Preparation And Mechanical Properties Of Short-Cut C_f/MoSi₂–SiC Composites

Based on the background of MoSi₂ based composites in the application of aerospace high-temperature structural materials, the short-cut Cf/MoSi₂-SiC composites have been made by the hot-pressing sintering method. Before the process, SiC-coating was prepared on the surface of short-cut carbon fibers by the in-situ reaction method and MoSi₂-15%SiC matrix powder was fabricated by the mechanical alloying method. The microstructure were studied by Scanning Electron Microscope (SEM) and Energy Dispersive Spectrometer (EDS). And the phase analysis was carried out on X-ray Diffraction (XRD). Then the mechanical properties were determined by three point bending experiment.The analysis of the microstructure and the phase including XRD, SEM and EDS is as follows: Coating phase isβ-SiC, and along with concentration of coating slurry increasing, SiC-coating got thicker while the peak ofβ-SiC got higher; In the matrix powder, there were plenty of -MoSi₂,β-MoSi₂ and little Mo, and mechanical alloying degree was greatly increased. The short-cut Cf/MoSi₂-SiC composites have great compact structures, in which there were -MoSi₂,β-SiC, C and Mo4.8Si3C0.6.We have studied effects of hot-pressing sintering, SiC-coating thickness and volume fraction of Cf on the phase composition and microstructure morphology. The results are as follows: with the increase of sintering temperature, both the damage of Cf surface and the content of second phase including Mo4.8Si3C0.6 got increased gradually. With the increase of the volume fraction of Cf, the distributional degree of carbon fibers and matrix minimized. In the interface of the short-cut Cf/MoSi₂-SiC composites whose Cf has no SiC-coating , there were a large number of white brittle phase named Mo4.8Si3C0.6, which made severe damage in Cf surface. SiC-coating can reduce the interface reaction effectively, and with the increase of concentration of SiC-coating slurry, Both the damage of Cf surface and the content of Mo4.8Si3C0.6 got great reduced.When the concentration of SiC-coating slurry increases to 50g/l and the volume fraction of Cf is 5%, the short-cut Cf/MoSi₂-SiC composites gained excellent distributional degree and interface state, fracture toughness , Vickers hardness and the bending strength of which reach the highest value of 5.566MPa·m1/2, 13.74GPa and 383.4MPa under the sintering process of 1450℃holinging for 1h, respectively, which are approximately 219.5%, 55.4% and 122.6% higher than MoSi₂ matrix which is sintered by hot-pressing also. When the sintering process change for another one, the bending strength, Vickers hardness and fracture toughness of short-cut Cf/MoSi₂-SiC composites still got the second high value of 5.44MPa·m1/2, 12.01GPa and 376.7MPa, respectively, which are approximately 213.9%, 35.6% and 117.6% higher than MoSi₂ matrix. The strengthening mechanism of the composites is fine-grain strengthening, dispersion strengthening and the second-phase strengthening. The sequence of the contribution to toughening is as follows: fine-grain toughening, crack deflection and the fracture mechanism of fiber with SiC-coating is debonding at first and then pull-out.The bending strength of the short-cut Cf/MoSi₂-SiC composites with two sintering process can reach the value of 544.9MPa and 521.4MPa at 800℃, respectively, which are approximately 36.2 % and 30.35% higher than MoSi₂ matrix. With the temperature increasing to 1000℃, the bending strength of the composites decreased, but is still higher than MoSi₂ matrix.