High temperature oxidation behavior of HVOF-sprayed unreinforced and reinforced molybdenum disilicide powders

Intermetallics like silicides are useful for protective coatings against high-temperature corrosion. Especially molybdenum disilicide which has a great potential as protective coating e.g. in aircraft engines and gas turbines in the temperature range between 1400 and 1800°C due to its high melting point and its low brittle-ductile transition temperature of approximately 800–1100°C. Four types of coatings were produced by high velocity oxyfuel spraying (HVOF): unreinforced MoSi2 with low porosity, unreinforced MoSi2 with high porosity, with silicon carbide reinforced MoSi2 and with alumina reinforced MoSi2. The coatings as sprayed were characterized by XRD, SEM and EDX. Microhardness and porosity were measured. The oxidation behavior of the coatings was determined at 500, 1000 and 1500°C. The influence of the heating rate was investigated during oxidation tests at 1000°C. The tests at 500°C showed that the pesting depends on the porosity of the coating. SiC as reinforcing phase seems to accelerate pesting, while alumina reduces this reaction. Unreinforced MoSi2 coatings form a protective SiO2 layer on the surface with a thickness below 10 μm during oxidation at 1500°C. The layer seems to be glassy with cristobalite inclusions. The microstructure of the coating changes to a high crystalline two-phase system of α-MoSi2 and hexagonal Mo5Si3.