Maurice Gonon

A polysilazane precursor for Si-C-N-O matrix composites

Abstract This paper reports the ability to use a commercial polysilazane † as matrix precursor for the manufacture of ceramic matrix composites. By pyrolysis under nitrogen in the temperature range 1000–1400 °C this polysilazane leads to an amorphous silicon carbonitride with a ceramic yield around 65 wt%. The rheological behaviour and the thermal decomposition of the polysilazane have been studied. Parameters for the impregnation and pyrolysis cycles required by the manufacture of composites are also proposed.

Manufacture of monolithic ceramic bodies from polysilazane precursor

Abstract A low temperature process for the manufacture of non-oxide monolithic ceramic bodies is presented. This process is based on the pyrolysis at temperatures between 1000 and 1400 °C of the polysilazane precursor. The different steps of the process are precisely described. The physical (porosity, density, crystalline phases) and mechanical properties (flexural strength, Youngs modulus, toughness and hardness) of the bodies obtained are studied versus the maximum temperature of the pyrolysis. The high temperature oxidation resistance in air is also presented.

Comparison of Two Processes for Manufacturing Ceramic Matrix Composites from Organometallic Precursors

Abstract A commercial polysilazane is used as a silicon carbonitride matrix precursor for the manufacture of ceramic matrix composites using bi-directional SiC Nicalon fabrics as reinforcing material. The objective is to develop a simple and fast process leading to materials able to compete with SiC/C/SiC composites obtained by the Chemical Vapour Infiltration (CVI) route. Two processes are investigated: (1) a ‘conventional’ process using the densification of a SiC fibre preform by several cycles of impregnation of the preform with the polymer followed by pyrolysis and (2) a ‘modified’ process consisting in a powder filling of the fibre preform prior to the precursor impregnation and pyrolysis. This paper describes the different steps of both processes. The materials obtained are characterised in terms of their porosity, microstructure and mechanical properties. ©

Determination and refinement of the crystal structure of M2SiAlO5N “B-phase” (M=Y, Er, Yb)

Abstract The crystal structure of the compound M 2 SiAlO 5 N “B-phase” (with M=Y, Yb, Er) is determined by Rietveld analysis of X-ray powder diffraction patterns. The pseudo α-wollastonite derived structure, often proposed in the literature [D.P. Thompson, The crystal chemistry of nitrogen ceramics, Materials Science Forum 47 (1989) 21–42], is used as a model for simulation of an X-ray diffraction pattern. The simulated pattern obtained with this model exhibits several peaks that are not observed on the experimental pattern. Moreover, when refining this model by Rietveld analysis, the final atom co-ordinates are strongly shifted with respect to their initial values and lead to aberrant bond lengths. A new model of the structure of B-phase is proposed: after refinement, the final reliability factors show that this new model is in very good agreement with experiments. According to this new model, the alternative layers of yttrium cations and (Si,Al)(O,N) 4 tetrahedra proposed in the pseudo α-wollastonite model is confirmed but the tetrahedra do not form rings as was initially suggested, but instead are randomly linked to each other.

Association of the CVI process and of the use of polysilazane precursor for the elaboration of ceramic matrix composites reinforced by continuous fibres

Abstract A new process route for ceramic composites, associating CVI of SiC and impregnation and pyrolysis cycles with a polysilazane precursor, has been developed. The interest of this process is, firstly, a shorter time of elaboration than for CVI alone and, secondly higher mechanical properties than those resulting from a conventional use of the precursor. Composites with mechanical properties comparable to those of composites entirely obtained by CVI of SiC have been realised. The flexual strength of these materials is about 400 MPa, whereas that of composites resulting from a conventional use of the same polysilazane does not exceed 200 MPa.