Abstract
The surface microstructure and growth mode of ceramic exercise a crucial influence to its overall properties. A surface composite method is used to manufacture alumina ceramic with composite surface layer. The alumina ceramic layer is reinforced by the method of co-sintering which is few in the study of structural ceramic surface modification. For the reason of liquid phase sintering, solid solution and in-situ new substances, the introduction of the second phase changes the growth mode of alumina ceramic, and further the microstructure and surface property.
The effect of single additive on surface microstructure and growth mode of alumina ceramic is investigated. Because the additives like MgO, SiO2, CaO, K2O and Na2O can produce liquid phase during sintering which is help to refine grains and stop the anisotropic growth of the grains, the obtained surface is mostly consisted by the equiaxed grains. But an excess amount of MgO can created MgAl2O4 Spinel on the surface of alumina. Because of the introduced SiO2, AM997 alumina obtains smooth grain boundaries and stops the anisotropic growth of the grains. As the amount of SiO2 increases, alumina grain size tends to be consistent. After CaO is reinforced on the surface of alumina, there is a microstructure of stripe grains alternating with equiaxial grains on the surface of AM997 alumina and a microstructure of anisotropic growth and abnormal growth for AKP50 alumina grains. With the sintering temperature of 1550℃, a lot of flaky grains are investigated on the surface of AKP50 alumina. And with the sintering temperature increasing to 1600℃, the ratio of length to diameter for the flaky grains increases from 3:1 to about 8:1. TiO2 and MnO tend to form solid solution after reinforced on the surface of alumina which can help to activate lattice, increase sintering power and coarsen alumina grains. ZrO2 coating with thickness of 10μm can be coated on the alumina cermic surface by this advance method with the good combination of ZrO2 and Al2O3, which is good to improve the surface toughness.
The effect of composite additives on the growth mode of alumina surface grains is investigated. The results indicate that MgO-SiO2 composite additives prompt the anisotropic growth of the AM997 alumina grains. It also shows that most grains are flaky grains and abnormal growth belongs to in-situ reaction which is good to form self-toughening microstructure. It also changes the growth mode of alumina grains located in the surface area with thickness about 200-300μm. For AM997 alumina,CaO-SiO2 composite additives can prompt the grains to anisotropic growth and create a great deal long column grains. Due to MgO-TiO2 composite additives, club-shaped,flaky and equiaxial grains are formed and the influenced depth is about 100μm. Due to MgO-TiO2-CaO composite additives, big equiaxial grains are formed on AM997 alumina surface, surround by club-shaped grains. With the sintering temperature of 1550℃, the ratio of length to diameter for the club-shaped grains is 3:1. And with the sintering temperature increasing to 1600℃, the ratio of length to diameter for the club-shaped grains will be more than 10:1.
The thermal shock resistance data is obtained from testing Al2O3 surface modified by TiO2 layer and aluminium titanate specimens. The thermal shock resistance has thus been improved due to the in-situ synthesized aluminium titanate coating, coarsening of alumina micro-grain structures and existence of submicron pores in the surface and sub-surface layers of the alumina ceramic.
Key Words: Alumina; Surface composite; Microstructure; Growth mode;Thermal shock resistance
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