Hard Carbon Coatings: The Way Forward
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risk of continued in-service failures occurring. In the following, we outline the present state of knowledge to facilitate a move toward the more appropriate application of hard carbon films.
What We Mean by Hard Carbon Diamond and Graphite Diamond is the hardest natural material known. It is not only very hard (typical Vicker hardness values quoted are around 10,000 Hv), but it also possesses a high elastic limit and yield stress3 because it contains a fully interconnected network with very strong C-C covalent bonds (7 eV) of the tetrahedric configuration of carbon (sp3). In addition this material shows good tribological properties (when polished) and is resistant to chemical attack, except in oxygen at high temperature (over 800°C). Otherwise it is a thermally stable material up to 1300°C under inert gases. Its properties seem close to ideal for many applications, but consideration must be given to the cost of production, including polishing. Another well-known state of carbon is graphite, which represents a thermodynamically more stable state than diamond, but is not very hard and has a much lower elastic limit. Its tribological properties differ from polished diamond. The interplaner bond strength of graphite is weak (0.8 eV), which can give low friction properties compared to dry metal contacts but not so low as polished diamond contacts. For example, the friction coefficient of graphite against polished Si3N4 is about 0.25 in argon and about 0.18 in humid air but rises up to 0.6 in pure oxygen as compared to 0.07 in argon and about 0.05 in humid air and up to 0.1 in pure oxygen (at temperatures
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