On the CVD of MoSi2:: an experimental study from the MoCl4-SiCl4-H2-Ar precursor with a view to the preparation of C/MoSi2/SiC and SiC/MoSi2/SiC microcomposites

The chemical vapour deposition of MoSi2 on plane substrates (graphite or sintered-SiC) and ceramic fibres has been studied from MoCl4SiCl4-H-2-Ar gas mixtures at 900 < T < 1400 degrees C and 2 < P< 40 kPa, according to an experimental approach. MoSi2 is deposited as single phase coatings for 2.5 < alpha = PSiCl4/PMoCl4 < 10 and 10 < beta = P-H2/(P-MoCl4 + P-SiCl4) < 20. The deposition process appears to be thermally activated, the thermal variations of the growth rate obeying one or two Arrhenius law(s) depending on P. It seems to remain rate controlled by heterogeneous surface reactions as long as the gas flow-rate is high enough. Deposits with a smooth surface aspect and homogeneous in thickness are obtained at low T, low P and high H-2-dilution. Nicalon/MoSi2/SiC and C(T300)/MoSi2/SiC microcomposites are prepared and tested in tensile loading at ambient and high temperatures. They exhibit a brittle or quasi-brittle mechanical behaviour, with no (or almost no) matrix microcracking before failure. It is anticipated that dense MoSi2 deposited by CVD may not be a suitable interphase material for SiC/SiC and C/SiC composite materials. (C) 1998 Kluwer Academic Publishers.