Oxidation behaviour of a pressureless sintered HfB2–MoSi2 composite

The thermal stability of a 80-vol.% HfB₂ + 20 vol.% MoSi₂ composite is tested under oxidizing environment. Oxidation tests are carried out in flowing synthetic air in a TG equipment from 1000 to 1400 °C with exposure time of 30 h. At temperatures ≥1200 °C the silica resulting from oxidation of molybdenum disilicide seals the sample surface, preventing hafnium diboride from fast degradation. Analysis of the kinetics is carried out through fitting of the thermogravimetric curves. Between 1200 and 1400 °C, the kinetic curves deviate from a parabolic behaviour, being more close to a logarithmic–parabolic behaviour.     

Oxidation Resistance of Alumina–Titanium Nitride and Alumina–Titanium Carbide Composites

The presence of TiC or TiN paritcles in an Al₂O₃ matrix affects the thermal stability of the composites in oxidizing environments. In isothermic oxidation tests at 700°, 800°, 900°, 1000°, and 1100°C for up to 20 h, two different oxidation regimes have been observed at T < 900°C and at 900°C ≤ T ≤ 1100°C. At low temperatures ( T < 900°C), the oxidation follows a phase-boundary reaction; the reaction product initially consists of aggregates of submicrometer needlelike TiO₂ rutile crystals that subsequently grow and coalesce. When a continuous TiO₂ rutile layer is formed ( T ≥ 900°C), the oxidation kinetics change to parabolic, and the diffusion of O₂ through a thick TiO₂ layer is proposed as the governing step.     

Factors influencing structural evolution in the oxide of hot-pressed Si3N4-Y2O3-SiO2 materials

X-ray diffraction, SEM, EDS and WDS studies were performed of the structure and morphology of the oxide developed on hot-pressed compositions of the Si₃N₄-Si₂N₂O-Y₂Si₂O₇ sub-system subjected to short-term (∼30 h) oxidation in air atT=1273 to 1673 K. Viscosity of the oxide glassy phase, ionic mobility and oversaturation degree of the atomic species involved were found to be the most important parameters in the control of structural evolution of the oxide. Clear links appear to exist between compositional parameters of the grain-boundary phase of the hot-pressed materials and the evolution of the crystal phases in the oxide. The ΣM_i/Y ratio (ΣM_i, total amount of metal impurities) in the grain-boundary phase, seems to play a major role in the transport properties of the oxide glassy phase.     

Sintering and Characterization of TiB2-B4C-Zr02 Composites

High density titanium diboride based composites have been densified through hot pressing. 10 vol.% B₄C + 10 vol.% 2Y-ZrO₂ have been used as reinforcing phases. Several properties, including fracture strength, fracture toughness, hardness, Young's modulus, thermal expansion and electrical resistivity were measured. Resulting data were correlated with sample microstructure and composition and were compared to the base-line matrix material.     

Fabrication and characterization of polymer-derived Si2N2O-ZrO2 nanocomposite ceramics

Amorphous Si-Zr-N-O powders, obtained by nitridation in an NH₃ flow of zirconium modified polycarbosilane, have been sintered to full density by hot pressing at 1500C. The resulting ceramic shows an extremely fine-grained microstructure composed of Si₂N₂O and ZrO₂ crystallites 20–30 nm in diameter. Thermal stability measured in air appears excellent up to 1300C for 48 h. Mechanical characterization pointed out good values of flexural strength (330 MPa), fracture toughness (4.1 MPam^0.5) and Weibull modulus.     

Oxidation of monolithic TiB2 and of Al2O3-TiB2 composite

In view of the susceptibility of TiB₂ to oxidation, the thermal stability of monolithic TiB₂ and Al₂O₃-TiB₂ composite was investigated. The temperature at which TiB₂ ceramic starts to oxidize is about 400°C, oxidation kinetic being controlled by diffusion up toT900°C and in the first stage of the oxidation at 1000 and 1100°C (up to 800 and 500 min, respectively), and by a linear law at higher temperatures and longer periods. Weight gains of Al₂O₃-TiB₂ composite can be detected only at temperatures above 700°C and the rate-governing step of the oxidation reaction is characterized by a one-dimensional diffusion mechanism atT=700 and 800°C and by two-dimensional diffusion at higher temperatures. The composition and morphology of the oxidized surfaces were analysed.