NITROGEN LUBRICATION FOR SLIDING BETWEEN CNX COATINGS AND CERAMIC BALLS N. UMEHARA Tokyo Metropolitan Institute of Technology, Asahigaoka 6-6, Hino 191-0065, JAPAN; e-mail: ume@tmit.ac.jp M. TATSUNO GE Yokogawa Medical Systems,LTD., Asahigaoka 4-7-127, Hino 191-9503, JAPAN K. KATO Tohoku University, Aobayama 01, Sendai 980-8529, JAPAN SUMMARY During the sliding between CNx and ceramics, applying nitrogen as environmental gas provided very low friction as the level of 0.01 in friction coefficient. After several friction tests and chemical analysis, we found specific results on nitrogen lubricated sliding between CNx coatings and ceramic balls. Lubrication effect of nitrogen for CNx coating sliding against ceramics ball was only occurred for a Si3N4 ball. In the case of low friction coefficient in nitrogen, there are few wear particles on the contact surfaces and an increase of sp2 bonding. In order to know the effect of the running-in process on the reduction of the friction, the effect of surface roughness on friction was investigated. It was shown that smooth surface in wear scar of ball provides low friction coefficient. Friction coefficient after running-in was proportional to the Ry value of wear scar of ball. Also smooth thin transferred layer was observed on the wear scar of balls with an AFM after sliding test. Those results showed the smoothing of wear scar of ball, the generating of the transferred film from CNx and sp2 rich structure are necessary for low friction.

Keywords: CNx, Ceramics, Nitrogen, Lubrication, Sliding

1 INTRODUCTION Carbon nitride (CNx) coatings possess possibility of many attractive properties especially for high hardness, good wear durability and low friction coefficient [1]. And amorphous carbon nitride coatings has already started to use for the coating of magnetic rigid disk [2],[3].

Basically, a-CNx was expected as new coating with high hardness and wear resistance. However, recently, we found that the sliding of CNx coating against Si3N4 ball in N2 gas indicated very low friction coefficient as 0.007 under dry condition, namely N2 gas played a role of lubrication [4]. We believed that super low friction in nitrogen was specific interesting tribological properties of CNx. Erdemir et al showed the ultra low friction as 0.001 in friction coefficient for hydrogenated DLC in nitrogen under dry condition [5]. However, we observed the similar phenomena for CNx.

Fig. 1 Schematic of Ion Beam Assisted Deposition

apparatus

In the present work, in order to know the lubrication mechanism of CNx in nitrogen, the lubrication effects of nitrogen gas for CNx coatings sliding against ceramic balls and mechanism of lubrication by nitrogen gas are investigated.

2 EXPERIMENTAL APPARATUS AND PROCEDURE

2.1 Coating Amorphous carbon nitride (CNx) thin coatings were coated on Si (111) wafer by Ion Beam Assisted Deposition (IBAD) method, which consisted of sputtered coating of carbon by argon-ion beam and simultaneous irradiation of nitrogen-ion as shown in Fig.1. Substrate material was Si (111) and thickness of coating was 100 nm. Other deposition conditions are shown in Table 1.

Fig. 2 Schematic of pin-on-disk type friction tester

Table 1 Coating condition of CNx

Table 2 Friction condition

2.2 Friction test The friction properties were evaluated by ball-on-disk tests in various ambiences under different conditions. Figure 2 shows the schematics of pin-on-disk type friction tester. This tester was set in the same chamber as that used by synthesis of CNx coatings. Therefore, we can have friction test after synthesis of coatings without the exposure of the specimen in air. Table 2 summaries the friction condition. The contact surfaces after sliding tests were observed by optical microscope.

3 EXPERIMENTAL RESULTS

3.1 The effect of mating material on nitrogen lubrication of CNx

Variations of friction coefficient with sliding distance under different mating material for ball as Si3N4, SiC, Al2O3 and high chromium steel (SUJ2) in nitrogen and vacuum were investigated. For all of mating materials, friction coefficient decreased with sliding distance, and reached to the low friction coefficient. However, only Si3N4 ball provided super low friction coefficient as 0.019 in nitrogen. After these experiments, we calculated the mean friction coefficient in stable friction condition as the representative friction coefficient under each sliding condition. Figure 3 shows the effect of ball materials on friction coefficient in nitrogen. Only the material combination between CNx coating and Si3N4 ball shows nitrogen lubrication effect. For SiC as ball, nitrogen did not show the specific reduction effect of friction coefficient. For Al2O3 and SUJ2, nitrogen made friction coefficient large. Si3N4 balls showed the lubrication effect of nitrogen for CNx a lot. In the following sections, the material of balls was decided as Si3N4.

Fig. 3 The effect of ball materials on friction coefficient

in vacuum and nitrogen

Fig. 4 Comparing between CNx and a-C in the

viewpoint of nitrogen lubrication

Fig. 5 variation of friction coefficient with sliding cycles

under different initial surface roughness Ry,i

3.2 The effect of the content of nitrogen in coating on nitrogen lubrication of carbon coating

Figure 5 shows the results of comparing between CNx and a-C in the viewpoint of nitrogen lubrication. It can be seen that a-C shows the low friction coefficient in vacuum and nitrogen both. On the other hand, friction coefficient of CNx in vacuum was relatively high, but its friction coefficient decreased a lot in nitrogen. It can be seen that the including nitrogen is important to reveal the nitrogen lubrication.

Fig. 6 (a) AFM image of Si3N4 ball(Ry,i=300nm)

before sliding

Fig. 6 (b) AFM image of Si3N4 ball (Ry,i=300nm) after

sliding in nitrogen

0

0.02

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0.06

0.08

0.1

0.12

0 20 40 60 80 100 120 140

Ry,i=30nmRy,i=150nmRy,i=300nmRy,i=600nmM

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Surface roughness of ball after sliding Ry, nm

Ambience:N2(7.4x104Pa)

Disk: a-CNx on Si(t=100nm)Ball: Si

3N4 (r=4mm)

Normal load: 1NSliding speed 0.26m/s

Fig. 7 Relationship between friction coefficient and

surface roughness of ball after sliding

3.3 Variation of friction coefficient with surface roughness of balls

Friction coefficient decreased with sliding distance and reached to the small value less than 0.01. Therefore it can be considered that the contact surface should be smoothed during sliding. Some Si3N4 balls with different initial surface roughness Ry,i were prepared and done friction test against CNx coatings in nitrogen. Figure 5 shows the variation of friction coefficient with sliding cycles for different balls with different initial surface roughness. Friction coefficient for rough ball is initially high, decreased suddenly and reached small value of friction coefficient. On the other hand, friction coefficient for the smooth balls was initially low, decreased gradually and reached to very small value less that 0.01.

Fig. 8 Raman spectroscopy of CNx

Fig. 9 The intensity ratio of D band to G band of CNx

coating after sliding under different load

In order to know the relationship between friction coefficient and surface roughness, profiles of wear scar of balls were observed with an AFM. Figure 6 (a) and (b) show the AFM images of ball before and after sliding in nitrogen. Friction coefficient reached to 0.007 for the ball in Fig.6 (b).

It can be seen that surface of ball changed to be smooth after sliding and coated with transferred layer. After obtaining very low friction coefficient, ball surfaces were smoothed and coated with very thin films as shown in Fig.6 (b). Figure 7 shows the relationship between

friction coefficient and surface roughness Ry on wear scar of ball after sliding. It can be seen that friction coefficient decreased with the decreasing of surface roughness in wear scar of balls.

On the basis of results of AFM observation results, it can be considered that smoothing and coating of ball should be necessary for very low friction.

3.4 Raman spectroscopy Figure 8 show the Raman spectra of CNx coating. Raman spectra of the wear scar of CNx are decomposed using a Gaussian fit. The change of structure is evaluated by the ratio of intensities of the D to G band (ID/IG) and the band position. The G band corresponds to graphite-like layers of sp2 micro domain, while the D band to small graphite crystallites (basal plane dimension in less than 20nm) [6][7]. Figure 9 shows the intensity ration of D band to G band of CNx coating after sliding under different load. It can be seen that the ID/IG of wear scar increased after the sliding in nitrogen. This was consistent with an increase in the number or size of small graphite domain.

4 DISCUSSION Low friction mechanism of CNx sliding against Si3N4 in nitrogen was discussed. For the sliding between CNx and Si3N4 in nitrogen, friction coefficient decreased from 0.35 to 0.019. After sliding test, we observed the wear debris on CNx surface even if final friction coefficient was 0.019. We thought such wear debris was generated when friction coefficient was large. There were nano-meter scale surface roughnesses for CNx and Si3N4. It can be considered that smoothing of friction surface during sliding was occurred as namely running-in process. In running-in process, large asperities were removed or deformed to smooth surface. Therefore, friction coefficient decreased with sliding distance.

From the results of Raman spectra, it was found that low friction coefficients are induced by the sp2 rich structure on the surface of wear scar which have a low shear strength. The reason why material structure was change to the material with graphite structure smoothly is that nitrogen avoided the agglomeration of wear debris to the both surfaces. It can be considered that applying adequate tangential stress to CNx change the structure from amorphous to the structure including sp2 structure.

5 CONCLUSIONS Lubrication effects of nitrogen for CNx coating sliding against ceramics pin was only occurred for a Si3N4 pin. In the case of low friction coefficient in nitrogen, there are very smooth surface of mating balls and an increase of sp2 bonding of CNx. One of the lubrication effects of nitrogen is the avoidance of the accumulation of wear particles on the sliding surfaces. A sp2 rich structure which has low shear strength bonds was considered to cause low friction.

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