Production of Ti powders by calcio-thermic reduction of TiO2

Abstract

The process of densification and development of the microstructure of mullite–ZrO2/Y2O3 ceramics from mixture of Al2O3, SiO2, ZrO2 and Y2O3 by gradually adding of α–β Si3N4 nanopowder from 1 to 5 wt% by traditional and spark plasma sintering were investigated by means of differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and some ceramic and mechanical properties. The processes of DTA for all samples are characterised by a low-pitched endo-effect, when gradual mullite formation and noticeable densification at temperatures of 1200–1400 °C is started. It is testified by shrinkage and density both for traditionally and by SPS-sintered samples. The influence of the Si3N4 additive on the density characteristics is insignificant for both sintering cases. For SPS samples, the density reaches up to 3.33 g/cm3, while for traditionally sintered samples, the value is 2.55 g/cm3, and the compressive strength for SPS grows with Si3N4 additives, reaching 600 N/mm2. In the case of traditional sintering, it decreases to approximately 100 N/mm2. The basic microstructure of ceramic samples sintered in a traditional way and by SPS is created from mullite (or pseudo-mullite) crystalline formations with the incorporation of ZrO2 grains. The microstructure of ceramic samples sintered by SPS shows that mullite crystals are very densely arranged and they do not have the characteristic prismatic shape. The traditional sintering process causes the creation of voids in the microstructure, which, with an increasing amount of Si3N4 additive, are filled with mullite crystalline formations.