Types of Abrasives | Commonly Used Abrasives

Abrasives are hard crystals or minerals used to polish, smooth, shape, and grind materials. Found both in nature and manufactured artificially, their exceptional hardness allows them to penetrate even the most rigid metals and alloys, making them indispensable in metalworking. Beyond metals, abrasives are equally effective with other hard materials such as stone, glass, and certain types of plastics.

However, their utility extends even to relatively softer materials like wood and rubber. Here, abrasives are valued for their ability to achieve high stock removal rates, provide long-lasting cutting ability, offer precise control, and facilitate fine finishing.

Characteristics of Abrasives

The primary characteristics defining an abrasive's performance are its hardness and controlled toughness/brittleness. Hardness allows the abrasive grain to cut or scratch the workpiece. Abrasives also exhibit controlled brittleness, meaning they predictably fracture under stress, exposing new, sharp cutting edges – a process known as self-sharpening.

Commonly used hard abrasive materials include:

• Alumina (Aluminum Oxide): A versatile and widely used abrasive.
• Carbide (Silicon Carbide, Boron Carbide): Extremely hard and sharp.
• Cubic Boron Nitride (CBN): A superabrasive, second only to diamond in hardness.
• Diamond: The hardest known natural material, a superabrasive.

Other materials, such as garnet, zirconia, glass, and even softer organic materials such as walnut shells or corn cobs, are used for specific, gentler abrasive applications.

Types of Abrasive Materials

Abrasives are broadly categorized by their origin:

1. Natural Abrasives

These are minerals or rocks found within the Earth's crust and used directly or with minimal processing. Examples include:

• Diamonds: The hardest natural abrasive, used for precision cutting and grinding of tough materials.
• Corundum: A natural crystal made of aluminum oxide.
• Garnet: A relatively softer abrasive, suitable for woodworking and specific polishing tasks.
• Quartz (Silica): A common and versatile mineral, often used in sandpapers and blasting media.

2. Artificial / Synthetic Abrasives

This group includes materials manufactured to possess very high and consistent hardness, often surpassing natural counterparts in performance predictability. Examples are:

• Synthetic Diamonds: Man-made diamonds, widely used in industrial applications.
• Silicon Carbide: Tough and sharp, used for grinding and abrasive papers.
• Aluminum Oxide: Produced at high temperatures, offering consistent properties for grinding wheels and coated abrasives.
• Boron Carbide: A rigid ceramic material.
• Various Aluminas: Different forms of manufactured aluminum oxide tailored for specific uses.

Forms and Applications of Abrasive Products

Abrasive materials, whether natural or synthetic, are processed and shaped into various forms to create the tools used across industries. These forms dictate how the abrasive interacts with the workpiece:

1. Bonded Abrasives

These are abrasive grains mixed with a bonding agent (such as vitrified glass, resin, rubber, or metal) and molded into solid shapes.

• Examples: Grinding wheels, cut-off wheels, mounted points, sharpening stones, abrasive sticks.

• Applications: It is used for heavy stock removal, precision grinding, deburring, cutting, and shaping of various materials. The bond wears away to expose new abrasive grains.

2. Coated Abrasives

These consist of abrasive grains bonded with an adhesive onto a flexible backing material.

• Examples: Sandpaper, sanding belts, sanding discs, flap discs, abrasive rolls.

• Applications: It is ideal for sanding, finishing, polishing, blending, and deburring across a wide range of materials, from wood to metal, and offers flexibility to conform to surfaces.

3. Loose/Blasting Abrasives

These are free-flowing abrasive grains used without a fixed backing or bond.

• Examples: Sandblasting media (e.g., garnet, aluminum oxide grit, glass beads), tumbling media (e.g., ceramic or plastic media with abrasive compounds), lapping powders.

• Applications: It is used for surface preparation, cleaning, deburring, rust removal, creating matte finishes, and precision lapping for very tight tolerances.

4. Superabrasive Tools

These are specialized tools that incorporate diamond or Cubic Boron Nitride (CBN) due to their extreme hardness.

• Examples: Diamond grinding wheels, CBN grinding wheels, diamond cutting tools, diamond wire-drawing dies.

• Applications: It is essential for grinding and machining tough materials that other abrasives cannot effectively cut, such as hardened steels, ceramics, carbides, and natural stone.

Commonly Used Abrasive Materials in Detail

Most abrasives utilized in industries today belong to the artificial group, with diamond being the notable exception among natural abrasives for high-performance applications.

1. Diamond:

• Origin: Natural (purest form of carbon, H=10 on Mohs scale) and Synthetic.
• Properties: The hardest known substance, perfectly colorless in its purest form, offering superior lifespan and resistance to wear, with distortion primarily occurring from impacts.
• Applications: Core drilling stones, rock and metal cutters, wire-drawing dies, precision grinding tools, and kit grindstones.
• Natural Sources: They have historically come from regions like the Belgian Congo, South Africa, Angola, Sierra Leone, Brazil, and Venezuela.

2. Carborundum (Silicon Carbide):

• Origin: Artificial/Synthetic.
• Properties: The toughest artificial abrasive after diamond, composed of silicon carbide and manufactured by fusing high-purity sand, coke, and sawdust under high electrical current (e.g., 2482 °C or 4500 °F in an electric arc furnace).
• Applications: Grinding wheels, abrasive papers and cloth, and gear sharpening blocks.

3. Boron Carbide:

• Origin: Artificial/Synthetic.
• Properties: Manufactured by heating boric oxide and coke together at 2600 °C (4712 °F). Its hardness level is similar to silicon carbide.
• Applications: Primarily used as a powdered abrasive for lapping and polishing hard materials, and in wear-resistant components.

4. Corundum:

• Origin: Natural mineral.
• Properties: It is a crystalline form of aluminum oxide (H=9 on the Mohs scale). Perfect, flawless crystals form valuable gems like sapphire and ruby, while imperfect varieties are used as abrasives.
• Applications: Historically used in abrasive papers, grinding wheels, and polishing compounds.

5. Quartz (Silica):

• Origin: Natural mineral.
• Properties: One of the most common natural minerals (H=7 on Mohs scale). Also known as silica (in its purest form).
• Applications: Commonly applied in various fields as pure quartz blocks, quartz sand (sandblasting), sandstone, quartzite, chert, and pumice (all different forms of silica in nature), primarily for less aggressive abrasive tasks.

Conclusion

Abrasives are essential tools across various industries, fundamental to processes like grinding, cutting, polishing, and smoothing. From the natural hardness of diamonds to the engineered precision of synthetic silicon carbide, understanding the different types of abrasive materials and their various forms—whether bonded into grinding wheels, coated onto sandpaper, or used as loose media—is crucial for selecting the right solution for any surface finishing challenge. These versatile materials enable the precise shaping, refining, and preparing of a vast array of materials, transforming raw components into finished products.