Orientador: Fernando Alvarez / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-11T05:33:37Z (GMT). No. of bitstreams: 1 Basso_RodrigoLeonardodeOliveira_D.pdf: 34307948 bytes, checksum: fd2817ddc7e94a48942bf4136fdb0472 (MD5) Previous issue date: 2007 / Resumo: Este trabalho apresenta um estudo sobre os efeitos da incorporação de carbono e nitrogênio na formação de uma camada monofásica, compacta e homogênea, composta por carbonitretos do tipo e -Fe2-3(C,N), na superfície da liga metálica AISI H13. Também são apresentados estudos sobre os efeitos da incorporação desses elementos nas propriedades mecânicas, químicas e na microestrutura da superfície da liga metálica. A incorporação desses elementos na superfície do material foi feita utilizando-se o tratamento termoquímico de nitrocarburização por plasma pulsado em atmosferas contendo N2+ H2+ CH4 em diferentes proporções. Além da composição da atmosfera de tratamento, também foram variadas a temperatura e o tempo de tratamento. De maneira a complementar o estudo do comportamento da liga metálica frente ao tratamento termoquímico a plasma, também foram preparadas amostras com atmosferas oxidantes contendo CO2. A análise da composição química da superfície das amostras foi feita através de espectroscopia de fotoelétrons e mostra que a incorporação de nitrogênio sofre grande influência da quantidade de gás contendo carbono, presente no plasma. Porém a incorporação desses elementos não é diretamente proporcional à concentração do gás contendo esse elemento. Ao contrário do esperado, aumentando-se suficientemente a concentração desses elementos na atmosfera de tratamento, não acarreta em aumento na concentração dos mesmos no material. Por outro lado a análise através de difração de raios X revelou a formação de estruturas cristalinas consistentes com o diagrama de fases do sistema Fe-C-N. Para menores concentrações de compostos de carbono no plasma, a formação de nitretos e carbonitretos é predominante enquanto que maiores quantidades desse elemento levam ao surgimento da fase cementita e -Fe3 C, indesejável por possuir baixa resistência mecânica. As propriedades mecânicas da superfície foram estudadas através de medidas de nano e micro-dureza e seus resultados indicam que, como esperado, amostras com maior quantidade de nitrogênio incorporada possuim maior dureza da superfície. Esse comportamento é condizente com a microestrutura observada por microscopia eletrônica de varredura que mostra o entrelaçamento dos grãos da estrutura do aço e o preenchimento de seus contornos com carbonitretos de ferro que dificultam a movimentação dos planos cristalinos do cristal aumentando sua dureza. Foi verificado que as concentrações de carbono e de nitrogênio contido no material exercem grande influência sobre a reatividade da superfície das amostras frente a processos corrosivos em meios contendo íons cloreto. Esse comportamento foi verificado através de medidas de corrosão em solução aquosa de NaCl a 0,9 vol.%. Novamente os resultados apontam que maior concentração de nitrogênio é responsável por conferir a superfície maior proteção contra corrosão. Esses resultados são atribuídos a presença de uma mono-fase compacta formada pela fase e - Fe2-3 (C,N) na superfície das amostras. Resultados menos significativos foram obtidos quando a superfície continha uma mistura de fases contendo g -Fe4 N, e -Fe2-3 N, a -Fe e CrN / Abstract: This work presents a study on the effect of the incorporation of carbon and nitrogen on he formation of a singlephase homogeneous layer, composed by e -Fe2-3(C,N) carbonitrides, on the top surface of a AISI H13 tool steel. We also present studies on the effect of the incorporation of these elements (C and N) in the microstructure and in the mechanical and chemical properties of the surface of the metallic alloy. The incorporation of these elements in the surface of the material was made using the plasma nitrocarburizing termochemical treatment in atmospheres containing N2 + H2+CH4 in different ratios. Beyond the composition of the treatment atmosphere, the temperature and the time of treatment had been also varied. In way to complement the study of the behavior of the metallic alloy we had been also prepared samples with oxidating atmospheres containing CO2. The chemical composition analysis of the sample¿s surface was made through X-ray photo-electron spectroscopy and pointed that nitrogen incorporation suffers great influence from the amount of gas containing carbon, on the plasma atmosphere. However the incorporation of these elements is not directly proportional to the concentration of the gas containing this element. In con-trast, increasing enough the concentration of these elements in the treatment atmosphere, it does not cause increase in the concentration of the same element in the material¿s surface. On the other hand the X-ray diffraction analysis showed the formation of crystalline structures, consistent with the phase diagram of Fe-C-N system. For lesser carbon concentrations in the plasma, the formation of nitrides and carbides are predominant whereas bigger amounts of this element lead to the formarion of the cementite phase (q -Fe3 C , undesirable for possessing low resistance mechanics). The mechanical properties of the surface had been studied by means of nano and microhardness and its results indicate that, as expected, samples with bigger amount of incorporated nitrogen possesses bigger hardness in its surface. This behavior is in agreement with the observed microstructure observed by scanning electron microscopy that show the interlacement of the grains of the steel structure and the fulfilling of its boundaries with iron carbonitrides that make it difficult the movement of the crystalline plans of the lattice increasing its hardness. It was verified that the nitrogen and carbon concentrations in the material¿s surface, are of great influence on the reactivity of the surface regarding to corrosive processes in clorine solutions. This behavior was verified by measurements of corrosion in NaCl aqueous solution. Again the results pointed that bigger nitrogen concentration is responsible for protecting the surface against corrosion process. These results are attributed to the presence of a compact and homogeneous e -Fe2-3(C,N) surface layer on the surface of the samples. Less significant results had been gotten when the surface contained a mixture of phases containing g -F e4N, e -Fe2-3 N, a -Fe e CrN / Doutorado / Física da Matéria Condensada / Doutor em Ciências

Low-temperature plasma nitrocarburizing treatments are applied to improve the surface properties of austenitic stainless steels by forming an expanded austenite layer without impairing the excellent corrosion resistance of the steel. Here, low-temperature active screen plasma nitrocarburizing (ASPNC) was investigated in an industrial-scale cold-wall reactor to compare the effects of two active screen materials: (i) a steel active screen with the addition of methane as a gaseous carbon-containing precursor and (ii) an active screen made of carbon-fibre-reinforced carbon (CFC) as a solid carbon precursor. By using both active screen materials, ASPNC treatments at variable plasma conditions were conducted using AISI 316L. Moreover, insight into the plasma-chemical composition of the H2-N2 plasma for both active screen materials was gained by laser absorption spectroscopy (LAS) combined with optical emission spectroscopy (OES). It was found that, in the case of a CFC active screen in a biased condition, the thickness of the nitrogen-expanded austenite layer increased, while the thickness of the carbon-expanded austenite layer decreased compared to the non-biased condition, in which the nitrogen- and carbon-expanded austenite layers had comparable thicknesses. Furthermore, the crucial role of biasing the workload to produce a thick and homogeneous expanded austenite layer by using a steel active screen was validated.

The effect of a controlled oxygen admixture to a plasma nitrocarburizing process using active screen technology and an active screen made of carbon was investigated to control the carburizing potential within the plasma-assisted process. Laser absorption spectroscopy was used to determine the resulting process gas composition at different levels of oxygen admixture using O2 and CO2, respectively, as well as the long-term trends of the concentration of major reaction products over the duration of a material treatment of ARMCO® iron. The short-term studies of the resulting process gas composition, as a function of oxygen addition to the process feed gases N2 and H2, showed that a stepwise increase in oxygen addition led to the formation of oxygen-containing species, such as CO, CO2, and H2O, and to a significant decrease in the concentrations of hydrocarbons and HCN. Despite increased oxygen concentration within the process gas, no oxygen enrichment was observed in the compound layer of ARMCO® iron; however, the diffusion depth of nitrogen and carbon increased significantly. Increasing the local nitrogen concentration changed the stoichiometry of the ε-Fe3(N,C)1+x phase in the compound layer and opens up additional degrees of freedom for improved process control.

Active screen plasma nitrocarburizing (ASPNC) increases the surface hardness and lifetime of austenitic stainless steel without deteriorating its corrosion resistance. Using an active screen made of carbon opens up new technological possibilities that have not been exploited to date. In this study, the effect of screen power variation without bias application on resulting concentrations of process gas species and surface modification of AISI 316L steel was studied. The concentrations of gas species (e.g., HCN, NH3, CH4, C2H2) were measured as functions of the active screen power and the feed gas composition at constant temperature using in situ infrared laser absorption spectroscopy. At constant precursor gas composition, the decrease in active screen power led to a decrease in both the concentrations of the detected molecules and the diffusion depths of nitrogen and carbon. Depending on the gas mixture, a threshold of the active screen power was found above which no changes in the expanded austenite layer thickness were measured. The use of a heating independent of the screen power offers an additional parameter for optimizing the ASPNC process in addition to changes in the feed gas composition and the bias power. In this way, an advanced process control can be established.