Composite Materials

Properties of Composite Materials

The properties of a composite material depend on its constituent materials. The main characteristic of composite materials, though, is that they are materials which are a combination of two or more different materials.

In a composite material, the constituent materials can play a role from either of these two:

1. Reinforcement material
2. Binder material or matrix

The individual materials are chosen to obtain something which can be used as a single material, and will have the combined properties of its constituent materials. But the composite materials which have been applied and accepted as standard in the industry, tend to have these common properties:

• High strength
• High resistance to fatigue (crack formation)
• Resistance to corrosion
• High strength-to-weight ratio - provides better performance per weight
• Flexible - the constituent materials can be tweaked to suit the needs.

Types of Composite Materials

Composite materials are made to obtain a material which can exhibit superior properties to the original materials. These combinations can be done however required so that the composite material fulfils the requirements. Based on the type of reinforcing material used, though, composite materials can be classified under 4 categories:

Based on these categories, there are many types of composite materials, some of which are listed below -

These are only some of the types of composite materials that are possible. As discussed earlier, materials can be combined in any assortment, to produce a composite material with any property. For example, plastic-coated paper is also a composite. It has more durability than normal paper, thanks to the plastic lamination. You can see this in playing cards. Papier-mâché is also a composite material! The paper acts as the reinforcing material that is spread through a matrix of an adhesive.

Mask made with Papier-Mâché | Photo by andreusK on Unsplash

Composite Materials: Examples

We've already mentioned some examples of composite materials, but we'll have a more in-depth look at a specific class of them: ceramics.

Ceramics as Composite Materials

Ceramics can be considered as composite materials when they consist of a ceramic matrix reinforced with a filler material. The matrix is typically made of a ceramic material such as silicon carbide or alumina, while the filler material can be made of a variety of substances such as metal or polymer fibers.

The filler material is added to enhance the mechanical properties of the ceramic, such as toughness, strength, and wear resistance. The resulting composite material has superior properties compared to the pure ceramic material. Ceramics as composite materials have a wide range of applications, including aerospace, automotive, and biomedical engineering.

Clay Ceramics

Clay is a mineral that is found in the earth. When clay is wet, it can be morphed into any shape. When the clay is heated in a furnace, it hardens. Bricks are an example of clay ceramics. You also might have seen earthen pots and utensils made of clay ceramics.

Glass as Ceramic Composite Materials

Glass finds a wide variety of uses in our daily lives. Did you know that the key component of glass is sand? The chemical composition of glass is silicon dioxide, which is what sand is made of. Sand is mixed with other materials and heated to a very high temperature until it melts. When it cools, the molecules arrange themselves in a crystalline structure that forms glass. Based on the materials that are mixed with sand, glass is of two types:

• Soda-lime glass
• Borosilicate glass

Soda-Lime Glass

Most of the glass that you come across is soda lime glass. It is used to make drinking glasses, glass bottles, glass windows and other common things of glass. Soda-lime is made by mixing sand with sodium carbonate and limestone before melting. When it is cooling down, it can be shaped as required. Soda-lime glass is not good for heating as it has a relatively low melting point. That's where borosilicate glass is useful.

Borosilicate Glass

Borosilicate glass has a higher melting point than soda-lime glass, and is, therefore, useful for heating. Borosilicate glass is made by mixing sand with boron trioxide (B₂O₃) and then melting. The test tubes, beakers and other chemistry laboratory equipment you will use in the chemistry lab are made from borosilicate glass. Borosilicate glass is also used to make some kitchenware.

Reinforced Concrete as a Composite Material

Earlier in this article, you read that in a composite material, the reinforcement material can be a fibrous material or fragments of a material, while the binder material surrounds the reinforcement material and holds it in place. The example of reinforced concrete would be an ideal example to better understand this.

Reinforced concrete is used in the construction of buildings, bridges and other such megastructures. Next time you see an ongoing construction of a building, notice the long metal rods coming out of blocks of cement.

Steel Reinforced Concrete | Technology Student

These rods are steel rods, which are reinforcing the concrete, and combined they are making a composite material. Reinforced concrete is used because it has properties which are not found in cement alone or steel alone:

• Concrete has good performance against compressive stresses, but not against tensile stresses
• Steel performs well under tensile stresses and poorly under compressive stresses.
• Reinforced concrete performs well under both kinds of loads.

Among fibre-reinforced polymer composites, Carbon fibre is rapidly gaining popularity as it shows incredible strength while being extremely lightweight. Its usage has trickled down from spacecrafts to automotive industry to sports equipment to safety products like bike helmets.

A Piece of Woven Carbon Fibres | Hiconsumption.com

A Propeller Made from Carbon Fibre Composite Hiconsumption.com

In carbon fibre composites, the reinforcing material is pieces of woven fibres of carbon which are 5-10 micrometres thick (10^-6 m). The binder material is epoxy resin. Epoxy resin is a viscous liquid which hardens when exposed to ultraviolet (UV) light.

Advantages and Disadvantages of Composite Materials

Despite their strong advantages (especially their improved properties and adaptability), composite materials also have disadvantages that in some cases can overpower their advantages.

Materials other than Composites: Polymers and Metals

To further highlight the properties and advantages of composite materials, let us look at some other materials such as polymers and metals. This will help emphasise the difference between composites and other materials and the importance of using composites instead of these materials.

Polymers

Plastics are the most common type of polymers in use. Polyethylene (more commonly known as polythene) is widely used in the making of carry bags (poly-bags). Polyethylene is made up of repeating units of C₂H₄ - the chemical formula of polyethylene is (C₂H₄)_n.

Polyethylene sheet being used as a wrapping for food | Photo by Wicki58 onUnsplash

Polymers can be naturally occurring in nature, or be synthetic. Cellulose, DNA, wool, and silk are examples of naturally occurring polymers. Synthetic polymers include Teflon, nylon, polyester, and epoxy.

Teflon is used as a non-stick coating on utensils. Nylon is a thermoplastic made from petroleum, usually manufactured as a fibre, and used to make clothes, seatbelts, airbags etc.

Epoxy resins are used as the binder material in fibre-reinforced composite materials such as carbon fibre and fibreglass. Properties such as:

• Adhesion to a wide range of materials,
• Ability to easily change physical properties with the process of curing,
• low contraction during curing/solidification,
• No volatile substances affect the material during curing.

make epoxy resins ideal candidate materials to be used as binder materials in fibre-reinforced composite materials.

Polymers are a broad class of materials in which materials with widely varying properties exist. Out of all polymers, epoxy resin is a type of polymer material which is used in the making of fibre-reinforced composite materials.

Metals

Metals are elementary substances. They occur in ores in the earth and need to be extracted before using in any other application. Metals are crystalline when solid. They are all good conductors of electricity and heat, are lustrous (shiny), and highly malleable. Metals are used as materials in their pure form (iron, copper, gold, silver) or by making alloys (steel, brass, bronze, white gold).

Metals are used in composite materials as either a reinforcing material or a binder material.

Syntactic foams and metal matrix composites use metals as the binder material, or the matrix. Some syntactic foams have a metal matrix littered with micro balloons of glass or plastic. Other syntactic foams use metal spheres as the reinforcing material in a matrix of epoxy or polymer.

Syntactic Foam | Alchetron

Metal matrix composites (MMC) is a class of composite materials which uses metals as the matrix through which fibres or particles of a reinforcing material will be spread out. Aluminium-based composites are most widely used in the automotive and aerospace sector, and microelectronics. The most common aluminium-based composite are aluminium composite panels (ACP).

ACP sheets are composed of alternating layers of aluminium sheets bonded to thermoplastic sheets. Titanium-based composites provide higher specific strength and stiffness while being significantly lighter than titanium alloys. They are used in jet propulsion systems.

Layers of Aluminium Composite Panel | Bespoke Composite Panels

Metal alloys provide better performance than their pure forms in terms of hardness, stiffness, corrosion resistance etc. Using metals in composite materials provides far more advantages than alloys as they can be lighter while having far better performance than alloys and pure metals.