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carbon composites to improve their wettability by copper
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Surface modification of carbon/carbon composites to improve their wettability by copper V. Casalegno *, M. Salvo, M. Ferraris Applied Science and Technology Department, Institute of Materials Physics and Engineering, Politecnico di Torino, Corso Duca degli Abruzzi 24, I-10129 Torino, Italy
A R T I C L E I N F O
A B S T R A C T
Article history:
This work proposes a simple and low cost method to modify the surface of undoped and
Received 28 October 2011
silicon-doped carbon/carbon composites (C/C) in order to widen their application field in
Accepted 13 January 2012
particular when joined to metals. The C/C surface was modified using W, Mo and Cr in
Available online 24 January 2012
an attempt to improve its wettability by copper. W, Mo and Cr powders are applied to C/C surface by the slurry technique and successively heat treated in Ar atmosphere to perform a solid state reaction with C/C. As a consequence, Cu wetting on C/C is improved and direct joining of C/C to Cu is feasible without any filler metal. The reactions between the C/C surface and the deposited metal powders were investigated by X-ray diffraction analysis, optical and electron microscopy. The best results in terms of wettability of molten copper on C/C have been achieved by modifying C/C with Cr. Cr forms a homogeneous carbide layer on the C/C surface with strong adhesion which provides a suitable substrate for molten copper. Because of the importance of surface wetting behaviour in certain applications (e.g. joining C/C to metals), C/C surface modification may enhance their use. 2012 Elsevier Ltd. All rights reserved.
1.
Introduction
There is a growing interest in carbon based materials and their use joined to metal alloys. In this respect, carbon science can help in modifying C/C surface in order to obtain a carbide coating wettable by molten metals (copper, in this case); this could have additional benefits, namely an improvement in metal/ carbon composite manufacturing and to avoid C/C degradation by strong reactivity as observed for the Cu alloy containing active elements and extensively studied in literature [1]. The aim of the C/C surface modification is the wettability improvement by molten metals, particularly by copper, since C/C must to be joined to metals in several devices, including hypersonic engines and liquid rocket propulsion systems [1]. The contact angle of molten copper on carbon substrates is comprised between 139 and 145 [2]. To the best of author’s knowledge, metallizing of C/C surface by direct reaction of W,
Mo and Cr powders has not been reported in literature, especially not involving simple deposition technique as slurry coating on C/C substrate. References [3,4] investigated C fibers metallization while our research activity focuses on the C/C surface modification. The surface modification of carbon fibre reinforced carbon composite (C/C) both Si-doped and undoped has been performed by chemical etching and solid state reaction with transition metals belonging to VIB group of the periodic table. Molybdenum, tungsten and chromium are used to modify the C/C surface and then the wettability by copper in order to directly join C/C to copper. The surface modification of C/C proposed in this paper can also be used as starting point for production of new composite materials, where a carbon to metal interface is required. As an example, one component of ITER [5], the divertor, consists of various combinations of joints between C/C, pure
* Corresponding author: Fax: +39 0115644699. E-mail address: [email protected] (V. Casalegno). 0008-6223/$ - see front matter 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2012.01.049
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Cu and Cu alloys heat sinks [6,7]. In all the cases wettability is a crucial problem. For C/C–Cu joints several technologies have been developed, like active metal casting technology, which includes special laser pre-treatment of the C/C surface followed by casting of pure Cu into C/C [8] or active metal brazing with alloys such as CuMn and CuSiAlTi [9] or pre-brazing casting of the C/C tiles by a commercial Ti-based alloy and then the joining by hot radial pressing to the CuCrZr pipe [10]. Other techniques focus on C/C surface metallization by Ti–Cu and then use of Ni–Cu–Mn braze filler [11]. The studies reported in literature on wettability of C/C mainly concern the wettability of graphite in its three different types: bulk polycrystalline, pyrolitic and vitreous carbon. Obviously, differences in bulk structure have effects on surface structure and properties, e.g. wetting behaviour is affected by the surface roughness. Moreover, wetting properties may diverge significantly from the intrinsic properties of carbon-based materials because the density of these substrates is very different and also because of the preferred orientation of graphitic hexagons with regard to the surface. Therefore, data for these systems reveal usually a wide trend range in contact angle and work of adhesion value [2]. In addition, wettability can be strongly affected by the content of oxygen and other impurities in the carbon-based materials (absorbed on the surface or in the bulk material). According to these assessments, reliable data must be obtained only by experimental work referred to a particular kind of C/C with its peculiar surface porosity, roughness, impurities, etc. As regards the wettability of metals on carbon materials, three different systems are known [12]: -non-reactive systems (metals in which the maximum solubility of carbon is lower than 1 ppm), -reactive systems in which metals can form carbides, -metals that dissolve large amounts of carbon without forming stables carbides; copper belongs to the first system. Several attempts have been carried out to improve wetting by non-reactive pure metals by the addition of C dissolving metals: for instance, Ni or Fe were added to Cu to decrease contact angle, but the results were not promising [13]. In contrast, improvements in wetting have been achieved by adding carbide-forming elements such as Cr or Ti. Addition of Cr to Cu above a critical value results in a sharp wetting transition caused by the formation of a continuous layer of wettable Cr carbides [2,14,15]. In the past [16] the effects of the alloying conditions of copper on the wetting of two types of carbon (graphite and vitreous carbon) were investigated. It was shown that the addition of small percentage of chromium in a copper alloy promotes wetting, while the same effect is obtained only with massive amounts of titanium, manganese or vanadium. Eustathopoulos et al. [2] show a periodicity in both contact angle and reaction product morphology, since alloying elements belonging to groups IV A, V A, VI A of the periodic table tend to have the most significant effects on the contact angles. Mortimer et al. [16] report about a significant effect of Cr; its addition to Cu forms a carbide reaction layer at carbon interface thus improving wetting behaviour. Many methods for joining carbon-based materials to metals foresee firstly the formation of a carbide layer and then the wetting of metal on the carbide substrate. Such carbides are often called
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‘‘metal-like’’ ceramics since they have partly metallic bonding that has effect on interfacial and adhesion properties with metals. Ramqvist [2] demonstrated that the wetting of Cu on C-based substrate is improved in presence of a metal-like compounds at the metal/ceramic interface. The essential condition for any alloying element (X) to promote the wetting on C surface by an inert metal (M) is that the interfacial energy between M and XC should be lower than the surface energy of carbon. Several carbide wetting experiments were carried out in the past [17,18]: pure copper and copper alloys alloyed with one or more carbide formers have been used; it was demonstrated that alloying with the carbide parent metal had only a slight effect on the wetting of most carbides. Some authors [19,20] demonstrated that an adequate surface modification of the C/C surface leads to improvements of the composites wettability by metals; the modification is carried out by using active metal alloys or sputtering methods, usually very expensive. In this work Cr, Mo and W were used to modify C/C in order to obtain carbide layers which can be wetted by molten copper. Such elements, in particular Cr and Mo, have been extensively used as ‘‘active elements’’ in brazing alloy for copper ‘‘active brazing’’. Typically, the brazing alloys used to join carbon–carbon composites to copper contain some wetting agents that may lead to the formation of brittle intermetallics (for example Ti5Si3, Ti2Cu) or compounds with a low melting point. This paper focuses on C/C modification, while details on copper joined to modified C/C can be found in [21,22]. C/C surface modification instead of C (graphite, etc.) substrate was investigated and the experimental activity focused on the use of transition metals belonging to VIB group to modify the C/C surface and get it ‘‘active’’, in order to perform C/C to Cu direct joining and avoiding the use of active metal brazing alloy for copper as described in [22,23]. Moreover, the ‘‘activation’’ of C/C surface has been obtained by using simple and low cost technique, i.e. slurry deposition of metallic modifier. Usually Mo, W and Cr are used as alloying elements in brazing alloy to promote wettability on C/C surface and not as pure metals directly reacting on C/C substrate.
2.
Experimental
2.1.
Materials and methods
The carbon/carbon composite materials were supplied by the French company SNECMA PROPULSION SOLIDE. The commercial names of the C/C utilized are C/C SEP NB31 and C/C SEP NS31; these two kinds of C/C are conventional 3D composites, densified through a CVI process at 1000 C followed by a graphitization heat treatment at high temperature. They are constituted by a NOVOLTEX preform with P55 ex-pitch fibers in the ydirection and ex-PAN fibers in the x-direction; the subsequent needling gives an orientation in the z direction. The C/C NS31 is a Si doped C/C; it is manufactured by an infiltration of liquid silicon leading partly to the formation of silicon carbide. NS31 contains about 8–10 at.% of silicon and its porosity is about 3–5%. Both C/C NB31 and C/C NS31
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were developed specifically for high thermal conductivity applications and nuclear environment [24]. Flat samples of both C/C were obtained by cutting larger blocks; they were then cleaned in an ultrasonic bath with ethanol to remove superficial impurities. The modified surface was chosen to be perpendicular to the high thermal conductivity direction, i.e. perpendicular to the pitch fibres of the composites. The wetting tests on modified C/C were performed on samples with dimensions 15 · 10 · 6 mm3 by using a heating microscopy (hot stage microscope Leitz GmbH AII) equipped with a Leica DBP (Ernst Leitz GMBH, Wetzlar, Germany) camera and a webcam; the samples were treated at 1100 C for 30 min in Ar atmosphere. The wetting behaviour was defined by contact angle measurements from pictures acquired by means of camera or webcam. The surface of some C/C NS31 was chemical etched in order to study the influence of Si as doping material in C/C on the wettability behaviour of the composite. A Philips 525 M type scanning electron microscope (SEM) equipped with an X-ray energy-dispersive spectrometer was used for microstructure observation of the samples and semi-quantitative composition analysis of some typical microzones at the interface. The reaction products between the composites and the transition metal were detected by X-ray diffraction (XRD) analysis (X’Pert Philips diffractometer, Cu Ka incident radiation).
2.2.
Surface modification
The carbon–carbon composite surface was modified by a solid state chemical reaction between the carbon substrate and Cr, Mo or W.
Table 1 summarizes the different process parameters, metal modifiers and XRD detected crystalline phases on C/C NS31 and C/C NB31. For the sake of completeness, mechanical test results performed on C/C/Cu presented in a previous paper [22] have been reported: they refer to single-lap test (adapted from ASTM C1292 and C1425) and they were carried out on at least five samples for each kind of joint; all samples were broken in the joined area; if not, results were not considered. Chromium and molybdenum were tested both on C/C NB31 and on silicon doped C/C NS31, while tungsten was used only on undoped C/C NB31. The metal was deposited on the C/C surface by a simple slurry technique, i.e. metal powder suspension in ethanol. The heat treatments were performed in a tubular oven in argon atmosphere (<50 mbar) or in a vacuum furnace (5 · 101 mbar) above 1000 C.
2.2.1.
Molybdenum
Mo powders (supplied by Sigma–Aldrich) are 99.99+% purity and particle size is less than 10 lm. The heat treatments were performed under vacuum or in Ar atmosphere at 1300 C and 1400 C with a dwell time of 1 or 2 h (heating rate: 10 C/min) (Table 1).
2.2.2.
Chromium
Cr powders (by Sigma–Aldrich) used for the modification of C/C NS31 and NB31 surface is 99+% purity and the particle size is less than 44 lm. The temperature and dwell time chosen for the solid state reaction treatment between C/C surface and Cr have been summarized in Table 1.
Table 1 – Summary of C/C surface modifications and wettability results: kind of C/C composites, metal modifier, heat treatment parameters (time and temperature), crystalline phases detected by XRD examination, qualitative observed contact angle of copper on modified C/C surface, apparent shear strength results, according to [21,22]. Single-lap shear tests have only been performed on most promising samples. C/C
Modifier
Sample ID
Dwell t (h)
T (C)
Phases C,Mo2C,SiC,Mo5Si3, Mo3Si MoO3,Mo C,Mo2C,SiC,Mo in some cases oxides C,Mo2C,Mo5Si3,Mo C,Mo2C,Mo C,Mo2C,SiC,Mo5Si3, Mo3Si,Mo Mo,Mo2C W,W2C,C W,WC,C W,WC,C C,Cr2C3 Cr7C3, Cr23C6 Cr7C3, Cr23C6 Cr3C2 C,Cr2C3 Cr7C3 Cr7C3, Cr23C6 Cr3C2, Cr7C3, Cr23C6 Cr7C3, Cr23C6 Cr7C3, Cr23C6, Cr3Si
NS31
Mo
L
1
1300
NS31
Mo
M
1
1400
NS31 etched NS31 NS31
Mo Mo, C (2:1) Mo, C (2:1)
P N O
1 1 1
1300 1300 1300
NB31 NB31 NB31 NB31 NB31 NB31 NB31 NB31 NB31 NB31 NB31 NB31 NS31 etched NS31
Mo W W W Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr
Q W1 W2 W3 A B C D E F G H R S
2 2 4 1 1 1 1 1 3 3 3 3 1 1
1400 1400 1400 1600 1100 1200 1300 1400 1100 1200 1300 1400 1300 1300
Cu contact angle (deg)
Average shear strength (MPa)
Refs.
5.3; machined: 10.8
[22]
30–35
[22]
30 30
[22] [22]
40
31.7, machined: 29.7
[21,22]
35.1 33.1, machined: 32.4
[21,22]
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2.2.3.
Tungsten
W powder were supplied by Sigma–Aldrich and they are 99.99+% purity; the particle size is less than 10 lm. A set of samples was heated to 1400 C; the dwell time was 1 h, 2 h, 4 h. A second set of samples was treated at 1600 C for 1 h; the heating rate was 10 C/min in any case (Table 1).
2.2.4.
Chemical etching of C/C SEP NS31
The C/C SEP NS31 samples were chemically etched by a solution of KOH in distilled water (44gr of KOH in 100 ml of water) at 85 C for 5 min. Measurements on the etched surfaces were done by XRD; the cross sections of C/C samples were analyzed by optical microscopy before and after etching.
3.
Results and discussion
3.1.
Chemical etching of C/C SEP NS31
The removal of silicon was carried out considering that the chemical etching should not induce excessive porosity at the interface or inside the composite to avoid the reduction of mechanical strength and thermal conductivity of the joint. The chemical etching of C/C NS31 was aimed to remove silicon from the faying surfaces to be joined to avoid reaction of Si with copper and metallic modifier: i.e. Cr, Mo, W and Cu can react with the infiltrated silicon to form silicides on the
Before etching
surfaces which are detrimental for the wettability and the adhesion at copper/C/C interface [25]. The chemical etching can partly remove silicon at the composite surface and reduce the formation of brittle intermetallic compounds at C/C NS31/Cu interface (for instance Cu5Si). The chemical reaction occurred at C/C surface during etching by KOH, as described in the experimental section, is supposed to be the following: Si þ 2KOH þ H2 O ! K2 SiO3 þ 2H2 In Fig. 1A, it is reported a cross section of a C/C NS31 before etching where it can be noticed Si infiltrated the carbon matrix and partially reacted to produce SiC; the same area was analyzed after chemical etching (Fig. 1B): the amount of silicon is clearly reduced but it is still detectable inside the composite; further areas were observed before and after chemical etching (Fig. 1C and D); the partial removal of silicon is noticeable. These micrographs give only a qualitative demonstration of the chemical treatment. The XRD pattern of C/C NS31 etched surface in comparison with the non-etched one is shown in Fig. 2. The peaks of the silicon carbide as well as C peaks were detected, but Si was not identified on the composite etched surface. The chemical etching was found to be effective to remove only silicon but not SiC from the C/C surface. In order to overcome this problem, some samples were etched in an ultrasonic bath: cracks propagate in SiC layer that embeds
After etching
Carbon fibres
A
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B
Carbon fibres
D
C SiC Si
Fig. 1 – Optical micrographs of C/C NS31 before (A and C) and after (B and D) chemical etching: reduced silicon amount can be seen inside red circles (figures A and B). SiC is still present after etching (figures C and D); the quality of the images is low because the samples were observed after chemical etching without any grinding or polishing treatment, in order to avoid any alteration of the surface. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 2 – XRD patterns of C/C NS31 before and after chemical etching.
silicon rich zones in C/C thus making silicon in direct contact with KOH and more easily etched. In this case the etching efficiency depends on the Si and SiC distribution and depth profile near the surface.
3.2.
Surface modification
Some general requirements must be fulfilled in order to modify efficiently the C/C surface: the modified layer should be chemically and thermally stable and the modified surface should be strongly bonded to the composite bulk. The presence of brittle intermetallics in Cu layer and of unreacted compounds must be avoided; they could melt at temperatures below or comparable to the operating temperature. Moreover, their presence as continuous layers in the reaction zones at the interface can be effective in lowering the strength of the joints.
3.2.1.
Molybdenum
In the Mo–C system at least six different phases have been identified [26] and the carbon solubility in molybdenum is low (0.007% at 1600 C and 0.02% at 2200 C). The only thermodynamically stable phases at room temperature are two types of Mo2C and c-MoC; the two different Mo2C phases display an ABAB stacking sequence of the metal planes with carbon in the octahedral sites. Order/disorder transformations of the carbon atoms give explanation for the differences between the Mo2C phases. In contrast, g-MoC1x and d-MoC1x are high-temperature phases which become stable at temperatures above about 1700 C, while the c 0 -MoC1x is considered to be a metastable phase at all compositions and temperatures. The synthesis of molybdenum carbides has been traditionally carried out by reacting metal or metal oxide with carbon or carbon containing gases [27]. It was patented in the past [28] the coating of carbon-based materials through molybdenum carbide produced by reacting a gaseous molybdenum compound with the surface of the carbonaceous material under a reducing atmosphere.
In the present paper, the molybdenum carbides were obtained by direct solid-state reaction at high temperature between the metal powder and the C/C surface (Table 1). Preliminary experiments carried out by the authors on C/C SEP N11 (not Si-doped composites) revealed that the detected phases after heat treatment (1500 C for 1 h) were: a-Mo2C, C (from C/C substrate) and unreacted Mo [29]. The limited quantity of unreacted Mo is not detrimental for copper wettability on C/C: the contact angle of molten copper on Mo is 30 at 1100 C in vacuum [2] but Mo, unbonded to C/C, leads to a weak interface, because it does not take part in any bonding to the composite surface. For C/C NS31 a lower process temperature compared to that used for C/C SEP N11 was used because 1500 C exceeds the Si melting point. Several experiments were performed varying the heating temperature between 1300 C and 1400 C, with a dwell time of 1 h. The XRD examination on the C/C surface showed the presence of unreacted Mo, Mo3Si, Mo5Si3, and Mo2C, due to reactions between Si and Mo that can produce molybdenum silicides at temperatures above 1200 C. In order to reduce the unreacted Mo, a second set of samples was prepared by deposition of a slurry made of a mixture of Mo powder and graphite powder (ratio 2:1 mol). The XRD characterization of the coating (obtained after a treatment at 1300 for 1 h) put in evidence the formation of Mo2C and silicides; unfortunately, unreacted C and Mo were still detected. Table 1 summarizes the composition of the coatings on the C/C surface obtained by different process conditions and the results of Cu-wettability tests on the coatings. The wetting experiments on samples ‘‘M’’, ‘‘L’’, ‘‘O’’ did not give promising results. The common feature of these samples is the presence in the C/C modified surface of silicides and SiC. It can be concluded that: the residual Si in the C/C matrix reacts with Mo at the temperature needed to form Mo carbide and forms silicides; furthermore, some C/C samples have SiC on the surface, and molten copper does not wet SiC in
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common experimental conditions (h = 128 C) [2]. Results on wetting of metals on SiC show the absence of wetting by moderate melting point metals presenting low reactivity with SiC, as copper. In fact, even if good wetting of Cu on SiC is expected according to [30], the wetting behaviour can be hindered by the presence of oxide or carbide layers on SiC substrates. The good result obtained on samples ‘‘N’’ can be explained by the random distribution of Si and SiC in doped C/C [31]; probably, in case of sample ‘‘N’’, the C/C surface has not a significant amount of Si and SiC (they do not appear at the interface), thus avoiding the reaction of Si with Mo. The C/C surface modification by Mo carried out after chemical etching on C/C NS31 showed a reduced amount of silicides at interface, but they were not completely absent (Table 1). While silicon carbide influences contact angle of molten copper on C/C surface, silicides have not influence on it; on the contrary they cannot provide strong interface between C/C and copper: the Cu wettability is not affected but the mechanical strength is reduced. Molten copper can react with Si and SiC starting at temperature lower than 600 C [25,32–34]; some XRD measurements performed on the cross-section of the joined C/C/Cu close to the interface, in Cu layer detected Cu5Si and Mo2C. On C/C NB31, best results were obtained with longer time and higher temperature (1400 C, 2 h): the XRD analysis performed on modified C/C NB31 surface shows that Mo2C and a residual quantity of unreacted graphite and metals were found (Fig. 3). Consequently, the optimized solid state reaction between Mo and C/C surface was performed at 1300 for 1 h for C/C NS31 (after chemical etching) and at 1400 C for 2 h for C/C NB31, under argon flow (40 mbar) in a tubular oven or in vacuum furnace; in these cases the measured contact angle of copper on the modified substrates was 30–35 [22]. The Mo carbide coating thickness ranges between 10 lm and 30 lm. The coating is homogeneous and no cracks are observed. The cross-section of a molybdenum carbide-modified C/C NS31 (etched by KOH)-Cu sample is reported in Fig. 4: the molybdenum carbide layer follows the irregular profile of the composite surface, the copper wets uniformly the carbide layer and infiltrates the surface porosity of the substrate.
Mo2C
Cu
C/C
Fig. 4 – Optical micrograph of C/C NS31/Cu sample; the Mo2C coating is clearly observable at interface between composite (previously etched by KOH) and copper.
Furthermore, the two interfaces (Cu-carbide and carbide-C/C) are continuous and without any cracks or pores. The same morphology was observed for C/C NB31.
3.2.2.
Tungsten
Tungsten carbides used for producing hard metal are conventionally synthesized by heating a mixture of metallic tungsten powder and carbon black in a graphite furnace under a flowing hydrogen atmosphere; other methods are based on a gas-solid reaction between a W source and a C-containing gas (for instance, mixture of H2 and CH4) [35]. In the last case, the better contact between the gas-solid reactants leads to lower temperatures (about 800–900 C) and higher reaction rates, if compared with the conventional method. The carburization reaction can be described as a solid state reaction between W and C; this reaction is remarkably dependent on the diffusion of C in W. Thus high temperatures (reaction temperatures in the range 1400–1800 C) and long times are necessary to obtain W carbides.
Fig. 3 – XRD pattern of C/C NB31 after modification with molybdenum at 1400 C for 2 h (Ar flow).
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According to the literature [36], carbon starts to react with tungsten to form W2C at 800 C but it is thermodynamically not stable at this temperature, in agreement with Okamoto et al. [37] that reports eutectoid decomposition of W2C at 1250 C; at higher temperatures (1600 C) WC is the stable carbide, even if it can be obtained at 1400 C for longer soaking time (4 h). Some C/C NB31 samples coated by W slurry were heated up to 1400 C and at 1600 C (the heating rate was 10 C/min). XRD measurement conducted on modified C/C NB31 surface, after each heat treatment is given in Fig. 5, where sample W1 was treated at 1400 C for 2 h, sample W2 at 1400 C for 4 h and sample W3 at 1600 C for 1 h; the first pattern refers to C/C NB31 without any surface modification. In any case unreacted graphite and free W were found. Samples treated at 1600 C for 1 h showed more homogeneous coating layer on C/C NB31, due to probable beginning
The modification of C/C NS31 and NB31 surface through the reaction of composite substrate with Cr was carried out to form chromium carbides on C/C surface, since Cu shows good wettability on chromium carbides. Wettability data of copper
Fig. 5 – XRD examination of W modified C/C NB31 surface; the first pattern refers to as received C/C NB31, sample W1 was treated at 1400 C for 2 h, sample W2 at 1400 C for 4 h and sample W3 at 1600 C for 1 h (Ar flow).
Fig. 6 – Patterns of samples A–D after Cr modification and as received C/C NB31.
of W powder sintering; that is the reason why Cu wetting tests were performed on this last kind of sample. In the following discussion the reference sample will be C/C NB31 treated at 1600 C for 1 h to obtain tungsten carbide coating. Good spreading of copper on W-modified C/C surface was achieved during wettability experiments; the copper wets uniformly the interface, but the adhesion is poor, because of the weak carbide interface at C/C surface and to the insufficient infiltration of copper in WC substrate; the contact angle value of pure liquid copper in contact with modified C/C is about 30 [22].
3.2.3.
Chromium
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Fig. 7 – XRD patterns of samples E–H after Cr modification and as received C/C NB31.
2303
on carbides are available for Cr3C2 [2] (h = 47–50) and for Cr7C3 [38] (h = 30). In [39–41] it is reported that three carbides have been identified within the C–Cr system: Cr23C6, Cr7C3 and Cr3C2; all the other reported carbides are mixture of these. The chromium carbides show more complex structures than Mo and W carbides: in fact, in the case of chromium the metal atoms are relatively small so that the metal lattice undergoes a major distortion when carbon is incorporated. The synthesis of chromium carbides is tricky since, due to the very high affinity of chromium with oxygen, there is a competition between the formation of chromium oxide and chromium carbide during the reaction process. Consequently, the heat treatment after slurry deposition of Cr powder on C/C surface gave more promising results in vacuum furnace than under Ar flow. Some authors [42] obtained a carbide layer on the surface of carbon substrates through chromium impregnation; the substrates react with Cr at 1460 C for 3 h forming Cr23C6 and Cr7C3 phases. In Ref. [43] it is reported that chromium carbides prepared by contacting Cr powders and solid carbon in a protective atmosphere (H2 or vacuum) were obtained starting from about 1225 C. Many authors [42,44] report synthesis of chromium carbide by direct reaction between Cr and Cr compounds with a carbon-containing gas (for instance methane); moreover, they refer to solid carbon or graphite or, more often, to graphite. Every set of samples was submitted to XRD examination after the treatment for the solid state reaction. Table 1 summarizes the different process parameters used for the formation of chromium carbides. In Figs. 6 and 7 the results of the analysis are summarized; it is reported, as a comparison, the XRD pattern of the asreceived C/C NB31. Reaction between C/C NB31 and Cr powder at 900 C and 1000 C for 1 h was investigated as well: from XRD analysis (not reported here) it can be noticed that no reaction happened and no carbides were obtained on carbon composite surface.
Fig. 8 – Optical micrographs of Cr carbides/C/C NB31 interface.
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In the case of surface modification at 1100 C both for 1 h and 3 h dwell time (Figs. 5 and 6, respectively), a residual quantity of unreacted graphite was found together with Cr carbides. The presence of residual graphite on
the surface can lead to an increase of the contact angle of molten copper on modified C/C; at temperatures above 1100 C (both for 1 h and for 3 h) no graphite was detected.
Cr
Point 1 Cr Cr Cr C
1 2 3 4 Full Scale 1805 cts Cursor: 0.097 (0 cts)
5
6
7
8
9
10
k 2
C
Cu
Cu
Point 2
Cu Cu
Cu
1 2 3 4 Full Scale 173 cts Cursor: 0.081 (0 cts)
5
6
7
8
9
10
keV 3
Cu
Cu
Point 3
Cu
Cu Cr Cr
1 2 3 4 Full Scale 3198 cts Cursor: 0.114 (2 cts)
Cr
5
Cu
Cr
6
7
8
9
10 keV
Fig. 9 – SEM image of the cross-section of the C/CN B31/Cr carbide/Cu interfaces and the corresponding elements mapping.
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5 0 ( 20 1 2 ) 2 2 9 6–23 0 6
According to the free energies of reactions between metallic chromium powder and graphite [39], firstly Cr23C6 is formed, then Cr7C3 and finally, at higher temperature (above 1300 C), Cr and C/C react to form Cr3C2. Even if the formation of these carbides is possible at temperature lower than 1100 C [26] it is clearly established that the kinetic aspect has strong influence on the reaction. The wetting experiments with copper were performed on A–H samples; on A and E samples copper did not wet, according to XRD analysis that revealed a large amount of unreacted carbon. On all the other samples, wetting experiments give more promising results; the contact angle of copper on C/C NB31 after solid state reaction with Cr was about 40 [22]. The Cr carbide coating on C/C obtained at lower temperature was less uniform in comparison with that obtained at upper temperature; at higher temperature and longer soaking time there is evidence of densification of chromium carbide on the C/C surface. Optical micrographs of Cr-coated C/C NB31 are shown in Fig. 8. The thickness of the coating is homogeneous along the cross-section; the interface between Cr carbide layer and the composite is continuous and independent from carbon fibers orientation (parallel or perpendicular to the surface); moreover it follows the morphology of the composite surface; the Cr-carbide layer thickness is quite constant (about 15–20 lm). The morphological analysis showed that the thickness of the chromium carbide layer does not grow significantly with an increase in reaction time: the thickness of the carbide is about 15 lm for sample sets B, C, D, F, G, H. Concerning the Cr-modification on Si-doped C/C, as in the case of molybdenum modification, Si infiltrated in doped-C/C takes part in reaction between Cr and C/C substrates. Actually, in case of C/C NS31, silicides (i.e. Cr3Si), were formed during solid state reaction, besides carbides (the C/C NS31 were processed with Cr slurry at 1300 C for 1 h, according to optimized parameters for C/C NB31, [21]). As a consequence, C/C NS31 composites were chemically etched in order to remove silicon from the surface. As reported in ref. [32,44] the interface reactions between SiC and Cr develop several phases at temperature above 900 C. The wetting tests of molten copper performed on etched C/C NS31 modified by Cr gave the same results as for C/C NB31; the measured contact angle was about 40. In case of not etched C/C NS31 molten copper can react with silicon on C/C surface; it is present as silicon carbide or as chromium silicides to form Cu5Si, as discussed for Momodified C/C NS31. Also in this case the contact angle is low, but the formation of intermetallics at C/C/Cu interface has not significant effect on the mechanical properties at C/C/Cu interface (Table 1). It can be noticed a morphological rearrangement of carbide coating on C/C when the copper spreads on the surface. In principle, the copper wets uniformly the carbide layer and infiltrates partially the coating (Fig. 9). In order to study the composition of the Cr-carbide coating and the copper infiltration in this layer, EDS analysis has been performed; the micrograph allows observing the ‘‘finger-like’’ pattern of chromium carbide into copper bulk.
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Moreover, the in depth formation of Cr-carbides resulted in a sort of mechanical joint, since the strength at interface relies also in substrate/metal interlocking. By EDS analysis it is not possible to check if the dendritic grey regions that are not in contact with C/C interface are constituted of Cr-carbide only or if it presents also Cr; in any case, unreacted pure Cr (if any) must be low, since it is not detected by XRD examination. In any case the presence of Cr is not detrimental for the joint with Cu; on the contrary it can solubilize in Cu.
4.
Conclusions
This study proposed a simple and low-cost method based on slurry coating to modify C/C surface through formation of Cr, Mo and W carbides in order to widen the C/C field of application. Metal carbide synthesis on C/C surface enhances C/C wettability by molten copper, thus enabling C/C to metal joining. The results obtained in this study can be summarized as follows: •
•
•
•
•
•
The surface modification of two different kinds of C/C (Si-doped and undoped) has been studied by analytical techniques and mechanical tests. The purpose to enhance the C/C wettability by molten copper has been obtained in all cases with Mo, W or Cr modification of C/C surface, with contact angles significantly lower than 90. Cr is the most promising metal to modify C/C surface because it reacts with C/C surface to form a homogeneous carbide layer that shows good adhesion to the composite substrate. The optimal process parameters to obtain the chromium carbide coating are 1300 C, 1 h; the temperature is lower than that used for W and Mo reaction and the heat treatment is shorter. The detected carbides are Cr23C6 and Cr7C3; this treatment also gave the best mechanical test results, according to data reported in Table 1. The role of silicon in chemical solid state reaction between Si-doped C/C and transition metals has been studied; Si and SiC react with metals to form silicides. They can be partially removed from the composite surface by a tailored chemical etching. The roughness of the C/C substrate has not been measured, but one can expect on composite materials a high roughness and therefore hysteresis phenomena during the wetting process. Further investigations will be addressed to this topic.
R E F E R E N C E S
[1] Moutis NV, Jimenez C, Azpiroz X, Speliotis Th, Wilhelmi C, Messoloras S, et al. Brazing of carbon–carbon composites to Nimonic alloys. J Mater Sci 2010;45:74–81. [2] Eustathopoulos N, Nicholas MG, Drevet B. Wettability at high temperature. Pergamon Ed; 1999.
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[3] Balani K, Agarwal A. Wetting of carbon nanotubes by aluminum oxide. Nanotechnology 2008;19:165701–9. [4] So KP, Lee IL, Duong DL, Kim TH, Lim SC, An KY, et al. Improving the wettability of aluminum on carbon nanotubes. Acta Mater 2011;59(9):3313–20. [5] Tivey R, D’Agata E, Chuyanov V, Heidl H. Overview of the engineering design of the ITER divertor improvements towards manufacture. Fus Eng Des 2005;75–79:447–50. [6] Pitts RA, Kukushkin A, Loarte A, Martin A, Merola M, Kessel CE. Status and physics basis of the ITER divertor. Phys Scripta 2009;T138:014001–10. [7] Merola M, Loesser D, Martin A, Chappuis P, Mitteau R, Komarov V, et al. ITER plasma-facing components. Fus Eng Des 2010;85(10–12):2312–22. [8] Schedler B, Huber T, Friedrich T, Eidenberger E, Kapp M, Scheu C, et al. Characteristics of an optimized active metal cast joint between copper and C/C. Phys Scripta 2007;T128:200–3. [9] Tivey R, Akiba M, Driemeyer D, Mazul I, Merola M, Ulrickson M. ITER R&D: vacuum vessel and in-vessel components: divertor cassette. Fus Eng Des 2001;55(2–3):219–29. [10] Visca E, Libera S, Mancini A, Mazzone G, Pizzuto A, Testani C. Pre-brazed casting and hot radial pressing: a reliable process for the manufacturing of CFC and W monoblock mock-ups. Fus Eng Des 2007;15–24:1651–6. [11] Ezato K, Dairaku M, Taniguchi M, Sato K, Suzuki S, Akiba M, et al. Development of ITER divertor vertical target with annular flow concept – II: development of brazing technique for CFC/CuCrZr joint and heating test of large-scale mock-up. Fus Sci Tech 2004;46(4):530–40. [12] Eustathopoulos N, Chatain D, Coudurier L. Wetting and interfacial chemistry in liquid metal-ceramic systems. Mater Sci Eng A 1991;135:83–8. [13] Arroyave R, Eagar TW. Metal substrate effects on the thermochemistry of active brazing interfaces. Acta Mater 2003;51:4871–80. [14] Dezellus O, Eustathopoulos N. Fundamental issues of reactive wetting by liquid metals. J Mater Sci 2010;45:4256–64. [15] Dezellus O, Eustathopoulos N. The role of van der Waals interactions on wetting and adhesion in metal/carbon systems. Scripta Mater 1999;40(11):1283–8. [16] Mortimer DA, Nicholas M. The wetting of carbon by copper and copper alloys. J Mater Sci 1970;5:149–55. [17] Mortimer DA, Nicholas M. The wetting of carbon and carbides by copper alloys. J Mater Sci 1973;8:640–8. [18] Abel P, Korenyi-Both A, Honecy F, Pepper S. Study of copper on graphite with titanium or chromium bond layer. J Mater Res 1994;9(3):617–24. [19] Libera S, Visca E. Junction process for a ceramic material and a metallic material with the interposition of a transition material. Patent WO/2006/024971, 2006. [20] Koppitz T, Pintsuk G, Reisgen U, Remmel J, Hirai T, Sievering R, et al. High-temperature brazing for reliable tungsten-CFC joints. Phys Scripta 2007;T128:175–81. [21] Appendino P, Ferraris M, Casalegno V, Salvo M, Merola M, Grattarola M. Direct joining of CFC to copper. J Nucl Mater 2004;329–333:1563–6. [22] Appendino P, Ferraris M, Casalegno V, Salvo M, Merola M. Proposal for a new technique to join CFC composites to copper. J Nucl Mater 2006;348:102–7.
[23] Ferraris M, Casalegno V, Salvo M. Process to join carbon based materials to metals and its applications. Patent WO2005/ 037734, 2005. [24] Pintsuk G, Compan J, Linke J, Majerus P, Peacock A, Pitzer D, et al. Mechanical and thermo-physical characterization of the carbon fibre composite NB31. Phys Scripta 2007;T128:66–71. [25] Xiao P, Derby B. Wetting of silicon carbide by chromium containing alloys. Acta Mater 1998;46(10):3491–9. [26] Moffatt WG. The handbook of binary phase diagrams. Gemini Publishing Corporation, USA; 1984. p. 55. [27] Pascal P. Nouveau traite´ de chimie mine´rale. In: Maisson C et al. Editeurs, Tome XIV. Paris; 1959. p. 728–32. [28] Weeks, Jr. Method for coating a carbonaceous material with a molybdenum carbide coating. US Patent 5914156, 1999. [29] Rado C, Kalogeropoulou S, Eustathopoulos N. Bonding and wetting in non-reactive metal/SiC systems: weak or strong interfaces? Mater Sci Eng 2000;A276:195–202. [30] Casalegno V. Joining of composite materials for nuclear fusion applications, PhD thesis. Torino, Italy: Politecnico di Torino; 2006. [31] Compan J, Hirai T, Pintsuk G, Linke J. Microstructural and thermo-mechanical characterization of carbon/carbon composites. J Nucl Mater 2009;386–388:797–800. [32] Bhanumurthy K, Schmid-Fetzer R. Interface reactions between silicon carbide and metals (Ni, Cr, Pd, Zr). Composites: Part A 2001;32:569–74. [33] Kock T, Brendel A, Bolt H. Interface reactions between silicon carbide and interlayers in silicon carbide–copper metal– matrix composites. J Nucl Mater 2007;362:197–201. [34] Liu GW, Muolo ML, Valenza A, Passerone A. Survey on wetting of SiC by molten metals. Ceram Int 2010;36:1177–88. [35] Medeiros FFP, De Oliveira SA, De Souza CP, Da Silva AGP, Gomes UU, De Souza JF. Synthesis of tungsten carbide through gas-solid reaction at low temperatures. Mater Sci Eng 2001;A315:58–62. [36] Hatano Y, Takamori M, Matsuda K, Ikeno S, Fujii K, Waranabe K. Solid state reaction between tungsten and amorphous carbon. J Nucl Mater 2002;307–311:1339–43. [37] Okamoto H. C–W (carbon–Tungsten). J Phase Eq 2008;29(6):543–4. [38] Hodaj F, Dezellus O, Barbier JN, Mortensen A, Eustathopoulos N. Diffusion-limited reactive wetting: effect of interfacial reaction behind the advancing triple line. J Mater Sci 2007;42:8071–82. [39] Udy MJ. Chromium. New York: Reinhold; London: Chapman & Hall; 1956. p. 114–7. [40] Sully AH, Brandes EA. Chromium. London: Butterworths; 1967. p. 310–1. [41] Pascal P. Nouveau traite´ de chimie mine´rale. Maisson C et al. Editeurs. Tome XIV. Paris; 1959. p. 363–9. [42] Yamamoto O, Hiura H, Sasmoto T, Inagaki M. Formation of carbide layer on carbon materials by silicon and chromium impregnation. Carbon 1996;34(5):689–91. [43] Oyama ST. The chemistry of transition metal carbides and nitrides. London: Chapman & Hall; 1996. p. 14–22. [44] Park JS, Landry K, Perepetzko JH. Kinetic control of silicon carbide/metal reactions. Mater Sci Eng 1999;A259:279–86.
5 0 ( 2 0 1 2 ) 2 2 9 6 –2 3 0 6
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Surface modification of carbon/carbon composites to improve their wettability by copper V. Casalegno *, M. Salvo, M. Ferraris Applied Science and Technology Department, Institute of Materials Physics and Engineering, Politecnico di Torino, Corso Duca degli Abruzzi 24, I-10129 Torino, Italy
A R T I C L E I N F O
A B S T R A C T
Article history:
This work proposes a simple and low cost method to modify the surface of undoped and
Received 28 October 2011
silicon-doped carbon/carbon composites (C/C) in order to widen their application field in
Accepted 13 January 2012
particular when joined to metals. The C/C surface was modified using W, Mo and Cr in
Available online 24 January 2012
an attempt to improve its wettability by copper. W, Mo and Cr powders are applied to C/C surface by the slurry technique and successively heat treated in Ar atmosphere to perform a solid state reaction with C/C. As a consequence, Cu wetting on C/C is improved and direct joining of C/C to Cu is feasible without any filler metal. The reactions between the C/C surface and the deposited metal powders were investigated by X-ray diffraction analysis, optical and electron microscopy. The best results in terms of wettability of molten copper on C/C have been achieved by modifying C/C with Cr. Cr forms a homogeneous carbide layer on the C/C surface with strong adhesion which provides a suitable substrate for molten copper. Because of the importance of surface wetting behaviour in certain applications (e.g. joining C/C to metals), C/C surface modification may enhance their use. 2012 Elsevier Ltd. All rights reserved.
1.
Introduction
There is a growing interest in carbon based materials and their use joined to metal alloys. In this respect, carbon science can help in modifying C/C surface in order to obtain a carbide coating wettable by molten metals (copper, in this case); this could have additional benefits, namely an improvement in metal/ carbon composite manufacturing and to avoid C/C degradation by strong reactivity as observed for the Cu alloy containing active elements and extensively studied in literature [1]. The aim of the C/C surface modification is the wettability improvement by molten metals, particularly by copper, since C/C must to be joined to metals in several devices, including hypersonic engines and liquid rocket propulsion systems [1]. The contact angle of molten copper on carbon substrates is comprised between 139 and 145 [2]. To the best of author’s knowledge, metallizing of C/C surface by direct reaction of W,
Mo and Cr powders has not been reported in literature, especially not involving simple deposition technique as slurry coating on C/C substrate. References [3,4] investigated C fibers metallization while our research activity focuses on the C/C surface modification. The surface modification of carbon fibre reinforced carbon composite (C/C) both Si-doped and undoped has been performed by chemical etching and solid state reaction with transition metals belonging to VIB group of the periodic table. Molybdenum, tungsten and chromium are used to modify the C/C surface and then the wettability by copper in order to directly join C/C to copper. The surface modification of C/C proposed in this paper can also be used as starting point for production of new composite materials, where a carbon to metal interface is required. As an example, one component of ITER [5], the divertor, consists of various combinations of joints between C/C, pure
* Corresponding author: Fax: +39 0115644699. E-mail address: [email protected] (V. Casalegno). 0008-6223/$ - see front matter 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2012.01.049
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5 0 ( 20 1 2 ) 2 2 9 6–23 0 6
Cu and Cu alloys heat sinks [6,7]. In all the cases wettability is a crucial problem. For C/C–Cu joints several technologies have been developed, like active metal casting technology, which includes special laser pre-treatment of the C/C surface followed by casting of pure Cu into C/C [8] or active metal brazing with alloys such as CuMn and CuSiAlTi [9] or pre-brazing casting of the C/C tiles by a commercial Ti-based alloy and then the joining by hot radial pressing to the CuCrZr pipe [10]. Other techniques focus on C/C surface metallization by Ti–Cu and then use of Ni–Cu–Mn braze filler [11]. The studies reported in literature on wettability of C/C mainly concern the wettability of graphite in its three different types: bulk polycrystalline, pyrolitic and vitreous carbon. Obviously, differences in bulk structure have effects on surface structure and properties, e.g. wetting behaviour is affected by the surface roughness. Moreover, wetting properties may diverge significantly from the intrinsic properties of carbon-based materials because the density of these substrates is very different and also because of the preferred orientation of graphitic hexagons with regard to the surface. Therefore, data for these systems reveal usually a wide trend range in contact angle and work of adhesion value [2]. In addition, wettability can be strongly affected by the content of oxygen and other impurities in the carbon-based materials (absorbed on the surface or in the bulk material). According to these assessments, reliable data must be obtained only by experimental work referred to a particular kind of C/C with its peculiar surface porosity, roughness, impurities, etc. As regards the wettability of metals on carbon materials, three different systems are known [12]: -non-reactive systems (metals in which the maximum solubility of carbon is lower than 1 ppm), -reactive systems in which metals can form carbides, -metals that dissolve large amounts of carbon without forming stables carbides; copper belongs to the first system. Several attempts have been carried out to improve wetting by non-reactive pure metals by the addition of C dissolving metals: for instance, Ni or Fe were added to Cu to decrease contact angle, but the results were not promising [13]. In contrast, improvements in wetting have been achieved by adding carbide-forming elements such as Cr or Ti. Addition of Cr to Cu above a critical value results in a sharp wetting transition caused by the formation of a continuous layer of wettable Cr carbides [2,14,15]. In the past [16] the effects of the alloying conditions of copper on the wetting of two types of carbon (graphite and vitreous carbon) were investigated. It was shown that the addition of small percentage of chromium in a copper alloy promotes wetting, while the same effect is obtained only with massive amounts of titanium, manganese or vanadium. Eustathopoulos et al. [2] show a periodicity in both contact angle and reaction product morphology, since alloying elements belonging to groups IV A, V A, VI A of the periodic table tend to have the most significant effects on the contact angles. Mortimer et al. [16] report about a significant effect of Cr; its addition to Cu forms a carbide reaction layer at carbon interface thus improving wetting behaviour. Many methods for joining carbon-based materials to metals foresee firstly the formation of a carbide layer and then the wetting of metal on the carbide substrate. Such carbides are often called
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‘‘metal-like’’ ceramics since they have partly metallic bonding that has effect on interfacial and adhesion properties with metals. Ramqvist [2] demonstrated that the wetting of Cu on C-based substrate is improved in presence of a metal-like compounds at the metal/ceramic interface. The essential condition for any alloying element (X) to promote the wetting on C surface by an inert metal (M) is that the interfacial energy between M and XC should be lower than the surface energy of carbon. Several carbide wetting experiments were carried out in the past [17,18]: pure copper and copper alloys alloyed with one or more carbide formers have been used; it was demonstrated that alloying with the carbide parent metal had only a slight effect on the wetting of most carbides. Some authors [19,20] demonstrated that an adequate surface modification of the C/C surface leads to improvements of the composites wettability by metals; the modification is carried out by using active metal alloys or sputtering methods, usually very expensive. In this work Cr, Mo and W were used to modify C/C in order to obtain carbide layers which can be wetted by molten copper. Such elements, in particular Cr and Mo, have been extensively used as ‘‘active elements’’ in brazing alloy for copper ‘‘active brazing’’. Typically, the brazing alloys used to join carbon–carbon composites to copper contain some wetting agents that may lead to the formation of brittle intermetallics (for example Ti5Si3, Ti2Cu) or compounds with a low melting point. This paper focuses on C/C modification, while details on copper joined to modified C/C can be found in [21,22]. C/C surface modification instead of C (graphite, etc.) substrate was investigated and the experimental activity focused on the use of transition metals belonging to VIB group to modify the C/C surface and get it ‘‘active’’, in order to perform C/C to Cu direct joining and avoiding the use of active metal brazing alloy for copper as described in [22,23]. Moreover, the ‘‘activation’’ of C/C surface has been obtained by using simple and low cost technique, i.e. slurry deposition of metallic modifier. Usually Mo, W and Cr are used as alloying elements in brazing alloy to promote wettability on C/C surface and not as pure metals directly reacting on C/C substrate.
2.
Experimental
2.1.
Materials and methods
The carbon/carbon composite materials were supplied by the French company SNECMA PROPULSION SOLIDE. The commercial names of the C/C utilized are C/C SEP NB31 and C/C SEP NS31; these two kinds of C/C are conventional 3D composites, densified through a CVI process at 1000 C followed by a graphitization heat treatment at high temperature. They are constituted by a NOVOLTEX preform with P55 ex-pitch fibers in the ydirection and ex-PAN fibers in the x-direction; the subsequent needling gives an orientation in the z direction. The C/C NS31 is a Si doped C/C; it is manufactured by an infiltration of liquid silicon leading partly to the formation of silicon carbide. NS31 contains about 8–10 at.% of silicon and its porosity is about 3–5%. Both C/C NB31 and C/C NS31
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were developed specifically for high thermal conductivity applications and nuclear environment [24]. Flat samples of both C/C were obtained by cutting larger blocks; they were then cleaned in an ultrasonic bath with ethanol to remove superficial impurities. The modified surface was chosen to be perpendicular to the high thermal conductivity direction, i.e. perpendicular to the pitch fibres of the composites. The wetting tests on modified C/C were performed on samples with dimensions 15 · 10 · 6 mm3 by using a heating microscopy (hot stage microscope Leitz GmbH AII) equipped with a Leica DBP (Ernst Leitz GMBH, Wetzlar, Germany) camera and a webcam; the samples were treated at 1100 C for 30 min in Ar atmosphere. The wetting behaviour was defined by contact angle measurements from pictures acquired by means of camera or webcam. The surface of some C/C NS31 was chemical etched in order to study the influence of Si as doping material in C/C on the wettability behaviour of the composite. A Philips 525 M type scanning electron microscope (SEM) equipped with an X-ray energy-dispersive spectrometer was used for microstructure observation of the samples and semi-quantitative composition analysis of some typical microzones at the interface. The reaction products between the composites and the transition metal were detected by X-ray diffraction (XRD) analysis (X’Pert Philips diffractometer, Cu Ka incident radiation).
2.2.
Surface modification
The carbon–carbon composite surface was modified by a solid state chemical reaction between the carbon substrate and Cr, Mo or W.
Table 1 summarizes the different process parameters, metal modifiers and XRD detected crystalline phases on C/C NS31 and C/C NB31. For the sake of completeness, mechanical test results performed on C/C/Cu presented in a previous paper [22] have been reported: they refer to single-lap test (adapted from ASTM C1292 and C1425) and they were carried out on at least five samples for each kind of joint; all samples were broken in the joined area; if not, results were not considered. Chromium and molybdenum were tested both on C/C NB31 and on silicon doped C/C NS31, while tungsten was used only on undoped C/C NB31. The metal was deposited on the C/C surface by a simple slurry technique, i.e. metal powder suspension in ethanol. The heat treatments were performed in a tubular oven in argon atmosphere (<50 mbar) or in a vacuum furnace (5 · 101 mbar) above 1000 C.
2.2.1.
Molybdenum
Mo powders (supplied by Sigma–Aldrich) are 99.99+% purity and particle size is less than 10 lm. The heat treatments were performed under vacuum or in Ar atmosphere at 1300 C and 1400 C with a dwell time of 1 or 2 h (heating rate: 10 C/min) (Table 1).
2.2.2.
Chromium
Cr powders (by Sigma–Aldrich) used for the modification of C/C NS31 and NB31 surface is 99+% purity and the particle size is less than 44 lm. The temperature and dwell time chosen for the solid state reaction treatment between C/C surface and Cr have been summarized in Table 1.
Table 1 – Summary of C/C surface modifications and wettability results: kind of C/C composites, metal modifier, heat treatment parameters (time and temperature), crystalline phases detected by XRD examination, qualitative observed contact angle of copper on modified C/C surface, apparent shear strength results, according to [21,22]. Single-lap shear tests have only been performed on most promising samples. C/C
Modifier
Sample ID
Dwell t (h)
T (C)
Phases C,Mo2C,SiC,Mo5Si3, Mo3Si MoO3,Mo C,Mo2C,SiC,Mo in some cases oxides C,Mo2C,Mo5Si3,Mo C,Mo2C,Mo C,Mo2C,SiC,Mo5Si3, Mo3Si,Mo Mo,Mo2C W,W2C,C W,WC,C W,WC,C C,Cr2C3 Cr7C3, Cr23C6 Cr7C3, Cr23C6 Cr3C2 C,Cr2C3 Cr7C3 Cr7C3, Cr23C6 Cr3C2, Cr7C3, Cr23C6 Cr7C3, Cr23C6 Cr7C3, Cr23C6, Cr3Si
NS31
Mo
L
1
1300
NS31
Mo
M
1
1400
NS31 etched NS31 NS31
Mo Mo, C (2:1) Mo, C (2:1)
P N O
1 1 1
1300 1300 1300
NB31 NB31 NB31 NB31 NB31 NB31 NB31 NB31 NB31 NB31 NB31 NB31 NS31 etched NS31
Mo W W W Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr
Q W1 W2 W3 A B C D E F G H R S
2 2 4 1 1 1 1 1 3 3 3 3 1 1
1400 1400 1400 1600 1100 1200 1300 1400 1100 1200 1300 1400 1300 1300
Cu contact angle (deg)
Average shear strength (MPa)
Refs.
5.3; machined: 10.8
[22]
30–35
[22]
30 30
[22] [22]
40
31.7, machined: 29.7
[21,22]
35.1 33.1, machined: 32.4
[21,22]
CARBON
2.2.3.
Tungsten
W powder were supplied by Sigma–Aldrich and they are 99.99+% purity; the particle size is less than 10 lm. A set of samples was heated to 1400 C; the dwell time was 1 h, 2 h, 4 h. A second set of samples was treated at 1600 C for 1 h; the heating rate was 10 C/min in any case (Table 1).
2.2.4.
Chemical etching of C/C SEP NS31
The C/C SEP NS31 samples were chemically etched by a solution of KOH in distilled water (44gr of KOH in 100 ml of water) at 85 C for 5 min. Measurements on the etched surfaces were done by XRD; the cross sections of C/C samples were analyzed by optical microscopy before and after etching.
3.
Results and discussion
3.1.
Chemical etching of C/C SEP NS31
The removal of silicon was carried out considering that the chemical etching should not induce excessive porosity at the interface or inside the composite to avoid the reduction of mechanical strength and thermal conductivity of the joint. The chemical etching of C/C NS31 was aimed to remove silicon from the faying surfaces to be joined to avoid reaction of Si with copper and metallic modifier: i.e. Cr, Mo, W and Cu can react with the infiltrated silicon to form silicides on the
Before etching
surfaces which are detrimental for the wettability and the adhesion at copper/C/C interface [25]. The chemical etching can partly remove silicon at the composite surface and reduce the formation of brittle intermetallic compounds at C/C NS31/Cu interface (for instance Cu5Si). The chemical reaction occurred at C/C surface during etching by KOH, as described in the experimental section, is supposed to be the following: Si þ 2KOH þ H2 O ! K2 SiO3 þ 2H2 In Fig. 1A, it is reported a cross section of a C/C NS31 before etching where it can be noticed Si infiltrated the carbon matrix and partially reacted to produce SiC; the same area was analyzed after chemical etching (Fig. 1B): the amount of silicon is clearly reduced but it is still detectable inside the composite; further areas were observed before and after chemical etching (Fig. 1C and D); the partial removal of silicon is noticeable. These micrographs give only a qualitative demonstration of the chemical treatment. The XRD pattern of C/C NS31 etched surface in comparison with the non-etched one is shown in Fig. 2. The peaks of the silicon carbide as well as C peaks were detected, but Si was not identified on the composite etched surface. The chemical etching was found to be effective to remove only silicon but not SiC from the C/C surface. In order to overcome this problem, some samples were etched in an ultrasonic bath: cracks propagate in SiC layer that embeds
After etching
Carbon fibres
A
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5 0 ( 20 1 2 ) 2 2 9 6–23 0 6
B
Carbon fibres
D
C SiC Si
Fig. 1 – Optical micrographs of C/C NS31 before (A and C) and after (B and D) chemical etching: reduced silicon amount can be seen inside red circles (figures A and B). SiC is still present after etching (figures C and D); the quality of the images is low because the samples were observed after chemical etching without any grinding or polishing treatment, in order to avoid any alteration of the surface. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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5 0 ( 2 0 1 2 ) 2 2 9 6 –2 3 0 6
Fig. 2 – XRD patterns of C/C NS31 before and after chemical etching.
silicon rich zones in C/C thus making silicon in direct contact with KOH and more easily etched. In this case the etching efficiency depends on the Si and SiC distribution and depth profile near the surface.
3.2.
Surface modification
Some general requirements must be fulfilled in order to modify efficiently the C/C surface: the modified layer should be chemically and thermally stable and the modified surface should be strongly bonded to the composite bulk. The presence of brittle intermetallics in Cu layer and of unreacted compounds must be avoided; they could melt at temperatures below or comparable to the operating temperature. Moreover, their presence as continuous layers in the reaction zones at the interface can be effective in lowering the strength of the joints.
3.2.1.
Molybdenum
In the Mo–C system at least six different phases have been identified [26] and the carbon solubility in molybdenum is low (0.007% at 1600 C and 0.02% at 2200 C). The only thermodynamically stable phases at room temperature are two types of Mo2C and c-MoC; the two different Mo2C phases display an ABAB stacking sequence of the metal planes with carbon in the octahedral sites. Order/disorder transformations of the carbon atoms give explanation for the differences between the Mo2C phases. In contrast, g-MoC1x and d-MoC1x are high-temperature phases which become stable at temperatures above about 1700 C, while the c 0 -MoC1x is considered to be a metastable phase at all compositions and temperatures. The synthesis of molybdenum carbides has been traditionally carried out by reacting metal or metal oxide with carbon or carbon containing gases [27]. It was patented in the past [28] the coating of carbon-based materials through molybdenum carbide produced by reacting a gaseous molybdenum compound with the surface of the carbonaceous material under a reducing atmosphere.
In the present paper, the molybdenum carbides were obtained by direct solid-state reaction at high temperature between the metal powder and the C/C surface (Table 1). Preliminary experiments carried out by the authors on C/C SEP N11 (not Si-doped composites) revealed that the detected phases after heat treatment (1500 C for 1 h) were: a-Mo2C, C (from C/C substrate) and unreacted Mo [29]. The limited quantity of unreacted Mo is not detrimental for copper wettability on C/C: the contact angle of molten copper on Mo is 30 at 1100 C in vacuum [2] but Mo, unbonded to C/C, leads to a weak interface, because it does not take part in any bonding to the composite surface. For C/C NS31 a lower process temperature compared to that used for C/C SEP N11 was used because 1500 C exceeds the Si melting point. Several experiments were performed varying the heating temperature between 1300 C and 1400 C, with a dwell time of 1 h. The XRD examination on the C/C surface showed the presence of unreacted Mo, Mo3Si, Mo5Si3, and Mo2C, due to reactions between Si and Mo that can produce molybdenum silicides at temperatures above 1200 C. In order to reduce the unreacted Mo, a second set of samples was prepared by deposition of a slurry made of a mixture of Mo powder and graphite powder (ratio 2:1 mol). The XRD characterization of the coating (obtained after a treatment at 1300 for 1 h) put in evidence the formation of Mo2C and silicides; unfortunately, unreacted C and Mo were still detected. Table 1 summarizes the composition of the coatings on the C/C surface obtained by different process conditions and the results of Cu-wettability tests on the coatings. The wetting experiments on samples ‘‘M’’, ‘‘L’’, ‘‘O’’ did not give promising results. The common feature of these samples is the presence in the C/C modified surface of silicides and SiC. It can be concluded that: the residual Si in the C/C matrix reacts with Mo at the temperature needed to form Mo carbide and forms silicides; furthermore, some C/C samples have SiC on the surface, and molten copper does not wet SiC in
CARBON
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common experimental conditions (h = 128 C) [2]. Results on wetting of metals on SiC show the absence of wetting by moderate melting point metals presenting low reactivity with SiC, as copper. In fact, even if good wetting of Cu on SiC is expected according to [30], the wetting behaviour can be hindered by the presence of oxide or carbide layers on SiC substrates. The good result obtained on samples ‘‘N’’ can be explained by the random distribution of Si and SiC in doped C/C [31]; probably, in case of sample ‘‘N’’, the C/C surface has not a significant amount of Si and SiC (they do not appear at the interface), thus avoiding the reaction of Si with Mo. The C/C surface modification by Mo carried out after chemical etching on C/C NS31 showed a reduced amount of silicides at interface, but they were not completely absent (Table 1). While silicon carbide influences contact angle of molten copper on C/C surface, silicides have not influence on it; on the contrary they cannot provide strong interface between C/C and copper: the Cu wettability is not affected but the mechanical strength is reduced. Molten copper can react with Si and SiC starting at temperature lower than 600 C [25,32–34]; some XRD measurements performed on the cross-section of the joined C/C/Cu close to the interface, in Cu layer detected Cu5Si and Mo2C. On C/C NB31, best results were obtained with longer time and higher temperature (1400 C, 2 h): the XRD analysis performed on modified C/C NB31 surface shows that Mo2C and a residual quantity of unreacted graphite and metals were found (Fig. 3). Consequently, the optimized solid state reaction between Mo and C/C surface was performed at 1300 for 1 h for C/C NS31 (after chemical etching) and at 1400 C for 2 h for C/C NB31, under argon flow (40 mbar) in a tubular oven or in vacuum furnace; in these cases the measured contact angle of copper on the modified substrates was 30–35 [22]. The Mo carbide coating thickness ranges between 10 lm and 30 lm. The coating is homogeneous and no cracks are observed. The cross-section of a molybdenum carbide-modified C/C NS31 (etched by KOH)-Cu sample is reported in Fig. 4: the molybdenum carbide layer follows the irregular profile of the composite surface, the copper wets uniformly the carbide layer and infiltrates the surface porosity of the substrate.
Mo2C
Cu
C/C
Fig. 4 – Optical micrograph of C/C NS31/Cu sample; the Mo2C coating is clearly observable at interface between composite (previously etched by KOH) and copper.
Furthermore, the two interfaces (Cu-carbide and carbide-C/C) are continuous and without any cracks or pores. The same morphology was observed for C/C NB31.
3.2.2.
Tungsten
Tungsten carbides used for producing hard metal are conventionally synthesized by heating a mixture of metallic tungsten powder and carbon black in a graphite furnace under a flowing hydrogen atmosphere; other methods are based on a gas-solid reaction between a W source and a C-containing gas (for instance, mixture of H2 and CH4) [35]. In the last case, the better contact between the gas-solid reactants leads to lower temperatures (about 800–900 C) and higher reaction rates, if compared with the conventional method. The carburization reaction can be described as a solid state reaction between W and C; this reaction is remarkably dependent on the diffusion of C in W. Thus high temperatures (reaction temperatures in the range 1400–1800 C) and long times are necessary to obtain W carbides.
Fig. 3 – XRD pattern of C/C NB31 after modification with molybdenum at 1400 C for 2 h (Ar flow).
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5 0 ( 2 0 1 2 ) 2 2 9 6 –2 3 0 6
According to the literature [36], carbon starts to react with tungsten to form W2C at 800 C but it is thermodynamically not stable at this temperature, in agreement with Okamoto et al. [37] that reports eutectoid decomposition of W2C at 1250 C; at higher temperatures (1600 C) WC is the stable carbide, even if it can be obtained at 1400 C for longer soaking time (4 h). Some C/C NB31 samples coated by W slurry were heated up to 1400 C and at 1600 C (the heating rate was 10 C/min). XRD measurement conducted on modified C/C NB31 surface, after each heat treatment is given in Fig. 5, where sample W1 was treated at 1400 C for 2 h, sample W2 at 1400 C for 4 h and sample W3 at 1600 C for 1 h; the first pattern refers to C/C NB31 without any surface modification. In any case unreacted graphite and free W were found. Samples treated at 1600 C for 1 h showed more homogeneous coating layer on C/C NB31, due to probable beginning
The modification of C/C NS31 and NB31 surface through the reaction of composite substrate with Cr was carried out to form chromium carbides on C/C surface, since Cu shows good wettability on chromium carbides. Wettability data of copper
Fig. 5 – XRD examination of W modified C/C NB31 surface; the first pattern refers to as received C/C NB31, sample W1 was treated at 1400 C for 2 h, sample W2 at 1400 C for 4 h and sample W3 at 1600 C for 1 h (Ar flow).
Fig. 6 – Patterns of samples A–D after Cr modification and as received C/C NB31.
of W powder sintering; that is the reason why Cu wetting tests were performed on this last kind of sample. In the following discussion the reference sample will be C/C NB31 treated at 1600 C for 1 h to obtain tungsten carbide coating. Good spreading of copper on W-modified C/C surface was achieved during wettability experiments; the copper wets uniformly the interface, but the adhesion is poor, because of the weak carbide interface at C/C surface and to the insufficient infiltration of copper in WC substrate; the contact angle value of pure liquid copper in contact with modified C/C is about 30 [22].
3.2.3.
Chromium
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5 0 ( 20 1 2 ) 2 2 9 6–23 0 6
Fig. 7 – XRD patterns of samples E–H after Cr modification and as received C/C NB31.
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on carbides are available for Cr3C2 [2] (h = 47–50) and for Cr7C3 [38] (h = 30). In [39–41] it is reported that three carbides have been identified within the C–Cr system: Cr23C6, Cr7C3 and Cr3C2; all the other reported carbides are mixture of these. The chromium carbides show more complex structures than Mo and W carbides: in fact, in the case of chromium the metal atoms are relatively small so that the metal lattice undergoes a major distortion when carbon is incorporated. The synthesis of chromium carbides is tricky since, due to the very high affinity of chromium with oxygen, there is a competition between the formation of chromium oxide and chromium carbide during the reaction process. Consequently, the heat treatment after slurry deposition of Cr powder on C/C surface gave more promising results in vacuum furnace than under Ar flow. Some authors [42] obtained a carbide layer on the surface of carbon substrates through chromium impregnation; the substrates react with Cr at 1460 C for 3 h forming Cr23C6 and Cr7C3 phases. In Ref. [43] it is reported that chromium carbides prepared by contacting Cr powders and solid carbon in a protective atmosphere (H2 or vacuum) were obtained starting from about 1225 C. Many authors [42,44] report synthesis of chromium carbide by direct reaction between Cr and Cr compounds with a carbon-containing gas (for instance methane); moreover, they refer to solid carbon or graphite or, more often, to graphite. Every set of samples was submitted to XRD examination after the treatment for the solid state reaction. Table 1 summarizes the different process parameters used for the formation of chromium carbides. In Figs. 6 and 7 the results of the analysis are summarized; it is reported, as a comparison, the XRD pattern of the asreceived C/C NB31. Reaction between C/C NB31 and Cr powder at 900 C and 1000 C for 1 h was investigated as well: from XRD analysis (not reported here) it can be noticed that no reaction happened and no carbides were obtained on carbon composite surface.
Fig. 8 – Optical micrographs of Cr carbides/C/C NB31 interface.
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5 0 ( 2 0 1 2 ) 2 2 9 6 –2 3 0 6
In the case of surface modification at 1100 C both for 1 h and 3 h dwell time (Figs. 5 and 6, respectively), a residual quantity of unreacted graphite was found together with Cr carbides. The presence of residual graphite on
the surface can lead to an increase of the contact angle of molten copper on modified C/C; at temperatures above 1100 C (both for 1 h and for 3 h) no graphite was detected.
Cr
Point 1 Cr Cr Cr C
1 2 3 4 Full Scale 1805 cts Cursor: 0.097 (0 cts)
5
6
7
8
9
10
k 2
C
Cu
Cu
Point 2
Cu Cu
Cu
1 2 3 4 Full Scale 173 cts Cursor: 0.081 (0 cts)
5
6
7
8
9
10
keV 3
Cu
Cu
Point 3
Cu
Cu Cr Cr
1 2 3 4 Full Scale 3198 cts Cursor: 0.114 (2 cts)
Cr
5
Cu
Cr
6
7
8
9
10 keV
Fig. 9 – SEM image of the cross-section of the C/CN B31/Cr carbide/Cu interfaces and the corresponding elements mapping.
CARBON
5 0 ( 20 1 2 ) 2 2 9 6–23 0 6
According to the free energies of reactions between metallic chromium powder and graphite [39], firstly Cr23C6 is formed, then Cr7C3 and finally, at higher temperature (above 1300 C), Cr and C/C react to form Cr3C2. Even if the formation of these carbides is possible at temperature lower than 1100 C [26] it is clearly established that the kinetic aspect has strong influence on the reaction. The wetting experiments with copper were performed on A–H samples; on A and E samples copper did not wet, according to XRD analysis that revealed a large amount of unreacted carbon. On all the other samples, wetting experiments give more promising results; the contact angle of copper on C/C NB31 after solid state reaction with Cr was about 40 [22]. The Cr carbide coating on C/C obtained at lower temperature was less uniform in comparison with that obtained at upper temperature; at higher temperature and longer soaking time there is evidence of densification of chromium carbide on the C/C surface. Optical micrographs of Cr-coated C/C NB31 are shown in Fig. 8. The thickness of the coating is homogeneous along the cross-section; the interface between Cr carbide layer and the composite is continuous and independent from carbon fibers orientation (parallel or perpendicular to the surface); moreover it follows the morphology of the composite surface; the Cr-carbide layer thickness is quite constant (about 15–20 lm). The morphological analysis showed that the thickness of the chromium carbide layer does not grow significantly with an increase in reaction time: the thickness of the carbide is about 15 lm for sample sets B, C, D, F, G, H. Concerning the Cr-modification on Si-doped C/C, as in the case of molybdenum modification, Si infiltrated in doped-C/C takes part in reaction between Cr and C/C substrates. Actually, in case of C/C NS31, silicides (i.e. Cr3Si), were formed during solid state reaction, besides carbides (the C/C NS31 were processed with Cr slurry at 1300 C for 1 h, according to optimized parameters for C/C NB31, [21]). As a consequence, C/C NS31 composites were chemically etched in order to remove silicon from the surface. As reported in ref. [32,44] the interface reactions between SiC and Cr develop several phases at temperature above 900 C. The wetting tests of molten copper performed on etched C/C NS31 modified by Cr gave the same results as for C/C NB31; the measured contact angle was about 40. In case of not etched C/C NS31 molten copper can react with silicon on C/C surface; it is present as silicon carbide or as chromium silicides to form Cu5Si, as discussed for Momodified C/C NS31. Also in this case the contact angle is low, but the formation of intermetallics at C/C/Cu interface has not significant effect on the mechanical properties at C/C/Cu interface (Table 1). It can be noticed a morphological rearrangement of carbide coating on C/C when the copper spreads on the surface. In principle, the copper wets uniformly the carbide layer and infiltrates partially the coating (Fig. 9). In order to study the composition of the Cr-carbide coating and the copper infiltration in this layer, EDS analysis has been performed; the micrograph allows observing the ‘‘finger-like’’ pattern of chromium carbide into copper bulk.
2305
Moreover, the in depth formation of Cr-carbides resulted in a sort of mechanical joint, since the strength at interface relies also in substrate/metal interlocking. By EDS analysis it is not possible to check if the dendritic grey regions that are not in contact with C/C interface are constituted of Cr-carbide only or if it presents also Cr; in any case, unreacted pure Cr (if any) must be low, since it is not detected by XRD examination. In any case the presence of Cr is not detrimental for the joint with Cu; on the contrary it can solubilize in Cu.
4.
Conclusions
This study proposed a simple and low-cost method based on slurry coating to modify C/C surface through formation of Cr, Mo and W carbides in order to widen the C/C field of application. Metal carbide synthesis on C/C surface enhances C/C wettability by molten copper, thus enabling C/C to metal joining. The results obtained in this study can be summarized as follows: •
•
•
•
•
•
The surface modification of two different kinds of C/C (Si-doped and undoped) has been studied by analytical techniques and mechanical tests. The purpose to enhance the C/C wettability by molten copper has been obtained in all cases with Mo, W or Cr modification of C/C surface, with contact angles significantly lower than 90. Cr is the most promising metal to modify C/C surface because it reacts with C/C surface to form a homogeneous carbide layer that shows good adhesion to the composite substrate. The optimal process parameters to obtain the chromium carbide coating are 1300 C, 1 h; the temperature is lower than that used for W and Mo reaction and the heat treatment is shorter. The detected carbides are Cr23C6 and Cr7C3; this treatment also gave the best mechanical test results, according to data reported in Table 1. The role of silicon in chemical solid state reaction between Si-doped C/C and transition metals has been studied; Si and SiC react with metals to form silicides. They can be partially removed from the composite surface by a tailored chemical etching. The roughness of the C/C substrate has not been measured, but one can expect on composite materials a high roughness and therefore hysteresis phenomena during the wetting process. Further investigations will be addressed to this topic.
R E F E R E N C E S
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