Research Of Mechanical Properties Of MoSi₂ Composites Doped With Carbon Nanotubes

Recently, with the rapid development of aerospace , higher service temperature are demanded by new structural materials. Refractory metal silicide and intermetallic compound have absorbed universal attention because of their high melting point, among which MoSi₂ are considered as the most promising high temperature structural material due to its high melting point, excellent antioxidant capacity and corrosion resistance. But MoSi₂ have low fracture toughness at room temperature and poor creep ability at high temperature. So some methods such as strengthening and toughening were used to enhance its fracture and creep abilities.Carbon nanotubes (CNTs) probably offer a kind of nanosize reinforcement which is lightweight, and has exceptional high axial strength. Tensile strength and Young's modulus of CNTs have been measured to be as high as 800 GPa and 1 TPa, respectively. Moreover, CNTs have been proven to be stable under temperatures as high as 2800℃under vacuum. In this work, CNTs is used to improve the inherent brittleness of MoSi₂.CNTs were dispersed with C₁₂H₂₅SO₃Na using ultrasonic, and then The Mo, Si, MoSi₂ powder were mixed with CNTs powder by wet-milling and then made into MoSi₂ matrix composite using vacuum hot pressing. The mechanical properties, the composition and ditribution of materials, and the microstructure of the composite were studied by mechanical analysis, X-ray diffraction (XRD), optical microscope (OM), and scanning electron microscope (SEM).The results show that CNTs are dispersed in alcohol solvents incorporated with C₁₂H₂₅SO₃Na by ultrasonic agitation to decrease its conglobation. The optimal fabrication process of CNT/MoSi₂ composite is: heating-up powder to 1350℃at the speed of 15℃/min, and insulating for 1 h, compressing powder slowly at room temperature, continuing heating-up powder to 1450℃at the speed of 20℃/min and insulating and maintaining pressure for 30 min and then furnace cooling to room-temperature. After this process, CNTs/MoSi₂ composite with relative density above 90% is obtained.XRD analysis showed that the material without CNTs is composed of MoSi₂. In the composite, a small amount of Mo_4.8Si₃C_0.6 and SiC appear and the amount of Mo_4.8Si₃C_0.6 and SiC phase increase with increasing the content of CNTs.The strength and toughness of the composites which fabricated by powders of Mo, Si and MoSi₂ system are improved significantly compared with MoSi₂ matrix. The strengthening effect of Mo, Si system is more significant than MoSi₂ system. The Vickers hardness, bending strength as well as compressive strength of the composites with the CNTs addition of 3wt%, compared with the monolithic MoSi₂, are enhanced 32%, 82% and 65%, respectively, while MoSi₂ system has better toughening function, the fracture toughness of composite added with 3wt.% CNTs becomes 1.34 times of the monolithic MoSi₂.The analysis of the microstructure and crack propagation of CNTs/MoSi₂ composite indicates that grain refining and second phase strengthening are the strengthening mechanisms of CNTs/MoSi₂ composite. While crack deflection at the CNTs/matrix interface is considered as the main toughening mechanism of carbon nanotube reinforced MoSi₂-based composites.