Tribology Letters 2 (1996) 13-21 13

Tribological and mechanical properties of carbon nitride thin coating prepared by ion-beam-assisted deposition

Andrei Khurshudov, Koji Kato and Daisuke Sawada Laboratory of Tribology, School of Mechanical Engineering, Tohoku University, Sendal980-77, Japan

Received 24 November 1995; accepted 15 January 1996

Carbon nitride thin films may become good competitors for diamond-like carbon, due to their high hardness, high wear resistance, and low friction coefficient. At present, there are only a few studies of the effect of CNx coating hardness and internal stress on its tribological proper- ties, such as coating life and frictional behaviour. This work deals with tribological and mechanical properties of a carbon nitride coating prepared by ion-beam-assisted deposition (IBAD). Friction coefficients in the range of0.10-0.12 were observed for the best CNx coatings sliding against silicon nitride under ambient conditions. A nonlinear correlation between coat- ing life and its internal stress and hardness was found.

Keywords: carbon nitride; ion-beam-assisted deposition; coating life; internal stress; hardness

1. I n t r o d u c t i o n

A theoretical prediction was made in 1989-90 by Liu and Cohen [1,2] about a hypothetical material,/3-C3N4, which has a structure similar to fl-SiaN4 and may be even harder than diamond. This prediction stimulated studies in the field of CNx thin coatings.

One of the most attractive practical areas for CNx coatings may be in tribologi- cal applications. However, there is still only a limited number of papers in which friction and wear of carbon nitride have been studied [3-8]. At present, little is known about the effect of CNx coating hardness and internal stress on its tribologi- cal properties, such as coating life and friction.

Usually, there is a correlation between high coating hardness and high internal compressive stress. It has been found, for example, that high hardness of amor- phous hydrogenated carbon (a-C:H) is always accompanied by high compressive stress [9,10].

A nonlinear correlation between the coating stress and hardness was found by Franceschini et al. [11]. They studied nitrogen-doped amorphous hydrogenated carbon films (a-C:H) produced by r.f. self-bias glow discharge from CH4-N2 mix- tures. Chung et al. [12] found a linear hardness increase with internal stress for amorphous CN~ films prepared by unbalanced magnetron sputter-deposition.

�9 J.C. Baltzer AG, Sdence Publishers

14 A. Khurshudov et aL / Tribological and mechanical properties of carbon nitride

Effect of hardness and internal stress on the life of amorphous hydrogenated CNx films, prepared by capacitively coupled r.f. plasma-assisted CVD from CH4- N2 gas mixtures, has been studied by Dekempeneer et al. [6]. Some results of the tri- botests were inconclusive, but a life increase with hardness and stress increase can be observed. Li et al. [7] synthesized a CNx coating using d.c. unbalanced magne- tron sputtering of graphite in a nitrogen-containing plasma. They have shown that wear resistance of carbon nitride coatings under lubricated sliding against steel is proportional to the coating hardness.

So, it can be concluded that high coating hardness and, correspondingly, high compressive internal stresses are desirable for the best tribological performance of CNx coatings. On the other hand, high stress in the coating decreases its adhesion to the substrate.

The best tribological coating, therefore, must combine high hardness with the optimum stress, providing both high wear resistance and high adhesion to the substrate.

The purpose of this work is to study the inter-relationship between hardness, internal stress and life of CNx coatings prepared by ion-beam-assisted deposition.

2. Experimental

2.1. COATING PREPARATION

Ion-beam-assisted deposition (IBAD) is a process in which the coating is bom- barded (concurrently with the deposition or sequentially) with a beam of energetic ions from a separate ion source [13]. There are several important phenomena that can enhance the performance of the coating. Firstly, an improvement in coating adhesion due to substrate sputter cleaning (prior to deposition) and formation of a mixed interlayer (during deposition) made of substrate atoms, deposited atoms and assisted ions. Secondly, a considerable densification of the coating [13] due to low energy ion-bombardment, especially in the 0.1-1.0 keV range.

The IBAD system used in the present study was developed by Hitachi Ltd., Japan. It consisted of a cryogenically pumped chamber, a sputter deposition source, a low and high energy "bucket" type ion source, and a substrate holder. The diameter of the area irradiated by the ion beam was about 80 mm. The substrate holder consisted of a water-cooled copper plate, which could be rotated (4 rpm) during the deposition.

The CNx coatings were deposited using simultaneous Ar ion beam sputtering of carbon and bombardment by nitrogen ions on 2" diameter polished Si(111) wafer substrates, which were held at room temperature. The background and operating pressures in the vacuum chamber were better than 1 • 10 -6 Torr and about 1.4 x 10 -4 Torr, respectively. A 99.95% pure carbon target was used in these

A. Khurshudov et al. / Tribological and mechanical properties o f carbon nitride 15

experiments. The energy of sputtered ions was 1 keV and the ion current 100 mA. The deposition rate of carbon was monitored via a calibrated quartz crystal oscil- lator and was about 1.4 nm/min. The energy of the assisted N + ions was varied in the 0.5-10 keV range, with the incident angle to the substrate of 45 ~ The nitro- gen ion current density was varied in the 10-50 # A / c m z range. Substrates were sputter cleaned prior to deposition by bombardment with nitrogen ions (5 min, 1 keV, 100 #A/cm2).

At a preliminary stage of the experiment, 20 compositions of CNx of 100 nm thickness were deposited on Si by varying the ion beam current density (10, 20, 30, 40 and 50 #A/cm 2) and the energy of the assisted ions (0.5, 1, 3, and 10 keV). In the region of high assisted ion energy and high ion beam current density, no coat- ings were formed as a result of the N + ion irradiation effect (eight cases).

2.2. COATING CHARACTERIZATION

After deposition of different CNx coatings, the chemical composition was evalu- ated by XPS (ULVAC PHI ESCA-5500). The measurements were performed using A1Ka (1486.5 eV) X-ray radiation and a pass energy of 187 eV.

The morphology of all the deposited coatings was studied by AFM. The hardness of the five selected CNx coatings and a Si wafer was evaluated

using a NEC nanohardness tester MHA-400 using continuous normal force-inden- tation depth recording [14]. The normal force ranged from 0 to 0.5 mN (0.05 g). Penetration depth was about 50 nm.

The internal stress in the coating on the Si wafer was calculated with the well- known formula:

cr = 4 E s ~ 6 / 3 ( 1 -- v)L2tf,

where Es (113 GPa) and v (0.42) are the elastic modulus and Poisson's ratio of the substrate, ts and tf are the thicknesses of the substrate and of the film, respectively, and ~ is the substrate bending. L is the length of the scanned substrate.

Such tribological parameters as coefficient of friction and coating life (number of cycles before failure) were evaluated in this work using a pin-on-disk type tester. No lubricant was used. Silicon wafers with 100 nm thick CNx coatings were glued to the steel disk and rotated to provide the sliding velocity of 0.4 m/s in contact with a Si3N4 ceramic ball. The radius of the ball was 3.5 mm, contact load was 2.5 N, and contact Hertzian stress was about 720 MPa. The diameter of the sliding trace on the disk specimen was usually 20 mm. Friction force was continuously recorded during the test. Tests were stopped in the case of sharp friction force increase. In all cases, damage (wear) of the coating was confirmed by observation in the optical microscope. All tests were done in laboratory air with a humidity of 40-60% RH.

16 A. Khurshudov et al. / Tribological and mechanical properties of carbon nitride

3. Results and discussion

Center-line average Ra for all coatings was in the range of 0.1-0.3 nm, peak-to- valley roughness was 1.3-2.6 nm, and root-mean-square roughness was 0.17-0.5 nm for the sampling area 2 x 2 #m 2. ESCA analysis showed the absence ofa Si sub- strate signal, which implies no significant holes or cracking of the film. No oxygen was found in the bulk material of the film.

3.1. H A R D N E S S

In order to evaluate an absolute value of the CNx hardness obtained, the hard- ness of the (111) Si wafer was measured in the same range of normal load. It was assumed that the Vickers hardness of silicon is about 10 GPa [15].

Fig. 1 summarizes the results. The tests showed that such deposition parameters

a

b

25

15

lo Si . . . . . . . .

5 Ion energy: 500 eV Film thickness: 100 nm Substrate: Si (111)

0 I l l l l l a l l n I | n n n l l | l l l n l n l n l l l ' l l l l l l i ' l l l l l l l l l

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Ion current density, gA/em 2

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0 I I I i I | i ' ' I , , , I , . . I . . .

0 2 4 6 8 10 12

Ion energy, keV

Fig. 1. Effect of assisted ion beam current density (a) and ion energy (b) on the hardness of CNx

coatings. Hardness ofSi(111) is given as a reference.

A. Khurshudov et al. I Tribological andmechanicalproperties of carbon nitride 17

as ion current density (fig. l a) and assisted ion energy (fig. lb) have a significant effect on hardness. Increase of both parameters resulted in a hardness decrease. Depending on the deposition parameters, the Vickers hardness of the CNx coating changed from ~ 10 GPa to ,,~ 22.4 GPa. The maximum hardness value was obtained for 20 # A / c m 2 and 0.5 keV of ion current density and ion energy, respectively.

This hardness value is far from that for diamond, but is quite comparable with the hardness range of 7.4-50 GPa, which is usually reported for DLC coatings [16].

3.2. INTERNAL STRESS

Internal stresses were also found to be strongly affected by the deposition param- eters (fig. 2). Both negative (compression) and positive (tension) stresses were

b

0.5

t~

0

-0.5

"d -1

-1.5

-2

-2.5

0.5

o

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wl \ / - - - 0 - - - 0.5 keV .Y ~-[-/ - -Q- - I keY o ~ L 3keV

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- -Q- - 20 ~A/cm 2 �9 ~. 30 gA/cm 2 [] 40 gA/cm 2 --~ 50 ISA/cm ~

2 4 6 8 10

Ion energy, keV

60

0 12

Fig. 2. Effect of assisted ion beam current density (a) and ion energy (b) on the internal stress in CNx coatings.

18 A. Khurshudov et al. / Tribological andrnechanical properties of carbon nitride

observed. No simple dependence of internal stress on ion current density (fig. 2a) and assisted ion energy (fig. 2b) was found. Internal stresses varied from -2.1 GPa to 0.8 GPa.

Fig. 3 presents the dependence of the CNx coating hardness on the internal stress. This dependence is not linear. The hardness value is relatively stable at high com- pressive stress, decreasing from about -1 GPa, and dropping down to the value of Si wafer in the tensile stress region.

3.3. F R I C T I O N A N D C O A T I N G L I F E

The friction coefficient of CNx sliding against Si3N4 was in the region of 0.10- 0.14. No correlation of friction with the deposition parameters or coating hardness and internal stress was observed.

Fig. 4 presents the dependence of the CNx coating life on ion beam current den- sity (a) and ion energy (b). The coating life was strongly affected by deposition con- ditions and varied from 1 to 200 000 sliding cycles. The wear of the silicon nitride ball was found to be small enough for contact stress decrease to be neglected, even for 200 000 cycles of sliding. Coatings which showed life longer than 100 000 (three coatings) or shorter than 1 000 (five coatings) were tested twice. Other coatings were tested once. For both dependencies (see fig. 4) the life value increased as the argument increased, passed through its maximum, and finally decreased.

Fig. 5 presents the dependence of the CNx coating life on the internal stress. It can be seen that both tensile and high compressive stress results in a drastic decrease in life. Sharp life increase corresponds to a narrow region between -0.5 GPa and 0 GPa. This can be explained from the viewpoint of surface or interfacial crack pro- pagation. Interaction of the moving slider with the coating leads to either tensile or compressive cracking with delamination (see fig. 5). In the case of high tensile

24 i

22 nsion

~ 14

, , i , , I i , I I0 -2.5 -2 -1.5 -1 -0.5 0 0.5 1

Internal stress, GPa

Fig. 3. CNx coat ing hardness dependence on in te rna l stress.

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20 A. Khurshudov et al. / Tribological andmechanical properties o f carbon nitride

"d

,-1

25o0o0

200000

150000

100000

50000

Compression Ol

. . . . . .

-2.5 -2 -1.5 -I -0.5 0 0.5 1 Internal stress, GPa

Tension

Fig. 5. CNx coating life dependence on internal stress.

the coefficient of friction about 0.10.0.12, was deposited at 1 keV and 20 #A/ cm 2 .

ESCA analysis of the coatings that provided life longer than 100000 cycles showed them to contain about 10 at% nitrogen.

4. C o n c l u s i o n s

Testing of CNx coatings deposited by IBAD has shown that: - There is no clear linear correlation between coating hardness, internal stress,

and coating life. - Coating life is strongly affected by internal stress. In the narrow region of inter-

nal stress values ranging from -0.5 GPa to 0 GPa a sharp increase of the coating life was observed.

- Friction coefficients in the range of 0.10-0.14 were observed for CNx coatings sliding against silicon nitride. Those coatings that provided the longest life (more than 100 000 cycles) showed a coefficient of friction about 0.10-0.12.

A c k n o w l e d g e m e n t

The authors would like to thank Mr. M. Tsurumi and Dr. M. Yanagisawa of the Department of Mechatronics and Manufacturing, NEC Corporation for their assistance in the nanohardness measurements, and Mr. K. Goto from the Center of Surface Analysis, Center Laboratory, ALPS Electric Co., Ltd. for assistance in ESCA examination.

A. Khurshudo v et al. / Tribological and mechanical properties of carbon nitride 21

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