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Preparation of ZrC nano-particles reinforced amorphous carbon composite coating by atmospheric pressure chemical vapor deposition
Applied Surface Science 255 (2009) 7142–7146
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation of ZrC nano-particles reinforced amorphous carbon composite coating by atmospheric pressure chemical vapor deposition W. Sun *, X. Xiong, B.Y. Huang, G.D. Li, H.B. Zhang, P. Xiao, Z.K. Chen, X.L. Zheng State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan, 410083, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 January 2009 Received in revised form 21 February 2009 Accepted 15 March 2009 Available online 24 March 2009
To eliminate cracks caused by thermal expansion mismatch between ZrC coating and carbon–carbon composites, a kind of ZrC/C composite coating was designed as an interlayer. The atmospheric pressure chemical vapor deposition was used as a method to achieve co-deposition of ZrC and C from ZrCl4–C3H6– H2–Ar source. Zirconium tetrachloride (ZrCl4) powder carrier was especially made to control accurately the flow rate. The microstructure of ZrC/C composite coating was studied using analytical techniques. ZrC/C coating shows same morphology as pyrolytic carbon. Transmission electron microscopy (TEM) shows ZrC grains with size of 10–50 nm embed in turbostratic carbon. The formation mechanism is that the growth of ZrC crystals was inhibited by surrounding pyrolytic carbon and kept as nano-particles. Fracture morphologies imply good combination between coating and substrate. The ZrC crystals have stoichiometric proportion near 1, with good crystalline but no clear preferred orientation while pyrolytic carbon is amorphous. The heating-up oxidation of ZrC/C coating shows 11.58 wt.% loss. It can be calculated that the coating consists of 74.04 wt.% ZrC and 25.96 wt.% pyrolytic carbon. The average density of the composite coating is 5.892 g/cm3 by Archimedes’ principle. Crown Copyright ß 2009 Published by Elsevier B.V. All rights reserved.
Keywords: ZrC Amorphous carbon Composite coating Chemical vapor deposition
1. Introduction Carbon–carbon composites are considered as one of the most promising candidate materials for high temperature structural applications because of their excellent high temperature mechanical properties. Nevertheless, the oxidation of carbon–carbon composites above 370 8C limits their application in oxygen containing environment. As a result, the development of reliable oxidation protection is crucial to utilizing the full potential of carbon–carbon composites [1,2]. Strife and Sheehan [3] have stated that, silicon-based ceramics coatings, like SiC and Si3N4, could meet the requirement below 1800 8C. As for higher temperature, the protective coatings are based on the use of refractory carbides including TaC, HfC and ZrC. So far, ZrC coatings have been widely applied for cutting tools [4], nuclear industry [5,6] as well as electronic devices [7,8]. But few literatures report ZrC coatings are used for carbon–carbon composites. Compared to TaC or HfC, ZrC has a lower density, but higher hardness, strength and modulus [9]. Preparation of low cost is another advantage. Therefore, our research pays more attention to ZrC ceramic coating system.
We find ZrC coatings are apt to crack due to the thermal expansion mismatch between ZrC and carbon–carbon composites. The literatures about SiC coatings for carbon–carbon composites relate the similar cracks [10,11]. The harmful cracks serve as the path through which oxygen can diffuse to the underlying substrate. Three main methods are employed to solve the problem. One is seal the cracks by sealing glass with appropriate viscosity [12,13]. Another is prepare an outer-layer forming a multi-layer coating system, such as SiC/mullite–Al2O3 [14], SiC/SiO2 [15], and SiC/yttyium silicate [16]. The last is develop the functionally gradient SiC/C composite coating to minimize the thermal stress caused by the carbon-carbon composites and SiC coating [17–19]. Following the third method above, a kind of ZrC/C composite coating is prepared on substrate by atmospheric pressure chemical vapor deposition in this study. The composite coating can be candidate for the interlayer between ZrC coating and substrate, or short-term oxidation protection directly. The formation mechanism and microstructures of the ZrC/C composite coating were studied. The behavior of heating-up oxidation was also analyzed. 2. Experimental procedures 2.1. Coating preparation
* Corresponding author at: State Key Laboratory of Powder Metallurgy, Central South University, Lu shan Nan lu, Changsha, Hunan, 410083, China. E-mail address: [email protected] (W. Sun).
As shown in Fig. 1, the atmospheric pressure chemical vapor deposition system is designed for composite coating preparation. It
0169-4332/$ – see front matter . Crown Copyright ß 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.03.045
W. Sun et al. / Applied Surface Science 255 (2009) 7142–7146
7143
Fig. 1. Schematic of atmospheric pressure chemical vapor deposition system. There was an improvement in precursor supply system.
is worth noting that an improvement has been made in precursor supply system. Conventional vaporizer, always with problems of vapor condensation and agglomeration [20], has been replaced by a special powder carrier, which can better control the throughput of solid precursor. The circular substrates are made of high purity graphite, with a size of F 30 mm 10 mm. Before deposition, all specimens were polished, degreased and alcohol cleaned. Then they were hanged in the middle of graphite calandria. Under argon atmosphere, elevated furnace to deposition temperature. Table 1 is the detailed processing parameters for deposition of ZrC/C composite coating. Zirconium tetrachloride (ZrCl4) was used as zirconium source and propene (C3H6) as carbon source. The deposition process can be described as [21]: ZrCl4 þ H2 ! ZrClx þ HClðx ¼ 0; 1; 2; 3Þ
(1)
C3 H6 ! ½C þ H2
(2)
½C ! C
(3)
ZrClx þ ½C ! ZrC þ xCl
(4)
Cl þ H2 ! HCl
(5)
Active [C], generated from pyrolysis of relatively excessive C3H6, not only meet the requirement of formation of ZrC crystal, but also form carbon film simultaneously. 2.2. Coating characterization The surface and fracture morphologies as well as element distribution of ZrC/C composite coating were investigated by
Fig. 2. SEM surface morphology of the ZrC/C composite coating. Non-uniform cells can be observed.
scanning electron microscopy (SEM; JEOL-6360LV, Tokyo, Japan), coupled with an energy dispersive X-ray analysis system (EDXA; PV7746121 ME, Sapphire, Tokyo, Japan). The inner morphology and crystal structure of composite coating exfoliated from substrate were studied by a LIBRA 200 FE Transmission Electron Microscopy (TEM) equipped with selected-area electron diffraction. Phase composition of coating was examined by a D/max 2550 VB + 18 kW rotating target X-ray diffraction instrument (XRD; Rigaku Ltd., Japan, Cu Ka radiation, l = 1.54056 A˚). Oxidation reaction of ZrC/C coating was studied by thermogravimetry-differential scanning calorimetry (TG-DSC; NETZSCH STA 409C, Selb, Germany). The density of the ZrC/C coating was determined by the Archimedes principle, using the water immersion method. 3. Results and discussion Surface morphologies of ZrC/C composite coating are shown in Fig. 2. The composite coating is close stacked by so many spherical shape cells. Fig. 2(b) shows that the cell diameters are in the range of 1–3 mm. In addition, some cells are merged together. These cells are very similar to that of deposited pyrolytic carbon reported in literature [22,23]. No obvious microcracks exist. This morphology is concerning to the different speed of nucleation for ZrC and pyrolytic carbon during the co-deposition
Table 1 Processing parameters for ZrC/C composite coating by atmospheric pressure chemical vapor deposition. Deposition temperature (8C)
Deposition pressure (Pa)
Deposition time (h)
ZrCl4 (>99%) (g min1)
C3H6 (>99.9%) (ml/min)
Carrier Ar (ml/min)
Dilute Ar (ml/min)
1350
1.01 105
3
1.8
100
400
400
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process [24]. Speed of nucleation (dN/dt) can be described as Eq. (6) [25], dN 4apg 2 ns ð pV P S ÞNA DG=kT pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ e dt 2pMRT
(6)
where a is the constant describing atomic absorption capacity on surface of nucleates, g* the critical radius of nucleus, ns the density of nucleus, PV the actual pressure of reactive gas, Ps the equilibrium vapor pressure of deposited solid phase, NA the Avogadro’s number, M the molar mass of deposited solid phase, R the gas constant, T the deposition temperature, DG* the critical Gibbs free energy for solid phase nucleation and K is the constant of reaction rate. Of these, exponential term influences mostly, so dN / eDG=kT dt
(7)
and
DG ¼
16pg 3 3DG2v
D Gv ¼
kT
lnð1 þ SÞ
(8)
(9)
V where V is the atomic volume, S the gas super-saturation.
Fig. 4. TEM image of ZrC/C composite coating exfoliated from substrate. The black grains are ZrC crystals.
Substituting Eq. (8) [25] and Eq. (9) [26] into Eq. (7), we have,
estimated by Scherred method [27,28]:
2 3 dN 3 2 / e16p V =3K Tln ð1þSÞ dt
Dh k l ¼
(10)
From Eq. (10), we know that dN/dt increases with the gas supersaturation. In experiment, active [C] decomposed from C3H6 is excessive to [Zr] from ZrCl4. So the nucleation of pyrolytic carbon is prior to that of ZrC. As can be seen from Fig. 3, pyrolytic carbon film was deposited firstly on the substrate, while ZrC is nucleated on pyrolytic carbon films. Being surrounded by pyrolytic carbon, the growth of ZrC was inhibited and kept as nano-particles. So the cells own composite structure of ZrC crystal embedding in pyrolytic carbon and shape the morphology as same as pyrolytic carbon. In the whole process, deposition of pyrolytic carbon is the control step. Further TEM studies verify the co-deposition model of ZrC/C composite coating. As can be seen in Fig. 4, the black grains, dispersed in gray disordered floccules, have a grain size ranged from 10 nm to 50 nm and aggregate somewhere. EDXA shows the black grains are composed of 48.3% C and 51.3% Zr, and the gray floccules 100% C. The black grains, having a ratio of C and Zr near 1, are regarded as stoichiometric ZrC crystals. This can also be verified from the results of diffraction spots. Fig. 5 is the fracture morphology of ZrC/C composite coating deposited on graphite substrate. The coatings are about 200 mm thick. There are no any microcracks or microholes in the coating and interface. The ZrC/C coating has a good combination with substrate. Like the graphite substrate, ZrC/C composite coating exhibits brittle fracture. The XRD pattern of ZrC/C composite coating exfoliated from graphite substrate is shown in Fig. 6. The composite coating exhibits a ZrC1.0 cubic phase and amorphous carbon. The narrow and sharp peaks indicate good crystalline of ZrC phase. The wide peak of amorphous carbon, departure from (2 0 0) of graphite 3.48, corresponds to turbostratic carbon. The crystallite size of ZrC1.0 is
Fig. 3. Formation mechanism of co-deposition of ZrC/C composite coating.
Kl bh k l cos u
(11)
where Dh k l is the thickness of crystallite perpendicular to crystal plane (h k l). K, value of crystal, changes with different shape and
Fig. 5. SEM fracture morphology of ZrC/C composite coating deposited on graphite substrate. ZrC/C coating has a good combination with substrate.
W. Sun et al. / Applied Surface Science 255 (2009) 7142–7146
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Fig. 6. XRD pattern of ZrC/C composite coating exfoliated from graphite substrate.
crystal plane (K = 1.557 in our calculation), l is the wavelength of X-ray, b is the half width of the peak, and u is the Bragg angle. The calculated size is 22.44 nm, which is in accordance with TEM studies. Preferred orientation of ZrC crystal was calculated with Haris method from the XRD results according to the following relationship [29,30]: TC ¼
ð1=nÞ
Ii =I0 P n
(12)
i¼1 ðIi =I0 Þ
where TC is texture coefficient for a given plane, Ii is the actual intensity of measuring crystal plane, I0 standard integrated intensity from the reference of American Society for Testing & Materials (ASTM) file, n is the number of the diffraction peaks. In our case, (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes are considered and TC values in Table 2 are the calculated texture coefficients. The small difference of TC values implies that ZrC crystal grown at 1350 8C tends to be random. No obvious preferred orientation of crystal is found. Fig. 7 is the selected-area electron diffraction pattern of ZrC/C composite coating taken at Fig. 4. As calibrated, the concentric rings are corresponding to (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of fcc-ZrC solid solution. The formation of concentric rings suggests that the composite coating contains large quantity of nano-ZrC crystals. The light halo in the middle of the rings represents amorphous carbon with disordered structure. The results match well with that of XRD pattern. The line scanning EDXA of ZrC/C composite coating was shown in Fig. 8. The results show that the distribution of carbon element is relatively steady, while the distribution of Zr element is fluctuating along the cross-section of the coating. The convex peaks of Zr indicate the non-uniform distribution of ZrC nano-particles in composite coating. Fig. 9 is TG-DSC analysis of ZrC/C composite coating (5.595 g) with a temperature rising rate of 5 K/min from 20 K to 1440 K, under oxygen atmosphere. Two exothermal peaks (679.2–
Fig. 7. The selected-area electron diffraction pattern of ZrC/C composite coating taken at above TEM image (Fig. 4).
698.0 8C, 729.3–788.2 8C) are observed, which corresponds to two oxidizing reactions: C þ O2 ! CO2 "
(13)
ZrC þ O2 ! ZrO2 þ CO2 "
(14)
The weight loss of the coating is 11.58% (from TG curve). And there are 88.42% (or 5.595 g 88.42%) resultant reserved. From Eqs. (13) and (14), the resultant is only ZrO2. Obtain molar fraction of ZrC in the coating is 0.04 mol. So the composite coating consists of 74.04 wt.% ZrC and 25.96 wt.% pyrolytic carbon. K value method [31] was employed to evidence the above result. To samples with two phases: WA ¼
IA IA þ IB =KAB
(15)
WB ¼
IB IB þ IA KAB
(16)
where I is the mean intensity of measuring phase and K is the intensity factor obtained from PDF cards. The intensity of ZrC and
Table 2 Texture coefficients of crystal planes for different planes of ZrC crystals. Planes
I0 Ii TC
(1 1 1)
(2 0 0)
(2 2 0)
(3 1 1)
100.0% 100.0% 0.75
82.0% 99.0% 0.91
62.0% 88.6% 1.07
50.0% 84.4% 1.25
Fig. 8. Line scanning EDA of ZrC/C composite coating deposited on graphite substrate.
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W. Sun et al. / Applied Surface Science 255 (2009) 7142–7146
cm3) approximately equals to actual density (5.892 g/cm3) obtained by Archimedes’ principle. Acknowledgements This research program is financially supported by the National Basic Research Program of China (No. 2006CB600908) and the National Hi-Tech Research Development Program of China (No. 2007AA03Z110). References
Fig. 9. TG-DSC curves of ZrC–C composite coating exfoliated from graphite substrate (O2, 5 K/min).
pyrolytic carbon obtained from XRD pattern (Fig. 6). According to Eqs. (15) and (16), the results of 78.90 wt.% ZrC and 21.10 wt.% amorphous carbon were obtained. Neglecting the influence of pores on the density of the composite coating, gain the density of composite coating, 6.017 g/cm3, which is near to the value of 5.892 g/cm3 measured by Archimedes’ principle. 4. Conclusions This paper reported a kind of ZrC–C composite coating grown on graphite substrate successfully fabricated by atmospheric pressure chemical vapor deposition. The process is proposed to be co-deposition of zirconium carbide and pyrolysis carbon, and the latter is the control step. The surface morphology of ZrC/C composite coating, presenting character of pyrolysis carbon, is closed stacked by un-uniform cells. TEM image shows the feature of zirconium carbide nano-sized crystals surrounding by turbostratic carbon. The ZrC/C coating has a good combination with substrate. Through TG-DSC curves, it is calculated that the weight percent of ZrC and amorphous carbon in composite coating are 74.04% and 25.96%, respectively. The theoretical density (6.017 g/
[1] J.E. Sheehan, K.W. Buesking, B.J. Sullivan, Annu. Rev. Mater. Sci. 24 (1994) 19. [2] M.E. Westwood, J.D. Webster, R.J. Day, F.H. Hayes, R. Taylor, J. Mater. Sci. 31 (1996) 1389. [3] J.R. Strife, J.E. Sheehan, Am. Ceram. Soc. Bull. 67 (1988) 369. [4] L.E. Toth, Transition Metal Carbides and Nitride, Academic Press, New York, 1971. [5] K. Minato, K. Fukuda, H. Sekino, Y. Nozawa, I. Takahashi, J. Nucl. Mater. 224 (1995) 85. [6] K. Minato, K. Fukuda, H. Sekino, Y. Nozawa, E. Oeda, J. Nucl. Mater. 252 (1998) 13. [7] W.A. Mackie, T.B. Xie, P.R. Davis, J. Vac. Sci. Technol. 13 (1995) 2459. [8] K. Edamoto, T. Anazawa, S. Tokumisu, A. Tanabe, R. Sekine, E. Miyazake, H. Kato, S. Otani, Solid State Commun. 97 (1996) 435. [9] A. Sayir, J. Mater. Sci. 39 (2004) 5995. [10] Q.G. Fu, H.J. Li, X.H. Shi, K.Z. Li, G.D. Sun, Scripta Mater. 52 (2005) 923. [11] J.F. Huang, X.R. Zeng, H.J. Li, X.B. Xiong, Y.W. Fu, Carbon 42 (2004) 1517. [12] F. Smeacetto, M. Ferraris, Carbon 40 (2002) 583. [13] F.J. Buchanan, J.A. Little, Carbon 33 (1995) 491. [14] J.F. Huang, X.R. Zeng, H.J. Li, X.B. Xiong, M. Huang, Carbon 41 (2003) 2825. [15] O.S. Kwon, S.H. Hong, H. Kim, J. Eur. Ceram. Soc. 23 (2003) 3119. [16] J.F. Huang, X.R. Zeng, H.J. Li, X.B. Xiong, M. Huang, J. Mater. Sci. 39 (2004) 7383. [17] J.I. Kim, W.J. Kim, D.J. Choi, J.Y. Park, W.S. Ryu, Carbon 43 (2005) 1749. [18] W. Kowbel, J.C. Withers, P.O. Bansons, Carbon 33 (1995) 415. [19] Y.L. Zhang, H.J. Li, Q.G. Fu, K.Z. Li, J. Wei, P.Y. Wang, Surf. Coat. Technol. 201 (2006) 3491. [20] C. Liu, B. Liu, Y.L. Shao, Z.Q. Li, C.H. Tang, Trans. Nonferr. Met. Soc. China 18 (2008) 728. [21] Y.G. Wang, Q.M. Liu, J.L. Liu, L.T. Zhang, L.F. Cheng, J. Am. Ceram. Soc. 91 (2008) 1249. [22] H.O. Pierson, M.L. Lieberman, Carbon 13 (1975) 159. [23] W. Benzinger, A. Becker, K.J. Huttinger, Carbon 3 (1996) 3957. [24] T. Ogawa, K. Ikawa, K. Iwamoto, J. Mater. Sci. 14 (1979) 125. [25] .E. Kaldis, Crystal Growth—Theory and Techniques, Plenum Press, New York, 1974 [26] J.P. Hirth, G.M. Pound, Condensation and Evaporation, Pregamon, London, 1963. [27] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, London, 1998. [28] K. Minato, K. Fukude, J. Mater. Sci. 23 (1998) 699. [29] R.Y. Korotkov, P. Ricou, A.J.E. Farran, Thin Solid Films 502 (2000) 79. [30] M.H. Staia, E.S. Puchi, D.B. Lewis, J. Cawley, D. More, Surf. Coat. Technol. 87 (1996) 432. [31] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, 1978.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation of ZrC nano-particles reinforced amorphous carbon composite coating by atmospheric pressure chemical vapor deposition W. Sun *, X. Xiong, B.Y. Huang, G.D. Li, H.B. Zhang, P. Xiao, Z.K. Chen, X.L. Zheng State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan, 410083, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 January 2009 Received in revised form 21 February 2009 Accepted 15 March 2009 Available online 24 March 2009
To eliminate cracks caused by thermal expansion mismatch between ZrC coating and carbon–carbon composites, a kind of ZrC/C composite coating was designed as an interlayer. The atmospheric pressure chemical vapor deposition was used as a method to achieve co-deposition of ZrC and C from ZrCl4–C3H6– H2–Ar source. Zirconium tetrachloride (ZrCl4) powder carrier was especially made to control accurately the flow rate. The microstructure of ZrC/C composite coating was studied using analytical techniques. ZrC/C coating shows same morphology as pyrolytic carbon. Transmission electron microscopy (TEM) shows ZrC grains with size of 10–50 nm embed in turbostratic carbon. The formation mechanism is that the growth of ZrC crystals was inhibited by surrounding pyrolytic carbon and kept as nano-particles. Fracture morphologies imply good combination between coating and substrate. The ZrC crystals have stoichiometric proportion near 1, with good crystalline but no clear preferred orientation while pyrolytic carbon is amorphous. The heating-up oxidation of ZrC/C coating shows 11.58 wt.% loss. It can be calculated that the coating consists of 74.04 wt.% ZrC and 25.96 wt.% pyrolytic carbon. The average density of the composite coating is 5.892 g/cm3 by Archimedes’ principle. Crown Copyright ß 2009 Published by Elsevier B.V. All rights reserved.
Keywords: ZrC Amorphous carbon Composite coating Chemical vapor deposition
1. Introduction Carbon–carbon composites are considered as one of the most promising candidate materials for high temperature structural applications because of their excellent high temperature mechanical properties. Nevertheless, the oxidation of carbon–carbon composites above 370 8C limits their application in oxygen containing environment. As a result, the development of reliable oxidation protection is crucial to utilizing the full potential of carbon–carbon composites [1,2]. Strife and Sheehan [3] have stated that, silicon-based ceramics coatings, like SiC and Si3N4, could meet the requirement below 1800 8C. As for higher temperature, the protective coatings are based on the use of refractory carbides including TaC, HfC and ZrC. So far, ZrC coatings have been widely applied for cutting tools [4], nuclear industry [5,6] as well as electronic devices [7,8]. But few literatures report ZrC coatings are used for carbon–carbon composites. Compared to TaC or HfC, ZrC has a lower density, but higher hardness, strength and modulus [9]. Preparation of low cost is another advantage. Therefore, our research pays more attention to ZrC ceramic coating system.
We find ZrC coatings are apt to crack due to the thermal expansion mismatch between ZrC and carbon–carbon composites. The literatures about SiC coatings for carbon–carbon composites relate the similar cracks [10,11]. The harmful cracks serve as the path through which oxygen can diffuse to the underlying substrate. Three main methods are employed to solve the problem. One is seal the cracks by sealing glass with appropriate viscosity [12,13]. Another is prepare an outer-layer forming a multi-layer coating system, such as SiC/mullite–Al2O3 [14], SiC/SiO2 [15], and SiC/yttyium silicate [16]. The last is develop the functionally gradient SiC/C composite coating to minimize the thermal stress caused by the carbon-carbon composites and SiC coating [17–19]. Following the third method above, a kind of ZrC/C composite coating is prepared on substrate by atmospheric pressure chemical vapor deposition in this study. The composite coating can be candidate for the interlayer between ZrC coating and substrate, or short-term oxidation protection directly. The formation mechanism and microstructures of the ZrC/C composite coating were studied. The behavior of heating-up oxidation was also analyzed. 2. Experimental procedures 2.1. Coating preparation
* Corresponding author at: State Key Laboratory of Powder Metallurgy, Central South University, Lu shan Nan lu, Changsha, Hunan, 410083, China. E-mail address: [email protected] (W. Sun).
As shown in Fig. 1, the atmospheric pressure chemical vapor deposition system is designed for composite coating preparation. It
0169-4332/$ – see front matter . Crown Copyright ß 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.03.045
W. Sun et al. / Applied Surface Science 255 (2009) 7142–7146
7143
Fig. 1. Schematic of atmospheric pressure chemical vapor deposition system. There was an improvement in precursor supply system.
is worth noting that an improvement has been made in precursor supply system. Conventional vaporizer, always with problems of vapor condensation and agglomeration [20], has been replaced by a special powder carrier, which can better control the throughput of solid precursor. The circular substrates are made of high purity graphite, with a size of F 30 mm 10 mm. Before deposition, all specimens were polished, degreased and alcohol cleaned. Then they were hanged in the middle of graphite calandria. Under argon atmosphere, elevated furnace to deposition temperature. Table 1 is the detailed processing parameters for deposition of ZrC/C composite coating. Zirconium tetrachloride (ZrCl4) was used as zirconium source and propene (C3H6) as carbon source. The deposition process can be described as [21]: ZrCl4 þ H2 ! ZrClx þ HClðx ¼ 0; 1; 2; 3Þ
(1)
C3 H6 ! ½C þ H2
(2)
½C ! C
(3)
ZrClx þ ½C ! ZrC þ xCl
(4)
Cl þ H2 ! HCl
(5)
Active [C], generated from pyrolysis of relatively excessive C3H6, not only meet the requirement of formation of ZrC crystal, but also form carbon film simultaneously. 2.2. Coating characterization The surface and fracture morphologies as well as element distribution of ZrC/C composite coating were investigated by
Fig. 2. SEM surface morphology of the ZrC/C composite coating. Non-uniform cells can be observed.
scanning electron microscopy (SEM; JEOL-6360LV, Tokyo, Japan), coupled with an energy dispersive X-ray analysis system (EDXA; PV7746121 ME, Sapphire, Tokyo, Japan). The inner morphology and crystal structure of composite coating exfoliated from substrate were studied by a LIBRA 200 FE Transmission Electron Microscopy (TEM) equipped with selected-area electron diffraction. Phase composition of coating was examined by a D/max 2550 VB + 18 kW rotating target X-ray diffraction instrument (XRD; Rigaku Ltd., Japan, Cu Ka radiation, l = 1.54056 A˚). Oxidation reaction of ZrC/C coating was studied by thermogravimetry-differential scanning calorimetry (TG-DSC; NETZSCH STA 409C, Selb, Germany). The density of the ZrC/C coating was determined by the Archimedes principle, using the water immersion method. 3. Results and discussion Surface morphologies of ZrC/C composite coating are shown in Fig. 2. The composite coating is close stacked by so many spherical shape cells. Fig. 2(b) shows that the cell diameters are in the range of 1–3 mm. In addition, some cells are merged together. These cells are very similar to that of deposited pyrolytic carbon reported in literature [22,23]. No obvious microcracks exist. This morphology is concerning to the different speed of nucleation for ZrC and pyrolytic carbon during the co-deposition
Table 1 Processing parameters for ZrC/C composite coating by atmospheric pressure chemical vapor deposition. Deposition temperature (8C)
Deposition pressure (Pa)
Deposition time (h)
ZrCl4 (>99%) (g min1)
C3H6 (>99.9%) (ml/min)
Carrier Ar (ml/min)
Dilute Ar (ml/min)
1350
1.01 105
3
1.8
100
400
400
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process [24]. Speed of nucleation (dN/dt) can be described as Eq. (6) [25], dN 4apg 2 ns ð pV P S ÞNA DG=kT pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ e dt 2pMRT
(6)
where a is the constant describing atomic absorption capacity on surface of nucleates, g* the critical radius of nucleus, ns the density of nucleus, PV the actual pressure of reactive gas, Ps the equilibrium vapor pressure of deposited solid phase, NA the Avogadro’s number, M the molar mass of deposited solid phase, R the gas constant, T the deposition temperature, DG* the critical Gibbs free energy for solid phase nucleation and K is the constant of reaction rate. Of these, exponential term influences mostly, so dN / eDG=kT dt
(7)
and
DG ¼
16pg 3 3DG2v
D Gv ¼
kT
lnð1 þ SÞ
(8)
(9)
V where V is the atomic volume, S the gas super-saturation.
Fig. 4. TEM image of ZrC/C composite coating exfoliated from substrate. The black grains are ZrC crystals.
Substituting Eq. (8) [25] and Eq. (9) [26] into Eq. (7), we have,
estimated by Scherred method [27,28]:
2 3 dN 3 2 / e16p V =3K Tln ð1þSÞ dt
Dh k l ¼
(10)
From Eq. (10), we know that dN/dt increases with the gas supersaturation. In experiment, active [C] decomposed from C3H6 is excessive to [Zr] from ZrCl4. So the nucleation of pyrolytic carbon is prior to that of ZrC. As can be seen from Fig. 3, pyrolytic carbon film was deposited firstly on the substrate, while ZrC is nucleated on pyrolytic carbon films. Being surrounded by pyrolytic carbon, the growth of ZrC was inhibited and kept as nano-particles. So the cells own composite structure of ZrC crystal embedding in pyrolytic carbon and shape the morphology as same as pyrolytic carbon. In the whole process, deposition of pyrolytic carbon is the control step. Further TEM studies verify the co-deposition model of ZrC/C composite coating. As can be seen in Fig. 4, the black grains, dispersed in gray disordered floccules, have a grain size ranged from 10 nm to 50 nm and aggregate somewhere. EDXA shows the black grains are composed of 48.3% C and 51.3% Zr, and the gray floccules 100% C. The black grains, having a ratio of C and Zr near 1, are regarded as stoichiometric ZrC crystals. This can also be verified from the results of diffraction spots. Fig. 5 is the fracture morphology of ZrC/C composite coating deposited on graphite substrate. The coatings are about 200 mm thick. There are no any microcracks or microholes in the coating and interface. The ZrC/C coating has a good combination with substrate. Like the graphite substrate, ZrC/C composite coating exhibits brittle fracture. The XRD pattern of ZrC/C composite coating exfoliated from graphite substrate is shown in Fig. 6. The composite coating exhibits a ZrC1.0 cubic phase and amorphous carbon. The narrow and sharp peaks indicate good crystalline of ZrC phase. The wide peak of amorphous carbon, departure from (2 0 0) of graphite 3.48, corresponds to turbostratic carbon. The crystallite size of ZrC1.0 is
Fig. 3. Formation mechanism of co-deposition of ZrC/C composite coating.
Kl bh k l cos u
(11)
where Dh k l is the thickness of crystallite perpendicular to crystal plane (h k l). K, value of crystal, changes with different shape and
Fig. 5. SEM fracture morphology of ZrC/C composite coating deposited on graphite substrate. ZrC/C coating has a good combination with substrate.
W. Sun et al. / Applied Surface Science 255 (2009) 7142–7146
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Fig. 6. XRD pattern of ZrC/C composite coating exfoliated from graphite substrate.
crystal plane (K = 1.557 in our calculation), l is the wavelength of X-ray, b is the half width of the peak, and u is the Bragg angle. The calculated size is 22.44 nm, which is in accordance with TEM studies. Preferred orientation of ZrC crystal was calculated with Haris method from the XRD results according to the following relationship [29,30]: TC ¼
ð1=nÞ
Ii =I0 P n
(12)
i¼1 ðIi =I0 Þ
where TC is texture coefficient for a given plane, Ii is the actual intensity of measuring crystal plane, I0 standard integrated intensity from the reference of American Society for Testing & Materials (ASTM) file, n is the number of the diffraction peaks. In our case, (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes are considered and TC values in Table 2 are the calculated texture coefficients. The small difference of TC values implies that ZrC crystal grown at 1350 8C tends to be random. No obvious preferred orientation of crystal is found. Fig. 7 is the selected-area electron diffraction pattern of ZrC/C composite coating taken at Fig. 4. As calibrated, the concentric rings are corresponding to (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of fcc-ZrC solid solution. The formation of concentric rings suggests that the composite coating contains large quantity of nano-ZrC crystals. The light halo in the middle of the rings represents amorphous carbon with disordered structure. The results match well with that of XRD pattern. The line scanning EDXA of ZrC/C composite coating was shown in Fig. 8. The results show that the distribution of carbon element is relatively steady, while the distribution of Zr element is fluctuating along the cross-section of the coating. The convex peaks of Zr indicate the non-uniform distribution of ZrC nano-particles in composite coating. Fig. 9 is TG-DSC analysis of ZrC/C composite coating (5.595 g) with a temperature rising rate of 5 K/min from 20 K to 1440 K, under oxygen atmosphere. Two exothermal peaks (679.2–
Fig. 7. The selected-area electron diffraction pattern of ZrC/C composite coating taken at above TEM image (Fig. 4).
698.0 8C, 729.3–788.2 8C) are observed, which corresponds to two oxidizing reactions: C þ O2 ! CO2 "
(13)
ZrC þ O2 ! ZrO2 þ CO2 "
(14)
The weight loss of the coating is 11.58% (from TG curve). And there are 88.42% (or 5.595 g 88.42%) resultant reserved. From Eqs. (13) and (14), the resultant is only ZrO2. Obtain molar fraction of ZrC in the coating is 0.04 mol. So the composite coating consists of 74.04 wt.% ZrC and 25.96 wt.% pyrolytic carbon. K value method [31] was employed to evidence the above result. To samples with two phases: WA ¼
IA IA þ IB =KAB
(15)
WB ¼
IB IB þ IA KAB
(16)
where I is the mean intensity of measuring phase and K is the intensity factor obtained from PDF cards. The intensity of ZrC and
Table 2 Texture coefficients of crystal planes for different planes of ZrC crystals. Planes
I0 Ii TC
(1 1 1)
(2 0 0)
(2 2 0)
(3 1 1)
100.0% 100.0% 0.75
82.0% 99.0% 0.91
62.0% 88.6% 1.07
50.0% 84.4% 1.25
Fig. 8. Line scanning EDA of ZrC/C composite coating deposited on graphite substrate.
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cm3) approximately equals to actual density (5.892 g/cm3) obtained by Archimedes’ principle. Acknowledgements This research program is financially supported by the National Basic Research Program of China (No. 2006CB600908) and the National Hi-Tech Research Development Program of China (No. 2007AA03Z110). References
Fig. 9. TG-DSC curves of ZrC–C composite coating exfoliated from graphite substrate (O2, 5 K/min).
pyrolytic carbon obtained from XRD pattern (Fig. 6). According to Eqs. (15) and (16), the results of 78.90 wt.% ZrC and 21.10 wt.% amorphous carbon were obtained. Neglecting the influence of pores on the density of the composite coating, gain the density of composite coating, 6.017 g/cm3, which is near to the value of 5.892 g/cm3 measured by Archimedes’ principle. 4. Conclusions This paper reported a kind of ZrC–C composite coating grown on graphite substrate successfully fabricated by atmospheric pressure chemical vapor deposition. The process is proposed to be co-deposition of zirconium carbide and pyrolysis carbon, and the latter is the control step. The surface morphology of ZrC/C composite coating, presenting character of pyrolysis carbon, is closed stacked by un-uniform cells. TEM image shows the feature of zirconium carbide nano-sized crystals surrounding by turbostratic carbon. The ZrC/C coating has a good combination with substrate. Through TG-DSC curves, it is calculated that the weight percent of ZrC and amorphous carbon in composite coating are 74.04% and 25.96%, respectively. The theoretical density (6.017 g/
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