4. Effect of Temperature on the Evolution of Diffusivity, Microstructure and Hardness of Nanocrystalline Nickel Films Electrodeposited at Low Temperatures (4 Pagini)

Effect of temperature on the evolution of diff usivity, micr ostructure and hardness of nanocrystalline nickel lms electrodeposited at low temperatures C.K. Chung ⁎, W.T. Chang, C.F. Chen, M.W. Liao Dep't of Mechanical Engineering, Center for Micro/Nano Science and Technology, National Cheng Kung University, Taiwan 701, ROC a b s t r a c t a r t i c l e i n f o Article history: Received 17 September 2010 Accepted 21 October 2010 Available online 28 October 2010 Keywords: Electrodeposition Nickel Hardness Atomic force microscopy Diffusion coef cient Most nickel (Ni) lms galvanostatically electrodeposite d at 40–50 °C exhibited low hardness about 4 GPa and rough surface. In this article, we have investigated Ni electrodeposition at low temperatures of 5 –20 °C in order to enhance the hardness and smoothness of lms and performed by potentiostatic mode instead of galvanostatic mode to avoid the low-temperature pre cipitation of electrolyte agent s. Effect of temperature on the evolution of diffusion coef cient, deposition rate, morphology and hardness was studied. Electrodepo- sition at low temperature without hard-element addition can reduce diffusion rate and produce the ne- grain, smooth morphology and dense lm together with compressive residual stress to enhance hardness up to 6.18 GPa at 5 °C. The growth and hardening mechanism of low-temperature electrodeposited Ni were further discussed in details. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Nickel (Ni) electrodeposition has attracted great interest these decades and become one popular surface treatment technology for the coatings or microelectromechnical system (MEMS) application [1]. High strength and good smoothness are both crucial issues in the deco rativ e or func tiona l coati ngs [2]. Two kin ds of met hod s are commonly used to improve strength of coatings in electrodeposition: one is performed by pulse voltage and the other is alloyed with hard elements. Pulse electrodeposition produces ne grains in deposits to enhance the har dne ss via Hal l–Petch strengthening [3,4]. The hardness can be improved from 3.90 GPa by direct-current electrodeposition to 4.87 GPa by pulse electrodeposition [3]. Hardness of co- deposited nickel–cobalt (Ni–Co) alloys can be enhance d to 8.72 GP a by adding 22.53 at.% Co to Ni during electrodeposition [5] . Also, the compressive residual stress of thin lms from ion bombardment or thermal annealing can enhance hardness of coatings [6,7]. Electrode- position performance at low electrolytic temperature generally has uniq ue prope rties of low diff usion coef cien t, low nucl eatio n rate and low deposition rate [8–10]. However, some problem of electrolyte agent precipitation at low temperature may occur in conventional galvanostatic electrodeposition. In this article, we have investigated the potentiostatic electrodeposition at low temperature to enhance smoothness and hardness of deposits and to eliminate the precipitation pro blem. Temper ature eff ect on the evo lut ion of dif fusion coef cie nt, sur fac e mor phology and har dne ss of deposi ts was correlated and discussed. 2. Experimental procedures The Ni lms were electrod epos ited on the thin Cr/A u-coa ted silicon (Si) substrates at low temperatures of 5 –20 °C which were con trolle d by a coo lin g sys teminside theisoth ermalcontainer. TheCr/ Au-coated Si(100) wafers were used and diced into 0.5 cm×0.5 cm chips. Then they were cleaned with acetone and isopropanol in an ultrasonic bath for 15 min. The Ni was used as the anode, Cr/Au- coated Si as cathode and saturated calomel electrode as reference electrode. Potentiostatic electrodeposition was employed at a con- stan t vol tage of 1 V by pote nti osta t (Jie han5000, Tai wan) and deposition duration was 30 min. The electrolytic composition con- sis ted of nic kelsulphamate 450 g/l , nic ke l chl ori de 4 g/l and bor ic aci d 40 g/l . The initial pH of ele ctr olyte was controlled at 3.8 . The morphology of deposits was measured by atomic force microscopy (AFM, Nanosruf Mobile S, Swiss) at a scanning area of 10×10 μ m 2 . Hardness of the deposits was measured by MTS Nano Indenter® with the continuous stiffness measurement (CSM) technique to obtain the hardness as a function of depth. The hardness values were selected from the 150–200 nm less than 10% of lm thickness to avoid the substrate effect on lm property. 3. Results and discussion The chro noamp erometry meth od is used for study ing the diff usion behavior of ions during Ni electrodeposition at various electrolytic temperatures by recording the evolution of current with time. Fig. 1 Materials Letters 65 (2011) 416–419 ⁎ Corresponding author. Tel.: +886 6 2757575x62111; fax: +886 6 2352973. E-mail address: [email protected] (C.K. Chung). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.10.064 Contents lists available at ScienceDirect Materials Letters j ou r nal home p a g e: www. e l s ev i e r. c o m/ locate/ma t l e t

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4. Effect of Temperature on the Evolution of Diffusivity, Microstructure and Hardness of Nanocrystalline Nickel Films Electrodeposited at Low Temperatures (4 Pagini)