Effects of C2H2 Flow Rate on Structure and Properties of Amorphous Carbon Films Prepared by Reactive Sputtering Graphite

Amorphous carbon film, which combines good chemical inertia, wear resistance and mechanical properties, has been applied in aerospace, automotive, magnetic storage, mechanical processing and other fields. sp3 ratio and H content significantly influence microstructure and tribological properties of amorphous carbon. Especially, the tribological behavior of amorphous carbon with different sp3 ratio and H content varies greatly in the run-in stage under vacuum and air, which means a lot for wear pair design in aerospace equipment components. This work aims to investigate the relationship between film structure and properties, so as to find the optimum processing window. Mirror-polished 9Cr18 pieces (diameter 35 mm× thickness 3 mm) were used as substrates to deposit films by means of pulsed DC reactive sputtering graphite in Ar atmosphere mixed with different amount of acetylene. All substrates were ultrasonic cleaned in alcohol for 20 minutes to remove oil on the surface followed by hot air drying, and then fixed on the substrate holder. Before deposition, degassing was carried out by vacuum heating at 200 ℃ until base pressure reached 3×10−3 Pa. After that, a plasma source with current of 50 A was applied to do Ar+ cleaning for 20 min, to further remove contaminants and activate substrate surfaces. 500 nm Cr was deposited below the amorphous carbon layer to improve the adhesion strength between the substrate and the film. For amorphous carbon layer deposition, 4 kW were applied on the graphite target while a -100 V negative bias was applied on the substrate. The argon flow rate was between 50-100 cm3/min, and the acetylene flow rate changed from 0 to 100 cm3/min to deposit amorphous carbon with various structures. In order to investigate effects of acetylene flow on microstructure, mechanical properties and tribological properties of films fabricated above, an X-ray photoelectron spectrometer and a Raman spectrometer were used to analyze microstructure, such as sp2, sp3, ID/IG, position and FWHM of D peak and G peak. Mechanical properties were characterized with a nanoindentor. The tribological properties were examined with a ball-disc tester, with load of 5 N, rotational speed of 1 m/s in air condition. The wear track on the film as well as the wear scar on the mated ball were tested with a white light interferometer and an optical microscope. The results showed that dense and uniform amorphous carbon film were prepared by reactive sputtering. Element analyzing demonstrated that all films contained a certain amount of O element (6.36%-13.86%) on the surface layer, O content of lower than 1% could be reached for most films after Ar+ etch. With the increase of acetylene flow rate, H, E and H3/E2 increased first and then decreased. When acetylene flow rate was 10 cm3/min, the hardness and elastic modulus reached maximum, which were 27.93 GPa and 233.55 GPa, respectively. Tribological results showed that the average friction coefficient was between 0.09-0.11 for all films, and the friction coefficient for the start-up stage decreased with the increase of H content. Wear resistance of 5 cm3/min sample was the best, the minimum wear rate was 0.72×10−16 m3/(N·m). The sp3/sp2 and H can be tailored by adjusting acetylene flow in reactive sputtering, so as to regulate mechanical properties and tribological properties of amorphous carbon films. © 2024 Chongqing Wujiu Periodicals Press. All rights reserved.