Zirconia/stainless steel composites have been prepared by a wet processing method with metal volume concentration ranging from 15% to 30%. The composites were characterized by electrical and mechanical measurements. The dependence of the electrical properties of these composites with the metal concentration presents a percolative behaviour with a metal-insulator transition, in addition to an increment of the capacity in the neighbourhood of a critical volume concentration. This value was found to be f c = 0.285, which is much higher than the theoretical value for randomly dispersed 3D composites (f c = 0.16). It has been found that the incorporation of stainless steel particles to zirconia matrix, increases the toughness and decreases both the hardness and the flexural strength. The enhancement of toughness is attributed to a crack deflection mechanism as a consequence of a weak ZrO2/stainless steel interface.

Apresentam-se os estudos relacionados ao processamento de pós de Si3N4, sistematizando-se cada etapa dos processos de conformação por colagem e colagem sob pressão. A avaliação das propriedades eletroforéticas e do comportamento reológico possibilitou a preparação de suspensões aquosas concentradas de Si3N4 estáveis e homogêneas. Os estudos cinéticos foram realizados em função da pressão aplicada e dos aditivos de sinterização. As análises de superfície dos pós (XPS) demonstraram que com o controle dos parâmetros estudados é possível a obtenção de peças a verde de Si3N4 com baixa oxidação superficial. Produziu-se cerâmicas densas de Si3N4 (96%DT) com boas propriedades mecânicas (E = 312 GPa, K IC = 6,2 MPa.m1/2 ), tanto a temperatura ambiente como em altas temperaturas (1200 e 1300°C), compatíveis com os valores reportados na literatura.

A systematic study on Si3N4 slip processing is presented. Stable well dispersed aqueous suspensions with high solid contents can be prepared with an adequate control of the electrophoretic and rheological properties. The influence of these parameters on the kinetics of slip casting and pressure slip casting processes is studied. X-ray photoelectron spectroscopy (XPS) analysis has shown that the optimisation of processing conditions as for example liquid vehicle, pH and the characteristics of the deflocculant, allows the control of the oxidation level at the particle surface. After sintering, dense Si3N4 compacts (> 96% TD) are obtained with good mechanical properties ( E = 312 GPa, K IC = 6.6 MPa.m1/2) at room temperature as well as at high temperature (1200-1300°C), in the same range of values reported in the literature.

alpha- Si3N4 can easily oxidize when exposed in air atmosphere or other oxidizing environment. The chemistry of Si3N4 particle surfaces can change by different synthesis routes and also by subsequent processing steps. To avoid this oxidation is a challenge when Si3N4 powder is shaped by slip casting process from aqueous suspension. This study shows the variations on Si3N4 powder surface after slip casting process using x-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HTEM) on the green compacts. The results show that the dispersant used during the preparation of the slurry can reduce the level of oxidation of the Si3N4 particles when processing in aqueous solutions.