Structure, mechanical and tribological properties of hydrogen-free amorphous carbon films deposited by dual-frequency mode pulsed-DC magnetron sputtering

• Main Author: Liu, Chang
• Other Authors: Leyland, Adrian ; Matthews, Allan
• Published: University of Sheffield 2017
• Subjects: 621.8
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.722765

Amorphous carbon (hydrogenated and hydrogen-free) films have attracted much attention during the past few decades due to their unique properties such as high hardness, low coefficient of friction (CoF) and low wear rate. Compared with the hydrogenated amorphous carbon films, hydrogen-free amorphous carbon (a-C) films, with higher hardness, are believed to have better tribological properties especially in humid environments. Different deposition parameters (or techniques) will strongly affect the microstructure of the a-C films and thus influence their mechanical and tribological properties. For magnetron sputtering, pulsing both the sputtering target and the substrate at significant different frequencies (dual frequency DC-pulsed magnetron sputtering) provides more advantages and flexibility of controlling the film microstructure. Therefore, the aim of this PhD thesis is to investigate the effect of substrate bias parameters (i.e. voltage and duty cycle) and silicon doping concentration (to increase the deposition rate and reduce the internal stress) on the mechanical and tribological properties obtained in non-hydrogenated a-C films by such deposition techniques. Four a-C films were deposited at different substrate negative bias voltages (50 V - 170 V), which have a predominantly sp2 bonded structure. Film hardness, reduced elastic modulus and internal stress of the a-C films all initially increased with increasing bias voltage to a maximum at 130 V and then decreased when the voltage was increased further. The average CoF when sliding against an SAE 52100 steel counterface is around 0.3 in humid environments. The specific wear rate did not change in an obvious way with the voltage (from 50 V to 130 V), but increased when the voltage was at 170 V. A further four a-C films were deposited at different substrate bias duty cycles (10% to 40%), which also show a predominantly sp2 bonded structure. Film hardness and reduced elastic modulus follow similar trends to the a-C films deposited at different bias voltages (i.e. first increasing and then decreasing beyond a certain threshold). The film compressive internal stress continuously increases with the substrate bias duty cycle. The average CoF under similar test conditions as those for a-C films deposited by different bias voltages is around 0.2-0.3. The a-C film deposited at 30% duty cycle shows the lowest wear rate and the 40% one shows the highest. The effect on the wear rate of the substrate bias duty cycle appears more pronounced than that of bias voltage. Doping with silicon in a-C film (18.2 at.%, 28.8 at.% and 41.5 at. %) led to an increase of the deposition rate, compared to the ‘pure’ a-C films. Doping with smaller silicon concentration ( < 20 at. %), although reducing hardness, will also reduce the compressive internal stress. However, a higher silicon concentration appears to increase the internal stress. a-C film (or a-C/a-C:Si film system) deposited by dual-frequency mode DC magnetron sputtering with excellent mechanical and tribological performance is a good candidate for the real tribological applications, especially on the stainless steel substrate.