Phase Equilibrium In SiC-Based Ceramic Systems At High Teperature

The phase diagram study of SiC-based materials is helpful to understand the phase equilibrium relationships and compatible regions in SiC-based reaction systems,and can provide theoretical foundations for the physical and chemical reactions at high temperature. The phase diagram is also an important basis for the composition design,process selection and performance prediction of SiC-based ceramics. So far, however,there have been rare reports on the phase diagrams of SiC-based systems, despite SiC is one of the important structural ceramics. Therefore, it is necessary to supply and complete the researches in this area right now. The present work tries to investigate the high temperature reactions and the phase relations in the systems which consist of SiC and sintering aids (SiO2, MgO, Al2O3, ZrO2) and in the systems of SiC-MO2(M=Ti,Zr)-RO(R=Mg, Ca, Sr, Ba). And the subsolidus phase diagrams of the above systems were obtained in this dissertation by experimental study and theoretical analysis. The main results obtained are listed as below:(1) In the investigation of phase relations of ternary SiC-SiO2-MgO, SiC-SiO2-Al2O3,SiC-SiO2-ZrO2 systems, it was found that SiC possesses strong chemical inertness and dose not react with SiO2, MgO, Al2O3 and ZrO2. Based on this, subsolidus phase diagrams of the ternary SiC-SiO2-MgO, SiC-SiO2-Al2O3, SiC-SiO2-ZrO2 systems were presented in this paper.(2) SiC was difficult to react with other members and compounds in ternary SiC-Al2O3-ZrO2, SiC-Al2O3-MgO systems and quaternary SiC-SiO2-Al2O3-ZrO2,SiC-SiO2-Al2O3-MgO systems due to its sluggish. And all phases were compatible with SiC in the quaternary systems, and subsolidus phase diagrams of the SiC-SiO2-Al2O3-ZrO2 system and the SiC-SiO2-Al2O3-MgO system were presented.(3) A new solid-state reaction was found in ternary SiC-ZrO2-RO (R=Mg, Ca, Sr,Ba) system. In fact,there is no chemical reactions between SiC and ZrO2, it means that the solid-state reaction would not occur if RO (R=Mg, Ca, Sr, Ba) does not participate the reactions. The mechanism of the solid state reaction may be favorably explained by the acid-base reaction theory, and the solid-state reaction process was investigated by means of thermodynamic calculation, chemical kinetics and experimental verification.(4) The displacement reaction of SiC+TiO2=TiC+SiO2 was found in the SiC-TiO2 system. However, ZrO2 and HfO2 can't react with SiC in the similar way.(5) The similar solid-state reactions were found in the systems which consist of SiC,(6) IV oxide MO2(M=Ti, Zr) and alkaline earth metal oxide RO (R=Mg, Ca, Sr, Ba).These reactions could be effective in producing UHT carbides MC(M=Ti, Zr) . And it will form the new reaction systems for reaction-sintering of the MC ceramics and MC/SiC composites.(7) Based on the research results of high temperature reactions in the SiC-MO2(M=Ti, Zr) -RO (R=Mg, Ca, Sr, Ba) systems, the ternary SiC-MO2-RO systems were expanded to the quinary SiC-SiO2-MC-MO2-RO systems, and the subsolidus phase diagrams of the quinary SiC-SiO2-ZrC-ZrO2-RO (R=Mg, Ca, Sr, Ba) and SiC-SiO2-TiC-TiO2-RO (R=Mg, Ca) systems were presented in this paper.