Investigation Of Braking Properties Of Carbon-Carbon Composites

Carbon-carbon composites(C/C) possess many excellent properties such as low density ,high specific strength and modulus, high rupture toughness , good thermal shock resistance, high heat of ablation , chemical inertness etc. So, they are ideal high temperature thermal structural materials in the application of aircraft and spacecraft. Especially for its excellent friction and wear properties, C/C composites have been widely used as barking materials both in military and civilian aircrafts. However, it has still not got through in our Country. This investigation mainly aims at the preparation technology of C/C braking materials,control of pyrolytic carbon structures of CVI,mechanical properties and fracture mechanisms,friction and wear properties and mechanisms . The main contents and the conclusions are as follows.1.Using needled-carbon fiber felts as the preform, C/C composites were fabricated mainly by chemical vapor infiltration(CVI) and subsidiarily by liquid resin or pitch impregnation. In order to improve the rate of CVI and control the micro-structure of pyrolytic carbon, the technology of acquiring ideal pyrolytic carbon structures through control of the depositional micro-atmosphere was brought forward and Directional Flow CVI technique was invented. As a result, C/C composites with ideal rough laminar microstructures were prepared.2. The bending strengths, inter-laminar shear strengths, lognitudinal and transverse compressive of C/C composites were tested and analysed. For C/C composites with different pyrolytic carbon micro-structure: the bending strengths and inter-laminar shear strengths of C/C composites with smooth laminar structures (SL) are clearly higher than those with rough laminar structures(RL) and SL+RL; the strengths increase with density and the higher the density, the more increase the strengths. Lognitudinal compressive samples of C/C composites are destroyed by delamination; transverse compressive samples are destroyed by shear or delamination according to densities. C/C composites with 30% fiber volume fraction have higher strength value than that of with 40% and 25%.3. Friction and wear properties of C/C composites were tested under simulating different aircraft braking conditions. C/C composites with mainly RL pyrolytic carbon possess excellent friction and wear properties,and its high stable friction coefficient and low wear rate can be kept at different braking pressures and speeds. It is because wear debris produced by RL pyrolytic carbon are easily distorted by frictional shearing and come into being a smooth film covering over the friction surface, which can be restored by itself and thus ensure the stable friction and wear properties.4. The study on final heat treatment temperatures shows: due to strong frictional shearing promoting the move of carbon atoms in the friction film,braking under high speeds and pressures results in the graphitization of the carbon atoms in the film. Thus, original graphitization degrees of RL structure pyrolytic carbon corresponding to different heat treatment temperatures, have not much influence on friction and wear properties at the braking speeds above 25m/s.5.The investigation of braking frictional characterization shows: friction coefficients are closely related with braking speeds or temperatures on the frictional surface. Under low temperature ( at braking speed 5m/s or static), the water vapor adsoipted will lead to low friction coefficients. When the temperature is high engough to make the frictional surface dehydrated (such as at braking speed lOm/s), the friction coefficient will increase very high. Then, friction coefficients will decrease due to the decrease of the shear strength of the frictional film as the continously increase of the braking speeds and temperatures.6.Experiment on real products with inertia braking system were conducted .The friction and wear properties of C/C brakes made at home are simlar to or better than that of original Dunlop under different braking conditions, thus C/C brakes made at home can better meet the requirements of aircrafts.