Abrasives

Abrasives:

An abrasive is a hard ceramic material that is used to abrade or wear away the surface of another material when the proper movement and pressure are applied.

Classification of abrasives:

Abrasives are classified into three types. They are as listed below:

• Natural abrasives
• Manufactured or synthetic abrasives
• Super abrasives

Natural abrasives: Natural abrasives are obtained from natural resources. Then it can be mined as well as processed for usage with small alteration. It includes flint, garnet, emery, crocus, Tripoli, diatomaeceous earth, pumice, sand, quartz etc.

Synthetic abrasives: Synthetic abrasives are produced by manmade which have well defined and controlled properties such as hardness, toughness and structure type. It includes Aluminum oxide, Silicon Carbide.

Super abrasives: Super abrasives are the abrasives having superior performance, longer life and extraordinary hardness. It includes Cubic Boron Nitride and Diamond

Properties:

As abrasives are chemically inert, they don’t react with material being abraded. Here we will see the different physical properties of abrasives.

• Hardness: Resistance of a material to a mechanical force. In terms of Mohs scale hardness, it is defined as resistance to being scratched
• Toughness: Resistance to fracture (Not able to cut easily)
• Fracture or break: Able to break with sharp edges
• Friable: Ability to crumble into small pieces

Mineral name	Mohs hardness
Talc	1
Gypsum	2
Calcite	3
Fluorite	4
Apatite	5
Feldspar	6
Quartz	7
Topaz	8
Corundum	9
Diamond	10

 

Major product divisions in abrasives:

In abrasives, there are three major product divisions and they are as follows:

• Loose abrasives
• Coated abrasives
• Bonded abrasives

Loose abrasives:

Loose abrasives are the abrasives which contributes no connecting structure between the grains. They are used for cleaning, tumbling, sandblasting, grinding/polishing and barrel finishing. The main use of sandblasting is to cleaning the spark plugs. For grinding/polishing, natural gems, glass products and dimension stone are some of the materials used. Natural gems are tumbled and parts of metal are barrel finished.

Coated abrasives:

Coated abrasives are the abrasives in which the abrasive particles are distributed on the surface of the backing material with the help of gums (polymeric resins). The types of coated abrasive products are as follows:

• Sheets
• Strips
• Belts
• Discs  
• Drums  

These coated abrasive products are used by hand, stationery machines and portable machines. The products of coated abrasives are used on different materials which include ceramic, wood, metal, plaster and leather. Pencil pointers and fingernail boards are the special uses of coated abrasives.

Bonded abrasives:

Bonded abrasives (bonding between the grains) are the abrasives in which the grains are held together in a matrix with the help of binders.  The six types of bonded abrasives are as follows:

• Vitrified bond (V)
• Silicate bond (S)
• Rubber bond (R)
• Shellac bond (E)
• Resinoid bond (B)
• Oxychloride bond (O)

Vitrified bond: Made up of feldspar and clays due to their fusibility. This bond is having good strength and not able to agglomerate easily. It is commonly used for sharpening.

Silicate bond: It is used wherever high finish is required. It is the weakest bond when compared with other types of bond. It is used in mild cutting operations since, this bond is damaged easily.

Rubber bond: It is the newest bond among the other bonds. It is made up of rubber and it is the strongest bond among other bonds. Due to its small amount of flexibility, this bond is used for thin wheels.   

Shellac bond: It is used in operations where thin wheels are required as it possess higher degree of safety. It is used to impart fine finish to camshafts, cutlery and rolls.

Resinoid bond: Made up of synthetic resins and plasticizer incorporated with abrasive grains. It possesses more shock resistance than the vitrified bond.

Oxychloride bond: Made up of abrasive grains mixed with oxides and chlorides of magnesium. This bond is used in disc grinding operation for making wheels and wheel segments.

Materials used in abrasives:

A material plays an essential role in producing a new product. To make an abrasive product, materials used are given below:

• Grain
• Bond
• Backing
• Reinforcement

Grain

Grain denotes the type of abrasive material. The abrasive grains composed of natural grains as well as synthetic grains. The most commonly used natural abrasive grains are: Flint, Garnet and Emery. For synthetic abrasive grains: Silicon Carbide and Aluminium Oxide.  The natural and synthetic abrasive grains are listed below:

Name of the grain	Composition
Flint	SiO2
Garnet	3FeO.Al2O3.SiO2
Emery	Al2O3
Corundum	Al2O3
Diamond	C
Pumice	Lava
Rottenstone	CaCO3
Tripoli	CaCO3
Tin oxide	SnO2
Cerium oxide	CeO2

Natural abrasive grains

Name of the grain	Composition
Silicon Carbide	SiC
Aluminium Oxide	Al2O3
Crocus	FeO
Rouge	FeO
Diamond	C

Synthetic abrasive grains

Bauxite, coke and fillings of iron are the grain materials used to make Aluminum oxide. For making Silicon Carbide, silica sand, coke, salt and saw dust are the grain materials used.

Bond

A bond is defined as the material which sticks or adheres the abrasive particles together/backing material. In coated abrasives products as well as bonded abrasives products, bonds are used. To create a bond between abrasive particles and the backing material, glues and synthetic resins are used.

In bonded abrasives products, both vitreous and non vitreous bonds are used. There are two types in bonded abrasive products. They are Vitreous and Nonvitreous.

Vitreous bonded products: The two basic shapes were made in vitreous bonded products are wheels and slipstones. For grinding, thicker wheels are used and for cutting, thinner wheels are used. The wheels are made in different types of sizes, shapes and hardness. The slipstones are used for shaping and hand grinding. Cylindrical, disc, rectangular are the shapes available in slipstones. The vitreous bonded products are used for grinding, sharpening and cutting purposes. During firing process, a vitreous material generates a glassy bond. For this purpose, flint, clay, feldspars and frits are used.

Nonvitreous bonded products: The two basic shapes were made in nonvitreous bonded products are wheels and discs. The discs are usually thicker than the wheels and the edge of thin wheels is used for cutting. The nonvitreous bonded products are used for cutting and grinding purposes. Sodium silicate, shellac, synthetic resins and compounds of rubber are used for nonvitreous bonds.  

Backing

The main purpose of backing is to provide stability for the coated abrasive products. Only the coated abrasive products are in need of a backing material. The backing materials are: Paper, cloth, fiber, combination of paper, cloth and fiber, wood, nylon, metal and plastic. Of these, the most commonly used backing materials are: Paper, cloth, fiber, combination of paper, cloth and fiber.

Reinforcement

Reinforcement is a material which can provide strength to the nonvitreous bonded products. In nonvitreous bonded products, reinforcement materials are used. The typical reinforcement materials are: Nylon, burlap fabric and fiberglass.