Ji-Young Byun

Growth kinetics of three Mo-silicide layers formed by chemical vapor deposition of Si on Mo substrate

Abstract The growth kinetics of three Mo-silicide layers formed by chemical vapor deposition (CVD) of Si on a Mo substrate using SiCl 4 -H 2 gas mixtures were investigated at temperatures between 950 and 1200 °C. Three Mo-silicide layers (Mo 3 Si, Mo 5 Si 3 , and MoSi 2 ) grew simultaneously with a parabolic rate law after an initial nucleation period, indicating the diffusion-controlled growth. The activation energy (130 kJ/mol) for the MoSi 2 layer were in a good agreement with the previous results having low activation energy (130±20∼157 kJ/mol), but its growth rate was higher than the previous results with high activation energy (209∼241±25 kJ/mol). A possible explanation about this difference may be the detrimental effect of impurities such as oxygen on the growth rate of the MoSi 2 layer. The activation energy (350 kJ/mol) for growth of the Mo 5 Si 3 layer was consistent with the prior values (297∼360 kJ/mol) obtained by annealing of the MoSi 2 /Mo diffusion couples, but its growth rate was an order of magnitude lower than the rate measured in the MoSi 2 /Mo diffusion couples. The activation energy (223 kJ/mol) for the growth of the Mo 3 Si layer was similar with the value (199 kJ/mol) obtained from annealed Mo 5 Si 3 /Mo diffusion couple at temperatures between 1250 and 1350 °C. This value was lower than the value (326 kJ/mol) reported at higher temperatures from 1500–1715 °C. This suggests that the rate-limiting step for growth of the Mo 3 Si layer is the grain boundary diffusion-controlled process at low temperatures but volume diffusion-controlled process at high temperatures. The growth rates of the Mo 3 Si layer measured at condition of the simultaneous parabolic growth of three Mo-silicide layers were approximately two orders of magnitude lower than the rates measured in the Mo 5 Si 3 /Mo diffusion couples. The differences in the growth rates of the Mo 5 Si 3 and Mo 3 Si layers depending on the type of diffusion couples were well explained by the multiple layer growth model.

Study on reaction and diffusion in the Mo–Si system by ZrO2 marker experiments

The dominant diffusing element in the MoSi2, Mo5Si3 and Mo3Si was found to be Si from marker experiments using ZrO2 particles. Based on these marker experiments, the formation mechanism of the Mo5Si3 and Mo3Si layers by reaction and diffusion was also discussed.

Formation of MoSi2–SiC composite coatings by chemical vapor deposition of Si on the surface of Mo2C layer formed by carburizing of Mo substrate

Abstract The formation process of MoSi 2 /β-SiC composite coating by chemical vapor deposition of Si on the Mo 2 C layer formed in advance on a Mo substrate by a carburizing process was investigated using optical microscopy, field-emission scanning electron microscopy cross-sectional transmission electron microscopy and X-ray diffraction. The composite coating was composed of equiaxed MoSi 2 grains with average size of 300 nm and the β-SiC particles with average size of 92 nm, which were mostly located at the grain boundaries of MoSi 2 . The morphology of β-SiC particles was oblate-spheroidal shape and volume percentage was approximately 19.3%. The number of cracks in the composite coating was smaller than that for the monolithic MoSi 2 coating by the reduced mismatch of thermal expansion coefficient with the Mo substrate. The formation mechanism of MoSi 2 /β-SiC composite coating was suggested on the basis of microstructure observation.

Effect of ammonia nitridation on the microstructure of MoSi2 coatings formed by chemical vapor deposition of Si on Mo substrates

Abstract The effect of ammonia nitridation on the microstructure of MoSi 2 coatings formed by chemical vapor deposition (CVD) of Si on Mo substrates at 1100 °C was investigated. The microstructure of monolithic MoSi 2 coating formed without a prior nitridation of Mo exhibited a typical columnar structure perpendicular to the Mo substrate. On the other hand, a two-step deposition process (ammonia nitridation followed by CVD of Si) produced a MoSi 2 /α-Si 3 N 4 composite coating, which consisted of equiaxed MoSi 2 grains with an average size of 0.5 μm, where the α-Si 3 N 4 particles with an average size of 120 nm were mostly located at the grain boundaries of MoSi 2 . The shape of α-Si 3 N 4 particles was oblate-spheroidal type and its volume percentage ranged from 12.9 to 17.7%. The density and average width of cracks in MoSi 2 layer formed by the mismatch of thermal expansion coefficients between the MoSi 2 coating and the Mo substrate was smaller for the two-step deposition process than that for CVD process only. The columnar MoSi 2 coating grew by the diffusion of Si and conversion of Mo 5 Si 3 phase to MoSi 2 phase at the interface of MoSi 2 /Mo 5 Si 3 , while the MoSi 2 /α-Si 3 N 4 composite coating grew by successive displacement reaction of Mo 2 N phase and Si. The growth of MoSi 2 grains was inhibited by the nanosize α-Si 3 N 4 particles.

Multilayer diffusional growth in silicon–molybdenum interactions

Growth kinetics of the Mo–silicide layers formed by chemical vapor deposition of Si on a Mo substrate from the SiCl4–H2 gas mixtures at 1000 °C was investigated using the ‘Wang’ analysis of multilayer diffusional growth. All of the three Mo–silicide phases, tetragonal-MoSi2, Mo5Si3 and Mo3Si in the Mo–Si binary phase diagram were observed by cross-sectional transmission electron microscopy, and obeyed a parabolic rate law indicating diffusion-controlled growth. The intrinsic growth rates of the Mo5Si3 and Mo3Si layers were estimated from their apparent growth rates measured in the Si/Mo diffusion couple. Good agreement was found with the reported values measured from apparent growth rates at the MoSi2/Mo and Mo5Si3/Mo diffusion couples, respectively.

The modification of microstructure to improve the biodegradation and mechanical properties of a biodegradable Mg alloy.

The effect of microstructural modification on the degradation behavior and mechanical properties of Mg-5wt%Ca alloy was investigated to tailor the load bearing orthopedic biodegradable implant material. The eutectic Mg/Mg2Ca phase precipitated in the as-cast Mg-5wt%Ca alloy generated a well-connected network of Mg2Ca, which caused drastic corrosion due to a micro galvanic cell formed by its low corrosion potential. Breaking the network structure using an extrusion process remarkably retarded the degradation rate of the extruded Mg-5wt%Ca alloy, which demonstrates that the biocompatibility and mechanical properties of Mg alloys can be enhanced through modification of their microstructure. The results from the in vitro and in vivo study suggest that the tailored microstructure by extrusion impede the deterioration in strength that arises due to the dynamic degradation behavior in body solution.

Simultaneous growth mechanism of intermediate silicides in MoSi2/Mo system

Abstract The simultaneous growth mechanism of the Mo 5 Si 3 and Mo 3 Si layers in the MoSi 2 /Mo diffusion couple was investigated using optical microscopy (OM), scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and X-ray diffraction (XRD). The MoSi 2 /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 Mo 5 Si 3 and Mo 3 Si layers was observed at an early annealing stage of the MoSi 2 /Mo diffusion couple. From the location of Kirkendall voids observed in the Mo 5 Si 3 layer and the change of columnar diameter and texture of Mo 5 Si 3 grains, simultaneous growth mechanism of the Mo 5 Si 3 and Mo 3 Si layers was found. The phase transformation of MoSi 2 into Mo 5 Si 3 and Si contributed to approximately 30% of the total thickness of Mo 5 Si 3 layer, but the remainder of Mo 5 Si 3 layer and the Mo 3 Si 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 Mo 5 Si 3 phase.

Formation of MoSi2–Si3N4 composite coating by reactive diffusion of Si on Mo substrate pretreated by ammonia nitridation

Abstract A MoSi 2 /α-Si 3 N 4 composite coating was formed on a Mo substrate by a displacement reaction of Mo 2 N and Si. The composite coating was composed of equiaxed MoSi 2 grains with average size of 0.5 μm and α-Si 3 N 4 particles with average size of 120 nm mostly located at the grain boundaries of MoSi 2 .

Effect of Cl/H input ratio on the growth rate of MoSi2 coatings formed by chemical vapor deposition of Si on Mo substrates from SiCl4–H2 precursor gases

Abstract Under chemical vapor deposition (CVD) conditions of Si on Mo substrate limited by mass transport of reactant gas species through a gas boundary layer, the effect of the Cl/H input ratio on the growth rate of MoSi 2 coating at 1200 °C was investigated using a horizontal hot-wall reactor and SiCl 4 –H 2 gas mixtures. The growth kinetic of the MoSi 2 coating obeyed a parabolic rate law irrespective of Cl/H input ratios. The growth rate of MoSi 2 coating initially increased with increasing Cl/H input ratio until the maximum growth rate was reached, and then decreased inversely proportional to the Cl/H input ratio at higher Cl/H input ratio. This suggests that there is a limit to the increase in growth rate for MoSi 2 coating due to the etching effect of Si with respect to the input ratio of Cl/H. The etching effect of the Cl/H input ratio on the growth rate of the MoSi 2 coating was explained by thermodynamic calculations based on the variation of silicon activity on the surface of the MoSi 2 coating. The mass balance between the Si flux supplied by the mass transport step and that consumed by solid-state diffusion to form MoSi 2 coating was considered to be responsible for the activity of silicon.

An ab initio study of the energetics for interfaces between group V transition metal Nitrides and bcc iron

An ab initio study was carried out on interface energies, misfit strain energies and electron structures at coherent interfaces between bcc Fe and Nitrides (XNs) (NaCl structure, X = V, Nb, Ta). The interface energies at relaxed interfaces Fe/VN, Fe/NbN and Fe/TaN were −0.051 J m−2, −0.226 J m−2 and −0.643 J m−2, respectively. The influence of bond energy was estimated using the discrete lattice plane/nearest neighbour broken bond model. It was found that the dependence of interface energy on the type of nitride was closely related to changes of the bond energies between Fe, X and N atoms before and after formation of the interfaces Fe/XN. The misfit strain energies in Fe/VN, Fe/NbN and Fe/TaN systems were −0.052, 0.178 and 0.005 eV per 16 atoms (Fe 8 atoms and XN 8 atoms). The misfit strain energy became larger when the difference in lattice parameters between the bulk Fe and the bulk XNs increased.