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Annealing effect on tribological property of arc-deposited TiN film on 316L austenitic stainless steel
SCT-19186; No of Pages 5 Surface & Coatings Technology xxx (2014) xxx–xxx
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Annealing effect on tribological property of arc-deposited TiN film on 316L austenitic stainless steel C.H. Hsu ⁎, K.H. Huang, M.R. Lin Department of Materials Engineering, Tatung University, Taipei 104, Taiwan
a r t i c l e
i n f o
Available online xxxx Keywords: Annealing Cathodic arc evaporation TiN Rutile-TiO2 Wear resistance
a b s t r a c t In this work, TiN film was deposited onto 316L austenitic stainless steel by a cathodic arc evaporation technique, and then the coated specimens were annealed at the different temperatures. Microstructural analysis, nanoindentation tests, and wear tests were performed for understanding the effect of annealing temperature on composition, microstructure, and mechanical behavior of the coatings. The results showed when the TiN film was annealed at 500 °C for 2 h, a dense Ti–N–O thin film of about 0.5 μm was produced in the outer-layer, consisting of Rutile-TiO2 and TiN phases. Such the annealed coating had the most uniform chemical composition as well as the highest H/E value in comparison to other unannealed and annealed coatings. 316L stainless steel with the optimal annealed coating not only reduced the friction coefficient from 0.68 to 0.25, but also remarkably improved the adhesion wear resistance. © 2014 Elsevier B.V. All rights reserved.
1. Introduction 316L austenitic stainless steel is an attractive engineering material due to its excellent corrosion resistance, oxidation resistance, and good formability [1]. Recently, it has also been used quickly in biocompatibility applications and high quality goods, such as surgery instruments, artificial joints, and precision watches. Despite these advantages, however, 316L stainless steel has low hardness to limit its specific applications which require good performance in wear resistance [2]. Thus, many studies have focused on surface modification of the material to improve its wear behavior, such as carburizing, laser surface alloying, and high-energy electron beam irradiation. [3–6]. In general, most of the methods are contributive to improve wear resistance. It is well known that PVD technology has become a popular method of surface treatment in the past several decades, because this method allows at low processing temperatures to deposit various hard coatings on metallic substrates [2]. For example, some literatures [7–10] reported that 316L stainless steel coated nitride films by sputtering such as TiN, CrN, ZrN, and TiAlN could enhance its surface property. In particular, titanium nitride (TiN) not only has a graceful golden yellow, but also exhibits high hardness, good thermal conductivity, and electrical conductivity. With such attractive properties, TiN becomes an important film material in engineering applications [11]. Furthermore, some researches utilized oxygen addition to form Ti–N–O film by sputtering for varying TiN coating color, structure, and hardness [12,13]. Impressively, color altered from golden yellow type to dark blue with an increase of oxygen addition in the Ti–N–O coatings. A previous study ⁎ Corresponding author. E-mail address: [email protected] (C.H. Hsu).
[14] also used another PVD method — cathodic arc evaporation (CAE) system with the different O2/N2 ratios to synthesize Ti–N–O coatings. The result showed that the O2/N2 ratio properly controlled at 0.25 could produce an optimal Ti–N–O film with a dense crystalline structure and effectively improve the wear resistance of AISI 304 stainless steel. According to the above information, we have noted that the effect of oxygen on the characterization of Ti–N–O film appears to be expected and interesting. Therefore, the present work aims to coat TiN films on AISI 316L stainless steel by CAE method along with annealing treatment at different temperatures. Coating structure and properties, such as surface roughness, adhesion, hardness, and elastic modulus were analyzed. In addition, the wear tests were carried out to evaluate the effect of annealing temperature on the friction coefficient and wear rate of the coated specimens. 2. Experimental procedures In this study, the substrates were made of commercial AISI 316L stainless steel (~195 HV), and had a circular shape with a diameter of 20 mm and a thickness of 5 mm in size for the wear tests. Prior to deposition, the substrates were polished, degreased, ultrasonically cleaned, rinsed with alcohol, and dried by warm air. The base vacuum was approximately 9 × 10− 3 Pa. The bombardments of argon ion at the bias of −700 V for 10 min were carried out to further ensure good adhesion of the deposited films. Two opposite titanium targets (99.99% purity) were mounted at both sides of the chamber, while the reactive gas of N2 was used to deposit TiN film. Before TiN coating, a pure Tiinterlayer was deposited for 10 min to enhance the adhesion between TiN and substrate. The distance between the target and substrate was 150 mm, and the working pressure during coating was about 2 Pa.
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Please cite this article as: C.H. Hsu, et al., Surf. Coat. Technol. (2014), http://dx.doi.org/10.1016/j.surfcoat.2014.02.001
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C.H. Hsu et al. / Surface & Coatings Technology xxx (2014) xxx–xxx
The other deposition parameters were also selected as follows: the N2 flow rate = 50 sccm, the substrate bias voltage = −150 V, the evaporation current = 60 A, the substrate temperature = 270 °C, the holder rotation rate = 4 rpm, and depositing time = 40 min. After coating, post annealing treatments of the TiN-coated specimens were separately carried out at 400, 500, and 600 °C for 2 h in an air furnace. According to the three temperatures, the annealed TiN coatings in this paper were marked as TiN(400), TiN(500), and TiN(600), respectively. A field emission scanning electron microscopy (FESEM, LEO 1530) was used at an accelerating voltage of 15 kV to observe the coating morphology and to measure the coating thickness. A glancing incidence X-ray diffractometer (XRD, Rigaku-TTTRAX III) was employed to identify the coating structure with a Cu-target Kα radiation at 40 kV and 30 mA and a glancing incident angle of 2°; the scanning angular (2θ) ranged from 20° to 80° at 2°/min. The chemical composition of the films was determined by the quantitative electron probe microanalysis (EPMA, JEOL JXA-8200) with the mode of line scanning for Ti, N, and O elements. A surface roughness analyzer (Mitutoyo SV-400) was applied to measure the average surface roughness (Ra value) for each specimen. Adhesion strength quality (ASQ) of the coatings was evaluated by using Rockwell-C indentation testing with a load of 1471 N [15]. The damage to the coatings was compared with a defined ASQ basis, where HF1–HF4 were acceptable adhesion and HF5–HF6 represented insufficient adhesion (HF is the German short form for adhesion strength). The nanoindentation experiments were performed using a nanoindenter (MTS XP system) with a diamond Berkovich tip. According to the Oliver and McHargue method [16], the values of hardness and elastic modulus for the coatings were obtained by analyzing load–displacement curves using a load of 15 mN. The averages of 20 indentations for each film were reported as the resultant values in this study. Friction and wear properties of all the specimens were evaluated in wear tests using a ball-on-disk tribometer (CSM Instruments, Switzerland). The counterface material was a 6 mm diameter WC– 6%Co ball, which had a hardness of 1780 HV and a surface roughness of approximately 0.2 μm (Ra). As frequently selected conditions for
a
coating tests [17,18], the applied load was kept at 5 N with a linear sliding speed of 0.2 m/s. All the tests were conducted without lubricant at an ambient temperature of 25 °C as well as 65% relative humidity (in the laboratory atmosphere). For each coating condition, the wear tests were performed three times. The relationship between the friction coefficient and the wearing time of about 2500 s (total travel distance: 500 m) was continuously recorded during the tests. Furthermore, the wear rate of specimen was determined from weight loss divided by total travel distance. The weight loss of each specimen after the wear test was measured with a micro-balance (±1 × 10−4 g). In addition, the worn surface of each specimen after the wear test was observed using SEM. 3. Results and discussion 3.1. Coating structure and composition This present work used FESEM to observe cross-sectional micrographs of the TiN-coated specimens with and without post-annealing, as shown in Fig. 1. It can be seen that the coating has a columnar morphology, consisting of around 1.4 μm TiN film and 0.4 μm Ti-interlayer, as shown in Fig. 1(a). After annealing at 400 °C for 2 h, the TiN film almost had no variation in morphology (Fig. 1(b)). When the annealing temperature was up to 500–600 °C, the outward of TiN-coated specimens generated a dense thin film. Moreover, the outer-layer in thickness had an increase while the thickness of TiN film lessened with raising annealing temperature, as shown in Fig. 1(c) and (d). Fig. 2 compares the crystal structures of the coatings before and after annealing in terms of the XRD patterns. The result showed that the unannealed film had the main peaks on (111), (200), (220), and (311) diffracting planes for the TiN film because the depositing reaction between nitrogen and titanium ions formed a NaCl-type structure [19]. The XRD pattern of TiN(400) was almost the same with the unannealed one. For the TiN(500) and TiN(600) films, there were some peaks of Rutile-TiO2 crystalline plane such as (110), (111), (211), and (002),
b TiN Ti
1 µm
c
1 µm
d
Ti-N-O TiN Ti 1 µm Fig. 1. SEM cross-sectional view of the coated specimens: (a) TiN, (b) TiN(400), (c) TiN(500), and (d) TiN(600).
Please cite this article as: C.H. Hsu, et al., Surf. Coat. Technol. (2014), http://dx.doi.org/10.1016/j.surfcoat.2014.02.001
1 µm
Relative Intensity (arb. unit)
3
TiN(311)
TiO2(002)
TiO2(211)
TiN(220)
TiN(600)
TiN(200)
TiO2(111)
TiO2(110)
TiN(111)
C.H. Hsu et al. / Surface & Coatings Technology xxx (2014) xxx–xxx
TiN(500)
20
30
40
20
30
40
TiN(311)
TiN(220)
TiN(111)
TiN(200)
TiN(400)
TiN
50
60
70
80
50
60
70
80
TiN
TiO2
2 Theta (degree) Fig. 2. XRD patterns of the TiN coated specimens before and after annealing.
occurring in the XRD pattern besides the TiN peaks. We inferred that the outer-layer formed on the TiN(500) and TiN(600) specimens should be a Ti–N–O film, mainly consisting of the Rutile-TiO2 and TiN phases. The result can be explained with equilibrium phase diagram of TiO2 binary system [20], that Rutile-TiO2 is a stable phase at high temperature up to 500–600 °C. Moreover, the TiO2 peaks in TiN(600) seem to be stronger than that in TiN(500). This is because the former has a thicker outer-layer as compared to the latter (0.9 vs. 0.5 μm). Coating composition of each TiN film with and without annealing analyzed by the EPMA method is listed in Table 1. The data are further compared in Fig. 3. For the unannealed and TiN(400) specimens, their TiN coating composition nearly had the same amount in Ti (51.4 vs. 51.6 at.%) and N (48.6 vs. 48.4 at.%) elements. The result indicated that TiN film showed the stable oxidation resistance under the annealing temperature of 400 °C. When the coated specimen was annealed at 500 °C, the content of oxygen in the film was slightly higher than that of nitrogen (33.1 vs. 29.7 at.%). To further raise the annealing temperature up to 600 °C, the oxygen content in the film greatly increased (53.6 at.%) while both the nitrogen content (12.2 at.%) and the titanium content (34.2 at.%) decreased substantially. The result denoted that some of the nitrogen atoms were substituted by oxygen atoms to form TiO2 in Ti–N–O outer-layer. That is, the affinity of titanium and oxygen is stronger than that of titanium and nitrogen [14]. In addition, we also found that thickness and composition of the
outer-layer seemed to be affected by the annealing temperature, in which the annealing temperature of 500 °C could obtain a thinner but more uniform outer-layer in this study.
3.2. Analysis of coating properties The coating properties, such as surface roughness, adhesion, hardness (H), elastic modulus (E), and H/E obtained in this study are listed in Table 1. In the surface roughness, the TiN coated specimen had a rougher surface than the uncoated one (Ra: 0.22 vs. 0.05 μm). The main reason for this phenomenon could be attributed to macroparticles deposited on the substrate by CAE process [21]. After annealing, the surface roughness of TiN film had a slight decrease at 500 °C (Ra: 0.20 μm) but increased at 600 °C (Ra: 0.33 μm). The result should be dependent upon atomic distribution in the Ti–N–O outer-layer. That is, the film with the more uniform composition tends to the lower Ra value. Indentation tests were performed by using the Rockwell-C hardness tests to evaluate the coating adhesion between film and substrate. According to the definition of ASQ as aforementioned [15], we found that all the TiN coated specimens had a clear and smooth indentation regardless of annealing, closely resembled HF1–HF2 grades. Therefore, the annealing for TiN film in this study appears to have no influence on the coating adhesion.
Table 1 Coating properties of the un-annealed and annealed TiN films in this study. Specimen
Un-annealed TiN TiN(400) TiN(500) TiN(600)
Atomic concentration (at.%) Ti
N
O
51.4 51.6 37.2 34.2
48.6 48.4 29.7 12.2
– – 33.1 53.6
Surface roughness, Ra (μm)
Adhesion strength, ASQ [15]
Hardness, H (GPa)
Elastic Modulus, E (GPa)
H/E
0.22 0.23 0.20 0.33
HF1 HF1 HF1 HF2
18.7 18.5 22.0 19.1
186.0 187.1 201.5 198.0
0.101 0.099 0.109 0.096
Please cite this article as: C.H. Hsu, et al., Surf. Coat. Technol. (2014), http://dx.doi.org/10.1016/j.surfcoat.2014.02.001
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C.H. Hsu et al. / Surface & Coatings Technology xxx (2014) xxx–xxx
20
100
Ti N O
80
18
16.6
16
Wear rate (X10-6g/m)
Atomic concentration (%)
90
70 60 50 40 30
14 12 10 8
6.6
6.2
6
20
4
10
2
0
12.6
3.8
0
TiN
400
500
600
AISI 316L
Annealing temperature (°C)
TiN
TiN(400)
TiN(500)
TiN(600)
Specimen
Fig. 3. Effect of annealing temperature on the averaged concentration of the Ti, N, and O elements in the coating.
Fig. 5. Comparison of wear rate among all the specimens after wear tests.
The values of hardness, elastic modulus, and H/E ratio measured by the nanoindenter with an indentation depth of approximately 100 nm are also listed in Table 1. The set shallow depth measured in the tests implies that the reported values predominately arise from the coating properties without the substrate effect. The result showed that the annealing temperature governed the coating hardness, elastic modulus, and H/E values. When the TiN-coated specimens were annealed at the temperature range of 500–600 °C, the coatings in H and E values had an increase due to the formation of Ti–N–O outer-layer. In particular, the TiN(500) specimen revealed the highest H (22.0 GPa), E (201.5 GPa), and H/E (0.109) values among all the annealed coatings. It can be attributed to the uniform compositional concentration obtained in its Ti–N–O outer-layer as aforementioned.
the lowest friction coefficient of about 0.25 among all the specimens. In wear rate, a comparison for all the specimens after wear tests is shown in Fig. 5. The result revealed that the AISI 316L stainless steel coated with TiN film had a decrease in the wear rate from 16.6 × 10−6 to 6.6 × 10−6 g/m. Furthermore, the wear rate of the coated specimens after annealing, ranking from the fastest to the slowest, was as follow: TiN(600) N TiN(400) N TiN(500). It is interesting to find that the TiN(500) specimen has the highest H/E value (0.109) to claim the lowest wear rate (3.8 × 10−6 g/m) among all the specimens. This result coincides with the concepts described by Leyland and Matthews [22], who reported that a high ratio of H/E could be indicative of good wear resistance in a disparate range of materials. Besides, the TiN(500) specimen has the well coating adhesion as aforementioned. Accordingly, we can presume that the wear-life of protective coatings mainly depends upon the combined functions of the friction coefficient, hardness, elastic modulus, and film adhesion. Fig. 6 further shows the SEM damage patterns of the specimens after a wear test of 500 m. A typical morphology of adhesive wear with wear debris was clearly seen in the uncoated specimen (Fig. 6(a)). This is because the substrate has a relative lower hardness as compared to counterface material (195 vs. 1780 HV). In adhesive wear mechanism, the two solids experienced relative motion during wear testing, with a bonding adhesion between contacting surfaces eventually resulting in the fracturing of the softer material [23]. The track of wear became abrasion wear accompanied by a few scratches arising from the increase of surface hardness, and a partial substrate was exposed in unannealed TiN and TiN(400) specimens (Fig. 6(b) and (c)). By contrast, there was only a few TiN area without any substrate exposed on the worn surface of TiN(500) (Fig. 6(d)). The wear phenomenon showed a typical abrasion wear behavior. This is because the Ti–N–O outer-layer in TiN(500) shows high H/E value and well adhesion. Though there was also a Ti–N–O outer-layer formed on TiN(600) after annealing, the H/E value of TiN(600) was lower than that of TiN(500), resulting in a large area of Ti–N–O film scraped to expose underlying TiN, and even substrate in TiN(600) (Fig. 6(e)).
3.3. Wear behavior In this study, a CSM tribometer was used to evaluate the wear behavior of AISI 316L stainless steel with and without annealed TiN coating. The relationship curves between the friction coefficient and the abrasion distance were plotted in Fig. 4. The result showed that the TiN-coated specimen had a lower friction coefficient than the uncoated one (0.49 vs. 0.67). After annealed at the temperature of 500–600 °C, the friction coefficient of TiN-coated specimens could be decreased to 0.25–0.37. In particular, the TiN(500) specimen displayed 1.0
0.8
Friction coefficient
AISI 316L
0.6
TiN(400)
TiN
TiN(600)
0.4
4. Conclusions
0.2
TiN(500)
0.0 0
100
200
300
400
500
Travel distance(m) Fig. 4. The curves of friction coefficient of all the specimens at a load of 5 N, sliding against a 6 mm WC + 6%Co ball counterface.
The TiN arc-coated 316L stainless steel after annealing at proper temperature could form a Ti–N–O outer-layer, containing Rutile-TiO2 and TiN phases. The outer-layer in thickness had an increase with raising the annealing temperature. Moreover, the properties of the outer-layer such as composition, hardness, elastic modulus, and H/E were also dependent upon the annealing temperature. When the annealing temperature was controlled at 500 °C, the formed outer-
Please cite this article as: C.H. Hsu, et al., Surf. Coat. Technol. (2014), http://dx.doi.org/10.1016/j.surfcoat.2014.02.001
C.H. Hsu et al. / Surface & Coatings Technology xxx (2014) xxx–xxx
5
a
Wear debris
b
c Substrate
TiN
d
e Ti-N-O TiN TiN
Fig. 6. Surface morphology of the specimens after ball-on-disk wear tests: (a) AISI 316L, (b) TiN, (c) TiN(400), (d) TiN(500), and (e) TiN(600).
layer had the most uniform composition, highest hardness, elastic modulus, and H/E values among all the annealing conditions. Such an annealed film remarkably reduced the friction coefficient of 316L stainless steel from 0.68 to 0.25, along with a low wear rate (3.8 × 10−6 g/m) to improve wear resistance. Acknowledgments The authors would like to thank the financial support of the National Science Council (Taiwan, ROC) under contract no. NSC 101-2221-E-036012. References [1] X.H. Chen, J. Lu, L. Lu, K. Lu, Scr. Mater. 52 (2005) 1039. [2] E.D.L. Heras, D.A. Egidi, P. Corengia, D. Gonzalez-Santamaria, et al., Surf. Coat. Technol. 202 (2008) 2945. [3] B.S. Suh, W.J. Lee, Thin Solid Films 295 (1997) 185. [4] C. Tassin, F. Laroudie, M. Pons, L. Lelait, Surf. Coat. Technol. 80 (1996) 207. [5] E. Yun, S. Lee, Surf. Coat. Technol. 200 (2006) 3478. [6] I. Boromei, L. Ceschini, A. Marconi, C. Martini, Wear 302 (2013) 899.
[7] J.A. Berrios, D.G. Teer, E.S. Puchi-Cabrera, Surf. Coat. Technol. 148 (2001) 179. [8] G. Aldrich-Smith, D.G. teer, P.A. Dearnley, Surf. Coat. Technol. 116–119 (2001) 1161. [9] J.A. Berrios-Ortiz, J.G. La Barbera-Sosa, D.G. Teer, E.S. Puchi-Cabrera, Surf. Coat. Technol. 179 (2004) 145. [10] B.Y. Man, L. Guzman, A. Miotello, M. Adami, Surf. Coat. Technol. 180–181 (2004) 9. [11] J.P. Bucker, K.P. Ackermann, F.W. Buschor, Thin Solid Films 122 (1984) 63. [12] F. Vaz, P. Cerqueira, L. Rebouta, S.M.C. Nascimento, E. Alves, Ph. Goudeau, J.P. Riviere, K. Pischow, J. de Rijk, Thin Solid Films 447–448 (2004) 449. [13] A. Rizzo, M.A. Signore, L. Mirenghi, T.D. Luccio, Thin Solid Films 517 (2009) 5956. [14] C.H. Hsu, K.H. Huang, Y.H. Lin, Wear 306 (2013) 97. [15] W. Heinke, A. Leyland, A. Matthews, G. Berg, C. Friedrich, E. Broszeit, Thin Solid Films 270 (1995) 431. [16] W.C. Oliver, C.J. McHargue, Thin Solid Films 161 (1988) 117. [17] A. Qzturk, K.V. Ezirmik, K. Kazmanli, M. Urgen, O.L. Eryilmaz, A. Erdemir, Tribol. Int. 41 (2008) 49. [18] J.J. Kang, C.B. Wang, H.D. Wang, B.S. Xu, J.J. Liu, G.L. Li, Surf. Coat. Technol. 203 (2009) 1927. [19] S. Paldey, S.C. Deevi, Mater. Sci. Eng. A 342 (2003) 58. [20] E.M. Levin, H.F. McMurdie, Phase Diagrams for Ceramists, The American Ceramic Society, Inc., 1975. 76. [21] C.H. Hsu, C.C. Lee, W.Y. Ho, Thin Solid Films 516 (2008) 4826. [22] A. Leyland, A. Matthews, Wear 246 (2000) 1. [23] K.G. Budinski, Surface Engineering for Wear Resistance, Prentice Hall, New Jersey, 1988. 17.
Please cite this article as: C.H. Hsu, et al., Surf. Coat. Technol. (2014), http://dx.doi.org/10.1016/j.surfcoat.2014.02.001
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Annealing effect on tribological property of arc-deposited TiN film on 316L austenitic stainless steel C.H. Hsu ⁎, K.H. Huang, M.R. Lin Department of Materials Engineering, Tatung University, Taipei 104, Taiwan
a r t i c l e
i n f o
Available online xxxx Keywords: Annealing Cathodic arc evaporation TiN Rutile-TiO2 Wear resistance
a b s t r a c t In this work, TiN film was deposited onto 316L austenitic stainless steel by a cathodic arc evaporation technique, and then the coated specimens were annealed at the different temperatures. Microstructural analysis, nanoindentation tests, and wear tests were performed for understanding the effect of annealing temperature on composition, microstructure, and mechanical behavior of the coatings. The results showed when the TiN film was annealed at 500 °C for 2 h, a dense Ti–N–O thin film of about 0.5 μm was produced in the outer-layer, consisting of Rutile-TiO2 and TiN phases. Such the annealed coating had the most uniform chemical composition as well as the highest H/E value in comparison to other unannealed and annealed coatings. 316L stainless steel with the optimal annealed coating not only reduced the friction coefficient from 0.68 to 0.25, but also remarkably improved the adhesion wear resistance. © 2014 Elsevier B.V. All rights reserved.
1. Introduction 316L austenitic stainless steel is an attractive engineering material due to its excellent corrosion resistance, oxidation resistance, and good formability [1]. Recently, it has also been used quickly in biocompatibility applications and high quality goods, such as surgery instruments, artificial joints, and precision watches. Despite these advantages, however, 316L stainless steel has low hardness to limit its specific applications which require good performance in wear resistance [2]. Thus, many studies have focused on surface modification of the material to improve its wear behavior, such as carburizing, laser surface alloying, and high-energy electron beam irradiation. [3–6]. In general, most of the methods are contributive to improve wear resistance. It is well known that PVD technology has become a popular method of surface treatment in the past several decades, because this method allows at low processing temperatures to deposit various hard coatings on metallic substrates [2]. For example, some literatures [7–10] reported that 316L stainless steel coated nitride films by sputtering such as TiN, CrN, ZrN, and TiAlN could enhance its surface property. In particular, titanium nitride (TiN) not only has a graceful golden yellow, but also exhibits high hardness, good thermal conductivity, and electrical conductivity. With such attractive properties, TiN becomes an important film material in engineering applications [11]. Furthermore, some researches utilized oxygen addition to form Ti–N–O film by sputtering for varying TiN coating color, structure, and hardness [12,13]. Impressively, color altered from golden yellow type to dark blue with an increase of oxygen addition in the Ti–N–O coatings. A previous study ⁎ Corresponding author. E-mail address: [email protected] (C.H. Hsu).
[14] also used another PVD method — cathodic arc evaporation (CAE) system with the different O2/N2 ratios to synthesize Ti–N–O coatings. The result showed that the O2/N2 ratio properly controlled at 0.25 could produce an optimal Ti–N–O film with a dense crystalline structure and effectively improve the wear resistance of AISI 304 stainless steel. According to the above information, we have noted that the effect of oxygen on the characterization of Ti–N–O film appears to be expected and interesting. Therefore, the present work aims to coat TiN films on AISI 316L stainless steel by CAE method along with annealing treatment at different temperatures. Coating structure and properties, such as surface roughness, adhesion, hardness, and elastic modulus were analyzed. In addition, the wear tests were carried out to evaluate the effect of annealing temperature on the friction coefficient and wear rate of the coated specimens. 2. Experimental procedures In this study, the substrates were made of commercial AISI 316L stainless steel (~195 HV), and had a circular shape with a diameter of 20 mm and a thickness of 5 mm in size for the wear tests. Prior to deposition, the substrates were polished, degreased, ultrasonically cleaned, rinsed with alcohol, and dried by warm air. The base vacuum was approximately 9 × 10− 3 Pa. The bombardments of argon ion at the bias of −700 V for 10 min were carried out to further ensure good adhesion of the deposited films. Two opposite titanium targets (99.99% purity) were mounted at both sides of the chamber, while the reactive gas of N2 was used to deposit TiN film. Before TiN coating, a pure Tiinterlayer was deposited for 10 min to enhance the adhesion between TiN and substrate. The distance between the target and substrate was 150 mm, and the working pressure during coating was about 2 Pa.
0257-8972/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2014.02.001
Please cite this article as: C.H. Hsu, et al., Surf. Coat. Technol. (2014), http://dx.doi.org/10.1016/j.surfcoat.2014.02.001
2
C.H. Hsu et al. / Surface & Coatings Technology xxx (2014) xxx–xxx
The other deposition parameters were also selected as follows: the N2 flow rate = 50 sccm, the substrate bias voltage = −150 V, the evaporation current = 60 A, the substrate temperature = 270 °C, the holder rotation rate = 4 rpm, and depositing time = 40 min. After coating, post annealing treatments of the TiN-coated specimens were separately carried out at 400, 500, and 600 °C for 2 h in an air furnace. According to the three temperatures, the annealed TiN coatings in this paper were marked as TiN(400), TiN(500), and TiN(600), respectively. A field emission scanning electron microscopy (FESEM, LEO 1530) was used at an accelerating voltage of 15 kV to observe the coating morphology and to measure the coating thickness. A glancing incidence X-ray diffractometer (XRD, Rigaku-TTTRAX III) was employed to identify the coating structure with a Cu-target Kα radiation at 40 kV and 30 mA and a glancing incident angle of 2°; the scanning angular (2θ) ranged from 20° to 80° at 2°/min. The chemical composition of the films was determined by the quantitative electron probe microanalysis (EPMA, JEOL JXA-8200) with the mode of line scanning for Ti, N, and O elements. A surface roughness analyzer (Mitutoyo SV-400) was applied to measure the average surface roughness (Ra value) for each specimen. Adhesion strength quality (ASQ) of the coatings was evaluated by using Rockwell-C indentation testing with a load of 1471 N [15]. The damage to the coatings was compared with a defined ASQ basis, where HF1–HF4 were acceptable adhesion and HF5–HF6 represented insufficient adhesion (HF is the German short form for adhesion strength). The nanoindentation experiments were performed using a nanoindenter (MTS XP system) with a diamond Berkovich tip. According to the Oliver and McHargue method [16], the values of hardness and elastic modulus for the coatings were obtained by analyzing load–displacement curves using a load of 15 mN. The averages of 20 indentations for each film were reported as the resultant values in this study. Friction and wear properties of all the specimens were evaluated in wear tests using a ball-on-disk tribometer (CSM Instruments, Switzerland). The counterface material was a 6 mm diameter WC– 6%Co ball, which had a hardness of 1780 HV and a surface roughness of approximately 0.2 μm (Ra). As frequently selected conditions for
a
coating tests [17,18], the applied load was kept at 5 N with a linear sliding speed of 0.2 m/s. All the tests were conducted without lubricant at an ambient temperature of 25 °C as well as 65% relative humidity (in the laboratory atmosphere). For each coating condition, the wear tests were performed three times. The relationship between the friction coefficient and the wearing time of about 2500 s (total travel distance: 500 m) was continuously recorded during the tests. Furthermore, the wear rate of specimen was determined from weight loss divided by total travel distance. The weight loss of each specimen after the wear test was measured with a micro-balance (±1 × 10−4 g). In addition, the worn surface of each specimen after the wear test was observed using SEM. 3. Results and discussion 3.1. Coating structure and composition This present work used FESEM to observe cross-sectional micrographs of the TiN-coated specimens with and without post-annealing, as shown in Fig. 1. It can be seen that the coating has a columnar morphology, consisting of around 1.4 μm TiN film and 0.4 μm Ti-interlayer, as shown in Fig. 1(a). After annealing at 400 °C for 2 h, the TiN film almost had no variation in morphology (Fig. 1(b)). When the annealing temperature was up to 500–600 °C, the outward of TiN-coated specimens generated a dense thin film. Moreover, the outer-layer in thickness had an increase while the thickness of TiN film lessened with raising annealing temperature, as shown in Fig. 1(c) and (d). Fig. 2 compares the crystal structures of the coatings before and after annealing in terms of the XRD patterns. The result showed that the unannealed film had the main peaks on (111), (200), (220), and (311) diffracting planes for the TiN film because the depositing reaction between nitrogen and titanium ions formed a NaCl-type structure [19]. The XRD pattern of TiN(400) was almost the same with the unannealed one. For the TiN(500) and TiN(600) films, there were some peaks of Rutile-TiO2 crystalline plane such as (110), (111), (211), and (002),
b TiN Ti
1 µm
c
1 µm
d
Ti-N-O TiN Ti 1 µm Fig. 1. SEM cross-sectional view of the coated specimens: (a) TiN, (b) TiN(400), (c) TiN(500), and (d) TiN(600).
Please cite this article as: C.H. Hsu, et al., Surf. Coat. Technol. (2014), http://dx.doi.org/10.1016/j.surfcoat.2014.02.001
1 µm
Relative Intensity (arb. unit)
3
TiN(311)
TiO2(002)
TiO2(211)
TiN(220)
TiN(600)
TiN(200)
TiO2(111)
TiO2(110)
TiN(111)
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TiN(500)
20
30
40
20
30
40
TiN(311)
TiN(220)
TiN(111)
TiN(200)
TiN(400)
TiN
50
60
70
80
50
60
70
80
TiN
TiO2
2 Theta (degree) Fig. 2. XRD patterns of the TiN coated specimens before and after annealing.
occurring in the XRD pattern besides the TiN peaks. We inferred that the outer-layer formed on the TiN(500) and TiN(600) specimens should be a Ti–N–O film, mainly consisting of the Rutile-TiO2 and TiN phases. The result can be explained with equilibrium phase diagram of TiO2 binary system [20], that Rutile-TiO2 is a stable phase at high temperature up to 500–600 °C. Moreover, the TiO2 peaks in TiN(600) seem to be stronger than that in TiN(500). This is because the former has a thicker outer-layer as compared to the latter (0.9 vs. 0.5 μm). Coating composition of each TiN film with and without annealing analyzed by the EPMA method is listed in Table 1. The data are further compared in Fig. 3. For the unannealed and TiN(400) specimens, their TiN coating composition nearly had the same amount in Ti (51.4 vs. 51.6 at.%) and N (48.6 vs. 48.4 at.%) elements. The result indicated that TiN film showed the stable oxidation resistance under the annealing temperature of 400 °C. When the coated specimen was annealed at 500 °C, the content of oxygen in the film was slightly higher than that of nitrogen (33.1 vs. 29.7 at.%). To further raise the annealing temperature up to 600 °C, the oxygen content in the film greatly increased (53.6 at.%) while both the nitrogen content (12.2 at.%) and the titanium content (34.2 at.%) decreased substantially. The result denoted that some of the nitrogen atoms were substituted by oxygen atoms to form TiO2 in Ti–N–O outer-layer. That is, the affinity of titanium and oxygen is stronger than that of titanium and nitrogen [14]. In addition, we also found that thickness and composition of the
outer-layer seemed to be affected by the annealing temperature, in which the annealing temperature of 500 °C could obtain a thinner but more uniform outer-layer in this study.
3.2. Analysis of coating properties The coating properties, such as surface roughness, adhesion, hardness (H), elastic modulus (E), and H/E obtained in this study are listed in Table 1. In the surface roughness, the TiN coated specimen had a rougher surface than the uncoated one (Ra: 0.22 vs. 0.05 μm). The main reason for this phenomenon could be attributed to macroparticles deposited on the substrate by CAE process [21]. After annealing, the surface roughness of TiN film had a slight decrease at 500 °C (Ra: 0.20 μm) but increased at 600 °C (Ra: 0.33 μm). The result should be dependent upon atomic distribution in the Ti–N–O outer-layer. That is, the film with the more uniform composition tends to the lower Ra value. Indentation tests were performed by using the Rockwell-C hardness tests to evaluate the coating adhesion between film and substrate. According to the definition of ASQ as aforementioned [15], we found that all the TiN coated specimens had a clear and smooth indentation regardless of annealing, closely resembled HF1–HF2 grades. Therefore, the annealing for TiN film in this study appears to have no influence on the coating adhesion.
Table 1 Coating properties of the un-annealed and annealed TiN films in this study. Specimen
Un-annealed TiN TiN(400) TiN(500) TiN(600)
Atomic concentration (at.%) Ti
N
O
51.4 51.6 37.2 34.2
48.6 48.4 29.7 12.2
– – 33.1 53.6
Surface roughness, Ra (μm)
Adhesion strength, ASQ [15]
Hardness, H (GPa)
Elastic Modulus, E (GPa)
H/E
0.22 0.23 0.20 0.33
HF1 HF1 HF1 HF2
18.7 18.5 22.0 19.1
186.0 187.1 201.5 198.0
0.101 0.099 0.109 0.096
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C.H. Hsu et al. / Surface & Coatings Technology xxx (2014) xxx–xxx
20
100
Ti N O
80
18
16.6
16
Wear rate (X10-6g/m)
Atomic concentration (%)
90
70 60 50 40 30
14 12 10 8
6.6
6.2
6
20
4
10
2
0
12.6
3.8
0
TiN
400
500
600
AISI 316L
Annealing temperature (°C)
TiN
TiN(400)
TiN(500)
TiN(600)
Specimen
Fig. 3. Effect of annealing temperature on the averaged concentration of the Ti, N, and O elements in the coating.
Fig. 5. Comparison of wear rate among all the specimens after wear tests.
The values of hardness, elastic modulus, and H/E ratio measured by the nanoindenter with an indentation depth of approximately 100 nm are also listed in Table 1. The set shallow depth measured in the tests implies that the reported values predominately arise from the coating properties without the substrate effect. The result showed that the annealing temperature governed the coating hardness, elastic modulus, and H/E values. When the TiN-coated specimens were annealed at the temperature range of 500–600 °C, the coatings in H and E values had an increase due to the formation of Ti–N–O outer-layer. In particular, the TiN(500) specimen revealed the highest H (22.0 GPa), E (201.5 GPa), and H/E (0.109) values among all the annealed coatings. It can be attributed to the uniform compositional concentration obtained in its Ti–N–O outer-layer as aforementioned.
the lowest friction coefficient of about 0.25 among all the specimens. In wear rate, a comparison for all the specimens after wear tests is shown in Fig. 5. The result revealed that the AISI 316L stainless steel coated with TiN film had a decrease in the wear rate from 16.6 × 10−6 to 6.6 × 10−6 g/m. Furthermore, the wear rate of the coated specimens after annealing, ranking from the fastest to the slowest, was as follow: TiN(600) N TiN(400) N TiN(500). It is interesting to find that the TiN(500) specimen has the highest H/E value (0.109) to claim the lowest wear rate (3.8 × 10−6 g/m) among all the specimens. This result coincides with the concepts described by Leyland and Matthews [22], who reported that a high ratio of H/E could be indicative of good wear resistance in a disparate range of materials. Besides, the TiN(500) specimen has the well coating adhesion as aforementioned. Accordingly, we can presume that the wear-life of protective coatings mainly depends upon the combined functions of the friction coefficient, hardness, elastic modulus, and film adhesion. Fig. 6 further shows the SEM damage patterns of the specimens after a wear test of 500 m. A typical morphology of adhesive wear with wear debris was clearly seen in the uncoated specimen (Fig. 6(a)). This is because the substrate has a relative lower hardness as compared to counterface material (195 vs. 1780 HV). In adhesive wear mechanism, the two solids experienced relative motion during wear testing, with a bonding adhesion between contacting surfaces eventually resulting in the fracturing of the softer material [23]. The track of wear became abrasion wear accompanied by a few scratches arising from the increase of surface hardness, and a partial substrate was exposed in unannealed TiN and TiN(400) specimens (Fig. 6(b) and (c)). By contrast, there was only a few TiN area without any substrate exposed on the worn surface of TiN(500) (Fig. 6(d)). The wear phenomenon showed a typical abrasion wear behavior. This is because the Ti–N–O outer-layer in TiN(500) shows high H/E value and well adhesion. Though there was also a Ti–N–O outer-layer formed on TiN(600) after annealing, the H/E value of TiN(600) was lower than that of TiN(500), resulting in a large area of Ti–N–O film scraped to expose underlying TiN, and even substrate in TiN(600) (Fig. 6(e)).
3.3. Wear behavior In this study, a CSM tribometer was used to evaluate the wear behavior of AISI 316L stainless steel with and without annealed TiN coating. The relationship curves between the friction coefficient and the abrasion distance were plotted in Fig. 4. The result showed that the TiN-coated specimen had a lower friction coefficient than the uncoated one (0.49 vs. 0.67). After annealed at the temperature of 500–600 °C, the friction coefficient of TiN-coated specimens could be decreased to 0.25–0.37. In particular, the TiN(500) specimen displayed 1.0
0.8
Friction coefficient
AISI 316L
0.6
TiN(400)
TiN
TiN(600)
0.4
4. Conclusions
0.2
TiN(500)
0.0 0
100
200
300
400
500
Travel distance(m) Fig. 4. The curves of friction coefficient of all the specimens at a load of 5 N, sliding against a 6 mm WC + 6%Co ball counterface.
The TiN arc-coated 316L stainless steel after annealing at proper temperature could form a Ti–N–O outer-layer, containing Rutile-TiO2 and TiN phases. The outer-layer in thickness had an increase with raising the annealing temperature. Moreover, the properties of the outer-layer such as composition, hardness, elastic modulus, and H/E were also dependent upon the annealing temperature. When the annealing temperature was controlled at 500 °C, the formed outer-
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C.H. Hsu et al. / Surface & Coatings Technology xxx (2014) xxx–xxx
5
a
Wear debris
b
c Substrate
TiN
d
e Ti-N-O TiN TiN
Fig. 6. Surface morphology of the specimens after ball-on-disk wear tests: (a) AISI 316L, (b) TiN, (c) TiN(400), (d) TiN(500), and (e) TiN(600).
layer had the most uniform composition, highest hardness, elastic modulus, and H/E values among all the annealing conditions. Such an annealed film remarkably reduced the friction coefficient of 316L stainless steel from 0.68 to 0.25, along with a low wear rate (3.8 × 10−6 g/m) to improve wear resistance. Acknowledgments The authors would like to thank the financial support of the National Science Council (Taiwan, ROC) under contract no. NSC 101-2221-E-036012. References [1] X.H. Chen, J. Lu, L. Lu, K. Lu, Scr. Mater. 52 (2005) 1039. [2] E.D.L. Heras, D.A. Egidi, P. Corengia, D. Gonzalez-Santamaria, et al., Surf. Coat. Technol. 202 (2008) 2945. [3] B.S. Suh, W.J. Lee, Thin Solid Films 295 (1997) 185. [4] C. Tassin, F. Laroudie, M. Pons, L. Lelait, Surf. Coat. Technol. 80 (1996) 207. [5] E. Yun, S. Lee, Surf. Coat. Technol. 200 (2006) 3478. [6] I. Boromei, L. Ceschini, A. Marconi, C. Martini, Wear 302 (2013) 899.
[7] J.A. Berrios, D.G. Teer, E.S. Puchi-Cabrera, Surf. Coat. Technol. 148 (2001) 179. [8] G. Aldrich-Smith, D.G. teer, P.A. Dearnley, Surf. Coat. Technol. 116–119 (2001) 1161. [9] J.A. Berrios-Ortiz, J.G. La Barbera-Sosa, D.G. Teer, E.S. Puchi-Cabrera, Surf. Coat. Technol. 179 (2004) 145. [10] B.Y. Man, L. Guzman, A. Miotello, M. Adami, Surf. Coat. Technol. 180–181 (2004) 9. [11] J.P. Bucker, K.P. Ackermann, F.W. Buschor, Thin Solid Films 122 (1984) 63. [12] F. Vaz, P. Cerqueira, L. Rebouta, S.M.C. Nascimento, E. Alves, Ph. Goudeau, J.P. Riviere, K. Pischow, J. de Rijk, Thin Solid Films 447–448 (2004) 449. [13] A. Rizzo, M.A. Signore, L. Mirenghi, T.D. Luccio, Thin Solid Films 517 (2009) 5956. [14] C.H. Hsu, K.H. Huang, Y.H. Lin, Wear 306 (2013) 97. [15] W. Heinke, A. Leyland, A. Matthews, G. Berg, C. Friedrich, E. Broszeit, Thin Solid Films 270 (1995) 431. [16] W.C. Oliver, C.J. McHargue, Thin Solid Films 161 (1988) 117. [17] A. Qzturk, K.V. Ezirmik, K. Kazmanli, M. Urgen, O.L. Eryilmaz, A. Erdemir, Tribol. Int. 41 (2008) 49. [18] J.J. Kang, C.B. Wang, H.D. Wang, B.S. Xu, J.J. Liu, G.L. Li, Surf. Coat. Technol. 203 (2009) 1927. [19] S. Paldey, S.C. Deevi, Mater. Sci. Eng. A 342 (2003) 58. [20] E.M. Levin, H.F. McMurdie, Phase Diagrams for Ceramists, The American Ceramic Society, Inc., 1975. 76. [21] C.H. Hsu, C.C. Lee, W.Y. Ho, Thin Solid Films 516 (2008) 4826. [22] A. Leyland, A. Matthews, Wear 246 (2000) 1. [23] K.G. Budinski, Surface Engineering for Wear Resistance, Prentice Hall, New Jersey, 1988. 17.
Please cite this article as: C.H. Hsu, et al., Surf. Coat. Technol. (2014), http://dx.doi.org/10.1016/j.surfcoat.2014.02.001