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The influence of boron contents on the microstructure and mechanical properties of Cr–B–N thin films
Vacuum 87 (2013) 191e194
Contents lists available at SciVerse ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
The influence of boron contents on the microstructure and mechanical properties of CreBeN thin films Jyh-Wei Lee a, b, *, Cheng Chih-Hong c, Hsien-Wei Chen d, Li-Wei Ho a, Jenq-Gong Duh d, Yu-Chen Chan d a
Dept. of Materials Engineering, Ming Chi University of Technology, Taiwan, ROC Center for Thin Films Technologies and Applications, Ming Chi University of Technology, Taiwan, ROC c Dept. of Mechanical Engineering, Tungnan University, Taiwan, ROC d Dept. of Materials Science and Engineering, National Tsing Hua University, Taiwan, ROC b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 July 2011 Received in revised form 22 October 2011 Accepted 29 February 2012
Seven CreBeN films with various boron contents ranging from 3.7 to 20.6 at.% were deposited by a bipolar asymmetric pulsed DC reactive magnetron sputtering system. The structures of thin films were characterized by X-ray diffraction (XRD). The surface and cross-sectional morphologies of thin films were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The surface roughness of thin films was explored by atomic force microscopy (AFM). A nanoindenter was used to evaluate the hardness and properties of thin films, respectively. The phases of each CreBeN coating were mainly CrN and amorphous BN regardless the boron content. It was found that the grain size decreased with increasing boron content. The hardness and elastic modulus of CreBeN thin films decreased with increasing boron concentration, which were mainly attributed to the nanostructure configuration of small CrN nanograins and large intergranular amorphous BN phase. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: CreBeN nanocomposite thin film Amorphous BN Nanoindentation Pulsed DC reactive magnetron sputtering
1. Introduction Surface engineering is an enabling technology to provide a modified surface on the interest with required properties and prolonged surface life. For the cutting tool and molding applications, nanocomposite thin films have been studied by researchers and industries due to its excellent mechanical properties [1,2]. Since boron is an effective element to produce a nanocomposite structure, the B contained chromium nitride thin films have been explored extensively in literature [3e8]. It is reported that the nitrogen flow rate ratio during sputtering has strong influence on the phase, microstructure and mechanical properties of the CreBeN coating [3e8]. However, the effect of boron content on the phase, microstructure and related mechanical properties of CreBeN coatings is seldom studied. In this work, a co-sputtering process using a CrB2 compound target and a pure Cr target was executed to deposit a series of CreBeN thin films containing different B concentrations by a pulsed DC magnetron sputtering. Through this process, the influence of boron content on the phase,
* Corresponding author. Dept. of Materials Engineering, Ming Chi University of Technology, No.84, Gongzhuan Rd., Taishan Dist., New Taipei City 24301, Taiwan, ROC. Tel.: þ886 2 29089899; fax: þ886 2 29084091. E-mail address: jeffl[email protected] (J.-W. Lee). 0042-207X/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2012.02.049
microstructure and mechanical properties of CreBeN coating were evaluated. 2. Experimental procedure The CreBeN thin films were deposited on a p-type (100) silicon wafer substrate by a bipolar asymmetry pulsed DC reactive magnetron sputtering system. A pure Cr (99.99 at.%) and a CrB2 compound target (33%Cre67%B, in at.%) were placed on the same side of the chamber. The diameter and thickness of each target were 76.2 mm and 6.0 mm, respectively. The substrate was placed between the Cr and CrB2 targets to deposit CreBeN thin films with various B contents. The different B contents of CreBeN thin films were obtained due to the difference of substrate position between the Cr and CrB2 targets. According to the substrate positions, thin films deposited on Si substrate are denoted as A1 to A7, in which the A1 was close to the Cr target and A7 was close to CrB2 target. A pulse frequency of 2 kHz was used during sputtering. A pulsing bias of 150 V and a pulse unit with 2 kHz frequency was applied to the substrates. No attempt was made to coordinate the pulse cycles. The substrate-to-target distance was kept at 100 mm. The base pressure was 1.6 103 Pa before deposition. Substrate surfaces were cleaned by Ar ion etching for 10 min at 1.2 Pa under 500 V substrate bias prior to the deposition process. The working pressure was 0.47 Pa with the same ratio of Ar and N2 flow rates.
J.-W. Lee et al. / Vacuum 87 (2013) 191e194
The surface topography and roughness of thin films were investigated by an atomic force microscopy (AFM, DI 3100, Bruker, USA). The cross-sectional morphologies of thin films were examined with a field emission scanning electron microscope (FE-SEM, JSM-6701, JEOL, Japan). Selected thin films were further analyzed by TEM to reveal the microstructures in details. Chemical compositions of thin films were analyzed with a field emission electron probe microanalyzer (FE-EPMA, JXA-8500F, JEOL, Japan). The phases of thin films were explored by a glancing angle X-ray diffractometer (GAXRD, Panalytical X’Pert Pro, Holland) with an incidence angle of 2 . Cu Ka radiation generated at 30 kV and 40 mA from a Cu target was used. The lateral grain size of each film was estimated by the Scherrer formula using the integral width of the Bragg reflection [9]. The Fourier transform infrared spectroscopy (FTIR, Spectrum One, PerkineElmer, USA) measurement was performed to explore the type of chemical bonds of the BN structure of each CreBeN thin film. The FTIR spectrum background of each coating was subtracted from the spectrum of uncoated Si wafer. The hardness and elastic modulus of thin films were investigated by means of a nanoindenter (TI-900, TriboIndenter, Hysitron, USA) using a Berkovich 142.3 diamond probe at a maximum applied load of 3 mN.
CrN(200) CrN(111)
CrN(220)
CrN(311)
A7
A6
Intensity (a.u.)
192
A5
A4
A3
A2
A1 20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
Diffraction Angle(2θ) Fig. 1. X-ray diffraction patterns of seven CreBeN coatings.
3. Results and discussion 3.1. Composition and microstructure of CreBeN thin films The chemical composition of each coating is listed in Table 1. Apparently, the boron content increases monotonically from 3.7 at.% (A1) to 20.6 at.% (A7), whereas the chromium content decreases from 39.8 at.% (A1) to 20.4 at.% (A7). On the other hand, the nitrogen content of each coating is almost the same, which is around 55 at.%. It is noticed that the oxygen content around 1.2 at.% is found for each coating. The argon content around 1.2 at.% of each coating is also detected. The existence of Ar and excess nitrogen contents are attributed to the 150 V bias voltage applied during deposition process. The X-ray diffraction patterns of coatings are shown in Fig. 1. Crystalline CrN phase can be observed for each coating. No XRD signals corresponding to BN and CrB2 phases are found. The peak intensities decrease and broaden as the boron content of coating increases. The calculated average grain size of each film is listed in Table 1. It is obvious that the grain size of CrN in the coating decreases with increasing boron content. A fine nanocrystalline structure with average grain size around 3 nm in diameter is achieved for the A7 coating containing 20.6 at.% B. Gorishnyy et al. [5] reported that the addition of nitrogen into the CreBeN films will induce the nucleation of BN and CrxN phases, and thus decreases the grain size. In this work, the grain refinement effect by boron addition in the CreBeN thin film is confirmed. Since the formation energy of BN, 250.3 kJ/mol, is favorable than that of CrN, 135 kJ/mol, Cr2N, 156 kJ/mol, and CrB,
92 kJ/mol at 473 K [9], the amorphous BN phase should be formed in the CreBeN coatings. Therefore, the FTIR technique was further employed to investigate the existence of amorphous BN phase in the coating. According to the FTIR absorption spectrum of coatings depicted in Fig. 2, the intensities of h-BN characteristic peaks at 780 cm1 increase with increasing boron content, indicating BN matrix changed its phase from amorphous to h-BN. Meanwhile, no characteristic peak of c-BN at 1060 cm1 is found [10,11]. 3.2. Microstructure of CreBeN thin films Based on the AFM analysis data, typical fine granular structure can be observed on the surface of each CreBeN film. The average surface roughness obtained by AFM measurement of each coating is listed in Table 1. In general, the average surface roughness, Ra, of CreBeN film is around 2 nm except the A7 coating containing
Table 1 The chemical composition, average grain size and surface roughness of CreBeN coatings. Sample
A1 A2 A3 A4 A5 A6 A7
Chemical composition (at. %) B
Cr
N
O
Ar
3.7 8.1 11.0 14.2 15.7 20.2 20.6
39.8 34.6 31.2 27.6 26.4 21.5 20.4
54.6 55.2 55.2 55.6 55.2 55.7 56.1
1.2 1.4 1.4 1.3 1.4 1.4 1.2
0.7 0.8 1.0 1.2 1.3 1.2 1.8
Average grain size (nm) 12.9 12.7 8.1 6.4 4.2 3.7 3.5
2.7 1.5 1.3 1.1 0.6 0.4 0.5
Average surface roughness (nm) 1.74 2.65 2.14 2.26 2.10 2.28 0.19 Fig. 2. The FTIR absorption spectrum of CreBeN coatings.
J.-W. Lee et al. / Vacuum 87 (2013) 191e194
20.6 at.% B. It is suggested that the smooth surface effect is caused by the nanocrystalline/amorphous structure of thin film containing higher boron contents. The cross-sectional FE-SEM images of A2 and A6 coatings are shown in Fig. 3. The thickness of each thin film is around 1000e1100 nm. For the A2 coating, a fine columnar feature is revealed, suggesting that the incorporation of boron into coatings could refine the size of columnar CrN grains. On the other hand, a featureless and glassy microstructure is observed in sample A6 (Fig. 3 (b)) due to large amount of B added to the coating to completely restrict the growth of columnar CrN grains. For nanocomposite nitride coatings, the growth of columnar grain can be retard by amorphous matrix consisted of incorporated silicon, boron, as well as carbon [12,13]. It is obvious that the addition of boron element into the CreBeN thin film has great influence on the microstructure evolution, which is altered from columnar to featureless structures. Further details of the microstructure of sample A7 is revealed by the cross-sectional TEM image as shown in Fig. 4, which contains 20.6 at.% boron. The two phases nanocomposite structure consisting of CrN nanograins around 3e4 nm in diameter surrounded by amorphous matrix, the h-BN phase is clearly seen. The distance between CrN grains is 1.5 0.9 nm analyzed by image process. It is believed that the distance between CrN grains is about the same for A6 coating due to very similar B content as compared with A7. } s et al. [7] explored the microstructure of the CreBeN thin Hegedu
193
Fig. 4. TEM micrograph of A7 coating.
films deposited at different nitrogen partial pressure by TEM analysis and reported that a nano crystalline structure with CrN nanograins around 1e3 nm in diameter embedded in the amorphous born nitride matrix was revealed. However, the distance between CrN grains for A1 to A5 CreBeN coatings is still difficult to quantitatively analyze in present work due to limited TEM analysis results. As compared with the grain size analysis and microstructure evolution of CreBeN thin films as shown in Table 1, Figs. 3 and 4, the influence of boron addition on the grain size refinement of CrN in the CreBeN films is obvious. The introduction of boron into the thin film deposition process shows a pronounced suppression of columnar growth and a strong nanocomposite structure forming ability. 3.3. Mechanical properties evaluation of CreBeN thin films
Fig. 3. Cross-sectional SEM morphologies of (a) A2 and (b) A6 coatings.
Fig. 5 illustrates the relationship between average hardness, elastic modulus and boron content for CreBeN thin films. Since the H/E ratio is an important factor to describe the resistance of materials against elastic strain to failure [14], the H/E ratio of each coating is also plotted in Fig. 5. The tendency for the H/E ratio with respect to the B content is almost the same as compared with those of hardness and modulus. For the CreBeN film containing only 3.7 at.% boron (A1), the highest hardness and elastic modulus around 22 and 210 GPa are obtained due to its dense and fine columnar structure. In addition, the solution hardening effect is also attributed to its higher hardness. It is obvious that the hardness, elastic modulus and H/E ratio of CreBeN coatings decreases with increasing boron contents. It is argued that the softening of CreBeN film with increasing boron content is caused by the increasing amount of soft amorphous h-BN phase. The lowest hardness and elastic modulus values, 14.1 1.0 and 186 7 GPa are observed in the A7 coating, which is possibly attributed to its highest B and N concentrations (20.6 at.% B and 56.1 at.% N) to form the largest volume fraction of soft h-BN phase than other coatings. As compared with the mechanical properties between A6 and A7, the higher hardness and elastic modulus values, 15.0 0.8 and 188 5 GPa are observed for the A6 coating due to its lower B and N contents (20.2 at.% B and 55.7 at.% N). The high oxygen
194
J.-W. Lee et al. / Vacuum 87 (2013) 191e194
0.11 H/E
0.10 0.09 0.07 240 220 200
Hardness (GPa)
180 24 22 20 18 16 14
Elastic modulus (GPa)
0.08
sputtering system. Only CrN and BN phases were found for each coating. The amount of amorphous h-BN phase increased with increasing boron content based on the FTIR analysis. It can be concluded that the boron element showed strong influence on the microstructure, grain size and mechanical properties of CreBeN thin films. The hardness, elastic modulus and H/E ratio decreased with increasing boron content, which was possibly attributed to the microstructure of CreBeN coatings containing small nanocrystallites and thick intergranular amorphous BN phase. Acknowledgment The authors gratefully acknowledge the financial support of the National Science Council, Taiwan through contracts No. NSC 99-2221-E-131-043.
2
4
6
8
10
12
14
16
18
20
22
Boron content (at. %) Fig. 5. The hardness, elastic modulus and H/E ratio versus B contents of CreBeN coatings.
contamination around 1.2 at.% for each CreBeN coating is another possible reason for its poor mechanical properties. Patscheider et al. [15] reported that the hardness of nanocomposite coating is influenced by a complex interaction of orientation of nanocrystallites, size of nanograins and the separation of the nanocrystallites by the surrounding amorphous phase. Peak hardness can be achieved when the percentage of amorphous phase is around 20% in the superhard nanocomposite nc-TiN/aSi3N4 thin film system, which consists of an appropriate nanostructure to hinder the growth and propagation of dislocations and microcracks [15]. The nanograins around 5 nm in diameter are surrounded by w0.5 nm thick a-Si3N4 phase in grain boundaries is the main reason for its high hardness [2]. According to the microstructure as shown in Fig. 4, it is suggested that the content of soft amorphous BN phase is rather high for the A7 coating, which makes the separation distance between nanograins, 1.5 0.9 nm, becomes too large. Accordingly, the nanocrystallites are too small and the intergranular amorphous BN phase is too thick, the resultant hardening effect is thus limited in this work. 4. Conclusions In this work, CreBeN thin films containing various boron concentrations ranging from 3.7 to 20.6 at.% were deposited successfully by a bipolar asymmetric pulsed DC reactive magnetron
References [1] Veprek S, Reiprich S, Li S. Superhard nanocrystalline composite materials: the TiN/Si3N4 system. Appl Phys Lett 1995;66:2640e2. [2] Veprek S, Zhang RF, Veprek-Heijman MGJ, Sheng SH, Argon AS. Superhard nanocomposites: origin of hardness enhancement, properties and applications. Surf Coat Technol 2010;204:1898e906. [3] Mitterer C. Borides in thin film technology. J Solid State Chem 1997;133: 279e91. [4] Aouadi SM, Namavar F, Tobin E, Finnegan N, Haasch RT, Nilchiani R, et al. Characterization of CrBN films deposited by ion beam assisted deposition. J Appl Phys 2002;91:1040e5. [5] Gorishnyy TZ, Mihut D, Rohde SL, Aouadi SM. Physical and mechanical properties of reactively sputtered chromium boron nitride thin films. Thin Solid Films 2003;445:96e104. [6] Kiryukhantsev-Korneev PhV, Pierson JF, Petrzhik MI, Alnot M, Levashov EA, Shtansky DV. Effect of nitrogen partial pressure on the structure, physical and mechanical properties of CrB2 and CreBeN films. Thin Solid Films 2009;517: 2675e80. } s É, Kovács I, Pécz B, Tóth L, Budna KP, Mitterer C. Transmission [7] Hegedu electron microscopy of nanocomposite CeBeN thin films. Vacuum 2008;82: 209e13. } s É, Kovács I, Tóth L, et al. Effect [8] Budna KP, Mayrhofer PH, Neidhardt J, Hegedu of nitrogen-incorporation on structure, properties and performance of magnetron sputtered CrB2. Surf Coat Technol 2008;202:3088e93. [9] Kubaschewski O, Alcock CB, Spencer PJ. Materials Thermochemistry. Pergamon Press; 1993. [10] Budak E, Bozkurt C. Synthesis of hexagonal boron nitride with the presence of representative metals. Physica B 2010;405:4702e5. [11] Xin H, Shi X, Lin C, Xu W, Zhong L, Zou S. Phase evolution in boron nitride thin films prepared by a dc-gas discharge assisted pulsed laser deposition. Thin Solid Films 1997;293:17e21. [12] Chekour L, Nouveau C, Chala A, Labidi C, Rouag N, Djouadi MA. Growth mechanism for chromium nitride films deposited by magnetron and triode sputtering methods. Surf Coat Technol 2005;200:241e4. [13] Zhang G, Wang L, Yan P, Xue Q. Structure and mechanical properties of CreBeN films deposited by reactive magnetron sputtering. J Alloys Compd 2009;486:227e32. [14] Leyland A, Matthews A. On the significance of the H/E ratio in wear control: a nanocomposite coating approach to optimised tribological behavior. Wear 2000;246:1e11. [15] Patscheider J, Zehnder T, Diserens M. Structureeperformance relations in nanocomposite coatings. Surf Coat Technol 2001;146e147:201e8.
Contents lists available at SciVerse ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
The influence of boron contents on the microstructure and mechanical properties of CreBeN thin films Jyh-Wei Lee a, b, *, Cheng Chih-Hong c, Hsien-Wei Chen d, Li-Wei Ho a, Jenq-Gong Duh d, Yu-Chen Chan d a
Dept. of Materials Engineering, Ming Chi University of Technology, Taiwan, ROC Center for Thin Films Technologies and Applications, Ming Chi University of Technology, Taiwan, ROC c Dept. of Mechanical Engineering, Tungnan University, Taiwan, ROC d Dept. of Materials Science and Engineering, National Tsing Hua University, Taiwan, ROC b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 July 2011 Received in revised form 22 October 2011 Accepted 29 February 2012
Seven CreBeN films with various boron contents ranging from 3.7 to 20.6 at.% were deposited by a bipolar asymmetric pulsed DC reactive magnetron sputtering system. The structures of thin films were characterized by X-ray diffraction (XRD). The surface and cross-sectional morphologies of thin films were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The surface roughness of thin films was explored by atomic force microscopy (AFM). A nanoindenter was used to evaluate the hardness and properties of thin films, respectively. The phases of each CreBeN coating were mainly CrN and amorphous BN regardless the boron content. It was found that the grain size decreased with increasing boron content. The hardness and elastic modulus of CreBeN thin films decreased with increasing boron concentration, which were mainly attributed to the nanostructure configuration of small CrN nanograins and large intergranular amorphous BN phase. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: CreBeN nanocomposite thin film Amorphous BN Nanoindentation Pulsed DC reactive magnetron sputtering
1. Introduction Surface engineering is an enabling technology to provide a modified surface on the interest with required properties and prolonged surface life. For the cutting tool and molding applications, nanocomposite thin films have been studied by researchers and industries due to its excellent mechanical properties [1,2]. Since boron is an effective element to produce a nanocomposite structure, the B contained chromium nitride thin films have been explored extensively in literature [3e8]. It is reported that the nitrogen flow rate ratio during sputtering has strong influence on the phase, microstructure and mechanical properties of the CreBeN coating [3e8]. However, the effect of boron content on the phase, microstructure and related mechanical properties of CreBeN coatings is seldom studied. In this work, a co-sputtering process using a CrB2 compound target and a pure Cr target was executed to deposit a series of CreBeN thin films containing different B concentrations by a pulsed DC magnetron sputtering. Through this process, the influence of boron content on the phase,
* Corresponding author. Dept. of Materials Engineering, Ming Chi University of Technology, No.84, Gongzhuan Rd., Taishan Dist., New Taipei City 24301, Taiwan, ROC. Tel.: þ886 2 29089899; fax: þ886 2 29084091. E-mail address: jeffl[email protected] (J.-W. Lee). 0042-207X/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2012.02.049
microstructure and mechanical properties of CreBeN coating were evaluated. 2. Experimental procedure The CreBeN thin films were deposited on a p-type (100) silicon wafer substrate by a bipolar asymmetry pulsed DC reactive magnetron sputtering system. A pure Cr (99.99 at.%) and a CrB2 compound target (33%Cre67%B, in at.%) were placed on the same side of the chamber. The diameter and thickness of each target were 76.2 mm and 6.0 mm, respectively. The substrate was placed between the Cr and CrB2 targets to deposit CreBeN thin films with various B contents. The different B contents of CreBeN thin films were obtained due to the difference of substrate position between the Cr and CrB2 targets. According to the substrate positions, thin films deposited on Si substrate are denoted as A1 to A7, in which the A1 was close to the Cr target and A7 was close to CrB2 target. A pulse frequency of 2 kHz was used during sputtering. A pulsing bias of 150 V and a pulse unit with 2 kHz frequency was applied to the substrates. No attempt was made to coordinate the pulse cycles. The substrate-to-target distance was kept at 100 mm. The base pressure was 1.6 103 Pa before deposition. Substrate surfaces were cleaned by Ar ion etching for 10 min at 1.2 Pa under 500 V substrate bias prior to the deposition process. The working pressure was 0.47 Pa with the same ratio of Ar and N2 flow rates.
J.-W. Lee et al. / Vacuum 87 (2013) 191e194
The surface topography and roughness of thin films were investigated by an atomic force microscopy (AFM, DI 3100, Bruker, USA). The cross-sectional morphologies of thin films were examined with a field emission scanning electron microscope (FE-SEM, JSM-6701, JEOL, Japan). Selected thin films were further analyzed by TEM to reveal the microstructures in details. Chemical compositions of thin films were analyzed with a field emission electron probe microanalyzer (FE-EPMA, JXA-8500F, JEOL, Japan). The phases of thin films were explored by a glancing angle X-ray diffractometer (GAXRD, Panalytical X’Pert Pro, Holland) with an incidence angle of 2 . Cu Ka radiation generated at 30 kV and 40 mA from a Cu target was used. The lateral grain size of each film was estimated by the Scherrer formula using the integral width of the Bragg reflection [9]. The Fourier transform infrared spectroscopy (FTIR, Spectrum One, PerkineElmer, USA) measurement was performed to explore the type of chemical bonds of the BN structure of each CreBeN thin film. The FTIR spectrum background of each coating was subtracted from the spectrum of uncoated Si wafer. The hardness and elastic modulus of thin films were investigated by means of a nanoindenter (TI-900, TriboIndenter, Hysitron, USA) using a Berkovich 142.3 diamond probe at a maximum applied load of 3 mN.
CrN(200) CrN(111)
CrN(220)
CrN(311)
A7
A6
Intensity (a.u.)
192
A5
A4
A3
A2
A1 20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
Diffraction Angle(2θ) Fig. 1. X-ray diffraction patterns of seven CreBeN coatings.
3. Results and discussion 3.1. Composition and microstructure of CreBeN thin films The chemical composition of each coating is listed in Table 1. Apparently, the boron content increases monotonically from 3.7 at.% (A1) to 20.6 at.% (A7), whereas the chromium content decreases from 39.8 at.% (A1) to 20.4 at.% (A7). On the other hand, the nitrogen content of each coating is almost the same, which is around 55 at.%. It is noticed that the oxygen content around 1.2 at.% is found for each coating. The argon content around 1.2 at.% of each coating is also detected. The existence of Ar and excess nitrogen contents are attributed to the 150 V bias voltage applied during deposition process. The X-ray diffraction patterns of coatings are shown in Fig. 1. Crystalline CrN phase can be observed for each coating. No XRD signals corresponding to BN and CrB2 phases are found. The peak intensities decrease and broaden as the boron content of coating increases. The calculated average grain size of each film is listed in Table 1. It is obvious that the grain size of CrN in the coating decreases with increasing boron content. A fine nanocrystalline structure with average grain size around 3 nm in diameter is achieved for the A7 coating containing 20.6 at.% B. Gorishnyy et al. [5] reported that the addition of nitrogen into the CreBeN films will induce the nucleation of BN and CrxN phases, and thus decreases the grain size. In this work, the grain refinement effect by boron addition in the CreBeN thin film is confirmed. Since the formation energy of BN, 250.3 kJ/mol, is favorable than that of CrN, 135 kJ/mol, Cr2N, 156 kJ/mol, and CrB,
92 kJ/mol at 473 K [9], the amorphous BN phase should be formed in the CreBeN coatings. Therefore, the FTIR technique was further employed to investigate the existence of amorphous BN phase in the coating. According to the FTIR absorption spectrum of coatings depicted in Fig. 2, the intensities of h-BN characteristic peaks at 780 cm1 increase with increasing boron content, indicating BN matrix changed its phase from amorphous to h-BN. Meanwhile, no characteristic peak of c-BN at 1060 cm1 is found [10,11]. 3.2. Microstructure of CreBeN thin films Based on the AFM analysis data, typical fine granular structure can be observed on the surface of each CreBeN film. The average surface roughness obtained by AFM measurement of each coating is listed in Table 1. In general, the average surface roughness, Ra, of CreBeN film is around 2 nm except the A7 coating containing
Table 1 The chemical composition, average grain size and surface roughness of CreBeN coatings. Sample
A1 A2 A3 A4 A5 A6 A7
Chemical composition (at. %) B
Cr
N
O
Ar
3.7 8.1 11.0 14.2 15.7 20.2 20.6
39.8 34.6 31.2 27.6 26.4 21.5 20.4
54.6 55.2 55.2 55.6 55.2 55.7 56.1
1.2 1.4 1.4 1.3 1.4 1.4 1.2
0.7 0.8 1.0 1.2 1.3 1.2 1.8
Average grain size (nm) 12.9 12.7 8.1 6.4 4.2 3.7 3.5
2.7 1.5 1.3 1.1 0.6 0.4 0.5
Average surface roughness (nm) 1.74 2.65 2.14 2.26 2.10 2.28 0.19 Fig. 2. The FTIR absorption spectrum of CreBeN coatings.
J.-W. Lee et al. / Vacuum 87 (2013) 191e194
20.6 at.% B. It is suggested that the smooth surface effect is caused by the nanocrystalline/amorphous structure of thin film containing higher boron contents. The cross-sectional FE-SEM images of A2 and A6 coatings are shown in Fig. 3. The thickness of each thin film is around 1000e1100 nm. For the A2 coating, a fine columnar feature is revealed, suggesting that the incorporation of boron into coatings could refine the size of columnar CrN grains. On the other hand, a featureless and glassy microstructure is observed in sample A6 (Fig. 3 (b)) due to large amount of B added to the coating to completely restrict the growth of columnar CrN grains. For nanocomposite nitride coatings, the growth of columnar grain can be retard by amorphous matrix consisted of incorporated silicon, boron, as well as carbon [12,13]. It is obvious that the addition of boron element into the CreBeN thin film has great influence on the microstructure evolution, which is altered from columnar to featureless structures. Further details of the microstructure of sample A7 is revealed by the cross-sectional TEM image as shown in Fig. 4, which contains 20.6 at.% boron. The two phases nanocomposite structure consisting of CrN nanograins around 3e4 nm in diameter surrounded by amorphous matrix, the h-BN phase is clearly seen. The distance between CrN grains is 1.5 0.9 nm analyzed by image process. It is believed that the distance between CrN grains is about the same for A6 coating due to very similar B content as compared with A7. } s et al. [7] explored the microstructure of the CreBeN thin Hegedu
193
Fig. 4. TEM micrograph of A7 coating.
films deposited at different nitrogen partial pressure by TEM analysis and reported that a nano crystalline structure with CrN nanograins around 1e3 nm in diameter embedded in the amorphous born nitride matrix was revealed. However, the distance between CrN grains for A1 to A5 CreBeN coatings is still difficult to quantitatively analyze in present work due to limited TEM analysis results. As compared with the grain size analysis and microstructure evolution of CreBeN thin films as shown in Table 1, Figs. 3 and 4, the influence of boron addition on the grain size refinement of CrN in the CreBeN films is obvious. The introduction of boron into the thin film deposition process shows a pronounced suppression of columnar growth and a strong nanocomposite structure forming ability. 3.3. Mechanical properties evaluation of CreBeN thin films
Fig. 3. Cross-sectional SEM morphologies of (a) A2 and (b) A6 coatings.
Fig. 5 illustrates the relationship between average hardness, elastic modulus and boron content for CreBeN thin films. Since the H/E ratio is an important factor to describe the resistance of materials against elastic strain to failure [14], the H/E ratio of each coating is also plotted in Fig. 5. The tendency for the H/E ratio with respect to the B content is almost the same as compared with those of hardness and modulus. For the CreBeN film containing only 3.7 at.% boron (A1), the highest hardness and elastic modulus around 22 and 210 GPa are obtained due to its dense and fine columnar structure. In addition, the solution hardening effect is also attributed to its higher hardness. It is obvious that the hardness, elastic modulus and H/E ratio of CreBeN coatings decreases with increasing boron contents. It is argued that the softening of CreBeN film with increasing boron content is caused by the increasing amount of soft amorphous h-BN phase. The lowest hardness and elastic modulus values, 14.1 1.0 and 186 7 GPa are observed in the A7 coating, which is possibly attributed to its highest B and N concentrations (20.6 at.% B and 56.1 at.% N) to form the largest volume fraction of soft h-BN phase than other coatings. As compared with the mechanical properties between A6 and A7, the higher hardness and elastic modulus values, 15.0 0.8 and 188 5 GPa are observed for the A6 coating due to its lower B and N contents (20.2 at.% B and 55.7 at.% N). The high oxygen
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0.11 H/E
0.10 0.09 0.07 240 220 200
Hardness (GPa)
180 24 22 20 18 16 14
Elastic modulus (GPa)
0.08
sputtering system. Only CrN and BN phases were found for each coating. The amount of amorphous h-BN phase increased with increasing boron content based on the FTIR analysis. It can be concluded that the boron element showed strong influence on the microstructure, grain size and mechanical properties of CreBeN thin films. The hardness, elastic modulus and H/E ratio decreased with increasing boron content, which was possibly attributed to the microstructure of CreBeN coatings containing small nanocrystallites and thick intergranular amorphous BN phase. Acknowledgment The authors gratefully acknowledge the financial support of the National Science Council, Taiwan through contracts No. NSC 99-2221-E-131-043.
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Boron content (at. %) Fig. 5. The hardness, elastic modulus and H/E ratio versus B contents of CreBeN coatings.
contamination around 1.2 at.% for each CreBeN coating is another possible reason for its poor mechanical properties. Patscheider et al. [15] reported that the hardness of nanocomposite coating is influenced by a complex interaction of orientation of nanocrystallites, size of nanograins and the separation of the nanocrystallites by the surrounding amorphous phase. Peak hardness can be achieved when the percentage of amorphous phase is around 20% in the superhard nanocomposite nc-TiN/aSi3N4 thin film system, which consists of an appropriate nanostructure to hinder the growth and propagation of dislocations and microcracks [15]. The nanograins around 5 nm in diameter are surrounded by w0.5 nm thick a-Si3N4 phase in grain boundaries is the main reason for its high hardness [2]. According to the microstructure as shown in Fig. 4, it is suggested that the content of soft amorphous BN phase is rather high for the A7 coating, which makes the separation distance between nanograins, 1.5 0.9 nm, becomes too large. Accordingly, the nanocrystallites are too small and the intergranular amorphous BN phase is too thick, the resultant hardening effect is thus limited in this work. 4. Conclusions In this work, CreBeN thin films containing various boron concentrations ranging from 3.7 to 20.6 at.% were deposited successfully by a bipolar asymmetric pulsed DC reactive magnetron
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