Citation

Heikinheimo E. Acta Polytechnica Scandinavica, Chemical Technology Series 1998; (256): X1-40.

Copyright

(Copyright © 1998)

DOI

unavailable

PMID

unavailable

Abstract

The solid state diffusion couple technique was used to study high-temperature chemical interactions in the systems Ti-Si, Ti-Al2O3, Al-SiC and Me-Si3N4 where Me = Mo, Ni, Fe. The reaction products were studied with light microscopy, x-ray diffraction, SEM + EDS and EPMA. The measured penetration curves in the metal were subjected to Matano-Boltzmann analysis for the determination of interdiffusion coefficients. Thermodynamic calculations were performed to predict the outcome of the interaction reactions. The experimental results were used to draw conclusions regarding the interaction mechanism. In the Ti-Si system, all the known suicides were seen in the diffusion couples. The formation sequence was as follows: TiSi2 and TiSi (< 1 h), Ti5Si4 (in 4 h), Ti3Si and Ti5Si3 (in 10 h). In the Ti-Al2O3 couples, aluminium and oxygen dissolved readily into titanium. In the quenched samples, there was an easily discernible crack running parallel to the Ti-Al2O3 interface in the Ti matrix, about 60 μm away from the interface. This crack coincided with an abrupt change in the oxygen content of Ti of about 5 at%, which agreed well with the α/β boundary in the Ti-O phase diagram. A partial isothermal cross-section of the Ti-Al-O ternary and the solubility of Al2O3 in β-Ti were calculated. In the ternary system Al-Si-C, all the corresponding binaries were calculated using available thermodynamic data. Isothermal cross sections of the ternary were calculated at 1273 and 2273 K. Measurements of Al-SiC couples annealed at 773...1273 K showed that in addition to the phases predicted by the thermodynamic analysis, ie. SiC, Al4C3 Si and Al(Si) solid solution, there was possibly present a ternary phase, the composition of which suggested 2Al4C3SiC, In the reaction between dense silicon nitride ceramic and the metals Mo, Ni and Fe, the extent of the reaction is controlled by a pressure build-up at the metal ceramic interface. Because of this, with Ni and Fe at 1373 K, the formation of brittle suicides is prevented. The reaction rate is determined by Me-Si interdiffusion in the solid solution. The reaction products away from the edges of the couple are independent of the surrounding atmosphere. The metalceramic-gas triple point effect was observed in the Mo-Si3N4 couples annealed at 1573 K, where transport of silicon took place via SiO gas, owing to low levels of oxygen present in the atmosphere. Oxide layer or sintering aid enrichment on the ceramic did not appear the hinder the metal-ceramic reaction. Interdiffusion D̃ in the Fe(Si) solid solution depends strongly on the Si concentration and on the degree of ordering. D̃ has a maximum at the composition Fe3Si, corresponding to the highest degree of ordering. This unexpected observation is probably caused by changes in the number of structural vacancies, but needs more experimental verification.

Language: en

Keywords

Aluminium; Aluminium oxide; Diffusion bonding; Diffusion couple; Iron; Metal-ceramic interaction; Molybdenum; Nickel; Silicon carbide; Silicon nitride; Thermodynamics; Titanium