
0.18 1.80 1.50
+
Table 11. Anodic Film Data. I. mA-cm-2
critical thickness, pm
20 27 35 35 50 50 75 75 100 100 150 200
54.0 20.4 16.6 22.5 19.0 18.5 15.8 15.8 14.2 13.8 14.7 12.2
no. of striae 2 4
12 12 16 16 14 20 22 24 22 32
The parametersare the current density1(mA*cm-2),the estimated critical oxide film thickness h (pm), and the measured number of striae m.
Figure 1. Optical and SEM micrographs showing the buckling of the oxide film and the pattern of striae along the periphery
of the electrode: (a, top) buckling of the anodic film, the oxide pores are enlarged in this area, I = 4 A-cm-2,t = 0.8 s; (b, middle) longitudinal striae along the perimeter of the oxide film, I = 100 mA*cm-2,t = 600 s; (c, bottom) enlarged view of a stria.
solution results (Table I). A satisfactory understanding of the role of glycerine and of ethyleneglycol is not possible at this time, because to our knowledge there is no information in the literature on the effects of these substances on mechanical and rheological properties of aluminum oxide gels. However, it should be recalled that glycerine is known to replace water in hydrated systems, strengthening the H-bond network, an ability which derives from its connectivity 6. For instance, surfactant critical micelle concentrations (cmc’s) show little change,
in the presence of glycerine.8 On the other hand, ethylene glycol has the same connectivity for H-bonding as water, which is equal to four (two on each hydroxyl groups). In addition to that, in ethylene glycol two connection sites are separated from the other two by an -0-C-C-0 chain, which is flexible and should thus impart flexibilityto the network. Now, regarding the solvation of the aluminum ions, glycerine is a tridentate ligand, which can be expected to be effective as a cross-linking agent in the oxide network. Glycerine addition to the solutionresulted in a Ay/Evalue increase of 1order of magnitude compared with the plain solutionvalue. Ethyleneglycol isjust bidentate and should not be as effective in making a denser network as glycerine; the measured A y / E value with the addition of ethylene glycol was 1.5 X 10-lo m, slightly smaller than the value obtained with the addition of glycerine. For various current densities, the time intervals corresponding to onset of failureby bucklingwere determined. We have evaluated the stress in the oxide film by counting the number of striae formed and using eq 1 and the parameters given in Table 11. From these data and eq 2, we calculated Ay/E for various films. The results shown in Figure 2 indicate that for low current densities (around 20 mA*cm-2,region I in Figure 2) the value of Ay/E is low, ca. -0.01 X m; this current density corresponds to the deposition condition for “soft” anodic coating^.^ As the film current density increases, there is a substantial increase in the Ay/E value up to ca. -0.2 X 10-lom where it reaches a region of slower increase;“hard”coatingsgare formed under such current density conditions. Finally, the third region (-200 mA*cm-2) corresponds to the current density range used for etching aluminum electrodes. Figure 2 shows that large current densities create films with large Ay/E values. We are now investigating the reasons for large values of Ay/E (large Ay, small E, or both). The fact that in region I1 (Figure 2) Ay/E shows little change with current density suggests that both Ay and E ~~
~
(8)Cantu,L.; Corti, M.;Sonnino,S.; Tettamanti,G.J.CoZZoidInterface Sci. 1987,116,384. Kothwala,P.; Deisai,A.; Bahador, D. TensideDeterg. 1985,22,1985. (9) Pletcher, D.; Walsh, F. C. Industrial Electrochemistry, 2nd ed.; Chapman and Hak London, 1990.
Letters
2100 Langmuir, Vol. 8, No. 9, 1992
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Figure 2. A-y/E ratios obtained from the experimental data and the measurednumberof striae ma function of the currentdensity. Other parameters were R = 2.3 mm and Y = 0.17.
are nearly independent of current density. Local current density fluctuations usually should arise, during anodization. These fluctuations may create nonuniformity of the film coatings,1° since the mechanical and interfacial (10) Teschke, 0.;Kleinke, M. U.; Galembeck, F. Langmuir 1989, 5, 844.
characteristics of the films formed in regions I and 111 depend on the anodizationcurrent density. F h f o d in region I1 do not show any mechgpical or interfacial dependence on current density and consequently are highly uniform. In conclusion, mechanical instability of anodic filmson aluminum rods takes place at various coatingthicknesses depending on the current density values used for film formation. T h e comparison of experimental values with the predictiona of a model baaed on Euler's conditions
(governinsthestabiliCyofstructuralelemeatsasthinshells when they fail by buckling) was done. Thb allows us to determine the ratio between the work of adheeion (of the oxide coating to the aluminum substrate) and Young's modulus of the film. On the more practical side, the use of this model gives a basis for underatanding the wellknown changes in the mechanical properties of anodic aluminum coatings with current density during coating formation.
Aetnowledgment. F.G.acknowledgee the support of FAPESP. Regis&ry No. AI, 74S90-6; HnSOa, 7664-93-9; glycerine, 6681-5; ethylene glycol, 107-21-1; aluminum oxide, 1961-28-1; aluminum oxide hydrate, 1333-84-2.
