Simultaneous growth mechanism of intermediate silicides in MoSi2/Mo system

The simultaneous growth mechanism of the Mo5Si3 and Mo3Si layers in the MoSi2/Mo diffusion couple was investigated using optical microscopy (OM), scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and X-ray diffraction (XRD). The MoSi2/Mo diffusion couple was made by chemical vapor deposition (CVD) of Si on the Mo substrate at 1100°C for 5 h and annealed at temperatures of 1250 and 1600°C, respectively, in an argon atmosphere. Simultaneous parabolic growth of the Mo5Si3 and Mo3Si layers was observed at an early annealing stage of the MoSi2/Mo diffusion couple. From the location of Kirkendall voids observed in the Mo5Si3 layer and the change of columnar diameter and texture of Mo5Si3 grains, simultaneous growth mechanism of the Mo5Si3 and Mo3Si layers was found. The phase transformation of MoSi2 into Mo5Si3 and Si contributed to approximately 30% of the total thickness of Mo5Si3 layer, but the remainder of Mo5Si3 layer and the Mo3Si layer with small thickness were formed by the diffusion reaction of the released Si and Mo substrate. These results indicated that Si was the only diffusing element in the Mo5Si3 phase.