Classification of materials

Materials classification:

Materials are classified in different ways. Commonly materials are classified into five groups:

  • 1.       Metals and Alloys
  • 2.       Polymers
  • 3.       Ceramics, glass, and glass-ceramics
  • 4.       Composites
  • 5          Semiconductors

Advanced materials:

  • 6.       Nanomaterials (new)
  • 7.       Biomaterials
  • 8.       Smart materials

Metals and Alloys:

Metals and alloys are usually considered solid materials with good electrical and thermal conductivities. They have relatively high strength, ductility, formability, stiffness, and shock resistance. Atoms in the metals and alloys are periodically arranged, and they are very dense when compared to ceramics and polymers. Some of the common metals and alloys include copper, aluminium, steels, cast iron, titanium, nickel, zinc, bronze, duralumin, etc. They are mainly considered for structural and load-bearing applications. Pure metals are occasionally used. For the majority of applications, combinations of metals known as alloys are widely used because it provides superior performance with desirable properties.

Figure: Objects made up of metals and alloys


Ceramics are inorganic crystalline materials between metallic and nonmetallic elements. Ceramics mostly existed as oxides, nitrides, and carbides. They are the most naturally occurring materials. Examples are beach sand, rocks and clay are naturally occurring ceramics. The mechanical properties of ceramics have stiffness, and strength is comparable to those of metals. On the other hand, they are extremely hard and brittle and high susceptible to fracture. But the advancement in ceramic technology has been improved and has resistance to fracture, these advanced ceramics are used for applications like cookware, cutlery, automobile parts, rocket nozzles, ceramic coatings in turbine engines etc. They are typically insulating materials and are more resistant to high temperatures and harsh environments than metals and polymers. Some of the known ceramic materials are alumina (Al2O3), Silicon carbides (SiC), Silica (SiO2).

    Figure: Objects made of ceramics

Glass and Glass-ceramics:

Glass is an amorphous non-crystalline transparent solid and widely used for decorative and technological purposes like window panels, tables, glassware, and optic applications like microscopy, lens technology, etc. It is formed by the rapid cooling of the molten silica. Naturally occurring glasses are volcanic glass. Obsidian is a common volcanic glass with high silica content, it forms when the lava cools rapidly.

Glass-ceramics are polycrystalline materials. It is formed by nucleating small crystals in the base glass through a special thermal process. Zerodur is a glass-ceramic material used for making mirror substrate for large telescopes like the Hubble space telescope. Glass and Glass-ceramics are usually processed by melting and casting.

Figure: Objects made of Glass and Glass-ceramics


Semiconductors are solid crystalline materials that lie between conductors and insulators. At room temperature and normal atmospheric conditions, they do not conduct electricity. They are extremely sensitive in their electrical characteristics even a small amount of impurity atoms makes them electrically conductive. Sometimes conditions like temperature, and light irradiations also make them electrically conductive. Some of the common semiconducting materials are silicon, germanium, gallium arsenide, etc. Semiconductors are the main reason for this information age. It revolutionizes the electronics and the computer industries over the few decades with the invention of transistors, diodes, and integrated circuits.

Figure: Objects made of semiconducting materials


Polymers are organic materials made up of chain linking subunits called monomers. Polymers are formed by the process known as polymerization, including rubber, adhesives, and plastics. They are not stiff as metals and ceramics nor as strong. Polymers are used in electrical insulation purposes, ropes, bulletproof vests, and clothes from everyday uses to the aerospace application but are not limited to use. It plays a vital role in every walk of life. Polymers have good strength to weight ratio. The main disadvantage of polymers is that they are not suitable for high-temperature applications. Even they are used in electronic devices like flexible electronics. Some examples of polymers are polyethylene, polyester, polycarbonates, silicones, etc.

Figure: Objects made of Polymers


The composite materials are formed from two or more materials and obtain properties that are entirely different from the parent materials. Distinct and unique characteristics will be obtained. The parent materials are metals, ceramics, and polymers. Usually, composites consist of two components 1. matrix, 2. reinforcement. Matrix is a continuous phase in composite, and reinforcement is a discontinuous phase. Some examples of naturally occurring composites are bone and wood. Furthermore, some known artificial composites are plywood, concrete, and fiberglass.

In concrete, the matrix is the cement, and reinforcement is the gravel (small stones) and iron rods. In the same way, fiberglass is made up of glass fiber embedded in resins like epoxy or polyester. Here, the polymers (epoxy or polyester) are the continuous matrix phase, and glass fiber is the reinforcement which is the discontinuous phase.

Figure: Objects made of composite materials

Advanced materials:

These are typically traditional materials such as metals, ceramics, composites, and semiconductors but the properties have been enhanced also artificially synthesized with high performance. They are highly expensive because of their sophisticated synthetic methods. These advanced materials are mainly used for the operation of sophisticated high-end devices of sensitive electronic components like computers, space crafts, aircraft, and military rockets. These advanced materials include semiconductors (discussed already), nanomaterials, biomaterials, and smart materials. They are referred to as “materials of the future or futuristic materials”


The discovery of semiconductors enabled great technological development in the past few decades. Because of the semiconductors, our life has been changed entirely. The invention of integrated circuits revolutionized the entire electronics and the computer industries. This leads us to the next step of mankind’s civilization, known as the “information age”


Nanomaterials in the definition are defined as the materials in which their size exists between 1 to 100nm in at least any one of the dimensions. ISO definition: the materials with any external dimension in the nanoscale or having an internal structure or surface structure in the nanoscale is known as nanomaterials. Nanoscale length range from 1 nm to 100 nm (1 nanometer = 10^-9 m).

In simple one billionth of a meter is called a nanometer. It is like comparing the diameter of marble with the diameter of the earth. Assume the earth is in the diameter of 1m, then the size of a marble is 1nm.

Usually, nanomaterials are synthesized by either a top-down or bottom-up approach (discussed later). These nanomaterials have fascinating properties and are also promising materials for futuristic technological approaches. The classification of nanomaterials is based on their size, not based on their chemistry like other materials.

Because of the unusual size of the materials, new properties have been found in the materials at the nanoscale. For example. Graphite is usually soft and slippery. They are used in pencils, lubricants, etc. It is also a good conductor of heat and electricity. They are in the shape of hexagonal honeycomb structures. On the other side graphene is a single layer of graphite material (2-dimensional material). It is one of the strongest materials known so far and about 200 times stronger than steel and lighter than a feather. It is an excellent conductor of heat and electricity and has several useful properties.

Figure: Graphene and their properties

Materials are even classified by their functional properties based on mechanical, structural, electrical, magnetic, optical, and biological functions. The flowchart of materials classified based on their functional properties is shown below.

Aerospace materials:

In the initial stage, the Wright brothers used wood and aluminium alloys for their flight. Now, aerospace materials include aluminium alloys, superalloys, plastics, amorphous silicon, SiO2 for space shuttles tiles, C-C composites, and many other lightweight materials.

Figure: Materials used in an airplane Source:


Biomaterials include bones, teeth are naturally occurring materials formed from a ceramic known as hydroxyapatite. Now artificial materials like titanium alloys, plastics, and non-magnetic stainless steel are used as a replacement for bones, organs, etc. Biomedical instruments like pacemakers made up of titanium and polymers, ultrasonic imaging systems made up of ceramics known as lead zirconium titanates, and MRI (magnetic resonance imaging) the magnets are made up of niobium-tin (Nb-Sn) based superconductors.

Figure: Biomaterial products Source: wikipedia

Electronic materials:

The invention of transistors and capacitors has entirely changed the electronics industry, which enabled dramatic technological developments in mankind. The electronic materials include silicon, germanium, gallium arsenide, aluminium, copper, tungsten, conducting polymers, piezoelectric materials etc. Ceramic materials like Barium titanate, manganese oxide, and tantalum oxide are used in capacitors and other electronic devices. For power transmission and micro/nanoelectronic devices copper, aluminium, silicon, gallium arsenic phosphide are used.

Figure: Classification of functional materials.

Energy materials:

The materials which are used in energy-related technologies like nuclear, solar, supercapacitors, batteries, fuel cells, etc are known as energy materials. Nuclear materials include plutonium and uranium dioxide. Energy storage and energy conversion materials include lithium-based oxides, zirconia, cadmium selenide/sulfide, crystalline silicon, amorphous silicon, lead tellurides/selenides, titania, and ruthenium oxide, graphene, graphene oxide, etc. In catalytic applications, Pt, Au, Pt/Rh are used.

Magnetic materials:

Electronic devices like hard disks and audio-video cassettes are made up of materials like ceramics, metallic and polymeric materials. In audio cassettes, gamma iron oxide particles are used as a coating and high purity iron particles in videotapes. Cobalt-platinum-tantalum-chromium (Co-Pt-Ta-Cr) alloys are used in computer hard disks. Magnetic ferrites are used to make inductors and components for wireless transmission components. Transformer cores are made up of steels based on iron and silicon.

Photonic or optical materials:

Optical materials usually consist of silica for making optical fibers. They are widely used in fiber optic communication systems, lasers, and other components. Similarly, alumina and yttrium aluminum garnets materials are used for making lasers. Crystalline and amorphous silicon for making photovoltaic devices, polymers, LEDs, and OLEDs are used to make display screens.

Figure: Photonic fibers

Smart materials:

The material which can respond to an external stimulus such as temperature, stress, humidity, or chemical conditions is known as smart materials. These are widely used for sensors and actuators applications. Some examples of smart materials are, lead zirconium titanate a voltage developed when subjected to stress. Shape memory alloys such as nitinol nickel-titanium alloy when it is deformed it reverted to their original shape when the temperature is applied to it.

Figure: Mechanism of shape memory alloy

Structural materials:

Structural materials are designed for load-bearing applications. Steels and concrete are used in making bridges and buildings. Using structural materials we can fabricate automotive components easily. Examples of these materials are steel, aluminium, plastics, composites glass etc.

Materials classified based on structure:

Based on the microstructure (arrangement of atoms, grains, crystallites) materials are classified as single-crystalline and polycrystalline.

Single crystal means they have the same periodic arrangement of atoms throughout the crystal. They do not have grain boundaries and grains. Isotropic properties in all directions.

Polycrystal means it consists of several grains and is separated by grain boundaries. The arrangement of atoms in each grain is in a different orientation. The properties of each grain will be different from each other.

 Figure: Orientation of atoms in single crystal and polycrystal.

Post a Comment

Post a Comment