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Duplex coating technique to improve the adhesion and tribological properties of CrAlSiN nanocomposite coating
SCT-21778; No of Pages 7 Surface & Coatings Technology xxx (2016) xxx–xxx
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Duplex coating technique to improve the adhesion and tribological properties of CrAlSiN nanocomposite coating Chun-Chi Chang, Jenq-Gong Duh ⁎ Department of Material Science and Engineering, National Tsing-Hua University, Hsinchu, Taiwan
a r t i c l e
i n f o
Article history: Received 8 August 2016 Revised 22 October 2016 Accepted in revised form 9 November 2016 Available online xxxx Keywords: CrAlSiN Nanocomposite Nitriding Adhesion Wear Friction
a b s t r a c t The study is intended to solve the adhesion problem of high residual stress nanocomposite CrAlSiN coating deposited on the AISI 304 substrate. The duplex coating system via low pressure plasma nitriding and RF magnetron sputtering is used. Nitriding experiments are carried out at low temperature 500 °C for 1 h, 2 h, and 5 h with RF power 150 W, 225 W, and 300 W. It is revealed that the substrate with 19.7 at.% nitrogen exhibits the outstanding adhesive strength. The specimen is characterized by X-ray diffraction, atomic force microscopy, and transmission electron microscopy. The strengthening mechanisms in adherence are addressed and can be related to the crystal structure, thermal expansion coefficient, surface roughness, and diffusion layer between the coating and the substrate. Tribological properties of the coatings on the substrate with various nitrogen contents are investigated. The coating on the substrate with 19.7 at.% nitrogen shows the best wear resistance which has almost zero wear volume. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Nitride coatings have been widely used in aerospace, automotive and cutting tool industries for extending the lifetime of working components [1]. However, the main drawback of nitride coatings is the poor adhesion between nitride coating and metallic substrate [2–3]. The quaternary CrAlSiN nanocomposite coating is a promising material which exhibits outstanding mechanical strength, oxidation resistance and wear resistance [4,5]. In these quaternary compound, Si tends to segregate as amorphous SiNx phase along the grain boundary creating nanocomposite microstructure. It is the very microstructure that makes the residual stress in CrAlSiN coating produced 4 GPa [6]. Large residual stress in nitride coating leads to not only superior hardness but also the detachment of coatings from the substrate, limiting its application [7]. In general, there are lots of surface treatment technologies to improve the adhesion of nitride coatings. The common way is to fabricate thin metal layer between the coating and the substrate or nitrogen diffusion layer into the metal substrate. For depositing TiN coatings on AISI 304 stainless steel and ASP 23 high speed steel, fabricating diffusion layers exhibited better coatings adhesion than metal interlayers [8]. Bending of the TiN coating in front of the scratch indenter induced tensile residual stresses in the TiN coating which become critical at a certain penetration depth. Fabricating diffusion layers reduced cohesive failure as compared with fabricating metal interlayers, which means that the ⁎ Corresponding author. E-mail address: [email protected] (J.-G. Duh).
critical indentation load is higher [9]. In a preliminary study, CrAlSiN nanocomposite coatings had better adhesion via nitriding process than deposition a metal layer on the AISI 304 stainless steel substrate. The nitrogen diffusion layer induced that the substrate had similar crystal structure as nitride coating, which decreased the strain force and increased the interfacial bonding strength. In literature, the effects of nitridation substrate on Cr-based nanocomposite coating system are rarely studied so far. There is limited discussion on the effects of the adhesion of CrAlSiN coatings with various nitriding process parameters. In this study, the duplex coating technique, including low pressure plasma nitriding and RF magnetron sputtering, is used to improve the adhesion of CrAlSiN coatings on AISI 304 substrate. The nitriding treatments are controlled by the working power and process time to figure out the optimum nitriding parameters. Furthermore, the tribological properties of coatings on the substrate with various nitrogen contents are investigated. 2. Experimental procedure The substrate used in this study was AISI 304 austenitic stainless steels with the following chemical compositions in wt%: C, 0.07; Si, 0.70; Ti, 0.14; Cr, 18.9; Ni, 9.2; and Fe in balance. Specimens (2 × 2 × 0.5 cm3) were ground, mirror-polished and cleaned with acetone before RF plasma nitriding process. Plasma nitriding process was integrated into the PVD coating device as a pretreatment chamber with equipped heating system to render substrate temperature up to 500 °C. This temperature was necessary for nitrogen diffusion in plasma nitriding process. The nitriding chamber was filled with 93%Ar + 7%H2
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Fig. 1. The top-view images of as-deposited CrAlSiN nanocomposite coating on AISI 304 substrates (a) without and (b) with nitriding treatment.
flux of 80 sccm and N2 flux of 20 sccm to 300 mTorr. In order to investigate the effects of nitriding parameters, the working power was adjusted from 150 to 300 W. The process time was ranged from 1 to 5 h. The CrAlSiN coatings were deposited by reactive RF magnetron cosputtering onto AISI 304 substrates after nitriding process with Cr0.5Al0.5 and Si targets. Firstly, the chamber was evacuated down to 8 × 10−4 Pa and heated to 300 °C for 3 h. During the reactive sputtering process, Ar and N2 flux were fixed at 20 and 10 sccm, respectively. The substrates were fixed on the rotational holder with a speed of 20 rpm. To obtain the largest hardness of CrAlSiN coating, the input power on Cr0.5Al0.5 target was fixed at 300 W and that on Si target was 120 W during sputtering. A negative substrate bias of 50 V was exerted on the substrates. By cross-sectional observations through scanning electron microscope (SEM, JSM-7600F, JEOL Instruments) at 15 keV, the coating thickness was measured to be around 2 μm. The quantitative compositions of the nitridation AISI 304 substrates were acquired by a field emission electron probe microanalyzer (JXA8500M, JEOL Instruments). Samples were cold mounted and grinded with 200, 400, 800, 1200, 2000, and 4000 mesh sandpapers. These samples were then polished with 1.0 and 0.3 μm alumina abrasives, and etched with Nital 4% solution. The crystallographic phase was analyzed by X-ray diffractometer (XRD, TTRAX, Rigaku Instruments) using Cu-Kα with a wavelength of 0.154 nm operated at 45 kV and 40 mA. The 2θ scan ranged from 30 to 70° with the incident angle of 2° and a step width of 0.02° at 2°/min. To study the adhesion of the coatings, the nanoscratch was performed by the scratch tester (Nano Scratch Tester, CSM Instruments) with a diamond tip. The load was from 5 mN to 1000 mN and the length was 3 mm. The surface roughness of the AISI
304 substrate after nitriding process was detected by the atomic force microscopy (AFM, Bruker Instruments) in contact mode with a commercial standard pyramidal Si3N4 tip. The microstructure of CrAlSiN coatings was evaluated by transmission electron microscopy (TEM, JEM-2010, JEOL Instruments) at 200 keV. The tribological behaviors of the coatings were evaluated by a ball-on-disc wear apparatus (High Temperature Tribometer, CSM Instruments) equipped with Al2O3 ball (φ = 6.3 mm). The wear test was conducted with a sliding speed of 2 × 10−2 m/s under a load of 1 N at ambient temperature within 3500 cycles. The radius of wear track was 5 mm and sliding distance was about 110 m. The wear rate, WR, was calculated using the following equation [10]:
WR ¼
2 t 3t 2 þ 4b 2πr 6bF n S
ð1Þ
where t is the depth of the wear track determined by a surface profilometer (Kosaka Surfcorder ET3000, Japan), b is the width of the wear track observed with an optical microscope, r is the radius of the wear track, Fn is the normal load and S is the sliding distance. 3. Results and discussion 3.1. Characterization of CrAlSiN coatings on nitridation substrate Fig. 1(a) and (b) show the top-view images of as-deposited CrAlSiN coatings on AISI 304 substrates with and without nitriding treatment.
Fig. 2. The XRD patterns of untreated and nitrided AISI 304 substrates.
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The gray part was the coating, while the white part was the substrate. The CrAlSiN nanocomposite coating deposited on the AISI 304 substrate without nitriding treatment was destroyed and stripped from the substrate. There existed substantial residual stress inside the nanocomposite coating and the bonding strength between the coating and the substrate was weak. On the contrary, the CrAlSiN nanocomposite coating was well adhered on AISI 304 after nitriding process. Fig. 2 shows the XRD patterns of untreated and nitrided specimens. After nitriding 2 h with a working power 150 W, the peak of γ-Fe (200) and γ-Fe (111) were transformed to γ′-Fe4N (111) as the concentration of interstitial N in the γ-Fe matrix increased. The peak identified as CrN (200) was also observed. With increasing process time and working power, another iron nitride phase ε-Fe2–3N precipitated in the matrix, which had a hexagonal symmetry with a primitive space lattice containing three atoms in the unit cell. This was in agreement with the observations in literatures [11]. The specimen after 5 h nitriding process with working power 300 W revealed the combination of ε-Fe2–3N, γ′-Fe4N and CrN phases. Fig. 3 shows that the thickness of nitrogen diffusion layers was 1.7, 2.5, and 4.0 μm, respectively, after 1, 2, and 5 h nitriding process with working power 300 W. The thickness of nitrogen diffusion layer increased with the nitriding process time. The sample after 1 h nitriding process with working power 150 W exhibited the same thickness of nitrogen layer as that after 2 h nitriding process with working power 300 W, since they possessed the identical nitrogen contents. 3.2. Adhesive strength of CrAlSiN coatings on nitridation substrates Fig. 4(a) and (b) show the critical load values of the coating on nitrided substrate with various nitriding parameters, in which Lc1 corresponds to the load inducing first cracking in the coating and Lc2 is the load inducing full delamination of the coating. The values of Lc1 and Lc2 increased with the process time and working power of nitriding process, indicating that the adhesive strength of nanocomposite CrAlSiN coating was improved via nitriding treatment on substrate. The CrAlSiN coating on the AISI 304 substrate after 5 h nitriding process with working power 300 W exhibited the largest value of Lc1 (345.1 mN) and Lc2 (803.1 mN), which was 2.4 and 1.4 times than the coating on the substrate after 2 h nitriding process with working power 150 W. Fig. 5(a) and (b) present the scratch scars where the load of tip is the maximum of CrAlSiN coatings. The coatings were too stiff to buckle leading to the formation of compressive shear cracks at the weak interface of the coating and substrate. As scratcher tips moved forward, the compressive shear stress induced cracks propagated and the wedge spallation of coatings occurred. This wedge spallation behavior was usually observed on brittle ceramic coatings which had weak interfacial adhesion to the substrate [12,13]. If the interfacial adhesion of coatings was large, the wedge spallation failure was slight and the spallation area was small. It appeared that the nitriding treatment with longer process time and larger working power would induce smaller rupture of the coating. 3.3. Adhesion strengthen mechanism 3.3.1. Reduction in structure incoherence Table 1 shows the chemical composition of AISI 304 substrates after nitriding process. The nitrogen contents of the nitrided substrate increased with the process time and working power of nitriding process. In heteroepitaxial growth, the material difference between the coating and substrate would imply a difference in their in-plane lattice parameters, which induced large strain on the overlayer crystal [14]. The AISI 304 substrate after 5 h nitriding process with working power 300 W had nitrogen embedded in the structure, which made the phase transformation from γ-Fe to the combination of ε-Fe2–3N, γ′Fe4N and CrN phases. The lattice parameters of FeN, CrN, AISI 304 and CrAlSiN are 0.433, 0.415, 0.359 and 0.422 nm, respectively [15–17],
Fig. 3. SEM cross-sectional images of samples after (a) 1 h, (b) 2 h and (c) 5 h nitriding process with working power 300 W.
indicating that FeN and CrN had similar lattice constant with nanocomposite CrAlSiN. The more nitrogen contents implanted into the substrate, the less incoherence occurred between the coating and substrate. Moreover, FeN, CrN and CrAlSiN are ionic compounds, yet the AISI 304 is metal. The bonding strength of ionic bonds is higher than metallic bonds. Therefore, the adhesive strength of the coating on the substrate after nitriding process was higher than which without nitriding process.
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Fig. 4. (a) The critical load values of the coating on nitrided substrate with various working power. (b) The critical load values of the coating on nitrided substrate with process time.
Fig. 5. (a) The scratch scar of the coating on the substrate after 2 h nitriding process with working power 150 W. (b) The scratch scar of the coating on the substrate after 5 h nitriding process with working power 300 W.
Table 1 The elemental composition of nitrided AISI 304 substrates. Concentration (at.%) Samples Nitrided 150 Nitrided 300 Nitrided 225 Nitrided 300 Nitrided 300
Fe W, 2 h W, 1 h W, 2 h W, 2 h W, 5 h
62.8 58.6 58.9 57.3 56.3
Cr ± ± ± ± ±
0.4 0.2 0.5 0.6 0.3
18.8 17.3 17.5 17.1 17.0
Si ± ± ± ± ±
0.2 0.1 0.6 0.6 0.5
0.5 0.3 0.4 0.4 0.4
Ni ± ± ± ± ±
0.06 0.06 0.05 0.03 0.04
3.3.2. Reduction in coefficient of thermal expansion mismatch The nitride coatings were normally deposited at high temperature and cooled to room temperature, resulting in significant residual stress from coefficient of thermal expansion (CTE) mismatch. If the metal substrate with large CTE was used, the nitride coatings with low thermal conductivity would tend to have delamination failures due to volumetric strain during cooling process. There is a large CTE mismatch between AISI 304 and CrN, where the CTE of AISI 304 and CrN are 18.4 × 10−6 and 2.3 × 10−6 /K, respectively [18,19]. The CTE of FeN is around 7.9 × 10−6 / K [20] which is between the value of AISI 304 and CrN. Hence, the nitridation of AISI 304 was favorable to decrease the CTE mismatch, avoiding the delamination of the CrAlSiN coating.
7.3 6.4 6.8 6.2 5.7
C ± ± ± ± ±
0.2 0.3 0.2 0.3 0.2
Mn
1.8 3.2 2.0 2.2 1.6
± ± ± ± ±
0.04 0.08 0.04 0.02 0.02
1.2 1.1 1.1 1.2 1.0
N ± ± ± ± ±
0.07 0.06 0.02 0.09 0.06
7.7 ± 0.1 13.2 ± 0.3 13.2 ± 0.1 15.5 ± 0.2 17.9 ± 0.5
depth within the evaluation length [21]. All values of Ra, Rq and Rmax increased with nitrogen contents in the substrate. Adhesive strength increased with surface roughness of substrate within about 50 μm [22]. On the other hand, for higher roughness (N50 μm) of the substrate, the adhesive strength was found to decrease with roughness [23]. The AISI 304 substrate after 5 h nitriding process with working power 300 W showed highest surface roughness Ra = 8.2 nm, Rq = 11.0 nm and Rmax = 193 nm, which was within the range of adhesion strengthening.
Table. 2 Surface roughness after nitriding process. Surface roughness (nm) Samples
3.3.3. Increment in surface roughness of substrate The surface roughness is also a critical issue on the adherence of the coating. Table 2 shows the values of Ra, Rq and Rmax. Ra is the average of absolute values of the roughness profile. Rq is the root mean square average of the roughness profile. Rmax is the largest single roughness
Nitrided 150 Nitrided 300 Nitrided 225 Nitrided 300 Nitrided 300
Ra W, 2 h W, 1 h W, 2 h W, 2 h W, 5 h
0.6 2.1 4.5 7.2 8.2
± ± ± ± ±
0.02 0.07 0.18 0.23 0.59
Rq
Rmax
0.8 ± 0.06 2.7 ± 0.11 5.8 ± 0.41 10.1 ± 0.27 11.0 ± 0.74
22.3 ± 1.2 36.6 ± 3.6 70.7 ± 5.9 113.0 ± 10.1 193.0 ± 12.2
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Fig. 8. The friction coefficient of CrAlSiN nanocomposite coatings as a function of nitrogen contents in the substrate at ambient temperature.
Fig. 6. The bright-field image of a cross-section of the as-deposited sample.
strength than Ti-based nitride coating [24]. The reason might be the formation of Cr-rich nitride layer between the substrate and the coating, where both the substrate and the coating had chromium contents. 3.4. Tribological properties of coating on nitrided substrate
3.3.4. Formation of inter-layer between the coating and the substrate Fig. 6 shows the bright-field image of a cross-section of the assputtered sample. There was a very thin (about 20 nm) layer between CrAlSiN nanocomposite coating and nitrided AISI 304 substrate. In Fig. 7, the thin layer was analyzed by EDS and three distinct regions were revealed. The region 1 which was about 3 nm near the substrate was formed because the iron atoms diffused out. The region 2 about 16 nm was chromium-rich layer, as the abundant chromium from AISI 304 steel and CrAlSiN coating accumulated in the boundary, resulting an increase in Cr concentration. Some diffusion of nitrogen into region 2 might also take place, yet it was rather difficult to precisely analyze the content of nitrogen by EDS. The region 3 about 2 nm near the coating was formed because of the Cr diffusion. In literature, the Cr-based nitride coatings deposited on nitrided steel generally had better adhesive
Fig. 8 shows the friction coefficient of CrAlSiN nanocomposite coatings as a function of nitrogen contents in the substrate at ambient temperature. The friction coefficient of all samples was about 0.8, indicating that the friction coefficient was not influenced by the nitridation of substrates. Fig. 9 presents the 2D profilometric curves of the wear tracks, in which the maximum wear depth was located at the middle of wear tracks and both sides had accumulation of wear debris. Fig. 10 presents the morphology of wear tracks through scanning electron microscope. The CrAlSiN coating on the substrate with the minimum nitrogen contents showed dark area at the wear scar because of the large gradient of depth, which indicated that there was large wear volume. As the nitrogen contents increased in the substrate, the gradient of depth decreased and wear scar was bright, implying the decreased wear
Fig. 7. EDS line scan analysis of the cross-section image.
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Fig. 9. The 2D profilometric curves of the wear tracks.
volume of the coating. The wear rate of coatings on substrates with 7.7, 13.2, and 15.5 at.% nitrogen contents was 3.1, 1.5, and 0.6×10−5 mm3/ N ∙ m, respectively. Moreover, the coating on the substrate with 17.9 at.% nitrogen content exhibited nearly no wear volume on the wear track. Therefore, the wear rate calculated from Eq. (1) was zero. The improvement in wear performance was attributed to good adhesive strength and high compressive stresses of the coatings. 4. Conclusion In this study, CrAlSiN nanocomposite coatings were successfully fabricated on nitrided AISI 304 substrate via duplex coating techniques. The CrAlSiN coating on the AISI 304 substrate after 5 h nitriding process with working power 300 W revealed the best adhesive strength. The
strengthening mechanism in adherence could be concluded as follows: (1) the reduction in structure incoherence between the coating and substrate owing to phase transformation from γ-Fe to the combination of ε-Fe2–3N, γ′-Fe4N and CrN phases, (2) the reduction in CTE mismatch between the coating and substrate, (3) the increasing surface roughness of the substrate, and (4) the formation of Crrich nitride layer in the boundary of coating and substrate. Moreover, the wear rate of the CrAlSiN coatings decreased as the nitrogen contents in the AISI 304 stainless steel increased. The coating on the substrate with 17.9 at.% nitrogen content exhibited the best wear resistance. Therefore, the CrAlSiN nanocomposite coatings deposited on the AISI 304 stainless steel by duplex coating techniques exhibited not only adhesion improvement but also wear resistance of the coating.
Fig. 10. SEM images of wear scars of CrAlSiN coating on the substrate after (a) 1 h nitriding process with working power 150 W and (b) 5 h nitriding process with working power 300 W.
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Acknowledgement The authors thank the financial support from the Ministry of Science and Technology, Taiwan under project 102-2221-E-007-045-MY3. The assistance in AFM analysis at National Taiwan University of Science and Technology, and the assistance in EPMA analysis in Precision Instrumentation Center at National Tsing Hua University are appreciated. References [1] M. Deepak, R. Khushwant, Comparison of mechanical properties and calculation of cutting force between uncoated tungsten carbide & Ti coated tip tungsten carbide with the help of lathe tool dynamometer, Int. J. Mod. Phys. Conf. Ser. 22 (2013) 645–653. [2] P. Richard, J. Thomas, D. Landolt, G. Gremaud, Combination of scratch-test and acoustic microscopy imaging for the study of coating adhesion, Surf. Coat. Technol. 91 (1997) 83–90. [3] Y.S. Hong, S.H. Kwon, T. Wang, D.I. Kim, J. Choi, K.H. Kim, Effects of Cr interlayer on mechanical and tribological properties of Cr-Al-Si-N nanocomposite coating, Trans. Nonferrous Met. Soc. Chin. 21 (2011) s62–s67. [4] C.C. Chang, H.W. Chen, J.W. Lee, J.G. Duh, Influence of Si contents on tribological characteristics of CrAlSiN nanocomposite coatings, Thin Solid Films 584 (2015) 46–51. [5] C.C. Chang, H.W. Chen, J.W. Lee, J.G. Duh, Development of Si-modified CrAlSiN nanocomposite coating for anti-wear application in extreme environment, Surf. Coat. Technol. 284 (2015) 273–280. [6] J.H. Yun, S.J. Heo, K.R. Kim, K.H. Kim, Synthesis and mechanical properties of CrMoCxN1 − x coatings deposited by a hybrid coating system, J. Vac. Sci. Technol. A 26 (2008) 146. [7] P. Holubar, M. Jilek, M. Sima, Present and possible future applications of superhard nanocomposite coatings, Surf. Coat. Technol. 133-134 (2000) 145–151. [8] M. Stappen, M. Kerkhofs, C. Quaeyhaegens, L. Stals, Introduction in industry of a duplex treatment consisting of plasma nitriding and PVD deposition of TiN, Surf. Coat. Technol. 62 (1993) 655–661. [9] P.J. Burnett, D.S. Rickerby, The mechanical properties of wear-resistant coatings: II: experimental studies and interpretation of hardness, Thin Solid Films 148 (1987) 51–65.
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Duplex coating technique to improve the adhesion and tribological properties of CrAlSiN nanocomposite coating Chun-Chi Chang, Jenq-Gong Duh ⁎ Department of Material Science and Engineering, National Tsing-Hua University, Hsinchu, Taiwan
a r t i c l e
i n f o
Article history: Received 8 August 2016 Revised 22 October 2016 Accepted in revised form 9 November 2016 Available online xxxx Keywords: CrAlSiN Nanocomposite Nitriding Adhesion Wear Friction
a b s t r a c t The study is intended to solve the adhesion problem of high residual stress nanocomposite CrAlSiN coating deposited on the AISI 304 substrate. The duplex coating system via low pressure plasma nitriding and RF magnetron sputtering is used. Nitriding experiments are carried out at low temperature 500 °C for 1 h, 2 h, and 5 h with RF power 150 W, 225 W, and 300 W. It is revealed that the substrate with 19.7 at.% nitrogen exhibits the outstanding adhesive strength. The specimen is characterized by X-ray diffraction, atomic force microscopy, and transmission electron microscopy. The strengthening mechanisms in adherence are addressed and can be related to the crystal structure, thermal expansion coefficient, surface roughness, and diffusion layer between the coating and the substrate. Tribological properties of the coatings on the substrate with various nitrogen contents are investigated. The coating on the substrate with 19.7 at.% nitrogen shows the best wear resistance which has almost zero wear volume. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Nitride coatings have been widely used in aerospace, automotive and cutting tool industries for extending the lifetime of working components [1]. However, the main drawback of nitride coatings is the poor adhesion between nitride coating and metallic substrate [2–3]. The quaternary CrAlSiN nanocomposite coating is a promising material which exhibits outstanding mechanical strength, oxidation resistance and wear resistance [4,5]. In these quaternary compound, Si tends to segregate as amorphous SiNx phase along the grain boundary creating nanocomposite microstructure. It is the very microstructure that makes the residual stress in CrAlSiN coating produced 4 GPa [6]. Large residual stress in nitride coating leads to not only superior hardness but also the detachment of coatings from the substrate, limiting its application [7]. In general, there are lots of surface treatment technologies to improve the adhesion of nitride coatings. The common way is to fabricate thin metal layer between the coating and the substrate or nitrogen diffusion layer into the metal substrate. For depositing TiN coatings on AISI 304 stainless steel and ASP 23 high speed steel, fabricating diffusion layers exhibited better coatings adhesion than metal interlayers [8]. Bending of the TiN coating in front of the scratch indenter induced tensile residual stresses in the TiN coating which become critical at a certain penetration depth. Fabricating diffusion layers reduced cohesive failure as compared with fabricating metal interlayers, which means that the ⁎ Corresponding author. E-mail address: [email protected] (J.-G. Duh).
critical indentation load is higher [9]. In a preliminary study, CrAlSiN nanocomposite coatings had better adhesion via nitriding process than deposition a metal layer on the AISI 304 stainless steel substrate. The nitrogen diffusion layer induced that the substrate had similar crystal structure as nitride coating, which decreased the strain force and increased the interfacial bonding strength. In literature, the effects of nitridation substrate on Cr-based nanocomposite coating system are rarely studied so far. There is limited discussion on the effects of the adhesion of CrAlSiN coatings with various nitriding process parameters. In this study, the duplex coating technique, including low pressure plasma nitriding and RF magnetron sputtering, is used to improve the adhesion of CrAlSiN coatings on AISI 304 substrate. The nitriding treatments are controlled by the working power and process time to figure out the optimum nitriding parameters. Furthermore, the tribological properties of coatings on the substrate with various nitrogen contents are investigated. 2. Experimental procedure The substrate used in this study was AISI 304 austenitic stainless steels with the following chemical compositions in wt%: C, 0.07; Si, 0.70; Ti, 0.14; Cr, 18.9; Ni, 9.2; and Fe in balance. Specimens (2 × 2 × 0.5 cm3) were ground, mirror-polished and cleaned with acetone before RF plasma nitriding process. Plasma nitriding process was integrated into the PVD coating device as a pretreatment chamber with equipped heating system to render substrate temperature up to 500 °C. This temperature was necessary for nitrogen diffusion in plasma nitriding process. The nitriding chamber was filled with 93%Ar + 7%H2
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Fig. 1. The top-view images of as-deposited CrAlSiN nanocomposite coating on AISI 304 substrates (a) without and (b) with nitriding treatment.
flux of 80 sccm and N2 flux of 20 sccm to 300 mTorr. In order to investigate the effects of nitriding parameters, the working power was adjusted from 150 to 300 W. The process time was ranged from 1 to 5 h. The CrAlSiN coatings were deposited by reactive RF magnetron cosputtering onto AISI 304 substrates after nitriding process with Cr0.5Al0.5 and Si targets. Firstly, the chamber was evacuated down to 8 × 10−4 Pa and heated to 300 °C for 3 h. During the reactive sputtering process, Ar and N2 flux were fixed at 20 and 10 sccm, respectively. The substrates were fixed on the rotational holder with a speed of 20 rpm. To obtain the largest hardness of CrAlSiN coating, the input power on Cr0.5Al0.5 target was fixed at 300 W and that on Si target was 120 W during sputtering. A negative substrate bias of 50 V was exerted on the substrates. By cross-sectional observations through scanning electron microscope (SEM, JSM-7600F, JEOL Instruments) at 15 keV, the coating thickness was measured to be around 2 μm. The quantitative compositions of the nitridation AISI 304 substrates were acquired by a field emission electron probe microanalyzer (JXA8500M, JEOL Instruments). Samples were cold mounted and grinded with 200, 400, 800, 1200, 2000, and 4000 mesh sandpapers. These samples were then polished with 1.0 and 0.3 μm alumina abrasives, and etched with Nital 4% solution. The crystallographic phase was analyzed by X-ray diffractometer (XRD, TTRAX, Rigaku Instruments) using Cu-Kα with a wavelength of 0.154 nm operated at 45 kV and 40 mA. The 2θ scan ranged from 30 to 70° with the incident angle of 2° and a step width of 0.02° at 2°/min. To study the adhesion of the coatings, the nanoscratch was performed by the scratch tester (Nano Scratch Tester, CSM Instruments) with a diamond tip. The load was from 5 mN to 1000 mN and the length was 3 mm. The surface roughness of the AISI
304 substrate after nitriding process was detected by the atomic force microscopy (AFM, Bruker Instruments) in contact mode with a commercial standard pyramidal Si3N4 tip. The microstructure of CrAlSiN coatings was evaluated by transmission electron microscopy (TEM, JEM-2010, JEOL Instruments) at 200 keV. The tribological behaviors of the coatings were evaluated by a ball-on-disc wear apparatus (High Temperature Tribometer, CSM Instruments) equipped with Al2O3 ball (φ = 6.3 mm). The wear test was conducted with a sliding speed of 2 × 10−2 m/s under a load of 1 N at ambient temperature within 3500 cycles. The radius of wear track was 5 mm and sliding distance was about 110 m. The wear rate, WR, was calculated using the following equation [10]:
WR ¼
2 t 3t 2 þ 4b 2πr 6bF n S
ð1Þ
where t is the depth of the wear track determined by a surface profilometer (Kosaka Surfcorder ET3000, Japan), b is the width of the wear track observed with an optical microscope, r is the radius of the wear track, Fn is the normal load and S is the sliding distance. 3. Results and discussion 3.1. Characterization of CrAlSiN coatings on nitridation substrate Fig. 1(a) and (b) show the top-view images of as-deposited CrAlSiN coatings on AISI 304 substrates with and without nitriding treatment.
Fig. 2. The XRD patterns of untreated and nitrided AISI 304 substrates.
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The gray part was the coating, while the white part was the substrate. The CrAlSiN nanocomposite coating deposited on the AISI 304 substrate without nitriding treatment was destroyed and stripped from the substrate. There existed substantial residual stress inside the nanocomposite coating and the bonding strength between the coating and the substrate was weak. On the contrary, the CrAlSiN nanocomposite coating was well adhered on AISI 304 after nitriding process. Fig. 2 shows the XRD patterns of untreated and nitrided specimens. After nitriding 2 h with a working power 150 W, the peak of γ-Fe (200) and γ-Fe (111) were transformed to γ′-Fe4N (111) as the concentration of interstitial N in the γ-Fe matrix increased. The peak identified as CrN (200) was also observed. With increasing process time and working power, another iron nitride phase ε-Fe2–3N precipitated in the matrix, which had a hexagonal symmetry with a primitive space lattice containing three atoms in the unit cell. This was in agreement with the observations in literatures [11]. The specimen after 5 h nitriding process with working power 300 W revealed the combination of ε-Fe2–3N, γ′-Fe4N and CrN phases. Fig. 3 shows that the thickness of nitrogen diffusion layers was 1.7, 2.5, and 4.0 μm, respectively, after 1, 2, and 5 h nitriding process with working power 300 W. The thickness of nitrogen diffusion layer increased with the nitriding process time. The sample after 1 h nitriding process with working power 150 W exhibited the same thickness of nitrogen layer as that after 2 h nitriding process with working power 300 W, since they possessed the identical nitrogen contents. 3.2. Adhesive strength of CrAlSiN coatings on nitridation substrates Fig. 4(a) and (b) show the critical load values of the coating on nitrided substrate with various nitriding parameters, in which Lc1 corresponds to the load inducing first cracking in the coating and Lc2 is the load inducing full delamination of the coating. The values of Lc1 and Lc2 increased with the process time and working power of nitriding process, indicating that the adhesive strength of nanocomposite CrAlSiN coating was improved via nitriding treatment on substrate. The CrAlSiN coating on the AISI 304 substrate after 5 h nitriding process with working power 300 W exhibited the largest value of Lc1 (345.1 mN) and Lc2 (803.1 mN), which was 2.4 and 1.4 times than the coating on the substrate after 2 h nitriding process with working power 150 W. Fig. 5(a) and (b) present the scratch scars where the load of tip is the maximum of CrAlSiN coatings. The coatings were too stiff to buckle leading to the formation of compressive shear cracks at the weak interface of the coating and substrate. As scratcher tips moved forward, the compressive shear stress induced cracks propagated and the wedge spallation of coatings occurred. This wedge spallation behavior was usually observed on brittle ceramic coatings which had weak interfacial adhesion to the substrate [12,13]. If the interfacial adhesion of coatings was large, the wedge spallation failure was slight and the spallation area was small. It appeared that the nitriding treatment with longer process time and larger working power would induce smaller rupture of the coating. 3.3. Adhesion strengthen mechanism 3.3.1. Reduction in structure incoherence Table 1 shows the chemical composition of AISI 304 substrates after nitriding process. The nitrogen contents of the nitrided substrate increased with the process time and working power of nitriding process. In heteroepitaxial growth, the material difference between the coating and substrate would imply a difference in their in-plane lattice parameters, which induced large strain on the overlayer crystal [14]. The AISI 304 substrate after 5 h nitriding process with working power 300 W had nitrogen embedded in the structure, which made the phase transformation from γ-Fe to the combination of ε-Fe2–3N, γ′Fe4N and CrN phases. The lattice parameters of FeN, CrN, AISI 304 and CrAlSiN are 0.433, 0.415, 0.359 and 0.422 nm, respectively [15–17],
Fig. 3. SEM cross-sectional images of samples after (a) 1 h, (b) 2 h and (c) 5 h nitriding process with working power 300 W.
indicating that FeN and CrN had similar lattice constant with nanocomposite CrAlSiN. The more nitrogen contents implanted into the substrate, the less incoherence occurred between the coating and substrate. Moreover, FeN, CrN and CrAlSiN are ionic compounds, yet the AISI 304 is metal. The bonding strength of ionic bonds is higher than metallic bonds. Therefore, the adhesive strength of the coating on the substrate after nitriding process was higher than which without nitriding process.
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Fig. 4. (a) The critical load values of the coating on nitrided substrate with various working power. (b) The critical load values of the coating on nitrided substrate with process time.
Fig. 5. (a) The scratch scar of the coating on the substrate after 2 h nitriding process with working power 150 W. (b) The scratch scar of the coating on the substrate after 5 h nitriding process with working power 300 W.
Table 1 The elemental composition of nitrided AISI 304 substrates. Concentration (at.%) Samples Nitrided 150 Nitrided 300 Nitrided 225 Nitrided 300 Nitrided 300
Fe W, 2 h W, 1 h W, 2 h W, 2 h W, 5 h
62.8 58.6 58.9 57.3 56.3
Cr ± ± ± ± ±
0.4 0.2 0.5 0.6 0.3
18.8 17.3 17.5 17.1 17.0
Si ± ± ± ± ±
0.2 0.1 0.6 0.6 0.5
0.5 0.3 0.4 0.4 0.4
Ni ± ± ± ± ±
0.06 0.06 0.05 0.03 0.04
3.3.2. Reduction in coefficient of thermal expansion mismatch The nitride coatings were normally deposited at high temperature and cooled to room temperature, resulting in significant residual stress from coefficient of thermal expansion (CTE) mismatch. If the metal substrate with large CTE was used, the nitride coatings with low thermal conductivity would tend to have delamination failures due to volumetric strain during cooling process. There is a large CTE mismatch between AISI 304 and CrN, where the CTE of AISI 304 and CrN are 18.4 × 10−6 and 2.3 × 10−6 /K, respectively [18,19]. The CTE of FeN is around 7.9 × 10−6 / K [20] which is between the value of AISI 304 and CrN. Hence, the nitridation of AISI 304 was favorable to decrease the CTE mismatch, avoiding the delamination of the CrAlSiN coating.
7.3 6.4 6.8 6.2 5.7
C ± ± ± ± ±
0.2 0.3 0.2 0.3 0.2
Mn
1.8 3.2 2.0 2.2 1.6
± ± ± ± ±
0.04 0.08 0.04 0.02 0.02
1.2 1.1 1.1 1.2 1.0
N ± ± ± ± ±
0.07 0.06 0.02 0.09 0.06
7.7 ± 0.1 13.2 ± 0.3 13.2 ± 0.1 15.5 ± 0.2 17.9 ± 0.5
depth within the evaluation length [21]. All values of Ra, Rq and Rmax increased with nitrogen contents in the substrate. Adhesive strength increased with surface roughness of substrate within about 50 μm [22]. On the other hand, for higher roughness (N50 μm) of the substrate, the adhesive strength was found to decrease with roughness [23]. The AISI 304 substrate after 5 h nitriding process with working power 300 W showed highest surface roughness Ra = 8.2 nm, Rq = 11.0 nm and Rmax = 193 nm, which was within the range of adhesion strengthening.
Table. 2 Surface roughness after nitriding process. Surface roughness (nm) Samples
3.3.3. Increment in surface roughness of substrate The surface roughness is also a critical issue on the adherence of the coating. Table 2 shows the values of Ra, Rq and Rmax. Ra is the average of absolute values of the roughness profile. Rq is the root mean square average of the roughness profile. Rmax is the largest single roughness
Nitrided 150 Nitrided 300 Nitrided 225 Nitrided 300 Nitrided 300
Ra W, 2 h W, 1 h W, 2 h W, 2 h W, 5 h
0.6 2.1 4.5 7.2 8.2
± ± ± ± ±
0.02 0.07 0.18 0.23 0.59
Rq
Rmax
0.8 ± 0.06 2.7 ± 0.11 5.8 ± 0.41 10.1 ± 0.27 11.0 ± 0.74
22.3 ± 1.2 36.6 ± 3.6 70.7 ± 5.9 113.0 ± 10.1 193.0 ± 12.2
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Fig. 8. The friction coefficient of CrAlSiN nanocomposite coatings as a function of nitrogen contents in the substrate at ambient temperature.
Fig. 6. The bright-field image of a cross-section of the as-deposited sample.
strength than Ti-based nitride coating [24]. The reason might be the formation of Cr-rich nitride layer between the substrate and the coating, where both the substrate and the coating had chromium contents. 3.4. Tribological properties of coating on nitrided substrate
3.3.4. Formation of inter-layer between the coating and the substrate Fig. 6 shows the bright-field image of a cross-section of the assputtered sample. There was a very thin (about 20 nm) layer between CrAlSiN nanocomposite coating and nitrided AISI 304 substrate. In Fig. 7, the thin layer was analyzed by EDS and three distinct regions were revealed. The region 1 which was about 3 nm near the substrate was formed because the iron atoms diffused out. The region 2 about 16 nm was chromium-rich layer, as the abundant chromium from AISI 304 steel and CrAlSiN coating accumulated in the boundary, resulting an increase in Cr concentration. Some diffusion of nitrogen into region 2 might also take place, yet it was rather difficult to precisely analyze the content of nitrogen by EDS. The region 3 about 2 nm near the coating was formed because of the Cr diffusion. In literature, the Cr-based nitride coatings deposited on nitrided steel generally had better adhesive
Fig. 8 shows the friction coefficient of CrAlSiN nanocomposite coatings as a function of nitrogen contents in the substrate at ambient temperature. The friction coefficient of all samples was about 0.8, indicating that the friction coefficient was not influenced by the nitridation of substrates. Fig. 9 presents the 2D profilometric curves of the wear tracks, in which the maximum wear depth was located at the middle of wear tracks and both sides had accumulation of wear debris. Fig. 10 presents the morphology of wear tracks through scanning electron microscope. The CrAlSiN coating on the substrate with the minimum nitrogen contents showed dark area at the wear scar because of the large gradient of depth, which indicated that there was large wear volume. As the nitrogen contents increased in the substrate, the gradient of depth decreased and wear scar was bright, implying the decreased wear
Fig. 7. EDS line scan analysis of the cross-section image.
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Fig. 9. The 2D profilometric curves of the wear tracks.
volume of the coating. The wear rate of coatings on substrates with 7.7, 13.2, and 15.5 at.% nitrogen contents was 3.1, 1.5, and 0.6×10−5 mm3/ N ∙ m, respectively. Moreover, the coating on the substrate with 17.9 at.% nitrogen content exhibited nearly no wear volume on the wear track. Therefore, the wear rate calculated from Eq. (1) was zero. The improvement in wear performance was attributed to good adhesive strength and high compressive stresses of the coatings. 4. Conclusion In this study, CrAlSiN nanocomposite coatings were successfully fabricated on nitrided AISI 304 substrate via duplex coating techniques. The CrAlSiN coating on the AISI 304 substrate after 5 h nitriding process with working power 300 W revealed the best adhesive strength. The
strengthening mechanism in adherence could be concluded as follows: (1) the reduction in structure incoherence between the coating and substrate owing to phase transformation from γ-Fe to the combination of ε-Fe2–3N, γ′-Fe4N and CrN phases, (2) the reduction in CTE mismatch between the coating and substrate, (3) the increasing surface roughness of the substrate, and (4) the formation of Crrich nitride layer in the boundary of coating and substrate. Moreover, the wear rate of the CrAlSiN coatings decreased as the nitrogen contents in the AISI 304 stainless steel increased. The coating on the substrate with 17.9 at.% nitrogen content exhibited the best wear resistance. Therefore, the CrAlSiN nanocomposite coatings deposited on the AISI 304 stainless steel by duplex coating techniques exhibited not only adhesion improvement but also wear resistance of the coating.
Fig. 10. SEM images of wear scars of CrAlSiN coating on the substrate after (a) 1 h nitriding process with working power 150 W and (b) 5 h nitriding process with working power 300 W.
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Acknowledgement The authors thank the financial support from the Ministry of Science and Technology, Taiwan under project 102-2221-E-007-045-MY3. The assistance in AFM analysis at National Taiwan University of Science and Technology, and the assistance in EPMA analysis in Precision Instrumentation Center at National Tsing Hua University are appreciated. References [1] M. Deepak, R. Khushwant, Comparison of mechanical properties and calculation of cutting force between uncoated tungsten carbide & Ti coated tip tungsten carbide with the help of lathe tool dynamometer, Int. J. Mod. Phys. Conf. Ser. 22 (2013) 645–653. [2] P. Richard, J. Thomas, D. Landolt, G. Gremaud, Combination of scratch-test and acoustic microscopy imaging for the study of coating adhesion, Surf. Coat. Technol. 91 (1997) 83–90. [3] Y.S. Hong, S.H. Kwon, T. Wang, D.I. Kim, J. Choi, K.H. Kim, Effects of Cr interlayer on mechanical and tribological properties of Cr-Al-Si-N nanocomposite coating, Trans. Nonferrous Met. Soc. Chin. 21 (2011) s62–s67. [4] C.C. Chang, H.W. Chen, J.W. Lee, J.G. Duh, Influence of Si contents on tribological characteristics of CrAlSiN nanocomposite coatings, Thin Solid Films 584 (2015) 46–51. [5] C.C. Chang, H.W. Chen, J.W. Lee, J.G. Duh, Development of Si-modified CrAlSiN nanocomposite coating for anti-wear application in extreme environment, Surf. Coat. Technol. 284 (2015) 273–280. [6] J.H. Yun, S.J. Heo, K.R. Kim, K.H. Kim, Synthesis and mechanical properties of CrMoCxN1 − x coatings deposited by a hybrid coating system, J. Vac. Sci. Technol. A 26 (2008) 146. [7] P. Holubar, M. Jilek, M. Sima, Present and possible future applications of superhard nanocomposite coatings, Surf. Coat. Technol. 133-134 (2000) 145–151. [8] M. Stappen, M. Kerkhofs, C. Quaeyhaegens, L. Stals, Introduction in industry of a duplex treatment consisting of plasma nitriding and PVD deposition of TiN, Surf. Coat. Technol. 62 (1993) 655–661. [9] P.J. Burnett, D.S. Rickerby, The mechanical properties of wear-resistant coatings: II: experimental studies and interpretation of hardness, Thin Solid Films 148 (1987) 51–65.
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