Unit 1 Topic: Engineering I. Reading skills Pre-reading tasks

II. Retell the text “Non-metallic Materials. Advantages and Limitations of Non-metallic Materials for Structural and Wear Application”.


III. Rendering


Glasses. Traditionally, glasses are defined as super-cooled liquids. This is because glasses do not behave like metals, ceramics, or even plastics on cooloing from the molten condition. Most glasses are considered to be just a very viscose liquid when it is a solid.
The properties of glasses very with composition, but some general statements can be made:

1. 
   Glasses are harder than many metals (400 to 600 kg/mm²).
   
2. 
   Glasses have low ductility; they are brittle.
   
3. 
   Glasses have a low coefficient of thermal expansion compared with many metals and plastics.
   
4. 
   Glasses have low thermal conductivity.
   
5. 
   Glasses can be good electrical insulators.
   
6. 
   Glasses are resistant to many acids, solvents, and chemicals.
   
7. 
   Glasses can be used at elevated temperature (up to 820^oC/1508^oF)
   
8. 
   Glasses are slowly attacked by water.
   

So, glasses have good mechanical properties, and a wide range of thermal, electrical and optical properties. They have many uses in machines.
Ceramic materials
Indeed, to most people the term “ceramic” means dishes, pottery, porcelain figurines, and the like. But these are not engineering ceramics.
Most of these products are made from natural clays). These clay products are quite weak and useless compared to most engineering ceramics. “Ceramic” materials are more commonly referred to as metalloids. Some common metalloids are boron, germanium, and silicon. Ceramics, formed by the combination of two metallic elements into a compound, are termed intermetallic compounds.
Natural abrasives
Speaking about non-metallic materials we must point out the following natural abrasives used in machine engineering: diamond, corundum, emery, oilstones.
Diamond is used extensively for grinding tungsten- and tantalum-carbide cutting tools, and also glass, stone and ceramics.
Corundum is a mineral composed chiefly of crystallized alumina. The present commercial source of corundum is South Africa (Northern Transvaal). It has been largely replaced by harder, more uniform and efficient, manufactured abrasives.
Emery, a cheap and impure form of natural corundum which has been used for centuries as an abrasive, has been largely superseded by manufactured aluminum oxide for grinding. It is still used to some extent in the metal- and glass-polishing trades. The principal deposits of commercial emery are located in Asia Minor and on the island of Naxos.
Oilstones. Most oilstones used today in metalworking shops are made of manufactured aluminum oxide. The hard variety is used for sharpening tools requiring a very fine edge, such as those of surgeons, engravers, and dentists.