Science Pictures

Carbon-Coated Silicon Carbide Fibres

copyright © ONERA 1996-2006
copyright © ONERA 1996-2006 - All rights reserved
1 - x20,000 enlargement of the surface
of a carbon-coated silicon carbide fibres
Rupture zone
DMSM & DMSC

Carbon-coated silicon carbide fibres are blended with a titanium alloy matrix to create a composite material, the thermal and mechanical properties of which are of particular interest in turbomachines of aeronautical engines.
See the transversal section of such a composite material:
www.onera.fr/photos-en/materials/015.php

The carbon coating of the fibre is carried out through a continuous PVD (physical vapor deposition) process. There are two phases in this process:

• At first, spherical carbon nuclei settle on the fibre.
• Subsequently, the deposit grows through planar carbonaceous molecules piling-up (Figure 3).

The resulting effect is a very rough surface, full of bumps and holes (Pictures 4 and 6)

Where do such pictures come from ?

To get such x100,000 enlarged pictures of the deposite, scanning electron microscopy has to be used. In this technique, the sample is scanned by a finely focused beam of energetic electrons (known as primary electrons). These electrons interact with the atomic electrons of the sample to generate other electrons with lower energy (known as secondary electrons). This signal is picked up to create the picture, in which the relief of the specimen is the major source of the observed contrast.

A field emission electronic microscope is required to produce the present pictures: because of its better resolution (1.5 to 3 nm), it allows high magnification (up to 200,000 times).

What is the use of the fibres ?

The fibres are blended, at high temperature, with a titanium alloy matrix. The resulting composite material has very high resistance and Young's modulus of elasticity when taken along the direction of the fibres (properties you look for when you build turbomachines).

The carbon coating has a double function:

• it protects titanium during the elaboration process of the composite,
• it acts as a mechanical lubricant, since the fibre can slip inside the carbon coating when the material is subjected to strain.

Pictures: Monique Raffestin (ONERA/DMSC)
Text: Marie-Hélène Vidal-Sétif (ONERA/DMSM)

2 - Spherical carbon nucleus,
composed of an agglomerate of nanometric entities
settled on the fibre

3 - Growth diagram of the carbon deposite
on a nucleus

4 - Piled-up carbonaceous molecules on the nuclei,
producing a very rough coating

5 - The fibre with its carbon coating

6 - Notice the book structure of the carbon coating

Last Update: April 20, 2004 - © ONERA 2009 - Terms of use