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

ELASTIC If, once removed the applied force, the material gains the initial state, such behavior is said to be ELASTIC

linear elastic behavior non linear elastic behavior (rubber) Anelastic behavior E elastic Hysteresis

LINEAR All materials, for small stresses, show a LINEAR 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 E T melt

Tensile measurements: fragile (brittle) materials break beyond the elastic limit (ceramics, glasses) ductile materials (metals, polymers): plastic deformation

Fragile Material Ductile material

Toughness 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 > 0,3-0,4T melt (Metals and ceramics) T > T g (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 c onductivity Resistance to t hermal shocks* * Not really important in biomaterials

THERMAL CAPACITY Attitude of a body to store heat Ratio between exchanged heat and change in temperature SPECIFIC HEAT When normalised to unit mass  SPECIFIC HEAT

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 ()=

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 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