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Rhenium used as an interlayer between carbon–carbon composites and iridium coating: Adhesion and wettability
Surface & Coatings Technology 235 (2013) 68–74
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Rhenium used as an interlayer between carbon–carbon composites and iridium coating: Adhesion and wettability Li'an Zhu a, Shuxin Bai a,⁎, Hong Zhang a, Yicong Ye a, Wei Gao b a b
Department of Materials Science and Engineering, College of Aerospace Science and Engineering, National University of Defence Technology, Changsha 410073, PR China Department of Chemical and Materials Engineering, The University of Auckland, PB 92019, Auckland 1142, New Zealand
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Article history: Received 20 April 2013 Accepted in revised form 4 July 2013 Available online 12 July 2013 Keywords: Rhenium coating Iridium coating Carbon/carbon composites Bond strength Wettability
a b s t r a c t Iridium (Ir) is one of the most promising materials as a protective coating on carbon/carbon (C/C) composites for very high temperature applications. However, the poor adhesion and thermal stress induced by thermal expansion mismatch between Ir coating and C/C substrate restrict their application. Rhenium (Re) has been selected as the interlayer material between the Ir coating and C/C substrate which can improve the adhesion of Ir coating and relieve the thermal stress. The Re coating was prepared on the C/C substrate by chemical vapor deposition (CVD). The Ir coating was electrodeposited on the C/C and Re/C/C substrates in molten salt. The morphology and microstructure of the Re and Ir coatings were studied by SEM and XRD. The bond strength of the coatings on C/C substrate was measured by coating-pull-off test. The molten salt wettability on the C/C and Re/C/C substrates was determined by drop shape method. The results showed that the CVD Re coating on the C/C substrate had a preferential orientation of b002 N with many fine regular cracks. The bond strength between the Re coating and C/C substrate was higher than 15 MPa before and after annealing treatment. The contact angles of molten salt on C/C and Re/C/C substrates at 500°C were 128.5° and 43.4°, respectively. The bond strength of as-deposited coatings increased from 3.2 MPa for Ir/C/C to 7.9 MPa for Ir/Re/C/C. After high temperature annealing treatment, however, the bond strength has been reduced by ~ 50%, probably due to the infiltration of molten salt into the C/C substrate. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Carbon/carbon (C/C) composites have received much attention in aerospace applications due to their high specific strength and modulus, high fracture toughness and thermal conductivity, low coefficient of thermal expansion (CTE), good thermal shock resistance, especially their excellent high-temperature strength up to 2200°C in oxygenfree environment [1,2]. However, C/C composites readily oxidize above 370°C and lose their strength with increasing temperature, which restricts their high-temperature applications [3]. Thus the oxidation protection of C/C composites is the key issue to their extensive use. Iridium (Ir) is considered as a promising material as an oxidation resistant coating for carbonaceous substrates due to its high melting point, excellent chemical stability, low oxygen permeability, good chemical compatibility and low carbon solubility below the eutectic temperature of 2100–2300°C [4–6]. Good adhesion of a compact and homogeneous Ir coating on the C/C composites will eliminate most of the problems encountered during oxidation at high temperature, such as inward diffusion of oxygen and outward diffusion of carbon. However, the thermal expansion mismatch of Ir and C/C (Ir: 6.2 × 10−6°C−1 [7] and C/C: 1.1 × 10−6°C−1 [8]) may cause ⁎ Corresponding author. Tel.: +86 731 84573180; fax: +86 731 84576578. E-mail address: [email protected] (S. Bai). 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.07.013
cracking and even spallation of Ir coating, considering the intrinsic brittleness of Ir [9]. In addition, because of the low solubility of carbon in Ir at temperatures of most preparation processes, the Ir coating can only mechanically adheres to the C/C substrate [10]. The common solution is to apply an interlayer between the Ir coating and C/C substrate to relieve the thermal stress and enhance the interface cohesion. However the interlayer material should be compatible with both Ir and C/C chemically and physically. Refractory metal rhenium (Re) is a potential candidate of the interlayer material. Because both Ir and C could dissolve into Re to form a solid solution phase instead of a brittle compound, the bonding force between Ir and C/C is increased. Although the CTE of Re (6.8 × 10−6°C−1 [8]) is close to that of Ir, the plasticity of the Re metal produced by chemical vapor deposition (CVD) is good. The utmost elongation of CVD Re at room temperature was reported to be over 15% [11,12]. Therefore, the thermal stress may be released through the plastic deformation of the interlayer. Rovang et al. [13,14] selected the CVD Re coating as the interlayer between the C/C composites and niobium coating, through which the gradation in CTE mismatch is established. This Re layer can carry a portion of the induced stress load, improving the coating adhesion and providing a carbon diffusion barrier. Hirai et al. [15] prepared a Re/W multilayer interlayer between the W coating and 2D-C/C composite substrate by physical vapor deposition (PVD). The soft Re
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Fig. 3. XRD pattern of the CVD Re coating.
top coating were prepared on C/C substrate by CVD and electrodeposition in molten salt, forming an Ir/Re/C/C composites. The improvement of the adhesion and thermal mismatch relief by applying the Re interlayer between Ir coating and C/C substrate were analyzed by a comparative study on the specimens of Ir/C/C and Ir/Re/C/C. Moreover, the wettability of the Re interlayer by molten salt was examined by measuring the contact angles of molten salt on the surface of Re coated C/C composites. Fig. 1. Schematic diagram of the apparatus for electrodeposition of Ir coating in molten salt.
2. Experimental coating is considered to provide a compliant interface between the C/ C and hard W layers in the coating system. Truszkowska et al. [16] proposed to use a Re coating as the interlayer between the C/C composite substrate and the W-Re focal track layer to fabricate a novel rotating anode for X-ray tube, for the purpose of relieving the thermal expansion mismatch stress between the C/C substrate and W–Re coatings. However, the effects of the Re interlayer between the C/C substrate and refractory coating on the adhesion and thermal stress relief have not been fully understood. Furthermore, as the Ir coating is produced by electrodeposition in molten salt, the wettability of Re interlayer by molten salt is very important for the quality of the Ir coating. Thus, it is necessary to examine the adhesion of Re interlayer on the C/C substrate and its wettability by molten salt. We have prepared a smooth, compact and adhesive Ir coating on a Re sheet by electrodeposition in chloride molten salt [17], and also discussed the deposition of a uniform Re coating on the graphite substrate by CVD [18]. Using the same method, the Re interlayer and Ir
2.1. CVD process of Re coating The 3D fine-woven-penetrated C/C composites (density: 1.85 g/cm3), with two different sizes of 20 mm × 15 mm × 3 mm and Φ25 mm × 3 mm, were used as the substrates in this study. Before the CVD process, the C/C substrates were polished using metallographic abrasive papers, ultrasonically cleaned in acetone and degassed at 1500°C for 1 h in a furnace vacuumed to the pressure below 3 × 10−3 Pa. The Re coating was prepared by thermal decomposition of ReCl5 produced by in-situ chlorination of Re powder (purity: 99.999%, Zhuzhou Kete Industries Co., Ltd). The details of the CVD device were reported in our previous work [18]. The CVD conditions were as follows: chlorination temperature 730°C; deposition temperature 1150°C; flow rate of chlorine (purity: 99.999%) 90 mL/min; flow rate of argon (purity: 99.999%) 500 mL/min; total pressure 30 kPa and deposition time 1 to 1.5 h. The deposition rate of Re coating under this condition is 35 ~ 40 μm/h.
Fig. 2. SEM micrographs of (a) top surface and (b) fracture cross section of the CVD Re coating.
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Table 1 Coating-pull-off test results. Sample
Ultimate tensile stress/MPa
Failure position
As-deposited Annealed (1600°C/1 h)
15.6 ± 1.9 16.1 ± 1.6
Adhesive/coating interface Adhesive/coating interface
the contact angle of molten salt on the Re/C/C substrates (Φ25 mm × 3 mm), which was measured by DSAHT (drop shape analysis system for high temperature) designed by Krüss Ltd., Germany. The salts were heated and melted under argon atmosphere at a heating rate of 15°C/min. 2.4. Characterization of the coating
2.2. Electrodeposition of Ir coating A schematic diagram of the apparatus for electrodeposition of Ir coating in molten salt is shown in Fig. 1. The electrodeposition was performed in a quartz chamber sealed by a rubber stopper. The electrodes and thermal couple fixed on the rubber stopper could be moved up and down without gas leakage. The electrolyte stored in a graphite crucible was heated in a vertical resistance furnace. The atmosphere above the molten salt was maintained by continuous blowing of the high-purity argon (99.999%) into the quartz chamber. The tail gas was treated by passing it through a saturated NaOH solution before emitted into air. A C/C substrate (or Re/C/C substrate) and two graphite plates (50 mm × 40 mm × 5 mm) were used as the cathode and anode, respectively (see Fig. 1). The cathode sample, connected to a carbon fiber, was immersed into the molten salt, while each of the anode plates, tied to a Fe–Cr alloy wire, was partly dipped into the electrolyte with a dipping area of ~25 cm2. The salts used for making electrolyte bath were dried at 150°C in a vacuum chamber for 5 h before being mixed and melted. The Ir coating was electrodeposited on the C/C (or Re/C/C) substrate in a ternary eutectic molten salt of NaCl–KCl–CsCl (29.8, 29.8 and 40.4 mol.%, respectively) containing 1.9 mol.% Ir ion at 580°C under the argon atmosphere. The Ir ions were introduced by adding iridium trichloride (purity: 99.99%, Shaanxi Kaida Chemical Engineering Co., Ltd) directly into the ternary molten salt solvent at room temperature. The current density of 40 mA/cm2 was used for 1 h to deposit all Ir coatings. 2.3. Testing procedures The adhesion of the coatings was examined by coating-pull-off and micro scratch tests. The coating-pull-off test was performed according to the standard of ISO 4624: 2002(E) [19], with a pulling speed of 0.5 mm/min. The micro scratch test was conducted using a micro scratch tester (CSM Ltd., Switzerland) with a diamond stylus (120° cone with a 200 μm diameter tip) under a continuously increasing load. The loading rate was 30 N/min and the scratch rate was 10 mm/ min. The test was terminated at the maximum load of 30 N. The microhardness (Vickers hardness) of the Re coating was measured by a microhardness tester (HXD-1000TC) with 50 g load. The wettability of the Re interlayer by molten salt was examined by determining
The morphologies of the Re and Ir coatings and coating/substrate interface were observed by Hitachi S-4800 scanning electron microscope (SEM) and FEI Quanta 200F environmental scanning electron microscope (ESEM). The chemical compositions of the residual inclusions at the interfaces were examined by energy dispersive spectroscopy (EDS). The phase identifications and growth directions of the Re and Ir coatings were conducted by X-ray diffraction (XRD, Rigaku D/ Max 2550VB+) using Ni-filtered Cu Kα radiation at a scanning rate of 5°/min and 2θ angle from 30° to 85°. 3. Results and discussion Fig. 2 shows the SEM micrographs of top surface and fracture cross section of the Re coating produced by CVD process. The Re coating was composed of columnar grains throughout the coating thickness with a flat topped tower-like surface topography. The XRD pattern of the Re coating is shown in Fig. 3, indicating a highly textured, ploycrystalline structure of the coating. The preferred growth direction of the Re coating could be determined by comparing the texture coefficients (TC(hkl)) of different crystal planes in Re coating. The texture coefficient of a crystal plane is calculated by comparing the measured intensity of the crystal plane with the corresponding recorded intensity in the Joint Committee on Powder Diffraction Standards (JCPDS) data file [20]. The higher the value of TC(hkl) is, the stronger the preferred orientation trend of b hklN is. From the calculation, TC(002), TC(101), TC(102) and TC(103) are 2.8117, 0.1021, 0.7395 and 0.3467 respectively, which indicate that the preferred orientation of the coating was b002N. Fine cracks can be seen in some regions, which have been discussed and reported elsewhere [21]. It was believed that the cracking is caused by the thermal stress generated due to thermal expansion mismatch between the substrate and coating during cooling of the coating deposited at high temperature. When the substrate is much thicker than the coating, the thermal stress σ could be estimated by the following formula [22]. In the present case, the thicknesses of the substrate and coating are 3 mm and 35 μm, respectively.
σ¼
Ec ðα s −α c Þ ΔT ð1−υc Þ
ð1Þ
Fig. 4. Microhardness measurement on the cross section of Re coatings annealed at 1600°C for different time: (a) measurement areas (Area 1 is adjacent to the C/C substrate and Area 2 is near the coating top surface.) and (b) measurement result.
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Fig. 5. Paving status of molten salt on (a) C/C and (b) Re/C/C substrates at 500°C.
where Ec and υc are the Young's modulus (460 GPa [23]) and Poisson's ratio (0.26 [24]) of the Re coating, respectively, αs and αc are the CTE of the C/C substrate (1.0 to 2.0 × 10−6°C−1 [25]) and Re coating (6.8 × 10−6°C−1 [26]), respectively; and ΔT is the temperature change from deposition temperature (1150°C) to room temperature. According to Eq. (1), the as-deposited Re coating suffers an average residual tensile stress of about 3360 to 4060 MPa, which is far beyond its ultimate strength (670 MPa [23]). Furthermore, the anisotropy of C/C's microstructure causes even larger difference in CTE between the Re coating and C/C substrate in some particular areas [27], e.g., the interfaces between the fiber bundles and carbon matrices. Therefore, many fine regular cracking occurred in the Re coating on C/C substrate. It seems that the cracking could not be eliminated by plastic deformation of the Re interlayer. The coating-pull-off test was performed on the as-deposited and vacuum annealed Re/C/C samples. The results are shown in Table 1. It was found that the rupture occurred at the adhesive/coating interface for both kinds of samples with ultimate tensile stresses of 15.6
and 16.1 MPa. It indicated that the bond strength between the Re coating and C/C substrate was higher than 15 MPa before and after annealing treatment. The excellent bonding between the Re coating and C/C substrate is attributed to the high reactivity and good compatibility between Re coating and C/C substrate. It was reported that C atoms starts to dissolve in Re at 527°C with an activation energy of ~ 2.0 eV [28]. Moreover, C atoms in Re exist in solid solution state rather than forming a brittle compound according to the Re–C phase diagram [29]. Thus, the bond strength of the as-deposited Re coating on C/C composites is quite good and would not deteriorate after high temperature annealing. Fig. 4 shows the microhardness that was measured in two areas on the cross section of the Re coatings annealed at 1600°C for different periods of time. Area 1 is adjacent to the C/C substrate and Area 2 is near the coating top surface. It was found that the hardness of the as-deposited Re coating was location dependent. The coating in Area 1 was harder than the coating in Area 2 (see Fig. 4 (b)). After annealing at 1600°C for 1 h, the hardness in the two areas of the coating became
Fig. 6. SEM micrographs of Ir coatings on (a), (b) C/C substrate, and (c), (d) Re/C/C substrate.
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Fig. 7. XRD patterns of the Ir coatings on (a) C/C and (b) Re/C/C substrates.
Table 2 Texture coefficients of the Ir coatings on different substrates. Substrate
TC(111)
TC(200)
TC(220)
TC(311)
C/C Re/C/C
0.5646 1.1285
0.0668 0.7809
2.3233 0.4744
1.0453 1.6163
the same, and remained unchanged with prolonged annealing. This indicates that C atoms diffuse quickly into the Re coating within an hour, reaching to the saturation and maximum hardness. Li and Zee [30] investigated the interdiffusion of Re and C at high temperatures and found that C prefers to diffuse into Re along its grain boundaries at low temperature and through lattice at temperature above 1500°C. Thus during annealing at 1600°C, C diffuses into the Re coating through both grain boundaries and lattices. The C diffusion, on one hand, enhances the bonding between the C/C substrate and Re coating, on the other hand, decreases the ductility of the Re coating, and thus weakens its potential for relieving the thermal stress. Therefore, the adhesion of Re interlayer on C/C composites is not significantly affected by the high-temperature environment. But meanwhile, the embrittlement of Re interlayer caused by diffusion of C element should be paid attention to. The paving status and contact angle of molten salt on C/C and Re/ C/C substrates at 500°C are shown in Fig. 5. To lower the effect of volatilization of molten salt on the quartz window for viewing, the measurement temperature was set a little higher than the melting point of the molten salt (~ 480°C). The contact angles of molten salt on the C/C and Re/C/C substrates were 128.5° and 43.4°, respectively. The paving status of molten salt turned from non-wetting on the C/ C substrate to good wetting on the Re/C/C substrate, indicating that
a remarkable wettability improvement of the molten salt has been achieved by applying the Re coating on C/C substrate. The wettability enhancement might be due to the different surface energy of the Re coating and C/C composites. The metal Re has a higher surface energy due to its strong metallic bond, while the C/C composites have a lower surface energy because of the weak molecular bond between the sheets of carbon atoms. It is known that the liquid generally has a low contact angle on the substance with a high surface energy, which indicates a better wetting between them. Generally, the wettability of the substrate by the electrolyte is closely related to the quality of electrodeposited coating. The poor wetting of the substrate usually leads to the coating defects such as pinholes and delamination. Therefore, better wetting of Re interlayer by the molten salt could improve the quality of the electrodeposited Ir coating. The surface morphologies and XRD patterns of the Ir coatings on the C/C and Re/C/C substrates are shown in Figs. 6 and 7. The surface of Ir coating on C/C substrate was relatively uniform, while the Ir coating on Re/C/C substrate showed the morphology that exactly copied the surface of the C/C composites (see Fig. 6 (a) and (c)). Some randomly distributed coarse cracks and pinholes were observed in the Ir coating on the C/C substrate. By contrast, many regular microcracks and holes, located at the interfaces between the fiber bundles and carbon matrices of the C/C composites, can be seen in the Ir coating on the Re/C/C substrate. Observation at a high magnification (Fig. 6 (b) and (d)) revealed the changing topographies of the Ir coating from taper shaped grains with straight edges on the C/C substrate to faceted grains on the Re/C/C substrate. The preferred orientations of the Ir coatings on two substrates were quite different as shown in Fig. 7. The calculated texture coefficients of the Ir coatings on different substrates are shown in Table 2. It could be concluded that the Ir coating on the C/C substrate had a noteworthy preferred growth direction of b 220N, while the Ir coating on the Re/C/ C substrate only had a slightly preferred orientation of b 311N. The SEM micrographs of the Ir coatings on the C/C and Re/C/C substrates after micro scratch test are shown in Fig. 8. The Ir coating on C/ C substrate broke up and flaked off near a crack when the stylus scratched across the crack. In contrast, the Ir coating on the Re/C/C substrate remained intact near a crack after the same scratch test (see Fig. 8 (b)). It indicated that a better adhesion of Ir coating on C/C substrate can be obtained when a Re interlayer is applied. The arc cracking occurred inside the scratch track for Ir coatings on both substrates, which may be attributed to the well-known intrinsic brittleness of Ir. The coating-pull-off test results of the as-deposited and annealed (1600°C/1 h) Ir coatings on the C/C and Re/C/C substrates are shown in Fig. 9, exhibiting the typical fracture surfaces on the lower puller end of the corresponding tests. The as-deposited Ir coating on
Fig. 8. SEM micrographs of the Ir coatings on (a) C/C and (b) Re/C/C substrates after micro scratch test.
L. Zhu et al. / Surface & Coatings Technology 235 (2013) 68–74
Fig. 9. The bond strength of as-deposited and annealed Ir coatings on different substrates.
C/C substrate was completely pulled off with a bond strength of 3.2 MPa, together with a small amount of non-uniformly distributed substrate material. As for the as-deposited Ir coating on the Re/C/C substrate, the bond strength was 7.9 MPa with the debonding between the Re interlayer and C/C substrate. It was noticed that an
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extensive amount of material in the superficial zone of the C/C substrate was pulled off together with the Ir/Re coating, indicating that the Re interlayer significantly improves the adhesion of the Ir coating. However, the bonding between Re interlayer and C/C substrate got weakened after applying the Ir coating compared with the above results (N 15 MPa) listed in Table 1. After high temperature annealing treatment, the bond strength of the Ir coatings on both types of substrates decreased significantly (~ 50%), and the amount of the substrate material stuck on the pulled-off coatings reduced notably (Fig. 9). It indicates that special events occurs on Re interlayer and underneath C/C substrate during electrodeposition of the Ir coating, which deteriorates the bond strength between the Re interlayer and C/C substrate. Considering the fracture position, it is probably due to the infiltration of molten salt into the superficial zone of C/C substrate and its effect on the coating/substrate interface. The effect of molten salt infiltration is further enlarged during the later annealing process. Studies have shown that liquids spontaneously penetrate into porous solid below a critical contact angle, which is related to the substrate pore structure [31,32]. For the C/C substrate, with plenty of pores and cracks inside, the critical contact angle is expected to be higher than that of the Re/C/C substrate with less pores and cracks. Taking both the critical contact angle and wettability of molten salt on the substrates into consideration, the C/C substrate (poor wettability but high critical contact angle) and Re/C/C substrate (low critical contact angle but good wettability) can both be infiltrated easily by the molten salt.
Fig. 10. SEM micrographs and EDS analyses of the fracture surfaces of (a) Ir/C/C and (b) Ir/Re/C/C samples after coating-pull-off test.
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Fig. 10 shows the SEM micrographs and EDS analyses on the fracture surfaces of the as-deposited Ir/C/C and Ir/Re/C/C samples after coating-pull-off test. The residual salts were detected in both samples, as shown in Fig. 10, on the surfaces of the C/C substrates, which confirm the above reasoning. Furthermore, it seemed that the salts were mainly distributed in the zones of the fiber bundles perpendicular to the substrate surface in C/C substrate. Such special distribution of the residual salts may be due to the gaps and cracks between the carbon fibers and adjacent pyrolytic carbon within the fiber bundles, which offer the convenient infiltration channels for the molten salt. However, the mechanism of the effect of molten salt on the C/C substrate and coating/substrate interface was not clear yet. More detailed studies are needed to verify above conclusions. The Re interlayer significantly improves the adhesion of Ir coating on C/C substrate. Unfortunately, the molten salt infiltration is not effectively prevented due to the cracked Re interlayer. Therefore, preparing crack-free Re interlayer on the C/C substrate is a key issue of future research in order to solve this problem. 4. Conclusions (1) The Re coating produced on C/C substrate by CVD was composed of b002 N oriented columnar grains with a flat topped tower-like surface topography. Many fine regular cracks were found in the Re coating due to the CTE mismatch between the Re coating and C/C substrate. (2) The bond strength between the Re coating and C/C substrate was higher than 15 MPa before and after annealing treatment (1600°C/1 h). The excellent bonding between the Re coating and C/C substrate is attributed to the high reactivity and good compatibility of Re with C/C composites. (3) The contact angles of the molten salt on the C/C and Re/C/C substrates at 500°C were 128.5° and 43.4°, respectively. Hence, applying a Re interlayer on the C/C substrate can significantly improve the wettability by molten salt, and thus the quality of the Ir coating produced by electrodeposition in molten salt. (4) The morphology and preferred growth direction of the Ir coating on Re/C/C substrate were quite different from those on the C/C substrate. The micro scratch test and coating-pull-off test confirm that a better adhesion of the Ir coating on C/C substrate can be obtained by Re interlayer. The coating bond strength of the as-deposited samples increased from 3.2 MPa for Ir/C/C to 7.9 MPa for Ir/Re/C/C. After an annealing at 1600°C for 1 h, a reduction of ~50% in the bond strengths for both kinds of samples was observed. (5) The bond strength between the Re interlayer and C/C substrate was weakened after electrodeposition of Ir coating. This is prob-
ably due to the infiltration of molten salt into the superficial zone of the C/C substrate and its effect on the coating/substrate interface. Acknowledgment This work has been supported by Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province. The authors gratefully acknowledge Dr. Jie Zeng for helping in the contact angle measurement of molten salt and Catherine Hobbis for helping in ESEM characterization. We would also like to thank the financial support of China Scholarship Council (CSC) and Dr. Tianping Zhu for proofreading. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
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Rhenium used as an interlayer between carbon–carbon composites and iridium coating: Adhesion and wettability Li'an Zhu a, Shuxin Bai a,⁎, Hong Zhang a, Yicong Ye a, Wei Gao b a b
Department of Materials Science and Engineering, College of Aerospace Science and Engineering, National University of Defence Technology, Changsha 410073, PR China Department of Chemical and Materials Engineering, The University of Auckland, PB 92019, Auckland 1142, New Zealand
a r t i c l e
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Article history: Received 20 April 2013 Accepted in revised form 4 July 2013 Available online 12 July 2013 Keywords: Rhenium coating Iridium coating Carbon/carbon composites Bond strength Wettability
a b s t r a c t Iridium (Ir) is one of the most promising materials as a protective coating on carbon/carbon (C/C) composites for very high temperature applications. However, the poor adhesion and thermal stress induced by thermal expansion mismatch between Ir coating and C/C substrate restrict their application. Rhenium (Re) has been selected as the interlayer material between the Ir coating and C/C substrate which can improve the adhesion of Ir coating and relieve the thermal stress. The Re coating was prepared on the C/C substrate by chemical vapor deposition (CVD). The Ir coating was electrodeposited on the C/C and Re/C/C substrates in molten salt. The morphology and microstructure of the Re and Ir coatings were studied by SEM and XRD. The bond strength of the coatings on C/C substrate was measured by coating-pull-off test. The molten salt wettability on the C/C and Re/C/C substrates was determined by drop shape method. The results showed that the CVD Re coating on the C/C substrate had a preferential orientation of b002 N with many fine regular cracks. The bond strength between the Re coating and C/C substrate was higher than 15 MPa before and after annealing treatment. The contact angles of molten salt on C/C and Re/C/C substrates at 500°C were 128.5° and 43.4°, respectively. The bond strength of as-deposited coatings increased from 3.2 MPa for Ir/C/C to 7.9 MPa for Ir/Re/C/C. After high temperature annealing treatment, however, the bond strength has been reduced by ~ 50%, probably due to the infiltration of molten salt into the C/C substrate. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Carbon/carbon (C/C) composites have received much attention in aerospace applications due to their high specific strength and modulus, high fracture toughness and thermal conductivity, low coefficient of thermal expansion (CTE), good thermal shock resistance, especially their excellent high-temperature strength up to 2200°C in oxygenfree environment [1,2]. However, C/C composites readily oxidize above 370°C and lose their strength with increasing temperature, which restricts their high-temperature applications [3]. Thus the oxidation protection of C/C composites is the key issue to their extensive use. Iridium (Ir) is considered as a promising material as an oxidation resistant coating for carbonaceous substrates due to its high melting point, excellent chemical stability, low oxygen permeability, good chemical compatibility and low carbon solubility below the eutectic temperature of 2100–2300°C [4–6]. Good adhesion of a compact and homogeneous Ir coating on the C/C composites will eliminate most of the problems encountered during oxidation at high temperature, such as inward diffusion of oxygen and outward diffusion of carbon. However, the thermal expansion mismatch of Ir and C/C (Ir: 6.2 × 10−6°C−1 [7] and C/C: 1.1 × 10−6°C−1 [8]) may cause ⁎ Corresponding author. Tel.: +86 731 84573180; fax: +86 731 84576578. E-mail address: [email protected] (S. Bai). 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.07.013
cracking and even spallation of Ir coating, considering the intrinsic brittleness of Ir [9]. In addition, because of the low solubility of carbon in Ir at temperatures of most preparation processes, the Ir coating can only mechanically adheres to the C/C substrate [10]. The common solution is to apply an interlayer between the Ir coating and C/C substrate to relieve the thermal stress and enhance the interface cohesion. However the interlayer material should be compatible with both Ir and C/C chemically and physically. Refractory metal rhenium (Re) is a potential candidate of the interlayer material. Because both Ir and C could dissolve into Re to form a solid solution phase instead of a brittle compound, the bonding force between Ir and C/C is increased. Although the CTE of Re (6.8 × 10−6°C−1 [8]) is close to that of Ir, the plasticity of the Re metal produced by chemical vapor deposition (CVD) is good. The utmost elongation of CVD Re at room temperature was reported to be over 15% [11,12]. Therefore, the thermal stress may be released through the plastic deformation of the interlayer. Rovang et al. [13,14] selected the CVD Re coating as the interlayer between the C/C composites and niobium coating, through which the gradation in CTE mismatch is established. This Re layer can carry a portion of the induced stress load, improving the coating adhesion and providing a carbon diffusion barrier. Hirai et al. [15] prepared a Re/W multilayer interlayer between the W coating and 2D-C/C composite substrate by physical vapor deposition (PVD). The soft Re
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Fig. 3. XRD pattern of the CVD Re coating.
top coating were prepared on C/C substrate by CVD and electrodeposition in molten salt, forming an Ir/Re/C/C composites. The improvement of the adhesion and thermal mismatch relief by applying the Re interlayer between Ir coating and C/C substrate were analyzed by a comparative study on the specimens of Ir/C/C and Ir/Re/C/C. Moreover, the wettability of the Re interlayer by molten salt was examined by measuring the contact angles of molten salt on the surface of Re coated C/C composites. Fig. 1. Schematic diagram of the apparatus for electrodeposition of Ir coating in molten salt.
2. Experimental coating is considered to provide a compliant interface between the C/ C and hard W layers in the coating system. Truszkowska et al. [16] proposed to use a Re coating as the interlayer between the C/C composite substrate and the W-Re focal track layer to fabricate a novel rotating anode for X-ray tube, for the purpose of relieving the thermal expansion mismatch stress between the C/C substrate and W–Re coatings. However, the effects of the Re interlayer between the C/C substrate and refractory coating on the adhesion and thermal stress relief have not been fully understood. Furthermore, as the Ir coating is produced by electrodeposition in molten salt, the wettability of Re interlayer by molten salt is very important for the quality of the Ir coating. Thus, it is necessary to examine the adhesion of Re interlayer on the C/C substrate and its wettability by molten salt. We have prepared a smooth, compact and adhesive Ir coating on a Re sheet by electrodeposition in chloride molten salt [17], and also discussed the deposition of a uniform Re coating on the graphite substrate by CVD [18]. Using the same method, the Re interlayer and Ir
2.1. CVD process of Re coating The 3D fine-woven-penetrated C/C composites (density: 1.85 g/cm3), with two different sizes of 20 mm × 15 mm × 3 mm and Φ25 mm × 3 mm, were used as the substrates in this study. Before the CVD process, the C/C substrates were polished using metallographic abrasive papers, ultrasonically cleaned in acetone and degassed at 1500°C for 1 h in a furnace vacuumed to the pressure below 3 × 10−3 Pa. The Re coating was prepared by thermal decomposition of ReCl5 produced by in-situ chlorination of Re powder (purity: 99.999%, Zhuzhou Kete Industries Co., Ltd). The details of the CVD device were reported in our previous work [18]. The CVD conditions were as follows: chlorination temperature 730°C; deposition temperature 1150°C; flow rate of chlorine (purity: 99.999%) 90 mL/min; flow rate of argon (purity: 99.999%) 500 mL/min; total pressure 30 kPa and deposition time 1 to 1.5 h. The deposition rate of Re coating under this condition is 35 ~ 40 μm/h.
Fig. 2. SEM micrographs of (a) top surface and (b) fracture cross section of the CVD Re coating.
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Table 1 Coating-pull-off test results. Sample
Ultimate tensile stress/MPa
Failure position
As-deposited Annealed (1600°C/1 h)
15.6 ± 1.9 16.1 ± 1.6
Adhesive/coating interface Adhesive/coating interface
the contact angle of molten salt on the Re/C/C substrates (Φ25 mm × 3 mm), which was measured by DSAHT (drop shape analysis system for high temperature) designed by Krüss Ltd., Germany. The salts were heated and melted under argon atmosphere at a heating rate of 15°C/min. 2.4. Characterization of the coating
2.2. Electrodeposition of Ir coating A schematic diagram of the apparatus for electrodeposition of Ir coating in molten salt is shown in Fig. 1. The electrodeposition was performed in a quartz chamber sealed by a rubber stopper. The electrodes and thermal couple fixed on the rubber stopper could be moved up and down without gas leakage. The electrolyte stored in a graphite crucible was heated in a vertical resistance furnace. The atmosphere above the molten salt was maintained by continuous blowing of the high-purity argon (99.999%) into the quartz chamber. The tail gas was treated by passing it through a saturated NaOH solution before emitted into air. A C/C substrate (or Re/C/C substrate) and two graphite plates (50 mm × 40 mm × 5 mm) were used as the cathode and anode, respectively (see Fig. 1). The cathode sample, connected to a carbon fiber, was immersed into the molten salt, while each of the anode plates, tied to a Fe–Cr alloy wire, was partly dipped into the electrolyte with a dipping area of ~25 cm2. The salts used for making electrolyte bath were dried at 150°C in a vacuum chamber for 5 h before being mixed and melted. The Ir coating was electrodeposited on the C/C (or Re/C/C) substrate in a ternary eutectic molten salt of NaCl–KCl–CsCl (29.8, 29.8 and 40.4 mol.%, respectively) containing 1.9 mol.% Ir ion at 580°C under the argon atmosphere. The Ir ions were introduced by adding iridium trichloride (purity: 99.99%, Shaanxi Kaida Chemical Engineering Co., Ltd) directly into the ternary molten salt solvent at room temperature. The current density of 40 mA/cm2 was used for 1 h to deposit all Ir coatings. 2.3. Testing procedures The adhesion of the coatings was examined by coating-pull-off and micro scratch tests. The coating-pull-off test was performed according to the standard of ISO 4624: 2002(E) [19], with a pulling speed of 0.5 mm/min. The micro scratch test was conducted using a micro scratch tester (CSM Ltd., Switzerland) with a diamond stylus (120° cone with a 200 μm diameter tip) under a continuously increasing load. The loading rate was 30 N/min and the scratch rate was 10 mm/ min. The test was terminated at the maximum load of 30 N. The microhardness (Vickers hardness) of the Re coating was measured by a microhardness tester (HXD-1000TC) with 50 g load. The wettability of the Re interlayer by molten salt was examined by determining
The morphologies of the Re and Ir coatings and coating/substrate interface were observed by Hitachi S-4800 scanning electron microscope (SEM) and FEI Quanta 200F environmental scanning electron microscope (ESEM). The chemical compositions of the residual inclusions at the interfaces were examined by energy dispersive spectroscopy (EDS). The phase identifications and growth directions of the Re and Ir coatings were conducted by X-ray diffraction (XRD, Rigaku D/ Max 2550VB+) using Ni-filtered Cu Kα radiation at a scanning rate of 5°/min and 2θ angle from 30° to 85°. 3. Results and discussion Fig. 2 shows the SEM micrographs of top surface and fracture cross section of the Re coating produced by CVD process. The Re coating was composed of columnar grains throughout the coating thickness with a flat topped tower-like surface topography. The XRD pattern of the Re coating is shown in Fig. 3, indicating a highly textured, ploycrystalline structure of the coating. The preferred growth direction of the Re coating could be determined by comparing the texture coefficients (TC(hkl)) of different crystal planes in Re coating. The texture coefficient of a crystal plane is calculated by comparing the measured intensity of the crystal plane with the corresponding recorded intensity in the Joint Committee on Powder Diffraction Standards (JCPDS) data file [20]. The higher the value of TC(hkl) is, the stronger the preferred orientation trend of b hklN is. From the calculation, TC(002), TC(101), TC(102) and TC(103) are 2.8117, 0.1021, 0.7395 and 0.3467 respectively, which indicate that the preferred orientation of the coating was b002N. Fine cracks can be seen in some regions, which have been discussed and reported elsewhere [21]. It was believed that the cracking is caused by the thermal stress generated due to thermal expansion mismatch between the substrate and coating during cooling of the coating deposited at high temperature. When the substrate is much thicker than the coating, the thermal stress σ could be estimated by the following formula [22]. In the present case, the thicknesses of the substrate and coating are 3 mm and 35 μm, respectively.
σ¼
Ec ðα s −α c Þ ΔT ð1−υc Þ
ð1Þ
Fig. 4. Microhardness measurement on the cross section of Re coatings annealed at 1600°C for different time: (a) measurement areas (Area 1 is adjacent to the C/C substrate and Area 2 is near the coating top surface.) and (b) measurement result.
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Fig. 5. Paving status of molten salt on (a) C/C and (b) Re/C/C substrates at 500°C.
where Ec and υc are the Young's modulus (460 GPa [23]) and Poisson's ratio (0.26 [24]) of the Re coating, respectively, αs and αc are the CTE of the C/C substrate (1.0 to 2.0 × 10−6°C−1 [25]) and Re coating (6.8 × 10−6°C−1 [26]), respectively; and ΔT is the temperature change from deposition temperature (1150°C) to room temperature. According to Eq. (1), the as-deposited Re coating suffers an average residual tensile stress of about 3360 to 4060 MPa, which is far beyond its ultimate strength (670 MPa [23]). Furthermore, the anisotropy of C/C's microstructure causes even larger difference in CTE between the Re coating and C/C substrate in some particular areas [27], e.g., the interfaces between the fiber bundles and carbon matrices. Therefore, many fine regular cracking occurred in the Re coating on C/C substrate. It seems that the cracking could not be eliminated by plastic deformation of the Re interlayer. The coating-pull-off test was performed on the as-deposited and vacuum annealed Re/C/C samples. The results are shown in Table 1. It was found that the rupture occurred at the adhesive/coating interface for both kinds of samples with ultimate tensile stresses of 15.6
and 16.1 MPa. It indicated that the bond strength between the Re coating and C/C substrate was higher than 15 MPa before and after annealing treatment. The excellent bonding between the Re coating and C/C substrate is attributed to the high reactivity and good compatibility between Re coating and C/C substrate. It was reported that C atoms starts to dissolve in Re at 527°C with an activation energy of ~ 2.0 eV [28]. Moreover, C atoms in Re exist in solid solution state rather than forming a brittle compound according to the Re–C phase diagram [29]. Thus, the bond strength of the as-deposited Re coating on C/C composites is quite good and would not deteriorate after high temperature annealing. Fig. 4 shows the microhardness that was measured in two areas on the cross section of the Re coatings annealed at 1600°C for different periods of time. Area 1 is adjacent to the C/C substrate and Area 2 is near the coating top surface. It was found that the hardness of the as-deposited Re coating was location dependent. The coating in Area 1 was harder than the coating in Area 2 (see Fig. 4 (b)). After annealing at 1600°C for 1 h, the hardness in the two areas of the coating became
Fig. 6. SEM micrographs of Ir coatings on (a), (b) C/C substrate, and (c), (d) Re/C/C substrate.
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Fig. 7. XRD patterns of the Ir coatings on (a) C/C and (b) Re/C/C substrates.
Table 2 Texture coefficients of the Ir coatings on different substrates. Substrate
TC(111)
TC(200)
TC(220)
TC(311)
C/C Re/C/C
0.5646 1.1285
0.0668 0.7809
2.3233 0.4744
1.0453 1.6163
the same, and remained unchanged with prolonged annealing. This indicates that C atoms diffuse quickly into the Re coating within an hour, reaching to the saturation and maximum hardness. Li and Zee [30] investigated the interdiffusion of Re and C at high temperatures and found that C prefers to diffuse into Re along its grain boundaries at low temperature and through lattice at temperature above 1500°C. Thus during annealing at 1600°C, C diffuses into the Re coating through both grain boundaries and lattices. The C diffusion, on one hand, enhances the bonding between the C/C substrate and Re coating, on the other hand, decreases the ductility of the Re coating, and thus weakens its potential for relieving the thermal stress. Therefore, the adhesion of Re interlayer on C/C composites is not significantly affected by the high-temperature environment. But meanwhile, the embrittlement of Re interlayer caused by diffusion of C element should be paid attention to. The paving status and contact angle of molten salt on C/C and Re/ C/C substrates at 500°C are shown in Fig. 5. To lower the effect of volatilization of molten salt on the quartz window for viewing, the measurement temperature was set a little higher than the melting point of the molten salt (~ 480°C). The contact angles of molten salt on the C/C and Re/C/C substrates were 128.5° and 43.4°, respectively. The paving status of molten salt turned from non-wetting on the C/ C substrate to good wetting on the Re/C/C substrate, indicating that
a remarkable wettability improvement of the molten salt has been achieved by applying the Re coating on C/C substrate. The wettability enhancement might be due to the different surface energy of the Re coating and C/C composites. The metal Re has a higher surface energy due to its strong metallic bond, while the C/C composites have a lower surface energy because of the weak molecular bond between the sheets of carbon atoms. It is known that the liquid generally has a low contact angle on the substance with a high surface energy, which indicates a better wetting between them. Generally, the wettability of the substrate by the electrolyte is closely related to the quality of electrodeposited coating. The poor wetting of the substrate usually leads to the coating defects such as pinholes and delamination. Therefore, better wetting of Re interlayer by the molten salt could improve the quality of the electrodeposited Ir coating. The surface morphologies and XRD patterns of the Ir coatings on the C/C and Re/C/C substrates are shown in Figs. 6 and 7. The surface of Ir coating on C/C substrate was relatively uniform, while the Ir coating on Re/C/C substrate showed the morphology that exactly copied the surface of the C/C composites (see Fig. 6 (a) and (c)). Some randomly distributed coarse cracks and pinholes were observed in the Ir coating on the C/C substrate. By contrast, many regular microcracks and holes, located at the interfaces between the fiber bundles and carbon matrices of the C/C composites, can be seen in the Ir coating on the Re/C/C substrate. Observation at a high magnification (Fig. 6 (b) and (d)) revealed the changing topographies of the Ir coating from taper shaped grains with straight edges on the C/C substrate to faceted grains on the Re/C/C substrate. The preferred orientations of the Ir coatings on two substrates were quite different as shown in Fig. 7. The calculated texture coefficients of the Ir coatings on different substrates are shown in Table 2. It could be concluded that the Ir coating on the C/C substrate had a noteworthy preferred growth direction of b 220N, while the Ir coating on the Re/C/ C substrate only had a slightly preferred orientation of b 311N. The SEM micrographs of the Ir coatings on the C/C and Re/C/C substrates after micro scratch test are shown in Fig. 8. The Ir coating on C/ C substrate broke up and flaked off near a crack when the stylus scratched across the crack. In contrast, the Ir coating on the Re/C/C substrate remained intact near a crack after the same scratch test (see Fig. 8 (b)). It indicated that a better adhesion of Ir coating on C/C substrate can be obtained when a Re interlayer is applied. The arc cracking occurred inside the scratch track for Ir coatings on both substrates, which may be attributed to the well-known intrinsic brittleness of Ir. The coating-pull-off test results of the as-deposited and annealed (1600°C/1 h) Ir coatings on the C/C and Re/C/C substrates are shown in Fig. 9, exhibiting the typical fracture surfaces on the lower puller end of the corresponding tests. The as-deposited Ir coating on
Fig. 8. SEM micrographs of the Ir coatings on (a) C/C and (b) Re/C/C substrates after micro scratch test.
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Fig. 9. The bond strength of as-deposited and annealed Ir coatings on different substrates.
C/C substrate was completely pulled off with a bond strength of 3.2 MPa, together with a small amount of non-uniformly distributed substrate material. As for the as-deposited Ir coating on the Re/C/C substrate, the bond strength was 7.9 MPa with the debonding between the Re interlayer and C/C substrate. It was noticed that an
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extensive amount of material in the superficial zone of the C/C substrate was pulled off together with the Ir/Re coating, indicating that the Re interlayer significantly improves the adhesion of the Ir coating. However, the bonding between Re interlayer and C/C substrate got weakened after applying the Ir coating compared with the above results (N 15 MPa) listed in Table 1. After high temperature annealing treatment, the bond strength of the Ir coatings on both types of substrates decreased significantly (~ 50%), and the amount of the substrate material stuck on the pulled-off coatings reduced notably (Fig. 9). It indicates that special events occurs on Re interlayer and underneath C/C substrate during electrodeposition of the Ir coating, which deteriorates the bond strength between the Re interlayer and C/C substrate. Considering the fracture position, it is probably due to the infiltration of molten salt into the superficial zone of C/C substrate and its effect on the coating/substrate interface. The effect of molten salt infiltration is further enlarged during the later annealing process. Studies have shown that liquids spontaneously penetrate into porous solid below a critical contact angle, which is related to the substrate pore structure [31,32]. For the C/C substrate, with plenty of pores and cracks inside, the critical contact angle is expected to be higher than that of the Re/C/C substrate with less pores and cracks. Taking both the critical contact angle and wettability of molten salt on the substrates into consideration, the C/C substrate (poor wettability but high critical contact angle) and Re/C/C substrate (low critical contact angle but good wettability) can both be infiltrated easily by the molten salt.
Fig. 10. SEM micrographs and EDS analyses of the fracture surfaces of (a) Ir/C/C and (b) Ir/Re/C/C samples after coating-pull-off test.
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Fig. 10 shows the SEM micrographs and EDS analyses on the fracture surfaces of the as-deposited Ir/C/C and Ir/Re/C/C samples after coating-pull-off test. The residual salts were detected in both samples, as shown in Fig. 10, on the surfaces of the C/C substrates, which confirm the above reasoning. Furthermore, it seemed that the salts were mainly distributed in the zones of the fiber bundles perpendicular to the substrate surface in C/C substrate. Such special distribution of the residual salts may be due to the gaps and cracks between the carbon fibers and adjacent pyrolytic carbon within the fiber bundles, which offer the convenient infiltration channels for the molten salt. However, the mechanism of the effect of molten salt on the C/C substrate and coating/substrate interface was not clear yet. More detailed studies are needed to verify above conclusions. The Re interlayer significantly improves the adhesion of Ir coating on C/C substrate. Unfortunately, the molten salt infiltration is not effectively prevented due to the cracked Re interlayer. Therefore, preparing crack-free Re interlayer on the C/C substrate is a key issue of future research in order to solve this problem. 4. Conclusions (1) The Re coating produced on C/C substrate by CVD was composed of b002 N oriented columnar grains with a flat topped tower-like surface topography. Many fine regular cracks were found in the Re coating due to the CTE mismatch between the Re coating and C/C substrate. (2) The bond strength between the Re coating and C/C substrate was higher than 15 MPa before and after annealing treatment (1600°C/1 h). The excellent bonding between the Re coating and C/C substrate is attributed to the high reactivity and good compatibility of Re with C/C composites. (3) The contact angles of the molten salt on the C/C and Re/C/C substrates at 500°C were 128.5° and 43.4°, respectively. Hence, applying a Re interlayer on the C/C substrate can significantly improve the wettability by molten salt, and thus the quality of the Ir coating produced by electrodeposition in molten salt. (4) The morphology and preferred growth direction of the Ir coating on Re/C/C substrate were quite different from those on the C/C substrate. The micro scratch test and coating-pull-off test confirm that a better adhesion of the Ir coating on C/C substrate can be obtained by Re interlayer. The coating bond strength of the as-deposited samples increased from 3.2 MPa for Ir/C/C to 7.9 MPa for Ir/Re/C/C. After an annealing at 1600°C for 1 h, a reduction of ~50% in the bond strengths for both kinds of samples was observed. (5) The bond strength between the Re interlayer and C/C substrate was weakened after electrodeposition of Ir coating. This is prob-
ably due to the infiltration of molten salt into the superficial zone of the C/C substrate and its effect on the coating/substrate interface. Acknowledgment This work has been supported by Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province. The authors gratefully acknowledge Dr. Jie Zeng for helping in the contact angle measurement of molten salt and Catherine Hobbis for helping in ESEM characterization. We would also like to thank the financial support of China Scholarship Council (CSC) and Dr. Tianping Zhu for proofreading. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
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