V. A. Lavrenko

Influence of oxidation on the composition and structure of the surface layer of hot-pressed boron carbide

The oxidation of hot-pressed boron carbide under isothermal conditions and under conditions of programmed heating up to 1500°C was investigated. Oxidized samples were studied by secondary-ion mass spectrometry, X-ray photoelectron spectroscopy, scanning electron microscopy, energy-dispersive X-ray microanalysis, X-ray diffraction, and other methods. It has been demonstrated that oxidation starts above 600°C and results in the formation of a thin transparent B2O3 film that is cracked after cooling. Up to 1200°C, the oxidation process is limited by the diffusion of reagents through the oxide layer; at higher temperatures, it is determined by the rate of chemical reaction of carbide with oxygen in the air. During boron carbide oxidation the etching of grain boundaries occurs, it results in strength degradation at higher temperatures.

COMPLEX INVESTIGATION OF HOT-PRESSED BORON CARBIDE*

Abstract In the present work we investigate the thermomechanical properties and oxidation resistance of hot-pressed boron carbide over a wide temperature range: 20–1200 °C. It was demonstrated that oxidation reduced the strength of the material. The possibility of using boron carbide as a structural material for components of various high-temperature devices has been evaluated.

High temperature oxidation of silicon carbide based materials

The process of high temperature oxidation of two silicon carbide based materials differing by methods of their production and properties has been studied up to 1500°C in air. The oxidation was performed under the isothermal conditions and at the programmed heat rate of 10° per minute. It was found that the oxidation resistance of the material was the function of the presence of extrinsic metals having close affinity for oxygen. It was also found that under heating up to 1500°C in air phase transitions occurred in the SiC surface layer.

Formation of Oxide Films on the Surface of Electroconductive Si3N4-TiN and Si3N4-TiB2 Materials under Electrochemical Polarization in a 3% NaCl Solution

It has been established that during electrochemical polarization, in a 3 % NaCl solution at 20 o C, at a rate of 5 mV/s, a very thin (~ 5 nm) protective oxide film is formed on the surface of a Si3N4-TiN ceramic. This film is constituted of three layers containing TiN0.7O0.3 in the inner part, TiO2 in the outer part and a TiO phase in the intermediate one. In the case of a Si3N4-TiB2 ceramic, treated in the same conditions, a 7-8 nm more friable oxide film was formed, consisting of rutile TiO2, oxynitride Si2N2O and of an oxygen-silicon nitride solid solution.

Relationship between the oxidation resistance and the high-temperature strength of silicon carbide materials

ConclusionsOxidation is the reason for the decrease in the ultimate strength of the self-bonded silicon carbide at temperatures exceeding 1000°C.Recrystallized silicon carbide possesses high oxidation resistance and retains its initial strength up to 1400°C.

Kinetics and mechanism of oxidation of sialons

ConlusionsOxidation at temperatures of up to 1300°C has no effect on the lattice constants of sialons. At 1000°C crystalline silica layers form on the surfaces of specimens which offer little protection against oxidation. Oxidation at 1300°C leads to the formation of a liquid silicate phase, which increases the oxidation resistance of the materials. However, the oxidation of a material containing more than 6% Al2O3 is accompanied by the formation of a blistered, foamed oxide film which readily disintegrates under any mechanical action.

High-temperature oxidation of a silicon nitride constructional ceramic

ConclusionsAn account is given of an investigation into the high-temperature oxidation of a silicon nitride base constructional ceramic. It is shown that the mechanism of the oxidation process for this ceramic depends on temperature. The ceramic is superior in oxidation resistance to similar materials studied earlier.