Composite Interfaces. Vol. 4. No. 5, pp. 287-298(1997) @vSP1997 Interfaces in metal matrix composites K. K. CHAWLA Department of Materials and Metallurgical Engineering, New Mexico institute of Mining and Technology, Socorro, New Mexico 87801, USA Received 23 June 1996: accepted 3 November 1996 Abstract-The interface region in a given composite has a great deal of importance in determining the ultimate properties of the composite. An interface is, by definition, a bidimensional region through which there occurs a discontinuity in one or more material parameters. In practice, there is always some volume associated with the interface region over which a gradual transition in material parameter(s)occurs. The importance of the interface region in composites stems from two main reasons: (i) the interface occupies a very large area in composites, and(ii) in general, the reinforcement and the metal matrix will form a system that is not in thermodynamic equilibrium. One can discuss the interface in a composite at various levels. An opti should be neither so simple that it covers only a few special cases nor so complex that it is not useful in designing composites from processing and applications points of view. In this paper. my objective is to give examples of interface microstructure in different metal matrix composite systems and suggest some ways of controlling the interface characteristics in order to control the properties of the composite. I shall give examples of the interface microstructure in different manx composites (particle and fiber reinforced as well as laminates)and discuss some of the ant implications various aspects of metal matrix composites, from the processing stage to ultimate perform Keywords: Metal matrix composite; interface: particle; fiber; reinforcement. 1 INTRODUCTION Metal matrix composites consist of a metal or an alloy as the continuous matrix and a reinforcement that can be particle, short fiber or whisker, ntinuous fib Table I provides examples of some important reinforcements used in metal matrix composites as well as their aspect(length/diameter) ratios and diameters. MMCs are really not new. Any heat treated steel or a two-phase metallic alloy is really a metal matrix composite. Hypereutectic Al-Si alloys represent a particulate metal matrix composite inasmuch as their microstructure consists of Si particles in an Al matrix. Metallurgists have controlled the shape and size of Si particles by means of alloy chemistry and solidification techniques. The new emphasis on MMCs involves mixing of reinforcement fiber or particles with a suitable metal matrix, generally larger volume fractions than ones found in steels and other alloys. In particular, it
Interfaces in metal matrix composites K. K. CHAWLA Department of Materials and Metallurgical Engineering, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA Received 23 June 1996; accepted 3 November 1996 Abstract_The interface region in a given composite has a great deal of importance in determining the ultimate properties of the composite. An interface is, by definition, a bidimensional region through which there occurs a discontinuity in one or more material parameters. In practice, there is always some volume associated with the interface region over which a gradual transition in material parameter(s) occurs. The importance of the interface region in composites stems from two main reasons: (i) the interface occupies a very large area in composites, and (ii) in general, the reinforcement and the metal matrix will form a system that is not in thermodynamic equilibrium. One can discuss the interface in a composite at various levels. An optimum one should be neither so simple that it covers only a few special cases nor so complex that it is not useful in designing composites from processing and applications points of view. In this paper, my objective is to give examples of interface microstructure in different metal matrix composite systems and suggest some ways of controlling the interface characteristics in order to control the properties of the composite. I shall give examples of the interface microstructure in different metal matrix composites (particle and fiber reinforced as well as laminates) and discuss some of the important implications on various aspects of metal matrix composites, from the processing stage to ultimate performance of the composite. Keywords: Metal matrix composite; interface; particle; fiber; reinforcement. 1. INTRODUCTION Metal matrix composites consist of a metal or an alloy as the continuous matrix and a reinforcement that can be particle, short fiber or whisker, or continuous fiber. Table 1 provides examples of some important reinforcements used in metal matrix composites as well as their aspect (length/diameter) ratios and diameters. MMCs are really not new. Any heat treated steel or a two-phase metallic alloy is really a metal matrix composite. Hypereutectic Al-Si alloys represent a particulate metal matrix composite inasmuch as their microstructure consists of Si particles in an Al matrix. Metallurgists have controlled the shape and size of Si particles by means of alloy chemistry and solidification techniques. The new emphasis on MMCs involves mixing of reinforcement fiber or particles with a suitable metal matrix, generally in larger volume fractions than ones found in steels and other alloys. In particular, it
KK Chawla Table 1 Typical reinforcements used in metal matrix composites Type Aspect ratio Diameter(um Examples Particle ~1-4 l-25 SiC, AlO3. BN. B4C Short fiber or whisker ~10-1000 SiC, AlzO3. Al2O3+ SiO, C Continuous fiber >1000 3-150 SiC, AlO3, C, B, w involves understanding and evaluating the characteristics of the interface region in order to obtain an optimum set of characteristics in the MMC The interface region in a composite is very important in determining the ultimate properties of the composite. An interface is the region through which there occurs a discontinu he or more material parameters such as elastic moduli, thermody namic parameters such as chemical potential, and the coefficient of thermal expansion The importance of the interface region in composites stems from two main reasons (i)the interface occupies a very large area in composites, and (ii)in general, the reinforcement and the matrix will form a system that is not in thermodynamic equi- There are many possible descriptions of the interface in a composite. An optimum one should be neither so simple that it covers only a few special cases nor so complex that it is not useful in designing composites from the points of view of processing and applications. In this paper, my objective is to give examples of interface microstructure in different metal matrix composite systems and suggest some means of controlling the interface characteristics in order to control the properties of the composite 2. WETTA BILITY An important parameter in regard to the interface is the wettability of reinforcement by the matrix. Wettability refers to the ability of a liquid to spread on a solid substrate Frequently, the contact angle between a liquid drop and a solid substrate is taken as a measure of wettability, a contact angle of 0 indicating perfect wetting while a contact angle of 180 indicating no wetting. Wettability is only a measure of the possibility of attaining an intimate contact between a liquid and a solid. good wetting is a necessary but not sufficient condition for strong bonding. One needs a good wetting even for purely mechanical bonding or weak van der Waals bonding, otherwise voids may form at the interface. Contact angle, 6, of the liquid metal matrix on the solid surface of the fiber is an important parameter to characterize the wettability. Commonly, the contact angle is measured by putting a sessile drop of the liquid on the flat surface of a solid substrate. When such a system is viewed from the side, the contact angle the angle between the tangents along three interfaces: solid/liquid, liquid/vapor, and solid/vapor. The angle 0 can be measured directly by a goniometer or calculated by using simple trigonometric relationships involving drop dimensions. In theory, one can use the following expression, called Youngs equation ysv= ysL nv cos 6
288 Table 1. Typical reinforcements used in metal matrix composites involves understanding and evaluating the characteristics of the interface region in order to obtain an optimum set of characteristics in the MMC. The interface region in a composite is very important in determining the ultimate properties of the composite. An interface is the region through which there occurs a discontinuity in one or more material parameters such as elastic moduli, thermodynamic parameters such as chemical potential, and the coefficient of thermal expansion. The importance of the interface region in composites stems from two main reasons: (i) the interface occupies a very large area in composites, and (ii) in general, the reinforcement and the matrix will form a system that is not in thermodynamic equilibrium. There are many possible descriptions of the interface in a composite. An optimum one should be neither so simple that it covers only a few special cases nor so complex that it is not useful in designing composites from the points of view of processing and applications. In this paper, my objective is to give examples of interface microstructure in different metal matrix composite systems and suggest some means of controlling the interface characteristics in order to control the properties of the composite. 2. WETTABILITY An important parameter in regard to the interface is the wettability of reinforcement by the matrix. Wettability refers to the ability of a liquid to spread on a solid substrate. Frequently, the contact angle between a liquid drop and a solid substrate is taken as a measure of wettability, a contact angle of 0° indicating perfect wetting while a contact angle of 180° indicating no wetting. Wettability is only a measure of the possibility of attaining an intimate contact between a liquid and a solid. Good wetting is a necessary but not sufficient condition for strong bonding. One needs a good wetting even for purely mechanical bonding or weak van der Waals bonding, otherwise voids may form at the interface. Contact angle, 0, of the liquid metal matrix on the solid surface of the fiber is an important parameter to characterize the wettability. Commonly, the contact angle is measured by putting a sessile drop of the liquid on the flat surface of a solid substrate. When such a system is viewed from the side, the contact angle is the angle between the tangents along three interfaces: solid/liquid, liquid/vapor, and solid/vapor. The angle 0 can be measured directly by a goniometer or calculated by using simple trigonometric relationships involving drop dimensions. In theory, one can use the following expression, called Young's equation
Interfaces in metal matrix composites where 8 is the contact angle and y is the surface energy per unit area. In very general terms, a small 6 indicates good wetting. In practice, it is rarely possible to obtain unique equilibrium value of 8. Most often, the ceramic reinforcement is rejected by the molten metal because of non-wettability or high contact angle. Sometimes the contact angle of a liquid drop on a solid substrate can be decreased by increasing the surface energy of the solid(ysv)or by decreasing the energy of the interface between the liquid and the solid(ysL). Thus, under certain circumstances, the wettability of a solid ceramic by a molten metal can be improved by making a small alloy addition to the matrix composition. An example of this is the addition of lithium to aluminum to improve the wettability in the alumina fiber/aluminum composite [1, 2]. However, besides wettability, there are other important factors such as chemical, mechanical thermal, and structural that affect the nature of bonding between reinforcement and matrix. As it happens, these factors frequently overlap and it may not always be possible to isolate these effects 3. MAJOR DISCONTINUITIES AT INTERFACES IN MMCS As we said above, at an interface there can occur a variety of discontinuities. The im- portant parameters that can show discontinuities in MMCs at a ceramic reinforcement/ metal matrix interface are as follows (i)Bonding. A ceramic reinforcement will have an ionic or a mixed ionic/covalent bonding while the metal matrix will have a metallic bonding (i)Crystallographic. The crystal structure and the lattice parameter of the matrix and the reinforcement will be different (iii) Moduli. Different elastic moduli of the matrix and the reinforcement. (iv) Chemical potential. The matrix and the reinforcement will not be in thermody namic equilibrium at the interface, i.e. there will be a driving force for a chemical reaction. Table 2 shows interfacial reaction products in some important MMC (v)Coefficient of thermal expansion(CTE). The matrix and the reinforcement will in general, have different CTE Interfacial reaction products in some important MMCs Reinforcement Matrix Reaction product(s) 二 TiC. Tis Si3 Al4C3 Alo MgO, MgAl2 O4(spinel) Al alloy Al alloy AlO3+ ZrO, Al alloy W None C Cu None None
289 where 9 is the contact angle and y is the surface energy per unit area. In very general terms, a small B indicates good wetting. In practice, it is rarely possible to obtain a unique equilibrium value of 9. Most often, the ceramic reinforcement is rejected by the molten metal because of non-wettability or high contact angle. Sometimes the contact angle of a liquid drop on a solid substrate can be decreased by increasing the surface energy of the solid (ysv) or by decreasing the energy of the interface between the liquid and the solid (ySL). Thus, under certain circumstances, the wettability of a solid ceramic by a molten metal can be improved by making a small alloy addition to the matrix composition. An example of this is the addition of lithium to aluminum to improve the wettability in the alumina fiber/aluminum composite [1, 2]. However, besides wettability, there are other important factors such as chemical, mechanical, thermal, and structural that affect the nature of bonding between reinforcement and matrix. As it happens, these factors frequently overlap and it may not always be possible to isolate these effects. 3. MAJOR DISCONTINUITIES AT INTERFACES IN MMCS As we said above, at an interface there can occur a variety of discontinuities. The important parameters that can show discontinuities in MMCs at a ceramic reinforcement/ metal matrix interface are as follows. (i) Bonding. A ceramic reinforcement will have an ionic or a mixed ionic/covalent bonding while the metal matrix will have a metallic bonding. (ii) Crystallographic. The crystal structure and the lattice parameter of the matrix and the reinforcement will be different. (iii) Moduli. Different elastic moduli of the matrix and the reinforcement. (iv) Chemical potential. The matrix and the reinforcement will not be in thermodynamic equilibrium at the interface, i.e. there will be a driving force for a chemical reaction. Table 2 shows interfacial reaction products in some important MMCs. (v) Coefficient of thermal expansion (CTE). The matrix and the reinforcement will, in general, have different CTE. Table 2. Interfacial reaction products in some important MMCs
K. K. Chawla 4. INTERFACIAL BONDING IN METAL MATRIX COMPOSITES We provide a summary of salient features of the interfacial region in some of the ost important metal matrix composites 4.1. Crystallographic nature In crystallographic terms, ceramic/metal interfaces in composites are, generally, inco- herent and high-energy interfaces. Accordingly, they can act as very efficient vacancy sinks, provide rapid diffusion paths, segregation sites, sites of heterogeneous precipita- tion, as well as sites for precipitate free zones. Among the possible exceptions to this are the eutectic composites [3] and the newer XDm type particulate composites [4] 4.2. Mechanical bonding Some bonding must exist between the ceramic reinforcement and the metal matrix for load transfer from matrix to fiber to occur. two main categories of bonding are mechanical and chemical. Mechanical keying effect between two surfaces can lead to bonding. Hill et al. [5] confirmed this experimentally for tungsten filaments in an aluminum matrix while Chawla and Metzger [6] observed mechanical gripping effects at AlO3/Al interfaces. The results of Chawla and Metzger [6] are shown in Fig. 1 in the form of the linear density of cracks in alumina as a function of strain in an alumina/aluminum composite for different degrees of interface roughness. The main message of this figure is that the crack density continues to increase to larger strain values in the case of a rough interface( deeply etched pits) vis-a-vis smooth or not very rough interface, i. e. the rougher the interface, the stronger the mechanical We can make an estimate of the radial stress at the fiber/ matrix interface due to roughness induced gripping [7] Emer E(1+m)+Em(1-) where E is Youngs modulus, v is Poisson's ratio, A is the amplitude of roughness, r is the radius of the fiber, and the subscripts m and f indicate matrix and fiber respectively. For a given composite, the compressive radial stress increases with the roughness amplitude and decreases with the fiber radius. Al uch an MMC, i.e. non-reacting components with a purely mechanical bond at the interface, is the filamentary superconducting composite consisting of niobium-titanium alloy filaments in a copper matrix 4.3. Chemical bon Ceramic/metal interfaces are generally formed at high temperatures. Diffusion and chemical reaction kinetics are faster at elevated temperatures. One needs to have knowledge of the chemical reaction products and, if possible, their properties. Molten
290 4. INTERFACIAL BONDING IN METAL MATRIX COMPOSITES We provide a summary of salient features of the interfacial region in some of the most important metal matrix composites. 4.1. Crystallographic nature In crystallographic terms, ceramic/metal interfaces in composites are, generally, incoherent and high-energy interfaces. Accordingly, they can act as very efficient vacancy sinks, provide rapid diffusion paths, segregation sites, sites of heterogeneous precipitation, as well as sites for precipitate free zones. Among the possible exceptions to this are the eutectic composites [3] and the newer XDTM type particulate composites [4]. 4.2. Mechanical bonding Some bonding must exist between the ceramic reinforcement and the metal matrix for load transfer from matrix to fiber to occur. Two main categories of bonding are mechanical and chemical. Mechanical keying effect between two surfaces can lead to bonding. Hill et al. [5] confirmed this experimentally for tungsten filaments in an aluminum matrix while Chawla and Metzger [6] observed mechanical gripping effects at A1203 /A1 interfaces. The results of Chawla and Metzger [6] are shown in Fig. 1 in the form of the linear density of cracks in alumina as a function of strain in an alumina/aluminum composite for different degrees of interface roughness. The main message of this figure is that the crack density continues to increase to larger strain values in the case of a rough interface (deeply etched pits) vis-a-vis smooth or not very rough interface, i.e. the rougher the interface, the stronger the mechanical bonding. We can make an estimate of the radial stress at the fiber/matrix interface due to roughness induced gripping [7] where E is Young's modulus, v is Poisson's ratio, A is the amplitude of roughness, r is the radius of the fiber, and the subscripts m and f indicate matrix and fiber, respectively. For a given composite, the compressive radial stress increases with the roughness amplitude and decreases with the fiber radius. An important example of such an MMC, i.e. non-reacting components with a purely mechanical bond at the interface, is the filamentary superconducting composite consisting of niobium-titanium alloy filaments in a copper matrix. 4.3. Chemical bonding Ceramic/metal interfaces are generally formed at high temperatures. Diffusion and chemical reaction kinetics are faster at elevated temperatures. One needs to have knowledge of the chemical reaction products and, if possible, their properties. Molten
Interfaces in metal matrix composites E ∝uo9omuo2 SMOOTH STEEP SDED PITS O PER cm GENTLY SLOPIN PERCENT ELONGATION Figure 1. Linear density of cracks in alumina as a function of strain in an alumina/aluminum composite for different degrees of interface roughness(from [4]) iron, nickel, titanium, low alloy steels, austenitic and ferritic stainless steels, nickel sed superalloys react with silicon containing ceramics to form eutectics, with the reaction products being mainly metal silicides and carbides. It is thus imperative to understand the thermodynamics and kinetics of reactions such that processing can be controlled and optimum properties can be obtained. We provide some examples Most metal matrix composite systems are nonequilibrium systems in the thermody- namic sense; that is, there exists a chemical potential gradient across the fiber/matrix interface. This means that given favorable kinetic conditions(which in practice means
291 Figure 1. Linear density of cracks in alumina as a function of strain in an alumina/aluminum composite for different degrees of interface roughness (from [4]J. iron, nickel, titanium, low alloy steels, austenitic and ferritic stainless steels, nickelbased superalloys react with silicon containing ceramics to form eutectics, with the reaction products being mainly metal silicides and carbides. It is thus imperative to understand the thermodynamics and kinetics of reactions such that processing can be controlled and optimum properties can be obtained. We provide some examples. Most metal matrix composite systems are nonequilibrium systems in the thermodynamic sense; that is, there exists a chemical potential gradient across the fiber/matrix interface. This means that given favorable kinetic conditions (which in practice means