Engineering Properties, Strategies and Emphasis of Metal Matrix Composites

By
Dr Thoguluva Raghavan Vijayaram
BE (Mechanical Engineering, Madurai Kamaraj University, India),
ME (Metallurgical Engineering, Bharathiyar University, India),
PhD (Mechanical Engineering, Universiti Putra Malaysia, Malaysia),
Rector Grant Researcher in Metallurgy (Genoa University, Italy),
Chartered Engineer (M123412-3, IIE, Calcutta, India)
MIIF, MISTE, MIIPE, MIE (Calcutta, India)
Senior Lecturer in Manufacturing Engineering and Researcher in Metallurgy,
Department of Manufacturing Process and System, Faculty of Manufacturing Engineering, UTeM, Universiti Teknikal Malaysia Melaka, Ayer Keroh, 75450 Melaka, Malaysia.
E-mail: thoguluva@utem.edu.my

A composite material is a combination of two or more chemically different materials with a distinct interface between them. The constituent materials maintain their separate identities microscopically in the composite, yet their combination produces properties and characteristics that are different from those of the constituents . One of these constituents forms a continuous phase and it is called as the matrix. The other major constituent is the reinforcement phase available in the form of fibers or as a particulate, in general, added to the matrix to improve or alter the matrix properties. Reinforcement by a particulate forms a discontinuous phase uniformly distributed throughout the matrix. Therefore, composites have improved mechanical properties such as strength and toughness when compared with monolithic materials.

Aluminium is the most popular matrix for the metal matrix composites. The aluminium alloys are quite attractive due to their low density, their capability to be strengthened by precipitation, their good corrosion resistance, high thermal and electrical conductivity, and their high vibration damping capacity. Aluminum matrix composites have been used since the 1920s and are now used in sporting goods, electronic packaging, armors and automotive industries. They offer a large variety of mechanical properties depending on the chemical composition of the aluminium matrix. They are usually reinforced by aluminium oxide, silicon carbide, silicon dioxide, carbon, graphite, boron, boron nitride, boron carbide etc. and aluminium nitride is also dispersed in the matrix. In the 1980s, transportation industries began to develop discontinuously reinforced aluminium matrix composites. They are very attractive for their isotropic mechanical properties and their low costs. The properties of composites of metal matrix composites are inevitably a compromise between the properties of the matrix and reinforcement phases. It is clear that the composition and properties of the matrix phase affect the properties of the composite both directly, by normal strengthening mechanisms, and indirectly, by chemical interactions at the reinforcement / matrix interface. Aluminium based composites, reinforced with ceramic particles, offer improvements over the matrix alloy: an elastic modulus higher than that of aluminium has a value of 70 GPa, a coefficient of thermal expansion which is closer to that of steel or of cast iron, a greater resistance to wear and an improvement in rupture stress especially at higher temperatures and possibly improved resistance to thermal fatigue.

In addition to the benefits listed above, there are decreases in elongation to failure and fracture toughness. Fortunately, the introduction of aluminium-silicon / ceramic composites seems to provide a good basis for manufacturing pistons which may be expected to meet the demands to withstand higher cylinder pressures, increased fuel injection rates and higher operating temperatures. Increasing the weight fraction percentage of silicon carbide particulates addition in the LM6 alloy matrix has increased the modulus, yield, and ultimate tensile stresses but a reduced strain to fracture. However, it is seen that that silicon content of the matrix has a dominant effect in reducing the fracture strain more than the increase in silicon carbide particulate addition. The strength of particulate metal matrix composites has been addressed by both a continuum mechanics, based upon shear-lag-type models, Eshelby’s equivalent inclusion technique, finite elements methods and a dislocation mechanics approach. However, the aluminium-12 % silicon materials have relatively higher fatigue strength compared with those contained 5% silicon. Endurance limit based on 107 cycles decreases with silicon carbide addition in the LM6 alloy-silicon carbide particulate composites. In the case of sillimanite particulate reinforced aluminium matrix composites, the hardness of the matrix alloy has increased from 57 to 85 HV due to the dispersion of the sillimanite particles in the matrix. This may be attributed to the significantly higher hardness value, 650 HV of the sillimanite particles, and the matrix becomes plastically constrained due to thermal residual stress in the presence of the sillimanite particles.

Many of the properties of particulate reinforced metal matrix composites like strength, ductility, and stiffness fall below the predicted values by the rule of mixtures. For 10% weight fraction addition of sillimanite particles in the aluminium-silicon alloy, the ultimate tensile strength has decreased marginally from 132 to 121 MPa and percentage elongation has decreased significantly from 2.25 to 1.42. Same properties results are found and similar to those of other aluminium-silicon alloy composites dispersed with other hard particles. This is primarily due to the mechanical type of bonding and ineffective load transfer from the matrix to the reinforcement. Matrix is the percolating metal/alloy/plastic/polymer/resin/ceramic forming the constituent of a composite in which other constituents are embedded. If the matrix is a metal, then it is called as a metal matrix and consecutively polymer matrix, if the matrix is a polymer and so on. In composites, the matrix or matrices have two important functions. Firstly, it holds the reinforcement phase in the place. Then, under an applied force, it deforms and distributes the stress to the reinforcement constituents. Sometimes the matrix itself is a key strengthening element. This occurs in certain metal matrix composites. In other cases, a matrix may have to stand up to heat and old. It may conduct or resist electricity, keep out moisture, or protect against corrosion. It may be chosen for its weight, ease of handling, or any of many other applications. Any solid that can be processed to embed and adherently grip a reinforcing phase is a potential matrix material.

Polymers and metals have been very successful in the role and inorganic materials such as glass, plaster, Portland cement, carbon, and silicon have been used as matrix materials with varying success. These later materials remain elastic up to their points of failure and characteristically exhibit low failure strains under tensile loading, but are strong under compression. One important consideration of matrix in composite production is how the constituents of a composite interact during fabrication and / or use. They should not react chemically or metallurgically in away that harms. In general, they should not have greatly different coefficients of linear thermal expansion. The area of contiguous contact between the matrix and the reinforcing material is called the interface, which in some ways is analogous to the grain boundaries in monolithic materials. In certain cases, however, the contiguous region is a distinct added phase, called an interphase. Examples are the coating on the glass fibers in reinforced plastics, and the adhesive that bonds the layers of a laminate together. When such an interphase is present, there are two interfaces-one between each surface of the interphase and its adjoining constituent. In still other composites, the surfaces of the dissimilar constituents interact to produce an interphase. Fabrication methods depend to a great degree on the matrix properties, and how the matrix affects the properties of the reinforcements. Some of the important matrices used normally in composites processing are metal, polymer, ceramic, glass, and carbon/graphite. In a composite, matrix is an important phase, which is defined as a continuous one. The important function of a matrix is to hold the reinforcement phase in its embedded place, which act as stress transfer points between the reinforcement and matrix and protect the reinforcement from adverse conditions. It influences the mechanical properties, shear modulus and shear strength and its processing characteristics. Reinforcement phase is the principal load-carrying member in a composite. Therefore, the orientation, of the reinforcement phase decides the properties of the composite.

Reinforcement materials must be available in quantity and at an economical rate. Recent researches are directed towards a wider variety of reinforcements for the range of matrix materials being considered, since different reinforcement types and shapes have specific advantages in different matrices. It is to be noted that the composite properties depend not only on the properties of the constituents, but also on the chemical interaction between them and on the difference in their thermal expansion coefficients, which both depend on the processing route. In high temperature composites, the problem is more complicated due to enhanced chemical reactions and phase instability at both processing and application temperature.

Reinforcement phases in metal matrix composites are embedded in the form of continuous reinforcement or discontinuous reinforcement in the matrix material. Continuous reinforcement phase is continuous in at least one direction through the composite. Continuous fibers or percolating open-celled foam is suitable examples of the continuous reinforcement phase type. Continuous fibers are cylindrical ingredient material produced continuously to form an essentially endless reinforcement in he composite, usually delivered on bobbins of multifilament tows, each tow consisting of may individual fibers of diameters typically in the range of 3 to 30 micron. According to the production process, such fibers are usually coated by a polymeric sizing and the tows may be slightly twisted. They are typically designated by a brand name, the number of fibers per tow and a symbol of the applied sizing. Monofilaments are endless reinforcement as continuous fibers, except for a larger diameter, typically greater than 100 micron. Monofilaments are generally produced by deposition onto a core fiber and are delivered as individual fibers instead of tows. Discontinuous reinforcement is a non- percolating constituent of a composite, taking the form of individual elements embedded in the matrix constituent as particulates, short fibers, and whiskers. Preforms produced from discontinuous reinforcements that are mechanically stabilized by a binder or by cold compaction are still considered discontinuous reinforcements. Particulates are roughly equiaxed reinforcement, usually of aspect ratio less than about five. They can be both mono-and polycrystalline, can take various shapes like spherical, angular, and plate-like and are typically greater than 1 micron in diameter.

Dispersoids are the same as particulates, except that the diameter is less than 1 micron, hence, being capable of providing orowan strengthening. Platelets are flat reinforcements of an aspect ratio (diameter to thickness) greater than two. Platelets of an aspect ratio less than 5 can be considered as a type of particulate. Short fibers are cylindrical reinforcements with a ratio of length to diameter greater than 5, but typically greater than 100, and with a diameter typically greater than 1 micron. Whiskers are elongated single crystals, typically produced with a length to diameter ratio greater than 10 and with a diameter typically less than 1 micron. Several refractory reinforcing phases are used in composites processing. They are refractory metals, carbides, nitrides, borides, oxides, sulfides, intermetallics, silicides, and silicates. Since refractory metal compounds such as carbides, nitrides, borides, silicide and oxides are known to be extremely hard and to keep their strength at elevated temperatures. There are different considerations in choosing reinforcement. The selecting criteria must be set up based on their properties, which are mainly influenced by the chemical composition, melting point, density, volume shrinkage, shape and size, crystal structure, free energy of formation, Young’s modulus, diffusivity and finally, availability and ease of production and use. The reinforcing phase may be a particulate or a fiber, continuous type or discontinuous type. Some of the important particulates normally reinforced in composite materials are tungsten carbide, titanium carbide, aluminium silicate, quartz, silicon carbide, silicon nitride, fly ash, alumina, graphite, glass fibers, titanium boride etc. The reinforcement second phase material is selected depending on the application during the processing of composites. The reinforcement phase is in the form of particulates and fibers generally. The size of the particulate is expressed in microns, micrometer. However, the discontinuous fiber is defined by a term called as ‘Aspect Ratio’. It is expressed as the ratio of length to the diameter of the fiber. To improve the wettabilty with the liquid alloy or metal matrix material, the reinforcement phase is always preheated.

Boron-reinforced aluminium metal matrix composite combines the outstanding strength, stiffness and low density of the boron fiber with fabrication and engineering reliability of an aluminium alloy. The overall improvement in modulus to density ratio of the boron fiber is almost six times that of any of the standard engineering materials, including steel, aluminium, molybdenum, and magnesium. This is advantageous in the prevention of micro buckling of fibers in the matrix under compression. Other important physical and mechanical properties of boron/aluminium composites include high electrical and thermal conductivity, ductility, and toughness, non-flammability and the ability to be coated, formed and heat treated, and joined.

The graphite / aluminium composites are very attractive because the composite can be designed with the coefficient of thermal expansion approaching zero. The extremely high stiffness of the graphite fibers makes possible a composite that is ideal for applications where precise pointing and tracking are required. These are well suited for start strut assemblies, especially in space structures that are subject to a wide range of temperature across them. Graphite aluminium is also applicable for stable instrument platforms, electronics, and thermal control devices such as heat pipes. Stiffness to weight is high since the material is 30 percent stiffer then aluminium with no thermal expansion. Dispersion of graphite particles in aluminium-silicon alloy provides the alloy with antifriction properties, good wear properties such as wear rate, seizure resistance, and P-V limits, high damping characteristics, and good machinability. As a result, most developmental activities on this class of composites have focused more on their microstructure and tribological characteristics than on mechanical properties.

The alumina dispersoid is thermodynamically stable in molten aluminium alloy containing no magnesium. As a result, wetting and bonding is achieved by changing the surface chemistry of the dispersoids or by alloy additions such as magnesium and nickel to the matrix melt. Additions of 3% weight fraction of alumina having a size of 100 micron to the aluminium alloy matrix has increased the hardness from 27BHN to 37BHN and the ultimate tensile strength from 75MPa to 93MPa, it reduced the ultimate tensile strength of aluminium-8 % silicon alloy from 157MPa to 123MPa. Zircon, which is relatively heavier than many ceramic particles, requires little or no vortex before introduction into the alloy melt. Hardness and abrasive wear resistance, ultimate tensile strength, and yield strength are increased with the amount of zircon addition in the aluminum matrix while the percentage of elongation is decreased.

Fly ash reinforced aluminium alloy matrix composites processed by casting vortex-mixing process showed better abrasion resistance and wear resistance than the monolithic aluminium and aluminium alloys. Specific abrasive wear rate of aluminium alloy with 3% weight fraction of fly ash is decreased with increasing load and increasing sliding velocity. The aluminium-alloy with 3% weight fraction of fly ash showed better resistance than the base alloy up to 24 N. Specific abrasive wear rates of the composite containing 3% fly ash decreased with the decreasing size of the abraded particles. Friction coefficients of the fly ash composites decreased with increasing time, load and size of the abrading particles. Fly ash alloy aluminium composites are significantly lighter when compared to steel.
Composites science and technology requires lose interaction of various disciplines such as structural design and analysis, materials science, mechanics of materials, and process engineering. The tasks of composites research are to investigate the basic characteristics of he constituents and composite materials, optimize the material for service conditions, develop effective and efficient fabrication procedures and understanding their effect on material properties and to determine material properties and predict the structural behavior by analytical procedures and hence to develop effective experimental methods for material characterization, stress analysis and failure analysis.

An important task is the non-destructive evaluation of material integrity, structural reliability, durability assessment, flaw criticality, and life prediction. New types of carbon fibers are being introduced with higher ultimate strains. Thermoplastic matrices are used under certain circumstances because they are tough and have low sensitivity to moisture effects, and are more easily amenable to mass production and repair. The use of woven fabric and short fiber reinforcement is receiving more attention. The design of structures and systems capable of operating at elevated temperatures has spurred intensive research in high temperature composites, such as metal/ceramic, ceramic/matrix, and carbon/carbon composites. The utilization of conventional and new composite materials is intimately related to the development of fabrication methods. The manufacturing process is one of the most important stages in controlling the properties and ensuring the quality of the finished product. The technology of composites, although still developing has reached a state of maturity. Prospects for the future are bright for a variety of reasons. Newer high volume applications, such as in the automotive industry, will expand the use of composites greatly.

About the Author
Dr.Thoguluva Raghavan Vijayaram, currently working as Senior Lecturer in the Faculty of Manufacturing Engineering at UTeM, Universiti Teknikal Malaysia Melaka, Malaysia. He hails from India and he has completed his PhD Research Degree in Mechanical Engineering (Metal Matrix Composites: Materials Engineering) from the Faculty of engineering, Universiti Putra Malaysia. He has published quality research papers in reputed International journals, National journals, International conference proceedings and in the Malaysian broadsheet. He has a wide range of work experience, both in academics and as well as in industry, consultancy and teaching and especially in research and development work. His areas of expertise include: Metallurgical Engineering, Mechanical Engineering and Manufacturing Engineering and his special areas of research interests are in the field of advanced casting technology and techniques, composite materials and processing, powder metallurgy, Ferrous and Non-Ferrous foundry metallurgy, solidification science and technology, solidification processing of metals, alloys and composites, microgravity solidification, squeeze casting, die casting die design, heat treatment, Metallography, microstructure-property correlation ship, new materials and process development, aerospace engineering materials, computer simulation of casting solidification, FEM analysis and advanced engineering mathematics. Besides, he is a prominent writer and possesses wider experience in editing technical papers, theses and dissertations.

Metallurgical Applications of Metal Matrix Composites (MMCs)
The Role of Electrochemical Machining (ECM) in Industrial Metallurgy
Application of Chemical Milling, Chemical Blanking and Photochemical Blanking in Metal Working Industries