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REFRACTORY METALS &tlARDMMERlAlS

ELSEVIER International Journal of Refractory Metals & Hard Materials 16 (1998) 95-98

WC-x%Co films prepared by arc ion plating

Li Yua, Zhujing Jin,” Yanqing Huang,b Tianjin Zhu,b Ichigo Yoshigawa’

“Institute of Corrosion and Protection of Metals, Academia Sinica, State Key Laboratory for Corrosion and Protection, Shenyang 110015, China bTokai University, Hiratsuka, Kanagawa 259-12, Japan

Received 23 September 1996; accepted 7 January 1998

Abstract

WC-x%Co films (x = 0, 4, 15) were obtained by arc ion plating (AIP). The deposition rate of the process and the surface microhardness of the films were measured. The microstructure of the films was analyzed by examining cross sections by scanning electron microscopy (SEM). It is demonstrated that the deposition of WCsr%Co films can be obtained by means of the proposed process. 0 1998 Elsevier Science Ltd. All rights reserved.

1. Introduction

The AIP process works in a vacuum chamber, where target materials are evaporated according to their evaporation and electronic characteristics. The evapor- ated particles, which mostly become ions under the high bias applied, are then quickly deposited on the substrate through the electronic cloud [1,2]. One of the characteristics of this method is its ability to make refractory metal or alloy films, which are hard- deposited on by other IP technologies using either the metal or an alloy as a target. Now, not only nitride and carbide films of Ti, but also nitride films of Hf, Zr, and Nb [3-51 have been deposited by this technology.

WC-X%CO is a hard alloy with good toughness, high hardness and wear-resistance even at high tempera- tures, so that it is a form of advanced intermetallic compound [6]. With regard to the surface modification of materials with hard films to extend their lifetime, it is worth investigating the fabrication process and the characteristics of WC-x%Co films. So far, little work on this subject has been reported; this paper presents some new results of such a study.

2. Experimental

The arc ion plating instrument is shown schematic- ally in Fig. 1. There is one target producing random arc and a stable sample fixture in the vacuum chamber. The maximum current allowed in the arc is 200 A.

0263-4368/98/$ - see front matter 0 1998 Elsevier Science Ltd. All rights reserved.

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WC-Co alloys, SUS304 stainless steel and glass with sizes of 10 x 10 x 5, 20 x 20 x 1.2 and 20 x 15 x 1.2 mm3, respectively, are used as a substrate. Before deposition, the samples were first cleaned in acetone and then fixed on the sample fixture. The round targets had a radius of 100 mm and were made from WC, WC-4%Co or WC-lS%Co alloys.

The parameters of the deposition processes are: arc current 180-200 A, substrate bias -5O-- 100 V, Ar gas pressure 0.133-1.33 Pa, substrate temperature 350 + 2o”C, and deposition time 20-60 min.

Reactive gas

Fig. 1. Arc ion plating instrument.

96 Li Yu et aLlInt. .I. of Refractory Metals & Hurd Materials 16 (1998) 95-98

The roughness of the film is measured by means of a SUFTEST.4 profilometer made in Japan. The adhesive properties are evaluated by scratch test in which a diamond stylus 0.02 mm in radius is drawn over the sample surface under a stepwise increasing load (normal force) until the film detaches. The critical load was determined by optical observation. The adhesion of the WC film has not been evaluated by scratch test because the size of the sample was not big enough as 60% of the film peeled off.

3. Results

The surface characteristics of the WC-x%Co alloy films prepared by AIP are different on the three substrates. The films deposited on a WC-Co substrate have a smooth surface and a stronger adhesion with only 10% of the films peeling off. The surface of the film deposited on SUS304 is smoother, but 30% of the films peeled off. The films on a glass substrate have the smoothest surface, but a poor adhesion and peel off nearly entirely.

This paper considers mainly the WC-x%Co films deposited on substrates of WC-Co alloys. Table 1 gives a comparison of the surface characteristics of the WC-x%Co films for x = 0, 4 and 15, respectively.

Figure 2 gives the change of the surface microhard- ness of the WC-x%Co film with increasing Co content.

Table 1 Characteristics of WC-x%Co films

Film Colour Roughness

(Ra, pm)

Adhesion (N)

WC WC-4%Co

WC-lS%Co

Heavy grey 0.19

Grey 0.19 32.0 Bright grey 0.17 40.5

4000

Q 0 TARGET E E h

P

P FILM

3000 - z

z \

s

2 2000- B 8 \ P 1

z 1000

0 5 10 15 20

Co CONTENT (%wt)

Fig. 2. Relationship between microhardness of AIP-WC-x%Co films

and Co content.

0.15 I

0.05 0 5 10 15

Co CONTENT (%wt)

Fig. 3. Relationship of deposition rate and Co content

Vickers’s hardness instrument has been used with a procedure of auto-loading, auto-maintaining 30 s and auto-unloading. The loads used are 100 g for the target and 25 g for the deposited films separately. Figure 2 shows that the surface microhardness of the films decreases with increasing Co content, while the differ- ence in microhardness between the target and the film is negligible.

Figure 3 shows the relation between the deposition rate and the cobalt content of the targets. The results indicate that under certain depositing conditions, the deposition rate increases with increasing Co content. In fact, the deposition rate is lowest (0.085 pmimin) when x = O%, and highest (0.13 pm/min) when x = 15%.

The morphology of cross sections and of the surface of the WC-x%Co films was observed by means of scanning electron microscopy (SEM), and the results are shown in Fig. 4. The structure of the WC-x%Co films changes from coarse columnar grains (Fig. 4a) to tine columnar grains (Fig. 4b) and then further to a compact structure (Fig. 4c) as the cobalt content increases. In addition, the WC-x%Co (x = 0, 4, 15) films have a rough surface, presenting dense droplets like nodules, but among them the surface of WC-lS%Co film is relatively planar (Fig. 4~).

4. Discussion

4.1. Effect of the Co content in the film

The previous results reveal that the cobalt content plays an important rote on the properties of the WC-x%Co films. The Co-WC phase diagram shown in Fig. 5 [7] indicates that the melting point of the alloy is reduced with increasing cobalt content. The melting point of WC-O%Co is about 28OO”C, while that of WC-lS%Co is about 2050°C. According to Mizikuchi [8], the arc current is directly proportional to the melting point of the target material. Thus, under the

Li Yu et aLlInt. J. of Refractory Metals & Hard Materials I6 (1998) 95-98 97

Fig. 4. SEM results of the WC-x%Co films prepared by arc ion plating. (a) WC, (b) WC-4%Co, (c) WC-15%Co.

same experimental conditions, the lower the melting point of the target material, the more stable the arc and plasma, and then the higher the ionization rate and the deposition rate. The present results show that the arc discharge and the plasma improve when the Co content of the target is increased. Following the obser- vation of the deposition processes, the schematic .arc discharge patterns of the three kinds of target materials are drawn by computer in Fig. 6. The results show that with by increasing the cobalt content, the plasma becomes stable. When the cobalt content is reduced to 0 (Fig. 6, x = 0), the quality of plasma

Fig. 5. Co-WC phase diagram.

becomes rather bad, sometimes producing out-arc. Thus, it may be concluded that the arc and the plasma become stable and the deposition rate higher when the cobalt content increases, owing to a reduction of the melting point of the WC-x%Co alloy.

According to Thornton’s figure [9], under certain gas pressures the microstructure of the deposited film depends on the ratio TIT,,,, where T and T, are the substrate temperature and the melting point of target materials. Calculations show that the microstructure of WC-x%Co films should locate in ‘Zone T’, which is a zone of transition from coarse columnar grains to fine columnar grains microstructure. SEM results prove that the microstructure of the WC-x%Co films is consistent with that expected for the ‘Zone T’ in Thornton’s figure (a somewhat different conclusion has been presented in Yu et al. [lo]).

4.2. Adhesion

A factor affecting the film/substrate adhesion is the thermal stress due to the difference between the expansion coefficients of the film and the substrate [ll]. Under the same deposition temperature, the larger the difference between the expansion coeffi- cients of the film and the substrate, the larger the

Fig. 6. Simulated graphs of arc discharge of WC-lS%Co target in different Co content.

Li Yu et aLlInt. J. of Refractory Metals & Hard Materials 16 (1998) 95-98

-

P-O.1 33Pa P=O.S3Pa P=2.67Pa

Fig. 7. Simulated graphs of arc discharge of WC-lS%Co target in different Ar content.

thermal stress at the film/substrate interface, and the worse their adhesion. When WC-Co alloys are used as the substrate material, the adhesive properties of the WC-.x%Co films are certainly better than those of stainless steel and glass substrates, since in the former case the expansion coefficients of the film and the substrate are very close to each other.

The results of the evaluation of the adhesive proper- ties reported in Table 1 show that the value of the critical load raises with increasing cobalt content. The reason for this may be that increasing the cobalt content of the target materials improves the arc discharge and the plasma, which in turn results in a high ionization rate.

4.3. Influence of Ar gas pressure

In the deposition process, the arc discharge of the target becomes worse (as shown in Fig. 7b, c) when Ar gas pressure is increasing. Only when the Ar gas pressure reduces to 0.133 Pa does the arc discharge and plasma become stable.

According to the plasma theory [12], electron colli- sion in the arc is the main source for electric ioniza- tion. In this process, the kinetic energy producing electric ionization E, is directly proportional to the electric field intensity E and to the mean free path I,:

e.E.l,mEi (1)

When e.E.1, < <Eiy electric ionization cannot occur because the electric ionization energy is too low; however, when e-Ed, > > E,, electric ionization can occur. According to eqn (1) a sufficiently high electric ionization energy could be reached under constant E only by lowering the residual pressure of the chamber, e.g. by increasing 1,. This is the reason why the arc

discharge and the plasma become stable when the Ar gas pressure reduces to 0.133 Pa.

5. Conclusions

When the deposition rate of WC-x%Co films increases, their microstructure changes from coarse columnar grains to fine columnar grains, which corre- sponds to the ‘Zone T’ of Thornton’s figure.

The adhesion of WC-x%Co films on WC-Co alloy substrates is higher than on stainless steel and glass substrates.

Under the present test conditions, when the Ar gas pressure is lowered to 0.133 Pa, the arc discharge and the plasma become stable.

References

[II

121 [31

[41

[51

[61

[71

PI

[91

[l~l

[“I

PI

Rickeby DS. Surface and Coatings Technology

1988;36:541-557.

Kawashita Anji. Metal Surface Technology 1984;35(1):10-15.

Randhawa H, Johnson PC. Surface and Coatings Technology

1987;3:303-318.

Boelens S, Veltrop H. Surface and Coatings Technology

1987;33:63-71.

Muenz WJ. Vat Sci Technol 1986;A4:2717.

Xiong Weihao, Hu Zhenhua, Cui Kun. Materials Review

1992;69( 15):24-29.

Japan Institute of Metals. Metal Material Textbook. Malu Jen

Corp., 1960, p. 272 (in Japanese). Mizikuchi, Seiken. Proceedings of the M. SC. in Department of

Engineering Tokai University, 1990 (in Japanese).

Thornton JAJ. Vat Sci Technol 1974;11:666. Yu Li, Jin Zhujing, Huang Yangqing, Zhu Tianjin, Yaoshigawa

Ichigo. Acta Metallurgica Sinica 1994;30(5):B229-B232. Ecketova L. Physics of the Films. Plenum Press and SNTL,

1977. Feinman J. Application of Plasma Technology in Metallurgical Industry. Peking Institute of Technology, Peking, 1989. Trans-

lated by Liu Shulin and Jin Youmin.