Cu nanoscale multilayer thin films

Surface & Coatings Technology 202 (2008) 5508–5511

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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t

Effect of bilayer period on CrN/Cu nanoscale multilayer thin films Youn J. Kim ⁎, Tae J. Byun, Ho Y. Lee, Jeon G. Han Center for Advanced Plasma Surface Technology, SungKyunKwan University, 300 ChunChun-dong, Jangan-gu, Suwon 440-746, Republic of Korea

A R T I C L E

I N F O

Available online 8 June 2008 Keywords: CrN/Cu Nanoscale multilayer Bilayer period

A B S T R A C T Thin films with a CrN/Cu multilayer structure comprising a superhard coating with a lamination of hard transition metal nitride and soft metal were synthesized by Closed Field Unbalanced Magnetron Sputtering (CFUBMS). The bilayer thickness (λ) of all of the laminated layers was controlled via the rotational speed of the substrate holder within the range from 4.7 to 47.1 nm. The structural characterization of the coatings was investigated using high resolution X-ray diffraction (HRXRD) and field emission transmission electron microscopy (FETEM). The preferred orientation of the CrN/Cu nanoscale multilayer thin films was CrN (200) and Cu (200). The surface morphology of the films was examined by atomic force microscopy (AFM). The mechanical properties of the coatings, as determined by nano-indentation testing and the ball-on-disc test varied with the bilayer thickness (λ). The multilayer with a bilayer thickness of 7.9 nm exhibited high hardness, high resistance to plastic deformation, and high wear resistance. © 2008 Elsevier B.V. All rights reserved.

1. Introduction

2. Experimental details

Since the early fundamental research on superlattice structured hard coatings in the late 1980s, rapid progress has been made in the production of nanoscale compositionally modulated coatings with hardness values exceeding 50 GPa. Films with these characteristics are typically known as superlattices or heterostructures. For example, most works reported so far describe heterostructures with a metal/ metal structure such as Cu/Ni [1,2], Cu/Fe [2,3], Al/Cu [4,5], and Al/Ag [5], and some ceramics such as TiC/Mo [7], TiN/NbN [8], and Ti/TiN [6,9] with a large range of hardness values varying approximately between 10 and 70 GPa in terms of their duplex properties, combined with high wear and lubrication, as well as long lasting wear and lubrication performance. The mechanical properties of multilayered films were found to be greatly affected by the bilayer thickness (λ). The conventional Hall–Petch relationship [10,11] or Koehler's theory [12] can be applied to explain the mechanism of hardness enhancement. However, a fundamental understanding of the mechanical and wear properties of multilayered coatings is still lacking. In this study, CrN/Cu nanoscale multilayer thin films comprised of a superhard coating with a lamination of hard transition metal nitrides were synthesized by Closed Field Unbalanced Magnetron Sputtering (CFUBMS). Their wear properties were evaluated using the ball-on-disc test against steel balls, after which the changes in their mechanical properties were compared in terms of the basic wear resistance behavior with respect to the change in microstructure of the multilayered systems.

2.1. Film deposition

⁎ Corresponding author. E-mail address: [email protected] (Y.J. Kim). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.06.028

All of the coatings were synthesized on Si wafers (100) by Closed Field Unbalanced Magnetron Sputtering (CFUBMS). CrN/Cu nanoscale multilayer films were reactively deposited from circular planar pure Cr (99.99%, φ=100 mm) and pure Cu (99.99%, φ=100 mm) targets, respectively. Prior to the growth of the nanoscale multilayer films, a Cr interlayer was deposited to enhance the adhesion between the film and the substrate. Concerning the nanoscale multilayer films, the period thickness (λ) was controlled via the rotational speed of the substrate holder. The thickness of all of the films was 1 μm. The detailed deposition conditions are listed in Table 1. 2.2. Evaluation of films To evaluate the crystal structure and formation behavior of the compounds, high resolution X-ray diffraction (HRXRD) studies were carried out using a D8 Discover X-ray diffractometer in parallel beam geometry, using CuKα radiation (λCuKα = 0.154186 nm) and operated at 40 kV and 40 mA. Field emission transmission electron microscopy (FETEM) was performed on a JEM2100F with an operating voltage of 200 kV. The surface morphology of the films was examined by atomic force microscopy (AFM) with a force sensor operated in contact mode. The mechanical characteristics of the films were determined by the nano-indentation and ball-on-disc tests. The hardness of the films was measured at a normal load of 30 mN using a commercially available nano-indentation instrument (Nano-indenter II) developed by MTS Instrument. To determine the wear resistance behavior of the nanoscale multilayer thin films, dry sliding tests were conducted with a

Y.J. Kim et al. / Surface & Coatings Technology 202 (2008) 5508–5511 Table 1 Deposition conditions of the CrN/Cu nanoscale multilayer thin film Deposition parameters

Conditions

Base pressure Working pressure Total working pressure Target power density

4 × 10− 3 Pa 0.39 Pa 0.48 Pa Cr: 13.5 W/cm2 Cu: 5.4 W/cm2 2, 4, 8, 12, 16, 20 rpm 100 mm −100 V Cr Si (100)

Jig rotation speed Distance between substrate and target Substrate bias Interlayer Substrate

sliding wear tester in air (temperature 298 K, humidity 35–38%). A load of 1.35 N was applied to the AISI 52100 steel ball (φ = 10 mm). The radius of the wear track and the sliding speed were 8.75 mm and 2.75 cm/s, respectively. The frictional force was detected by a load cell and recorded.

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Fig. 2 (a) and (b) shows the FETEM images of the CrN/Cu nanoscale multilayer thin films with bilayer thicknesses of 47.1 nm and 7.9 nm, respectively. When the bilayer thickness was 47.1 nm, the thin film had an uneven interface and layer by layer growth. The Selected Area Electron Diffraction (SAED) pattern confirmed that the crystal orientations of the films were CrN (200) and Cu (200). However, in the case where the bilayer thickness is 7.9 nm, the thin films have a linear interface and locally mixed layers and the crystal orientations of the films are Cu (200) and Cu (111) according to the SAED pattern. The indications given by the HRXRD analysis for the CrN/Cu nanoscale multilayer thin films were supported by the FETEM results. The surface morphology of the CrN/Cu nanoscale multilayer thin films is shown in Fig. 3. The Rp-v and Rms roughness values of the films were found to range from 40 nm to 145 nm and from 4 nm to 13 nm, respectively. As the bilayer thickness increased, the roughness value of the film was increased. This was due to the high level of crystallization and the increase of the crystallite size which was calculated using the Scherrer formula [13]. 3.2. Mechanical and tribological properties of CrN/Cu nanoscale multilayer thin films

3. Results and discussion 3.1. Microstructure of CrN/Cu nanoscale multilayer thin films Fig. 1 shows the HRXRD patterns of the synthesized CrN/Cu nanoscale multilayer thin films with different bilayer thicknesses. The crystal orientations of the CrN/Cu nanoscale multilayer films with a bilayer thickness of 47.1 nm were CrN (200) and Cu (200). In this case, Cr and Cu existed in the form of individual layers having a high degree of crystallization. When the bilayer thickness of the CrN/Cu nanoscale multilayer thin films was decreased their crystal orientations were changed, as shown by the fact that the peak intensity of Cu (200) was reduced and the CrN (200) peak was shifted toward the center of the 2θ positions for CrN (200) and Cu (111) referred to in the JCPDS. This means that the Cu and CrN layer existed in the form of locally mixed layers. These results were also confirmed by the FETEM results presented in Fig. 2 (a) and (b).

The mechanical properties of the synthesized nanoscale multilayer thin films are shown in Fig. 4. The hardness (range: 9 to 15 GPa), Young's Modulus (about 200 GPa) and resistance to plastic deformation (range: 1.5 × 10− 2 to 5.3 × 10− 2 GPa) of the CrN/Cu nanoscale multilayer thin films were confirmed to be influenced by the bilayer thickness. Especially, the film with a bilayer thickness of 7.9 nm had high mechanical properties. This means that there is a critical bilayer thickness needed to obtain good mechanical properties. Fig. 5 shows the depth profiles of the wear track on the CrN/Cu nanoscale multilayer thin films. When the bilayer thickness is below 10 nm, in which case the film has low plastic deformation resistance and hardness, the wear tracks are deeper and narrower. However, in the case where the bilayer thickness is over 10 nm, in which case the film has high plastic deformation resistance and hardness, there is Fe debris which originates from the impact of the steel ball on the films.

Fig. 1. HRXRD patterns of the CrN/Cu nanoscale multilayer thin films.

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varied with the bilayer thickness. When the bilayer thickness is over 10 nm, the Si substrate can be observed after the ball-on-disc test and adhesive wear occurred in the wear track of the thin film due to the low plastic deformation. Also, metal oxides were formed on the ball. However, in the case where the bilayer thickness was below 10 nm, abrasive wear on the film and ball occurred, due to the high plastic deformation resistance of the film. Moreover, there were no oxide or other materials on the thin films. It was also found that the wear volume of the synthesized CrN/Cu

Fig. 2. FETEM images of the CrN/Cu nanoscale multilayer thin films. (a) 47.1 nm bilayer thickness. (b) 7.9 nm bilayer thickness.

Fig. 4. Mechanical properties of the CrN/Cu nanoscale multilayer thin films.

Fig. 3. Surface morphology of the CrN/Cu nanoscale multilayer thin films.

A wear image of the CrN/Cu nanoscale multilayer thin film and steel ball after the ball-on-disc test is presented in Fig. 6. As regards the wear volume of the ball, the wear diameter and shape of the counterparts

Fig. 5. Wear depth profiles of the wear track on the CrN/Cu nanoscale multilayer thin films.

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Fig. 6. Wear volume, wear images and steel ball images of the CrN/Cu nanoscale multilayer thin films.

nanoscale multilayer thin film (range: 0 to 2.6×10− 2 mm3) was described by the Archard Equation (wear volume 1/H) [14]. 4. Conclusion We synthesized CrN/Cu nanoscale multilayer thin films comprising a superhard coating with a lamination of hard transition metal nitride and soft metal by CFUBMS. The microstructural, mechanical, and tribological properties of all of the synthesized films depended on the bilayer thickness (λ). When the thin films with a bilayer thickness (λ) N10 nm having a low plastic deformation resistance were subjected to the wear test, the Si substrate was observed, abrasive wear occurred and abrasive Fe existed on the ball. However, the high plastic deformation resistance of the thin films with a bilayer thickness (λ) b10 nm was confirmed by the fact that the wear volume of the films was decreased and abrasive wear occurred on the film. In the case of the counterpart, no metal oxide compounds were observed, but only Fe debris. The critical bilayer thickness (λ) maximizing the mechanical and tribological properties needs to be determined. Acknowledgement The authors are grateful for the financial support provided by the Korea Science and Engineering Foundation through the Center for Advanced Plasma Surface Technology (CAPST) at Sungkyunkwan University.

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