Effect Of Type Of Rare-Earth On Microstructure And Properties Of α-Sialon Ceramics

α-Sialons stabilized with different rare-earth cations or multi-cations were prepared by a two-step hot sintering. The relationship between microstructure and properties was systematically investigated. The nucleation and growth mode of elongatedα-sialon grains, as well as the self-toughening mechanism was revealed. The effect of type of rare-earth oxides on the oxidation behavior of materials was studied and the oxidation mechanism was also discussed.The experimental results show that Yb-, Y- and Dy-α-sialon ceramics consist of onlyα-sialon phase, while a few M'(R2Si3-xAlxO³⁺xN4-x), evenβ-sialon and 21R-AlN polytypoid are also found in large cation doped ceramics. With the increase of ionic radius, more elongated grains form and the aspect ratio tends to increase. The toughness increases, but the hardness decreases slightly.Sc₂O₃ with high melting point was incorporated into sialon ceramic. Onlyβ-sialon as main crystalline phase exists together with a small amount of 12H-AlN polytypoid. The formation of scandium containing AlN-polytypoid can provide a valid way to reduce the amount of grain boundary phase. The Sc-sialon exhibits a typically low hardness of 16 GPa due to itsβ-sialon phase assemblage. Moreover, the equiaxed morphology ofβ-sialon grains, which is discovered for the first time, also results in a relatively lower fracture toughness and strength, 541 MPa and 3.8 MPa?m1/2, respectively. Whereas, sialon ceramics doped with high melting point Lu2O3 mainly consist ofα-sialon phase with a few intergranular phase J'(Lu4Si2-xAlxO7+xN2-x). Lu-α-sialons possess very high hardness of over 21 GPa. With increasing the Lu2O3 content, the amount of intergranular phase increases, and the number and aspect ratio of elongatedα-sialon grains increase, too. When the extra content of Lu2O3 goes beyond 4wt%, the radial size ofα-sialon grains decreases obviously and the length decreases slightly. Lu-α-sialons exhibit an enhanced toughness and strength with the peak value of 4.7 MPa?m1/2 and 620.2 MPa for the material containing 4 wt% extra Lu2O3. The post-heat treatment results in an anisotropic growth of Lu-α-sialon grains and a decreased solubility of Lu³⁺. The intergranular phases tend to distribute at the triple-grain pockets and the solubility of Lu³⁺ increases. The toughness and strength are further improved.It is originally demonstrated that mixed cation scandium/lutetium dopedα-sialon was prepared. The addition of Lu2O3 in the composition promotes the Sc³⁺ to enter theα-sialon structure, and leads the producedα-sialon with elongated-grain morphology. The ScLu-α-sialon mainly consists ofα-sialon phase with small quantity ofβ-sialon, 12H' and J'. It possesses a high hardness, toughness and flexural strength with the values of 20.4 GPa, 5.2 MPa?m1/2 and 652.5 MPa, respectively.HREM analysis shows that no amorphous layer at theα-sialon/intergranular phase interface exists, but there is only a ¹ nm amorphous layer betweenα- andα-sialon grains which cannot be eliminated completely even after heat treatment in single-cation dopedα-sialon ceramics. In multi-cation ScLu-α-sialon, there is no amorphous layer atα/α-sialon andα/β-sialon interfaces.(S)TEM analyses indicate thatα-sialon always nucleates on initial unsolvedα-Si3N4 and then grows epitaxially on it. And the initial precipitation on theα-Si3N4 particle is rich in rare-earth and Al than the subsequent precipitation.This mechanism of heterogeneous nucleation and growth is applicable for majority of Re-α-sialon systems. HREM analysis shows that the core and shell have the same structure and crystallographic orientation, and the interface is coherent. The misfit dislocations and the contrast variation caused by the misfit strain are observed due to the difference in compositions and lattice constants. The anisotropic growth ofα-sialon grains are mainly resulted from the different growth mechanisms between the prisms and the base surface. The interfacial reaction controlled kinetics on the prisms and diffusion-controlled kinetics on the base surface (001) are believed to take effect, respectively. The formation of elongatedα-sialon grains facilitates self-toughening mechanism successfully, such as pullout of elongated grains and deflection and bridging of crack. The toughness is greatly enhanced as well as the strength. The high temperature strength of Lu- and ScLu-α-sialon ceramics are very high and can reach over 550 MPa even at 1400 oC. They possess very good oxidation resistance, with thinner dense oxidation layer and parabolic rate constants K≈2.5×10-6 - 4.2×10-6 mg2/(cm4?s) at 1300oC, which is one order of magnitude lower than that of Y-α-sialon. One rare-earth depleted zone forms beneath the oxidation layer due to the oxidation reactions and the outward diffusion of cations into the scale. The rare-earth depleted layer contacts closely with the oxidation layer and theα-sialon matrix.