Engineering materials are materials that are used to engineer new technologies. Or simply put, any matter that is used to create devices that are tangible is an engineering material. Now, there are thousands of engineering materials out there and each has properties that are suited to a specific requirement. So, in order to select the best material for an engineering project, understanding the basic classification of engineering materials is essential.
As almost all of the devices, machines and structures are made of solid materials. So, all engineering materials are also solid on room temperature.
Basic Classification of Engineering Materials
The topmost or basic classification is into:
- POLYMERS or PLASTICS
Metals are materials that are held together by metallic bonding.
The materials in this group are composed of one or more metallic elements (containing metallic bonding) such as iron, aluminum, copper, titanium, gold, silver etc. and often small amounts in traces of non-metallic elements like carbon, oxygen and nitrogen.
METALLIC BONDING : As, said earlier metallic atoms are interlocked tightly with some non-localized electrons, that are electron that are not bound to any specific atom but hover from one atom to other, leaving the other part of atom positively charged, hence called metal ions.
These non-localized electrons that have the ability to flow from atom to other give metallic atoms, one of their characteristic property, good electrical conductivity.
Metals can be further classified into two categories, Ferrous Metals and Non-Ferrous Metals.
Ferrous is from ferrum, which is the latin term for iron.
So, ferrous metals are the one that are composed of iron primarily with some traces of other elements usually carbon.
The reason for having an entire category for just iron based metals is the vast applications of them.
Ferrous Metals can be further classified into two categories depending on the amount carbon content in them.
- These metals with carbon content greater than 1.5% are called irons.
- Ferrous metals with carbon content less than 1.5% are called Steels.
Below is details of all these in the descending order of carbon content:
PIG IRON: Also, called as crude iron, is obtained by smelting iron ore in a blast furnace and contains 3.8-4.7% carbon content, along with silica and other impurities, which make it very brittle and not suitable for many applications.
CAST IRON: Cast iron contains carbon content greater than 2% and less than that of pig iron. As the name suggests, cast iron is suitable for casting and therefore has a lot of applications.
WROUGHT IRON: Wrought contains less than 2% of carbon content and is somewhat equivalent to steels in terms of strength. However, wrought iron differs from steels because it contains a lot of other undesirable impurities which are filtered for production of steel.
Steels furtherly are categorized into four sub-categories: Which are:
- Carbon Steels.
- Alloy Steels.
- Tool Steels.
- Stainless Steels.
HIGH CARBON STEEL: These steels contain carbon content varying from 0.65% to 1.5%. They get hard and tough by heat treatment, however their weld-ability is poor.
MEDIUM CARBON STEEL: They contain 0.3% to 0.6% of carbon content. They are moderately strong and have comparatively good weld-ability than high carbon steel.
MILD/LOW STEEL: They contain carbon content of 0.15% to 0.25%. They have good weld-ability and also their production cost is low.
Alloy steels are steels that contain a variety of elements with 1% to 50% by weight to increase its mechanical properties.
Technically, all steels are alloys as they contain carbon, but not all steels are alloy steels.
Other elements may be chromium, vanadium, molybdenum, manganese and nickel etc.
The element with which these are alloyed depends on the properties needed for a specific application.
For example, for railway tracks, manganese is added for high strength.
Tool steels are steels from which majority of tools are made, but not all tools are made of tool steels necessarily.
Tools steels are further classified into four categories:
WATER QUENCHED: Steels that are quenched in water after heat treatment.
OIL QUENCHED: Steels that are quenched in oil after heat treatment.
AIR QUENCHED: Steels that are quenched in air after heat treatment.
HIGH SPEED STEEL: High speed steel is commonly used for cutting tools. It contains tungsten, molybdenum or cobalt also in various ratios.
These steels are extremely corrosion resistant because they contain upto 13% chromium.
Stainless steels can be categorized into:
Austenitic Stainless Steel: Stainless steel that has austinitic microstructure,
Martensitic Stainless Steel: Stainless steel that has martensitic microstructure,
Duplex Steel: Stainless steel that has microstructure of both ferrite and austinitic grains.
Austenite, martensite and ferrite are microstructures of steels.
Non-ferrous metals are metals other than Iron and their alloys. However, in some iron may be present by in very minor amount.
For engineering purposes, usually six non-ferrous metals and their alloys are used for various application. These are aluminum, tin, copper, nickel, zinc and magnesium.
Some other non-ferrous metals are also important to modern world but, they are not produced in large quantities. These are vanadium, tungsten, molybdenum, antimony, titanium and more.
Polymer atoms are bounded to each other by covalent bonds in long chains, where small units called monomers join up to make chains of plastic, called as polymers.
Mono —–> One
Poly —-> Many
Mers ——> Units
The process by which mers join up to form polymers is called as polymerization.
Polymers are solid at room temperature, and have low melting points and hence can be shaped to various objects quite easily.
Polymers can be further divided into three types:
- THERMOSETTING POLYMERS
Thermoplastics typically behave as ductile materials.
The name “thermoplastics” means that they get soft upon heating and get hard again upon cooling, making them very easy to be shaped.
Also, thermoplastics are easily recylcable.
Examples are polystyrene, Teflon, nylon, polyethane etc.
These materials do not soften on heating but instead begin to decompose.
Polymer chains of these polymers cross-link to each other and because of that they are hard, tough and brittle.
Thermosetting polymers are best for casting and moulding into components. Also, they have good corrosive resitance.
Examples are PVC (poly-vinyl-chloride), epoxy resins, polyester resins etc.
These are commonly known as rubbers. They have elastic deformation of greater than 200%.
Examples are natural rubbers, styrene-butadiene block copolymers, polyisoprene, polybutadiene etc.
Ceramics are compounds of metallic and non-metallic materials. They have:
- High compressive strength
- Low thermal expansion
- High elasticity
- High Wear Resistance
- Low electrical and thermal conductivity
Ceramics are used for tiles, pottery, sanitary wares and more.
Ceramics can divided into two types of basis of structure: Crystalline and Non-Crystalline or Amorphous Ceramics.
Crystalline ceramics are materials that are not easily processed. They are usually processed into various shapes in powder form.
They have crystalline structure, meaning their atoms are tightly arranged and are not possible to move in a specific direction easily,
Clay, bricks, porcelain etc are crystalline ceramics.
Non-Crystalline Ceramics (Glass)
These are commonly called as glass. They are amorphous and melt and then can be cooled to be formed into various shapes.
However, glass can become partially crystalline by heat treatment but majority portion remains amorphous.
Examples are soda-lime glass, borosilicate glass etc.
Composite materials are combinations of previous three classes. It can be metal-polymer, polymer-ceramic or any other combination.
Majority of composite materials are artificial, however some natural composites also exist. Wood is one of most prominent composite materials.
Composite materials have huge range of applications from aerospace to medical industry due to their huge range of properties that can be molded for any specific application.
Composite materials are comprised of two phases: matrix phase and dispersed phase.
These two phases have to be of different kinds.
Matrix phase is the one that is surrounding or enveloping the other phase, i.e dispersed phase.
Properties of composite depend of constituents of these two phases and their ratio of magnitude.
Dispersed phase atoms can be any orientation.
Composite materials can be further classified into three types depending upon the nature of dispersed phase: Particle-Reinforced, Fiber-Reinforced and Structural Composites.
In these type of composites, dispersed phase is in particle or powder form. Particles can be of very small, sometimes less than 0.25 microns and can be of any nature.
Added particles add strength to the material or if needed can give various other desired properties. For example, using particles of conductive metals in plastic’s matrix can produce a material that is conductive but also has properties of plastics.
Concrete is a good example of particle-reinforced composites.
These composites are of three components. Matrix phase, fibers which are dispersed phase and third, an interface region.
Here, the dispersed phase is in the form of fibers. Usually, rice husk, rice hull or plastic is used.
Fiber dispersed phase gives them flexibility and increases their ultimate strength.
Kevlar is a very common example of these composites.
Structural composites are combinations of other composites and homogeneous materials.
Properties of structural composites not only depend on constituent materials but also on the geometrical design of each structural element.
Mostly they are in form laminated sheets of various composite materials, or sometimes one material sandwiched between two others to yield desired properties.
TREE OF ENGINEERING MATERIALS
Classification of engineering materials are mapped in the form of a tree below: