Ceramics, Refractory And Abrasive Materials
INTRODUCTION
There are several situations in practice where high temperatures are involved. Besides creating such temperatures engineers are called for using materials that are least affected by such temperatures so that usefulness and integrity of equipment may be maintained. Furnaces for heating and melting metals and alloys, boilers for evaporating water, combustion chambers for burning liquid and gaseous fuels and ladles for holding liquid metals are a few examples where high temperatures essentially exist. Reentry vehicles (entering earth’s atmosphere from space) and blades of gas turbine are also the examples of structure and parts subjected to high temperature. Thus engineering practice demands production and use of materials that will be able to withstand such temperatures while maintaining their properties, integrity & dimensional stability. Refractories are those materials that might withstand such temperatures. Most of refractory materials are carbides oxides, nitrides and borides.
It must be noted down that most elementary metals cannot be used for the purposes of high temperature applications because they themselves melt or become soft. However, there are a few metals that melt at much higher temperature and they from high melting alloys.
Such metals that melt above 2450oC are classified as refractory metals, molybdenum, niobrium tantalum and tungsten are examples.Abrasive machining processes are used either for removing very hard surfaces or for producing very smooth and nearly finished surfaces or both. Abrasive wheels are built up of extremely small hard abraiding particles, each of whereas acts as a small cutter and hence removes a very fine chip. The abrasive materials utilized for making abrasive tools are diamond, silicon carbide and aluminium oxide.
Several inorganic materials known for temperature resistance are very brittle and can carry only small tensile stresses. They are utilized for their thermal and electrical resistances and are capable of being formed in several shapes and sizes. Such as materials are ceramics, cements & glasses. Fortunately their tensile properties might be improved by fibre reinforcement.
Objectives
After studying this unit, you should be able to
- recognize properties that are required to be present in a material for high temperature applications,
- understand basic structure of materials utilized at high temperature,
- know materials with refractoriness,
- know elemental materials along high melting point,
- understand abrasive machining and why it is required,
- recognize materials with abraiding property,
- know structure of abrasive tools & wheels, and
- understand structure, properties & application of glasses.