Materials Science and Engineering A259 (1999) 279–286

Kinetic control of silicon carbide/metal reactions

J.S. Park a,*, K. Landry a, J.H. Perepezko a

a Department of Materials Science and Engineering, Uni6ersity of Wisconsin, 1509 Uni6ersity A6e., Madison, WI 53706, USA

Abstract

The kinetic features governing SiC/metal reactions have been investigated to identify the operating mechanisms and diffusioncharacteristics. With respect to the contact metal components, the reaction characteristics were identified by two separatemodes-formation of silicides and free carbon (Mode I), or formation of carbides and silicides (Mode II). The analysis wasconfirmed by comparing SiC/Ni and SiC/Cr reactions. The diffusion pathways for both reactions were examined in terms of achemical potential framework. The application of the analysis has been extended to in-situ interface reactions of SiC/Cu/Ni andSiC/Cr/Ni. New reaction modes representing SiC/metal reactions are considered as well as a strategy to control the reactionpathway. © 1999 Elsevier Science S.A. All rights reserved.

Keywords: Interfacial reactions; Reaction pathway; Kinetics; SiC/metal; Diffusion

1. Introduction

The control of interface reactions between reinforce-ment materials and matrix is a significant issue andshould be considered together with individual materialsproperties in a successful composite design [1]. Amongthe potential reinforcement materials, SiC is an attrac-tive candidate with superior properties such as corro-sion resistance, high modulus and high strength up tohigh temperatures [2]. A fair number of studies aboutthe SiC/metal interface reactions have been reportedand include investigations on the thermal compatibilityand/or the solid state reactions [3–5]. The reactionproduct sequence and interface morphology in SiC/metal reactions are mainly dependent on the contactmaterials. For example, the products of the SiC/Nireaction show a periodic morphology composed ofalternating carbon and silicides [6], but those of theSiC/Cr reaction reveal planar layers of carbides andsilicides [7]. While the characterization of individualSiC/metal interface reactions provides useful informa-tion, this is not sufficient to interpret the mechanismgoverning the SiC/metal reactions and reaction modes.Some reviews about SiC/metal reactions regarding tothe phase equilibria and phase evolution have beenpublished [8–10]. However, it is still necessary to definegoverning factors and reaction pathways. In fact, the

compiled data basis for the SiC/metal interface reac-tions can offer a guideline to control the interfaceproperties and phase evolution as well [11]. For exam-ple, when the reaction products obtained during inter-diffusion processing are not favorable in terms ofmaterial properties, a modification of the diffusionpathway can be designed by employing a constructeddatabase to identify a suitable phase combination selec-tion [12]. In this paper, the diffusion pathway andphase equilibria of the Si–C–Metal (Me) ternary sys-tems are systematically investigated to provide a guide-line for controlling SiC/metal reactions. Also, bothSiC/Ni and SiC/Cr reactions have been comparedbased upon the phase evolution and a chemical poten-tial framework. Finally, a strategy has been devised forcontrolling the reaction pathway of SiC/Ni with Cr andCu biased reactions.

2. Phase equilibria of Si–C–X systems

Since the phase evolution during diffusion in bulkmaterials is affected by the phase equilibria, it is usefulto investigate the isothermal phase diagrams relevant toSiC/Me reactions. Based upon published isothermalphase diagrams of ternary Si–C–X (X: Ni [13], Ti [14],Mo [15], etc.) systems, two general points can be madeconcerning these systems. First, the solubility of sili-cides and carbides is limited. In the case of the Si–C–* Corresponding author.

0921-5093/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.

PII S0921-5093(98)00899-5

J.S. Park et al. / Materials Science and Engineering A259 (1999) 279–286J.S. Park et al. / Materials Science and Engineering A259 (1999) 279–286280

Fig. 1. The schematic isothermal phase diagrams and reaction pathway of SiC/X(I) and X(II).

Ni ternary system (850°C), a limited solubility of sili-cides and carbide (SiC) has been reported [16]. Sec-ond, if a ternary phase exists, it is in equilibrium withone of the carbides and silicides. For example, in theSi–C–Mo system (1200°C), the ternary phase(Mo5Si3C) is in equilibrium with the Mo2C andMo5Si3 phases [17]. From this comparison, the Si–C–X ternary systems can be separated into type I andtype II based upon the shape of the isothermal phasediagram. The main features of type I and type II arethe following. Type I: (1) There is no compound in theMe–C binary system. (2) A three phase region (SiC–C–Silicide) exists in the ternary isothermal phase dia-gram. Type II: (1) At least one compound exists in the

Me–C binary system. (2) If a ternary phase exists, it isin equilibrium with a silicide and a carbide (no ternaryphase is reported in the Si–C–W [18] and Si–C–V[19] systems. Throughout this paper, X(I) will indicatea type I element and X(II) a type II element. In orderto simplify the description of the Si–C–X systems,two schematic ternary isothermal phase diagrams ofSi–C–X systems with the diffusion pathway are illus-trated in Fig. 1. The temperature of these diagrams isassumed to be under the melting temperature of anybinary or ternary phase. Since the reaction pathway isaffected by the phase equilibria, it is meaningful toexamine the diffusion pathway based upon this infor-mation.

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3. Diffusion pathway of Si–C–X systems

3.1. SiC/X(I) interface reactions

This reaction mode includes interdiffusion reactionsproducing free carbon and silicides. The formation ofsilicides is associated with producing free carbon inorder to satisfy the mass balance. A typical example isthe SiC/Ni reaction, in which periodic products aredeveloped. The reaction pathway of the SiC/Ni (type I)reaction is indicated on the schematic isothermal phasediagram as solid arrows (Fig. 1a). As free carbon formsnext to SiC [20], the diffusion pathway moves from theSiC to the carbon side of the phase diagram [6]. Then,it is shifted towards the silicide side in order to satisfythe mass balance requirements.

3.2. SiC/X(II) interface reactions

This reaction mode stands for the formation of car-bides and silicides as reaction products. The formationof carbides as products is observed in many systemssuch as SiC/Ti [14] and SiC/Cr [7]. The reaction path-way is marked by solid arrows, as shown in Fig. 1(b).In this case, carbon acts as a dominant componentcompared to other elements as indicated by the produc-tion of carbides as reaction products. The productformed next to SiC is a ternary phase or a carbidephase depending on the component diffusion kinetics.Again, as a result of mass balance requirements, reac-tion products such as silicides and carbides areproduced.

In the SiC/Me reactions, the properties of the contactmaterials affect the final diffusion pathway and mor-phology. In the case of the SiC/X(I) reactions, the finalreaction products are silicides and free carbon. In thecase of the formation of free carbon during SiC/X(I)such as the SiC/Ni combination, the reaction pathwayshould involve the formation of silicides in order tosatisfy the mass balance. If the contact material is astrong carbide former (affinity of C and metal is high)such as Mo [22], free carbon is not obtained as aproduct but both carbides and silicides. It should benoted that in the diffusion zone the neighboring phaseof the XC carbide or ternary phase is always a silicide.As summarized in Fig. 2, it seems that two governingfactors—the phase equilibria and mass balance require-ments—affect the formation of the product sequence.

4. SiC/Me reaction mode

Based upon the above considerations, the reactionmodes representing the SiC/Me systems have been for-mulated as follows:

Me + SiC� silicides + C(graphite) (type I)

Me + SiC�silicides + carbides + (MexSiyCz)

(type II)

The silicides with free carbon mixtures are productsof SiC/X(I) reactions, and the silicides with carbidelayers are the products of SiC/X(II) reactions. Aternary phase (MexSiyCz) is included in parenthesis togeneralize the reaction path to include ternary systemswith no reported ternary phase such as the Si–C–Wsystem. Since the decomposition of SiC is related to thethermodynamics of the reaction between SiC and Me, itis worthwhile to examine the formation energy of thecarbides. The free energies of selected carbides havebeen examined in the temperature range from 800 to1200°C [23], as shown in Fig. 3. The free energy of theSiC phase has the highest value, and that of the Cr23C6

phase the lowest value among the examined carbides.Therefore, with only thermodynamic information, it isclear that the SiC phase is not stable with many metalcombinations.

Fig. 2. The schematic figures showing the sequence of the productswith respect to the contact materials.

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Fig. 3. The free energy of formation of carbides with a temperaturerange from 1073 to 1473 K.

SiC is decomposed and reacts with Ni to form sili-cides and free carbon [20].

5.2. SiC/Cr reactions

The product phase sequence of SiC/Cr in the tem-perature range between 1000 and 1200°C has beenreported as SiC/Cr5Si3C/Cr7C3+Cr3Si/Cr7C3/Cr23C6/Cr [7]. It should be noted that the carbide layers areproduced next to the Cr side, indicating that the dif-fusion distance of carbon from the SiC is larger thanthat of Si from the SiC.

5.3. Diffusion pathway analysis

Since the formation of product phases is affectedby the chemical potential, it is useful to consider theobserved diffusion pathway within the chemical po-tential framework [24]. While the chemical potentialdiagram of the SiC/Cr reaction is accessible [7], thatof the SiC/Ni reaction is not well-defined yet [21].Therefore, chemical potential diagrams with respect toSi, C and Ni have been initially constructed basedupon the free energy values of each phase, assumingthat each phase is a line compound (i.e. the solubilityof each phase is limited). The method for producingchemical potential diagrams was employed from apreviously reported routine [24,25]. The diffusionpathway of SiC/Ni was marked in the stability dia-grams (Fig. 5). While the chemical potential of Si andNi decreases, that of carbon increases and decreasesthrough the reaction layers by forming free carbon.In the case of the SiC/Cr reaction, it has been re-ported that the chemical potentials of Si, C and Crdecrease through the reaction layer (Fig. 6). Since fortype I reactions, the chemical potential of carbon in-creases and decreases through the reaction zone due

5. Investigation of the diffusion pathway of the SiC/Niand SiC/Cr system

In order to establish if there is confirmation for theproposed reaction classification, SiC/Ni (type I) andSiC/Cr (type II) interface reactions have been exam-ined in detail to identify the complete diffusion path-way. Since both reactions have been well defined inprevious works [6,7], efforts have been concentratedon defining the main distinguishing points. It hasbeen reported that the formation of a periodic mor-phology is due to the difference of individual elementmobility. Indeed, in the SiC/Ni system, Ni is themain moving component, while carbon is immobile[6]. Since the knowledge of the kinetic behavior ofcomponents during periodic phase formation is im-portant for understanding the reaction kinetics, SiC/Ni (type I) reaction couples were prepared andinvestigated.

5.1. SiC/Ni reactions

The BSE image of a SiC/Ni reaction couple an-nealed at 900°C for 40 h is shown in Fig. 4. Periodiclayers of silicides and carbon were developed as prod-ucts. The observed product sequence is Ni/Ni3Si/Ni5Si2+C/Ni2Si+C/SiC. The reaction products(silicides and carbon) are observed on both sides (SiCand Ni) of the initial interface (laboratory referenceframe). This does not mean that carbon necessarilydiffuses out of the SiC side. The presence of theproducts (silicides and carbon) on the Ni side may bean effect of the volume expansion of the reactionzone. During the reaction, Ni diffuses into SiC and

Fig. 4. BSE image of SiC/Ni reaction couple annealed at 900°C for 40h.

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Fig. 5. The chemical potential diagrams with diffusion pathway of the Si–C–Ni system: (a) with respect to carbon; (b) with respect to nickel; and(c) with respect to silicon.

to the production of free carbon from SiC, the iden-tification of the chemical potential variation is alsoa characteristic feature. Therefore, the chemical poten-

tial behavior appears to be related to the kinetic differ-ence of reaction modes between Ni (type I) and Cr(type II).

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Fig. 6. The chemical potential diagrams with diffusion pathway of the Si–C–Cr system : (a) with respect to carbon; (b) with respect to chromium;and (c) with respect to silicon [7].

6. The diffusion pathway of SiC/Ni with Cr or Cubiased layer.

The formation of periodic layers during the phaseevolution of the SiC/Ni reaction can influence the inter-face behavior due to the free carbon layer. When thereaction extent can be controlled, a thin interfacedebonding layer can have a beneficial effect on theoverall fracture toughness of a composite [26]. How-ever, it has also been reported that the reaction kineticsof SiC/Ni is fast during isothermal annealing at (andabove) 1000°C, resulting in a severe degradation of SiC[27,28]. In order to examine the effect of a biasing layer(excess flux), a selected biasing interlayer was insertedinto a SiC/Ni diffusion couple subsequently annealed at900°C for 40 h. Cu (X(I)) and Cr (X(II)) have beenselected as interlayers based upon the proposed reactionclassification. Since the SiC/Ni reaction represents atype I reaction, the application of a type II componentsuch as Cr would change the carbon flux by producingcarbides based upon the above discussion. Indeed whena Cu interlayer (25 mm) is inserted in a SiC/Ni reactioncouple, the products are obtained with a periodic layer

morphology (Fig. 7). The reaction product sequence isNi/Ni2Si(Cu)+C/Ni5Si2(Cu)+C/SiC. However, whena Cr interlayer (Cr was sputter-deposited with a nomi-nal thickness of 10 mm) is placed in a SiC/Ni reactioncouple, the morphology of the reaction product layersis planar (Fig. 8). The product sequence identified byEPMA is Ni/Cr/Cr23C6/Cr7C3/Cr5Si3C/SiC. The reac-tion products are distinctly biased by the excess flux ofCr. Also, it should be noted that the reaction layerswere observed with a planar shape providing clearevidence for the effectiveness of the kinetic biasingstrategy [11]. This indicates that the interface reactionproperties and diffusion pathway can be controlled byestablishing a semi-empirical database to allow for theselection of a kinetic bias interlayer.

7. Summary

The establishment of characteristic phase equilibriaof Si–C–X systems appears as an important factor tointerpret the reaction modes in bulk material SiC/Mereactions. The SiC/Me interface reactions have been

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Fig. 6. (Continued)

identified as type I or type II based upon the formationof carbon and silicides or the formation of carbides andsilicides, respectively. This can provide a database andmodel analysis to understand the phase evolution dur-ing SiC/metal interdiffusion reactions. The classificationof reaction modes has also been supported by examin-

ing SiC/Ni and SiC/Cr reactions, which represents typeI and type II modes, respectively. While the chemicalpotential of carbon increases and decreases through thereaction zone in the SiC/Ni reaction, it decreases con-

Fig. 8. BSE image of SiC/Cr(10 mm)/Ni reaction couple annealed at900°C for 40 h.

Fig. 7. BSE image of SiC/Cu(25 mm)/Ni reaction couple annealed at900°C for 40 h.

J.S. Park et al. / Materials Science and Engineering A259 (1999) 279–286J.S. Park et al. / Materials Science and Engineering A259 (1999) 279–286286

tinuously through the reaction layers in the SiC/Crreaction. Therefore, the variation of chemical potentialof carbon can also be a guideline for the classificationof SiC/Me reactions. This information provides a base-line to control the diffusion pathway by applying thebias reaction strategy. For the Cr biased SiC/Ni reac-tion, the observed morphology of the reaction layer isplanar instead of periodic, which indicates that theidentification of the SiC/metal reaction type canprovide a key for designing composite interface proper-ties by controlling component fluxes.

Acknowledgements

The support of ONR (N00014-92-J-1554) is grate-fully acknowledged.

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