Elsevier

Surface and Coatings Technology

Volumes 177–178, 30 January 2004, Pages 409-414
Surface and Coatings Technology

Preparation of metal (W, Mo, Nb, Ti) containing a-C:H films by reactive magnetron sputtering

https://doi.org/10.1016/j.surfcoat.2003.09.017Get rights and content

Abstract

We discuss the preparation of metal containing hydrogenated amorphous carbon (a-C:H) thin films by means of reactive magnetron sputtering with pulsed d.c. power of a metal target (W, Mo, Nb, Ti) and r.f. bias using different gas mixtures of methane and argon. The obtained samples comprised a thickness between 100 and 600 nm and a low internal stress when little quantities of metal were incorporated. The chemical composition was analysed by X-ray Photoelectron Spectroscopy (XPS). Depth profile composition was qualitatively displayed by secondary ion mass spectrometry (SIMS). The optical characterization of the films was carried out by transmittance measurements in the visible range and showed a relationship between the optical band gap and the composition. Structural information was obtained by X-ray diffraction (XRD), whose results showed a shift of the Bragg peaks from carbide crystallites as the metal amount increased. Peak width calculations situated particle sizes in the nanometric range. Surface topography from atomic force microscopy (AFM) measurements is also discussed. Surface energy was measured by the contact angle technique. The internal stress of the films was obtained by profilometry and a relationship with their structure was found. The results are compared with those corresponding to metal-free (pure) a-C:H films.

Introduction

The outstanding mechanical properties of diamond-like carbon (DLC) films have been investigated for many years [1]. Hard, low-friction and wear-resistant DLC films, either in the form of amorphous carbon (a-C) or amorphous hydrogenated carbon (a-C:H), have been used as protective coatings in a great variety of industrial applications [2]. However, the main drawback of DLC coatings relies on their trend to delaminate due to a high compressive stress, which limits the thickness of adhesive films. Therefore, ways to enhance DLC film adherence to substrates maintaining acceptable mechanical properties are required. One of the methods used to obtain well-adhered coatings consists in depositing a metallic buffer layer or an interfacial a-Si:H layer [3] on the substrate. Other reported solutions to reduce the stress are based on the use of composition-graded films or multi-layer structures [4], [5].

Recently, research on DLC has investigated the addition of metal atoms to the DLC network in order to improve the mechanical performance of coatings [6]. This depends on the used metal and the structure of the film. Furthermore, metal additions make the DLC films structure to be conductive, giving rise to interesting electric transport properties [7] useful in many protective non-insulating materials applications.

Metal containing DLC (Me-DLC) films are commonly prepared by reactive sputtering of a metal target in an argon–hydrocarbon gas mixture [8] or by simultaneous sputtering of metallic and graphite targets [9]. Most of the published Me-DLC work has reported the use of only one target material, such as Ti [10], [11], Cr [12], W [13], Mo [7] and Nb [14], whereas very few studies compare the use of different metal materials [15].

The present work reports the preparation of Me-DLC coatings by combining r.f. plasma-enhanced chemical vapour deposition (PECVD) and pulsed d.c. magnetron sputtering of several target materials (molybdenum, titanium, tungsten and niobium) at different gas mixtures of methane and argon. The Me-DLC film properties were analysed by optical transmittance, XPS, SIMS, XRD, AFM, contact angle with water and profilometry. Our study also shows a relationship between surface roughness, composition and preparation technique of the films. Finally, the influence of size and density of metal inclusions on the internal stress of the amorphous matrix is discussed.

Section snippets

Experimental details

Me-DLC (Me: Mo, Ti, W, Nb) films were deposited by reactive sputtering of a metal target placed on a 3-inch diameter magnetron cathode in an argon–methane atmosphere. Glass and c-Si substrates were placed on a water-cooled holder, 10 cm away from the metal target, that was r.f. (13.56 MHz) powered to attain a negative bias voltage of −200 V. An asymmetric bipolar-pulsed d.c. power supply (ENI RPG-50) was used to drive the magnetron cathode. The pulsed d.c. signal consisted of a reverse small

Deposition rate

The deposition rate of different metal containing a-C:H films is presented in Fig. 1 as a function of R. The addition of a small amount of methane in the gas mixture (R up to 3%) leads to an increase in the deposition rate, which is more important in the case of W samples. However, in W-DLC films the deposition rate slightly decreases from R=0 to 0.03, probably due to the reactivity of W with CH4 on the target surface or in the gas phase. For the Ti, Mo and W samples, a further increment in R

Conclusions

Films of metal containing amorphous carbon have been produced by reactive magnetron sputtering using a pulsed d.c. power from different metal targets (W, Mo, Nb, Ti) and r.f. bias using different gas mixtures of methane and argon. In these experiments, we have changed the properties of the films from metallic behaviour to a DLC one by varying the methane flow rate from 0 up to 25% in the reactive plasma process. XPS measurements provide surface composition. Complementary chemical analysis by

Acknowledgements

This work was supported by the CICYT of Spain (project MAT2000-1014-CO2-02 and MAT2002-04263-CO4). The authors acknowledge Serveis Cientı́fico-Tècnics (SCT) of the Universitat de Barcelona for their assistance on XPS, AFM and XRD analysis; and PAS of the Universitat de Barcelona for SIMS measurements. One of the authors (C.C.) acknowledges financial support of the D.G.U. of the M.E.C.D. for a postgraduate scholarship.

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