Paul Goursat

Pyrolysis of polyvinylsilazane precursors to silicon carbonitride

The pyrolysis of liquid or solid polyvinylsilazanes, cross-linked according to four different routes, was studied in the 20–1400°C temperature range. The similar behaviour and the high ceramic yields (⋍85%) are related to the reactivity of the functional groups (CHCH2, SiH, NH) which lead to highly cross-linked solids before the mineralization step. The major gases evolved above 450°C are hydrogen, methane and ethylene. During the thermal treatment SiN bonds are unaffected. The silicon carbonitride samples are dense and amorphous up to 1400°C. The thermal stability of the microtexture is due to the high carbon content.

Raman spectra of SiYAlON glasses and ceramics

Abstract Silicon nitrides are ceramics that exhibit excellent properties for application at high temperatures in advanced heat engines as well as in oxidizing environments. Their densification with Y2O3 and Al2O3 as sintering aids leads to a two-phase material, called β′-Sialon, containing β′-Si3N4 grains where Si and N are partially substituted by Al abd O, and a mainly glassy intergranular phase which controls the high temperature behaviour of these ceramics. Nitrogen glasses, with compositions close to that of the grain boundary phase, have been synthesized to study the intrinsic properties of this secondary phase. Raman spectra obtained from both SiYAlON ceramics and glasses are reported and compared. Raman scattering spectroscopy appears to be an appropriate technique to investigate crystallization of nitrogen glasses. Further, this technique is very sensitive to the occurrence of free silicon in silicon nitride based materials. Residual silicon, resulting from incomplete nitridation or from decomposition during processing, leads to a narrow and strong Raman band located at about 520 cm−1.

Superplastic Forming of an α-Phase Rich Silicon Nitride

The deformation behavior of fine-grained α-phase rich silicon nitride materials has been studied between 1550°C and 1615°C, both in compression and in tension. First, it is shown that higher the α-phase content, better the superplastic forming ability. A large tension-compression flow asymmetry was evidenced. For instance, shear-thickening flow shows up in compression whereas shear-thinning is observed in tension. Furthermore, much higher flow stresses and hardening rates are reported in compression than in tension. Elongation of more than 80% were achieved for strain rates between 2.5 and 5×10−5S−1. In the light of our results and of the abundant literature dealing with the high temperature deformation in silicon nitride, a sketch of the different deformation stages is proposed, which emphasizes the tension-compression flow asymmetry. Starting from the promising results obtained at the laboratory scale, the feasibility for net-shaping of a real part was demonstrated by hot-forging of a parabolic shell.

Neutron diffraction study of the amorphous phase structure in silicon carbonitride ceramics obtained by pyrolysis of a polyvinylsilazane

Abstract The atomic structure of silicon carbonitride ceramics derived from the pyrolysis of a polyvinylsilazane precursor has been studied by means of neutron diffraction. It is shown that the pair correlation function G(r) of the amorphous phase substituting up to 1400°C exhibits well defined peaks in the medium range order domain along the 0·3-2 nm range. This spectrum can be interpreted as a mixture of siliconcarbonitride phases with an average composition close to SiN 0·98 C 0·25 O 0·06 and 1·25 free carbon. The local chemical and structural orders vary continuously in the material from pure aromatic carbon to nanocrystals of Si 3 N 4 , through the ternary SiCN solid solution which is believed to have a structure intermediate between the amorphous and crystalline states.

Microstructure and mechanical behaviour of self-reinforced Si3N4 and Si3N4-SiC whisker composites

Abstract Monolithic Si3N4 and Si3N4-SiC whisker composites were fabricated by hot pressing or hot isostatic pressing. They were sintered in the 1600–1800°C temperature range with 6 wt% Y2O3 and 3 wt% Al2O3 as additives. Morphological aspects of whiskers were statistically determined by image analysis and different matrix microstructures were observed after chemical etching. Then, a correlation was established with mechanical properties. When the same sintering conditions are used, the composite rupture stress increases or decreases with respect to that of the corresponding monolithic Si3N4 matrices. The increase is attributed to an effective load transfer mechanism which involves stress concentration at fibre-matrix interfaces. These interfaces can become the new critical defects in the microstructure when the whiskers are too large. The resistance to short cracks was determined by indentation. The single edge precracked beam (SEPB) method allowed characterization of the resistance to long crack propagation. The toughness increases both with the aspect ratio of whiskers and/or elongated β-Si3N4 grains and with the precrack length (R-curve effect). The improvement is mainly due to the bridging of crack borders by acicular shapes or ligaments of unbroken matter. The R-curve corresponds to the enlargement of the active clamping zone as the crack extends. At a given precrack length the fracture toughness is strongly dependent upon the potential diameter of bridges, whereas the R-curve steepness rises with the density of clamping sites.

Carbon-fiber-reinforced YMAS glass-ceramic-matrix composites—IV. Thermal residual stresses and fiber/matrix interfaces

Unidirectional composites consisting of a glass-ceramic matrix reinforced with continuous carbon fibers, fabricated by hot-pressing, exhibit different fracture behavior according to the sintering thermal cycle. The thermal residual stresses in composites are determined from the mismatch in coefficients of thermal expansion and the thermomechanical characteristics of both fibers and matrix. An ultrasonic technique is used to determine the temperature at which, on cooling, residual stresses induce microcracking in the matrix. The influence of the mechanical stresses and physico-chemical reactions between the fibers and the matrix on the nature and the strength of the fiber/matrix interface is discussed.

Carbon-fibre-reinforced (YMAS) glass-ceramic matrix composites. I. Preparation, structure and fracture strength

Unidirectional continuous carbon-fibre-reinforced glass-ceramic matrix composites are fabricated for dry sliding applications. The microstructure of the matrix has been characterized by X-ray diffraction and the microtexture and structure of the fibres (pitch-based and PAN-based) have been studied by transmission electronic microscopy. The different fracture behaviours of the composites are described (three-point bend test and fracture surface observation by scanning electronic microscopy). Due to the thermal expansion mismatch between the fibres and the matrix, a network of microcracks appears in the composites on cooling after hot-pressing. The relationship between the microcrack spacing and the fracture behaviour suggests modifications of the fibre-matrix bond.

Fibre reinforced silicon nitride composites

Abstract Three possible processing routes to obtain silicon nitride reinforced with continuous fibres are identified and demonstrated. Slip-infiltrated, HIed carbon fibre reinforced material, slip-infiltrated, SiC fibre reinforced nitrided Si3N4, and polysilazane solution infiltrated pyrolysed composites with SiC fibres have been successfully fabricated. Possible fibre/matrix reactions are discussed on the basis of scanning electron microscopy observations and the bend fracture behaviour of the composites. All materials exhibited non-brittle fracture and are thus potentially interesting composites for further development.

Thermal behaviour of SiCN nanopowders issued from laser pyrolysis

Abstract The thermal behaviour of two SiCN nanopowders ( C N = 0.87 and C N = 0.22 ), issued from laser pyrolysis, is studied under two different atmospheres (He, N2) by means of thermogravimetry coupled with mass spectrometry, and X-ray diffraction. In the 25–1200 °C temperature range, the same behaviour in both helium and nitrogen is observed for the two SiCN nanopowders: very little weight loss occurs because of the evolution of adsorbed water, residual synthesis gases and also methane, hydrogen and carbon monoxide. However, above 1200 °C, the significant weight loss associated with the release of nitrogen and oxide species, under helium, is related to the decomposition process; whereas under nitrogen the weight gain results from the nitriding process. These two phenomena are more important for an amorphous powder. A comparison of the hotpressing of Si3N4 UBE and SiCN nanopowders with sintering aids (Y2O3, Al2O3) indicates that the α to β Si3N4 transformation rate is increased by the small grain size and by the presence of SiC in SiCN nanopowders.

Influence of microstructural changes on the oxidation resistance of silicon carbonitrides derived from a polyvinylsilazane

Abstract The behaviour of silicon carbonitrides derived from a polyvinylsilazane (ViSiHNH)n and obtained under different conditions of pyrolysis is investigated in oxidizing atmospheres. Various experimental techniques (TGA, MS, microprobe and XRD) were used to study the degradation of this ceramic product. The microstructural changes and the formation of porosity at high temperature are thought to modify the oxidation mechanisms. In the same way, higher temperature and longer duration of pyrolysis allow the organization of carbon crystallites which are readily oxidized. The combustion of the carbon (CO/CO2 departure) is followed by the oxidation of the silicon carbonitride (CO, CO2, N2 and NOx departure).