Creep Behavior and Its Influence on the Mechanics of Electrodeposited Nickel Films

Transcription

1 9 J. Mater. Sci. Technol., Vol.25 No.1, 29 Creep Behavior and Its Influence on the Mechanics of Electrodeposited Nickel Films Zengsheng Ma, Shiguo Long, Yong Pan and Yichun Zhou Key Laboratory of Low Dimensional Materials & Application Technology of Ministry of Education, Faculty of Materials, Optoelectronics and Physics, Xiangtan University, Xiangtan 41115, China [Manuscript received August 13, 27, in revised form October 18, 27] In order to improve the accuracy and comparability of hardness and elastic modulus measurements in nanoindentation, an evaluation of the creep behavior and its influence on the mechanical properties of the electrodeposited nickel film has been conducted. The influence of loading time and hold period on the hardness and elastic modulus results at maximum load µn has also been examined. It is found that with increasing the loading time, the creep value is decreased. However, the creep value is increased when the hold period is increased. The elastic modulus results are more reliable if the hold period is longer. If the hold period is long enough, the loading time has no remarkable effect on the hardness and elastic modulus measured. KEY WORDS: Nanoindentation; Hardness; Elastic modulus; Creep 1. Introduction Nanoindentation is now widely used to measure the mechanical properties, such as hardness, elastic modulus, and adhesion, with the emphasis on nickel films due to its nanometer displacement resolution. A conventional nanoindentation analysis method is based on the assumption that the material behaves in an elastic-plastic manner. However, many materials such as nickel film can also exhibit a visco-elastic and visco-plastic response, which is commonly termed creep. In a nanoindentation test, this is usually observed as an increase in depth during a hold period at a maximum load in the load-displacement data. This creep behavior depends on the material and normally diminishes to very low values within several seconds. The creep rate may reach values of several nanometers per second at the beginning of the hold period and reduces to less than 1 nm/s after a couple of seconds. It not only influences the maximum depth but also the gradient of the upper part of the unloading curve, which is used for the calculation of contact stiffness and elastic modulus. Therefore, the creep can have a remarkable influence on the elastic modulus and hardness. How this influence strongly depends on the loading rate and the hold period, as well as on the creep rate of the material. The creep tests have been carried out with some microcrystalline and nanocrystalline metals and alloys produced by electrodeposition. Electrodeposited nickel film has been subject to extensive scientific investigations in the areas of grain growth kinetics [1 3], mechanical properties [4 12] and creep behavior [13 17]. To date, a through investigation on the creep behavior of electrodeposited nickel film during nanoindentation is yet lacking. One reason may be that high accuracy is required as the depth change in a period of is normally less than some 1 nm. Furthermore, for productivity reasons, the aim is to keep the measurement time as shorter as possible, and therefore, a hold period of 1 s or even shorter is often used. In the following content, it will be shown that this may not be Corresponding author. Prof.; Ph.D.; address: zhouyc@xtu.edu.cn (Y.C. Zhou). enough for materials with a higher creep rate. In particular, the influence of the creep rate on the elastic modulus measurement is crucial. In this article the creep behavior of electrodeposited nickel film is investigated. The influence of the hold period at maximum load and the loading rate on the elastic modulus and hardness results are also analyzed. 2. Experimental A carbon steel sheet of.3 mm thick was used as substrate. A uniform nickel film of 3. µm thick was deposited by electrodepositing method on both sides of the steel sheet. The film was obtained with nickel sulphate electrolyte, which was composed of 25 g NiSO 4 6H 2 O, 5 g NiCl 2 6H 2 O, and 35 g H 3 BO 3 per liter. Pure nickel was used as the anode. The ph value was adjusted with sulfuric acid to 4. at 42 C. A conventional rotating disc electrode was used for electrodeposition. Before electroplating, pretreatments were necessary to get rid of the impurities. The pretreatment procedure of substrates are shown in Fig. 1 (rinsing samples with deionized water). The nanoindentation measurements on nickel film specimens shown in this paper were performed at room temperature using a TriboIndenter from Hysitron Inc., USA with a three-sided pyramidal Berkovich diamond indenter under the same conditions. The load and displacement resolutions of the machine were 1 nn and.1 nm, respectively. It allows, therefore, very accurate measurements. Nanoindentation-creep experiments were performed at fixed maximum force (P max ) following ramp loading using a three-stage (trapezoidal) loading protocol in Fig. 2. Loading was performed at constant loading rates (κ=p max /t l ) where t l is the loading time and the hold time (t h ) was fixed at under each maximum load level (P max ). The nickel film sample was tested to varying maximum loads in the range of 1 9 µn at a fixed loading time of t l =1 s. In addition, tests were performed to the maximum load levels P max = µn with varying rise time (t l =3, 5, 1, 2, 3, 4, ) and varying hold time (t h =, 3, 5, 1, 2, 3, 4, ), respectively. In all cases, 1

2 J. Mater. Sci. Technol., Vol.25 No.1, Fig. 1 Technical flow chart of electroless plating P max Load, P t l Time, t t l +t h Fig. 2 Schematic trapezoidal load-time (P -t) input function for time dependent indentation experiments. P max denotes the maximum force, t l the loading time, and t h the creep hold time, all of which are variable in the current experiments indentations were done at a certain load and averaged. If a certain result deviates too much from the others, this result was excluded from the average. The aim of the average is to minimize the noise of the creep curves and hence to improve the reliability of measurement. The tests have been performed at different maximum loads for the nickel film. The hardness and elastic modulus calculations were done according to the Oliver and Pharr method [18] using a tip shape correction. For electrodeposited nickel film, when the maximum applied load was 9 µn, the maximum indentation depth was more or less 4 nm, or less than 15% of the film thickness. Thus, the influence of the lower carbon steel substrate should be minimal [19] Fig. 3 Comparison of the load-displacement curves obtained from the nanoindentation experiments performed at the maximum loads ranging from 1 to 9 µn Depth change / nm P max / N Time / s Fig. 4 Comparison of the creep curves under different maximum loads 3. Results and Discussion 3.1 Creep behavior Figure 3 shows the typical load-displacement curves obtained for the different maximum loads ranging from 1 to 9 µn on the electrodeposited nickel film. For consistency, only load-displacement profiles that mapped onto the same master curve were admitted as valid measurements. Significant creep displacements were observed in all the samples during the hold time of the nanoindentation tests. The relations between depth change (h h, where h is the instantaneous indentation depth and h the initial indentation depth.) and time (t t, where t is the instantaneous time and t (or t l ) is the time when the creep process is started.) for the electrodeposited nickel film under different maximum loads (P max ) are plotted in Fig. 4. The curves show an immediate rise in displacement upon varying the load from 1 to 9 µn. The data show that for every experiment there is an initial sharp rise in creep depth in the early part of the creep segment, followed by a region showing a smaller rate of increase in creep depth. The

3 92 J. Mater. Sci. Technol., Vol.25 No.1, 29 general profile of these curves is similar to the strain vs time plot obtained for the uniaxial tensile creep testing of bulk materials that exhibit power-law creep behavior. The initial stage in Fig. 4 corresponds to transient creep and after this initial displacement, the descent of the indenter continues but the rate of descent decreases to attain a steady state value. From Fig. 4, we also can see that the creep behavior can be well resolved, even below the several nanometers limit. Electrodeposited nickel film is a low creeping material and the depth change within is more or less 31 nm at 9 µn load. It can further be seen that the creep is continuously increasing with increasing maximum load. 3.2 Influence of the hold period on elastic modulus and hardness results The creep is closely connected with plastic deformation. Therefore, the load step within the plastic regime during a nanoindentation experiment is accompanied by a small amount of creep. This has an influence on the rate of the loading as well as on the hold period. How strong this influence is, depends on the displacement rate to creep rate ratio. Both parameters depend on each other. If the displacement rate is high during loading, the creep at maximum load starts with a higher creep rate than that for an indentation with lower displacement rate. The maximum indentation depth at a certain load is therefore influenced by: (1) the hold period at maximum load; and (2) the displacement rate during loading. To make the indentation experiments comparable, both parameters should be chosen in such a way that their influence on the hardness and elastic modulus results can be neglected. We may consider first the influence of the hold period on hardness and elastic modulus results for electrodeposited nickel film. Figure 5 compares the load-displacement curves for different hold periods but equal loading time t l =1 s. While the loading curves agree well, the unloading curves begin at higher depths for longer hold periods. Figure 6 shows a magnification of the load-displacement curves at maximum load. It can be clearly seen that the unloading curve shows a bowing towards higher depth if the hold period is too short. The creep value amounts to approximately 12 nm for the hold period t h =4 s at a maximum load of µn. The creep rate within the first after reaching the maximum load was 1. nm/s, while after a hold period of 4 s, the creep rate is reduced to approximately.3 nm/s in Fig. 6. The inclination of the unloading curve at maximum load is used for the calculation of the elastic modulus, and thus any bowing leads to a high hardness and elastic modulus error. Investigation of the hardness and elastic modulus as a function of the hold period has been performed on the electrodeposited nickel film, as shown in Fig. 7. It is evident that the hardness decreases with increasing hold period. The hardness and elastic modulus change amounts to approximately 8.5% and 53.8%, respectively. These results also show that only after holding or longer, can the elastic modulus error due to creep be neglected. However, the hardness is even more influenced by creep than the elastic modulus. The hold period needs to be at least 2 s to avoid the creep effect s 1 s 2 s 4 s 1 s Fig. 5 Comparison of the load-displacement curves for different hold periods at maximum load µn s 1 s 2 s 4 s 1 s Fig. 6 Magnification of the unloading range at maximum load µn from Hardness / GPa s Hardness Elastic modulus Holding period at maximum load / s Elastic modulus / GPa Fig. 7 Hardness and elastic modulus as a function of the hold period at maximum load µn with the loading time t l =1 s 3.3 Influence of the loading time on elastic modulus and hardness results The second investigation considers the influence of the loading time, which is related to the displacement rate. Figure 8 shows the load-displacement curves for three different loading times and a zero hold period at a maximum load of µn. It is evident that with increasing loading time the maximum displacement is also increased. The difference in the displacement rates during loading causes a difference of approximately 23 nm in the maximum depth. This can be interpreted as follows: with an increase in displacement rate, the velocity of the indenter increases, so it will penetrate into the test sample more deeply due to the inertia effect. However, an increase of the hold

4 J. Mater. Sci. Technol., Vol.25 No.1, s 3 s 3 1 s 2 1 Hardness / GPa Holding period s Fig. 8 Comparison of load-displacement curves from measurements with different loading time on electrodeposited nickel film. The hold period was s s Holding time Fig. 9 Comparison of the load-displacement curves from measurements with different loading time on nickel film. The depth difference caused by different loading times disappears Depth change / nm s Time / s Fig. 1 Comparison of creep curves under the maximum load µn for electrodeposited nickel film with the hold period at different loading time period reduces the displacement difference between measurements with different loading times in Fig. 9. After a hold period of, the displacement difference is reduced to only 1.5 nm, which corresponds to the measurement error. From Fig. 1, we can see that with a decrease in loading time (or an increase in displacement rate), the creep deformation is increased; moreover, the trend for the curve for a load rate of /3 µn/s is different from those of the other two curves. This may be attributed to two reasons. First, at the lowest load rate, the strain rate is also the lowest and a longer time is needed to reach the hold load, / s Fig. 11 Hardness of electrodeposited nickel film as a function of the duration of the hold period and loading time at maximum load µn Elastic modulus / GPa / s s Fig. 12 Elastic modulus of electrodeposited nickel film as a function of the duration of the hold period and loading time at maximum load µn so creep deformation may also occur during the loading period [2], and then the subsequent creep during the hold period will decrease. Second, the dislocation substructure formed beneath the indenter due to the indentation stress may be different at different loading strain rates, and this substructure will certainly affect the subsequent creep behavior [21]. The influence of the loading time on the hardness is given in Fig.11. The increase of depth, due to creep, results in a lower hardness for a higher loading time and a longer hold period. If the hold period is long enough, the loading time has no influence on the hardness longer. Using a loading time of the hardness difference, relative to a zero hold period, is 9.74% for hold period and 13.25% for hold period. The investigation of the influence of hold period and loading time on the elastic modulus shows that a zero hold period results in unacceptable elastic modulus results in Fig. 12. Independent of the loading time, they are much too high. In contrast, there is nearly no elastic modulus difference for unloading after a hold period of 3 and. The elastic modulus decreases slightly for a longer loading time. 4. Conclusions (1) Constant-load nanoindentation experiments were carried out at room temperature to investigate the creep behavior of electrodeposited nickel film with different loading time and different hold periods. An increase of the loading time could reduce the creep

5 94 J. Mater. Sci. Technol., Vol.25 No.1, 29 value. On the contrary, an increase of the hold period could enhance the creep value. (2) The loading time and the hold period of the load can have a strong influence on hardness and elastic modulus. To the electrodeposited nickel film, it was shown that using a zero hold period results in unacceptable elastic modulus results due to the bowing of the unloading curves. The elastic modulus error, due to creep, of more than 32.14% may arise. This error becomes negligible for hold periods of 3 s or longer. For the variation of the hold period, the hardness and elastic modulus change amounts to approximately 8.5% and 53.8%, respectively. These results also show that, in that case, only after around, can the elastic modulus error due to creep be neglected. However, the hardness is even more influenced by creep than the elastic modulus on electrodeposited nickel film. The hold period needs to be at least 2 s to avoid the creep effect. (3) The investigation considers the influence of the loading time, which is related to the displacement rate. The difference in the displacement rates during loading causes a difference of approximately 23 nm in the maximum depth. After a hold period of, the displacement difference is reduced to only 1.5 nm. For a sufficient hold period, the elastic modulus and the hardness decrease slightly for a longer loading time. Acknowledgements This work was supported by the National Nature Science Foundation of China (Grants Nos , and ), the Fok Ying Tong Education Foundation (No.1414) and the National Science Found for Distinguished Young Scholars of China (No ). REFERENCES [1 ] N. Wang, Z. Wang, K.T. Aust and U. Erb: Acta Mater., 1997, 45, [2 ] U. Klement, U. Erb, A.M. El-Sharik and K.T. Aust: Mater. Sci. Eng. A, 1995, 23, 177. [3 ] Y.M. Wang, S. Chen, Q.M. Wie, E. Ma, T.G. Nieh and A. Hamza: Scripta Mater., 24, 51, 123. [4 ] L. Lu, S.X. Li and K. Lu: Scripta Mater., 21, 45, [5 ] F.D. Torre, H. Van Swygenhoven and M. Victoria: Acta Mater., 22, 5, [6 ] R. Schwaiger, B. Moser, M. Dao, N. Chollacoop and S. Suresh: Acta Mater., 23, 51, [7 ] F. Ebrahimi, G.R. Bourne, M.S. Kelly and T.E. Matthews: Nanostruct. Mater., 1999, 11, 343. [8 ] A.F. Zimmerman, G. Palumbo, K.T. Aust and U. Erb: Mater. Sci. Eng. A, 22, 328, 137. [9 ] K.S. Kumar, S. Suresh, M.F. Chisholm, J.A. Horton and P. Wang: Acta Mater., 23, 51, 387. [1] H. Iwasaki, K. Higashi and T.G. Nieh: Scripta Mater., 24, 5, 395. [11] E. Thiele, R. Klemm, L. Hollang, C. Holste, N. Schell, H. Natter and R. Hempelmann: Mater. Sci. Eng. A, 25, 39, 42. [12] V. Sklenicka, K. Kucharova, M. Pahutova, G. Vidrich, M. Svoboda and H. Ferkel: Rev. Adv. Mater. Sci., 25, 1, 171. [13] W.M. Yin, S.H. Whang, R. Mirshams and C.X. Xiao: Mater. Sci. Eng. A, 21, 31, 18. [14] W. M. Yin and S.H. Whang: Scripta Mater., 21, 44, 569. [15] R.S. Kottadan and A.H. Chokshi: Scripta Mater., 25, 53, 887. [16] A.S.M.A. Haseeb: Comput. Mater. Sci., 26, 37, 278. [17] V. Sklenicka, K. Kucharova, M. Pahutova, G. Vidrich, M. Svoboda and H. Ferkel: Mater. Sci. Eng. A, 27, 462, 269. [18] W.C. Oliver and G.M. Pharr: J. Mater. Res., 1992, 7, [19] A.K. Bhattacharya and W.D. Nix: Int. J. Solids Struct., 1988, 24, [2] S. Yang, Y.W. Zhang and K.Y. Zeng: J. Appl. Phys., 24, 95, [21] V. Raman and R. Berriche: J. Mater. Res., 1992, 7, 627.