Microstructure, mechanical and tribological properties of CrSiC coatings sliding against SiC and Al2O3 balls in water

Microstructure, mechanical and tribological properties of CrSiC coatings sliding against SiC and Al2O3 balls in water

Accepted Manuscript Title: Microstructure, mechanical and tribological properties of CrSiC coatings sliding against SiC and Al2 O3 balls in water Auth...

3MB Sizes 15 Downloads 74 Views

Accepted Manuscript Title: Microstructure, mechanical and tribological properties of CrSiC coatings sliding against SiC and Al2 O3 balls in water Author: Zhiwei Wu Fei Zhou Kangmin Chen Qianzhi Wang Zhifeng Zhou Jiwang Yan Lawrence Kwok-Yan Li PII: DOI: Reference:

S0169-4332(16)30137-4 http://dx.doi.org/doi:10.1016/j.apsusc.2016.01.276 APSUSC 32508

To appear in:

APSUSC

Received date: Revised date: Accepted date:

30-11-2015 19-1-2016 30-1-2016

Please cite this article as: Z. Wu, F. Zhou, K. Chen, Q. Wang, Z. Zhou, J. Yan, L.K.-Y. Li, Microstructure, mechanical and tribological properties of CrSiC coatings sliding against SiC and Al2 O3 balls in water, Applied Surface Science (2016), http://dx.doi.org/10.1016/j.apsusc.2016.01.276 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

*Highlights (for review)

Highlights ●CrSiC coatings with Si content of 2.0-7.4 at.% were deposited via adjusting the TMS flow.

ip t

● The amorphous structure in the CrSiC coatings was presented. ●No obvious fluctuations of hardness (about 13 GPa) were observed with TMS flow.

cr

●CrSiC/SiC tribopairs showed better tribological performance than CrSiC/Al2O3

Ac

ce pt

ed

M

an

us

tribopairs.

Page 1 of 38

*Graphical Abstract (for review)

Graphical Abstract CrSiC coatings were speculated to be X-ray amorphous(Fig.1). Although the hardness of coatings fluctuated slightly (13.2~13.8 GPa), the CrSiC coatings showed poor wear resistance due to the decline of the crack resistance and toughness. Moreover, the friction coefficient (0.24~0.31) and the wear rate (2.97~7.66×10-6 mm3/Nm) of CrSiC/SiC trobopairs were lower than those of CrSiC/Al2O3 tribopairs (Figs.2 and 3). (220) Si

(200)

ip t cr

CrSiC-20

us

Intensity (a.u.)

CrSiC-30

Fig.1

30

40

50

60

CrSiC/Al2O3 tribopairs

0.3 0.2 0.1 0.0

10

20 TMS flows (sccm)

30

Ac

Fig.2 Mean steady friction coefficients of CrSiC coatings sliding against ceramic balls in deionized water.

90

CrSiC coating against SiC balls SiC balls CrSiC against Al2O3 balls Al2O3 balls

3

Specific wear rate(mm /Nm)

0.5

ed

0.6

-4

10

ce pt

Mean steady friction coefficient ()

CrSiC/SiC tribopairs

0.4

80

X-ray diffraction spectra of the CrSiC coatings with various TMS flows.

0.8 0.7

70

2()

M

20

an

CrSiC-10

-5

10

-6

10

-7

10

10

20 TMS flows/sccm

30

Fig.3 specific wear rates of CrSiC coatings and ceramic balls.

Page 2 of 38

*Manuscript

Microstructure, mechanical and tribological properties of CrSiC coatings sliding against SiC and Al2O3 balls in water Zhiwei Wua,b,c, Fei Zhoua,b*, Kangmin Chend, Qianzhi Wange, Zhifeng Zhouf, Jiwang Yane , Lawrence Kwok-Yan Lif a

cr

ip t

State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China b College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics and Jiangsu Key Laboratory of Precision and Micro-Manufacturing Technology, Nanjing, 210016,China c College of Mechanical and Electrical Engineering, Nanjing Forestry University, Nanjing 210037, China d Center of Analysis, Jiangsu University, Zhenjiang, 212013,China e Department of Mechanical Engineering, Keio University, Yokohama, 2238522, Japan f Advanced Coatings Applied Research Laboratory, Department of Mechanical and Biomedical Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, China

us

Abstract: CrSiC coatings with different silicon contents were prepared using unbalanced

an

magnetron sputtering via adjusting trimethylsilane (Si(CH3)3H) flows. Their phase structure, bonding structure, microstructure and hardness were characterized by X-ray diffraction (XRD),

M

X-ray photoelectrons spectroscopy (XPS), a field emission scanning electron microscope (FESEM) and nano-indenter, respectively. The tribological properties of CrSiC coatings sliding against SiC

ed

and Al2O3 balls were investigated in water. The results showed that the CrSiC coatings were speculated to be X-ray amorphous. Although the hardness of coatings fluctuated slightly (13.2~13.8

ce pt

GPa), the coatings showed poor wear resistance due to the decline of the crack resistance and toughness. Moreover, the friction coefficient (0.24~0.31) and the wear rate (2.97~7.66×10-6 mm3/Nm) of CrSiC/SiC trobopairs were lower than those of CrSiC/Al2O3 tribopairs.

Ac

Keywords: CrSiC coatings; Surface analysis; Hardness; Sliding friction; Wear; water lubrication 1. Introduction

Recently, water-lubrication systems have already been proposed to replace oil-lubrication systems in the fields of food & medicine factories, drainable pumps and hydraulic systems because of pollution caused by oil-lubrication systems. But water lubrication systems present some technical problems for metallic materials, such as lubricity, corrosion and reliability [1]. In view of the above *

Corresponding author. Tel./Fax: +86 25 84893083. E-mail address: [email protected](F. Zhou) 1

Page 3 of 38

situation, the surface modification of stainless steels via depositing hard coatings was an effective method. At present, carbon-based coatings and transition metal nitride-based coatings are the promising candidates for the sliding parts in water hydraulic systems, the journal bearings and mechanical face seals for water pump [2, 3]. Table1 in Ref.[4] shows that the CrN-based coatings

ip t

have been proved to be suitable in water lubrication. However, their low hardness and high friction

cr

coefficients would limit their wide application in industry. Although the carbon-based coatings (such as Diamond, DLC, a-CNx and GLC) have exhibited low friction coefficient and low wear rate

us

as sliding against ceramic and steel balls in water [2, 5-8], their poor adhesion strength to substrate,

an

high internal stress and weak thermal stability also worsen their application life [5, 9-10]. According to the results in Refs.[11-14, 16-17], it is clear that doping transition metal elements (Ti,

M

Cr etc.) into the carbon-based coatings could effectively reduce the internal stress and improve the

ed

adhesion strength of coatings. As seen in Table 1, Refs.[9, 14-16] have reported that the low friction coefficient (µ=0.1~0.3) was obtained when the Cr-DLC coatings with low Cr content slid against

ce pt

steel in the 40~50% humidity. Zhou’s groups [12-13] also reported that Cr/a-C films sliding against different mating balls (stainless steel, Al2O3, SiC and Si3N4) presented good water lubrication characteristics. A low friction coefficient (μ=0.07~0.3) was obtained with low Cr content (≤ 4.9

Ac

at.%), while the friction coefficient and wear rate all increased with increase of Cr content (12.0~14.1 at.%) due to serious abrasive wear. Similarly, Keunecke et al. [18] also pointed out that the CrC/a-C:H coatings with high Cr contents (Cr﹥50 at.%) provided poor wear resistance under oil lubrication. In addition, the influences of crystalline structures (amorphous structure, nanocomposite and crystalline composite) in Cr-C system on the mechanical and tribological properties of CrC films have been investigated [19-22]. For example, Jellad et al.[23] have reported the crystalline Cr3C2 film (partially crystallized) exhibited a higher hardness (24~25 GPa) than 2

Page 4 of 38

amorphous Cr3C2 (18 GPa). Though the corrosion and oxidation resistance of a-CrC/a-C coatings were improved via increasing a-C content to form dense microstructure [19-21], their corresponding hardness (6.9~13 GPa) usually was lower than that of crystalline CrC coatings (18~25 GPa) [19, 23-24]. Furthermore, because the high friction coefficient (0.68~1.04) was obtained when the CrC x

ip t

coatings slid against steel ball in air [17], the CrCx coatings could not satisfy the application

cr

requirements under extreme conditions. Due to surface graphitization during sliding, the nc-CrxCy/a-C:H coatings displayed low friction coefficient [25-27], but the friction property of

us

hydrogen containing carbon-based film is susceptible to temperature [28]. Thus, how to further

an

improve the mechanical and tribological properties of CrC-based coatings are key factors to the application of engineering.

M

In recent years, it has been found that the silicon was one of the most effective alloying

ed

elements to improve mechanical and tribological properties of coatings [29-32]. According to Refs.[33-35], the hydration reaction of silicon-contained compounds was easily occurred to form

ce pt

the Si(OH)x gel on the friction surface as sliding in water, and the Si(OH)x gel as self-lubricating transfer layer was beneficial to improve the wear and friction properties of coatings in water [31-33]. For example, Refs.[29-31] have reported that the mechanical properties, corrosion resistance and

Ac

oxidation resistance of CrN coatings were enhanced via introducing silicon into coatings. Furthermore, the friction and wear properties of CrSiN and Si-DLC coatings were also improved due to the tribochemical reaction in water, which was attributed to the formation of silicon nitride or silicon carbide phase [31-32]. For example, Geng et al. [31] have reported the friction coefficient of CrSiN coatings (μ=0.35) with Si/(Cr+Si) of 8.9 at.% was lower than that of CrN coatings (μ=0.46) as sliding against WC balls in water, and their wear resistance was also improved. In our previous reports [4], the CrSiCN with 2.1 at.% Si showed low friction coefficient of 0.11 and wear rate 3

Page 5 of 38

8.4×10−8 mm3/Nm as sliding against SiC balls in water. Ziegele et al. [36] have compared the friction behavior of single layer SiC, CrC and multilayer CrC/SiC coatings against a fixed SAE 52100 steel ball in air, the friction coefficients of all multilayer coatings kept at around 0.2 and was lower than that of single layer, whilst higher wear was obtained as compared with single layer.

ip t

Bertóti et al. [37-38] have studied the composition and chemical structure of CrSiC coatings as a

cr

function of silicon content, and reported that the hardness and Young's modulus of ternary CrSiC coatings with the Si content of 12.8~25.7 % varied between 13~16 GPa and 120~140 GPa. the

us

CrSiC films presented a significant elasticity and scratch resistance by nanoscratch tests, and the

an

silicide (CrxSi) might been formed at relatively high Cr and low Si content [39]. However, as a function of Si content, transition metal nitride or carbide coatings with low silicon content such as

M

CrSiN (Csi=4.1~6.7 at.%) [29, 40], TiSiC (Csi=9~16 at.%) [41], CrSiCN (Csi=0~3.4 at.%) [4] etc.

ed

presented better mechanical properties that were attributed to the solid solution hardening and the formation of nanocomposite microstructure. But with further increasing Si content, superfluous

ce pt

amorphous silicides (a-Si3N4, a-SiC) were formed at the grain boundary of nanocrystallites that weaken the grain boundary strength and the wear resistance of coatings. In addition, the compressive stress in the CrSiC coatings reduced with an increase in Si content, while their crack

Ac

resistance became poor when the Si content was higher than 3.5 at.% [42]. However, the microstructure and tribological behavior of CrSiC coatings with low Si content in water lubrication have not yet been studied.

In here, the CrSiC coatings with low silicon contents were deposited on Si (100) wafers and 316L stainless steel disks using unbalanced magnetron sputtering via adjusting trimethylsilane [(CH3)3SiH or TMS] flow, and the influence of TMS flow on the microstructure, mechanical properties and tribological properties of CrSiC coatings in water was outlined. 4

Page 6 of 38

2. Experimental details 2.1. Deposition of CrSiC coatings CrSiC coatings were deposited on Si (100) wafers and 316L stainless steel substrates simultaneously by closed-field unbalanced magnetron sputtering system (UDP-650, Teer Coatings

ip t

Limited, UK). The stainless steel disks (Φ30 mm×4 mm) were polished to reach Ra=30 nm of

cr

surface roughness by a metallographic polishing machine (UNIPOL-820) [43-44]. Then the substrates were cleaned ultrasonically in deionizer water and ethanol for 20 min. The deposition

us

parameters of CrSiC coatings were listed in Table 2. The substrates were sputter cleaned with Ar

an

plasma generated by glow discharge at a bias voltage of −450 V for 30 min, and then a pure Cr transitional layer (~0.5 μm) was first deposited onto the substrates with a bias voltage −80 V for

M

15min; After that, the CrSiC coatings were deposited by co-sputtering of two Cr targets and four

ed

graphite targets at different TMS flow rates controlled by a MKS mass flow meter. During deposition, the flow rate of Ar working gas was fixed at 30 sccm via adjusting a MKS mass flow

ce pt

controller. The substrate temperature was approximately 150 ℃due to ion bombardment. 2.2. Characterization of CrSiC coatings

The crystal phase structure of CrSiC coatings were characterized by X-ray diffractometer

Ac

(XRD) (Ultima IVUltima IV, Japan) with Cu Kα radiation source (λ=0.1542 nm) with a scan rate of 10° per minute from 20o to 80o. The bonding structures of CrSiC coatings were determined using X-ray photoelectron spectroscopy (XPS, VG ESCALAB 220i-XL). The hardness (H) and Young's modulus (E) of coatings were the average value of the 36 times’ measurements, which was measured using nano-indentation tester (ENT-1100a, Elionix Co. Ltd., Japan) with an indentation penetration depth of 100 nm. The adhesion strength of CrSiC coatings on Si wafer was measured using a scratch tester (WS-2005, Scratch Tester, China) equipped with a hemispherical diamond tip 5

Page 7 of 38

(R=0.2 mm), and the applied load, sliding speed and wear track length were set as 40 N, 40 N/min, 3 mm, and the scratch tests were repeated for three times to obtain the average value. As one of oxide ceramics and non oxide ceramic materials, Al2O3 and SiC balls are commonly used as mating materials, and their surface roughness was 88.5 and 52.8 nm, the hardness was 22

ip t

and 16.5 GPa, and the elastic modulus was 430 and 370 GPa respectively. The diameter of ceramic

cr

balls was 8 mm. The tribological properties of CrSiC coatings sliding against SiC balls and Al2O3 balls in deionized water were investigated using a ball-on-disc tribo-meter, the normal load was 2 N

us

and the sliding speed was 0.1 m/s, the total sliding distance was 500 m. The friction tests were

an

repeated for three times to obtain the mean value of friction coefficients and wear rates. After friction test, the wear diameters of mating balls and the cross-section area of wear tracks were

M

measured by optical microscope (XJZ-6, China) and Micro-XAMTM non-contact optical

ed

profilometer (ADE Phase-Shift, USA) respectively, and then the specific wear rates for balls and coatings could be calculated [4]. To know the wear mechanism, the surface topography,

ce pt

cross-section micrographs and wear track of CrSiC coatings were observed using a field emission scanning electron microscope (SEM) (JEOL-JSM-7001F) equipped with EDS (Inca Energy 350, Oxford, UK).

Ac

3. Results and discussion

3.1. Microstructure of CrSiC coatings with different Si content Table 3 showed the composition variation of CrSiC coatings with different TMS flows. With an increase in the TMS flows, the content of Si increased from 2.0 at.% to 7.4 at.%, and the Cr content decreased gradually from 73.1 at.% to 66.1 at.%. While the carbon content fluctuated slightly in the range of 24.9 to 27.8 at.%. Fig.1 shows the X-ray diffraction patterns of CrSiC coatings at different Si contents. The CrSiC coatings only present a preferred diffraction peak at 44o. Although this 6

Page 8 of 38

diffraction peak might be calibrated to Cr23C6 (511) and the interlayer Cr (210) in Ref.[42], the Cr23C6 phase with large unit cells was formed difficultly at low deposited temperature. This indicated that the diffraction peak at 44o was attributed to the interlayer Cr (210). Andersson et al.[19] have reported a mixture of Cr23C6, Cr7C3, Cr2O3 and Cr phases were determined by XRD

ip t

after annealed at 800 ℃ with a carbon content of 25 at.%. Refs.[45-46] have pointed out that the

cr

diffraction peak mainly represented metallic chromium signals of Cr (110) from Cr interlayer. Thus no crystalline phase of CrSiC was detected by XRD. Jansson et al.[47] indicated that the amorphous

us

films could be obtained via adding non-metal element (B or Si) at reduced temperatures during

an

sputtering process. Similarly, the amorphous C-Si-Cr films were formed by doping Si elements from TMS vapor during sputtering process [37-38]. The grain sizes usually were reduced at a

M

critical Si concentration after adding Si into nc-MCx/a-C film, thus the carbide grains could not be

ed

detected due to smaller size than the X-ray coherence length [47]. Based on above analysis and presented diffractograms, the CrSiC coatings could be speculated to be X-ray amorphous.

ce pt

In order to reveal the bonding structures of Si, C and Cr in CrSiC coatings, the XPS spectra were deconvoluted using XPS 4.1 software and illustrated in Fig.2. The Cr2p spectra could be fitted into five peaks located at 574.1~574.3 eV, 574.9~575.2 eV, 576.6~576.8 eV, 583.7 eV, 585.1~585.6

Ac

eV, 586.8~587.1 eV. The peaks located at 574.1~574.3 eV were correspond to either Cr-Cr or Cr-C bonds (Cr3C2) [13, 38, 48], since the Cr2p for metallic Cr and chromium carbide showed very similar binding energies around 574.2 eV. The bonding energy of 583.7 eV was equal to the available data of Cr7C3 [13, 18]. Thus the bonding energy at 574.9~575.2 eV was attributed to Cr-Si/Cr-C bond [48-49]. Similarly, Jansson et al.[47] have indicated that two-phase or possibly three-phase film was formed easily in the ternary Me-Si-C films. This shows that at least one carbide or one silicide phase was formed in the Me-Si-C system. The rests of Cr2p peaks were 7

Page 9 of 38

assigned to chromium oxide [13]. The Si2p spectra could be deconvoluted into two peaks located at 98.9~99.4 eV and 101.5~101.7 eV (Fig.2b), which were assigned to Si-Si/Si-Cr and C-Si bonds, respectively [37, 52-53]. As seen in Fig.2a and 2b, the binding energies of Cr2p and Si2p all presented positive shift with increase of Si content. This indicated the formation of metal silicide

ip t

[38, 50]. Tengstrand et al.[52] also observed the positive shift of Si2p in TiSiC coatings with

cr

increase of Si content, and pointed out the bond transformation from Si-C bonds to Si-Ti bonds with an increase in the Si content. As seen in Tables 4 and 5, the total fraction of chromium oxide

us

decreased with increase of Si content, which was related to the formation of stable chromium

an

silicon carbide coatings.

The C1s spectra could be deconvoluted into five peaks located at 282.9 eV, 283.3~283.7 eV,

M

284.8 eV, 285.2~286.1 eV and 288.3~288.5 eV in Fig.2c. The bond energy of 282.9 eV was equal to

ed

the available data for C-Si/C-Cr [38]. The bond energy located at 283.3~283.7 eV was corresponded to C-Cr bonds [51, 54]. The peaks centered at 284.8 eV, 285.2~286.1 eV and

ce pt

288.3~288.5 eV were corresponded to sp2C–C, sp3C–C and C-O bonds, respectively [13, 38, 48]. As seen in Table 6, the total fraction of sp3C-C decreased from 17.6 at.% to 3.7 at.% with an increase in Si content, while the total fraction of C-Si bonds increased from 5.8 at.% to 18.8 at.%,

Ac

thus a larger number of sp3C–C bonds were broken to form amorphous SiC. In addition, the total fractions of C-C bonds (sp2+sp3) were above 60 at.%, which was attributed to high carbon concentration from the decomposition of TMS. While Högström et al.[20] believed that post-deposition oxidization of Cr-C bonds into Cr-O bond and liberated C (C-C) might lead to an over estimation of the amorphous carbon fraction. In addition, the oxygen bonded to Cr and C could be detected, which indicated the coating surfaces were contaminated in ambient air [38]. Fig.3 showed the surface and cross-section SEM images. The thickness of the CrSiC coatings 8

Page 10 of 38

and pure Cr transitional layer were about 1.2 μm and 0.5 μm respectively. The honeycomb plane was shown in Fig.3a, and the slender columnar structures like featheriness were illustrated in Fig.3b. With an increase in Si content, the CrSiC-20 coating showed dense microstructures with small grains on surface (Fig.3c), while micropore could be observed on CrSiC-30 coating (Fig.3e), and

ip t

the columnar structure gradually became indistinct and could not been distinguished.

cr

3. 2. Mechanical and water lubricated tribological properties of CrSiC coatings

Table 3 shows the nano-hardness and Young's modulus of CrSiC coatings at various Si contents.

us

The hardness of three coatings was 13.8, 13.2 and 13.6 GPa, and no distinct alterations could be

an

observed. Only the Young's modulus value of CrSiC-30 decreased to 262.0 GPa. In general, the mechanical property closely responded to the microstructure and phase condition of coatings. Their

M

similar amorphous microstructure resulted in slight fluctuations of hardness. The adhesion strength

ed

of CrSiC coatings was detected by a scratch tester in Fig.4, and the first critical load (Lc1) was determined by sudden appearance of acoustic emission signals which represented first microcrack

ce pt

of coating [31], and the higher critical load (Lc2 ) was extracted from the optical images of scratch that implied the delamination of coatings [55]. According to acoustic emission signals and ruler on the optical micrographs of scratch surface in Fig.4, the first acoustic emissions were detected at 6 N

Ac

whilst the first ridge spallation marked by white arrow was observed at 15 N for CrSiC-10 coating. And the first acoustic emissions were detected at 5N while the first ridge spallation was happened in the load 14 N for CrSiC-20 coating. But for the CrSiC-30 coating, the load for ridge spallation decreased to 11 N, and the penetrating phenomenon was happened immediately. It was obvious that the coatings became more brittle with an increase in the Si content. According to “Scratch Crack Propagation Resistance” (CPRS= Lc1·(Lc2-Lc1)) [55], the parameter CPRS could be used to evaluate the toughness of coatings. The CPRS values of three coatings were ranked as 55.2 N2, 45.0 N2, 30.1 9

Page 11 of 38

N2, which indicated the decreasing toughness of coatings. That result was in agreement with Ref.[42], the radial cracks were easily present at impression corners of CrSiC-20 and CrSiC-30 coatings due to the brittleness of the SiC [36]. The friction behavior of CrSiC coatings sliding against SiC and Al2O3 balls in deionized water

ip t

was illustrated in Fig.5. When the Si content increased from 2.0 to 7.4 at.%, the initial friction

cr

coefficient was effectively reduced (Fig.5a). As sliding against SiC balls, the friction coefficient increased gradually during the running-in stage, and then reached the steady value after sliding

us

200m. But the friction curves fluctuated up and down as sliding against Al2O3 balls except for

an

CrSiC-10 coating. As compared with the friction coefficient of 316L substrate sliding against Al2O3 (μ=0.52), it could be inferred that the CrSiC-20 and CrSiC-30 coatings have been worn out at the

M

sliding distance of 220 m and 120 m separately, which was one of the reasons for the fluctuation of

ed

the friction curve. As seen in Fig.5a, the friction coefficient increased and even was higher than that of 316L/Al2O3 tribopairs. This observation was attributed to decreasing toughness (CPRS) and poor

ce pt

crack resistance of coatings [42]. In the different mating balls case, the mean-steady friction coefficient (0.24~0.31) for CrSiC/SiC tribopairs was lower than that (0.47~0.70) of CrSiC/Al2O3 tribopairs (Fig.5b). Similarly, the wear rates for coatings sliding against SiC balls were lower than

Ac

those sliding against Al2O3 balls (Fig.5c), which was consistent with the depth of wear track in Fig.5d. There were obvious plowing grooves on the wear track of CrSiC coatings as sliding against SiC balls, and the depths of wear track increased from 0.7 μm to 1.7 μm, which was higher than the thickness of CrSiC (~1.2 μm). This indicated that the CrSiC-30 was partially worn out. However, as sliding against Al2O3 balls, the depth of wear track became deeper (1.5~2.0 μm) and reached to stainless steel substrate. Fig.6 shows the SEM images with the corresponding EDS patterns of wear tracks for CrSiC 10

Page 12 of 38

coatings sliding against SiC balls. It is clear that the CrSiC-10 coating confronted slight wear, while the scratch lines became clear gradually on the wear track of other two coatings. As seen in Fig.6 (b, e and h), more serious plough and plastic deformation were observed. Especially, the delamination of coating marked by white arrows was also presented on the wear track of CrSiC-20. In addition,

ip t

the Fe element was detected in EDS patterns (Figs.6c, 6f, 6i). Taken the cross-sectional images in

cr

Fig.5 into account, the Fe element might come from 316L steel substrate. This indicated that the CrSiC coating was peeled off partly or completely. Fig.7 showed the optical microscopes of wear

us

scar on SiC balls, there were many obvious scratch grooves on the wear scar covered with

an

incidental black wear debris. As seen in Fig 5c, the specific wear rates of mating balls were lower than those of coatings. This indicated that the transfer of tribolayer from coating to ball was

M

occurred, and then the mating balls were protected against wear.

ed

Fig.8 shows the SEM images with the corresponding EDS patterns of wear tracks for CrSiC coatings sliding against Al2O3 balls. With an increase in Si content, the scratches on the wear track

ce pt

became obvious. As seen in Fig.8 (b, e, h), the wear track was smooth with minor scratches on CrSiC-10, while the fatigue cracks marked by white arrows become severe on the other two coatings. Furthermore, the Fe elements were also detected in EDS analysis (Fig.8(c, f, i)). Taken the

Ac

cross-sectional images in Fig.5d into account, the CrSiC coatings were wore out. In fact, the fatigue cracks on the wear track were produced by stainless steel substrate. Fig.9 showed the optical microscopes of wear scar for Al2O3 balls; only the wear scar of ball sliding with CrSiC-10 coating was obvious black in Fig.9a. Meanwhile, the corresponding friction coefficient (μ=0.47) was lower than that of other two coatings (μ=0.67, 0.70). That indicated that the tribolayer of coating on the friction interface could effectively reduce the friction coefficient. 3. 3. Discussion 11

Page 13 of 38

When the Si content in the CrSiC coatings increased from 2.0 at.% to 7.4 at.%, no crystalline phase was detected in the CrSiC coating. However, the analysis of XPS indicated that there were many different chemical bonds such as Cr-C, Cr-Si, Si-C and C-C in the CrSiC coatings. This indicated that the CrSiC coatings consisted of amorphous nanocomposites with noncrystalline CrC

ip t

and CrSi embedded in a-C and a-SiC matrix. As seen in Table 6, the total volume fraction of sp3C-C

cr

bonds decreased from 13.6 at.% to 3.7 at.%, while the total fraction of C-Si/C-Cr and C-Cr bonds increased from 15.3 at.% to 31.9 at.%. This pointed out that a larger number of C sites were

us

substituted by Si and Cr atoms to form silicon carbide and chromium carbide phase when the Si

an

content increased. Bertóti et al.[38] have reported that the formation of carbides was preferred in the CrSiC system according to standard enthalpy of formation of selected compounds. Tengstrand et

M

al.[52] pointed out that the Si atoms were more likely to replace the C atoms rather than Ti atoms in

ed

the TiSiC coatings. Furthermore, the hardness of CrSiC coatings fluctuated slightly around 13 GPa with variable Si content. The same phenomenon was reported by Bertóti [38], and then they

ce pt

indicated that the carbon matrix was the decisive role in influencing the hardness. In fact, the CrSiCN coatings with low Si content (2.1 at.% and 2.4 at.% ) all showed high hardness (21.3 GPa and 19.1GPa), which was attributed to solid solution hardening and the formation of nanocomposite

Ac

structures [4, 43]. But when the Si content increased to 5.4~9.8 at.%, the CrSiCN coatings became amorphous structure, and then their hardness all gradually decreased to about 13 GPa. This pointed out that the mechanical properties of CrSiC coatings were governed by the amorphous structure such as a-SiC/a-C matrix. For the CrSiC/SiC tribopairs, the friction coefficient reached the lowest value 0.24 when the Si content was 2.0 at.%. That was probably related to the occurrence of hydro-tribochemical reaction on the contact surface in water. As seen in Fig.6a, the white wear debris on the wear track of CrSiC 12

Page 14 of 38

coatings might be colloidal silica resulted from hydration reaction. Refs.[56-57] indicated the tribochemical reaction was easily occurred as self-mated Si3N4 or SiC under water lubrication. Especially, the activation energy of tribochemical reaction in water was estimated to be as small as 1/6–1/8 of that for static reaction [57]. Similarly, the same lubrication effect also could be obtained

ip t

by doping silicon element into the CrN-based and DLC coatings owing to the formation of silicon

cr

nitride or silicon carbide phase [4, 31-32, 43]. The tribochemical reaction might occur as followed in Eqs.(1-4). According to thermodynamic calculations of the phase equilibrium within the interface,

us

the water did not react with SiC in the presence of oxygen [58]. In here, the oxygen in water was

an

reacted with SiC to form silica as shown in Eqs.(2) [58]. Subsequently, the silicon oxide could be hydrated by water molecules to form SiO2·xH2O gels at the contact zone as shown in Eqs.(3) [59].

M

If the hydration reaction between SiC ceramic and water occurred directly, which could be expressed as Eqs.(4).

ed

CrSiC  H 2O  Cr2O3  Si(OH )4  CH 4  CO2 SiC  2O2  SiO2  CO2 G f

ce pt

298

 589kJ / mol

298 SiO2  2H2O  Si(OH )4 G f  273kJ / mol

(1) (2) (3)

298 SiC  4H 2O  Si(OH )4  CH4 G f  598.9kJ / mol (4)

Ac

Where ΔGf298 is the reaction Gibbs free energy of formation at 298 K. According to Eqs.(3-4), it was concluded that the hydration reaction between tribomaterials and water occurred at room temperature [31-32, 35]. In addition, the O elements were detected in wear track by EDS patterns, this indicated that silica was formed on the friction surface. Similarly, in order to detect the formation of silica gel, the wear track of CrSiCN coating as sliding against SiC balls in water had been detected by XPS [43]. These results indicated that the binding energies of Si-O bond were 103 eV and 532.6 eV in Si2p and O1s spectra, which were equal to the available data of Si(OH)4 [60] 13

Page 15 of 38

and SiO2 (gel de silica) [61]. But when the Si content was higher than 2.0 at.%, the friction coefficient and wear rate of coatings all increased, and the deep scratch grooves could be observed on the wear track (Fig.5d). Actually, when the CrSiC coatings were pressed at a maximum indentation load of 1000 mN, only a few pile-up steps near the impression edge of CrSiC-10

ip t

coating was observed, but for the CrSiC-20 and CrSiC-30 coatings, there were many obvious radial

cr

cracks that grew from 3.4 μm to 6.7 μm [42]. This indicated that the crack resistance of CrSiC coatings decreased when the Si content increased. Moreover, the fraction of Si-C bonds increased

us

from 52.3 at.% to 77.2 at.% with increasing the Si content. This showed that the CrSiC coatings

an

became brittle, and then microfracture occurred easily around the indentation in a certain degree [36]. This phenomenon could be proved in Fig.4, the failure and ridge spallation of CrSiC coatings

M

happened at a lower load when the Si content increased, so the toughness of coatings (CPRs)

ed

decreased from 55.2 N2 to 30.1 N2. Although the tribochemical reaction between CrSiC and water could be enhanced at high Si content [32], the wear debris could break the continuity of Si(OH)4 gel,

ce pt

and then the lubrication effect of Si(OH)4 gel would be worsen. But for the CrSiC/Al2O3 tribopairs, a low friction coefficient (μ=0.47) was obtained when the Si content was 2.0 at.%, and there was minor plowing grooves on the corresponding wear track. As the Si content increased to 3.5~7.4

Ac

at.%, the friction coefficient increased to 0.67 and 0.70, and the corresponding wear track appeared serious fatigue crack. These results were attributed to not only the reduced toughness of coatings (CPRs) but also the hydration reaction rate of mating balls with water. According to the Gibb’s free energy (ΔG=-25.9 kJ/mol and -21.6 kJ/mol) for the tribochemical reaction of Al2O3 with water, the reaction rate to form aluminum trihydroxide and aluminum hydroxide was very low [62]. Thus the hydration reaction was mainly occurred for CrSiC coating with water, which accelerated the wear of coatings and result in high wear rate. As a consequence, the thickness of silica gel layer formed was 14

Page 16 of 38

about nano-scale. When serious plough grooves were appeared on wear track, the continuity of silica gel was interrupted, and then the lubrication effect was weaken. Thus, the mechanical wear was dominated. On the other hand, the total fraction of C-C bonds reduced with an increase in the Si content, and then the lubrication effect of amorphous carbon also became poor, which caused

ip t

high friction coefficients. In addition, the crack resistance, toughness and the a-C fraction of CrSiC

cr

coatings with high Si content decreased, so the micro-crack and fracture easily happened on the wear track and the lubrication effect became worse, and then the friction coefficient became high

us

and the wear resistance of CrSiC coatings became poor. Comprehensive analysis of friction

an

performance, the friction properties of CrSiC coating as sliding against SiC and Al2O3 balls were weakened with an increase of Si content from 2.0 at.% to 7.4 at.%, and no turning point could be

M

obtained. The result was attributed to the amorphous structure of coatings that reduced the

ed

toughness and crack resistance of coatings. As a speculation, it is an effective method to improve the friction properties of CrSiC coatings via changing the crystalline structure of CrSiC coatings

4. Conclusions

ce pt

with reducing the Si content.

(1) When the TMS flows increased from 10 to 30 sccm, the Si concentration increased from 2.0

Ac

to 7.4 at.%, and the amorphous structure of CrSiC coatings were presented. Though the CrSiC coatings’ hardness fluctuated slightly around 13 GPa, the corresponding toughness decreased with increase of Si content.

(2) With an increase in Si content, the friction coefficient and wear rate all increased due to the decline of resistance crack and toughness of coatings. (3) The CrSiC/SiC tribopairs showed better tribological performance than CrSiC/Al2O3 tribopairs owing to the synergistic effect of low toughness of coatings and hydration reaction rate of 15

Page 17 of 38

mating balls with water. Acknowledgment This work was supported by National Natural Science Foundation of China (Grant No. 51375231), The Research Fund for the Doctoral Program of Higher Education (Grant

ip t

No.20133218110030). A Project Funded by Priority Academic Program Development of Jiangsu

cr

Higher Education Institutions (PAPD). We would like to acknowledge them for their financial support.

us

References

an

[1] A. Tanaka, M. Suzuki and T. Ohana, Friction and wear of various DLC films in water and air Environments, Tribol. Lett.17 (2004)917-924.

M

[2] C.S. Abreu, M. Amaral, F.J. Oliveira, J.R. Gomes, R.F. Silva, HFCVD nanocrystalline diamond coatings for tribo-applications in the presence of water, Diamond Relat. Mater.18(2009) 271-275.

ed

[3] F. Zhou, Q.Z. Wang, Low-friction behaviors of hard solid coatings in water environments, in: Q.J. Wang, Y.W. Chung (Eds.), Encyclopedia of Tribology, Springer Science+Business Media, New

ce pt

York, 2013, pp. 2020-2023.

[4] Z.W. Wu, F. Zhou, Q.Z. Wang, Z.F. Zhou, J. Yan , L. Li, Influence of trimethylsilane flow on the microstructure, mechanical and tribological properties of CrSiCN coatings in water lubrication,

Ac

Appl. Surf. Sci. 355(2015)516-530.

[5] H. Ronkainen, M.S. Varjus, K. Holmberg, Friction and wear properties in dry, water- and oil-lubricated DLC against alumina and DLC against steel contacts, Wear 222(1998)120-128. [6] F. Zhou, K. Adachi, K. Kato, Friction and wear properties of a-CNx coatings sliding against ceramic and steel ball in water, Diamond Relat. Mater.14(10) (2005)1711-1720.

16

Page 18 of 38

[7] F. Zhou, X. Wang, K. Adachi,K. Kato, Influence of normal load and sliding speed on the tribological property of amorphous carbon nitride coatings sliding against Si 3N4 balls in water, Surf. Coat. Technol. 202(15)(2008) 3519-3528. [8] Y.X. Wang, L.P. Wang, Q.J. Xue, Improvement in the tribological performances of Si3N4, SiC and WC by graphite-like carbon films under dry and water-lubricated sliding conditions, Surf. Coat.

ip t

Technol. 205(2011) 2770-2777.

cr

[9] W. Dai, G.S. Wu, A. Y. Wang, Preparation, characterization and properties of Cr-incorporated DLC films on magnesium alloy, Diamond Relat. Mater.19 (2010) 1307–1315.

us

[10] G. Gassner, P.H. Mayrhofer, J. Patscheider, C. Mitterer, Thermal stability of nanocomposite

an

CrC/a-C:H thin films, Thin Solid Films 515 (2007) 5411–5417.

[11] Y. X. Wang, L.P. Wang, G. A. Zhang, S. C. Wang, RJK. Wood, Q.J. Xue, Effect of bias voltage

M

on microstructure and properties of Ti-doped graphite-like carbon films synthesized by magnetron sputtering, Surf. Coat. Technol. 205(2010) 793–800.

ed

[12] Q.Z. Wang, F. Zhou, X.D. Ding, Z.F. Zhou, C.D. Wang, W.J. Zhang, L.K.Y. Li, S.T. Lee, Structure and water-lubricated tribological properties of Cr/a-C coatings with different Cr contents,

ce pt

Tribol. Int.67(2013)104-115.

[13] Q.Z.Wang, F. Zhou, X.D. Ding, Z.F. Zhou, W.J. Zhang, L.K-Y. Li, Influences of ceramic mating balls on the tribological properties of Cr/a-C coatings with low chromium content in water

Ac

lubrication, Wear 303(2013)354–60.

[14] V.Singh, J. C.Jiang, E.I. Meletis, Cr-diamond like carbon nanocomposite films: Synthesis, characterization and properties, Thin Solid Films 489 (2005) 150-158. [15] W. Dai, P.L. Ke, A.Y. Wang, Microstructure and property evolution of Cr-DLC films with different Cr content deposited by a hybrid beam technique, Vacuum 85 (2011) 792-797. [16] W. Dai, A.Y. Wang, Synthesis, characterization and properties of the DLC films with low Cr concentration doping by a hybrid linear ion beam system, Surf. Coat. Technol. 205 (2011) 2882-2886. 17

Page 19 of 38

[17] Y.L. Su, T.H. Liu, C.T. Su, T.P. Cho, Effect of chromium content on the dry machining performance of magnetron sputtered CrxC coatings, Mater.Sci.Eng.A364 (2004) 188-197. [18] M. Keunecke, K. Bewilogua, J. Becker, A. Gies, M. Grischke, CrC/a-C:H coatings for highly loaded, low friction applications under formulated oil lubrication, Surf. Coat. Technol. 207 (2012) 270-278.

ip t

[19] M. Andersson, J. Högström, S. Urbonaite, A. Furlan, L. Nyholm, U. Jansson, Deposition and characterization of magnetron sputtered amorphous Cr-C films, Vacuum 86 (2012) 1408-1416.

cr

[20] J. Hőgstrőm, M. Andersson, U. Jansson, F. Bjőefors, L. Nyholm, On the Evaluation of

us

Corrosion Resistances of Amorphous Chromium-Carbon Thin-Films, Electrochim. Acta 122 (2014) 224-233.

an

[21] M. Magnuson, M. Andersson, J. Lu, L. Hultman, U. Jansson, Electronic structure and chemical

M

bonding of amorphous chromium carbide thin films, J. Phys. :Condens. Matter 24 (2012) 225004 1-7.

ed

[22] K. Nygren, M. Andersson, J. Högström, W. Fredriksson, K. Edström, L. Nyholm, U. Jansson, Influence of deposition temperature and amorphous carbon on microstructure and oxidation

ce pt

resistance of magnetron sputtered nanocomposite CrC films, Appl. Surf. Sci. 305 (2014) 143-153. [23] A. Jellad, S. Labdi, C. Malibert, G. Renou, Nanomechanical and nanowear properties of Cr3C2 thin films deposited by rf sputtering, Wear 264 (2008) 893-898.

Ac

[24] K. Nygren, M. Samuelsson, A. Flink, H. Ljungcrantz, Å. Kassman Rudolphi, U. Jansson, Growth and characterization of chromium carbide films deposited by high rate reactive magnetron sputtering for electrical contact applications, Surf. Coat. Technol. 260 (2014) 326-334. [25] K. Nygren, M. Andersson, J. Högström, W. Fredriksson, K. Edström, L. Nyholm, U. Jansson, Influence of deposition temperature and amorphous carbon on microstructure and oxidation resistance of magnetron sputtered nanocomposite CrC films, Appl. Surf. Sci. 305 (2014)143-153. [26] G. Gassner, J. Patscheider, P. H. Mayrhofer , S. Šturm, C. Scheu, C. Mitterer, Tribological Properties of Nanocomposite CrCx/a-C:H Thin Films, Tribol. Lett. (2007) 27:97-104. 18

Page 20 of 38

[27] S. Zimowski, T. Moskalewicz, M. Kot, B. Wendlerc, A. Czyrska-Filemonowicz, Microstructure, mechanical and tribological properties of the nc-CrxCy/a-C and nc-CrxCy/a-C:H nanocomposite coatings on oxygen-hardened Ti-6Al-4V alloy, Surf. Interface Anal. 2012, 44, 1225-1228.

CrC/a-C:H thin films, Thin Solid Films 515 (2007) 5411-5417.

ip t

[28] G. Gassner, P.H. Mayrhofer, J. Patscheider, C. Mitterer, Thermal stability of nanocomposite

cr

[29] J.L. Lin, B. Wang, Y.X. Ou, W. Sproul, I.D. John, J. Moore, Structure and properties of CrSiN nanocomposite coatings deposited by hybrid modulated pulsed power and pulsed DC magnetron

us

sputtering, Surf. Coat. Technol. 216(2013)251-258.

[30] L. Castaldi, D. Kurapov, A. Reiter, V. Shklover, P. Schwaller, J. Patscheider, High temperature

an

phase changes and oxidation behavior of Cr-Si-N coatings, Surf. Coat. Technol. 202 (2007)

M

781-785.

[31] Z.G. Geng, H.X. Wang, C.B. Wang, L.P. Wang, G.G. Zhang, Effect of Si content on the

ed

tribological properties of CrSiN films in air and water environments, Tribol. Int.79(2014)140-150. [32] X.Y. Wu, M. Suzuki, T. Ohana, A. Tanaka, Characteristics and tribological properties in water

ce pt

of Si-DLC coatings, Diamond. Relat. Mater. 17 (2008) 7-12. [33] F. Zhou, X. L.Wang, K. Kato, Z. D. Dai, Friction and wear property of a-CNx coatings sliding against Si3N4 balls in water, Wear 263 (7-12) (2007) 1253-1258.

Ac

[34] F. Zhou, K. Adachi, K. Kato, Wear-mechanism map of amorphous carbon nitride coatings sliding against silicon carbide balls in water, Surf. Coat. Technol. 200 (2006) 4909-4917. [35] Q.Z. Wang, F. Zhou, K. Chen, M. Wang, T. Qian, Friction and wear properties of TiCN coatings sliding against SiC and steel balls in air and water, Thin Solid Films 519 (2011) 4830-4841. [36] H. Ziegele , C. Rebholz, A. A. Voevodin , A. Leyland, S. L. Rohde, A. Matthews, Studies of the tribological and mechanical properties of laminated CrC-SiC coatings produced by r.f. and d.c. Sputtering, Tribol. Int. 30(12)(1997)845-856. 19

Page 21 of 38

[37] I. Bertóti, A. Tóth, M. Mohai, J. Szépvölgyi, Chemical structure and mechanical properties of Si-containing a-C:H and a-C thin films and their Cr- and W-containing derivatives, Surf. Coat. Technol.206 (2011) 630-639. [38] I. Bertóti, M. Mohai, K. Kereszturi, A. Tóth, E. Kálmán, Carbon based Si- and Cr-containing thin films: Chemical and nanomechanical properties, Solid State Sci.11 (2009) 1788-1792.

ip t

[39] X.W. Yin, L.F. Cheng, L.T. Zhang, Y.D. Xu, C. You, Microstructure and oxidation resistance of

cr

carbon/silicon carbide composites infiltrated with chromium silicide, Mater.Sci.Eng.A, 290(2000) 89-94.

us

[40] M.D. Bao, L. Yu, X.B. Xu, J.W. He, H.L. Sun, D.G. Teer. Microstructure and wear behaviour of

an

silicon doped Cr–N nanocomposite coatings, Thin Solid Films 517 (2009) 4938-4941. [41]S. Hassani, J.E. Klemberg-Sapieha, L. Martinu. Mechanical, tribological and erosion behaviour

M

of super-elastic hard Ti-Si-C coatings prepared by PECVD, Surf. Coat. Technol.205 (2010) 1426-1430.

ed

[42] Q.Z. Wang, Z.W. Wu, F. Zhou, J. Yan, Comparison of crack resistance between ternary CrSiC

ce pt

and quaternary CrSiCN coatings via nanoindentation, Mater. Sci. Eng., A 642(2015)391-397. [43] Z.W. Wu, F. Zhou, K.M. Chen, Q.Z. Wang, Z.F. Zhou, J.W. Yan, L.K. Li, Friction and wear properties of CrSiCN coatings with low carbon content as sliding against SiC and steel balls in water, Tribol. Int. 94(2016)176-186.

Ac

[44] Q.Z. Wang, Z.W. Wu, F. Zhou, H. Huang, K. Niitsu, J. Yan, Evaluation of crack resistance of CrSiCN coatings as a function of Si concentration via nanoindentation, Surf. Coat. Technol.272 (2015) 239-245.

[45] J. Romero, A. Lousa, E. Mart′ınez, J. Esteve, Nanometric chromiumy chromium carbide multilayers for tribological applications, Surf. Coat. Technol. 163-164 (2003) 392-397. [46] J. Romero, E. Mart′ınez, J. Esteve, A. Lousa, Nanometric chromium nitride/ chromium carbide multilayers by r.f. magnetron sputtering, Surf. Coat. Technol. 180-181 (2004) 335-340. 20

Page 22 of 38

[47] U. Jansson, E. Lewin, Sputter deposition of transition-metal carbide films-A critical review from a chemical perspective, Thin Solid Films 536 (2013) 1-24. [48] Q.Z. Wang, F. Zhou, X. Ding, Z. Zhou, C. Wang, Microstructure and water-lubricated friction and wear properties of CrN(C) coatings with different carbon contents, Appl. Surf. Sci. 268 (2013) 579-587.

ip t

[49] D. Caputo, G. de Cesare, M. Ceccarelli, A. Nascetti, M. Tucci, L. Meda, M. Losurdo, G. Bruno,

cr

Characterization of chromium silicide thin layer formed on amorphous silicon films, J. Non-Cryst. Solids 354 (2008) 2171-2175.

us

[50] P.L. Tam, Y. Cao, L. Nyborg, XRD and XPS characterisation of transition metal silicide thin films, Surf. Sci. 606 (2012) 329-336.

an

[51] K. Petkov , V. Krastev, T. Marinova, XPS analysis of thin chromium films, Surf. Interface Anal.

M

18(1992)487-490.

[52] O. Tengstrand, N. Nedfors, B. Alling, U. Jansson, A. Flink, P. Eklund, L. Hultman,

ed

Incorporation effects of Si in TiCx thin films, Surf. Coat. Technol. 258 (2014) 392-397. [53] D-S. Han, P. K. Song, K-M. Cho, Y. H. Park, K.H. Kim, Synthesis and mechanical properties

(2004) 446-451.

ce pt

of Ti-Si-C films by a plasma-enhanced chemical vapor deposition, Surf. Coat. Technol.188-189

[54] C. Schmidt, H. Oetzmann, P. Hess, R. Nowak, XPS characterization of chromium films

Ac

deposited from Cr(CO)6 at 248 nm, Appl. Surf. Sci.43 (1989) 11-16. [55] S. Zhang, D. Sun, Y. Fu, H. Du, Toughness measurement of thin films: a critical review, Surf. Coat. Technol. 198(2005)74-84. [56] M. Chen, K. Kato, K. Adachi, The comparisons of sliding speed and normal load effect on friction coefficients of

self-mated

Si3N4

and

SiC under water lubrication, Tribol. int. 35

(2002) 129-135.

21

Page 23 of 38

[57] J.G. Xu, K. Kato, Formation of tribochemical layer of ceramics sliding in water and its role for low friction, Wear 245 (2000) 61-75. [58] F. Zhou, Y.Y. Yuan, X.L. Wang, M.L. Wang. Influence of nitrogen ion implantation fluences on surface structure and tribological properties of SiC ceramics in water-lubrication, Appl. Surf. Sci.

ip t

255 (2009) 5079-5087.

cr

[59] H. Tomizawa, T. E. Fischer. Friction and Wear of Silicon Nitride and Silicon Carbide in Water: Hydrodynamic Lubrication at Low Sliding Speed Obtained by Tribochemical Wear, ASLE Trans.

us

30 (1987) 41-46.

an

[60] T.L. Barr. An ESCA Study of Termination of the Passivation of Elemental Metals, J. Phys. Chem. 82(16) (1978):1801-1810.

M

[61] B.F. DzhurinskII, D. Gati, N.P. Sergushin, V.I. Nefedov, YA. V. Salyn. Simple and

Org. Chem. 20(1975): 2307-2314.

ed

coordination compounds. An X-ray photoelectron spectroscopic study of certain oxides, Russ. J.

ce pt

[62] D. Amutha Rani, Y. Yoshizawa, H. Hyuga, K. Hirao, Y. Yamauchi. Tribological behavior of ceramic materials (Si3N4, SiC and Al2O3) in aqueous medium, J. Eur. Ceram. Soc. 24(2004)

Ac

3279-3284.

22

Page 24 of 38

Figure captions Fig.1 X-ray diffraction spectra of the CrSiC coatings with various TMS flows. Fig. 2 XPS spectra of the CrSiC coatings with various TMS flows.

ip t

Fig. 3 Surface micrographs and cross-sectional microstructure of CrSiC coatings deposited with

cr

various TMS flows.

Fig. 4 Acoustic emission signals (a) and optical micrographs (b) of CrSiC films by a scratch tester.

us

Fig.5. Friction behaviors of CrSiC coatings sliding against ceramic balls in deionized water (a), the

an

mean steady friction coefficient (b), specific wear rates of tribopairs (c) and 2-D profiles of wear track (d).

M

Fig.6. SEM images (a, d, g), local amplified images (b, e, h) and corresponding EDS analysis of

ed

wear track (c, f, i) for the CrSiC coatings sliding against SiC balls. Fig.7. Optical microscopes of wear scar for SiC balls.

ce pt

Fig.8. SEM images (a, d, g), local amplified images (b, e, h) and corresponding EDS analysis of wear track (c, f, i) for the CrSiC coatings sliding against Al2O3 balls.

Ac

Fig.9. Optical microscopes of wear scar for Al2O3 balls.

23

Page 25 of 38

Figure

(220) Si

(200)

CrSiC-20

ip t

Intensity (a.u.)

CrSiC-30

30

40

50

60

2()

70

80

90

us

20

cr

CrSiC-10

Ac

ce pt

ed

M

an

Fig.1 X-ray diffraction spectra of the CrSiC coatings with various TMS flows.

Page 26 of 38

Cr-O Cr(Cr-C)

Cr-O

Cr-Si/ Cr(Cr-C) Cr-C

CrSiC(20)

Si2p

CrSiC-20

CrSiC-10

585

582

579

Binding energy(eV)

576

573

570 105

3

(c)

C-O

sp C-C sp2C-C

103

102 101 100 Binding energy(eV)

99

98

97

C-Cr C-Cr/C-Si

C1s

an

CrSiC-30

3

CrSiC-20

M

sp C-C

ed

Intensity(a.u.)

104

us

588

cr

CrSiC(10)

591

Si-Si/Si-Cr

C-Si

CrSiC-30

Intensity(a.u.)

CrSiC(30)

ce pt

CrSiC-10

290

289

288

287 286 285 284 Binding energy(eV)

283

282

Fig.2. XPS spectra of the CrSiC coatings with various TMS flows.

Ac

Intensity(a.u.)

(b)

Cr2p

ip t

(a)

Page 27 of 38

ip t cr us an M ed ce pt

Ac

Fig.3. Surface and cross-sectional microstructure of CrSiC coatings deposited with various TMS flows.

Page 28 of 38

(a) Lc2=11N

CrSiC-30

Lc1=5.1N Lc2=14N

CrSiC-20

Lc1=5.0N Lc2=15.2N

CrSiC-10

5

10

15

20

25

30

35

40

ed

M

an

us

Load,N

cr

0

ip t

Lc1=6.0N

Fig. 4 Acoustic emission signals (a) and optical micrographs (b) of CrSiC films by a scratch

Ac

ce pt

tester.

Page 29 of 38

1.0

CrSiC-20/Al2O3

0.9

CrSiC-30/Al2O3

0.8

316L/Al2O3

0.8

CrSiC-10/SiC CrSiC-20/SiC CrSiC-30/SiC

0.7

worn out worn out

0.7 0.6 0.5 0.4 0.3 0.2 0.1

0.6 0.5 0.4 0.3 0.2 0.1

100

200 300 Sliding distance (m)

400

10

500

Al2O3 balls

-6

5

M

10

10

an

-5

10

30

Mating with SiC balls Mating with Al2O3 balls CrSiC-30 CrSiC-30 CrSiC-20 CrSiC-20 CrSiC-10 CrSiC-10

15

Depth of wear track ( m)

3

Specific wear rate(mm /Nm)

CrSiC coating against SiC balls SiC balls CrSiC against Al2O3 balls

20 TMS flows (sccm)

us

(d)

(c) -4

CrSiC/SiC tribopairs CrSiC/Al2O3 tribopairs

0.0

0

10

(b)

ip t

CrSiC-10/Al2O3

cr

(a) Mean steady friction coefficient ( )

Friction coefficient (

1.1

-7

10

0

20 TMS flows (sccm)

30

ed

10

0

50

100 150 200 250 Width of wear track (m)

300

350

Ac

ce pt

Fig.5. Friction behaviors of CrSiC coatings sliding against ceramic balls in deionized water (a), the mean steady friction coefficient (b), specific wear rates of tribopairs (c) and 2-D profiles of wear track (d).

Page 30 of 38

140

(c)

(b)CrSiC-10

Cr

CrSiC-10

120

Intensity(a.u.)

100 80

Fe O Cr

60 40

Cr

20 C

Si

0 0 140

1

(f)

2

5

7

ip t

100

Cr

Fe O Cr

60 40

cr

80

20 C 1

2

3 4 Energy (keV)

5

us 140

Cr

Si

0 0

(h)CrSiC-30

6

CrSiC-20

120

Intensity(a.u.)

3 4 Energy (keV)

6

7

(i) CrSiC-30

120

60 40

Fe O Cr

Si

20 C

M

Cr

80

an

Intensity(a.u.)

100

0 0

Cr Fe 1

2

3 4 Energy (keV)

5

6

7

Ac

ce pt

ed

Fig.6. SEM images (a, d, g), local amplified images (b, e, h) and corresponding EDS analysis of wear track (c, f, i) for the CrSiC coatings sliding against SiC balls.

Fig.7. Optical microscopes of wear scar for SiC balls.

Page 31 of 38

120

(c)

CrSiC-10 Counterpart Al2O3 balls Cr

100

Intersity(a.)

80 60 Cr Fe O

40 20

Cr

Si

C

Fe

0 0

1

2

3

4

5

6

7

8

Energy(keV) 100

(f)

CrSiC-20 Counterpart Al2O3 balls

ip t Cr

60 Cr Fe O

40

cr

Intersity(a.)

80

20 C

Fe

Cr

Si

0 0

1

2

3

4

5

Fe

6

7

8

us

Energy(keV)

100

(i)

CrSiC-30 Counterpart Al2O3 balls

Cr Fe O

60

an

Intersity(a.)

80

Cr

40

Fe

20

Cr Si

C

Fe

M

0

0

1

2

3

4

Energy(keV)

5

6

7

8

Ac

ce pt

ed

Fig.8. SEM images (a, d, g), local amplified images (b, e, h) and corresponding EDS analysis of wear track (c, f, i) for the CrSiC coatings sliding against Al2O3 balls.

Fig.9. Optical microscopes of wear scar for Al2O3 balls.

Page 32 of 38

ip t

Table

Table 1 References related to mechanical and tribological properties of CrC-based coatings

Wang et al[12,13]

V. Singh et al [14]

a-C/Si3N4 Cr(a-C)/Si3N4 Cr(a-C)/Al2O3 Cr(a-C)/SiC DLC/440 steel

Normal load(N)

20.0 12.0~13.0 11.6~11.7 20

2

3

12

0 <4.8 >12 1.49~8.42

17.5 13.5 <13.5 8~12

Cr-DLC/440 steel Cr-DLC/SUJ-2

8.42~40.11

DLC/SUJ-2

0

Cr-DLC/SUJ-2

2.34~12.1 12.1~31.5 0

17.5

Cr-DLC/SUJ-2

-

12~15

CrxC/1045steel

18.2

5.6

68.8~84.5

8.6~24.2

25~40 40~70

10~12 12-14

DLC/SUJ-2

CrxC/1045steel

CrC(a-C:H)/100Cr-6 CrC(a-C:H)/100Cr-6

Ac c

M. Keunecke et al [18]

-

Cr-DLC/SUJ-2

Su et al [17]

﹥12 -

ep te

Dai et al [9,15-16]

Cr-DLC/SUJ-2

Lubricant

In water

cr

Speed(m/s)/ frequence(Hz)

us

Cr(a-C)/SUS

Hardness/ GPa

an

a-C/SUS

Cr content (at.%) 0 3~4.9 11.98~14.09 0

M

Tribopairs

d

Refs

0.1

2.5

0.1

Humidity 42±7%

3

0.1

Humidity 40~50%

0.1

Humidity 40~50%

1

1

0.2

Humidity 40~50%

100

50Hz

In air

30

0.1

Oil

Friction coefficient <0.1 <0.1 0.2<μ<0.3 0.08 0.08 0.07 0.11 <0.1 0.1<μ<0.14 ﹥0.2 ≈0.1

Coatings’ wear rate (mm3/Nm) <1×10-6 <1×10-6 2.5~6.1×10-5

≈1×10-7 ≈1×10-7 ﹥1×10-7 8.9×10-9

-

0.15~0.35

1.4×10−7

-

0.4

Wear out

<5

≈0.3

-

7.5

0.4

Wear out

≈0.17

WCr(a-C) ≥ Wa-C

−8

2.6×10

Critical Load/N 22.0 22.5~25 22~26.4 -

5~7.5 -

0.2

3.0×10−8

-

0.25

Wear out

0.68~1.04

-

﹥70 32~70

0.1<μ<0.12 0.1<μ<0.12

10−7 ﹥10

-7

-

Page 33 of 38

Corresponding values

Base pressure Working pressure Purity of Cr target Cr targets (2) current Purity of C target C targest (4) current Substrate temperature Substrate bias voltage Rotational velocity of substrate Typical deposition rate TMS flows

4×10−4 Pa 0.23 Pa 99.9% 4.0A 99.99% 1.0A 150℃ -80V 10 rpm 0.9 μm/h 10, 20, 30 sccm

Ac

ce pt

ed

M

an

us

cr

Process parameters

ip t

Table 2 Deposition conditions for CrSiC coatings by closed-field unbalanced magnetron sputtering system

Page 34 of 38

Table 3 Composition variation and mechanical properties for CrSiC coatings with various TMS flows. Cr at.%

Si at.%

C at.%

Hardness/GPa

Modulus/GPa

CrSiC-10 CrSiC-20 CrSiC-30

73.1±0.07 68.8±0.48 66.1±0.01

2.0±0.04 3.5±0.11 7.4±0.36

24.9±0.03 27.8±0.57 26.6±0.37

13.8±0.8 13.2±0.6 13.6±1.7

271.0±10.5 271.0±10.6 262.0±19.4

Ac

ce pt

ed

M

an

us

cr

ip t

Coatings

Page 35 of 38

CrSiC-30

18.5

576.8

Cr-O(Cr2O3)

583.7

Cr-C(Cr7C3)

585.6

Cr-O(Cr2O3)

587.1 574.2

Cr-O(Cr2O3) Cr/Cr-C(Cr3C2)

52.0 30.2

575.0

Cr-Si/Cr-C

17.0

576.6

Cr-O(Cr2O3)

583.6

Cr-C(Cr7C3)

585.2

Cr-O(Cr2O3)

586.8 574.3

Cr-O(Cr2O3) Cr/Cr-C(Cr3C2)

52.8 33.3

574.9

Cr-Si/Cr-C

21.0

576.7 583.7

Cr-O(Cr2O3) Cr-C(Cr7C3) Cr-O(Cr2O3)

ed

585.1

Cr-O(Cr2O3)

45.7

Ac

ce pt

587.0

cr

575.2

us

29.5

Cr-Si/Cr-C

an

CrSiC-20

Cr/Cr-C(Cr3C2)

M

CrSiC-10

574.1

ip t

Table 4 Calculated concentration of different bonds from Cr2p XPS results at different Si content Peak position Fraction of Coatings Bonds (eV) bonds (at.%)

Page 36 of 38

101.7

C-Si

47.7 52.3 27.0 73.0 22.8 77.2

Ac

ce pt

ed

M

an

CrSiC-30

Si-Si/Si-Cr C-Si Si-Si/Si-Cr C-Si Si-Si/Si-Cr

cr

CrSiC-20

98.9 101.5 99 101.7 99.4

us

CrSiC-10

ip t

Table 5 Calculated concentrations of different bonds from Si2p XPS results at different Si content Peak position Fraction of Coatings Bonds (eV) bonds (at.%)

Page 37 of 38

ip t

283.3

C-Cr

5.8

284.8

2 3

61.4

us

sp C-C sp C-C

17.6

288.5 282.9 283.5 284.8 285.4 288.3 282.9

C-O C-Si/C-Cr C-Cr sp2C-C sp3C-C C-O C-Si/C-Cr

5.7 8.9 18.2 63.6 5.8 3.5 13.1

283.7

C-Cr

18.8

284.8

2

60.5

286.1

3

sp C-C

3.7

288.4

C-O

3.9

an

285.2

sp C-C

Ac

ce pt

CrSiC-30

9.5

M

CrSiC-20

C-Si/C-Cr

ed

CrSiC-10

282.9

cr

Table 6 Calculated concentrations of different bonds from C1s XPS results at different Si content Peak position Fraction of Coatings Bonds (eV) bonds (at.%)

Page 38 of 38