Trans. Nonferrous Met. Soc. China 23(2013) 2957-2961

Improvement of adhesion properties of TiB₂ films on 316L stainless steel by Ti interlayer films

Mu-jian XIA, Hong-yan DING, Guang-hong ZHOU, Yue ZHANG

Jiangsu Provincial Key Laboratory for Interventional Medical Devices, Huaiyin Institute of Technology, Huai’an 223001, China

Received 6 September 2012; accepted 17 May 2013

Abstract: The periodic [Ti/TiB₂]_n (n=1, 2, 3) multilayered films were prepared on the substrate of AISI 316L stainless steel by magnetron sputtering to enhance the adhesion of TiB₂ films based on the remarkable mechanical performance of layered films. The influence of periods on microstructure, adhesion and hardness of [Ti/TiB₂]_n multilayered films was studied. X-ray diffraction (XRD) analysis shows that the monolayer TiB₂ films exhibit (001) preferred orientation, and the preferred orientation of [Ti/TiB₂]_n multilayered films transfers from (001) to (100) with the increase of periods. The cross-sectional morphology of each film displays homogeneity by field emission scanning electron microscopy (FESEM). The hardness of the films measured via nanoindention changes from 20 to 26 GPa with the increase of periods. These values of hardness are a bit lower than that of the monolayer TiB₂ films which is up to 33 GPa. However, the [Ti/TiB₂]_n multilayered films present a considerably good adhesion, which reaches a maximum of 24 N, in comparison with the monolayer TiB₂ films according to the experimental results.

Key words: multilayered films; adhesion; TiB₂ films; magnetron sputtering; nano-hardness

1 Introduction

Titanium diboride (TiB₂), which is well known to be a ceramic compound, possesses numbers of excellent mechanical, physical, and chemical properties, such as high hardness, good wear and corrosion resistance, high electrical and thermal conductivity, and good chemical stability [1,2]. Such superior properties of TiB₂ make it to be investigated extensively for various applications of hard coatings, corrosion-resistance coatings, and diffusion barrier films used for microelectronic devices [3,4]. The monolayer TiB₂ films are deposited by various PVD methods, among which magnetron sputtering is considered an easier approach to prepare TiB₂ films owing to its relatively high deposition rate and low deposition temperature applied to the substrate [5,6]. However, it is difficult to produce the monolayer TiB₂ films with good mechanical integrities which are suitable for commercial and engineering applications [7]. The main problem is that the sputtered TiB₂ films are too brittle and have a poor adhesion due to the high residual stress [8]. The adhesion of TiB₂ coatings sputtered on high-speed steel is lower than 5 N [7,9]. The high residual stress generated in films can influence their properties directly, e.g. adhesion, fatigue strengthen, bond strength, tribological properties, etc [10-12]. Furthermore, excessive residual stress existed in the films may cause the formation of defects and delamination at the interface. In fact, compared with other mechanical properties, the adhesion directly affects operating life of the films, especially for which applied in tribological application.

Recently, many attempts have been made to enhance the adhesion of monolayer TiB₂ films. BERGER et al [12] found that the adhesion of TiB₂ coatings deposited by switching the substrate bias from negative to positive was significantly improved by nearly 60% in comparison with the films without applying bias. PANICH and SUN [13] reported that applying a bias on the TiB₂ coatings could enhance the adhesion. In addition, BOHWAN et al [14] also found that the nitride layer could greatly improve the adhesion of TiB₂ films sputtered on H13 steel.

The structure of nacre consisted of inorganic and organic constituents. Many researchers reported that the soft organic layer of nacre could erase the stress derived from the hard inorganic layer [15]. For example, the adhesions of TiN and AlN were greatly enhanced owing to the fabrication of Ti/TiN [16] and Al/AlN [17]. Consequently, in this work, the metallic titanium film was considered a soft layer, and was attempted to strengthen the adhesion of TiB₂ films based on this unique structure of nacre. Besides, the metallic titanium, which had a similar crystal structure with TiB₂, may decrease the mismatch between TiB₂ films and substrate, and combine with a hard layer TiB₂ to fabricate the bilayerd or multilayered films. The microstructure, hardness and adhesion of the monolayer TiB₂ films and [Ti/TiB₂]_n multilayered films (n was the period of films ) were investigated. Furthermore, the effects of periods on adhesion of multilayered films with various periods were discussed in details.

2 Experimental

A monolayer TiB₂ films and [Ti/TiB₂]_n multilayered films were fabricated by magnetron sputtering. For the sputtering targets, a sintered TiB₂ target (99.9%) and a metallic Ti target (99.95%) of 50 mm in diameter were used. The Ti and TiB₂ targets were placed in the chamber at a substrate-magnetron distance of 50 mm. AISI 316L stainless steel (SS) was selected as the substrate specimens and cut to the dimensions of 10 mm×10 mm×1 mm. The specimens were grinded with 2000 grit water-proof silicon carbide papers, and then were treated by the electrochemical polishing. Subsequently, the specimens were ultrasonically cleaned in ultrasonic baths of acetone and ethanol. Finally, under the sputtering power of 60 W, the substrates were cleaned by Ar⁺ (a high purity of 99.9%) etched at 4 Pa for 10 min prior to the deposition processes. The deposition chamber was evacuated to the pressure of 5×10^-5 Pa before deposition. During deposition of all films, the Ar gas flow with 20 mL/min was introduced into the vacuum chamber. For the monolayer TiB₂ films, it was deposited by a r.f. power of 120 W, a working pressure of 0.7 Pa and a substrate temperature of 350 °C. For the [Ti/TiB₂]_n multilayered films, Ti layer and TiB₂ layer were successively prepared on AISI 316LSS. A d.c. power of 40 W and a substrate temperature of 150 °C were applied to the deposition of Ti layer (25 nm). TiB₂ layer (125 nm) was prepared by the processing parameters of TiB₂ films. [Ti/TiB₂]_n multilayered films were alternatively fabricated by the mechanical shutter, and were deposited with n=1, 2, 3.

The film structures were characterized by Thermo ARL X’TRA X-ray diffraction (XRD) equipped with Cu K_α radiation, and the measurement angle (2θ) was in the range from 20° to 80° with 0.02° interval. The cross-sectional morphology of films was observed by Hitachi Limited S4800ΙΙ field emission scanning electron microscopy (FESEM). Hysitron Triblab nanoindentation with a Berkovich indenter was employed to determine the hardness of films using load—displacement curve. The principle of nanoindentation was specifically introduced in Ref. [18]. For the purpose of statistics and reliability, three indentations were made in each experiment to find the average hardness results. Since a critical load for the onset of a film failure was generally accepted as a relatively accurate measurement to estimate the adhesion, scratch test was recommended to investigate the adhesion. The adhesion was evaluated from the point where an acoustic emission profile and corresponding friction coefficient had an abrupt change during a scratch test.

3 Results and discussion

3.1 Crystalline structures

The XRD patterns of a monolayer TiB₂ films and [Ti/TiB₂]_n multilayered films with various periods deposited on AISI 316LSS substrate are shown in Fig. 1. The peaks of AISI 316LSS (γ-Fe phases) were obvious. The monolayer TiB₂ films had the diffraction peaks of (001) and (100), but the intensity of (001) peak was higher than that of (100) peak. This indicates that the microstructure of TiB₂ films exhibited a preferred (001) orientation of TiB₂ crystalline. This preferred orientation was also formed when sputtered on Si (100) substrate [19]. The phenomenon can be interpreted that the films on substrates generally grow with the plane of the lowest surface free energy parallel to the film surface.

From Fig. 1, it can be also found that the [Ti/TiB₂]_n multilayered films with various periods clearly exhibited diffraction peaks, and the mutual relation of magnitude changed with the increase of periods. For Ti layers, the intensity of (100) peaks gradually strengthened with the increase of periods. Whereas, the intensity of (101) peaks weakened and appeared to be opposite with that of (100) peaks. For TiB₂ layers, the intensity of (100) peak started to strengthen with the increase of periods, and it had a higher intensity than (001) peaks. This may be caused by the change in a degree of the preferred Ti (100) orientation which favored the epitaxial growth of TiB₂ (100) peaks. All the above results suggested that [Ti/TiB₂]_n multilayered films with 3 periods exhibited preferred Ti (100) and TiB₂ (100) orientation. The preferred orientation was also found in the sputtered Ti/TiN multilayered films [20].

Fig. 1 XRD patterns of monolayer TiB₂ film and [Ti/TiB₂]_n multilayered films deposited on AISI 316LSS with various periods

3.2 Cross-sectional morphology

Figure 2 presents the cross-sectional images of the monolayer TiB₂ films and [Ti/TiB₂]_n multilayered films. From Fig. 2, it can seen that the [Ti/TiB₂]_n multilayered films were composed of Ti layers (25 nm) and TiB₂ layers (125 nm) which were deposited alternatively. FESEM analysis of all the films displayed a very good morphology, and the cross-section possessed extremely dense and homogeneous.

3.3 Hardness

Figure 3(a) presents the load—penetration depth curves from nanoindentation measurement of [Ti/TiB₂]_n multilayered films together with the monolayer TiB₂ films. The hardness values were calculated according to the corresponding load—penetration data, as shown in Fig. 3(b). It was observed that the hardness of multilayered films strongly depended on the periods. In the case of [Ti/TiB₂]₁, a relatively low hardness of 20 GPa was obtained. With the increase of periods, the hardness enhanced to a maximum value of 26 GPa that was found in [Ti/TiB₂]₃ multilayered films. It was similar to the hardness of TiB₂ films sputtered on nitrided AISI H13 steel ranging from 20 to 30 GPa [14]. The hardness of multilayered films was enhanced with the increase of periods due to the more interfaces generated between Ti and TiB₂ layers. As we all know that there were many imperfections at the interfaces between Ti and TiB₂ because of the crystal lattice mismatch. Meanwhile, the [Ti/TiB₂]_n multilayered films with more periods had more interfaces, and the dislocation motioned more difficulty [21]. Furthermore, the grain size decreased in TiB₂ films with the increase of period enhanced the hardness of the multilayered films [22].

However, compared with the hardness of the monolayer TiB₂ films up to 33 GPa, the weakness of multilayered films hardness could be attributed to the effect of adjacent Ti soft layer. As a result of the diffusion of atoms between Ti layer and the adjacent TiB₂ layer, the interfaces of multilayered films turned to be compositionally gradient zone, which leaded to the lost of hardness enhancement. On the contrary, the significant hardness enhancement was observed in the Ti/TiB₂ multilayered films with the modulation period ranging from 1.1 to 9.8 nm in comparison with the Ti and TiB₂ monolayer films [23]. As known to all, the multilayered films with small modulation period had less time for ion bombardment at each target, which resulted in the incomplete growth process. Then the sharp interfaces were generated in films. Therefore, the crack propagation was reduced, and the hardness of multilayer films with small modulation period was enhanced.

Fig. 2 Cross-sectional morphologies of a monolayer TiB₂ film (a), multilayered films [Ti/TiB₂]₁ (b), [Ti/TiB₂]₂ (c) and [Ti/TiB₂]₃ (d)

Fig. 3 Load—penetration depth curves (a) and hardness (b) of monolayer TiB₂ film and [Ti/TiB₂]_n multilayered films with n=1, 2, 3

3.4 Adhesion

Variations of the critical load obtained from scratch test of the monolayer TiB₂ films and [Ti/TiB₂]_n multilayered films are shown in Fig. 4. It was clearly found that the critical load of all multilayered films is higher than that of the monolayer TiB₂ films with the adhesion of 9.5 N. This phenomenon was also found by HAN et al [24] in Ti/TiN films. One possible reason was that the soft Ti layer could prevent the propagation of cracks generated in a brittle TiB₂ layer into a deeper layer. On the other hand, the enhancement of adhesion may be the result of the change of residual stress caused by ion bombardment during deposition. It was well accepted that the residual stress of films sputtered on diverse substrates presented difference, and the films sputtered with a higher stress also showed a poor adhesion strength. Consequently, the stress state of the monolayer TiB₂ films directly deposited on AISI 316LSS must be different from that prepared on Ti films. The interfaces of multilayered films were favorable to decrease the compressive stress, and the [Ti/TiB₂]_n films with compressive stress states of alternating component layers processed a relative lower compressive stress compared with the monolayer TiB₂ films. This tendency was also found in Al/AlN multilayered films [17].

Fig. 4 Critical load of monolayer TiB₂ film and [Ti/TiB₂]_n multilayered films with n=1, 2, 3

Additionally, it was clearly found that the critical load of multilayered films enhanced with the increase of periods. This enhancement may be related to the stress and periods of multilayered films. The more the periods were, the more the interfaces were between Ti and TiB₂, which can generally change the direction of the initial crack when it penetrated deep into the films [25]. The dislocations propagation of multilayered films which had more interfaces was easy to be inhibited. It also resulted in the failure of multilayered films during the scratch process [26,27].

On the other hand, the interface between a soft Ti layer and a brittle TiB₂ layer can store a part of total elastic energy during scratch loading, and the corresponding elastic energy at the interface between multilayered films and substrate was reduced [28]. With the increase of periods, the multilayered films presented more interfaces which can store more elastic energy and decline the stress effectively.

4 Conclusions

1) XRD patterns shows that the monolayer TiB₂ films exhibits the preferred orientation of (001), and the multilayered films with 3 periods presents the preferred orientation of TiB₂ (100) and Ti (100).

2) The cross-sectional morphology of each film processes homogeneous.

3) The interpretation for this hardness enhancement in [Ti/TiB₂]_n multilayered films are based on interfaces acting on dislocations, which will contribute to the increase of hardness of the multilayer films. The hardness of multilayered films varies from 20 to 26 GPa with the increase of periods, but it has a little lost compared with the monolayer TiB₂ films (up to 33 GPa).

4) Such multilayer films with more periods will be quite favorable for commercial application in the future. A good adhesion of the [Ti/TiB₂]₃ multilayered films is achieved with a maximum adhesion of 24 N, which is nearly three times 9.5 N of the adhesion of the monolayer TiB₂ films.

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Ti层对316L不锈钢表面TiB₂薄膜结合力的影响

夏木建，丁红燕，周广宏，章 跃

淮阴工学院 江苏省介入医疗器械研究重点实验室，淮安 223001

摘  要：基于多层膜优异的力学性能，采用磁控溅射法在316L不锈钢基体表面沉淀[Ti/TiB₂]_n (n=1,2,3)多层膜以增强TiB₂薄膜的膜基结合强度。研究周期数对多层膜的结构、硬度及结合力的影响。结果表明：TiB₂单层膜表现为(001)方向的织构。随着周期数的增加，多层膜的织构方向由(001)转变为(100)；多层膜的硬度从20 GPa增加到26 GPa，但略低于TiB₂单层膜的硬度(33 GPa)；相对于单膜的膜基结合力(9.5 N)，多层膜表现出较好膜基结合力，最大结合力可达24 N。

关键词：多层膜；结合力；TiB₂薄膜；磁控溅射；纳米硬度

(Edited by Chao WANG)

Foundation item: Project (51175212) supported by the National Natural Science Foundation of China

Corresponding author:Hong-yan DING; Tel/Fax: +86-517-83559130; E-mail: xjmhyit@yahoo.com.cn

DOI: 10.1016/S1003-6326(13)62820-4