Study On Grain Growth Model And Properties Of Alumina Ceramics Produced By Surface Composite Method

The surface microstructure and growth mode of ceramic exercise a crucialinfluence to its overall properties. A surface composite method is used to manufacturealumina ceramic with composite surface layer. The alumina ceramic layer isreinforced by the method of co-sintering which is few in the study of structuralceramic surface modification. For the reason of liquid phase sintering, solid solutionand in-situ new substances, the introduction of the second phase changes the growthmode of alumina ceramic, and further the microstructure and surface property.The effect of single additive on surface microstructure and growth mode ofalumina ceramic is investigated. Because the additives like MgO, SiO₂, CaO, K2O andNa2O can produce liquid phase during sintering which is help to refine grains and stopthe anisotropic growth of the grains, the obtained surface is mostly consisted by theequiaxed grains. But an excess amount of MgO can created MgAl2O4Spinel on thesurface of alumina. Because of the introduced SiO₂, AM997alumina obtains smoothgrain boundaries and stops the anisotropic growth of the grains. As the amount ofSiO₂increases, alumina grain size tends to be consistent. After CaO is reinforced onthe surface of alumina, there is a microstructure of stripe grains alternating withequiaxial grains on the surface of AM997alumina and a microstructure of anisotropicgrowth and abnormal growth for AKP50alumina grains. With the sinteringtemperature of1550℃, a lot of flaky grains are investigated on the surface of AKP50alumina. And with the sintering temperature increasing to1600℃, the ratio of lengthto diameter for the flaky grains increases from3:1to about8:1. TiO₂and MnO tendto form solid solution after reinforced on the surface of alumina which can help toactivate lattice, increase sintering power and coarsen alumina grains. ZrO₂coatingwith thickness of10μm can be coated on the alumina cermic surface by this advancemethod with the good combination of ZrO₂and Al2O3, which is good to improve thesurface toughness. The effect of composite additives on the growth mode of alumina surface grainsis investigated. The results indicate that MgO-SiO₂composite additives prompt theanisotropic growth of the AM997alumina grains. It also shows that most grains areflaky grains and abnormal growth belongs to in-situ reaction which is good to formself-toughening microstructure. It also changes the growth mode of alumina grainslocated in the surface area with thickness about200-300μm. For AM997alumina,CaO-SiO₂composite additives can prompt the grains to anisotropic growth and createa great deal long column grains. Due to MgO-TiO₂composite additives, club-shaped,flaky and equiaxial grains are formed and the influenced depth is about100μm. Dueto MgO-TiO₂-CaO composite additives, big equiaxial grains are formed on AM997alumina surface, surround by club-shaped grains. With the sintering temperature of1550℃, the ratio of length to diameter for the club-shaped grains is3:1. And with thesintering temperature increasing to1600℃, the ratio of length to diameter for theclub-shaped grains will be more than10:1.The thermal shock resistance data is obtained from testing Al2O3surfacemodified by TiO₂layer and aluminium titanate specimens. The thermal shockresistance has thus been improved due to the in-situ synthesized aluminium titanatecoating, coarsening of alumina micro-grain structures and existence of submicronpores in the surface and sub-surface layers of the alumina ceramic.