The influence of textured interface on DLC films prepared by vacuum arc

Accepted Manuscript The influence of textured interface on DLC films prepared by vacuum arc

X. Liu, W. Zhang, G. Sun PII: DOI: Reference:

S0257-8972(18)30609-1 doi:10.1016/j.surfcoat.2018.06.034 SCT 23485

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

29 March 2018 15 June 2018 20 June 2018

Please cite this article as: X. Liu, W. Zhang, G. Sun , The influence of textured interface on DLC films prepared by vacuum arc. Sct (2018), doi:10.1016/j.surfcoat.2018.06.034

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The influence of textured interface on DLC films prepared by vacuum arc

X. Liu1

W.Zhang1

G. Sun1

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[email protected]

G．Sun

[email protected]

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X．Liu

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Science and Technology on Power Beam Processes Laboratory, Beijing Aeronautical Manufacturing Technology Research Institute,No.1 Dongjunzhuang Road, Beijing, 100024, China

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Abstract

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Vacuum arc was used to getprepare the Cr/CrN/DLC films on different textured surfaces fabricated prepared by femto-laser, and the influences of textures with different

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patterns, sizes, and orientations on the structures, adhesion and mechanical properties of Cr/CrN/DLC films were investigated. The physical and chemical characteristics of theDLC

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films were studied by scanning electron microscopy (SEM), Raman. Morphology instrument was used to test the chemical structure and surface topography. Adhesive strength and

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anti-wear properties were measured by scratch tester and SRV micro-tester. The results showed that micro-structures improve the contents of sp3 in DLC film, and also affect the

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adhesive strength and anti-wear properties of DLC films by increasing interval from 150 to 350 μm, which was processed by femto-laser under 0.96 power density, 400 mm/min processing speed, 150-350 μm separation.. The experiment certificated that DLC film on stripe structure with 250-350 μm interval showed best properties, the wear rate of DLC film with 250 μm interval was lower than 1.35×10-6mm3/Nm, and the adhesion strength of film was up to 52 N on stripe micro-structure separated around 350 μm. The results showed that femto-second ultrafast laser has little influence on substrate, the textured surface improved the

ACCEPTED MANUSCRIPT sp3 contents in DLC films with suitable size. Compared to the DLC film without micro-structure, the best anti-wear properties of DLC film with textured interface with 250 μm interval were enhanced up to 50 times.

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Keywords: Vacuum arc, Texturing interface, DLC films, Mechanical properties

Introduction DLC films have been studied for good properties such as low friction coefficient, great

ACCEPTED MANUSCRIPT mechanical property, hardness, chemical inertness and high thermal conductivity, which are very attractive for industrial applications such as tribological, anti-corrosion, and gas barrier [1-3]

. In spite of their excellent mechanical and tribological properties, DLC films show

insufficient adhesion to steel substrates. Principal reason for poor adhesion and anti-wear

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properties was ascribed to the generation of strong residual stress, which limit the practical application, especially in the bearing and gear parts[4-6].

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In recently years, combining texture[7,8] with solid lubricant film to form

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multiple-dimensioned solid lubricant film was considered to be a useful method to avoid

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shortages of films, such as poor adhesion, high internal stress, et al. The main effect of micro-texture was storage of lubricants and abrasive dusts, and the micro-structure could

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decrease the contact area under face-to-face contact, reduce the friction. The research about micro-structure started from 1960s, Wakuda fabricated different pit-structures on Si3N4

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surface, and found the micro-structure with 15% texture density, 40 μm diameter could significantly improved the anti-wear properties[9]. Andersson used YAG laser to texture on the

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steel surface, and found the small texture with low density and deep size could decrease the friction coefficient [10]. Voecodin and Zabinski prepared 0.5%～50% micro-holes on TiCN

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coating surfaces, and anti-wear properties of TiCN film increased by one order of magnitude[11]. Under the similar principle, Basnyat used ion etching to prepare pits on TiAlCN film, and then filled with MoS2 and MoSx-Ag solid lubricants by magnetron sputtering, which supply the continuous lubricant, and obviously reduce wear properties than films without structures[12].Various texturing techniques, such as laser surface texturing, anodic oxidation, photolithography, electro machining, sol-gel method, have been developed for preparing micro-structures. Among them, the laser surface texturing (LST) seems to be

ACCEPTED MANUSCRIPT more encountered method of surface texturing because of its versatility, fast adaptability, and cleanness of environment[13,14]. However, the processing precision, recast layer and processing defects might be strongly influenced by heat affect of the traditional Laser processing methods. In recently years, ultra-fast laser(femto-second and picosecond laser) rapidly

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developed, which showed ultra-short pulse width, ultra-strong peak power, which might avoid the influence of linear absorption, energy transfer and diffusion in tradition laser processing,

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the ultra-fast laser obviously reduced the processing defect, recast layer and heat-affected

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better performance in micro-structure processing.

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zone[15,16]. The ultra-fast laser could prepare ultra high precision micro-structure, showing

The concept of enhancing the tribological properties of DLC by surface texturing is not

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entirely new, many efforts have already been made in other studies[17,18]. But ultra-fast laser processing has been seldom applied in these kinds of reports. In this study, different texturing

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interfaces were prepared between DLC films and substrates by femto-laser, and the influences of the pattern, size and orientation of textures were investigated. The aim of this work was to

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further improve the knowledge of the relation between surface textures by femto-laser processing and the DLC films, including their orientation and the tribological properties of

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sliding surfaces under nonlubricated boundary conditions.

2. Experimental Details

Femto-laser was applied to prepare textures onto steel surfaces, and vacuum arc was used to prepare Cr/CrN/DLC films on textured surface. GCr15 (0.95~1.05% C, 0.15~0.35% Si, 1.4~1.65% Cr, 0.25~0.45% Mn) steel samples were prepared by grinding, polishing, and ultrasonic cleaning 30 min in alcohol. While the substrates were located in the chamber,

ACCEPTED MANUSCRIPT sputter-etched was applied for 30 minutes in argon prior to deposition. The influence of femto-laser processing on steel substrates were investigated by varying laser power density around 0~2 J/cm2, scanning speed from 10 to 400 mm/min. Three different kinds of patterns were prepared by femto-laser, including pit, stripe and lattice

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ordered, , texture interval was controlled around 150 μm, 250 μm, 350 μm, the processed marking width varied between 150 to 250 μm by multiple scanning..

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Cr/CrN/DLC films were deposited on textured surface in a vacuum arc system, which is

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composed of vacuum, rotation, heating and deposition parts. Diameters of Cr targets were

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460×130 mm, with 99.99% purity, respectively. A rotated sample holder was located in the center of the chamber and the distance between targets and substrate was 130 mm. The base

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pressure of the vacuum chamber was kept below 6×10-4 Pa. Argon (99.999%), nitrogen (99.999%) and C2H2 were introduced into the chamber through mass flow controllers, which

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were used as the sputtering and reactive gases, respectively. The deposition pressure was 0.2~0.4 Pa, Ar 200 sccm, C2H2 500 sccm, thickness was controlled by deposited time and

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target powers. Two Cr target currents were kept at 1.5 A, respectively, and pulse bias (Vp=100 V) was applied to the substrate during film growth. DLC film was prepared with high C2H2

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flow, the thickness was controlled by deposition time and target power. The parameters were listed in table 1.

Table.1 The deposition parameters of Cr/CrN/DLC films parameters

Cr layer

CrN layer

CrCN layer

0.2

0.2

0.25

100

100

100

0

0

100

Pressure（Pa）

Bias（V）

The flow of C2H2（sccm）

Cr-DLC layer 0.4

100

500

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300

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100

100

100

100

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10

30

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The flow of N2（sccm）

The flow of Ar（sccm）

Current of Cr targets（A）

Deposited time（min）

0

100

100

40

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The physical and chemical characteristics of the films were studied by scanning electron microscopy(SEM) and Raman. Morphology instrument was used to test the chemical

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structure and surface topography. Ahesive strengths were measured by scratch tester, 0-60 N,

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and anti-wear properties were tested by SRV micro-tester, load 20 N, 20 Hz, test time 1200 s.

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3. Results and discussion

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Processing depth and width would be influenced by power density and scanning speed, figure 1 showed that the width varied from 20 to 150 μm with power density from 0.18 to

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0.96 J/cm2, the depth changed from 210 to 4 μm with increasing scanning speed from 10 to 400 mm/min. Meanwhile, it was found that typical defects increased while the power

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increased up to 1.42 J/cm2, as shown in figure 2, which showed typical recast layer and processing defects. In order to approve the heat affection, EDS was used to test elements in

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the concave areas fabricated by Laser processing under different power density. In figure 2, the results illustrated that the O element contents increased with the increase of power density, but less O element was presented while the power density was lower than 0.96 J/cm2, which was adequate for the surface texture. Meanwhile, textures with 3 or 4 micrometer depth would be perfect to form textured Cr/CrN/DLC film around 5 μm thickness. So fabrication of pit, stripe and lattice patterns were under 0.96 J/cm2, 400 mm/s, and different concave areas densities around 40%, 60%, 100% were shown in figure 3,

it was found that the concave

ACCEPTED MANUSCRIPT areas increase with the decrease of intervals, while 100% concave area presented with 150 μm processing intervals. Meanwhile sharp edge appeared as shown in figure 3. It means little defects and heat-affected zones were presented during the procedure, showing little effect on the substrates, which was tested and verified in figure 2. Different concave widths around 150

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μm~180 μm also were presented in figure 3, which were fabricated by laser scanning 2-3

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times with 50 μm apart.

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Fig. 1 Processing depth and width of femto-laser with scanning speed and fluence

Fig. 2 SEM images of the processed substrates by femto-laser with different power

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density under 400 mm/min speed

Fig. 3 Different structures on steel substrates by femto-laser processing

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Cr/CrN/DLC films were deposited on different micro-structures for 1.5 h., As shown

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in 4 and 5, bulk grains were found on the concave areas and around the edge of the processing boundary, and it was found that film flaked off the groove edge at narrow interval. In figure 5,

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many chips were also found on the concave areas, which meant the high stress during the

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growth of film. With the increase of interval distance, the film filled the textured surface without flaking off. Meanwhile, little influence of film growth was found with increasing

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processing width. But on the concave areas, grains were much bigger than the grains on plane

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areas due to the variation of substrate morphology influenced by laser processing. According to the theory of film nucleation and growth, the variation of substrate morphology and impurities would similarly influence free-energy by affecting terms related to either surface and volume electrostatic, chemical, etc., energy, which affected the grain size.[19]

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Fig. 4 DLC films on different structures

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Fig. 5 DLC films on different stripe structures(a separation 150 μm 350μm c scanning 2 times d scanning 3 times)

b separation

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XRD patterns of DLC films on steel substrates with different micro-structures were presented. The films deposited on steel substrates showed no diffraction angles besides the

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diffraction peaks arisen from the substrates (marked as Fe-Cr in the figure), indicating the formation of DLC films with amorphous structure[20].

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Fig. 6 XRD pattern of DLC films on micro-structured surface

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The films were also analyzed by Raman spectrum, which showed sp3 and sp2

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information of carbon. The spectrums of DLC films on different micro-structures were shown in figure 7, and two un-symmetry peaks were found between 1000 and 2000 cm-1, which

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could be fitted into D peaks around 1370 cm-1 and G peaks around 1560 cm-1[21,22], it meant that typical DLC films were fabricated. With the decrease of integral area ratios between D

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peak and G peak(ID/IG), sp3 contents(Diamond phase) increased, which meant the film was more closed to the properties of diamond[23,24]. Table 2 showed the information of D peaks

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and G peaks on base of Gaussian fitting. It was found the diamond phase increased with the increase of intervals between 150～300 μm, decreased at 350 μm interval, but was still higher

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than the DLC film without structures. Meanwhile, the diamond phase increased with the increase of concave width by increasing scanning times, then decreased. The variation of sp3 contents might be influenced by the change of electric field distribution influenced by grooves of textured substrates[25], which lead to the ion energy variation during the film deposition.

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Fig. 7 Raman spectrum of DLC films on micro-structured surface

ACCEPTED MANUSCRIPT Table 2 Information of DLC Raman spectrum on different micro-structured surface

Stripe

Lattice

Peak position Without 150 μm 250 μm 350 μm 2 scanning 3 scanning 150 μm 250 μm 350 μm

1407.1 1350.6 1372.5 1383.2 1374.5 1370.7 1388.3 1381.9 1394.8

D Peak Half Peak area width of peak 392.6 79.4 220 50.1 233.9 45.8 298.7 71.1 235.7 35.2 261.8 62.4 311.9 72.8 319.7 71.3 314.3 76.3

Peak position 1558.8 1588.9 1576.9 1581.1 1577.9 1575.6 1576.7 1583.7 1582.8

G Peak Half width of peak 111.8 178.9 133.2 122.7 132.5 140.4 111.1 136.0 104.9

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Samples

Peak area

ID/IG

20.6 49.9 39.8 28.9 22.8 37.6 27.2 28.7 23.7

3.85 1.0 1.15 2.4 1.54 1.7 2.68 2.48 3.21

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To test adhesion strength of DLC films with structures, scratch test was carried out. The

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adhesion properties of films could be characterized by scratch curves and scratch track pictures shown in Fig.8. According to scratch tracks, the appearance of the chips at the track

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edge indicated the peeling of the films, it was found that large area of films flake off near the

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scratch track at the track edge related to the brittleness of the film without structures. Meanwhile, it was found that films discontinuous stripped off textured substrates at intervals

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over 250 μm, and showed better adhesion strength, which illustrated that structures hindered

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continuous crack propagation, caused by releasing stress during film deposition. With the given interval of stripe and lattice microstructures, the adhesive strength obviously increased

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by increasing the intervals around 350 μm.

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Fig. 8 The scratch curves of DLC films on micro-structured surface Anti-wear properties of DLC films and steel substrate were analyzed by SRV test under

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surface-surface contact with 20 N load, 20 times per minute, which lasted for 20 minutes, the wear tracks were shown in figure 9, figure 10 and figure 11, containing the SEM pictures and

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white light interference topographies. The information of wear width, depth and calculated abrasion rates were shown in table 3. It was found that the substrate was obviously worn with

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wear depth 14.8 μm, wear width 1.54μm, and abrasion rate was calculated to be 341×10-6 mm-3/N·m. Meanwhile, the wear depth and width of DLC film without micro-structure were

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2.45 μm, 1.56 μm, respectively, and abrasion rate was around 57.33×10-6 mm-3/N·m. Under the similar wear condition, the anti-wear properties were obviously improved while interval increased up to 250 μm in stripe and lattice structure, but decreased with the interval up to 250 μm. The noticed lowest abrasion rate was around 1.35×10-6 mm-3/N·m on striped structure. In the SEM images, it was found abrasive dusts fill the processing gaps, showing little worn marks on the plane surface, while the DLC film without structure showed obviously abrasion with flaking fragments. The improvement of anti-wear properties of DLC

ACCEPTED MANUSCRIPT film with specified micro-structure could result from the storage effects of debris and lubricants, which limited the damage during face-face wear[25,26]. Meanwhile, the improved sp3 contents of film lead to better mechanical properties, and also brought better anti-wear

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properties.

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Fig. 9 Wear tracks on different structures by white light interference topographies

Fig. 10 Wear tracks on different structures by SEM

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topographies

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Fig. 11 Wear Depth on different stripe structures by white light interference

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Sample

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Table 3 Wear information of DLC on different micro-structure

Stripe 150 µm Stripe 250 µm Stripe 350 µm Stripe 2 scanning Stripe 3 scanning DLC Substrate

4. Conclusion

Wear Depth (µm) Damage 0.15 0.4 0.32 0.38 2.45 14.8

Wear Width (mm) Damage 0.6 0.7 0.65 0.7 1.56 1.54

Wear rate (10-6mm-3/N·m) Damage 1.35 4.2 3.12 3.99 57.33 341.9

ACCEPTED MANUSCRIPT Vacuum arc was used to prepare DLC films on different micro-structured surfaces of GCr15 stainless steel by femto-laser processing. The influences of pattern, separation and width of micro-structures were investigated. The results were obtained as follows: 1）Different texturing on GCr15 steels can be obtained by femto-laser without influence

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on mechanical properties and elementary composition of substrates . 2）The micro-structure improved the contents of sp3 in DLC film under 0.96 J/cm2 power

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maximum sp3 value was between 150 and 250 μm；

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density, 400 mm/min processing speed, which increased with the structure size, and the

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3）DLC films on 250 μm stripe and lattice structure showed the best anti-wear properties, wear rate 1.35×10-6 mm3/Nm, which was up to 50 times than DLC film without structure.

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From these results, the given stripe or lattice micro-structure (around 250-350 μm separation, 100 μm width, 4 μm depth) prepared by femto-laser plays a main role on the

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chemical structure and mechanical properties of DLC films. The proper micro-structure resulted in the enhancement of mechanical and anti-wear properties, indicating that DLC

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of the films.

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films on micro-structure is very effective to improve the mechanical and anti-wear properties

ACCEPTED MANUSCRIPT References

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[1]Bai L C, Zhang G A, Wang L P,et al. Tribological mechanism of hydrogenated amorphous carbon film against pairs: a physical description. Journal of Applied Physics, 2011,110,:33521 [2]R.Hauert, K. Thorwarth, G. Thorwarth, an overview on diamond-like carbon coatings in medical applications, Surf. Coating Technol. 233(2013)119-130. [3]J.Dufil, F.Faverjon, C.Heau, C.Donnet, S.Benayoun, S.Valette, Evaluation of a variety of a-C:H coatings on PEEK for biomedical implants, Sruf.Coat. Technol. 313(2017) 96-106 [4] [5]Staedler T, Schiffmannn K. Corrleation of nanomechanical and nanotribological behavior of thin DLC coatings on different substrates, Surface and coatings technology. 482( 2001)1125-1129 [6] A.P.Carapeto, A.P.Serro, B.M.F.Nunes,et al.Characterization of two DLC coatings for joint prosthesis: the role of albumin on the tribological behavior, Surf.Coat.Technol. 204(2010) 3451-3458. [7]C.Chouquet, J.Gavillet, C.Ducros, F.Sanchette, Effect of DLC surface texturing on friction and wear during lubricated sliding, Mater.Chem.Phs. 134(2010) 367-371 [8]Gualtieri E, Borghi A, Calabri L et al. increasing nanohardness and reducing friction of nitride steel by laser surface teturing, Tribol Int. 42(2009) 699-705

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[9] Wakuda M，Yamauchi Y，Kanzaki S, et al. Effect of surface texturing on friction reduction

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between ceramic and steel materials under lubricated soliding contact, Wear. 254(2013) 356-363 [10] Andersson P, Koskinen J, Varjus S, et al. Microlubrication effect by laser-textured steel surfaces, Wear. 262(2007) 369-373 [11] Voevodin A A, Zabinski J S. Laser surface texturing for adaptive solid lubrication, Wear. 261(2006) 1285-1292 [12]Basnyat P, luster B, Muratore C, et al, Surface texturing for adaptive solid lubrication, Surf Coat Technol. 203(2008) 73-79 [13]Dongqing He, SHaoxian Zheng, Jibin Pu, et al, Improving tribological properties of titanium alloys by laser surface textuing and diamond-like carbon film, Tribl.Int. 82(2015) 20-27 [14] DanielBraunChristianGreinerJohannesSchneiderPeterGumbsch, Efficiency of laser surface texturing in the reduction of friction under mixed lubrication, Tribol. Int. 77(2014)142-147 [15]Sanguk Park, Yunseok Kim, Joonho You, SeungWoo Kim, Damage-free cutting of chemically strengthened glass by creation of sub-surface cracks using femtosecond laser pulses, CIRP Annals. 66(2017) 535-538 [16]Birgit Angelika Schmidt, Simone Pentzien, Andrea Conradi. Femtosecond and nanosecond laser decontaminations of biocidal-loaded wooden artworks, Applied Physics A. 123(2017) 696 [17]Dumitru G, Romano V, Weber H P, et al, Lasre treatment of tribological DLC films, Diamond Relat Mater. 12(2003) 1034-1040 [18]Kononenko T V, Pimenov S M, Dumitru G, et al, laser-induced spallation in diamond-like carbon films, Appl Phys A: Mater Sci Process. 79(2004) 543-549.

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[19] Milt Ohring, Materials Science of Thin Films, Second ed., Elsevier, Singapore,1991. [20] Ferrari A C, Robertson J, Resonant Raman spectroscopy of disordered, amorphous, and diamond like carbon, Physical Review B. 64(2001) 075414-1-075414-13. [21] M. Lubwama, B. Corcoran, K.V. Rajani, C.S. Wong, J.B. Kirabira, A. Sebbit, K.A. McDonnell, D. Dowling, K. Sayers, Raman analysis of DLC and Si-DLC films deposited on nitrile rubber, Surf. Coat. Technol. 232 (2013) 521–527. [22] Zhang X W, Ke N, Cheung W Y,et al, Synthesis and structure of nitrogenated tetrahedral amorphous carbon thin films prepared by a pulsed filtered vacuum arc deposition, Diamond and Related Materials. 12(2003) 1-7. [23] A.Habibi,S.M.Mousavi Khoie,F.Mahboubi,M.Urgen, Raman spectroscopy of thin DLC film deposited by plasma electrolysis process, Surf. Coat. Technol. 309( 2017) 945-950 [24] Wang Y X, Wang L P, Xue Q J, Improvement in the tribological behaviour of DLC films under water lubrication by surface texturing, Tribol Lett. 41(2011) 439-449. [25]Pettersson U, Jacobson S, Influence of surface texture on boundary lubricated sliding contacts, Tribol. Int. 36(2003) 857-864 [26]Pettersson U, Jacobson S, Friction and wear properties of micro textured DLC coatd surfaces in boundary lubricated sliding, Tribol Lett. 17(2004) 553-559

ACCEPTED MANUSCRIPT Table.1 The deposition parameters of Cr/CrN/DLC films

The flow of C2H2（sccm） The flow of N2（sccm） The flow of Ar（sccm） Current of Cr targets（A）

0.2

0.2

0.25

100

100

100

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CrCN layer

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Cr layer

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Samples

Peak area

Peak position

Half width of peak

Peak area

Without

1407.1

392.6

79.4

1558.8

111.8

20.6

3.85

150 μm

1350.6

220

50.1

1588.9

178.9

49.9

1.0

250 μm

1372.5

233.9

45.8

1576.9

133.2

39.8

1.15

350 μm

1383.2

298.7

71.1

1581.1

122.7

28.9

2.4

2 scanning

1374.5

235.7

35.2

1577.9

132.5

22.8

1.54

3 scanning

1370.7

261.8

62.4

1575.6

140.4

37.6

1.7

150 μm

1388.3

311.9

72.8

1576.7

111.1

27.2

2.68

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1381.9

319.7

71.3

1583.7

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28.7

2.48

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Peak position

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G Peak

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Wear Width (mm)

Wear rate (10-6mm-3/N·m)

Stripe 150 µm Stripe 250 µm Stripe 350 µm Stripe 2 scanning Stripe 3 scanning DLC Substrate

Damage 0.15 0.4 0.32 0.38 2.45 14.8

Damage 0.6 0.7 0.65 0.7 1.56 1.54

Damage 1.35 4.2 3.12 3.99 57.33 341.9

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List of figure captions Fig.1. Processing depth and width of femto-laser with scanning speed and fluence Fig.2.: SEM images of the processed substrates by femto-laser with different power density under 400 mm/min speed Fig.3.: Different structures on steel substrates by femto-laser processing Fig.4.: DLC films on different structures Fig.5.: DLC films on different stripe structures(a separation 150 μm b separation 350μm c scanning 2 times d scanning 3 times) Fig.6.: XRD pattern of DLC films on micro-structured surface Fig.7.: Raman spectrum of DLC films on micro-structured surface Fig.8.: Scratch curves of DLC films on micro-structured surface Fig.9.: Wear tracks on different structures by white light interference topographies

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Fig.10.: Wear tracks on different structures by SEM Fig.11.: Wear Depth on different stripe structures by white light interference topographies

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ACCEPTED MANUSCRIPT Highlights

1. Femto-second ultrafast laser has little influence on substrate with different separations were prepared under the 0.97J/cm2, 400 mm/s.

PT

2. The micro-structure improved the contents of sp3 in DLC film under 0.96 power

maximum value was between 150 and 250μm.

RI

density, 400mm/min processing speed, which increased with the structure size, and the

SC

3. DLC films on 250μm stripe and lattice structure showed the best properties, adhesion

NU

strength 52N, wear rate 1.35×10-6mm3/Nm, which was up to 50 times than DLC film

AC

CE

PT E

D

MA

without structure.