Study On Service Performance Modification Of C_sf/SiC Ceramic Matrix Composites

The brake pad is one of the most critical safety components of aircraft,, the materials performances are required more and more strictly with more and more extreme environment. Silicon carbide?SiC? ceramic is a hot topic material in the research of aircraft brake pad. In order to improve the toughness and maintain the SiC friction properties of SiC ceramics, the hot pressing sintering method was adopted to prepare C_sf/SiC ceramic matrix composites without adding additives in the paper, using SiC ceramic as matrix, short carbon fiber as reinforcing material, nanometer carbon powder and SiC as fillers. The effects of fiber dispersion, fiber content and sintering temperature on the intrinsic properties and service performance modification of the composites were studied. The toughening mechanism, friction and wear mechanism, oxidation resistance and thermal shock mechanism of C_sf/SiC composites were explored in theory.The change of surface morphology of fibers fitted the matrix, and the physical friction between fibers and matrix could be increased in air oxidation of 480-490 ? and 45 min heat treatment. The surface groove density of the fibers were larger with obvious etching effect in nitric acid by liquid phase method. The dispersion of carbon fiber in the powder was better, the damage degree of the fiber was lower with using HEC dispersant in 48 h ball milling.C_sf/SiC composites were sintered in different process conditions. The density of composites was decreased with the increase of fiber content, and the relative density was above 98 % when the fiber content was less than 30 vol.%. The hardness of composites was also decreased with the increase of fiber content. When the fiber content was 55 vol.%, the hardness reached the minimum value of 17.1 GPa. The composites were mainly composed of 6H-SiC and C, in which the 6H-SiC crystal was not changed by the presence of fibers. Fibers in the matrix were uniform with no agglomeration. But when the fiber content was too high, the fibers began falling off. The bending strength of composites were increased first and then decreased with the increase of fibers content. 10 vol.% fiber content reached the maximum value of 423 MPa. The fracture toughness were increased with increasing of fiber content, and reached the maximum value of 4.9 MPa·m1/2 in 55 vol.%. The toughening mechanism was mainly composed of fiber debonding, fiber pullout, fiber breakage and crack propagation.The density and hardness of composites were increased first and then decreased with increasing of sintering temperature. The densification reached the maximum value of 98 % at 2000 ?, and the hardness was 20.7 GPa at 2050 ?. C_sf/SiC composites were still dominated by 6H-SiC in different sintering temperature. Fibers were easy to be graphite at high temperature, and the degree graphite were evident with temperature increasing. The bending strength and fracture toughness of composites were increased first and then decreased with the increase of sintering temperature, the bending strength reached the maximum value of 396 MPa at 2050 ?, fracture toughness reached the maximum value of 4.6 MPa·m1/2 at 2000 ?.The friction and wear properties and thermal properties of C_sf/SiC composites were characterized. The friction coefficient of composites were decreased with the increase of fibers. When the fiber content was 55 vol.%, the friction coefficient reached the minimum value of 0.23. Wear rates of composites were decreased with the increase of fiber content, and wear rate reached minimum value of 2.65×10-6 mm3/N·m when the fiber content was 30 vol.%. The sintering temperature had little effect on the friction coefficient of composites. Composites mainly showed brittle spalling and abrasive wear with low fiber content, too many fibers would lead to the increase of wear rate of composites. Composites surface had a layer of 3-4 ?m Si O2 film at high temperature, which made composites good oxidation resistance. The thermal shock times of C_sf/SiC composites were increased with the increase of fiber content. 55 vol.% fiber content reached 52 times. After 5 thermal shock cycles, the bending strength of composites were decreased 20.5 % sharply, but strength decreased with the increase of times gradually. The fracture toughness of composites were decreased with the increase of thermal shock times, but not obvious. After 20 thermal shock cycles, the fracture toughness decreased only 16 %. The toughness of the composites was still dominated by fiber debonding, fiber pullout, fiber breakage and crack propagation, and composites had good thermal shock resistance.