Microstructures and Growth of SiO2 scales formed on MoSi2

摘要

In advanced jet engines and gas turbines, development of ultra-high temperature structural materials is indispensable. Molybdenum disilicide (MoSi2) has a high melting point (2303 K) and a relatively low density (6.24 g/cm3), and it undergoes brittle-to-ductile transition at a temperature of about 1173 K, in addition to excellent resistance to oxidation at temperatures above 1073 K. Therefore, MoSi2 is a promising candidate material for ultra-high temperature applications. It is well known that the excellent resistance of MoSi2 to oxidation at temperatures above 1073 K is due to the formation of a protective SiO2 scale due to selective oxidation of Si. On the other hand, at low temperatures, especially at around 773 K, MoSi2 shows accelerated oxidation behavior due to simultaneous oxidation of Mo and Si.The microstructures of scales formed on MoSi2 at high temperatures in air were observed by TEM and crystallization of amorphous SiO2 scales was investigated. At 1273 K and 1373 K,the scales had a structure consisting of amorphous SiO2 with small amounts of fine MoO3 grains. The scales at 1573 K and 1773 K had a structure consisting of amorphous and crystalline SiO2. Growth rate of the scale formed at 1773 K appreciably increased with increase in the crystallization of amorphous SiO2. It was thought that the increase in the oxidation rate at 1773 K was caused by a change in the diffusion mechanism from O2 diffusion to lattice diffusion of O2- through SiO2. In addition, the diffusion coefficient of oxygen was estimated from the growth rate of SiOoxygen was estimated from the growth rate of SiO2 scale.