Microstructural and mechanical investigations of tungsten carbide films deposited by reactive RF sputtering

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Abstract

Tungsten carbide thin films were deposited on (100) Si single crystal substrates by reactive RF sputtering from a tungsten target in Ar–CH4 mixture. Under 1% of methane, the formation of a mixture of nanocrystalline WC1−x presenting a partial (200) texture and W2C phases was observed. The film deposited under 2% of CH4 shows a maximum hardness of 22 GPa (a value comparable to the bulk WC reference material), these coatings also exhibit the lowest friction coefficient of 0.1.

For coatings performed with CH4 percentage higher than 3%, a progressive amorphization of the layers was observed. At 5% of CH4 concentration, the films become essentially amorphous. Simultaneously the hardness decreases continuously to achieve a minimum value of 13 GPa.

Introduction

Transition metal carbides like tungsten carbide present high interest due to many of their specific physical and mechanical properties [1]. Tungsten carbide exhibits high melting point, extreme hardness, low coefficient of friction, chemical inertness, oxidation resistance and good electrical conductivity. These properties make this material an ideal candidate for industrial applications like wear-resistant coatings, cutting and drilling tools. In addition, the rare combination of strong bonding and metallic conductivity makes tungsten carbide an attractive material for high temperature contacts or as a conductive protective layer in sensor applications.

According to the W–C phase diagram investigated by Rudy [2], [3] WC is the sole thermodynamically stable phase at room temperature. Nevertheless, thin films often present WC1−x [4], [5], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21] and W2C [4], [6], [7], [9], [22], [23], [24] crystalline structure, the WC phase being the less observed one [4], [5], [6], [25], [26]. Moreover, the x value can reach 0 which is lower than the one reported in the phase diagram. This decrease of the x value indicates that the incorporation of the carbon in the WC1−x lattice is higher than that predicted by the phase diagram and it induces an increase of the lattice parameter [4].

The aim of this work is to study the deposition of W–C coatings by RF sputtering from a W target using various methane/argon gas mixture, and to correlate structural and mechanical properties of these films to their stoichiometry.

Section snippets

Deposition process

W–C layers were deposited on (100) oriented silicon wafers by reactive RF sputtering from pure tungsten target (5 N) in Ar/CH4 gas mixture. The sample was placed 7 cm away from the target, and heated to 150°C. The overall sputtering pressure was 0.25 Pa and the sputtering RF power was 200 W leading to a − 400 V self-bias. Various methane partial pressures were investigated: 1%, 2%, 3%, 4%, or 5% of total pressure. The corresponding samples have been called a, b, c, d and e, respectively. The

Composition and structure

The W and C contents in the films were determined by RBS and NRA. The carbon concentration in the films increases from 40% to 82% with the methane concentration varying in the 1% to 5% of total gas pressure. The film compositions are reported in Table 1. The increase in the carbon content was accompanied by a brutal reduction of the film densities from 15.8 (close to WC one (15.7 ) and WC1−x one (17.2)) for the sample a to 7.3 for the sample e. Nevertheless the films density never reaches the

Conclusion

Nanocrystallized W–C films have been produced by reactive RF sputtering, using a tungsten target and a methane gas.

Structural and mechanical properties are related to the nature and the proportion of the present phases:

  • Sample performed at 1% of CH4 is constituted of a mixture of WC1−x and W2C phases and presents a hardness of 19.5 GPa.

  • For PCH4 = 2%, the sample exhibits a pure WC1−x phase and have the maximum hardness of 22 GPa.

  • For higher PCH4 a progressive amorphization of the films occurs which

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