In
1998, six CNRS laboratories from many disciplines came together with
Snecma and Messier-Bugatti to launch the "Friction of Carbon-Carbon
Composites" research program, to provide a better understanding
of the mechanisms of friction and wear in carbon-carbon composites,
which are used in aircraft brakes and in some Formula 1 and industrial
vehicles.
Curiously,
wear on carbon-carbon composite brakes is much more significant during
low power braking, when the brake temperature reaches about 150ºC
(for example, during taxiing on the runway) than during emergency
braking when the brake temperature reaches about 2000ºC (for
example, during a rejected take-off). The research group has made
great strides in understanding the mechanisms governing the maximum
in wear rate, which appears to be physico-chemical in origin.
The
group is also studying the mechanical properties of carbon-carbon
composites. Although they are made of carbon alone, their mechanical
and tribological properties vary with the environment, fiber arrangement,
and the type of heat treatment applied to the pyrocarbon matrices
surrounding the fibers, and regulate the wear mechanisms that give
rise to some of the particles directly or indirectly responsible for
wear. Conventional tribometric studies should solve the problem, but
substantial difficulties are encountered in scaling up from laboratory
to real brake behavior.
Other
problems that complicate the picture are the effects of the debris
that accumulates in the disk material on wearing, the behavior of
the debris being sensitive to contact geometry and dimensions, to
the environment and to the adsorption and desorption kinetics of the
surrounding gases.
The
group has made significant advances in this difficult field by pooling
the skills of the participating laboratories to further optimize the
mechanical and tribological properties of future composite materials.
