The role of water vapour on the oxidation of two Ln-Si-Al-O-N glasses (Ln=Y, La)

The oxidation behaviour of LnSiAlON (Ln=Y, La) glasses was studied at different temperatures (990-1150 °C) and under different water vapour pressures (360-2690 Pa). These results were also compared with those obtained under O₂, N₂/H₂O or O₂/H₂O mixtures. When glasses are treated under a N₂/H₂O mixture, optical and SEM observations show porous scales. Transformations of the reaction rate data and a kinetic model show that there is only one limiting process occurring during oxidation. This rate limiting step is the progress of the chemical reaction at the internal interface. Determination of the pressure law dependence and thermodynamics calculations of water vapour molecules dissociation at the investigated temperatures allow us to suggest that the mechanism of oxidation corresponds to decomposition of water molecules on the oxynitride glass surface.     

Sur les intégrales rationnelles de l'équation de Kummer

Sans résumé     

Mechanical properties of H. P. or H. I. P. SiC-whisker reinforced Si3N4-matrix composites

Abstract

Composites (SiCw/Si₃N₄m) were fabricated by H. P. or H. I. P. The mechanical properties of the materials (strength, toughness) show that H. I. P. is a suitable process for complex shapes.     

Oxidation Behaviour of A Boron Carbide Based Material in Dry and Wet Oxygen

The oxidation behaviour of a B₄C based material was investigated in a dry atmosphere O₂(20 vol.%)-CO₂(5 vol.%)-He and also in the presence of moisture H₂O (2.3 vol%) as boron oxide is very sensitive to water vapour. The mass changes of samples consisting of a chemical vapour deposit of B₄C on silicon nitride substrates were continuously monitored in the range 500–1000°C during isothermal experiments of 20 h. The stability of boron oxide formed by oxidation of B₄C was also studied in dry and wet atmospheres to explain the kinetic curves. In both atmospheres, oxidation is diffusion controlled at 700 and 800°C and enhanced by water vapour. At 900°C and higher temperatures, boron oxide volatilisation and consumption by reaction with water vapour modifies the properties of the oxide film and the material is no more protected. At 600°C, B₄C oxidation is weak but the process remains diffusion controlled in dry conditions as boron oxide volatilisation is negligible. However, in the presence of water vapour, B₂O₃ consumption rate is significant and mass losses corresponding to this consumption and to the combustion of the excess carbon are observed. This revised version was published online in August 2006 with corrections to the Cover Date.