Optical and tribological properties of heat-treated diamond-like carbon
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NTRODUCTION
Diamond-like carbon, or DLC films, are metastable amorphous materials, which, since first deposited by Aisenberg and Chabot in 1971,1 have been prepared by a variety of methods including DC or RF plasma assisted CVD, sputtering, and ion beam deposition, from a variety of carbon bearing, solid, or gaseous source materials.2"5 DLC films contain sp2, sp3, and even sp1 coordinated carbon atoms in a disordered network. All DLC deposition processes are characterized by the interaction of energetic ions with the surface of the growing film. The metastable structure of DLC films most likely originates from the thermal and pressure spikes produced by impinging energetic species at the surface.6 These films are characterized by extreme hardness, measured to be in the range 30009000 kg/mm2,2 a generally low friction coefficient, between 0.017 and 0.28,8 as well as very high internal stresses.7'9"11 The films typically have high optical transparency over a wide spectral range,4 high electrical resistivity, and chemical inertness to both acids and alkalis. The high hardness and chemical inertness of the DLC films make them good candidates as wearresistant protective coatings for metals, optical or electronic components, and particularly for applications where layers of order of 100 nm thickness or less are required. Depending upon the precursor materials employed for film deposition, many of these films contain a significant amount of hydrogen. As a function of deposition method, the hydrogen content in DLC films J. Mater. Res., Vol. 5, No. 11, Nov 1990
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has been shown to vary between less than 10%12 up to 50%.13 The hydrogen content critically determines film structure, e.g., the ratio between the carbon atoms in the difference coordinations, and thus the film properties. Hydrogen content is also the key to obtaining a wide optical gap (£opt) and high electrical resistivity, as it passivates the dangling bonds in the amorphous structure.3 The properties of DLC films are also known to depend strongly upon both the deposition method and the specific deposition conditions.4 Although the properties of DLC films tend to be independent of the exact hydrocarbon precursor5 employed, this is not strictly true (e.g., Ref. 14). Deposition is generally done at substrate temperatures, Td < 300 °C, because it was found15 that, for substrate temperatures above 325 °C, the films became graphitic in nature (£ opt -> 0 eV, o--» 102 O"1 cm"1) and softened significantly. Thermal stability has been previously examined for some DLC films. Dischler et al.16 reported changes in the 2900 cm"1 IR absorption peak, with the change occurring subsequent to a 300 °C anneal. Kanazawa and Ebihara17 found a large decrease in the optical gap of a-C: H (DLC) even after annealing at 200 °C. Couderc and Catherine14 reported complete removal of hydrogen from DLC films deposited at 0.5 Torr after annealing at 600 °C, while hydrogen was still present in films deposited at 0.1 Torr after annealing them
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