metals & alloys

7/31/2019 Metals & Alloys

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ByDr Rashid Hassan

Head of DepartmentScience of Dental Materials Department

W.M.D.CABBOTTABAD

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METAL:

A substance that is a good conductor of heat andelectricity and when polished becomes a good

reflector of light. ALLOY:

A crystalline structure with metallic propertiesthat is composed of two or more chemicalelements atleast one of which is a metal.

METALLOIDS:Few elements that behave like metals andsometimes non metals.

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A microscopic single crystal in a microscopicstructure of a metallic material.

Stable cluster of atoms of a new phase that isformed within a present phase.

A heterogeneous, physically distinct andmechanically separable region of a metalmicrostructure.

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Ability of a material to deform plastically

under a tensile stress before fracture. e.g.metal drawn readily into long thin wires.

The ability of a material to sustain plasticdeformation, without fracture undercompression.


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Gold is the most ductile and malleable puremetal, followed by silver.

Platinum is ranked third in ductility. Copper ranks third in malleability.


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HARDNESS:

In mineralogy, relative hardness of a substance isbased upon its ability to resist scratching.

In metallurgy and mostly in all other disciplines,hardness is defined as resistance to indentation.

Designated as KNOOP HARDNESS NUMBER.

BRINELL HARDNESS NUMBER.

VICKERS HARDNESS NUMBER.

ROCKWELL HARDNESS NUMBER.


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Shapes produced by indentors

On materials

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KNOOP HARDNESS

TEST

BRINELL & ROCKWELLHARDNESS TEST

VICKERSHARDNESS TEST


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All metals are hard and lustrous at roomtemperature except mercury

Many uses in dentistry.1. In the construction of instruments.2. Wires for orthodontics.3. Crowns.4. Inlays.5. Dental implants6. Denture base material.7. Filling material.

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1. CASTING.

2. COLD WORKING.

3.AMALGAMATION.

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CRYSTALLINE IN SOLID STATE. METALS ARE HEATED UPTO THE MOLTEN STATE AND

THEN COOLED. SOLIDIFICATION OCCURS AT SPECIFIC SITES CALLED

NUCLEI. NUCLEI ARE FORMED FROM IMPURITIES. CRYSTALS GROW AND FORM DENDRITES. CRYSTAL GROWTH CONTINUES TLL THE MATERIAL HAS

BECOME SOLID AND ALL DENDRITES COME INCONTACT. EACH CRYSTAL IS KNOWN AS GRAIN AND THE AREA

BETWEEN THE TWO GRAINS IN CONTACT IS CALLEDGRAIN BOUNDRY

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IN CRYSTALLINE STATE, EACH GRAIN HAVEAPPROXIMATELY THE SAME DIMENSION IN

EACH DIRECTION FROM THE CENTRALNUCLEUS. EQUIAXIAL GRAIN STRUCTURE IS

FORMED. ANY CHANGE IN THE CRYSTAL STRUCTURE

CAN CAUSE CHANGES IN MECHANICALPROPERTIES.

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Atoms within a grain are arranged in a regular 3 dimensional lattice.

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NOBLE METALS:

Metals which have high resistance to

oxidation,corrosion & dissolution in organicsolvents. (Au, Pt, Ir, Os, Ag, Pd, Rh, Ru) BASE METALS:

Metals that undergo oxidation & corrosioneasily are base metals. (Fe, Co, Ni, Al, Ti, Cretc)

METALLOIDS: (discussed previously)

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1) METALLIC BONDING:2) FORM + IONS IN SOLUTION:3) GOOD CONDUCTORS OF HEAT &

ELECTRICITY:4) VERY DUCTILE & MALLEABLE:5) OPAQUE:6) WHITE COLOUR:7) HIGH DENSITY:8) HIGH FRACTURE TOUGHNESS:9) HIGH MELTING & BOILING TEMPERATURES:

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Cast dental alloys can be classified accordingto following 5 categories

1) According to use:2) According to major elements:3) According to nobility:4) According to principal 3 elements:5) According to dominant phase system:

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a) All metal inlays.

b) Crowns and bridges.c) Metal ceramic prosthesis.d) Posts and cores.e) Removable partial dentures.f) Implants.

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a) Gold based.

b) Palladium based.c) Silver based.d) Nickel based.e) Cobalt based.f) Titanium based.

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a) High noble.

b) Noble.c) Predominantly base metal.

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a) Au-Pd-Ag.

b) Pd-Ag-Sn.c) Ni-Cr-Be.d) C0-Cr-Mo.e) Ti-Al-V.f) Fe-Ni-Cr.

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a) Single phase (isomorphous)

b) Eutectic.c) Peritectic.d) Intermetallic.

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Divided into 2 types

Noble metal casting alloys

e.g. Gold alloys.Base metal casting alloyse.g. Cobalt chromium, Nickel

chromium alloys.

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GOLD ALLOYS Gold is alloyed with other metals (silver,

copper, palladium). Alloying improves

Surface hardness.

Strength.

Low melting point.

Homogeneous casting by finer grain size.

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PROPORTIONING OF GOLD(CARATAGE or FINENESS OF GOLD ALLOYS)Carat and fineness numbers donate the

proportion of gold in an alloy.CARAT GRADING:Carat number of an alloy shows how many 24thof metal by weight is gold.18 carat gold means18 out of 24 parts is gold.FINENESS GRADING:Fineness number for an alloy is 10 times thepercentage of gold within it. E.g. 75% gold alloyis 750 fine.

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Two types of heat treatments.

SOFTENING HEAT TREATMENT:

HARDENING HEAT TREATMENT:

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SOFTENING HEAT TREATMENT

Alloy is placed in an electric furnace for

10 min at a temperature of 700 C. Quenching in water is done next.

Ductility increases.

Tensile strength and hardness aredecreased.

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HARDENING HEAT TREATMENT(AGE HARDENING)

Done in 3 ways:

1) Heating alloy at a specific temp for a definitetime before quenching in water. Aging temp isusually 200-450 C. Aging time is 10-15 min.

2) Heating the alloy at 700 C and allowed to cool

slowly and gradually at room temperature.Crystal structure transforms properly by thismethod.

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3) Alloy is allowed to cool in a furnace

from 450-250 C over 30 min. Then

quenching is done in water.Hardness is increased but alloy

becomes brittle.

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According to ISO gold alloys are classified as Type 1: Low strength - For castings subjected

to very low stresses (inlays), minimum yieldstrength of 80 Mpa and minimum percentelongation is 18%

Type 2: Medium strength - For castings

subjected to moderate stresses (inlays,onlays & full crowns), minimum yieldstrength of 190 Mpa & minimum percentelongation is 10%

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Type 3: High strength - For castings subjected tohigh stresses (onlays, thin copings, pontics,crowns and saddles), minimum yield strength is270 Mpa and minimum percent elongation is 5%

Type 4: Extra high strength - For castingssubjected to very high stresses (saddles, bars,

clasps, certain single units and cast partialdenture frameworks), minimum yield strength360 Mpa & minimum percent elongation is 3%

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METAL CERAMIC ALLOYS A group of noble metal alloys that bind with

the porcelain.

Large variation in chemical composition yetfew common properties.Potential to bind with dental porcelain.Coefficient of thermal expansion similar to

dental porcelain.High melting temperature to allow application

and fusion of low fusing porcelain. Indium, tin and iron present in these alloys

provide oxide film.

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GOLD Major component in

gold alloys. Provides tarnish &

corrosion resistance. Adds to colour and

ease of casting.

Contributes toductility.

Provides strength anddensity.

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COPPER Increases strength

and hardness of

alloy. Improves ductility. In excessive amount

it reddens the alloy.

If more than 20%copper tarnish andcorrosion resistanceis lowered.

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ZINC Prevents oxygen

absorption. Lower the melting range of

alloy.SILVER

Tends to whiten the alloy. Improves ductility. Increases hardness and

strength. Reduces tarnish resistance. Gives green colour to

porcelain when used withmetal ceramic alloys.

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PLATINUM Effective hardener of gold. Raises the melting point of

alloy.

Increases corrosionresistance. Whitens the alloy. Replaced by palladium due to

high cost.PALLADIUM

Increases tarnish resistanceof silver. 1% palladium needed for 3%

silver. Reduces the density of alloy Whitens the alloy.

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INDIUM, TIN & IRON Harden the metal ceramic

palladium alloys.

Iron is most effectivehardener.

Ions of indium, tin and irondiffuse to the surface of

alloy during firing ofporcelain and adds achemical bond to themetal ceramic bonding.

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GALLIUM Compensates for the

decreased coefficient ofthermal expansion that

occurs due to lack of silverin metal ceramic alloy.

Fine grain size in noblemetal crown bridgealloys is obtained by

adding small amountsof IRIDIUMRUTHENIUM orRHENIUM

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Denture base metal alloysmust have

High modulus ofelasticity.

High elastic limit.CLASSIFICATION OFBASE METAL ALLOYS

Cast Co-Cr alloys:used for making

Partial dentures.Crown and bridges.Removable metallic

restorations.

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Cast Ni-Cr alloys:Used for making Partial dentures.Crowns and bridges.

Removable metallicrestorations.

Stainless steel(wrought alloys)

Used for making Endodontic instruments.Orthodontic bands and

brackets. Preformed crowns.

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Cobalt chromiumnickel alloysFor makingorthodontic wires.

Nickel titanium alloysFor makingorthodontic wires.

Beta titanium alloys

For makingorthodontic wires.

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INTRODUCTION Pure gold was very popular as a filling

material prior to amalgam. Introduced in U.S.A by ROBERT

WOOFENDALE in 1766 on his arrival fromENGLAND.

However, its use became widespread in U.S.Aonly towards the beginning of 19th century.

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Gold is most noble of all metals. Rarely tarnishes in oral cavity.

In pure form, gold is very soft (25 BHN) The properties of ductility, malleability & lack

of oxide layer formation allows it to be usedas a direct filling material.

Currently, direct filling gold is not used aswidely as it was used in the past.

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INDICATIONS

1) Pits & small class I

restorations.2) For repair of casting

margins.

3) For class II, class V &

class VI restorations.4) Repairing of

perforated gold

crowns

CONTRAINDICATIONS

1) Teeth with very large

pulp chambers.2) Periodontically

weakened teeth.

3) Handicapped

patients.4) Root treated teeth.

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Direct filling gold is based upon its physical form & manufacturingprocess.

1) Foil (Fibrous gold) Sheet Cohesive. Non cohesive.

Rope. Cylinder. Laminates. Platinized.2) Electrolytic precipitate (Crystalline gold) Mat. Mat foil. Gold-calcium alloy.

3) Granulated gold (Encapsulated powder)

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There are 3 processes involved

1) Desorbing or Degassing.

Electric annealing. Flame desorption.

2) Compaction. Hand mallet.

Pneumatic vibratory condensers.

Electrically driven condensers.3) Finishing.

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1) Strength.2) Hardness.

3) Density.

4) Effect of voids.5) Tarnish & corrosion.

6) Biocompatibility.

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Tarnish & corrosion

resistant. Good mechanical

properties.

Goodbiocompatibility.

Poor aesthetics.

High coefficient ofthermal expansion.

Difficult to

manipulate.

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metals & alloys

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