Mullite–ZrO2/Y2O3 Based Ceramics with Si3N4 Additive

The wide use of mullite ceramics can be explained by its observable high temperatura properties such as high thermal and pressure stability in harsh oxidizing environments, favorable thermal shock, low thermal expansion and thermal conductivity. This study is focused on modification of the structure/phase composition and properties of mullite-ZrO2/Y2O3 ceramics by Si3N4 additive 1 to 10 wt.% to show the effect on the thermal and mechanical properties, as well as corrosion stability to acidic and basic aqueous solutions at elevated temperatures. The starting mixtures of powders were prepared by planetary ball-milled process, obtained grain size of particles was in range 55-100nm. Samples were prepared as disks or cylinders and subjected to sintering in nitrogen atmosphere at temperature1400C as well as there spark plasma sintering were used. The sintering degree after firing was characterized by the density and shrinkage. Microstructure and phase composition of sintered samples was analysed using SEM (Nova Nano 650) and XRD (D8 Advance Bruker). The compressive strength was determined by Toni-technic model 2020, modulus of elasticity befor and after thermal shock - on the basis of the standard ASTM EN 1876-01. For determination of corrosion stability 1N NaOH- or H2SO4 solutions at temperature 60C were used. It was determined that microstructure of conventionally sintered at 1400oC ceramic samples with 1-5 wt.% Si3N4 additive is dense and is formed from mullite-corundum crystals with inclusions of ZrO2 cub. particles equally dispersed in mullite matrix. By increaing of Si3N4 additive to 5% as is shown on XRD the relative intensity of ZrO2 cub. peaks decrease. Density of samples conventionally sintered in nitrogen atmosphere and by SPS differs significantly. Density of ceramic samples compacted by spark plasma sintering is in the 3.25–3.55 g/cm3 range whereas for conventionally up to 2.5-2.9 g/cm3. There is shown that the Si3N4 additive not more than 5 wt.% to mullite-ZO2Y2O3 ceramics lead to formation of stable ZrO2 cubic phase by conventional sintering in nitrogen atmosphere at 1400C and sequential to growth of density and compressive strength. Values of compressive strength greatly is dependent on used sintering method. Indifferently these values is in range 350-390MPa for SPS sintered samples