Interaction of CVD Silicon with Molybdenum Substrates

Rice, M. J.; Sarma, Kalluri R.

doi:10.1149/1.2127637

Cited by 23 publications

(4 citation statements)

References 6 publications

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“…The phase diagram of Mo–Si confirms that all three phases are possible to evolve at temperatures around 1200 °C. The study of Yoon et al provides evidence that silicon is not able to adhere over longer annealing times to the Mo–silicide phases due to the large mismatch of the CTE (Si: 3.8 × 10 −6 K −1 , MoSi 2 : 9.5 × 10 −6 K −1 ) . In contrast to the scenario under vacuum described earlier, under oxygen containing atmosphere such as lab air the silicon coating on the TZM substrate behaves differently in the current study.…”

Section: Discussioncontrasting

confidence: 63%

Magnetron Sputtered Silicon Coatings as Oxidation Protection for Mo‐Based Alloys

et al. 2020

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Mo‐based alloys with solidus temperatures around and above 2000 °C are attractive high‐temperature structural materials for future applications in the hot section of gas turbines. However, their oxidation behavior is poor due to pesting starting at 600 °C and nonprotective oxide growth at temperatures above 1000 °C. To ensure a sufficient oxidation resistance over a wide temperature range, protective coatings become inevitable. Herein, silicon coatings have been applied by magnetron sputtering on Mo‐9Si‐8B and on titan–zirconium–molybdenum alloy (TZM). The coating architecture is designed to minimize the intercolumnar gaps and porosity, thereby increasing the density. Specimens are tested at 800 and 1200 °C in air isothermally for up to 300 h. The focus is put on the chemical reactions at the coating–substrate interface, the phase formation, and the evolution of the thermally grown oxide. An initially globular SiO2 evolves into a uniform SiO2 layer providing excellent oxidation protection. The investigations reveal a rather slow interdiffusion between the coating and the alloys when tested in air. At the coating–substrate interface exclusively, the Mo3Si phase develops. Finally, the phase formation at the coating–substrate interface is studied in detail for various heat treatments in air and vacuum.

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Section: Discussioncontrasting

confidence: 63%

Magnetron Sputtered Silicon Coatings as Oxidation Protection for Mo‐Based Alloys

et al. 2020

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“…Solid-state diffusion-controlled process.-Generally, the treating temperatures for formation of MoSi 2 coating by pack siliconizing or CVD are considered to be very high ͑Ͼ900°C͒, so it is commonly accepted that at such high temperatures the rate-limiting step for growth of MoSi 2 coating is a solid-state diffusion process of Si through MoSi 2 coating from the surface to the interface of MoSi 2 /Mo rather than a chemical reaction process of Si with Mo to form MoSi 2 phase. [31][32][33] It is well known that Si nonstoichiometry in MoSi 2 phase is below about 0.2 atom %, 34 and thus the diffusivity of Si through MoSi 2 phase was independent of composition, which is reasonable for diffusion in a line compound, and the dominant diffusion element in the MoSi 2 phase is Si. 19,31,32,35 From Fick's first law, the Si flux (J Si s ) consumed by the solid-state diffusion process of Si from the surface of the MoSi 2 coating to the MoSi 2 /Mo interface is given by…”

Section: Theoretical Backgroundmentioning

confidence: 99%

“…[31][32][33] It is well known that Si nonstoichiometry in MoSi 2 phase is below about 0.2 atom %, 34 and thus the diffusivity of Si through MoSi 2 phase was independent of composition, which is reasonable for diffusion in a line compound, and the dominant diffusion element in the MoSi 2 phase is Si. 19,31,32,35 From Fick's first law, the Si flux (J Si s ) consumed by the solid-state diffusion process of Si from the surface of the MoSi 2 coating to the MoSi 2 /Mo interface is given by…”

Section: Theoretical Backgroundmentioning

confidence: 99%

Growth Kinetics of MoSi[sub 2] Coating Formed by a Pack Siliconizing Process

Yoon

Lee

Kim

et al. 2004

J. Electrochem. Soc.

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Growth kinetics of MoSi 2 coating formed by pack siliconizing process of Mo substrate was investigated on the basis of the dependence of its growth rate on Si content in the pack powders. Pack siliconizing was carried out using ͑1-70͒ wt % Si-5 wt % NaF-bal SiC packs at 1100°C in a hydrogen atmosphere. The chemical vapor deposition ͑CVD͒ of Si on Mo substrate was also performed at 1100°C using SiCl 4 -H 2 gas mixture to determine the maximum growth rate of MoSi 2 coating obtainable in the bulk Si/Mo diffusion couple. The growth kinetics of the MoSi 2 coating formed by the pack siliconizing process obeyed a parabolic rate law irrespective of compositions of pack powders. The growth rates of MoSi 2 coating increased with increasing Si content in the pack powders up to 40 wt % Si but attained a constant value in the pack powders with over 40 wt % Si, which was equal to that obtained by the CVD process. The rate-limiting step for growth of MoSi 2 coating formed by the pack siliconizing process was subject to theoretical considerations. Three models such as a gas-phase diffusion controlled model, a solid-state diffusion controlled model, and a dynamic equilibrium model of gas-phase diffusion and solid-state diffusion were evaluated. The theoretically predicted results based on the dynamic equilibrium model and the experimentally obtained results were found to be in good agreement.The pack cementation process has been used for many years to produce corrosion-and wear-resistant coatings on the surface of metals and alloys. It is a relatively inexpensive, highly versatile, easily handled, and commercially feasible diffusion coating process for enriching the surface of metals and alloys with aluminum ͑aluminizing͒, 1-3 silicon ͑siliconizing͒, 4-6 and chromium ͑chromizing͒. 7-8 Pack cementation can generally be considered as a self-generated chemical vapor deposition ͑CVD͒ process carried out in a porous medium under isothermal conditions. The cementation pack consists of a mixture of powders of a source alloy ͑master alloy͒ containing the element to be transported, a halide salt activator, and a relatively inert filler ͑SiC, Al 2 O 3 , and SiO 2 , etc.͒. 9 After proper surface preparation, the substrate to be coated is buried in the pack powders, which is placed in a retort and heated isothermally under an inert or reducing atmosphere.It is generally believed that pack cementation proceeds in a series of seven steps: ͑i͒ chemical reaction step of the activator with master alloy in the pack, which determines the equilibrium vapor pressure of the active gas species; ͑ii͒ gas-phase diffusion of the active gas species from the pack to the surface of the substrate, driven by chemical potential gradients in the gas phase; ͑iii͒ a chemical reaction step to deposit the coating element from the active gas species at the surface of the substrate; ͑iv͒ solid-state interdiffusion of the coating element and substrate element; ͑v͒ the chemical reaction step to form coating; ͑vi͒ desorption of the reaction product gas species from the g...

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“…During the diffusion processes, the supersaturation of lattice vacancies develops into an interior pore which becomes the inner part of the final hollow structure. − This strategy has been extensively used for noble and transition metals, metal oxides, and sulfides. ,, However, the Kirkendall effect for Si or Ge has been rarely investigated because their diffusivities were expected not as high as the Kirkendall effect induced. Although a few trials for Kirkendall effect of Si or Ge were reported, − they found the Kirkendall effect mostly in only thin film structure, and the mechanism of the effect for such elements has remained elusive. Especially, the Kirkendall effect of Si element has not been apparent hitherto.…”

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confidence: 99%

Hollow Silicon Nanostructures via the Kirkendall Effect

Son

Choi³

et al. 2015

Nano Lett.

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The Kirkendall effect is a simple, novel phenomenon that may be applied for the synthesis of hollow nanostructures with designed pore structures and chemical composition. We demonstrate the use of the Kirkendall effect for silicon (Si) and germanium (Ge) nanowires (NWs) and nanoparticles (NPs) via introduction of nanoscale surface layers of SiO2 and GeO2, respectively. Depending on the reaction time, Si and Ge atoms gradually diffuse outward through the oxide layers, with pore formation in the nanostructural cores. Through the Kirkendall effect, NWs and NPs were transformed into nanotubes (NTs) and hollow NPs, respectively. The mechanism of the Kirkendall effect was studied via quantum molecular dynamics calculations. The hollow products demonstrated better electrochemical performance than their solid counterparts because the pores developed in the nanostructures resulted in lower external pressures during lithiation.

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Interaction of CVD Silicon with Molybdenum Substrates

Cited by 23 publications

References 6 publications

Magnetron Sputtered Silicon Coatings as Oxidation Protection for Mo‐Based Alloys

Magnetron Sputtered Silicon Coatings as Oxidation Protection for Mo‐Based Alloys

Growth Kinetics of MoSi[sub 2] Coating Formed by a Pack Siliconizing Process

Hollow Silicon Nanostructures via the Kirkendall Effect

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