phys. stat. sol. (c) 4, No. 3, 740– 743 (2007) / DOI 10.1002/pssc.200673765
Nanostructured metal/oxide coatings F. Muktepavela*, 1, G. Bakradze1, and S. Stolyarova2 1 2
Institute of Solid State Physics, University of Latvia, 8 Kengaraga, Riga 1063, Latvia Technion, Solid State Institute, Technion-City, Haifa 32000, Israel
Received 7 July 2006, revised 19 September 2006, accepted 25 September 2006 Published online 9 March 2007 PACS 68.35.Gy, 68.35.Np, 68.37.-d, 68.65.-k Al based nanocomposite coatings on Cu and glass substrates were obtained in the conditions of severe shear stresses by deformation scheme similar to that of friction. Structure, composition and micromechanical properties were investigated using AFM, XRD, SIMS, electron, optical microscopy and precision microindentation techniques. Coatings are characterized by the high microhardness and good adhesion to the substrates. This is determined by the formation of nanostructured Al-oxide composite stabilized by the presence of oxidized interlayers, which are barriers for the grain growth and intermetallic phase formation. The annealing in vacuum leads to the development of oxygen redistribution processes on interphase boundaries in coatings according to the metal-oxygen interaction thermodynamics. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction As it was shown in some experimental [1, 2] and theoretical works [3] a strong adhesion could be observed on metal/oxide interfaces fabricated by surface activated bonding at room temperature. One of the activation methods is the plastic deformation of near-surface layers. Earlier [4] we have shown, that during plastic deformation of metals (Al, Mg, In, Pb) on the surface of oxides (MgO, glass etc.) at room temperature a level of chemical adhesion in the regions of the maximum shear stress could be reached. The effect was significant for metals with the high affinity towards oxygen (Al, In, Mg). On the other hand, it is known, that severe plastic deformation is one of the method for obtaining nanostructured metals with good complex of mechanical properties due to the formation of dislocationdisclination substructure with the grain size of 10-100 nm. According to the empirical Hall-Petch equation, the dispersion of crystalline structure at T≤0.5·Tm promotes essential strengthening of the material: σy = σo + k / d , where σy is the yield strength, σo is the intrinsic strength impeding the motion of dislocations, k is a constant, and d is the grain size. However, nanostructure of metals obtained by severe plastic deformation is not thermally stable [5]. We have chosen a method of rotating wire brush [6] to obtain nanostructured Al based coatings on different substrates (glass, Cu). During such treatment, Al particles are cut out from a bulk Al specimen with steel wires and transferred to the substrate under the conditions of severe dynamic shear deformation and intensive oxidation, which might favour the formation of oxidized interlayers and therefore ensure the thermal stability of nanostructure. Based on our previous investigations [4], which have shown that strong chemical adhesion between the contacting metal and oxide occurs only in the regions of the maximum shear stresses, we can expect a strong adhesion both inside the composite coatings and on the coating/substrate interface. The dynamic procedure of coating formation and particle interaction in the conditions of severe shear deformation show similarity to the microscopic act of friction, therefore this method can justifiably be designated as a microtribological one. *
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The aim of the present work was to obtain Al-oxide nanostructured coatings on glass and Cu substrates by microtribological method and to investigate their microstructure, micromechanical properties and thermal stability. The last was necessary to reveal the role of oxidized layers as barriers for grain growth or secondary phase formation on the interfaces that is especially important for Al/Cu system which is characterized by the complex state diagram. 2 Experimental Aluminium based coatings on boron-silica glass and copper substrates (Al/glass, Al/Cu systems) were obtained by the method of rotating wire brush [6]. The procedure is performed in ambient atmosphere at room temperature. The surfaces of glass and Cu were preliminarily degreased and mechanically cleaned. The thickness of the coatings obtained was 1-3 µm for Al/glass and 5-50 µm for Al/Cu systems. Micromechanical properties of the coatings were investigated by Vickers microhardness methods. Used microhardness tester is insensitive to the vibrations and is suitable for accurate surface hardness measurements over a wide load range from 0.14×10–3 to 2N. The indentation depth was calculated as 1/7 of the impression diagonal. The structure and chemical composition of coatings were investigated using AFM (Nanoscope), XRD (DRON-3M, Cu-Kα1), SIMS (Ar+, E = 6 keV), and SEM with EDX (Zeiss EVO 50XVR) equipments. All annealing experiments were carried out in high vacuum (10–6 Pa) at temperatures 373-823 K for 20 min. 3 Results and discussion Figure 1 shows the change of microhardness in depth of obtained Al coatings on glass and Cu substrates. The hardness values of Al coatings on different substrates noticeably exceed those of initial metal (3 times). As it can be seen, the microhardness of Al coating varies within the coating depth. Microhardness H, GPa
10
Cu
Al 11000
0.1100 0
deformed Al annealed Al
1 2 3 Penetration depth h, µm
(a)
175
Al
150
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0,1
4
200
glass
125 Io 0
1 2 Penetration depth h, µm
I+, arb.units
Microhardness H, GPa
10000 10
100 3
75
(b)
Fig. 1 Dependence of microhardness (H, GPa) on the indentor penetration depth (h, µm) for Al coatings on Cu (a) and glass (b) substrates in as-obtained state. Graph (b) also shows the depth profile of SIMS-signal intensity for Al+ ions in the Al coating on glass (Io – intensity of non-oxidized Al SIMS signal).
The hardness values of Al/Cu and Al/glass for the indentation depth up to 1-2 µm are related to the coating material. The increase in microhardness values at higher indentation depths (>2 µm) is caused by the influence of hard substrates. As it is seen, at the indentation depth of 1µm the microhardness of Al/glass and Al/Cu is about 1.4 GPa; this value is not comparable with that for the deformed (about 420 MPa) or annealed (about 250 MPa) Al, and with the microhardness of aluminium or copper oxides (Al2O3 – 20 GPa, Cu2O – 2.01 GPa, CuO – 2.05 GPa). In the near-surface layers (0.1–0.5 µm) the microhardness of all investigated Al coatings is about 4–4.5 GPa. This significant hardening can be explained by higher concentration of alumina in the near-surface layers. The results obtained allow assumption to be made that during severe shear deformation a new heterogeneous small-grained material is formed, which is stabilized by the presence of oxidized interlayers. To verify this hypothesis, the chemical compound of the coatings was experimentally level-by-level traced by the SIMS method. In Fig. 1b, as an example, the result obtained for Al/glass system is given, which shows the presence of oxidized Al
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F. Muktepavela et al.: Nanostructured metal/oxide coatings
throughout the coating. The SIMS result validates the microhardness measurements and testifies to the presence of oxidized metal in coatings with oxygen concentration gradient, which determines compositional non-homogeneity of the coatings. EDX studies have revealed the presence of Cu on the surface of Al coating on Cu (Fig. 2a). These results show that the obtained coatings contain oxidized phases: Al-O in case Al/glass and Al-O, Cu-O in case Al/Cu.
(a)
(b)
Fig. 2 The AFM images of Al coatings on Cu (a) and glass (b) substrates in as-obtained state. Image (a) also presents an EDX spectrum of Al coating.
Structural studies using electron and atomic force microscopy (Fig. 2) have shown that the obtained coatings are nanostructured with a grain size from 20 to 300 nm. This result was confirmed by the XRD data, whereas the grain size was calculated through broadening of diffraction peaks, using initial annealed and deformed metals as a standard. The nanostructure of investigated coatings is not homogeneous due to the dynamic procedure of coating formation. Thus, the high microhardness values of Al coatings are caused not only by the presence of oxidized interlayers, but also by nanostructured state. No indications of coating detachment from the substrates were observed, this testifies to the good adhesion between coatings and substrates. From both SIMS and microhardness (Fig. 1b) data we can estimate the mutual diffusion coefficient (D) through the Al/glass interface. Assuming that the interfacial reaction zone is equal to the diffusion length (x ≈ 1.0 µm), and coating obtaining time (t ≈ 60 s), then according to the equation D = x 2 / t , D is equal to about 2·10–10 cm2/s. This result seems to be reasonable and is in good agreement with diffusion coefficients in Al/quartz systems obtained in wetting studies [7]. High value of D in our experiments could be explained with the high density of defects in oxide surface and nanostructured state of metal [8]. Thus, the formation of reaction zone on metal-oxide interfaces occurs due to the shear stress induced defects formation in oxide, nanostructure formation in metal and oxygen diffusion activity at developed grain boundaries of nanostructure. This phenomenological model agrees with theoretical approaches [9]. As it was shown above, high microhardness of Al based coating is caused by the nanocomposite structure Al-O and Al-O-Cu. However, XRD studies have shown presence of only Al peaks in Al/glass system and Al and Cu peaks in system Al/Cu; and no presence of stoichiometric oxides has been revealed in as-obtained coatings (Fig. 3). During annealing of Al/Cu samples in vacuum a peak characteristic for Al2O3 appears in XRD patterns beginning with the annealing temperature of 373 K. After annealing at 823 K in XRD patterns (Fig. 3) additional peaks characteristic for stoichiometric copper oxides appear (CuO and Cu2O) and increase of microhardness is observed (Fig. 4). As the annealing experiments were performed in vacuum, the influence of atmosphere oxygen is excluded. At the same time, remarkable is that no formation of intermetallic compounds was revealed after annealing of Al/Cu at such high temperatures. This indicates that oxygen redistribution processes occur only along internal Al-O-Cu interfaces. As the energy of Al2O3 formation is much lower than that for Cu2O or CuO (-1674×10–6 J/kmol vs. -168×10–6 J/kmol and -157×10–6 J/kmol, respectively), the formation of Al2O3 is energetically more favourable even at low annealing temperatures. If we estimate additive microhardness, considering contributions of all components (Al, Cu, and all oxides), we get the microhardness value close to the ex-
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perimental. This evidences, that the oxidized interlayers in the obtained Al/Cu nanocomposite coatings have the size close to that of grains (20-300 nm). Thus, the presence of oxidized interlayers is obstacle for both the grain growth and new phase formation in this system. 2,5
10
1,0
Cu Al
0,5
Al
Al
0,0
-0,5
Cu Cu
after annealing @ 823K
Cu
Al
Al2O3
CuO Al2O3
Al2O3
Al2O3
1,5
Cu2O
Relative intensity
2,0
Cu Al
Cu
before annealing
30
40
50 60 70 80 90 100 Diffraction angle 2θ
Fig. 3 XRD patterns of Al coating on copper substrate in asobtained state and after annealing in vacuum at 823 K.
Microhardness H, GPa
Al
Al Cu
1 annealed Cu annealed Al
0
1 2 3 4 Penetration depth h, µm
5
Fig. 4 Dependence of microhardness (H, GPa) on the indentation depth (h, µm) for Al coating on Cu substrates after annealing in vacuum at 823 K.
At the same time, no changes in the microhardness and in the structure in the system Al/glass after annealing at high temperatures were detected. This could be explained by the presence of stable thin oxidized interlayers, which form in the process of coating formation. Results show, that the system Al/glass can be used in wide interval of temperatures, maintaining its properties and structure. However, there are temperature limitations for the use of the system Al/Cu. 4 Conclusions Using phenomenon of micromechanical mass transfer by wear nanocomposite Al-Ocoatings on Cu and glass substrates have been obtained in the conditions of severe shear stresses by deformation scheme similar to that of friction. Microhardness of obtained coatings is by the factor of 3 higher than that for the source metals due to the presence of oxidized interlayers in coatings. Coatings show strong adhesion to the substrates because of shear stress induced formation of reaction zone on the interface metal/substrate. The annealing in vacuum leads to the oxygen redistribution along internal interphase boundaries keeping the role of oxidized interlayers as barriers for grain growth. Al/glass system is more stable in wide interval of temperatures, than the Al/Cu system, in which a possibility of different oxides (Cu2O, CuO, Al2O3) formation exists in the accordance with thermodynamics of metaloxygen interaction. Acknowledgement Authors would like to express deep thanks to Dr. E.Tamanis for XRD measurements.
References [1] J.T. Klomp and A.J.C. Van de Ven, J. Mater. Sci. 15, 2483 (1980). [2] T. Akatsu, N. Hosoda, T. Suga, and M. Rühle, Mater. Sci. Forum 329, 294 (1999). [3] Yu.F. Zhukovskii, E.A. Kotomin, P.W.M. Jacobs, A.M. Stoneham, and J.H. Harding, J. Phys.: Condens. Matter 12, 55 (2000). [4] F. Muktepavela, G. Bakradze, E. Tamanis, S. Stolyarova, and N. Zaporina, phys. stat. sol. (c) 2, 339 (2005). [5] S.C. Tjong and H. Chen, Mater. Sci. Eng. R 45, 1 (2004). [6] V.I. Kacheev, Device for Applying Coatings WO 95/10392. [7] E.P. Trifonova, V. Lasarova, L. Spassov, and N. Efremova, Cryst. Res. Technol. 34, 391 (1999). [8] L.N. Paritskaya, Yu. Kaganovskii, and V.V. Bogdanov, Solid State Phenomena 101/102, 123 (2005). [9] M. Backhaus-Ricoult, Philos. Mag. A 81, 1759 (2001).
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