René Pailler

Conversion mechanisms of a polycarbosilane precursor into an SiC-based ceramic material

The pyrolysis of a PCS precursor has been studied up to 1600 °C through the analysis of the gas phase and the characterization of the solid residue by thermogravimetric analysis, extended X-ray absorption fine structure, electron spectrocopy for chemical analysis, transmission electron microscopy, X-ray diffraction, Raman and Auger electron spectroscopy microanalyses, as well as electrical conductivity measurements. The pyrolysis mechanism involves three main steps: (1) an organometallic mineral transition (550 < Tp < 800 °C) leading to an amorphous hydrogenated solid built on tetrahedral SiC, Si02 and silicon oxycarbide entities, (2) a nucleation of SiC (1000 < Tp < 1200 °C) resulting in SiC nuclei (less than 3 nm in size) surrounded with aromatic carbon layers, and (3) a SiC grain-size coarsening (Tp > 1400 °C) consuming the residual amorphous phases and giving rise simultaneously to a probable evolution of SiO and CO. The formation of free carbon results in a sharp insulator-quasimetal transition with a percolation effect.

Oxidation-resistant carbon-fiber-reinforced ceramic-matrix composites

Abstract A multilayer Si–B–C ceramic matrix has been developed to improve the oxidation resistance and the lifetime in an oxygen environment of carbon-fiber-reinforced ceramic-matrix composites. This concept has been applied to a multidirectional fibrous carbon preform. The paper deals with the processing, the mechanical behaviour and the oxidation resistance of a carbon-fiber-reinforced multilayer ceramic-matrix composite. The efficiency of this matrix is compared to that of the classical anti-oxidation systems based on an external coating usually employed to reduce the oxygen permeation. The efficiency of the oxidation protection of the multilayer ceramic matrix is still evident in spite of the complex architecture of the fibrous preform and the damage of the matrix. The experimental results show that a strong improvement in the lifetime of such composites under thermomechanical loading and in an oxygen environment is obtained when a multilayer matrix is used to protect the carbon reinforcement against oxidation.

Thermal stability of a PCS-derived SiC fibre with a low oxygen content (Hi-Nicalon)

The oxygen free Si–C fibre (Hi-Nicalon) consists of β-SiC nanocrystals (≈5nm) and stacked carbon layers of 2–3nm in extension, in the form of carbon network along the fibre. This microstructure gives rise to a high density, tensile strength, stiffness and electrical conductivity. With respect to a Si–C–O fibre (Nicalon NL202), the Si–C fibres have a much greater thermal stability owing to the absence of the unstable SiOxCy phase. Despite its high chemical stability, it is nevertheless subject to a slight structural evolution at high temperatures of both SiC and free carbon phases, beginning at pyrolysis temperatures in the range 1200–1400°C and improving with increasing pyrolysis temperature and annealing time. A moderate superficial decomposition is also observed beyond 1400°C, in the form of a carbon enriched layer whose thickness increases as the pyrolysis temperature and annealing time are raised. The strength reduction at ambient for pyrolysis temperatures below 1600°C could be caused by SiC coarsening or superficial degradation. Si–C fibres have a good oxidation resistance up to 1400°C, due to the formation of a protective silica layer.

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.

SiC filament/titanium matrix composites regarded as model composites

Two types of large diameter SiC CVD filaments have been investigated on both chemical and mechanical standpoints: a 100μm filament with a tungsten core (from SNPE) and three 140μm filaments with carbon cores and surface coatings (from AVCO). On the basis of microprobe (X-ray, Auger and Raman), X-ray diffraction and SEM analyses, it appears that the former is made of a single homogeneous stoichiometric SiC deposit while the latter are mainly made of two concentric shells (the inner being a SiC+C mixture and the outer almost pure SiC). All the C-core filaments had received a surface coating (either pure pyrocarbon or SiC+C mixture) presumably to protect the brittle SiC deposit against abrasion due to handling in opposition to the W-core filament which seems to have no surface coating at all. The W-core filament, although smaller in diameter, is weaker than the C-core filaments (average UTS of 3 and 4 GPa respectively for a 40 mm gauge length). However, its strength distribution is much narrower (Weibull moduli of 7–8 and 2–5 respectively). Failures of most filaments appear to have a multimodal character.

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.

Synthesis of highly tailored ceramic matrix composites by pressure-pulsed CVI

Abstract Pressure-pulsed chemical vapor infiltration (P-CVI) is a new processing technique to produce ceramic matrix composites (CMCs) (C/C, C/SiC and SiC/SiC) from gaseous precursors with highly tailored interphases and matrices. P-CVI can be used to control the microtexture of pyrocarbon deposited from hydrocarbon in a porous body. It is also used to form (PyC–SiC)n or (BN–SiC)n interphases with elementary layer thickness of a few nanometers. Finally, P-CVI is an efficient way to produce self-healing matrices comprising mechanical fuse layers (PyC or BN) and glass-former layers (B13C2 or/and SiC). The resulting model or real composites display an oxidation resistance, in air at 600–700°C and under load, which is improved by two orders of magnitude.

The fibre/matrix interfacial shear strength in titanium alloy matrix composites reinforced by silicon carbide or boron CVD filaments

Abstract The mechanical adhesion between CVD filaments (B, SiC) and titanium matrices was studied. Because a single fiber composite was chosen for this purpose, the critical length measurement and the shear strength were calculated using a statistical analysis. The study indicated the role played by the surface treatments of the fibers on the reinforcement/matrix adhesion. The conclusions obtained on model materials are in agreement with the results obtained on real composites.

Modern boron and SiC CVD filaments: A comparative study

Abstract Large diameter filaments (100–150 μm in diameter) made by chemical vapor deposition (CVD) of two ceramic materials (i.e. boron and SiC) on a heated tungsten or carbon core are compared from a mechanical and chemical standpoint. The most interesting of the filaments studied have received a rather thick surface coating (1–3 μm) which is made of boron carbide for B(W) filaments and a sequence of pyrocarbon and silicon carbide layers for SiC filaments. The mechanical behavior of the filaments in tension is explained on the basis of a Weibull statistics approach as well as a fracture analysis. Failure appears to be mainly controlled by surface defects, a feature which emphasizes the protective role played by the coating. Annealing at high temperatures (i.e. 800–950°C) in the presence of titanium shows that coated filaments have superior behavior. The coating acts in fact as a consumable sacrificial material, the strength of the filament remaining unchanged as long as the coating is not totally consumed by chemical reaction with titanium. Modern CVD filaments appear to be the most suitable ceramic reinforcements from a fundamental point of view.

Structure of pyrocarbon infiltrated by pulse-CVI

Abstract Pulse chemical vapor infiltration (P-CVI) was developed to study both the texture and density of infiltrated pyrocarbon. In this work model pores were machined in graphite with various diameters. Compared to classical chemical vapor infiltration (I-CVI) where the deposition of pyrocarbon is mainly controlled by temperature, pressure and precursor type, P-CVI provides an additional controlling parameter: the residence time (tR). For same pressure, temperature and mother molecules, tR variations were seen to produce texture changes. The comparison of the infiltration within the different pores has shown that two deposition mechanisms exist with a continuum in between, one of which was favored by very low residence time (heterogeneous reactions). In this case infiltration was pore-size-insensitive with an homogeneous texture in-depth. But the anisotropy was never found to be very high. The second is supposed to occur by diffusion and “condensation” of higher molecular weight species (resulting from homogeneous reactions). The deposited carbon is then characterized by a texture gradient: texture is highly anisotropic at the entrance of the pore and is increasingly disorganized in depth (pore-sizesensitive). Between these two extremes, (i.e. at intermediate tR), it was possible to combine the interest of the heterogeneous mechanism with a gentle maturation (homogeneous reactions). This was obtained at low pressure and low temperature, producing a highly anisotropic pyrocarbon that was infiltrated without gradient. In this case pyrocarbon structure is characterized by a large layer size (L2 > 10 nm), a high preferred orientation and, surprisingly, a reduced turbostratic coherency.