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 ELASTICELASTIC

linear elastic behavior

non linear elastic

behavior (rubber)

Anelastic behavior

Eelastic

Hysteresis

All materials, for small stresses, show a LINEARLINEAR elastic behavior (Hooke’s law)

σ = E ε

E = elastic modulus(Young modulus,

dimensions of a pressure)

Curiously, the cause (load) is on the abscissa scale)

Covalent or ionic solids

Metals

Polymers

EE TTmeltmelt

Tensile measurements:

fragile (brittle) materials break beyond the elastic limit (ceramics, glasses)

ductile materials (metals, polymers): plastic deformation

Fragile Material

Ductile material

ToughnessToughness

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

CREEPCREEP

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

HARDNESSHARDNESS

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

kgK

J

mdT

dQC

When normalised to unit mass SPECIFIC SPECIFIC HEATHEAT

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 (Coefficient of linear thermal expansion ()=)=

Tl

l

TTl

llt

t

0

00

0 )()(

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

lv

sl

sv

BIOGLASS

BIOGLASS

SILANIZED

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