Adding Boron Carbide Properties Of Carbon / Carbon Composites

C/C composites are applied to the field of aviation and spaceflightand so on, because of their excellent high temperature mechanicalperformance and thermal physical performance and tribological property,but have some defects, for example tend to be oxidized at oxidableatmosphere of over 400℃. In order to overcome those defects andimprove its synthetical properties and spread their application fields, thematrix inhibited C/C composites by adding ceramic particles were studiedin this paper. The preforms with boron carbide were prepared by addingB₄C powder into integrated carbon felts. C/C composites were made ofthe preforms with B₄C additive which were densified by CVI (chemicalvapor infiltration) and following resin infiltration/carbonization. Themicrostructure feature and graphitization degree of the composites hadbeen studied by optical microscope, SEM, XRD and so on. The oxidationbehaviors, mechanical performance and tribological properties of thecomposites were also investigated, and the influencing factors of them arediscussed. The conclusions could be drawn as follows:(1) Most parts of boron carbide particles distributed in net mat layersof the preforms dispersedly. Boron carbide particles were surrounded bypyrolytic carbon, thus which seem like bud or worm. Also no interspaceswere found between boron carbide and pyrolytic carbon. Most of boroncarbide particles were dispersedly distributed in net mat layers of thepreforms. After CVI, Boron carbide particles were surrounded bypyrolytic carbon, which seemed like bud or worm. Also no interspaceswere found between boron carbide and pyrolytic carbon.(2) Boron carbide can accelerate graphitization of C/C composites.With the increase of boron carbide content and/or the raise of HTTtemperature, the graphitization degree of the composites rose. Hightemperature was indispensable to catalytic graphitization of boron carbide.When the temperature of treatment was lower, only the mechanism of"solid soluble B catalytic graphitization effect" worked; when thetemperature of treatment was increased, not only the mechanism of "solidsoluble B catalytic graphitization effect" was strengthened, but also other mechanisms of catalytic graphitization were induced, thus, thegraphitization degree of the composites was obviously improved.(3) At 2000℃HTT, oxidation resistance of composites with boroncarbide was less than that of without carbide boron. With the raise ofHTT temperature, oxidation resistance of composites with B₄C wasimproved. At 2500℃HTT, oxidation resistance of the C/C compositeswith B₄C was been significantly better than those without carbide boron,the reasons were that graphitization degree and graphite crystallite size ofthe composites had been obviously increased due to addition of boroncarbide and parts of boron atoms could insert into crystal lattice ofgraphite as substitutional boron atoms to reduce carbon reactivity, inaddition, B₂O₃ film and B₂O₃ clusters which could inhibit oxidationformed during oxidation.(4) The composites with B₄C had bending strength of 80～130MPaand transverse compressive strength of 90～150MPa and longitudinalcompressive strength of 60～110MPa. The addition of boron carbide cutdown the mechanical performance of C/C composites. And the moreaddition, the poorer mechanical performance. With the raise of HTTtemperature, the bending strength and compressive strength of C/Ccomposites with B₄C declined. The bending failure mode of C/Ccomposites with B₄C additive was brittle fracture. Especially thecomposites with more addition of boron carbide and/or higher HTTtemperature showed the more remarkable brittle fracture.(5) Tribological properties of the composites were various withdifferent HTT temperature. At 2000℃HTT, the worn mechanism wasgrain-abrasion, the abrasion of the composites was very severe, and thevalue of its friction coefficient was higher. When the HTT temperaturewas 2300℃, smooth lubrication film came into being at worn surface, asa result, abrasive resistance of the composites was improved dramaticallyand the value of friction coefficient was lower. However at 2500℃HTT,the wear rate and the value of friction coefficient were higher than thoseat 2300℃. Raise of braking pressure reduced friction coefficient of thecomposites while increased friction stability slightly and wear rate of thecomposites. The optimum tribological properties of C/C composites with B₄C were shown with coefficient of friction of 0.32, stability factor of0.60, linear loss of 1.9μm, ana mass loss of 8.69 mg.