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Biomaterials > Lecture 6 > Flashcards

Lecture 6 Flashcards

1
Q

Definition of a ceramic

A
  • Solid
  • Inorganic, non metallic
  • Synthetic
  • Process requires a stage at High Temperature (mainly at solid state)
  • Compound of electropositive* elements combined mainly with C,O,N
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2
Q

Electropositive

A

ability of an atom to donate electron(s)

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3
Q

Some common applications of ceramics

A

load bearing applications (dental, femoral), bone repair/reconstruction, coatings

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4
Q

Al2O3

A

Alumina

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5
Q

ZrO2

A

Zirconia (YSZ)

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6
Q

ZTA

A

Zirconia toughened alumina

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7
Q

HA

A

hydroxyapatite

Ca10(PO4)6(OH)2

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8
Q

TCP

A

Tricalcium phosphate

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9
Q

Bioactive glasses

A

SiO2, Na+, Ca2+

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10
Q

IMPORTANT ceramics take away message

A

Iono-covalent atomic bonds, stronger than metal bonds

β†’ implication on chemical stability (+) and mechanical properties (+ & -)

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11
Q

3 classes of bioceramics

A

Nearly inert, bio active, resorbable

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12
Q

Nearly inert class

A

Fixation modes: morphological (if dense) biological (if porous)
Compounds: Carbon: LTI amorphous, Al2O3, ZrO2, YSZ, ZTA, LTI

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13
Q

Bio active class

A

Fixation modes: interfacial bonding

Comounds: HA, bioglasses

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14
Q

Resorable

A

Fixation modes: replacement

Compounds: HA+TCP, TCP, Calcium Phosphate, Calcium Sulfate

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15
Q

Difference between morphological and biological faxation for nearly intert

A

Morph: dense, no direct bonding, non adherent growth of fibrous tissue into surface irregularities.
Biol: porous, ingrowth of tissue. Pores > 100um. Provides blood supply but lowers strength. Used as coating.

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16
Q

Diff between anions and cations

A

Big anions tend to build a closed packed structure. Smaller cations fill available sites

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17
Q

Microstructure

A

how grains are assembled, which defects vs single crystal.

Controlled by processing. Allow us to play with physical and mechanical properties.

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18
Q

Important mechanical properties of ceramic for femoral ball healds

A

Hardness, no plastic/elastic deformation, no creep, malleable, fatigue resistant

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19
Q

Important properties of Alumina nad Zirconia

A

High strength and stiffness. Very low deformation. At 37C, no plastic deformation. Brittle failues, low fracture energy.

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20
Q

Plastic deformation occurs by

A

shear of the crystal lattice along preferential planes and directions

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21
Q

Stress needed to break atomic bonds in one time is far too high !
Slip occurs at much lower stresses by an other mechanism

A

Shear is progressive shear line =dislocation

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22
Q

Burger’s vector

A
  • describes the magnitude and direction of lattice distortion
  • define the stress field caused by the dislocation
  • is a lattice vector (periodicity of the lattice)
23
Q

What happens in iono-covalent ceramics ?

A

1) Strong bonds -> Shear modulus is HIGH
2) Ionic compounds -> LONGER Burgers vectors. Lower number of slip planes.
IMPORTANT: Fewer slip systems. Dislocations are not mobile body temp

24
Q

Ceramics break at

A

sigma(experimental) &laquo_space;sigma(theoretical)

25
Q

What are the origin or glass and ceramic failure?

A

extrinsic DEFECTS

26
Q

Ceramic characteristics

A
  • High stiffness (Higher than stainless steel)
  • No ductility
  • Fail at stresses &laquo_space;cleavage stresses
  • Due to extrinsic defects
  • Around defect : stress concentration without plastic relaxation
  • Catastrophic failure
27
Q

For hip replacements, ceramics must be mounted

A

in compression

28
Q

m : Weibull modulus

A

characterizes the stress (defect) distribution. Small m is unreliable. Large m is reliable.

29
Q

How to have a reliable ceramic?

A

High median stress + high weibull modulus (m)

30
Q

CONSEQUENCES

A
  • No intrinsic value for strength
  • Defect population (size) introduced during processing will dictate strength distribution
  • Probabilistic approach
  • Avoid large pieces (higher proba to find a large defect)
  • Reduce flaw size and flaw size dispersion (Processing)
  • Use in compression πœŽπ‘…,π‘π‘œπ‘šπ‘= 10 to 15 x πœŽπ‘…,π‘‘π‘’π‘›π‘ π‘–π‘œπ‘›
31
Q

Ceramics fail because of EXTRINSIC defects, but…

A

the ability of a ceramic under stress to withstand
the extension of a crack is an INTRINSIC property
called fracture TOUGHNESS

32
Q

formula for toughness

A

sigma(R) * sqrt(a) = cste

33
Q

Ceramics and glasses and ___ toughness

A

LOW

34
Q

To increase ceramic strength we must

A

decrease defect sizes, increase toughness

35
Q

Grain boundary

A

accommodation region around the contact between two grains of distinct crystal orientations

36
Q

GB is often

A

weaker than the grain

defect size ~ grain size

37
Q

Increase toughness by

A

phase transformation. (monoclinic, tetragonal, cubic, etc.) Volume increases

38
Q

Tetragonal and Cubic phases are at RT

A

metastable

39
Q

Stress field ->

A

phase transformation -> increase particle volume -> crack closure -> hinders crack propagation. Therefore toughness increased.

40
Q

To improve Failure Stress :

A

reduce grain size and control process (reduce defects)

41
Q

To improve Toughness (2 methods) :

A

Increase fracture energy by

  • crack bridging
  • transformation toughening
42
Q

ceramics loaded below this critical stress in a moist environment can undergo a

A

delayed fracture

43
Q

Activated diffusion at crack tips

A

Hydroloysis breaks atmoic bonds, crack propagation

44
Q

Tetragonal Zirconia is more sensitive to

A

cycling

45
Q

Zirconia aging

A

due to tetragonal to monoclinic transformation before crack happens

46
Q

Hydroxyapatite

A

Bioactive. Very close to mineral phase of bone. Osteoblasts adhere to HA coating

47
Q

TriCalcium Phosphate

A

Resorbable. Osteoconductive: gives Ca, P to the medium ,

helps bone formation

48
Q

Silicate glasses: Network formers

A

SiO4 tetrahedra
ionocovalent bonds
form the network

49
Q

Silicate glasses: Network modifiers

A

large cations break the network -> ionic bonds lower fusion temp. and viscosity

50
Q

processing of ceramics

A

Powders, milling, batching, mixing, forming, drying, firing -> sintering, finishing

51
Q

Sintering

A

coalesce into a solid or porous mass by means of heating (and usually also compression) without liquefaction. Since ceramics are brittle, there are few machining possibilites.

52
Q

Nanoceramics for bio-applications

A

biological probes for imaging cellular activity, targeting agents, local delivery of therapeutic agents, hyperthermia … Must be

  • non toxic, non viral
  • biocompatible
  • stable
53
Q

ceramic nanoparticles examples

A
  • Quantum dots as probes
  • Magnetic nanoparticles
  • TiO2 for photo-catalysis

Biomaterials (10 decks)

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