Influence of Processing Conditions on the Microstructure and Mechanical Properties of Sintered Yttrium Oxides

High-purity (99.9%) yttrium oxide (Y₂O₃) powder has been consolidated, either by vacuum sintering (VS) or by hot isostatic pressing (HIP). As the dense polycrystalline ceramics contained point defects, mainly oxygen vacancies, oxidation treatment was applied at 1500°C for 2 h. The two materials were compared with respect to mechanical, elastic, and thermal properties, and this work demonstrated that the HIPed oxide had the best performance. The beneficial effect of the HIP technique was mainly due to the achievement of a slightly higher densiflcation at a much lower applied temperature. This technique prevented any grain coarsening and allowed us to obtain a more homogeneous structure. Consequently, improvements in microhardness, flexural strength, and thermal shock parameters were significant. Nevertheless, the elastic parameters and the toughness values of the two grades were very close to each other.     

High-Temperature Corrosion of AlN-Based Composite Ceramic in Air and in Combustion Products of Commercial Fuel. Part 1. Corrosion of Ceramic Composites in the AlN – SiC System in Air and in Combustion Products of Kerosene and Diesel Fuel

We have established that scale formed upon oxidation of ceramic composites in the AlN – SiC system in air at temperatures up to 1550°C contains mullite 3Al₂O₃·2SiO₂ as the major phase of the outer layer, which provides its high protective properties. The inner layer of the scale contains β-SiO₂, α-Al₂O₃, and a fairly small amount of the oxynitride Al₁₀N₈O₂. In dry air, even at 1500°C with a long holding time (50 h), material of 50 mass% AlN – 50 mass% SiC retains extremely high corrosion resistance. We have shown that upon prolonged (up to 240 h) oxidation of the indicated ceramic in the combustion atmosphere of S- and Na-containing fuels (kerosene and marine diesel fuel) at 1200-1300°C, along with mullite in the scale we see formation (due to reaction of β-cristobalite with Al₂O₃ and gaseous Na₂O and NaOH) of low-viscosity silicate glass Na₂SiO₃ and NaAlSiO₄. Together with impurity Fe₂O₃ and gaseous Na₂SO₄, it partially destroys the protective mullite layer and leads to degradation of the protective properties of the scale.     

High-Temperature Corrosion of AlN-Based Composite Ceramic in Air and in Combustion Products of Commercial Fuel. Part 2. Corrosion of Ceramic Composites in the AlN – SiC – TiB2 System in Air and in Combustion Products of Kerosene and Diesel Fuel

Corrosion resistance has been examined for composites in the AlN – SiC – TiB₂ ternary system. They have high corrosion resistance in air up to 1500°C because mullite and β-tialite are formed in the outer layer of scale. Their corrosion resistance substantially exceeds that of other ceramic materials because of the formation of the mullite phase on prolonged exposure (up to 200 h) to gaseous combustion products from kerosene and diesel fuel in the presence of sea salt at 900 and 1000°C. Also, AlN – SiC – TiB₂ ceramics have higher bending strength than does AlN – TiB₂ ceramic.     

Features of corrosion resistance of AlN–SiC ceramics in air up to 1600°C

The kinetics and mechanism of (80% AlN + 20% SiC) and (50% AlN + 50% SiC) powders and ceramics oxidation in air up to 1600°C were studied with the aid of TG, DTA, XRD, EPMA, SEM and metallographic analysis methods. The ceramics samples were obtained by hot pressing fine-dispersion AlN and SiC powders with an average particle size of 1 μm at 1800°C for 2 h. This ensures a fine-grain material structure with a uniform distribution of phase components. It was shown that in a nonisothermal regime, a three-stage oxidation mechanism takes place. It was established that the scale formed consists of three oxide layers. In the inner layer, Al₁₀N₈O₂ oxynitride and β-SiO₂ (cristobalite) phases were observed; in the intermediate layer, β-SiAlON was found for samples with a relatively low SiC content whereas -Al₂O₃ was present in samples with a greater SiC content. The outer layer contains 3Al₂O₃.2SiO₂ (mullite) as a main phase, the latter ensuring highly protective properties of the scale. The materials investigated can be considered as having extremely high resistance to corrosion up to 1550°C.     

Mechanical properties of hot isostatically pressed zirconium nitride materials

The influence of powder properties on the sintering behaviour, the microstructural development and the mechanical properties of hot isostatically pressed (HIPed) zirconium nitrides were investigated. The results show that the densification behaviour is dependent on the powder characteristics, more precisely the grain morphology and size, and oxygen, carbon and metallic impurity contents. The mechanical properties are controlled mainly by the amount of porosity and the presence of a complex intergranular phase in direct relation to the purity of the starting powder. The differences in the fracture strength and toughness between the two grades are perceptible. On the other hand, the thermal shock resistance to fracture initiation, as well as the elastic parameters of both dense materials, are similar.