Papers by Author: Y.T. Pei

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Authors: Y.T. Pei, P. Huizenga, Damiano Galvan, Jeff T.M. de Hosson
Abstract: Advanced TiC/a-C:H nanocomposite coatings have been produced via reactive deposition in a closed-field unbalanced magnetron sputtering system. In this paper, we report on the tribological behavior of TiC/a-C:H nanocomposite coatings in which ultra-low friction is tailored with superior wear resistance, being two properties often difficult to achieve simultaneously. In-situ monitoring the wear depth at nanometer scale reveals that the self-lubricating effects are induced by the formation of transfer films on the surface of ball counterpart. In addition, the CoF of TiC/a-C:H nanocomposite coatings decreases with decreasing relative humidity. This phenomenon can be interpreted in terms of water molecule interactions with the wear track. The influence of the volume fraction and grain size of TiC nanocrystallites on the coating properties has been examined. A superior wear resistance at a level of 10-17 m³/(N m lap) has been achieved under the condition of super-low friction and high toughness, both of which require fine TiC nano-particles (e.g. 2 nm) and a wide matrix separation that must be comparable to the dimensions of the nano-particles.
Authors: Damiano Galvan, Y.T. Pei, Jeff T.M. de Hosson, Albano Cavaleiro
Abstract: A commercial RF-sputtering deposition rig was employed to deposit H-free diamond-like carbon (DLC) coatings. The influence of alloying elements such as Ti and Si on the structure, mechanical and tribological properties of the coatings was investigated. The coating was observed in cross section and in plan view with SEM, TEM and AFM. Because of the highly-ionized plasma generated by the RF-powered glow discharge, ion bombardment suppresses the formation of a columnar structure regardless of the composition of the coatings. The method produces featureless microstructures and smooth surfaces. TEM investigations confirm that no crystalline phases form in the coatings regardless of the presence of considerable concentrations of Ti and Si. Tribological tests were performed with a high-temperature tribometer in a ball-on-disk configuration, using coated disks and different materials for the ball countepart. At ambient T the sliding friction coefficient decreases as the concentration of alloying elements increases. Nevertheless, high-T tribotests with a constant thermal load showed that the presence of alloying elements decreases the thermal stability of the coatings. For each coating a temperature exists above which a sudden increase of friction coefficient is observed, with subsequent detachment and failure of the coatings. The mechanism of disruption of the self-lubrication effects is identified and the influence of the alloying elements on the thermal degradation of tribological performance of the coatings is discussed.
Authors: Y.T. Pei, Damiano Galvan, Jeff T.M. de Hosson
Abstract: TiC/a-C:H nanocomposite coatings have been deposited by magnetron sputtering and are composed of 2-5nm TiC nanocrystallites well separated by amorphous hydrocarbon (a-C:H) of about 2nm separation width. A transition from columnar to glassy microstructure has been observed with increasing substrate bias or carbon content. Micro-cracks induced by nanoindentation or wear tests readily propagate through the column boundaries whereas the coatings without a columnar microstructure show supertough behavior. The nanocomposite coatings exhibit hardness of 5~20 GPa, superior wear resistance and strong self-lubrication effects with a friction coefficient of 0.05 in air and 0.01 in nitrogen under dry sliding against uncoated bearing steel balls. Especially, the transitions from low to ultralow friction or the reverse are repeatedly switchable if the atmosphere is cycled between ambient air and nitrogen. The lowest wear rate is obtained at high humidity.
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