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Proceedings of ATEM’11

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ATEM’11, September 19-21, 2011, Kobe, Japan

Microstructural Characterization of Nanocrystalline Nickel Thin Films by X-Ray

Diffraction*

Keiseke Tanaka**, Masashi Sakakibara**, Hiroto Tanaka** and Hirohisa Kiamchi**

**Department of Mechanical Engineering, Meijo University 1-501 Shiogamaguchi, Tempaku-ku, Nagoya, Japan

E-mail:[email protected] Abstract Nickel nanocrystalline thin films with various grain sizes were produced by electrodeposition using sulfamate solution with different brightener contents at two temperatures. The grain size of thin films determined by X-ray diffraction ranged between 9 and 68 nm. The effect of the grain size on the yield strength in tension tests can be divided into two regions: region A with the grain size larger than about 50 nm and region B with the grain size smaller than about 20 nm. The region with the grain size between 20 to 50 nm is the transitional region. The yield stress increased in proportion to the inverse square root of the grain size in region A, while in region B in proportion to the inverse of the grain size. In region A, ordinary slip deformation within the grain is operating, while grain-boundary generation of dislocations is predominant in region B. The change of X-ray diffraction profile during loading and unloading was examined for two kinds of films with the grain sizes of 9 and 68 nm. The full width at half maximum (FWHM) of the diffraction profile increased with plastic strain in both films. After unloading, it came back to the initial value for filmｓ with 9 nm grain size, while it recovered only partially for films with 68 nm grain size. Dislocations disappeared after unloading in the former film, and some dislocations remain in the latter film.

Key words: Nanocrystal, Nickel Thin Films, Electrdeposition, X-Ray Diffraction, Grain Size, Tensile Deformation, Full Width at Half Maximum

1. Introduction

Nanocrystallization is one of the most promising technique to improve the strength of metallic thin films which are now being widely used for micro-electro-mechanical systems. The electrodeposition method will be the easiest method to obtain a homogeneous nanocrystalline structure in thin films (1),(2). The strength properties of nanocrystals are controlled by the materials microstructures and the X-ray diffraction method is very powerful tool for microstructural characterization. The mechanisms of plastic deformation depend on the grain size (3). For metals with grain sizes in the micrometer and larger size range, plastic deformation takes place by the intragranular dislocation motion. For submicrometer or nanoscale grained metals, the grain boundary emission of dislocation or the grain-boundary sliding appear to operate. The relation between the strength and the grain size may also changes depending on the grain-size range.

In the present paper, nickel nanocrystalline thin films were produced by electrodeposition using sulfamate solution (4),(5). Various kinds of thin films with different grain sizes ranging from submicrometer to nanometer were produced by changing the elecrodeposition conditions. X-rays diffraction techniques are used to characterize the

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microstructures. The tensile properties were correlated to the grain size determined by X-ray diffraction. The change of the broadening of the X-ray diffraction profiles was measured during loading and unloading of thin films to understand the mechanisms of plastic deformation.

2. Electrodeposition of Nickel Thin Films

Nickel thin films were produced by electrodeposition using sulfamate solution. A polished stainless steel plate was used for a cathode and a pure nickel plate for an anode. Table 1 shows the composition of the sulfamate solution with a addition of surfactant. Table 2 summarizes the conditions of elctrodeposition. The amount of brightener additive was changed at six levels from 0 to 2 g/L. The temperature of the bath was kept constant at 313K or 328 K in a hot-water circulating bath. The pH value of the solution was maintained at 3.7 to 4.2 by adding sulfamate acid. The solution was stirred by a magnetic stirrer to avoid pit formation. The current density was kept constant at 25 mA/cm2 by using a constant current supply, and the deposition period was 28 min. The thickness of the thin film was around 10 µm. After deposition, thin films were removed from the cathode and subjected to the X-ray characterization of microstructures and to the tension test as free-standing films. In the following, each film is designated by the brightener content and the solution temperature.

3. X-Ray Characterization of Nickel Thin Films

The texture of thin films was estimated from the X-ray diffraction profiles obtained by a wide angle θ-2θ step-scanning in focusing-beam goniometer equipped with a monochromater of Cu-Ka radiation. Figure 1 shows the diffraction profiles obtained from four films. The intensity of 111diffraction has the largest for films with larger brightener contents as seen in (c) and (d), 200 diffraction intensity is the maximum for films with less brightener content and without brightener as seen in (a) and (b). The intensity ratio of former two films resembles that obtained from power with random orientation except that 311 diffraction was not observed in thin films. The latter two films have a strong 200 fiber texture.

Table 1 Composition of sulfamate solution.

Table 2 Condition of electrodeposition.




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The grain size of films was determined by X-ray line broadening of 200 diffraction.

The diffraction profile was approximated by Cauchy function and the grain size, D, was determined by the integral width, β, of the diffraction profile by the following Scherrer equation:

( )cosD λ β θ= (1) where λ is the wave length and θ is the diffraction angle.

Figure 2 shows 200 diffraction taken from two films: CC(328K) and CCally(2.0g/L, 313K). The profile of the latter film (b) is much broader than the former one (a). The value of the grain size determined by Eq. (1) is summarized in Table 3 where the texture of thin films is also indicated. Films with small grain sizes show random orientation, while films

Fig. 1 X-ray diffraction profiles of nickel thin films.

Fig. 2 X-ray 200 diffraction of thin films.




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with larger grain sizes have a strong 200 fiber texture. The boundary is around 20 nm in grain size. Figure 3 shows the change of the grain size with the brightener content. The addition of brightener quickly reduced the grain size from around 100 nm to 10 nm. The lower temperature of deposition gave a smaller grain size. The smallest is 9.0 nm of CCally (2g/L,313K) film. The X-ray value agrees well with TEM observation showing a fairly uniform distribution of the grain size with the average of 15 nm(5). The thin films made without brightener at 328K has a large grain size. EBSD observation shows the grain size of 670nm which is larger than the value determined by X-rays(4). This difference may comes from the neglect of the microstrain contribution to the broadening. It will be necessary to separate the profile broadening into two components: particle size broadening and strain broadening by using Williamson-Hall method or Warren-Averbach Fourier analysis.

4. Tensile Properties of Nickel Thin Films

Tensile tests were conducted using dumbbell type specimens with a width of 4 mm and a gage length of 10 mm. The thickness was adjusted to have 10 µm and the exact thickness was determined from the fracture surface by SEM. For each thin film, three specimens stretched at the crosshead speed of 0.1 mm/min, and the elongation was measured by a laser dimension measurement equipment (KEYENCE，LS-7600， LS-7030M).

Figure 4 shows the stress strain curves of four kinds of films. The tensile fracture took place at the maximum load without any load drop. The 0.2% offset stress is taken as the yield stress. Both yield stress and the tensile strength increase with the grain size. The elongation to fracture tends to decrease with decreasing grain size down to about 15 to 20 nm, and then increases with decreasing grain size.

Table 3 Grain size determined by X-ray diffraction.

Fig. 3 Change of grain size with brightener content.




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Fig. 4 Stress-strain relation of thin films.

Fig. 6 Relation between 0.2% offset stress and grain size.

Fig. 5 0.2% offset stress plotted against inverse of grain size.




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The yield stress is plotted against the inverse square root of the grain size in Fig. 5, where the data for wrought nickel reported by Thompson(6) and for electrodeposited nickel by Ebrahimi(2) are also plotted. For ordinary size grained materials whose grain size is larger than about 50 nm, the following Hall-Petch relation is satisfied as seen in the figure:

0 2 0. Yk Dσ σ= + (2) where σ0 is the frictional stress for dislocation motion and is about 20 MPa. The yield stress tends to increase with decreasing grain size even below 50 nm. For ordinary grained films, the plastic deformation takes place by dislocation motion inside the grain and the yield strength increases following Hall-Petch relation. On the other hand, in nano-grain region, the generation of dislocations from the grain boundary is predominant, and the yield stress increases linearly with the inverse of the grain size as in the model proposed by Asaro et al.(3) In Fig. 6, the yield stress is plotted against the grain size in log-log diagram. For nano-grain region with the grain size lower than about 20 nm, the straight line with a slope of -1 approximates the experimental data. In this paper, the region with grain sizes larger than of about 50 nm is called region A and the region below 20 nm is called region B. The region with the grain size 20 to 50 nm is the transitional region. The influence of the orientation on the plastic deformation is not yet clear and requires the future study. It is also interesting to note that the topography of tensile fracture surfaces of nano-grained films is distinctly different from the chisel edge fracture of ordinary grained films. Figure 7 (a) shows a chisel edge fracture following necking in a film CC(328K), while Fig. 7(b) shows flat fracture surface without necking in thin films of of CCally (0.5g/L, 313K) film. Grain boundary sliding may also be responsible for enhanced ductility in nano-grain region.

5. In-situ Observation of Plastic Deformation by X-Ray Diffraction

Using synchrotron X-rays at the beam line BL02B1 of SPring-8, the change of X-ray diffraction profile was measured during loading and unloading of two kinds of thin films, CC(328K) and CCally(2.0g/L, 313K) films, which have large and small grain sizes. The energy level used was 12 keV.

Figure 8 shows the change of full-width at the half maximum (FWHM) of 111 diffraction taken from CCally film as a function of the applied strain, where the change of the stress is also shown as a function of strain. The FWHM value remains unchanged at low strain where the stress strain relation is linear indicating elastic deformation. When the stress strain relation becomes nonlinear, FWHM begins to increase sharply as a function of plastic strain. During unloading, the value reduced quickly and returned to the initial value

Fig. 7 SEM micrograph of tensile fracture surface.




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after complete unloading as reported by Budrovic et al.(7) The change of FWHM during loading and unloading corresponds to the micromechanism of plastic deformation. In nano-grained films, the plastic deformation is caused by the dislocation generation from the grain boundary and no dislocation remains within the grain after unloading.

In ordinary size grained metals, dislocation motion within grains is the predominant mechanism of plastic deformation and dislocations remains within the grain after unloading. The profile broadening increase with plastic strain during loading, while remains unchanged during unloading as reported by Akiniwa et al.(8) for copper films with the grain size 10 µm. Figure 8 shows the change of 200 diffraction of CC film. The FWHM value of 200 diffraction decreases during unloading, but it does not back to the initial value. Dislocations produced by plastic deformation remain partially after unloading. This behavior of FWHM is somehow in between nan-grained and ordinary grained films.

Fig. 8 Changes of FWHM and stress with strain for CCally (2.0g/L, 313K).

Fig. 9 Changes of FWHM and stress with strain for CC (328K).




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6. Conclusions

(1) Nickel nanocrystalline thin films produced by electrodeposition with different brightener contents had the grain size ranging between 9 and 68 nm. Films with grain size below about 20 nm had a random orientation, while those with grain sizes larger than about 20 nm had 200 fiber texture. (2) The effect of the grain size on the yield strength in tension tests can be divided into two regions: region A with the grain size smaller than about 20 nm and region B with the grain size larger than about 50 nm. The region with the grain size between 20 to 50 nm is the transitional region. The yield stress increased in proportion to the inverse of the grain size in region A, while in region B in proportion to the inverse square of the grain size. In region A, ordinary slip deformation within the grain is operating, while grain-boundary generation of dislocations is predominant in region B. (3) The change of X-ray diffraction profile during loading and unloading was examined for two kinds of films with the grain sizes of 9 and 68 nm. The full width at half maximum (FWHM) of the diffraction profile increased with plastic strain in both films. After unloading, it came back to the initial value for films with 9 nm grain size, while it recovered only partially for films with 68 nm grain size. Dislocations disappeared after unloading in the former film, and some dislocations remain in the latter film.

References

(1) El-Sherik, AM, Erb, U. Synthesis of Bulk Nanocrystalline Nickel by Pulsed Electrodeposition, Journal of Materials Science, Vol. 30: (1995); pp. 5743-5749. (2) Ebrahimi, F, Bourne, GR, Kelly, MS, Matthews, TE. Mechanical Properties of Nanocrystalline Nickel Produced by Electrodeposition, Nanocrystalline Materials,Vol. 11, No. 3: (1999), pp. 343-350. (3) Asaro, R. J., Krysl, P., Kad, B., Deforamtion Mechanism Transitions in Nanoscale FCC Metals, Philosophical Magazine Letters, Vol. 83, No. 12 (2003), pp. 733-743. (4) Tanaka, K., Isokawa, Y., Asano, H., Kimachi, H., Fatigue Properties of Nano-Crystalline Nickel Electrodeposited Thin Films, Journal of the Society of Materaisl Science, Japan, Vol. 59, No. 4 (2010), pp. 315-321. (5) Tanaka, K, Asano, H, Kimachi, H. Fatigue Properteies of Nanocrystalline Nickel Electrodeposited Thin Films, Proceedings of 18th European Conference on Fracture, 2010, ESIS ans DVM. (6) Thompson, A. W. , Effect of Grain Size on Work Hardening in Nickel, Acta Metallurica, Vo. 25 (1977), pp. 83-86. (7) Budrovic, Z., Van Swygenhoven, H., Derlet, P. M., Van Petegem, S., Schmitt, B., Plastic Deforamtion with Reversible Peak Broadening in Nanocrystalline Nickel, Science, Vol. 304 (2004), pp. 273-276.. (8) Akiniwa, Y., Kimura, H., Suzuki, T., Evaluation of Deformation Beahvior of Cu Thin Films Sputterd on Polyimide Films by X-Ray Method, The Transaction of The Japan Society of Mechanical Engineers, Vol. 75, No. 749 (2009), pp. 103-109.



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