[100]-Textured diamond films for tribological applications

D| AND RE ELSEVIER

OHD TED TIR|AL$

Diamond and Related Materials 6 ( 1997] 381-385

[ 100]-Textured diamond films for tribological applications Y. Avigal

a,,, O. G l o z m a n b, I.

E t s i o n ~, G . H a l p e r i n ¢, A. H o f f m a n ~.b

a Solid State Institute, Technion - - Israel Institute of Technology, Haifa, 32000, Israel b Chemistry Department, Technion - - Israel Institute of Technology, Haifa, 32000, Israel Mechanical Engineering Department, Technion - - Israel Institute of Technology, Haifa, 32000, Israel

Abstract Application of CVD diamond films as hard coating on tools may offer, in addition to their mechanical properties, also low friction coefficient. In this sense [100]-textured films are potentially advantageous over non-textured films due to their lower roughness. In the present work we demonstrate the capability to deposit [100]-textured diamond films on steel substrates coated with a nitrided chromium thin interlayer, and assess their tribological properties. The [100]-textured diamond films were depo;ited by MW-PACVD using a methane-hydrogen gas mixture with the addition of minute amounts of nitrogen in the ppm level. Continuous, textured films of good adhesion, and improved tribological properties were obtained. Scanning electron microscopy (SEM), Raman spectroscopy and friction coefficient measurements were used for characterization of the deposited films. © 1997 Elsevier Science S.A.

Keywords: [100J-texture; Chromium nitride-coated steel substrate; Tribology; Friction coefficient

1. Introduction Diamond-coated steel is of high potential for various applications including those where the low friction coefficient is an advantage. However, due to anaorphous carbon tbrmation at the steel surface under diamond deposition conditions, the deposited diamond films suffer from poor adhesion to steel [1 ]. At an earlier stage of our study of diamond deposition on steel, we successfully solved the adhesion problem [2,3] through the use of a nitrided chromium interlayer, which serves both as a buffer layer for carbon and iron inter-diffusion and, to a certain extent, as a matching layer for the widely differing expansion coefficients of diamond and steel [2,3]. In the present study an effort was made to improve the tribological properties of the diamond coating on steel. It can be expected that both the 'planar' [100]-textured films [4,5] and the submicron size or the so called nano-crystalline films [6] would have lower friction coefficients than that of randomly oriented films. Also, we expected that rounded edges of the [100] facets would reduce their friction coefficient value over straight edges. Although textured diamond film growth on silicon is already a well-estab* Corresponding author. 0925-9635/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PH S0925-963 5 (96) 00625-5

lished method [4,5], its deposition on metals has not been reported to the best of our knowledge. In the present study we have tried to control the morphologies of the diamond films on chromium nitridecoated steel so as to attain both [ 100J-textured films and submicron crystalline films, and to assess their tribological properties in comparison with randomly oriented films.

2. Experimental procedures 2.1. Diamondfihn deposition The diamond films have been grown in a home-made tubular type MW-PACVD reactor. Randomly oriented films were deposited at 950QC and 50Torr using 0.5:99.5 at.% methane:hydrogen gas mixture. The temperature measurements were done using a one-wave.length IR optical pyrometer. We have studied the effect of nitrogen addition to the gas phase on the texture and size of the crystallites.

2.2. Substrate preparation procedure Most of the tribological measurements were done on film~ which were grown on silicon substrates. This is

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due to the better planarity ~md smoothness of the silicon wafers in comparison with the steel substrates, which enable more accurate tribological measurements. The [ 100]-oriented p-type, boron-doped silicon wati~rs were pretreated ultrasonically in 30-40-1am diamond slurry to enhance diamond nucleation. The steel substrates were pretreated as reported in detail elsewhere [2,3]. A 20-l.tm thick chromium film was electroplated on the steel and then nitrided at 900°C for 1 h under ammonia flow. Eventually, the chromium nitride-coated steel substrates were treated ultrasonically in diamond slurry in an identical way as was done for the silicon substrates to enhance diamond nucleation.

2. 3. Friction coefficient measurements The friction coefficient measurements were done using a special test rig as illustrated schematically in Fig. 1. In this test rig the sample (2) is attached to the sample holder (1) on which a normal load is imposed. Underneath, and in contact with the sample, a tungsten carbide rod (3), 50 mm in length and 3.2 mm in diameter) moves horizontally along its long axis, at a constant speed of 0.6 mm s-~. The friction force is measured by the load cell (6), amplified by an amplifier (7) and signal processed by a computer (8). For each measurement the rod is being rotated slightly about its axis and the sample is moved sideways to form a fresh contact area. A load of I N has been chosen, as higher loads caused material transfer from the tungsten carbide rod to the diamond surface.

8

7

/

3. Results and discussion

3.1. [ lO0]-Textured fihns growth on chromium nitridecoated steel We have tried to control the surface morphology of the diamond films on the chromium nitride-coated steel substrate through the addition of small quantities of nitrogen to the gas mixture. We have found tttat, with the addition of minute quantities of nitrogen, it is possible to grow [ 100]-textured films on the chromium nitride-coated steel under the same conditions as it was reported to grow on silicon [7,8]. With increasing the nitrogen concentration it was found that the [ 100] facets get rounded. With increasing the nitrogen concentration even further, submicron crystalline films are deposited. Table 1 summarizes the deposition conditions that we have found most suitable for the production of the various film morphologies stated. SEM images of the films grown simultaneously for 3.5 h on silicon and chromium nitride-coated steel substrates with the addition of 30 ppm nitrogen to the gas mixture are shown in Fig. 2. [ 100]-textured film is grown on both substrates. The slightly higher density of crystal-

Table 1 Diamond deposition conditions Deposited film morphology

CH4:H2 (%)

Randomly oriented [ 100]:ti~xlurc, straight edges [ 100J-Texture, rounded shapes Submicron crystalline

0.5

....

950

50

1.5

3(I

811(}

50

1.5

100

800

50

1.5

> 600

800

50

6

N~.:H2 Temperature (ppm) ( C )

Pressure (Torr)

5

I I

.

2

Fig. I. Scheme of a test rig for measurements of friction coefficient. 1, holder; 2, sample; 3, tungsten carbide counterpart; 4, slide table; 5, holder; 6, load cell; 7, amplifier; 8, computer; 9, motor-reducer.

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Y. Avigal et al. / Diamond and Related Materials 6 (1997) 381-385

1344

a) Steel substrate

Illl

Inll r~

Silicon substrat¢

i

1200

IA0

b,

|4100

|5~00

1600

1700

R a m a n shift, cm "! Fig. 3. Raman spectra of the films specified in Fig. 2.

effect of the partially carburized chromium nitride interlayer as discussed elsewhere [2, 3].

3.2. Friction co~:fficient ell'cot o./ surface morphoh~gv (b) Fig. 2. SEM images of [lO0]-textured fihns simultaneously grow,a for 3.5 h on (I) silicon substrate. (2) chromium nitride-coated steel substrate. The deposition conditions are summarized in Table 1.

lite growth on the coated steel substrate indicates higher nucleation density on the nitrided chromium interlayer than on silicon. Micro-Raman spectra of the [100]-textured diamond film on the coated steel and on silicon substrates were measured using an Ar laser (514.5 nm) with an output power of 5 mW and laser beam diameter of 2 lam. The Rattan spectrum shown in Fig. 3(a) for the coated steel substrate exhibits a shifted diamond line at 1344cm -I and a shoulder or line SlZ,lit around 1339 cm-~. Both positive shift and split, which does not exist for the film grown on the silicon substrate (Fig. 3(b)) indicate a large compressive stress. Still, there is no spontaneous delamination of the fiIm. This good adhesi~on is attributed to the excellent buffering

The SEM images of the four difl'crenl iiim morphologies grown according to the deposition conditions summarized in Table i are shown in Fig. 4 ( a d ) . Table 2 summarizes the average friction coefficient values and range of statistical variation measured tbr each film. In addition to the first four films ( Fig. 4(a~d )) grown for 5 h, a textured film grown for 10 h was added for comparison with the other films (Fig. 4(e)). As can be seen from Table 2, the randomly oriented film having the highest topography, mainly due to [1 ! 1] facets, has the highest friction coefficient of all five films. The clear, visual smoothness of the [100J-textured films induces a sharp decrease in the fi'iction coefficient. The films with rounded [100] facets were found to have higher friction cocfficie,ts than the one havfilg straight edges. This is contrary to our expectations, and might be due to pits existing on the periphery of the rounded crystallites, in comparison with the [100] straight-edged crystallites. For the same deposition time, the submicron crystalline surface has the lowest friction coefficient. However, heavy material transfer occurred from the tungsten carbide to the submicron crystalline diamond

E Avigal et al. / Diamond and Related lllaterials 6 (1997) 381-385

384

(a)

.......

(b)

(d)

(0 ....

Fig 4. SEM images of the films which underwent the friction coefficient measurements: (a) randomly oriented polycrystalline film; (b) [100]textured film (-,. 5/am thick); (c) [100]-textured film with rounded shaped facets: (d) submicron crystalline film; (e) [100]-textured fihn (,,. l0 pm thick).

surface. This phenomenon was not found !br the other type of film~. The cause of this material transfer should be further studied. With increasing thickness of the [100]-textured films the average facet size increases as seen in Fig. 4. Also, the average tilt angle of the [100] facet is known

to decrease with thickness. Both effects can be expected to reduce the friction coefficient of the films in comparison with the thinner [100]-textured films. This is indeed the case where the measured value for the thicker [100]textured film is even smaller than that for the submicron crystalline film.

Y. Avigal et aL / Diamoml and Related Materials 6 (1997) 381-385 Table 2 Results of friction coefficient measurements Substrate morphology

a. Randomly oriented b. [ 100]-Textured, straight edges, 5 ~tm thick c. [ 100]-Textured, straight edges, 10 ~tm thick d. [ 100]-Textured, rounded shape e. Submicron crystalline

Friction coefficient Average value

90% confidence interval of average

0.414 0.288

0.385 +0.443 0.269 + 0.307

0.246

0.226 + 0.266

0.340

0.314+0.366

0.254

0.229 +0.279

385

(3) The wide use of steel, and the good adhesion, hardness and smoothness of [ 100]-textured diamond films make it an attractive combination for tribological applications.

Acknowledgement The financial assistance of the Israel Ministry of Art and Science (Grand Number 100004) to carr) out this work is acknowledge.

References 4. Conclusions

( 1 ) [ 100]-Textured diamond films are grown on nitrided chromium-coated steel under the same conditions suitable for [100]-texture growth on silicon, but with higher nucleation density. (2) As expected, [100]-textured films have much lower friction coefficients than those of non-textured films. Submicron-crystalline films have even a slightly lower friction coefficient; however, they suffer from the problem of material transfer from the tribological counter-part.

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