Mechanical and Tribological Properties of a-GeC x Films Deposited by DC-Magnetron Sputtering
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ABSTRACT Amorphous carbon-germanium films were grown by dc-magnetron sputtering at different argon plasma pressures ranging from 0.17 and 1.4 Pa. The water-cooled sample holder was grounded. The film thickness were typically 0.5 gim. The ratio between germanium and carbon atomic concentration ranges up to 2.8, as measured by Rutherford backscattering spectrometry (RBS). Elastic recoil detection technique was used to measure hydrogen contamination. The film hardness was measured by nanoindentation techniques and the internal stress was determined by the bending of the substrate. The incorporation of Ge reduces both the film hardness and the internal stress. Hardness and internal stress increases when the films are deposited in lower pressures. Atomic Force Microscopy (AFM) was used to measure the surface roughness, which was found to be insensitive to the pressure and to the Ge content. A possible influence of the thickness on the morphology of pure carbon films is discussed. The friction coefficient measured by AFM is independent on the film composition within experimental errors. INTRODUCTION Amorphous carbon films, the so-called diamond-like carbon films (DLC), are of high technological interest due to their properties concerning hardness, chemical inertness, high
electrical resistivity, infrared transparency, etc. Among them all, the high hardness of these films has attracted the attention mostly [1]. On the other hand, the high compressive internal stress found in these films is an important limitation to the use of thick DLC films as protective coatings, since they tend to delaminate. An approach commonly employed to reduce the internal stress was the use of DLC alloys, such as a-CN.:H films, since the incorporation of nitrogen tends to relax the carbon network [2]. Actually, these films are used as protective coatings of hard disks. Other DLC alloys, such as a-SiCx:H, also present some improvements in which concerns mechanical and tribological properties [3]. The structure of DLC films can be described as distorted benzenic rings, formed by sp2 bonds, bonded to each other through s9 bonds. While the sp2 bonds control the opto-electronic characteristics, the sp3 bonds are closely related to their mechanical properties [1]. DLC films grown by dc-magnetron sputtering have been investigated before, these films can reach hardness of about 20 GPa and compressive internal stresses up to 2 GPa [1,4]. In a recent publication, Mounier and Pauleau show that the intrinsic level in amorphous carbon films, a-C films, deposited by both conventional and unbalanced magnetron sputtering was essentially dependent on the energy of the positive ions impinging the surface of the growing film [5]. In this work, mechanical and tribological properties of amorphous carbon-germanium (a-GeC.) films grown by dc-magnetron sputtering were investigated. Despite a lot of work done on the opto-electronic properties of these films, only few results concerning mechanical and tribological properties have been published. After a first report on
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