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Deposition characteristics of titanium coating deposited on SiC fiber by cold-wall chemical vapor deposition
Materials Chemistry and Physics xxx (2016) 1e8
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Deposition characteristics of titanium coating deposited on SiC fiber by cold-wall chemical vapor deposition Xian Luo*, Shuai Wu, Yan-qing Yang, Na Jin, Shuai Liu, Bin Huang State Key Lab of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, PR China
h i g h l i g h t s Both thermodynamic analysis and experimental studies were adopted in this work. The transformation paths of TiCl4 to Ti is: TiCl4 / TiCl3 / Ti, or TiCl4 / TiCl3 / TiCl2 / Ti. Typical deposited Ti coating on SiC fiber contained two distinct layers. Deposition temperature is important on deposition rate and morphologies. Appropriate argon gas flow rate has a positive effect on smoothing of the coating.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 April 2016 Received in revised form 1 August 2016 Accepted 12 September 2016 Available online xxx
The deposition characteristics of titanium coating on SiC fiber using TiCl4-H2-Ar gas mixture in a coldwall chemical vapor deposition were studied by the combination of thermodynamic analysis and experimental studies. The thermodynamic analysis of the reactions in the TiCl4-H2-Ar system indicates that TiCl4 transforms to titanium as the following paths: TiCl4 / TiCl3 / Ti, or TiCl4 / TiCl3 / TiCl2 / Ti. The experimental results show that typical deposited coating contains two distinct layers: a TiC reaction layer close to SiC fiber and titanium coating which has an atomic percentage of titanium more than 70% and that of carbon lower than 30%. The results illustrate that a carbon diffusion barrier coating needs to be deposited if pure titanium is to be prepared. The deposition rate increases with the increase of temperature, but higher temperature has a negative effect on the surface uniformity of titanium coating. In addition, appropriate argon gas flow rate has a positive effect on smoothing the surface morphology of the coating. © 2016 Elsevier B.V. All rights reserved.
Keywords: Metals Chemical vapor deposition (CVD) Coatings Microstructure Thermodynamic properties
1. Introduction Titanium coating has been prepared for a variety of applications, including corrosion-resistant, abrasion-resistant, electrodes or electronic contacts et al. [1e3]. There are many techniques to prepare titanium coating, which can be mainly divided into two kinds: physical and chemical ones. Physical methods are mostly used to deposit titanium film in different coating systems [4e8], such as magnetron sputtering [9e13], pulse biased arc ion plating [14,15], plasma immersion ion implantation and deposition [16], pulsed laser deposition [17], or cold spraying [18]. Chemical methods are much less used to prepare titanium film [19e22], and the relationships between deposition parameters and characteristics of
* Corresponding author. E-mail address: [email protected] (X. Luo).
the coating are still unclear. Up to now, chemical vapor deposition (CVD) has got great development as a technique to prepare metal films for numerous applications in the electronics and coating industries [23]. Thin titanium films have been obtained by conventional CVD, plasma enhanced chemical vapor deposition (PECVD), or laser chemical vapor deposition (LCVD). For instance, Tan [24] prepared a Ti coating on SiC-coated boron fibers via CVD using TiCl4-H2 mixture at 1040 C under atmospheric pressure. The titanium was found to react with the fiber surface, and the carbon-rich surface results in the formation of a protective TiC layer which could impede the diffusion of Ti into the SiC coating. Sang [25] applied Ti-I2 powder mixture to prepare Ti coating in alumina foam by hot-wall CVD method. Their results showed that the obtained titanium coating had a high purity, and showed a good coverage and uniform distribution on the skeleton surface of the foam. Hedaiatmofidi [26]
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Please cite this article in press as: X. Luo, et al., Deposition characteristics of titanium coating deposited on SiC fiber by cold-wall chemical vapor deposition, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.09.041
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used PECVD method to deposit titanium film at 470 Ce530 C with TiCl4-H2-Ar gas mixture. It was found that increasing hydrogen flow rate resulted in a decrease in oxygen and chlorine contents of the film. Moreover, applied plasma voltage had a severe effect on nanohardness of the coating, and pressure of the deposition chamber had a negative effect on titanium characteristics. Chou [27] prepared titanium film directly from the dissociation of TiBr4 using a CO2 laser beam in an argon atmosphere. Their results indicate that the coatings were relatively pure (90%) and uniform. In addition, both the film thickness and deposition rate of the films have been studied as a function of partial vapor pressure of TiBr4, irradiation time, and chamber temperature, respectively. Anyhow, few researchers have studied depositing titanium film employing TiCl4-H2-Ar system [26], and it mostly focused on experimental studies in order to prepare titanium coating with optimum parameters. Currently there is little information available concerning the thermodynamic analysis of the CVD reactions in TiCl4-H2-Ar system, but these theoretical data are vital to achieve a titanium coating which predicts the formation mechanism of the deposit. Accordingly, the theoretical and experimental analyses on the deposition characteristics of the coating are necessary to understand the phenomena involved in the CVD process. In the present work, thermodynamic analysis was carried out based on the TiCl4-H2-Ar system in a cold-wall CVD chamber. Meanwhile, titanium coating was deposited systematically using TiCl4-H2-Ar as a reaction system under a temperature range from 1000 C to 1200 C. The deposition characteristics of titanium coating were studied through studying the microstructure of the deposits prepared by cold-wall CVD process under different temperatures and gas flow rates. 2. Experimental 2.1. Preparation of the coating TiCl4 (purity 99.99 wt%) was adopted as Ti resource, and high purity H2 (purity 99.99 wt%) was used as dilution and carrier gas which delivers TiCl4 from the bubbler to the reactor, meanwhile high purity Ar (purity 99.99 wt%) was used as dilution gas. The deposition substrate was a 100 mm diameter SiC fiber with a tungsten core of about 16 mm in diameter. To avoid the effect of surface contamination on the deposition process of titanium coating, the surface of SiC fiber was cleaned by alcohol and dried in a drying oven which was set at 30 C for 30 min before starting deposition experiments. Fig. 1 shows a schematic of the experimental system used for the preparation of titanium coating. The experimental system contains a mixture of TiCl4, H2 and Ar reagents in which all kinds of reagents were delivered through float flow meters. Reagents were carried to the reactor through rubber tubes. The deposition reactor was a horizontal, cold-wall deposition chamber. The substrate was heated to certain temperature by controlling a DC power device, and the temperature of the substrate was measured using a WGG2 optical pyrometer. Typical deposition process parameters are listed in Table 1. After deposition, the deposited coatings were cooled to room temperature by air cooling. The reactant TiCl4 was carried into the reactor by H2. According to the Dalton's Law of Partial Pressures, the relation between the amount of TiCl4 and the carrier gas H2 can be described by the following formula:
. RT FT ¼ 3:06 102 PT FH
(1)
where FT refers to the flow of TiCl4 in the reactor, ml/min; PT refers
to saturated vapor pressure of TiCl4 at a certain temperature, Pa; FH refers to the flow of H2, ml/min; R refers to the gas constant, J/ (mol K); and T refers to the temperature of TiCl4, C. In this work, T was controlled to 100 C. 2.2. Characterization methods After the coating was deposited on SiC fiber, the surface and cross-sectional morphologies of the deposited coating were observed using scanning electron microscopy (SEM, Zeiss SUPRA 55, Germany). In order to acquire average thickness and calculate deposition rate, ten different positions around cross-sectional edge were measured by SEM. The distributions of ten positions are uniform around cross-sectional edge. Deposition rate was calculated through Eqs. (2) and (3):
Sa ¼ ðS1 þ S2 þ S þ … þ S10 Þ=10
(2)
V ¼ Sa =t
(3)
In Eqs. (2) and (3), Sa refers to average thickness of deposits, mm; Si (i ¼ 1, 2, 3 … 10) refers to thicknesses of deposits at different positions on substrates, mm; V refers to average deposition rate, mm/ min; and t refers to deposition time, min. The phase composition was characterized by X-ray diffraction (XRD, X'Pert Pro, Philip), and X-ray energy dispersive spectrometer (EDS, Oxford INCA Energy 350) was used to qualitatively analyze the chemical composition of the deposited coating. 3. Thermodynamic analysis It is known that CVD reaction process is governed by both thermodynamics and kinetics. Thermodynamic analysis has great significances for guiding and determining a practical CVD process [28], which is accomplished by calculations of the thermodynamic equilibrium of the CVD reactions. Thermodynamic analysis would provide useful information about the characteristics and behaviors of the reactions, and can help optimize deposition conditions. In general, a complete thermodynamic calculation must include all chemical reactions involved in the actual process. Through the calculated results, researchers can analyze and predict the feasibility of the practical chemical reactions. Therefore, before starting the experiment, we attempted to make a complete understanding and an optimal prediction of the CVD reactions in the TiCl4-H2-Ar system through thermodynamic investigation. In this work, HSC Chemistry 5.1 chemical reaction and equilibrium software was used. From the reactor in Fig. 1, it is easy to know that a temperature gradient would be produced from substrate to the cold-wall when the substrate is heated. In the cold-wall CVD chamber, gas sources may transform into other forms when adequate energy is provided for corresponding reactions at certain temperatures. Table 2 shows physical characteristics of different titanium chlorides, including their colors, molar mass, melting points and boiling points, which indirectly indicate their lowest energy state at certain temperatures. For example, TiCl4 is gaseous while TiCl3 is solid at 200e1300 C. Titanium chlorides mainly include three kinds, TiCl4, TiCl3 and TiCl2, whereas TiCl is ruled out because of its unstable and easy decomposition characteristics. As temperature changes, TiCl4 may decompose into subchlorides such as TiCl3 and TiCl2 before it reaches the substrate, since heated substrate will radiate a large amount of heat to provide enough energy for chemical reaction. In the TiCl4-H2-Ar system, TiCl4 and H2 are the reactant gases. The main reaction equations that will probably take place are shown as the following:
Please cite this article in press as: X. Luo, et al., Deposition characteristics of titanium coating deposited on SiC fiber by cold-wall chemical vapor deposition, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.09.041
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Fig. 1. Schematic of the horizontal cold-wall reactor used to deposit coating using TiCl4-H2-Ar system.
Table 1 Typical process parameters of CVD titanium coating. Substrate
TiCl4 (ml/min)
H2 (ml/min)
Ar (ml/min)
T ( C)
T (min)
SiC fiber
300
600
300
1000
30
groups: (4) and (5e13). It can be seen that, with the increase of temperature, the DrG of the former increases and the Logk of the former decreases, but the trends of the latter are just the opposite. It is well known that, if the DrG is below zero and the Logk is over
Table 2 Physical properties of titanium chlorides. Chemical formula
Color
Molar mass (g/mol)
Melting point ( C)
Boiling point ( C)
TiCl4 TiCl3 TiCl2
Colorless Purple Brown
189.71 154.26 118.80
25 >440, decomposition 1035
136
H2(g) þ 2TiCl4(g) ¼ 2TiCl3(s) þ 2HCl(g)
(4)
H2(g) þ TiCl4(g) ¼ TiCl2(s) þ 2HCl(g)
(5)
2H2(g) þ TiCl4(g) ¼ Ti(s) þ 4HCl(g)
(6)
H2(g) þ 2TiCl3(s) ¼ 2TiCl2(s)þ 2HCl(g)
(7)
3H2(g) þ 2TiCl3(s) ¼ 2Ti(s) þ 6HCl(g)
(8)
H2(g) þ TiCl2(s) ¼ Ti(s) þ 2HCl(g)
(9)
1500
2TiCl3(s) ¼ TiCl2(s) þ TiCl4(g)
(10)
4TiCl3(s) ¼ Ti(s) þ 3TiCl4(g)
(11)
3TiCl2(s) ¼ Ti(s) þ 2TiCl3(s)
(12)
zero, the chemical reaction can occur from the viewpoint of thermodynamics, and if the smaller the DrG is as well as the greater the Logk is, the more easily the reaction might occur. From the variation trends of DrG and Logk, the reaction (4) may occur in thermodynamics at a lower temperature below 300 C when its DrG is below zero and its Logk is over zero. As for the reactions (5e13), when the temperature is higher than 900 C, the DrG of the reactions (10) and (11) are below zero and the Logk of the reactions (10) and (11) is over zero. The reaction (13) has the same variation trend as the reactions (10) and (11) when temperature is over 1100 C. Therefore, the reactions (10, 11) and (13) can occur in thermodynamics at temperatures over 900 C and 1100 C, respectively. Furthermore, the reaction (11) will be easier to occur than the reactions (10) and (13). Thus one may prediect that TiCl4 would reduce gradually with the increase of temperature near the substrate, which means that the reactants transform as follows: TiCl4 / TiCl3 / Ti, or TiCl4 / TiCl3 / TiCl2 / Ti.
2TiCl2(s) ¼ Ti(s) þ TiCl4(g)
(13)
4. Results and discussion
Effects of temperatures on the Gibbs free energy (DrG) and chemical equilibrium constants (Logk) of reactions (4)e(13) are shown in Fig. 2. The reactions (4)e(13) might be divided into two
4.1. Growing process of titanium film From the above thermodynamic analysis, it is known that TiCl4
Please cite this article in press as: X. Luo, et al., Deposition characteristics of titanium coating deposited on SiC fiber by cold-wall chemical vapor deposition, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.09.041
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60
(a)
48 ΔrG (kJ/mol)
36 24 12 0 -12 4 8 12
-24 -36 800
5 9
6 10
7 11
13
900
1000
1100
1200
1300
T (°C)
6
(b)
4 2
4 8 12
5 9 13
6 10
7 11
Fig. 3. A schematic diagram of the steps that occurred during the CVD process.
25000 Intensity (counts)
Logk
0 -2 -4 -6 -8 -10 -12
800
900
1000
1100
1200
1300
TiCl3 TiCl3·6H2O
20000
TiO2
15000 10000 5000 0
T (°C)
10
Fig. 2. The thermodynamic relationships: (a) Effects of temperature on DrG of reactions (4)e(13); (b) Effects of temperature on Logk of reactions (4)e(13).
20
30
40
50
60
70
2θ (degree) Fig. 4. XRD pattern of the deposited purple powder.
gas has two react ways to transform into titanium. First of all, TiCl4 is directly degraded to TiCl3 by hydrogen, and its reaction temperature is near and under 300 C. Since there is a large temperature range from the fiber surface to the wall of the reaction chamber, TiCl3 is likely to be generated by TiCl4 and hydrogen. Then TiCl3 decomposes to produce titanium in two possible paths, one is to form titanium directly, the other is to decompose to TiCl2 first and then TiCl2 decomposes to titanium. These gas reactions took place near or on the heated SiC fiber as the heated filament continuously provides sufficient energy. According to the experiment conditions, the dynamic process of the CVD titanium can be schematically shown in Fig. 3. In Fig. 3, the deposition process is separated into the following four stages to clarify the growing process of titanium film: 1) TiCl4 is degraded to TiCl3 by hydrogen; 2) The decomposition of TiCl3: TiCl3 / Ti or TiCl3 / TiCl2 / Ti; 3) Nucleation and growth of titanium atoms to form a continuous coating in the surface of SiC fiber; 4) Gaseous by-products of the reactions diffuse away from the surface through the boundary layer. In this work, we observed that purple powder formed and deposited onto the wall of the reaction chamber during gas reaction. Fig. 4 shows XRD pattern of the purple powder, which reveals that the purple powder is mainly TiCl3, and with a small amount of TiCl3$6H2O and TiO2. The appearance of TiCl3$6H2O and TiO2 was caused by the hydrolysis and oxidation reaction of TiCl3 in the air, as TiCl3 is apt to absorb water vapor and react with oxygen [29]. The hydrolysis and oxidation reaction equations are: TiCl3 þ 6H2O / TiCl3$6H2O
(14)
3TiCl3 þ TiCl3$6H2O þ O2 / 4TiO2 þ 12HCl
(15)
Obviously, the results support the above thermodynamics analysis that TiCl4 may decompose into TiCl3 first. However, no TiCl2 was found in the whole deposition process. The formed TiCl2 may be decomposed quickly in gases since it is extremely unstable or its yield is little. 4.2. Surface and interfacial characterization of titanium coatings Typical titanium coating was deposited at 1000 C for 30 min, and detailed process parameters are listed in Table 1. Fig. 5a and b show the SEM images of the surface and cross-sectional morphology of titanium coating on SiC fiber, respectively. It is seen that the surface structure of the titanium coating is compact and uniform. Fig. 5c shows the magnified morphology of the area marked in Fig. 5b. The average thickness of the titanium coating is 3.5 mm. There are two distinct layers, the layer 1 having a granular morphology while the layer 2 having a columnar morphology. In order to know the differences between the two layers, EDS analysis was carried out. Fig. 5d shows the EDS line scanning results from SiC fiber to the surface of the coating where an arrow is marked in Fig. 5c. CVD SiC can be stoichiometric, C-rich or Si-rich, depending on the deposition conditions [30]. The SiC fiber used in this work is a carbon-rich fiber, and the original Si:C ratio close to surface is about 60:40 [31]. From inner to surface of the SiC fiber, the atomic percentage of
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Fig. 5. Typical titanium coating: (a), (b) SEM images of the surface and cross-sectional morphology of typical titanium coating, respectively; (c) The detailed morphology of the area marked by a white square in (b); (d) EDS line scanning result of the area marked by the red arrow in (c). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
4.3.1. Deposition rate Usually, a CVD process includes diffusion of reactants through boundary layer, adsorption of reactants on substrate, chemical reaction taking place near substrate or on substrate, desorption of adsorbed species, and diffusion out of by-products [28]. In this work, the CVD titanium process of the TiCl4-H2-Ar system is symbolically exhibited in Fig. 3. The whole process often determines the deposition rate, yet it requires adequate activation energy to make it happen. Therefore, temperature has a great influence on the deposition rate since it is the energy provider. The relationship of them can be calculated by Arrhenius equation:
(16)
where V is the deposition rate, mm/min; A is the pre-exponential factor; R is the gas constant; Ea is the apparent activation energy of these processes, kJ/mol; and T is the deposition temperature, K. Fig. 6 shows the relationship between temperature and deposition rate. The process parameters are listed in Table 3. According to the Arrhenius formula, the deposition rate has an exponential increase with the increase of temperature. It can be seen from Fig. 6 that the deposition rate of titanium coating increases as the temperature increases from 1000 to 1200 C. Moreover, it increases
20
6.5
1/T (10-4/K) 7.0 7.5
8.0 4
16 2 12 0
8
-2
4 0
Ln deposition rate
4.3. Effect of temperature on the deposition of titanium
Ea V ¼ A$exp RT
Deposition rate (μm/min)
carbon changed from 60% to nearly 70%, which indicates that carbon atoms in SiC fiber have diffused from inner to surface after the Ti deposition process. As to the deposition layer, in layer 1, no very little silicon diffusion exists, and the atomic ratio of Ti and C closed to the atomic percentage composition of TiC. So the layer 1 mainly contains TiC. But it may also contain ternary phases like Ti3SiC2 [32] or Ti5Si3Cx [33], since Ti can react with SiC to form this compounds in the deposition condition. In layer 2, the atomic percentage of carbon sharply decreases to lower than 30% while that of titanium quickly increases to more than 70%. The atomic content of titanium and carbon in layer 2 remained relatively stable. It is clear that a part of titanium close to the fiber has reacted to form the layer 1, which could hinder the diffusion of carbon atoms in some extent. So the carbon amount in layer 2 was much lower than layer 1. However, the layer 2 still had some carbon diffused in. If a pure titanium coating is to be prepared, a coating which can hinder the diffusion of carbon atoms needs to be deposited on SiC fiber firstly.
-4 1000
1100 T (°C)
1200
Fig. 6. The relationship between temperature and deposition rate.
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Table 3 Process parameters of CVD titanium coating at different temperatures. Substrate
TiCl4 (ml/min)
H2 (ml/min)
T ( C)
t (min)
SiC fiber
300
600
1000e1200
5e30
faster at the temperature range of 1100e1200 C. The maximum deposition rate is about 16 mm/min at 1200 C, while the minimum deposition rate is only about 0.1 mm/min at 1000 C. This phenomenon can be explained as we take the logarithm of the Arrhenius Eq. (16). Then Eq. (16) is transformed into the form: LnV ¼ LnA Ea/RT. The relation of LnV and 1/T is also shown in Fig. 6 (the blue cure). In the graph, the absolute value of the slope Ea/R at 1100e1200 C is lower than that at 1000e1100 C. The lower slope Ea/R means the lower Ea, therefore, the CVD process would be easier to occur at 1100e1200 C, which helps improve deposition rate.
4.3.2. Surface morphology The temperature also has great influences on nucleation and growth of the deposited titanium. From the viewpoint of nucleation dynamics [34], temperature T impacts on radius of critical nucleus rc and critical nucleation energy DrGmax. Their relationships are that
rc is in proportion to 1/T, and DrGmax is in proportion to 1/T2. Both rc and DrGmax decrease with the increase of temperature, which means that the nuclei formation is easier at higher temperature, and the thickness of deposits and the deposition rate will increase. At the same time, the faster nuclei formation of atoms makes them clusters and grow together, therefore, the surface structure changes. Fig. 7 shows SEM images of the surface morphology of titanium coatings which were prepared at different temperatures. When deposition temperature was 1000 C, the surface is relatively smooth, and the grain size of the coating is even, as shown in Fig. 7a. It is due to the very slow deposition rate as well as the uniform atomic diffusion and growth on the deposition surface. However, microcracks formed on the surface because of thermal residual stresses, as shown in Fig. 7b. When the coating was prepared from high temperature to room temperature, thermal residual stresses were generated due to the large difference of thermal expansion coefficients between SiC and titanium. When the preparing temperature was 1100 C, the grain of the coating had a relatively complete growth even though the coating surface is not smooth, as shown in Fig. 7c. Meanwhile, small granular crystals can be observed on the surface, as shown in Fig. 7d. This is due to the fact that the increased deposition rate, the faster nucleation and the lower effect of atomic diffusion make the deposited atoms cluster
Fig. 7. SEM images of the surface of titanium coatings which were formed at certain temperatures in different magnifications: (a) 1000 C at a low magnification; (b) 1000 C at a higher magnification; (c) 1100 C at a low magnification; (d) 1100 C at a higher magnification; (e) 1200 C at a low magnification; (f) 1200 C at a higher magnification.
Please cite this article in press as: X. Luo, et al., Deposition characteristics of titanium coating deposited on SiC fiber by cold-wall chemical vapor deposition, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.09.041
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Fig. 8. Surface morphology of titanium coating with certain flow rate of argon gas in different magnifications: (a), (b) the argon gas flow rate is 300 ml/min; (c), (d) the argon gas flow rate is 1000 ml/min.
and grow together. When deposition temperature was 1200 C, the generating speed of titanium atoms as well as the nucleation rate in the chemical reaction was very fast, which made atoms cluster more easily. Meanwhile, the obvious thermal effect on surface gave rise to adsorptions and growth of the atoms. It is inferred that a large number of new titanium atoms nucleation and further growth in perpendicular to the direction of the coating induced the obvious changes in the morphology of titanium coating, which presents cellular bulges in Fig. 7e. The magnified image of Fig. 7f shows that the cellular bulges consisting of many nanoscale particles were sporadically distributed on the coating surface.
4.4. Effect of argon gas on microstructure of titanium coating In previous studies, argon gas added to the reaction chamber usually acts different roles with certain depositional conditions in various coating deposition systems. Zhang and co-authors [35] drew a conclusion that argon gas can decrease grain size and improve film qualities in a CVD diamond system. Zhang and coauthors [36] also revealed that argon gas can change grain preferred-orientation and decrease reaction rate in a CVD TaC system. For the CVD titanium system, argon gas never participates in gas phase reaction. The main roles of argon gas are to dilute the concentration of the reaction gases, to make an intensive mixing of gases, and to bring out the residual gases. This indicates that argon gas has a certain effect on the CVD titanium process which was previously illuminated to have a significant impact on the morphology of the deposited coating. Fig. 8 shows the surface morphology of titanium coating in different magnifications with different argon gas flow rates. The main process parameters were the same as Table 1, but the argon gas flow rate was altered. Fig. 8a shows a flat and smooth surface morphology when the argon gas flow rate was 300 ml/min, whereas Fig. 8c shows a coarse surface morphology with numerous obvious protrusions when the argon gas flow rate was increased up to 1000 ml/min. At higher magnifications (Fig. 8b and d), the obvious difference between Fig. 8a and c can be seen in detail, as
the bud shaped morphology shows in Fig. 8d. It is clear that a proper increase of argon gas flow rate in the reaction chamber could promote the grain refinement and make coating surface uniform. However, high argon gas flow rate would make the surface rough since it reduces the reaction gas which is adsorbed on the fiber surface and lowers the surface temperature.
5. Conclusions Titanium coating was deposited on SiC fiber under different deposition parameters in a TiCl4-H2-Ar system in order to reveal their effects on the coating characteristics. The thermodynamic analysis of the CVD reactions indicates that TiCl4 is transformed to titanium in two ways: TiCl4 / TiCl3 / Ti, or TiCl4 / TiCl3 / TiCl2 /vTi. In the cold-wall CVD process, typical deposited coating contains two distinct layers: the layer close to SiC fiber forms mainly TiC, and the following Ti layer. It is clear that a diffusion barrier coating is needed to be deposited firstly if pure titanium is to be made. The deposition rate can be improved with the increase of temperature, especially when temperature is higher than 1100 C. However, high temperature has a negative effect on the surface uniformity of titanium coating. At 1000 C, the surface is relatively smooth. When temperature is 1100 C, the coating surface becomes rough and small particular crystal morphology can be observed. When temperature is increased up to 1200 C, the surface presents cellular bulges which consist of many nanoscale particles. Argon gas flow rate with an appropriate value has a smoothing effect on the surface morphology of the coating.
Acknowledgments Thanks are given to the financial supports of the Natural Science Foundation of China (No. 51201134, 51271147), the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (Grant No. 115-QP-2014) and the Fundamental Research Funds for the Central Universities (3102014JCQ01023).
Please cite this article in press as: X. Luo, et al., Deposition characteristics of titanium coating deposited on SiC fiber by cold-wall chemical vapor deposition, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.09.041
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Deposition characteristics of titanium coating deposited on SiC fiber by cold-wall chemical vapor deposition Xian Luo*, Shuai Wu, Yan-qing Yang, Na Jin, Shuai Liu, Bin Huang State Key Lab of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, PR China
h i g h l i g h t s Both thermodynamic analysis and experimental studies were adopted in this work. The transformation paths of TiCl4 to Ti is: TiCl4 / TiCl3 / Ti, or TiCl4 / TiCl3 / TiCl2 / Ti. Typical deposited Ti coating on SiC fiber contained two distinct layers. Deposition temperature is important on deposition rate and morphologies. Appropriate argon gas flow rate has a positive effect on smoothing of the coating.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 April 2016 Received in revised form 1 August 2016 Accepted 12 September 2016 Available online xxx
The deposition characteristics of titanium coating on SiC fiber using TiCl4-H2-Ar gas mixture in a coldwall chemical vapor deposition were studied by the combination of thermodynamic analysis and experimental studies. The thermodynamic analysis of the reactions in the TiCl4-H2-Ar system indicates that TiCl4 transforms to titanium as the following paths: TiCl4 / TiCl3 / Ti, or TiCl4 / TiCl3 / TiCl2 / Ti. The experimental results show that typical deposited coating contains two distinct layers: a TiC reaction layer close to SiC fiber and titanium coating which has an atomic percentage of titanium more than 70% and that of carbon lower than 30%. The results illustrate that a carbon diffusion barrier coating needs to be deposited if pure titanium is to be prepared. The deposition rate increases with the increase of temperature, but higher temperature has a negative effect on the surface uniformity of titanium coating. In addition, appropriate argon gas flow rate has a positive effect on smoothing the surface morphology of the coating. © 2016 Elsevier B.V. All rights reserved.
Keywords: Metals Chemical vapor deposition (CVD) Coatings Microstructure Thermodynamic properties
1. Introduction Titanium coating has been prepared for a variety of applications, including corrosion-resistant, abrasion-resistant, electrodes or electronic contacts et al. [1e3]. There are many techniques to prepare titanium coating, which can be mainly divided into two kinds: physical and chemical ones. Physical methods are mostly used to deposit titanium film in different coating systems [4e8], such as magnetron sputtering [9e13], pulse biased arc ion plating [14,15], plasma immersion ion implantation and deposition [16], pulsed laser deposition [17], or cold spraying [18]. Chemical methods are much less used to prepare titanium film [19e22], and the relationships between deposition parameters and characteristics of
* Corresponding author. E-mail address: [email protected] (X. Luo).
the coating are still unclear. Up to now, chemical vapor deposition (CVD) has got great development as a technique to prepare metal films for numerous applications in the electronics and coating industries [23]. Thin titanium films have been obtained by conventional CVD, plasma enhanced chemical vapor deposition (PECVD), or laser chemical vapor deposition (LCVD). For instance, Tan [24] prepared a Ti coating on SiC-coated boron fibers via CVD using TiCl4-H2 mixture at 1040 C under atmospheric pressure. The titanium was found to react with the fiber surface, and the carbon-rich surface results in the formation of a protective TiC layer which could impede the diffusion of Ti into the SiC coating. Sang [25] applied Ti-I2 powder mixture to prepare Ti coating in alumina foam by hot-wall CVD method. Their results showed that the obtained titanium coating had a high purity, and showed a good coverage and uniform distribution on the skeleton surface of the foam. Hedaiatmofidi [26]
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used PECVD method to deposit titanium film at 470 Ce530 C with TiCl4-H2-Ar gas mixture. It was found that increasing hydrogen flow rate resulted in a decrease in oxygen and chlorine contents of the film. Moreover, applied plasma voltage had a severe effect on nanohardness of the coating, and pressure of the deposition chamber had a negative effect on titanium characteristics. Chou [27] prepared titanium film directly from the dissociation of TiBr4 using a CO2 laser beam in an argon atmosphere. Their results indicate that the coatings were relatively pure (90%) and uniform. In addition, both the film thickness and deposition rate of the films have been studied as a function of partial vapor pressure of TiBr4, irradiation time, and chamber temperature, respectively. Anyhow, few researchers have studied depositing titanium film employing TiCl4-H2-Ar system [26], and it mostly focused on experimental studies in order to prepare titanium coating with optimum parameters. Currently there is little information available concerning the thermodynamic analysis of the CVD reactions in TiCl4-H2-Ar system, but these theoretical data are vital to achieve a titanium coating which predicts the formation mechanism of the deposit. Accordingly, the theoretical and experimental analyses on the deposition characteristics of the coating are necessary to understand the phenomena involved in the CVD process. In the present work, thermodynamic analysis was carried out based on the TiCl4-H2-Ar system in a cold-wall CVD chamber. Meanwhile, titanium coating was deposited systematically using TiCl4-H2-Ar as a reaction system under a temperature range from 1000 C to 1200 C. The deposition characteristics of titanium coating were studied through studying the microstructure of the deposits prepared by cold-wall CVD process under different temperatures and gas flow rates. 2. Experimental 2.1. Preparation of the coating TiCl4 (purity 99.99 wt%) was adopted as Ti resource, and high purity H2 (purity 99.99 wt%) was used as dilution and carrier gas which delivers TiCl4 from the bubbler to the reactor, meanwhile high purity Ar (purity 99.99 wt%) was used as dilution gas. The deposition substrate was a 100 mm diameter SiC fiber with a tungsten core of about 16 mm in diameter. To avoid the effect of surface contamination on the deposition process of titanium coating, the surface of SiC fiber was cleaned by alcohol and dried in a drying oven which was set at 30 C for 30 min before starting deposition experiments. Fig. 1 shows a schematic of the experimental system used for the preparation of titanium coating. The experimental system contains a mixture of TiCl4, H2 and Ar reagents in which all kinds of reagents were delivered through float flow meters. Reagents were carried to the reactor through rubber tubes. The deposition reactor was a horizontal, cold-wall deposition chamber. The substrate was heated to certain temperature by controlling a DC power device, and the temperature of the substrate was measured using a WGG2 optical pyrometer. Typical deposition process parameters are listed in Table 1. After deposition, the deposited coatings were cooled to room temperature by air cooling. The reactant TiCl4 was carried into the reactor by H2. According to the Dalton's Law of Partial Pressures, the relation between the amount of TiCl4 and the carrier gas H2 can be described by the following formula:
. RT FT ¼ 3:06 102 PT FH
(1)
where FT refers to the flow of TiCl4 in the reactor, ml/min; PT refers
to saturated vapor pressure of TiCl4 at a certain temperature, Pa; FH refers to the flow of H2, ml/min; R refers to the gas constant, J/ (mol K); and T refers to the temperature of TiCl4, C. In this work, T was controlled to 100 C. 2.2. Characterization methods After the coating was deposited on SiC fiber, the surface and cross-sectional morphologies of the deposited coating were observed using scanning electron microscopy (SEM, Zeiss SUPRA 55, Germany). In order to acquire average thickness and calculate deposition rate, ten different positions around cross-sectional edge were measured by SEM. The distributions of ten positions are uniform around cross-sectional edge. Deposition rate was calculated through Eqs. (2) and (3):
Sa ¼ ðS1 þ S2 þ S þ … þ S10 Þ=10
(2)
V ¼ Sa =t
(3)
In Eqs. (2) and (3), Sa refers to average thickness of deposits, mm; Si (i ¼ 1, 2, 3 … 10) refers to thicknesses of deposits at different positions on substrates, mm; V refers to average deposition rate, mm/ min; and t refers to deposition time, min. The phase composition was characterized by X-ray diffraction (XRD, X'Pert Pro, Philip), and X-ray energy dispersive spectrometer (EDS, Oxford INCA Energy 350) was used to qualitatively analyze the chemical composition of the deposited coating. 3. Thermodynamic analysis It is known that CVD reaction process is governed by both thermodynamics and kinetics. Thermodynamic analysis has great significances for guiding and determining a practical CVD process [28], which is accomplished by calculations of the thermodynamic equilibrium of the CVD reactions. Thermodynamic analysis would provide useful information about the characteristics and behaviors of the reactions, and can help optimize deposition conditions. In general, a complete thermodynamic calculation must include all chemical reactions involved in the actual process. Through the calculated results, researchers can analyze and predict the feasibility of the practical chemical reactions. Therefore, before starting the experiment, we attempted to make a complete understanding and an optimal prediction of the CVD reactions in the TiCl4-H2-Ar system through thermodynamic investigation. In this work, HSC Chemistry 5.1 chemical reaction and equilibrium software was used. From the reactor in Fig. 1, it is easy to know that a temperature gradient would be produced from substrate to the cold-wall when the substrate is heated. In the cold-wall CVD chamber, gas sources may transform into other forms when adequate energy is provided for corresponding reactions at certain temperatures. Table 2 shows physical characteristics of different titanium chlorides, including their colors, molar mass, melting points and boiling points, which indirectly indicate their lowest energy state at certain temperatures. For example, TiCl4 is gaseous while TiCl3 is solid at 200e1300 C. Titanium chlorides mainly include three kinds, TiCl4, TiCl3 and TiCl2, whereas TiCl is ruled out because of its unstable and easy decomposition characteristics. As temperature changes, TiCl4 may decompose into subchlorides such as TiCl3 and TiCl2 before it reaches the substrate, since heated substrate will radiate a large amount of heat to provide enough energy for chemical reaction. In the TiCl4-H2-Ar system, TiCl4 and H2 are the reactant gases. The main reaction equations that will probably take place are shown as the following:
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Fig. 1. Schematic of the horizontal cold-wall reactor used to deposit coating using TiCl4-H2-Ar system.
Table 1 Typical process parameters of CVD titanium coating. Substrate
TiCl4 (ml/min)
H2 (ml/min)
Ar (ml/min)
T ( C)
T (min)
SiC fiber
300
600
300
1000
30
groups: (4) and (5e13). It can be seen that, with the increase of temperature, the DrG of the former increases and the Logk of the former decreases, but the trends of the latter are just the opposite. It is well known that, if the DrG is below zero and the Logk is over
Table 2 Physical properties of titanium chlorides. Chemical formula
Color
Molar mass (g/mol)
Melting point ( C)
Boiling point ( C)
TiCl4 TiCl3 TiCl2
Colorless Purple Brown
189.71 154.26 118.80
25 >440, decomposition 1035
136
H2(g) þ 2TiCl4(g) ¼ 2TiCl3(s) þ 2HCl(g)
(4)
H2(g) þ TiCl4(g) ¼ TiCl2(s) þ 2HCl(g)
(5)
2H2(g) þ TiCl4(g) ¼ Ti(s) þ 4HCl(g)
(6)
H2(g) þ 2TiCl3(s) ¼ 2TiCl2(s)þ 2HCl(g)
(7)
3H2(g) þ 2TiCl3(s) ¼ 2Ti(s) þ 6HCl(g)
(8)
H2(g) þ TiCl2(s) ¼ Ti(s) þ 2HCl(g)
(9)
1500
2TiCl3(s) ¼ TiCl2(s) þ TiCl4(g)
(10)
4TiCl3(s) ¼ Ti(s) þ 3TiCl4(g)
(11)
3TiCl2(s) ¼ Ti(s) þ 2TiCl3(s)
(12)
zero, the chemical reaction can occur from the viewpoint of thermodynamics, and if the smaller the DrG is as well as the greater the Logk is, the more easily the reaction might occur. From the variation trends of DrG and Logk, the reaction (4) may occur in thermodynamics at a lower temperature below 300 C when its DrG is below zero and its Logk is over zero. As for the reactions (5e13), when the temperature is higher than 900 C, the DrG of the reactions (10) and (11) are below zero and the Logk of the reactions (10) and (11) is over zero. The reaction (13) has the same variation trend as the reactions (10) and (11) when temperature is over 1100 C. Therefore, the reactions (10, 11) and (13) can occur in thermodynamics at temperatures over 900 C and 1100 C, respectively. Furthermore, the reaction (11) will be easier to occur than the reactions (10) and (13). Thus one may prediect that TiCl4 would reduce gradually with the increase of temperature near the substrate, which means that the reactants transform as follows: TiCl4 / TiCl3 / Ti, or TiCl4 / TiCl3 / TiCl2 / Ti.
2TiCl2(s) ¼ Ti(s) þ TiCl4(g)
(13)
4. Results and discussion
Effects of temperatures on the Gibbs free energy (DrG) and chemical equilibrium constants (Logk) of reactions (4)e(13) are shown in Fig. 2. The reactions (4)e(13) might be divided into two
4.1. Growing process of titanium film From the above thermodynamic analysis, it is known that TiCl4
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60
(a)
48 ΔrG (kJ/mol)
36 24 12 0 -12 4 8 12
-24 -36 800
5 9
6 10
7 11
13
900
1000
1100
1200
1300
T (°C)
6
(b)
4 2
4 8 12
5 9 13
6 10
7 11
Fig. 3. A schematic diagram of the steps that occurred during the CVD process.
25000 Intensity (counts)
Logk
0 -2 -4 -6 -8 -10 -12
800
900
1000
1100
1200
1300
TiCl3 TiCl3·6H2O
20000
TiO2
15000 10000 5000 0
T (°C)
10
Fig. 2. The thermodynamic relationships: (a) Effects of temperature on DrG of reactions (4)e(13); (b) Effects of temperature on Logk of reactions (4)e(13).
20
30
40
50
60
70
2θ (degree) Fig. 4. XRD pattern of the deposited purple powder.
gas has two react ways to transform into titanium. First of all, TiCl4 is directly degraded to TiCl3 by hydrogen, and its reaction temperature is near and under 300 C. Since there is a large temperature range from the fiber surface to the wall of the reaction chamber, TiCl3 is likely to be generated by TiCl4 and hydrogen. Then TiCl3 decomposes to produce titanium in two possible paths, one is to form titanium directly, the other is to decompose to TiCl2 first and then TiCl2 decomposes to titanium. These gas reactions took place near or on the heated SiC fiber as the heated filament continuously provides sufficient energy. According to the experiment conditions, the dynamic process of the CVD titanium can be schematically shown in Fig. 3. In Fig. 3, the deposition process is separated into the following four stages to clarify the growing process of titanium film: 1) TiCl4 is degraded to TiCl3 by hydrogen; 2) The decomposition of TiCl3: TiCl3 / Ti or TiCl3 / TiCl2 / Ti; 3) Nucleation and growth of titanium atoms to form a continuous coating in the surface of SiC fiber; 4) Gaseous by-products of the reactions diffuse away from the surface through the boundary layer. In this work, we observed that purple powder formed and deposited onto the wall of the reaction chamber during gas reaction. Fig. 4 shows XRD pattern of the purple powder, which reveals that the purple powder is mainly TiCl3, and with a small amount of TiCl3$6H2O and TiO2. The appearance of TiCl3$6H2O and TiO2 was caused by the hydrolysis and oxidation reaction of TiCl3 in the air, as TiCl3 is apt to absorb water vapor and react with oxygen [29]. The hydrolysis and oxidation reaction equations are: TiCl3 þ 6H2O / TiCl3$6H2O
(14)
3TiCl3 þ TiCl3$6H2O þ O2 / 4TiO2 þ 12HCl
(15)
Obviously, the results support the above thermodynamics analysis that TiCl4 may decompose into TiCl3 first. However, no TiCl2 was found in the whole deposition process. The formed TiCl2 may be decomposed quickly in gases since it is extremely unstable or its yield is little. 4.2. Surface and interfacial characterization of titanium coatings Typical titanium coating was deposited at 1000 C for 30 min, and detailed process parameters are listed in Table 1. Fig. 5a and b show the SEM images of the surface and cross-sectional morphology of titanium coating on SiC fiber, respectively. It is seen that the surface structure of the titanium coating is compact and uniform. Fig. 5c shows the magnified morphology of the area marked in Fig. 5b. The average thickness of the titanium coating is 3.5 mm. There are two distinct layers, the layer 1 having a granular morphology while the layer 2 having a columnar morphology. In order to know the differences between the two layers, EDS analysis was carried out. Fig. 5d shows the EDS line scanning results from SiC fiber to the surface of the coating where an arrow is marked in Fig. 5c. CVD SiC can be stoichiometric, C-rich or Si-rich, depending on the deposition conditions [30]. The SiC fiber used in this work is a carbon-rich fiber, and the original Si:C ratio close to surface is about 60:40 [31]. From inner to surface of the SiC fiber, the atomic percentage of
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Fig. 5. Typical titanium coating: (a), (b) SEM images of the surface and cross-sectional morphology of typical titanium coating, respectively; (c) The detailed morphology of the area marked by a white square in (b); (d) EDS line scanning result of the area marked by the red arrow in (c). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
4.3.1. Deposition rate Usually, a CVD process includes diffusion of reactants through boundary layer, adsorption of reactants on substrate, chemical reaction taking place near substrate or on substrate, desorption of adsorbed species, and diffusion out of by-products [28]. In this work, the CVD titanium process of the TiCl4-H2-Ar system is symbolically exhibited in Fig. 3. The whole process often determines the deposition rate, yet it requires adequate activation energy to make it happen. Therefore, temperature has a great influence on the deposition rate since it is the energy provider. The relationship of them can be calculated by Arrhenius equation:
(16)
where V is the deposition rate, mm/min; A is the pre-exponential factor; R is the gas constant; Ea is the apparent activation energy of these processes, kJ/mol; and T is the deposition temperature, K. Fig. 6 shows the relationship between temperature and deposition rate. The process parameters are listed in Table 3. According to the Arrhenius formula, the deposition rate has an exponential increase with the increase of temperature. It can be seen from Fig. 6 that the deposition rate of titanium coating increases as the temperature increases from 1000 to 1200 C. Moreover, it increases
20
6.5
1/T (10-4/K) 7.0 7.5
8.0 4
16 2 12 0
8
-2
4 0
Ln deposition rate
4.3. Effect of temperature on the deposition of titanium
Ea V ¼ A$exp RT
Deposition rate (μm/min)
carbon changed from 60% to nearly 70%, which indicates that carbon atoms in SiC fiber have diffused from inner to surface after the Ti deposition process. As to the deposition layer, in layer 1, no very little silicon diffusion exists, and the atomic ratio of Ti and C closed to the atomic percentage composition of TiC. So the layer 1 mainly contains TiC. But it may also contain ternary phases like Ti3SiC2 [32] or Ti5Si3Cx [33], since Ti can react with SiC to form this compounds in the deposition condition. In layer 2, the atomic percentage of carbon sharply decreases to lower than 30% while that of titanium quickly increases to more than 70%. The atomic content of titanium and carbon in layer 2 remained relatively stable. It is clear that a part of titanium close to the fiber has reacted to form the layer 1, which could hinder the diffusion of carbon atoms in some extent. So the carbon amount in layer 2 was much lower than layer 1. However, the layer 2 still had some carbon diffused in. If a pure titanium coating is to be prepared, a coating which can hinder the diffusion of carbon atoms needs to be deposited on SiC fiber firstly.
-4 1000
1100 T (°C)
1200
Fig. 6. The relationship between temperature and deposition rate.
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Table 3 Process parameters of CVD titanium coating at different temperatures. Substrate
TiCl4 (ml/min)
H2 (ml/min)
T ( C)
t (min)
SiC fiber
300
600
1000e1200
5e30
faster at the temperature range of 1100e1200 C. The maximum deposition rate is about 16 mm/min at 1200 C, while the minimum deposition rate is only about 0.1 mm/min at 1000 C. This phenomenon can be explained as we take the logarithm of the Arrhenius Eq. (16). Then Eq. (16) is transformed into the form: LnV ¼ LnA Ea/RT. The relation of LnV and 1/T is also shown in Fig. 6 (the blue cure). In the graph, the absolute value of the slope Ea/R at 1100e1200 C is lower than that at 1000e1100 C. The lower slope Ea/R means the lower Ea, therefore, the CVD process would be easier to occur at 1100e1200 C, which helps improve deposition rate.
4.3.2. Surface morphology The temperature also has great influences on nucleation and growth of the deposited titanium. From the viewpoint of nucleation dynamics [34], temperature T impacts on radius of critical nucleus rc and critical nucleation energy DrGmax. Their relationships are that
rc is in proportion to 1/T, and DrGmax is in proportion to 1/T2. Both rc and DrGmax decrease with the increase of temperature, which means that the nuclei formation is easier at higher temperature, and the thickness of deposits and the deposition rate will increase. At the same time, the faster nuclei formation of atoms makes them clusters and grow together, therefore, the surface structure changes. Fig. 7 shows SEM images of the surface morphology of titanium coatings which were prepared at different temperatures. When deposition temperature was 1000 C, the surface is relatively smooth, and the grain size of the coating is even, as shown in Fig. 7a. It is due to the very slow deposition rate as well as the uniform atomic diffusion and growth on the deposition surface. However, microcracks formed on the surface because of thermal residual stresses, as shown in Fig. 7b. When the coating was prepared from high temperature to room temperature, thermal residual stresses were generated due to the large difference of thermal expansion coefficients between SiC and titanium. When the preparing temperature was 1100 C, the grain of the coating had a relatively complete growth even though the coating surface is not smooth, as shown in Fig. 7c. Meanwhile, small granular crystals can be observed on the surface, as shown in Fig. 7d. This is due to the fact that the increased deposition rate, the faster nucleation and the lower effect of atomic diffusion make the deposited atoms cluster
Fig. 7. SEM images of the surface of titanium coatings which were formed at certain temperatures in different magnifications: (a) 1000 C at a low magnification; (b) 1000 C at a higher magnification; (c) 1100 C at a low magnification; (d) 1100 C at a higher magnification; (e) 1200 C at a low magnification; (f) 1200 C at a higher magnification.
Please cite this article in press as: X. Luo, et al., Deposition characteristics of titanium coating deposited on SiC fiber by cold-wall chemical vapor deposition, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.09.041
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Fig. 8. Surface morphology of titanium coating with certain flow rate of argon gas in different magnifications: (a), (b) the argon gas flow rate is 300 ml/min; (c), (d) the argon gas flow rate is 1000 ml/min.
and grow together. When deposition temperature was 1200 C, the generating speed of titanium atoms as well as the nucleation rate in the chemical reaction was very fast, which made atoms cluster more easily. Meanwhile, the obvious thermal effect on surface gave rise to adsorptions and growth of the atoms. It is inferred that a large number of new titanium atoms nucleation and further growth in perpendicular to the direction of the coating induced the obvious changes in the morphology of titanium coating, which presents cellular bulges in Fig. 7e. The magnified image of Fig. 7f shows that the cellular bulges consisting of many nanoscale particles were sporadically distributed on the coating surface.
4.4. Effect of argon gas on microstructure of titanium coating In previous studies, argon gas added to the reaction chamber usually acts different roles with certain depositional conditions in various coating deposition systems. Zhang and co-authors [35] drew a conclusion that argon gas can decrease grain size and improve film qualities in a CVD diamond system. Zhang and coauthors [36] also revealed that argon gas can change grain preferred-orientation and decrease reaction rate in a CVD TaC system. For the CVD titanium system, argon gas never participates in gas phase reaction. The main roles of argon gas are to dilute the concentration of the reaction gases, to make an intensive mixing of gases, and to bring out the residual gases. This indicates that argon gas has a certain effect on the CVD titanium process which was previously illuminated to have a significant impact on the morphology of the deposited coating. Fig. 8 shows the surface morphology of titanium coating in different magnifications with different argon gas flow rates. The main process parameters were the same as Table 1, but the argon gas flow rate was altered. Fig. 8a shows a flat and smooth surface morphology when the argon gas flow rate was 300 ml/min, whereas Fig. 8c shows a coarse surface morphology with numerous obvious protrusions when the argon gas flow rate was increased up to 1000 ml/min. At higher magnifications (Fig. 8b and d), the obvious difference between Fig. 8a and c can be seen in detail, as
the bud shaped morphology shows in Fig. 8d. It is clear that a proper increase of argon gas flow rate in the reaction chamber could promote the grain refinement and make coating surface uniform. However, high argon gas flow rate would make the surface rough since it reduces the reaction gas which is adsorbed on the fiber surface and lowers the surface temperature.
5. Conclusions Titanium coating was deposited on SiC fiber under different deposition parameters in a TiCl4-H2-Ar system in order to reveal their effects on the coating characteristics. The thermodynamic analysis of the CVD reactions indicates that TiCl4 is transformed to titanium in two ways: TiCl4 / TiCl3 / Ti, or TiCl4 / TiCl3 / TiCl2 /vTi. In the cold-wall CVD process, typical deposited coating contains two distinct layers: the layer close to SiC fiber forms mainly TiC, and the following Ti layer. It is clear that a diffusion barrier coating is needed to be deposited firstly if pure titanium is to be made. The deposition rate can be improved with the increase of temperature, especially when temperature is higher than 1100 C. However, high temperature has a negative effect on the surface uniformity of titanium coating. At 1000 C, the surface is relatively smooth. When temperature is 1100 C, the coating surface becomes rough and small particular crystal morphology can be observed. When temperature is increased up to 1200 C, the surface presents cellular bulges which consist of many nanoscale particles. Argon gas flow rate with an appropriate value has a smoothing effect on the surface morphology of the coating.
Acknowledgments Thanks are given to the financial supports of the Natural Science Foundation of China (No. 51201134, 51271147), the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (Grant No. 115-QP-2014) and the Fundamental Research Funds for the Central Universities (3102014JCQ01023).
Please cite this article in press as: X. Luo, et al., Deposition characteristics of titanium coating deposited on SiC fiber by cold-wall chemical vapor deposition, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.09.041
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Please cite this article in press as: X. Luo, et al., Deposition characteristics of titanium coating deposited on SiC fiber by cold-wall chemical vapor deposition, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.09.041