Tribological Behavior of Nanocomposite Diamondlike Carbon-Aluminum Films
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Tribological Behavior of Nanocomposite Diamondlike Carbon-Aluminum Films E. Liu1, J.X. Gao1, A.P. Zeng1, B.K. Tay2, X. Shi2 1 Centre for Mechanics of MicroSystems, School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 2 School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798 ABSTRACT Tetrahedral amorphous carbon (ta-C) contains a large percentage of sp3 carbon bonding. However, high internal stresses develop in ta-C films with a high sp3 content, which limits their applications for wear protection. In this study, Al containing ta-C films were produced using Filtered cathodic vacuum arc (FCVA) technique. The structure of films was studied using MicroRaman spectroscopy in terms of aluminum content. The pure carbon ta-C films generally contained a relatively high residual compressive stress, which was related to its sp3 content, amorphous structure and preparation conditions. For the Al containing ta-C films, the stress reduction is significant with increase of aluminum content in the film. A decrease of the mechanical properties of the ta-C:Al nanocomposite films was noticed with the decrease of the internal stress in the films. The tribological behavior of the films was measured using ball-ondisk tribometer. The wear rate and friction coefficient were determined correspondingly. In the ball-on-disk testing, different loads are applied to the sapphire counterbody. All the tests were performed in ambient air (RH50%) and at room temperature (22 °C). It was noted that the friction coefficient of ta-C:Al films increased at the beginning of the testing before reaching a peak value of about 0.25. After the peak, the friction coefficient dropped until reaching a steady state value. The original surface roughness of the counterfaces, surface smoothening due to successive wear, and wear debris produced during the testing are all responsible for the tribological behavior of ta-C:Al films. INTRODUCTION Diamond films have many unique properties, which are suitable for various applications. However, diamond films generally require relatively high substrate temperature during deposition, which limits the choice of substrate materials. In addition, polycrystalline diamond films have relatively rough surfaces due to the facets of diamond grains, which has limited their applications in tribology and contact mechanics. As an alternative, diamondlike carbon films can be deposited on any substrate materials and have a much smoother surface, specifically, tetrahedral amorphous carbon (ta-C) films which contain a large percentage of sp3 carbon bonding. Filtered cathodic vacuum arc (FCVA) has been reported to be an effective method of producing high quality ta-C films containing up to 85% sp3 carbon bonding [1], which leads to a hardness up to 70 GPa [2]. However, high compressive stresses have also been reported, which limits the applications of ta-C films as a protective coating on cutting tools or mechanical parts
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