Low friction ta-C films with hydrogen reservoirs

Diamond and Related Materials 10 Ž2001. 1030᎐1035

Low friction ta-C films with hydrogen reservoirs J. Koskinena,U , H. Ronkainena , S. Varjus a , T. Muukkonena , K. Holmberg a , T. Sajavaarab a

VTT Manufacturing Technology, P.O. Box 1703, FIN-02044 VTT Espoo, Finland b Accelerator laboratory, Uni¨ ersity of Helsinki, Helsinki, Finland

Abstract Diamond-like tetrahedral amorphous carbon Žta-C. films, which contain no hydrogen and high content of sp 3 bonds have a very high hardness and elastic modulus Žup to 600 GPa.. They have also excellent tribological properties in atmospheres that contain humidity Ž50% RH. or also when submerged in water. However, in dry atmospheres the co-efficient of sliding friction against bearing steel, for instance, increases to values of over 0.7. In this paper ta-C films were modified by adding hydrogen to the films. The ta-C films were deposited by using non-filtered pulsed vacuum arc deposition in the ambient pressure of hydrogen and hydrocarbon gases. A novel layered structure containing titanium layers with hydrogen and carbon have also been grown by using filtered metal arc in combination with the pulsed carbon arc. The hydrogen content and the composition of the films were measured by using time-of-flight elastic recoil detection analysis ŽTOF-ERDA.. The hardness of the films was measured by nanoindentation. The tribological performance of the coatings were determined by using pin-on-disk tests. The counter part material was steel ŽAISI 52100., the normal load applied 5 N and the sliding velocity 0.02 mrs. In the modified films the hydrogen content was up to 16 at.% in ta-C and up to approximately 50 at.% in the titanium layers. A co-efficient of friction of approximately 0.2 at dry atmosphere was measured for hydrogen-containing ta-C and also for layered structures where a hydrogen-containing Ti layer was buried under a hydrogen free ta-C layer. The results indicate that the lowered co-efficient of friction is a result of hydrogen transport to the contact surfaces with the aid of surface diffusion through the pinholes of the ta-C layer. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: ta-C; Hydrogen content; Dry sliding; Tribology

1. Introduction Carbon films deposited by arc evaporation process possess a high fraction of sp 3 bonds and are nearly free of hydrogen. Such amorphous films are called tetrahedral amorphous carbon films Žta-C. w1x and they reveal properties close to those of diamond. The typical properties of the ta-C films are high

U

Corresponding author. Tel.: q358-9-456-5413; fax: q358-9-463118. E-mail address: [email protected] ŽJ. Koskinen..

hardness and elastic modulus. Thus, they provide excellent wear resistance and low coefficient of friction against most other materials such as metals and ceramics when the tests are performed in normal air Žrelative humidity RH approx. 50%. or even in water w2,3x. However, the co-efficient of friction has been reported to be high when the films are tested in vacuum or in dry conditions w4,5x. This is opposite to what has been reported of the tribological properties of hydrogenated amorphous carbon films Ža-C:H. w6,7x. The a-C:H films have a significantly lower friction in dry conditions compared to, e.g. normal air RH 50%. There are several industrial applications which operate

0925-9635r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 0 . 0 0 4 8 8 - X

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in dry conditions such as pneumatic process controls, plastic moulds, space applications, etc., in which the excellent tribological properties of ta-C cannot be exploited. The effect of the ambient atmosphere to the friction has been investigated quite intensively in the case of crystalline diamond w8,9x. The role of adsorbed gas species on the surface has been observed to actively terminate the dangling bonds and thus creating a passive surface. This reduces the adhesion of the diamond surface to the counter surface and results into a lower friction. On the other hand, the outmost atomic layers of a clean diamond surface are reconstructed to a graphitic bonding w9x. The presence of a graphitic phase on the surface has been assumed to be the actual cause of the low friction of diamond and diamond-like materials. It has been well documented that in dry conditions graphite is not a low friction material, but the molecular impurities Žhumidity, hydrogen, etc.. reduce the shearing force of the uppermost basal planes of graphite w10x. In this paper we present results where the ta-C films are modified in order to provide lower friction against steel in dry conditions. Coatings which are doped with hydrogen and also multilayer films with hydrogen incorporated are demonstrated in a unique manner. Since titanium forms hydrides w11x an idea of using hydrogen doped titanium as a sublayer in the films was investigated. The high local flash temperatures in the sliding contact might be a mechanism, which releases hydrogen from the structure in sufficient amounts possibly lowering the friction. The friction measurements with different film compositions are also discussed.

in order to reduce the amount of droplets to the sample. The deposition rate was approximately 1 nmrs. In order to dope the films gas was fed into the deposition chamber by a needle valve. The amount of gas was controlled by monitoring the gas pressure during the deposition. In the carbon film doping with hydrogen H 2 gas pressures were in the range of 1.0᎐27 = 10y2 Pa and methane CH 4 pressure was 1.8= 10y1 Pa. Titanium films were doped with hydrogen Ž2.7= 10y2 Pa. and with methane Ž3.7= 10y1 Pa.. The addition of H 2 did not change the deposition rate but the addition of CH 4 lowered the deposition rate to approximately 0.4 nmrs. The substrate material was steel ŽAISI 52100. with R a of approximately 0.05.

2. Experimental details

2.3. Nanohardness testing

2.1. Film deposition

The hardness of the ta-C films was measured by using a NanoTest550 device. A Berkovich tip was used and the calibration was done by using a quartz sample. Measurements were made at loads from 3 to 40 mN by using a loading rate varying from 0.01 to 0.9 mNrs, respectively. A pure ta-C film denoted C177 Židentical to sample CC6. and sample C166 were measure. The samples were indented at maximum plastic depths with selected values varying from 36 to 265 nm. Each such indentation was repeated 10 times. The hardness was analysed by using the Oliver and Pharr method w14x.

The pulsed vacuum arc-discharge source is flanced to a 150-l vacuum chamber evacuated to a base pressure of approximately 3 = 10y4 Pa by using an oil diffusion pump. The cathode is a cylindrical graphite electrode Ždiam. 30 mm. and the anode is a copper cylinder Ždiam. 150 mm.. The arc is ignited with an electrical spark between ignition electrodes at a frequency of 5 Hz. The capacitor bank Ž2600 ␮F. is discharged yielding a current pulse with a maximum current of approximately 3 kA and a half width of 250 ␮s. The distance from the sample to the cathode was approximately 300 mm. The nominal deposition rate was approximately 0.5 nmrs and an accumulation of approximately 1.6= 10 15 atrcm 2 during each pulse was obtained. Titanium deposition was obtained by using a continuos arc source with 90⬚ degree bending of the plasma

2.2. Film composition measurements The composition of the coatings was measured using time-of-flight elastic recoil detection analysis ŽTOFERDA. by using the 5 MV EGP-10-II tandem accelerator of the accelerator laboratory of the University of Helsinki. In the measurements 34 MeV 127 I 6q beam was used. The detection angle Žscattering angle. was 40⬚ and the sample was tilted 20⬚ relative to the beam direction. The beam electric current varied between 5 and 12 nA. The energy spectra of the recoils were calculated from the time-of-flight signal and the energy detector was used for mass separation. For hydrogen the detection efficiency of the time-of-flight detector was reduced but valid data were obtained by dividing every energy channel with its known efficiency. The atomic concentrations were calculated with known geometry and ZBL-stopping powers w12,13x.

2.4. Tribological testing The friction and wear performance of the coatings was assessed by pin-on-disc tests. The tests were carried out using polished balls Ž ␾10 mm. made of bearing steel AISI 52100 Ž100 Cr 6.. The tests were performed

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in dry synthetic air Ž80% N2r20% O 2 . at the temperature of 21 " 2⬚C. Prior to the start of the test the chamber covering the tribometer was rinsed with synthetic air for 8 h. The normal force was 5 N, which created the initial Herzian contact pressure of 0.8 GPa. The sliding velocity was 0.02 msy1 and the sliding distance 24 m. The coefficient of friction was recorded during the tests. The tests were carried out two times and the friction values reported are given as mean values of the results. The disc wear was not measured, since no wear could be detected due to the short sliding distance. The wear surfaces of the pins and the discs were studied by optical microscopy. The slow sliding speed for the experiments was chosen due to the reason that the ta-C films delaminated during the run-in period at higher speeds in the dry atmosphere tests.

3. Results and discussion Carbon films with different amounts of hydrogen were produced. As an example two depth distribution spectra are shown in Fig. 1. The film compositions are summarised in Table 1. The hydrogen content in carbon layers varied between values of less than 0.1᎐16 at.%. The use of CH 4 for hydrogen doping is clearly more efficient resulting into higher hydrogen contents when compared to H 2 gas. The doped titanium layer contained approximately 50% of hydrogen when H 2 was used. In case of CH 4 there was roughly an equal amount of carbon and titanium in the layers Žsamples C165 and C168. with approximately 13᎐16 at.% of hydrogen, which suggests that the layer is hydrogenated

titanium carbide. However, no chemical analysis of the Ti-C bonding was carried out. The accuracy of the composition measurements was in some cases limited by overlapping of signals of the layered structures, for example in the determination of carbon content in the mid layer in sample Nr. C165. The accuracy of the layer thickness measurements is dependent of the stopping powers used in the TOFERDA. It was estimated to be at the surface roughly 5 nm and in the deepest layer approximately 20 nm. The nanohardness measurements are shown in Fig. 2. The hardness values are quite similar in both pure and hydrogen doped ta-C films. The error bars resulting from the statistical variation of the repeated indentations are larger than the difference of the hardness values of the different films. However, all measured hardness values of the hydrogenated ta-C are systematically lower than those of pure ta-C. Based on the results it is concluded that the hydrogenated ta-C is 10᎐20% lower in hardness than the pure ta-C. The friction coefficient measurements are shown in Table 1. The maximum value during the run-in period ŽCO max. and the value at the end of the experiment ŽCOF end. are shown. The addition of hydrogen to the top layer of the carbon coatings clearly reduced the co-efficient of friction. In general, the co-efficient of friction rises to a relatively high value at the beginning of the experiment. In the case of ‘low’ friction the maximum value of co-efficient of friction is reached after approximately 1᎐5 m of sliding beyond which the co-efficient of friction starts to reduce. The value of 0.2᎐0.3 is then reached after a 5᎐10-m of sliding distance. The addition of hydrogen reduced the co-efficient of friction in dry atmosphere as expected. However, this effect has

Table 1 Sample No.

Coating typer layer thickness wnmx

H-content in hydrogenated ta-C wat.%x

H-content in Ti wat.%x

COF max

COF end

Note

CC6

ta-C 500 Žta-C. q ŽH2 :ta-C.r 330 q 240 Žta-C. q ŽH2 :ta-C.r 240 q 240 Žta-C. q ŽCH4 :ta-C.r 250 q 160 Žta-C. q ŽTi. q Žta-C. q ŽTi. q Žta-C.r 280 q 30 q 70 q 30 q 80 Žta-C. q ŽH2 :Ti. q Žta-C.r 300 q 100 q 80 Žta-C. q ŽCH4 :Ti. q Žta-C.r 300 q 60 q 80 Žta-C. q ŽCH4 :Ti. q ŽCH4 :ta-C.r 290 q 190 q 100

- 0.1

᎐

0.85

0.72

Mild wear

0.5

᎐

0.73

0.60

Mild wear

8

᎐

0.81

0.50

Mild wear

16

᎐

0.76

0.28

Mild wear

0.5

0.70

0.60

Wear into substrate

-1

52 " 3

0.66

0.25

Wear through top layers

1

16 " 2

0.61

0.22

Mild wear

13

0.66

0.25

Mild wear

C170 C173 C166 C174 C172 C165 C168

0.2

16

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Fig. 2. Nanohardness values as a function of plastic depth measured from: ŽI. pure ta-C ŽC177.; and Ž䢇. sample containing 16 at.% of hydrogen C166.

Fig. 1. Depth distributions of: Ža. sample C166; and Žb. sample C168 obtained from TOF-ERDA measurements. A constant density of 2.65 grcm3 was used through the sample.

not been demonstrated with arc deposited ta-C coatings before.

The wear track on the coating surface did appear as polishing of the surface roughness of the coatings. During the run-in period of the wear experiment the surface particles were removed and pinholes decorated the wear track as seen in Fig. 3. These effects were seen for every sample and thus it is probable that the contact mechanisms of the tribological contact are quite similar in all samples. Likewise a hardness change does not seem to be a probable explanation to the observed lowering of friction. The reduction of co-efficient of friction measured for samples which contained hydrogen in an interlayer but not on the top layer was an interesting new observation. At least the sample C165 demonstrated that significantly lower co-efficient of friction may be obtained in a case where the hydrogen containing layer is not in a direct tribological contact to the counter face. Similar

Fig. 3. Optical micrograph of: Ža. the wear track; and Žb. the counter body of sample C165.

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to the hydrogen doped ta-C films, no wear of the films in terms of an average thinning of the coatings was observed Žexcept in samples C172 and C174.. This was evident because there was now colour change before and after the wear test in the wear track. Since the ta-C film is transparent a change of film thickness of over 50 nm would easily be detected. Thus, it is concluded that no wear through the hydrogen free ta-C top layers occurred. The results suggest that by some mechanism hydrogen is transported to the contact surfaces resulting to a lower co-efficient of friction. Due to the high activation energy of hydrogen diffusion in ta-C Ž Ea s 2 eV. the transport by bulk diffusion through the ta-C film does not seem probable w15x. On the other hand, the ta-C films grown from the pulsed vacuum arc with no filtering are decorated by defects due to particle bombardment to the growing film w16x. The pin holes possibly connect the tribological contact to the hydrogen reservoir and from there hydrogen is transported to the contact surface by surface diffusion of hydrogen or hydrogen containing species. The transport of hydrogen follows the creation of pin holes during the run-in period of the sliding. After run-in hydrogen starts to effect into the formation of the tribolayers and consequently lowers the co-efficient of friction. This sequence may be seen in co-efficient of friction measurements as shown in Fig. 4. Interestingly the hydrogen containing Ti layer with no carbon ŽC172. also lowered the co-efficient of friction even though the surface ta-C was partially damaged. Possibly the load is still carried by the fragments of the top ta-C coating and the hydrogenated titanium layer provides hydrogen.

In samples C165 and C168 titanium was doped with CH 4 and the samples were worn only mildly without penetrating through the thin ta-C film on the surface. This is probably due to the higher hardness of the Ti-C bonds within the titanium film. In case of titanium layers which did not contain carbon, more severe wear of the coating was observed. In case of sample C172 the top layer was damaged partially and in the sample C174 a catastrophic wear into the substrate material was observed. The transport of titanium from the sublayer to the tribological contact might also be a contributing factor to the lowering of the co-efficient of friction. The buried pure titanium layers did not lower the co-efficient of friction in sample C174. However, it must be noted that the sample was worn and the soft titanium layers might have caused the damage of the layered structure already at an early stage of the wear test. Thus, possibly the surface analysis of the tribological contact might give an answer to the role of titanium to the lowering of the co-efficient of friction. This work is under way.

4. Summary and conclusion Hard and wear resistant ta-C coatings have been modified by doping hydrogen into the coatings. Hydrogen could be doped either directly to the ta-C film or by growing layered structures with hydrogen reservoirs in buried layers. Hydrogen reduces the co-efficient of friction in ta-C from value of approximately 0.8 down to approximately

Fig. 4. Co-efficient of friction of sample: Ža. CC6; and Žb. C165 in dry sliding.

J. Koskinen et al. r Diamond and Related Materials 10 (2001) 1030᎐1035

0.25 in dry conditions. Imbedded hydrogenated layers has an effect of lowering the co-efficient of friction even though this hydrogenated layer is not in direct contact to the tribological contact. A plausible explanation could be the transport of hydrogen to the contact surfaces via surface diffusion from the defects and pinholes of the top surface with the assistance of the local elevated flash temperatures on the tribological contact. Further work to reveal the hydrogen transport mechanisms is under way. However, the role of possible titanium transport to the contact surface is not yet clear. The innovative hydrogenated ta-C films offer a novel approach to develop highly wear resistant and low friction ta-C coatings for numerous engineering applications which can be used in non humid conditions.

w2x w3x w4x

w5x w6x w7x

w8x

Acknowledgements

w9x w10x w11x

The financial support for the work was received from Finnish industry and National Technology Agency of Finland ŽTEKES..

w12x w13x

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w14x w15x

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