I've only made a few posts so I don't feel the need to order a Personal Title and help support PPRuNe
posted 18th November 2003 12:27
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.
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