The effect of silicon on thermal stability and wear behavior of diamond-like carbon films
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1. Introduction Diamond-like carbon (DLC) films have been studied extensively and also utilized industrially for their superior properties such as high hardness, low friction and wear, chemical inertness to acid and alkalis, optical transparency etc.. One of the greatest disadvantages of these films is that they cannot retain their superior characteristics at a higher temperature because of the conversion of sp 3 -bonded carbon to sp 2-bonded one, which limits the application of these films. The friction and wear characteristics of the DLC film were investigated for different test environments[ I], anneal treatment[2], contact load[3], sliding speed[4J and different sliding contact materials[5]. It was reported that the wear of DLC films is not directly related to the value of their friction coefficient[6] and indicated that various deposition conditions will lead to different wear life of films. Carbon atoms have both threefold and fourfold coordinated networks depending on the film composition while silicon atoms are arranged in a fourfold coordinated configuration. Introducing silicon to a-C:H to improve properties such as friction coefficient becomes interesting[7]. In this study, the effect of silicon addition on the thermal stability of DLC films in ambient air was studied. The pin-on-disk wear was used to investigate the wear behavior of DLC film coated SKD61 with a contact material of SUJ2 steel ball. 2. Experimental procedures The Si-containing diamond-like carbon films were deposited using a parallelplate capacitively coupled 13.56 MHz R.F. plasma system[9]. The substrates of SKD61 steel disks, silicon (100) and KBr disks of 1.5 X 1.5cm2 and 1cm in diameter respectively were chosed. The flow rates the feed gases of CH 4 and Ar were fixed at 60 sccm and 80 sccm respectively and the SiH 4 flow rate was changed from 0 to 3.0 sccm. The R. F. power was at 50W to 200W and the substrate temperature was 1I0O0 C 339 Mat. Aes. Soc. Symp. Proc. Vol. 555 ©1999 Materials Research Society
to 400'C to deposit the films for 2ltm of thickness. Some of these films were then annealed up to 600'C in ambient air for one hour, and the others were ready for wear test. The relative content of silicon used in this study was defined as [Si]/[Si]+[C]. Sliding tests were carried out using a pin-on-disk tribometer for DLC films coated SKD61 steel on a counterbody of SUJ2 steel ball of 5.9mm in diameter. The thickness of the films coated on SKD61 steel was 2.0tO.2 4m and the surface roughness (Ra) was 80_±20 nm. The contact forces were 30N and 40N. The constant sliding speed was 0.75m/s and the total sliding distance was 600m in an ambient air at room temperature(27 0 C) with relative humidity(RH) of 30-50%. 3. Results and discussion Thermal stability 3.1 FTIR analysis The FTIR transmittance spectra are shown in fig.1 which reveals the bond change of the silicon containing DLC coatings annealed at different temperatures. The CHn group in the wavenumber range from 2800 cm 1 to 3100 cm-' shows that by raising the annealing temperature
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