Influence of Interfacial Structure Development on the Fracture Mechanism and Bond Strength of Aluminum/Copper Bimetal Plate

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1 Materials Transactions, Vol. 47, No. 4 (26) pp to 1239 #26 The Japan Institute of Metals EXPRESS REGULAR ARTICLE Influence of Interfacial Structure Development on the Fracture Mechanism and Bond Strength of uminum/copper Bimetal Plate Chih-Yuan Chen 1; *, Hao-Long Chen 2 and Weng-Sing Hwang 1 1 Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan, R. O. China 2 Department of Electronic Engineering, Kao Yuan University, Kaohsiung County 821, Taiwan, R. O. China The aim of this article is to study the influence of interfacial structure development at interface on the fracture mechanism and the bond strength of cold roll bonded / bimetal plate. The / bimetal plates are produced by cold roll bonding and then sintered at different conditions. The bond strength of the / bimetal plate increases generally to maximum values and then decreases to low values with increasing sintering temperature and time. Interfacial structures develop with increasing sintering temperature and time. The main interfacial layers are 2,, 3 4 and 4 9. The formation and thickening of those intermetallic compounds promotes cracks propagation and weakens the bond strength of the bimetal plates. The fracture mechanism transforms from ductile to brittle cleavage with the development of interfacial structures. While the bond strength of the material starts to decrease, no obvious Kirkendall effect of void formation is observed in the present study. (Received January 17, 26; Accepted February, 26; Published April 15, 26) Keywords: bimetal, cold roll bonding, intermetallic compound, bond strength, fracture mechanism 1. Introduction *Graduate Student, National Cheng Kung University Clad metal materials have become increasingly popular for industrial applications in recent years, such as /, / SUS, /Ag, /Ni, Steel/Ti, etc. 1 4) They sometimes possess enhanced mechanical properties and corrosion resistance. Metallurgical bonding to is widely used as a transition piece in high direct-current bus systems to transmit electricity. 5) Thus the bonding properties of the material have a great influence on its electrical properties. From the viewpoint of the metallurgy in the bonding process, and are incompatible metals because of high diffusion affinity to each other at temperatures above 1 C that results in the formation of intermetallic compounds that are brittle, and with high electric resistance on their interfaces. 6,7) Therefore, fusion-welding processes are not applicable for bonding of to. The clad metal can be produced by some solid-state welding such as explosion welding, friction stir welding, diffusion bonding, or co-axial extrusion. However, the most economic and productive manufacturing process for large size flat clad metal sheets and foils is roll bonding. Roll bonding is a well-established and widely used solid state welding process to join dissimilar metals. Compared with hot roll bonding, cold roll bonding has several advantages, including more uniform individual layer thickness ratio, good surface quality and lower oxidation during bond process, which makes it suitable to be applied on some ductile metals like and. 8,9) After the roll bonding process, a sintering treatment is usually applied to enhance the bonding strength of the materials. The post sintering treatment generally governs both the bond strength and mechanical properties. 10,11) Some previous investigations have revealed that the process parameters have a crucial influence on the final properties of the / bimetal plate. Those process parameters including surface preparation, rolling reduction, rolling temperature, sintering temperature and time, significantly affect the bond strength. According to some previous studies on the mechanism producing metallic bonding in the roll bonding process, surface scratch-brushing is a particularly effective form of surface preparation for cold rolling because of its higher surface decontaminating effect and the production of suitable surface roughness. Higher cold reduction provides plastic deformation energy to create more effective bonding areas in cold roll bonding, 12,13) and the bond strength could be enhanced at suitable roll bonding temperature. 14) Some studies on the roll bonding of / bimetal laminates show that the interfacial reactions that occur in the sintering process, named interfacial phase transformations and Kirkendall void formation, dominate the variation of bond strength. As the sintering time and/or temperature increase, the Kirkendall effect becomes more significant with the agglomeration of voids, which leads to the formation of weak layers in the interface region ) In this study, / bimetal plates are made using cold roll bonding. The development of interfacial structure is performed and identified at different sintering conditions. The objectives of the present study are to investigate the influence of the development of the intermetallic compound on mechanical properties during sintering treatment under various temperatures and times. The relation between the interfacial development and fracture mechanism provides valuable information in controlling process parameters. 2. Experimental Procedure 2.1 Fabrication of / bimetal plates The metal sheets used in this study were pure copper (C110) with the primary dimensions of :8 mm, and aluminum (AA1050) mm, which were shown in Table 1. Cold roll bonding process was carried out using a laboratory rolling mill with a roll road capacity of 2 tons. The roll diameter was 4 mm and the roll speed was about 10 m min 1. The uminum and Copper sheets were polished both mechanically and chemically in the

2 Influence of Interfacial Structure Development on the Fracture Mechanism and Bond Strength of uminum/copper Bimetal Plate 1233 Table 1 Specification of aluminum and copper plate. Grade Chemical composition (mass%) Yielding Plate thickness (mm) (MPa) Before bonding After bonding , Si+Fe<0.95, Zn0.1, other< C110 >99.9, O 2 < (1) Surface treatment (2) Stack (3) Cold roll bonding (4) Post-sintering Brittle oxide/hardening layer (by scratchbrushing) Tension force (RD) Normal Roll pressure A B Tension force Oxide/hardening layer fracture Virgin metal surface exposure Adhesive bonding Diffusion bonding Normal Roll force (ND) Extrude of virgin metal Stage 1 Virgin metal contact and form bonding Fig. 1 Schematic illustration of cold roll bonding. Stage 2 Stage 3 Peel force Fig. 3 Schematic diagram of film theory for cold roll bonding. 15mm Fig. 2 air to obtain a clean surface before cold roll bonding. The unbonded sheet was packed and rolled at room temperature, as shown in Fig Material characterization The / bimetal samples produced were heat treated at different temperatures of 250, 3, 4, 5 C, for different holding times between min. The temperature deviation of the annealing furnace was 2 C. The bond strength of the / bimetal plates was measured using a peel test. Test samples were machined as shown in Fig. 2. Peel test was performed using an Instron tensile testing machine with a 50 KN load cell. Tests were carried out using a crosshead speed of 10 mm min 1. The peel strength was taken as the average peel force divided by the bond width. Break-off peel strength could be identified as follows: p ¼ RD TD F break-off L wide sample Peel force Where p was break-off peel strength (N cm 1 ), F was the breaking-off force (N), L is the width of test sample (cm). After the peel test, the relationship between peel strength and the characteristics of fracture morphology were investigated for the / bimetal plates at cold reduction of 57%. After materials were separated by peel force, the surface Dimensions of the peel test sample. ð1þ morphology of the various samples was observed through a Field Emission Scanning Electron Microscope (FESEM, Hitachi S-41). The formation of intermetallic phases in the interface was analyzed by X-ray diffraction technique (GID, Rigaku D/MAX25) on the fracture surfaces of the aspeeled specimens of -side and -side at a scanning speed of 3 min 1. The development of interfacial structures was identified using Wavelength-Dispersive Spectrometer (WDS). The route of crack propagation for the various samples were observed using FESEM to investigate the fracture mechanism. 3. Results and Discussion 3.1 Bonding of aluminum and copper plate Previous studies on the solid state bonding process of metals, take film theory as the major bonding theory to explain the formation of initial joints. The reactions between the metal plates involve a three-stage process of (1) development of physical contact, (2) activation of surfaces in contact and (3) interaction within the metals being joined. 12) Figure 3 shows the schematic diagram of film theory for cold roll bonding. Brittle work hardening layers and oxide contaminants formed by surface treatment will fragment under high pressure in a roll mill, and then fresh, virgin metals extruded through the cracks and contact within the range of mutually attractive force along the interface. A bond (normally a mechanical bond) is thus obtained through interfacial mechanical locking and atomic affinity between the two metals. In the present study, the virgin aluminum and copper extrude through cracks and contact to develop stable joints, as shown in Fig. 4. uminum is only found to adhere to copper at the cracks. In order to develop a stronger mechanical bonding, it is necessary to increase the number of effect joint areas. Figures 5 (c) show SEI and BEI images of -side specimens at different cold reductions of 45, 57 and 66%. The darker parts are cracks for effect bonding (virgin and adhere at cracks), and more evident while cold reduction increased from 45 to 66%.

3 1234 C.-Y. Chen, H.-L. Chen and W.-S. Hwang -side Brittle/hardening layer non-bonded area -side Table 2 Relationship between break-off peel strength available bonding areas. Cold reduction (%) Break-off peel strength (N/cm) Available bonding areas (%) Virgin metal jointed at cracks Fig. 4 Fracture morphology of as-rolled specimen -side and -side for a cold reduction of 57%. Higher reductions are found to generate a greater amount of heat and create more cracks at the contact surfaces of both metals being joined, which benefit bond strength, 9) as shown in Table 2. Peel strength, σ/n. cm -1 1 Break-off peel strength 250 o C-30min 5 o C-30min 5 o C-min 450 o C-30min 3 o C-30min as-rolled 3.2 Bonding strength evolution of / bimetal plate After the initial bonding produced by cold roll bonding, post heat treatments are applied to quicken the development of metallurgical joints and create a common lattice structure. Peel test is carried out to investigate how the bond strength varied with different sintering conditions. The peel strength (N cm 1 ) at different sintering conditions was determined. Figure 6 shows the typical peel strength vs. crosshead displacement curves at a cold reduction of 57%. The tendency of all test curves drops with increasing crosshead displacement, which is attributed to the fact that the bond strength of the / bimetal plate is smaller than the stiffness of the and plate. While cracks propagate, the crack tip deviates away from peel axial and forms a lever to peel the bimetal plate, as shown in Fig. 6. Figure 7 shows the variation of break-off peel strength with respect to sintering time at the sintering temperatures of 250 to 5 C. It is apparent that the samples sintered at 250 C generally have higher break-off peel strength than those sintered at 3, 4, and 5 C. As the sintering time increases, break-off peel strength of the / bimetal plate increases to a 0 Crack tip Crosshead dispalcement, d/mm Peel force Peel Crosshead displacement maximum values and then decreases. With increasing sintering temperature, the maximum value decreases and sintering period decreases. This may be attributed to the higher relaxation of the mechanical locking for materials sintered at higher temperature. Peel Peel Form a lever to peel sample Crack tip Fig. 6 The typical peel strength vs. crosshead displacement curve, and schematic diagram of peel strength decreased with crack propagation. (c) 2 µ m 2 µ m 2 µ m Fig. 5 Available bonding areas (cracks) at different cold reductions 45%, 57%, (c) 66%.

4 Influence of Interfacial Structure Development on the Fracture Mechanism and Bond Strength of uminum/copper Bimetal Plate 1235 Break-off peel strength, σ p /N. cm o C 3 o C 4 o C 5 o C Holdingtime, t/min Fig. 7 Variation of break-off peel strength of / bimetal strip for different sintering temperatures with times. Interface thickness, d/µm o C 3 o C 4 o C 5 o C Holding time, t/min Fig. 9 The variation of interface thickness with annealing time at different sintering temperatures: 250, 3, 4, 5 C. ~0 ~1.1 µ m ~1.4 µ m ~2.1µ m (c) (d) 5 µ m 10 µ m 10 µ m 10 µ m (e) ~5.0 µ m ~5.6 µ m ~15.4 µ m ~17.9 µ m (f) (g) (h) 10 µ m 10 µ m 10 µ m 10 µ m Fig. 8 Interface development of / bimetal plates at different sintering conditions: as-rolled, 3 C 15 min, (c) 3 C 30 min, (d) 3 C 90 min, (e) 4 C 15 min, (f) 4 C 30 min, (g) 5 C 15 min, (h) 5 C 30 min. 3.3 Development of interfacial phases Figures 8 (h) shows interfacial structure developments of / bimetal plates undergo different sintering conditions. As the sintering time and temperature increase, the interface continues to thicken and develop different microstructure layers. It is understood that both and atoms are thermally activated in the post sintering treatment process. At a sintering temperature of 3 C, the interface develops some intermetallic layers, as shown in Figs. 8 (d). With increasing the sintering temperatures higher than 4 C, the interface continues to thicken and consists of several different structures, as shown in Figs. 8(e) (h). Figure 9 shows that the interface thickens as the sintering time increases under various sintering temperatures. The overall thickness of interface increases dramatically at sinter temperatures higher than 3 C. WDS is applied to identify the interfacial phases developed at the sintering process. Figures 10 and show the interfacial phases development of the / bimetal plate sintered at 5 C for 30 and 1 min. The development of the interface phases could be identified on a line at points A, B, C,..., P, Q to specify the composition of each intermetallic layer. According to the equilibrium phase diagram of the - system, it can be seen that several - phases are stable at the studied temperature range of 3 5 C. WDS analyses performed across the interface provided the chemical composition of the interface structure. The - phases corresponding to the measured atomic concentrations are categorized to be 2 (), ( 2 ), 3 4 ( 2 ), 4 9 ( 1 ). The results of this analysis are presented in Table 3. The formation of interfacial phases is affected by their energies of formation. According to previous study, 18) the formation energies of 4 9 and 2 are 0.83 ev and 0.78 ev, and there is a greater diffusivity of in than that of in. Thus the first reaction product 2 is presumed, and then the next reaction phase is 4 9. and 3 4 are next. The same results could be observed in the present study. Figure 11 shows the relation between the interface thickness and break-off shear strength. Referring to the figure, break-off shear strength of the materials increases generally to maximum values and then decreases to low values with increasing the intermetallic width, and maximum values decrease with increasing sintering temperature. The analysis results shows that the decrease of break-off peel strength is not directly proportional to the intermetallic width, which reveals that there are some other mechanisms affect the bond strength of this material during sintering process.

5 1236 C.-Y. Chen, H.-L. Chen and W.-S. Hwang A B C D Q mass% mass% Distance, d/µm Distance, d/µm Fig. 10 WDS study of intermetallic compounds development of / bimetal plate at different sintering conditions: 5 C 30 min, 5 C 1 min. Table 3 WDS quantitative analysis at points A, B, C,..., P, Q to identify the composition of each intermetallic compound for / bimetal plate sintered at 5 C for 1 min. Analysis point mass% of mass% of Nearest intermetallic compounds A B C D E () F G H ( 2 ) I J ( 2 ) K L ( 1 ) M N O P Q Characteristics of fracture mechanism at different sintering conditions Figures 12 and 13 show the fracture morphology at surfaces of / peel tested samples. At the as-rolled condition, areas of ductile fracture can be separated by unbonded regions which consist of original scratch-brushed surface morphology. The fracture surface micrograph of the -side specimen shows parts of pure aluminum transfer from the -side to -side surface. Ductile dimple cups can be observed from part of the ductile fracture areas. With Break-off peel strength, σ p /N. cm o C 3 o C 4 o C 5 o C Interface thickness, d/µm Fig. 11 The relation between the interface thickness with break-off shear strength at different sintering temperatures: 250, 3, 4, 5 C. increasing the sintering temperatures, ductile fracture and scratch-brushed surfaces disappear gradually, and some brittle cleavage fracture morphology becomes more inevitable. Brittle cleavage fracture covers almost the entire fracture surface when the sintering temperature rises up to 4 C. The interfacial layers break under peel force during crack propagation process, as shown in Figs. 12(c) (d) and 13(c) (d). The interfacial layers form and thicken gradually across the interface, and the characteristic of fracture morphology changes from ductile damage to brittle cleavage with increasing the sintering temperatures. Thus the development of interfacial structures dominates the phenomenon of the transformation of fracture mechanism. X-ray diffraction is applied to investigate chemical composition on the fracture surfaces of both -side and -side specimens. Figures 14 and show the X-ray diffractographs of both -side and -side fracture surface specimens at different sintering conditions. At a sintering

6 Influence of Interfacial Structure Development on the Fracture Mechanism and Bond Strength of uminum/copper Bimetal Plate o C-30min 4 o C-30min 75 µ m 75 µ m 3 o C-30min (c) (d) o o o o o o o o o θ 75 µ m 75 µ m o C-30min Fig. 12 Fracture morphology of -side specimens after different sintering conditions: as-rolled, 3 C 30 min, (c) 4 C 30 min, (d) 5 C 30 min. 4 o C-30min 3 o C-30min o o o o o o o o o θ Fig. 14 X-ray diffraction spectra measured on the specimens of side, and -side for different sintering conditions: as-rolled, 3, 4, 5 C for 30 min. -rich phase ( 4 9 ) 75 µ m 75 µ m (c) (d) -rich phase ( 2 ) 5 µ m Fig. 15 The cross section image (SE image) of the fractured sample sintered at 3 C for 30 min shows the route of crack propagation. 75 µ m 75 µ m Fig. 13 Fracture morphology of -side specimens after different sintering conditions: as-rolled, 3 C 30 min, (c) 4 C 30 min, (d) 5 C 30 min. temperature of 3 C for 30 min, aluminum and copper are detected on - and -side fracture specimens respectively, which is attributed to the fact that the thickness of the interfacial layers is too thin to be detected, as shown in Fig. 8(c). As the sintering temperature rises up to 4 C, intermetallic compounds of 2 and 4 9 thicken and can be detected on the fracture surfaces, which become more obvious when sintered at 5 C. The diffraction peaks of and gradually weaken. The results of this analysis show the deep relationship of the development of interfacial structures with fracture morphology of the / bimetal plate. With the development of interfacial structures, a ductile to brittle behaviour is observed. Figure 15 shows the cross section image of the fractured sample annealed at 3 C for 30 min. A line tracing along the fracture surface indicates the part where cracks propagate at the interfacial layer, and the remaining cracks propagate along the boundary between interfacial layer and base material. With increasing sintering temperature and/or time, all of the cracks propagate at the interfacial layers. Most cracks propagated through the zone between the 4 9 and 2 layers, and the remaining cracks attack some local parts of the 4 9 and 2 layers. When the sample

7 1238 C.-Y. Chen, H.-L. Chen and W.-S. Hwang Parts of & 3 4 fall off -rich phase ( 2 ) -rich phase ( 4 9 ) 15µ m Fig. 16 The cross section image (SE image) of the fractured sample sintered at 5 C for 30 min shows the route of crack propagation. Bond strength increasing Bonding areas increase with the atoms diffusion Bond strength superimposed Effect of the interface development Effect of mechanical lock (relaxation of residual stress) Annealing time & temperature 2-layers (3 o C-30min) 1~2um -rich phase ( 4 9 ) -rich phase ( 2 ) Crack propagation formation of 2 & 4 9 interfacial layers develop and thicken Formation of Kirkendall voids Fig. 18 The combined effects of interface reactions on the variation of the bond strength. 4-layers (5 o C-30min) 15~um 3 4 -rich phase ( 4 9 ) -rich phase ( 2 ) Crack propagation Fig. 17 Schematic diagram of the cracks propagation for different interfacial structure initial periods of interface development, interface fully develop and thicken. sintered at 5 C for 30 min, this phenomenon becomes more obvious and some fragments fall off, as shown in Fig. 16. The analysis results show that the crack usually propagates at the interfacial layers, and it is reasonable to expect that the bond strength between the intermetallic layer and base materials is higher than that between interfacial layers. Therefore, at the as-rolled condition, the ductile fracture of the material occurs at the effect joint areas of / interface. As the sintering temperature and/or time increase, the interface continues to develop and thicken. The brittle interfacial structure gradually dominates the fracture morphology. The summarized results of the able mentioned analysis are shown in Fig. 17. In the present study, the interface development affects the bond strength and fracture characteristics. In the initial periods of sintering treatment, few intermetallic compounds form, and the as-rolled condition will dominate the interfacial behaviour. The fracture morphology of the / bimetal plate is ductile fracture in parts. As the materials sintered at higher temperatures and/or prolonged time, hard and brittle intermetallic compounds develop and continue to thicken across the interface. Thus the brittle cleavage morphology becomes more visible. The peel test curves of samples sintered at higher temperatures show the zig-zag profile, which is attributed to the fracture of intermetallic layers with the cracks propagation under peel force (Figs. 12 and 13). The analysis results show that formation and thickening of the interfacial layers promotes crack propagating under peel force, which weakens the bond strength of the material. Some studies on the bond strength and interfacial development of / complex laminates have shown that three major diffusion controlled processes, involving phase transformation, Kirkendall effect of void formation and the dissolution of the interfacial oxide layer occur during the sintering process. In the sintering process, both and atoms will be thermally activated, and the diffusivity of in is greater than that of in. The non-equilibrium diffusivity between and promotes the Kirkendall effect of void formation, which is supposed to weaken the bond strength of the material with prolonged sintering ) However, no obvious voids are observed form the fractographs shown in Figs. 12 and 13, when the bond strength of the material starts to decrease in the present study. In a study of Kirkendall voids formation in aluminum-copper system, 15) the Kirkendall voids would be found to gather at the boundaries between the copper-rich layers and the copper base metal after prolonged sintering at high temperatures. According to the investigation of the fracture mechanism and the development of interfacial structure (Fig. 17), the major cracks propagate through the path near -side intermetallic compounds 2 and the zone of, 3 4 intermetallic layers, and no obvious Kirkendall voids occurred along the route of crack propagation in the studied sintering conditions. Moreover, it is believed that the Kirkendall voids will be found to have influence on bond strength at higher sintering temperatures and/or for prolonged periods. According the results obtained in the present study, it is believed that the development of brittle intermetallic compounds dominates the bond strength and fracture mechanisms. As the sintering temperatures and/or time increase, the formation and thickening of those brittle intermetallic compounds promotes the crack propagation and weaken bond strength of the material. The phenomenon deteriorates when and 3 4 start to develop and thicken. It is believed that the variation of bond strength is the compromise result between some interface reactions including mechanical bonding (virgin metals joint, mechanical lock), interfacial structure development and defects formation. In the initial periods of sintering process, the bond strength increases generally because of the formation of some hard intermetallic compounds and the bonding areas increase with the atoms diffusing across the interface, even if the mechanical lock decreases with the relaxation of residual stress. However, the development of intermetallic layers and defects will have greater influence on the bond strength for higher sintering temperature and/or for prolonged periods. The summarized result for the above effects is depicted in Fig. 18.

8 Influence of Interfacial Structure Development on the Fracture Mechanism and Bond Strength of uminum/copper Bimetal Plate Conclusions The cold roll bonding technique applied to aluminum on copper has been investigated. Microscopic examination of the interfacial structures at different sintering conditions has been undertaken to assess the fracture mechanism. The following conclusions may be drawn from this study: (1) For cold roll bonded / bimetal plate, the variation of bond strength is the compromise result between some interface reactions including mechanical bonding, interfacial structure development and defects formation. The bond strength will reach to maximum value under suitable sintering condition. (2) Interfacial structures of / bimetal plates develop with different sintering conditions. With increasing annealing temperature and/or time, the first reaction product 2 is presumed, and then the next reaction phase is 4 9., and 3 4 are next. (3) The development of / interfacial structure dominates the bond strength and fracture mechanism. The formation and thickening of and 3 4 brittle intermetallic compounds promotes crack propagating and then damages bond strength. The Fracture mechanism is brittle cleavage. (4) No obvious Kirkendall void formation is observed when the bond strength of the material starts to decrease in the present study. Acknowledgements This work has been supported by the National Science Council in Taiwan (NSC E-6-041), for which the authors are grateful. REFERENCES 1) V. A. tekar, S. K. Banerjee, B. N. Ghose and J. Bhattacharya: NML Technical Journal 23 (1981) ) N. Bay: Met. Const. (1986) ) Koga and Shinji: Journal of the Japan Welding Society 72 (23) ) A. Yahiro, T. Masui, T. Yoshida and D. Doi: ISIJ International 31 (1991) ) M. Abbasi, M. T. Salehi and A. K. Taheri: Z. Metallkd. 92 (21) ) M. Braunovic and N. eksandrov: J. IEEE Trans. Paper No. CHMT (1993) ) V. L. A. Silveria and A. G. Mury: J. Microstruct. Sci. 14 (1987) ) M. Abbasi, A. K. Taheri and M. T. Salehi: J. loys Compd. 319 (21) ) D. Pan, K. Gao and J. Yu: Material Science and Technology 5 (1989) ) J. A. Cave and J. D. Williams: Journal of the Institute of Metals 101 (1973) ) N. Bay: Welding Research Supplement, May 1983, ) L. R. Vaidyanath and D. R. Milner: British Welding Journal (19) ) R. C. Pendrous, A. N. Bramley and G. Pollard: Metals Technology 11 (1984) ) X. K. Peng, R. Wuhrer, G. Heness and W. Y. Yeung: J. Mate. Sci. 34 (1999) ) X. K. Peng, R. Wuhrer, G. Heness and W. Y. Yeung: J. Mater. Sci. 34 (1999) ) X. K. Peng, R. Wuhrer, G. Heness and W. Y. Yeung: J. Mater. Sci. 35 () ) E. B. Hannech, N. Lamoudi, N. Benslim and B. Makhloufi: Surface Review and Letters 10 (23) ) H. G. Jiang, J. Y. Dai, H. Y. Tong, B. Z. Ding, Q. H. Song and Z. Q. Hu: J. Apll. Phys. 74 (1993)