Engineering Materials: Properties and Classification

In this article we will discuss about the properties and classification of engineering materials used in industries.

Properties of Materials:

Material selection should be based mainly upon the properties of material. The designer must decide the properties required of a material for a part under design and then weight the properties of candidate material. There are literally hundreds of properties that are measured in laboratories for the purpose of comparing materials, but we shall concentrate on the more important ones, e.g., chemical, physical, mechanical, dimensional properties.

1. Chemical Properties:

Chemical properties are material characteristics that relate to the structure of a material and its formation from the elements.

The important chemical properties are:

i. Composition,

ii. Structure,

iii. Corrosion Resistance.

i. Composition:

Composition of material can be determined by analytical chemistry. In metals, composition usually means percentage of the various elements that make up the metal. In material selection, composition is a fundamental consideration. The designer should always have at least some idea of what a material is made of.

ii. Structure:

Studies of the structure of the materials are one of the most useful tools of the metallurgist. Structure of a material generally refers to its micro-structure, i.e. the components seen when the metal is examined under a microscope. Microstructure is invaluable to the materials engineer in solving problems and in understanding material responses to treatments. Microstructure studies are also an important tool in analyzing as to why a part failed in service.

iii. Corrosion Resistance:

Corrosion is a gradual, chemical or electrochemical attack on a metal by its surroundings so that the metal is converted into an oxide, salt or some other compound. It may be brought about by almost unlimited number of factors of corrosive media such as air, industrial atmosphere soils, acids, bases and salt solutions. It may also occur at elevated temperature in media which are inert when near or below room temperature.

2. Physical Properties:

Material used in industry possess some definite physical characteristics which render them suitable for particular use and their choice for a particular use is governed mostly by these properties. Physical properties are characteristics of materials that are determined by nature.

Some important physical properties are described as follows:

i. Density:

Density is a measure of the mass per unit volume and can be interpreted in various ways as:

a. Crystallographic density—the ideal density that would be calculated from continuous defect-free crystal lattice of the material,

b. Theoretical density same as crystallographic density but taking into account solid solutions and multiple phases.

c. Bulk density—the measured density of a material which includes all lattice defects, phases and fabrication porosity.

ii. Specific Heat:

Specific heat is the quantity of heat in calories required to raise the temperature of 1 gm of a material by one degree centigrade. Different substances possess different specific heats. Water has the highest specific heat.

iii. Specific Gravity:

Specific gravity is the relative mass of a certain volume of a material compared to mass of the same volume of water. Consequently, it is the ratio of densities of the material to that of water.

iv. Porosity:

Porosity is the ratio of volume occupied by pores to the volume of a material. Low porosity of a material makes it light in mass. More porous material is better insulating materials i.e. thermal conductivity of a material can be decreased by increasing its porosity.

v. Refractoriness:

Refractoriness is the property due to which a material is able to withstand high temperature without fusing e.g. silica, alumina and fireclay.

vi. Thermal Conductivity:

Thermal conductivity is the ability of a material to conduct heat from the hot end to the cold end-silver is the best conductor of heat. A material that conducts heat very badly is called heat insulator. The insulating materials are frequently employed to prevent the loss of heat.

vii. Electrical Conductivity:

Electrical conductivity is the ability of a material to conduct electricity. A material which allows electric current to pass through it with minimum loss is called a good conductor of electricity. Silver is the best electrical conductor and copper is the next one. Material which offers great resistance to the flow of electric current is called electrical insulator or dielectric.

viii. Magnetic Property:

Materials which are attracted by a magnet are magnetic in nature, e.g. Fe, Ni, Co, Cr, Mn, Gd. Materials which are repelled feebly by a magnet are called diamagnetic e.g. Cu, Zn, Pb, Sb, Bi etc.

ix. Colour:

Colour results from the absorption of a relatively narrow wavelength of radiation within the visible region of the em spectrum (0.4 to 0.7 mm). For this type of absorption, transition of electrons must occur.

The melting point, boiling point and linear coefficient of expansion are some of other physical properties.

3. Dimensional Properties:

Dimensional properties form an important selection factor. The available size, shape and tolerances on materials are important in selecting a material. Other dimensional properties include surface roughness, waviness and micro topography. The microscopic characteristics of a surface are called micro topography and it includes various surface characteristics such as surface layout, roughness height, roughness width, waviness height, waviness width etc.

Total surface profile, which is the net of the surface roughness and waviness, is usually measured using a profilometer a device that electronically measures surface texture with a stylus. The profilometers yield contour maps, single-line surface profiles and roughness data.

4. Mechanical Properties:

Mechanical properties are the characteristics of a material that are displayed when a force is applied to the material. They usually relate to the elastic and plastic behaviour of the material. Important mechanical properties are hardness, modulus of an elasticity, yield strength, tensile strength, percent elongation, reduction in area, impact, fatigue and creep strengths and wear resistance.

Mechanical properties are of foremost importance in selecting materials for structural machine components. There are many tests to measure mechanical properties such as hardness test, tensile test, bend test, impact test, fatigue test and creep test.

Classification of Engineering Materials:

Solid materials have been conveniently grouped into three basic classification; metals, ceramics, and polymers. This scheme is based primarily on chemical makeup and atomic structure, and most materials fall into one distinct grouping or another, although there are some intermediates. In addition, there are three other groups of important engineering materials—composites, semiconductors and biomaterials.

1. Metals:

Metals have large numbers of non-localized electrons; that is, these electrons are not bound to particular atoms. Many properties of metals are directly attributable to these electrons. Metals are extremely good conductor of electricity and heat and are not transparent to visible light; a polished metal surface has a lustrous appearance.

Furthermore, metals are quite strong, yet deformable, which accounts for their extensive use in structural applications. Metals are classified as ferrous metals (e.g. carbon steel, alloy steel, stainless steel, tool and die steel etc.) and non-ferrous metals (e.g. Al, Mg, Cu, Ni, Ti, refractory metals, Be, Zr, super alloys). Alloying elements together has led to a large variety of metallic materials that can be designed for a specific property.

2. Ceramics:

Ceramics are compounds between metallic and nonmetallic elements; they are mostly oxides, nitrides, and carbides. The wide range of materials that falls within this classification includes ceramics that are composed of clay minerals, cement, and glass. These materials are typically insulative to the passage of electricity and heat, and are more resistant to high temperatures and harsh environments than metals and polymers. With regards to mechanical behaviour, ceramics are hard but very brittle. MgO, SiC, BaTiO₃, Silica, Glasses, Concrete, Cement, Ferrites, Garnets, Al₂O₃, Granite, Calcite, Magnesite etc. are ceramics.

3. Polymers:

A polymer is a large molecule built up by the repetition of small, simple chemical units. In some cases the repetition is linear, much as a chain is built up from its links. In other cases the chains are branched or interconnected to form 3D networks. The repeat unit of the polymer is usually equivalent or nearly equivalent to the monomer, or starting material from which the polymer is formed.

Polymers include the familiar plastic and rubber materials. Many of them are organic compounds that are chemically based on carbon, hydrogen, and other nonmetallic elements; furthermore, they have very large molecular structures. These materials typically have low densities and may be extremely flexible. PVC, PTFE, Polythene, Terylene, Nylon, Rubber etc. are polymers.

4. Composites:

A number of composite materials have been engineered that consist of more than one material type. Fiberglass is a familiar example, in which glass fibers are embedded within a polymeric material. A composite is designed to display a combination of the best characteristics of each of the component materials. Fiberglass acquires strength from the glass and flexibility from the polymer. Many of the recent material development have involved composite materials. These are also known as engineered materials.

5. Semiconductors:

Semiconductors have electrical properties that are intermediate between the electrical conductors and insulators. Furthermore, the electrical characteristics of these materials are extremely sensitive to the presence of minute concentration of impurity atoms. The semiconductors have made possible the advent of integrated circuit that has totally revolutionized the electronics and computer industries over the past few decades. Semiconducting materials include Si, Ge, CdS, CdSe, GaAs etc.

6. Biomaterials:

Biomaterials are employed in components implanted into the human body for replacement of diseased or damaged body parts. These materials must not produce toxic substances and must be compatible with body tissues (i.e., must not cause adverse biological reactions). All of the above materials—metals, ceramics, polymers, composites and semiconductors—may be used as biomaterials.

7. Advanced Materials:

Materials that are utilized in high-technology (or high-tech) applications are sometimes termed advanced materials. By high technology we means a device or product that operates or functions using relatively intricate and sophisticated principles; examples include electronic equipment (VCRs, CD players, etc.), computers, fiber-­optic systems, spacecraft, aircraft and military rocketry.

These advanced materials are typically either traditional material whose properties have been enhanced or they are newly developed, high-performance materials. Furthermore, they may be of all material types (e.g., metals, ceramics, polymers), and are normally relatively expensive.

These materials are used for lasers, integrated circuits, magnetic information storage, liquid crystals displays (LCDs), fiber optics and the thermal protection system for the space shutter orbiter. This category includes materials like prozeelectrics, ferroelectrics, high temperature superconductors, super refractories, magnetic alloys, shape memory alloys etc.