An investigation into the effect of process parameters on carbon film physical properties produced by different deposition techniques
This thesis reports research to investigate the effect of process parameters on the physical properties of carbon films produced using different deposition techniques. The deposition techniques utilised are a FAB source, an r.f./d.c. EBPVD system and a hot filament CVD system. The findings of this study indicate that the films produced by the FAB source technique are relatively hard (-2000 HK at 25g). Occasionally some films showed a hardness up to 8000HK due, in part, to elastic recovery during testing. FAB source films are transparent in the infrared, however they suffer from a relatively poor transmission in the visible. The optical band gap is about 1eV. The refractive index is around 2.2 and their hydrogen content is low. When the carbon content in the source gas was reduced by mixing hydrogen with the hydrocarbon used, the transparency, the content of bonded hydrogen in the films and the optical band gap increased slightly but the hardness, adhesion, refractive index and deposition rate decreased. The impact energy has a dominant influence on the film properties. This has been demonstrated by changing the sample orientation.@ FAB source films have good wear and friction properties with only a small sign of localised failure after pin on disc sliding tests for 100 metres. A study on titanium nitride/DLC multilayer films showed that the adhesion at the TiN/DLC interface can be improved (and therefore the wear performance). The best adhesion and wear results were obtained by utilising an over-stoichiometric composition of titanium nitride. FAB source films have been deposited on PET and were shown to be as good as that of sputtered gold on PET, and much superior to evaporated titanium on PET. The transparency of the coated PET has been improved by optimising the deposition conditions. The properties of films grown by plasma-assisted evaporation of graphite depends on the carrier gas used, substrate type and bias voltage (under both d.c and r.f. conditions). Films grown on tool steel in a butane r.f. plasma have a hardness around 4000 HK (25g). The growth rate on glass under similar conditions is very low, but the film is completely transparent with an optical band gap of 2.6 eV. When argon is used as the carrier gas, no film is formed on tool steel, but on glass a completely transparent film is produced. Nanohardness measurement showed that r.f EBPVD films are extremely hard (33-44 GPa) and the lowest elastic recovery was obtained for the non-hydrogenated film (11% against 44% for the a-C:H film). Pin on disc tests of r.f. a-C:H films exhibited a typical film wear of 48x 10⁻⁶mm3/Nm against steel, which represents a modest reduction in wear compared to an uncoated steel substrate. All d.c. EBPVD films showed poor performance in the pin on disc machine due to their poor adhesion. The concentration of bonded hydrogen (αs) in d.c. films is up to five times more than that of FAB source films. Their relatively low refractive index reflects a polymeric type of film. They were more brittle and usually less well adhered than those produced by other methods. Their optical band gap varied between 1eV to 3.5eV. The concentration of total hydrogen (bonded or eventually non-bonded) has been found by coworkers to be 33% in dc EBPVD films and 25% in FAB source films. The decrease of the concentration of bonded hydrogen with cathode voltage and the absence of such correlation for the total hydrogen in dc/EBPVD an FAB source films, suggests the presence of nonbonded hydrogen in the films. One of the achievements of the work is that an empirical relationship is developed, linking the optical band gap with the amount of bonded hydrogen in the film, the relative carbon to hydrogen flow input ratio, and other plasma electrical parameters for carbon coatings produced using a thermionically enhanced plasma-assisted d.c. PVD process. It explains observed properties in terms of process parameters and highlights the difficulties in obtaining consistent coatings on sample surfaces at different locations and orientations in the deposition chamber. Finally a new system is described for the deposition of hard carbon films. This new system allows the generation of very high ionisation levels and produces films with hardnesses over 8000 HK (200g) (thickness - 2.5μm,substrate; tool steel). The deposition rate is higher than previously reported for such hard films (2-3μm/hr) in a 0.5-1% CH₄ in H₂ plasma. SEM microscopy showed particles having octahedral shapes. Raman spectroscopy indicates a OLC structure with some disordered graphite present. Infrared spectrophotometry showed very little evidence of bonded hydrogen in the films. The optical band gap is 2 eV for a film deposited on silica.