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DMS > Ceramics > Flashcards

Ceramics Flashcards

1
Q

What are 2 types of sintered cermaicc

A

Alumina

-Leucite reinforced

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2
Q

What happens in sintering

A

Pressed powdered form which is moulded and fired

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3
Q

What is sintered ceramic combined with in final restoration

A

glassy translucent porcelain

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4
Q

How good is aluminas bond to the glass

A
  • Bonds well

- Coefficent of thermal expansion is similar between glass and alumina

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5
Q

What is the benefit of large alumina crystals being present?

A

They act as a crack deflecting mechanism

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6
Q

Disadvangtages of alumina (sintered)

A

shrinkage of 10-20%

-Opacity of alumina

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7
Q

Why is heat pressing better than sintering?

A
  • Improves crystal dispersion
  • Higher crystallinity
  • Decreased crystal size
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8
Q

How is heat pressed and sintered restorations similar

A

-They both have a veneering glass porcelain layer

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9
Q

2 Types of Heat pressed ceramics

A
  • Lithium disilicate

- Leucite reinforced

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10
Q

What is the thermal coefficient of leucite reinforced compared to glass? (sintered)

A

3x higher (mismatch)

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11
Q

What happens to the restoration when it is cooled with the mismatch

A

leucite cools quicker than the glass and this puts residual stress on the glass, but this acts as a CRACK DEFLECTING mechanism

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12
Q

Benefit of Leuite reinforced sintered

A

-Aesthetic

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13
Q

What is the thermal coefficient of Lithium disilcate compared to glass (Heat pressed)

A
  • There is a mismatch between the 2.

lithium cools quicker and this puts residual stress on the glass which acts as a crack deflecting mechanism

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14
Q

Why is the crack delfection in lithum disilicate better than lecuite (heat pressed)

A

-As the crystals are elongated and interlocked in lithum disilicate

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15
Q

Why is heat pressed leucite reinforced better than sintered?

A
  • Less operator variability
  • Stronger as improved crystallinity and better dispersion of crystal
  • Higher flexural strength
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16
Q

What materials can we soft machine?

A

Alumina
Zirconia
Spinel

17
Q

Why do we design the alumina oversized in soft machining process?

A

To accommodate for shrinkage

18
Q

What materials can we hard machine

A
  • Felspar porcelain
  • Lithium Disilcate
  • Leucite reinforced
19
Q

What makes a metal ceramic restoration

A

-Metal framework with 2+ layers of ceramic

20
Q

What is the first layer of the MCC

A

=-Ceramic which contains opacifying oxide. This helps bond to metal (ceramic oxide +metal oxide)
-Also aesethically hides the metal

21
Q

What are the additional layers on the MCC

A

-Translucent porcelains for aesthetics and colour match

22
Q

Why is the restoration vibrated after the porcelain has been applied

A

to condense the material which reduces shrinkage and increases packing factor

23
Q

Why is a vacuum of 0.01 mPa needed?

A

To decrease porosity

24
Q

Characteristics of what makes the metal ceramic restoration

A
  • Metal must have higher melting temp than the ceramic (so when ceramic is applied metal doesnt melt)
  • Metal should not lose its shape when the ceramic is sintered or it can reduce fit
  • metal have higher thermal coeffiecnet of expansion than ceramic
  • Metal should have chemical bond via oxides and mechanical bond via bur roughening of surface
  • Metal to have high elastic modulus to mitigate the stress the ceramic experiences
  • Veneering porcelain to have low fusion temperature so that it doesn’t distort in sintering
25
Q

5 Key properties of ceramics

A
  • High Strength but fracture toughness is lower than metals
  • Optically favourable
  • Stable over long period
  • Doesnt react with acids,alkalis liquids and gases
  • High harness
26
Q

Advantages and disadvantages of feldspar

A

A- Translucent, matches shade/lustre and addition of crystal phases improves mechanical properties,

D- low strength and prone to mechanical failure

27
Q

What is different about theoretic strength of ceramics and IRL

A

Theoretical strength = High as it can handle tensile loads

IRL= It is never defect free so the tensile strenth can vary.

28
Q

What limits strength in ceramics

A
  • Defect size

- Distribution of defect

29
Q

Define fracture toughness

A

Measures the resistance to a crack propagating through a material under tensile stress

30
Q

Why are glassy ceramics brittle

A

Due to the microstructure, they experience little plastic deformation before failing

31
Q

Why are crystals added to glassy ceramics

A

-To toughen the glassy matrix, for a crack to progagate it will have to take a path around the glass particles as it doesnt propgate through the crystals.

32
Q

Whats the problem with adding crystals to the matrix

A
  • Shrinkage during sintering, so dimensional accuracy goes down
  • decreased transparency

DMS (6 decks)

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