Tailoring Composite Materials

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Tailoring Composite Materi


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Introduction

One of the advantage composite materials is the abimaterial is the strength can be directed in a certain directiowant. This is called “Tailoring Properties” and this is oproperties of composites as compared to other conventional

Beside strong, rigid, and lightweight composites also corrosion resistance and has resilience also to dynamic loads.

Bellow are several types of processes “ Tailoring Materials ”.


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1 Tailoring by Component Selection

1.1 Polymer-Matrix CompositesEpoxy is by far the most widely used polymer

structural composites. This is due to the strong adhesivenessin addition to the long history of its use in composites. Epoxycharacterized by having two or more epoxide groups permole

An epoxy is a thermosetting polymer that cures upon mwith a catalyst (also known as a hardener). This curing procesreaction that involves polymerization and crosslinking.


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1.2 Cement-Matrix Composites

Component selection for cement-matrcomposites involves the useof admixtures, are additives included in the cement mix. Tadditives serve various functions, as describ

below:


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• Water reducing agent – a minor additive to increase the workamix.

• Polymer (Such as latex) – to decrease liquid permeabilitystrength.

• Fine particles (such as silica fume) – to decrease liquid permeastrength and drying shrinkage, and to increase the moduluabrasion resistance.

• Short fiber (such as steel fiber) – to increase the flexural toughne

Continuous fibers are not suitable for inclusion in a cealthough they can be applied prior to cement pouring and canreinforcement. Both processing and material costs are high, Ipenetration of the cement mix into the small spaces between mdifficult. On the other hand, macroscopic steel rebars are similar continuous microfibers and are commonly used to reinforce concr


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1.3 Metal-Matrix Composites

Metal-Matrix Composite is composite material with atconstituent parts, one being a metal necessarily, the othe

may be a different metal or another material, such as ceramor organic compound.

Metal-Matrix Composites usually consist of a low-densuch as aluminum or magnesium, reinforced with particulatof a ceramic material, such as silicon carbide or graphite.

with unreinforced metals, MMCs offer higher specific strstiffness, higher operating temperature, and greater wear reswell as the opportunity to tailor these properties for a application.


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Metals and ceramics tendto have very different in theirproperties, as shown in Table 6.2.Metals are electrically andthermally conductive. The thermalconductivities of aluminum andcopper (Table 6.2) are higher thanthose of any of the ceramics listed,while the electrical resistivities of aluminum and copper aremuchlower (by many orders of

magnitude) than those of any of the ceramics listed. However, mostmetals exhibit a high coefficient of thermal expansion (CTE) and a lowelastic modulus compared toceramics.


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Due to its low cost and high elastic modulus, silicon carbide is thcommonly used to reinforce metals. SiC is also used as an abrassandpaper). There are numerous polymorphs of SiC, but the mopolymorph is α-SiC,which has a hexagonal crystal structure (similar t

less common polymorph is β-SiC, which exhibits the zinc blende crystal Silicon carbide is available in particle and whisker forms. A whisk

fiber that can be essentially a single crystal.


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These picture explainSEMphotographs of mechanically polishedsections of aluminum-matrix composites

containing 10 vol% siliconcarbide whiskers. aHigh-magnification view, b low-magnificationview. The whiskers are essentially randomlyoriented; the whisker diameter is 1.4 μmandthe whisker length is 18.6 μm.


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Compared to silicon carbide,titaniumdiboride has a higher modulus but alower thermal conductivity (Table 6.2). The highmodulus makes titanium diboride a highlyeffective reinforcing material. The addition of TiB2 to a metal greatly increases the stiffness,hardness and wear resistance and decreases theCTE, while it reduces the electrical and thermalconductivity much less than the addition of mostother ceramic fillers.

These picture explain Optical microscopephotographs of a copper-matrix compositecontaining: a 15 vol% TiB2 platelets; b 60 vol%TiB2 platelets.


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2 Tailoring by Interface Modification

2.1 Interface Bond Modification

a. General Concepts

Effective reinforcement requires good bonding between the filler anespecially for short fibers. For an ideally unidirectional composite (i.e., oncontinuous fibers all aligned in the same direction) containing fibers with a mmuch higher than that of the matrix, the longitudinal tensile strength is quiteof the fiber –matrix bonding, but the transverse tensile strength and the flex

(for bending in the longitudinal or transverse directions) increase with incrmatrix bonding.On the other hand, excessive fiber –matrix bonding can causewith a brittle matrix (e.g., carbon and ceramics) to become more brittle, afiber –matrix bonding causes cracks to propagate linearly in the direction perthe fiber –matrix interface without being deflected to propagate along this intcase of a composite with a ductile matrix (e.g., metals and polymers), a cracthe brittle fiber tends to be blunted when it reaches the ductile matrix, ev

fiber –matrix bonding is strong.


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b. Filler Surface Treatment

The surface treatment of a filler is usual

valuable method of improving the bonding between the filler thematrix. If the filler is carbon fiber, surface treatments involve oxidatreatments and the use of coupling agents, wetting agents, and/or siz(coatings). Carbon fibers need treatment for both thermosetting thermoplastic polymers. As the processing temperature is usually highethermoplastic polymers than for thermosetting polymers, the treatmmust be stable to a higher temperature (300 –400◦C) when a thermopla

polymer is used.


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c. Use of an Organic Coupling Agent

1. Organic Coupling of Inorganic Components

The use of an organic molecule (known as a coupling agent; also known

tether) to covalently link an inorganic particle and an inorganic matrix enhancbetween the inorganic particle and the matrix, thus resulting in a nano composstrength. The chosen molecule must have functional groups at its two ends treact with the surface of the inorganic particle and the matrix material. Silane are most commonly used as the organic molecules. An example is a nanoccement as the matrix and silica fume as the inorganic particles.

2. Organic Coupling of an Inorganic filler and a Polymer Matrix

An organic coupling agent can be used to improve the bond between an inoa polymer matrix. For example, a silane coupling agent is effective at enhabetween boron nitride particles and an epoxy matrix. Boron nitride (BN) is a cewith some ionic character in the covalent bond between boron and nitrogen.


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2.2 Interface Composition Modificationa. Filler –Matrix Interface Composition Modification

Ceramic –Metal and Carbon –Metal Interface Composition Modification

Due to the low CTEs of ceramics compared to metals, ceramic fillers

used in metal-matrix composites in order to achieve a composite with a low liquid metals tend not to wet ceramic surfaces well (i.e., a liquid metal tends ceramic surface), thus making the liquid metal infiltration process difficult. Tproblem, the ceramic surface may be modified by applying a coating that the liwet sufficiently well.

Reinforcement-Cement Interface Composition Modification

The bond strength can be enhanced by adding polymeric admixtures suthe formof an aqueous particulate dispersion) and methylcellulose (in the form osolution, since methylcellulose is water soluble) to the cement or concrete mix. T

only resides in the cement matrix but it also lines the fiber cement interface, therthe fiber –matrix bond. The bond strength can also be enhanced by adding a fine the mix, such as silica fume, which is around 0.1μm in particle size. The fine partipore size in the cementmatrix and at the reinforcement –cement interface. The preduction at the interface results in the strengthening of the interface.


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b. Interlaminar Interface Composition Modification

For continuous fiber polymer-matrix composites, the interface between adjaof fibers is a polymer-rich region. It is the weak link in the composite, as shown by thedelamination is a common type of damage in these composites. The composition of t

can be modified by filling this region with a nanofiller, as illustrated in Fig. 6.13.

Figure6.13. Thermal conductivity of cross ply

carbon fiber polymer matrix composites with

and without various interlaminar interface

modifications. WO – without carbon black and

without vehicle. VE – with vehicle and without

carbon black. The vehicle is ethylene glycol

monoethyl ether. The carbon black wt.% refers

to the concentration of carbon black in the

dispersion involving the vehicle. The dispersion is

applied onto the fiber epoxy prepreg surface

prior to consolidation of the laminae to form a

composite.


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2.3 Interface Microstructure Modification

Chemical methods tend to provide microscopic roughness (lowwith a high density of hillocks in the resulting topography), whereasmethods tend to provide macroscopic roughness (high roughness,

density of hillocks in the resulting topography). Both a high roughnesdensity of hillocks promote mechanical interlocking. Thus, both typeshave their advantages and disadvantages. Chemical methods are usuamicroscopic reinforcements (e.g., carbon fiber), whereas mechanical musually used for macroscopic reinforcements (e.g., steel rebars).


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3 Tailoring by Surface Modification

The coating of carbon materials to protect them fromis used in this section to illustrate methods of surface comodification. In the absence of oxygen, carbon materials havhigh-temperature resistance. For example, the carbcomposites used for the nose cap of a Space Shuttle can 1,600°C, whereas more advanced carbon –carbon compo

withstand 2,200°C. In contrast, superalloys can only withstanand also have high densities.


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4 Tailoring by Microstructure Control

a. Crystallinity Control

There are two kinds of crystallinity control, first is crystallinity copolymer-matrix composites and crystallinity control for carbon-matrix c

b. Porosity Control

Porosity or void fraction is a measure of the void (i.e. "empty") smaterial, and is a fraction of the volume of voids over the total volume,

and 1, or as a percentage between 0 and 100%.

The porosity canbe reduced by tailoring the process and compossilicon carbide whisker copper-matrix composites, the use of the coatedmethod of powder metallurgy in place of the admixture method of powmetallurgy for composite fabrication reduces the porosity at a given fillefraction, as shown in Fig. 1.10.


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• Figure 1.10. Variation in theporosity, hardness and compressiveyield strength of silicon carbidewhisker copper-matrix compositesfabricated by powder metallurgy.Circles, coated filler method.Triangles, admixture method.


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5 Tailoring Organic Inorganic Nanosc

Hybridization

Nanocomposites with Organic Solid Nanoparticles DispeInorganic Matrix.

Nanocomposites with dispersed organic nanoparticinorganic matrix can be attractive for toughening andreduction, since the organic component is relatively tougnanosize may allow it to fill the pores that tend to be pre

inorganic material.


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Nanocomposites with an Organic Component DisperseInorganicMatrixWhere the Organic Component is Added as a Liquid

Instead of using an organic component in the form of particles, onorganic component in the form of a liquid solution obtained by dissolvincomponent in a solvent.

Methylcellulose is a water-soluble polymer and water is needed for anyway. Thus, the water used to dissolve the methylcellulose can be counted of the water in the cement mix. Table 6.9 shown us that Methylcellulose can mthe tensile strength, tensile modulus , and tensile ductility.


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Nanocomposites Made by Inorganic Component Exfoliation and Organic Component Adsorption

In general, adsorption is a process that occurs when a ga

accumulates on the surface of a solid (adsorbent), forming a film of matoms (the adsorbate).

The overall process is known as exfoliation –adsorption. Wheasmontmorillonite) is used as the inorganic component, the platelets ananoclay, and each nanoplatelet tends to consist of a silicate monmodified clay is known as organoclay, which is then immersed in a liquid

the organic component (or its resin) that serves as the matrix for tcomposite


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TERIMA KASIH

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Tailoring Composite Materials