Oxidation Behavior And Mechanism Of Si₃N₄-MoSi₂ And Si₃N₄-SiC Multiphase Ceramics

Si₃N₄-MoSi₂ and Si₃N₄-SiC multiphase ceramics were prepared by hot-pressing sintering method. 5wt.%Y2O3 and 5wt.%La2O3 were used as sintering additives. Relative density, flexural strength, hardness, electrical conductivity and thermal expansion coefficient were tested. The oxidation behavior of the two ceramics was studied using isothermal temperature oxidation method. Oxidation products, the surface morphology of oxidized samples and the microstructure of oxide layer were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and X-ray photoelectron Spectroscopy (XPS), and the oxidation mechanism of the two multiphase ceramics were discussed.The relative density, flexural strength and Vickers hardness of Si₃N₄-MoSi₂ ceramics increase with the content of MoSi₂. In contrast, the thermal expansion coefficient decreases with the increase of MoSi₂ content. The conduction threshold value of the Si₃N₄-MoSi₂ ceramic is estimated to be 30⁴5wt.%MoSi₂. The change of the relative density, flexural strength and Vickers hardness variety of Si₃N₄-SiC ceramics with the SiC content is very similar to the Si₃N₄-MoSi₂ ceramics except that Si₃N₄-SiC ceramics are nonconductor.As for Si₃N₄-MoSi₂ ceramics, when held at 1100℃, there is no significant weight change. While when held At 1300℃, the oxidation kinetic curves approximately follow parabolic laws, and the weight gain increases with the increase of the content of MoSi₂. The main reason for their weight gain is the formation of SiO2 and Mo5Si3 during the oxidation process. At 1500℃, during the initial oxidation period, positive logarithmic law dominates the oxidation process, and then minus parabolic law is strengthened with increasing the oxidation time, and after 15h, the minus logarithmic law begins to govern the whole oxidation process. The outward diffusion of Mo element and volatilization in the form of MoO3 is responsible for the change of oxidation kinetics from weight gain to weight loss at 1500℃.When the Si₃N₄-30wt.%SiC and Si₃N₄-45wt.%SiC composites are held at 1100℃, there is no significant weight change. However, when held at 1300℃...