FATIGUE PROPERTIES OF NANOCRYSTALLINE NICKEL ELECTRODEPOSITED THIN FILMS. Tempaku-ku, Nagoya, , Japan

Transcription

1 FATIGUE PROPERTIES OF NANOCRYSTALLINE NICKEL ELECTRODEPOSITED THIN FILMS K. Tanaka 1), H. Asano 2) and H. Kimachi 1) 1) Department of Mechanical Engineering, Meijo University, Shiogamaguchi, Tempaku-ku, Nagoya, , Japan 2) Graduate School, Meijo University, Nagoya, , Japan, Japan ABSTRACT Three types of nickel thin films with different grain sizes were produced by electrodeposition using sulfamate solution: CC films were made under constant current, PC films under pulse current, and CC-ally films under constant current with brightener additive. The grain size was smaller in the order of CC, PC, and CC-ally films down to nanometers. The fracture strength and yield strength in tension tests followed the Hall-Petch relation, and the elongation was largest for CC-ally films. The fatigue strength increased with decreasing grain size, following the Hall-Petch relation. The resistance to fatigue crack propagation decreased for nanocrystalline films. The threshold stress intensity factor was the smallest for PC and CC-ally films. In the intermediate-rate range, the propagation rate increased with decreasing grain size when compared at the same stress intensity factor. The fatigue fracture surface near the threshold consisted of granular features whose size decreased with decreasing grain size. At high stress intensity factors, striations were observed on the fracture surface of CC films, while only fine granular feature was observed for CC-ally films. On the specimen surface near the fatigue fracture surface of CP and CC-ally films, the small grain boundary fracture facets were observed at high stress intensity factors. For CC films, slip bands were seen together with the grain boundary fracture facets. KEYWORDS Nickel thin film, nanocrystals, electrodeposition, tensile properties, fatigue strength, fatigue crack propagation, Hall-Petch relation, grain-size effect INTRODUCTION Nanocrystallization is one of the most promising technique to improve the fatigue 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 uniform nanocrystalline structure in thin films [1,2]. In comparison with hardness and tensile properties, fatigue properties of nanocrystalline metals are not well understood. Ultrafine-grained copper made by equal-channel angular processing (ECAP) has a high fatigue resistance at an intermediate life region, but the improvement of the fatigue strength diminishes near the fatigue limit because of grain growth by cyclic deformation [3]. Since nickel may not show grain growth by cyclic deformation during fatigue, the improvement of the fatigue strength due to nanocrystallization remains even near the fatigue threshold [4]. In the present paper, nickel nanocrystalline thin films were produced by electrodeposition using sulfamate solution. Three types of thin films with different grain sizes ranging from submicrometer to nanometer were produced under different elecrodeposition conditions. The grain-size effect on the tensile properties and fatigue strength was investigated. Fractures surfaces were examined by scanning electron microscopy (SEM) to understand micromechanisms of fracture.

2 ELECTRODEPOSITION OF NICKEL THIN FILMS Nickel thin films were produced by electrodeposition using sulfamate solution. Either constant or pulsed current was used for electrodeposition. A polished stainless plate was used for a cathode and a pure nickel plate for an anode. Table 1 shows the composition of the sulfamate solution. Three kinds of thin films were produced: CC films were made under constant current, PC films under pulse current, and CC-ally films under constant current with brightener additive [5]. Table 2 summarizes the conditions of elctrodeposition. The temperature of the bath was kept constant in a hot-water circulating bath and the current condition was controlled by a programmable direct-current source. Table 1: Composition of sulfamate solution. Table 2: Electrodeposition condition. After deposition, thin films were removed from the cathode and subjected to the characterization of the fatigue properties as free-standing films. The microstructure of thin films was examined by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). SEM micrographs of the film surface are shown in Fig. 1. Granular structures with the size of 1mm are seen in Fig. (a) of CC films, while PC films have much smaller granular structures. CC-ally films are bright and have no such structures. Inverse pole figure (IPF) maps of EBSD of CC and PC films are shown in Figs. 2 and 3, respectively. The average grain size measured from IPF maps is 670 nm and 80 nm for CC and PC films. For CC-ally films, EBSD observation was not successful because of the grain size was below the resolution limit of EBSD. Figure 4 shows the TEM bright images and electron diffraction. The grain size is fairly uniform and the average grain size is 15 nm, and the grain orientation is random.

3 Fig. 1: SEM micrographs of electrodeposited surface. Fig. 2: IPF map of CC film. Fig. 3: IPF map of PC film. Fig. 4: TEM image and electron diffraction of CC-ally film

4 TENSILE PROPERTIES 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 films, three specimens stretched at the crosshead speed of 0.1mm/min, and the elongation was measured by a laser dimension measurement equipment (KEYENCE,LS-7600, LS-7030M). Figure 5 shows stress-strain relations for three types thin films, together with a rolled nickel thin films having a thickness of 10 µm as a reference. The tensile properties are summarized in Table 4, where the values indicate the mean of three specimens. The fracture takes place at the maximum load. The fracture strength was equal to the maximum stress and the yield strength was the stress at the deviation of the linearity of the stress-strain relation. Young s modulus may depend on the orientation of thin films. The fracture strength was lowest for rolled thin films. For electrodeposited films, the strength increases in the order of CC, PC, CC-ally films, and the amount of the strength increase of CC-ally films is about 2.2 times. Vickers hardness also shows a similar increase. It is interesting to note that the elongation is largest for CC-ally with smallest grain size. The yield strength and fracture strength are plotted against the square root of the grain size in Fig. 6, where the grain size is the values measured by EBSD and TEM. It can be conclude that Hall-Petch relation is obtained down to the grain size of 15 nm. Ebrahimi et al. [2] also reported Hall-Petch relation down to about 10nm for the flow stress calculated from Vickers hardness. Fig. 5: Stress-strain curves for thin films. Table 4: Mechanical properties of thin films.

5 Fig. 6: Hall-Petch plot for yield stress, tensile strength and fatigue limit. FATIGUE STRENGTH Dumbbell-type specimens were machined from free-standing thin films which has thickness of 10 µm and a width of 4 mm. Fatigue testing was conducted in a servo-electromagnetic fatigue testing machine (Shimadzu MMT-100N-10) under the stress ratio of R=0.1 at a frequency of 20 Hz. The relation between the applied stress amplitude and the number of cycles to failure is shown in Fig. 7, where the S-N date for the rolled nickel film of 10 µm thickness was also indicated for reference. In comparison with rolled thin films, the fatigue limit corresponding to 10 7 cycles increases 1.7 times for CC films, 2 times for PC films and 3 times for CC-ally films. The fatigue limit of CC-ally films reaches to 757 MPa and is about 2 times larger than that reported by Hanlon et al. [4] for nanocrsytalline nickel of nm grain size. Not only grain size but also some other microstructural feature is expected to be responsible for the improvement of the fatigue strength. The fatigue limit is plotted against the inverse square root of the grain size in Fig. 5, where the yield strength and the tensile strength determined by tensile tests are also indicated. The fatigue limit increased with decreasing grain size, following Hall-Petch relation down to 15nm as well as the yield strength and the tensile strength. The slope of the straight line relation for the fatigue limit is close to that for the yield strength. Together with a great improvement near the fatigue threshold, even at an intermediate life region, some improvement of fatigue strength is seen in Fig. 6. This shows a clear contrast to the fatigue strength of ECAPed copper whose improvement near the fatigue threshold disappears because of grain growth due to cyclic deformation [3].

6 Fig. 7: Relation between stress range and number of cycles to fracture. FATIGUE CRACK PROPGATION Fatigue crack propagation tests were conducted using single edge notched films with 5 mm in width and 10 µm in thickness under a stress ratio of the minimum to maximum stress R=0.1. The stress intensity factor was calculated by the modified crack-closure integral method of the finite element method under the experimental boundary condition of gripping. The near-threshold crack propagation behavior was determined by the load-shedding method. Fig. 8: Relation between fatigue crack propagation rate and stress intensity range.

7 Figure 8 shows the relation between the crack propagation rate and the stress intensity range. The resistance to fatigue crack propagation is reduced for nanocrystalline films, CC-ally and PC, and the threshold stress intensity factor of these materials were lower than that of CC and rolled films with larger grain size. A similar trend of inferior resistance to crack propagation of nanocrystalline nickel was also reported by Hanlon et al. [5]. Figure 9 shows SEM micrographs of the fracture surfaces (left two columns) and the film surface near the fracture line (right column) for CC, PC, and CC-ally films. The crack propagation direction is from left to right. Granular features are seen on the fracture surface near the threshold in Figs. 9(a), (d), and (g), and the size decreases with decreasing grain size. At high stress intensity range, striations are detected in Fig. 9(b) for CC films. For CC-ally films, a granular feature seen in Fig. 9(h) is similar to the feature near the threshold Fig. 9(g). The feature of the fracture surface for PC films is in between and striation-like feature is not evident. Fig. 9: SEM micrographs of fatigue fracture surface and side surface

8 On the specimen surface near the fracture surface at high stress intensities, slip deformations are evident in Fig. 9(c) for CC film and grain boundary fracture facets are seen in Fig. 9(f) for PC film. On the other hand, there is not trace of slip deformation of grain boundary fracture for CC-ally film as seen in Fig. 9(i). The roughness of the fracture surface is low for PC and CC-ally films compared with CC films. A small amount of roughness may result in a smaller amount of crack closure, and consequently less amount of resistance to crack propagation. The crack propagation mechanisms for CC films are not different from microcrystalline materials, while other mechanisms related to grain boundary will be operating for nanocrystalline materials. CONCLUSIONS Nickel nanocrystalline thin films were successfully produced by electrodeposition using sulfamate solution. Three kinds of thin films with different grain sizes were produced: CC films were made under constant current and had the grain size 670µm, PC films under pulse current and the grain size 80nm, and CC-ally films under constant current with grain refinement additive and the grain size 15nm. The fatigue limit increased with decreasing grain size, following the Hall-Petch relation down to 15 nm. The resistance to fatigue crack propagation decreased for nanocrystalline films. The threshold stress intensity factor was the smallest for PC and CC-ally films. The fatigue fracture surface near the threshold consisted of granular features whose size decreased with decreasing grain size. At high stress intensity factors, striations were observed on the fracture surface of CC films, while only fine granular feature was observed for CC-ally films. On the specimen surface near the fatigue fracture surface of CP and CC-ally films, the small grain boundary fracture facets were observed at high stress intensity factors. For CC films, slip bands were seen together with the grain boundary fracture facets. REFERENCES [1] El-Sherik, A.M.; Erb, U.: Synthesis of Bulk Nanocrystalline Nickel by Pulsed Electrodeposition Journal of Materials Science, 30 (1995) pp [2] Ebrahimi, F.; Bourne, G.R.; Kelly, M.S.; Matthews, T.E.: Mechanical Properties of Nanocrystalline Nickel Produced by Electrodeposition, Nanocrystalline Materials,11 (1999) No. 3, pp [3] Kimura, H.; Kojima, Y.; Akiniwa, Y.; Tanaka, K.; Ishida, T.; Fatigue Damage Mechanism of Nanocrystals in ECAP-Processed Copper Investigated by EBSD and AFM Hybrid Method Key Engineering Materials, (2007) pp [4] Hanlon, T.; Tabachnikova, E.D.; Suresh, S.: Fatigue Behavior of Nanocrystalline Metals and Alloys International Journal of Fatigue, 27 (2005) pp [5] Kishimoto, K.; Yoshioka, S.; Kobayakawa, K.; Sato, Y.: Effects of Various Additives on the Characteristics of Electroplated Nickel Thin Films Journal of the Surface Finishing Society of Japan, 54 (2003) No. 10, pp Corresponding author: K. Tanaka; ktanaka@ccmfs.meijo-u.ac.jp