Ž .Surface and Coatings Technology 131 2000 121]126

TiCN coatings on aluminum alloy formed by MO-PACVD

S.K. Kima,U, T.H. Kima, J. Wohleb, K.-T. Riec¨aSchool of Materials and Metallurgical Engineering, Uni ersity of Ulsan, Ulsan 680-749, South Korea

bFraunhofer Institut fur Schicht-und Oberflachentechnik, Bienroder Weg 54E, D-38108 Braunschweig, Germany¨ ¨cInstitut fur Oberflachentechnik und Plasmatechnische Werkstoffentwicklung, Technische Uni ersitat Braunschweig, Bienroder Weg 53,¨ ¨ ¨

D-38108 Braunschweig, Germany

Abstract

ŽTiCN layers were deposited on an aluminum alloy Si 0.5%, Fe 0.5%, Cu 4.3%, Mn 0.6%, Mg 1.5%, Cr 0.1%, Zn 0.25%, bal..Al by using diethylaminotitanium, hydrogen and nitrogen with the pulsed d.c. PACVD process. The effect of process parameters

such as precursor evaporation temperature, duty ratio, frequency, voltage, H rN gas ratio on the properties of the TiCN layer2 2was investigated. The layer thus obtained had high hardness and a low friction coefficient. Detailed results on the hardness,surface morphology, WDS analysis, wear test and scratch test of these layers are presented. Q 2000 Elsevier Science B.V. Allrights reserved.

Keywords: TiCN coating; Aluminum alloy; Metal-organic compound; Wear resistance

1. Introduction

The use of aluminum alloys in industry such asautomotive and aerospace is increasing due to their lowdensity. However, the low surface hardness restrictstheir application. A wear resistant coating on aluminumalloys would overcome these restrictions. The coatingshould be performed at low temperatures due to thelow hardening temperature of these alloys.

Wear-resistant TiCN coatings are currently producedby plasma assisted chemical vapor deposition methodat a temperature of approximately 4508C. The titaniumprecursor is usually TiCl . However, the coatings pro-4duced at this low temperature contain chlorine which

U Corresponding author. Tel.: q82-52-2592228; fax: q82-52-2591688.

Ž .E-mail address: skim@uou.ulsan.ac.kr S.K. Kim .

causes deterioration of its mechanical properties andw xincreased stresses induced in it 1,2 . The use of metal-

organic compounds as precursors is a promising ap-proach to replace TiCl and to lower the temperature4

w xof the PACVD process 3]5 . Several works have beendone already on steel substrates and some light metalsw x6]8 .

In this paper, the effect of precursor evaporationtemperature, duty ratio, pulse frequency, plasma power,H rN gas ratio on the formation of TiCN layer on an2 2aluminum alloy substrate using diethylaminotitanium isreported. Wear and adhesion test results and the mea-sured activation energy value were also presented.

2. Experimental

The process was carried out in a standard PACVDapparatus equipped with a pulsed d.c. power supply.

0257-8972r00r$ - see front matter Q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S 0 2 5 7 - 8 9 7 2 0 0 0 0 8 3 1 - 8

( )S.K. Kim et al. r Surface and Coatings Technology 131 2000 121]126122

Table 1Process parameters

Ž .Evaporation temperature 8C 65]90Ž .Substrate temperature 8C 150]250

Ž .Gas pressure mbar 1.8]2.5Ž .Duty % 19]95

Ž .Frequency kHz 7.3]20Ž .Voltage V 450]650

Ž .Coating duration h 1]4

The gaseous atmosphere was electrically excited be-cause the process was enhanced by a glow discharge.The metal-organic compound was evaporated in anevaporator and fed into the chamber by the carrier gas

Fig. 1. SEM micrograph of fractured TiCN coating on aluminum.

Fig. 2. Effect of duty on hardness of TiCN layer.

Ž . Ž90 sccm of H . The process gases 10 sccm of N 202 2,.sccm of Ar flowed continuously through the chamber

and were controlled by mass flow controllers. Theevaporator temperature was varied in the range of65]908C at a 28 interval. The power supply generated apulsed d.c. voltage of up to 650 V with a frequency ofup to 20 kHz. The process parameters are given inTable 1.

The aluminum alloy used as substrate material wasŽAl 2024 Si 0.5%, Fe 0.5%, Cu 4.3%, Mn 0.6%, Mg

w Ž . Ž . ŽFig. 3. Surface topography of TiCN layers obtained at various duties a: 19% 13.3 msr56.7 ms ; b: 55% 38.5 msr31.35 ms ; c: 80% 56 msr14. Ž .xms ; d: 93% 65.1 msr24.9 ms .

( )S.K. Kim et al. r Surface and Coatings Technology 131 2000 121]126 123

.1.5%, Cr 0.1%, Zn 0.25%, bal. Al . The coated layercomposition was determined by wavelength-dispersive

Ž .spectroscope analysis WDS . The microstructure wasinvestigated by X-ray diffraction and scanning electronmicroscope. The hardness of the coatings was mea-sured using a Vickers microhardness tester with a loadof 25 g. The adhesion was measured by the scratch testand the friction coefficient by a pin-on-disc type weartester.

3. Results and discussions

Below an evaporator temperature of 728C, the sam-ples were only partially coated. Above 828C, the coat-ings spalled. Good layers were obtained at temperaturerange of 74]788C. Fig. 1 shows a typical SEM micro-graph of fractured TiCN coating on an aluminum alloyobtained in this research. The evaporated amount ofprecursor for 2 h deposition time was approximately 2g.

The effect of the duty on the hardness of the coat-ings is shown in Fig. 2. As can be seen in Fig. 2, ahardness of 1100 Hv was obtained at 55% duty. Thesurface topography of the coatings obtained at variousduties is shown in Fig. 3. As can be seen, a dense finedome structure was obtained at 55% duty which re-sulted in the highest hardness. The dome structure was

Fig. 4. Effect of frequency on the hardness of the TiCN layer.

not developed at 19% duty and coarse dome structureswere developed at 80 and 93% duties. High duty al-lowed increased plasma power which resulted incoarsely developed dome structures.

Fig. 4 shows the effect of frequency on the hardnessof the coatings. A hardness of 1100 Hv was obtainedwith 14.2 kHz frequency. The layers had a lower hard-ness when using frequencies of 7.3, 10.8, and 20 kHz.The surface topography of the coatings obtained atvarious frequencies is shown in Fig. 5. A dense fine

Ž .Fig. 5. Surface topography of TiCN layers obtained at various frequencies a: 7.3 kHz; b: 10.8 kHz; c: 14.2 kHz; d: 20 kHz .

( )S.K. Kim et al. r Surface and Coatings Technology 131 2000 121]126124

dome structure was obtained only with 14.2 kHz. Thedome structure was not developed at 7.3 kHz and acoarse dome structure was developed at 10.8 and 20kHz.

The effect of the voltage on the hardness of thecoatings is shown in Fig. 6. With voltage increasing, thehardness of the coating decreased. Fig. 7 shows surfacetopography of coatings obtained at various voltage.With voltage increasing, a coarse dome structure wasdeveloped which caused the low hardness of the coat-ing.

Each TiCN layer obtained after varying duty, fre-quency, voltage was analyzed by WDS to see whether itwas possible to correlate observed hardness withchemical composition such as carbon and nitrogen con-tent. The composition was nearly constant for all lay-ers. So, a correlation was not possible. A typical analy-sis of the TiCN layers was: Ti 55%, C 20%, N 20%, O5%. Oxygen was incorporated as an impurity

The effect of the hydrogen to nitrogen gas ratio inthe chamber on the hardness of the coatings is shownin Fig. 8. An increasing nitrogen content reduces thehardness of coatings. Increased nitrogen content leadsto a bad dissociation of the precursor. More undissoci-ated precursor molecules were incorporated in thelayer which resulted in the low hardness.

The activation energy of the TiCN deposition reac-Žtion was calculated from the measured rate data Fig.

.9 . An activation energy of 40.67 kJrmol was obtainedwhich was a rather low value. At low temperatureranges, differences in coating rates at different temper-

Fig. 6. Effect of voltage on the hardness of the TiCN layers.

atures were relatively low which was responsible forthis low activation energy.

SEM micrographs of wear tracks on the TiCN coat-ing and aluminum alloy are shown in Fig. 10. The wearload and wear speed were 50 g, 60 rev.rmin, respec-tively. Silicon nitride was used as a pin. Friction coef-ficients of 0.1]0.15 was obtained for the TiCN coating.The number of rotations for the TiCN coating was halfof that for the aluminum alloy. As can be seen in Fig.

Ž .Fig. 7. Surface topography of TiCN layers obtained at various voltages a: 450 V; b: 550 V; c: 650 V .

( )S.K. Kim et al. r Surface and Coatings Technology 131 2000 121]126 125

Fig. 8. Effect of H rN gas ratio on the hardness of TiCN layers.2 2

10, quite a large amount of wear occurred with thealuminum alloy while a relatively small amount of wearoccurred with the TiCN coating.

Fig. 11 shows the optical micrograph of a scratchtrack of the TiCN coating. The critical load of approxi-mately 18 N was obtained. Scratch tests were per-formed for TiCN layers obtained after varying duty,frequency, voltage; the critical load was nearly thesame for all layers. Therefore, the adhesion of theTiCN layer was not dependent on duty, frequency,voltage within the range investigated in this study as faras the scratch test reflects the adhesion.

4. Conclusions

TiCN coatings were grown on an aluminum alloyŽFig. 10. SEM micrographs of wear tracks a: Al alloy 5000 rotation;

.b: TiCN coating 10 000 rotation .

Fig. 9. Arrhenius plot of ln k vs. 1rT.

( )S.K. Kim et al. r Surface and Coatings Technology 131 2000 121]126126

Fig. 11. Optical micrograph of scratch track of TiCN layer.

with d.c. pulsed PACVD using diethylaminotitanium,hydrogen, nitrogen and argon in the temperature rangeof 150]2508C. The evaporator temperature should bein the range of 74]788C. High hardness of coatings wasobtained at 54% duty, 14.2 kHz frequency, and 450 V.Coarse dome structures were obtained at increasedduty, high frequency, and high voltage. An activationenergy of 40.67 kJrmol was obtained within the tem-perature range of present experiment. TiCN coatingsshowed a good wear resistance. A critical load of TiCNcoating was approximately 18 N and adhesion was notdependent on duty, frequency, voltage within the rangeinvestigated in this study.

Acknowledgements

This work was supported by Korea Science and Engi-neering Foundation under the Korean-GermanCooperative Research Program.

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