REPOSITORY > RESULTS

Abstract

The scientific backgrounds of technologies involved in the production of headlamps for the automotive industry are elaborated in this thesis. The experimental work was performed using an industrial plasma reactor with a volume of approximately 5 m3. The reactor enables three different operations, i.e., plasma pretreatment of workpieces made from polymers, deposition of a thin metal film by evaporation, and deposition of a protective coating resembling poly(dimethylsiloxane) by plasma polymerization. The pretreatment removes any surface impurities and functionalises the polymer surfaces with polar functional groups. The surface finish is a result of the interaction of radicals with organic material, in particular, OH and O. Pretreatment enables the good adhesion of metal film on the polymer substrate due to the interface between the layers. The properties of the interface were determined by XPS and SIMS depth profiling. The metallisation is performed in ultrahigh vacuum conditions by evaporation of liquid metal. An appropriate range of parameters enables the deposition of aluminum films with a thickness of approximately 100 nm that are almost completely free from impurities as revealed by depth profiles. The aluminum film is then coated with a protective layer. Plasma polymerisation was used as the deposition technique using hexamethyl disiloxane (HMDSO) as the precursor at a pressure of several Pa, the residual atmosphere was composed predominantly of water vapor. Both XPS and SIMS depth profiling showed an appropriate interface between the metal film and protective coating as well as the composition and structure of the coating. The plasma parameters during deposition of the protective coating were measured systematically at various processing parameters by residual gas analysis (RGA) and optical emission spectroscopy (OES). The results of these complementary techniques enabled the determination of key reactions leading to the formation of coatings with the desired properties. Fragmentation of the precursor in plasma conditions was found rather insufficient as explained by an extremely low plasma density, in the order of 1014 m-3, which was elaborated from measured discharge parameters. A predominant reaction in plasma conditions was the subtraction of H atoms from the HMDSO and partial association of these atoms to hydrogen molecules by heterogeneous surface recombination. Fragmentation also caused the formation of CH radicals as determined by OES. The radicals recombined into stable molecules either in the plasma reactor or on the walls between the reactor and differentially pumped RGA. The major products of the CH association were light hydrocarbons as determined by RGA. The association was explained by surface reactions between adsorbed CH radicals and hydrogen atoms rather than by gas-phase reactions. The collision frequency in the range of investigated pressures was found to be too low to enable significant association in the gas phase. The association was probability rather large since neither XPS nor SIMS showed a measurable amount of pure hydrocarbons or hydrogenated carbon in the deposited films. The results of experimental work enabled faster deposition without reducing the performance of the protective coatings.