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Probing the Mechanical Behavior of Nanostructured Materials Th rough Micropillar Compression Experiments

Alican Tuncay Alpkaya¹, Amir Motallebzadeh², Sezer Özerinç¹

¹Middle East Technical University, Department of Mechanical Engineering, Ankara, Turkey²Koç University, Surface Science and Technology Center, İstanbul, Turkey

Abstract

Recent developments in focused ion beam (FIB) milling and nanomechanical sensing technologies have enabled the fabrication of microspecimens and their mechanical testing at the small scale. One of the most recent micromechanical testing techniques is micropillar compression, where a microcylindrical specimen is milled by FIB and it is tested under uniaxial compression using a nanoindenter. In this study we utilize this technique to investigate the mechanical behavior of Cu-Nb nanolayers. The samples were prepared by magnetron sputtering on single crystal silicon wafers and their microstructure was investigated by X-ray diffraction and electron microscopy. The micropillars with 1 μm diameter and 2.5 μm height were fabricated by FIB milling and tested using a nanoindenter with a flat diamond punch. The micropillar compression technique enables the direct measurement of the elastic modulus and the yield strength of the pillars, and provide insight to the high strength of nanostructured materials.

1. Introduction

Nanostructured materials offer superior mechanical properties when compared to conventional alloys [1]. In order to utilize these new generation materials in engineering applications, the relationship between their microstructures and mechanical properties should be better understood. Many nanostructured materials are synthesized in the form of thin films due to the ease of generating model nanostructures and alloys in desired compositions [2]. Since the thin film geometry does not allow conventional mechanical testing, micromechanical characterization techniques are necessary for the characterization studies.

Nanoindentation is a common technique for obtaining the hardness and elastic modulus of a thin film [3]. In this technique, a diamond indenter penetrates into the film, and the load on the sample and the displacement of the indenter into the surface are continuously monitored. Analysis of the unloading segment of the data provides the hardness and the elastic modulus of the sample [3].

While nanoindentation is widely used for testing nanomaterials, a recent technique called micropillar compression has enabled the more systematic characterization of the mechanical properties. In this technique, a microcylindrical specimen is prepared by using a focused ion beam [4]. Then a nanoindenter equipped with a flat diamond punch compresses the micropillar. The measured load and displacement is directly converted to a stress-strain curve, from which, elastic modulus, yield strength, strain hardening and other mechanical behavior can be systematically quantified.

In this work, we applied both nanoindentation and micropillar compression techniques to nanocrystalline copper and nanolayered copper-niobium (Cu-Nb) thin films. Nanolayered Cu-Nb is a promising nanomaterial that has outstanding strength and thermal stability [5]. We investigated the effect of layer thickness on the strength of these nanolayers and observed good agreement with literature findings. The successful synthesis of the nanolayers and their mechanical testing have provided evidence about the effectiveness of our approach and procedures, and will enable further characterization of other type of novel nanolayered metals with alloying additions.

2. Experimental Procedure

Pure Cu – pure Nb nanolayers and pure nanocrystalline Cu films were deposited on oxidized single crystal silicon substrates by means of magnetron sputtering. The base pressure in the chamber prior to sputtering was Torr and Ar pressure during sputtering was Torr. Cu and Nb deposition rates were 1.7 Å/s and 1.5 Å/s, respectively. In the nanolayered samples, Cu and Nb layers had equal thickness, and the individual layer thickness varied from 5 to 100 nm in different samples. The total film thickness of all the samples were 1 𝜇𝑚 for nanoindentation studies. For micropillar compression, an additional Cu-Nb sample of 𝜇𝑚 thickness was prepared. This sample had a layer thickness of 50 nm.

An Agilent G200 nanoindenter equipped with a diamond Berkovich tip measured the hardness of the films at room temperature. The continuous stiffness measurement method

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provided the hardness as a function of penetration depth [6]. First 250 nm of the data was used to obtain the hardness of the films to minimize the substrate effect [7]. A minimum of 16 indents were performed on each specimen.

A Rigaku Ultima-IV diffractometer performed X-ray diffraction measurements on the samples in grazing incidence mode at 1°. Grain size was measured using Scherrer equation.

An FEI Nova 600 Nanolab focused ion beam fabricated the micropillars. Ga+ ions sputtered away the material locally in the desired geometry. First, an ion beam with a relatively high current of 6.3 nA milled a circular pattern of 25 μmouter diameter and 6 μm inner diameter. Machining of the micropillar was completed by gradually reducing the size of the milling pattern and the beam current, down to 100 pA. The Agilent G200 nanoindenter compressed the micropillars at a constant displacement rate of 3 nm/s using a 10 μm diameter flat diamond punch.

3. Results and Discussion

Figure 1 shows the cross-section scanning electron microscopy (SEM) images of some of the samples cut by a focused ion beam. The alternating layers are clearly visible due to the contrast. There exist some waviness, especially in the thinner layers. We attributed this to the grainy structure of individual layers introducing roughness, and also to the residual stress present in each layer.

Figure 1. Cross-section SEM images of nanolayered Cu-Nb with layer thicknesses of (a) 100 nm, (b) 10 nm.

Figure 2 shows XRD data of pure Cu and pure Nb together with Cu-Nb nanolayers. Cu-Nb nanolayered sample shows both Cu and Nb peaks as expected, indicating that both layers are polycrystalline. Figure 3 shows the average Cu grain size as a function of layer thickness of the Cu-Nb nanolayers. The grain size decreases with decreasing layer thickness, a trend commonly observed in nanolayered metals [8].

Figure 4 shows the hardness of Cu-Nb nanolayers as a function of layer thickness. Hardness of the samples monotonically increase with decreasing layer thickness. We observe good agreement between our results and the

literature [9], especially for the samples that are prepared using the same technique of physical vapor deposition.

Figure 2. XRD results of nanocrystalline pure copper, pure niobium and nanolayered Cu-Nb with a layer thickness of 100 nm.

Figure 3. Variation of Cu grain size as a function of layer thickness as determined from Cu (111) peaks using Scherrer equation.

Figure 4. Hardness of Cu-Nb nanolayers as a function of layer thickness. Literature values are based on Cu-Nb nanolayers prepared by, PVD: physical vapor deposition [9], and ARB: accumulative roll bonding [11].

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Increasing strength with decreasing layer thickness can generally be attributed to Hall-Petch strengthening. In this strengthening mechanism, as the grain size or layer thickness decreases, the number of dislocations piling up at the boundaries or interfaces decreases, and higher stresses are required for plastic deformation. However, when the grain size or layer thickness is below 100 nm, dislocation pile-up is no longer significant and deviations are observed from the ideal Hall-Petch behavior [10]. A modified confined layer slip has been developed to correctly predict the hardness of sub-100 nm nanolayers [9].

In order to probe the mechanical properties of nanolayers more systematically, we proceeded with micropillar compression experiments. Figure 5 shows a nanolayered Cu-Nb micropillar prepared by focused ion beam, before and after compression testing. SEM image of the compressed micropillar shows that the softer copper layers plastically flowed outwards, as a result of being squeezed between the harder Nb layers. It should be noted that this type of detailed observation of plasticity is usually not possible in nanoindentation without detailed cross-section analyses.

Figure 6 shows the stress-strain behavior of the compressed micropillar. The yield strength of the micropillar is around 1.3 GPa. This is in agreement with our nanoindentation results when a Tabor factor of 3 is used, that is, H / y 3 [12]. In addition, the results are close to that reported by Mara et al. [13]. There is also considerable strain hardening, as opposed to the strain softening caused by the deformation morphology in the work of Mara et al. [13].More work will be necessary to investigate the sources of this strain hardening.

Figure 5. SEM images of a Cu-Nb micropillar with 50 nm layers, (a) before compression, and (b) after compression.

Figure 6. Stress-strain behavior of the Cu-Nb nanolayered micropillar shown in Figure 5.

4. Conclusion

We have demonstrated the advanced mechanical characterization of nanolayered metallic thin films through nanoindentation and micropillar compression. Nanoindentation results are in agreement with literature values for similar nanolayers and we have obtained consistent results in applying the micropillar compression technique on Cu-Nb nanolayers.

Based on the successful synthesis and mechanical characterization of pure metallic nanolayers, future work will focus on the investigation of the mechanical properties of nanolayered metals upon alloying additions, whose effects on mechanical properties are currently not known.

Acknowledgment

This research is supported by TÜB TAK 3501 CAREER Award under Project No. 116M429 and by METU BAP under Project No: BAP-03-02-2017-004. We would like to thank METU Central Laboratory, Koç University KUYTAM, and Bilkent University UNAM for their support with the sample preparation and measurements. We would like to thank Prof. Robert Averback (University of Illinois) for useful discussions.

References

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[7] R. Saha, and W. D. Nix, Acta Materialia, 50 (2002) 23–38.[8] Z. Fan, S. Xue, J. Wang, K. Y. Yu, H. Wang, and X. Zhang, Acta Materialia, 120 (2016) 327–336. [9] A. Misra, J. P. Hirth, and R. G. Hoagland, Acta Materialia, 53 (2005) 4817–4824. [10] J. T. Schiøtz Francesco D. Jacobsen. Karsten W., Nature, 391 (1998) 561. [11] I. J. Beyerlein, N. A. Mara, J. Wang, J. S. Carpenter, S. J. Zheng, W. Z. Han, R. F. Zhang, K. Kang, T. Nizolek, and T. M. Pollock, JOM, 64 (2012) 1192–1207. [12] D. Tabor, The Hardness of Metals, OUP Oxford, 2000. [13] N. A. Mara, D. Bhattacharyya, P. O. Dickerson, R. G. Hoagland, and A. Misra, Materials Science Forum, (2010).