Flashcards in Dental Materials Deck (293)
What are the 6 uses of alumina?
2. Filler in cements
3. Reinforcement of restorations
5. Maxillofacial reconstruction
What are the 3 crystalline forms of zirconia?
1. Monoclinic: low temp
2. Tetragonal: med temp
3. Cubic: high temp
What are the properties of zirconia?
1. Elastic modulus: GPa < alumina
2. Flexural toughness: MPa > alumina
3. Fracture toughness: 6-13 > alumina
As chemically stable as alumina
What is the transformation toughening of zirconia?
Load induced transformation of tetragonal to monoclinic
What are the uses of zirconia?
Similar to alumina
Additives (CaO, MgO, Y2O3) stabilise in either tetragonal or cubic forms
Y2O3 partially stabilised used as high strength core for crown and bridge
What are the properties of hydroxyapatite?
Biological HA contains: F-, CO3-, Mg2+, Na+; not homogenous
Ca/P ratio 1.67
What are the 5 uses of HA?
1. RC filler
2. Bone filler
3. Tooth root
4. Glass ceramic restorative
5. Bioactive coating
What are the 4 uses of porcelains?
1. Artificial teeth
4. Crowns and bridges
What are the 5 advantages of porcelains?
1. Excellent aesthetics
2. Relatively inert
3. Low thermal expansion coefficient: similar to tooth
4. High MP
5. High elastic modulus
Describe the components and composition of porcelains
Feldspar: albite, orthoclase
Modern porcelains mainly feldspar and quartz
Kaolin (4%) only in high temp fusing type
Ratio soda to potash: high K red. fusing temp but less effect on viscosity than Na
What are fluxes?
Additives to porcelains that red. fusing temp
Include glass formers: added as carbonates
Boric oxide added as borax
Describe boric oxide
Added to porcelains to red. fusing temp
Boron anomaly: red. fusing temp w/o inc. thermal expansion
What are some aesthetic additives for porcelains?
Metal oxides for colour, opacity, fluorescence
Cobalt for blue
Chrome/tin for pink
Titanium/zirconium for opacity
Terbium/europium/cerium for fluorescence
What are the 3 porcelains that may be required for an aesthetic restoration?
1. Core/opaque: mask cement interface or metal alloy core
2. Body/gingival dentine: bulk colour build up
3. Enamel: highly translucent
What are the 5 uses of metals?
1. Partial dentures and clasps
2. Inlays and onlays
3. Direct filling material
5. Crowns and bridges
Describe a metallic bond
Metal atoms lost outer electron (valence) to form cations
Lost electrons able to flow around cations in sea of electrons
What are the 3 methods in which metals can be made?
1. Casting: crowns, partial dentures
2. Cold working: wires, clasps
3. Amalgamation: amalgam
Describe a pure metal cooling curve
High temp, no time: liquid state
W/ time and temp dec. ~50% solidification begins, in liquid and solid state
Further temp. dec. cause solidification w/ time
What causes the plateau in a pure metal cooling curve?
Balance between the latent heat of fusion and cooling
Describe the 5 stages of solidification of a metal
1. Small nuclei act as centres for crystal growth
2. Small dendrites grow from nuclei
3. Dendrites continue to grow
4. Space between dendrites fill in
5. Complete: little evidence of dendrite structure remains
Describe the grain boundaries of metals
Ill defined: almost amorphous, random structure
Describe grain structure of metals
Fast cool = small size; slow cool = large size
Key to mechanical performance of metals and alloys
Dependent on conditions at solidification: casting into cold mould
@ RT what are the 3 general crystal structures of metals?
1. Body-centred cubic
2. Face-centred cubic
3. Close-packed hexagonal
Define wrought alloy
Cast alloys that have been formed by mechanical processes (cold working) e.g. rolling, hammering, forging, drawing
Describe wrought alloys
Above yield stress
Grains become elongated resulting in springiness
Under go work (strain) hardening
Describe the general properties of metals
1. All polycrystalline
2. Good strength, high elastic modulus (80-200 GPa)
3. Good conductors heat and electricity
4. Lustre: shiny if polished
- 3 4 result of metallic bonding
5. Some (Au) resist corrosion
6. Alloys: better, more controlled mechanical properties
Describe the biocompatibility of metals
All metals are potentially toxic but some are essential
Toxicity is conc. dependent
Essential: Cu, Mg, Ni, Zn
Non-essential: Ag, Au, Li, Pb, Hg, Sn
Describe the reactivity of metals
Some v reactive: K, react w/ water
Ti, Cr: react w/ O2 but form passive oxide layer; thin, impermeable, prevent further oxidation
Define chemical and electrochemical corrosion?
Chemical: direct combination of metal and non-metal (including oxidation)
Electrochemical: different metals in an electrolyte (saliva)