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Flashcards in Dental Materials Deck (293)
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241

What are the 6 uses of alumina?

1. Abrasives
2. Filler in cements
3. Reinforcement of restorations
4. Implants
5. Maxillofacial reconstruction
6. Orthopaedics

242

What are the 3 crystalline forms of zirconia?

1. Monoclinic: low temp
2. Tetragonal: med temp
3. Cubic: high temp

243

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

244

What is the transformation toughening of zirconia?

Load induced transformation of tetragonal to monoclinic
3-5% expansion

245

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

246

What are the properties of hydroxyapatite?

Biological HA contains: F-, CO3-, Mg2+, Na+; not homogenous
Ca/P ratio 1.67

247

What are the 5 uses of HA?

1. RC filler
2. Bone filler
3. Tooth root
4. Glass ceramic restorative
5. Bioactive coating

248

What are the 4 uses of porcelains?

1. Artificial teeth
2. Veneers
3. Inlays
4. Crowns and bridges

249

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

250

Describe the components and composition of porcelains

Clay: kaolin
Feldspar: albite, orthoclase
Crystalline quartz

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

251

What are fluxes?

Additives to porcelains that red. fusing temp
Include glass formers: added as carbonates
Boric oxide added as borax

252

Describe boric oxide

Glass former
Added to porcelains to red. fusing temp
Boron anomaly: red. fusing temp w/o inc. thermal expansion

253

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

254

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

255

What are the 5 uses of metals?

1. Partial dentures and clasps
2. Inlays and onlays
3. Direct filling material
4. Orthodontics
5. Crowns and bridges

256

Describe a metallic bond

Metal atoms lost outer electron (valence) to form cations
Lost electrons able to flow around cations in sea of electrons

257

What are the 3 methods in which metals can be made?

1. Casting: crowns, partial dentures
2. Cold working: wires, clasps
3. Amalgamation: amalgam

258

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
Plateau
Further temp. dec. cause solidification w/ time

259

What causes the plateau in a pure metal cooling curve?

Balance between the latent heat of fusion and cooling

260

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

261

Describe the grain boundaries of metals

Ill defined: almost amorphous, random structure
Attract impurities
More reactive

262

Describe grain structure of metals

Fast cool = small size; slow cool = large size
~0.1mm
Key to mechanical performance of metals and alloys
Usually equiaxed
Dependent on conditions at solidification: casting into cold mould

263

@ RT what are the 3 general crystal structures of metals?

1. Body-centred cubic
2. Face-centred cubic
3. Close-packed hexagonal

264

Define wrought alloy

Cast alloys that have been formed by mechanical processes (cold working) e.g. rolling, hammering, forging, drawing

265

Describe wrought alloys

Above yield stress
Grains become elongated resulting in springiness
Under go work (strain) hardening

266

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

267

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

268

Describe the reactivity of metals

Some v reactive: K, react w/ water
Least Au

Ti, Cr: react w/ O2 but form passive oxide layer; thin, impermeable, prevent further oxidation

269

Define chemical and electrochemical corrosion?

Chemical: direct combination of metal and non-metal (including oxidation)

Electrochemical: different metals in an electrolyte (saliva)

270

What does corrosion lead to and what does this cause?

Degradation and release of ions:
- structural breakdown
- migration of ions around body
- cytotoxicity or allergic phenomena
- tissue discolouration