Zirkonya esaslı refrakterlerin proses ve analizleri

During recent years, significant progress has been done in the development of engineering ceramic materials. A new generation of ceramics has been developed which are expected to find wide use in applications at high temperatures. Among these materials, zirconia (ZrO2) is an attractive candidate for high temperature applications because of its high melting point and excellent corrosion resistance. Unfortunately the tetragonal to monoclinic phase change is a martensitic transformation with a volume increase of about 3–5% so that if a component is cooled through the transformation temperature it becomes heavily microcracked, decreasing the Young modulus and the strength, although it increases the resistance to catastrophic failure. In this study some dopants were added to both monoclinic and tetragonal stabilized zirconia in order to mechanical properties improvement. Raw materials based on zirconia were studied to synthesize refractory material for metallurgical applications. Fused MgO stabilised Zirconia and chemicaly precipitated monoclinic zirconia were supplied and characterized using XRD, SEM and some mechanical tests. Fused MgO stabilised Zirconia powders with addtion of TiO2, SiC and Si3N4 were subjected to various test to determine the suitable doppant content for improving mechanical properties. Various mixtures with different composition were prepared in dry form. Particle size distribution is checked according to Dinger&Funk theory. Samples were pressed and sintered in air, nitrogen and argon atmosphere. The effect of TiO2, SiC and Si3N4 addition on the mechanical properties of monoclinic zirconia and tetragonal stabilized zirconia during the sintering and in-situ stabilization with dead burnt zirconia. The composition of monoclinic zirconia and MgO was prepared with 0, 5, 10, 15% TiO2, Si3N4 aor SiC addition by weight percent respectively. MgO content was kept unchanged. The powder with PVA as a binder was mixed and reduced in size by wet milling. Milling procedure was kept same in order not to change grain size then the powder was shaped by press with the same procedure as before. The samples were then sintered at 1400 and 1600˚C with the same procedure applied before for in-situ stabilization. After sintering; - XRD - SEM - Density and apparent porosity - Hardness - Bending strength - Fracture toughness As a conclusion; In zirconia containing Si3N4 specimens, monoclinic phase was decreased and tetragonal, cubic and zirconium oxide nitride phase were formed (β:Zr7O8N4). Decreasing densification specimens can be related to low powder density of silicon nitride (3.44 g/cm3). Flexural strength, hardness and fracture toughness values are increased by increasing Si3N4 content. XRD analysis revealed Mg2Zr5O12 structure and tetragonal phase formed only in 5 wt% TiO2 containing specimens. Doping TiO2 to zirconia was destabilized zirconia and titanium dioxide reacted with MgO. Density of all samples decreased with increasing of TiO2. This decreased density can be due to the grain growth and increasing amount of intergranular porosity. Hardness was reduced with increasing TiO2 content, and it can be explained by increasing amount of porosities. Zirconia containing 5 wt % TiO2 was showed relatively high flexural strength and fracture toughness at lower sintering temperature. Doping TiO2 and sintering at 1400°C without ageing process presented better mechanical properties for both monoclinic and tetragonal powder. Also increasing TiO2 over 5 wt% changed microstructure, where increased hardness, flexural strength and fracture toughness values. In the case of silicon carbide specimens, according to X-ray diffraction patterns and strength values sintering temperature was not enough high and specimens were not densificated.