Lecture 6 Flashcards
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
Electropositive
ability of an atom to donate electron(s)
Some common applications of ceramics
load bearing applications (dental, femoral), bone repair/reconstruction, coatings
Al2O3
Alumina
ZrO2
Zirconia (YSZ)
ZTA
Zirconia toughened alumina
HA
hydroxyapatite
Ca10(PO4)6(OH)2
TCP
Tricalcium phosphate
Bioactive glasses
SiO2, Na+, Ca2+
IMPORTANT ceramics take away message
Iono-covalent atomic bonds, stronger than metal bonds
→ implication on chemical stability (+) and mechanical properties (+ & -)
3 classes of bioceramics
Nearly inert, bio active, resorbable
Nearly inert class
Fixation modes: morphological (if dense) biological (if porous)
Compounds: Carbon: LTI amorphous, Al2O3, ZrO2, YSZ, ZTA, LTI
Bio active class
Fixation modes: interfacial bonding
Comounds: HA, bioglasses
Resorable
Fixation modes: replacement
Compounds: HA+TCP, TCP, Calcium Phosphate, Calcium Sulfate
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.
Diff between anions and cations
Big anions tend to build a closed packed structure. Smaller cations fill available sites
Microstructure
how grains are assembled, which defects vs single crystal.
Controlled by processing. Allow us to play with physical and mechanical properties.
Important mechanical properties of ceramic for femoral ball healds
Hardness, no plastic/elastic deformation, no creep, malleable, fatigue resistant
Important properties of Alumina nad Zirconia
High strength and stiffness. Very low deformation. At 37C, no plastic deformation. Brittle failues, low fracture energy.
Plastic deformation occurs by
shear of the crystal lattice along preferential planes and directions
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
Burger’s vector
- 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 ?
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
sigma(experimental) «_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 «_space;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
