Study of W/WC films produced by plasma assisted vacuum arc discharge

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phys. stat. sol. (c) 2, No. 10, 3758 – 3761 (2005) / DOI 10.1002/pssc.200461818

Study of W/WC films produced by plasma assisted vacuum arc discharge R. Ospina, P. Arango, Y. C. Arango, E. Restrepo, and A. Devia* Universidad Nacional de Colombia Sede Manizales, Km. 9 vía al aeropuerto Campus La Nubia, Manizales, Colombia Received 11 October 2004, revised 30 May 2005, accepted 30 May 2005 Published online 29 July 2005 PACS 61.10.Nz, 68.37.Ps, 81.05.Je, 81.15.Ef, 82.80.Pv W/WC films were grown by the PAPVD repetitive pulsed vacuum arc technique on 304 stainless steel substrates. To produce the coatings, a target of W with purity of 99.9999% was used. The system is composed by a reaction chamber with two opposite electrodes placed inside it. The target is located on the cathode and the samples on the anode. A pulsed power supply is used to generate the discharge. For the production of the W layer, the chamber was filled with Ar gas at a pressure of 3 mbar, and the voltage of the discharge was 270 V with 3 pulses. WC films were grown in an atmosphere of methane at 3 mbar and a voltage discharge of 275 V with 4 pulses. The active and passive times of the discharge were 1 s and 0.5 s, respectively. XRD technique was employed to study the coatings, to study the present phases and the crystallographic orientation of the films, the XRD analyses were carried out varying the temperature of the systemcoating-substrate between room temperature and 600 °C, when the WC coatings are degradated, leaving just the tugsten. XPS analyses present the apparition of WC, WO and WO2 compounds. AFM analyses allowed to measure the morphological properties and the thickness around 3 µm. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1

Introduction

Different coatings have recently attracted increasing interest because of the possibilities of synthesizing materials with unique physical–chemical properties [1, 2]. Highly sophisticated surface related properties, such as optical, magnetic, electronic, catalytic, mechanical, chemical and tribological properties can be obtained by advanced coatings, making them attractive for industrial applications in high speed machining [3, 4], tooling [4], optical applications [5] and magnetic storage devices [6]. Tungsten carbide WC thin films have been classically used as protective hard coatings due to their good mechanical properties [7] such as high hardness and corrosion resistance and low wear properties, that are sustained up to 400 C [8, 9]. Recently, technological interest in this material has increased because of its use in WC-C composite coatings [9, 10]. Erosion by solid particle impingement reduces the life of mechanical components used in the aerospace industry. Consequently, multilayered coatings deposited by physical vapor deposition methods have been developed to protect some critical components against abrasive erosion. A promising coating is based on a multilayer arrangement, alternating ductile layers (tungsten) with harder layers (a solid solution of carbon into tungsten) [11]. The cathodic vacuum arc is widely used to produce thin film deposition. The deposited material comes from highly ionized plasma, with ions having high energies [12]. That is the reason because many thin films have been deposited by this technique [13, 14].

*

Corresponding author: e-mail: [email protected] © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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The aim of this work is to study the production of WC thin films on stainless steel substrates with an interlayer of W. XRD analyses were carried out increasing the temperature from room up to 600 °C, studying the evolution of the structure. Morphology (grain size and roughness) and chemical composition of the coatings as a function of the substrate temperature were done.

2

Experimental setup

The experimental setup is made up by a vacuum system which includes the reaction chamber with two opposite electrodes inside it. The cathode consists of a tungsten target with 99.999% of purity and the anode is a substrate which will be coated. The samples were polished and deeply cleaned with an ultrasonic cube. A high power supply designed to generate pulses with different active and passive times, was employed to produce the discharge between the electrodes. The cathode is made up by the target of W and the anode is composed by samples of stainless steel. For the coatings production, first a vacuum of 10-5 mbar is done inside the chamber. W layer was produced in an environment of argon at 3 mbar of pressure, with a voltage of 270 V. WC films were grown with 5 pulses using methane as gas of work at a pressure of 3 mbar and a voltage of 275 V. Both coatings W and WC were grown at 1s and 0.5 s as passive and active time respectively. Part of the system is widely described in other works [15, 16]. For the X-ray characterization, a difractometer Bruker AXS, model D8 Advance, geometry of parallel beams, monochromatic of graphite, which has a chamber used to vary the temperature between 0 °C and 1000 °C was employed. For the XPS Analysis, a Thermo VG Scientific Escalab 250 with a monochromatic source of Al radiation (Kα= 1486.6 eV), hemispherical energy analyzer between 10 and 1200 eV and a multi-technical chamber was used. For morphological chacterization (thickness and grain size) a scanning probe microscopy (SPM), Park Scientific Instruments in model AutoProbe CP, in AFM mode was employed.

W(211)

W2C(302)

W2C(113)

W2C(300)

WC(110)

W(200)

W2C(112)

WC(101) S

Intensity (a.u.)

200

WC(100)

250

W2C(002) W(110) W2C(111) S S

Results and discussion W2C(110)

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RT

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100 ºC 200 ºC

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300 ºC 400 ºC

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0

500 ºC 600 ºC

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60 2θ

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Fig. 1 XRD patterns of W/WC coatings as a function of the heating temperature.

© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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R. Ospina et al.: Study of W/WC films

200000

W4f7/2

180000 160000

Intensity (a.u.)

W4f5/2

140000 120000 100000 80000 60000 40000 20000 0

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33 32 31 Binding Energy (eV)

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Fig. 2 XPS narrow spectra of W4f peak for the coating at room temperature.

a)

b)

Fig 3 AFM images of the W/WC coatings; a) at room temperature, b) after heating at 600 °C.

The XRD patterns for W/WC films done varying the temperature in air environment are shown in Fig. 1. FCC W oriented in planes (110), (200) and (211) with a lattice parameter of 3.165 Ǻ, hexagonal WC with lattice parameters a = 5.171 Ǻ and c = 4.719 Ǻ oriented in planes (100), (101), (110) and W2C in planes (110), (111), (002), (112), (300), (113) and (302) with lattice parameters a = 2.889 Ǻ and c = 2.823 Ǻ are present in the bilayer, indicating a very polycrystalline material. Analyses displayed a progressive degradation of the WC phases by the increase of the temperature, while the peaks corresponding to the substrate and the metallic W were maintained. No oxides were formed with the increase of the temperature, as it is reported in the literature for these kinds of materials [17]. When the heating temperature is lower than 700 °C the oxidation of the alloy is not serious, but when the heating temperature is at 800 °C the oxidation quickens, and when the heating temperature is more than 900 °C, the oxidation rate remarkably increases. C. Louro and A. Cavaleiro reported results of oxidation for WC produced by sputtering techniques. For 600 °C oxidation temperature, the coatings present incipient oxidation with weight gains of less than 0.25 mg cm2. As expected, when the temperature increases, greater oxidation rates were observed [18].

© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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XPS narrow spectrum of the sample for W4f and C1s was carried out and they are present in Fig. 2. The W 4f region displays two peaks 33.65 eV, and 31.55 eV assigned to W4f7/2 and W4f5/2, respectively, which correspond to C-W bonds, other materials produced by these techniques like TiN and TiC normally show oxides formation during the growth of the films. Both XRD and XPS analyses allow to conclude that W and WC are not easy to oxide. Figure 3 shows AFM images of the W/WC before and after the heating. There are notorious changes in the morphology. In Fig. 3a) the grain formation is appreciable while in Fig. 3b) the grain structure has almost totally disappeared, and in some regions, the layers generated during the growth process are observable, this is in accordance to the XRD analyses which shows a film degradation with the temperature.

4 Conclusions W/WC bilayers have been prepared by a repetitive pulsed vacuum arc system. The produced films have not presented any oxide phase as it was shown by the XRD and XPS analysis. The coatings were studied by means of X-ray diffraction varying the temperature of the system coating-substrate, observing a degradation of the WC when the temperature was increased while the W film was kept during all the process. XPS analyses displayed W4f peaks with binding energy corresponding to W-C bonds, but this spectrum did not present WO formation during the growth process. AFM images taken before and after the temperature changes presented morphological changes, associated to the degradation of the films.

Acknowledgements This work was supported by Instituto Colombiano para el Desarrollo de la Ciencia y la Tecnología (CONCIENCIAS) under projet RC 566-2002 and la División para la Investigación de la Universidad Nacional de Colombia Sede Manizales (DIMA).

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