Weathering resistance of thin plasma polymer films on pre-coated steel

O trabalho apresentado teve origem no projecto de investigação “Tailored Thin Plasma Polymers Films for Surface Engineering of Coil Coated Steel”, financiado pelo Programa Europeu ECSC Steel Research. Sistemas de aço galvanizado pré-pintado em banda à base de poliéster e poliuretano foram submetidos a um processo de polimerização por plasma onde um filme fino foi depositado de modo a modificar as propriedades de superfície. Foram usados reactores de cátodo oco, microondas e rádio frequência para a deposição do polímero fino. Os sistemas preparados foram analisados de modo a verificar a influência do processo de polimerização por plasma na alteração das propriedades barreira dos sistemas pré-pintados em banda. Foi estudado o efeito dos diferentes passos do processo de polimerização por plasma, bem como o efeito de diferentes variáveis operatórias. A mistura precursora foi variada de modo a modificar as propriedades da superfície de modo a poder vir a obter maior hidrofobicidade, maior resistência a marcas digitais, bem como maior facilidade de limpeza. Os testes foram conduzidos em solução de NaCl 0,5 M. Para o trabalho foram usadas técnicas de análise da morfologia da superfície como Microscopia de Força Atómica e Microscopia Electrónica de Varrimento. As propriedades electroquímicas dos sistemas foram estudadas por Espectroscopia de Impedância Electroquímica. A estrutura dos filmes gerados no processo de polimerização por plasma foi caracterizada por Microscopia de Transmissão Electrónica. A modificação das propriedades ópticas devido ao processo de polimerização por plasma foi também obtida.

Asymmetric rolling of 5182 aluminium alloy and interstitial free steel sheets

This Ph.D. research focuses on asymmetric rolling (ASR), as an alternative method for improving mechanical responses of aluminium-magnesium alloy and interstitial free (IF) steel regarding industrial requirements. Aluminium alloys are attractive materials in various industries due to their appropriate properties such as low density and corrosion resistance; however, their low formability has limited their applications. As formability of aluminium alloys can be improved through texture development, part of this dissertation is dedicated to producing the desired crystallographic texture with the ASR process. Two types of ASR (i.e. reverse and continuous asymmetric rolling) were investigated. The impact of shear deformation imposed by ASR processes on developing the desirable texture and consequently on mechanical behaviours was observed. The developed shear texture increased the normal and also planar anisotropy. Texture evolution during plastic deformation as well as induced mechanical behaviour were simulated using the “self-consistent” and Taylor models. Interstitial free (IF) steel was the second material selected in this dissertation. Since IF steel is one of the most often used materials in automotive industries it was chosen to investigate the effect of shear deformation through ASR on its properties. Two types of reverse and continuous asymmetric rolling were carried out to deform IF steel sheets. The results of optical microscopy and atomic force microscopy observations showed no significant difference between the grains’ morphology of asymmetric and conventionally rolled samples, whereas the obtained results of transmission electron microscopy indicated that fine and equiaxed dislocation cells were formed through the asymmetric rolling process. This structure is due to imposed shear deformation during the ASR process. Furthermore, the mechanical behaviour of deformed and annealed sheets was evaluated through uniaxial tensile tests. Results showed that at low thickness reductions (18%) the asymmetric rolled sample presented higher stress than that of the conventionally rolled sheet; while for higher thickness reductions (60%) the trend was reversed. The texture analyses indicated that intense rolling texture components which developed through 60% thickness reduction of conventional rolling cause a relatively higher stress; on the contrary the fine structure resulting from ASR appears to be the source of higher stress observed after pre-deformation of 18%.