Material Science Engineering Viva Questions
Material Science Engineering Viva Questions, Viva Questions on Material Science Engineering, Introduction to Material Science Engineering, Engineering Viva Questions, Material Science Viva Questions, Short Answer Questions on Material Science Engineering
Mechanical Properties of Materials Viva Questions
Short Answer Type Questions
Q.1. What is material science?
Ans. Material Science refers to that branch of applied science concerned with investigating the relationship existing between the structure of materials and their properties, and it concerns the interdisciplinary study of materials for entirely practical purposes. Material science has developed rapidly during the last twenty years. The new approach of material science has paid off handsomely in many ways and thus has solved the problems in the selection of the right materials in complex situations.
Q.2. Briefly discuss the classification of engineering materials.
Ans. The engineering materials may be classified into three broad groups according to their mode of occurrence:
- Metals and Alloys
- Organic Polymers
Metal is an elemental substance. An alloy is a homogeneous mixture of two or more metals or a metal and a non-metal. Among the solid materials, metals and alloys predominate because of their useful characteristics of hardness, strength, rigidity, formability, machinability, weldability, conductivity and dimensional stability. In general, metals have good electrical and thermal conductivity.
Ceramics are materials consisting of phases. A phase is a physically separable and chemically homogeneous constituent of a material. These are themselves compounds of metallic and non-metallic elements. All metallic compounds, rocks, minerals, glass, glass fibre, abrasives and all fired clays are ceramics. Ceramics are inorganic crystalline materials and probably the most natural materials.
Organic materials are those materials derived directly from carbon. They usually consist of carbon chemically combined with hydrogen, oxygen or other non-metallic substances, and their structures are, in many instances, fairly complex. The organic materials are prepared by a polymerization reaction in which small molecules are chemically combined into long-chain molecules. Plastics and synthetic rubbers are common organic engineering materials.
Q.3. Explain the structure of materials in detail.
Ans. Depending on the level, the structure of materials is classified as follows:
- Crystal structure
- Electronic structure
- Nuclear structure
1. Macrostructure of a material refers to the structure as observed with the naked eye or under low magnification. The internal symmetry of atomic arrangements in a crystalline material may reflect in the external form of a crystal such as quartz. The individual crystals of crystalline material can be visible, as in brass. Standard procedures are used for macro-examination to reveal flaws and segregations in a material. The length scale is ≈ > 100 nm.
2. Microstructure generally refers to the structure as observed under the optical microscope. The microscope can magnify a structure up to about 1500 times linear, without loss of resolution of details of the structure. The limit of resolution of the human eye is about 0.1 mm, that is, the eye can distinguish two lines as separate lines, only when their distance of separation is more than 0.1 mm. This optical microscope can resolve details up to a limit of about 0.1 mm (10-7m).
3. Substructure refers to the structure obtained by using a microscope with a much higher magnification and resolution than the optical microscope. In an electron microscope, a magnification of 1000000 times linear is possible. By virtue of the smaller wavelength of electrons as compared to visible light, the resolving power also increases correspondingly, so that much finer details show up in the electron microscope. We can obtain additional information on very fine particles or on crystal imperfections such as dislocations. Another modern microscope is the field ion microscope. It produces images of individual atoms and imperfections in atomic arrangements.
4. Crystal structure refers to the details of the atomic arrangement within a crystal. It is usually sufficient to describe the arrangement of a few atoms within what is called a unit cell. The crystal consists of a very large number of unit cells forming regularly repeating patterns in space. The main technique used for determining the crystal structure is the X-ray diffraction,
5. Electronic structure of a solid refers to the electrons in the outermost orbital of individual atoms that constitute the solid. Spectroscopic techniques are very useful in determining the electronic structure. It refers to the arrangement within the crystal.
6. Nuclear structure is studied by nuclear spectroscopic techniques, such as nuclear magnetic resonance (NMR) and Mossbauer studies.
The levels of structure that are of greatest interest in material science are the microstructure, the substructure and the crystal structure. The chemical, mechanical, electrical and magnetic properties are among the most important engineering properties.
Q.4. Explain the essential properties of steel.
Ans. Steel is an alloy of iron and carbon which contains carbon up to a maximum of 1.5%. The ingredient that exerts the most influence on steel is carbon. Hence to manufacture a particular type of steel, it is important to get the right proportion of carbon.
In the manufacture of steel, the carbon may be added to wrought iron by crucible process, or the carbon may be burnt of pig iron by Bessemer and open-hearth process to get the required percentage of carbon. The essential difference between cast iron and steel is in the amount of carbon contained in the constituency of the metal.
Pure iron is a soft metal, and as the carbon content increases, the metal becomes harder and tougher.
A series of steels are made by increasing the carbon up to 1.5%. Up to the content of 1.5%, all the carbon is present in chemical combination with the iron, and none of it exists in its free graphite state. It, however, the carbon is increased above 1.5%, a stage soon arrives when no more can be contained in the combined state and any excess must be present as free graphite.
At this stage, the metal merges into the group termed cast iron, and we may go on increasing the carbon content up to about 4.5% whilst producing a range of cast iron. Thus, for a material to be classed as steel, there must be no free graphite in its composition. It is true that other elements are present in small quantities to have certain desired properties in the steel, but carbon is by far the most important modifying element in it.
Q.5. Define the term polymers and also the classification of polymers.
Ans. Mer is a Greek word. It means a unit. Monomer stands for a single unit and polymer for many units joined together. A polymer usually has thousands bonded by a chemical reaction. In other words, a mer (literary unit) is the repetition unit of a large molecule and is comparable to the unit cell of crystals. A monomer (literary, one unit) is a small molecule that may combine with others to form a large molecule. A polymer (literary, many units) is a large molecule that may be made up of many small units.
The polymers are broadly classified as follows:
Thermoplasts have the property of increasing plasticity with increasing temperature. They are long-chain polymers held together by secondary bonds. Such materials deform easily under pressure particularly at high temperatures because weak van der Waal’s forces are easily overcome at high temperatures.
Thermosets have a three-dimensional network of primary bonds, as polymerization proceeds in all directions in this case. The chemical reaction which creates a three-dimensional network is called curing. The curing process is assisted by pressure, heat etc. They are, therefore, hard and rigid at room temperature and do not soften on heating.
Long-chain polymers are further divided as follows:
Q.6. Write a short note on semiconductor materials.
Ans. It is evident, conductivity is a relative term. Most metals have resistivity, which is reciprocal of conductivity. Many materials lie between the extremes of conductors and insulators. Approximately, they are known as semiconductors. They include materials such as silicon, germanium and galena (PBS), which was the crystal of the early radio crystal sets. The conducting properties of semiconductors originate from imperfections or minutes quantity of chemical impurities introduced into the crystal.
The conductivity of semiconductors can be modified appreciably by slight variations of these compositions. Silicone is a good example of a semiconductor that is used in transistors and other similar devices. Since silicon has the same structure as diamond each silicon atom has a coordinate number of four and shares electrons covalently with each of its neighbours. Silicon structure has negligible conductivity because it has no partially filled electron energy bands.
A semiconductor is a solid crystalline material, whose electrical conductivity is intermediate between that of a conductor and an insulator. A semiconductor is neither a good conductor nor a good insulator. Copper is a good conductor, its conductivity is high. Glass is a good insulator, its resistivity is high. But germanium is a semiconductor. It has neither good conductivity nor good resistivity.
Q.7. Write a short note on “SF6 as a dielectric material”.
Ans. When sulphur is burned in an atmosphere of fluorine, sulphur hexafluoride is formed. It is used as a good dielectric and has many advantages. It is non-flammable and has remarkable high dielectric strength. It has superior cooling properties to those of air and nitrogen. Its dielectric strength increases at high pressure and may even become equal to that of mineral transformer oil.
Apparatus insulated with sulphur hexafluoride gas is higher in weight than those insulated with liquid dielectric. It is used in transformers and electrical switches. Having many, advantages, it possesses certain disadvantages also. The presence of sulphur in molecules, under certain conditions, can evolve corrosion of the contacting surfaces. To increase high dielectric strength, the gas is to be used at high pressure which will require a sealed tank capable of withstanding the high pressure. It has high chemical stability at normal pressure and at temperatures up to 100°C.