Surface & Coatings Technology 228 (2013) S159–S163

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Surface & Coatings Technology

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Investigating the microstructure and mechanical behaviors of DLC films on AISI52100bearing steel surface fabricated by plasma immersion ion implantationand deposition

Hongxi Liu a,⁎, Qian Xu a, Chuanqi Wang a, Xiaowei Zhang a, Baoyin Tang b

a School of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, Chinab State Key Laboratory of Advanced Welding Production Technology, Harbin Institute of Technology, Harbin 150001, China

⁎ Corresponding author.E-mail address: piiiliuhx@sina.com (H. Liu).

0257-8972/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.surfcoat.2012.06.071

a b s t r a c t

a r t i c l e i n f o

Available online 1 July 2012

Keywords:Plasma immersion ion implantation anddeposition (PIIID)MicrostructureDiamond-like carbon (DLC) filmBearing steelMechanical property

The microstructure and mechanical properties of diamond-like carbon (DLC) films fabricated on anAISI52100 bearing steel substrate surface by plasma immersion ion implantation and deposition (PIIID)were studied. Atomic force microscope (AFM) observation reveals that the DLC film has an extremely smoothsurface, and high uniformity and efficiency of space filling over large areas. Raman spectroscopy analysis in-dicates that DLC films are mainly constituted by amorphous and crystalline phases, with a variable ratio ofsp2/sp3 carbon bonds, and sp3 bond content of more than 10%. The maximum nanohardness and elastic mod-ulus of the DLC film are 40 GPa and 430 GPa, and increase by 263.6% and 95.5%, respectively. The friction andwear results exhibit that the friction coefficient against an AISI52100 steel ball decreases from 0.87 to 0.20.The corrosion polarization curves in a 3.5% saturated NaCl solution shows that the corrosion resistance ofDLC/AISI52100 samples is much better than that of bearing steel substrate. Compared with the bare bearingsteel, the surface mechanical property of the DLC/AISI52100 sample is improved significantly.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

A bearing is a versatile engineering component in various ma-chines and equipment or instruments. It has been widely used inaerospace, nuclear, automotive and other special industries [1,2].But once the bearing is subjected to a high cyclic loading and highlevels of abrasion, the fatigue, friction and wear, corrosion, plastic de-formation or fracture failure will be brought at any time [3]. When abearing fails, it will affect the machine and reliability of the wholesystem. Therefore, prolonging the service life and reliability of thebearing is a hot research topic for domestic and foreign scientists.

Previous studies have revealed that bearing failure occurs mainly onthe surface or in the near-surface region [4,5]. Therefore, improving thecomprehensive performance of a bearing by surface treatment is anideal access to prolong its service life and working reliability. With thedevelopment of high property bearing steels, more and more surfacemodification and film deposition techniques, such as physical vapor de-position (PVD), ion implantation, plasma assisted chemical vapor depo-sition (PACVD), ion beam assisted deposition (IBAD), plasma immersionion implantation (PIII), also known as plasma source ion implantation(PSII) and plasma immersion ion implantation and deposition (PIIID)have been studied [6–12]. In all these surface modification techniques,plasma immersion ion implantation and deposition (PIIID) has aroused

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great interest as an alternative to conventional ion beam implantationbecause of its low temperature operation, excellent adhesion, controlledfilm deposition thickness, non-line-of-sight process, conformal implan-tation of complex three-dimensional surfaces, batch processing, and re-duced space and equipment cost compared with the conventionalbeam-line ion implantation [13–15].

In recent years, the number of literature regarding plasma immer-sion ion implantation and deposition has increased [16–22]. Howev-er, most of these reports focused mainly on increasing surfacehardness and corrosion resistance, improving friction and wear be-haviors, or changing the surface chemical components of substratematerials. Papers rarely study the effect on friction and wear behav-iors of bearing steel components treated by PIII or PIIID process. Inthe paper, diamond-like carbon (DLC) films synthesized by PIIIDtechnique has been investigated for improving the friction and wearbehavior, corrosion resistance and surface comprehensive mechanicalproperties of the AISI52100 bearing steel. For comparison, a barebearing material was also discussed under the same conditions.

2. Experimental procedures

2.1. Material preparation

Samples of AISI52100 bearing steel in quenched and temperedstates (HRC61~65) were used in this study. The principal chemicalcomposition is (wt.%): C 0.95–1.05, Si 0.15–0.35, Mn 0.20–0.40,

S160 H. Liu et al. / Surface & Coatings Technology 228 (2013) S159–S163

Cr 1.30–1.65 and Fe in balance. The cylindrical samples were ma-chined to a diameter of 15 mm and thickness of 3 mm. Before ion im-plantation and film deposition, one side of each coupon was groundwith SiC abrasive paper nos. 400, 800, 1200 and 1500 grits sequen-tially, followed by polishing with fine diamond paste to a certain sur-face roughness (the size of surface roughness and the SiC abrasivepaper model were selected according to the requirements of differentsamples, see Table 1), followed by an ultrasonic clean of 20 min inacetone and alcohol, then kept in an electric-dryer to prevent the sur-face from pollution again, and then put it into the vacuum chamber.

2.2. Fabrication of DLC film

Fabrication of diamond-like carbon (DLC) films was carried out inour multi-purpose plasma immersion ion implantation and deposi-tion facility [23]. The vacuum chamber was evacuated to a base vacu-um of 5.0×10−3 Pa, and then argon ion sputtering was introducedinto the chamber to remove undesirable oxide and contaminationlayers. Carbon plasma was generated by pulsed cathodic arc plasmasource with S-shape curved magnetic duct. The RF source power is600 W, the metal source main arc pulse width is 1000 μs, and themain arc current is 120 A. Implantation pulse width is 60 μs, pulsebias voltage is 25 kV, and substrate temperature is less than 150 °C.Experimental details are shown in Table 1.

2.3. Characterization methods

After PIIID, the microstructure and mechanical properties of thesamples were evaluated and compared with the AISI52100 bearingsteel substrate material. The as-deposited films were characterizedby a micro-Raman spectrometer (T64000 Raman spectrometer) atan excitation wavelength of 514.5 nm and argon laser power of300 mW, and the wave number range was 800–2000 cm−1. Themicro hardness and elastic modulus were measured using thenano-indentation system UMIS-2000 (CSIRO). A trigonal diamond in-denter (Berkovich-type indenter) with a total included angle of142.3° was used for all the measurements. The tip radius of the in-denter was approximately 50 nm and the load and depth resolutionwere 1 μN and 0.03 nm, respectively. The root-mean-square (RMS)roughness and surface morphology were observed by Nanoscope®IIIa(contact mode) atomic force microscopy (AFM) over sampling areasof 5 μm×5 μm. During pin-on-disk (CJS-IIIA tester) experiments,the radius of the sliding track was 3 mm; the upper ball was madeof silicon carbide (SiC) ceramic, 4 mm in diameter at ambientenvironment (room temperature 22 °C, humidity 40%±2%). Thespecimen was rotating at a constant sliding speed of 300 rpm; thecontact load was 0.3 N. No lubricant was used in the abrasion test.The corrosion behavior was determined by potential dynamicpolarization (Potential/Galvanostat Model 273) in 3.5% NaCl saturatedsolution (Hg/HgCl reference electrode). The potential was changed ata rate of 10 mV/s within the range of −1000 mV to 0 mV.

3. Results and discussion

3.1. Raman spectroscopy analysis

Raman spectroscopy is an effective way to characterize a C\C bondstructure of the diamond-like carbon films. Generally, the Raman peak

Table 1Detail parameters of the DLC film synthesized by PIIID.

Sample no. D0 D1 D2 D3

Implanted pulse width (μs) – 30 60 90Treated time (h) – 4 4 4Surface roughness (μm) – 0.06±0.005 0.06±0.005 0.06±0.005

of DLC is composed of two broad peaks, namely D peak (disorderpeak) and G peak (graphite peak). According to literature [24], thehigh frequency band (G band centered around 1560 cm−1) in the DLCRaman spectra has been assigned to the graphite-like sp2-bondedcarbon and the low frequency bond (D band centered around1350 cm−1) has been assigned to the sp3-bonded phase. That is tosay, the location of the G peak is proportional to the sp2 content in theDLC film. It is believed that the content of sp3 bonds or the value ofsp3/sp2 determines the main performance of the DLC film. The smallerthe integral intensity ratio of ID/IG, the higher the content of sp3 bondsin the DLC films, and the DLC is more similar to a diamond.

Fig. 1 shows the Raman spectra curves of the DLC film in differentimplantation pulse width conditions. From these figures, we can seethat different Raman spectra of DLC films in the 1100–1170 cm−1

scope all have a non-symmetric wide scattering peak, correspondingto typical DLC Raman spectra. For the PIIID DLC films, the G peak wasshifted to a lower frequency from the graphite position at 1580 cm−1,indicating that the structure of the PIIID DLC film is not graphite but alarge amount of graphite-like sp2 bonds, together with diamond-likesp3 bonds. The ID/IG value was calculated by fitting two Gaussian curvesand also showed in Fig. 1. According to these curves, the ID/IG value ofsample D1 is the biggest among the three kinds of implantation pulsewidth. It means that there is more sp2 bond content in the DLC film. Incontrast, the ID/IG values of samples D2 and D3 are 0.65 and 0.47, respec-tively, less than that of sample D1. It indicates that there is more sp3

bond content in the DLC film of the D2 and D3 samples. Literature[25] describes at least 10% sp3 bonds in the DLC film, if the Raman spec-tra have a wide peak near 1570 cm−1. Therefore, the wide peak of1550 cm−1 and 1560 cm−1 found in Fig. 1 shows that the sp3 bondcontent should be more than 10%. According to literature [6], the mainreason of the high content of sp3 bonds in DLC film samples D2 andD3 is that the ion beam energy range is easy to achieve the transitionfrom π hybridized orbital to σ hybridized orbital (i.e. from sp2 bond tosp3 bond), thus contributing to the increase of the sp3 content.

3.2. AFM surface morphology

An AFM image, as-grown DLC film on AISI52100 bearing steel sub-strate (sample D2), measured in a non-contact mode is shown inFig. 2. From this image, a quantitative assessment of the surfaceroughness can be made. The film surface is columnar, the grain issmall, and the structure is compact and well-proportioned. The aver-age and root-mean square surface roughness (RMS) values of the DLCfilm, corresponding to a surface area of 25 μm2, are about 12.5 nmand 14.9 nm, respectively. But the average and root-mean square sur-face roughness (RMS) values of the blank AISI52100 steel substrateare about 60.7 nm and 75.8 nm, respectively. These indicate thatthe fabricated DLC film is extremely smooth and the coating processcertainly does not result in any measurable increase in surface rough-ness. In addition, we can see a small amount of surface protuberancein AFM 2D and 3D images. This is because there exist inevitably largegraphite particles dispersed in the film surface during the plasma im-mersion ion implantation and deposition process.

3.3. Nanohardness and elastic modulus

The nano-hardness and elastic modulus relationship curves of DLC/AISI52100 samples in different surface roughness conditions are shown

D4 D5 D6 D7 D8

60 60 60 60 603 5 4 4 40.06±0.005 0.06±0.005 0.53±0.005 0.44±0.005 0.34±0.005

0 200 400 600 800 1000 1200 1400 1600 1800 20000

8

16

24

32

40

48

D6

D7

D8

D2

Har

dnes

s / G

Pa

Displacement into surface / nm

a

0 200 400 600 800 1000 1200 1400 1600 1800 20000

50

100

150

200

250

300

350

400

450

500

D8 D7

D6

D2

Ela

stic

mod

ulus

/ G

Pa

Displacement into surface / nm

b

Fig. 3. Nanohardness (a) and elastic modulus (b) as a function of displacement into thesurface in different surface roughness conditions treated by PIIID.

800 1000 1200 1400 1600 1800 2000

Ram

an in

tens

ity /

(a.u

.)

ID/IG=1.12

ID/IG=0.471580

1560

1550

D1

D2

D3

Wavenumber / (cm-1)

ID/IG=0.65

Fig. 1. Raman spectra curves of the PIIID DLC films in different implanted pulse widthconditions.

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in Fig. 3. The indentation depth into the film surface is about 2000 nm.The nanohardness and elastic modulus of the AISI52100 bearing steelsubstrate are 11 GPa and 220 GPa, and the DLC film thickness is2000 nm. It is believed that substrate materials have a little effect onthe measurements for film mechanical properties, if the indentationdepth is in the 1/10–1/5 film thickness scope, namely there is no ISE(indentation size effect). When the indentation depth is about 0.05–0.30 μm, the nanohardness of the DLC film sample is about 22 GPa,30 GPa, 36 GPa and 40 GPa, corresponding to the substrate surfaceroughness of 0.53 μm, 0.44 μm, 0.34 μm and 0.06 μm, respectively.The maximum nanohardness of treated samples increases by 263.6%than that of bare substrate. The elastic modulus has little difference inthe roughness of 0.53 μm, 0.40 μm and 0.34 μm, but for the polishedsample, it's about 430 GPa, increased by 95.5%. Therefore, the rough-ness has a certain influence on the mechanical properties of the DLC/AISI52100 samples, that is to say, the rougher the substrate surface,the worse the mechanical properties of the DLC film specimen.However, it must be emphasized that when making indentation mea-surements on extremely thin films such as the DLC film describedhere, substrate effects come into play and strongly influence the resultsof the nanohardness and elastic modulus. This can be seen from therapid decline in the nanohardness values when the indenter movescontinuously from the film into the substrate.

3.4. Friction and wear behavior

Fig. 4 depicts the friction coefficient as a function of sliding cyclesfor the treated and untreated samples during sliding test. Variationsin the friction coefficient with the sliding cycles for different implan-tation pulse width samples are shown in Fig. 4a and different surfaceroughness samples are shown in Fig. 4b. From Fig. 4a, we can see that

Fig. 2. Three dimensional (a) and two dimensional (b) AF

the friction coefficient of the bare sample (D0) quickly reaches a rel-atively high value of 0.87 at about 500 sliding cycles, but the frictioncoefficient of all PIIID samples remains lower at the early stage ofthe test, and gradually increases with the number of sliding cycles.The wear life of samples D1 and D3 is about 2000 cycles and 4000 cy-cles, respectively. By contrast, the friction coefficient of sample D2 re-mains about 0.2 when the sliding cycle is more than 10,000. FromFig. 4b, it also can be seen that the friction coefficient of the treatedsamples is much less than that of the AISI52100 bearing steel sub-strate. At the beginning, the friction coefficient is smaller becausesome oxide layer exists in the surface, then the friction coefficient

M surface morphology of the PIIID DLC film sample.

Sliding cycles/×103

Sliding cycles/ ×103

0 1 2 3 4 5 6 7 8 9 10 11 12 130.0

0.2

0.4

0.6

0.8

1.0b

D8

D7D6

D2

Fric

tion

coef

ficie

nt

D0

a

Fig. 4. Friction coefficient curves of samples in different implantation pulse widths(a) and different surface roughness (b) conditions treated by PIIID.

Fig. 5. Corrosion polarization curves of DLC film samples treated by PIIID.

Fig. 6. Surface corrosion images of bearing steel substrate and DLC/AISI52100 samplesat different magnifications.

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increases when the oxide frazzled. The sliding cycles are about 2000,8000 and 9000, corresponding to samples D6, D7 and D8. But for thepolished sample D2, when the sliding cycle is more than 12,000, thefriction coefficient is about 0.2 all the same. It indicates that substratesurface roughness has much effect on the friction coefficients of theDLC/AISI52100 samples, the rougher the surface, the shorter thewear life. This is because the rough surface itself is the crack sourceand easily produces stress concentration. But for the PIIID sample,the probability of crack nucleation and deformation of the materialsurface in force action can be reduced, because of improving the ma-terial surface hardness. Meanwhile, some DLC is converted to graph-ite during the sliding, which can effectively reduce the frictioncoefficient because of solid lubrication, and improve the wear resis-tance of the treated samples.

3.5. Corrosion resistance of DLC samples

Fig. 5 shows the relationship curves between current density (icorr)and corrosion potential (Ecorr) for untreated and DLC/AISI52100 couponsin the 3.5% NaCl saturated solution. The untreated sample shows a rapidcurrent density increase resulting from the formation of dichromate(Cr2O7

−) and chromate (Cr42−) together with the evolution of oxygen inthe transpassive region. The corrosion potential for the untreated sub-strate (D0)was−0.75 V (Hg/HgCl) and that for the PIIID samplewas ap-proximately−0.68 V (D4),−0.57 V (D1),−0.55 V (D2),−0.49 V (D3)and −0.38 V (D5), respectively. The corrosion polarization curves of alltreated samples shift to the top left. It indicates that corrosion resistanceof the DLC/AISI52100 samples can be improved obviously. In particular,

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for the DLC/AISI52100 samples, the current density wasmore than threeorders of magnitude smaller than that of the untreated sample through-out the anodic region. The corrosion resistance increases with theimplantation pulse width and implantation time, and the sample D5 ex-hibits the best corrosion behavior.

In order to compare the corrosion resistance of bearing steel sub-strate and DLC/AISI52100 samples, the optical corrosion microscopyof samples D0, D2 and D5 is shown in Fig. 6. From these figures, wecan see a lot of corrosion pits almost all over the surface area of thesubstrate at 10 times magnification. By contrast, for the DLC/AISI52100 samples, even at 500 times magnification, the sample sur-face is still uniform with only a few corrosion pits and the corrosionarea is small. This indicates that the DLC/AISI52100 sample not onlyhas good corrosion resistance, but also has very low porosity andexcellent compactness.

4. Conclusions

The surface properties of the AISI52100 bearing steel substrate can beimproved obviously by diamond-like carbon films (DLC), fabricated byPIIID technique. The DLC film has good uniformity and density, and thesp3 bond content is more than 10%. The maximum nanohardness andelastic modulus of the DLC/AISI52100 specimens increased by 263.6%and 95.5%, and the friction coefficient decreased from 0.87 to 0.2,which are depending on the substrate surface roughness, implantationpulse width and implantation time. The optical PIIID parameters are im-plantation pulse width of 60 μs, implantation time of 5 h and surfaceroughness of 0.06 μm. Therefore, the DLC film synthesized on anAISI52100 bearing steel surface by PIIID is a promising technology for im-proving the mechanical performance and corrosion behavior of bearingsteel materials.

Acknowledgment

This research was supported by the National Nature Science Founda-tion of China (Grant No. 51165015) andAnalysis & Testing Foundation ofKunming University of Science and Technology (Grant No. 2011008).

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