Growth behavior and microstructure of oxide scale formed on MoSi2 coating at 773 K

Growth behavior and microstructure of oxide scale formed on MoSi 2 coating by cyclic oxidation testing in air at 500 °C were investigated using field emission scanning electron microscopy, cross-sectional transmission electron microscopy, glancing angle x-ray diffraction, and x-ray photoelectron spectroscopy. MoSi 2 coating was prepared by chemical vapor deposition of Si on a Mo substrate at 1100 °C for 5 h using SiCl 4 –H 2 precursor gas mixtures. After the incubation period of about 454 cycles, accelerated oxidation behavior was observed in MoSi 2 coating and the weight gain increased linearly with increasing oxidation cycles. Microstructural analyses revealed that pest oxide scale was formed in three sequential processes. Initially, nanometer-sized crystalline Mo 4 O 11 particles were formed with an amorphous SiO 2 matrix at MoSi 2 interface region. Inward diffusing oxygen reacted with Mo 4 O 11 to form Mo 9 O 26 nano-sized particles. At final stage of oxidation, MoO 3 was formed from Mo 9 O 26 with oxygen and growth of MoO 3 took place forming massive precipitates with irregular and wavy shapes. The internal stress caused by the growth of massive MoO 3 precipitates and the volatilization of MoO 3 was attributed to the formation of many lateral cracks into the matrix leading to pest oxidation of MoSi 2 coating.

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