Synthesis, Microstructure And Properties Of MoSi₂ Matrix Composites

Molybdenum disilicide（MoSi2） has been extensively investigated as a promising material for ultrahigh-temperature applications due to its high melting point and excellent high temperature resistance to oxidation. The limitations of MoSi2 monolithic material such as low fracture toughness at room temperature（2-3 MPa.m1/2）, poor creep resistance at high temperature and the “pest oxidation” behavior at approximately 500 °C have restricted its use in structural applications. Most promising routes reported to improve fracture toughness are（1） developing composites and（2） addition of alloying elements. In this thesis,（MoX）Si₂-Al₂O₃（X=Nb, Co, Ni） ceramics were synthesized by combustion synthesis（CS）, in which the alloying elements Nb, Co and Ni and the second-phase particles were introduced by thermite-reaction. MoSi2-MoB-SiC multiphase ceramics were synthesized by combustion synthesis from Mo, Si, and B4 C powders.For（MoX）Si₂-Al₂O₃（X=Nb, Co, Ni） ceramics, with the increase of Al2O3 formation, the combustion temperature and propagation velocity of combustion wave are increasing because of thermal reaction is a high exothermic reaction. X ray diffraction results show that the combustion products include major phase MoSi2, trace Mo5Si3 and Al2O3. For MoSi2-MoB-SiC multiphase ceramics, the combustion temperature and propagation velocity of combustion wave decrease with the increasing of B4 C addition. The combustion products are three-phase mixture of MoSi2, MoB and SiC, and the relative content of MoB increase with the increasing of B4 C addition.The milled composite powders were sintered by hot-pressing（HP） to achieve bulk materials. For（Mo X）Si₂-Al₂O₃（X=Nb, Co, Ni） hot-pressed ceramics, the Nb, Co and Ni alloyed MoSi2 ceramics（（MoX）Si2（X=Nb, Co, Ni）） has similar bending strength, lower Vickers hardness and higher fracture toughness with MoSi2. With the addition of Al2O3, the bending strength, the Vickers hardness and fracture toughness increased gradually. Take an example of（MoNb）Si₂-Al₂O₃, the（MoNb）Si2-10 mol%Al2O3 shows the highest mechanical properties: bending strength 742 MPa, Vickers hardness 11.3 GPa and fracture toughness 3.4 MPa.m1/2. In addition, Mo5Si3 phase has a very strong attraction to alloying elements（Nb, Co and Ni）. For MoSi2-MoB-SiC hot-pressed ceramics, with the increase of B4 C addition, the bending strength has a slight improvement, The Vickers hardness and fracture toughness increased gradually. MoSi2-0.4MoB-0.1SiC shows the highest mechanical properties: bending strength 537 MPa, Vickers hardness 12.5 GPa and fracture toughness 3.8 MPa.m1/2.In（MoX）Si₂-Al₂O₃（X=Nb, Co, Ni） ceramics, Al2O3 can enhance the mechanical properties as the second-phase particles, meanwhile, Al2O3 can inhibit the formation of brittle phase SiO2. In MoSi2-MoB-SiC ceramics, the fine grain SiC crystalline distribution in the SiO2 phase which can strengthen the brittle phase SiO2 to enhance the mechanical properties of the hot-pressed ceramics.