Carbon brakes

Could someone please point me to a link where educative material on
carbon brakes is available? A Google search came up with not much. I
hear that these brakes don't wear either by pressure or temperature but
on the actual number of times they're applied!

Thanks in advance,

Ramapriya

In article .com,
wrote:

Could someone please point me to a link where educative material on
carbon brakes is available? A Google search came up with not much. I
hear that these brakes don't wear either by pressure or temperature but
on the actual number of times they're applied!

http://en.wikipedia.org/wiki/Disc_brake

I heard that also, but didn't know it was carbon brakes. I recall some
airlines prefer fewer longer brake applications rather than more shorter
ones.

wrote:
Could someone please point me to a link where educative material on
carbon brakes is available? A Google search came up with not much. I
hear that these brakes don't wear either by pressure or temperature but
on the actual number of times they're applied!

Thanks in advance,

Ramapriya

Just found this stuff somewhere, in case any of you is interested in a
read...

[Cite]

Carbon fibre-based brakes are made out of so-called carbon-carbon
composite material that consists of two components: weaved carbon cloth
and a solid carbon (and other things) matrix. The carbon matrix is
formed by Chemical Vapour Infiltration (CVI) method. According to this
method, a vapour of gases (such as CCl4) are introduced across the
[heated] weaved carbon cloth and react on the surface of these fibres
with formation of solid carbon phase which has ceramic and silicon in
it. This solid carbon phase is the one that plays important role in
friction and wear performance of the brakes. The CVI coating is a few
microns thick and is fine grained and harder than similar materials
produced using conventional ceramic fabrication processes.

There is a general wear mechanism for carbon-carbon aircraft brakes
proposed in 1988. According to this mechanism, there are two types of
wear:

Type I. This type of wear happens at low energy conditions, such as
aircraft taxiing, or when low pressure is applied during braking. At
these conditions, a particulate powdery wear debris are formed. The
worn particles cause abrasive wear which is the most damaging mode in
terms of brake wear - it's like applying a sand paper over the brakes.
The particles are mostly formed by carbon matrix, not carbon fibres.

Type II. This type of wear is at high energy conditions, such as in
aircraft landing, or when high pressure is applied during braking. The
difference is that at these conditions, a smooth friction film is
formed on the brakes which serves as a solid self-lubricant. This film
protects the brakes, therefore the brakes wear less. Of coarse, the
braking efficiency suffers, meaning that the friction coefficient is
lower for brakes that have formed such a film.

The mechanism of formation of this film is not completely clear, even
though its existence was proved many times by many researchers.
Usually, the following explanation is offered: under higher braking
energy condition, higher pressure and temperature assist deformation of
wear particles to form a debris film. The particles do not melt though,
but plastically deform (carbon does not melt). Nobody will say anything
more definitive about this film formation, although there have been a
lot of research done on density, crystalline structure, porosity,
microscopy and X-Ray diffraction of these films. However Malhotra did
work on silicon barriers on carbon-carbon and also on ceramics, which
makes me think that these are important parts of it. Murdie, Don,
Wright at the Materials Technology Center (MTC) at Southern Illinois
University, USA (later CAFS / Centre for Friction Studies) did most of
the work on the aircraft braking systems, thanks initially to those
kind folks from BF Goodrich and Aircraft Braking Systems (and
thereafter the sponsors look like a who's who of industry).

Because carbon is oxidised in air at temperatures as low as 500 degC,
the extensive research aimed to improve the oxidation resistance of
carbon-carbon composites. CAFS researchers studied the oxidation of
carbon-carbon materials.

There were two commonly accepted ways to protect these materials
against oxidation. The first method makes use of oxidation-resistant
coatings, such as SiC (silicon carbide). The major problem with this
method is the fact that coatings usually induce stresses and often lead
to crack formation. The other method of protecting C-C composites was
by using matrix inhibitors, such as boron or boron carbide. They reduce
the carbon oxidation by spreading a sealant borate glass within the
composite. However, due to their low melting point, such inhibitors
introduce temperature limitations for composite applications and are
effective only after an appreciable fraction of carbon has been
gasified. In the braking process and at high humidity, a carbon
composite loses much of its friction property and becomes greasy--more
like a lubricant. This could be a problem operationally [he says
tongue-in-cheek].

What MTC did, but won't tell anyone, was to develop a third method of
protecting against oxidisation - by making sure the carbon didn't get
too hot. The nano-composite material uses ceramic particles to protect
carbon from high heat in an oxidising environment. I reckon that there
is silicon in the coating as well. The silicon forms the film and the
ceramic retards the heat transfer to the carbon fibres. Their
dynamometer testing (because they had one) showed that the
ceramic-enhanced carbon composite had about a 20-fold higher
coefficient of friction than a standard carbon composite. For certain
friction applications, ceramic doped carbon materials exhibit more
braking capability.

The upper limit comes because as temperature and braking energy rise
even higher (like in a rejected take-off), the silicon friction film
would break into chunks due to shear stresses and thus expose the
ceramic-carbon mix to higher friction and so the wear rate would
increase again. The other bad thing that could happen at extremely high
energy braking is the ceramic cannot sufficiently slow the heat
transfer to the carbon fibres and the carbon heats over 500 degC and
starts to oxidise. This is especially critical if the temperature of
the total brake system exceeds 1000 degC - because that means that
the thermal gradient is too steep, the ceramic can't offer enough
protection, the carbon fibres get hotter, and the oxidation of the
carbon fibres leads to a very rapid degradation of the brakes.

In conclusion, high braking pressure leads to a lower brake wear, but
only up to a limit. The lower brake wear is due to the formation of the
silicon film at high energy braking which serves as a lubricant and
protector of the ceramic/carbon brake material. The formation mechanism
of the film is the subject of scientific debates, but it's known that
it does not form at low energy braking, and it is destroyed at
extremely high energy braking.

[End cite]

Ramapriya