Dental Ceramics Flashcards
1
Q
Ceramics
A
- inorganic, non-metallic
- frequently a compound formed by combination of a metallic and non-metallic element
- oxides, borides, carbides, nitrides
- minerals
2
Q
Applications of Ceramics in Dentistry
A
- ceramics for metal crowns and fixed partial dentures
- ceramic inlays, onlays, veneers, abutments, crowns, FPDs
- ceramic denture teeth
- ceramic orthodontic brackets
3
Q
Ceramics: low/high melting point
A
high
4
Q
Ceramics: low/high thermal and electrical conductivity
A
low
5
Q
Ceramics: low/high compressive strength and stiffness
A
high
6
Q
Ceramics: brittle/non-brittle
A
brittle
7
Q
Ceramics: inert/non-inert
A
inert
8
Q
Ceramics: insoluble/insoluble
A
insoluble
9
Q
Ceramics: ionic/covalent
A
mostly ionic, both include both
10
Q
Ceramics: crystalline/amorphous
A
both
11
Q
Ceramics: stronger compression/tension
A
compression
12
Q
What controls tensile strength?
A
size of cracks or defects
13
Q
Flaws in ceramics
A
- fabrication defects: includsions and voids during sintering
- surface cracks: during machining and grinding
14
Q
Surface smoothing
A
- strengthens ceramics
- glaze/polish to remove or reduce size/number of surface flaws
- glazed porcelain is stronger in flexural strength than unglazed (bridges surface flaws, prevents crack propagation)
- porcelain with highly polished surface has comparable strength to glazed
- glazed/polished surfaces=less abrasive
15
Q
Dispersion Strengthening
A
- strengthens ceramics
- crystal phase is added to a glassy phase
- added crystals help block cracks from growing
- adding dispersion strengthening makes it tougher for crack to get through=requires more energy
16
Q
Residual surface/compressive stresses
A
- cracks grow in tension but not in compression
- methods
* *mismatch of coefficient of thermal expansion
* *thermal tempering
* *ion exchange (place ceramic in molten solution bath–>sub larger ions for sodium ions–>exerts compressive around it on surface)
17
Q
Zirconia-based All Ceramic Restorations
A
- increased fracture toughness compared to other dental ceramics
- Yttrium oxide stabilizes tetragonal structure
* *normally a monoclinic phase is stable at room temperature
18
Q
Transformation toughening
A
- tetragonal to monoclinic phase transformation in response to increase stress (occurs at tip of a crack))
- phase change–>increased accompanied volume of crystals–>residual compressive stress
- increased fracture toughness due to crack
19
Q
Abrasion
A
- ceramic restorations may wear opposing enamel
- one factor is hardness
* *porcelain>enamel>dentin>posterior composite resin
20
Q
Metal-ceramic restorations
A
- introduced 1950s-60s
- AKA porcelain-fused-to metal, ceramo metal, ceramic-metal, porcelain bonded to metal
- metal substructure and a porcelain veneer
- provides a strong foundation (metal) with good esthetics (porcelain)
21
Q
Layers of metal-ceramic restorations
A
- metal substructure
- oxide layer
- opaque porcelain layer
- dentin porcelain veneer
- enamel porcelain veneer
- external glaze
22
Q
Function of the metal substructure in the metal-ceramic restorations
A
- increases strength and support for the more brittle porcelain
- casting provides fit of restoration to the tooth
- forms oxides that bond chemically to porcelain
- restores tooth contour to original form and function
23
Q
Requirements of the metal substructure in metal-ceramic restorations
A
- produce surface oxides for chemical bonding to porcelain
- thermal compatibility (slightly greater than coefficient of thermal expansion)
- melting range higher than fusing range of porcelain
- no distortion at firing temperatures of porcelain (sag resistance)
- easy to handle (melt, cast, finish, polish)
- biocompatible
24
Q
Porcelain requirements of metal-ceramic restorations
A
lower fusing porcelain
- coefficient of thermal expansion
- fuse below melting range of alloy
25
Oxide layer of metal-ceramic restorations
1. base metals on the surface oxidize (sometimes called degassing)
2. oxides of gallium, indium, tin, iron
26
Application of opaque porcelain in metal-ceramic restorations
1. creates a bond (mostly a direct chemical bond)
| 2. sometimes mechanical retention, compression bonding, van der Waal forces
27
Porcelain/metal bond in metal-ceramic restorations
1. metal contracts more
2. metal "pulls" the porcelain with it and creates a "residual" compressive force
3. protective=cracks grow under tensile forces, not compressive
4. interface between metal and ceramic is under compression
28
Mismatch of CTE between ceramic/metal in metal-ceramic restorations
1. inner metal has a greater CTE than outer material
2. not just ceramic-metal restorations, works with layers of porcelain also
3. limit to materials slightly larger than CTE
29
Major component of porcelain
1. feldspar: forms a glassy matrix
2. silica /quartz: high fusion temperature; stabilizes the porcelain build-up at high temperature; strengthens porcelain
3. kaolin: used as a binder to increase moldability of the unfired porcelain; opaque; used in small amounts
30
Porcelain Fluxes/Pigments/Opacifiers
1. fluxes: sodium carbonate/lithium carbonate; lowers porcelain sintering temps
2. pigments: added metal oxides
3. opacifiers: cerium, zirconium, titanium
31
Porcelain Glass Modifiers
1. potassium, sodium, calcium, other oxides
2. break up the 3D silica tetrahedra to a more linear chain
* *increase porcelain's CTE
* *lowers softening temperature of glass
* *lowers viscosity
3. TOO MANY modifiers=decreased chemical durability
32
Leucite
1. heating feldspar forms Leucite (crystalline phase) and glassy phase with amorphous structure
2. amount helps control coefficient of thermal expansion and strength of porcelain
33
How is porcelain applied to metal?
1. porcelain supplied as fine powder
2. mixed wit liquid
3. applied to metal structure in layers
4. condensation: packing suspended particles as tightly as possible via removal of liquid
34
sintering/Firing
1. heating a densely packed powder to a temperature slightly below melting point
2. causes edges of particles to fuse together
35
High Fusing Porcelain
>1300C
36
Medium Fusing Porcelain
1101-1300C
37
Low Fusing Porcelain
850-1100C
38
Ultra-low Fusing Porcelain
<850C
39
Crystalline Phases of All-Ceramic Restorations
1. wide variety of crystalline phases as reinforcing agents
* *alumina
* *leucite
* *mica
* *lithia disilicate
* *magnesia-alumina spinel
2. up to 90% by volume
3. composition, amount, and particle size influence mechanical and optical properties
40
Fabrication Types of All-Ceramic Restorations
1. condensed and sintered
2. castable glass-ceramics
3. heat pressed
4. slip cast
5. machined
41
Castable glass-ceramics
1. low-cost waxing process
| 2. crystal phase nucleation and growth in post-casting process
42
Heat-pressed ceramics
1. heat and external pressure to sinter and shape
2. avoid large pores
3. ex: Empress
43
Slip-Cast ceramics
1. condensing (packing) an aqueous porcelain mix (slip) onto a porous refractory die (purpose=remove water)
2. sintered on die, coated with slurry of glass-phase layer, fired
3. glass melts, infiltrates the ceramic core
44
What are the advantages of slip-cast ceramics?
1. reduced porosity and flaws
2. higher strength
3. long-processing time
4. greater toughness
5. opaque
45
Machined All-Ceramic Restorations
1. CAD/CAM (computer assisted design/computer assisted machine) is usually used
2. prepared tooth is optically scanned, image is computerized
3. restoration is machined from ceramic blocks by computer-controlled milling machine
46
Ceramic Denture Teeth
1. medium/high fusing porcelain
| 2. polymer-based teeth (alternative)
47
Advantages of Ceramic Denture Teeth
1. hardness: porcelain >enamel>dentin>acrylic
2. 10-20 times ABRASION RESISTANCE of polymer-based
3. greater dimensional stability (forces, water)
4. excellent shade stability
5. superior esthetics
6. able to withstand high heat
48
Disadvantages of Ceramic Denture Teeth
1. harder to adjust and polish
2. polymer-based have greater flexural and impact strength
3. possible clicking sound in use
