CrSiN multilayer coatings by cathodic arc ion-plating

Applied Surface Science 314 (2014) 581–585

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Synthesis and characterization of AlTiSiN/CrSiN multilayer coatings by cathodic arc ion-plating B. Yang a,∗ , C.X. Tian a,b , Q. Wan a , S.J. Yan b , H.D. Liu a , R.Y. Wang a , Z.G. Li a , Y.M. Chen a , D.J. Fu b a b

School of Power & Mechanical Engineering, Wuhan University, 430072 Wuhan, China Accelerator Laboratory, Department of Physics, Wuhan University, 430072 Wuhan, China

a r t i c l e

i n f o

Article history: Received 10 December 2013 Received in revised form 17 May 2014 Accepted 24 May 2014 Available online 7 July 2014 Keywords: AlTiSiN/CrSiN SiH4 flowrate Adhesive force Microhardness Tribological performance

a b s t r a c t AlTiSiN/CrSiN multilayer coatings were deposited on Si (1 0 0) and cemented carbide substrates using Cr, AlTi cathodes and SiH4 gases by cathodic arc ion plating system. The influences of SiH4 gases flowrate on the structural and mechanical properties of the coatings were investigated, systematically. AlTiSiN/CrSiN coatings exhibit a B1 NaCl-type nano-multilayered structure in which the CrSiN nano-layers alternate with AlTiSiN nano-layers with multiple orientations of crystal planes indicated by XRD patterns and TEM. Si contents of the coatings increase with increasing SiH4 flowrate. The hardness of the coatings increases to the maximum value of 3500 Hv0.05 with increasing SiH4 flowrate from 20 to 40 sccm and then decreases with further addition of SiH4 gases. A higher adhesive force of 73 N is obtained at the flowrate of 48 sccm. The coatings exhibit different tribological performance when the mating materials were varied from Si3 N4 to cemented carbide balls and the variation of friction coefficients of the coatings against Si3 N4 influenced by SiH4 flowrate are not obvious as against cemented carbide balls. © 2014 Elsevier B.V. All rights reserved.

1. Introduction TiN is the most popular hard tool coating used on different machining environment in industrial scale for nearly 30 years. However, TiN coating is limited to use in high temperature less than 500 ◦ C because of the oxidation. Al is incorporated into TiN for forming TiAlN coatings and improving the oxidation resistance up to 800 ◦ C [1–9]. Furthermore, Cr is added into TiAlN coating for obtaining a multi-element Ti-Al-Cr-N compound or forming a multilayer structure, such as AlTiN/CrN [10], to improve the wear and oxidation resistance, by combining the good oxidation resistance of AlTiN and excellent wear resistance of CrN. In the attempt of adding Cr into AlTiN, previous research has revealed that a slight addition of Cr will cause greater improvement in cutting performance for machining hardened steels [11,12]. Incorporating Si into ternary or quaternary compounds is also a new way in fabricating super hard tool coatings for improving the hardness and structure stability at elevated temperature above 1100 ◦ C, which leads to the

∗ Corresponding author. Tel.: +86 27 6877 2253; fax: +86 27 6877 2253. E-mail address: [email protected] (B. Yang). http://dx.doi.org/10.1016/j.apsusc.2014.05.166 0169-4332/© 2014 Elsevier B.V. All rights reserved.

development of TiSiN, TiAlSiN, CrAlSiN and TiCrAlSiN nanocomposite coatings used in dry cutting [13–18]. As a newly developing super hard composite coating, TiCrAlSiN coating shows a multilayered structure in which nano-crystalline TiCrN layers alternate with amorphous AlSiN layers and exhibits high hardness of 43 GPa [17]. Usually, most of TiCrAlSiN coatings are now deposited by metal and alloy targets (Ti, Cr, SiAl, TiAlSi or TiCrAlSi) under nitrogen ambient by arc ion plating [17,19–22]. However, the industrial scale doping uniformity of Si, generated from the solid target, into the composite PVD coatings is still a challenging work. SiH4 gases are the most popular Si source used in chemical vapor deposition, which exhibits good doping performance and has been used widely for depositing thick hard coatings onto the cemented carbide or another heat-resisting alloy steel [13,18,23]. Few researches are now focused on the utilization of silane (SiH4 ) as the Si source to synthesize the TiSiNbased composite coatings in lower temperature PVD methods [24]. In this study, AlTiSiN/CrSiN nano-multilayer coatings were deposited by arc ion platings under silane ambient using Cr and AlTi alloy targets. The effects of silane flowrate on the structure and properties of AlTiSiN/CrSiN coatings were investigated systematically.

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B. Yang et al. / Applied Surface Science 314 (2014) 581–585

Table 1 Deposition parameters for AlTiSiN/CrSiN coatings.

(200)

(111) Value

Target material Substrate bias voltage (V) Reaction gas SiH4 flowrate (sccm) Base pressure (Pa) Working pressure (Pa) Arc current (A) Cr targets AlTi targets Substrate temperature (◦ C) Ratation speed (rpm) Deposition time (min) Cr interlayer AlTiSiN/CrSiN coating

AlTi (67 at.% Al), Cr −200 N2 , SiH4 20, 30, 40, 48, 60, 78 7.0 × 10−3 3.3 70 70 300 4 10 30

Intensity (arb.units)

Parameters

(220)

78 sccm 60 sccm 48 sccm 40 sccm 30 sccm Cr 20 sccm CrN

AlTiN

35

40

45

50

55

60

65

2 (deg.) 2. Experiment details Fig. 1. XRD patterns of AlTiSiN/CrSiN coatings with varied SiH4 flowrate.

2.1. Deposition AlTiSiN/CrSiN coatings were deposited onto Si (1 0 0) and cemented carbide substrates by a cathode arc ion plating system. The Si substrates were coated for the measurements of XRD, EDS, AFM and the preparation of TEM samples, while the cemented carbide substrates were used for the analysis of mechanical properties. The equipment used for the deposition in this study has been depicted in a previous paper [25]. Four Cr and four AlTi (67:33 at.%) alloy targets were mounted alternatively on the opposite sides of chamber wall. Before multilayer coatings deposition, the substrates were etched in argon glow discharge plasma for 20 min and then Cr interlayer was deposited for 10 min. SiH4 gases carried by nitrogen with a ratio of 9:1 (N2 :SiH4 , in volume) were varied from 20 to 78 sccm during the deposition for obtaining different Si content. In addition, another pure N2 gases were fed into the chamber from another mass flowrate controller to balance the pressure at a fixed value of 3.3 Pa. The substrates holder spins as it moves around the axis of the chamber. A multilayer structure can be obtained when the substrates rotate in the deposition chamber and face the AlTi target and Cr target, sequentially. The distance from the axis of the substrates holder to targets was approximately 250 mm. The samples were fixed on the edge of the 140 mm disc mounted on the substrates holder. A temperature controller was applied to control the heater by which the temperature of the substrate was fixed at about 300 ◦ C. The more deposition details were summarized in Table 1. 2.2. Characterization The crystallinity and microstructure were analyzed by using symmetrical geometry X-ray diffraction (XRD) performed on a D8 advanced X-ray diffractometer with a Cu K␣ radiation (0.15418 nm). The microstructure observation of the coatings was carried out on JEM 2010 transmission electron microscope (TEM). The chemical compositions were measured by an EDAX genesis 7000 energy dispersive spectrometer (EDS) operated at 12 kV (with standard atomic number, absorption and fluorescence (ZAF) correction). The surface roughness was analyzed using an atomic force microscope (AFM) (Shimadzu SPM-9500J3) operated in the tapping mode with a measuring area of 10 × 10 ␮m2 . On each sample, there are three places were selected at random location and three numbers of root mean square (rms) were calculated values for an average. The hardness was measured using an HX-1000 microhardness tester with a load of 50 g and taking the average of 10 values. The wear behavior was evaluated through sliding tests using a

conventional ball-on-disc wear tester (MS-T3000). Si3 N4 and cemented carbide ball (3 mm in diameter) were used as counterpart materials. The sliding tests were conducted with a sliding speed of 0.02 m/s under a load of 4 N at room temperature (around 30 ◦ C) and relative humidity (70–75% RH) condition. The adhesion was evaluated on a computer controlled scratch tester (WS-2002) equipped with an acoustic detector and a Rockwell diamond probe (R = 200 ␮m). The constant scratching velocity was 2.8 mm/min and the applied force increased from 0 to 90 N with a loading rate of 50 N/min, continuously. 3. Results and discussion Fig. 1 reveals the crystal structure of the deposited coatings with varied SiH4 flowrate. The XRD patterns show that AlTiSiN/CrSiN coatings exhibit a B1 NaCl-type structure and multiple orientations of crystal planes, corresponding to (1 1 1), (2 0 0) and (2 2 0) of AlTiN/CrN. The small peak of interlayer Cr also appears at 2 of 44.1◦ . The dash lines and dots lines indicate the Bragg angles of corresponding planes from the standard reference AlTiN and CrN samples (JCPDS 38-1420 and 11-0065), respectively. No obvious diffraction peaks of Si or Si alloy are observed, which is consistent with previous reports on TiSiN nanocomposite coatings prepared by Vepˇrek and Reiprich [26], suggesting that Si is present in an amorphous state. It is worth noting that the diffraction peaks of multilayer coatings locate at the positions between AlTiN and CrN phases. This phenomenon may be the results of the interaction between AlTiN and CrN phases in the coatings deposition. The diffraction intensity of (1 1 1) and (2 2 0) planes decreases with the increase of SiH4 flowrate from 20 to 78 sccm and the diffraction peaks of that are almost disappeared at the maximum SiH4 flowrate. However, the diffraction peak of (2 0 0) is intensified gradually with the increase of SiH4 flowrate from 20 to 48 sccm, then reduced diffraction intensity and broadening (2 0 0) peak are exhibited while further increase of SiH4 flowrate (the FWHM of (2 0 0) peak increases from 0.81 to 1.13◦ with the increase of SiH4 flowrate from 48 to 78 sccm). Furthermore, the weaker intensity and larger FWHM of diffraction peaks of 78 sccm SiH4 flowrate are believed to originate from the refinement of grain size [27]. There is no obvious diffraction peaks shift of the coatings with increasing Si content, which may be due to that the Si atoms were not dissolved into the coatings lattice [28,29]. The chemical compositions of AlTiSiN/CrSiN coatings as a function of SiH4 flowrate are shown in Fig. 2. It can be seen that the Si contents increase with increasing SiH4 flowrate, which means more Si were generated from the decomposition of SiH4 . The metal

B. Yang et al. / Applied Surface Science 314 (2014) 581–585

50

N

Cr

Al

Ti

583

160

Si

45

140

35 30

RMS (nm)

Atomic ratio (at.%)

40

25 20 15

120

100

10 5 0 15 20 25 30 35 40 45 50 55 60 65 70 75 80

80

20

30

50

60

70

80

SiH4 flowrate (sccm)

SiH4 flowrate (sccm) Fig. 2. Effect of SiH4 flowrate on chemical composition of coatings.

40

Fig. 4. Variation in RMS roughness measured from AFM images of AlTiSiN/CrSiN coatings as a function of SiH4 flowrate.

3.8

Deposition rate ( m/h)

3.6 3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0

20

30

40

50

60

70

80

SiH4 flowrate (sccm) Fig. 5. Deposition rate of AlTiSiN/CrSiN coatings as a function of SiH4 flowrate. Fig. 3. Cross-sectional HR-TEM micrograph and selected area electron diffraction pattern of AlTiSiN/CrSiN coating deposited at 40 sccm SiH4 flowrate.

contents decrease with increasing SiH4 flowrate from 40 to 78 sccm and the variation values are less than 5%. Moreover, N contents in the coatings are lower than 50 at.%, from which it can be seen that the as-deposited coatings are slightly understoichiometric. Fig. 3 shows a typical cross-sectional TEM micrograph of the AlTiSiN/CrSiN coatings deposited at 40 sccm SiH4 and the corresponding selected area electron diffraction (SAED) pattern with selected area size of 200 nm in diameter. The bright field TEM micrograph of the coatings shows a well defined multilayer structure. The alternating AlTiSiN and CrSiN layers are perpendicular to the growth direction, which are shown as the bright and dark interval fringes with a bilayer thickness of 9.6 nm. It should be noted that 9.6 nm is valid only for the coatings deposited at 40 sccm SiH4 and the bilayer thickness for the other coatings will show the same tendencies with respect to their deposition rate considering the same deposition time and rotation speed. From the image it can be seen there exists a sharp interfaces which indicate less intermixing between the alternating layers. The diffraction rings correspond to diffraction lines of (1 1 1), (2 0 0) and (2 2 0), which is consistent with the XRD results shown in Fig. 1. Moreover, the diffraction rings of (3 1 1), (2 2 2), (4 0 0) and (3 3 1) also could be observed. The root mean square (RMS) roughness measured from AFM images of the AlTiSiN/CrSiN coatings as a function of the SiH4 flowrate is shown in Fig. 4. The RMS of the coatings increases to 153 nm as the increasing flowrate up to 40 sccm, then decreases with further addition of the SiH4 gases. It is known that if the more

excessive reactive N2 are fed into the chamber, a thin layer of nitride will be formed on the target surface and results in the “target poisoning” [30]. The melting point of nitride is much higher than that of AlTi alloy [31], while the melting point of CrN is much lower than pure Cr [32]. Usually, the coatings deposited by AlTi alloy target shows the highest number of droplets emission compared with Cr, Ti and Al target [33,34]. From above mention it can be seen that the increasing SiH4 addition will reduce the N2 flowrate because of the fixed total pressure, which will decrease the formation of poisoning layer on the target surface. Therefore, the more droplets emission of the AlTi target will increase the RMS of the coatings. Meanwhile, more hydrogen decomposed from SiH4 will etch the surface of the coatings with further increasing SiH4 flowrate, which will result in the decreasing RMS roughness. The deposition rates of AlTiSiN/CrSiN coatings, which were calculated via the coating thickness and deposition time, are shown in Fig. 5. The results indicate that the deposition rate increased initially and then decreased with further addition of SiH4 gases, which is believed to be mainly related with the decrease of N2 gas and the decomposition of SiH4 . With increasing the SiH4 flowrate from 20 to 48 sccm, Si ions generated from the decomposition of SiH4 may be the one reason for the increase of deposition rate, while the subdued target poisoning may be another possible reason. However, with further SiH4 addition, the excessive hydrogen decomposed from SiH4 may etch the as-deposited coating and reduce the deposition rate [23]. Fig. 6 shows the micro-hardness of AlTiSiN/CrSiN coatings as a function of SiH4 flowrate and silicon content. The hardness

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B. Yang et al. / Applied Surface Science 314 (2014) 581–585 Silicon ratio (at.%)

2

4

6

8

3500

Micro-hardness (Hv0.05)

0.7 10

12

Against cemented carbide Against Si3N4

14

Silicon ratio SiH4 flowrate

3000

2500

2000

Average friction coefficient

0

0.6 0.5 0.4 0.3 0.2 0.1 20

20

30

40

50

60

70

80

SiH4 flowrate (sccm)

30

40 50 60 SiH4 flow rate (sccm)

70

80

Fig. 8. Average friction coefficient of AlTiSiN/CrSiN coatings as function of SiH4 flowrate against cemented carbide and Si3 N4 ball.

Fig. 6. Micro-hardness of CrAlTiSiN coatings depending on SiH4 flowrate and silicon content.

increases from 3000 Hv0.05 to 3500 Hv0.05 as SiH4 flowrate increase from 20 to 40 sccm and the Si content increases from 1.41 at.% to 4.11 at.%. However, in our research, the hardness dramatically decreased from 3500 Hv0.05 to 1900 Hv0.05 when the corresponding Si content increase from 4.11 at.% to 12 at.% and the SiH4 flowrate increase to 78 sccm. In general, the addition of Si elements will lead to the refinement of crystal grain size as well as the increase of grain boundaries which will hinder the dislocation mobility, therefore, the hardness of coatings will increase. Furthermore, excess Si elements will result in the increase of amorphous phase which could be the reason of decreasing hardness. The results are different from other report partly due to the difference of the coating deposition process [28]. Fig. 7 shows the correlation of the typical acoustic emission (AE) signals, the frictional force (FF) and the applied force during scratch testing in the three different samples deposited at 40, 48 and 78 sccm. The adhesive strength of coatings is affected by a number of factors – composition and stress. For example, the multilayer coatings possess a high adhesive force of 70 N at the condition

of 48 sccm SiH4 flowrate, while the coatings show satisfactory deposition rate and hardness. The smaller surface roughness may be responsible for better adhesive force of the coatings deposited at 48 and 78 sccm (as shown in Fig. 4). The average friction coefficients of AlTiSiN/CrSiN multilayer coatings against cement carbide and Si3 N4 balls are shown in Fig. 8 as the function of the SiH4 flowrate. As SiH4 flowrate increases from 20 sccm to 78 sccm, the average friction coefficient of the coating increases from 0.5 to 0.55 against the Si3 N4 ball and from 0.1 to 0.35 against the cement carbide ball, respectively. The varied average friction coefficient of AlTiSiN/CrSiN coatings with increasing SiH4 flowrate from 20 to 48 sccm may be attributed to the combined effect of hardness and surface roughness. However, as further increase of SiH4 flowrate to 78 sccm, the increasing friction coefficient may be affected by the rapid deterioration of hardness predominantly. The AlTiSiN/CrSiN coatings show a relatively lower friction coefficient against the cemented carbide ball compared with the Si3 N4 ball. This may be attributed to that the harder Si3 N4 ball will easily flake away the AlTiSiN/CrSiN coatings than the cemented carbide ball in the wear process.

Fig. 7. The frictional force and acoustic emission signal as a function of applied load in a scratch test of AlTiSiN/CrSiN coatings deposited at different SiH4 flowrate.

B. Yang et al. / Applied Surface Science 314 (2014) 581–585

4. Conclusion The AlTiSiN/CrSiN coatings have been deposited onto Si (1 0 0) and cemented carbide substrates by a cathodic arc ion plating system. The results showed that the AlTiSiN/CrSiN coatings exhibit a multilayered structure in which the AlTiSiN nano-layers alternate with CrSiN nano-layers. The flowrate of SiH4 exhibited greater effects on the hardness of the coatings. The maximum hardness of 3500 Hv0.05 is obtained at the flowrate of 40 sccm with 4.11 at.% Si contents. The multilayer coatings possess a relatively high adhesive force of 70 N at 48 sccm SiH4 flowrate. The coating shows relatively lower friction coefficient against cemented carbide than Si3 N4 . Acknowledgments This work was supported by the National Natural Science Foundation of China under contract No. 50905130 and 11275141, the Doctoral Fund of Ministry of Education of China for New Teacher under contract No. 20090141120067 and the International Cooperation Program of the Ministry of Science and Technology of the People’s Republic of China under contract No. 2011DFR50580. References [1] O. Knotek, T. Leyendecker, J. Solid State Chem. 70 (1987) 318. [2] O. Knotek, M. Atzor, A. Barimani, F. Jungblut, Surf. Coat. Technol. 42 (1990) 21. [3] T. Leyendecker, O. Lemmer, S. Esser, J. Ebberink, Surf. Coat. Technol. 48 (1991) 175. [4] H. Ichimura, A. Kawana, J. Mater. Res. 8 (1993) 1093. [5] G. Fox-Rabinovich, B. Beake, J. Endrino, S. Veldhuis, R. Parkinson, L. Shuster, M. Migranov, Surf. Coat. Technol. 200 (2006) 5738. [6] S. Harris, Wear 254 (2003) 723.

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