Metal Matrix Composites: Processing and Interfaces || Characterization and Modification of Composite Interfaces

8

Characterization and Modification of Composite Interfaces

BENJI MARUYAMA* Center for Materials Science and Engineering

University of Texas Austin, Texas

ENRIQUE V. BARRERA Department of Mechanical Engineering and Materials Science

Rice University Houston, Texas

L. RABENBERG Center for Materials Science and Engineering

University of Texas Austin, Texas

I. Introduction 182 II. Chemical Interaction 183

A. Analysis Using Microscopy 184 B. Analysis Using AES, RBS, and XPS 186 C. Analysis Using EXAFS 188 D. Evaluation of Interface Chemical Interaction 191

1. Thermodynamics 192 2. Kinetics 192 3. Active Sites 193 4. Catalysis: Oxidation Model of Carbide Formation 194

III. Wetting 196 A. Wetting Experiments 197

1. Sessile Drop Technique 197 2. Thin-Film Stability 198 3. Resistive Filtration Breakthrough Pressure 199

B. Evaluation of Wetting Behavior 200 1. Wetting in the Aluminum/Graphite System 201 2. Modification Strategies 201

IV. Bonding 202 A. Mechanical/Macroscopic Bonding 203

1. Peel Test 204 2. Pullout Test 206

* Currently an NRC Postdoctoral Research Associate at the Naval Research Laboratory, Washington, D.C.

181 Copyright © 1991 by Academic Press, Inc.

All rights of reproduction in any form reserved. ISBN 0-12-341832-1

182 B. MARUYAMA, E.V. BARRERA, AND L. RABENBERG

3. Mechanical Testing/EXAFS 207 4. Compact Tension Test 208

B. Atomistic/Microscopic Bonding 209 1. EXAFS 209 2. XPS and AES 210

C. Evaluation of Bonding 211 1. Work of Fracture 211 2. Reactions as a Probe of Bonding 212 3. Modification Strategies 213

Summary and Conclusions 213 References 214

I. Introduction

A composite interface is engineered by a cyclic process of synthesis, characterization, and modification based on probable causes of, and solu-tions to, the problems that are encountered in composite fabrication and service. In general, the components of a composite are chosen based on their intrinsic macroscopic properties, in conjunction with suitable assumptions regarding the averaging of these properties. The interface becomes important in the fabrication of the composite, as a mechanism for load transfer, and as a possible source of failure processes during the service life of the composite. If at any point the interface presents difficulties or does not perform its required roles, it is examined, and modifications based on the perceived origin of the problem are proposed and possibly implemented. This cycle is repeated as new problems arise or when improvements are demanded.

This chapter emphasizes the characterization and modification of metal matrix composite interfaces. Our approach is somewhat unconventional; instead of detailing a list of experimental techniques appropriate for interface characterization, we concentrate on phenomena specific to the interface that are worthy of characterization. These phenomena dictate the choice of characterization techniques and modification options.

The primary role of the interface between the matrix and the reinforcement phase in a metal matrix composite is to transfer the load from matrix to reinforcement. The interface may also be expected to serve as a mechanical fuse or as a diffusion/reaction barrier. However, the interface rarely, if ever, satisfies all these design criteria; instead, it commonly presents an array of such new phenomena as nonuniform adhesion, reaction products, reinforce-ment degradation, and generation of long-range residual stresses. Thus, the actual behavior of the interface reflects a complex interplay of several phenomena, each of which may be poorly understood in isolation.

8 MODIFICATION OF COMPOSITE INTERFACES 183

In this chapter we have chosen to distill this complex interplay of phenomena down to the three essential, related processes of chemical interaction, wetting, bonding, and the interaction among these processes with a view toward understanding the resulting mechanical properties. Regardless of the mode of fabrication, these are the atomistic processes that establish the interface. Some degree of wetting, either spontaneous or by means of applied pressure, is necessary to establish intimate contact between a liquid matrix and the reinforcement phase. If the matrix and reinforcement phase are in intimate contact, some chemical bonding normally occurs. That is, some electrons are transferred to localized states between the surface atoms of the reinforcement phase and the metal. This establishes a bond between metal and reinforcement atoms: it may or may not disrupt other bonds within the reinforcement. If electrons are transferred to the interface region to the extent that bonding within the reinforcement is weakened, there is a strong possibility that intermediate-phase reaction products will form, and a simple sharp interface will convert to an extended interfacial zone of reaction products. All these phenomena have been observed in metal matrix composites and may occur simultaneously in a single composite system.

Experimental techniques used to probe interfacial phenomena, modifica-tion strategies, and their rational will be discussed. This is not intended to be a survey of the field. It is intended to be a representation of the current understanding as well as an indication of future trends.

II. Chemical Interaction

Chemical interaction at a metal matrix composite interface is defined in the present discussion as mass transfer across the interface. This occurs when components from the matrix and reinforcement dissociate and interdiffuse to form new phases or solutions. Chemical interaction results in a new distribution of elements near the interface, the creation and elimination of phases, and a new interface morphology. Chemical interaction is char-acterized by determining the distribution and redistribution of elements, by quantifying the distribution of phases and identifying any new phases that have formed, and by determining the morphology of the interface.

Commercial metal matrix composite interfaces are composed of many components: oxides, carbides, wetting agents, diffusion barriers, compliant layers, sacrificial reaction layers, matrix second-phase particles, segregants, and contaminants. Each component can be identified, quantified, and eva-luated as to its effect on the composite interface mechanical behavior vis-à-vis chemical interaction.


In such reactive composite systems as aluminum or titanium with carbon or silicon carbide fibers, carbides and intermetallic phases form as a result of chemical interaction. The mechanical properties of the composite are degraded by an excess of these phases and their effects on the interface morphology. Because these carbides and intermetallics are brittle and irregu-larly shaped, they reduce the strength of the interface (the strength of the bond between reinforcement and matrix). They reduce the strength of the reinforcement by growing into it and forming stress concentrations in the reinforcement.

Diffusion barriers are applied to the reinforcement to reduce the extent of the reaction. Diffusion barriers substances are typically insoluble metals or refractory materials, such as Ta, TiC, and TiN [/]. Since wetting is also a problem for SiC and carbon fiber reinforcements, such wetting agents such as TiB2 [2] and Si02 [3] are used. Reactivity and wetting are also affected by matrix alloy composition, and therefore the effect of alloying is also determined.

Compliant layers are sometimes applied to fiber surfaces to relieve stresses due to thermal expansion mismatch and to act as crack arrestors. From the viewpoint of chemical interaction, the effectiveness of the compliant layer depends on its integrity. It cannot be effective if it forms a different (brittle) phase by reacting with the reinforcement or matrix.

Finally, the composition of the matrix can affect the reactivity of the interface. These are the major issues that motivate investigations of chemical interaction at metal matrix composite interfaces. In the following section, techniques used in these investigations and some results are discussed.

A. Analysis Using Microscopy

Transmission electron microscopy (TEM), scanning electron microscopy (SEM), and their associated analytical techniques, such as energy dispersive spectroscopy (EDS) and optical microscopy, are techniques commonly used to characterize chemical interaction. These are the workhorse techniques for interface characterization. Each is used to quantify the distribution of phases and to determine the interface morphology. All but optical microscopy can be used to determine the elemental distribution. TEM has the ability to identify the crystalline state of interface interphases by employing selected area diffraction (SAD) and microdiffraction capabilities, and has the highest spatial resolution.

TEM is used extensively to analyze the structure of metal matrix composite interfaces [4-6\. The high spatial resolution is ideal for the analysis of the early stages of chemical reaction.


Using TEM, Fu et al [7] determined the existence of aluminum oxide at the interface of graphite/aluminum composites. Electron diffraction patterns identified the oxide phase as gamma alumina. They also determined that aluminum carbide forms on the matrix side of the interfacial oxide (see Fig. 1). This work allows several important observations to be made. It identifies carbon, rather than aluminum, as the diffusing species. Furthermore, the carbon that dissociated from the graphite fiber does so not in the presence of aluminum metal or aluminum carbide but in the presence of gamma alumina. This point will be discussed further in the next section.

Motoki et al. [8, 9] and Khan [10] have used TEM to study a model aluminum/graphite reaction couple. Both groups found that the thermal carbon used to simulate graphite reacted at a lower temperature than did commercial graphite fibers. Motoki determined that the surface oxide formed on aluminum exposed to 10"2 torr was enough to diminish the reactivity relative to thermal carbon exposed to 10"3 torr pressure [8],

Khan [70] has performed quantitative experiments on reaction kinetics. Using SEM to analyze aluminum/graphite couples, he showed that the reaction rate of aluminum carbide formation at graphite/aluminum interfaces is parabolic or diffusion limited in the thickness range of 1 to 10 microns with heat treatment temperatures of 650 to 800°C and times of 1 to 10 h.

As will be discussed in the section on wetting, reaction between the wetting liquid and the solid are important in determining the wetting behavior. The

.'■V

iÈÊ^m

FIG. 1. TEM dark-field microstructure of heat-treated specimen showing Al matrix, graph-ite fiber reinforcement, y-Al203 (fine grain) and Al4C3 (course grain) interface phases. Note that the A14C3 forms on the matrix side of the A1203, indicating that carbon diffuses to the aluminum rather than vice versa. (Copyright ASTM. Reprinted with permission.)


degree of reaction during sessile drop experiments is investigated by SEM of a solidified and sectioned sessile drop sample [77].

Many have investigated the effect of matrix alloying elements on the reactivity of the interface [72, 75]. Most frequently investigated are the addition of silicon and magnesium to aluminum for consolidation with silicon carbide fibers [77]. Magnesium is alloyed to the aluminum to improve wetting and matrix mechanical properties. Silicon is alloyed to the aluminum to prevent reaction between the matrix and silicon carbide reinforcement. A 5% silicon/aluminum alloy is thermodynamically stable with respect to silicon carbide. Magnesium and lithium are alloyed to aluminum for con-solidation with aluminum oxide fibers [6, 14]. These alloys improve the wetting behavior and enhance the matrix/reinforcement bond. After reaction, the matrix/reinforcement couple is mechanically tested to determine the reduction in strength. Then the matrix can be dissolved from the reinforce-ment, and the degree of reaction can be determined by changes in the surface morphology of the reinforcement. Alternatively, the sample is cross-sec-tioned; SEM is used to determine reactant thickness and morphology, and EDS is used to determine composition [12].

B. Analysis Using AES, RBS, and XPS

Less commonly used techniques are Auger electron spectroscopy, (AES), Rutherford backscattering spectroscopy (RBS), and x-ray photoelectron spectroscopy (XPS). These spectroscopic techniques are all sensitive to light elements, in particular carbon and oxygen, and each has a depth resolution of 2nm (AES, XPS) [75, 16] to 20 nm (RBS) [77, 18]. Although AES and XPS are surface techniques—that is, they generally sample only the first few nanometers of the sample surface by sequentially removing layers of atoms via ion sputtering—one can obtain characteristic information as a function of depth through the interface. Also, both AES and XPS are used to determine the chemical state of an element (e.g., metal, oxide, carbide). RBS can be used to analyze buried interfaces—that is, interfaces that exist at least 10 or 20 nm below the sample surface.

These less commonly used techniques have two major advantages over the microscopic techniques. The first is that they are sensitive to light elements, carbon and oxygen being the two most important. Second, the resolution of the techniques allows information about the interface to be gained on an atomistic scale. As our understanding of the interface continues to broaden, it is clear that these techniques will contribute important information.

Using AES/SAM, Marcus et al. [19] were able to determine the composi-


tion of an interfacial oxide of an aluminum/graphite composite. By fracturing the composite in situ and analyzing an area that had fractured at the oxide, they found that the oxide contained both aluminum and magnesium. Finello also confirmed that aluminum carbide forms on the matrix side of the interfacial oxide in graphite/aluminum composites [20].

XPS and RBS are not suited for analysis of real composite interfaces because the spot size of the incident probes (x-rays for XPS and He ions for RBS) is larger than the diameter of a reinforcing material. However, RBS and XPS analysis can be performed on model composite materials made in a planar configuration. For example, a thin film of metal representing the matrix can be deposited onto a substrate of silicon carbide representing the reinforcement [21].

X-ray photoelectron spectroscopy is a well-established surface analysis technique [22]. A spectrum is composed of characteristic peaks of elements present in the first few monolayers (up to 50 Â). These peaks contain information on the chemical state of an element. The information lies in the degree of shifting of the characteristic peak due to a change in valence and the associated outer electron screening. Polymer matrix composite scientists have used XPS extensively to study bonding at the interface [23], and recently this has been expanded to metal on polymer systems [24-26].

XPS is just beginning to be exploited as a tool to analyze chemical interactions at metal matrix composite interfaces [75, 27]. Bermudez [25] has studied the interaction of aluminum, and Hasegawa et al. [21] the interaction of titanium with silicon carbide. These works focus on the initial stages of reaction, where the thickness of the reactant can range from a few monolayers to less than one monolayer of reactant. Interestingly, Bermudez determined that the Al//?-SiC interface did not react, even after heat treatment at 1050°C. It was also determined that aluminum carbide formation was greater on a sputter-damaged, carbon-rich surface than on clean, silicon terminated (001) /?-SiC, leading to the hypothesis that normal SiC fibers react because their surfaces are damaged or dirty. These results on clean, well-ordered surfaces are in contrast to reaction experiments, performed on poorly characterized surfaces of commercially available silicon carbide fibers, which found significant reactions at only 600°C [28, 29]. This underscores the importance of careful characterization to the clear understanding of chemical interaction.

Recently Maruyama et al. [30] have determined that the formation of aluminum carbide at glassy carbon/aluminum interfaces is catalyzed by the presence of as little as 500 Langmuirs (10 "6 torr-s) of water vapor. This implies that the formation of aluminum carbide in carbon/aluminum compo-sites may be catalyzed because of the levels of water vapor found in typical consolidation environments.


Zhong and Ohuchi [31] have used XPS to study the nickel/sapphire interface. Interestingly, they found that the nickel was able to partially reduce the alumina, even though nickel is much less active toward oxygen than aluminum.

All of the XPS work was done by depositing several monolayers of metal onto a clean substrate in ultra-high vacuum. The resolution and cleanliness of these experiments allow insight into the fundamental processes in compo-site interface reactions. This technique should prove invaluable to funda-mental characterization of the interface.

RBS is a nondestructive, thin-film technique that can profile the distribu-tion of elements normal to the surface of a sample [17, 18, 32]. It requires the depth of the interface below the sample surface to be uniform across the sample. Everett et al [33] used RBS to study chemical interaction between aluminum and graphite. They determined interaction as measured by a change in interface sharpness. They found that the graphite basal plane edges are much more reactive than basal plane faces. They also found that a decrease in oxygen present at the interface as a result of baking the graphite was associated with a decrease in aluminum/graphite interaction. Since the oxygen was probably in the form of water or hydroxide (RBS cannot detect hydrogen), it is possible that the water vapor catalyzed the reaction, con-firming the XPS work of Maruyama et al described in Section II.D.4 and in references [30, 34].

C. Analysis Using EXAFS

Extended x-ray absorption fine structure spectroscopy (EXAFS) is a unique tool which allows the characterization of the atomic neighborhood of a specific element. Using it, one can determine the identity and interatomic distance between the absorbing atom and the nearest and next-nearest neighboring atoms in a nondestructive manner. Of special interest is that, with an appropriate sample configuration, the atomic bonding and chemical interaction at an interface can be determined. In contrast to AES, XPS, and SIMS, EXAFS can be used for surface analysis, bulk analysis, or the analysis of buried interfaces. Buried interfaces up to several hundred nanometers below a sample surface can be analyzed. For a detailed review of the EXAFS technique, refer to Stern [35] and Koeningsberger and Prins [36]. The technique is briefly summarized here.

If an incident photon has sufficient energy to induce a core electron of a target atom to make a transition to a higher energy state, it can be absorbed by that atom. A lower-energy photon will not be absorbed, and the


absorption of photons with energies higher than the characteristic energy have an exponentially decaying probability of being absorbed. Figure 2 is a plot of absorption versus photon energy in the titanium A>edge energy range. Edge structure is a result of interaction between atoms. The incident photon interacts with the potential well formed by atoms near the absorbing atom. This produces oscillations in the absorption edge. The near-edge (0 to 50 eV) structure originates from interaction of the incident photon with conduction and valence bands and weakly bound electrons. Fine structure (>50eV) is associated with nearest, second-, and third-nearest-neighbor atomic interactions. Analysis of the fine structure, combined with comparison of experimental spectra to spectra from standards, is then used to give information on the bond lengths, coordination number, and nearest-neigh-bor identity. Glancing-angle x-ray reflectivity is a related technique used to determine interface sharpness and roughness of planar thin-film samples [37, 38, 39, 40].

Chemical interaction at copper/aluminum interfaces has been studied by Heald et al. [40]. Using glancing-angle reflectivity and glancing-angle EXAFS, they observed CuAl2 formation of 20 to 25 À at 160°C. Lamble et al. [39] studied the reaction of tungsten/carbon multilayers, using glancing-angle EXAFS. They detected WC and W2C formation in the 1 to 2 nm range as well as the crystallization of W.

The same sort of experiment can be done in metal matrix composite systems. For example, any reinforcement coating, such as titanium on

.2 2.6 a D

I-1.8 X

Ti Bulk

t4 L Pre-edge Background

4700 4800 4900 5800 GI HO 'J20Ü 5300 5400

Photon Energy (eV) FIG. 2. Ti EXAFS with pre-edge, edge background, and EXAFS identified.


alumina fibers in an aluminum matrix or titanium deposited as titanium diboride on graphite fibers in an aluminum matrix, would be interesting systems to examine. The chemical interaction of these coatings with the reinforcement or matrix can be analyzed. Several sample configurations are possible. One is to use a bulk composite in which the target atom exists only at the interface, such as TiB2 at a carbon/aluminum interface (see Fig. 3a). Another is to use a model system consisting of multilayers of matrix and reinforcing material [37] (see Fig. 3b). This configuration increases the interface region volume and allows directional techniques such as glancing-angle EXAFS [35, 41] and polarization-dependent EXAFS [42] to be em-ployed.

Ti coated fiber

(a)

Incident X-Rays

Matrix Material

Reinforcing Material

(b) FIG. 3(a). Composite sample with titanium at the interface. The Ti is used as a target atom

for EXAFS analysis, (b) Multilayer composite sample for EXAFS analysis. Alternating layers of matrix and reinforcement increase the relative contribution of the interface to the EXAFS signal.


Finally, by surface-sensitive EXAFS techniques (SEXAFS), the surface of a sample can be analyzed [36,43,44]. This technique should prove a powerful tool in the analysis of fracture surfaces of composites. SEXAFS analysis of the fracture surface of a metal matrix composite, or any interesting fracture surface, fractured in vacuo will allow the characterization of the composition and local environment of the fracture surface and give information on reaction and interdiffusion at the interface along the fracture path (see also bonding section). This line of research, as well as that described in the EXAFS bonding section, is currently being pursued by the authors. Preliminary experiments on interface reactivity and atomic interfacial bonds have been performed on aluminum/vanadium/graphite and aluminum/gadolinium/ graphite model composites [45].

D. Evaluation of Interface Chemical Interaction

In the idealized case of a planar, atomically sharp interface, one can imagine that chemical interaction is accomplished simply by an atom hopping across the interface. In reality, the process is much more complex. The chemical reaction in the aluminum/graphite system will serve to illustrate the complexity of a real composite system. Two observations allow several conclusions to be drawn concerning the reaction process: At the fiber/matrix interface, there always exists a layer of oxide of finite thickness [19], and aluminum carbide is known to form on the matrix side of the interfacial oxide [20]. The first step in the reaction is to dissociate carbon atoms from the surface of the graphite. Note that this dissociation must occur in the presence of the oxide (i.e., while the surface carbon atom is bonded to the oxide), since the interface oxide is adjacent to the graphite. Second, each carbon atom must diffuse through any oxide and previously formed carbide. Diffusion through the oxide probably results in an inter-mediate aluminum oxycarbide phase [46]. Finally, when the carbon has reached unreacted, metallic aluminum, it can react to form aluminum carbide. Thus, to characterize chemical interaction, the diffusing species, the path by which diffusion occurs (e.g., bulk, grain boundary, etc.), intermediate phases, and the process by which the matrix or reinforcement atoms are dissociated must be known.

As complex as a real composite interface is, in order to create an effective interface modification strategy, an understanding of the fundamental factors which control interface chemical interaction must be achieved. The funda-mental considerations for any chemical reaction at the interface are the thermodynamics, the kinetics, and the reaction mechanisms. Most composite


systems are not thermodynamically stable. Attaining an equilibrium state involves chemical interaction, and the driving force for the interaction is the change in chemical potential across the interface. The overall kinetics of the reaction and the path the reaction takes to equilibrium are governed by the rate at which the individual reaction steps can occur. The reaction mechanism will be influenced by active sites for reaction and possible catalytic influences.

1. THERMODYNAMICS

The available thermodynamic data, which include phase diagrams and Ellingham diagrams, give a limited amount of information. They indicate whether a reaction is favorable, show the solid solubility limits of one component in another, and give stoichiometry of any new phases. Sometimes it is possible to predict the influence of alloying elements and impurities based on phase diagrams. The thermodynamics of the system, however, do not determine the rate of reaction or the path to equilibrium.

2. KINETICS

The rate at which a reaction proceeds is governed by the rate of the individual steps of the reaction. These steps involve atomic motion and can be thought of as the breaking and forming of new bonds (i.e., when carbon dissociates from graphite, carbon/carbon bonds are broken and carbon/ aluminum and/or carbon/oxygen bonds are formed). The next step, diffusion, can be thought of as repeated breaking and forming of the same bonds. An energy barrier must be overcome in order to break chemical bonds or squeeze between atoms. The height of the energy barrier, or the activation energy, influences the rate of a reaction.

For reaction steps that occur sequentially (i.e., dissociation, diffusion, reaction), the reaction rate is determined by the slowest step. For example, if diffusion is the slowest, or rate-determining step, then the rate at which reaction product is formed will be the rate of diffusion. For reaction steps that can occur concurrently (e.g., diffusion through bulk material and diffusion through grain boundaries), the reaction rate is determined by the fastest step (e.g., if overall mass transport is faster through the grain boundaries, then the reaction rate will be dominated by grain boundary diffusion). As a second example, consider a system where dissociation can occur by catalyzed and uncatalyzed means. If the activation barrier of the catalyzed reaction is sufficiently small and the attempt frequency is suffi-ciently large, the dissociation rate will be dominated by the catalyzed reaction; i.e., the rate of reaction will be limited by the catalyzed reaction rate.

Some information about the reaction process can be gained by analyzing


the extent of reaction as a function of time. If the rate of change of thickness of a reaction layer at a planar interface is constant—that is, a plot of thickness versus time is linear—then the reaction kinetics are interface-controlled. The rate of reaction does not depend on the reaction thickness; it depends on some process occurring at the reinforcement/reactant interface (e.g., dissocia-tion of the reinforcement) or the reactant/matrix interface (e.g., combining of the reinforcement component with the matrix component to form re-actant). If the change of thickness of a reaction layer at a planar interface exhibits a square root dependence with time—that is, the rate depends on the thickness of the reactant layer—then the reaction kinetics are diffusion-controlled. The rate of increase of the reactant thickness is limited by the rate at which a component can diffuse through the reactant. Since the reactant is continuously increasing, the distance to diffuse and the association time both increase, thereby increasing the time to diffuse across the interface. For discussion of diffusion-controlled growth of a uniform reaction layer at a cylindrical interface, refer to [75].

The first stages of a reaction at a planar reaction front will be interface-controlled, since the concentration gradient of the diffusing species will be large, the distance to diffuse is short, and therefore diffusion will be rapid. At some point, the distance over which the diffusing component must diffuse will become large enough that the reaction at the reaction front has the potential to proceed faster than the diffusing component can diffuse to the reaction front. The important point to remember is that all such reactions begin with interface-controlled reaction kinetics, at least for a short time, and eventually turn to diffusion-limited growth [47].

Current modification strategies for reducing reactions at metal matrix composite interfaces rely exclusively on reducing the rate of diffusion. Limited success has been achieved via this strategy; thus, the purpose of discussing interface- and diffusion-controlled growth is to recall the fact that, even though the rate of diffusion has been found to limit the reaction rate in most experiments on composite systems [8-10], it is still possible that the extent of reaction can be reduced by reducing the rate of interface-controlled steps (i.e., dissociation of a reinforcement component). In fact, the diffusion-controlled growth measured by Khan [70] was for aluminum carbide thick-nesses greater than 1 micron. At this thickness, a 10-micron-diameter graphite fiber would be totally degraded.

3. ACTIVE SITES

A typical surface is heterogeneous. Atoms on a surface can occupy various types of sites, such as the position at the edge of a step or the positions in a flat surface. Individual atoms on a surface can have different atomic species


as neighbors. At an interface, atoms can have various interactions with atoms on the adjoining surface or interphase. These various types of sites will have differing bond strengths and differing bond configurations. This difference in bonding can cause a difference in reactivity between different sites.

For example, graphite has an extremely anisotropic structure. It is well established that basal-plane edge atoms are much more reactive than basal-plane face atoms [33]. By measuring the relative amount of aluminum carbide formation on basal-plane edge and basal-plane face surfaces, it can be implied that the basal-plane edge atoms dissociate more readily than basal-plane face atoms. The basal-plane edge atoms are active sites for dissociation. Typical active sites are grain boundaries and areas of local disorder. Inhomogeneous reactivity can lead to an inhomogeneous distribution of reaction products that can lead to stress concentrations.

4. CATALYSIS: OXIDATION MODEL OF CARBIDE FORMATION

It is possible that the reactivity of a site is influenced by a chemical species at the interface. If the chemical species lowers the energy barrier for dissociation of atoms from the surface of either the matrix or reinforcement, it catalytically enhances the reactivity of the surface atom. In order for a catalyst to be effective, it must have a good turnover and a good turnover rate. The turnover is the total number of times the catalyst can act on the surface before becoming ineffective. The turnover rate, in this context, is the rate at which the catalyst dissociates and releases the surface atom. Chemical species that catalyze a reaction can originate from a number of sources, such as interface segregants or adsorbed impurity gases.

The oxidation model of aluminum carbide formation was developed to describe the bond structure and reactivity of aluminum/graphite interfaces [30, 34]. The oxidation model predicts that active oxygen, present at an aluminum/carbon interface will catalyze the formation of aluminum carbide. The model and prediction are based on an analogy between the gas-phase oxidation of graphite and the formation of aluminum carbide at alumi-num/carbon interfaces. The oxidation of graphite in air or in an oxygen atmosphere has been well characterized. The basic steps to the reaction are as follows (the reaction C(gr) + O -► CO will be described).

Following the discussion of Long and Sykes [48], the oxidation of graphite occurs in three steps. Step 1 : oxygen is adsorbed onto a surface carbon atom, forming a double bond. Step 2: the carbon/oxygen pair localizes a resonant electron forming a strong double bond. Note that the carbon bond between the carbon/oxygen pair is at the expense of the double bond that exists between the surface carbon atom and the underlying graphite. This state is diagrammed in Fig. 4. Step 3: the two single bonds between the surface


0 O 1 I I

Il I II I I

(a) (b) FIG. 4. Bonding configuration of oxygen to the surface of graphite. In state (a) the surface

carbon atom remains strongly bound to the underlying bulk graphite. In state (b) the bond between the oxygen atom and the surface carbon is so strong that it reduces the strength of the bond between the surface carbon atom and the underlying bulk graphite, thus facilitating dissociation of the carbon oxygen pair as carbon monoxide.

carbon atom and the underlying graphite are now served and the reaction is complete.

Step 2 is important. In the rearrangement of bonds from state (a) to state (b) in Fig. 4, the carbon/oxygen pair has become strong enough to overwhelm the bond between the surface carbon atom and the underlying graphite. This is a weak link in the chain of bonds across the interface as described above. In state (a), however, a well-mediated bond exists across the interface with no weak link.

In order to set up an analogy between the oxidation in an oxygen atmosphere and the formation of aluminum carbide at aluminum/graphite interfaces, two observations need to be made. Oxygen is always present at graphite/aluminum interfaces as an interfacial oxide [79], and aluminum carbide forms on the aluminum side of the interfacial oxide [7].

The formation of aluminum carbide at graphite/aluminum interfaces occurs in three steps: First, the bonds between the carbon atoms on the surface of the graphite must be broken. Second, the dissociated carbon atom diffuses through the intermediate aluminum oxide layer and other inter-phases and reaches unreacted aluminum metal. Third, the carbon and aluminum react to form aluminum carbide.

By analogy, if oxygen is able to bond to an atom on the surface of graphite so as to weaken the bond between the surface carbon atom and the underlying graphite at an aluminum/graphite metal matrix composite inter-face, as it does during the gas-phase oxidation of graphite, then the presence of the oxygen will catalyze the dissociation of carbon atoms from the surface of the graphite. This increases the interface-controlled growth rate and hastens the onset of diffusion-controlled growth. Therefore, active oxygen will catalyze the formation of aluminum carbide at aluminum/graphite interfaces.


If the bond between the interfacial oxygen and the surface carbon atoms remains strong enough to weaken the room-temperature bond strength of the bond between the surface carbon atom and the underlying graphite, then a weak link has been created in the interface bond strength. That is, an easy fracture path exists between the first layer of carbon atoms of the graphite and the remaining graphite.

Recently, Maruyama et al. [30] have confirmed the oxidation model by showing that water vapor-dosed aluminum/glassy carbon interfaces exhi-bited catalytic reaction rates.

III. Wetting

The degree of wetting at a metal matrix composite interface is the propensity of a liquid matrix to increase its area of contact with a reinforcing material during composite consolidation. The wetting characteristics of a matrix-reinforcement system are of interest in any synthesis technique which involves a liquid or semiliquid matrix, such as casting, and, in general, where interface surface energies are involved. The equilibrium wetting state between a liquid, solid, and gaseous environment is most often described by Young's equation for the mechanical equilibrium of surface tension (see Fig. 5):

yLY cos Θ = y s y - y S L . (3.1)

The y's are the surface energies of the three different interfaces (surface energy and surface tension will not be distinguished in the present discussion), namely the liquid/solid, the solid/vapor, and the liquid/vapor interfaces. The system minimizes its free energy by maximizing low-surface-energy interfacial area at the expense of high-energy interfacial areas, consistent with the constraints imposed by the gross geometry of the system. If ysv > ySL, then Θ will be less than 90°, and the liquid can reduce the energy or the system by spreading, thereby creating more solid/liquid and liquid/vapor interfaces at the expense of the solid/vapor interface. The liquid is then said to wet the

- Y s v

FIG. 5. Surface tension at a solid/liquid interface. Various surface tensions are identified.


solid. If ysv < ySL, then the system can reduce its free energy by reducing its solid/liquid and liquid/vapor interface area and increasing its solid/vapor interface area. The liquid is then said to be in a nonwetting condition. The experimentally observable variables of the wetting phenomenon are the measured wetted interface area and the contact angle at the intersection of the solid, liquid, and vapor phases.

Recall that Young's equation is based on equilibrium conditions and does not describe the kinetics of the formation of the equilibrium interfaces nor the path the system takes to equilibrium during dynamic, nonequilibrium wetting. A discussion of dynamic wetting processes is given by Aksay et al. [49\ Apart from the fluidity of the liquid, the kinetics of wetting depend on chemical interaction in the system (i.e., dissociation, diffusion, and reaction). The driving force for wetting is the change in chemical potential across the interface. The dynamic wetting angle will be reduced from the equilibrium value if the interaction proceeds faster than the liquid flows, and will increase if the liquid must wait for the reaction to proceed before wetting becomes favorable.

A. Wetting Experiments

1. SESSILE DROP TECHNIQUE

The sessile drop technique is used quite commonly to determine wetting behavior, and has been applied extensively to metal matrix composite systems. It serves to illustrate many of the issues concerning wetting in metal matrix composite systems.

The sessile drop technique is designed to measure wetting in a geometrical fashion. A drop of liquid representing the matrix alloy is allowed to come to or near equilibrium with a flat solid substrate representing the reinforce-ment in the presence of a gaseous environment representing that in which composite consolidation will occur. The wetting angle of Eq. (3) is measured optically, either directly or by calculating the angle by using an equation for the shape of a sessile drop.

In practice, sessile drop experiments conducted using molten aluminum as the liquid and graphite or alumina as a substrate are carried out from 600 to 1200°C in such inert atmospheres as argon, helium, or a vacuum. The time allowed for stabilization of the drop morphology ranges from 5 min to several hours [77].

Unfortunately, it is extremely difficult to characterize the wetting behavior of aluminum. Molten aluminum forms an oxide skin very rapidly. This oxide interferes with wetting by intervening between the molten aluminum and the


substrate. Any new solid/liquid interface that forms must do so in the presence of this oxide (see Fig. 6). This problem also exists for titanium and other such reactive metals. Because the formation of the surface oxide depends very strongly on the oxygen content of the gas environment, there is a large variation in the reported results of sessile drop experiments found in the literature [50]. However, most investigators [77, 49-51\ find 950°C to be the approximate transition temperature of aluminum from nonwetting to wetting behavior. Reported temperatures for spontaneous wetting of aluminum to graphite using the sessile drop technique vary from 800 to 1100°C. Eustathopoulos et al. [57] found that the contact angle of aluminum was unaffected by the nature of the substrate (reactive or unreactive) for aluminum on carbon and aluminum on sapphire.

Brennen and Pask found 950°C as the transition temperature for alu-minum on sapphire at 6 x 10"5 torr. Laurent et al. [77] found a nonwetting to wetting transition temperature of 740°C for aluminum on silicon carbide at 1 x 10 ~8 torr. Admirably, they quantified the partial pressure of oxygen in their system as 1 x 10"1 9 torr at room temperature. Eustathopoulos et al. [57] found that alloying the aluminum with elements that were active toward the surface aluminum oxide could significantly affect the wetting behavior. This was especially true for samples that had formed a relatively thick oxide of over 100 Â. The effect of chemical reaction on the contact angle must be considered. If a reaction occurs in a nonplanar fashion, then the apparent wetting angle will be less than the actual contact angle [49]. Therefore, some postanalysis of the sessile drop experiment should be done to check this.

2. THIN FILM STABILITY

Thin-film stability experiments consist of coating a substrate, representing the reinforcement, with a continuous thin film of metal representing the

Surface Aluminum Liquid Aluminum Oxide

( ' ) Area to be wetted

Graphite FIG. 6. Wetting of oxidized liquid aluminum to graphite: In order to increase its wetted

area, the molten aluminum must find a way to wet the graphite despite the intervening surface oxide.


matrix, heating the sample to a specified temperature in a controlled atmosphere, and observing whether the thin film breaks up into droplets of nonwetting metal. Typical test conditions are temperatures of 200°C above the metal liquidus, vacuum from 10~2 to 10"5 torr, and time at temperature of 5 to 60 min. Surface roughness and other factors make it difficult to quantify wetting angles from this type of experiment. In general, the result of this type of experiment is a yes or no answer. It has the advantage that it is simple and can be performed on nonplanar surfaces, which allows commercial fibers to be used. Also, reactions between the coating materials and commercial fibers can be analyzed as a reaction experiment.

Kimura et al [52] evaluated the compatibility of graphite coated with aluminum alloyed with various elements. The fibers were tensile-tested, and the wetting behavior was observed with scanning electron microscopy. They encapsulated their samples at 5 x 10"5 torr and heat-treated them for 30 min at 800°C. The first series they tested had solute concentrations below the maximum solid solubility limit. They found that pure aluminum, Al-0.5 at.% Mn, 1 at.% Mg, 1 and 2.3 at.% Ge, 0.5 at.% Cr, 2.5 at.% Cu, 1 at.% Si, and 5 at.% Ge exhibited poor wetting; that is, the coating broke into small droplets. Only Al-5 at.% Mg showed wetting behavior. The second series of alloys had solute concentration above their solid solubility limit. One at.% Ga and 1 at 5 at.% Sn all exhibited nonwetting behavior, and only 5 at.% Si exhibited wetting behavior. The third series of alloys had alloying elements that were nearly insoluble in aluminum. They were Al-1 at.% In, 1 at.% Tl, and 1 at.% Pb. They all exhibited wetting behavior, apparently by reducing

Although the thin-film stability technique is simple and relatively un-ambiguous, certain considerations must be maintained. The technique of using an artificially wetted surface may be considered a best-case scenario for wetting, because liquid surface oxide problems encountered in casting are circumvented, and any resistance to the wetting/nonwetting transition would favor wetting. Control of the environment is still important, as with the sessile drop technique. Encapsulation of the coated fibers, as performed by Kimura et al certainly leads to a poorly characterized environment.

3. RESISTIVE FILTRATION BREAKTHROUGH PRESSURE

Cornie et al [53] have devised a technique to measure wetting by measuring the critical pressure at which a liquid matrix will infiltrate a filter composed of well-characterized reinforcing material. The critical pressure is related to wetting by the equation [53]

Pc = (2yLV cos 0)/r, (3.2)


where r is the capillary radius (the size of the opening through which the matrix must flow) and Pc is the critical pressure.

This technique has several advantages. First, it is most reflective of chemical and physical environments of commercial composites. Second, a quantitative value for the degree of wetting can be obtained; this allows critical evaluation of experiments carried out under different conditions. Third, it can be dynamic in the sense that experiments can be carried out in a time short enough to reflect wetting kinetics in commercial consolidation techniques. One difficulty with this technique is using data generated from resistive filtration experiments to evaluate consolidation processes in which a continuous fiber is drawn through a melt. That is, if the critical pressure is measured while the liquid is static and has not yet begun to move, this is not the same dynamic process that occurs during continuous casting.

B. Evaluation of Wetting Behavior

Experiments devised to understand the wetting behavior of fiber and matrix can be placed into three categories. The sessile drop experiment measures wetting near equilibrium, at an advancing (growing) solid/liquid interface. The thin-film stability experiment measures wetting near equi-librium at a receding interface (also called dewetting [54\). The breakthrough pressure for filter resistance is related to dynamic, forced wetting of a liquid to a solid. Large degrees of fiber pullout observed on fracture specimens, fiber channeling (an uneven distribution of reinforcement and matrix), and voids at the interface are also indications of very poor wetting behavior. However, these features are rarely reported in the literature. Because wetting is a dynamic process, the method by which wetting behavior is measured can affect the results of an experiment. Therefore, it is important to be aware of the rate and direction of the wetting process whenever scrutinizing a wetting experiment.

The chemical environment in which wetting occurs is also very important. As has been stressed throughout this chapter, the vapor environment in which wetting occurs is crucial to the observed wetting behavior. Of particular importance is the oxygen content of the environment. Other components of the chemical environment are the presence of surface oxides on either the matrix or reinforcement materials, alloying elements in the matrix, and any surface modifications such as wetting layers, diffusion barrier, wetting agents, or surface treatments such as oxygen cleaning or sputter cleaning.


1. WETTING IN THE ALUMINUM/GRAPHITE SYSTEM

Aluminum/graphite will serve to display some of the above-mentioned effects because it is a system in which chemical perturbations play a large role in determining wetting behavior. First, aluminum is recognized as being highly active toward oxygen. It forms a thin, adherent oxide layer in any but the most benign environments, and this layer is known to interfere with wetting. Wetting studies have shown that the wetting behavior of aluminum on graphite is sensitive to the thickness of the surface oxide [55]. Thus, the amount by which the liquid aluminum surface, and therefore yLV, is modified depends on the amount of oxidation that occurs. Also, if the aluminum liquid is to increase its liquid/solid interface area, it must do so by making new liquid/solid interface with an oxidized liquid aluminum surface (see Fig. 6).

Since the new solid/liquid interface will form in the presence of the surface oxide, the solid/liquid surface energy will be different from that of oxide-free aluminum/graphite. Finally, aluminum and graphite can react to form aluminum carbide, A14C3. Since yS(carbide)L is distinct from yS(graphite)L, the wetting properties of liquid aluminum depend on the degree of carbide formation, and if, hypothetically speaking, carbide must form before wetting is favorable then the rate at which aluminum wets graphite may be de-termined by the rate at which the carbide reaction can proceed (for more discussion on solid/liquid reactions and wetting see Ref. [49]).

2. MODIFICATION STRATEGIES

Although time, temperature, and chemical environment are the controll-able variables in the wetting process, the time and temperature at which wetting occurs is usually limited by the occurrence of deleterious chemical interaction. Modification of the chemical environment is the key to enhanced wetting behavior. The most successful modification strategies are oxidation control, matrix alloying, wetting layers, and surface fluxes.

Oxidation of the surface of the liquid during wetting can significantly reduce the propensity for wetting. Aluminum and titanium, being strong oxide formers, are especially susceptible to this problem. Control of this problem by controlling the gas environment is difficult since it is difficult to maintain oxygen partial pressure sufficiently low in commercial consolida-tion processes. Alloying the matrix with a surface oxide-active species, such as magnesium, is another important factor. Such an alloying element can destabilize the oxide skin, allowing better wetting.

Coating the reinforcement with a material that is wetted by the matrix avoids any difficulty in wetting the matrix to the reinforcement. This has been accomplished with various materials (Ni, TiC, etc.). However, one of the most successful wetting agents, TiB2, is not such a coating and leads to


the last category, surface fluxes. TiB2 is applied via CVD to carbon and silicon carbide fibers as a wetting agent. Interestingly, aluminum does not wet bulk TiB2 as well as it does the CVD TiB2 coated fibers. Recently, Wu [56] determined that CVD deposited TiB2 contains significant amounts of chlorine, and that good wetting behavior in the C/Al system was associated with higher chlorine concentration. He hypothesized that the enhanced wetting was due to a fluxing action of the chlorine, which destabilized the surface aluminum oxide and allowed clean aluminum metal to wet clean carbon fiber. The ability of fluxes to enhance wetting has been demonstrated by Rocher et al. [57]. They precipitated fine particles of transition metal/alkali fluoride, such as K2ZrF6, onto carbon and silicon carbide fibers before infiltrating with molten aluminum. They attributed the enhanced wetting behavior to the fluxes' ability to dissolve the surface aluminum oxide, thereby making it fluid. Once fluid, the oxide would withdraw, allowing clean aluminum metal to wet clean fiber. The fluxing approach is attractive because it is effective, easily done, and does not add mass or the problems associated with coatings to the reinforcement.

IV. Bonding

Bonding is used in two different contexts in this chapter. In the first, bonding is used in a mechanical sense to mean the macroscopic strength of the interface, the stress required to separate matrix and reinforcement. In this context, intrinsic materials properties such as ductility, notch sensitivity, fracture toughness, and extrinsic or sample preparation-dependent para-meters, such as interface roughness, mechanical locking, and residual stress, are important and enter into any description of the mechanical behavior of the interface.

In the second context, bonding is used in a chemical or atomistic sense to mean the strength of the atomic bonds between atoms across the interface from matrix to reinforcement. This is related to the chemistry of the interface. Bonding is accomplished by an exchange of electrons, and the type of exchange determines the character of the bond (ionic, metallic, covalent; sp2, sp3; signa and pi bonding). The electronic configuration, atomic coordina-tion, and elements engaged in the bond are important considerations. Interface segregants, contaminants, and reaction can affect the strength of the individual atomic bonds at the interface, which ultimately reflects on the macroscopic strength.

The macroscopically measured strength of the interface changes as a result


of the rearrangement of atomic bonds. The strength of the interface can be degraded in a number of ways. The interface can become weak and brittle as a result of chemical interaction, and inhomogeneities such as reaction products and partially debonded or poorly wetted areas can serve as crack nucleation sites. Once nucleated, a crack can propagate along the interface or into a brittle reinforcement. Fine et al. [58] note that one should design where undesirable phases do not occur. This is to say that the interface should be designed with phases in equilibrium with each other; otherwise the structure may coarsen during fabrication or at the temperature of application. Furthermore, chemical reactivity can cause interface degrada-tion in the form of environmental attack or corrosion. Thus, interface oxidation and the galvanic coupling at the interface become important. One method of dealing with corrosion of composite materials is to apply coatings. Some researchers are considering the use of chromâtes or cerium additives to inhibit the chemical attack [59, 60]. There is significant work being done in this area and will not be the subject of this chapter. This sction of the chapter is concerned with techniques for mechanical testing of MMC interfaces, as well as characterization of composite interfaces on an atomic scale. Several novel and newly proposed techniques will be discussed.

A. Mechanical/Macroscopic Bonding

There are a significant number of mechanical testing techniques for determining the mechanical properties of commercial and model composites. Many are derived from early tests on adhesives for wood, plastics, and metals [67]. These tests, which include lap, peel, impact, puncture, and basic tensile, were quite apt at evaluating whether failure occurred in the matrix, adhesive, or matrix/adhesive interface. They demonstrated that the interface was a focal point of the mechanical integrity of a composite and thus lay the groundwork for more sophisticated experiments. These tests, while generic in nature, cannot all be applied to MMC systems. For example, a fiber pullout test, which measures interface bond strength, much like the early lap tests, is only meaningful for composites for which the fracture stress of the fiber is very large relative to the stress to cause debonding. A pullout test of a carbon fiber from a polymer matrix is well designed since the carbon fiber is pulled from the matrix intact, although there are cases where the carbon fiber may fail in a polymer matrix as a result of flaws in the carbon fiber or the presence of a rough surface. For an MMC such as SiC/Al, the pullout test is not as easily applied since fracture of the fiber in the matrix is more likely than in a polymer matrix. However, the pullout test is still applicable


because the length of the fiber pulled out can be related to the interface bond strength. This and other mechanical tests, including the peel test, fragmentation test, tensile, residual stress, and fracture toughness tests, will be discussed.

1. PEEL TEST

The peel test serves a number of uses, from quality control on bonded materials in the aerospace industry to measuring adhesion of thin films on dielectric materials in the electronics and semiconductor industries. The test involves pulling apart a thin, flexible strip of material bonded to a rigid substrate in the same manner that tape is pulled from a dispenser. The peel force is the force needed to separate the adhered thin layer from the substrate. Ideally, this force is simply related to the adhesion strength of the joint. In reality, the test is much more complicated.

Consider for a moment the complexity of this simple test and what contributes to the total peel force. First, it is necessary to adhere to the top layer in order to peel it off. This requires an adhesive that is stronger than the total peel force of the film. If residual stresses occur in the adhesive connection, the peel force must include its contribution, PA. The deposited film/substrate can have residual stresses, PR, and the flexible layer sees plastic and elastic strengthening during the test, PP and PE. Finally, the force to pull the interface apart, Pl9 is that which is hoped to be measured. Therefore, the total measured peel force, PT, is

PT = PA + PR + PP + PE + PI (4.1) when the thin film is in tension. Since Pt is desired, it is necessary to know or control the other forces. Finally, if the two layers separate by failure in an interphase, the measured peel force is related to the strength of that interphase, as opposed to an adhesion strength. With this level of complexity, some investigators feel it is an impossible task to measure Pl [62]. Still, investigators have developed models to explain the loading conditions of the peel test. One such model by Kim et al. [63] uses finite element analysis to model a thin elastoplastic film on an elastic substrate. An energy balance was used to equate experimentally measured peel forces to the specific fracture energy. The model does not consider variations in wetting, bonding, or chemical interaction along the interface. Although this omission can be considered a shortcoming of this model, it would be a very difficult venture to treat them properly and quantitatively.

A new technique for measuring the adhesion strength via a modified peel test is proposed. It seems feasible to relate the local deformation in the thin film, if it occurs, to the force necessary for bonding at the interface. Therefore,


by conducting a standard peel test and then measuring changes in dimensions of the film, one can more directly measure the stress to debond the interface. To understand the change in dimensions that can occur during peeling, consider the inverse case of orthogonal cutting, where a thin surface layer is delaminated by a cutting tool which creates compressive forces in the plane of the film. Figure 7 shows a schematic representation of orthogonal cutting (a) [64] in comparison to the peel test (b). Note that the thickness of the cutting, h2, can be used to determine various parameter in the cutting process. For orthogonal cutting, h2 (the final thickness of the film), is greater

Cutting Direction

(a)

Peel Direction

(b)

FIG. 7. Proposed modified peel test in which the change in thickness upon peeling is used to determine the interface bond strength, (a) Comparison of this idea to orthogonal cutting, in which the compressive force the cutting tool exerts on the delaminating film causes it to increase in thickness (after Schey [64]). (b) The tensile stresses in the film which occur in a peel test cause the film to decrease in thickness.


than hc (the original thickness of the film), due to plowing and the compres-sive nature of the technique. A peel test, on the other hand, would result in a final thickness less than the original thickness (h2 < hc), since the removed film is in tension. The interface debond strength can be calculated by measuring the change in dimensions and using the bulk-film properties to calculate Pr It is expected that when h2 = hc, failure would occur in the interface at a stress below the yield strength of the film. If h2 < Ac, then the film has work-hardened before the interface failed, and the final dimensions of the thin film can be used to equate PP-PE to Pj for a well-defined system. This technique has the advantage that the residual stresses due to the adhesive bond to the film have no bearing on the measured change in dimensions. However, residual stresses between the substrate and film must still be taken into account. Measuring residual stresses has always been an interest of the materials scientist. The x-ray technique [65, 66] is one method, but new advances in this area would certainly influence all the above-mentioned mechanical test techniques and appropriate theoretical and numerical models.

2. PULLOUT TEST

The pullout test [67-70] measures the shear stress necessary to debond the matrix/reinforcement interface. Also a factor in the measurement is the internal frictional stress. This stress is the result of the interaction of mechanical interlocking with residual stresses and Poisson effects. Pullout tests can be conducted either on single fibers or bundles of fibers. The critical length lc is defined as that length of fiber embedded in a matrix for which a transition of failure mode from fiber pullout to fiber fracture occurs. Models incorporate this parameter by measuring the fiber pullout length with respect to the critical length. It is also recognized as a design factor for composite systems; that is, the fiber should be long enough to attain its ultimate strength under loading before interface debonding occurs.

Consider the stresses present in a fiber pullout test [77], when, for example, a load is applied to the end of a fiber in a block of elastic material, the maximum interfacial shear stress occurs at the surface of the block. If the matrix is ductile, it will deform near the fiber surface to resist its extraction from the matrix. When the load applied to the fiber exceeds that of the interfacial bond at the fiber/matrix interface, cracks will form and propagate down the fiber. Frictional contact may be maintained by the presence of residual stresses and affected by Poisson effects. At some embedded length, the stress applied to the protruding length of fiber will reach the fiber fracture stress. Thus, the stress σ required to extract the fiber from the matrix is [71]

σ = Ατχ/d, (4.2)


where d is the fiber diameter and τ is the shear strength of the interface. When the ultimate tensile strength of the fiber, afu, is reached, x — IJ2, and 4 is the critical length of the fiber. Fibers embedded in the matrix of length shorter than IJ2 will be extracted, while fibers of greater length will fracture.

For a pullout test, care should be given to determining whether there is incomplete adhesion or mechanical interlocking is present. Also, the surfaces of the reinforcements must be prepared so that variations in surface rough-ness do not affect the results. The reason for this is that stress concentrations can occur at the surface of the matrix where the fiber/matrix interface is exposed and where the interfacial shear stress will be a maximum. These stress concentrations can affect the initiation of fracture and, in turn, the debond strength [72]. Concern should also be given to notch sensitivity, which can dictate the fracture path. Finally, as a demonstration of the wide range of applicability of the fiber pullout test, Piggot and Dai [67] conducted pullout tests on steel rods embedded in epoxy contained within steel tubes, and within polymer tubes. They concluded that both configurations demon-strated linear elastric fracture mechanics.

Other tests which measure a critical length include the single-fiber tension test and the various fragmentation tests [73]. For all measurements of interface bond strength, since it is very unlikely that surface roughness, mechanical interlocking, and residual stresses can be controlled, it is impor-tant that some method of asseessing their contribution be made.

3. MECHANICAL TESTING/EXAFS

One way to handle the problem of surface flaws or notch sensitivity is to locally monitor the loading condition at the interface in each of these different tests. Tracer elements (low concentration elements) at the interface offer an interesting approach to do this. It is proposed that structural characteriza-tion of the interface by EXAFS of tracer elements placed at the interface concurrent with a mechanical test (Fig. 8) will probe the stress stage of the interface and identify the fracture path in the interface. Figure 9a is a hypothetical plot of the radial distribution function of atoms neighboring the target titanium tracer atom in a hypothetical graphite/aluminum MMC with titanium at the interface. For recognizable strain levels the peaks (atom displacements) will shift and broaden, indicating bond stretching (Fig. 9b). It is possible that changes occurring at the interface during loading and fracture are detectable using EXAFS, although they are likely to be small. Following fracture, the newly created survaces can be analyzed using SEXAFS (surface-sensitive EXAFS) to precisely determine the fracture path. Loss of intensity corresponding to a given element after fracture would correspond to a change in the local neighborhood (i.e., fracture on one side


X-rays

O O O O O O O^

Titanium atoms Carbon Fiber

(o o o o o o o

FIG. 8. Proposed tensile test of aluminum/carbon fiber-reinforced composite with Ti tracer atoms placed at the interface to monitor strain state and fracture path by EXAFS.

or the other of the Ti tracer atoms). According to our hypothetical re-presentation of such an experiment in Fig 9c, since the aluminum peak has diminished, fracture has occurred at the Ti/Al interface instead of at the Ti/C interface. This testing configuation seems feasible for a pullout and single-fiber tensile test, as well as for the peel test. One can also include acoustic emission to monitor the fracture events, whereby the debonding stress can be determined more accurately. This is also an example of the need to couple analytical techniques, microscopic and spectroscopic, to the mechanical test. Coupling has also been done between bend tests and simultaneous determi-nation of stress using x-ray diffraction and strain gauges [74].

4. COMPACT TENSION TEST

Stress concentrations in MMCs influence the mechanical properties of the composite [72, 75~\. They, in turn, are related to the fracture properties of

Titanium Near Neighbor Distribution (Distance)

FIG. 9. Model EXAFS plot of Ti tracer experiment: (a) initial condition: (b) during loading, not peak shifts and broadening; (c) after fracture, note reduction in Al peak.


fracture toughness of the composite interface by adding an additional driving force for fracture. Fracture toughness testing using compact tension test specimens are ideal for measuring K. However, the crack geometry must be well designed to limit crack formation in the reinforcement by reducing its susceptibility to crack growth. This means that when the notch is cut in the specimen along the interface, care should be taken to prevent surface cracks or flaws in the/reinforcement, which is usually more notch-sensitive than the matrix. The maximum crack sensitivity must be along the interface. Even when the notch is not cut, the geometry of the crack tip must be exactly in the plane of the interface.

A number of models have been developed which describe fracture at an MMC interface [75-79]. The most applicable fracture criterion for metal matrix composites is that in which the fracture path is governed by the strain energy release rate equaling the material's resistance to cracking [75]. Since it is known that the weakest point of the material is not always the first to fail, fracture along the weakest path is not as accurate. A number of numerical models for fracture toughness of interfaces in MMCs are also available [75-79].

B. Atomistic/Microscopic Bonding

For the purposes of this discussion, atomistic bonding means the chemical bonds which are formed between individual atoms. The important informa-tion to be gained is the strength of the bond, the elements engaged in the bond, and the type of bond. The elements present at the interface can be determined from spectroscopic data which are characteristic of each element. Perturbations on the characteristic spectra can give information on the type of bond or the associated element forming the bond.

1. EXAFS

Extended x-ray absorption fine structure spectroscopy has been described earlier. It is particularly well suited to the analysis of the bonding at metal matrix composite interfaces on an atomistic scale. EXAFS analysis can determine the chemical state of a target atom by identifying the nearest and next-nearest neighbors. EXAFS is also used to determine the nearest and next-nearest-neighbor distances and the coordination number of atoms around the target atom. Thus, EXAFS is extremely useful when trying to determine the strength of atomic bonds at an interface.

Recently, EXAFS has been used to examine interfaces. Barrera et al. [80] examined titanium monolayers at nickel/nickel interfaces using EXAFS.


Since titanium was present only at the interface, the fine structure associated with the titanium edge was characteristic of the interface structure. They were able to determine that titanium deposited using RF sputtering existed in an oxide state, and titanium deposited using electron beam evaporation existed in a metallic state.

The same sort of EXAFS experiments used to investigate chemical interaction can be used to investigate the bonding state of the interface (e.g., use EXAFS to determine whether a Ti tracer atom placed at a graphite/alu-minum interface exists as a metal, oxide, carbide boride, or intermetallic state). The same sample configurations (discussed earlier) can be used. By determining the type of bonding, the bond lengths, and the atomic coordina-tion, one can gain important insight concerning the strength and mechanical behavior of an interface. That is, if the bond is metallic (e.g., Ti/Ti or Ti/Al), then the bond may be more ductile than an ionic bond (e.g., Ti/O). Since EXAFS can also be used to determine disorder [36], the degree of disorder at an interface can also be determined.

Another promising procedure is fracture-EXAFS analysis of fracture surfaces. Although not limited to composite interfaces, fracture of a com-posite along an interface and SEXAFS analysis of the fracture surface using partial yield [36] will yield information on the composition, atomic structure, bonding, and the path along which fracture occurs. Heretofore, analysis of the first 3 nm of a fracture surface was limited to AES and XPS. SEXAFS of fracture surfaces will allow investigation of the atomic structure; co-ordination, bond lengths, and the identity of nearest and next-nearest neighboring atoms.

2. XPS AND AES

XPS and AES (introduced in Section IIC) are surface techniques which are extremely well suited to the characterization of interfaces. The value of XPS lies in its ability to characterize the local bonding state of an atom. One can determine, for example, whether aluminum deposited onto silicon carbide bonds to silicon atoms or carbon atoms [81]. Many metal matrix composite model interfaces can be deposited in situ, and, therefore, clean, fundamental work is readily performed. This is useful in studying wetting, bonding, and chemical reactivity. Hasegawa et al [21] were able to determine that titanium bonded to silicon carbide at very low temperature. Maruyama et al [30] determined that hydrated alumina can affect the bonding of aluminum to carbon. In the future, XPS is likely to play an important role in the understanding of atomistic bonding of MMC interfaces.

AES has been used to identify the fracture path along metal matrix composite interfaces [79]. By analyzing the fiber and matrix sides of a fracture surface, the path by which fracture occurs can be determined (i.e., through


the matrix, reinforcement, interfacial oxide, etc.). AES is also used to analyze the distribution of light elements normal to an interface [82]. On the one hand, determination of the chemical state of an element can be more difficult with AES as compared to XPS; on the other, the spot size of the probe is much smaller, thereby allowing AES analysis of real composite materials.

C. Evaluation of Bonding

1. WORK OF FRACTURE

Ultimately, there must be some connection between microscopic/chemical and macroscopic/mechanical bonding. This can be accomplished by applying fundamental theories to bonding at the interface. As an example, the load transferred across the matrix/reinforcement interface can be related to the work of fracture, Wf\

Wf = 7A + 7B - TAB, (4.3)

where yA and yB are surface energies per unit area for newly created surfaces A and B, and yAB is the interface energy per unit area. Work of fracture can be derived from an atomic potential energy function such as a Lennard-Jones potential. In such a potential, the slope of the potential (U) versus r curve (curves (a) of Fig. 10) gives the stress-displacement curve (curve (b) of Fig. 10), and

Wf = σ dr. (4.4)

Tensile Stress

Potential Energy

Interatomic Distance

FIG. 10. (a) Plot of potential energy (U) versus interatomic distance (r): potential; strengthened potential; (b) plot of σ versus r.

weakened


Bonding, like wetting, is subject to chemical perturbations. Chemical pertrbations on the interface include changes in the elemental composition of the interface and reaction of the matrix and reinforcement. Also to be considered are the effects of various phases present in the interface region, such as oxides, wetting agents, and diffusion barriers on bonding. Thus a change in the interface due to chemical perturbations or new phases can be modeled by changes in the atomic potential energy function (see Fig. 10).

2. REACTIONS AS A PROBE OF BONDING

Most chemical reactions consist of the transport of atoms or molecules and rearrangement of bonds between atoms. An interface reaction between fiber and matrix can be described as the breaking of bonds between the atom that is to diffuse and the stationary species, followed by the diffusion of the mobile species across the interface, and completed by the forming of a new compound. Imagine a perfectly sharp planar interface: A atoms on one side and B atoms on the other. Assume that the bond between two atoms affects only their nearest neighbors. This is a one-dimensional problem and can be represented schematically as follows:

A—A—A—A—A—a—ß—B—B—B—B—B.

There are five energetically different bonds in this simplistic model. Two are the respective bonds between A and B atoms in the bulk material far from the interface. The other bonds are interface bonds: the a—ß bond across the interface, the A—a bond between the surface A atom and the underlying A bulk material, and the ß—B bond between the surface B atoms and the underlying B bulk material. All of these bonds will affect each other, except the bulk A and B bonds. The bond between the surface a and surface ß atoms at the interface will affect the bond between the surface ß atom and the underlying B atom, just below the surface, as well as the bond between the surface a atom and the underlying A atom, just below the surface.

There are several important ramifications of this representation. Ideally, one desires a smooth transition in bond strength from bulk A—A bonding to bulk B—B bonding. One might naively think that the strength of the interface should be maximized by maximizing the strength of the a—ß bond. However, if the a—ß bond is so strong that it reduces the strength of the neighboring ß—B bond or A—a bond, then a weak link has been created in the chain of bonds across (normal to) the interface [83, 84]. If this interface were stressed in tension normal to the interface and the ß—B bond were the weak link (neglecting ductility), then the fracture path would be between the surface layer ß and the bulk B material.

Experimental work shows that a small amount of reaction at the interface


is associated with an increase in the strength of the interface, whereas a large amount of reaction weakens the interface by forming a brittle second phase, thereby degrading the fiber, etc. [70]. It is a commonly held, heuristic belief that a metal matrix composite interface should be reactive enough to ensure good interface bonding, but not so reactive as to degrade the composite by brittle phase formation or chemical dissolution of the reinforcement. It is suggested that a strong reaction is undesirable not only because of potential degradation due to reaction but also because an overwhelmingly strong potential to react may incur a weak bond between the surface of a component and its bulk. To illustrate this point, refer to the oxidation model of carbide formation discussed in the chemical reaction section. One conclusion derived from the oxidation model is that the strength of the bond between the carbon atoms on the surface of a graphite fiber in a graphite/aluminum composite and the underlying atoms in the bulk of the graphite fiber are weakened by the overwhelmingly strong bond of those surface carbon atoms to oxygen atoms of the interfacial aluminum oxide. It is proposed that a weak link exists between the surface carbon atoms and the bulk graphite fiber, and that fracture can easily occur along the path between them. Although this effect has not been demonstrated conclusively, supporting evidence is re-ported by Marcus et al. [19] and Finello [20].

3. MODIFICATION STRATEGIES

A most important strategy for attaining a strong bond at a composite interface is to minimize interface reactions and degradation of the interface. Equally important is to ensure good wetting throughout the composite. A third strategy addresses the problem of residual stresses and easy propaga-tion of cracks along the interface or through the interface and into the reinforcement. The strategy is to apply a compliant layer to the reinforcement to relieve stresses and blunt any cracks. These compliant layers comprise nickel [2] and other such ductile metals.

IV. Summary and Conclusions

Wetting, bonding, and chemical interaction are the principal processes which govern the interface/interphase properties of any composite. Macro-scopically, the effects of mechanical interlocking and residual stresses must also be considered. The complexity of the interface makes any fundamental analytical investigation of the individual terms virtually impossible. There-fore one must turn to fundamental studies of model systems to simplify and


isolate the important aspects. Thus, the design of model metal matrix composite systems and the choice of analytical techniques are discussed as a guide to the interested reader.

In this chapter, we have outlined traditional measurements of wetting, bonding, and chemical reactivity. Wetting precedes bonding, which can ultimately lead to chemical reaction products. The interrelationship of wetting, bonding, and chemical reactivity complicate any model study that attempts to evaluate them separately. On the other hand, knowledge about one aspect of interface creation can supplement the other such that an integrated approach is best. We have stressed the importance of modifica-tions of interface structure and stoichiometry, such as oxidation and segre-gation, in relation to interface properties. We have also outlined the use of such new analytical techniques as XPS and EXAFS, which may prove quite fruitful in the future. Some atomistic theories on the state of the interface have been presented. Their ramifications on bonding and chemical reactivity stress the importance of understanding the chemistry of the interface as related to atomistic bond strength and catalyzed reactions.

As a closing comment, we feel that metal matrix composite technology has matured to a point where an understanding of their manufacture and performance requires fundamental theories and experiments be created and executed.

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Metal Matrix Composites: Processing and Interfaces || Characterization and Modification of Composite Interfaces