Ceramics,
in particular ceramic fibers, are rapidly becoming the material of
the third millennium, particularly in the aviation and space industries.
Their superior refractory properties and lightness means that they
can replace metals as the material of choice in turbines, which will
have to comply with new noise and pollution regulations starting in
2015. However, the inexorable spread of their use is being hampered
by their fragility: their defects (rare pores, grain boundaries) as
well as ionic covalent bonding facilitate crack formation and propagation
and makes breakage only statistically predictable. These problems
can be overcome by using composite matrix ceramics, CMCs, which improve
microstructure by combining two or more types of ceramic in the same
material and carefully controlling the interface between them, producing
a reliable, resilient material.
The
properties of such composites, usually a fiber preform and a matrix,
can only be optimized by monitoring the structure on both a micromechanical
and physico-chemical basis. Existing methods have not proved satisfactory,
and so the "Laboratoire de dynamique, interactions et réactivité"
(LADIR, Dynamics, Interactions and Reactivity Laboratory) and the
"Office national d'études et de recherches aérospatiales"
(ONERA, National Office for Aerospace Research) have combined to study
the possibilities of Raman spectroscopy in the study of ceramics.
Working with NASA, they have developed a new procedure for mechanical
and physico-chemical analysis of future composite materials.
This
new procedure, micro-Raman stress imaging, can measure stress in CMC
fibers, thereby helping in the design of composites. It can non-destructively
determine fiber break strength, and can analyze the nanostructure
of fiber coatings at the fiber-matrix interface.
The
procedure can be applied to ceramic and metal matrix composites and
will also be capable of analyzing old, corroded and stressed materials.
