Question

1. Explain how an understanding of the 4 classes of materials and their general properties aid in the initial selection or elimination of materials suitable for a specific application. I.E. First outline some of the main general properties of each class, DO NOT GIVE EXAMPLES OF COMPONENTS AFTER EACH CLASS SECTION! Then describe how you would apply that knowledge to choose a suitable material for ONE specific component. ONLY ONE COMPONENT SHOULD BE PRESENTED AFTER YOU HAVE OUTLINED THE GENERAL PROPERTIES. I.E, SAY WHY ONE CLASS COULD NOT BE USED AND WHY ANOTHER CLASS CAN BE USED.

Answer 1

As we all know different problems needed to be solved require use of many type of materials.different applications require different types of materials.consider a problem of making a window for a house situated near sea. due to the content of salt in the atmospheric air around the house , we cannot use a metal window because metals react with salt content and moisture content in the air and will be affected by corrossion.so we can eliminate the option of using meats of this perticular application and use non reactive materials like ceramics or polymers for the same.this is how the selection of material is done, so generally if we know the properties of each kind of materials available it will be easy for us to select the best amoung those.

The main properties of four classes of materials are:

(1) METALS

There are many metals which you are familiar with - copper pipes and wire, aluminium saucepans and cast iron stoves. Metals may be mixed with other elements especially other metals to produce Alloys which will have improved properties. Heat treatment can also be used to change the properties of alloys e.g. hardening and tempering of high carbon steel.

All metals are good conductors of heat and electricity. Copper is a particularly good conductor but is not very strong, it is also fairly dense. Aluminium is a good conductor has a low density and when alloyed has a high tensile strength. Some alloys such as pewter and zinc alloys have a low melting point and can be easily formed by casting or moulding but they have a low tensile strength.Metals in common use are corrosion resistant except iron and steel which rust quickly. Corrosion resistance is achieved by electroplating to add a layer of corrosion resistant material such as chromium or zinc, painting, plastic coating, and coating with an oil or grease. The alloy stainless steel is very rust resistant.

(2)POLYMERS

Polymers are made from long chain molecules which may have cross linking bonds affecting flexibility/stiffness.

There are three groups of polymer:-

Thermoplastics which may be reformed with heat. e.g. PVC, HIPS, nylon, polycarbonate, PET, acrylic.

Thermosetting plastics which once moulded or formed cannot be reformed by heat. e.g. Melamine(MF), epoxy resin, Urea formaldehyde (UF).

Elastomers - rubbers long chain elastic molecules. e.g. neoprene, natural rubber. Used for car tyres and elastic bands.

(3)CERAMICS

This class of material includes plates and cups, bricks, earthenware pots, engineering ceramics, glasses [glasses are non-crystalline and not normally classed as ceramics], and refractory (furnace) materials. Ceramics are made by heating together materials such as silica, chalk and clays. Other chemicals may be included to act as flux and to change colour etc.

Engineering ceramics are ideally suited for high performance applications where a combination of properties such as wear resistance, hardness, stiffness and corrosion resistance are important. In addition to these properties, engineering ceramics have relatively high mechanical strength at high temperatures. They are good electrical insulators, They often have a close thermal expansion coefficient to metals (they can be bonded to metals - e.g. carbide tipped tools).

Ceramics have been regarded as hard but brittle, however modern ceramics have been developed which are viable alternatives to metals and their alloys in many applications - engineering ceramic parts and components are more durable and have longer life-spans under given operational conditions. Ceramic cutting tools, for instance, require less sharpening or replacement due to wear, and will last at least 60 to 100 times longer than steel blades.

Engineering ceramics are chemically resistant to most acids, alkalis and organic solvents and can withstand high temperatures. Metals weaken rapidly at temperatures above 816 degrees C while engineering ceramics retain a good degree of their mechanical properties at much higher temperatures.

(4)COMPOSITES

They are mixtures of materials which give improved properties. One of the materials is the matrix or binding chemical and the other is the reinforcer. A good example is GRP - glass reinforced polyester(plastic) resin. where the glass fibres increase the strength of the polyester resin. Carbon fibre reinforced epoxy resin is stronger and lighter than steel.

APPLICATION

COLUMN USED FOR BRIDGES ACROSS THE RIVER:

In the above application if we use bare Metals we can see that due to the contact between water and the metal the metal will start to corrode and the bridge will fail.now if use polymers , since the bridge is needed to be taken very high load and since polymers have less strength it cannot be used here also . since ceramic is very brittle it also cannot be use for this application.

In the above application we need the material to be strong as metals but not reactive with water, so we use concrete (composite) which is having metal (usually steel ) reinforcement and thus problem is solved.