Bonding structure and tribological properties of DLC films synthesized by dual ion beam sputtering

Vacuum/volume

Pergamon

45lnumber S/pages 977 to 980/1994 Copyright 0 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207x/94$7.00+.00

Bonding structure and tribological properties of DLC films synthesized by dual ion beam sputtering Xiaoming Tsinghua received

He, Wenzhi University,

11 March

Li and Hengde

Beijing,

700084,

Li, Department PR China

of Materials

Science

and Engineering,

1993

Results of DLC films synthesized on Si(l7 I), glass and AI.9 52100 bearing steel by dual ion beam sputtering at room temperature are presented. The carbon films deposited by ion beam sputtering were simultaneously bombarded by CH”’ with energies of 0.2-25 keV. lnfra-red and Raman spectroscopy revealed that films consisted of amorphous carbon with the structure of mixed sp3+sp2 bonding. It appears that carbon films bombarded by CH”’ with low energies of 200400 eV have more sp3 bonding and high microhardness. Tribologicalinvestigation of DL C films on AlSl52 100 steel under a wear load of 15 N showed a constant friction coefficient of 0.7 and roughly stable wear factor of (0.436+ 0.06) x 1O-6 mm3 N-’ m-’ and, when the load surpassed 15 N, the friction coefficients and wear factor increased with the increase of load.

I. Introduction Hard or diamond-like carbon (DLC) films are being studied extensively because of their superior properties. In addition to electrical and thermal properties that are attractive for electronic applications’, their high hardness [Hr = 6600 kgf mm-’ (ref 2)]. low coefficient of friction (,u = 0.02 in dry Nz or vacuum’), low wear rate’ and high thermal conductivity make them attractive for tribological applications. Amorphous hydrogenated carbon (a-C: H) film synthesized by direct ion beam deposition was found to be extremely suitable for the reduction of friction coefficient and wear for AM 52100 steels. Some authors2.5 ’ pointed out that the content of hydrogen in the films critically determined their properties. Dual ion beam sputtering (DIBS) deposition, belonging to the ion beam assisted deposition (IBAD) in which the film preparing process is used, in conjunction with ion implantation, can produce, at a low substrate temperature, films such as TIN, Tic’ and DLC7.Y that have strong adhesion to substrates. However, the tribological properties of DLC films prepared by DIBS have not been extensively investigated yet. In this work, DLC films are synthesized by DIBS at room temperature, in which the films deposited by Ar+ beam sputtering were simultaneously bombarded by CH”+ with energies of 0.225 keV. More concern was given to the influence of bombarding energy on the bonding structure of DLC films. The DLC films exhibiting obvious characteristics of diamond were selected for tribological tests, to study properties such as coefficient of friction, wear factor and the surface morphology. 2. Experimental procedure The thin film deposition system is equipped with three broadbeam Kaufman ion sources (one is used for ion beam sputtering deposition, and the others to produce high and/or low energy bombarding ions), one rotatable water-cooled sample holder, and one rotatable water-cooled target holder. In this study, a pure

graphite target was sputtered by Ar+ to deposit carbon films. Prior to the experiment, substrates such as Si(1 1 I), glass and AISI 52100 steel were cleaned by ion beam bombardment. Pure CH, was introduced through the bottom of the high or the low energy ion sources below the filament and CH”+ ions were extracted through a two-electrode multiple aperture system. Both the high energy ions of 2-25 keV and the low energy ions of 2O(t400 eV were separately used to bombard carbon films during deposition. Table 1 shows the conditions for the deposition. A series of carbon films was prepared by changing the bombarding energies of CH “-I- ions. The films were characterized by IR and Raman spectroscopy. The microhardness was measured by a diamond indenter with a load of 10 g for the carbon films bombarded by CH”+ with energies of 3-25 keV and a load of 20 g for the films bombarded with energies of 200-400 eV. The thickness of the films for hardness measurement is larger than 600 nm. Wear tests of DLC lilms were performed without lubrication in a ball-on-disk test system. DLC films for measuring the tribological properties were synthesized on the substrate of AISI 52100 steel (HRC 61) which was shaped into disks with a diameter of 20 mm and thickness of 1.5 mm. The ball (also hardened

Table 1. Typical experimental Substrates Base pressure Sputtering energy Sputtering ion currents Bombarding energies Bombarding

ion currents

Partial pressure of CH, Working pressure Deposition temperature

deposition

condition

Si(l1 I) wafer, glass and 52100 steel 4 x 10eh torr 3.5 keV 130.- 150 mA 200,400 eV 3, 6. 10, 15, 25 keV 5 mA (high energy) 10 mA (low energy) (6--g) x 10e5 torr (1.2-1.4) x lo-“ torr room temperature

977

Xiaoming

He et al: Bonding

structure of DLC films

AISI 52100), 10 mm in diameter, held in a penlike ball holder, was moved back and forth across the surface of the disk at about IO cycles per second, creating a track length of 2 mm. Testing conditions included wear loads of 2-56 N, relative humidity of about 50% and room temperature. During wear process, the friction coefficient can be monitored and evaluated on a chart recorder as a function of wearing time. The volumetric wear of the disk was determined by measuring the wear track or groove by means of a surface profilometer. 3. Results and discussion 3.1. Infra-red and Raman spectroscopy. The CC bondings of carbon films were measured by Raman spectroscopy. Figure 1 shows the spectra for DLC films deposited at different bombarding energies of CH”+. In Raman shift analyses, the Raman spectrum of diamond consists of a single line at 1332 cm.- ’ and that of graphite corresponds to a line at 1580 cm- ‘. There are two broad peaks in Raman spectrum for DLC film : the broad peak at 152OO1536 cm--’ stands for bond-angle disorder sp2 bonding; and the peak at about 1283 cm- ’ for the sp’ bonding which represents the diamond characters of DLC films”. It was obvious in Figure I that by decreasing the bombarding energy of CH”+, the Raman spectra gradually changed from a single broad peak at about 140&l 550 cm- ’ to two separated peaks set at about 1500-l 550 cm- ’ and about 1300 cm- ‘. This shows clearly that DLC films deposited at low bombarding energies of CH”+ would have more fourfold coordinated bonds as the same in diamond. The CH stretching vibration of DLC films deposited on glass were also examined by IR spectra in Figure 2. It is well known that both sp’ bonding vibration in 2850-2960 cm ’ and sp2 bonding vibration in 3000-3095 cm- ’ could be observed in IR spectra’, which reflects the existing situation of C-H in amorphous hydrogenated DLC films. Figure 2 shows that DLC films deposited with ion bombardment in a range of 0.2-25 keV arc all composed of sp’+sp’ bonding. However, the C-H with .pp’ bonding is remarkable for the DLC films synthesized with low bombarding energies (< 1 keV), which again indicates that the films possess more sp3 bonds in their structure*.

1300

Raman

1500

shift (cm-t)

Figure 1. Raman spectra of diamond-like carbon tilms prepared by DIBS at various

CH”+ bombarding

energies.

The Knoops microhardness was measured on DLC films deposited on a Si(ll1) wafer. Table 2 lists the microhardness of the carbon films as related to the bombarding energies, with other conditions identical. It shows that the films synthesized by high energy bombardment have relatively low hardness and, in contrast, the carbon films bombarded by low energy ions exhibit high microhardness. The hardness value of the carbon film bombarded by CH”+ of 200 eV is 5100 kgf mm *, which belongs

58

29 t 21

54 1

18

50 46 .s z

23

2 .-

21

Pg

19

34

11

30

42

38

9

2s ~

153400

3000

2600

103400

3000

253400

2600

3000

Wave number (cm-‘)

(a) 15 keV Figure 2. Infra-red spectra for the carbon films bombarded 978

(b) 6 keV at the energies of: (a) 15 keV

(c) 200 eV

; (6) 6 keV

: (c)

200 eV.

2600

Xiaoming He et a/: Bonding structure of DLC films 0.7

Table 2. Hardness Bombarding energy (eV) Knoops hardness kgfmm~

of carbon 200

films prepared

300

400

3000

Wear load

0

by dual ion beam deposition 6000

10,000

15,000

25,000

AISI 52100 0.6 DLC film

log ’ 20g

1550 5100

3250

1630

2180

1560

1600

2720

to the ultra-hard film, according to ref 11. Since more sp’ bonds exist in the films synthesized under low energy CH”+ bombardment, as IR and Raman spectroscopies showed above, the much higher hardness obtained at 200 eV is a natural result 3.2. Tribological properties. DLC films were synthesized on AI.91 52100 steel samples by DIBS with CH”+ bombarding energy of 200 eV and their tribological properties were investigated. All tests were conducted on the films with thickness of about 30& 600 nm. The coefficients of friction as a function of sliding time or the

Cycles

x

10-3 12

6

18

I

I

0 0.3

-

0.2

-

A

0.1

1 L

0

10 Sliding

20

30

time (mm)

Figure 3. p vs sliding time for loads of 2 N (curve A), 5 N (curve B), 15 N (curve C) for DLC films deposited on the 52100 disks and 15 N (curve D) for uncoated 52100 disk.

(a) Figure 4. SEM micrographs

10N

0

I

I

I

I

I

I

10

20

30

40

50

60

Wear loads (N)

Figure 5. Friction wear load.

coefficient

values of all tests as a function

of increasing

number of sliding cycles are shown in Figure 3. For comparison, the data for the uncoated AISI 52100 disk are also illustrated in it. It was found that with wear loads less than 15 N, the coefficients of friction basically maintained a constant value of about 0.1, which was not influenced by the changes of wear load (from 2 to 15 N) and was much less than the coefficient of selffriction of AISI 52100 steel. Figure 4 shows the wear track morphology on the films produced by the ball reciprocating motion after 30 min of running. When the wear was carried under a load of 10 N only a very shallow wear groove formed on the film and the substrate was not at all exposed [Figure 4(a)]. When the wear load was increased to 20 N, the film in the center of the wear groove has been partially removed. exposing the substrate [Figure 4(b)], but the process of friction could still keep on steadily with a coefficient of 0. II. However, if the wear load surpassed 20 N, the coefficient increased slowly with increasing load and once the load surpassed 56 N, the coefficient abruptly incrcascd to 0.68, which reflected that the wear was now applied to the substrate itself and represented a typical adhesive wear [Figure 4(c)]. The changing trend of the friction coefficients of DLC film as a function of the wear loads is illustrated in Figure 5. The increase in the friction coefficients with the wear load means that the friction has been working on the surface where DLC film, in some locations, has been removed by wear abrasion.

500 x

of wear track produced

by a steel ball sliding under a wear load of: (a) 10 N, 30 min

; (b) 20 N, 25 min ; (c) 56 N, 11min. 979

Xiaoming He et a/: Bonding structure of DLC films Table 3. The wear properties of DLC films

Load (N)

2

5 72

IO 72

under different loads I5 88.8

20

I5

60

(uncoated) 36

Sliding distance (m)

12

Groove wear volume x IO6 (mm’)

15.62 129.84 358.98 599.42 960.25 21.232.25

Wear ractor 0.108 0.361 x IOh (mm’ N ’ m- ‘)

0.499

0.45

0.8

39.32

The wear properties summarized from the test series are listed in Table 3. The wear groove dimensions, determined by the su&ce profilometer, have been used to calculate the volume of film removed, as well as the wear factors. The wear factor is defined as the volume of material removed per unit applied load per unit sliding distance and is expressed as mm.’ N- ’ m- ‘. It was found that the wear factor under the load of 2 N is lower than that in rcf 5 and its wear groove for 30 min is too shallow to be observed by SEM, even if the magnification is quite large (> 1000 x ). The wear factors in the load range of 5-I 5 N do not exhibit the striking increase with increasing wear load but seem to keep at a roughly stable value of (0.436f0.06) x IOmh mm’ N - ’ m- ‘, which reflects a relatively stable step of wearing. All above wear factor values arc specified to the range of mild wear in unlubricated sliding” and indicate that DLC films have an excellent wear resistant characteristic. When the wearing load is 20 N, the wear factor obviously rises but is still much lower than the value of uncoated 52100 steel (Table 3). According to ref 13, the magnitude of wear would decrease significantly in lubricated conditions. As mentioned above, DLC films consisted of mixed sp’+sp’ bonding which was proved by IR and Raman spectroscopy, so it is not difficult to understand that the wearing test can steadily go on, even if the carbon film is partially removed during the wearing process [see Figure 4(b)]. The graphite constituent related to sp’ bonding in DLC film can act as a solid lubricant 14.15and effectively reduce the wear rate and maintain a stable wearing on the partly broken DLC film. The DLC films with suitable graphite sp’ bonding would be advantageous for their tribological applications, but Lhe best ratio of sp” to sp2 bonding still needs detailed investigation. Conversely, the wear loads applied to our ball-on-disk tribological tests are considerably larger than that in somes.‘S.‘6. It is estimated that the load of 2. I N in the ball-on-disk will result in a contact pressure of more than 3 x lo* N m- ‘, which represents an already very severe test I’. Therefore, the test with load of 15 N would be a much more severe wearing condition. Furthermore, the larger the wear load, the higher the impact stress of the ball upon DLC films. Since the coefficients of friction can still maintain a value of 0.1 with the stable wear factor of (0.436kO.06) x 10m6 mm3 N ’ m ’ at a high load of 15 N, it also implies that DLC films have a good impact toughness and adhesive force with AISI 52100 steel, All these demonstrate that DLC films prepared by DIBS with U-I”+ bombardment of 200 eV exhibit excellent tribological properties and should be suitable for improving the antiwcar properties of AISI 52100 steel. In addition, it should be pointed out that the content of H in DLC films has an important influence on the property. The lower hydrogen concentration would render the DLC films denser and 980

harder than films with higher hydrogen concentrationss.7. When pure methane is used to synthesize DLC films by ion beam deposition with bombarding energies of 500--1000 eV. the films were detected to contain approximately 30% hydrogen’. In the present work, the pure methane was also used for bombardment by CH”+ ions, but the graphite target was simullaneously bombarded by Ar+ for sputtering carbon atoms toward the substrate. Therefore, a decrease of hydrogen concentration in films should be expected. The extremely high microhardness and excellent tribological results show that the DLC films prepared by DIBS in this way possess good mechanical properties.

4. Conclusion (I) Amorphous DLC films synthesized by DIBS with CH”+ bombarding in energy of 0.2-25 keV are composed of mixed ~p’+sp~ bonding structure. Decreasing bombarding energy enables DLC films to contain more sp” bonds and become much harder. (2) When the wear load is less than or equal to 15 N, the tribological properties of DLC films are not influenced by the wear load and the wearing process can be carried out with the friction coefficient of 0.1 and the wear pdctor of (0.436_+0.06) x 10 ’ mm3 N-’ m-‘. Once the wear load is larger than 15 N, both the friction coefficient and the wear factor rise with the increase of the load. (3) The wearing process can be lasted steadily on a partially broken carbon film. This may be attributed 10 the self-lubrication of graphitic constituents related to spz bonding in DLC films. All experimental results prove that DLC films deposited on AISI 52 IO0 steel have excellent tribological propertics. Acknowledgements This work is a part of ‘863’ project. The authors also wish to express their thanks to the Natural Science Foundation of China for financial support. References ‘V J Kapoor, J Vuc Sci Technol, 4, 1013 (1986). ‘K Tanka, M Okada, T Kohno, M Yanokura and M Aratani, Nucl Insfrum Mel, B58.34 (1991). ) R Memming, H J Tolle and P E Wierenga, Thin Solid Films, 143, 3 1 (1986). 4 F Jansen and M A Machonkin, Thin Solid Films, 140,227 (1986). ’ R S Bhattacharya. R L C Wu and C S Yust, Nucl Instrum Mcth. B59/60, 1383 (1991). “T Hioki. Y Itoh, A Itoh. S Hibi and J Kawamoto, Surface Coating Technof, 46, 233 (1991). ‘B V Spitsyn, L L Bouilov and B V Derjayuin, Prog Cryslul Growth Charuct, 17,79 (1988). ‘Zhang Min, Wenzhi Li and Hengde Li, Nucl Ins&urn Meth, B59/60, 1358 (1991). ‘Zhang Min, Wenzhi Li, Mingling Dong, Fuzhai Cui and Hengde Li. Vucuum, 43(10), 945 (1992). ‘“H Tsai and D B Bogy, J Vuc Sci Technof, S(6), 3287 (1987). ” P Rodhammer 12th International Plasma Srminur ‘89 Proc. Rcutle, Tirol, Austria, Vb. Cl, p 661 (1989). lZGao Caiqiano, Tribologicul Metallurgy, p 82. Harbin Institute ofTechnology, Harbin (1980) (in Chinese). ‘I J F Archard, In Wear Control Hundhook (Edited by M B Peterson and W 0 Winer), p 35. ASME, New York (1980). 14M Braun, Nucf fnstnun Meth. B39,544 (1989). I5R Ochsner, A Kluge, L Freg and H Ryssel, Nucl Insrrunz Merh, B59/M), 793 (1991). ILK Kobs, H Dimigen, C J M Dcnissen. E Gerritsen, J Politiek, R Oechsner. A Kluge and H Ryssel. Nucllnsrrum Muth, BS9/60,746 (1991).