Abrasive is a hard material. It can be used to cut or wear away other materials. Small abrasive particles are used in grinding wheels. They are called abrasive grains. Abrasives may be classified into two types.
These are produced by uncontrolled forces of nature. These are found from mines. The following are the natural abrasives.
a. Sandstone or solid quartz
Natural abrasives lack uniformity of properties and reliability has been largely replaced by manufactured or artificial abrasives.
These are manufactured under controlled conditions in closed electric furnace in order to avoid the introduction of impurities and to achieve necessary temperature for the chemical reaction to take place. These abrasives have better cutting properties and higher efficiency than natural abrasives. The various manufactured abrasives are
a. Aluminium oxide
b. Silicon carbide
c. Artificial Diamond
d. Boron carbide
e. Cubic boron nitride.
It is the crystalline form of aluminium oxide. This abrasive carries very hard and tough grains having sharp cutting edges. It is manufactured by fusing mineral Bauxide in an electric arc furnace mixed with coke and iron scrap. Here, iron scrap acts as a flux. After fusing, it is crushed, washed and treated with alkalis. Again, it is washed and finally, ground.
Aluminium oxide is tough and less brittle. It is used for grinding materials of high tensile strength like high speed steel, malleable iron, wrought iron etc.
It is made from Silicon dioxide, coke, sawdust and salt. The ingredients are thoroughly mixed and heated in an electric furnace about 2000 C for 34 Hours. The mass under the action of intense heat fuses and the following chemical reaction takes place.
The silicon carbide mass is crushed, washed and treated with alkalis. It is again washed and finely ground into small particles. Silicon carbide is hard and brittle. It is used for grinding materials of low tensile strength like grey cast iron, brass, copper, aluminium etc. The common trade name for this abrasive is ‘Silicon carbide’ ‘Carborundum’ ‘Crystolon’ and ‘Electron’ etc. It is represented by the letter C.
In general, the physical properties of aluminium oxide are compared with silicon carbide as follows:
1. Silicon carbide is harder than aluminium oxide.
2. Aluminium oxide can withstand greater stresses than silicon carbide.
3. Aluminium oxide is tougher than silicon carbide.
Artificial diamond is a form of pure carbon which is mainly used for truing and dressing other grinding wheels for sharpening carbide tools, and for processing glass, ceramics and stone.
It is harder than silicon carbide but not as hard as diamond. It is produced from coke and boric acid at tremendously high temperatures in an electric furnace. Boron carbide is mainly used for grinding and lapping very hard metals, hard alloys, glass and jewels.
Cubic boron nitride
It is a never synthetic abrasive that is harder than either aluminium oxide or silicon oxide. It is a combination of boron and nitrogen. Boron Cutters, grinding tool-steel, punch-press dies, grinding some hardenable stainless steel and for internal grinding of all ferrous metals.