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The microstructure and mechanical properties of nitrogen and boron contained ZrCuAlNi thin film metallic glass composites
Surface & Coatings Technology 237 (2013) 276–283
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
The microstructure and mechanical properties of nitrogen and boron contained ZrCuAlNi thin film metallic glass composites Tzu-Pin Hsiao a, Yung-Chin Yang a,⁎, Jyh-Wei Lee b,c,⁎⁎, Chia-Lin Li d, Jinn P. Chu e a
Department of Materials and Mineral Resources Engineering, Institute of Materials Science and Engineering, National Taipei University of Technology, Taipei, Taiwan Department of Materials Engineering, Ming Chi University of Technology, Taipei, Taiwan Center for Thin Film Technologies and Applications, Ming Chi University of Technology, Taipei, Taiwan d Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei, Taiwan e Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan b c
a r t i c l e
i n f o
Available online 18 September 2013 Keywords: Zr-based thin film metallic glass composite Adhesion Boron Nitrogen ZrN
a b s t r a c t In this work, nitrogen contained and B doped ZrCuAlNi thin film metallic glass composites (TFMGCs) were grown by magnetron co-sputtering process. Boron element was doped to evaluate its influence on the microstructure and mechanical properties of nitrogen contained TFMGCs. The supercooled liquid region of TFMG was determined by the differential scanning calorimetry (DSC) analysis. The microstructure was analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The phase structure was confirmed by using X-ray diffraction (XRD). The mechanical properties were measured by nanoindentation. The scratch tester and tests were employed to evaluate the adhesion and tribological properties. The surface roughness was determined by atomic force microscopy (AFM). It was concluded that the surface roughness, microstructure, glassforming ability and mechanical behaviors of TFMGCs were strongly influenced by the nitrogen and boron doping. The ZrN nanocrystallites were embedded in the amorphous matrix. The hardness, H/E ratio, and indentation toughness increased with increasing boron concentration. Excellent adhesion quality was also confirmed for all coatings. The Zr-based thin film metallic glass composite containing 6.4 B–32.6 N (in at.%) with optima hardness, 16.2 GPa, adequate indentation toughness and excellent adhesion quality was achieved in this work. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The amorphous bulk metallic glass (BMG) materials have attracted considerable scientific and commercial attention since 1960s due to their unique characteristics, such as high elastic energy, high hardness, good wear and corrosion resistance [1,2]. However, due to the rapid propagation of shear bands, the brittleness nature and catastrophic failure of BMGs restricted their application at ambient [1,2]. Recently, the size-dependent properties of BMGs were reported by Kumar et al. [3]. As the effective thickness (smallest dimension) of metallic glass material was less than 100 nm, the deformation mode changed from shear localization to homogeneous deformation [3]. Since the fast quenching rate can be achieved by the physical vapor deposition process, the metallic glass in thin film form, the so-called thin film metallic glass (TFMG) is considered as candidate to solve the brittleness problem of BMGs at ambient temperature. The Zr-based metallic glass thin film has been widely studied because of their good mechanical, tribological, fatigue property,
⁎ Corresponding author. ⁎⁎ Correspondence to: J.-W. Lee, Department of Materials Engineering, Ming Chi University of Technology, Taipei, Taiwan. E-mail addresses: [email protected] (Y.-C. Yang), jeffl[email protected] (J.-W. Lee). 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.09.015
corrosion resistance, and excellent adhesion [4–8]. Meanwhile, the addition of Ta element can improve the mechanical properties of Zr-based TFMG by reducing the density of free volume and retarding the shear band formation [9]. The hardness and elastic modulus increased up to the maximum values of 7.5 GPa and 141 GPa when Ta was introduced into the Ta–Zr–Cu–Al–Ag TFMG [9]. The addition of metalloid elements, especially B, in BMG materials has been studied in literature [10–17]. It is reported that B element had a positive effect on thermal stability of Zr base glassy alloys, while keeping the same glass-forming ability (GFA) as the base BMG [10–14]. The B element to decrease the fragility and improve the glass forming ability of Ni-rich BMG was also explored [15]. The glass formation ability of Be–B thin film can be improved as the B concentration was higher than 11 at.% [16]. On the other hand, the nitrogen was also added as the metalloid element in the BMG and TFMG system. For example, Hasegawa and coworkers [18] synthesized the nitrogen doped ZrCuNiAl BMG ribbon using Zr-based glassy alloy and AlN powder as starting materials. The Zr-based BMG ribbon had higher hardness due to its dispersed crystalline ZrN microparticles embedded in the glassy matrix. Some N2 pores and nitrogen atoms dissolved as interstitial atom in the glassy structure were also observed. Liebermann [19] discovered that the doping of 0.5 at.% nitrogen in the Fe–Cr-based BMG ribbon can increase its thermal stability as much as 31 K due to the nitrogen induced changes in the precipitation
T.-P. Hsiao et al. / Surface & Coatings Technology 237 (2013) 276–283
ZrN (111) ZrN (200) ZrN (220) ZrN (311)
Table 1 Sputtering parameter for Zr-based TFMGCs. Sample #
ZNB0
ZNB1
ZNB2
ZNB3
ZrCuAlNi target power (W) Boron target power (W) Background pressure (Pa) Deposition pressure (Pa) Input gas (sccm) Ar N2 Substrate bias voltage (V) Deposition time (h) Deposition temperature (°C) Surface roughness, Rms (nm)
200 – 8.0 × 10−6 0.4
200 50 3.2 × 10−5
200 125 3.9 × 10−5
200 200 4.0 × 10−5
1.92
0.20
ZNB3 Intensity (arb. units)
40 2 −100 3 25 2.16
277
ZNB2
ZNB1
0.40
ZNB0 20
of crystalline phases. In previous work [20], the thermal and mechanical properties of the brand new nitrogen doped Zr–Cu–Ni–Al–Ag–Ta TFMG were investigated. We discovered that both the hardness and the thermal stability were increased due to the formation of large amount of bigger size nitrogen-centered clusters. The energy barrier for flow stress was thus increased by the nitrogen doping. In addition, the plasma emission monitoring (PEM) technique was employed to fabricate the Zr–Ni–Al–Si TFMG in previous study [21]. The Zr-based thin film containing 13.8 at.% N having a microstructure consisting of ZrN nanograins embedded in the amorphous matrix provided a combination of high hardness, good wear resistance and excellent adhesion properties. Although the addition of B and N metalloid elements showed positive effects on the thermal/ mechanical properties of BMG and TFMG materials, the effect of adding both B and N elements on the microstructure and thermal and mechanical properties of Zr–Cu–Al–Ni thin film metallic glass has not yet been explored. In this work, the boron and nitrogen doped Zr–Cu–Al–Ni TFMGs were grown by co-sputtering using commercial ZrCuNiAl alloy target and pure B target. The microstructure, mechanical properties and strengthening mechanism of nitrogen and boron contained Zr–Cu–Al– Ni thin film metallic glasses were studied.
2. Experimental procedure In this study, a magnetron sputtering system was used to deposit Zr– Cu–Al–Ni–B thin film metallic glasses with the addition of nitrogen on three kinds of substrates, including p-type (100)Si wafer, glass plates, and polished and hardened AISI 420 stainless steel disks. The commercial Zr–Cu–Al–Ni alloy target (55% Zr–30% Cu–10% Al–5% Ni, in at.%, provided by Solar Applied Materials Technology Corporation, Taiwan) and 99.9 wt.% boron target were connected to a pulsed direct current (DC) and radio frequency (RF) power supplies, respectively. Both targets were 6 in. in diameter and located on the top of the substrate holder. The commercial Zr–Cu–Al–Ni alloy cathode was operated in poisoned mode during sputtering process using an Ar:N2 flow ratio of 40:2. The distances between centers of two targets and target-tosubstrate were 26 and 15 cm, respectively. The substrates were rotated in a speed of 10 rpm to keep the uniformity of chemical composition. The operating parameters for coating are presented in Table 1.
30
40
50
60
70
80
Fig. 1. The X-ray diffraction patterns for the Zr-based TFMGCs.
The cross-sectional morphologies of thin films were examined with a field emission scanning electron microscope (FE-SEM, JSM-6701, JEOL, Japan) and by transmission electron microscopy (TEM, JEOL, JSM-2100, Japan). The chemical composition of thin films was 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 (GA-XRD, PANalytical, X'pert, Holland) with an incidence angle of 1°. Cu Kα radiation generated at 30 kV and 40 mA from a Cu target was used. The thermal behavior of the film was determined by using a differential scanning calorimeter (DSC, NETZSCH DSC 404F3, Germany) in Ar at a heating rate of 40 K/min. The temperature resolution of DSC is 0.1 °C. A calibration of the DSC was held by heating the empty crucible to deduct the background signal. The DSC film sample was delaminated from the glass without the aid of any chemical solutions. The nanoindentation hardness, H, and elastic modulus, E, of the thin films were investigated by means of a nanoindenter (TI-900, TriboIndenter, Hysitron, USA) by using a Berkovich 142.3° diamond probe at different loads to achieve a fixed indentation depth of 80 nm. The loading rates were 3.3 to 6.0 μN/s. Eight indentation tests were made for each coating. The hardness and elastic modulus of each indent were determined on the basis of the Oliver and Pharr method [22]. The elastic modulus, E, was expressed as follows: 1 1−v2 1−v2i þ ¼ Er E Ei
Table 2 Chemical composition of the Zr-based TFMGCs (in at.%). Element
Zr Cu Al Ni B N O
Sample # ZNB0
ZNB1
ZNB2
ZNB3
29.1 18.3 7.1 4.0 0 32.3 9.3
35.8 16.0 7.0 4.7 0.4 35.1 1.2
33.5 17.1 5.9 4.3 6.4 32.6 0.3
33.4 17.6 5.4 4.1 12.3 26.8 0.5
90
2 Theta (degree)
Fig. 2. The DSC thermograms for the Zr-based TFMGCs.
ð1Þ
278
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where Er and ν are the reduced elastic modulus and Poisson's ratio, respectively, for the thin films under test, and Ei (1140 GPa) and νi (0.07) are the corresponding parameters of the diamond indenter. The Poisson's ratio ν for Zr55Cu30Ni5Al10 BMG was 0.375 [23]. The Vickers microindentation tester was adopted to evaluate the indentation toughness, Kc, according to the following eqn. [24]: rffiffiffiffi P E KC ¼ δ 3=2 H c
ð2Þ
where P is the applied indentation load and δ is an indenter geometry constant, equal to 0.016, for a Vickers diamond pyramid indenter. E and H are the elastic modulus and the hardness of thin film derived from nanoindentation tests. And c is the average of the radial crack lengths occurring on the thin films, which were measured using a SEM. The criterion for a valid crack is taken as c ≥2a, where a is half of the diagonal length of the indent. The adhesion strength quality of the thin films was evaluated by Daimler Benz Rockwell-C (HRC-DB) tester [25]. A scratch test (Scratch Tester, J & L Tech. Co., Korea), with up to the maximum load of 50 N was adopted to explore the adhesion properties of the thin films. 3. Results and discussion 3.1. Composition and phase characterization The chemical compositions of four coatings are listed in Table 2. The Zr content increased from 29.1 at.% to 35.8 at.% with oxygen concentration decreased from 9.3 at.% to 0.5 at.%. Apparently, the oxygen contents decreased significantly with the addition of boron. The exact reason for the decreasing tendency of oxygen content by boron addition is still unknown. Further investigation on this phenomenon is needed. It should be noticed that since the oxygen impurity level of the commercial target is less than 0.01 at.%, the oxygen content measured in the film is due to the residual atmosphere inside the chamber. The nitrogen decreased monotonically from 32.3 at.% to 26.8 at.% when B increased from 0 at.% to 12.3 at.%. On the other hand, The Cu, Al, and Ni concentrations were not influenced significantly by the boron doping. Fig. 1 shows the X-ray diffraction patterns of thin films in this work. The obvious crystallinity diffraction peaks corresponding to the ZrN phase are discovered for the ZNB0 and ZNB1, respectively. Interestingly, the lower intensity broad peaks are observed for ZNB2 and ZNB3 when more boron was contained in thin films. It is noticed that no detectable AlN or Si3N4 phase was found for each coating, implying that the reactive sputtering deposition reaction occurred mainly between nitrogen and Zr element due to its high concentration. From the viewpoint of free energy at 300 K, the Zr–N is −80.3 kcal/mol and B–N is −53.7 kcal/mol, respectively. The enthalpy for B–N, Al–N, Ni–N and Zr–N was −28, −92, −69,
and −233 kJ/mol, respectively [26]. Since the formation energy favors Zr–N phase, the ZrN phase can be produced easily. According to the XRD analysis, the average grain size values calculated by the Scherrer formula [27] were 7.6, 5.7, 4.4 and 3.4 nm, respectively for the ZNB0, ZNB1, ZNB2 and ZNB3 coatings. According to the XRD analysis, it indicates that Zr element reacted with nitrogen atoms easily during the reactive sputtering process. The grain growth of ZrN crystallites were restricted by the amorphous matrix and the increasing amount of B element. The DSC results for four coatings are illustrated in Fig. 2. The Tg and Tx were not found for all coatings implying the absence of the glass transition and crystallization phenomena with boron and nitrogen addition in the work. According to the AFM surface morphologies and measured surface roughness, Rms, as listed in Table 1, of TFMGs, the surface roughness decreased from 2.16 nm to 0.2 nm as the B content increased from 0 (ZNB0) to 6.4 at.% (ZNB2), although some surface contamination particles can be observed on the surface of ZNB2. Since more nano-sized particles are found on the surface of ZNB3, the surface roughness thus increases up to 0.4 nm. Apparently, the B addition can decrease the surface roughness effectively. The fractured cross-sectional morphologies of the TFMGs deposited on Si wafers are shown in Fig. 3. The typical columnar structure is observed for the nitrogen doped ZNB0. The dense and featureless microstructures are observed for ZNB1, ZNB2, and ZNB3 as B doping. Meanwhile, the thickness of the coating increased with the addition of the boron. It is important to point out that the cross sectional morphology of nitrogen doped TFMG was altered from columnar to featureless with only 0.4 at.% boron addition. The higher resolution TEM bright-field image and the selected area electron diffraction (SAED) pattern for ZNB2 are shown in Fig. 4(a) and (b), respectively. The ringlike diffraction patterns indicate the existence of ZrN nanograins. The diffusing ring is also implying the existence of amorphous TFMG matrix in Fig. 4(b). The TEM bright field image and dark-field image for ZNB2(111), (200) and (220) are shown in Fig. 4(c)–(f), respectively. It is obvious that a lot of ZrN nanocrystallites are evenly distributed in the TFMG matrix, which can be named as the thin film metallic glass composite (TFMGC). Very similar findings were reported in previous study [21]. In literature [10–15], the positive effect on the glass-forming ability was found for the boron doped Zr-based BMG. However, different tendency is observed according to the DSC data in this work. It is argued that the larger amount of ZrN nanocrystallites embedded in the μm thick glassy thin film matrix is the possible reason for the disappearance of Tg and Tx temperature in Fig. 2.
3.2. Mechanical property evaluation Fig. 5 depicts the relationships among the hardness, elastic modulus, H/E ratio, indentation toughness and boron target power. The hardness
Fig. 3. The cross-sectional FE-SEM images of the Zr-based TFMGCs.
T.-P. Hsiao et al. / Surface & Coatings Technology 237 (2013) 276–283
and elastic modulus are 9.3 GPa and 192 GPa for the ZNB0 TFMGC. These values increase as more boron is doped in the system. The hardness and elastic modulus increase up to the maximum values of 16.2 GPa and 216 GPa, respectively, for the 6.4 at.% B contained ZNB2 TFMGC. The H/E
279
ratio, the resistance of materials against elastic strain to failure, displays an upward trend with increasing boron doping amount. The maximum H/E ratio reaches up to 0.08 for the ZNB2 TFMGC. It is reported that denser atomic packing and stronger atomic interaction can be achieved by the
Fig. 4. The TEM analysis of ZNB2 (a) bright field image at high magnification and (b) the selected area electron diffraction pattern, (c) bright field image and dark-field images for ZrN (d) (111), (e) (200) and (f) (220).
T.-P. Hsiao et al. / Surface & Coatings Technology 237 (2013) 276–283
1 0.8
H/E
0.1
0.6
0.08
Indentation toughness (MPa-m1/2)
280
0.06
240 220
Hardness (GPa)
200 20
180
Elastic modulus (GPa)
0.04
16 12 8 0
50
100
150
200
Boron target power (W) Fig. 5. The relationships among the hardness, elastic modulus, H/E ratio, indentation toughness and boron target power.
addition of much smaller boron atoms in metallic glass materials [15]. Thus, the decreasing free volume and higher hardness caused by the denser atomic packing is clearly observed in this work. The increasing tendency for the hardness and indentation toughness versus the B target power is clearly observed in Fig. 5. The surface morphologies of indentation craters after the indentation toughness measurement for the ZNB0 and ZNB3 TFMGCs are illustrated in Fig. 6. The radial cracks can be recognized clearly. The value of indentation toughness is 0.75 MPa(m)−0.5 for the ZNB0 as shown in Fig. 5. The indentation toughness increased with increasing boron content and reached to the maximum value of 1.00 MPa(m)−0.5 for the ZNB2. Therefore, the indentation toughness is effectively enhanced by the addition of boron in TFMGCs. The Daimler–Benz Rockwell-C (HRC-DB) adhesion test [24] is an easy and commercially adopted method to evaluate the adhesion quality of coatings. The load of 1471 N was applied to cause a coating damage adjacent to the boundary of the indentation. The damage to the coatings was compared with the adhesion strength quality maps HF1–HF6. In general, the adhesion strength quality HF1 to HF4 defines a sufficient adhesion, whereas HF5 and HF6 represent insufficient adhesion obtained from
the HRC-DB test. The surface morphologies and enlarged image of indentation craters for each TFMGC is shown in Fig. 7(a)–(d). No radial cracking or delamination is observed on the indentation crater of ZNB0, ZNB1 and ZNB3 (Fig. 7(a),(b),(d)) indicating its excellent adhesion quality, which is classified as HF1. In Fig. 7(c), some radial cracks are found on indentation crater for ZNB2, but it is still classified as HF1. These results indicated its excellent adhesion. In this work, excellent adhesion quality, HF1, is obtained for each TFMGC with the addition of nitrogen or boron/nitrogen elements. The backscattered electron images (BEI) of scratch track morphologies for four coatings are shown in Fig. 8(a)–(d), respectively, to distinguish the adhesion failures of the coating. It can be observed that a lot of cracks occurred on the scratch track, but no delamination of the coating inside or adjacent to the scratch track was observed for all thin films even in the end of scratch track as shown in Fig. 9(a)–(d). Meanwhile, it is clear that the “ductile tensile cracking” [28] is found for the ZNB0 (Fig. 8(a)) and the “conformal cracking” [28] is discovered for the ZNB1, ZNB2, and ZNB3 as shown in Fig. 8(b)–(d), respectively. Interestingly, the “ductile tensile cracking” changed to “conformal cracking” as the B doped into the ZNB1, ZNB2, and ZNB3. Although the “ductile tensile cracking” is a characteristic of ductility material and the “conformal cracking” is correlated to the brittleness material, the excellent adhesion property is achieved for all coatings in this work. Caron et al. [29] reported that the internal structure of metallic glass plays a key role on its mechanical properties. This structure change can be explained by the free volume [30] and the configuration potential energy [31]. Therefore, the increasing mismatch in atomic size by the much smaller boron atoms and the stronger atomic interaction can produce denser atomic packing, higher viscosity of liquid, less free volume and the better mechanical behaviors in the Zr-based ZrN and B doped TFMGCs. It can be concluded that, according to the hardness, toughness, HRC-DB and scratch test results, the B doping into the nitrogen contained TFMGC can increase its hardness, toughness and adhesion properties effectively. 4. Conclusion In this work, the ZrN dispersed thin film metallic glass composite (TFMGC) materials with different boron contents were fabricated by co-sputtering process using ZrCuAlNi alloy target and B target. The TFMGCs with B and N contents ranging from 0 at.% to 12.3 at.% and 26.8 at.% to 35.1 at.% were achieved. The ZrN nanocrystallites were embedded in the amorphous matrix. The decreasing surface roughness was observed for the TFMGC with increasing B content. The cross sectional morphology changed from columnar to fine and featureless as B doped into the TFMGCs. On the other hand, the hardness, H/E ratio, and indentation toughness increased with increasing boron doping concentration. Excellent adhesion quality was also confirmed through the scratch and HRC-DB tests. The optima hardness reaching 16.2 GPa,
Fig. 6. The surface indentation crater morphologies of (a) ZNB0 and (b) ZNB3 after the indentation toughness measurement.
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281
Fig. 7. The secondary electron (SE) and backscattered electron (BSE) images and inserted higher magnification images of indentation craters for (a) ZNB0, (b) ZNB1, (c) ZNB2 and (d) ZNB3.
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adequate indentation toughness and excellent adhesion quality were achieved for the Zr-based thin film metallic glass composite containing 33.5 Zr–17.1 Cu–5.9Al–4.3 Ni–6.4 B–32.6 N (in at.%). Acknowledgment The authors gratefully acknowledge the financial support of the National Science Council, Taiwan through contract no NSC 101-2221-E131-023-MY3 and the partial financial support from Solar Applied Materials Technology Corporation, Taiwan. References [1] [2] [3] [4] [5] [6] [7]
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Fig. 8. The backscattered electron images of the scratch tracks for all coatings.
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Fig. 9. The secondary electron and backscattered electron images of the end of scratch tracks for (a) ZNB0, (b) ZNB1, (c) ZNB2 and (d) ZNB3.
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The microstructure and mechanical properties of nitrogen and boron contained ZrCuAlNi thin film metallic glass composites Tzu-Pin Hsiao a, Yung-Chin Yang a,⁎, Jyh-Wei Lee b,c,⁎⁎, Chia-Lin Li d, Jinn P. Chu e a
Department of Materials and Mineral Resources Engineering, Institute of Materials Science and Engineering, National Taipei University of Technology, Taipei, Taiwan Department of Materials Engineering, Ming Chi University of Technology, Taipei, Taiwan Center for Thin Film Technologies and Applications, Ming Chi University of Technology, Taipei, Taiwan d Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei, Taiwan e Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan b c
a r t i c l e
i n f o
Available online 18 September 2013 Keywords: Zr-based thin film metallic glass composite Adhesion Boron Nitrogen ZrN
a b s t r a c t In this work, nitrogen contained and B doped ZrCuAlNi thin film metallic glass composites (TFMGCs) were grown by magnetron co-sputtering process. Boron element was doped to evaluate its influence on the microstructure and mechanical properties of nitrogen contained TFMGCs. The supercooled liquid region of TFMG was determined by the differential scanning calorimetry (DSC) analysis. The microstructure was analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The phase structure was confirmed by using X-ray diffraction (XRD). The mechanical properties were measured by nanoindentation. The scratch tester and tests were employed to evaluate the adhesion and tribological properties. The surface roughness was determined by atomic force microscopy (AFM). It was concluded that the surface roughness, microstructure, glassforming ability and mechanical behaviors of TFMGCs were strongly influenced by the nitrogen and boron doping. The ZrN nanocrystallites were embedded in the amorphous matrix. The hardness, H/E ratio, and indentation toughness increased with increasing boron concentration. Excellent adhesion quality was also confirmed for all coatings. The Zr-based thin film metallic glass composite containing 6.4 B–32.6 N (in at.%) with optima hardness, 16.2 GPa, adequate indentation toughness and excellent adhesion quality was achieved in this work. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The amorphous bulk metallic glass (BMG) materials have attracted considerable scientific and commercial attention since 1960s due to their unique characteristics, such as high elastic energy, high hardness, good wear and corrosion resistance [1,2]. However, due to the rapid propagation of shear bands, the brittleness nature and catastrophic failure of BMGs restricted their application at ambient [1,2]. Recently, the size-dependent properties of BMGs were reported by Kumar et al. [3]. As the effective thickness (smallest dimension) of metallic glass material was less than 100 nm, the deformation mode changed from shear localization to homogeneous deformation [3]. Since the fast quenching rate can be achieved by the physical vapor deposition process, the metallic glass in thin film form, the so-called thin film metallic glass (TFMG) is considered as candidate to solve the brittleness problem of BMGs at ambient temperature. The Zr-based metallic glass thin film has been widely studied because of their good mechanical, tribological, fatigue property,
⁎ Corresponding author. ⁎⁎ Correspondence to: J.-W. Lee, Department of Materials Engineering, Ming Chi University of Technology, Taipei, Taiwan. E-mail addresses: [email protected] (Y.-C. Yang), jeffl[email protected] (J.-W. Lee). 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.09.015
corrosion resistance, and excellent adhesion [4–8]. Meanwhile, the addition of Ta element can improve the mechanical properties of Zr-based TFMG by reducing the density of free volume and retarding the shear band formation [9]. The hardness and elastic modulus increased up to the maximum values of 7.5 GPa and 141 GPa when Ta was introduced into the Ta–Zr–Cu–Al–Ag TFMG [9]. The addition of metalloid elements, especially B, in BMG materials has been studied in literature [10–17]. It is reported that B element had a positive effect on thermal stability of Zr base glassy alloys, while keeping the same glass-forming ability (GFA) as the base BMG [10–14]. The B element to decrease the fragility and improve the glass forming ability of Ni-rich BMG was also explored [15]. The glass formation ability of Be–B thin film can be improved as the B concentration was higher than 11 at.% [16]. On the other hand, the nitrogen was also added as the metalloid element in the BMG and TFMG system. For example, Hasegawa and coworkers [18] synthesized the nitrogen doped ZrCuNiAl BMG ribbon using Zr-based glassy alloy and AlN powder as starting materials. The Zr-based BMG ribbon had higher hardness due to its dispersed crystalline ZrN microparticles embedded in the glassy matrix. Some N2 pores and nitrogen atoms dissolved as interstitial atom in the glassy structure were also observed. Liebermann [19] discovered that the doping of 0.5 at.% nitrogen in the Fe–Cr-based BMG ribbon can increase its thermal stability as much as 31 K due to the nitrogen induced changes in the precipitation
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ZrN (111) ZrN (200) ZrN (220) ZrN (311)
Table 1 Sputtering parameter for Zr-based TFMGCs. Sample #
ZNB0
ZNB1
ZNB2
ZNB3
ZrCuAlNi target power (W) Boron target power (W) Background pressure (Pa) Deposition pressure (Pa) Input gas (sccm) Ar N2 Substrate bias voltage (V) Deposition time (h) Deposition temperature (°C) Surface roughness, Rms (nm)
200 – 8.0 × 10−6 0.4
200 50 3.2 × 10−5
200 125 3.9 × 10−5
200 200 4.0 × 10−5
1.92
0.20
ZNB3 Intensity (arb. units)
40 2 −100 3 25 2.16
277
ZNB2
ZNB1
0.40
ZNB0 20
of crystalline phases. In previous work [20], the thermal and mechanical properties of the brand new nitrogen doped Zr–Cu–Ni–Al–Ag–Ta TFMG were investigated. We discovered that both the hardness and the thermal stability were increased due to the formation of large amount of bigger size nitrogen-centered clusters. The energy barrier for flow stress was thus increased by the nitrogen doping. In addition, the plasma emission monitoring (PEM) technique was employed to fabricate the Zr–Ni–Al–Si TFMG in previous study [21]. The Zr-based thin film containing 13.8 at.% N having a microstructure consisting of ZrN nanograins embedded in the amorphous matrix provided a combination of high hardness, good wear resistance and excellent adhesion properties. Although the addition of B and N metalloid elements showed positive effects on the thermal/ mechanical properties of BMG and TFMG materials, the effect of adding both B and N elements on the microstructure and thermal and mechanical properties of Zr–Cu–Al–Ni thin film metallic glass has not yet been explored. In this work, the boron and nitrogen doped Zr–Cu–Al–Ni TFMGs were grown by co-sputtering using commercial ZrCuNiAl alloy target and pure B target. The microstructure, mechanical properties and strengthening mechanism of nitrogen and boron contained Zr–Cu–Al– Ni thin film metallic glasses were studied.
2. Experimental procedure In this study, a magnetron sputtering system was used to deposit Zr– Cu–Al–Ni–B thin film metallic glasses with the addition of nitrogen on three kinds of substrates, including p-type (100)Si wafer, glass plates, and polished and hardened AISI 420 stainless steel disks. The commercial Zr–Cu–Al–Ni alloy target (55% Zr–30% Cu–10% Al–5% Ni, in at.%, provided by Solar Applied Materials Technology Corporation, Taiwan) and 99.9 wt.% boron target were connected to a pulsed direct current (DC) and radio frequency (RF) power supplies, respectively. Both targets were 6 in. in diameter and located on the top of the substrate holder. The commercial Zr–Cu–Al–Ni alloy cathode was operated in poisoned mode during sputtering process using an Ar:N2 flow ratio of 40:2. The distances between centers of two targets and target-tosubstrate were 26 and 15 cm, respectively. The substrates were rotated in a speed of 10 rpm to keep the uniformity of chemical composition. The operating parameters for coating are presented in Table 1.
30
40
50
60
70
80
Fig. 1. The X-ray diffraction patterns for the Zr-based TFMGCs.
The cross-sectional morphologies of thin films were examined with a field emission scanning electron microscope (FE-SEM, JSM-6701, JEOL, Japan) and by transmission electron microscopy (TEM, JEOL, JSM-2100, Japan). The chemical composition of thin films was 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 (GA-XRD, PANalytical, X'pert, Holland) with an incidence angle of 1°. Cu Kα radiation generated at 30 kV and 40 mA from a Cu target was used. The thermal behavior of the film was determined by using a differential scanning calorimeter (DSC, NETZSCH DSC 404F3, Germany) in Ar at a heating rate of 40 K/min. The temperature resolution of DSC is 0.1 °C. A calibration of the DSC was held by heating the empty crucible to deduct the background signal. The DSC film sample was delaminated from the glass without the aid of any chemical solutions. The nanoindentation hardness, H, and elastic modulus, E, of the thin films were investigated by means of a nanoindenter (TI-900, TriboIndenter, Hysitron, USA) by using a Berkovich 142.3° diamond probe at different loads to achieve a fixed indentation depth of 80 nm. The loading rates were 3.3 to 6.0 μN/s. Eight indentation tests were made for each coating. The hardness and elastic modulus of each indent were determined on the basis of the Oliver and Pharr method [22]. The elastic modulus, E, was expressed as follows: 1 1−v2 1−v2i þ ¼ Er E Ei
Table 2 Chemical composition of the Zr-based TFMGCs (in at.%). Element
Zr Cu Al Ni B N O
Sample # ZNB0
ZNB1
ZNB2
ZNB3
29.1 18.3 7.1 4.0 0 32.3 9.3
35.8 16.0 7.0 4.7 0.4 35.1 1.2
33.5 17.1 5.9 4.3 6.4 32.6 0.3
33.4 17.6 5.4 4.1 12.3 26.8 0.5
90
2 Theta (degree)
Fig. 2. The DSC thermograms for the Zr-based TFMGCs.
ð1Þ
278
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where Er and ν are the reduced elastic modulus and Poisson's ratio, respectively, for the thin films under test, and Ei (1140 GPa) and νi (0.07) are the corresponding parameters of the diamond indenter. The Poisson's ratio ν for Zr55Cu30Ni5Al10 BMG was 0.375 [23]. The Vickers microindentation tester was adopted to evaluate the indentation toughness, Kc, according to the following eqn. [24]: rffiffiffiffi P E KC ¼ δ 3=2 H c
ð2Þ
where P is the applied indentation load and δ is an indenter geometry constant, equal to 0.016, for a Vickers diamond pyramid indenter. E and H are the elastic modulus and the hardness of thin film derived from nanoindentation tests. And c is the average of the radial crack lengths occurring on the thin films, which were measured using a SEM. The criterion for a valid crack is taken as c ≥2a, where a is half of the diagonal length of the indent. The adhesion strength quality of the thin films was evaluated by Daimler Benz Rockwell-C (HRC-DB) tester [25]. A scratch test (Scratch Tester, J & L Tech. Co., Korea), with up to the maximum load of 50 N was adopted to explore the adhesion properties of the thin films. 3. Results and discussion 3.1. Composition and phase characterization The chemical compositions of four coatings are listed in Table 2. The Zr content increased from 29.1 at.% to 35.8 at.% with oxygen concentration decreased from 9.3 at.% to 0.5 at.%. Apparently, the oxygen contents decreased significantly with the addition of boron. The exact reason for the decreasing tendency of oxygen content by boron addition is still unknown. Further investigation on this phenomenon is needed. It should be noticed that since the oxygen impurity level of the commercial target is less than 0.01 at.%, the oxygen content measured in the film is due to the residual atmosphere inside the chamber. The nitrogen decreased monotonically from 32.3 at.% to 26.8 at.% when B increased from 0 at.% to 12.3 at.%. On the other hand, The Cu, Al, and Ni concentrations were not influenced significantly by the boron doping. Fig. 1 shows the X-ray diffraction patterns of thin films in this work. The obvious crystallinity diffraction peaks corresponding to the ZrN phase are discovered for the ZNB0 and ZNB1, respectively. Interestingly, the lower intensity broad peaks are observed for ZNB2 and ZNB3 when more boron was contained in thin films. It is noticed that no detectable AlN or Si3N4 phase was found for each coating, implying that the reactive sputtering deposition reaction occurred mainly between nitrogen and Zr element due to its high concentration. From the viewpoint of free energy at 300 K, the Zr–N is −80.3 kcal/mol and B–N is −53.7 kcal/mol, respectively. The enthalpy for B–N, Al–N, Ni–N and Zr–N was −28, −92, −69,
and −233 kJ/mol, respectively [26]. Since the formation energy favors Zr–N phase, the ZrN phase can be produced easily. According to the XRD analysis, the average grain size values calculated by the Scherrer formula [27] were 7.6, 5.7, 4.4 and 3.4 nm, respectively for the ZNB0, ZNB1, ZNB2 and ZNB3 coatings. According to the XRD analysis, it indicates that Zr element reacted with nitrogen atoms easily during the reactive sputtering process. The grain growth of ZrN crystallites were restricted by the amorphous matrix and the increasing amount of B element. The DSC results for four coatings are illustrated in Fig. 2. The Tg and Tx were not found for all coatings implying the absence of the glass transition and crystallization phenomena with boron and nitrogen addition in the work. According to the AFM surface morphologies and measured surface roughness, Rms, as listed in Table 1, of TFMGs, the surface roughness decreased from 2.16 nm to 0.2 nm as the B content increased from 0 (ZNB0) to 6.4 at.% (ZNB2), although some surface contamination particles can be observed on the surface of ZNB2. Since more nano-sized particles are found on the surface of ZNB3, the surface roughness thus increases up to 0.4 nm. Apparently, the B addition can decrease the surface roughness effectively. The fractured cross-sectional morphologies of the TFMGs deposited on Si wafers are shown in Fig. 3. The typical columnar structure is observed for the nitrogen doped ZNB0. The dense and featureless microstructures are observed for ZNB1, ZNB2, and ZNB3 as B doping. Meanwhile, the thickness of the coating increased with the addition of the boron. It is important to point out that the cross sectional morphology of nitrogen doped TFMG was altered from columnar to featureless with only 0.4 at.% boron addition. The higher resolution TEM bright-field image and the selected area electron diffraction (SAED) pattern for ZNB2 are shown in Fig. 4(a) and (b), respectively. The ringlike diffraction patterns indicate the existence of ZrN nanograins. The diffusing ring is also implying the existence of amorphous TFMG matrix in Fig. 4(b). The TEM bright field image and dark-field image for ZNB2(111), (200) and (220) are shown in Fig. 4(c)–(f), respectively. It is obvious that a lot of ZrN nanocrystallites are evenly distributed in the TFMG matrix, which can be named as the thin film metallic glass composite (TFMGC). Very similar findings were reported in previous study [21]. In literature [10–15], the positive effect on the glass-forming ability was found for the boron doped Zr-based BMG. However, different tendency is observed according to the DSC data in this work. It is argued that the larger amount of ZrN nanocrystallites embedded in the μm thick glassy thin film matrix is the possible reason for the disappearance of Tg and Tx temperature in Fig. 2.
3.2. Mechanical property evaluation Fig. 5 depicts the relationships among the hardness, elastic modulus, H/E ratio, indentation toughness and boron target power. The hardness
Fig. 3. The cross-sectional FE-SEM images of the Zr-based TFMGCs.
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and elastic modulus are 9.3 GPa and 192 GPa for the ZNB0 TFMGC. These values increase as more boron is doped in the system. The hardness and elastic modulus increase up to the maximum values of 16.2 GPa and 216 GPa, respectively, for the 6.4 at.% B contained ZNB2 TFMGC. The H/E
279
ratio, the resistance of materials against elastic strain to failure, displays an upward trend with increasing boron doping amount. The maximum H/E ratio reaches up to 0.08 for the ZNB2 TFMGC. It is reported that denser atomic packing and stronger atomic interaction can be achieved by the
Fig. 4. The TEM analysis of ZNB2 (a) bright field image at high magnification and (b) the selected area electron diffraction pattern, (c) bright field image and dark-field images for ZrN (d) (111), (e) (200) and (f) (220).
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1 0.8
H/E
0.1
0.6
0.08
Indentation toughness (MPa-m1/2)
280
0.06
240 220
Hardness (GPa)
200 20
180
Elastic modulus (GPa)
0.04
16 12 8 0
50
100
150
200
Boron target power (W) Fig. 5. The relationships among the hardness, elastic modulus, H/E ratio, indentation toughness and boron target power.
addition of much smaller boron atoms in metallic glass materials [15]. Thus, the decreasing free volume and higher hardness caused by the denser atomic packing is clearly observed in this work. The increasing tendency for the hardness and indentation toughness versus the B target power is clearly observed in Fig. 5. The surface morphologies of indentation craters after the indentation toughness measurement for the ZNB0 and ZNB3 TFMGCs are illustrated in Fig. 6. The radial cracks can be recognized clearly. The value of indentation toughness is 0.75 MPa(m)−0.5 for the ZNB0 as shown in Fig. 5. The indentation toughness increased with increasing boron content and reached to the maximum value of 1.00 MPa(m)−0.5 for the ZNB2. Therefore, the indentation toughness is effectively enhanced by the addition of boron in TFMGCs. The Daimler–Benz Rockwell-C (HRC-DB) adhesion test [24] is an easy and commercially adopted method to evaluate the adhesion quality of coatings. The load of 1471 N was applied to cause a coating damage adjacent to the boundary of the indentation. The damage to the coatings was compared with the adhesion strength quality maps HF1–HF6. In general, the adhesion strength quality HF1 to HF4 defines a sufficient adhesion, whereas HF5 and HF6 represent insufficient adhesion obtained from
the HRC-DB test. The surface morphologies and enlarged image of indentation craters for each TFMGC is shown in Fig. 7(a)–(d). No radial cracking or delamination is observed on the indentation crater of ZNB0, ZNB1 and ZNB3 (Fig. 7(a),(b),(d)) indicating its excellent adhesion quality, which is classified as HF1. In Fig. 7(c), some radial cracks are found on indentation crater for ZNB2, but it is still classified as HF1. These results indicated its excellent adhesion. In this work, excellent adhesion quality, HF1, is obtained for each TFMGC with the addition of nitrogen or boron/nitrogen elements. The backscattered electron images (BEI) of scratch track morphologies for four coatings are shown in Fig. 8(a)–(d), respectively, to distinguish the adhesion failures of the coating. It can be observed that a lot of cracks occurred on the scratch track, but no delamination of the coating inside or adjacent to the scratch track was observed for all thin films even in the end of scratch track as shown in Fig. 9(a)–(d). Meanwhile, it is clear that the “ductile tensile cracking” [28] is found for the ZNB0 (Fig. 8(a)) and the “conformal cracking” [28] is discovered for the ZNB1, ZNB2, and ZNB3 as shown in Fig. 8(b)–(d), respectively. Interestingly, the “ductile tensile cracking” changed to “conformal cracking” as the B doped into the ZNB1, ZNB2, and ZNB3. Although the “ductile tensile cracking” is a characteristic of ductility material and the “conformal cracking” is correlated to the brittleness material, the excellent adhesion property is achieved for all coatings in this work. Caron et al. [29] reported that the internal structure of metallic glass plays a key role on its mechanical properties. This structure change can be explained by the free volume [30] and the configuration potential energy [31]. Therefore, the increasing mismatch in atomic size by the much smaller boron atoms and the stronger atomic interaction can produce denser atomic packing, higher viscosity of liquid, less free volume and the better mechanical behaviors in the Zr-based ZrN and B doped TFMGCs. It can be concluded that, according to the hardness, toughness, HRC-DB and scratch test results, the B doping into the nitrogen contained TFMGC can increase its hardness, toughness and adhesion properties effectively. 4. Conclusion In this work, the ZrN dispersed thin film metallic glass composite (TFMGC) materials with different boron contents were fabricated by co-sputtering process using ZrCuAlNi alloy target and B target. The TFMGCs with B and N contents ranging from 0 at.% to 12.3 at.% and 26.8 at.% to 35.1 at.% were achieved. The ZrN nanocrystallites were embedded in the amorphous matrix. The decreasing surface roughness was observed for the TFMGC with increasing B content. The cross sectional morphology changed from columnar to fine and featureless as B doped into the TFMGCs. On the other hand, the hardness, H/E ratio, and indentation toughness increased with increasing boron doping concentration. Excellent adhesion quality was also confirmed through the scratch and HRC-DB tests. The optima hardness reaching 16.2 GPa,
Fig. 6. The surface indentation crater morphologies of (a) ZNB0 and (b) ZNB3 after the indentation toughness measurement.
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281
Fig. 7. The secondary electron (SE) and backscattered electron (BSE) images and inserted higher magnification images of indentation craters for (a) ZNB0, (b) ZNB1, (c) ZNB2 and (d) ZNB3.
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adequate indentation toughness and excellent adhesion quality were achieved for the Zr-based thin film metallic glass composite containing 33.5 Zr–17.1 Cu–5.9Al–4.3 Ni–6.4 B–32.6 N (in at.%). Acknowledgment The authors gratefully acknowledge the financial support of the National Science Council, Taiwan through contract no NSC 101-2221-E131-023-MY3 and the partial financial support from Solar Applied Materials Technology Corporation, Taiwan. References [1] [2] [3] [4] [5] [6] [7]
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
Fig. 8. The backscattered electron images of the scratch tracks for all coatings.
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Fig. 9. The secondary electron and backscattered electron images of the end of scratch tracks for (a) ZNB0, (b) ZNB1, (c) ZNB2 and (d) ZNB3.
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