The Research Of Preparation Of Mullite Whisker From Andalusite

Mullite has high strength, excellent creep resistance, low coefficient of thermalexpansion and good thermal shock resistance; therefore it can be used under high temperature,wear and oxidation conditions. Accordingly, mullite whiskers are excellent compositereinforcement, which can be used to improve the comprehensive performance of the metal orceramic matrix composites. Compared with non-oxide whiskers, mullite whisker has excellentresistance to oxidation and can be used in the high temperature oxidation conditions.Preparation of mullite whiskers from andalusite and application of mullite whiskers inceramic matrix composite were studied in this paper.Mullitization of andalusite had shown that until at1500℃can the andalusite completelyconvert into mullite and β-quartz phase. The addition of alumina in andalusite helpedconsume excess SiO₂and promoted the process of mullitization of andalusite.Effects of admixtures on mullitization of andalusite also had been studied. Thetemperature of mullitization can be reduced with addition of a certain amount of aluminumfluoride. Increased the content of aluminum fluoride appropriately, mullitization of andalusitemineral occurred relatively at lower temperature. There is a relationship between aspect ratioof mullite grain and concentration of aluminum fluoride in the gas phase. Mullite whiskerswith a diameter of0.5-2.0μm（aspect ratio of20-40） were prepared successfully under thecondition of M3（68wt%andalusite+32wt%alumina）, plus6wt%of aluminum fluoridesintered at1400℃for4hours from andalusite. However, the size of mullite whiskers is notuniform and there are some impurities. So the further optimization process is needed. Thepossible growth mechanisms of mullite whisker may be VS（Vapor-Solid） growth mechanismwhen aluminum fluoride was added.Significant change in morphology of mullite was not found by adding magnesium oxidein the process of mullitization of andalusite. Short mullite grains with relative uniform sizewere obtained by adding some boron oxide, which was effective in decreasing the aspect ratioof mullite particle.Application of mullite whiskers had been studied. The bending strength of ceramicmaterials reinforced by mullite whisker had been improved to some extent. When the amountof mullite whisker was20wt%, the maximum bending strength105MPa was obtained.Thermal shock resistance of the materials also had been enforced. The times material canwithstand most to the thermal shock was26when addition of mullite whiskers was30wt%. The study demonstrated that mullite whiskers indeed have played a role in strengthening andimproving the thermal shock resistance of the material.