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Fundamental Considerations on Plasma Sprayed Ceramic Coating (Report IV) : Effects of Mullite Addition on Alumina

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The effects of mullite addition to commercial alumina on lowering porosity and suppression of the transformation of metastable alumina were investigated. Apparent porosity of sprayed alumina coat with some mullite decreases from 8.5% to 6.0% with increasing mullite content up to 20wt%. Glassy phases in sprayed-mullite and alumina containing some mullite were identified as mullite glass from the results of DTA and X-ray diffraction analysis. Further, it was detected that the mullite glass exothermally recrystallized at about 950℃. From the results of X-ray diffraction analysis on specimens heat-treated at various temperature, it was indicated that the transformation of γ-Al_2O_3 formed after plasma spraying was inclined to be retarded. The results of X-ray diffraction analysis also suggested that silica dissolved in γ-Al_2O_3, which was produced by the decomposition of mullite in plasma-spraying, gave suppressive effect on the transformation of γ-Al_2O_3 to α-Al_2O_3.



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