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Rhenium coating prepared on carbon substrate by chemical vapor deposition
Applied Surface Science 261 (2012) 390–395
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Rhenium coating prepared on carbon substrate by chemical vapor deposition Yonggang Tong ∗ , Shuxin Bai, Hong Zhang, Yicong Ye College of Aerospace and Materials Engineering, National University of Defense Technology, Changsha 410073, PR China
a r t i c l e
i n f o
Article history: Received 22 April 2012 Received in revised form 28 July 2012 Accepted 5 August 2012 Available online 13 August 2012 Keywords: Rhenium coating Graphite C/C composite Chemical vapor deposition
a b s t r a c t Rhenium coating was prepared on the isotropic graphite and C/C composite by chemical vapor deposition. The phase composition and microstructure of the coating were studied by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. The results show that the rhenium coating on the isotropic graphite was compact and crack-free with a preferential growth orientation of 0 0 2 while two kinds of cracks were found in the coating on the C/C composite. The formation of the two cracks in the coating on the C/C composite was mainly attributed to the residual stress resulting from the mismatch in the thermal coefficient of expansion and the anisotropy of the C/C composite. Furthermore, the bond strength between the rhenium coating and the substrate was tested by the coating-pull-off method. The graphite substrate fractures with the tensile strength about 4.5 MPa for the coating on the graphite, while the rupture is at the interface of coating-adhesive for the coating on the C/C substrate with the tensile strength of 15 MPa. © 2012 Elsevier B.V. All rights reserved.
1. Introduction As one of the most promising high temperature materials, rhenium has attracted much attention for its unique combination of properties including exceptionally high melting point, high modulus and strength at elevated temperatures, good ductility and corrosion resistance [1,2]. In recent years, the interest in rhenium has grown considerably due to its widespread use in high temperature applications. Rhenium coated iridium combustion chambers taking advantage of rhenium’s high melting point and high mechanical properties at elevated temperatures have been fabricated and used for satellites and liquid rocket engines [3,4]. Rhenium also can be used in producing solid rocket motor nozzles in hot-gas solid propellant applications due to the superior erosion resistance and space nuclear reactor design as a barrier between the uranium nitride nuclear fuel and the niobium alloy cladding [5]. However, rhenium is one of the heaviest materials (density of 21 g/cm3 ) and extremely expensive [7], which limits its widespread applications. It is known that the cost of operating a spacecraft with heavier components escalates rapidly, and the efficiency degenerates rapidly to the unacceptable levels [8,9]. The volume increasing of the heavy rhenium parts decreases the efficiency and the lifetime of the spacecraft. Thus, great efforts should be made to decrease the component weight. Carbon based materials, such as C/C composite and graphite, are lightweight with excellent strength at elevated temperatures [10–12]. A rhenium
∗ Corresponding author. Tel.: +86 731 4576147; fax: +86 731 4574791. E-mail address: [email protected] (Y. Tong). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.020
coating on the carbon material can produce a rhenium like part that is lighter and cheaper than the solid rhenium. In addition, the ablation resistance of the carbon material can be greatly improved by the rhenium coating [6]. The aim of this work is to develop a material with both lightweight and good high temperature properties by producing carbon materials coupled with rhenium coating. Rhenium coating was prepared on the carbon substrate (including the isotropic graphite and C/C composite) by chemical vapor deposition (CVD) and the microstructure and the bond strength of the rhenium coating on carbon substrate have been characterized. 2. Experimental 2.1. Preparation of the rhenium coating The apparatus for CVD rhenium coating is shown in Fig. 1. A quartz tube was fabricated as the reaction chamber, where both the chlorination of rhenium powder and CVD of rhenium coating took place. Rhenium powder (99.999%, Zhu Zhou Kete Industries Co., Ltd) was placed in a quartz cup and heated up to 730 ◦ C using a vertical furnace. The substrate materials including the isotropic graphite and C/C composite (density, 1.8 g/cm3 ), which were cut, polished, ultrasonically cleaned with ethanol and dried at 100 ◦ C for 4 h, were fixed on the substrate holder and heated using a high-frequency induction furnace. The temperature of the substrate materials measured using an infrared thermometer (Raytek MR1SBSF) was controlled to be 1150 ◦ C. The flow rates of chlorine (99.999%) as reagent and argon (99.999%) as carrier gas determined by mass flow meter were 90 ml/min and 500 ml/min, respectively.
Y. Tong et al. / Applied Surface Science 261 (2012) 390–395
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and carbon/sulfur analyzer (LECO, CS 600). The bond strength between the coating and substrate was tested by coating-pulloff test according to ISO 4624:2002(E) [13]. The tensile rate was 0.5 mm/min. 3. Results and discussion
Fig. 1. The apparatus for CVD rhenium coating.
The total pressure of the reaction chamber maintained by vacuum pump was about 0.076 MPa. 2.2. Characterization and tests of rhenium coatings The phase identification was conducted by X-ray diffraction (XRD, Rigaku D/Max 2550VB) using a Ni-filtered Cu K␣ radiation at a scanning rate of 5◦ /min and scanning from 20◦ to 80◦ of 2. The surface and cross section morphologies of rhenium coating were observed by a Hitachi S-4800 scanning electron microscope (SEM), at accelerating voltages in the range 5–25 kV. The chemical composition of the coating was determined by energy dispersive spectroscopy (EDS), inductively coupled plasma atomic emission spectroscopy (ICP-AES, IRIS Advantage 1000), oxygen/nitrogen/hydrogen analyzer (LECO, TCH 600)
Fig. 2 shows the macroscopical photographs of the substrates and the as-coated specimens. It was found that successive rhenium coatings were obtained on the C/C composite and the isotropic graphite by CVD. No detectable delamination and cracks were observed in the coating deposited on both substrates. Fig. 3 shows the XRD patterns of the coatings on the C/C composite and the graphite. It is indicated that both the coatings are composed of pure rhenium. The scan data from the coating on the C/C composite exhibits strong 2 peaks at 40.58◦ , 43.02◦ , 56.50◦ and 75.27◦ respectively, corresponding to the (0 0 2), (1 0 1), (1 0 2) and (1 0 3) peaks of rhenium according to the standard d-values taken from JCPDS (05-0702), whereas 2 peaks at 40.58◦ was just seen in the scan data from the coating on the graphite. It is indicated that the rhenium coatings have a highly textured, polycrystalline structure with preferential growth orientation. The preferential growth orientation of the coatings can be determined using a texture coefficient (TC(h k l) ) that can give the information of the strongest reflection along the (h k l) plane under different conditions. This factor can be calculated via the following relation [14]: TC(h
k l)
=
I
1/N
k l) (h
/I0(h k l)
I(h k l) /I0(h k l)
(1)
where TC(h k l) is the texture coefficient of the (h k l) plane, I(h k l) is the measured intensity of (h k l) plane, I0(h k l) is the corresponding
Fig. 2. The macroscopical photographs of the carbon materials and the as-coated specimens: (a) isotropic graphite; (b) as-coated isotropic graphite; (c) C/C composite; and (d) as-coated C/C composite.
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Fig. 3. XRD patterns of the rhenium coatings deposited on (a) C/C composite and (b) isotropic graphite. Table 1 TC(h k l) calculation results of the coating on C/C composite and isotropic graphite.
TC(h k l) C/C composite TC(h k l) isotropic graphite
(0 0 2)
(1 0 1)
(1 0 2)
(1 0 3)
2.71 1
0.07 0
0.36 0
0.82 0
recorded intensity in JCPDS data file, and N is the number of preferred growth directions. The TC(h k l) calculation results of the coatings on both substrates are shown in Table 1. The texture analyzed indicates that the rhenium has a strong (0 0 2) component for the coatings on both substrates. SEM micrographs of the rhenium coatings on the graphite and the C/C composite are shown in Figs. 4 and 5, respectively. The
coating on the graphite is compact and crack-free, appearing as nearly equiaxed grains of about 5 m size on the coating surface (Fig. 4(c)). The fracture surface of the coating on the graphite (Fig. 4(d)) shows that the rhenium coating has a columnar structure with average grain diameter of about 5 m, which is consistent with the result of the coating surface observation. Different from the coating on the graphite, the coating on the C/C composite owning the similar crystal diameter has some voids and cracks (see Fig. 5(b–d)), the reasons for which will be discussed later. Additionally, it can been seen that all the grains in the coating on the graphite present regular hexagonal crystal planes approximately paralleled to the substrate surface, while some other crystal planes appearing in the coating on the C/C composite unparallel to the substrate surface except the most regular hexagonal crystal planes approximately parallel to the substrate surface. It is believed that the regular hexagonal crystal plane approximately paralleled to the substrate surface is (0 0 2) of the HCP rhenium crystal, which can be confirmed by the XRD analysis results. For one hand, the preferential growth orientation of the coating on both substrates is 0 0 2. For the other, only diffraction peaks of (0 0 2) crystal plane was seen for the coating on the graphite, while diffraction peaks of some other crystal planes appear in the XRD patterns for the coating on the C/C composite. Movchan and Demchishin [15] found that the microstructures of metal coatings could be represented as a function of T/Tm by three zones, where T and Tm indicated the substrate temperature and the melting point of the coating material, respectively. Zone I (T/Tm < 0.3) consists of tapered crystallites with a characteristic domed surface; zone II (0.3 < T/Tm < 0.45–0.5) consists of columnar grains with smooth surface; zone III (T/Tm > 0.45–0.5) consists of equiaxial grains and a bright surface. The rhenium coating was deposited at 1150 ◦ C and T/Tm has a value of 0.36. The fracture surfaces of the rhenium coatings on both isotropic graphite and C/C composite have a columnar structure, corresponding to the zone II, which is consistent with the three-zone model.
Fig. 4. SEM micrographs of the rhenium coating on the isotropic graphite substrate (a–c) surface and (d) cross-section fracture.
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Fig. 5. SEM micrographs of the rhenium coating on the C/C composite substrate (a–c) surface and (d) cross-section fracture.
EDS analysis of the coatings on both substrates is shown in Fig. 6. It is indicated that both coatings are composed of rhenium. No detectable other elements were observed. Further composition analysis results of the coatings by ICP-AES, oxygen/nitrogen/hydrogen analyzer and carbon/sulfur analyzer are shown in Table 2. The rhenium powder used in the preparation of the rhenium coating was analyzed as well for comparison. As can be seen, the coating’s purity is much higher than that of the powder, which indicated that the CVD process is an effective method for the rhenium purification.
Fig. 6. EDS analysis of the coatings on (a) C/C composite and (b) isotropic graphite.
Table 2 Composition analysis results of the rhenium coating and powder: As, B, Cd, Co, Mg, Ni, Sn, V and Ti of 0.0001% in the coating and powder: Al, Ca, P, Pb and Si of 0.0010% in the coating and powder. Element
Cr Cu Fe Mn Mo Zn C S O N H Re
Content (%) Rhenium powder
Rhenium coating
0.0001 0.0041 0.044 0.0001 0.0010 0.0001 0.031 0.0020 1.53 0.12 0.062 98.1997
0.0005 0.0045 0.0020 0.0010 0.0001 0.0005 0.015 0.0010 0.0010 0.0018 0.0006 99.9661
Fig. 7. The voids and cracks distribution in the rhenium coating on the C/C composite.
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Fig. 8. SEM micrographs of the C/C composite.
Fig. 7 shows that some voids and cracks distribute in the coating on the C/C composite. It is observed that some voids locate in the coating on the inter fiber bundles of the C/C composite substrate, showing the same distribution of pores in the C/C composite substrate (see Fig. 8). Two kinds of cracks can be observed in the coating (marked as A and B in Fig. 7). A cracks distribute along the interface of the inter fiber bundles while B cracks locate in the coating on the intra fiber bundles. The directions of B cracks are mostly vertical to the direction of the carbon fiber axis. The crack propagation and path in the coating can also be observed in Fig. 7. The A cracks usually combine the voids and the B cracks. The width of A cracks is larger than that of the B cracks, which indicates the crack origination of A and the crack propagation from A to B cracks (as indicated by arrows). Such crack propagation and path in the coating can also be confirmed by the crack close in the B cracks. Compact and crack-free rhenium coating was deposited on the graphite substrate while the coating on the C/C substrate has some cracks. The formation of cracks should be attributed to the residual stress thermally induced by the difference of the coefficient of thermal expansion (CTE) between the coating and substrate. The CTE of rhenium, 6.9 ppm/K [6], is much larger than that of the C/C composite (less than 1.5 ppm/K) [16]. When the coating deposited at elevated temperature (1150 ◦ C in our work) is cooling down to the room temperature, the mismatch in thermal expansion between the coating and the substrate results in a large residual tensile stress in the rhenium coating. If the tensile residual stress exceeds a critical value, micro-cracking will tend to form perpendicularly. A simple equation has been proposed to estimate the thermal stress due to the CTE mismatch, which can be simplified if the coating thickness is much smaller than the substrate thickness. Considering force balance, the residual stress in the coating was given by the following formula [17]: cT =
Ec (˛s − ˛c )T Ec (˛s − ˛c )T ≈ (1 − c ) (1 − c ) + (2tc Ec /ts Es )(1 − s )
(2)
where ˛ is the CTE, T is the temperature change, h is the thickness, E is the Young’s modulus, is the Poisson ratio, subscripts c and s refer to the coating and substrate, respectively. The CTE of the isothermal graphite, 5.8 ppm/K [18], is much closer to the rhenium (6.9 ppm/K) than that of the C/C composite. The residual stress on the isothermal graphite is much less than that on the C/C composite according to Eq. (2). This may be the main reason that crack-free rhenium coating is obtained on the isothermal graphite while the coating on the C/C composite has some micro-cracks. It is reported that the stress within coatings near the interfacial defects and borders can be substantially different [19,20]. Stress concentration is often found at the interfacial defects and borders, which can subsequently promote the micro-crack formation in the coating. C/C composite is composed of carbon fibers,
pyrolytic carbon and some pores. The cross-section micrographs of the C/C composite (Fig. 8) indicate that some voids and gaps exist in the inter fiber bundles. Stress concentration is prone to happen there. Thus, micro-cracks prefer to forming at the voids and borders in the rhenium coating on the C/C composite when the coating is cooling down from the deposited temperature to the room temperature. The distribution of A and B cracks in the coating on the C/C composite is referable to the anisotropy of the C/C composite. Two direction carbon fiber bundles can be observed on the C/C composite surface with one paralleled and the other vertical to the C/C composite surface (marked as I and II, respectively in Fig. 8). Some voids and gaps exist in the inter fiber bundles while the intra fiber bundles are dense without voids and gaps. It is known that the radial CTE of carbon fiber is 7 ppm/K, close to the CTE of rhenium coating, while the axial CTE is −0.1 to −1.1 ppm/K [21]. Thus, the residual stress resulted from the thermal mismatch in region I is much larger than that in region II. When cracks are initially formed in the coating, they firstly propagate along the voids and the gaps in the inter fiber bundles where stress concentration is often caused. After the crack tips contact the intra fiber bundle regions, they vertically propagate into region II due to the larger thermal mismatch. With the crack propagation, the crack propagation energy is consumed and some cracks are closed in region II (see Fig. 7). Therefore, A and B cracks are distributed as indicated in Fig. 7. Results of the coating-pull-off tests of rhenium coatings on the graphite and the C/C composite are listed in Table 3. The test result of the coating on the graphite is about 4.5 MPa with the fracture of graphite substrate, which demonstrates that the bond strength between the coating and the graphite substrate is higher than 4.5 MPa. Thus, it is impossible to obtain the exact bond strength of the rhenium coating to the graphite substrate due to the low substrate strength. Compared with the coating on the graphite, the rupture is at the interface of coating-adhesive for the coating on the C/C composite substrate with the tensile strength of 15 MPa. It is indicated that the bond strength between the rhenium coating and C/C composite substrate is higher than 15 MPa. Other adhesives with stronger adhesive force are needed in order to gain a more accurate result of bond strength of the coating to C/C substrate.
Table 3 Results of the coating-pull-off tests. Substrate
Test stress, MPa
Rupture style
Bond strength, MPa
Isotropic graphite C/C composite
4.5 15
The substrate Interface of coating-adhesive
>4.5 >15
Y. Tong et al. / Applied Surface Science 261 (2012) 390–395
4. Conclusion Rhenium coating was prepared on the isotropic graphite and C/C composite by CVD. The rhenium coating on the isotropic graphite was compact and crack-free with a preferential growth orientation of 0 0 2 while a certain amount of cracks were found in the coating on the C/C composite. Two kinds of cracks existed in the coating on the C/C composite. One distributed along the interface of inter fiber bundles of the C/C composite and the other existed in the coating on the intra fiber bundles, which are mostly vertical to the direction of fiber axis. The formation of the cracks in the coating on the C/C composite was mainly contributed to the residual stress resulted from the thermal expansion coefficient mismatch and the anisotropy of the C/C composite. The bond strength test result of the coating on the graphite is about 4.5 MPa with the fracture of the graphite substrate, while the rupture is at the interface of coating-adhesive for the coating on the C/C substrate with the tensile strength of 15 MPa. References [1] J.C. Carlen, B.D. Bryskin, Rhenium a unique rare metal, Material Manufacture Process 9 (6) (1994) 1087–1104. [2] B.D. Bryskin, Rhenium and its alloys, Advanced Materials & Processes 142 (3) (1992) 22–27. [3] S. Chen, C.Y. Hu, J.M. Yang, J.M. Guo, D.G. Deng, Study on interdiffusion of iridium/rhenium, Rare Metal Materials and Engineering 35 (1) (2006) 17–20. [4] C.G. Liu, J. Chen, H.Y. Han, A long duration and high reliability liquid apogee engine for satellites, Acta Astronautica 55 (2004) 401–408. [5] E.D. Sayre, T.J. Ruffo, D.C. Wadekamper, M. Kangilaski, in: B.D. Bryskin (Ed.), Rhenium and Rhenium Alloys, TMS, Warrendale, PA, 1997, pp. 261–273. [6] D. Mittendorf, Economical erosion-resistant rhenium coating on carbon substrates, AIAA Paper No. 2000-3559, 2000.
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[7] B.J. Koeppel, G. Subhash, Influence of cold rolling and strain rate on plastic response of powder metallurgy and chemical vapor deposition rhenium, Metallurgical and Materials Transactions A 30 (1999) 1999–2461. [8] R.H. Tuffias, B.E., Williams, R.B. Laplan, Method of forming a composite structure such as a rocket combustion chamber, US patent 5,855,828 (1995). [9] R.H. Tuffias, B.E., Williams, R. B. Kaplan, Lightweight, inexpensive radiationcooled advanced composite combustion chambers, AIAA Paper No. 95-2400, 1995. [10] D.D. Jayaseelan, R.G. Sá, P. Brown, W.E. Lee, Reactive infiltration processing (RIP) of ultra high temperature ceramics (UHTC) into porous C/C composite tubes, Journal of the European Ceramic Society 31 (2011) 361–368. [11] Y.J. Wang, H.J. Li, Q.G. Fu, H. Wu, D.J. Yao, B.B. Wei, Ablative property of HfC-based multilayer coating for C/C composites under oxy-acetylene torch, Applied Surface Science 257 (2011) 4760–4763. [12] P.M. Zhu, H.Y. Chen, Z.H. Li, Study on oxidation resistance of graphite materials by impregnation, Carbon Technique 27 (3) (2008) 39–41 (in Chinese). [13] ISO 4624:2002(E), Paints and varnishes-pull-off test for adhesion, 11 pp. [14] P. Patil, P. Chigare, S. Sadale, T. Seth, D. Amalnerkar, R. Kawar, Thicknessdependent properties of sprayed iridium oxide thin films, Material Chemistry and Physics 80 (3) (2003) 667–675. [15] B.A. Movchan, A.V. Demchishin, Study of the structure and properties of thick vacuum condensates of nickel, titanium, tungsten, aluminum oxide and zirconium dioxide, Fizika Metallov i Metallovedenie 28 (1969) 653–660. [16] J.M. Su, H. Cui, R.Z. Li, S.J. Zhou, Z.A. Su, Y.D. Li, Structure and properties of the new needled carbon cloth C/C composite, New Carbon Materials 15 (2) (2000) 11–15. [17] V. Teixeira, Mechanical integrity in PVD coatings due to the presence of residual stresses, Thin Solid Films 392 (2001) 276–281. [18] K. Mumtaz, J. Echigoya, H. Enoki, T. Hirai, Y. Shindo, Thermal cycling of iridium coating on isotropic graphite, Journal of Materials Science 30 (1995) 465–472. [19] J.T. Drake, R.L. Williamson, B.H. Rabin, Finite element analysis of thermal residual stresses at graded ceramic metal interface. Part II: Interface optimization for residual stress reduction, Journal of Applied Physics 74 (2) (1993) 1321–1326. [20] A.G. Evans, J.W. Hutchinson, The thermomechanical integrity of thin films and multilayers, Acta Metallurgica et Materialia 43 (7) (1995) 2507–2530. [21] Y.D. Xu, L.T. Zhang, L.F. Cheng, D.T. Yan, Microstructure and mechanical properties of three-dimensional carbon/silicon carbide composites fabricated by chemical vapor infiltration, Carbon 36 (7–8) (1998) 1051–1056.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Rhenium coating prepared on carbon substrate by chemical vapor deposition Yonggang Tong ∗ , Shuxin Bai, Hong Zhang, Yicong Ye College of Aerospace and Materials Engineering, National University of Defense Technology, Changsha 410073, PR China
a r t i c l e
i n f o
Article history: Received 22 April 2012 Received in revised form 28 July 2012 Accepted 5 August 2012 Available online 13 August 2012 Keywords: Rhenium coating Graphite C/C composite Chemical vapor deposition
a b s t r a c t Rhenium coating was prepared on the isotropic graphite and C/C composite by chemical vapor deposition. The phase composition and microstructure of the coating were studied by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. The results show that the rhenium coating on the isotropic graphite was compact and crack-free with a preferential growth orientation of 0 0 2 while two kinds of cracks were found in the coating on the C/C composite. The formation of the two cracks in the coating on the C/C composite was mainly attributed to the residual stress resulting from the mismatch in the thermal coefficient of expansion and the anisotropy of the C/C composite. Furthermore, the bond strength between the rhenium coating and the substrate was tested by the coating-pull-off method. The graphite substrate fractures with the tensile strength about 4.5 MPa for the coating on the graphite, while the rupture is at the interface of coating-adhesive for the coating on the C/C substrate with the tensile strength of 15 MPa. © 2012 Elsevier B.V. All rights reserved.
1. Introduction As one of the most promising high temperature materials, rhenium has attracted much attention for its unique combination of properties including exceptionally high melting point, high modulus and strength at elevated temperatures, good ductility and corrosion resistance [1,2]. In recent years, the interest in rhenium has grown considerably due to its widespread use in high temperature applications. Rhenium coated iridium combustion chambers taking advantage of rhenium’s high melting point and high mechanical properties at elevated temperatures have been fabricated and used for satellites and liquid rocket engines [3,4]. Rhenium also can be used in producing solid rocket motor nozzles in hot-gas solid propellant applications due to the superior erosion resistance and space nuclear reactor design as a barrier between the uranium nitride nuclear fuel and the niobium alloy cladding [5]. However, rhenium is one of the heaviest materials (density of 21 g/cm3 ) and extremely expensive [7], which limits its widespread applications. It is known that the cost of operating a spacecraft with heavier components escalates rapidly, and the efficiency degenerates rapidly to the unacceptable levels [8,9]. The volume increasing of the heavy rhenium parts decreases the efficiency and the lifetime of the spacecraft. Thus, great efforts should be made to decrease the component weight. Carbon based materials, such as C/C composite and graphite, are lightweight with excellent strength at elevated temperatures [10–12]. A rhenium
∗ Corresponding author. Tel.: +86 731 4576147; fax: +86 731 4574791. E-mail address: [email protected] (Y. Tong). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.020
coating on the carbon material can produce a rhenium like part that is lighter and cheaper than the solid rhenium. In addition, the ablation resistance of the carbon material can be greatly improved by the rhenium coating [6]. The aim of this work is to develop a material with both lightweight and good high temperature properties by producing carbon materials coupled with rhenium coating. Rhenium coating was prepared on the carbon substrate (including the isotropic graphite and C/C composite) by chemical vapor deposition (CVD) and the microstructure and the bond strength of the rhenium coating on carbon substrate have been characterized. 2. Experimental 2.1. Preparation of the rhenium coating The apparatus for CVD rhenium coating is shown in Fig. 1. A quartz tube was fabricated as the reaction chamber, where both the chlorination of rhenium powder and CVD of rhenium coating took place. Rhenium powder (99.999%, Zhu Zhou Kete Industries Co., Ltd) was placed in a quartz cup and heated up to 730 ◦ C using a vertical furnace. The substrate materials including the isotropic graphite and C/C composite (density, 1.8 g/cm3 ), which were cut, polished, ultrasonically cleaned with ethanol and dried at 100 ◦ C for 4 h, were fixed on the substrate holder and heated using a high-frequency induction furnace. The temperature of the substrate materials measured using an infrared thermometer (Raytek MR1SBSF) was controlled to be 1150 ◦ C. The flow rates of chlorine (99.999%) as reagent and argon (99.999%) as carrier gas determined by mass flow meter were 90 ml/min and 500 ml/min, respectively.
Y. Tong et al. / Applied Surface Science 261 (2012) 390–395
391
and carbon/sulfur analyzer (LECO, CS 600). The bond strength between the coating and substrate was tested by coating-pulloff test according to ISO 4624:2002(E) [13]. The tensile rate was 0.5 mm/min. 3. Results and discussion
Fig. 1. The apparatus for CVD rhenium coating.
The total pressure of the reaction chamber maintained by vacuum pump was about 0.076 MPa. 2.2. Characterization and tests of rhenium coatings The phase identification was conducted by X-ray diffraction (XRD, Rigaku D/Max 2550VB) using a Ni-filtered Cu K␣ radiation at a scanning rate of 5◦ /min and scanning from 20◦ to 80◦ of 2. The surface and cross section morphologies of rhenium coating were observed by a Hitachi S-4800 scanning electron microscope (SEM), at accelerating voltages in the range 5–25 kV. The chemical composition of the coating was determined by energy dispersive spectroscopy (EDS), inductively coupled plasma atomic emission spectroscopy (ICP-AES, IRIS Advantage 1000), oxygen/nitrogen/hydrogen analyzer (LECO, TCH 600)
Fig. 2 shows the macroscopical photographs of the substrates and the as-coated specimens. It was found that successive rhenium coatings were obtained on the C/C composite and the isotropic graphite by CVD. No detectable delamination and cracks were observed in the coating deposited on both substrates. Fig. 3 shows the XRD patterns of the coatings on the C/C composite and the graphite. It is indicated that both the coatings are composed of pure rhenium. The scan data from the coating on the C/C composite exhibits strong 2 peaks at 40.58◦ , 43.02◦ , 56.50◦ and 75.27◦ respectively, corresponding to the (0 0 2), (1 0 1), (1 0 2) and (1 0 3) peaks of rhenium according to the standard d-values taken from JCPDS (05-0702), whereas 2 peaks at 40.58◦ was just seen in the scan data from the coating on the graphite. It is indicated that the rhenium coatings have a highly textured, polycrystalline structure with preferential growth orientation. The preferential growth orientation of the coatings can be determined using a texture coefficient (TC(h k l) ) that can give the information of the strongest reflection along the (h k l) plane under different conditions. This factor can be calculated via the following relation [14]: TC(h
k l)
=
I
1/N
k l) (h
/I0(h k l)
I(h k l) /I0(h k l)
(1)
where TC(h k l) is the texture coefficient of the (h k l) plane, I(h k l) is the measured intensity of (h k l) plane, I0(h k l) is the corresponding
Fig. 2. The macroscopical photographs of the carbon materials and the as-coated specimens: (a) isotropic graphite; (b) as-coated isotropic graphite; (c) C/C composite; and (d) as-coated C/C composite.
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Fig. 3. XRD patterns of the rhenium coatings deposited on (a) C/C composite and (b) isotropic graphite. Table 1 TC(h k l) calculation results of the coating on C/C composite and isotropic graphite.
TC(h k l) C/C composite TC(h k l) isotropic graphite
(0 0 2)
(1 0 1)
(1 0 2)
(1 0 3)
2.71 1
0.07 0
0.36 0
0.82 0
recorded intensity in JCPDS data file, and N is the number of preferred growth directions. The TC(h k l) calculation results of the coatings on both substrates are shown in Table 1. The texture analyzed indicates that the rhenium has a strong (0 0 2) component for the coatings on both substrates. SEM micrographs of the rhenium coatings on the graphite and the C/C composite are shown in Figs. 4 and 5, respectively. The
coating on the graphite is compact and crack-free, appearing as nearly equiaxed grains of about 5 m size on the coating surface (Fig. 4(c)). The fracture surface of the coating on the graphite (Fig. 4(d)) shows that the rhenium coating has a columnar structure with average grain diameter of about 5 m, which is consistent with the result of the coating surface observation. Different from the coating on the graphite, the coating on the C/C composite owning the similar crystal diameter has some voids and cracks (see Fig. 5(b–d)), the reasons for which will be discussed later. Additionally, it can been seen that all the grains in the coating on the graphite present regular hexagonal crystal planes approximately paralleled to the substrate surface, while some other crystal planes appearing in the coating on the C/C composite unparallel to the substrate surface except the most regular hexagonal crystal planes approximately parallel to the substrate surface. It is believed that the regular hexagonal crystal plane approximately paralleled to the substrate surface is (0 0 2) of the HCP rhenium crystal, which can be confirmed by the XRD analysis results. For one hand, the preferential growth orientation of the coating on both substrates is 0 0 2. For the other, only diffraction peaks of (0 0 2) crystal plane was seen for the coating on the graphite, while diffraction peaks of some other crystal planes appear in the XRD patterns for the coating on the C/C composite. Movchan and Demchishin [15] found that the microstructures of metal coatings could be represented as a function of T/Tm by three zones, where T and Tm indicated the substrate temperature and the melting point of the coating material, respectively. Zone I (T/Tm < 0.3) consists of tapered crystallites with a characteristic domed surface; zone II (0.3 < T/Tm < 0.45–0.5) consists of columnar grains with smooth surface; zone III (T/Tm > 0.45–0.5) consists of equiaxial grains and a bright surface. The rhenium coating was deposited at 1150 ◦ C and T/Tm has a value of 0.36. The fracture surfaces of the rhenium coatings on both isotropic graphite and C/C composite have a columnar structure, corresponding to the zone II, which is consistent with the three-zone model.
Fig. 4. SEM micrographs of the rhenium coating on the isotropic graphite substrate (a–c) surface and (d) cross-section fracture.
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Fig. 5. SEM micrographs of the rhenium coating on the C/C composite substrate (a–c) surface and (d) cross-section fracture.
EDS analysis of the coatings on both substrates is shown in Fig. 6. It is indicated that both coatings are composed of rhenium. No detectable other elements were observed. Further composition analysis results of the coatings by ICP-AES, oxygen/nitrogen/hydrogen analyzer and carbon/sulfur analyzer are shown in Table 2. The rhenium powder used in the preparation of the rhenium coating was analyzed as well for comparison. As can be seen, the coating’s purity is much higher than that of the powder, which indicated that the CVD process is an effective method for the rhenium purification.
Fig. 6. EDS analysis of the coatings on (a) C/C composite and (b) isotropic graphite.
Table 2 Composition analysis results of the rhenium coating and powder: As, B, Cd, Co, Mg, Ni, Sn, V and Ti of 0.0001% in the coating and powder: Al, Ca, P, Pb and Si of 0.0010% in the coating and powder. Element
Cr Cu Fe Mn Mo Zn C S O N H Re
Content (%) Rhenium powder
Rhenium coating
0.0001 0.0041 0.044 0.0001 0.0010 0.0001 0.031 0.0020 1.53 0.12 0.062 98.1997
0.0005 0.0045 0.0020 0.0010 0.0001 0.0005 0.015 0.0010 0.0010 0.0018 0.0006 99.9661
Fig. 7. The voids and cracks distribution in the rhenium coating on the C/C composite.
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Fig. 8. SEM micrographs of the C/C composite.
Fig. 7 shows that some voids and cracks distribute in the coating on the C/C composite. It is observed that some voids locate in the coating on the inter fiber bundles of the C/C composite substrate, showing the same distribution of pores in the C/C composite substrate (see Fig. 8). Two kinds of cracks can be observed in the coating (marked as A and B in Fig. 7). A cracks distribute along the interface of the inter fiber bundles while B cracks locate in the coating on the intra fiber bundles. The directions of B cracks are mostly vertical to the direction of the carbon fiber axis. The crack propagation and path in the coating can also be observed in Fig. 7. The A cracks usually combine the voids and the B cracks. The width of A cracks is larger than that of the B cracks, which indicates the crack origination of A and the crack propagation from A to B cracks (as indicated by arrows). Such crack propagation and path in the coating can also be confirmed by the crack close in the B cracks. Compact and crack-free rhenium coating was deposited on the graphite substrate while the coating on the C/C substrate has some cracks. The formation of cracks should be attributed to the residual stress thermally induced by the difference of the coefficient of thermal expansion (CTE) between the coating and substrate. The CTE of rhenium, 6.9 ppm/K [6], is much larger than that of the C/C composite (less than 1.5 ppm/K) [16]. When the coating deposited at elevated temperature (1150 ◦ C in our work) is cooling down to the room temperature, the mismatch in thermal expansion between the coating and the substrate results in a large residual tensile stress in the rhenium coating. If the tensile residual stress exceeds a critical value, micro-cracking will tend to form perpendicularly. A simple equation has been proposed to estimate the thermal stress due to the CTE mismatch, which can be simplified if the coating thickness is much smaller than the substrate thickness. Considering force balance, the residual stress in the coating was given by the following formula [17]: cT =
Ec (˛s − ˛c )T Ec (˛s − ˛c )T ≈ (1 − c ) (1 − c ) + (2tc Ec /ts Es )(1 − s )
(2)
where ˛ is the CTE, T is the temperature change, h is the thickness, E is the Young’s modulus, is the Poisson ratio, subscripts c and s refer to the coating and substrate, respectively. The CTE of the isothermal graphite, 5.8 ppm/K [18], is much closer to the rhenium (6.9 ppm/K) than that of the C/C composite. The residual stress on the isothermal graphite is much less than that on the C/C composite according to Eq. (2). This may be the main reason that crack-free rhenium coating is obtained on the isothermal graphite while the coating on the C/C composite has some micro-cracks. It is reported that the stress within coatings near the interfacial defects and borders can be substantially different [19,20]. Stress concentration is often found at the interfacial defects and borders, which can subsequently promote the micro-crack formation in the coating. C/C composite is composed of carbon fibers,
pyrolytic carbon and some pores. The cross-section micrographs of the C/C composite (Fig. 8) indicate that some voids and gaps exist in the inter fiber bundles. Stress concentration is prone to happen there. Thus, micro-cracks prefer to forming at the voids and borders in the rhenium coating on the C/C composite when the coating is cooling down from the deposited temperature to the room temperature. The distribution of A and B cracks in the coating on the C/C composite is referable to the anisotropy of the C/C composite. Two direction carbon fiber bundles can be observed on the C/C composite surface with one paralleled and the other vertical to the C/C composite surface (marked as I and II, respectively in Fig. 8). Some voids and gaps exist in the inter fiber bundles while the intra fiber bundles are dense without voids and gaps. It is known that the radial CTE of carbon fiber is 7 ppm/K, close to the CTE of rhenium coating, while the axial CTE is −0.1 to −1.1 ppm/K [21]. Thus, the residual stress resulted from the thermal mismatch in region I is much larger than that in region II. When cracks are initially formed in the coating, they firstly propagate along the voids and the gaps in the inter fiber bundles where stress concentration is often caused. After the crack tips contact the intra fiber bundle regions, they vertically propagate into region II due to the larger thermal mismatch. With the crack propagation, the crack propagation energy is consumed and some cracks are closed in region II (see Fig. 7). Therefore, A and B cracks are distributed as indicated in Fig. 7. Results of the coating-pull-off tests of rhenium coatings on the graphite and the C/C composite are listed in Table 3. The test result of the coating on the graphite is about 4.5 MPa with the fracture of graphite substrate, which demonstrates that the bond strength between the coating and the graphite substrate is higher than 4.5 MPa. Thus, it is impossible to obtain the exact bond strength of the rhenium coating to the graphite substrate due to the low substrate strength. Compared with the coating on the graphite, the rupture is at the interface of coating-adhesive for the coating on the C/C composite substrate with the tensile strength of 15 MPa. It is indicated that the bond strength between the rhenium coating and C/C composite substrate is higher than 15 MPa. Other adhesives with stronger adhesive force are needed in order to gain a more accurate result of bond strength of the coating to C/C substrate.
Table 3 Results of the coating-pull-off tests. Substrate
Test stress, MPa
Rupture style
Bond strength, MPa
Isotropic graphite C/C composite
4.5 15
The substrate Interface of coating-adhesive
>4.5 >15
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4. Conclusion Rhenium coating was prepared on the isotropic graphite and C/C composite by CVD. The rhenium coating on the isotropic graphite was compact and crack-free with a preferential growth orientation of 0 0 2 while a certain amount of cracks were found in the coating on the C/C composite. Two kinds of cracks existed in the coating on the C/C composite. One distributed along the interface of inter fiber bundles of the C/C composite and the other existed in the coating on the intra fiber bundles, which are mostly vertical to the direction of fiber axis. The formation of the cracks in the coating on the C/C composite was mainly contributed to the residual stress resulted from the thermal expansion coefficient mismatch and the anisotropy of the C/C composite. The bond strength test result of the coating on the graphite is about 4.5 MPa with the fracture of the graphite substrate, while the rupture is at the interface of coating-adhesive for the coating on the C/C substrate with the tensile strength of 15 MPa. References [1] J.C. Carlen, B.D. Bryskin, Rhenium a unique rare metal, Material Manufacture Process 9 (6) (1994) 1087–1104. [2] B.D. Bryskin, Rhenium and its alloys, Advanced Materials & Processes 142 (3) (1992) 22–27. [3] S. Chen, C.Y. Hu, J.M. Yang, J.M. Guo, D.G. Deng, Study on interdiffusion of iridium/rhenium, Rare Metal Materials and Engineering 35 (1) (2006) 17–20. [4] C.G. Liu, J. Chen, H.Y. Han, A long duration and high reliability liquid apogee engine for satellites, Acta Astronautica 55 (2004) 401–408. [5] E.D. Sayre, T.J. Ruffo, D.C. Wadekamper, M. Kangilaski, in: B.D. Bryskin (Ed.), Rhenium and Rhenium Alloys, TMS, Warrendale, PA, 1997, pp. 261–273. [6] D. Mittendorf, Economical erosion-resistant rhenium coating on carbon substrates, AIAA Paper No. 2000-3559, 2000.
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