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Thermodynamic analysis of silicon deposition on ASTM P92 and AISI 4340 steels
Surface & Coatings Technology 205 (2010) 325–331
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Thermodynamic analysis of silicon deposition on ASTM P92 and AISI 4340 steels P.A. Hernández, S. Mato, M.P. Hierro, F.J. Pérez ⁎ Grupo de Investigación de Ingeniería de Superficies y Materiales Nanoestructurados Nº910627, Universidad Complutense de Madrid, Facultad de Ciencias Químicas, 28040 Madrid, Spain
a r t i c l e
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Article history: Received 14 May 2010 Accepted in revised form 23 June 2010 Available online 7 July 2010 Keywords: P92 steel 4340 steel Silicon coating CVD-FBR THERMO-CALC
a b s t r a c t Chemical Vapor Deposition in a Fluidized Bed Reactor (CVD-FBR) has numerous advantages compared to other surface modification techniques like good heat and mass transfer during the deposition process, temperature uniformity and high degree of mixing between the gases and fluidized particles involved. This technique has been used to obtain silicon coatings on ferritic steels, which improves the limited corrosion resistance of the substrate. Analysis of optimum conditions by means of numerical simulations for the CVD process of silicon deposition on the AISI 4340 and ASTM P92 steels has been presented in this work. The temperature and pressure effects, the silicon conversion factor starting from the precursors formed, as well as the possible equilibria taking place in the formation of the silicon coating have been studied based on the thermodynamic calculations carried out on the THERMO-CALC software tool. © 2010 Elsevier B.V. All rights reserved.
1. Introduction During the last decades, researchers have made great efforts to develop new techniques to deposit protective thin coatings on substrates with good mechanical properties but with low resistance to corrosion. The most reliable techniques that have been incorporated to industrial processes because of their high versatility and allowance of rapid growth of thin layers are Physical Vapor Deposition (PVD), Chemical Vapor Deposition Assisted by Plasma (PECVD) and Chemical Vapor Deposition (CVD). The latter in particular has been successfully scale up to the industry because of the following characteristics: • Capability to produce high purity coatings. • Production of uniform coatings with good reproducibility, reasonable adherence and high deposition rate. • Control of the crystalline structure and coating morphology by controlling the deposition parameters. • Easy adjustment of the atomic deposition ratio [1,2]. Among all the chemical vapor deposition techniques there is a variant, the CVD in fluidized bed reactor (CVD-FBR), which consists on a bed of solid particles that are fluidized by introducing a constant flux of gas in the reactor. CVD-FBR variant combines the advantages of CVD mentioned before with those related to the fluidized bed, including good heat and mass transfer in the deposition process, temperature uniformity and high degree of mixing between the gases and the fluidized particles. This technique can be even more effective if a prior thermodynamic analysis is carried out to design the optimum working conditions. That ⁎ Corresponding author. Tel.: + 34 91 39 44215; fax: + 34 91 39 33457. E-mail address: [email protected] (F.J. Pérez). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.06.054
task can be undertaken thanks to commercially available computational tools which provide an excellent support in the study of processes governed mainly by thermodynamic laws. Although this is not the case for CVD, where diffusion and mass transport phenomena have an important role in the process, the computational method helps us to understand the relationships between composition, microstructure and conditions of the system. This means that it is possible to perform qualitative predictions like the phases that can be formed in the process and their compositions at certain given conditions of temperature and pressure. Silicon based coatings on ferritic steels have been developed by various methods in order to improve corrosion and erosion resistance [3,4] or as adhesion promoter for polymer films [5]. When such coatings are produced by CVD the interfacial problems associated to any coating/substrate system with different thermomechanic properties are overcome, since CVD coatings are the result of a surface modification generated by diffusion of the deposited material into the base material [6]. Therefore, the study of this process helps to solve usual adhesion problems related to residual stresses and thermal coefficient mismatch. The main objective of this work is to predict the best conditions for the deposition of a silicon based coating on AISI 4340 and ASTM P92 steels in a fluidized bed reactor by the CVD technique. To achieve this goal, thermodynamic calculations are carried out using the Gibbs energy minimization method with the thermodynamic simulation tool THERMO-CALC. 2. Thermodynamics considerations Many studies can be found in the literature which used thermodynamically calculated equilibrium states to anticipate or explain the experimental data obtained in a chemical process.
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Table 1 Compositions of the ferritic steel substrates used in this work (% in weight). Material
Cr%
V%
Ni%
Mo%
Mn%
Si%
W%
C%
Fe%
ASTM P92 AISI 4340
9.07 1.5
0.2 –
0.06 1.5
0.46 0.2
0.47 0.5
0.02 0.3
1.78 –
0.1 0.34
87.84 95.66
For instance, in relation with the process of interest here, Sirtl and Hunt describe the Si–H–Cl system by means of the standard enthalpy of formation, considering the initial partial pressure of precursors as starting point of the calculations and subsequently finding the equilibrium concentrations of the product species as a function of temperature [7,8]. Another usual thermodynamic function employed with similar purposes is the Gibbs free energy of the reaction, ΔG. It is worth remembering that at given conditions of temperature and pressure the occurrence of a compound can be predicted regarding the value of ΔG. The feasibility of the reaction of formation of the compound is estimated by the energy transfer of the system from an initial situation to the final, being favorable when that is negative, i.e, ΔG = ∑j ΔGj products
∑i ΔGi reactives
where ΔGj and ΔGi are respectively the Gibbs free energies of products and reactives involved in the reaction, and j and i are the number of them. Additionally, for a given temperature and pressure, a system is at equilibrium when the total Gibbs free energy of the reaction is at a minimum. Then, a possible approach to thermodynamic calculations consists in obtaining the coefficients that minimize the expression of the Gibbs free energy in terms of temperature, pressure and chemical composition. THERMO-CALC software performs such calculations in order to predict the composition of the most stable solid and gaseous phases and compounds formed at the system equilibrium fulfilling also the law mass action equations. Predictions of the variations of composition of the stable phases with temperature, pressure and composition of the initial state of the system can be also obtained. However, any software application would result useless without reliable and updated experimental databases. THERMO-CALC employed in its calculations high quality databases, as those provide by the Scientific Group Thermodata Europe, which are the strength of this software compared to similar applications also used for thermodynamic simulations [9]. The simulations presented here were carried out using SSUB3 Table 2 Chemical species calculated in the equilibrium of the Si/Ar/H2/HCl/Fe/Cr system at 101325 Pa and 500 °C. Gas Ar H2 SiH3Cl SiHCl3 SiCl4 HCl SiH2Cl2 SiCl2 SiH4 SiCl3 SiHCl H FeCl2 SiCl SiH3 CrCl3 Cl CrCl2 Si2H6
(mol/L)
Gas −1
7.44 × 10 2.46 × 10− 1 9.14 × 10− 3 5.38 × 10− 4 1.83 × 10− 4 1.04 × 10− 4 1.17 × 10− 5 1.72 × 10− 8 1.11 × 10− 8 4.11 × 10− 11 2.07 × 10− 12 4.47 × 10− 13 1.33 × 10− 14 3.41 × 10− 15 9.59 × 10− 16 2.01 × 10− 16 1.94 × 10− 16 1.16 × 10− 16 8.81 × 10− 17
SiH2 SiH FeCl3 Cl2 CrCl Si FeCl CrCl4 Fe2Cl4 Fe Cr Si2 HFe CrCl5 HCr CrCl6 Fe2Cl6 Cr2 Fe2
(mol/L) − 17
5.13 × 10 4.13 × 10− 20 3.82 × 10− 21 1.61 × 10− 21 8.70 × 10− 22 2.32 × 10− 23 2.04 × 10− 23 1.89 × 10− 24 1.79 × 10− 25 5.72 × 10− 26 3.09 × 10− 26 7.38 × 10− 30 1.47 × 10− 30 1.00 × 10− 30 1.00 × 10− 30 1.00 × 10− 30 1.00 × 10− 30 1.00 × 10− 30 1.00 × 10− 30
Solid
(mol)
Si2Cr Si Fe Si
5.10 × 102 1.70 × 10− 1 3.12 × 10− 1
Table 3 Chemical species calculated in the equilibrium of the Si/Ar/H2/HCl/Fe system at 101325 Pa and 500 °C. Gas
(mol/L)
Gas
(mol/L)
Solid
(mol)
Ar H2 SiH3Cl SiHCl3 SiCl4 HCl SiH2Cl2 SiCl2 SiH4 SiCl3 SiHCl H FeCl2 SiCl SiH3 CrCl3 Cl CrCl2 Si2H6
7.65 × 10− 1 2.25 × 10− 1 8.58 × 10− 3 6.26 × 10− 4 2.37 × 10− 4 1.04 × 10− 4 1.23 × 10− 5 1.96 × 10− 8 9.32 × 10− 9 5.00 × 10− 11 2.12 × 10− 12 4.28 × 10− 13 1.52 × 10− 14 3.64 × 10− 15 9.59 × 10− 16 2.44 × 10− 16 2.08 × 10− 16 1.33 × 10− 16 6.81 × 10− 17
SiH2 SiH FeCl3 Cl2 CrCl Si FeCl CrCl4 Fe2Cl4 Fe Cr Si2 HFe CrCl5 HCr CrCl6 Fe2Cl6 Cr2 Fe2
4.71 × 10− 17 3.96 × 10− 20 4.65 × 10− 21 1.83 × 10− 21 9.29 × 10− 22 2.32 × 10− 23 2.17 × 10− 23 2.45 × 10− 24 2.33 × 10− 25 5.72 × 10− 26 3.09 × 10− 26 7.39 × 10− 30 1.41 × 10− 30 1.00 × 10− 30 1.00 × 10− 30 1.00 × 10− 30 1.00 × 10− 30 1.00 × 10− 30 1.00 × 10− 30
Si Fe Si
3.26 × 101 3.46 × 101
for gas precursors reactions and SOL2 for substrate definition. Those databases contain assessed thermodynamic data on enthalpy of formation, entropy and temperature dependence of the heat capacity for the gaseous and condensed compound considered in the system studied in this work. In that way, the study of the feasibility of gaseous and solid phases by means of THERMO-CALC at given conditions should provide a guideline to define the optimum deposition conditions in the CVD-FBR process. 3. Description of simulated process and input parameters Briefly, the process to simulate here consists in a chemical vapor deposition on a steel surface carried out in a fluidized bed reactor. In this particular case, the bed is composed of Si (donor) and Al2O3 (inert filler material) powders, whose particles of ~150 μm of diameter are fluidized by a mixture of Ar, H2, and HCl gases introduced at the bottom of the reactor. This reactive gas mixture in contact to the silicon powder at an appropriate temperature leads to the formation of gas precursors that are transported through the reactor and diffused to the surface of the substrate to be coated. Then, the precursors are adsorbed onto the heated surface and subsequently decomposed, to finally form the silicon based coating. A complete description of the process has been already presented somewhere else [6,10]. Prior to the deposition process, input parameters were introduced in the Gibbs energy minimizer software THERMO-CALC in order to provide
Fig. 1. Variation of partial pressure of gaseous species present in the process of Si deposition in terms of the pressure reactor.
P.A. Hernández et al. / Surface & Coatings Technology 205 (2010) 325–331
Fig. 2. Partial pressure variation with temperature of the gaseous species present in the deposition process of Si on steel ASTM P92.
327
Fig. 5. Solid species present in the deposition process of Si on steel AISI 4340.
– and the concentration of the main elements of substrate, in this particular case Fe and Cr for P92 steel, and Fe for 4340 steel, according to their chemical composition (Table 1). The simulation will provide the concentration of products in the final gaseous and solid phases for which the minimum free Gibbs energy condition is satisfied. In the deposition process to be simulated a relation of gases of 69.6 vol.% Ar, 28 vol.% H2 and 2.4 vol.% HCl react with the silicon metal powder at 101,325 Pa of pressure in a temperature range between 0
Fig. 3. Solid species present in the deposition process of Si on steel ASTM P92.
theoretical information about the optimum conditions to achieve the Si deposition. The necessary parameters for the thermodynamic calculations are: – the ratio of gases in the Ar/H2/HCl mixture volume, – molar ratio of solid particles of the donor and the filler powders, i.e. Si and Al2O3 respectively,
Fig. 4. Partial pressure variation with temperature of the gaseous species present in the deposition process of Si on steel AISI 4340.
Fig. 6. Gibbs free energy of formation of species a) for P92 steel, b) for 4340 steel.
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compounds present in the gaseous phase for both steels, resulting from the reaction of the reactive gases with the silicon powder particles, are SiH3Cl, SiHCl3, SiCl4, SiH2Cl2 and SiCl2. In the solid phase Si and FeSi form, as well as Si2Cr for the steel with Cr content. Noticeably the formation of FeSi2 is not predicted by THERMO-CALC calculations although it is present as stable phase in the Fe–Si phase diagram. The reason is that in this work silicon and silicides deposition on steels by CVD-FBR is achieved through the decomposition of chlorosilanes precursors gases on the steels surfaces, not being the reaction for the formation of that specific phase thermodynamically favored. In addition to the reactive gases that remain after the reaction and the main precursors or chemical species directly responsible for the deposition of silicon, other possible precursors, according to the literature [10], appear in the calculation. It is common to assume that a gaseous phase is significant only if its partial pressure exceeds 10− 6 Pa, therefore, phases which have partial pressures below this value have been neglected in the graph simulations presented in this work. On the other hand, pressure can be an important parameter in chemical reactions. However, the study of the effect of the total pressure inside the reactor on the formation or stability of the precursor gases (Fig. 1) suggests its contribution to this particular process is almost negligible. Therefore, the CVD deposition can be run at atmospheric pressure providing a great operational flexibility and a simple experimental set up.
4.1. Thermodynamic simulations for steel ASTM P92
Fig. 7. Possible solid phases that were formed as a function of moles of silicon in the reactive bed a) for P92, b) for 4340 steels.
and 1000 °C. The selection of the input simulation parameters was based on previous works on silicon depositions by CVD-FBR [11]. 4. Thermodynamic results and discussion In Tables 2 and 3 are shown the gaseous and solid species calculated and its molar concentration when the equilibrium is established at 500 °C, for P92 and 4340 steels respectively. The major products
Fig. 2 shows the influence of temperature on the equilibrium compositions of the gas phase at 101,325 Pa for a system where the reactive elements are Si, H, Cl and Ar, and the main elements of the substrate are Fe and Cr with a molar ratio of Si:Fe:Cr:Ar:HCl: H2 =0.36:0.12:0.02:69:2.40:28. The partial pressures of the main precursors were calculated. For values beyond 10− 6 Pa the precursors are SiH3Cl, SiHCl3, SiCl4, SiH2Cl2, SiCl2 and SiHCl, among which the most representative gaseous reactants in the temperature range studied were SiH3Cl, SiHCl3 and SiCl4. These results are consistent with those obtained by Bolivar et al. [12] and Sanjurjo et al. [13]. Importantly, a plateau is observed in the temperature range corresponding to the CVD working temperatures for most of these precursors. Also of relevance, this graph shows the formation of iron chlorides in very low concentrations and only at temperatures above 1000 °C. This means that the thermodynamic calculations do not anticipate a significant formation of compounds which would indicate an attack of
Fig. 8. Gibbs free energy variation with temperature of several reactions of precursors that could take place in equilibrium.
P.A. Hernández et al. / Surface & Coatings Technology 205 (2010) 325–331
329
Fig. 9. Gibbs free energy variation with temperature of several reactions that could deposit Si.
the steel substrate during the coating process due to the presence of the HCl as reactive gas during the coating process. Fig. 3 shows the possible solid species to be formed as a function of temperature. The main solid phase which is expected to deposit is FeSi although Si and the intermetallic Si2Cr are also likely to form. Noticeably the formation of these solids is not affected by temperature for the range considered in this work. 4.2. Thermodynamic simulations for steel AISI 4340 In Fig. 4 the influence of the temperature on the equilibrium compositions of the solid and gaseous phases is presented at a constant pressure of 101,325 Pa. The reactive elements considered are Si, H, Cl and Ar, and the main element of the substrate is Fe solely, since the concentration of the alloying elements of the 4340 steel is for all of them lower than 2% in weight. The input molar ratio for the calculations is Si:Fe: Ar:HCl:H2 =0.36:0.18:69:2.4:28. According to their partial pressures, the predominant species result to be SiH3Cl, SiHCl3, SiCl4, SiH2Cl2 and SiCl2. Similarly to the results obtain for the P92 steel, the values of partial pressures are almost constant all over the temperature range characteristic of CVD, starting to decrease only when temperature rises to 1000 °C. However, unlike to P92 steel, the SiHCl gas phase and iron chlorides do not come into view for this steel even at the highest temperatures, finding instead the gaseous species SiH3. Furthermore, according to the simulation obtained for the solid phase disclosed in Fig. 5, the deposition of silicon is thermodynamically favorable, as well as an iron silicide like FeSi, in the complete temperature range considered. 4.3. Study of the stability of the solid phases Regarding Fig. 6 (a) and (b), where the change in the Gibbs free energy of formation of solid species on the P92 and the 4340 steels is Table 4 Gibbs free energy of formation of SiCl2 at 500 °C. Chemical reactions
ΔG (kJ), T = 500 °C
(a) 2Si + 6HCl ↔ SiCl 4 + SiCl2 + 3H2 (b) SiHCl3 ↔ SiCl2 + HCl (c) SiHCl + SiH2Cl2 ↔ SiH3Cl2 + SiCl2 Si + 2HCl ↔ SiCl2 + H2 SiH2Cl2 ↔ SiCl2 + H2 SiCl4 + Si ↔ 2SiCl2 SiCl4 + H2 ↔ SiCl2 + 2HCl SiH3Cl + SiCl4 ↔ SiCl2 + Si + 3HCl SiH3Cl + HCl ↔ SiCl2 + 2H2
− 163.07 121 − 100.6 5.67 47.3 169 163 199 40.34
presented against temperature, it can be deduced that the stability of iron silicide and the deposition of silicon increase with temperature for both steels. Nevertheless, in the case of the P92 steel, which has higher chromium content, the formation of silicon and iron silicide competes with the formation of chromium silicides which shows a lower stability than those. In addition, a simulation was performed to calculate the possible solid phases that may deposit on both substrates varying the molar fraction of silicon in the donor of the reactive bed (Fig. 7). It can be seen in these figures that, at a certain molar fraction of silicon in the bed, the phase representing the ferritic bulk material with body centered cubic structure is not longer stable and iron is more prone to be combined with silicon. Likewise, Fig. 7 shows that the concentration of FeSi phase has a maximum at approximately 0.17 mol of silicon for both steels decreasing up to that concentration in benefit of an increase of pure silicon deposition. At this point of the discussion it is convenient to remember that CVD deposition is a non-equilibrium process and that THERMO-CALC calculations are based on the assumption that the system to consider is at its equilibrium. This means that although a phase results thermodynamically stable following the THERMO-CALC simulation it will be not necessarily present. Since diffusion phenomena are favored at the working temperature, silicon atoms are expected to diffuse into the steel substrate forming, instead pure Si or FeSi, iron silicides with different stoichiometry. 4.4. Thermodynamic studies of precursor gases and products formation Now, considering the precursors formed by the direct reaction of the Si donor with the reactive gases HCl and H2, which were obtained as result of the previous thermodynamic calculations, it is possible to propose the candidates reactions that are responsible for the Si deposition. To achieve this, first, the feasibility of the reactions for the precursors formation as a function of the working temperature is studied by means of the variation of the Gibbs free energy of each reaction, in correspondence to temperature (Fig. 8). Regarding that figure it can be concluded that the formation of all the possible precursors is expected at the temperature range of operation of the CVD-FBR reactor, i.e. from 300 to 600 °C, except for SiCl2 which has a positive value of Gibbs free energy of formation. However, since the above simulations predict its presence, this suggests the possibility of its formation not through the direct interaction between Si, HCl and H2, but through an intermediate reaction carried out in the gaseous phase.
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Fig. 10. Gibbs free energy variation with temperature of several reactions that could form SiCl2.
Next, the precursors reactions leading to the silicon deposition are studied. As observed in Fig. 9 the only reactions of the gaseous species predicted by THERMO-CALC with negative values of the Gibbs free energy for the temperature range of interest are those where SiCl2 is involved, i.e. reactions (d) and (e) (the last one previously described by Petit et al. [14]). Now, the free Gibbs energies at 500 °C of the reactions which could lead to the formation of SiCl2 taking into account the precursors predicted by THERMO-CALC in the gaseous phase are considered in Table 4. Then, knowing that this is the main precursor for the formation of silicon, the most probable reaction to occur regarding Table 4 is: 2Si + 6HCl←→SiCl4 + SiCl2 + 3H2
ΔG = −163kJ
The input temperature for the calculations was 500 °C since that is a representative temperature for the process of interest here. Additionally, the variation with temperature of the free Gibbs energy of these reactions has been disclosed in Fig. 10 in order to study their feasibility in a range of temperature. In this way, reaction (b), already proposed by Petit et al. [14] as initiation reaction at the CVD deposition temperature of silicon layers, is only favored at temperatures above 1200 °C when the ΔG of the reaction reaches negative values. Since in CVD-FBR the working temperature is between 300 °C and 650 °C this reaction is ruled out.
Likewise, reaction (c), proposed by Swihart et al. [15] to explain dichlorosilane decomposition as precursors for the CVD deposition of silicon, has negative value of ΔG in the entire temperature range considered in the figure, resulting therefore also feasible at the conditions of the CVD-FBR deposition. Thus, the reactions (b) and (c) can be suggested to be the responsible of the main precursors formation, meanwhile reactions (d) and (e) would lead to silicon deposition on the steels substrates. On the other hand, if the reactions involved in the equilibrium deposition starting from precursors and ending up in the solid compound FeSi are considered (Fig. 11), it is found that only SiCl2, SiH2Cl2 and SiH3Cl reacting with Fe could lead to deposit FeSi on the steel substrate, since those are the reactions with negative values of ΔG. At 500 °C the most probable reaction would be that of SiCl2, as occurs in the case of the pure silicon deposition (Table 5). Finally, Fig. 12 (a) and (b) shows the conversion of the main precursors to the Si and FeSi phases following the calculations carried out by THERMO-CALC. The number of Si moles deposited by the interaction of the main gaseous precursors, SiH3Cl, SiCl2 and SiH2Cl2, with the substrate do not show dependence on temperature, having a greater factor of conversion for the first two mentioned precursors (Fig. 12 (a)). Also, the formation of FeSi takes place at the entire range of temperatures considered (Fig. 12 (b)). However, for values between 400 °C and 900 °C the SiH2Cl2 precursor shows no production of FeSi
Fig. 11. Gibbs free energy variation with temperature of the reactions of several precursors that could form FeSi.
P.A. Hernández et al. / Surface & Coatings Technology 205 (2010) 325–331 Table 5 Gibbs free energy of formation of Si by several precursors at 500 °C. Chemical reactions
ΔG (kJ), T= 500°C
(d)2SiCl2 ↔ Si + SiCl4 (e)SiCl2+ H2 ↔ Si + 2HCl SiHCl3 + H2 ↔ Si + 3HCl SiH2Cl2 ↔ Si + 2HCl SiCl4 + 2H2 ↔ Si + 4HCl
− 169 − 5.12 [14] 116.999 42.18 –
331
and ASTM P92. According to the analysis and discussion of results obtained by THERMO-CALC, the following conclusions were drawn: – The main gaseous species expected to be generated when the donor reacts with the reactive gases are SiH3Cl, SiHCl3, SiCl4, SiH2Cl2 and SiCl2. – The solid species expected to be deposit on the substrate surface are Si, FeSi and, in the case of the P92 steel, Si2Cr. – No chemical attack to the substrate is expected during the deposition process, since the presence of chloride associated with the elements of the substrate material is negligible in the range of operating temperatures. – The final precursor directly responsible for the deposition of silicon on the substrates would be SiCl2. – The CVD-FBR process can be carried out at atmospheric pressure and temperatures in the range of 500 to 650 °C. Further experimental studies are needed to validate the above conclusions and to clarify the role of thermodynamic considerations in the deposition process of Si by CVD-FBR.
Acknowledgements The authors want to express their gratitude to the Spanish “Ministerio de Ciencia e Innovación” for the financial support to this work under the projects CSD2008-00023 and ENE2008-06755-C0202/CON.
References
Fig. 12. Conversion of solid phases in function of temperature starting from several precursors a) for Si and b) for FeSi.
and only SiCl2 and SiH3Cl are associated to a good conversion precursors to solid phases independently of temperature in the studied range. This is in agreement with the previous conclusions obtained showing that SiH3Cl is the main precursor since it is involved in the formation of SiCl2 and subsequently in the formation of the coating. On the other hand, since the stability of the ferritic structure of the substrate imposes a temperature limitation, and previous works pointed out that at temperatures higher than 650 °C CVD depositions result in porous coatings with serious spallation problems [16,17], working temperatures in a range of 500 to 650 °C are advised. 5. Conclusions Thermodynamic approaches have been made using the program THERMO-CALC in order to find the optimum working conditions for the deposition of silicon by CVD-FBR on two ferritic steels, AISI 4340
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Further reading [18] J.M. Brossard, M.P. Hierro, L. Sanchez, F.J. Bolivar, F.J. Perez, Surf. Coat. Technol. 201 (2006) 2475. [19] J.M. Brossard, M.P. Hierro, J.A. Trilleros, M.C. Carpintero, L. Sanchez, F.J. Bolivar, F.J. Perez, Surf. Coat. Technol. 201 (2007) 5743. [20] A. Sanjurjo, K. Lau, B. Wood, Surf. Coat. Technol. 54 (55) (1992) 219. [21] C. Klam, J.P. Millet, H. Mazille, J.M. Grass, J. Mater. Sci. 26 (1991) 4945. [22] A. Sanjurjo, B. Wood, K. Lau, G. Krishnan, Scripta Met. Mater. 31 (8) (1994) 1019. [23] F.J. Perez, M.P. Hierro, C. Carpintero, F. Pedraza, C. Gomez, Surf. Coat. Technol. 140 (2001) 93.
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Thermodynamic analysis of silicon deposition on ASTM P92 and AISI 4340 steels P.A. Hernández, S. Mato, M.P. Hierro, F.J. Pérez ⁎ Grupo de Investigación de Ingeniería de Superficies y Materiales Nanoestructurados Nº910627, Universidad Complutense de Madrid, Facultad de Ciencias Químicas, 28040 Madrid, Spain
a r t i c l e
i n f o
Article history: Received 14 May 2010 Accepted in revised form 23 June 2010 Available online 7 July 2010 Keywords: P92 steel 4340 steel Silicon coating CVD-FBR THERMO-CALC
a b s t r a c t Chemical Vapor Deposition in a Fluidized Bed Reactor (CVD-FBR) has numerous advantages compared to other surface modification techniques like good heat and mass transfer during the deposition process, temperature uniformity and high degree of mixing between the gases and fluidized particles involved. This technique has been used to obtain silicon coatings on ferritic steels, which improves the limited corrosion resistance of the substrate. Analysis of optimum conditions by means of numerical simulations for the CVD process of silicon deposition on the AISI 4340 and ASTM P92 steels has been presented in this work. The temperature and pressure effects, the silicon conversion factor starting from the precursors formed, as well as the possible equilibria taking place in the formation of the silicon coating have been studied based on the thermodynamic calculations carried out on the THERMO-CALC software tool. © 2010 Elsevier B.V. All rights reserved.
1. Introduction During the last decades, researchers have made great efforts to develop new techniques to deposit protective thin coatings on substrates with good mechanical properties but with low resistance to corrosion. The most reliable techniques that have been incorporated to industrial processes because of their high versatility and allowance of rapid growth of thin layers are Physical Vapor Deposition (PVD), Chemical Vapor Deposition Assisted by Plasma (PECVD) and Chemical Vapor Deposition (CVD). The latter in particular has been successfully scale up to the industry because of the following characteristics: • Capability to produce high purity coatings. • Production of uniform coatings with good reproducibility, reasonable adherence and high deposition rate. • Control of the crystalline structure and coating morphology by controlling the deposition parameters. • Easy adjustment of the atomic deposition ratio [1,2]. Among all the chemical vapor deposition techniques there is a variant, the CVD in fluidized bed reactor (CVD-FBR), which consists on a bed of solid particles that are fluidized by introducing a constant flux of gas in the reactor. CVD-FBR variant combines the advantages of CVD mentioned before with those related to the fluidized bed, including good heat and mass transfer in the deposition process, temperature uniformity and high degree of mixing between the gases and the fluidized particles. This technique can be even more effective if a prior thermodynamic analysis is carried out to design the optimum working conditions. That ⁎ Corresponding author. Tel.: + 34 91 39 44215; fax: + 34 91 39 33457. E-mail address: [email protected] (F.J. Pérez). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.06.054
task can be undertaken thanks to commercially available computational tools which provide an excellent support in the study of processes governed mainly by thermodynamic laws. Although this is not the case for CVD, where diffusion and mass transport phenomena have an important role in the process, the computational method helps us to understand the relationships between composition, microstructure and conditions of the system. This means that it is possible to perform qualitative predictions like the phases that can be formed in the process and their compositions at certain given conditions of temperature and pressure. Silicon based coatings on ferritic steels have been developed by various methods in order to improve corrosion and erosion resistance [3,4] or as adhesion promoter for polymer films [5]. When such coatings are produced by CVD the interfacial problems associated to any coating/substrate system with different thermomechanic properties are overcome, since CVD coatings are the result of a surface modification generated by diffusion of the deposited material into the base material [6]. Therefore, the study of this process helps to solve usual adhesion problems related to residual stresses and thermal coefficient mismatch. The main objective of this work is to predict the best conditions for the deposition of a silicon based coating on AISI 4340 and ASTM P92 steels in a fluidized bed reactor by the CVD technique. To achieve this goal, thermodynamic calculations are carried out using the Gibbs energy minimization method with the thermodynamic simulation tool THERMO-CALC. 2. Thermodynamics considerations Many studies can be found in the literature which used thermodynamically calculated equilibrium states to anticipate or explain the experimental data obtained in a chemical process.
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Table 1 Compositions of the ferritic steel substrates used in this work (% in weight). Material
Cr%
V%
Ni%
Mo%
Mn%
Si%
W%
C%
Fe%
ASTM P92 AISI 4340
9.07 1.5
0.2 –
0.06 1.5
0.46 0.2
0.47 0.5
0.02 0.3
1.78 –
0.1 0.34
87.84 95.66
For instance, in relation with the process of interest here, Sirtl and Hunt describe the Si–H–Cl system by means of the standard enthalpy of formation, considering the initial partial pressure of precursors as starting point of the calculations and subsequently finding the equilibrium concentrations of the product species as a function of temperature [7,8]. Another usual thermodynamic function employed with similar purposes is the Gibbs free energy of the reaction, ΔG. It is worth remembering that at given conditions of temperature and pressure the occurrence of a compound can be predicted regarding the value of ΔG. The feasibility of the reaction of formation of the compound is estimated by the energy transfer of the system from an initial situation to the final, being favorable when that is negative, i.e, ΔG = ∑j ΔGj products
∑i ΔGi reactives
where ΔGj and ΔGi are respectively the Gibbs free energies of products and reactives involved in the reaction, and j and i are the number of them. Additionally, for a given temperature and pressure, a system is at equilibrium when the total Gibbs free energy of the reaction is at a minimum. Then, a possible approach to thermodynamic calculations consists in obtaining the coefficients that minimize the expression of the Gibbs free energy in terms of temperature, pressure and chemical composition. THERMO-CALC software performs such calculations in order to predict the composition of the most stable solid and gaseous phases and compounds formed at the system equilibrium fulfilling also the law mass action equations. Predictions of the variations of composition of the stable phases with temperature, pressure and composition of the initial state of the system can be also obtained. However, any software application would result useless without reliable and updated experimental databases. THERMO-CALC employed in its calculations high quality databases, as those provide by the Scientific Group Thermodata Europe, which are the strength of this software compared to similar applications also used for thermodynamic simulations [9]. The simulations presented here were carried out using SSUB3 Table 2 Chemical species calculated in the equilibrium of the Si/Ar/H2/HCl/Fe/Cr system at 101325 Pa and 500 °C. Gas Ar H2 SiH3Cl SiHCl3 SiCl4 HCl SiH2Cl2 SiCl2 SiH4 SiCl3 SiHCl H FeCl2 SiCl SiH3 CrCl3 Cl CrCl2 Si2H6
(mol/L)
Gas −1
7.44 × 10 2.46 × 10− 1 9.14 × 10− 3 5.38 × 10− 4 1.83 × 10− 4 1.04 × 10− 4 1.17 × 10− 5 1.72 × 10− 8 1.11 × 10− 8 4.11 × 10− 11 2.07 × 10− 12 4.47 × 10− 13 1.33 × 10− 14 3.41 × 10− 15 9.59 × 10− 16 2.01 × 10− 16 1.94 × 10− 16 1.16 × 10− 16 8.81 × 10− 17
SiH2 SiH FeCl3 Cl2 CrCl Si FeCl CrCl4 Fe2Cl4 Fe Cr Si2 HFe CrCl5 HCr CrCl6 Fe2Cl6 Cr2 Fe2
(mol/L) − 17
5.13 × 10 4.13 × 10− 20 3.82 × 10− 21 1.61 × 10− 21 8.70 × 10− 22 2.32 × 10− 23 2.04 × 10− 23 1.89 × 10− 24 1.79 × 10− 25 5.72 × 10− 26 3.09 × 10− 26 7.38 × 10− 30 1.47 × 10− 30 1.00 × 10− 30 1.00 × 10− 30 1.00 × 10− 30 1.00 × 10− 30 1.00 × 10− 30 1.00 × 10− 30
Solid
(mol)
Si2Cr Si Fe Si
5.10 × 102 1.70 × 10− 1 3.12 × 10− 1
Table 3 Chemical species calculated in the equilibrium of the Si/Ar/H2/HCl/Fe system at 101325 Pa and 500 °C. Gas
(mol/L)
Gas
(mol/L)
Solid
(mol)
Ar H2 SiH3Cl SiHCl3 SiCl4 HCl SiH2Cl2 SiCl2 SiH4 SiCl3 SiHCl H FeCl2 SiCl SiH3 CrCl3 Cl CrCl2 Si2H6
7.65 × 10− 1 2.25 × 10− 1 8.58 × 10− 3 6.26 × 10− 4 2.37 × 10− 4 1.04 × 10− 4 1.23 × 10− 5 1.96 × 10− 8 9.32 × 10− 9 5.00 × 10− 11 2.12 × 10− 12 4.28 × 10− 13 1.52 × 10− 14 3.64 × 10− 15 9.59 × 10− 16 2.44 × 10− 16 2.08 × 10− 16 1.33 × 10− 16 6.81 × 10− 17
SiH2 SiH FeCl3 Cl2 CrCl Si FeCl CrCl4 Fe2Cl4 Fe Cr Si2 HFe CrCl5 HCr CrCl6 Fe2Cl6 Cr2 Fe2
4.71 × 10− 17 3.96 × 10− 20 4.65 × 10− 21 1.83 × 10− 21 9.29 × 10− 22 2.32 × 10− 23 2.17 × 10− 23 2.45 × 10− 24 2.33 × 10− 25 5.72 × 10− 26 3.09 × 10− 26 7.39 × 10− 30 1.41 × 10− 30 1.00 × 10− 30 1.00 × 10− 30 1.00 × 10− 30 1.00 × 10− 30 1.00 × 10− 30 1.00 × 10− 30
Si Fe Si
3.26 × 101 3.46 × 101
for gas precursors reactions and SOL2 for substrate definition. Those databases contain assessed thermodynamic data on enthalpy of formation, entropy and temperature dependence of the heat capacity for the gaseous and condensed compound considered in the system studied in this work. In that way, the study of the feasibility of gaseous and solid phases by means of THERMO-CALC at given conditions should provide a guideline to define the optimum deposition conditions in the CVD-FBR process. 3. Description of simulated process and input parameters Briefly, the process to simulate here consists in a chemical vapor deposition on a steel surface carried out in a fluidized bed reactor. In this particular case, the bed is composed of Si (donor) and Al2O3 (inert filler material) powders, whose particles of ~150 μm of diameter are fluidized by a mixture of Ar, H2, and HCl gases introduced at the bottom of the reactor. This reactive gas mixture in contact to the silicon powder at an appropriate temperature leads to the formation of gas precursors that are transported through the reactor and diffused to the surface of the substrate to be coated. Then, the precursors are adsorbed onto the heated surface and subsequently decomposed, to finally form the silicon based coating. A complete description of the process has been already presented somewhere else [6,10]. Prior to the deposition process, input parameters were introduced in the Gibbs energy minimizer software THERMO-CALC in order to provide
Fig. 1. Variation of partial pressure of gaseous species present in the process of Si deposition in terms of the pressure reactor.
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Fig. 2. Partial pressure variation with temperature of the gaseous species present in the deposition process of Si on steel ASTM P92.
327
Fig. 5. Solid species present in the deposition process of Si on steel AISI 4340.
– and the concentration of the main elements of substrate, in this particular case Fe and Cr for P92 steel, and Fe for 4340 steel, according to their chemical composition (Table 1). The simulation will provide the concentration of products in the final gaseous and solid phases for which the minimum free Gibbs energy condition is satisfied. In the deposition process to be simulated a relation of gases of 69.6 vol.% Ar, 28 vol.% H2 and 2.4 vol.% HCl react with the silicon metal powder at 101,325 Pa of pressure in a temperature range between 0
Fig. 3. Solid species present in the deposition process of Si on steel ASTM P92.
theoretical information about the optimum conditions to achieve the Si deposition. The necessary parameters for the thermodynamic calculations are: – the ratio of gases in the Ar/H2/HCl mixture volume, – molar ratio of solid particles of the donor and the filler powders, i.e. Si and Al2O3 respectively,
Fig. 4. Partial pressure variation with temperature of the gaseous species present in the deposition process of Si on steel AISI 4340.
Fig. 6. Gibbs free energy of formation of species a) for P92 steel, b) for 4340 steel.
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compounds present in the gaseous phase for both steels, resulting from the reaction of the reactive gases with the silicon powder particles, are SiH3Cl, SiHCl3, SiCl4, SiH2Cl2 and SiCl2. In the solid phase Si and FeSi form, as well as Si2Cr for the steel with Cr content. Noticeably the formation of FeSi2 is not predicted by THERMO-CALC calculations although it is present as stable phase in the Fe–Si phase diagram. The reason is that in this work silicon and silicides deposition on steels by CVD-FBR is achieved through the decomposition of chlorosilanes precursors gases on the steels surfaces, not being the reaction for the formation of that specific phase thermodynamically favored. In addition to the reactive gases that remain after the reaction and the main precursors or chemical species directly responsible for the deposition of silicon, other possible precursors, according to the literature [10], appear in the calculation. It is common to assume that a gaseous phase is significant only if its partial pressure exceeds 10− 6 Pa, therefore, phases which have partial pressures below this value have been neglected in the graph simulations presented in this work. On the other hand, pressure can be an important parameter in chemical reactions. However, the study of the effect of the total pressure inside the reactor on the formation or stability of the precursor gases (Fig. 1) suggests its contribution to this particular process is almost negligible. Therefore, the CVD deposition can be run at atmospheric pressure providing a great operational flexibility and a simple experimental set up.
4.1. Thermodynamic simulations for steel ASTM P92
Fig. 7. Possible solid phases that were formed as a function of moles of silicon in the reactive bed a) for P92, b) for 4340 steels.
and 1000 °C. The selection of the input simulation parameters was based on previous works on silicon depositions by CVD-FBR [11]. 4. Thermodynamic results and discussion In Tables 2 and 3 are shown the gaseous and solid species calculated and its molar concentration when the equilibrium is established at 500 °C, for P92 and 4340 steels respectively. The major products
Fig. 2 shows the influence of temperature on the equilibrium compositions of the gas phase at 101,325 Pa for a system where the reactive elements are Si, H, Cl and Ar, and the main elements of the substrate are Fe and Cr with a molar ratio of Si:Fe:Cr:Ar:HCl: H2 =0.36:0.12:0.02:69:2.40:28. The partial pressures of the main precursors were calculated. For values beyond 10− 6 Pa the precursors are SiH3Cl, SiHCl3, SiCl4, SiH2Cl2, SiCl2 and SiHCl, among which the most representative gaseous reactants in the temperature range studied were SiH3Cl, SiHCl3 and SiCl4. These results are consistent with those obtained by Bolivar et al. [12] and Sanjurjo et al. [13]. Importantly, a plateau is observed in the temperature range corresponding to the CVD working temperatures for most of these precursors. Also of relevance, this graph shows the formation of iron chlorides in very low concentrations and only at temperatures above 1000 °C. This means that the thermodynamic calculations do not anticipate a significant formation of compounds which would indicate an attack of
Fig. 8. Gibbs free energy variation with temperature of several reactions of precursors that could take place in equilibrium.
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329
Fig. 9. Gibbs free energy variation with temperature of several reactions that could deposit Si.
the steel substrate during the coating process due to the presence of the HCl as reactive gas during the coating process. Fig. 3 shows the possible solid species to be formed as a function of temperature. The main solid phase which is expected to deposit is FeSi although Si and the intermetallic Si2Cr are also likely to form. Noticeably the formation of these solids is not affected by temperature for the range considered in this work. 4.2. Thermodynamic simulations for steel AISI 4340 In Fig. 4 the influence of the temperature on the equilibrium compositions of the solid and gaseous phases is presented at a constant pressure of 101,325 Pa. The reactive elements considered are Si, H, Cl and Ar, and the main element of the substrate is Fe solely, since the concentration of the alloying elements of the 4340 steel is for all of them lower than 2% in weight. The input molar ratio for the calculations is Si:Fe: Ar:HCl:H2 =0.36:0.18:69:2.4:28. According to their partial pressures, the predominant species result to be SiH3Cl, SiHCl3, SiCl4, SiH2Cl2 and SiCl2. Similarly to the results obtain for the P92 steel, the values of partial pressures are almost constant all over the temperature range characteristic of CVD, starting to decrease only when temperature rises to 1000 °C. However, unlike to P92 steel, the SiHCl gas phase and iron chlorides do not come into view for this steel even at the highest temperatures, finding instead the gaseous species SiH3. Furthermore, according to the simulation obtained for the solid phase disclosed in Fig. 5, the deposition of silicon is thermodynamically favorable, as well as an iron silicide like FeSi, in the complete temperature range considered. 4.3. Study of the stability of the solid phases Regarding Fig. 6 (a) and (b), where the change in the Gibbs free energy of formation of solid species on the P92 and the 4340 steels is Table 4 Gibbs free energy of formation of SiCl2 at 500 °C. Chemical reactions
ΔG (kJ), T = 500 °C
(a) 2Si + 6HCl ↔ SiCl 4 + SiCl2 + 3H2 (b) SiHCl3 ↔ SiCl2 + HCl (c) SiHCl + SiH2Cl2 ↔ SiH3Cl2 + SiCl2 Si + 2HCl ↔ SiCl2 + H2 SiH2Cl2 ↔ SiCl2 + H2 SiCl4 + Si ↔ 2SiCl2 SiCl4 + H2 ↔ SiCl2 + 2HCl SiH3Cl + SiCl4 ↔ SiCl2 + Si + 3HCl SiH3Cl + HCl ↔ SiCl2 + 2H2
− 163.07 121 − 100.6 5.67 47.3 169 163 199 40.34
presented against temperature, it can be deduced that the stability of iron silicide and the deposition of silicon increase with temperature for both steels. Nevertheless, in the case of the P92 steel, which has higher chromium content, the formation of silicon and iron silicide competes with the formation of chromium silicides which shows a lower stability than those. In addition, a simulation was performed to calculate the possible solid phases that may deposit on both substrates varying the molar fraction of silicon in the donor of the reactive bed (Fig. 7). It can be seen in these figures that, at a certain molar fraction of silicon in the bed, the phase representing the ferritic bulk material with body centered cubic structure is not longer stable and iron is more prone to be combined with silicon. Likewise, Fig. 7 shows that the concentration of FeSi phase has a maximum at approximately 0.17 mol of silicon for both steels decreasing up to that concentration in benefit of an increase of pure silicon deposition. At this point of the discussion it is convenient to remember that CVD deposition is a non-equilibrium process and that THERMO-CALC calculations are based on the assumption that the system to consider is at its equilibrium. This means that although a phase results thermodynamically stable following the THERMO-CALC simulation it will be not necessarily present. Since diffusion phenomena are favored at the working temperature, silicon atoms are expected to diffuse into the steel substrate forming, instead pure Si or FeSi, iron silicides with different stoichiometry. 4.4. Thermodynamic studies of precursor gases and products formation Now, considering the precursors formed by the direct reaction of the Si donor with the reactive gases HCl and H2, which were obtained as result of the previous thermodynamic calculations, it is possible to propose the candidates reactions that are responsible for the Si deposition. To achieve this, first, the feasibility of the reactions for the precursors formation as a function of the working temperature is studied by means of the variation of the Gibbs free energy of each reaction, in correspondence to temperature (Fig. 8). Regarding that figure it can be concluded that the formation of all the possible precursors is expected at the temperature range of operation of the CVD-FBR reactor, i.e. from 300 to 600 °C, except for SiCl2 which has a positive value of Gibbs free energy of formation. However, since the above simulations predict its presence, this suggests the possibility of its formation not through the direct interaction between Si, HCl and H2, but through an intermediate reaction carried out in the gaseous phase.
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Fig. 10. Gibbs free energy variation with temperature of several reactions that could form SiCl2.
Next, the precursors reactions leading to the silicon deposition are studied. As observed in Fig. 9 the only reactions of the gaseous species predicted by THERMO-CALC with negative values of the Gibbs free energy for the temperature range of interest are those where SiCl2 is involved, i.e. reactions (d) and (e) (the last one previously described by Petit et al. [14]). Now, the free Gibbs energies at 500 °C of the reactions which could lead to the formation of SiCl2 taking into account the precursors predicted by THERMO-CALC in the gaseous phase are considered in Table 4. Then, knowing that this is the main precursor for the formation of silicon, the most probable reaction to occur regarding Table 4 is: 2Si + 6HCl←→SiCl4 + SiCl2 + 3H2
ΔG = −163kJ
The input temperature for the calculations was 500 °C since that is a representative temperature for the process of interest here. Additionally, the variation with temperature of the free Gibbs energy of these reactions has been disclosed in Fig. 10 in order to study their feasibility in a range of temperature. In this way, reaction (b), already proposed by Petit et al. [14] as initiation reaction at the CVD deposition temperature of silicon layers, is only favored at temperatures above 1200 °C when the ΔG of the reaction reaches negative values. Since in CVD-FBR the working temperature is between 300 °C and 650 °C this reaction is ruled out.
Likewise, reaction (c), proposed by Swihart et al. [15] to explain dichlorosilane decomposition as precursors for the CVD deposition of silicon, has negative value of ΔG in the entire temperature range considered in the figure, resulting therefore also feasible at the conditions of the CVD-FBR deposition. Thus, the reactions (b) and (c) can be suggested to be the responsible of the main precursors formation, meanwhile reactions (d) and (e) would lead to silicon deposition on the steels substrates. On the other hand, if the reactions involved in the equilibrium deposition starting from precursors and ending up in the solid compound FeSi are considered (Fig. 11), it is found that only SiCl2, SiH2Cl2 and SiH3Cl reacting with Fe could lead to deposit FeSi on the steel substrate, since those are the reactions with negative values of ΔG. At 500 °C the most probable reaction would be that of SiCl2, as occurs in the case of the pure silicon deposition (Table 5). Finally, Fig. 12 (a) and (b) shows the conversion of the main precursors to the Si and FeSi phases following the calculations carried out by THERMO-CALC. The number of Si moles deposited by the interaction of the main gaseous precursors, SiH3Cl, SiCl2 and SiH2Cl2, with the substrate do not show dependence on temperature, having a greater factor of conversion for the first two mentioned precursors (Fig. 12 (a)). Also, the formation of FeSi takes place at the entire range of temperatures considered (Fig. 12 (b)). However, for values between 400 °C and 900 °C the SiH2Cl2 precursor shows no production of FeSi
Fig. 11. Gibbs free energy variation with temperature of the reactions of several precursors that could form FeSi.
P.A. Hernández et al. / Surface & Coatings Technology 205 (2010) 325–331 Table 5 Gibbs free energy of formation of Si by several precursors at 500 °C. Chemical reactions
ΔG (kJ), T= 500°C
(d)2SiCl2 ↔ Si + SiCl4 (e)SiCl2+ H2 ↔ Si + 2HCl SiHCl3 + H2 ↔ Si + 3HCl SiH2Cl2 ↔ Si + 2HCl SiCl4 + 2H2 ↔ Si + 4HCl
− 169 − 5.12 [14] 116.999 42.18 –
331
and ASTM P92. According to the analysis and discussion of results obtained by THERMO-CALC, the following conclusions were drawn: – The main gaseous species expected to be generated when the donor reacts with the reactive gases are SiH3Cl, SiHCl3, SiCl4, SiH2Cl2 and SiCl2. – The solid species expected to be deposit on the substrate surface are Si, FeSi and, in the case of the P92 steel, Si2Cr. – No chemical attack to the substrate is expected during the deposition process, since the presence of chloride associated with the elements of the substrate material is negligible in the range of operating temperatures. – The final precursor directly responsible for the deposition of silicon on the substrates would be SiCl2. – The CVD-FBR process can be carried out at atmospheric pressure and temperatures in the range of 500 to 650 °C. Further experimental studies are needed to validate the above conclusions and to clarify the role of thermodynamic considerations in the deposition process of Si by CVD-FBR.
Acknowledgements The authors want to express their gratitude to the Spanish “Ministerio de Ciencia e Innovación” for the financial support to this work under the projects CSD2008-00023 and ENE2008-06755-C0202/CON.
References
Fig. 12. Conversion of solid phases in function of temperature starting from several precursors a) for Si and b) for FeSi.
and only SiCl2 and SiH3Cl are associated to a good conversion precursors to solid phases independently of temperature in the studied range. This is in agreement with the previous conclusions obtained showing that SiH3Cl is the main precursor since it is involved in the formation of SiCl2 and subsequently in the formation of the coating. On the other hand, since the stability of the ferritic structure of the substrate imposes a temperature limitation, and previous works pointed out that at temperatures higher than 650 °C CVD depositions result in porous coatings with serious spallation problems [16,17], working temperatures in a range of 500 to 650 °C are advised. 5. Conclusions Thermodynamic approaches have been made using the program THERMO-CALC in order to find the optimum working conditions for the deposition of silicon by CVD-FBR on two ferritic steels, AISI 4340
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