D. Mocaer

Si-C-N ceramics with a high microstructural stability elaborated from the pyrolysis of new polycarbosilazane precursors

Novel polycarbosilazanes (PCSZs) were prepared by stepwise synthesis and thermal crosslinking of polysilasilazane (PSSZ) copolymers. Their pyrolysis under inert gas, producing Si-C-N ceramics, was investigated up to 1600 °C by analyses performed on the solids (elemental analysis; EPMA; TGA, density; 1H, 13C and 29Si solid state NMR, i.r. XRD, electrical conductivity) and analyses of the evolved gases (gas chromatography and mass spectrometry). From 250 to 450 °C, a first strong weight loss was observed, which was due to the formation and elimination of low-boiling-point oligomers. The weight loss closely depends on the cross-linking degree of the ceramic precursor resulting from the PSSZ/PCSZ conversion. Then, the organic/inorganic transition took place between 500 and 800 °C, proceeding via evolution of gases (mainly H2 and CH4) and yielding a hydrogenated silicon carbonitride. This residue remained stable up to 1250 °C although it progressively lost its residual hydrogen as the temperature was raised. Then, crystallization occurred between 1250 and 1400 °C, yielding β-SiC crystals surrounded by free-carbon cage-like structures. Finally, above 1400 °C, the remaining amorphous Si-C-N matrix underwent a decomposition process accompanied by nitrogen evolution and a second substantial weight loss. At 1600 °C, the pyrolytic residue was a mixture of β-SiC and free carbon. So, the amorphous silicon carbonitride resulting from the pyrolysis of PCSZ precursors was found to be appreciably more thermally stable than the previously reported Si-C-O ceramic obtained by pyrolysis of polycarbosilane precursors.

Composition-microstructure-property relationships in ceramic monofilaments resulting from the pyrolysis of a polycarbosilane precursor at 800 to 400 °C

A 15 μm monofilament was extruded from a Yajimas type molten polycarbosilane, stabilized by addition of oxygen and heat-treated at 800 to 1400 °C under an argon atmosphere. Two important phenomena occur during pyrolysis. At 500 to 750 °C, an organic-inorganic state transition takes place with a first weight loss. It yields an amorphous material stable up to about 1100 °C. At this temperature, its composition is close to Si4C5O2. It can be described as a continuum of SiC4 and/or SiC4−xOx tetrahedral species (and possibly contains free carbon), with a homogeneity domain size less than 1 nm. The amorphous filament exhibits a high strength and semi-conducting properties. Above 1200 °C, a thermal decomposition of the amorphous material takes place with an evolution of gaseous species thought to be mainly SiO and CO, an important cross-section shrinkage and the formation of 7 to 20 nm SiC crystals which are surrounded with a poorly organized turbostratic carbon. The amorphous-crystalline state transition results in a drop in the tensile failure strength and an increase, by four orders of magnitude, in the electrical conductivity which becomes temperature independent. The former effect is due to the crystallization of the filament and the latter to a percolation phenomenon related to the intergranular carbon. The low stiffness is also due to the presence of carbon. It is anticipated that this transition is mainly related to the decomposition of the silicon oxycarbide species. Finally, a 40 to 50 nm layer of turbostratic carbon is formed at the filament surface at 1200 to 1400 °C whose origin remains uncertain. It is thought to be mainly responsible for the formation of the carbon interphase in the high-temperature processing of ceramic matrix composites.

Thermal behavior of polymer-derived ceramics. I. Si-C and Si-C-O systems from both commercial and new polycarbosilane (PCS) precursors

Abstract Si-C- and Si-C-O-based ceramics from new or commercial polymeric precursors are compared. The emphasis is upon the structural and textural features as observed by high-resolution transmission electron microscopy (HRTEM), and their evolution with an increasing heat-treatment temperature up to 1800°C. Specifically, the behavior of an increasing excess of carbon—with regard to the Si-C stoichiometry—is revealed, and the consequences on the nucleation and the growth of the SiC-grains and the electrical properties are discussed.

Tensile testing at high temperatures of ex-PCS Si-C-O and ex-PCSZ Si-C-N single filaments

A microtensile tester consisting mainly of an induction-heated furnace, a 0–2 N load cell, a 0.1/1 μm sensitivity straining device and hot grips has been designed and used to test ceramic single ceramic filaments at 25–1600°C under vacuum (0.1 Pa) or in controlled atmospheres. Both failure strength and Youngs modulus were measured with an isothermal gauge length of 30 mm. A system compliance correction was applied for each test temperature and material. The apparatus was used to characterize an ex-poly-carbosilane Si-C-O fibre (Nicalon NLM-202) and an ex-polycarbosilazane Si-C-N experimental single filament almost free of oxygen (γ-ray curing). Both materials exhibit a significant strength loss at 1200–1600°C when tested under vacuum, assigned to a decomposition process with an evolution of gaseous species (SiO/CO or N2) and the formation of a mechanically weak decomposition surface layer. Conversely, the Si-C-N filament undergoes no strength loss when tested in an atmosphere of nitrogen (P=100 kPa) at 1200°C, the decomposition being impeded by the external nitrogen pressure. In all cases, no significant decrease in Youngs modulus was observed.

Thermal behavior of polymer-derived ceramics. IV. SiCNO fibers from an oxygen-cured polycarbosilazane

Abstract Fibered SiCNO-based ceramics from a polycarbosilazane precursor (copolymer) are studied, then compared to other ceramic materials previously prepared and described in other papers. The emphasis is upon the structural and textural features as observed by high-resolution transmission electron microscopy and their evolution with an increasing heat-treatment temperature up to 1600°C. Both argon or nitrogen atmospheres were used. Specifically, the peculiar behavior of the material during the amorphous to crystalline transition until degradation is revealed. The consequences on the nucleation and the growth of the SiC grains, and the electrical and mechanical properties are discussed.