SiCf-SiBC composites: microstructural investigations of the as-received material and creep tested composites under an oxidative environment.

SiCf-SiBC composites fabricated by Snecma Propulsion Solide (St Médard en Jalles, France) were investigated by SEM and HRTEM in the as-received state and after creep tests performed in air, in a temperature range 1423-1573 K, under 170 and 200 MPa. These composites are reinforced by Hi-Nicalon fibres (Nippon Carbon). A pyrocarbon interphase was first deposited on the fibres. The matrix was then deposited on the fibrous preform by several chemical vapour infiltrations (CVI). As a result the matrix is multilayered and based on the Si-B-C ternary system. This matrix is self-sealing: this is due to the presence of boron inducing the formation of a sealant glass if the material is heated in an oxidative environment. This glass will protect fibres and fibre/matrix interphases against oxidation. Hi-Nicalon fibres as well as the different matrix layers were studied by HRTEM and EDX. Some investigations were carried out on the creep-tested specimens in order to characterize modifications observed in the different constituents of the composites, particularly at the interfaces between the matrix layers and at the fibre/matrix interface. It was shown that several matrix layers crystallized during the creep tests. Moreover, a thin silica layer was observed at the pyrocarbon/matrix interfaces. Differences between the behaviour of the same type of material creep tested under neutral atmosphere are discussed.

A synthetic aragonite-based bioceramic: influence of process parameters on porosity and compressive strength.

We investigate the influence of process parameters such as weight fraction and particle size of pore-former, and isostatic pressure, on porosity and compressive strength of non-sintered porous calcium carbonate biomaterials compacted at high pressure in uniaxial or isostatic mode. Experiment design and results analysis are performed according to a two-level 2k factorial design method (FDM). Results indicate that only the weight fraction of pore-former (wt fpf) influences significantly the porosity and the compressive strength. The porosity P, is described by a linear function of wt fpf, and the compressive strength sigma(comp), by an exponential one. For materials compacted under uniaxial pressing: P (vol%) = 33.7 + 85.4 (wt fpf) and sigma(comp) (MPa) = 28.8e(-9.2(wt fpf)) with 0.1 < or = wt fpf < or = 0.3. For materials compacted in isostatic mode: P (vol%) = 33.9 + 82.1 (wt fpf) and sigma(comp) (MPa) = 24.0e(-7.0(wt fpf)) with 0.15 < or = wt fpf < or = 0.35. The pore-former particle size has no significant influence on both properties. The increase in isostatic pressure provides slightly lower porosity and better compressive strength. For a fixed fraction of pore-former, isostatic pressing leads to a better compressive strength than uniaxial pressing. This study indicates that, for a constant amount of pore former, the size of macropores can be adjusted to reach optimal bone-ingrowth without change in compressive strength.