In-situ Synthesis,Microstructure And Properties Of MoSi₂ Matrix Composites

Molybdenum disilicide?MoSi2?is a very competitive and potential candidate for high temperature structure materials,and has received wide applications in high temperature environment,due to the high melt point and excellent resident oxidation.However,the actual applications of MoSi2 are limited to three problems:low fracture toughness at room temperature,poor strength at high temperature,as well as the“pest oxidation”behavior at approximately 500°C.The alloying approaches and addition of ceramic particle-reinforcements are very effective methods to overcome the disadvantages of MoSi2 materials.In this thesis,in-situ synthesis of xMoSi2-0.4MoB-0.1SiC?x=1,0.75,0.5,0.25?,xMoSi2-0.2ZrB2-0.1SiC?x=1.0,1.5,2.0?and xMoSi2-0.2TiB2-0.1TiC?x=1.0,1.5,2.0?were fabricated by self–propagation high–temperature synthesis,while multiphases?MoB,SiC or ZrB2,SiC or TiB2,TiC?are introduced into MoSi2 matrix materials as toughening and strengthening methods.In xMoSi2-0.4MoB-0.1SiC system,the combustion synthesis temperatures decreased with the decreasing MoSi2 molar ratio.In xMoSi2-0.2ZrB2-0.1Si C and xMoSi2-0.2TiB2-0.1TiC system,the combustion synthesis temperatures increased with the decreasing MoSi2 molar ratio.The combustion synthesis mechanism of these systems is solution-precipitation.The xMoSi2-0.4MoB-0.1SiC?x=1,0.75,0.5,0.25?and xMoSi2-0.2ZrB2-0.1Si C ceramics were densified by hot-pressing and spark plasma sintering,respectively.The XRD results show that MoSi2,MoB,ZrB2 and Ti B2 are dominating phases,while SiC and TiC are secondary phases.The intensities of MoB,ZrB2,SiC,TiB2 and TiC phases increased with the decreasing MoSi2 molar ratio.Multiphase MoSi2 matrix materials showed good mechanical properties,such as the xMoSi2-0.4MoB-0.1SiC composites showed fracture toughness,Vickers hardness and bending strength improved with the incremental volume fraction of MoB and SiC phase.The maximum bending strength,Vickers hardness and fracture toughness were 827 MPa and 15.2 GPa,7.0 MPa m^1/2,respectively.The 1.0MoSi2-0.2ZrB2-0.1SiC composites showed that fracture toughness and Vickers hardness improve with the incremental volume fraction of ZrB2 and SiC phase.The maximum Vickers hardness and fracture toughness were14.0 GPa and 5.5 MPa m^1/2,respectively.The multiphase MoB and SiC,ZrB2 and SiC,TiB2 and TiC as second phase strengthening are successfully introduced into MoSi2 matrix materials through in-situ synthesis and SHS method.Meanwhile,the multiphases can significantly enhance the mechanical properties of MoSi2 matrix materials.