Effect Of Processing Conditions On The Phase Constituent And Microstructure Of Mullite Ceramics

Mullite ceramics were fabricated using alumina micropowder, high purity SiO2 powder, coal gangue, molten mullite, barium carbonate and calcium carbonate as starting materials by reaction sintering. Influence of firing temperature, soaking time and the raw material system on the phase constituent and microstructure of samples were investigated by XRD, SEM and mechanical property test. Conclusions as follows:(1)BaCO3 was introduced as flux to promote the generation of liquid phase and the in-situ growth of mullite grain in the preparation of self-toughened mullite ceramic. At the same time celsian and mullite are chemical compatibility at high temperature.(2)Mullite ceramics in which the major phase was mullite and minor phases were monocline celsian, hexagonal celsian, little corundum and quartz were prepared with alumina micropowder, high purity SiO2 powder, barium carbonate and coal gangue as starting materials by reaction sintering. The content of mullite in mullite ceramics increased and the content of corundum decreased with the temperature increasing.(3)There exists a wide temperature interval between the forming temperature of the primary mullite and secondary mullite, so the microstructure of self-toughned mullite ceramics can be controlled by controlling the development and grain size of the primary mullite and the secondary mullite through controlling firing condition.(4)The densification of the samples prepared by hot-press sintering was more complete than the samples prepared by pressure less sintering. The samples became denser, apparent porosity decreased with the increase of firing temperature and prolong of firing time. The existence of closed pores in samples results in low densification in the case of lower apparent porosity.(5)The formation amount of mullite of the samples introduced molten mullite was more than the samples with coal gangue, and the distribution of mullite grain was more uniform. The flexural Strength was higher due to higher volume density of the samples with molten mullite. When increasing the sintering temperature and the soaking time, the development of the mullite crystal grain was more complete and toughening effect was better. Because there existed more closed pores in samples, the flexural strength and fracture toughness of samples were low.