Tribological improvements of polished chemically vapor deposited diamond filmsby fluorinationS. Miyake Citation: Applied Physics Letters 65, 1109 (1994); doi: 10.1063/1.112113 View online: http://dx.doi.org/10.1063/1.112113 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/65/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Tribological properties of nanocrystalline diamond films deposited by hot filament chemical vapordeposition AIP Advances 2, 032164 (2012); 10.1063/1.4751272 Effects of thermal annealing on the structural, mechanical, and tribological properties of hard fluorinatedcarbon films deposited by plasma enhanced chemical vapor deposition J. Vac. Sci. Technol. A 22, 2321 (2004); 10.1116/1.1795833 Low energy carbonaceous and graphite phases on the surfaces of thermochemically polished chemicalvapor deposited diamond films J. Appl. Phys. 87, 4553 (2000); 10.1063/1.373101 Tribological properties of polished diamond films J. Appl. Phys. 74, 4174 (1993); 10.1063/1.354421 Tribological properties of diamond films grown by plasmaenhanced chemical vapor deposition Appl. Phys. Lett. 54, 2006 (1989); 10.1063/1.101197

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Triboilogical improvements of polished chemically vapor deposited diamond films by fluorination

S. Miyake Nippon Institute of Technology, 4-IGakeundai, Miyashiro-machi, Saitama 345, Japan

(Received 16 February 1994; accepted for publication 5 July 1994)

In an effort to realize wear resistant and lubricating surfaces, polished chemically vapor deposited (CVD) diamond films are fluorinated. The effect of these films on improving frictional properties is then investigated. On fluorinated diamond surfaces, G-F bonds are formed and carbon and oxygen contents decrease. Fluorination can reduce the surface energy, as evaluated by contact angle to the water. The relationship between friction coefficient and initial Hertzian stress of polished diamond film can be expressed as one curve designated as the “master curve” independent of tip radius and material. Fluorination decreases friction coefficient and change of friction coefficient of polished CVD diamond with increasing load and opposing tip radius.

Microtribology is a key technology in development of micromachines and magnetic recording head-media interfaces.’ In these fields, atomic-scale wear and minute friction fluctuations degrade the equipment performance. Surface modification seems to be a useful countermeasure to improve microtribological properties.

In microtribology, the surface material required is not the usual sacrificial solid lubricant but a wear-resistant lubricat- ing film. A practical surface material composition model to reduce atomic-scale wear “to realize microtribological zero wear” was suggested.’

Superior atomic-scale lubricities can be obtained by re- ducing the atomic interaction between the sliding surfaces. This requires a top surface material with a low surface en- ergy such as GI? Wear-resistant properties were expected to require superhard materials such as diamond. Fluorinated diamond film was suggested as the ideal surface material.“’

Diamond is the hardest material and has been investi- gated with respect to its tribological properties. Its excellent properties, such as low friction and high wear resistance were reported.3’4 Fluorination effects of diamond were stud- ied by various methods,5 but these tribological properties were not reported. In contrast, fluorination effects of dia- mondlike carbon (DLC) film on microtribological properties were studied, which revealed the effect of reduced mi- crowear and friction.“‘”

In this study, to improve the lubricity, polished chemi- cally vapor deposited (CVD) diamond films were fluorinated using CF4 plasma discharge. The effects of fluorination on frictional properties were examined using a reciprocating tri- bometer.

Diamond films were deposited on a rectangular silicon nitride surface by the thermal filament CVD method using CH4 and Ha 1:lOO mixed gas, and then polished to better than 5 nm surface roughness, as confirmed by an atomic force microscope.

To obtain a low surface energy, the diamond surfaces were fluorinated by exposure to CF, plasma. Diamond speci- mens were placed on the electrode connected to an rf supply (13.56 MHz). CF4 was used as a gas, and the flow rate was controlled by a needle valve near 6 X 10m2 Torr during fluo- rination. The mean gas flow rate was 10 ml/min. Fluorination

was performed at 1.3 W/cm’ rf target power for 10 min. Sliding tests were performed with a reciprocating sliding

tester in air at humidity of 35%-50% and temperature of 15-20 “C. A plate specimen coated with diamond film was moved using an oil-pressure-cylinder-type actuator. The op- posing diamond tip was slid against the plate specimen. Op- posing specimens were diamond tips (point radius R of 0.02, 0.5, 1.0) and a stainless-steel ball (SUS44OC, point radius R of 3.2). Friction forces were measured by strain gauges at- tached to the plane spring.

To reveal the composition and structure of the fluori- nated diamond film surface, micro-Raman, micro-Fourier transform infrared (FI’IR) and x-ray spectroscopy (XI’S) analyses were performed.

From the micro-Raman spectroscopy, the diamond peak at 1380 cm-’ was clearly obtained from fluorinated diamond film. This peak was similar to that of untluorinated diamond Elms. Fluorination did not change the crystallinity of CVD diamond film, as determined from the evaluation of Raman spectroscopy.

Micro-FTIR analysis was performed, and the wave num- ber resolution was 4 cm-r. FTIR spectra of diamond film and fluorinated diamond film are shown in Fig. 1. Superfi- cially C-F combination is formed by fluorination. The strong peak near 1100 cm ml is attributed to C-F stretching in CF, groups.7

3 .- CI f -e s 3 .- 2 2 -

1500 1400 1300 1200 1100 1000 900 Wave number (cm-')

FIG. 1. Micro-FTIR spectra of diamond and tiuorinated diamond films.

Appl. Phys. Lett. 65 (9), 29August 1994 QQQ3-6951/94/65(9)/11 Q9/3/$6.Q0 0 1994 American Institute of Physics 1109 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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c1 loo z -- cl .-

z zs 0 10-l

c% .- % .- I2

1 o-’

Load (N)

FIG. 2 Friction coefficient dependence. of diamond fi lms on load. FIG. 3. Friction coefficient dependence of fluorinated diamond 6hns on load.

XPS measurement was performed in the incident angles between the x-ray beam and the analyzed surface were 20” and 90”. Fluorinated diamond film surfaces are composed of carbon, fluorine, and oxygen. Carbon and fluorine show mul- tiple chemical states. The concentrations of carbon and oxy- gen decrease with fluorination. The fluorine concentrations are nearly constant at 24%-27% in these surfaces at a 20” incident angle and 5% at a 90” incident angle. The penetra- tion depth of x rays is nearly 2 nm at a 90” incident angle, and that at an incident angle of 20” is nearly 0.5 nm.* The Cls XI’S spectrum showed that the relative concentrations of fluorinated carbon species such as CF,, CF,, and CF were larger for the 20” incident angle, in comparison with the 90” incident angle. This is confirmed by the fact that the fluorine content at the surface is higher than that of film.

Fluorination is expected on the basis of the decrease in surface energy. To confirm the effect of fluorination on sur- face energy, the instantaneous contact angles of water on the fluorinated diamond film were measured. The contact angle of polished diamond film was 60”, and that of the fluorinated tilm, produced in the CF, plasma, was 85”. The surface en- ergy of diamond film decreases with fluorination.

cient is low, nearly 0.07-0.11, and the change of friction coefficient with load and tip radius is small. The specific shear strength of the interface between the diamond film and tip decreases upon fluorination. If the specific shear strength s of the interface between the tluorinated diamond film and tip were constant, with respect to initial Hertzian stress, the friction coefficient of fluorinated diamond film would also have a similar dependence on load. However the friction coefficient of fluorinated diamond film is nearly constant un- der these load conditions. In addition, the macroscopic elas- ticity dependence on load does not change upon fluorination of a thin layer composed of several atoms. It is concluded that specific shear strength s of the interface between the fluorinated diamond tilm and tip decreases with decreasing load.

The friction of diamond in air is generally low. The fric- tion coefficient is not constant but increases as the load decreases.3 The friction properties of CVD diamond films are shown in Fig. 2 as measured by a reciprocating-type friction tester. The friction coefficient with 3.2R SUS ball is 0.12 at a load of 0.5 N and decreases to 0.08 at a load of 5 N. As an initial approximation, ,u=KW-~” may be written for lower load range. This is interpreted simply as arising from an adhesion mechanism; the area of contact is given by A = KI W2’3. Assuming, as an upper limit, that the true area is the same as the geometric, F-sA = sK,W~‘~, where s is the specific shear strength of the interface such that the coeffi- cients of friction are proportional to W-f’3.3 With a higher load, the power of W increases from -4.

Dependencies on initial Hertzian stress of diamond and fluorinated diamond films are shown in Fig. 4. The relation- ships between the friction coefficient and initial Hertzian stress of unfluorinated diamond film and fluorinated diamond film can be expressed as single curves, so-called “master curves” independent of the opposing tip radius and tip ma- terial.

Comparing fluorinated diamond Urn and untluorinated diamond film, fluorination markedly reduces the friction co-

-e 5 .- 0 .-

% s

s .- w 0 .- I=

2

.The friction coefficient shows the tendency to decrease with decreasing radius and increasing load. However, in the case of the smallest radius R=0.02 diamond tip, the friction coefficient shows the tendency to increase with load.

The friction coefficient dependence of a tluorinated CVD diamond film is shown in Fig. 3. The friction coeffi-

10-l 1 8 6

I 2 345681 2 345681 2 345681 2 345681 2

IO2 lo3 lo4 105 lo6 Hertzian stress iMPa)

FIG. 4. Frktion coefficient dependence on initial Hertzian contact stress.

1110 Appl. Phys. L&t., Vol. 65, No. 9, 29 August 1994 S. Miyake

10-l 0

Load'(N) 10’

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efficient and the changes of friction coefficient of diamond film with initial Hertzian stress.

The friction coefficient of CVD diamond shows a mini- mum value near p=2X104 MPa. Furthermore, the friction coefficient shows the tendency to increase with increasing initial Hertzian stress. Surface damage is observed with ini- tial Hertzian stress higher than that under this minimum fric- tion coefficient condition of diamond film. With higher stress, the contact area increase rapidly, and Hertzian stress changes from the initial value. Therefore, the friction coeffi- cient of diamond film increases and fluctuates with increase of initial Hertzian stress. In contrast, the surface damage of fluorinated diamond film is very slight under the same slid- ing conditions, therefore, there was little change due to the initial Hertzian stress.

In conclusion, on tluorinated polished diamond surfaces C-F bonds were formed and carbon and oxygen contents decreased. Fluorination can reduce the surface energy, as evaluated by contact angle to the water. The relationships

between friction coefficient and initial Hertzian stress of dia- mond film and fluorinated diamond film can be expressed as single curves so-called “master curves.” Fluorination de- creases friction and change of friction coefficient of CVD diamond with increasing load and opposing tip radius.

This work was partially supported by a Grant-in-Aid (No. 04650072) for Scientific Research from the Ministry of Education, Science and Culture.

‘S. Miyake and R. Kaneko, Thin Solid Films 212, 2.56 (1992). ‘S. Miyake, R. Kaneko, Y. Kikuya, and I. Sugimoto, Trans. ASME J Tribol.

113, 384 (1991). “F P Bowden and D. Tabor, The Friction and Lubrication of Solids Part 2 . .

(Clarendon, Oxford, 1954). 4M. N. Gardos and B. L. Soriano, J. Mater. Res. 5, 2599 (1990). ‘D. E. Patterson, R. H. Hauge, and J. Margrave, Mater. Res. Sot. Symp.

Proc. 140, 351 (1989). 6S. Miyake, T. Miyamoto, and R. Kaneko, Wear 168, 155 (1993). 71. Sugimoto and S. Miyake, J. Appl. Phys 67, 4083 (1990). *S. Tanuma, C. T. Powell, and D. R. Renn, Surf. Inter. Anal. 17, 927

(1991).

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