- Email: [email protected]
Improving high-temperature tribological characteristics on nanocomposite CrAlSiN coating by Mo doping
Surface & Coatings Technology 349 (2018) 752–756
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Improving high-temperature tribological characteristics on nanocomposite CrAlSiN coating by Mo doping Heng Taoa, M.T. Tsaib, Hsien-Wei Chenc, J.C. Huangc, Jenq-Gong Duha,
T
⁎
a
Department of Materials Science and Engineering, National Tsing-Hua University, Hsinchu, Taiwan Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung, Taiwan c Institute for Advanced Study, Department of Materials Science and Engineering, City University of Hong Kong, Kowloon, Hong Kong b
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanocomposite CrAlSiN Molybdenum Tribological characteristic High temperature Mechanical property
Recently, nanocomposite coatings have been widely used for protective coatings at high temperature owing to the superior mechanical strength over a broad operating temperature. However, there is still room for improvement on lubricating behavior of the coatings. Molybdenum is a promising candidate to further enhance the lubricating properties because of the layer crystal feature oxide. Therefore, this study attempts to investigate the influence of Mo assisting in mechanical and tribological properties. The Mo doped CrAlSiN coatings were deposited onto Inconel alloy 718 by radio frequency reactive magnetron co-sputtering. The tribological properties of CrAlSiN and CrAlMoSiN coatings were evaluated at 600 °C by ball-on-disc wear test. The reduced friction coefficient of coatings with increasing Mo content was revealed. The improvement of wear resistance correlated with incorporating the formation of oxide into high-temperature tribological motion. Additionally, the parameter of the H3/E*2 regarded as the indicator of plastic deformation resistance was utilized to estimate the antiwear property. The wear rate of CrAlMoxSiN coatings was roughly inversely related to the H3/E*2 ratio and the lowest wear rate existed in the coating with Mo of 14.5 at.% contents. It was demonstrated that anti-wear and lubricating capability of the coatings at elevated temperature could be improved by doping Mo. The merit could be used to provide strong probability for designing advanced high-temperature coatings via adjusting both of mechanical strengthening and oxide modifying by beneficial elements tuning in connection with properties of interest.
1. Introduction Friction, the resistance to motion, has been a huge challenge for mankind throughout history. For instance, in passenger cars, one-third of the fuel energy is used to overcome friction in the engine, transmission, tires, and brakes [1]. The turbine, lift rods face challenging needs of surface protection similarly. The rods work in a frictional contact mode against a stack of seal rings at elevated temperature (540 °C) [2]. Therefore, how to reduce friction loss and to prolong wear life at high temperature by a protective coating become a critical issue of interest for engineering applications. The metastable face-centered cubic structure of CrAlN hard coatings with high hardness [3,4], outstanding thermal stability [5] and oxidation resistance [6,7] have been widely used for protective coatings over the past two decades. To further enhance the mechanical and anti-wear properties, the silicon incorporated CrAlSiN nanocomposite coating was proposed [8]. CrAlSiN nanocomposite coatings with stronger mechanical properties as compared with CrAlN coatings were confirmed owing
⁎
to grain refinement and denser structure by silicon doping [9,10]. However, the main drawback of the coating is limited by a higher friction coefficient and poor wear rate at elevated temperature than that at room temperature due to the oxidation reaction of Si which is unfavorable to lubricating [11,12]. The vitiating factors make the thin film worn out in a short time and also cause lots of energy consumption during the sliding. For improving the tribological performance, the transition metal oxide Magnéli phase with layered crystal structure provides an alternative way, including Ti, V, Pb, Mo, W, and Zn [13]. For molybdenum oxide, the MoO3 phase slips more easily and provides a lubricating interface owing to the attenuated Van der Waals force between each layer crystal [14]. It is expected that high temperature anti-wear application would be modified and improved by Mo doping. The objective of this study is to deposit CrAlMoxSiN coatings with various Mo concentrations on Inconel alloy 718 for better anti-wear capability at 600 °C. Inconel alloy 718 with high thermal stability and mechanical strength at elevated temperature is commonly used for components which have to be operated at elevated temperature wear
Corresponding author at: R405, Dep. Mater. Sci. & Eng., No. 101, Section 2, Kuang-Fu Road, Hsinchu City 30013, Taiwan E-mail address: [email protected] (J.-G. Duh).
https://doi.org/10.1016/j.surfcoat.2018.03.086 Received 8 December 2017; Received in revised form 15 March 2018; Accepted 23 March 2018
Available online 28 March 2018 0257-8972/ © 2018 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 349 (2018) 752–756
H. Tao et al.
condition [15]. In this study, the mechanical, tribological and chemical characteristics were evaluated and discussed. The surface morphology, depth profile, and components in wear track were also observed. The high-temperature tribological behavior and correlated the coating characteristics as critical factors were addressed for the anti-wear performance.
Table 1 The elemental composition of CrAlMoxSiN coatings. Composition (at.%)
CASN CAMSN1 CAMSN2 CAMSN3
2. Material and methods The CrAlMoxSiN coatings were deposited on Inconel alloy 718 and silicon (100) substrate by radio frequency reactive magnetron cosputtering with three separated targets, including Cr0.4Al0.6 alloy, Mo (99.99%), and Si (99.99%) with 50 mm in diameter. In the beginning, the substrates were fixed on the rotational holder around 25 rpm. The chamber was evacuated down to 4 × 10−6 Torr and heated to 400 °C within 160 min. Before the reactive sputtering, CrAlN interlayer of 100 nm was deposited under N2 and Ar flux of 20 and 10 sccm. The CrAlMoxSiN coatings with thickness of 1.2 μm were fabricated under 4 × 10−2 Torr and 50 V bias for 90 min. In order to fix the composition of multi-component except for Mo, the input power of CrAl target was fixed at 200 W and Si target was adjusted with Mo target varied from 0 to 120 W. The quantitative elemental composition of CrAlMoxSiN coatings with various Mo concentrations was verified by field-emission electron probe micro-analyzer (JXA-8500F, JEOL, Japan). The hardness (H) and reduced elastic modulus (E*) were measured by nanoindentation (Nano Hardness Tester, Anton Paar, Austria) with a maximum loading of 3mN and holding segment of 5 s. Besides, the maximum indentation depth was controlled below one-tenth of the coating thickness to prevent the substrate effect. The tribological behaviors of coatings were evaluated by ball-ondisc wear apparatus (High Temperature Tribometer, Anton Paar, Austria) with Al2O3 ball of 6 mm in diameter. The wear test was conducted under the condition of 3.2 cm/s liner speed and a loading fixed at 3N for sliding length of 100 m at 600 °C. For each sample, the depth and the width of wear track, t and b, were determined by surface profilometer (ET3000, Kosaka Surfcorder, Japan). Then, the wear volume could be calculated from Eq. (1) [16],
WS =
t (3t2 + 4b2)2π∙r 6b
WS FN ∙S
Al
Mo
Si
N
19.3 17.9 15.8 14.5
25.8 24.8 22.0 19.3
– 3.9 10.0 14.5
7.8 7.6 7.8 8.2
47.1 45.8 44.4 43.5
Fig. 1. The hardness and the H3/E*2 ratio of CrAlMoxSiN coatings.
of maintaining the superior mechanical strength, the Si content would be fixed at around 8 at.% in CrAlMoxSiN coating. The composition of Cr/Al ratio in the coatings was retained at 1.5, which is same as that of the target. Fig. 1 shows the hardness and the H3/E*2 ratio of CrAlSiN and CrAlMoxSiN coatings via averaged ten testing points by nanoindentation in each coating. The average hardness of all coatings was above 29 GPa. With the addition of Mo contents up to 14.5 at.%, the hardness still maintained at around 31 GPa. It is demonstrated that the Mo doping show no adverse effect on mechanical strength and the softer crystalline MoN phase would not pack a lot into the coatings [19,20]. The slight difference in hardness values between CrAlSiN and CrAlMoxSiN coatings suggested that the mechanical characteristic of asdeposited coating was not reduced apparently with further addition of Mo up to 14.5 at.%. As a rule of thumb, the dominating factor of anti-wear capability is the H3/E*2 ratio rather than hardness only. Actually, the E* value has a strong influence on the durability of coating. The external force on the coating could be dissipated over a wider elastic strained and further prevent the larger wear volume with a lower reduced elastic modulus of the coating [10]. In addition, anti-wear capability is also regarded as a type of resistance to plastic deformation. Tsui and Pharr purposed that the yield pressure (Py) on elastic/plastic plate contact as determined by the Eq. (3) could be used as an indicator of plastic deformation resistance [21],
(1)
where the r is the radius of the wear track. The wear rate, WR, was calculated using Eq. (2):
WR =
Cr
(2)
where the S is the sliding distance and the FN is the nominal load. The morphology and chemical bonding characterizations of the wear tracks were analyzed by scanning electron microscopy (JSM-7600F, JEOL, Japan) and electron spectroscopy for chemical analysis (ESCA, PHI 1600, PHI, USA) with an Mg Kα radiation source. 3. Results and discussion 3.1. Elemental composition and mechanical characteristics of as-deposited coatings
Py = 0.78r 2
The elemental compositions of CrAlSiN and CrAlMoxSiN coatings with different Mo contents are listed in Table 1. With the increasing input power of Mo target, the Mo content increased from 0 to 14.5 at.%. Moreover, another critical issue is the concentration of Si which would severely affect the degree of crystallinity of the coating. In literature, the influence of Si contents on mechanical property of CrAlSiN nanocomposite coating has been revealed [17,18]. The hardness of coatings would yield a peak with Si contents around 9 at.% owing to the appropriate amount of amorphous silicon nitride phase. For the purpose
where the r is the radius of contact sphere, and E* = E/(1 − v2). Therefore, the H3/E*2 ratio in the formula of the Py plays an important role in the tribological behavior. According to Fig. 1, the hardness of CASN to CAMSN3 coating were 31.9, 30.7, 29.0 and 31.2, respectively. The tendency in the H3/E*2 ratio is in agreement with that of hardness. That is, the mechanical characteristics of CrAlMoxSiN coatings were nearly close to CrAlSiN coating and the coatings showed superior mechanical strength simultaneously. 753
H3 E∗2
(3)
Surface & Coatings Technology 349 (2018) 752–756
H. Tao et al.
friction coefficient. Therefore, if a coating exhibits a lower friction coefficient, both the friction force and total force are smaller. Namely, the less destructive force onto the coating, the less wear volume and wear rate could be revealed in the coating with lower friction coefficient. Accordingly, CAMSN3 coating with the lowest wear rate could be partly attributed to the lowest friction coefficient, which made the less destructive force onto the coating smaller. Although the anti-wear capability does not completely go hand in hand with friction coefficient, there is still something correlated with each other. It could be concluded that the H3/E*2 ratio and friction coefficient mutually determine the wear resistance of coating. 3.3. Anti-wear mechanism of CrAlMoxSiN coatings After the wearing process, the profile and topography of the wear track are worthy to be explored. According to Fig. 4, the 2D wear track profilometric curves of CrAlMoxSiN coatings are contoured by the surface profilometer. The wear depth of CASN and CAMSN3 is less than 1 μm and the wear volume of CAMSN2 is the largest among all coatings, revealing consistency with the higher wear rate. During the wearing test, some debris would form between ball and coating, and some of them would aggregate beside the wear track. Especially in CAMSN1, the debris assembling beside or even down to the wear track result in the extremely narrow wear width. The wear mechanism is associated with the contact type of interface and the intrinsic characteristic of the wearing matter. It could be observed by the wear track morphology of CrAlMoxSiN coatings in Fig. 5. For CASN and CAMSN1 coatings, some ribbon like ploughing took place in sliding, forming long wear micro-scars which is a typical case of severely abrasive wear. Additionally, there is a distinctly different topography in CAMSN2 and CAMSN3 coatings. For adhesive wear, the contact interface between two surfaces under plastic contact has enough adhesive bonding strength to resist relative sliding, and large plastic deformation caused by dislocation is introduced [22]. Some flake-like wear particles in CAMSN2 and CAMSN3 coatings imply the adhesive wear. The tribological behavior gradually transfers from abrasive wear to adhesive wear with the addition of Mo. The oxide formation on the worn surface which could act as a lubricating layer was found to significantly affect the friction characteristic of the coating. By identifying the binding energy with ESCA analysis for each coating, the semi-quantitative distribution of the oxides below the track surface could be verified. The peaks of chromium oxide, aluminum oxide, molybdenum oxide and silicon oxide with different signal strength had been identified as shown in Fig. 6. After calculating the integral area of the peaks, the content distribution of oxides is listed in Table 2. As mentioned before, the transition metal oxide Magnéli phases are traditionally described as crystallographic shear structures and some of the phases feature a layered crystal structure based on deformed metal-oxide octahedral [13]. The more molybdenum oxide on the track surface implied that the stronger lubricating behavior for the coating could be showed up. In CrAlMoxSiN coatings, the amount of molybdenum oxide on the worn surface increased from 0 to 21% with rising Mo contents. As molybdenum oxide reached a certain ratio over all the oxides, it could decrease the friction coefficient significantly. Additionally, the anti-oxidation capability at high temperature is another critical issue for protective coatings. The coating could be prevented from the further oxidation due to the formation of chromium oxide and aluminum oxide, which is commonly regarded as a protective layer for anti-oxidation application. In summary, a stable and adequate amount of oxide on the coating surface provides not only a self-lubricating characteristic but also a feasible resistance to oxidation.
Fig. 2. The friction coefficient of CrAlMoxSiN coatings at 600 °C.
3.2. Tribological behaviors at evaluated temperature The friction coefficient of CrAlMoxSiN coatings with different Mo content against Al2O3 ball at 600 °C are shown in Fig. 2. The friction coefficients of CrAlMoxSiN coatings reduce with increasing Mo contents. The CrAlSiN coating without Mo exhibited the highest friction coefficient around 0.6 among all coatings. After doping Mo in the coating, the values of friction coefficient almost remained constant in CAMSN1 with 3.9 at.% Mo and CAMSN2 with 10.0 at.% Mo. With Mo doping contents up to 14.5 at.%, the self-lubricating characteristic showed up noticeably in CAMSN3 and the friction coefficient dropped to the minimum of 0.52. The self-lubricating behavior could appear while the lubricating Mo reached a certain amount. Fig. 3 presents the wear rate of the coatings at 600 °C. CAMSN3 coating showed the lowest wear rate around 2.1 × 10−6 mm3 N−1 m−1 among all coatings and the difference between the other three coatings was slight. As mentioned before, the wear rate tendency of CASN, CAMSN1 and CAMSN2 was inversely related to the H3/E*2 ratio, implying that the H3/E*2 ratio could be an indicator of anti-wear capability. However, there was an exception on CAMSN3 with the lowest wear rate. It could be speculated that the resistance of plastic deformation is not the only factor to influence the wear resistance in the wearing process. During the ball-on-disc wearing test, there are two kinds of force onto the coating, including normal force and friction force. The former is perpendicular to the coating surface and the latter is parallel to that. From the formula of friction force (Ff = μ × FN), the normal force is fixed at a constant and the friction force reduces with decreasing
4. Conclusion In this study, CrAlMoxSiN coatings with different Mo contents were fabricated by radio frequency reactive magnetron co-sputtering. The
Fig. 3. The wear rate of CrAlMoxSiN coatings at 600 °C. 754
Surface & Coatings Technology 349 (2018) 752–756
H. Tao et al.
Fig. 4. Two-dimensional profilometric curves of the wear tracks: (a) CASN, (b) CAMSN1, (c) CAMSN2, and (d) CAMSN3.
hardness of the coating only slightly reduced by 2 to 3 GPa and the H3/ E*2 ratio also maintained the same level in CrAlSiN and CrAlMoxSiN coatings. Both CrAlSiN and CrAlMoSiN system as a nanocomposite coating revealed superior mechanical properties. For high-temperature tribological testing, all coatings showed an impressive performance of wear rate which was below 8 × 10−6 mm3 N−1 m−1, and CAMSN3 had the lowest average wear rate of around 2 × 10−6 mm3 N−1 m−1. The friction coefficient also depended on the Mo contents, and CAMSN3
presented the lowest one of 0.52. This indicates enhanced lubrication as the Mo contents attain a certain level. In conclusion, there are two key points for this study. One is that the mechanical strength of CrAlMoxSiN coating is still retained at a high level, and the other is that the tribological behavior of CrAlSiN nanocomposite coating could be significantly improved at elevated temperature by Mo doping. This suggests that high-temperature performance of nanocomposite coatings could be further improved via composition tuning by doping of
Fig. 5. Wear track morphology of (a) CASN, (b) CAMSN1, (c) CAMSN2, and (d) CAMSN3. 755
Surface & Coatings Technology 349 (2018) 752–756
H. Tao et al.
Fig. 6. ESCA analysis for O 2 s at the wear track of (a) CASN, (b) CAMSN1, (c) CAMSN2, and (d) CAMSN3. Table 2 Content distribution of oxides on the wear track.
[2] W. Wang, Surf. Coat. Technol. 177–178 (2004) 12–17. [3] Q. Wang, F. Zhou, J. Yan, Surf. Coat. Technol. 285 (2016) 203–213. [4] C. Rebholz, H. Ziegele, A. Leyland, A. Matthews, Surf. Coat. Technol. 115 (1999) 222–229. [5] H. Willmann, P.H. Mayrhofer, A.E. Reiter, L. Hultman, C. Mitterer, Scr. Mater. 54 (2006) 1847–1851. [6] J.L. Endrino, G.S. Fox-Rabinovich, A. Reiter, S.V. Veldhuis, R. Escobar Galindo, J.M. Albella, J.F. Marco, Surf. Coat. Technol. 201 (2007) 4505–4511. [7] Y.Y. Chang, W.T. Chiu, J.P. Huang, Surf. Coat. Technol. 303 (2016) 18–24. [8] C.B. Liu, W. Pei, F. Huang, L. Chen, Vacuum 125 (2016) 180–184. [9] S. Zhang, L. Wang, Q. Wang, M. Li, Surf. Coat. Technol. 214 (2013) 160–167. [10] C.C. Chang, H.W. Chen, J.W. Lee, J.G. Duh, Thin Solid Films 584 (2015) 46–51. [11] T. Polcar, T. Vitu, J. Sondor, A. Cavaleiro, Plasma Process. Polym. 6 (2009) S935–S940. [12] M. Oliveira, S. Agathopoulos, J.M.F. Ferreira, J. Eur. Ceram. Soc. 25 (2005) 19–28. [13] A.A. Voevodin, C. Muratore, S.M. Aouadi, Surf. Coat. Technol. 257 (2014) 247–265. [14] Y. Wang, J.W. Lee, J.G. Duh, Surf. Coat. Technol. 303 (2016) 12–17. [15] C.J. Wang, S.M. Chen, Surf. Coat. Technol. 201 (2006) 3862–3866. [16] S. Ma, J. Procha'zka, P. Karva'nkova', Q. Ma, X. Niu, X. Wang, D. Ma, K. Xu, S. Veprˇek, Surf. Coat. Technol. 194 (2005) 143–148. [17] C.C. Chang, H.W. Chen, J.W. Lee, J.G. Duh, Surf. Coat. Technol. 284 (2015) 273–280. [18] H.W. Chen, Y.C. Chan, J.W. Lee, J.G. Duh, Surf. Coat. Technol. 205 (2010) 1189–1194. [19] P. Hones, N. Martin, M. Regula, F. L'evy, J. Phys. D. Appl. Phys. 36 (2003) 1023–1029. [20] J.Y. Xiang, F.B. Wu, Surf. Coat. Technol. 332 (2017) 161–167. [21] A. Leyland, A. Matthews, Wear 246 (2000) 1–11. [22] B. Bhushan, Modern Tribology Handbook, Two Volume Set, USA, 2000.
Content distribution of oxides (wt%)
CASN CAMSN1 CAMSN2 CAMSN3
Cr2O3
Al2O3
MoO3
SiO2
50 48 50 45
31 28 26 28
– 11 16 21
19 13 8 6
adequate beneficial elements. Acknowledgment This study was financially supported by the Ministry of Science and Technology, Taiwan under project MOST-105-2221-E-007-027-MY3. The author also thanks for EPMA analysis by Precision Instrument Center at National Tsing-Hua University and nano hardness test at Ming-Chi University of Technology. References [1] K. Holmberg, P. Andersson, A. Erdemir, Tribol. Int. 47 (2012) 221–234.
756
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Improving high-temperature tribological characteristics on nanocomposite CrAlSiN coating by Mo doping Heng Taoa, M.T. Tsaib, Hsien-Wei Chenc, J.C. Huangc, Jenq-Gong Duha,
T
⁎
a
Department of Materials Science and Engineering, National Tsing-Hua University, Hsinchu, Taiwan Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung, Taiwan c Institute for Advanced Study, Department of Materials Science and Engineering, City University of Hong Kong, Kowloon, Hong Kong b
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanocomposite CrAlSiN Molybdenum Tribological characteristic High temperature Mechanical property
Recently, nanocomposite coatings have been widely used for protective coatings at high temperature owing to the superior mechanical strength over a broad operating temperature. However, there is still room for improvement on lubricating behavior of the coatings. Molybdenum is a promising candidate to further enhance the lubricating properties because of the layer crystal feature oxide. Therefore, this study attempts to investigate the influence of Mo assisting in mechanical and tribological properties. The Mo doped CrAlSiN coatings were deposited onto Inconel alloy 718 by radio frequency reactive magnetron co-sputtering. The tribological properties of CrAlSiN and CrAlMoSiN coatings were evaluated at 600 °C by ball-on-disc wear test. The reduced friction coefficient of coatings with increasing Mo content was revealed. The improvement of wear resistance correlated with incorporating the formation of oxide into high-temperature tribological motion. Additionally, the parameter of the H3/E*2 regarded as the indicator of plastic deformation resistance was utilized to estimate the antiwear property. The wear rate of CrAlMoxSiN coatings was roughly inversely related to the H3/E*2 ratio and the lowest wear rate existed in the coating with Mo of 14.5 at.% contents. It was demonstrated that anti-wear and lubricating capability of the coatings at elevated temperature could be improved by doping Mo. The merit could be used to provide strong probability for designing advanced high-temperature coatings via adjusting both of mechanical strengthening and oxide modifying by beneficial elements tuning in connection with properties of interest.
1. Introduction Friction, the resistance to motion, has been a huge challenge for mankind throughout history. For instance, in passenger cars, one-third of the fuel energy is used to overcome friction in the engine, transmission, tires, and brakes [1]. The turbine, lift rods face challenging needs of surface protection similarly. The rods work in a frictional contact mode against a stack of seal rings at elevated temperature (540 °C) [2]. Therefore, how to reduce friction loss and to prolong wear life at high temperature by a protective coating become a critical issue of interest for engineering applications. The metastable face-centered cubic structure of CrAlN hard coatings with high hardness [3,4], outstanding thermal stability [5] and oxidation resistance [6,7] have been widely used for protective coatings over the past two decades. To further enhance the mechanical and anti-wear properties, the silicon incorporated CrAlSiN nanocomposite coating was proposed [8]. CrAlSiN nanocomposite coatings with stronger mechanical properties as compared with CrAlN coatings were confirmed owing
⁎
to grain refinement and denser structure by silicon doping [9,10]. However, the main drawback of the coating is limited by a higher friction coefficient and poor wear rate at elevated temperature than that at room temperature due to the oxidation reaction of Si which is unfavorable to lubricating [11,12]. The vitiating factors make the thin film worn out in a short time and also cause lots of energy consumption during the sliding. For improving the tribological performance, the transition metal oxide Magnéli phase with layered crystal structure provides an alternative way, including Ti, V, Pb, Mo, W, and Zn [13]. For molybdenum oxide, the MoO3 phase slips more easily and provides a lubricating interface owing to the attenuated Van der Waals force between each layer crystal [14]. It is expected that high temperature anti-wear application would be modified and improved by Mo doping. The objective of this study is to deposit CrAlMoxSiN coatings with various Mo concentrations on Inconel alloy 718 for better anti-wear capability at 600 °C. Inconel alloy 718 with high thermal stability and mechanical strength at elevated temperature is commonly used for components which have to be operated at elevated temperature wear
Corresponding author at: R405, Dep. Mater. Sci. & Eng., No. 101, Section 2, Kuang-Fu Road, Hsinchu City 30013, Taiwan E-mail address: [email protected] (J.-G. Duh).
https://doi.org/10.1016/j.surfcoat.2018.03.086 Received 8 December 2017; Received in revised form 15 March 2018; Accepted 23 March 2018
Available online 28 March 2018 0257-8972/ © 2018 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 349 (2018) 752–756
H. Tao et al.
condition [15]. In this study, the mechanical, tribological and chemical characteristics were evaluated and discussed. The surface morphology, depth profile, and components in wear track were also observed. The high-temperature tribological behavior and correlated the coating characteristics as critical factors were addressed for the anti-wear performance.
Table 1 The elemental composition of CrAlMoxSiN coatings. Composition (at.%)
CASN CAMSN1 CAMSN2 CAMSN3
2. Material and methods The CrAlMoxSiN coatings were deposited on Inconel alloy 718 and silicon (100) substrate by radio frequency reactive magnetron cosputtering with three separated targets, including Cr0.4Al0.6 alloy, Mo (99.99%), and Si (99.99%) with 50 mm in diameter. In the beginning, the substrates were fixed on the rotational holder around 25 rpm. The chamber was evacuated down to 4 × 10−6 Torr and heated to 400 °C within 160 min. Before the reactive sputtering, CrAlN interlayer of 100 nm was deposited under N2 and Ar flux of 20 and 10 sccm. The CrAlMoxSiN coatings with thickness of 1.2 μm were fabricated under 4 × 10−2 Torr and 50 V bias for 90 min. In order to fix the composition of multi-component except for Mo, the input power of CrAl target was fixed at 200 W and Si target was adjusted with Mo target varied from 0 to 120 W. The quantitative elemental composition of CrAlMoxSiN coatings with various Mo concentrations was verified by field-emission electron probe micro-analyzer (JXA-8500F, JEOL, Japan). The hardness (H) and reduced elastic modulus (E*) were measured by nanoindentation (Nano Hardness Tester, Anton Paar, Austria) with a maximum loading of 3mN and holding segment of 5 s. Besides, the maximum indentation depth was controlled below one-tenth of the coating thickness to prevent the substrate effect. The tribological behaviors of coatings were evaluated by ball-ondisc wear apparatus (High Temperature Tribometer, Anton Paar, Austria) with Al2O3 ball of 6 mm in diameter. The wear test was conducted under the condition of 3.2 cm/s liner speed and a loading fixed at 3N for sliding length of 100 m at 600 °C. For each sample, the depth and the width of wear track, t and b, were determined by surface profilometer (ET3000, Kosaka Surfcorder, Japan). Then, the wear volume could be calculated from Eq. (1) [16],
WS =
t (3t2 + 4b2)2π∙r 6b
WS FN ∙S
Al
Mo
Si
N
19.3 17.9 15.8 14.5
25.8 24.8 22.0 19.3
– 3.9 10.0 14.5
7.8 7.6 7.8 8.2
47.1 45.8 44.4 43.5
Fig. 1. The hardness and the H3/E*2 ratio of CrAlMoxSiN coatings.
of maintaining the superior mechanical strength, the Si content would be fixed at around 8 at.% in CrAlMoxSiN coating. The composition of Cr/Al ratio in the coatings was retained at 1.5, which is same as that of the target. Fig. 1 shows the hardness and the H3/E*2 ratio of CrAlSiN and CrAlMoxSiN coatings via averaged ten testing points by nanoindentation in each coating. The average hardness of all coatings was above 29 GPa. With the addition of Mo contents up to 14.5 at.%, the hardness still maintained at around 31 GPa. It is demonstrated that the Mo doping show no adverse effect on mechanical strength and the softer crystalline MoN phase would not pack a lot into the coatings [19,20]. The slight difference in hardness values between CrAlSiN and CrAlMoxSiN coatings suggested that the mechanical characteristic of asdeposited coating was not reduced apparently with further addition of Mo up to 14.5 at.%. As a rule of thumb, the dominating factor of anti-wear capability is the H3/E*2 ratio rather than hardness only. Actually, the E* value has a strong influence on the durability of coating. The external force on the coating could be dissipated over a wider elastic strained and further prevent the larger wear volume with a lower reduced elastic modulus of the coating [10]. In addition, anti-wear capability is also regarded as a type of resistance to plastic deformation. Tsui and Pharr purposed that the yield pressure (Py) on elastic/plastic plate contact as determined by the Eq. (3) could be used as an indicator of plastic deformation resistance [21],
(1)
where the r is the radius of the wear track. The wear rate, WR, was calculated using Eq. (2):
WR =
Cr
(2)
where the S is the sliding distance and the FN is the nominal load. The morphology and chemical bonding characterizations of the wear tracks were analyzed by scanning electron microscopy (JSM-7600F, JEOL, Japan) and electron spectroscopy for chemical analysis (ESCA, PHI 1600, PHI, USA) with an Mg Kα radiation source. 3. Results and discussion 3.1. Elemental composition and mechanical characteristics of as-deposited coatings
Py = 0.78r 2
The elemental compositions of CrAlSiN and CrAlMoxSiN coatings with different Mo contents are listed in Table 1. With the increasing input power of Mo target, the Mo content increased from 0 to 14.5 at.%. Moreover, another critical issue is the concentration of Si which would severely affect the degree of crystallinity of the coating. In literature, the influence of Si contents on mechanical property of CrAlSiN nanocomposite coating has been revealed [17,18]. The hardness of coatings would yield a peak with Si contents around 9 at.% owing to the appropriate amount of amorphous silicon nitride phase. For the purpose
where the r is the radius of contact sphere, and E* = E/(1 − v2). Therefore, the H3/E*2 ratio in the formula of the Py plays an important role in the tribological behavior. According to Fig. 1, the hardness of CASN to CAMSN3 coating were 31.9, 30.7, 29.0 and 31.2, respectively. The tendency in the H3/E*2 ratio is in agreement with that of hardness. That is, the mechanical characteristics of CrAlMoxSiN coatings were nearly close to CrAlSiN coating and the coatings showed superior mechanical strength simultaneously. 753
H3 E∗2
(3)
Surface & Coatings Technology 349 (2018) 752–756
H. Tao et al.
friction coefficient. Therefore, if a coating exhibits a lower friction coefficient, both the friction force and total force are smaller. Namely, the less destructive force onto the coating, the less wear volume and wear rate could be revealed in the coating with lower friction coefficient. Accordingly, CAMSN3 coating with the lowest wear rate could be partly attributed to the lowest friction coefficient, which made the less destructive force onto the coating smaller. Although the anti-wear capability does not completely go hand in hand with friction coefficient, there is still something correlated with each other. It could be concluded that the H3/E*2 ratio and friction coefficient mutually determine the wear resistance of coating. 3.3. Anti-wear mechanism of CrAlMoxSiN coatings After the wearing process, the profile and topography of the wear track are worthy to be explored. According to Fig. 4, the 2D wear track profilometric curves of CrAlMoxSiN coatings are contoured by the surface profilometer. The wear depth of CASN and CAMSN3 is less than 1 μm and the wear volume of CAMSN2 is the largest among all coatings, revealing consistency with the higher wear rate. During the wearing test, some debris would form between ball and coating, and some of them would aggregate beside the wear track. Especially in CAMSN1, the debris assembling beside or even down to the wear track result in the extremely narrow wear width. The wear mechanism is associated with the contact type of interface and the intrinsic characteristic of the wearing matter. It could be observed by the wear track morphology of CrAlMoxSiN coatings in Fig. 5. For CASN and CAMSN1 coatings, some ribbon like ploughing took place in sliding, forming long wear micro-scars which is a typical case of severely abrasive wear. Additionally, there is a distinctly different topography in CAMSN2 and CAMSN3 coatings. For adhesive wear, the contact interface between two surfaces under plastic contact has enough adhesive bonding strength to resist relative sliding, and large plastic deformation caused by dislocation is introduced [22]. Some flake-like wear particles in CAMSN2 and CAMSN3 coatings imply the adhesive wear. The tribological behavior gradually transfers from abrasive wear to adhesive wear with the addition of Mo. The oxide formation on the worn surface which could act as a lubricating layer was found to significantly affect the friction characteristic of the coating. By identifying the binding energy with ESCA analysis for each coating, the semi-quantitative distribution of the oxides below the track surface could be verified. The peaks of chromium oxide, aluminum oxide, molybdenum oxide and silicon oxide with different signal strength had been identified as shown in Fig. 6. After calculating the integral area of the peaks, the content distribution of oxides is listed in Table 2. As mentioned before, the transition metal oxide Magnéli phases are traditionally described as crystallographic shear structures and some of the phases feature a layered crystal structure based on deformed metal-oxide octahedral [13]. The more molybdenum oxide on the track surface implied that the stronger lubricating behavior for the coating could be showed up. In CrAlMoxSiN coatings, the amount of molybdenum oxide on the worn surface increased from 0 to 21% with rising Mo contents. As molybdenum oxide reached a certain ratio over all the oxides, it could decrease the friction coefficient significantly. Additionally, the anti-oxidation capability at high temperature is another critical issue for protective coatings. The coating could be prevented from the further oxidation due to the formation of chromium oxide and aluminum oxide, which is commonly regarded as a protective layer for anti-oxidation application. In summary, a stable and adequate amount of oxide on the coating surface provides not only a self-lubricating characteristic but also a feasible resistance to oxidation.
Fig. 2. The friction coefficient of CrAlMoxSiN coatings at 600 °C.
3.2. Tribological behaviors at evaluated temperature The friction coefficient of CrAlMoxSiN coatings with different Mo content against Al2O3 ball at 600 °C are shown in Fig. 2. The friction coefficients of CrAlMoxSiN coatings reduce with increasing Mo contents. The CrAlSiN coating without Mo exhibited the highest friction coefficient around 0.6 among all coatings. After doping Mo in the coating, the values of friction coefficient almost remained constant in CAMSN1 with 3.9 at.% Mo and CAMSN2 with 10.0 at.% Mo. With Mo doping contents up to 14.5 at.%, the self-lubricating characteristic showed up noticeably in CAMSN3 and the friction coefficient dropped to the minimum of 0.52. The self-lubricating behavior could appear while the lubricating Mo reached a certain amount. Fig. 3 presents the wear rate of the coatings at 600 °C. CAMSN3 coating showed the lowest wear rate around 2.1 × 10−6 mm3 N−1 m−1 among all coatings and the difference between the other three coatings was slight. As mentioned before, the wear rate tendency of CASN, CAMSN1 and CAMSN2 was inversely related to the H3/E*2 ratio, implying that the H3/E*2 ratio could be an indicator of anti-wear capability. However, there was an exception on CAMSN3 with the lowest wear rate. It could be speculated that the resistance of plastic deformation is not the only factor to influence the wear resistance in the wearing process. During the ball-on-disc wearing test, there are two kinds of force onto the coating, including normal force and friction force. The former is perpendicular to the coating surface and the latter is parallel to that. From the formula of friction force (Ff = μ × FN), the normal force is fixed at a constant and the friction force reduces with decreasing
4. Conclusion In this study, CrAlMoxSiN coatings with different Mo contents were fabricated by radio frequency reactive magnetron co-sputtering. The
Fig. 3. The wear rate of CrAlMoxSiN coatings at 600 °C. 754
Surface & Coatings Technology 349 (2018) 752–756
H. Tao et al.
Fig. 4. Two-dimensional profilometric curves of the wear tracks: (a) CASN, (b) CAMSN1, (c) CAMSN2, and (d) CAMSN3.
hardness of the coating only slightly reduced by 2 to 3 GPa and the H3/ E*2 ratio also maintained the same level in CrAlSiN and CrAlMoxSiN coatings. Both CrAlSiN and CrAlMoSiN system as a nanocomposite coating revealed superior mechanical properties. For high-temperature tribological testing, all coatings showed an impressive performance of wear rate which was below 8 × 10−6 mm3 N−1 m−1, and CAMSN3 had the lowest average wear rate of around 2 × 10−6 mm3 N−1 m−1. The friction coefficient also depended on the Mo contents, and CAMSN3
presented the lowest one of 0.52. This indicates enhanced lubrication as the Mo contents attain a certain level. In conclusion, there are two key points for this study. One is that the mechanical strength of CrAlMoxSiN coating is still retained at a high level, and the other is that the tribological behavior of CrAlSiN nanocomposite coating could be significantly improved at elevated temperature by Mo doping. This suggests that high-temperature performance of nanocomposite coatings could be further improved via composition tuning by doping of
Fig. 5. Wear track morphology of (a) CASN, (b) CAMSN1, (c) CAMSN2, and (d) CAMSN3. 755
Surface & Coatings Technology 349 (2018) 752–756
H. Tao et al.
Fig. 6. ESCA analysis for O 2 s at the wear track of (a) CASN, (b) CAMSN1, (c) CAMSN2, and (d) CAMSN3. Table 2 Content distribution of oxides on the wear track.
[2] W. Wang, Surf. Coat. Technol. 177–178 (2004) 12–17. [3] Q. Wang, F. Zhou, J. Yan, Surf. Coat. Technol. 285 (2016) 203–213. [4] C. Rebholz, H. Ziegele, A. Leyland, A. Matthews, Surf. Coat. Technol. 115 (1999) 222–229. [5] H. Willmann, P.H. Mayrhofer, A.E. Reiter, L. Hultman, C. Mitterer, Scr. Mater. 54 (2006) 1847–1851. [6] J.L. Endrino, G.S. Fox-Rabinovich, A. Reiter, S.V. Veldhuis, R. Escobar Galindo, J.M. Albella, J.F. Marco, Surf. Coat. Technol. 201 (2007) 4505–4511. [7] Y.Y. Chang, W.T. Chiu, J.P. Huang, Surf. Coat. Technol. 303 (2016) 18–24. [8] C.B. Liu, W. Pei, F. Huang, L. Chen, Vacuum 125 (2016) 180–184. [9] S. Zhang, L. Wang, Q. Wang, M. Li, Surf. Coat. Technol. 214 (2013) 160–167. [10] C.C. Chang, H.W. Chen, J.W. Lee, J.G. Duh, Thin Solid Films 584 (2015) 46–51. [11] T. Polcar, T. Vitu, J. Sondor, A. Cavaleiro, Plasma Process. Polym. 6 (2009) S935–S940. [12] M. Oliveira, S. Agathopoulos, J.M.F. Ferreira, J. Eur. Ceram. Soc. 25 (2005) 19–28. [13] A.A. Voevodin, C. Muratore, S.M. Aouadi, Surf. Coat. Technol. 257 (2014) 247–265. [14] Y. Wang, J.W. Lee, J.G. Duh, Surf. Coat. Technol. 303 (2016) 12–17. [15] C.J. Wang, S.M. Chen, Surf. Coat. Technol. 201 (2006) 3862–3866. [16] S. Ma, J. Procha'zka, P. Karva'nkova', Q. Ma, X. Niu, X. Wang, D. Ma, K. Xu, S. Veprˇek, Surf. Coat. Technol. 194 (2005) 143–148. [17] C.C. Chang, H.W. Chen, J.W. Lee, J.G. Duh, Surf. Coat. Technol. 284 (2015) 273–280. [18] H.W. Chen, Y.C. Chan, J.W. Lee, J.G. Duh, Surf. Coat. Technol. 205 (2010) 1189–1194. [19] P. Hones, N. Martin, M. Regula, F. L'evy, J. Phys. D. Appl. Phys. 36 (2003) 1023–1029. [20] J.Y. Xiang, F.B. Wu, Surf. Coat. Technol. 332 (2017) 161–167. [21] A. Leyland, A. Matthews, Wear 246 (2000) 1–11. [22] B. Bhushan, Modern Tribology Handbook, Two Volume Set, USA, 2000.
Content distribution of oxides (wt%)
CASN CAMSN1 CAMSN2 CAMSN3
Cr2O3
Al2O3
MoO3
SiO2
50 48 50 45
31 28 26 28
– 11 16 21
19 13 8 6
adequate beneficial elements. Acknowledgment This study was financially supported by the Ministry of Science and Technology, Taiwan under project MOST-105-2221-E-007-027-MY3. The author also thanks for EPMA analysis by Precision Instrument Center at National Tsing-Hua University and nano hardness test at Ming-Chi University of Technology. References [1] K. Holmberg, P. Andersson, A. Erdemir, Tribol. Int. 47 (2012) 221–234.
756