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Title: Si-DLC coatings optimized for low wear and low friction applications
Author: Hofmann, Dieter
ISNI:       0000 0004 7223 6741
Awarding Body: Sheffield Hallam University
Current Institution: Sheffield Hallam University
Date of Award: 2014
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Diamond-like carbon (DLC) has excellent properties for the use as tribological coating such as high hardness, high wear resistance and a low friction coefficient. Nanolayer systems of hydrogenated diamond-like carbon (a-C:H) and silicon doped hydrogenated diamond-like carbon (Si-DLC or a-C:H:Si) have been studied. The objectives of this work are to develop layer combinations which allow combining low abrasive wear, low friction properties, and improved temperature stability for the coatings. A literature survey on sputtering, plasma enhanced chemical vapor deposition (PECVD) and carbon based materials with a focus on a-C:H and Si-DLC coatings is given. This survey includes a brief overview of the 60 years history of DLC coating. In the second part of the thesis, the experimental set up for the layer preparation by a magnetron based deposition method and a basic description of the process are presented. Using sputtering from a solid SiC target as a source of the Si for the Si-DLC instead of the commonly used PECVD process with Si-containing precursors Si-DLC/a-C:H nanolayers were deposited at high deposition rates. For the coatings, the influence of the acetylene gas flow, the bias voltage, and the hydrogen and silicon concentration are discussed. Undoped a-C:H coatings with high indentation hardness above 40 GPa and very low abrasive wear rates of 0.6 x 10[-15] m[3]/(Nm) and low hydrogen content of about 11 at.% were deposited. Si-DLC/a-C:H nanolayer coatings with high hardness of 20 to 30 GPa and a high temperature stability up to 500 °C were prepared. For these nanolayer films low friction coefficients of 0.06 to 0.11 and high abrasive wear rates of > 2.5 x 10[-15] m3/(Nm) were achieved at high silicon contents above 15 at.%. On the other hand nanolayer coatings with low silicon contents of less than 10 at.% showed low wear rates below 1.7 x 10[-15] m3/(Nm) combined with higher friction coefficients of 0.12 to 0.15. In order to combine the low abrasive wear rate and the low friction coefficient at first a Si-DLC/a-C:H layer with low Si-concentration followed by a second layer with high Si-concentration was deposited. Due to the low surface free energy of Si-DLC coatings of 30 to 35 mN/m the wetting behavior for some lubricants may deteriorate. In this case a further optimization of the nanolayer system is required.
Supervisor: Hovsepian, Papken Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available