| In this paper, the effects of NH4F content and calcining process on phase transition of Al2O3 and morphology of a-Al2O3,the effect of fluoride(AlF3、NH4F) on the density, microstructure and thermal shock property of high purity alumina ceramic prepared by cold and hot pressure method, the influence of molding process and sintering process on wear resistance of high purity alumina ceramic balls, the diameter distribution of the green spheres prepared by granulation method and the high purity alumina ceramic balls have been investigated. The results are as follows:The introduction of NH4F could remarkably decrease the phase transition temperature of a-Al2O3. With the increase of NH4F content, the complete transition temperature of α-Al2O3 decreases gradually. α-Al2O3 could be achieved from Al(OH)3 powder with 5.0mass%NH4F content calcined at 900℃ for 2.5 h, which was 200-250℃ lower than the conventional transition temperature. At same phase transition temperature, the complete transition time of α-Al2O3 also decreases with the NHUF content. With the increase of NH4F content from 1.0mass% to 5.0mass%, the morphology of α-Al2O3 changes from the cluster-shaped particles to regular hexagonal plate-like shape. And the hexagonal platelets of α-Al2O3 "melted" to granular shape when calcined at 1200℃ or 1300℃ for a longer time. However, the’melting’ phenomenon does not observed through the same calcining processes in the disks samples pressed at 25 MPa pressure.The NH4F contents have great effects on the density and microstructure of high purity alumina ceramic. In pressureless sintering, with the increase of NH4F content from 2.0mass% to 4.0mass%, the density of high purity alumina ceramic increases from 3.18 g/cm3 to 3.71 g/cm3, and then decreases when NH4F content exceeds 4.0mass%, the density of high purity alumina ceramic with 5.0mass% NH4F decreases to 3.70 g/cm3. Meanwhile, the morphology of alumina grains changes from platelet shaped crystals to polygonal crystals. With the increase of sintering temperature, the densification degree of high purity alumina ceramic also increases. The density of sample increases from 3.09 g/cm3 to 3.67 g/cm3 as the sintering temperature arises from 1450℃ to 1600℃. With the increase of sintering time, the density of high purity alumina ceramic increases from 3.61 g/cm3 to 3.67 g/cm3. But when the sintering time exceedes 2 h, there are some grains growing up abnormally in the sample, and the density of high purity alumina ceramic sintered for 2.5 h decreases to 3.62 g/cm3.AIF3 can remarkably change the crystal growth habit of alumina. In hot-pressing sintering, with the increase of AlF3 content, the morphology of alumina grains changes from spherical grains to platelet shaped crystals, and eventually the high purity alumina ceramic with embedded plate microstructure is obtained. With the increase of AIF3 content, the density, flexural strength and thermal shock resistance all first decrease and then increase. When the mass fraction of AIF3 is 3.0%, the high purity alumina ceramic with density of 3.59 g/cm3, flexural strength of 243.6 MPa and residual flexural strength of 88.3% is obtained, in which the thermal shock resistance increases 18% compared with that of alumina ceramic with non-added AIF3.With the increase of rotating speed and rotating time, the percent of green spheres with small size increases. With the increase of sintering temperature or the extension of sintering time in pressureless sintering, the density of high purity alumina ceramic balls gradually increases, but the wear rate decreases substantially. At the same time, the particle size of alumina powder has great effects on the density of high purity alumina ceramic balls. High purity alumina ceramic balls with the density of 3.57 g/cm3 and wear rate of 3.09%, respectively, are prepared at 1600℃ for 2 h with high purity alumina powder milled for 5 h as the raw material. |
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