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Silicon coating on ferritic steels by CVD-FBR technology
Surface & Coatings Technology 201 (2006) 3953 – 3958 www.elsevier.com/locate/surfcoat
Silicon coating on ferritic steels by CVD-FBR technology F.J. Bolívar a , L. Sánchez a , S.A. Tsipas a , M.P. Hierro a , J.A. Trilleros b , F.J. Pérez a,⁎ a
b
Grupo de Investigación de Ingeniería de Superficies y Materiales Nanoestructurados, Spain Grupo de Investigación en Ingeniería Metalúrgica, Universidad Complutense de Madrid, Facultad de Ciencias Químicas, 28040 Madrid Spain Available online 9 November 2006
Abstract Silicon protective coatings were deposited on ferritic steels (9–12% Cr) by chemical vapour deposition by means of fluidized bed reactor (CVD-FBR). The process was performed at temperatures below 580 °C with the use of Silicon donator powder and hydrogen chloride (HCl) of activator. Thermodynamic calculations were made before the experimental study to investigate the conditions for the formation of gas precursors for the deposition of the Si coating. The samples were examined by means of optical microscopy (OM), Scanning Electron Microscopy (SEM), Energy Dispersion Spectroscopy (EDS) as well as X-ray diffraction, and the results, show the formation of dense, homogenous and thin coatings consisting of Fe3Si. © 2006 Published by Elsevier B.V. Keywords: Silicon deposition; Chemical vapour deposition; Silicides; Ferritic steel
1. Introduction The use of surface coatings is an important alternative as a means of extending the performance of materials in a wide range of applications. Materials exposed to high temperature in an oxidizing environment may suffer from corrosion and/or oxidation, with subsequent depletion in alloy elements, affecting their mechanical properties. The application of protective coatings is therefore interesting to shield the materials from such corrosive environment. Addition of silicon to metals and alloys, including steels, generally increases the corrosion, oxidation and erosion resistance[1]. Addition of silicon to alloys increases oxidation resistance. Unfortunately, addition of silicon also changes the mechanical properties of the alloys and increases the cost of the materials [2]. However, application of silicon coatings can overcome this limitation, because it can improve the corrosion resistance of the steels without changing the mechanical properties of the bulk materials [3–5]. Many coating techniques have been used to obtain Si coating such as chemical vapour deposition (CVD), physical vapour deposition (PVD), pack cementation and ion implantation [6]. Chemical vapour
⁎ 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 © 2006 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2006.08.101
deposition is one of the most important processes to obtain Si coating, especially in the microelectronic industry. The aim of this work is to study the possibility of obtaining Si coating on ferritic steel with content of Chromium between (9–12% Cr) by means of chemical vapour deposition in fluidized bed reactor. The initial parameters of the process were obtained by thermodynamic simulation realized with Thermocalc software [7]. Then, the results were used to select the initial conditions for the experiment to produce Si coating, at low temperature under atmospheric pressure. 2. Experimental 2.1. Substrate materials The composition of ferritic steel used as base materials for Si diffusion coating is presented in Table 1. The samples (10 mm × 20 mm × 2.5 mm) were polished from 240-grit SiC paper up to 600-grit SiC paper, and then ultrasonically cleaned in alcohol, dried and weighed prior to coating experiment. 2.2. Coating procedure 2.2.1. Thermodynamic calculation Thermodynamic studies of phase equilibrium (composition, partial pressure) during CVD were performed using the
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Table 1 Ferritic steel composition wt.%
Fe
Si
Cr
Ni
Mn
Mo
Other
P-91
89.3
0.40
9
0.38
0.54
0.36
Nb, C, Al, V, N, Co, P, O (wt.% b 0.4)
Thermocalc computer program [7] to determine the feasibility of metal deposition, and to provide a useful guideline to optimise deposition condition. Calculations are based on the free Gibbs energy minimisation code and mass conversion rule. SSUB3 and SOL2 databases (Scientific Group Thermodata Europe) were used to define chloride and hydride precursors and substrate respectively. 2.2.2. Experimental conditions The characteristics of the fluidized bed reactor used in this study have been presented in previous works [8]. The bed was composed of powder mixture of Si (99.5% purity, grain size 200 μm) as donor. The bed particles were fluidized using a high Ar flow (1.6 L min− 1). Si-chloride precursors were generated at the deposition temperature by reaction between silicon donor powder and an input HCl/H2 gas mixture (1/20). Coatings were deposited in the temperature range of 520 at 580 °C for 1 h. The HCl was only supplied for 15 min, and the extra 45 min the samples were subsequent heat treatment at the same deposition temperature. 2.3. Characterisation techniques As deposited coatings were characterised by optical microscopy (OM), scanning electron microscopy (SEM JEOL JM-6400) and energy dispersive X-ray spectroscopy (EDS). Standard metallographic preparation was performed on samples prior to cross-section observations. X-ray diffraction analyses (Philips X′PERT MPD, K α(Cu) radiation) were carried out to identify phase composition of the coated samples. 3. Results
also included in the system to consider interaction between gas phase and BCC substrate. The input conditions used for calculation were guided by previous experimental studies [9,10]. Between 300 °C and 800 °C, the main Si chlorides precursors, Si hydrides and chlorosilanes precursors SiH xCl4−x (partial pressure N 1 × 10− 4 Pascal) obtained form these calculations were ClH3Si, Cl3HSi, H4Si, Cl2H2Si, Cl4Si Cl2Si, ClHSi. The partial pressures of the main Si precursors determinated at 500 °C and 600 °C are presented in Table 2. ClH3Si is the main and most stable Si precursor present in all the temperature range studied; the concentration of this phase does not change significantly with the increasing temperature. According to thermodynamical estimations, partial pressures of other precursors are typically below 0.1 Pascal. Although, this pressure is too low for the conventional CVD process; but in this FBR-CVD even those low partial pressures could play an important role in the deposition processes. These calculations provide interesting information about gaseous chloride and hydride precursors formed in the reactor starting from solid donor and about dependence of these partial pressures on temperature. These thermodynamic calculations are an important guide for selecting the experimental conditions. These conditions were selected on the basis of: equilibrium distribution of the Si species that favoured the silicon deposition (Fig. 1) that increases with temperature. Moreover, the microstructural properties of the ferritic steels were considered, because this type of steel suffers microstructural changes at temperatures upper 700 °C. In addition to this, at high temperatures, it has also been reported as problems of adhesion [2,9]. The conditions selected for the experiment are shown in the Table 2. 3.2. Coatings characterisation 3.2.1. SEM and EDS analysis Fig. 2 shows the surface morphology of the Si coating on the ferritic steels (P-91). The sample was treated at 520 °C for 1 h in a 14 vol.% H2 (g) and 1.2 vol.% HCl(g) gas mixture. In this figure, it can be observed that the coating formed is homogenous, and small nodular grains grow on the surface. The EDS
3.1. Thermodynamics approximation Thermodynamic calculations of the vapour pressure of gaseous species of silicon in equilibrium conditions were performed with the purpose to find all possible gaseous and condensed phases that can form the combinations of the element involved in the following chemical system Si(s)/Ar(g)/HCl(g)/H2(g). Fe and Cr were Table 2 Coating conditions for deposition of silicon on ferritic steels Coating conditions
Pressure Time HCl H2
Temperatures 520 °C
550 °C
580 °C
101325 Pascal 1h 2.4 vol.% 28 vol.%
101325 Pascal 1h 2.4 vol.% 28 vol.%
101325 Pascal 1h 2.4 vol.% 28 vol.%
Fig. 1. Thermodynamic calculations of the system formed by w(Si) = 14 g with relation to H2/HCl = 12/1 Si precursors.
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Fig. 2. SEM surface morphology and EDS microanalysis of the Si coating on ferritic steels P-91 under a reactive gas mixture of 28 vol.% H2 and 2,4 vol.% HCl at 520 °C. The HCl was only supplied for 15 min, and the extra 45 min the samples were subsequent heat treatment at the same deposition temperature.
microanalysis shown in Fig. 2 revealed that the coating is principally composed of Si, Cr and Fe. According to the atomic percentages found by means of EDS figure in 2 and in agreement with phase diagram [11] suggest that the possible phase formed on the surface of steel is Fe3Si. According to the obtained results, XRD and EDS analysis suggest that the coating formed at temperatures upper 520 °C get spalled easily. At high temperature, the formation of Si-rich silicides is promoted and the coatings formed by this type of silicides have poor adhesion. Fig. 3 shows the surface morphology of the spalled Si coating on the ferritic steels (P-91). The sample was treated at 580 °C for 1 h in a 14 vol.% H2 (g) and 1.2 vol.% HCl(g) gas mixture. The EDS microanalysis shows that the coating is principally composed of Si, Cr and Fe (Fig. 3). According to the atomic percentages found by means of EDS figure in 3 and in agreement with phase diagram [11] suggest that the possible phase formed on the surface of steel is FeSi. Fig. 4 shows the cross-sections of the silicon coating on a ferritic steels. In this figure it is possible to observe that the coating is very irregular and porous. The thickness of the silicon coating is approximately 8 μm. Fig. 4 also shows the corresponding composition of the coating as a function of the distance from interface. This in line profile shows that the coating mainly
consists of Si, Fe and Cr. The percentages of Si in the coating is the range of 12–25 at.%, Fe is in the range of 48–65 at.%, and Cr is in the range of 5–12 at.% approximately. The atomic percentages suggest that the coating is mainly composed of the phase Fe–Si (Fe3Si), in agreement with phase diagram. According to this diagram DO3 structured Fe3Si, exist from 11 at.% Si to about 30 at.% depending on the temperature, see Fig. 5 [11]. 3.2.2. XRD analysis Fig. 6a shows the XRD diffraction pattern corresponding to a Si coating on ferritic steels and Fig. 6b the XRD diffraction pattern of the spalled coating. In Fig. 6a it is possible to observe the characteristic peaks of the Fe–Si phase Fe3Si as well as αFe. The presence of the Fe3Si is consistent with the results obtained by EDS analysis. Fig. 6b shows the XRD pattern of the spalled coating revealing that it consists of Fe3Si. 4. Discussion 4.1. Thermodynamic analysis According to the thermodynamic results an equilibrium between HCl, solid particles of Si, Fe–Cr steel and the surrounding gas phase can be approached closely in the bed.
Fig. 3. SEM surface morphology and EDS microanalysis of the spalled Si coating on ferritic steels P-91 under a reactive gas mixture of 28 vol.% H2 and 2.4 vol.% HCl at 580 °C. The HCl was only supplied for 15 min, and the extra 45 min the samples were subsequent heat treatment at the same deposition temperature.
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Fig. 4. Cross-section of Si coated substrate obtained after 1 h at 52O °C and EDS microanalysis of the Si coating on ferritic steels P-91 under a reactive gas mixture of 28 vol.% H2 and 2.4 vol.% HCl. The HCl was only supplied for 15 min, and the extra 45 min the samples were subsequent heat treatment at the same deposition temperature.
The main precursors formed in this system were ClH3Si, Cl3HSi, H4Si, Cl2H2Si, Cl4Si Cl2Si, ClHSi. Therefore, the possible equilibrium that takes place in the formation of the Si coating includes: Si þ HClðgÞ þ H2ðgÞ ⇔SiH3 ClðgÞ ΔG ¼ −35:2kJ
ðReaction1Þ
Si þ 3HClðgÞ ⇔SiHCl3ðgÞ þ H2ðgÞ ΔG ¼ −116kJ
ðReaction2Þ
Si þ 2HClðgÞ ⇔SiH2 Cl2ðgÞ ΔG ¼ −42:2kJ
ðReaction3Þ
Si þ 4HClðgÞ ⇔SiCl4 ðgÞ þ 2H2ðgÞ ΔG ¼ −158kJ
ðReaction4Þ
Si þ 2HClðgÞ ⇔SiCl2 ðgÞ þ H2ðgÞ ΔG ¼ −5:12kJ
ðReaction5Þ
Si þ 2H2ðgÞ ⇔SiH4ðgÞ ΔG ¼ 102kJ
ðReaction6Þ
They considered that the possible mechanism of the position of Si consists of the two steps: Adsorption of SiCl2(g) at substrate surface followed by reduction of SiCl2(g) with deposition of Si and desorption of HCl. This reaction is reversible (Reaction 8) and it can lead to etching of silicon coating for high SiHCl3 amount, which diminishes the deposition rate of Si. In the CVD-FBR process, according to thermodynamic calculations, different silicon halides are obtained by a reaction between silicon powder and a mixture of hydrochloride and hydrogen. The formation of the coating is probably due to the interaction of different precursors, such as SiHCl3 and SiCl2. Some authors proposed that above 1000 °C, these compounds are responsible for the formation of the layer. Thermodynamically, SiH3Cl is more stable than the other precursors in all the range of temperatures studied but its lifetime is very short. Therefore, its participation in the coating may be less important. However, in the CVD-FBR the precursors are generated very close to the
This approximation leads to consider that the coatings formation is due to a competitive mechanism between precursors. According to Sanjurjo et al. [12] SiCl2, SiH2Cl2 and SiHCl3 are more reactive species than SiCl4 and help in the deposition process at low temperatures. However, there are other more reactive species, such as SiH3Cl that may also be produced. Their lifetime however is very short, and so their participation in the processes of the formation of the coating may be less important. Petit and Zeman [13] suggest the principal reactions that occurring during process of formation of silicon coating are the following: SiHCl3ðgÞ ⇔SiCl2 ðgÞ þ HClðgÞ ΔG ¼ −121kJ
ðReaction7Þ
SiCl2ðgÞ þ H2ðgÞ ⇔Si þ 2HClðgÞ ΔG ¼ −5:12kJ
ðReaction8Þ
SiHCl3ðgÞ þ HClðgÞ ⇔SiCl4ðgÞ þ H2ðgÞ ΔG ¼ −42:4kJ (Reaction 9)
Fig. 5. Iron–silicon phase diagram [11].
F.J. Bolívar et al. / Surface & Coatings Technology 201 (2006) 3953–3958
Fig. 6. XRD Pattern of P-91 substrate a) coated; b) spalled coated.
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SiH4 gas and Fe as substrate, at temperatures below of 600 °C is due to diffusion of Fe along the grain boundaries and along defects which leads to the formation of spherical nodules of Fe3Si. However, when the temperature is higher than 800 °C the diffusion of Si into the bulk of Fe is dominant and therefore, no spherical nodules appear on the surface. This situation is in agreement with the present results and explains the morphology of the coating and the spallation, when the deposition was performed at higher temperature or for longer time, because when the silicon concentration increases in the coating and the coating becomes less adherent. When the deposition is performed at temperatures lower than 600 °C, the possible mechanism of coating formation is due to outward diffusion of Fe; then Fe3Si forms on the surface and grows in the form of few grains. The Si contained with increase of temperature or time deposition. Coatings with higher Si content spalled easily. 5. Conclusions
substrate. As a consequence, SiH3Cl could play an important role in the deposition of Si coating even if the lifetime is short. 4.2. Characterization of Si coating The composition of the Si coating was determined by EDS measurements on the surface and in incross-section. The results of this analysis show that the coating is composed at Si, Fe and Cr. The concentration of silicon on the surface reaches approximately 25 at.% and Fe and Cr reach approximately 55 and 20 at.% respectively. Therefore, it is possible to postulate that Fe3Si is the phase formed on the surface, because these atomic percentages correspond to this phase, as predicted by Fe–Si phase diagram shown in Fig. 5. This result was confirmed by the XRD analysis showing the characteristic peaks corresponding to Fe3Si. Sanjurjo et al. [2] found similar results, observing first the formation of α-Fe and then the formation of Fe3Si; the formation of this phase was possible after 5 min of treatment in the CVD-FBR reactor at 500 °C. They also observed that phases rich in Si, such as FeSi, are formed at higher temperatures or for longer times. Perez-Maraino et al. [4] obtained similar results as Sanjurjo; they deposited a Si coating on AISI 316 stainless steel by means of the CVD-FBR reactor at low temperatures, 450 °C and 500 °C. The coating obtained consisted mainly of Fe3Si. However, when the experiment was performed at higher temperature such as 675 °C, the formation of Si-rich silicides rich (such as FeSi and Fe5Si3) was promoted and this leads to the formation of a coating that spalled easily. Nevertheless, the deposition of silicon on steels was studied in the present work with results very similar to those obtained by Ref. [4], for other type of steels with different type of the microstructures. Therefore, in the mechanism of growth of the Si coating at the temperature factors such as temperature and time of the reactions is more important than the microstructure of the steels. Consequently, it is possible that the growth mechanism of the coating is very similar to the mechanism proposed by Rebhan et al. [14]. They proposed that the growth of the Si coating obtained by means of CVD using as precursors
Silicon coating were developed using CVD-FBR technology on ferritic steel with chromium content of 9–12 wt.%. The process was realized at low temperature in order to conserve the microstructure properties of the material. The thermodynamic study was used as a guide to select the initial condition for performing Si deposition on a low temperature (b700 °C). Based on these calculations, the temperature range selected for the experiment was 500 °C to 600 °C. The best results were obtained for temperature of 520 °C during 1 h with only 15 min of input of HCl. Diffusion coatings obtained on ferritic steel were approximately 8 μm thick and constituted principally of Fe3Si. The coatings obtained at temperatures higher than 600 °C easily spalled and are constituted principally of FeSi and Fe3Si. The coatings obtained at temperatures upper 520 °C easily spalled and are constituted principally of FeSi and Fe3Si. Acknowledgements The authors want to express their gratitude to the Spanish Ministerio de Educación y Ciencia for the financial support of this work. Project ENE-2005-08494-C02-02. References [1] J.T.K. Clark, A.I. Foster, M.L. Sims, M.A.M. Swidzinski, D. Young, Chemical Vapour Deposition, Eindhoven, Netherlands, 1983. [2] A. Sanjurjo, Samon Hettiarachchi, K.H. Lau, Philip Cox, Bernard Wood, Surf. Coat. Technol. 54/55 (1992) 224. [3] S.S. Ionayoshi, S. Tsukahara, A. Kinbara, Vacuum 53 (1999) 281. [4] J. Perez Mariano, J. Elvira, F. Plana, C. Colominas, Surf. Coat. Technol. 200 (18/19) (2006) 5606. [5] A. Sanjurjo, Bernard Wood, Kai Lau, Gopala Krishnan, Scr. Metall. Mater. 31 (8) (1994) 1019. [6] J. Porcayo-Calderón, E. Brito Figueroa, J.G. Gonzaléz-Rodriguez, Mater. Lett. 38 (1999) 45. [7] Thermocalc, Thremocalc Software, Foundation of Computational Thermodynamics, Stockholm, Sweden, 1995–2003. [8] F.J Pérez, M.P. Hierro, F. Pedraza, C. Gomez, M.C. Carpintero, J.A. Trilleros, Surf. Coat. Technol. 122 (1999) 281.
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[9] F.J. Pérez, M.P. Hierro, M.C. Carpintero, F. Pedraza, C. Gomez, Surf. Coat. Technol. 140 (2001) 93. [10] F.J. Pérez, M.P. Hierro, M.C. Carpintero, C. Gomez, F. Pedraza, Surf. Coat. Technol. 160 (2002) 87. [11] A. Il´Inskii, S. Slyusarenko, O. Slukhovskii, I. Kaban, W. Hoyer, J. NonCryst. Solids 306 (2002) 90. [12] A. Sanjurjo, K. Lau, B. Wood, Surf. Coat. Technol. 54/55 (1992) 219.
[13] Agnés Petit, Miro Zeman, High temperature CVD depostion of thin polycristalline silicon layer, p, http://retina.et.tudelft.nl/data/artwork/ publication/679241359.pdf. [14] M. Rebhan, R. Meier, A. Plagge, M. Rohwerder, M. Stratmann, Appl. Surf. Sci. 178 (2001) 194.
Silicon coating on ferritic steels by CVD-FBR technology F.J. Bolívar a , L. Sánchez a , S.A. Tsipas a , M.P. Hierro a , J.A. Trilleros b , F.J. Pérez a,⁎ a
b
Grupo de Investigación de Ingeniería de Superficies y Materiales Nanoestructurados, Spain Grupo de Investigación en Ingeniería Metalúrgica, Universidad Complutense de Madrid, Facultad de Ciencias Químicas, 28040 Madrid Spain Available online 9 November 2006
Abstract Silicon protective coatings were deposited on ferritic steels (9–12% Cr) by chemical vapour deposition by means of fluidized bed reactor (CVD-FBR). The process was performed at temperatures below 580 °C with the use of Silicon donator powder and hydrogen chloride (HCl) of activator. Thermodynamic calculations were made before the experimental study to investigate the conditions for the formation of gas precursors for the deposition of the Si coating. The samples were examined by means of optical microscopy (OM), Scanning Electron Microscopy (SEM), Energy Dispersion Spectroscopy (EDS) as well as X-ray diffraction, and the results, show the formation of dense, homogenous and thin coatings consisting of Fe3Si. © 2006 Published by Elsevier B.V. Keywords: Silicon deposition; Chemical vapour deposition; Silicides; Ferritic steel
1. Introduction The use of surface coatings is an important alternative as a means of extending the performance of materials in a wide range of applications. Materials exposed to high temperature in an oxidizing environment may suffer from corrosion and/or oxidation, with subsequent depletion in alloy elements, affecting their mechanical properties. The application of protective coatings is therefore interesting to shield the materials from such corrosive environment. Addition of silicon to metals and alloys, including steels, generally increases the corrosion, oxidation and erosion resistance[1]. Addition of silicon to alloys increases oxidation resistance. Unfortunately, addition of silicon also changes the mechanical properties of the alloys and increases the cost of the materials [2]. However, application of silicon coatings can overcome this limitation, because it can improve the corrosion resistance of the steels without changing the mechanical properties of the bulk materials [3–5]. Many coating techniques have been used to obtain Si coating such as chemical vapour deposition (CVD), physical vapour deposition (PVD), pack cementation and ion implantation [6]. Chemical vapour
⁎ 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 © 2006 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2006.08.101
deposition is one of the most important processes to obtain Si coating, especially in the microelectronic industry. The aim of this work is to study the possibility of obtaining Si coating on ferritic steel with content of Chromium between (9–12% Cr) by means of chemical vapour deposition in fluidized bed reactor. The initial parameters of the process were obtained by thermodynamic simulation realized with Thermocalc software [7]. Then, the results were used to select the initial conditions for the experiment to produce Si coating, at low temperature under atmospheric pressure. 2. Experimental 2.1. Substrate materials The composition of ferritic steel used as base materials for Si diffusion coating is presented in Table 1. The samples (10 mm × 20 mm × 2.5 mm) were polished from 240-grit SiC paper up to 600-grit SiC paper, and then ultrasonically cleaned in alcohol, dried and weighed prior to coating experiment. 2.2. Coating procedure 2.2.1. Thermodynamic calculation Thermodynamic studies of phase equilibrium (composition, partial pressure) during CVD were performed using the
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Table 1 Ferritic steel composition wt.%
Fe
Si
Cr
Ni
Mn
Mo
Other
P-91
89.3
0.40
9
0.38
0.54
0.36
Nb, C, Al, V, N, Co, P, O (wt.% b 0.4)
Thermocalc computer program [7] to determine the feasibility of metal deposition, and to provide a useful guideline to optimise deposition condition. Calculations are based on the free Gibbs energy minimisation code and mass conversion rule. SSUB3 and SOL2 databases (Scientific Group Thermodata Europe) were used to define chloride and hydride precursors and substrate respectively. 2.2.2. Experimental conditions The characteristics of the fluidized bed reactor used in this study have been presented in previous works [8]. The bed was composed of powder mixture of Si (99.5% purity, grain size 200 μm) as donor. The bed particles were fluidized using a high Ar flow (1.6 L min− 1). Si-chloride precursors were generated at the deposition temperature by reaction between silicon donor powder and an input HCl/H2 gas mixture (1/20). Coatings were deposited in the temperature range of 520 at 580 °C for 1 h. The HCl was only supplied for 15 min, and the extra 45 min the samples were subsequent heat treatment at the same deposition temperature. 2.3. Characterisation techniques As deposited coatings were characterised by optical microscopy (OM), scanning electron microscopy (SEM JEOL JM-6400) and energy dispersive X-ray spectroscopy (EDS). Standard metallographic preparation was performed on samples prior to cross-section observations. X-ray diffraction analyses (Philips X′PERT MPD, K α(Cu) radiation) were carried out to identify phase composition of the coated samples. 3. Results
also included in the system to consider interaction between gas phase and BCC substrate. The input conditions used for calculation were guided by previous experimental studies [9,10]. Between 300 °C and 800 °C, the main Si chlorides precursors, Si hydrides and chlorosilanes precursors SiH xCl4−x (partial pressure N 1 × 10− 4 Pascal) obtained form these calculations were ClH3Si, Cl3HSi, H4Si, Cl2H2Si, Cl4Si Cl2Si, ClHSi. The partial pressures of the main Si precursors determinated at 500 °C and 600 °C are presented in Table 2. ClH3Si is the main and most stable Si precursor present in all the temperature range studied; the concentration of this phase does not change significantly with the increasing temperature. According to thermodynamical estimations, partial pressures of other precursors are typically below 0.1 Pascal. Although, this pressure is too low for the conventional CVD process; but in this FBR-CVD even those low partial pressures could play an important role in the deposition processes. These calculations provide interesting information about gaseous chloride and hydride precursors formed in the reactor starting from solid donor and about dependence of these partial pressures on temperature. These thermodynamic calculations are an important guide for selecting the experimental conditions. These conditions were selected on the basis of: equilibrium distribution of the Si species that favoured the silicon deposition (Fig. 1) that increases with temperature. Moreover, the microstructural properties of the ferritic steels were considered, because this type of steel suffers microstructural changes at temperatures upper 700 °C. In addition to this, at high temperatures, it has also been reported as problems of adhesion [2,9]. The conditions selected for the experiment are shown in the Table 2. 3.2. Coatings characterisation 3.2.1. SEM and EDS analysis Fig. 2 shows the surface morphology of the Si coating on the ferritic steels (P-91). The sample was treated at 520 °C for 1 h in a 14 vol.% H2 (g) and 1.2 vol.% HCl(g) gas mixture. In this figure, it can be observed that the coating formed is homogenous, and small nodular grains grow on the surface. The EDS
3.1. Thermodynamics approximation Thermodynamic calculations of the vapour pressure of gaseous species of silicon in equilibrium conditions were performed with the purpose to find all possible gaseous and condensed phases that can form the combinations of the element involved in the following chemical system Si(s)/Ar(g)/HCl(g)/H2(g). Fe and Cr were Table 2 Coating conditions for deposition of silicon on ferritic steels Coating conditions
Pressure Time HCl H2
Temperatures 520 °C
550 °C
580 °C
101325 Pascal 1h 2.4 vol.% 28 vol.%
101325 Pascal 1h 2.4 vol.% 28 vol.%
101325 Pascal 1h 2.4 vol.% 28 vol.%
Fig. 1. Thermodynamic calculations of the system formed by w(Si) = 14 g with relation to H2/HCl = 12/1 Si precursors.
F.J. Bolívar et al. / Surface & Coatings Technology 201 (2006) 3953–3958
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Fig. 2. SEM surface morphology and EDS microanalysis of the Si coating on ferritic steels P-91 under a reactive gas mixture of 28 vol.% H2 and 2,4 vol.% HCl at 520 °C. The HCl was only supplied for 15 min, and the extra 45 min the samples were subsequent heat treatment at the same deposition temperature.
microanalysis shown in Fig. 2 revealed that the coating is principally composed of Si, Cr and Fe. According to the atomic percentages found by means of EDS figure in 2 and in agreement with phase diagram [11] suggest that the possible phase formed on the surface of steel is Fe3Si. According to the obtained results, XRD and EDS analysis suggest that the coating formed at temperatures upper 520 °C get spalled easily. At high temperature, the formation of Si-rich silicides is promoted and the coatings formed by this type of silicides have poor adhesion. Fig. 3 shows the surface morphology of the spalled Si coating on the ferritic steels (P-91). The sample was treated at 580 °C for 1 h in a 14 vol.% H2 (g) and 1.2 vol.% HCl(g) gas mixture. The EDS microanalysis shows that the coating is principally composed of Si, Cr and Fe (Fig. 3). According to the atomic percentages found by means of EDS figure in 3 and in agreement with phase diagram [11] suggest that the possible phase formed on the surface of steel is FeSi. Fig. 4 shows the cross-sections of the silicon coating on a ferritic steels. In this figure it is possible to observe that the coating is very irregular and porous. The thickness of the silicon coating is approximately 8 μm. Fig. 4 also shows the corresponding composition of the coating as a function of the distance from interface. This in line profile shows that the coating mainly
consists of Si, Fe and Cr. The percentages of Si in the coating is the range of 12–25 at.%, Fe is in the range of 48–65 at.%, and Cr is in the range of 5–12 at.% approximately. The atomic percentages suggest that the coating is mainly composed of the phase Fe–Si (Fe3Si), in agreement with phase diagram. According to this diagram DO3 structured Fe3Si, exist from 11 at.% Si to about 30 at.% depending on the temperature, see Fig. 5 [11]. 3.2.2. XRD analysis Fig. 6a shows the XRD diffraction pattern corresponding to a Si coating on ferritic steels and Fig. 6b the XRD diffraction pattern of the spalled coating. In Fig. 6a it is possible to observe the characteristic peaks of the Fe–Si phase Fe3Si as well as αFe. The presence of the Fe3Si is consistent with the results obtained by EDS analysis. Fig. 6b shows the XRD pattern of the spalled coating revealing that it consists of Fe3Si. 4. Discussion 4.1. Thermodynamic analysis According to the thermodynamic results an equilibrium between HCl, solid particles of Si, Fe–Cr steel and the surrounding gas phase can be approached closely in the bed.
Fig. 3. SEM surface morphology and EDS microanalysis of the spalled Si coating on ferritic steels P-91 under a reactive gas mixture of 28 vol.% H2 and 2.4 vol.% HCl at 580 °C. The HCl was only supplied for 15 min, and the extra 45 min the samples were subsequent heat treatment at the same deposition temperature.
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Fig. 4. Cross-section of Si coated substrate obtained after 1 h at 52O °C and EDS microanalysis of the Si coating on ferritic steels P-91 under a reactive gas mixture of 28 vol.% H2 and 2.4 vol.% HCl. The HCl was only supplied for 15 min, and the extra 45 min the samples were subsequent heat treatment at the same deposition temperature.
The main precursors formed in this system were ClH3Si, Cl3HSi, H4Si, Cl2H2Si, Cl4Si Cl2Si, ClHSi. Therefore, the possible equilibrium that takes place in the formation of the Si coating includes: Si þ HClðgÞ þ H2ðgÞ ⇔SiH3 ClðgÞ ΔG ¼ −35:2kJ
ðReaction1Þ
Si þ 3HClðgÞ ⇔SiHCl3ðgÞ þ H2ðgÞ ΔG ¼ −116kJ
ðReaction2Þ
Si þ 2HClðgÞ ⇔SiH2 Cl2ðgÞ ΔG ¼ −42:2kJ
ðReaction3Þ
Si þ 4HClðgÞ ⇔SiCl4 ðgÞ þ 2H2ðgÞ ΔG ¼ −158kJ
ðReaction4Þ
Si þ 2HClðgÞ ⇔SiCl2 ðgÞ þ H2ðgÞ ΔG ¼ −5:12kJ
ðReaction5Þ
Si þ 2H2ðgÞ ⇔SiH4ðgÞ ΔG ¼ 102kJ
ðReaction6Þ
They considered that the possible mechanism of the position of Si consists of the two steps: Adsorption of SiCl2(g) at substrate surface followed by reduction of SiCl2(g) with deposition of Si and desorption of HCl. This reaction is reversible (Reaction 8) and it can lead to etching of silicon coating for high SiHCl3 amount, which diminishes the deposition rate of Si. In the CVD-FBR process, according to thermodynamic calculations, different silicon halides are obtained by a reaction between silicon powder and a mixture of hydrochloride and hydrogen. The formation of the coating is probably due to the interaction of different precursors, such as SiHCl3 and SiCl2. Some authors proposed that above 1000 °C, these compounds are responsible for the formation of the layer. Thermodynamically, SiH3Cl is more stable than the other precursors in all the range of temperatures studied but its lifetime is very short. Therefore, its participation in the coating may be less important. However, in the CVD-FBR the precursors are generated very close to the
This approximation leads to consider that the coatings formation is due to a competitive mechanism between precursors. According to Sanjurjo et al. [12] SiCl2, SiH2Cl2 and SiHCl3 are more reactive species than SiCl4 and help in the deposition process at low temperatures. However, there are other more reactive species, such as SiH3Cl that may also be produced. Their lifetime however is very short, and so their participation in the processes of the formation of the coating may be less important. Petit and Zeman [13] suggest the principal reactions that occurring during process of formation of silicon coating are the following: SiHCl3ðgÞ ⇔SiCl2 ðgÞ þ HClðgÞ ΔG ¼ −121kJ
ðReaction7Þ
SiCl2ðgÞ þ H2ðgÞ ⇔Si þ 2HClðgÞ ΔG ¼ −5:12kJ
ðReaction8Þ
SiHCl3ðgÞ þ HClðgÞ ⇔SiCl4ðgÞ þ H2ðgÞ ΔG ¼ −42:4kJ (Reaction 9)
Fig. 5. Iron–silicon phase diagram [11].
F.J. Bolívar et al. / Surface & Coatings Technology 201 (2006) 3953–3958
Fig. 6. XRD Pattern of P-91 substrate a) coated; b) spalled coated.
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SiH4 gas and Fe as substrate, at temperatures below of 600 °C is due to diffusion of Fe along the grain boundaries and along defects which leads to the formation of spherical nodules of Fe3Si. However, when the temperature is higher than 800 °C the diffusion of Si into the bulk of Fe is dominant and therefore, no spherical nodules appear on the surface. This situation is in agreement with the present results and explains the morphology of the coating and the spallation, when the deposition was performed at higher temperature or for longer time, because when the silicon concentration increases in the coating and the coating becomes less adherent. When the deposition is performed at temperatures lower than 600 °C, the possible mechanism of coating formation is due to outward diffusion of Fe; then Fe3Si forms on the surface and grows in the form of few grains. The Si contained with increase of temperature or time deposition. Coatings with higher Si content spalled easily. 5. Conclusions
substrate. As a consequence, SiH3Cl could play an important role in the deposition of Si coating even if the lifetime is short. 4.2. Characterization of Si coating The composition of the Si coating was determined by EDS measurements on the surface and in incross-section. The results of this analysis show that the coating is composed at Si, Fe and Cr. The concentration of silicon on the surface reaches approximately 25 at.% and Fe and Cr reach approximately 55 and 20 at.% respectively. Therefore, it is possible to postulate that Fe3Si is the phase formed on the surface, because these atomic percentages correspond to this phase, as predicted by Fe–Si phase diagram shown in Fig. 5. This result was confirmed by the XRD analysis showing the characteristic peaks corresponding to Fe3Si. Sanjurjo et al. [2] found similar results, observing first the formation of α-Fe and then the formation of Fe3Si; the formation of this phase was possible after 5 min of treatment in the CVD-FBR reactor at 500 °C. They also observed that phases rich in Si, such as FeSi, are formed at higher temperatures or for longer times. Perez-Maraino et al. [4] obtained similar results as Sanjurjo; they deposited a Si coating on AISI 316 stainless steel by means of the CVD-FBR reactor at low temperatures, 450 °C and 500 °C. The coating obtained consisted mainly of Fe3Si. However, when the experiment was performed at higher temperature such as 675 °C, the formation of Si-rich silicides rich (such as FeSi and Fe5Si3) was promoted and this leads to the formation of a coating that spalled easily. Nevertheless, the deposition of silicon on steels was studied in the present work with results very similar to those obtained by Ref. [4], for other type of steels with different type of the microstructures. Therefore, in the mechanism of growth of the Si coating at the temperature factors such as temperature and time of the reactions is more important than the microstructure of the steels. Consequently, it is possible that the growth mechanism of the coating is very similar to the mechanism proposed by Rebhan et al. [14]. They proposed that the growth of the Si coating obtained by means of CVD using as precursors
Silicon coating were developed using CVD-FBR technology on ferritic steel with chromium content of 9–12 wt.%. The process was realized at low temperature in order to conserve the microstructure properties of the material. The thermodynamic study was used as a guide to select the initial condition for performing Si deposition on a low temperature (b700 °C). Based on these calculations, the temperature range selected for the experiment was 500 °C to 600 °C. The best results were obtained for temperature of 520 °C during 1 h with only 15 min of input of HCl. Diffusion coatings obtained on ferritic steel were approximately 8 μm thick and constituted principally of Fe3Si. The coatings obtained at temperatures higher than 600 °C easily spalled and are constituted principally of FeSi and Fe3Si. The coatings obtained at temperatures upper 520 °C easily spalled and are constituted principally of FeSi and Fe3Si. Acknowledgements The authors want to express their gratitude to the Spanish Ministerio de Educación y Ciencia for the financial support of this work. Project ENE-2005-08494-C02-02. References [1] J.T.K. Clark, A.I. Foster, M.L. Sims, M.A.M. Swidzinski, D. Young, Chemical Vapour Deposition, Eindhoven, Netherlands, 1983. [2] A. Sanjurjo, Samon Hettiarachchi, K.H. Lau, Philip Cox, Bernard Wood, Surf. Coat. Technol. 54/55 (1992) 224. [3] S.S. Ionayoshi, S. Tsukahara, A. Kinbara, Vacuum 53 (1999) 281. [4] J. Perez Mariano, J. Elvira, F. Plana, C. Colominas, Surf. Coat. Technol. 200 (18/19) (2006) 5606. [5] A. Sanjurjo, Bernard Wood, Kai Lau, Gopala Krishnan, Scr. Metall. Mater. 31 (8) (1994) 1019. [6] J. Porcayo-Calderón, E. Brito Figueroa, J.G. Gonzaléz-Rodriguez, Mater. Lett. 38 (1999) 45. [7] Thermocalc, Thremocalc Software, Foundation of Computational Thermodynamics, Stockholm, Sweden, 1995–2003. [8] F.J Pérez, M.P. Hierro, F. Pedraza, C. Gomez, M.C. Carpintero, J.A. Trilleros, Surf. Coat. Technol. 122 (1999) 281.
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[9] F.J. Pérez, M.P. Hierro, M.C. Carpintero, F. Pedraza, C. Gomez, Surf. Coat. Technol. 140 (2001) 93. [10] F.J. Pérez, M.P. Hierro, M.C. Carpintero, C. Gomez, F. Pedraza, Surf. Coat. Technol. 160 (2002) 87. [11] A. Il´Inskii, S. Slyusarenko, O. Slukhovskii, I. Kaban, W. Hoyer, J. NonCryst. Solids 306 (2002) 90. [12] A. Sanjurjo, K. Lau, B. Wood, Surf. Coat. Technol. 54/55 (1992) 219.
[13] Agnés Petit, Miro Zeman, High temperature CVD depostion of thin polycristalline silicon layer, p, http://retina.et.tudelft.nl/data/artwork/ publication/679241359.pdf. [14] M. Rebhan, R. Meier, A. Plagge, M. Rohwerder, M. Stratmann, Appl. Surf. Sci. 178 (2001) 194.