Date of Award

2014

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

Applied sciences, Carbon coatings, Adhesion, Diamond like carbon, Friction, Wear

Supervisor

Alpas, Ahmet

Rights

info:eu-repo/semantics/openAccess

Abstract

The objective of this study was to gain insight into the friction, aluminum adhesion, and wear mechanisms of diamond-like carbon (DLC) coatings, and to provide guidelines for coating design and development. Mechanisms that control the tribological behaviour of DLC coatings and the effects of dopants (i.e. hydrogen (H-DLC), and tungsten (W-DLC)) against aluminum alloys were investigated under various environments and test temperatures. The effects of temperature and an oxygen-rich environment on dopant-free DLC, H- DLC, and W- DLC were investigated. Experimental analyses of dopant-free DLC showed that, when it was tested in an atmosphere consisting of 50% oxygen and 45% moisture, a high COF of 0.6 observed during the running-in against aluminum was eliminated compared to environment without moisture. At elevated temperatures, presence of hydrogen reduced the COF of H-DLC (e.g., to 0.06 at 200 oC). W-DLC coatings provided a low COF of 0.18 and minimized aluminum adhesion at temperatures ranging between 400 oC and 500 oC, which was attributed to the formation of a tungsten oxide film. Additionally, DLC coatings were found to generate a low COF at subzero temperatures (-196 oC), with W-DLC and H-DLC generating a COF of 0.18. The work of adhesion (W ad ) was determined using a nano-indentation pull-off force method. In this way, insight was gained into the nature of atomic interactions contributing to tribological mechanisms at elevated temperatures. The results showed that the adhesion of the diamond tip against all four samples tested (H-DLC, dopant-free DLC, W-DLC, and aluminum) decreased with temperature. At 25 °C, no aluminum adhesion was observed on the diamond tip, due to OH passivation of the diamond surface in agreement with the low COF of 0.12 for the dopant-free DLC coating. The elimination of meniscus forces due to adsorbed water molecules on the sample surface was identified as an important factor contributing to the adhesion at room temperature. The results also confirmed that the hydrogen in the H-DLC mitigated interatomic interactions at the surface and reduced Wad to as low as 0.01 J/m 2 at 200 oC. At 25 °C, there was no aluminum adhesion observed on the diamond tip, due to OH passivation of the diamond surface in agreement with the low COF of 0.12 for the dopant-free DLC coating.

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