Alain Guette

LPCVD and characterization of boron-containing pyrocarbon materials

Pyrocarbon materials containing various amounts of boron have been prepared by LPCVD from BC13C3H8H2 precursor mixtures. By increasing the BCl3(C3H8 + BCl3) ratio up to 85%, the incorporation of boron can reach 33 at.%. A small amount of boron (e.g. 8 at.%) highly enhances the structural anisotropy of pyrocarbon, as evidenced by optical microscopy, X-ray diffraction and transmission electron microscopy (selected area diffraction and lattice fringes techniques). X-ray photoelectron spectroscopy has shown that a large fraction of the boron atoms are included by substitution in the carbon layers; the remaining boron atoms belong to a boron-rich amorphous part of the material. As the boron content increases beyond 8 at.%, the structural anisotropy of the boron-rich pyrocarbon decreases, due to the limited growth and stacking of the carbon layers. Also, amorphous boron-rich regions are more and more abundant as the total amount of boron increases. The oxidation resistance of the C(B) materials is better than that of pure pyrocarbon. This is mainly due to the improvement of the structural organization for the low boron content materials and to the coating of the whole material with a stable boron oxide for materials with a higher boron content.

A theoretical and experimental approach to the active-to-passive transition in the oxidation of silicon carbide: Experiments at high temperatures and low total pressures

Active-to-passive oxidation transition in chemical vapour deposited β-SiC was investigated in the temperature range 1300≤T≤1700°C under low total pressures (100≤Ptot≤800 Pa) and relatively high linear gas flow rates (10≤Vgas≤60 m s1) by thermogravimetric analysis. For given T, Ptot and Vgas, the oxygen partial pressure at the transition, PtO2, corresponds to the value where the mass-loss rate per unit area of the oxidized sample, R, is maximum. Logarithms of PtO2 are linear functions of reciprocal temperature for given Ptot, and Vgas. Vgas has a significant influence on the position of the transition log(PtO2)–T-1 line. PtO2 is also slightly affected by an increase of Ptot from 100 Pa to 800 Pa. In passive oxidation at high temperatures (>1500°C), large bubbles form in the silica film which is then disrupted leading to a loss of material. In active oxidation, R significantly depends on Vgas: the kinetics is diffusion or mass transfer controlled under the conditions investigated in the present study. In both active and passive oxidation regimes, a mass loss of the test specimen is always observed; an explanation is proposed.

The concept of a strong interface applied to SiC/SiC composites with a BN interphase

Abstract Strong interfaces have been shown to allow improvement of the mechanical properties of ceramic matrix composites (CMC). The concept of a strong interface has been established in SiC/SiC composites with pyrocarbon (PyC) or multilayered (PyC/SiC) fiber coatings (also referred to as interphases). The present paper reports an attempt directed at applying the concept of a strong interface to SiC/SiC composites with a BN coating (referred to as SiC/BN/SiC). Fiber bonding and frictional sliding were investigated by means of push-out tests performed on 2D-composites as well as on microcomposite samples, and tensile tests performed on microcomposites. The stress–strain behavior of the SiC/BN/SiC composites and microcomposites is discussed with respect to interface characteristics and location of debonding either in the coating or in the fiber/coating interface.

Tribological behaviour of hard coatings deposited by arc-evaporation PVD

Hard coatings deposited by arc-evaporation PVD are adequate solutions for increasing the lifetime of tools and components commonly employed in many different industrial applications. The present paper reports a comparative study of the tribological properties of the most employed hard coatings like TiN, TiCN, TiAlN, CrN and ZrN. In this study, microhardness tests were carried out by using a microindenter. Friction and wear tests were carried out by using a ball-on-disk tribometer and an optical profilometer. AUGER and XPS techniques were employed to measure the stoichiometry and thickness of the different films.

Characterization of nearly stoichiometric SiC ceramic fibres

A comparative study of the chemical composition and microstructure of Hi-Nicalon, Hi-Nicalon type S, Tyranno SA, Sylramic and Carborundum fibres has been conducted. This analysis has confirmed results already published but has also evidenced some original features. The Hi-Nicalon type S fibre has a near stoichiometric composition but it still contains some oxygen (≈1 at. %) and free carbon (≈2 at. %). The expected near stoichiometric composition of both the Tyranno SA and the Sylramic fibres is only effective near the edge region, while the core of the fibres contains some amount of free carbon (e.g., up to ≈14 at. % and ≈6 at. % respectively in large diameter fibres) as well as some residual oxygen (≈0.5 at. %). The composition of the Carborundum fibre is very close to stoichiometric SiC except rare and localised free carbon or B4C inclusions. The properties of the different fibres, some of them still beeing at a development stage, are discussed from a chemical and a phase composition point of view, on the basis of what is known about their respective preparation process.

Micro/minicomposites: a useful approach to the design and development of non-oxide CMCs

Abstract Micro (one single filament) and mini (one single fiber tow) non-oxide composites (C/C; C/SiC and SiC/SiC) with simple (PyC or BN) or complex interphases [C (B) or (PyC-SiC) n multilayers] are fabricated in a short time by CVD/CVI. The fiber/matrix interfacial zone is characterized by AES and TEM. Tensile tests are used to assess the mechanical properties and the Weibull statistical parameters of both the fiber and matrix, as well as the fiber–matrix interfacial parameters (τ i ; l d ; G ic ). The tensile stress–strain behavior has been modelled. The tensile curves exhibit the same features as those previously reported for real nD-composites. Lifetime at high temperatures in air is characterized through static/cyclic fatigue tests and modelled. It is improved by replacing conventional pyrocarbon by highly engineered interphases. The micro/mini composite approach is used in the optimization of processing conditions and to derive parameters necessary for the modelling of the thermomechanical and chemical behavior of composites with more complex fiber architectures.

Interface characterization by TEM, AES and SIMS in tough SiC (ex-PCS) fibre-SiC (CVI) matrix composites with a BN interphase

The fibre-matrix interfacial zone formed during the isothermal/isobaric chemical vapour infiltration processing of SiC fibres (ex-polycarbosilane)/boron nitride/SiC matrix composites has been analysed by TEM/electron energy loss spectroscopy, Auger electron spectroscopy, and secondary ion mass spectroscopy. In the composites, the boron nitride interphase (deposited from BF3-NH3) is made of turbostratic boron nitride, almost stoichiometric but containing some oxygen (less than 5 at %). The boron nitride layer stacks are randomly orientated except in the very vicinity of the fibre surface where they lie almost parallel to the substrate. The long chemical vapour infiltration treatment at 1000 °C used to infiltrate the SiC matrix acts as an annealing treatment for the metastable ex-polycarbosilane fibres which gives rise to the growth of an SiO2/carbon amorphous double layer at the boron nitride/fibre interface. Deflection of microcracks arising from the failure of the brittle SiC-matrix occurs at the boron nitride/SiO2 interface considered to be the weaker link in the matrix/boron nitride interphase/SiO2/carbon/fibre sequence. It is suggested that the combination of the boron nitride layered interphase and SiO2/carbon fibre decomposition products might play an important role in determining the propagation path of microcracks in the fibre/matrix interfacial zones and could be responsible, at least to some extent, for the non-brittle behaviour of such composites.

Sur le diborure et le tétraborure de magnésium. Considérations cristallochimiques sur les tétraborures

Abstract MgB 2 and MgB 4 have been prepared at high temperature in sealed molybdenum vessels from mixtures of the elements. The utilization of an excess of metal in the vessel generates a pressure of magnesium vapor which inhibits thermal decomposition of the compounds during the synthesis. The structure of MgB 4 has been established from single crystal data collected on an automatic diffractometer. MgB 4 is orthorhombic, space group Pnam with a = 5.464; b = 7.472; c = 4.428A and Z = 4. The structure of MgB 4 , is based on chains of boron pentagonal pyramids in which the averaged BB bond is 1.787 A. Interchain BB bonds of 1.730 A are responsible for the three-dimensional boron framework. The Mg atoms, located in tunnels, form zigzag chains. The structure of MgB 4 is compared to those of ThB 4 and CrB 4 ; it is concluded that the size of the metal atom plays an important role in the nature of the boron framework which exists. In MgB 4 , the fundamental unit of the boron skeleton is a pentagonal pyramid; a new feature, in boron-rich borides where this type of coordination polyhedron was previously found only in B 12 icosahedra.

Effect of ageing treatments at high temperatures on the microstructure and mechanical behaviour of 2D nicalon/C/SiC composites. 1: Ageing under vacuum or argon

Abstract Two-dimensional Nicalon/C/SiC composites processed by chemical vapour infiltration have been aged at 1100–1500 °C under vacuum or argon. The composites experience a weight loss Δm m 0 related to the decomposition of the SiO2xC1 − x phase of the fibres and to side-reactions between the fibre decomposition gaseous products (particularly SiO) and carbon (from the fibre and the interphase), as assessed by electron probe microanalysis, and Auger electron spectroscopy analyses. This decomposition/reaction process results in an evolution of the mechanical behaviour (characterized through tensile/push-out tests) correlated with the weight loss. Mild ageing conditions ( Δm m 0 ) result in a weakening of the fibrematrix bonding due to some interfacial decohesion with a decrease in both the proportional limit and failure strength, as well as an increase in the failure strain. More severe ageing conditions ( Δm m 0 ≈ 2% ) yield a partial consumption of the interphase with the formation of large SiC-crystals on the fibre surface weakening the fibres, the composite still exhibiting a non-linear mechanical behaviour. Finally, for extremely severe ageing conditions ( 3 Δm m 0 ), the interphase is totally consumed and the fibres undergo a pronounced decomposition/crystallization process, the composite becoming brittle with a very low load-bearing capability.

Oxidation resistance of SiC/SiC micro and minicomposites with a highly crystallised BN interphase

Abstract A three dimensionally ordered hex-BN is deposited by LPCVD from the BF3–NH3 system at relatively low temperature. This coating was studied in term of crystallisation by X-ray diffraction, and transmission electron microscopy. SiC/SiC microcomposites with such a BN interphase were produced in a first step. Their mechanical behaviour was determined under tensile loading at room temperature. They exhibited a wide non-linear stress–strain domain similar to that commonly observed during the damage of a ceramic matrix composite. Further, static fatigue tests at 700°C in air were carried out on a few microcomposites. The long lifetimes provide evidence that such a BN-based interphase brings a real improvement in the oxidation resistance. In a second step, from these latter results, minicomposites were prepared with a complex interphase consisting of a few layers of BN, which included a highly crystallised layer. Similar mechanical tests at room temperature and static fatigue tests at high temperature in air were performed. Despite the change of geometry (single fibre to a tow), damage capability and improved oxidation resistance are observed in comparison with minicomposites with a carbon interphase.