Influence of the Process Parameters on the Properties of Sputtered Aluminum-Zirconium Sheets

Transcription

1 Influence of the Process Parameters on the Properties of Sputtered Aluminum-Zirconium Sheets J. Kovac, B. Köhler, Y. Ramdani, J. Henry, H.-R. Stock, R. von Bargen, A. Mehner, H.-W. Zoch Stiftung Institut für Werkstofftechnik, Bremen, Germany 1 Introduction The Collaborative Research Center 747 (SFB 747) aims to develop and optimize micro cold forming processes for the fabrication of small parts for micro-electronic or micro-mechanical systems. Among these processes, micro deep drawing is particularly appropriate for the mass fabrication of parts with complex shapes [1]. In classical macroscale deep drawing processes, a sheet of metal is deformed into a three dimensional object by the action of a punch descending into a mating die. Micro deep drawing acts the same way, however downscaling the dimension of the resulting products to the sub-millimeter scale involves to downscale the thickness of the sheet as well. Hence, in classical deep drawing, the sheet thickness can be close to 1 mm while deep drawing processes currently uses sheets with thicknesses below 50 µm. Rolled sheets of common materials as pure copper, pure aluminum or stainless steel are available in the required thickness. For certain applications micro components with high mechanical strength are required as for example in a micro valve, where parts have to sustain high fluid pressure. Aluminum alloys may be interesting since they may present good mechanical properties due to precipitation hardening and simultaneously certain ductility. Among binary aluminum alloys, aluminumscandium alloys have the highest strengthening rate relatively to the alloying element concentration as well as other interesting properties such as anti-recrystallization properties, a good weldability and resistance to corrosion [2]. The disadvantage of such alloys is the high price of scandium. On the contrary, zirconium is a much cheaper element which incorporated to aluminum results in similar effects as scandium [3]. Indeed, the small solubility of zirconium in aluminum results in the precipitation of Al 3 Zr phase. At the thermodynamic equilibrium the Al 3 Zr phase presents a D0 23 (tetragonal) structure which does not result in considerable hardening. Nevertheless, during aging of aluminum-zirconium alloys, prior to the equilibrium structure, the precipitates appear first under a metastable L1 2 (cubic) structure. Due to the high coherency of the precipitate with the aluminum matrix, the material achieves a high strength though the strengthening potential seems more limited compared to aluminumscandium alloys [3]. It is however difficult to roll sheets of high strength material to the required thickness. As an alternative to rolling, dc magnetron sputtering was successfully used to produce aluminum-scandium as well as aluminum zirconium sheets [4]. It was shown that the properties of such sheets can be significantly influenced by the deposition and post-treatment parameters, namely the substrate and the aging temperature. In the present papers, the influence of other processing parameters such as the target power, the substrate motion amplitude and the pressure were investigated.

2 2 Experimental 2.1 Sheet preparation The sample deposition took place in a dc magnetron Sputtering unit (Leybold Z700) fitted with a mm³ alloyed aluminum-zirconium target (4 wt% Zr). During the deposition, the substrate ( mm³, DC01) steel was moved periodically to homogenize the coating thickness as described by Kovac et al. [5]. The sheets were produced using different sets of Target power, Argon flow (corresponding to different working pressures) and substrate motion amplitude as specified in table 1. Glow discharge optical spectroscopy (GDOS) measurements were performed on every sample and showed that the sheets have a constant zirconium content of 3.8 wt%. In order to produce sheets with comparable thicknesses the deposition times were varied for every sample. A relative thickness profile of sheets produced with motion amplitudes of 15 and 23, respectively are shown in figure1. Due to a better distribution of the vapor flow, the sheet produced with an amplitude of 23 presents a more homogeneous thickness distribution than the sheet produced with an amplitude of 15. After deposition, the samples were immersed into a 69 % nitric acid to dissolve the steel substrate without damaging the aluminum-zirconium film. Figure 1: Relative thickness profile of samples A (15 ) and B (23 ) Table 1. Deposition parameters of the different samples Sample designation A B C D E F G H Target power (kw) Target voltage (V) Current (A) Ar-flow (sccm) Working pressure (10-3 mbar) Motion amplitude ( ) Thickness (µm) Deposition time (h) Rate (µm/h)

3 2.2 Analyze methods mm² rectangular tensile specimens were cut in the central area of each sheet and there edges were reinforced with steel backings ( mm²) [4]. The prepared specimens were clamped in a tensile tester designed for thin sample testing and deformed under a constant strain rate of s -1 in order to measure their tensile properties. Scanning electron microscopy and atomic force microscopy were used to investigate the morphology and roughness of the different samples. 3.1 Morphology and roughness The cross section, free surface and contact surface of the different samples were observed with scanning electron microscopy. All the samples have a typical close packed columnar structure as shown in figures 2 and 3. The free surface presents a certain roughness due to the conical shape of the growing crystallite. In comparison, the contact surface (i.e. the surface in contact with the substrate) is smoother and presents long stripes corresponding similarly to the substrate surface. These long stripes are due the cold rolling process employed to produce the steel sheet used as substrate. Figure 2: SEM micrographs of the cross section (a), the free surface (b) and contact surface (c) of sample E The variation of the deposition parameters does not change the columnar microstructure of the sheets. However, the samples sputtered at 3 kw seem to have a higher density on the free surface as shown exemplary for samples A and E in figure3. Figure 3: SEM micrographs of the free surface of samples B (a) and F (b)

4 Table 2. Average grain size and roughness on the free surface of the different samples Sample designation A B C D E F G H Grain Size [µm] , Ra [nm] The average grain size and the surface roughness of the different samples are summarized in table 2. No significant changes in grain size occur. Hence, the grain size is supposed to remain relatively constant for all the samples. In opposition, the roughness shows significant differences. In particular the samples sputtered by the higher target power of 3 kw (E, F, G, H) show a smoother surface compared to the samples sputtered by 1 kw. This might be linked to the better density of samples produced by 3 kw. 3.2 Tensile Properties In every sample the influence of the target power, the pressure and the motion amplitude on the ultimate tensile strength (UTS), the yield stress (YS) and the elongation at fracture (A%) of the different samples were investigated. The results are shown in figure 4. Figure 4: Tensile strength (R m ), yield stress (Rp 0.01 ) and elongation at fracture (A) of the different samples The aluminum-zirconium sheets achieve very high tensile strength (> 600 MPa) and yield stress (> 500 MPa) but have a relatively poor ductility (< 2%). The measurement of the Young s modulus gave a constant value of approximately 69 GPa which is comparable to the Young s modulus of pure aluminum. The process parameter having the most significant influence on the tensile properties is the target power. Actually, both yield stress and tensile strength of specimen E, F, G and H (deposited by 3 kw) are higher than tensile strength and yield stress of the other samples deposited with a target power of 1 kw. When comparing samples A and B, C and D, G and H, it can be seen that higher motion amplitude tends to de-

5 crease the tensile strength and yield stress of aluminum-zirconium sheets. However, the same behavior is not observed when comparing samples E (15 ) and F (23 ) which could signify that the influence of the substrate motion amplitudes is subjected to possible interactions with other parameters. The comparison between samples A, B and C, D and E, F and G, H indicates that the increase of the pressure (due to the higher argon flow of 150 sccm) does not result in particular change of tensile properties at least in the considered pressure range (4-7 Pa). Finally, the elongation does not show any clear behavior when the process parameters are changed. 4 Discussion The different variations of tensile properties observed are unlikely to result from sold solution hardening or precipitation hardening since a constant chemical composition of 3.8 wt% Zr was measured with GDOS in the different samples and since transmission electron microscopy observations have shown that after deposition the only phase in aluminum-zirconium sheet is an oversaturated solution of zirconium in aluminum phase and therefore no precipitates are present, as shown before in [4]. It is also unlikely that the grain size influences the mechanical behavior of the different specimens since the average grain size did not show considerable variations. This leads to the assumption that the variation of tensile properties observed are due to varying crystal defects density. Indeed, increasing the target power results in a rise of both the deposition rate and the kinetic energy of the sputtered aluminum and zirconium atoms. Higher deposition rates and kinetic energies of the sputtered species lead to the formation of higher stress within the sheets, as shown by different authors [6-7]. If the stress exceeds the yield stress of the material, work hardening takes place, resulting in a higher dislocation density which in turns increases the yield stress and tensile stress. On the opposite, when using a broader substrate motion amplitude the mean deposition rate and kinetic energy of the vaporized atoms drops. Actually, in sputtering the deposition rate as well as the kinetic energy of the gaseous species are not homogeneously distributed and therefore tend to be higher in the direction normal to the target. Using broader motion amplitudes leads to a better spatial distribution of the deposition rate and kinetic energy of sputtered species on the substrate but the mean values are lower compared to those reached with smaller motion amplitudes. This could explain the tendency of observing lower tensile properties when using higher substrate motion amplitude. 5 Conclusions In this paper the influence of the processing parameters on the properties of thin aluminumzirconium sheets produced by dc magnetron sputtering was investigated. It has been shown that the aluminum-zirconium sheets reach very high yield stress and tensile strength but relatively low ductility. The strongest influence came from the increase of the target power resulting in significantly higher tensile properties. An increase of the substrate motion amplitude

6 tends to decrease the tensile strength and yield stress. The observed variations of the tensile properties of the aluminum-zirconium sheets are probably due to a variation of stress induced dislocation densities resulting from different deposition rates and kinetic energies of the sputtered species. 6 Acknowledgement The authors gratefully acknowledge the financial support by the DFG (German Research Foundation) for projects A1, A2 and B4 within the Collaborative Research Center SFB 747 Micro Cold Forming Processes, Characterization and Optimization. 6 References [1] Vollertsen, F., Hu, Z., Schulze Niehoff, H., Theiler, C., Journal of Material Processing Technology 2004,151, [2] Royset, J., Ryum, N., International Material Review 2005, 50, [3] Knipling, K. E., Seidman, D. N., Dunand, D. C., Acta Materiala 2011, 59, [4] Kovac J., Stock, H.-R., Köhler, B., Bomas, H., Zoch, H.-W., Surface and Coating Technology 2013, 215, [5] Kovac, J., Stock, H.-R., Zoch, H.-W., Journal of Surface Engineered Materials and Advanced Technology 2012, 2, [6] Amin-Ahmadi, B., Idrissi, H., Galceran, M., Colla, M. S., Raskin, J.P., Pardoen, T., Godet, S., Schryvers, D., Thin Solid Films 2013, 539, [7] Pauleau, Y., in Handbook of Thin Films Vol. 1. (Ed.: Elsevier), H. S. Nalwa, Los Angeles,CA,USA, 1992, chapter 9