By

Dr. RASHID HASSAN

BDS(de’ Mont) MSc (QMUL.UK)

Hod Science of Dental Materials

Abrasion

• Smoothening of rough surface before polishing.

• The wearing away of a substance or structure through a mechanical process, such as grinding, rubbing or scraping.

• Each point/edge of an abrasive acts as an individual cutting blade.

• Hardness, strength, ductility & thermal conductivity affect the process of abrasion.

• Types of Abrasion:

A two body process.

A three body process.

Desirable Characteristics of an

Abrasive

It should be irregular in shape.

Abrasive should be harder than the worked it abrades.

They should have a high impact strength.

Should have attrition resistance, so that it does not wear.

Abrasion Vs. Cutting

Cutting: – Blades or cutting edges are

regularly arranged. – Removal of the material is

corresponding to this regular arrangement.

Abrasion:

– They have many abrasive points, which are not arranged in an ordered pattern.

– Innumerable random scratches are produced.

Rate of Abrasion

1. Size of the abrasive particle.

• Large particles deeper scratches & Work fast.

• Course abrasive are used on highly rough surface. fallowed by finer course surface.

2. The pressure of the abrasive agent.

• Heavy pressure • Deeper scratches and fast work.

• Less control over abrasion process.

3. Speed at which the abrasive particle moves across the surface being abraded.

• Higher the speed inc rate of abrasion.

Classification on The Basis of Use

1. Finishing Abrasives.

• Are hard, coarse abrasives.

• Used for initially to develop contour and remove gross irregularities.

• e.g. coarse stones.

2. Polishing Abrasives.

• Have finer particle size and are less hard than finishing ones.

• Used for smoothening surfaces after finishing abrasive.

• e.g. polishing cakes, pumice.

3. Cleansing Abrasives. • Are soft materials with

small particle size • Used to remove deposits

that adhere to enamel or restorative material.

Types of Abrasives

1) Emery.

2) Aluminium oxide.

3) Garnet.

4) Pumice.

5) Kieselgurh.

6) Tripoli.

7) Rouge.

8) Tin oxide.

9) Chalk.

10) Chromic oxide.

11) Sand.

12) Carbides.

13) Diamond.

14) Zirconium silicate.

15) Zinc oxide.(mixed with

alcohol to polish dental amalgam)

Types of Abrasives

1. EMERY

• Consists of natural oxide of aluminium called corundum.

• Impurities present (iron oxide) act as an abrasive too.

• Greater the alumina = finer the grade of emery.

2. ALUMINIUM OXIDE

• Prepared from bauxite (impure aluminium oxide)

• Have very fine grain size.

Types of Abrasives

3. GARNET • Composed of minerals having

similar physical properties & crystalline form.

• Silicates of aliminium, cobalt, iron, magnesium & maganese form the mineral.

• Garnet is coated on paper or cloth with glue.

4. PUMICE • Highly siliceous material of

volcanic origin. • Depending on particle size can

be used as both abrasive & polishing agent.

• Use ranges from smoothening dentures to polishing teeth in oral cavity.

Types of Abrasives

5. KIESELGURH • Siliceous remains of minute

aquatic plant (diatoms)

• Coarser form is diatomaceous earth.

• Excellent as mild abrasive & polishing agent.

6. TRIPOLI • Originates from certain porous

rocks first found in North Africa near Tripoli.

• Substitute for kieselgurh.

• Mild abrasive & polishing agent.

Types of Abrasives

7. ROUGE • Used in cake form. • Fine red powder composed of

iron oxide. • Impregnated on paper or cloth

(crocus cloth) • Excellent polishing agent for

gold & noble metal alloys. • Dirty to handle.

8. TIN OXIDE • Putty powder used as a

polishing agent for teeth & metallic restorations in mouth.

• Mixed with water, alcohol & glycerin & used as a paste.

Types of Abrasives

9. CHALK • Calcium carbonate prepared

by precipitation method. • Various physical forms

available for various techniques.

• Used in dentifrices as polishing agent.

10. CHROMIC OXIDE

• Relatively hard abrasive.

• Used as a polishing agent for stainless steel.

Types of Abrasives

11. SAND

• Sand & other forms of quartz are used as abrasive agent.

• In powder form used in sandblasting.

• Use as sand paper is a common example.

12. CARBIDES • Silicone carbide & boron

carbide are used as effective abrasive agents.

• Both silicone & boron are heated at a high temperature for fusion with carbon.

• Most of the stone burs used for cutting tooth structure are made of silicon carbide.

Types of Abrasives

13. DIAMOND

• Hardest & most effective abrasive material.

• Diamond burs & discs are the most popular among dental profession.

14. ZIRCONIUM SILICATE • Occurring in nature as zircon. • Used as a polishing agent in

grounded form. • Used as a constituent in

prophylactic pastes. • Also used as abrasive

impregnated polishing strips & discs.

Polishing

Production of a smooth mirror like surface without much loss of any external form. Polishing agents remove

material from the surface molecules by molecules.

In the process scratches and irregularities are filled in by powder particles.

Non-Abrasive Polishing

1. Composite Glazing:

• A layer of clear transparent liquid (unfilled resin) is applied over the restoration and cured.

2. Glazing ceramic:

• Finished restoration is placed at high temp. surface layer melts and produce a smooth glass-like surface.

Non-Abrasive Polishing Contd..

3. Electrolytic polishing: • Reverse of electroplating.

• Alloy to be polished is made as anode of an electrolytic cell.

• When current passed surface layer of anode is dissolved away bright surface.

Anode

Dr. RASHID HASSAN

BDS(de’ Mont) MSc (QMUL.UK)

Hod Science of Dental Materials

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