GeO2/SiO2-glasses from gels to increase the oxidation resistance of porous silicon containing ceramics

Abstract GeO 2 /SiO 2 -glasses with relatively high expansion coefficients (between 5 and 7.3 × 10 −6 ° C −1 ) were prepared by hydrolysis of metal alkoxides (tetramethoxy silane+tetraethoxy germane as well as mixtures of the silane with GeO 2 -powder) with subsequent heat treatment up to 1000°C. It is shown that the preparation of thin films on glasses and ceramics is possible but also that this technique can be used for liquid infiltration of porous SiC- and Si 3 N 4 -materials to increase the oxidation behaviour in the temperature range 1000 to 1400°C. In the case of molybdenumdisilicide as a ceramic material it is shown that the catastrophic inner oxidation in the low temperature range (600 to 800°C) of porous specimens (tested with plasma-sprayed MoSi 2 -layers on refractory metals) can be hindered if these materials are infiltrated and heat treated.