A tribological and mechanical study of ion assisted diamond-like carbon thin films
Holiday, Peter Stuart
Engineering design and manufacture
Thesis or dissertation
- © 1992 Peter Stuart Holiday. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder.
Amorphous hydrogenated carbon (a-C:H), diamond & diamond-like (DLC) thin films are some of the many terms used when referring to the generic group of coatings based on hard carbon. They are an emerging technological area within the surface coating discipline and are being increasingly used to improve the efficiency of a wide range of engineering components. In addition, the unique and extreme characteristics of these films result in unequalled material properties, such that in many cases a wide range of new and superior performance devices have only recently begun to be realised.
This study focuses on hydrogenated & non-hydrogenated diamond-like thin films deposited by various plasma based, hybrid and beam deposition techniques. The wear resistant and low friction properties of these films are of great importance in many of the potential application areas and has attracted particular interest in recent years. Therefore the major thrust of this research has been on the tribological aspect, particularly in relation to other advanced ceramic coatings, and to highlight the applicability of endurance wear tests used to evaluate diamond-like films. The main findings have been:-
a} That carbon can be deposited by several techniques in a hard amorphous phase, the properties of which depend heavily upon the conditions, substrate choice and method of deposition. For a particular technique, material properties can be made to be repeatable by a good understanding of deposition process control.
b} The use of plasma based hybrid PVD and beam methods have resulted in a considerably improved structural performance of the films over those produced by the direct evaporation of graphite. The introduction of a hydrocarbon gas into the plasma at the synthesis stage has also been shown to provide further improvements in the physical properties which has correspondingly led to an enhancement in the tribological behaviour. The levels of hydrogen, whether in an unbonded or bonded form, included in the film after deposition has been demonstrated to affect the mechanical and optical properties of the considerably.
c) The wear resistant and frictional performance of these coatings has been shown to be variable, depending upon the method and conditions of deposition as well as test parameters such as humidity, surface roughness, film structure, adhesive strength and oxide/impurity formation. In some cases the tribological performance was found to be excellent. The presence of the diamond-like carbon coating has been shown to be beneficial in reducing wear between contacting bodies experiencing relative movement by encouraging the formation of a carbon transfer layer on the surface of the counterface material which acts as a zone of low shear and provides a physical barrier to tribo-chemical interactions. Under certain conditions, such tribo-chemical interactions can occur readily at the interface, facilitating the formation of strong interfacial bonding and increased wear.
d) The inclusion of metallic elements into the carbon matrix has been shown to enhance the wear resistant properties of the film to only a small extent, although at the expense of a deterioration in the friction coefficient. The most beneficial effect of doping carbon films with metal species has been the improved resistance to thermal degradation.
e) Thin intermediate layers of titanium nitride have also been shown to produce a remarkable improvement in both wear resistance and frictional performance of the diamond-like carbon films to an extent which appears to be related to the level of stoichiometry of the titanium nitride. The main mechanism behind this increased performance appears to be due largely to an enhancement in adhesive strength at the diamond-like carbon/titanium nitride junction, with an increase in load support being provided as a secondary benefit.
f) A critical assessment of the available techniques and methodology available for testing hard carbon films has been made and in some cases methods have been found to be either entirely inappropriate or appropriate only when suitable precautionary measures have been taken. These difficulties largely stem from the exacting demands of thin, hard layers of diamond-like carbon due to its unique and extreme mechanical, electrical and optical properties.
- Department of Engineering, Design and Manufacture, The University of Hull
- Matthews, Allan
- Sponsor (Organisation)
- Science and Engineering Research Council (Great Britain)
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- 21 MB