Nowadays, the civil aeronautic industry is subjected to many standards aiming to the reduction of its environmental impact. This leads industries to elaborate new materials, lighter and more efficient, to replace for example parts of the turbojets which are currently made of nickel based super-alloys. Thermostructural composites, and more specifically ceramic matrix composites (CMC), are promising materials in this domain due to their low density, mechanical properties and their resistance to oxidation in a wide range of temperatures. The present work has been focused on the optimisation of a new matrix material by an innovating elaboration process, to be able to reach temperature in use greater than 1450°C. 3D woven preforms made with Hi-Nicalon S fibres were used. Depending on the porosity size distribution, filling by submicron powders suction required a powder suspension with a controlled particle size range. Wet ball milling led to particle size between 0.6 and 0.8 µm, which allowed the filling of 50 to 55% of the initial porosities with a 18 vol% charged suspension. Then, the preforms were densified by a pressurised heat treatment. Heating rate and pressure were controlled by self-regulation programs. The synthesis reaction has also been controlled and monitored by an acquisition program of the temperature at the surface of the material. The different characterization techniques that have been used, allowed a better understanding of the physical and chemical properties of the prepared samples, and thus a study of the influences of the manufacturing parameters. Scanning electron microscopy (SEM) observations showed that the fibres were not damaged despite the high temperature during treatments. Moreover, a good cohesion between matrix and consolidation has been observed. They also revealed that for low heating rate, non-reaction zones in the matrix could be seen. At constant pressure, different heating rates were tested. The X-ray diffraction (XRD) analysis showed an increase in the mass content of the desired compound while heating rate was increasing. Thermograms recorded during treatments showed three phenomena linked to the pressure variation:-Greater the pressure, lower the initiation temperature of the reaction -The range of the exothermal peak is proportional to the gas pressure-Greater the pressure, the more intense and quicker the reactionFinally, an optimal combination of pressure and heating rate leading to the synthesis of the desired matrix compound could have been determined to achieve a 95% in mass in function of the preform type. Mechanical characterisation and wet oxidation aging are still to be carried out to have a better understanding of the behaviour of the composite materials that have been made.