Microstructure, Friction And Wear Mechanisms Of C/C Composites

Owing to their low density, high specific strength and modulus, high rupture toughness, good thermal shock resistance, high heat of ablation, self-lubricating and low-wear capability, C/C composites have been widely used as aircraft brake materials. Their microstructure and properties have always been the research focus of scholars. However, the friction and wear mechanisms of C/C composites have not been understood.Based on the background of several kinds of C/C composites used in air braking disc, the detailed microstructures of the bulk materials, thermal properties, micro-mechanical properties, friction and wear properties and mechanisms, microstructure of friction surface layer were investigated by transmission electron microscope. The main research contents and results are as follows.(1) The thermal conductivity and coefficient of thermal expansion of C/C composites with resin-derived carbon, smooth laminar pyrocarbon, regenerative laminar pyrocarbon and rough laminar pyrocarbon added with resin-derived carbon were tested and analyzed. For C/C composites impregnated with resin:the thermal conductivity decreased with increasing temperature. For samples with no resin-derived carbon:the thermal conductivity increased with increasing temperature. For sample with regenerative laminar pyrocarbon, in the direction of parallel to carbon cloth, the concentrated distribution of large pores is help to reduce the CTE, it posses the lowest CTE and thermal stress under the same temperature, which is help to stable the friction coefficient and reduce the wear rate.(2) The influence of detailed microstructures of C/C composites on their micro-mechanical properties was studied. Deformation models for matrix carbon with different texture degree were established. Isotropic resin-derived carbon displayed elastic deformation, it shown the highest elastic modulus and hardness. While high textured pyrocarbon displayed plastic deformation, and shown relative lower elastic modulus and hardness. Micro-mechanical properties of matrix carbon with other texture degree between them, and they decreased with increasing texture degree.(3) Friction and wear properties of C/C composites were tested under simulating aircraft braking condition. C/C composites with resin-derived carbon, rough laminar pyrocarbon or regenerative laminar pyrocarbon show high stable friction coefficient, which consistent with the typical characteristics of the friction coefficient curve of C/C composite as brake material. Friction coefficient of C/C composites with smooth laminar pyrocarbon has no significant variation with changing the braking condition. Friction coefficient of C/C composites with pure or partial resin-derived carbon decreased first and increased afterwards. Resin-derived carbon shows high hardness and it is difficult to grind, compact and pile up under high pressure. So that there is no lubricant layer was formed, shear force and friction coefficient increased. C/C composites with resin-derived carbon show the highest wear rate under each braking condition.(4) Samples are distinguished into three types according to the microstructure of matrix carbon around the carbon fiber. Three wear models are established. When the texture degree of matrix carbon decreased from high to low, failure occurs first at the highly textured matrix carbon or at the interface between the carbon fiber and highly textured matrix carbon. For C/C composites with pyrocarbon added with resin-derived carbon, failure occurs at the interface between the carbon fiber and highly textured pyrocarbon, highly textured matrix carbon and interface between ptrocarbon and resin-derived carbon. When the texture degree of matrix carbon increased from low to high, failure mainly occurs at the high textured pyrocarbon, and wear occurs layer by layer, the wear rate is low.(5) The difference between friction layer and friction film are discussed for the first time. Friction layer with different thickness was formed on the bulk material of each sample under each braking condition. The top surface of the wear debris is flat and consists mainly of compacted nanoparticles of less than400nm in size. On the contrary, the underside of the wear debris is very rough and is mainly composed of equiaxed particles, chopped carbon fibers, and some granular particles larger than20μm. The nanosized particle consisted of amorphous carbon and a few small crystallites with thicknesses of less than2nm. Less organized carbon is present between the better-organized crystalline regions. On the contrary, the microsized particle was characterized as anisotropic carbon, with crystallites of thickness larger than10nm.(6) In addition to the friction layer, a friction film with a thickness of less than200nm has been observed for the first time. It covers the top surface of the friction layer. The friction film is characterized by a laminar structure, the belts of which have been revealed to be highly oriented graphene sheets, and the distance between two adjacent belts is about150-300nm. The crystallite thickness of highly oriented graphene sheets are larger than20nm, the surrounding carbon belt was characterized as having a less ordered structure.(7) Temperature field of C/C composite with asperity was studied by means of finite element simulate and experiment. The highest temperature of asperities is much higher than that of worn plane, which is one of the main factors in leading to stress graphitization on worn surface. This result was verified by RMS.