Metal matrix composites types

There are three kinds of metal-matrix composites distinguished by type of reinforcement particle-reinforced MMCs, short fiber- or whisker-reinforced MMCs, and continuous fiber- or sheet-reinforced MMCs. Table 1 provides examples of some important reinforcements used in metal-matrix composites as well as their aspect (length/diameter) ratios and diameters. 

Most fiber-matrix composites (FMCs) are named according to the type of matrix involved. Metal-matrix composites (MMCs), ceramic-matrix composites (CMCs), and polymer-matrix composites (PMCs) have completely different structures and completely different applications. Oftentimes the temperatnre at which the composite mnst operate dictates which type of matrix material is to be nsed. The maximum operating temperatures of the three types of FMCs are listed in Table 1.27. 

Metals and ceramics (claylike materials) are also used as matrices in advanced composites. In most cases, metal matrix composites consist of aluminum, magnesium, copper, or titanium alloys of these metals or intermetallic compounds, such as TiAl and NiAl. The reinforcement is usually a ceramic material such as boron carbide (B4C), silicon carbide (SiC), aluminum oxide (A1203), aluminum nitride (AlN), or boron nitride (BN). Metals have also been used as reinforcements in metal matrices. For example, the physical characteristics of some types of steel have been improved by the addition of aluminum fibers. The reinforcement is usually added in the form of particles, whiskers, plates, or fibers. 

Depending upon what type of host matrix material is used in creating the composite material, the composites may be classified into three classes (1) polymer-matrbe composites, (2) metal-matrix composites, and (3) ceramic-matrix composites. We discussed the characteristics of matrix materials earlier when we covered metals and plastics. 

The Saffil fiber which contains 4% of silica is produced by the blow extrusion of partially hydrolyzed solutions of some aluminum salts with a small amount of silica, in which the liquid is extruded through apertures into a high velocity gas stream. The fiber contains mainly small 8-alumina grains of around 50 run but also some a-alumina grains of 100 run. The widest use of the Saffil type fiber in composites is in the form of a mat which can be shaped to the form desired and then infiltrated with molten metal, usually aluminium alloy. It is the most successful fiber reinforcement for metal matrix composite. 

Metal matrix nanocomposites are those having metal as the continuous phase or matrix and other nanoparticles like carbon nanotube as the reinforced materials. These types of composites can be classified as continuous and noncontinuous. One of the more important nanocomposites is Carbon nanotube reinforced metal matrix composite, which is an emerging new material with the high tensile strength and electrical conductivity of carbon nanotube materials. In addition to carbon nanotube metal matrix composites, boron nitride reinforced metal matrix composites and carbon nitride metal matrix composites are the new research areas on metal matrix nanocomposites [9,10]. 

Composite materials can also be broadly classified based simply on the matrix material used. This is often done more for processing than for performance purposes. Thus there are polymer-matrix composites (PMCs), ceramic-matrix composites (CMCs), or metal-matrix composites (MMCs). The last type is an advanced composite uncommon in biomedical applications and is mostly used for high-temperature applications. 

FRCs can be classified based on matrix and fibres. Based on fibre source, FRCs may be natural fibre reinforced and synthetic fibre reinforced. Based on fibre length, they can be continuous fibre reinforced and discontinuous fibre reinforced. But FRCs are generally classified based on matrix component. Thus according to the types of matrices stated earlier, composites are of three types (i) ceramic matrix composites (CMCs), (ii) metal matrix composites (MMCs) and (iii) organic matrix composites (OMCs). Organic matrix is subdivided into two classes, namely polymer matrix and carbon matrix. A short description of all these types of composites are discussed below. 

In composites, the matrix can be either polymeric, ceramic or metallic, hence, polymer matrix composites (PMC), ceramic matrix composites (CMC) or metal matrix composites (MMC). Obviously, the latter two structures are used for high temperature applications (>315 °C), where PMC are usually inadequate. In addition, MMC with proper electrical and thermal conductivities are also used in heat dissipation/electronic transmission applications. In addition to the general types of composites, some specific composites can also be of the type ceramic/metal/polymer or carbon matrix (CMC) or even hybrid composites (HC). 

An alternative basket type has been developed and licensed which is constructed out of Metamic-HT, a metal matrix composite made by embedding nanoparticles of aluminum oxide and fine boron carbide powder on the grain boundaries of aluminum resulting in improved structural strength properties at elevated temperatures. This allows the basket to serve as both a structural component and a neutron absorber. 

The next generation of fibre-reinforced materials were the metal matrix composites (MMC) of ductile alloys reinforced with hard carbon or ceramic fibres and whiskers. The MMC offer serious advantages over other types of composites, mainly in the following aspects ... 

T. W. Clyne, An Introductory Overview of MMC Systems, Types, and Applications, Comprehensive Composite Materials, Vol. 3 Metal Matrix Composites, T. W. Clyne, Vol. Ed., Anthony Kelly and Carl Zweben, Editors-in-Chief, Pergamon Press, Elsevier Science Ltd., Oxford, 2000. 

Metal-Matrix Service temperatures are higher for metal-matrix composites (MMCs) than for Composites polymer-matrix composites. MMCs also use a variety of fiber and whisker types. 

Those basic matrix selection factors are used as bases for comparing the four principal types of matrix materials, namely polymers, metals, carbons, and ceramics, listed in Table 7-1. Obviously, no single matrix material is best for all selection factors. However, if high temperatures and other extreme environmental conditions are not an issue, polymer-matrix materials are the most suitable constituents, and that is why so many current applications involve polymer matrices. In fact, those applications are the easiest and most straightforward for composite materials. Ceramic-matrix or carbon-matrix materials must be used in high-temperature applications or under severe environmental conditions. Metal-matrix materials are generally more suitable than polymers for moderately high-temperature applications or for modest environmental conditions other than elevated temperature. 

The composition of the codeposition bath is defined not only by the concentration and type of electrolyte used for depositing the matrix metal, but also by the particle loading in suspension, the pH, the temperature, and the additives used. A variety of electrolytes have been used for the electrocodeposition process including simple metal sulfate or acidic metal sulfate baths to form a metal matrix of copper, iron, nickel, cobalt, or chromium, or their alloys. Deposition of a nickel matrix has also been conducted using a Watts bath which consists of nickel sulfate, nickel chloride and boric acid, and electrolyte baths based on nickel fluoborate or nickel sulfamate. Although many of the bath chemistries used provide high current efficiency, the effect of hydrogen evolution on electrocodeposition is not discussed in the literature. 

The effect of dispersoids on the mechanical properties of metals has already been described in Section 5.1.2.2. In effect, these materials are composites, since the dispersoids are a second phase relative to the primary, metallic matrix. There are, however, many other types of composite materials, as outlined in Section 1.4, including laminates, random-fiber composites, and oriented fiber composites. Since the chemical nature of the matrix and reinforcement phases, as well as the way in which the two are brought together (e.g., random versus oriented), vary tremendously, we shall deal with specific types of composites separately. We will not attempt to deal with all possible matrix-reinforcement combinations, but rather focus on the most common and industrially important composites from a mechanical design point of view. 

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