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Surface & Coatings Technology

Deposition of CrN–MoS2 thin films by D.C. magnetron sputtering

S.K. Kim*, B.C. Cha

School of Materials Science and Engineering, University of Ulsan, Ulsan 680-749, Korea

Available online 13 September 2004

Abstract

As technology advances, there is a demand for development of hard, solid lubricant coatings. CrN–MoS2 films were deposited on SKD 11

tool steel by co-deposition of MoS2 with CrN using a D.C. magnetron sputtering process. The influence of the Cr interlayer thickness, the N2/Ar

inlet gas ratio, the deposition temperature, the amount ofMoS2 in the film, and the bias voltage on the mechanical and the structural properties of

the films were investigated. The critical load increased with the increase of the Cr interlayer thickness. The hardness of the film increased with

the decrease of nitrogen content in the gas and with the increase of the deposition temperature. The films show less crystallinity with the increase

of the MoS2 content in the films. The hardness of the film reached maximum level at the negative substrate bias potential of �100 V and

decreased with a further increase of the bias potential. The thickness of the film remained the same with the increase of the bias voltage.

D 2004 Elsevier B.V. All rights reserved.

Keywords: CrN–MoS2 thin film; Co-deposition; D.C. magnetron sputtering

1. Introduction

As technology advances, there is a demand for better

wear resistant coatings to extend the lifetime of steel

machine parts, cutting tools and dies. Also, there is a need

for development of a coating that enables less usage of

liquid lubricant since the liquid lubricant is expensive and

poses an environmental concern for disposal. Solid

lubricant such as MoS2 [1–4] has been exploited a lot to

replace the liquid lubricant. The development of the scheme

of hard, solid lubricated coatings is intriguing. One way to

achieve this scheme is to deposit a soft lubricated film on

the hard films [5]. Another way is to incorporate a solid

lubricant in a hard coating [6]. Incorporation of MoS2 in a

TiN matrix by D.C. magnetron co-deposition has been

studied by Gilmore et al. [6]. Carrera et al. [5] reported

CrN/MoS2 coatings. They deposited CrN film first and

MoS2 film was subsequently deposited.

In this study, CrN–MoS2 films were deposited on SKD

11 tool steel by co-deposition of solid MoS2 within a CrN

0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.surfcoat.2004.08.013

* Corresponding author. Tel.: +82 52 259 2228; fax: +82 52 259 1688.

E-mail address: skim@ulsan.ac.kr (S.K. Kim).

matrix. Influence of process parameters such as the N2/Ar

input gas ratio, the deposition temperature, thickness of

interlayer, the deposition pressure and the bias voltage on

the chemical and physical properties of CrN–MoS2 films

were investigated.

2. Experimental procedures

CrN–MoS2 films were produced using an unbalanced

D.C. magnetron sputtering equipment. Two circular

sputter sources were fixed to the lid of the chamber. A

chromium target (99.99% pure) with a diameter of 76.2

mm and a MoS2 target (99% pure) of the same size were

attached to the sputter sources. A sample holder, which

could be rotated to enable voltage bias, was located at the

center of the chamber. The substrate to target distance

was 60 mm.

After the chamber was evacuated to 1.3�10�4 Pa using a

rotary pump and a diffusion pump, argon was introduced to

maintain a working pressure. The SKD11 steel (1.5%C,

11.5% Cr, 0.8% Mo, 0.9%V, Fe bal.) specimens were

polished and degreased ultrasonically in alcohol. Before

deposition, the specimens were plasma etched for 40 min

188–189 (2004) 174–178

Fig. 1. Effect of N2/Ar gas ratio and temperature on the hardness of CrN–MoS2 Films.

S.K. Kim, B.C. Cha / Surface & Coatings Technology 188–189 (2004) 174–178 175

with 600 mA (cathode surface area: 45.6 cm2) at a pressure

of 114 Pa and 380 V. Then, the CrN–MoS2 film was

deposited on a SKD11 steel substrate.

To determine the effect of nitrogen partial pressure, the N2/

Ar gas ratio of inlet gases was varied from 0.2 to 0.5. By

varying the current ratio of MoS2 over Cr, the influence of

MoS2 content of the film on the structural and mechanical

properties of CrN–MoS2 films was investigated.

Fig. 2. X-ray diffractograms of CrN–MoS2 films deposited with various of N2/Ar g

Field emission scanning electron microscope (JEOL,

JSM-820) was used to observe the surface and cross-section

morphology of the films. The hardness of the film was

measured by a nano-indentor and a Vicker’s hardness tester.

The loads used in Vicker’s hardness measurement and nano-

indentation were 25 g and 500 mN, respectively. Adhesion

was evaluated by a scratch tester (Revetest, CSEM). When

wear resistance was measured by a ball-on-disc type wear

as ratio of inlet gases at room temperature ((a) 0.2, (b) 0.3, (c) 0.4, (d) 0.5).

Fig. 3. SEM micrographs of the surface morphology of CrN–MoS2 films obtained at various N2/Ar ratio in room temperature ((a) N2/Ar: 0.2, (b) N2/Ar: 0.3, (c)

N2/Ar: 0.4, (d) N2/Ar: 0.5).

Fig. 4. SEM micrographs of the surface morphology of CrN–MoS2 films obtained at various N2/Ar ratio in 100 8C ((a) N2/Ar: 0.2, (b) N2/Ar: 0.3, (c) N2/Ar:

0.4, (d) N2/Ar: 0.5).

S.K. Kim, B.C. Cha / Surface & Coatings Technology 188–189 (2004) 174–178176

Fig. 5. SEM micrographs of the surface morphology of CrN–MoS2 films obtained at various N2/Ar ratio in 200 8C ((a) N2/Ar: 0.2, (b) N2/Ar: 0.3, (c) N2/Ar:

0.4, (d) N2/Ar: 0.5).

S.K. Kim, B.C. Cha / Surface & Coatings Technology 188–189 (2004) 174–178 177

tester, the test condition was 100 rev/min, 3 N load, 40–50%

relative humidity.

Fig. 6. EPMA analysis of CrN–MoS2 films deposited various with MoS2currents.

3. Results and discussion

The influence of Cr interlayer thickness on the adhesion

of the CrN–MoS2 film was determined. After 40-min sputter

etching, the interlayer was formed at various deposition

times at 7.9�10�1 Pa, and 210 W. Deposition rate was

about 0.1 Am/min. The critical load increased with increas-

ing thickness. Approximately 7 N of critical load was

obtained with a 0.5-Am Cr interlayer. With increasing the

interlayer thickness, the critical load increased reaching 14

N with a 3-Am Cr interlayer. In the case of films having

good adhesion to the substrate, the load should increase with

film thickness. Hence, in the present conditions, results

indicate that a thicker Cr interlayer would improve the CrN–

MoS2 adhesion properties. CrN–MoS2 composite film was

deposited after the Cr interlayer formation. The hardness

levels of the CrN–MoS2 films deposited at various N2/Ar

gas ratios of inlet gases and various temperatures are shown

in Fig. 1. Hardness increased with the decrease of nitrogen

content in the gas although there is some deviation and with

the increase of deposition temperatures. The highest hard-

ness levels were observed at the N2/Ar gas ratio of 0.2 and

deposition temperature of 200 8C. X-ray diffractograms of

CrN–MoS2 films that were deposited using various N2/Ar

gas ratios of inlet gases at 258C are shown in Fig. 2. It can

be noticed that CrN phase developed more at high N2/Ar gas

ratio which could be responsible for the low hardness of the

film. Similar tendency was observed at X-ray diffractograms

obtained at higher deposition temperatures.

The surface morphology of CrN–MoS2 films deposited at

different temperatures and with different N2/Ar gas ratios are

shown in Figs. 3, 4, and 5, respectively. As the deposition

Fig. 7. Hardness of the CrN–MoS2 films deposited at various bias potentials.

S.K. Kim, B.C. Cha / Surface & Coatings Technology 188–189 (2004) 174–178178

temperature increased, a dome structure in the film devel-

oped. This accounts for the increase of the film hardness with

the increase of the temperature. Similar trends in the

development of dome structures in the films deposited with

increasing temperatures were previously observed win the

TiCN films deposited by MO-PACVD [7] and NbN films

deposited by D.C. magnetron sputtering process [8]. MoS2content in the composite film was varied by varying the

current applied to the MoS2 target. EPMA analysis of CrN–

MoS2 films deposited with variousMoS2 currents is shown in

Fig. 6. Hardness of the CrN–MoS2 films deposited with

various MoS2 currents was measured. The hardness of the

film decreased up to 150 mA and then increased slightly.

XRD diffractograms of these films indicated that at low

current values, the film was crystalline with a preferred

orientation of CrN(111), CrN(200) and CrN(220). The film

became more amorphous with the increase of current

resulting in the increase of the hardness. Wear test of these

films was performed. Films deposited with currents of 50 and

100mA showed good wear resistance. CrN–MoS2 filmswere

deposited with the change of the bias voltage from �50 to

�250 V. Surface morphology and the cross-section of each

films deposited with different bias voltage were studied.

Generally, the thickness of the film decreased with the

increase of the bias voltage due to the formation of fine grains

with less voids caused by resputtering [9]. This usually

happens with hard film such as NbN [8] and TaN [10]. In this

case, the thickness of the films was similar. Inclusion of the

soft amorphous phase of MoS2 in the films could be

responsible for these different phenomena. The dome

structures were coarse at the beginning. They became fine

with the increase of the bias voltage to �100 V and the fine

dome structures remained with further increase. The hardness

of these films is shown in Fig. 7. An increased substrate bias

voltage raises the kinetic energy of the Ar+ ions and

chromium particles. Bombardment of the growing film with

highly energized chromium particles and Ar+ ions causes a

dense structure which resulted in the increase of the hardness

of the films. Generally, a too high bias potential causes a

change of the structure which results in the decrease of the

hardness of the film. In the present work, the hardness of the

films decreased with further increase of the bias voltage.

However, the change of the structure was not noticed by

observing the surface morphology. XRD diffractograms of

CrN–MoS2 films deposited with different bias voltages were

studied. Peaks of each phase decrease with increased bias

voltage which means less crystallinity. This could be the

reason of low hardness of the films with the increased bias

voltage.

4. Conclusions

CrN–MoS2 films were deposited on SKD 11 tool steel by

co-deposition of MoS2 with CrN using a D.C. magnetron

sputtering method. With increasing the interlayer thickness,

the critical load increased within the range studied. Hard-

ness of the films increased with the decrease of nitrogen

content in the inlet gas and with the increase of deposition

temperature. The films became amorphous with the increase

of the MoS2 content in the films. As the substrate bias

potential was increased, hardness level of the film increased

reaching a maximum value at �100 V and then decreased.

The thickness of the film remained the same with the

increase of the bias voltage. Inclusion of MoS2 could be

responsible for this different behavior comparing those of

hard films such as NbN and TaN.

Acknowledgement

This work was supported by Grant No. R-11-2000-086-

0000-0 from the Center of Excellency Program of the Korea

Science and Engineering Foundation and Ministry of

Science and Technology.

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