Metals, alloys and casting

Question 1

All metals are crystalline. What does crystallinity mean?

Answer 1

Metals have regularity, a repeating pattern

Question 2

How many different space lattices can a metal form and which is the most common?

Answer 2

14 space lattices

Simple cubic is most common

Question 3

Properties of metals

Answer 3

• Strong bonds (high MP +BP)
• Good stiffness (modulus of elasticity)
• Ductile (Can be pulled into wires)
• Malleable (can be beaten into sheets)
• Conduct heat and electricity
• Opaque (non-aesthetic)

Question 4

What is an alloy?

Answer 4

An alloy is two or more metals that are mutually soluble in each other in a molten state. Formation of alloys results in enhanced properties.

Question 5

What do alloys of a fine grain structure have?

Answer 5

The least permanent deformation during service e.g. higher yield stress values

Question 6

Properties of alloys that are important

Answer 6

• Colour: aesthetics
• Grain size: size of crystals of alloy influenced by cooling rate heat and determines properties like tensile strength
• Phase structure: level of mixing
• Corrosion
• Biocompatibility
• Thermal properties: melting range, thermal expansion
• Strength
• Economic - cost
• Porcelain bonding

Question 7

Precious metal

Answer 7

Containing metals of high economic value such as gold, platinum, palladium, silver
How expensive they are

Question 8

Noble metal

Answer 8

A precious metal that is resistant to tarnish. This excludes “silver” by definition
Chemical property

Question 9

Low gold alloys

Answer 9

Alloys containing <75% gold (less than 50 a/o gold) which means that gold atoms represent less than every other atom

Question 10

Gold-substitute alloys

Answer 10

Precious alloys that do not contain gold

Question 11

Base-metal alloys

Answer 11

Alloys not containing precious metals to impart their corrosion

Question 12

How are classes of alloys classified according to ADA 2003?

Answer 12

• High noble alloys (40% wt Gold, 60% wt noble)
• Titanium and titanium alloys (85% wt Ti) for implants
• Noble alloys (25% wt e.g. pallidium)
• Base metal alloys (<25% wt noble e.g. nickel, chromium)

Question 13

How are high noble alloys (Gold-Au) classified?

Answer 13

Types 1-4 from soft to extra hard. The less gold, the stronger the alloy will be. From type 1-4 the ductility, elongation at break and corrosion will decrease whilst hardness, tensile strength, proportional limit and modulus of elasticity will increase.

Question 14

How can gold be measured?

Answer 14

Carat value (24 is maximum carat gold)
Fineness value (value goes up to 1000 and is more valuable than carat to work out % of gold)

Question 15

Uses of type 1-4 of high noble alloys

Answer 15

Type 1 (non-heat hardenable): inlay, single surface
Type 2 (non-heat hardenable): inlay, onlay (no thin sections)
Type 3 (heat hardenable): onlay, crown (potential for fracture)
Type 4 (heat hardenable): crown, bridge

Question 16

Use of gold in alloy

Answer 16

Corrosion resistance

Question 17

Use of copper in alloy

Answer 17

Hardness

Question 18

Use of silver in alloy

Answer 18

Counteract reddish colour of copper

Question 19

Use of pallidium in alloy

Answer 19

Increase MP and hardness

Question 20

Use of platinum in alloy

Answer 20

Increase MP (harder than Pd)

Question 21

Use of zinc in alloy

Answer 21

Prevents oxidation during melting (O2 getter)

Question 22

What are the 2 types o f corrosion?

Answer 22

Chemical corrosion = reaction of metal with oxygen
Electrochemical corrosion: need anode, cathode, circuit and electrolyte. Different electrochemical corrosion categories: galvanic corrosion, local galvanic corrosion, concentration cell corrosion and stress concentration

Question 23

How are noble alloys different from high noble alloys?

Answer 23

Noble alloys have higher hardness and yield strength compared to high noble but lower melting range.
Pd-Cu-Ag very strong - used for cast and are 25% stiffer than any high noble alloy.

Question 24

Base metal alloys

Answer 24

Consist of Ni-Cr and Co-Cr alloys

80% of RPD are from Co-Cr alloys

Question 25

Composition of base metal alloys

Answer 25

Cobalt 35-65%, Chromium 11-26% Nickel 0-30% Molybdenum little amount

Question 26

What are the properties of each components of base metal alloys?

Answer 26

Co and Ni are simialr and result in hard alloys. Ni slightly enhances ductility [percent elongation] Mo provides strength and hardness to Co-Cr alloys Cr gives corrosion resistance but it fails in high concentration of Cl-

Question 27

Advantages of base metal alloys

Answer 27

+ High modulus of elasticity (they can be used in thin section) + Hardness + strength + low density

Question 28

How are base metal alloys classified for fixed restorations?

Answer 28

Type 1: low strength applications e.g. inlays Type 2: medium strength e.g. large inlays, onlays, crowns Type 3: high strength e.g bridge pontics Type 4: extra high strength for very high stress applications e.g. long span bridges and implant superstructures

Question 29

What is biocompatibility?

Answer 29

Ability of a material to perform with an appropriate host response in a specific situation

Question 30

Titanium alloy composition

Answer 30

Ti6Al4V Ti 6% Aluminium and 4% vanadium with coating hydroxyapatite Al is a stabiliser and V is B stabiliser

Question 31

Why titanium?

Answer 31

Lightweight, biocompatible, corrosion resistant (dynamic inert oxide layer), strong and low priced

Question 32

Titanium grades

Answer 32

Grades 1-4 from yeild strength 25,000 --> 70,000

Question 33

Why is implant dentistry becoming more common?

Answer 33

- Demographics and increasing wealth: ageing population with increasing wealth - Growing pt awareness of benefits of implants over conventional solutions, increasing importance of estehtics - Increasing acceptance of implants: long term clinical success of implant treatment - Rising number of trained dentists

Question 34

Different types of implant materials

Answer 34

Titanium Zirconia (aesthetics, no allergies, more holistic, biofilm no plaque accumulation, periimplantitis, chipping of crown) TiZr (stronger than pure titanium and has excellent osteointegration properties)

Question 35

Lost wax casting technique

Answer 35

A wax pattern is heated, vaporised and subsequently replaced by a metal. 1. impression 2. Die production (positive replica) 3. Waxing on die (Wax restoration) 4. Wax investment (sprue) 5. Investing 6. Wax burnout 7. casting

Question 36

Porcelain-Fused to metal: metal important requirements

Answer 36

Metal must have the potential to bond to the dental porcelain, need oxide forming elements; possess coefficient thermal contraction compatible with dental porcelains; sufficiently high solidus temp to permit application of low-fusing porcelains

Question 37

How does porcelain bond to metal?

Answer 37

- Physical bonding nil - Mechanical bonding: extensive - surface roughness from sandblasting determines degree of mechanical inter-digitation - Chemical bonding: occurs by oxide mixing

Question 38

What are the mechanical failures for porcelain-to-metal fracture and porcelain fractures?

Answer 38

Porcelain-to-metal fracture 1. Fractures originating at the metal surface: surface metal contamination, incomplete degassing, under fired opaque porcelain, improper metal thickness, incorrect metal conditioner, reused metal alloy. 2. Fracture at the opaque and entering the bulk porcelain: porosity at the opaque layer, cracks at the opaque layer, incomplete opaque bonding from firing at too low temp. Porcelain fractures 1. Design or procedural errors: too little bulk of metal, sharp edges of porcelain, improper margin design 2. Malocclusion or impact stresses 3. Thermal contraction incompatibility: built in stresses generate cracks at pores, thermal fatigue propagates cracks

Question 39

Repair systems for porcelain to metal fractures

Answer 39

1. Silane and acrylic e.g. fusion, George Taub products 2. silane + composite e.g. PULPDENT porcelain repair kit 3. Silane and composite e.g. MIRAGE PFM Repair