all-ceramics part 2

Question 1

T/F: the actual strength of a ceramic material is always lower than its theoretical strength

Answer 1

true

Question 2

low locally applied stresses can result in what?

Answer 2

very high effective local stresses

stress magnification by flaws

Question 3

sintered powders can introduce ______ in the ceramic

Answer 3

voids

Question 4

what can be done to reduce the voids formed by sintered powders?

Answer 4

casting and pressing

Question 5

how can glazing increase the strength of ceramics?

Answer 5

1) self glazing ceramics can “heal” flaws”

2) surface glazes can be designed to cool and provide SURFACE COMPRESSION

Question 6

ceramics are weaker when fractures occur in _____

Answer 6

water

Question 7

what can be done to reduce the weakness of ceramics in water?

Answer 7

fuse the ceramic to metal foil (Captek)

Question 8

what does crystalline reinforcement do?

Answer 8

introduces an increase crystalline content in the glassy matrix (helps DEFLECT cracks)

Question 9

what are examples of crystalline reinforced glass ceramics?

Answer 9

(the “hot-pressed” glass ceramics)

1) leucite-reinforced glass ceramics (empress)
2) lithium disilicate glass ceramics (eMax)

Question 10

what are the characteristics of empress (leucite-reinforced) and eMax (lithium disilicate) ceramics?

Answer 10

• moderate strength
• good translucency
• can be used in full thickness
• no shrinkage (good fit)

Question 11

what type of crystalline reinforced ceramic is used only for cores

Answer 11

dense sintered alumina

high strength, but low translucency

Question 12

what 4 all-ceramic systems are used at OSU?

Answer 12

1) leucite reinforced glass-ceramic
2) lithium disilicate glass-ceramic
3) full contour zirconia
4) porcelain fused to zirconia

Question 13

what are the advantages of full contour with surface staining fabrication?

Answer 13

1) less labor intensive (cheaper)
2) can have excellent outcomes
3) full contour strength of material

Question 14

why would the minimal cutback layered with porcelain technique be more expensive?

Answer 14

requires more labor- must layer the porcelain

Question 15

what is the most labor intensive (and expensive) fabrication technique for ceramic crowns? what is the other disadvantage of this technique?

Answer 15

full cutback with layered porcelain

strength of material is reduced

Question 16

what are the advantages of isostatically pressed zirconia powders?

Answer 16

• stronger than chalk form

- easy to machine

Question 17

what are the advantages/disadvantages of unsintered zirconia? what about sintered zirconia?

Answer 17

unsintered- has a 20% shrinkage rate, but is easy to mill

sintered- harder to mill, but is stronger than unsintered

Question 18

what is the effect of adding yttrium to zirconia?

Answer 18

small amounts (3%) will cause powders to stabilize in the tetragonal phase

Question 19

what is “YTZP”?

Answer 19

Yttrium stabilized Tetragonal Zirconia Polycrystalline

Question 20

T/F: long term failure mechanisms for zirconia crowns are still not known

Answer 20

true

Question 21

what causes the low temperature degradation of zirconia?

Answer 21

zirconia is METASTABLE- it can transform over time, and create flaws that lead to failure

Question 22

how are leucite reinforced glass-ceramics, and lithium disilicate glass-ceramics, cemented?

Answer 22

cemented the same as any other silica based crown

• etch with HF acid
• apply SILANE agent
• dual cure resin cement
• apply adhesive to tooth

Question 23

why is cementing zirconia different than cementing metal?

Answer 23

zirconia does not have a rough inner surface

- metal has higher mechanical resistance

Question 24

how does “Transformation Toughening” help the strength of zirconia crowns

Answer 24

Normally, zirconia crystals are in Tetragonal form

Once cracked- the lack of crystal constraint allows them to switch phases- the crystal network expands and closes the cracks