Transcript Diapositiva 1

Properties of materials
The behaviour of a given material is
characterised by the response to a
stimulus.
• Mechanical properties (behaviour
under a set of forces)
• Physical properties (behaviour under
action of temperature, electrical or
magnetic fields or radiation)
• Chemical properties (behaviour
under the action of chemicals)
Mechanical properties studied as:
• time –independent
• time-dependent
• temperature-dependent
Applying a force to a structure causes a stress
bringing about a strain.
STRESS or TENSION : the ration between force
F and the surface A to which is applied (Nm-2 o Pa).
 = F/A
Three main types of stress: TENSILE,
COMPRESSION and SHEAR
If, once removed the applied force, the
material gains the initial state, such
behavior is said to be ELASTIC
Hysteresis
Eelastic
linear elastic
behavior
non linear elastic
behavior (rubber)
Anelastic
behavior
All materials, for small stresses, show a
LINEAR elastic behavior (Hooke’s law)
σ=Eε
E = elastic modulus
(Young modulus,
dimensions of a pressure)
Curiously, the
cause (load) is on
the abscissa scale)
Covalent or ionic solids
Metals
Polymers
E
Tmelt
Materiale
Temp Fusione
E
[° C]
TiC Carburo di Ti
3160
310
Al2O3 Allumina
2045
370
W
3410
393
Fe
1536
207
Cu
1083
111
Al
660
70
Pb
327
14
Polietilene
130
1
Tensile measurements:
fragile (brittle) materials break
beyond the elastic limit (ceramics,
glasses)
ductile materials (metals, polymers):
plastic deformation
Fragile
Material
Ductile
material
Toughness
measures the energy a material can store before breaking
Area under the curve!
Indeed, a corrected curve should be used…
striction
Another measure of the cohesive strength of
the material: tenacity
Charpy pendulum
Time dependent mechanical properties:
Creep
Fatigue
CREEP
A constant static load may cause deformation
Not so important at ambient temperature, i.e. with
biomaterials
Relevant process when T > 0,3-0,4Tmelt (Metals and
ceramics)
T > Tg (Polymers and glasses)
FATIGUE
Degration in mechanical properties when a
material is subjected to cyclic stresses
Samples are subjected to different loads, and the
number of cycles cause breakdown is measured at
each load
Often, a limit value for the load (FATIGUE LIMIT) is
observed
HARDNESS
Property of the external layers of a material: resistance to
scratching (Mohs’ scale), to abrasion and to plastic deformation
upon compression.
Measure: i) formation of an indentation by applying a static
constant load for a definite time; ii) evaluation of the dimension
Rockwell Method
Ultimate
Tensile
strength
Relationship
between
hardness and
UTS
THERMAL PROPERTIES
OF MATERIALS
Thermal capacity*
Thermal expansion*
Thermal conductivity
Resistance to thermal shocks*
* Not really important in biomaterials
THERMAL CAPACITY
Attitude of a body to store heat
Ratio between exchanged heat and change in
temperature
When normalised to unit mass  SPECIFIC
HEAT
dQ
J
C

m dT kgK
THERMAL CONDUCTIVITY
Attitude of a body to transfer heat
The thermal conductivity coefficient is defined through
Fourier’s law: the heat flux across a unit surface is
proportional to the temperature gradient (with inverted
sign)
THERMAL EXPANSION
Usually all solids expand when heated
Coefficient of linear thermal expansion ()=
(lt  l0 )
  (Tt  T0 )
l0
l
 T
l0
Chemical characterization
Often surface only
Others:
• HRTEM
• Adsorption (porous systems)
Contact angle:
Measures the wettability of a surface by a
liquid
Usually water or aqueous solutions
(hydrophobicity/hydrophilicity)
Also the surface tension of the solid
BIOGLASS
glv
BIOGLASS
SILANIZED
gsv
q
gsl
Ways of measuring contact angles
ESCA
Highly energetic X-rays cause expulsion of the electrons
of the inner cores, which have different binding
energies, so allowing chemical determination
Infrared Spectroscopy: functional groups
in a molecule are recognized through
their vibrational features
A well developed technique, very powerful…
Versions of the
technique for
surface
analysis
Scanning tunneling microscope
The end