As a high-temperature material, mullite has the characteristics of high softening point under load, good creep resistance and chemical resistance, low thermal expansion coefficient, and good thermal stability. When there is no external substance, mullite is easy to form at the grain boundary. The glass phase affects the high temperature performance of the material; when forming the corundum-mullite composite material with corundum, it can reduce the formation of the glass phase and significantly improve the mechanical properties. The corundum-mullite composite material concentrates both corundum and mullite. The advantages of this single-phase material, it has excellent high temperature strength, creep resistance, thermal shock resistance and higher use temperature (1650 ℃), its chemical stability is good, and it is not easy to react with the burned product, especially Suitable for firing soft magnetic (ferrite) materials and electronic insulating ceramics. At present, high-temperature push-slab kilns often use corundum-mullite kiln furniture. Compared with foreign products, domestic push-slab bricks have a lower life and stability Not good, the wear resistance and bending strength during application are not ideal, and it is easy to wear and fracture during use, especially the thermal shock stability and creep are not ideal, which are the main reasons for the poor performance of the push plate. The structure determines the properties. Since corundum, mullite particles and fine powder will not participate in the reaction during the firing process, the properties and structure of the corundum-mullite material are mainly determined by the content of silica powder and α-Al2O3 powder and the firing temperature. Decision. Therefore, it is of practical significance to study the influence of micronized powder and firing temperature on the high-temperature performance of corundum-mullite materials. At present, the research on corundum-mullite materials at home and abroad is mostly single-factor analysis, which is related to actual control. There is a big gap. Based on the optimized design of the particle phase composition and gradation, this paper controls the microstructure of the corundum-mullite composite ceramics through the orthogonal test of silica micropowder, alumina micropowder and firing temperature to high-temperature strength. , In order to improve the high temperature performance of multiphase ceramics.
experiment
1.1 Raw materials
The average particle size of α-Al2O3 micropowder and white corundum is below 5μm; the SiO2 micropowder is taken from Elkem, Norway, with a mass fraction of 98.3%, and its average particle size is 5.917μm; the particles used are tabular corundum, white corundum and electric Melt mullite has two particle size specifications: 0～1mm and 1～3mm.
1.2 Determination of experimental factors
If the influence of impurities on the properties of corundum-mullite materials is ignored or the influence of impurities on the properties of corundum-mullite materials is considered to be the same, since corundum, mullite particles and fine powder will not participate in the reaction during the firing process, It can be considered that the performance of corundum-mullite material is mainly determined by the mass fraction of silica powder and α-Al2O3 powder and the firing temperature. According to the previous test results and literature [9], the orthogonal condition can be determined as: w(α-Al2O3 Micropowder) are 7%, 9%, 11%, respectively; w (SiO2 micropowder) are 3%, 3.5%, 4%, respectively; the firing temperature is 1600, 1650, 1700°C, respectively.
1.3 Multiphase ceramic formula
The m (corundum): m (mullite) in the binding phase is approximately 75: 25, and the mass fraction of the binding phase is 36% to 38%. The final ingredient composition contains Al2O3 with a mass fraction of 70% to 81% and SiO2 with a mass fraction of 19 %～30%.
In this study, by adjusting the mass fraction and firing temperature of SiO2 micropowder and α-Al2O3 micropowder, the microstructure of corundum-mullite composite ceramics was controlled to achieve the purpose of improving the high-temperature strength of composite ceramics. According to the classic continuous accumulation theory, Andreasen uses U(Dp)=100.(Dp/Dpmax)q represents the density distribution, where U(Dp) is the cumulative percentage under the sieve (%), Dpmax is the maximum particle size, and q is the Fuller index. The test shows that when q= The accumulation of continuous graded particles at 0.33～0.50 has a smaller void ratio. In this study, q=0.45, so that the particle phase used has a denser packing structure. Among them, the composition of 1#～9# particles is 1～3mm Corundum phase, the mass fraction is 47%; 0～1mm fused mullite, the mass fraction is 15%.
1.4 Experimental method
The powder used as the binding phase is mixed uniformly with a ball mill, and the mixing time is 12h. The particle phase is mixed evenly according to the designed formula, and an appropriate amount of polyvinyl alcohol is added to stir, and then the binding phase is added, and the material is discharged after mixing evenly. It is formed by a press. After the formed samples are dried, they are fired at 1600, 1650 and 1700°C respectively, and the holding time is 4h.
The physical and mechanical properties of the fired samples are carried out in accordance with relevant national standards. The thermal stability test adopts the water-cooling method. The 25mm×25mm×125mm sample is directly used for the test. The high-temperature furnace is heated to 1100°C, and the sample is placed in After raising the temperature to 1100°C again within the period of time, keep it for 30 minutes, take it out and place it in flowing room temperature water (about 20°C) to quickly cool for 3 minutes, and use the percentage of residual strength of the sample to characterize the thermal stability of the product. Creep resistance test conditions In order to keep the temperature at 1600℃ in the air for 25h. The high-temperature flexural strength is tested with a 25mm×25mm×125mm sample, and the test condition is 3h at 1400℃ in the air. The S-570 scanning electron microscope (SEM) is used to observe the heat The microstructure morphology of the fractured surface of the sample before and after the impact.
in conclusion
(1) SiO2 micropowder, α-Al2O3 micron Thermal shock stability and creep have the greatest impact, followed by α-Al2O3 micropowder and silicon micropowder; the best test conditions are: w (α-Al2O3 micropowder) = 11%, w (SiO2 micropowder) = 3%, firing At a temperature of 1650°C, the sample properties under this condition are: bulk density 2.96g/cm3, porosity 18.5%, flexural strength loss percentage 30%, creep percentage 0.99%.
(2) α-Al2O3 micropowder, SiO2 micropowder and firing temperature will have a greater impact on the bonding state between the particles and the matrix, as well as the mullite, pores and residual α-Al2O3 in the matrix, which will have a greater impact on the thermal expansion coefficient, elastic modulus and Thermal conductivity also has an impact, which ultimately affects the thermal shock resistance of the material.
(3) The fracture of corundum mullite material at room temperature is controlled by the crack propagation process, while at high temperature it is controlled by the creep mechanism.
