dental casting alloys

what are noble metals?

metals with a good metallic surface and retain their surfaces in dry air. they have good resistance to oxidation, tarnish and corrosion during heating, casting, soldering or when used in the mouth

what are base metals?

common metals that are not considered precious, like copper, tin or zinc. they typically have poor tarnish or corrosion resistance

what are some base metals capable of forming?

a passive oxide layer that protects it from corrosion, which is found in metals such as chromium and titanium

what is the composition of noble metal alloys?

noble metal ingredients make up 25% or more of the total weight, with the remaining amount being base metal ingredients

what is the composition of base metal alloys?

base metal ingredients make up more than 75% of the total weight, with the remaining amount being noble metal ingredients

what characteristics of pure gold makes it undesirable for cast dental restorations and appliances?

it is soft and ductile, so it is not used for that reason

how is gold classified based on the gold content?

it can either be classified based on:

1. carat specification: the number of the parts of pure gold in 24 parts of alloy

2. fineness: number of parts per thousand of pure gold in the alloy (pure gold is 1000 fine)

how is gold classified based on the color?

1. yellow gold, which is yellow due its high gold content, which is greater than 60% (>60%)

2. white gold, which appears white with gold content (>50%) modified by alloying metals

how is gold classified based on the gold proportioning?

1. conventional gold: greater than 50% (>50%)

2. low gold (economic): less than 50% (<50%)

how is gold classified based on the function?

1. type I (soft): small inlays, easily burnished and is subjected to a very slight amount of stress, such as in gingival and interproximal inlays

2. type II (medium): inlays subjected to a moderate amount of stress such as thick ¾ crowns, and full crowns

3. type III (hard): inlays subjected to high stresses, such as thin ¾ crowns, full crowns and short span bridges

4. type IV (extra hard): inlays subjected to very amounts of stress, like partial denture frameworks and long span bridges (partial denture type)

what other precious metal alloys are used for inlays, crowns and bridges?

1. silver-palladium alloys, which has comparable properties to type III and IV gold alloys

2. palladium-silver alloys

what problems is there with alternative alloys to gold?

• a lower density thus requiring a greater centrifugal force for casting

• low burnishability

• their tendency to absorb gases during melting and casting leads to porosity

• with all the problems listed, it requires specialized equipment for melting and casting, making it technique sensitive

what are the 3 types of base metal alloys?

1. cobalt-chromium alloys, introduced in 1928

2. nickel-chromium alloys, introduced after cobalt-chromium

3. titanium alloys, a more recent discovery

they were introduced to dentistry to substitute gold alloys type III and IV

what are the advantages of base metal alloys?

• increased strength

• low price

• high tarnish and corrosion resistance

what are the disadvantages of base metal alloys?

• increased melting temperature

• increased hardness

• decreased density

• technique sensitive

what is the resistance to tarnish and corrosion like for type III and IV gold alloys?

excellent due to the presence of noble metals, which have high electrode potential

what is the resistance to tarnish and corrosion like for base metal alloys (CoCr, NiCr, Ti + Ti alloys)?

excellent due to the presence of passive oxide layer, which is caused by Cr₂O₃ in CoCr and NiCr alloys and TiO₂ in commercially pure Ti alloys.

what will Titanium do?

it will repassivate in nanoseconds if the passive layer is scratched or damaged

why is the melting range in base metal alloys decreased?

because of certain alloying elements, such as beryllium (Be), lower the melting temperature of the alloy.

how does density affect alloys?

it affects alloy castability. high density alloys will flow faster into the mold forming complete casting more easily than low density alloys would, like base metal alloys, making them technique sensitive

how does casting shrinkage affect the dental alloys?

thermal shrinkage affects the selection of the investment, which will expand to compensate for it

what investment are selected for gold alloys type III and IV?

gypsum bonded investment (1.6%)

what investment is selected for gold alloys type III and IV?

gypsum bonded investment

what investment is selected for base metal alloys (CoCr, NiCr, Ti + Ti alloys)?

silicate or phosphate bonded investment

what does the modulus of elasticity affect in dental alloys?

the thickness of the restoration.

how does the modulus of elasticity affect gold alloys type III and IV?

they have a lower modulus of elasticity, meaning they are more flexible and less stiff, causing it to require thicker sections to achieve the same rigidity that a base metal can provide in thinner sections

how does the modulus of elasticity affect base metal alloys?

they have a higher modulus of elasticity, making them stiff and less flexible, meaning they can be applied in thin sections while still resisting deformation

what is the elongation percentage and hardness of gold alloys type 3 and 4?

8-20% elongation % and 250VHN. they are easy to finish and polish but they retain surface finish for a shorter time

what is the elongation percentage and hardness of base metal alloys?

1-2% elongation % and 350 VHN, 1/3 greater than gold alloys. they are difficult to finish and polish, but retain surface finish for longer

why are gold alloys tougher than base metal alloys, even though base metal alloys are stronger?

toughness is based on strength and ductility, because even though they are weaker than base metal alloys, they are significantly more ductile than them, meaning they can bend and deform without breaking, making them tougher overall, because they absorb more energy before fracturing

is casting difficult for gold alloys type 3 and 4?

no, because generally, gold alloys are not technique sensitive for casting

is casting difficult for base metal alloys?

yes, because they are technique sensitive due to:

• decreased density, making it difficult to cast

• increased melting temperature, making it difficult to cast

• increased hardness, making it difficult to finish and polish

why should casting of Titanium alloys be done in a well-controlled vacuum?

to avoid:

• formation of thick, oxygen enriched, hard surface oxide layer, which decreases ductility

• nitrogen incorporation in the alloy. if more than 0.1 nitrogen is included, a brittle casting is obtained

what is the investment used for in gold alloys type 3 and 4?

gypsum bonded investment or phosphate bonded investment mixed with water.

what is the casting ring used for in gold alloys type 3 and 4?

it is a metallic ring which supports the investment during casting

what is the type of melting crucible used for in gold alloys type 3 and 4?

carbon or ceramic crucible

what is the casting machine used for gold alloys type 3 and 4?

air pressure casting machine

what is the casting machine used for base metal alloys?

for CoCr & NiCr alloys:
- centrifugal casting machines

a more recent on is spark erosion, where a series of sparks are released to shape a piece of titanium

what are some health hazards of base metal alloys?

1. beryllium is carcinogenic and toxic to the technician, as inhalation of it can lead to fibrosis of the lungs. beryllium-free alloys were developed to combat this

2. nickel is a common allergen

how can uses of dental alloys be divided?

based on casts:

1. fixed partial denture

2. crown

3. removable partial denture, etc

based on wroughts

1. dental wires

2. instruments

3. orthodontic brackets

4. stainless steel crowns, etc

what are wrought stainless steel alloys?

the steel part is alloy of iron and carbon (I.S.S). the stainless steel part are alloys of iron and carbon that contain chromium (>11%) nickel, manganese, and other metals. they are used in orthodontic appliances, endodontic instruments, stainless steek crowns, denture clasps, etc.

why must the chromium amount be between 12-28% in stainless steel alloys?

for optimal corrosion resistance.

Cr<12%: not enough to form the chromium oxide passive layer

Cr>28%: formation of chromium-carbides (δ phase), causing embrittlement of the steel

why does chromium resist corrosion so well?

because of the formation of a chromium oxide layer on the surface of the metal, known as a passive layer, which prevents the further reaction with the metal below the surface. it is characterized as being thin, strongly adherent, impervious, lustrous and not visible even at high magnifications

what is sensitization?

also known as weld decay. if stainless steel alloys are overheated to around 650 °C, known as the critical temperature, chromium will react with carbon, forming chromium carbides which precipitate at the grain boundaries, causing embrittlement of the alloy and loss of passivity, so the alloy will undergo corrosion and tarnish