Oxidation of the intermetallics MoSi2 and TiSi2 - A comparison

The oxidation behavior of two MoSi2 variants, one Mo-rich and one Si-rich, and TiSi2 was investigated between 1000 and 1400 degrees C in air, oxygen and an 80/20-Ar/O-2 mixture. A protective SiO2 scale develops on MoSi2 in all atmospheres in the temperature range investigated The SiO2 modification changes around 1300 degrees C from tridymite to cristobalite. This change in SiO2 modification seems to cause an enhanced formation of SiO2 and evaporation of MoO3. The SiO2 grows at the MoSi2-scale interface. In ail a two-layer scale glows on TiSi2 between about 1000 and 1200 degrees C with an inner inwards growing fine-grain mixture of SiO2 + TiO2 and an outer outward-growing TiO2 partial layer. TiN formation in the transient oxidation is responsible for the formation of the inner mixed partial layer because in N-free atmospheres a scale of a SiO2 matrix with some Ti oxide precipitates inside is formed. A one-layer scale structure similar as that in N-free atmosphere is found on TiSi2 in nir at T > 1200 degrees C. In oxygen the TiO2 precipitates glow as needles mostly oriented perpendicular to the surface. Due to the faster oxygen transport in TiO2 compared with SiO2, these TiO2 needles act as ''oxygen pipes,'' causing an enhanced oxidation of TiSi2 in front of these needles. The SiO2 scale dissolves about 1-2% TiO2. This doping causes a mixed oxygen and Si transport with the consequence that the SiO2 scale on TiSi2 grows partly by oxygen transport inwards and Si transport outwards. The SiO2 modification is cristobalite over the entire temperature range investigated.