Posted: November 16th, 2022

Catalytic Oxidation of Carbon-Carbon Composite of Aircraft Brakes

Catalytic Oxidation of Carbon-Carbon Composite of Aircraft Brakes

Summary
Carbon-carbon composites have proved to be better frictional materials than any other materials such as steel applicable for making aircraft brake system. However, there is one main problem that challenges the use of carbon-carbon composites in aircraft brake systems, that is, catalytic oxidation from de-icers used in aerospace and runways mostly during winters. As a result, a plethora of studies has been conducted on the effects of the de-icers on the carbon/carbon composites oxidation. From these studies, important findings have been made, which have been very impactful to the industry. Some of the studies are still underway because a solution that completely eradicates the problem has not yet been realized. There is still need to look for solutions that would mitigate the recurring challenges to the lowest possible limit. The best approach in handling this issue is through the exploration of the effects of the di-icy materials on catalytic oxidation of carbon-carbon composites. This research is aimed at exploring the various effects of manganese acetate on the catalytic oxidation of carbon-carbon composites. The investigation focuses specifically on the rate of oxidation of carbon-carbon composites in manganese acetate prisms, data computation of the carbon/carbon composites weight loss during the oxidation, and characterization of the oxidation for understanding and expansion of knowledge in the field of catalytic oxidation of carbon-carbon composites.

Introduction
De-icy chemicals find a great deal of usage on the aircraft runways and aerodynamic surfaces. During winter, the snow and ice that accumulate on the ground surface hinder the in-flight operations and landing of the aircraft (Friedman & Shi 2009, p.226). The resulting impacts may be devastating. Di-icy chemicals have come to rescue the situation. The application of these chemicals is indispensable because they enhance safety operation conditions at the airport runways and in the course of in-flight (Bevilacqua, Babutskyi & Chrysanthou 2015, p.862; Wu, Pantano & Radovic 2001, p.10). The ice lowers the friction coefficient, and under such circumstances, the aircraft tires will fail to have a grip on the runway surface. In such a situation, there is no option other than the removal of the ice from the runways to enable landing which can only be achieved through the use de-icy elements. The first de-icer material to be used for de-icing operations was urea (Chauhan et al. 2009, p.6). According to Bevilacqua, Babutskyi & Chrysanthou (2015, p.862), in the 1990s, urea got replaced with the alkali metals and acetates to overcome the problems that urea contributed. Urea contributed negatively regarding the environmental concerns (Chand, 2000 p. 1303). It threatened the aquatic life through the amount of toxins that were being released out into the water bodies. Its chemical oxygen and biochemical oxygen demand values are extremely high thereby depriving the aquatic life off the available oxygen content. The aircraft’s braking systems come hand in hand with the de-icers (Xuetao et al. 2010 p. 344; Shen et al. 2011, p.107; Wang et al. 2011, p.1280). The urea was replaced with calcium and potassium salts and acetates (Wu & Radovic 2005, p.333; Walker & Booker 2000, p.41; Wu 2002, p.282).
Steel brakes were used in aircrafts before carbon-carbon composites were adopted. The major problem experienced with the steel brakes was high heat development in instances of normal and aborted landings. Steel has poor heat dissipation. The carbon-carbon composite brakes possess superior quality such as surviving abortive stops, stopping capability in normal landings and endure much longer as compared to the steel brakes (Bevilacqua, Babutskyi and Chrysanthou 2015, p.862). The mechanism of disk braking generates a considerable amount of frictional heat which has to be dissipated (DuviviFFer, Robin‐Brosse & Naslain 2007, p.227). Apart from the heat resistance and high-temperature stability, carbon-carbon composites have excellent thermal conductivity (Feng et al 2015, p.608) low thermal expansion, and favourable friction coefficient with is independent of temperature variation. Carbon also has a lower density than steel (Serp & Figueiredo 2009 p.16; Figueiredo & Pereira 2010, p.4). Integrating the low density and its higher specific heat capacity as compared to steel leads to the realization of 60% weight saving for a similar system that operates within a given operational temperature range. The cost of carbon-carbon composites are also cost effective and have the capability of delivering on landings as twice as that steel braking systems. The maximum braking requirement in commercial aircraft needs dissipates roughly 45 kJ/mm2 of the kinetic energy within a period of 30 seconds (Bevilacqua, Babutskyi and Chrysanthou 2015, p.862).
Carbon-carbon composites are made from PAN fibres. The PAN fibres are knitted in three-dimensional to form a cloth-like structure which then undergoes a carbonation process (Saito et al. 2011, p.3829; Farhan, Wang & Li 2015, p.9765; Lachaud et al. 2006, p.3; Lachaud et al. 2006, 10). Before carbonation process, the fibres are taken through a stabilization process through which the materials are altered chemically from the linear atomic bonding to ladder bonding (thermally stable). Once the PAN fibres are stabilized, they are heated in a furnace to a gas mixture that does oxygen free (Junhe, Jin, & Liuhua 2007, p.17; Shin et al. 2006, p.544; Breuer 2016, p.61). The absence of oxygen offers no room for burning of fibres at the high temperature of the furnace (Fan et al. 2007, p.293; Figueiredo, Pereira, Freitas,. and Órfão, 2007 p. 3041; Pico & Steinmann 2016, p.160). The pressure inside the furnace is kept higher than the outside pressure, and the entry points are sealed to ensure that no oxygen enters the furnace. The heating of the fibres at a high temperature of between 1000 0C and 3000 0C resulting into the losing of non-carbon atoms, few carbon atoms, and the gasses present. After the carbonation process, the carbonized fibres are cut into particular moulds and numerous layers are laid to form give a carbon preform disc with the specification of weight for producing aircraft brake of a specific that suits a particular aircraft (Gaur, Sharma, and Verma, 2005 p.3041; Wei, Liu & Fan 2014, p.64). The discs are then compressed before loading them into a predetermined position within the furnace. They are then inductively heated to a temperature of about 1000 0C (Cho et al. 2014, p.806). The step is followed by the introduction of gas for infiltration of the disc pre-forms and carbon is deposited in the gaps between the fibres (Cao et al. 2013, p.24; Liu et al. 2008, p.2). Cycles of multiple infiltrations are conducted till the designed density is reached (Cho et al., 2014, p.763). The range of the accepted standard density falls between 1750 kg/m3 and 1850 kg/m3 (Francois, Joly, Kapsa, and Jacquemard, 2007 p.124). The discs are then machined according to the design to form carbon-carbon composite brakes.

Figure 1: C-C composite fibers matrix (Bevilacqua, Babutskyi and Chrysanthou 2015, p.862)
Carbon-carbon composites are applied in various areas. However, the most common are the aircraft brake system (Krenkel & Berndt 2005, p.179; Krenkel, Heidenreich, & Renz 2002, p.432). The composites are designed in a manner that they form frictional materials utilized in aerospace (Krenkel 2003, p.589). The carbon/carbon composites are also seen as potential materials for structural applications at high temperatures exceeding 2000 0C. In the aircraft brake system, the carbon-carbon composites are designed to operate safely, and decelerate and stop the aircraft at times of landing (Blanco et al 1997, p.5). During landing, a lot of kinetic energy gets converted to heat energy (Fitzer & Manocha 2012, p.267; Aly-Hassan et al. 2003, p. 2074; Bertrand et al. 2006, p.2). This heat energy must be dissipated within the required time to avoid complications (Savage, 2012 p. 25; Fitzer & Manocha 2012, p.267). Carbon-carbon composites perform these duties due to its desirable qualities such as low friction coefficient and excellent thermal conductivity.

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