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

Lecture 6 Flashcards by Alex Chappell

Definition of a ceramic

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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Electropositive

ability of an atom to donate electron(s)

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Some common applications of ceramics

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

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Al2O3

Alumina

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ZrO2

Zirconia (YSZ)

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ZTA

Zirconia toughened alumina

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hydroxyapatite

Ca10(PO4)6(OH)2

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TCP

Tricalcium phosphate

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Bioactive glasses

SiO2, Na+, Ca2+

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IMPORTANT ceramics take away message

Iono-covalent atomic bonds, stronger than metal bonds

→ implication on chemical stability (+) and mechanical properties (+ & -)

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3 classes of bioceramics

Nearly inert, bio active, resorbable

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Nearly inert class

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

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Bio active class

Fixation modes: interfacial bonding

Comounds: HA, bioglasses

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Resorable

Fixation modes: replacement

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

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Difference between morphological and biological faxation for nearly intert

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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Diff between anions and cations

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

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Microstructure

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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Important mechanical properties of ceramic for femoral ball healds

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

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Important properties of Alumina nad Zirconia

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

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Plastic deformation occurs by

shear of the crystal lattice along preferential planes and directions

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

Shear is progressive shear line =dislocation

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Burger’s vector

Study These Flashcards

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

What happens in iono-covalent ceramics ?

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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

Ceramics break at

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sigma(experimental) &laquo_space;sigma(theoretical)

What are the origin or glass and ceramic failure?

extrinsic DEFECTS

Ceramic characteristics

- High stiffness (Higher than stainless steel) - No ductility - Fail at stresses << cleavage stresses - Due to extrinsic defects - Around defect : stress concentration without plastic relaxation - Catastrophic failure

For hip replacements, ceramics must be mounted

in compression

m : Weibull modulus

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

How to have a reliable ceramic?

High median stress + high weibull modulus (m)

CONSEQUENCES

- 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 𝜎𝑅,𝑐𝑜𝑚𝑝= 10 to 15 x 𝜎𝑅,𝑡𝑒𝑛𝑠𝑖𝑜𝑛

Ceramics fail because of EXTRINSIC defects, but...

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

formula for toughness

sigma(R) * sqrt(a) = cste

Ceramics and glasses and ___ toughness

LOW

To increase ceramic strength we must

decrease defect sizes, increase toughness

Grain boundary

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

GB is often

weaker than the grain | defect size ~ grain size

Increase toughness by

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

Tetragonal and Cubic phases are at RT

metastable

Stress field ->

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

To improve Failure Stress :

reduce grain size and control process (reduce defects)

To improve Toughness (2 methods) :

Increase fracture energy by - crack bridging - transformation toughening

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

delayed fracture

Activated diffusion at crack tips

Hydroloysis breaks atmoic bonds, crack propagation

Tetragonal Zirconia is more sensitive to

cycling

Zirconia aging

due to tetragonal to monoclinic transformation before crack happens

Hydroxyapatite

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

TriCalcium Phosphate

Resorbable. Osteoconductive: gives Ca, P to the medium , | helps bone formation

Silicate glasses: Network formers

SiO4 tetrahedra ionocovalent bonds form the network

Silicate glasses: Network modifiers

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

processing of ceramics

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

Sintering

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.

Nanoceramics for bio-applications

biological probes for imaging cellular activity, targeting agents, local delivery of therapeutic agents, hyperthermia … Must be - non toxic, non viral - biocompatible - stable

ceramic nanoparticles examples

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

Lecture 6 Flashcards

(53 cards)