In-situ Synthesis,Microstructure And Properties Of MoSi₂-MoB-ZrO₂ Matrix Composites

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?2030°C?,low density?6.24 g/cm³?and excellent resident oxidation.However,the actual application of MoSi₂ is limited to the problems of low fracture toughness at room temperature,poor strength at high temperature,as well as the“pest”behavior at approximately 500°C.In this thesis,MoSi₂-MoB-ZrO₂ multiphase ceramics were synthesized by combustion synthesis from Mo,Si,B,Zr,and MoO₃.In the high MoSi₂ content system,The second phase dispersion strengthening phase was successfully introduced into the MoSi₂ matrix material by self-propagating combustion synthesis,thereby achieving the purpose of multiphase composite strengthening of MoSi₂.The two phases of MoB and ZrO₂ can play a role of double tougheningandstrengtheningbysparkplasmasintering.Taking8MoSi₂-4MoB-3ZrO₂ as an example,the Vickers hardness is 12.6±0.07 GPa and the fracture toughness is 4.54±0.44 MPa.m^1/2,which is 1.29 and 1.67 times of MoSi₂,respectively.In xMoSi₂-MoB-3ZrO₂?x=5,4,3?system,the combustion synthesis temperature increases from 1946 K to 3877 K with the decreasing MoSi₂ molar ratio.xMoSi₂-MoB-3ZrO₂ multiphase ceramics were by spark plasma sintering.It is found that Si gasification produces a large amount of Mo₅Si₃,which synergizes with MoB and ZrO₂ to improve the hardness and fracture toughness of the composite ceramics.Compared to MoSi₂ ceramics,Taking 4MoSi₂-MoB-3ZrO₂ as an example,the Vickers hardness is 11.8±0.13 GPa and the fracture toughness is 7.8±0.49 MPa.m^1/2,which is1.29 and 2.89 times of MoSi₂,respectively.The microstructure and oxidation behavior of xMoSi₂-MoB-3ZrO₂ system coating for Mo substrate using spark plasma sintering was investigated.The results showed that the oxidation life of MoSi₂ coating at 1400 C is less than 80 h.The porosity and the amounts of cracks of the coatings are decreased with the decreasing MoSi₂ molar ratio.After oxidation,the composite coatings are made up of ZrSiO₄ and SiO₂ phases.Compared with 5MoSi₂-MoB-3ZrO₂ and 4MoSi₂-MoB-3ZrO₂,3MoSi₂-MoB-3ZrO₂ coating oxide film and interface diffusion layer are thinner and have the least quality loss.In contrast,the 3MoSi₂-MoB-3ZrO₂ composite coating has the best oxidation resistance.