Microstructure evolution of Ti-based and Cr cathodes during arc discharging and its impact on coating growth
Abstract: This thesis explores the microstructure evolution of cathodes with various material compositions and grain sizes during cathodic arc evaporation processes as well as the impact on the arc movement, and the microstructure and properties of the deposited nitride coatings. The studied cathode material systems include conventionally metal forged Ti and Ti -Si cathodes, novel Ti3SiC2 MAX-phase cathodes, and dedicatedly designed powder-metallurgical Ti-Si and Cr cathodes with different grain size. The microstructure and chemical composition of the virgin and arced cathodes together with the microstructure and mechanical properties of the deposited coatings were analyzed with various characterization techniques, including x-ray diffractometry, x-ray photoelectron spectroscopy, elastic recoil detection analysis, scanning electron microscopy, focused ion beam sample preparation technique, transmission electron microscopy, energy dispersive x-ray, electron energy loss spectroscopy, and nanoindentation.In general, a converted layer forms on the cathode surfaces during cathodic arc evaporation. The thickness, the microstructure and the chemical composition of such layer are dependent on the composition and the grain size of the virgin cathodes, the nitrogen pressure, and the cathode fabrication methods.For Ti based materials, the converted layer is 5-12 ?m thick and consists of nanosized nitrided grains caused by the high reactivity of Ti to the ambient nitrogen gas. In comparison, the Cr cathode is covered with a 10-15 ?m converted layer with micrometer/sub-micrometer sized grains. Only very limited amounts of nitrogen are detected within the layer due to the low reactivity of Cr to nitrogen.For Ti-Si cathodes, the existence of multiple phases of Ti and Ti5Si3 with different work function renders preferential arc erosion on the Ti5Si3 phase during discharging. The preferential erosion generates higher roughness of the Ti-Si cathode surface compared with Ti. By increasing the grain size of the virgin Ti-Si cathodes from ~8 ?m to ~620 ?m, the average roughness increases from 1.94±0.13 ?m to 91±14 ?m due to the amplified impact of preferential erosion of the enlarged Ti5Si3 grains. The variation of the preferential erosion affects the arc movement, the deposition rate, and the macroparticle distribution of the deposited Ti-Si-N coatings.A novel Ti3SiC2 MAX phase is used as cathode material for the growth of Ti-Si-C-N coating. During arcing, the cathode surface forms a converted layer with two sublayers, consisting of a several-micrometer region with a molten and resolidified microstructure followed by a region with a decomposed microstructure. The microstructure and hardness of the deposited Ti -Si-C-N coatings is highly dependent on the wide range of coating compositions attained. In the coatings with abundance of N, the combined presence of Si and C strongly disturbs cubic phase growth and compromises their mechanical strength. At a nitrogen pressure of 0.25-0.5 Pa, 45-50 GPa superhard (Ti,Si)(C,N) coatings with a nanocrystalline feathered structure were obtained.By increasing the grain size of the elemental Cr cathodes from ~10 ?m and ~300 ?m, the grain structure of the converted layer on the cathode surface varies from equiaxed grains to laminated grains after evaporating in a nitrogen atmosphere. When evaporated with a stationary fixture, the worn Cr cathode surface contains an organized pattern of deep ditches in the surface. The formation of such patterns is enhanced by increasing the cathode grain size. The fixture movement, which is either stationary or single rotating, affects the phase composition, the droplet density and the microstructure of the deposited Cr-N coatings, which consequently determines the mechanical properties of the coatings.
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