Composite Materials – Complete Guide

As you know, composite materials are one of four groups of engineering and manufacturing materials. We think that these materials are the most interesting. Because they have very distinct properties and the technology of composite materials is the most rapidly developing around conventional materials such as metals, ceramics, and polymers. Here, you can find out the general information about these materials, their general characteristics, and the advantages/disadvantages of these materials. 

What is Composite Material?

The most distinct characteristic of composite materials is hidden in their name. The word ‘composite’ is probably derived from the action we call ‘composing’. Yes, these materials are the composition of different materials that have different properties, in different phases. Different properties of these different phases of different materials show superior physical and mechanical properties if we compare them with conventional materials when we combine them as these materials.

At the end of this passage, you will understand that also some of the materials that we explained in conventional material groups are these materials. For example, tungsten carbides can be counted as composite material because it includes cemented carbide phase. Also, some thermoplastic polymers include additives and fillers that we can say as these materials. Fillers inside the thermoplastics are in a different phase from the main matrix polymer material. You know that elastomers that we combine with carbon black are also these materials.

Composite materials have a profound place in engineering and high-tech applications. They have a very important place in the engineering material world. In general, these materials consist two of phases or components inside them. One of these phases is the ‘matrix’ that gives the general shape and bulk of composite material. Matrix materials in composites can be metals, ceramics, or polymers. The other component in these materials that we call as ‘secondary phase’ or ‘reinforcement phase’ that we add to the matrix to give reinforcement.

Types of Composites

Composite materials are classified according to their matrix compound. But their reinforcing phase can be metals, polymers, or ceramics again. But we define them according to their matrix compound;

• Ceramic Matrix Composites(CMC): This category of them is not common in them. We do not use ceramics commonly in them as matrix materials. Silicon carbide and aluminum oxide are the most important examples of this group. Concrete is another important example of this group that we make reinforcement generally with steel rebars.
• Metal Matrix Composites(MMC): In this group of them, we use ceramics generally as a reinforcing phase in metals. Cemented carbides are the most important example in this category.
• Polymer Matrix Composites(PMC): In aerospace, we use this kind of composite generally. We use Thermoset resins widely as a matrix in this category. And we use fiber reinforcements generally as a secondary phase. Epoxies and polyesters are the most known examples of this group.

The Embedded or reinforcement phase can be hidden inside the matrix phase in these materials. But they have a very important duty. The matrix phase generally transmits the external loading on that reinforcement phase. So these materials can withstand types of loadings that the matrix can not withstand by itself.

Ceramic Matrix Composites

Using ceramic materials as the matrix in these materials is a very tough business. We use various kinds of materials in the reinforcement phase which are generally fiber-reinforcement. We use general materials such as matrix materials, titanium carbide, boron carbide, silicon nitride, boron nitride, and alumina.

Production of ceramic matrix composites is very hard, but the production of metal matrix composites with short fibers is possible. We treat the short fibers as powders to apply them to these ceramics. Long fibers are much better in terms of mechanical properties, but production of them is not easy and very costly.

Ceramics have very straightforward advantages in engineering such as high stiffness, hardness, hot hardness, low density, and compressive strength. But ceramics has also drawbacks which are limiting the use of ceramics in engineering; thermal cracking, low toughness, and bulk tensile strength. 

With the applications of ceramic matrix composite materials, we sustain the advantageous properties of ceramics. We try to eliminate the drawbacks of ceramic materials with the application of the reinforcing phase.

The most important application of ceramic matrix composites is the cutting tool production which they are competitive with cemented carbide cutting tools in machining.

Polymer Matrix Composites

Polymer matrix composites which we abbreviate PMCs are a very important type of composite materials. There are various kinds of composite parts that we produce with polymer matrix composites in today’s technology. Here, you can find out the general expression of the polymer matrix composites. 

As you know that these materials constitute two or more different phases or materials in the same place. In general, the first phase we call a matrix that hosts reinforcing material. The reinforcing phase or material is the second phase that gives additional physical or mechanical properties to the matrix.

Polymer Matrices in PMCs

In PMCs, we use polymer materials as matrices. The most used polymer matrices in PMCs are thermosetting materials. Phenolics and epoxies are the most used types of matrix materials in PMCs. We use Thermoplastic materials also in PMCs. But they are not common matrix materials in PMCs. We use them mostly as reinforcement. 

In general, a wide variety of elastomer products that we can say as these materials, because of the addition of carbon black.

Types of Reinforcing Agents in Polymer Matrix Composites

Reinforcing agents can be in various kinds of shapes. And also we can make them from various kinds of materials such as ceramics, metals, or polymers. In terms of shapes, you can find reinforcing agents in polymer matrix composites as; fibers and flakes, and particles. For various purposes in various applications, we use different shapes of reinforcing agents with different materials. 

Fibers as Reinforcing Agents

Fiber structures are the most common type of reinforcing agent in polymer matrix composites. There are various kinds of materials are available as fiber; carbon-based materials, glass fibers, and polymer-fibers. 

Fibers can be also in different shapes; 

• Yarn: In the yarn shape of reinforcing agents, continuous they twist the fibers in form but they are not woving them. 
• Rovings: Unlike yarns, rovings are in the form of parallel orientation. There is no twisted form between continuous fibers or filaments. 
• Cloths: Cloths are the woven structure of yarns from continuous filaments. 
• Mat: This shape of reinforcing agents constitutes the randomly oriented short-sized filaments. 

Flakes And Particles As Reinforcing Agents

Flakes are thinner types of particles that we use as reinforcements in polymer matrix composites. The use and production of flakes and particles as reinforcement is generally easier than the other types of reinforcement. Also, there are lots of kinds of materials that they use as flakes and particles in the reinforcement of PMCs. 

Fiber-Reinforced Composites

Fiber-reinforced composite materials have a very important place in engineering. One important example of these fiber-reinforced composites is fiber-reinforced metal matrix composites. Here, we briefly explain these fiber-reinforced composite materials and their applications and properties below.

In fiber-reinforced metal matrix composites, low-density metals such as use aluminum, titanium, and magnesium as metal matrices. Also, in these metal matrices, fiber reinforcements are aluminum oxide and silicon carbide.

As in another fiber-reinforced material, we obtain maximum tensile strength in the direction of fiber reinforcements. Also, we obtain the maximum modulus of elasticity in this direction. According to the application, we adjust the alignment of fiber reinforcements. When we increase the fibers inside the metal matrix, we increase these mechanical properties also.

We obtain a very good strength-to-weight ratio with fiber-reinforced metal matrix composites. Also, these materials are very good thermal and electrical conductors.

General applications of fiber-reinforced metal matrix ones are high-temperature turbine parts and aircraft components.

Fiber structures are very important reinforcement phase structures in composite materials. Here, we will explain the general properties of fiber reinforcing agents in them.

What is Fiber Reinforcement Used in Composites

We produce fibers in these materials generally in very thin filament form. In general applications of these materials, we produce fibers in diameters ranging from 0.0025mm to 0.13mm. The mechanical strength enhancement of composite materials is very straightforward. Especially in tensile reinforcement in them, fibers are very vital.

In general mechanics of materials, the filament form of a material is much stronger than its bulk form. And also with the reduced diameter of filaments, the tensile strength of the material increases. Because of this fact, filament shapes of materials which we call ‘fibers’ that used in the reinforcement of them.

Continuous and discontinuous fibers are also available in composite applications. In theory, continuous fibers are possible, but the production of them is very hard. So, discontinuous fibers are also an application of fibers in them. For discontinuous fibers, the L/D ratio can be around 100. ‘Whiskers’ is an example of discontinuous fibers which has a hair-like structure and a very low diameter which is around 0.001 mm. This means that they are superior in applications where tensile stress is prominent.

In general applications, the diameter of used fibers starts from 0.0025mm to 0.13mm.

Reinforcement Application Types of Fibers

Three types of reinforcement applications for fiber reinforcements in composites. The first one is ‘one-dimensional reinforcement’ that fibers orient in tensile stress application direction. So reinforcement for this condition is provided by fiber reinforcement. Matrix of composite material transmits the stress on fiber reinforcement to overcome this tensile stress.

The second type of reinforcement is ‘planar reinforcement’ in which we accomplish the reinforcement with fibers in a two-dimensional plane. Consider the two-dimensional tensile stress application on these them. Planar fiber reinforcement probably overcomes these stresses.

The third one is ‘three-dimensional reinforcement’ where we accomplish fiber design in these kinds of composites to overcome three-dimensional tensile stress applications.

Materials as Fiber Reinforcements?

They use a wide variety of materials as fiber reinforcements in these materials. But material type also can change according to the matrix material that we use in composite material. We use these materials generally in fiber reinforcements;

• Metal Fibers: Some metals are very good in tensile strength. So we use these metals as both continuous and discontinuous fibers in them.
• Boron Fibers: Boron has a very good modulus of elasticity value, but is high in cost. Composite materials that we produce with boron fiber reinforcements have a very good strength/weight ratio. But, we use them in aircraft and aerospace applications because of their high cost.
• Carbon Fibers: Along with their very good stiffness, low thermal expansion and low-density properties are very attractive for carbon fiber reinforced them. We use Graphite and amorphous carbon generally for carbon fiber reinforcements.
• Ceramic Fibers: Low density and modulus metals reinforce with ceramic fibers. We use Silicon Carbide(Sic) and aluminum oxide(Al2O3) types of ceramics as fiber reinforcements.
• Glass Fibers(Fiberglass): Glass is the most used material as fiber reinforcement in these materials. We use E-glass and S-glass in fiber reinforcement applications. S-Glass is an expensive glass fiber reinforcement, but its tensile strength and modulus elasticity are best around other fiber materials. E-Glass is strong and low cost but its mechanical properties are lower than other fiber materials.

• Kevlar 49: It is a special polymer fiber reinforcement material. The density of Kevlar 49 is low and the best strength-to-weight ratio composite that we produce with this fiber material.

Fiber reinforcement is very important in design. It has a very specific technique and you can develop yourself in this area.

Particle and Flake Reinforcements in Composites

We make reinforcement in composite materials in various ways. One of these ways is the reinforce these materials. Here, we will take a look at the general characteristics of particle and flake-reinforced materials.

The strengthening mechanism changes with the used particle size in them. Sizes of particles change on both microscopic and macroscopic basis. With the increasing size of particles inside the composite, the mechanism of reinforcement for matrix material changes.

If they use the small particle sizes in particle-reinforced composite materials, the main strengthening mechanism is not load-carrying by the reinforcement phase. The main mechanism is that the microscopic particles act as prohibitions of dislocation motions in the matrix phase of composite materials. These dislocations lead to the failure of the material, if we prevent it by an action, the material can get strong. In common, the proportions of microscopic particles inside matrix materials are around 15% or low.

Also…

But if the particle size increased to macroscopic sizes, the proportion of the particle reinforcement phase also increases. It increases up to 25% or more with the increased size of reinforcement particles. Also, the mechanism of reinforcement changes into that load carrying by reinforcement particles in which load is transmitted by matrix phase, just like in fiber-reinforced composites. This is the main difference between the small particle reinforcement phase and the big particle reinforcement phase in them.

Flakes are also a very important type of reinforcement in composite materials. The difference between flakes in terms of shape, compared with particles, flakes are in the form of two-dimensional platelets. The sizes of platelets change from 0.01 to 1 millimeter and the thicknesses of these flakes changes between 0.001 to 0.005 millimeters.

The materials of particle and flake reinforcements can be metals and ceramics. The distribution of reinforcement particles inside the matrix phase is generally random, but obtained mechanical and physical properties are generally isotropic.

Sizes of small particles or microscopic particles can be around 1 micrometer.

Application Examples of Particle and Flake Reinforcement in Composites

The most important and known application of particle-reinforced composites is the Tungsten Carbide(WC). This is a type of cemented carbide that we hold tungsten carbide in a cobalt binder. Here, cobalt binder is the matrix phase and tungsten carbide particles are particle reinforcements.

An important application of flake reinforcement in them; we use flake-shaped mica and talc in various polymers as reinforcement. We use polymers as matrix materials. This application of flakes, adds stiffness and strength to plastic materials and plastic molding compounds.

These are the general characteristics of particle and flake reinforcement in these materials.

Honeycomb, Foam, and Laminar Composite Structures

These structures of composite materials can not fit into conventional particle-reinforced and fiber-reinforced composite material groups. Honeycomb, foam, and laminar structures of composites are very different in structure, and here, we explain these honeycomb, foam, and laminar composite structures below.

Laminar Composite Structures

As you understand from its name, laminar composite structures are laminated structures of different materials or the same materials to obtain much better mechanical properties. In conventional ones, there are primary and secondary phases. But in these kinds of these materials, there are no exact primary or secondary phases. We mechanically bring different materials together as laminae to obtain the required mechanical characteristics.

The laminated structure is visible by the naked eye, unlike other matrix-reinforcement composites.

Honeycomb and foam structures are the sub-groups of laminar structure composites. In foam structure, we produce the core thick layer from thermoset foams and we add other layers on both sides of the core foam layer to obtain a stiff and strong material. With this technique, we produce very good materials in terms of strength/weight ratio.

The logic of honeycomb structure is like foam structure composites in which the core of the material that we produce from honeycomb-shaped materials. Layers have added both sides of honeycomb structures to obtain a sandwich shape, in which the strength-to-weight ratio is very important.

We call these laminar structures of these materials ‘sandwich structures’ also.

Laminar Composite Examples

• Snow Skis: They produce snow skis in the laminar structure that includes metal layers and phenolic plastic layers.
• Windshield Glass: They add Glass layers to both sides of the transparent plastic sheet.
• Fiber-reinforced Polymers: These layered fiber-reinforced materials find various applications such as boat hulls and aircraft applications.
• Plywood Applications: Sheet structures that they produce from wood parts that bond together to obtain a stiff structure.
• Printed Circuit Boards: For electrical conductivity and insulation properties, layers of polymers and coppers produce and bond together.
• Tires: we combine rubber-reinforced carbon black and rubber-impregnated fabrics as a laminar structure to obtain automotive tires.

Infiltrated Phase in Composite Materials

The common reinforcement of infiltrated composite type is the filler which we use generally in polymers. The matrix phase is a sponge-like structure when we add the filler inside it. The shape of the sponge-like structure is because of the filler material that we use as reinforcement.

The materials of these fillers are generally metals. And these filler forms of metals that we produce generally with powder metallurgy techniques. When we add these metal powders inside polymer matrix materials, we produce one of them.

There are lots of kinds of applications of this kind of them; such as bearings that we use in machinery, we use gear products in dynamic applications.

Oil-impregnated sintered PM is one of the most known examples of infiltrated matrix phase composites.

Interphase Structures between Phases of Composites

Interphase must occur and generally occur between the phases of composite materials to obtain a solid material. This interphase is a very important thing to form, because of obtaining one-piece material parts. Here, we explain the general interphase mechanisms that occur in them, between matrix and reinforcement phases.

The primary phase in these materials that we call the matrix phase and the second phase we call the reinforcement phase. The interphase is the bonding between these different phases inside these materials. There are three bonding mechanisms for these interface structures in composite materials;

• The first type of interphase in these materials is what we call direct bonding. There are no additives to obtain bonding between matrix and reinforcement phases, or there is no tertiary phase between primary and secondary phases to obtain bonding. In this case, there is a direct bonding between primary and secondary phases in these materials. There is no straightforward interphase between primary and secondary phases or indirect bonding.
• In some composite applications, we add some kinds of agents to obtain bonding between matrix and reinforcement. We can consider them as an adhesive between the primary and secondary phases of these materials.
• The third mechanism of interphase forming between matrix and reinforcement is the formation of a solution between them. They must not be soluble in each other completely. They must solve in a percentage to obtain this bonding interface between primary and secondary phases in these materials.

Application Examples of These Interphase Structures in Composites

For example in fiber-glass reinforced thermosets, we use glass fibers as the reinforcing phase. We use thermosets as the matrix phase. We obtain the interphase between these phases with a coating of glass fibers before the production of fiber-glass reinforced thermosets. This is a very good example of the second type of interphase formation between primary and secondary phases in composite materials.

Another example of interphase that occurs as the solution of two phases is cemented carbides. In high sintering temperatures, the solution of interphases occurs between particle reinforcement phases and matrix phases. These phases are not soluble completely in each other.

Curing of Produced Composite Parts; Prepregs, FRP Laminates

When we produce them from the thermosetting matrix, there is a time interval required for curing. Curing is the hardening and stiffening of a composite matrix with the occurrence of cross-linking between molecules. 

These cross-linking can occur at room temperature but also we apply additional heat. If we apply additional heat, the curing process will take less time if we compare it to room temperature. 

Also, if we think about the very big composite products that they produce in hand lay-up, spray-up processes, heat application to these parts can be a tough process. 

If the composite parts are middle-sized if we compare them with big parts such as boat hulls, we can apply heat accurately in special ovens. This heat application can shorten the required times from days to hours, even minutes. 

Also, they apply the infrared light application to produce parts to obtain a cured product. 

Autoclave Molding 

To shorten the required curing times of composite parts, they design and develop special autoclaves. Autoclaves are very big-size cylindrical containers that contain high pressure and temperature. These big cylinders have doors on both sides. 

They place big parts inside these autoclaves for curing applications. 

We use autoclaves generally in the aircraft industry. 

Transfer Molding Of Polymer Matrix Composites

They use Transfer molding originally for the molding of thermosetting polymers. In principle, we prepare thermosetting polymer charge in a container and transferred it to closed molds by hydraulic rams. Inside these closed molds, the curing of thermosetting takes place. 

To produce polymer matrix composites, we make some adjustments in the conventional transfer molding process, according to the PMC products. Here, we explain several of these types of transfer molding processes to produce polymer matrix composites.

Different Types of PMC Transfer Molding Methods 

As you understand from its name, polymer matrix composites are produced with thermosetting matrix materials. So, transfer molding operations can be adapted to the production of polymer matrix composites. But the management of the reinforcing agent is very important. 

In one of the techniques, perform mats are placed in the lower half of transfer molding molds. And like in conventional methods, the resin is transferred after the closure of mold halves. High-quality boat hulls, bathtubs swimming pools and products like them can be produced with this method. 

In some applications, high-density thermosetting matrices are used. And continuous fiber-reinforcements are applied instead of preform mats. This kind of production application is common in aerospace. 

Another application is that two layers and laid on foam reinforcements and placed into the transfer molding mold. And resin is transferred inside the mold. When the curing is established, rigid sandwich structures are produced which are lightweight. 

Automated Tape Laying Machines in Composite Part Production

Tape lying techniques are generally used in the aviation industry to accelerate the laying processes of FRP laminates to produce parts. Because hand lay-up and spray-up can be very slow for serial production manner. And the requirement of the labor force and mastery is a problem for very big industries. 

So, fully-automated tape lying machines are developed to satisfy these requirements of big companies. Here, we summarize the working principle of automated tape-laying machines. 

How Do Automated Tape Laying Machines Work?

As you understand from its name, automated laying machines are programmable machines that end effector of the machine head is a special laying gun. In this laying gun, FRP is formed as the tape is fed from feedstock and wrapped inside the machine head. And a special knife on the end effector cuts the tape in the required places. 

The path of the robotic machine is programmed with computers. This programming is very tough because of the very accurate programming required to obtain high-quality and uniform products. 

The main advantage of these automated tape laying machines is the no or very less requirement of the workforce, which will save human beings from the hazardous volatile environment. 

What are the Advantages of Composite Materials?

• In general, mechanical and physical properties that are not attainable with metals, ceramics, and polymers, can be attained with the composition of different materials.
• The toughness and fatigue properties of these materials are superior to conventional materials.
• Generally, these materials are stiff and strong and light in weight compared with other engineering materials. This property of composites makes them the best material group in terms of strength-to-weight ratio.
• Corrosion characteristics of composite materials are much better than other materials.
• Better surface characteristics and smoothness properties of these materials are achievable compared with other materials such as metals.

What are the Disadvantages of Composite Materials?

After reading of advantages of these materials above, you can probably think that ‘We must use composites in every application!’. But the situation is not like that… Take a look at the disadvantages of composites.

• Polymer-based composites are prone to chemical attacks which are not desirable in such applications.
• Shaping processes for these materials are costly, so this makes composite materials expensive for such applications.
• Also, the production of composites is expensive generally. We need high technologyhigh-technology production techniques to use to produce composites.
• This can be the greatest drawback of these materials that, the properties that are obtained by composing different materials in different phases, differ according to the direction that they are measured. For example, in each direction of loadings, these materials do not show the same strength characteristics.

It can be interesting information about composites that the most known natural composite material is wood. If you think about the structure of wood, it has a specially designed inner structure to show different properties to maintain a tree’s life.

Conclusion

As you see above, the topic of these materials is a very common area that different kinds of sub-topics are available. It is not an easy area. But this guide will be a very good starting point for you.

In general, we are using matrix and reinforcement phases in composite materials. So, understanding the general mechanisms is very important for it.

Finally, do not forget to leave your comments and questions below about the composite materials below. Your precious feedbacks are very important to us.

Reference: Elsevier.

FAQs About Composite Materials