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Active metal brazing of graphite foam-to-titanium joints made with SiC-Coated foam
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Original Article
Active metal brazing of graphite foam-to-titanium joints made with SiCCoated foam M. Singha, C.E. Smitha, R. Asthanab,*, A.L. Gyekenyesia a b
Ohio Aerospace Institute, 22800 Cedar Point Road, Cleveland, OH, 44142, USA Department of Engineering & Technology, University of Wisconsin-Stout, Menomonie, WI, 54751, USA
ARTICLE INFO
ABSTRACT
Keywords: Carbon foam Brazing Titanium Joint strength
Medium-density, high-conductivity carbon foams were joined to titanium using a two-step process that first exposed foam to SiO vapor at 1450 °C for 30 min. under vacuum followed by vacuum brazing Ti using Cusil-ABA to the sides of prismatic foam pieces along the ‘with-rise’ (WR) or foaming direction and the ‘against rise’ (AR) or transvers direction. Well-bonded joints with braze-infiltrated foam and Ti-rich interfaces formed along WR and AR. The un-bonded foam was stronger along WR (785 kPa) than along AR (277 kPa) as were the joints made using coated and uncoated foams. Foam thickness minimally affected joint strength along WR but along AR, joints with thick foam were 58 % stronger. The coating marginally (9 %) lowered joint strength along WR but led to a nearly 50 % strength drop along AR. The experimental foam is more robust and amenable to coating and joining along foaming direction than transverse to it.
1. Introduction Porous materials such as carbon foams have generated considerable interest for thermal management and energy storage applications [1–3]. Foams can be designed for density, pore size, conductivity and other attributes for multifunctional applications. Carbon foams offer benefits such as low density, large surface area, high ligament conductivity (> 1700 W/m.K) and high bulk conductivity (∼175 W/m.K) besides chemical inertness and thermal stability. Thermal management and energy storage applications require graphite foams to be integrated with other materials for effective heat transfer. Carbon foams have been soldered to metals such as aluminum for low (< 300 °C) use-temperatures [4]; however, to our knowledge, brazing of foam-to-metal joints for higher usetemperatures at which the large mismatch in the coefficients of thermal expansion (CTE) of bonded materials could generate severe thermal stresses, has not been demonstrated. Although high-temperature brazing of various carbon-based materials (e.g., carboncarbon and carbon-silicon carbide) to metals such as Ti, Inconel, and Cu-clad-Mo has been demonstrated [5–11], integration of foams, particularly, new and emerging varieties of highly conductive graphitic foams, via brazing for elevated temperature use has, to our knowledge, not been explored.
In a recent study [12], the effect of density of GTIH graphite foams®1 [3] on the tensile strength of foam/Ti joints brazed using a Ag-Cu-Ti alloy was evaluated. The tests revealed that joints were stronger than the foams (failure occurred within foam rather than in joint), and the strength increased with increasing foam density. GTIH foams are grown using a proprietary procedure that allows different faces of the grown material to be identified as either ‘with-rise’ (WR), which is the direction of foaming in the manufacturing process, or against rise (AR), which is the direction transverse to foaming direction. In a previous study [12], foams sectioned at arbitrary orientations to expose random surfaces were brazed to titanium and the effect, if any, of growth direction of foam on joint strength was not considered. Unpublished data produced by some of the present authors has indicated large scatter regarding the tensile strengths of unbonded foams, especially in tests oriented in the WR direction where density variations are more pronounced. The absolute values of the tensile strength of un-bonded foam were similar to the compressive strength values as documented in ref. [13]. These considerations suggested that foaming direction might also have an effect on foam-tometal bond strength. In the present work, the effect of foam orientation on foam/Ti joint strength was studied by testing the joints that had Ti bonded either to the WR surface or to the AR surface of the foam. Additionally, the effect
Corresponding author. E-mail address: [email protected] (R. Asthana). 1 The GTIH foam is a product of GrafTech International, Inc., Parma, OH, currently under development. The foam is not yet commercially available. ⁎
https://doi.org/10.1016/j.jeurceramsoc.2019.12.048 Received 31 July 2019; Received in revised form 21 December 2019; Accepted 23 December 2019 0955-2219/ © 2019 Published by Elsevier Ltd.
Please cite this article as: M. Singh, et al., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.12.048
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
M. Singh, et al.
Table 1 Selected Properties of Braze and Substrate Materials used for Brazing. Braze
Composition (wt%)
TL, C
E, GPa
YS, MPa
UTS, MPa
CTE×106, K−1
% El
k, W/mK
Cusil-ABA® Ti
63Ag-35.3Cu-1.75Ti Commercial purity
815
83 105
271 480
346 550
18.5 9.7
42 15
180 17.2
E: Young’s modulus, YS: yield strength, UTS: tensile strength, CTE: coefficient of thermal expansion, %El: percent elongation, k: thermal conductivity, ® Morgan Advanced Ceramics, Hayward, CA.
Fig. 1. Schematic of cut foam surfaces relative to the foaming direction for scanning electron microscopic examination.
conductivity of 460−700 W.m.K-1 . The foams were carefully sectioned into rectangular prisms to expose surfaces that were either parallel or perpendicular to the foaming direction and identified as ‘WR’ or ‘AR’ samples. The rectangular prisms had a cross-section of 12.7 mm x 12.7 mm and lengths of either 25.4 mm or 12.7 mm, thereby yielding two aspect ratios (L/W) of 2.0 and 1.0 whose influence on mechanical strength of foam/Ti joints was evaluated. The density of the individual cut foam samples ranged from 250−300 kg.m −3 and matched with the bulk density values provided by the manufacturer.
of foam surface modification by SiO vapor deposition and reaction to form SiC on the strength of SiC-coated foam-to-Ti joints was investigated. The infiltration of foam with SiO vapor was done to produce surfaces comprised of thin layers of reaction-formed SiC on carbon without causing significant detriment to the outstanding thermal characteristics of the foam. 2. Experimental procedure The porous graphite foams of a narrow range of densities (260−270 kg.m −3) were obtained from GrafTech International, Inc., Parma, OH. The foam specimens came from two larger billets which were produced in different batches; both billets had a bulk density of 260−270 kg.m −3. As stated earlier, the GTIH foams are grown using a proprietary procedure that allows different faces of the grown material to be identified as either ‘with-rise’ (WR), which is the foaming direction, or against rise (AR). Some other salient characteristics of the foam are: solid volume fraction (6.7–14.2 %), pores per inch (20–40), average pore size (1270−635 μm), a bulk conductivity of 100−175 W.m.K -1 along WR and a ligament
2.1. Coating of foam A solid-vapor coating procedure involving heating and evaporation of metastable solid SiO under vacuum conditions was used to coat graphite foam with SiO. SiO is expected to react with carbon to form SiC and gaseous CO. Induction heating was used to raise the temperature of solid silicon monoxide to 1450 °C in a vacuum furnace with graphite foam exposed to rising SiO vapor. No specific orientation of the foam relative to the direction of rising SiO vapors was used for coating deposition.
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Fig. 2. SEM views of SiC-coated foam at different locations on cut plane A-A (Fig. 1) perpendicular to the AR direction.
After an exposure of 30 min. to SiO vapors at 1450 °C, the furnace was shut down and coated foam allowed to cool to room temperature.
assembly was heated in an atmosphere-controlled furnace to the brazing temperature (typically 15−20 °C above the braze liquidus) under vacuum (10−6-10-5 torr) and isothermally held for 5 min. at the brazing temperature followed by slow cooling to room temperature.
2.2. Brazing procedure A commercial Ag-Cu-Ti active braze, Cusil-ABA in powder form obtained from Morgan Advanced Ceramics, Hayward, CA, was used for joining. The composition, liquidus and solidus temperatures, and selected physical and mechanical properties of Cusil-ABA are given in Table 1. Although pure Ag and Cu do not wet graphite and display large values of contact angle θ (∼137°-140°) with graphite, Ti additions to Ag and Cu are known to improve the wettability via carbide forming reactions [14]. Commercially pure Ti plates (1.52 mm thick) from Ti Metal, Inc., were sliced into two sizes: 25.4 mm x 25.4 mm cross-section (used in joints for mechanical testing), and 25.4 mm x 12.5 mm (used in joints for microscopy). All materials were ultrasonically cleaned in acetone for 15 min. prior to brazing. The braze foils were sandwiched between the metal and the composite, and a normal load of 0.30-0.40 N was applied to the assembly. Braze powders were mixed with glycerin to make a thick paste with dough-like consistency, and the paste was applied using spatula to the surfaces to be joined. The
2.3. Microscopy The foam/Ti joined samples for microscopy were created using smaller (25.4 mm x 12.5 mm x 12.5 mm) pieces of foam than those used to make the joints for strength testing. The joined samples were visually examined, then mounted in epoxy, ground and polished on a Buehler automatic polishing machine, and examined using optical microscopy (Olympus DP 71 system) and scanning electron microscopy (SEM) (JEOL, JSM-840A) coupled with energy dispersive x-ray spectroscopy (EDS). The elemental composition across joints was accessed with the EDS and presented as relative atomic percentage among the alloying elements at point markers on SEM images. Uncoated and Si-coated foam were also examined under the SEM for pore morphology and evidence of coating deposition.
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Fig. 3. SEM views of SiC-coated foam on cut plane C-C (Fig. 1) parallel to the AR direction.
2.4. Mechanical testing
were then used to secure the sample in place. To eliminate any alignment concerns, two universal joints were used in the fixture; one above and one below the sample. This way, if the specimen ends were not cut perfectly perpendicular, or if the sample did manage to shift during the epoxy cure time, then uniaxial loading would still be maintained. Samples were loaded in crosshead control, at a rate of 0.64 mm/min. The joint strength was measured as a function of the foam thickness (25.4 and 12.7 mm), surface modification (coated and Si-coated), and foam orientation (e.g., brazing of Ti to WR and AR faces).
The strength of the foam/metal joints was characterized mechanically by tensile testing at room temperature with an Instron 55R4502 universal testing machine. Both brazed and un-brazed foams (density: 260−270 kg.m−3) were tested for comparison. Additionally, tension test along WR and AR directions was also carried out on un-bonded low-density (127−156 kg.m−3) foam to evaluate the effect of density and foam orientation on foam strength. For the tension test, specimens were adhered to aluminum tabs using two-part epoxy. An alignment rig was used to ensure that the tabs and foam remained in position while the epoxy cured. For brazed specimens, the titanium end plates were adhered to the tabs. The tabbed specimen was then carefully inserted into a top steel sleeve on the Instron machine and lowered into the bottom sleeve (a schematic of the test set-up is shown in Fig. 9b under Results and Discussion). Pins
3. Results and discussion 3.1. Structure of coated and uncoated foams The SiO vapor from solid SiO in the reactor condenses on the foam where it reacts with the carbon to form SiC. Strictly, the foam surface
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Fig. 4. Uncoated foam on cut plane C-C (Fig. 1) parallel to the AR direction.
that directly faces the rising SiO vapor should form more SiC than surfaces that are not directly in the path of SiO vapor. Fig. 1 shows a schematic of the coated surfaces relative to the foaming direction that were examined using the SEM and EDS. Fig. 2 shows the SEM views of several locations on cut plane A-A perpendicular to the AR direction (marked in Fig. 1). The EDS elemental analysis confirmed the presence of Si on foam surface; however, as carbon concentration could not be accessed via EDS, a semi-quantitative assessment of the relative atomic percentages of the elements was not feasible. Fig. 3 shows the SEM views of the carbon foam on cut plane C-C parallel to the AR direction (marked in Fig. 1) where extensive SiC formation was noted. As the orientation of each individual foam piece relative to the direction of rising SiO vapor within the coating reactor was not tracked, it was not possible to identify the faces on which maximum deposit should have occurred. Fig. 4 shows uncoated foam on cut plane C-C parallel to the AR direction for comparison with coated foam. While growth kinetics and mechanisms of SiC formation do not constitute focus of our study, it should be mentioned
that the SiO vapor coating procedure used here is similar to other solidvapor methods to grow SiC that utilize either Si vapor from solid Si to react with carbon [15] or vaporize bulk SiC by heating under reduced pressure to form SiC crystals at nucleation sites that contain a catalyst (e.g., lanthanum [16]). Other processes also have been used to grow SiC and include hydrogen reduction of silane compounds on carbon [17], controlled decomposition of mixtures of chlorosilanes, CO and CH4 [18], reaction of the silica and carbon present in rice hulls [19], and evaporation of molten Si and its reaction with carbon [20–22]. 3.2. Microstructure and composition of Foam/Ti joints Fig. 5 shows optical photomicrographs of foam/titanium joints created using the Cusil-ABA braze for both coated (Fig. 5a-c) and uncoated (Fig. 5d-f) foams. Sound metallurgical bonding and braze penetration in the foam are evident in both types of foam. This confirms that Cusil-ABA has good wettability on both graphite [13] and silicon carbide [23,24] which promotes bonding. A prominent but diffuse multilayer interaction zone (e.g., Fig. 5c & f) has developed between Ti
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Fig. 5. Optical photomicrographs of (a)-(c) SiC-coated foam/Ti joints (cut surface along AR was joined to Ti), and (d)-(f) uncoated foam/Ti joint (cut surface along WR was joined to Ti).
substrate and Cusil-ABA. Representative secondary electron SEM images of the joints and EDS elemental composition scans are shown in Figs. 6 and 7 for coated (Figs. 6 & 7) and uncoated (Fig. 8) foams. Carbon ligaments are intimately bonded to Cusil-ABA and the latter shows characteristic twophase eutectic structure with well-defined Ag-rich and Cu-rich areas. A distinct reaction layer separates the Ag-Cu eutectic region and the carbon in SiC-coated foam (Fig. 6b). The EDS scans (Fig. 6c and 7e) across the Cusil-ABA/carbon interface show large concentrations of titanium in the reaction layer together with the presence of some Si (∼10 % max., Fig. 6c) and Ag in the reaction layer. The high Ti concentration areas in the EDS scan correspond to a low Ag concentration and vice versa. It is conceivable that the reaction layer (Fi. 6b) might be a silicide of Ti, a carbide of Ti or a ternary TixSiyCz compound that bonds well to both braze and the carbon. Additionally, Cu-Ti intermetallic phases would probably also form during cooling and solidification of the braze. The large Ti concentrations within the solidified braze layer at the interface represent the combined effect of Ti segregation out of the Cusil-ABA, consistent with the wettability enhancing effects of Ti, as well as Ti supplied by the
dissolution of the metal substrate in molten braze. As EDS scans could not track carbon, it was not possible to determine if carbon from the foam had also dissolved in braze. Fig. 7 shows the secondary electron SEM images of uncoated foam/Ti joint made using Cusil-ABA. Although intimate metallurgical bonding is evident between foam and braze, no reaction layers could be identified at the interface; the Ag-Cu eutectic phases appear to be in direct physical contact with the carbon in the foam. Furthermore, Si was not found at the Ti-enriched interface which suggests probable formation of a thin TiC layer that facilitated bonding but was too thin to be detected at the magnifications used (x2000). This is different from the coated foam/Ti joints (Fig. 6a) where reaction layers were visible at the same magnification (Fig. 6a). 3.3. Joint strength Fig. 8(a) & (b) shows a schematic of the sample configuration in tension tests and Fig. 8(c) shows representative stress-strain plots of coated and uncoated foam/Ti joints along WR and AR. The mechanical test results are summarized in Fig. 9 and Table 2. All brazed samples that were tension tested broke in the foam region indicating that the foam was the
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Fig. 6. (a) & (b) Secondary electron SEM views of SiC-coated foam/Ti joints (cut surface along AR was joined to Ti) showing braze penetration in cracks of exfoliated graphite, and (c) EDS scans showing relative atomic percentages of Si, Ti, Cu and Ag across the braze/foam reaction layer.
Fig. 7. (a) – (d) Secondary electron SEM views of an uncoated foam/Ti joint (cut surface along WR was joined to Ti), and (e) EDS scans showing relative atomic percentages of Si, Ti, Cu and Ag across the braze/foam interface region.
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However, results from mechanical testing (Fig. 9 and Table 2) show that brazing did not influence the strength in either a beneficial or a detrimental way. The scatter for brazed and un-brazed data was similar. Different foam orientations led to a significant difference in strength. The foam tested along WR had much higher strength than foam tested along AR. For example, the as-received (un-bonded) medium-density foam had strength of 785.32 kPa along WR and strength of 277.14 kPa along AR. Some un-bonded low-density (127−156 kg.m−3) foams were also tested and they also exhibited a similar behavior relative to WR and AR directions (Table 2). The strength values for low-density foams along WR and AR were 587.7 kPa and 115.8 kPa, respectively. The higher strength along WR than AR can be explained by the fact that the carbon ligaments are slightly more aligned in the foaming direction, which presumably leads to better load carrying capability. The results also show that the mediumdensity (MD) foam is stronger than low-density (LD) foam along both AR (277.14 kPa (MD) versus 115.84 kPa (LD)) and WR (785.32 (MD) versus 587.7 kPa (LD)). This result is corroborated by our earlier study [12] in which a wider range of foam density was studied. The results summarized in Table 2 show that both coated and uncoated foams brazed to Ti are stronger along WR than along AR. In uncoated foam/Ti joints, the effect of the foam thickness was minimal along WR; the short (12.7 mm thick) foam had only 8 % lower strength than the long (25.4 mm thick) foam. However, along AR, the short (12.7 mm) uncoated foam brazed to Ti had significantly (58 %) lower strength than the long (25.4 mm) foam. Silicon carbide coating marginally (9 %) lowered the strength of foam in joints along WR. However, along AR, there was nearly 50 % decrease in strength in joints made using coated foam compared to joints made using uncoated foam. The results, therefore, show that the experimental GTIH foam is more robust and amenable to secondary processing such as coating and joining along WR than along AR.
Fig. 8. (a) & (b) Schematic diagrams showing setup for mechanical testing of foam/Ti joints, and (c) shows representative stress-strain plots of foam/Ti joints made using uncoated and SiC-coated foam along AR and WR directions.
4. Summary and conclusions 1 Silicon monoxide vapor deposition on carbon foam at 1450 °C with 30 min. exposure led to formation of SiC on carbon surface. No modification of the pore morphology of pristine foam was evident following SiC deposition. 2 SiC-coated and uncoated carbon foams, vacuum brazed to titanium using a Ag-Cu-Ti active braze alloy, Cusil-ABA, showed that joints were defect-free, had well-bonded carbon/braze interfaces, and exhibited Ti segregation at the interface, as well as braze penetration within the foam. A prominent and well-defined reaction layer containing both Ti and Si developed in joints made using SiC-coated foams; no such reaction layer was detected in joints made using uncoated foam although large Ti concentrations appeared at the interface probably indicating a thin TiC layer had formed. 3 The as-received (un-bonded) medium-density foam is stronger along
Fig. 9. Tensile strength of as- produced, brazed, and coated foam/Ti joints. All of the samples broke in the foam itself, rather than in the braze region.
weakest link. Typical fracture behavior is shown in Fig. 10 for foam tested in the with-rise (WR) and against-rise (AR) directions. The fracture was usually very close to the braze region, possibly due to stress concentration. Table 2 Summary of Tension Test Results on Foam/Ti Joints. Orientation
Coating
Braze
Length of Foam in joint, mm
Avg. Bulk Density, kg/m3
Avg. Str. ± SD (kPa)
% Deviation in Mean Strength
AR WR AR WR AR WR AR WR AR WR
None None None None None None None None SiC SiC
None None None None CusilABA CusilABA CusilABA CusilABA CusilABA CusilABA
25.4 25.4 25.4 25.4 25.4 25.4 12.7 12.7 25.4 25.4
266 266 127 156 266 266 271 271 266 266
277.14 ± 50.04 785.32 ± 172.85 115.84 ± 7.79 587.66 ± 101.44 568.19 ± 244.5 878.31 ± 120.95 239.48 ± 91.49 809.1 ± 10.24 285.22 ± 70.76 796.82 ± 195.29
18 22 6.7 17.3 43 13.8 38.2 1.3 24.8 24.5
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Fig. 10. (a) Graphite foam specimen brazed to titanium plates with Cusil-ABA paste, and (b) & (c) typical failure of brazed foam specimens subjected to tensile loading.
WR (785.32 kPa) than long AR (277.14 kPa). This is true also of lowdensity (127−156 kg.m−3) foam (587.7 kPa versus 115.8 kPa). Additionally, the medium-density (MD) foam is stronger than lowdensity (LD) foam along both AR (277.14 kPa (MD) versus 115.84 kPa (LD)) and WR (785.32 (MD) versus 587.7 kPa (LD)). 4 Both coated and uncoated foams brazed to Ti are stronger along WR than along AR. This is true regardless of the test-piece thickness (25.4 mm or 12.7 mm). In foam/Ti joints, the effect of the foam thickness was minimal along WR; the short foam was only 8 % less strong than the long foam. However, along AR, foam of shorter (12.7 mm) thickness brazed to Ti had significantly lower strength (58 % lower) than the foam having larger (25.4 mm) thickness. 5 SiC coating on foam marginally (9 %) lowered the strength of the foam in WR joints. However, along AR, there was 49.8 % decrease in strength in joints made using coated foam compared to joints made using uncoated foam. All foam/Ti joints were stronger than the foam which was the weakest link. 6 Overall, the experimental foam used here is more robust and amenable to secondary processing such as coating and joining along WR than along AR.
22, 2006). [3] http://www.graftech.com/. [4] R.W. Smith, R.R. Redd, Active solder joining of thermal management and electronic packaging, in: J.J. Stephens, K. Scott Weil (Eds.), Brazing and Soldering, ASM and AWS, 2006, pp. 79–82. [5] M. Singh, R. Asthana, Characterization of brazed joints of carbon-carbon composites to Cu-clad-Mo, Compos. Sci. Tech. 68 (14) (2008) 3010–3019. [6] M. Singh, G.N. Morscher, T.P. Shpargel, R. Asthana, Active metal brazing of titanium to high-conductivity carbon-based sandwich structures, Mater. Sci. Eng. A 498 (1-2) (2008) 31–36. [7] M. Singh, R. Asthana, T.P. Shpargel, Brazing of ceramic-matrix composites to Ti and Hastealloy using Ni-base metallic glass interlayers, Mater. Sci. Eng. A 498 (1-2) (2008) 19–30. [8] M. Singh, R. Asthana, T.P. Shpargel, Brazing of C-C composites to Cu-clad Mo for thermal management applications, Mater. Sci. Eng. A 452-453 (2007) 699–704. [9] G.N. Morscher, M. Singh, T.P. Shpargel, R. Asthana, A simple test to determine the effectiveness of different braze compositions for joining Ti tubes to C/C composite plates, Mater. Sci. Eng. A 418 (1-2) (2006) 19–24. [10] M. Singh, T.P. Shpargel, G.N. Morscher, R. Asthana, Active metal brazing and characterization of brazed joints in titanium to carbon-carbon composites, Mater. Sci. Eng. A 412 (2005) 123–128. [11] R. Asthana, M. Singh, N. Sobczak, Wetting behavior and interfacial microstructure of palladium and silver-based braze alloys with ceramic matrix composites, J. Mater. Sci. 45 (16) (2010) 4276–4290. [12] M. Singh, R. Asthana, A.L. Gyekenyesi, C.E. Smith, Bonding and integration of titanium to graphitic foams for thermal management applications, Int. J. Appl. Ceram. Tech. 9 (4) (2012) 657–665. [13] C.E. Smith, A.L. Gyekenyesi, M. Singh, J. Bail, Compressive behavior of conductive graphite foams”, J. Mater. Eng. Perf. 21 (8) (2012) 1703–1707. [14] N. Sobczak, J. Sobczak, P. Rohatgi, M. Ksiazek, W. Radziwill, J. Morgiel, Interaction between Ti or Cr containing copper alloys and porous graphite substrates, in: N. Eustathopoulos, N. Sobczak (Eds.), Proc. Int. Conf. High-Temperature Capillarity, Foundry Research Institute, Krakow (Poland), 1997, pp. 145–152. [15] G. Yang, R. Wu, Y. Pan, J. Chen, R. Zhai, L. Wu, J. Lin, Direct observation of the growth process of silicon carbide nanowhiskers by vapor–solid process, Physica E Low. Syst. Nanostruct. 39 (1) (2007) 171–174. [16] W.F. Knippenberg, G. Verspui, R.C. Marshall, J.W. Faust, Jr.C.E. Ryan (Eds.), Silicon Carbide – 1972, Univ. of South Carolina Press, 1973, p. 108. [17] C.E. Ryan, I. Berman, R.C. Marshall, D.P. Considine, J.J. Hawley, J. Crystal Growth 1 (1967) 255. [18] N.J. Parratt, Fiber Reinforced Materials Technology, Van Nostrand Reinhold, London, 1972. [19] I.B. Cutler, Production of silicon carbide from rice hulls, US Patent 3 (August 754) (1973) 76. [20] J.J. Shyne, J.V. Milewski, Method of growing silicon carbide whiskers, US Patent 3 (November) (1971) 622272. [21] W.F. Knippendburg, G. Verspui, Mater. Res. Bull. 4 (1969) 33. [22] Q. Zhou, S. Dong, Y. Ding, Z. Wang, Z. Huang, D. Jiang, Ceram. Int. 35 (6) (2009) 2161–2169. [23] M.G. Nicholas, S.D. Peteves, N. Eustathopoulos (Ed.), Proceedings of HighTemperature Capillarity, Slovak Academy of Sciences, Slovakia, 1994, p. 18. [24] R. Asthana, M. Singh, N. Sobczak, Wetting behavior and interfacial microstructure of palladium and silver-based braze alloys with ceramic matrix composites, J. Mater. Sci. 45 (16) (2010) 4276–4290.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was funded through U.S. Air Force Contract FA8650-05-D5807, Prime Contractor: Universal Technology Corporation (UTC). The authors would like to thank Dr. Ajit K. Roy for technical suggestions. We would also like to thank Mr. Joseph Manter, Program Manager/Vice President for Materials & Manufacturing, UTC and other project partners. References [1] J.W. Klett, Mesophase pitch-based carbon foam: effects of precursor on thermal conductivity, Proceedings of the 23rd Annual Conference on Ceramic, Metal and Carbon Composites, Materials, and Structures, January 25–28, Cocoa Beach, 1999, pp. 657–674. [2] POCO HTC web site: http://www.poco.com/us/Thermal/htc.asp#thermal (August
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Contents lists available at ScienceDirect
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Original Article
Active metal brazing of graphite foam-to-titanium joints made with SiCCoated foam M. Singha, C.E. Smitha, R. Asthanab,*, A.L. Gyekenyesia a b
Ohio Aerospace Institute, 22800 Cedar Point Road, Cleveland, OH, 44142, USA Department of Engineering & Technology, University of Wisconsin-Stout, Menomonie, WI, 54751, USA
ARTICLE INFO
ABSTRACT
Keywords: Carbon foam Brazing Titanium Joint strength
Medium-density, high-conductivity carbon foams were joined to titanium using a two-step process that first exposed foam to SiO vapor at 1450 °C for 30 min. under vacuum followed by vacuum brazing Ti using Cusil-ABA to the sides of prismatic foam pieces along the ‘with-rise’ (WR) or foaming direction and the ‘against rise’ (AR) or transvers direction. Well-bonded joints with braze-infiltrated foam and Ti-rich interfaces formed along WR and AR. The un-bonded foam was stronger along WR (785 kPa) than along AR (277 kPa) as were the joints made using coated and uncoated foams. Foam thickness minimally affected joint strength along WR but along AR, joints with thick foam were 58 % stronger. The coating marginally (9 %) lowered joint strength along WR but led to a nearly 50 % strength drop along AR. The experimental foam is more robust and amenable to coating and joining along foaming direction than transverse to it.
1. Introduction Porous materials such as carbon foams have generated considerable interest for thermal management and energy storage applications [1–3]. Foams can be designed for density, pore size, conductivity and other attributes for multifunctional applications. Carbon foams offer benefits such as low density, large surface area, high ligament conductivity (> 1700 W/m.K) and high bulk conductivity (∼175 W/m.K) besides chemical inertness and thermal stability. Thermal management and energy storage applications require graphite foams to be integrated with other materials for effective heat transfer. Carbon foams have been soldered to metals such as aluminum for low (< 300 °C) use-temperatures [4]; however, to our knowledge, brazing of foam-to-metal joints for higher usetemperatures at which the large mismatch in the coefficients of thermal expansion (CTE) of bonded materials could generate severe thermal stresses, has not been demonstrated. Although high-temperature brazing of various carbon-based materials (e.g., carboncarbon and carbon-silicon carbide) to metals such as Ti, Inconel, and Cu-clad-Mo has been demonstrated [5–11], integration of foams, particularly, new and emerging varieties of highly conductive graphitic foams, via brazing for elevated temperature use has, to our knowledge, not been explored.
In a recent study [12], the effect of density of GTIH graphite foams®1 [3] on the tensile strength of foam/Ti joints brazed using a Ag-Cu-Ti alloy was evaluated. The tests revealed that joints were stronger than the foams (failure occurred within foam rather than in joint), and the strength increased with increasing foam density. GTIH foams are grown using a proprietary procedure that allows different faces of the grown material to be identified as either ‘with-rise’ (WR), which is the direction of foaming in the manufacturing process, or against rise (AR), which is the direction transverse to foaming direction. In a previous study [12], foams sectioned at arbitrary orientations to expose random surfaces were brazed to titanium and the effect, if any, of growth direction of foam on joint strength was not considered. Unpublished data produced by some of the present authors has indicated large scatter regarding the tensile strengths of unbonded foams, especially in tests oriented in the WR direction where density variations are more pronounced. The absolute values of the tensile strength of un-bonded foam were similar to the compressive strength values as documented in ref. [13]. These considerations suggested that foaming direction might also have an effect on foam-tometal bond strength. In the present work, the effect of foam orientation on foam/Ti joint strength was studied by testing the joints that had Ti bonded either to the WR surface or to the AR surface of the foam. Additionally, the effect
Corresponding author. E-mail address: [email protected] (R. Asthana). 1 The GTIH foam is a product of GrafTech International, Inc., Parma, OH, currently under development. The foam is not yet commercially available. ⁎
https://doi.org/10.1016/j.jeurceramsoc.2019.12.048 Received 31 July 2019; Received in revised form 21 December 2019; Accepted 23 December 2019 0955-2219/ © 2019 Published by Elsevier Ltd.
Please cite this article as: M. Singh, et al., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.12.048
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Table 1 Selected Properties of Braze and Substrate Materials used for Brazing. Braze
Composition (wt%)
TL, C
E, GPa
YS, MPa
UTS, MPa
CTE×106, K−1
% El
k, W/mK
Cusil-ABA® Ti
63Ag-35.3Cu-1.75Ti Commercial purity
815
83 105
271 480
346 550
18.5 9.7
42 15
180 17.2
E: Young’s modulus, YS: yield strength, UTS: tensile strength, CTE: coefficient of thermal expansion, %El: percent elongation, k: thermal conductivity, ® Morgan Advanced Ceramics, Hayward, CA.
Fig. 1. Schematic of cut foam surfaces relative to the foaming direction for scanning electron microscopic examination.
conductivity of 460−700 W.m.K-1 . The foams were carefully sectioned into rectangular prisms to expose surfaces that were either parallel or perpendicular to the foaming direction and identified as ‘WR’ or ‘AR’ samples. The rectangular prisms had a cross-section of 12.7 mm x 12.7 mm and lengths of either 25.4 mm or 12.7 mm, thereby yielding two aspect ratios (L/W) of 2.0 and 1.0 whose influence on mechanical strength of foam/Ti joints was evaluated. The density of the individual cut foam samples ranged from 250−300 kg.m −3 and matched with the bulk density values provided by the manufacturer.
of foam surface modification by SiO vapor deposition and reaction to form SiC on the strength of SiC-coated foam-to-Ti joints was investigated. The infiltration of foam with SiO vapor was done to produce surfaces comprised of thin layers of reaction-formed SiC on carbon without causing significant detriment to the outstanding thermal characteristics of the foam. 2. Experimental procedure The porous graphite foams of a narrow range of densities (260−270 kg.m −3) were obtained from GrafTech International, Inc., Parma, OH. The foam specimens came from two larger billets which were produced in different batches; both billets had a bulk density of 260−270 kg.m −3. As stated earlier, the GTIH foams are grown using a proprietary procedure that allows different faces of the grown material to be identified as either ‘with-rise’ (WR), which is the foaming direction, or against rise (AR). Some other salient characteristics of the foam are: solid volume fraction (6.7–14.2 %), pores per inch (20–40), average pore size (1270−635 μm), a bulk conductivity of 100−175 W.m.K -1 along WR and a ligament
2.1. Coating of foam A solid-vapor coating procedure involving heating and evaporation of metastable solid SiO under vacuum conditions was used to coat graphite foam with SiO. SiO is expected to react with carbon to form SiC and gaseous CO. Induction heating was used to raise the temperature of solid silicon monoxide to 1450 °C in a vacuum furnace with graphite foam exposed to rising SiO vapor. No specific orientation of the foam relative to the direction of rising SiO vapors was used for coating deposition.
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Fig. 2. SEM views of SiC-coated foam at different locations on cut plane A-A (Fig. 1) perpendicular to the AR direction.
After an exposure of 30 min. to SiO vapors at 1450 °C, the furnace was shut down and coated foam allowed to cool to room temperature.
assembly was heated in an atmosphere-controlled furnace to the brazing temperature (typically 15−20 °C above the braze liquidus) under vacuum (10−6-10-5 torr) and isothermally held for 5 min. at the brazing temperature followed by slow cooling to room temperature.
2.2. Brazing procedure A commercial Ag-Cu-Ti active braze, Cusil-ABA in powder form obtained from Morgan Advanced Ceramics, Hayward, CA, was used for joining. The composition, liquidus and solidus temperatures, and selected physical and mechanical properties of Cusil-ABA are given in Table 1. Although pure Ag and Cu do not wet graphite and display large values of contact angle θ (∼137°-140°) with graphite, Ti additions to Ag and Cu are known to improve the wettability via carbide forming reactions [14]. Commercially pure Ti plates (1.52 mm thick) from Ti Metal, Inc., were sliced into two sizes: 25.4 mm x 25.4 mm cross-section (used in joints for mechanical testing), and 25.4 mm x 12.5 mm (used in joints for microscopy). All materials were ultrasonically cleaned in acetone for 15 min. prior to brazing. The braze foils were sandwiched between the metal and the composite, and a normal load of 0.30-0.40 N was applied to the assembly. Braze powders were mixed with glycerin to make a thick paste with dough-like consistency, and the paste was applied using spatula to the surfaces to be joined. The
2.3. Microscopy The foam/Ti joined samples for microscopy were created using smaller (25.4 mm x 12.5 mm x 12.5 mm) pieces of foam than those used to make the joints for strength testing. The joined samples were visually examined, then mounted in epoxy, ground and polished on a Buehler automatic polishing machine, and examined using optical microscopy (Olympus DP 71 system) and scanning electron microscopy (SEM) (JEOL, JSM-840A) coupled with energy dispersive x-ray spectroscopy (EDS). The elemental composition across joints was accessed with the EDS and presented as relative atomic percentage among the alloying elements at point markers on SEM images. Uncoated and Si-coated foam were also examined under the SEM for pore morphology and evidence of coating deposition.
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Fig. 3. SEM views of SiC-coated foam on cut plane C-C (Fig. 1) parallel to the AR direction.
2.4. Mechanical testing
were then used to secure the sample in place. To eliminate any alignment concerns, two universal joints were used in the fixture; one above and one below the sample. This way, if the specimen ends were not cut perfectly perpendicular, or if the sample did manage to shift during the epoxy cure time, then uniaxial loading would still be maintained. Samples were loaded in crosshead control, at a rate of 0.64 mm/min. The joint strength was measured as a function of the foam thickness (25.4 and 12.7 mm), surface modification (coated and Si-coated), and foam orientation (e.g., brazing of Ti to WR and AR faces).
The strength of the foam/metal joints was characterized mechanically by tensile testing at room temperature with an Instron 55R4502 universal testing machine. Both brazed and un-brazed foams (density: 260−270 kg.m−3) were tested for comparison. Additionally, tension test along WR and AR directions was also carried out on un-bonded low-density (127−156 kg.m−3) foam to evaluate the effect of density and foam orientation on foam strength. For the tension test, specimens were adhered to aluminum tabs using two-part epoxy. An alignment rig was used to ensure that the tabs and foam remained in position while the epoxy cured. For brazed specimens, the titanium end plates were adhered to the tabs. The tabbed specimen was then carefully inserted into a top steel sleeve on the Instron machine and lowered into the bottom sleeve (a schematic of the test set-up is shown in Fig. 9b under Results and Discussion). Pins
3. Results and discussion 3.1. Structure of coated and uncoated foams The SiO vapor from solid SiO in the reactor condenses on the foam where it reacts with the carbon to form SiC. Strictly, the foam surface
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Fig. 4. Uncoated foam on cut plane C-C (Fig. 1) parallel to the AR direction.
that directly faces the rising SiO vapor should form more SiC than surfaces that are not directly in the path of SiO vapor. Fig. 1 shows a schematic of the coated surfaces relative to the foaming direction that were examined using the SEM and EDS. Fig. 2 shows the SEM views of several locations on cut plane A-A perpendicular to the AR direction (marked in Fig. 1). The EDS elemental analysis confirmed the presence of Si on foam surface; however, as carbon concentration could not be accessed via EDS, a semi-quantitative assessment of the relative atomic percentages of the elements was not feasible. Fig. 3 shows the SEM views of the carbon foam on cut plane C-C parallel to the AR direction (marked in Fig. 1) where extensive SiC formation was noted. As the orientation of each individual foam piece relative to the direction of rising SiO vapor within the coating reactor was not tracked, it was not possible to identify the faces on which maximum deposit should have occurred. Fig. 4 shows uncoated foam on cut plane C-C parallel to the AR direction for comparison with coated foam. While growth kinetics and mechanisms of SiC formation do not constitute focus of our study, it should be mentioned
that the SiO vapor coating procedure used here is similar to other solidvapor methods to grow SiC that utilize either Si vapor from solid Si to react with carbon [15] or vaporize bulk SiC by heating under reduced pressure to form SiC crystals at nucleation sites that contain a catalyst (e.g., lanthanum [16]). Other processes also have been used to grow SiC and include hydrogen reduction of silane compounds on carbon [17], controlled decomposition of mixtures of chlorosilanes, CO and CH4 [18], reaction of the silica and carbon present in rice hulls [19], and evaporation of molten Si and its reaction with carbon [20–22]. 3.2. Microstructure and composition of Foam/Ti joints Fig. 5 shows optical photomicrographs of foam/titanium joints created using the Cusil-ABA braze for both coated (Fig. 5a-c) and uncoated (Fig. 5d-f) foams. Sound metallurgical bonding and braze penetration in the foam are evident in both types of foam. This confirms that Cusil-ABA has good wettability on both graphite [13] and silicon carbide [23,24] which promotes bonding. A prominent but diffuse multilayer interaction zone (e.g., Fig. 5c & f) has developed between Ti
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Fig. 5. Optical photomicrographs of (a)-(c) SiC-coated foam/Ti joints (cut surface along AR was joined to Ti), and (d)-(f) uncoated foam/Ti joint (cut surface along WR was joined to Ti).
substrate and Cusil-ABA. Representative secondary electron SEM images of the joints and EDS elemental composition scans are shown in Figs. 6 and 7 for coated (Figs. 6 & 7) and uncoated (Fig. 8) foams. Carbon ligaments are intimately bonded to Cusil-ABA and the latter shows characteristic twophase eutectic structure with well-defined Ag-rich and Cu-rich areas. A distinct reaction layer separates the Ag-Cu eutectic region and the carbon in SiC-coated foam (Fig. 6b). The EDS scans (Fig. 6c and 7e) across the Cusil-ABA/carbon interface show large concentrations of titanium in the reaction layer together with the presence of some Si (∼10 % max., Fig. 6c) and Ag in the reaction layer. The high Ti concentration areas in the EDS scan correspond to a low Ag concentration and vice versa. It is conceivable that the reaction layer (Fi. 6b) might be a silicide of Ti, a carbide of Ti or a ternary TixSiyCz compound that bonds well to both braze and the carbon. Additionally, Cu-Ti intermetallic phases would probably also form during cooling and solidification of the braze. The large Ti concentrations within the solidified braze layer at the interface represent the combined effect of Ti segregation out of the Cusil-ABA, consistent with the wettability enhancing effects of Ti, as well as Ti supplied by the
dissolution of the metal substrate in molten braze. As EDS scans could not track carbon, it was not possible to determine if carbon from the foam had also dissolved in braze. Fig. 7 shows the secondary electron SEM images of uncoated foam/Ti joint made using Cusil-ABA. Although intimate metallurgical bonding is evident between foam and braze, no reaction layers could be identified at the interface; the Ag-Cu eutectic phases appear to be in direct physical contact with the carbon in the foam. Furthermore, Si was not found at the Ti-enriched interface which suggests probable formation of a thin TiC layer that facilitated bonding but was too thin to be detected at the magnifications used (x2000). This is different from the coated foam/Ti joints (Fig. 6a) where reaction layers were visible at the same magnification (Fig. 6a). 3.3. Joint strength Fig. 8(a) & (b) shows a schematic of the sample configuration in tension tests and Fig. 8(c) shows representative stress-strain plots of coated and uncoated foam/Ti joints along WR and AR. The mechanical test results are summarized in Fig. 9 and Table 2. All brazed samples that were tension tested broke in the foam region indicating that the foam was the
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Fig. 6. (a) & (b) Secondary electron SEM views of SiC-coated foam/Ti joints (cut surface along AR was joined to Ti) showing braze penetration in cracks of exfoliated graphite, and (c) EDS scans showing relative atomic percentages of Si, Ti, Cu and Ag across the braze/foam reaction layer.
Fig. 7. (a) – (d) Secondary electron SEM views of an uncoated foam/Ti joint (cut surface along WR was joined to Ti), and (e) EDS scans showing relative atomic percentages of Si, Ti, Cu and Ag across the braze/foam interface region.
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However, results from mechanical testing (Fig. 9 and Table 2) show that brazing did not influence the strength in either a beneficial or a detrimental way. The scatter for brazed and un-brazed data was similar. Different foam orientations led to a significant difference in strength. The foam tested along WR had much higher strength than foam tested along AR. For example, the as-received (un-bonded) medium-density foam had strength of 785.32 kPa along WR and strength of 277.14 kPa along AR. Some un-bonded low-density (127−156 kg.m−3) foams were also tested and they also exhibited a similar behavior relative to WR and AR directions (Table 2). The strength values for low-density foams along WR and AR were 587.7 kPa and 115.8 kPa, respectively. The higher strength along WR than AR can be explained by the fact that the carbon ligaments are slightly more aligned in the foaming direction, which presumably leads to better load carrying capability. The results also show that the mediumdensity (MD) foam is stronger than low-density (LD) foam along both AR (277.14 kPa (MD) versus 115.84 kPa (LD)) and WR (785.32 (MD) versus 587.7 kPa (LD)). This result is corroborated by our earlier study [12] in which a wider range of foam density was studied. The results summarized in Table 2 show that both coated and uncoated foams brazed to Ti are stronger along WR than along AR. In uncoated foam/Ti joints, the effect of the foam thickness was minimal along WR; the short (12.7 mm thick) foam had only 8 % lower strength than the long (25.4 mm thick) foam. However, along AR, the short (12.7 mm) uncoated foam brazed to Ti had significantly (58 %) lower strength than the long (25.4 mm) foam. Silicon carbide coating marginally (9 %) lowered the strength of foam in joints along WR. However, along AR, there was nearly 50 % decrease in strength in joints made using coated foam compared to joints made using uncoated foam. The results, therefore, show that the experimental GTIH foam is more robust and amenable to secondary processing such as coating and joining along WR than along AR.
Fig. 8. (a) & (b) Schematic diagrams showing setup for mechanical testing of foam/Ti joints, and (c) shows representative stress-strain plots of foam/Ti joints made using uncoated and SiC-coated foam along AR and WR directions.
4. Summary and conclusions 1 Silicon monoxide vapor deposition on carbon foam at 1450 °C with 30 min. exposure led to formation of SiC on carbon surface. No modification of the pore morphology of pristine foam was evident following SiC deposition. 2 SiC-coated and uncoated carbon foams, vacuum brazed to titanium using a Ag-Cu-Ti active braze alloy, Cusil-ABA, showed that joints were defect-free, had well-bonded carbon/braze interfaces, and exhibited Ti segregation at the interface, as well as braze penetration within the foam. A prominent and well-defined reaction layer containing both Ti and Si developed in joints made using SiC-coated foams; no such reaction layer was detected in joints made using uncoated foam although large Ti concentrations appeared at the interface probably indicating a thin TiC layer had formed. 3 The as-received (un-bonded) medium-density foam is stronger along
Fig. 9. Tensile strength of as- produced, brazed, and coated foam/Ti joints. All of the samples broke in the foam itself, rather than in the braze region.
weakest link. Typical fracture behavior is shown in Fig. 10 for foam tested in the with-rise (WR) and against-rise (AR) directions. The fracture was usually very close to the braze region, possibly due to stress concentration. Table 2 Summary of Tension Test Results on Foam/Ti Joints. Orientation
Coating
Braze
Length of Foam in joint, mm
Avg. Bulk Density, kg/m3
Avg. Str. ± SD (kPa)
% Deviation in Mean Strength
AR WR AR WR AR WR AR WR AR WR
None None None None None None None None SiC SiC
None None None None CusilABA CusilABA CusilABA CusilABA CusilABA CusilABA
25.4 25.4 25.4 25.4 25.4 25.4 12.7 12.7 25.4 25.4
266 266 127 156 266 266 271 271 266 266
277.14 ± 50.04 785.32 ± 172.85 115.84 ± 7.79 587.66 ± 101.44 568.19 ± 244.5 878.31 ± 120.95 239.48 ± 91.49 809.1 ± 10.24 285.22 ± 70.76 796.82 ± 195.29
18 22 6.7 17.3 43 13.8 38.2 1.3 24.8 24.5
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Fig. 10. (a) Graphite foam specimen brazed to titanium plates with Cusil-ABA paste, and (b) & (c) typical failure of brazed foam specimens subjected to tensile loading.
WR (785.32 kPa) than long AR (277.14 kPa). This is true also of lowdensity (127−156 kg.m−3) foam (587.7 kPa versus 115.8 kPa). Additionally, the medium-density (MD) foam is stronger than lowdensity (LD) foam along both AR (277.14 kPa (MD) versus 115.84 kPa (LD)) and WR (785.32 (MD) versus 587.7 kPa (LD)). 4 Both coated and uncoated foams brazed to Ti are stronger along WR than along AR. This is true regardless of the test-piece thickness (25.4 mm or 12.7 mm). In foam/Ti joints, the effect of the foam thickness was minimal along WR; the short foam was only 8 % less strong than the long foam. However, along AR, foam of shorter (12.7 mm) thickness brazed to Ti had significantly lower strength (58 % lower) than the foam having larger (25.4 mm) thickness. 5 SiC coating on foam marginally (9 %) lowered the strength of the foam in WR joints. However, along AR, there was 49.8 % decrease in strength in joints made using coated foam compared to joints made using uncoated foam. All foam/Ti joints were stronger than the foam which was the weakest link. 6 Overall, the experimental foam used here is more robust and amenable to secondary processing such as coating and joining along WR than along AR.
22, 2006). [3] http://www.graftech.com/. [4] R.W. Smith, R.R. Redd, Active solder joining of thermal management and electronic packaging, in: J.J. Stephens, K. Scott Weil (Eds.), Brazing and Soldering, ASM and AWS, 2006, pp. 79–82. [5] M. Singh, R. Asthana, Characterization of brazed joints of carbon-carbon composites to Cu-clad-Mo, Compos. Sci. Tech. 68 (14) (2008) 3010–3019. [6] M. Singh, G.N. Morscher, T.P. Shpargel, R. Asthana, Active metal brazing of titanium to high-conductivity carbon-based sandwich structures, Mater. Sci. Eng. A 498 (1-2) (2008) 31–36. [7] M. Singh, R. Asthana, T.P. Shpargel, Brazing of ceramic-matrix composites to Ti and Hastealloy using Ni-base metallic glass interlayers, Mater. Sci. Eng. A 498 (1-2) (2008) 19–30. [8] M. Singh, R. Asthana, T.P. Shpargel, Brazing of C-C composites to Cu-clad Mo for thermal management applications, Mater. Sci. Eng. A 452-453 (2007) 699–704. [9] G.N. Morscher, M. Singh, T.P. Shpargel, R. Asthana, A simple test to determine the effectiveness of different braze compositions for joining Ti tubes to C/C composite plates, Mater. Sci. Eng. A 418 (1-2) (2006) 19–24. [10] M. Singh, T.P. Shpargel, G.N. Morscher, R. Asthana, Active metal brazing and characterization of brazed joints in titanium to carbon-carbon composites, Mater. Sci. Eng. A 412 (2005) 123–128. [11] R. Asthana, M. Singh, N. Sobczak, Wetting behavior and interfacial microstructure of palladium and silver-based braze alloys with ceramic matrix composites, J. Mater. Sci. 45 (16) (2010) 4276–4290. [12] M. Singh, R. Asthana, A.L. Gyekenyesi, C.E. Smith, Bonding and integration of titanium to graphitic foams for thermal management applications, Int. J. Appl. Ceram. Tech. 9 (4) (2012) 657–665. [13] C.E. Smith, A.L. Gyekenyesi, M. Singh, J. Bail, Compressive behavior of conductive graphite foams”, J. Mater. Eng. Perf. 21 (8) (2012) 1703–1707. [14] N. Sobczak, J. Sobczak, P. Rohatgi, M. Ksiazek, W. Radziwill, J. Morgiel, Interaction between Ti or Cr containing copper alloys and porous graphite substrates, in: N. Eustathopoulos, N. Sobczak (Eds.), Proc. Int. Conf. High-Temperature Capillarity, Foundry Research Institute, Krakow (Poland), 1997, pp. 145–152. [15] G. Yang, R. Wu, Y. Pan, J. Chen, R. Zhai, L. Wu, J. Lin, Direct observation of the growth process of silicon carbide nanowhiskers by vapor–solid process, Physica E Low. Syst. Nanostruct. 39 (1) (2007) 171–174. [16] W.F. Knippenberg, G. Verspui, R.C. Marshall, J.W. Faust, Jr.C.E. Ryan (Eds.), Silicon Carbide – 1972, Univ. of South Carolina Press, 1973, p. 108. [17] C.E. Ryan, I. Berman, R.C. Marshall, D.P. Considine, J.J. Hawley, J. Crystal Growth 1 (1967) 255. [18] N.J. Parratt, Fiber Reinforced Materials Technology, Van Nostrand Reinhold, London, 1972. [19] I.B. Cutler, Production of silicon carbide from rice hulls, US Patent 3 (August 754) (1973) 76. [20] J.J. Shyne, J.V. Milewski, Method of growing silicon carbide whiskers, US Patent 3 (November) (1971) 622272. [21] W.F. Knippendburg, G. Verspui, Mater. Res. Bull. 4 (1969) 33. [22] Q. Zhou, S. Dong, Y. Ding, Z. Wang, Z. Huang, D. Jiang, Ceram. Int. 35 (6) (2009) 2161–2169. [23] M.G. Nicholas, S.D. Peteves, N. Eustathopoulos (Ed.), Proceedings of HighTemperature Capillarity, Slovak Academy of Sciences, Slovakia, 1994, p. 18. [24] R. Asthana, M. Singh, N. Sobczak, Wetting behavior and interfacial microstructure of palladium and silver-based braze alloys with ceramic matrix composites, J. Mater. Sci. 45 (16) (2010) 4276–4290.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was funded through U.S. Air Force Contract FA8650-05-D5807, Prime Contractor: Universal Technology Corporation (UTC). The authors would like to thank Dr. Ajit K. Roy for technical suggestions. We would also like to thank Mr. Joseph Manter, Program Manager/Vice President for Materials & Manufacturing, UTC and other project partners. References [1] J.W. Klett, Mesophase pitch-based carbon foam: effects of precursor on thermal conductivity, Proceedings of the 23rd Annual Conference on Ceramic, Metal and Carbon Composites, Materials, and Structures, January 25–28, Cocoa Beach, 1999, pp. 657–674. [2] POCO HTC web site: http://www.poco.com/us/Thermal/htc.asp#thermal (August
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