Nanoscience: Mechanical Properties

Olivier Nguon CHEM *7530/750 Feb 21st 2006

Outline  I. Classic Mechanical Properties  II. Nanostructured Materials  III. Conclusions and Applications

Tensile test  Determination of mechanical properties  Stress: σ = F/S  Strain: ε = Δl / l 0

Tensile Test curve Stress, σ (Mpa) Max stress : tensile strength Max elasticity: Yield strength Necking Fracture Strain, ε (%) Elastic deformation Plastic deformation Typical Tensile Test curve or Strain Stress curve

Elastic Deformation  Hooke’s law: σ = E ε  E = Young modulus (Pa) Modulus = slope  Stiffness of material Strain  Non linear models exist (visco-elastic behaviour)

Mechanical properties  Yield strength: maximum stress before permanent strain  Tensile strength: maximum stress  Ductility: measure of deformation (L f – L o )/ L o  Toughness: ability to absorbe energy: area under curve

Hardness  Resistance to plastic deformation  Measure of depth or size of indentation

II. Nanostructured materials

Nanoparticles  Conventional materials: Grain size micron to mm  Nanoparticles increase grain boundaries  Influence on mechanical properties: Increased hardness, yield strength, elastic modulus, toughness

Comparison tensile curves  Comparison: Al Mg cryomilled (20 nm) Al Mg ultra fine grain (80 nm) Al Mg coarse (2 mm)  Cryomilling: Milling in liquid N 2  Ultrafine grain: electrodeposition B. Han,

Red.Adv.Mater.Sci

;

9

(2005) 1-16

Mechanical properties of nanomaterials compared to coarse grain materials  Higher Young modulus and tensile strength (to 4 times higher)  Lower plastic deformation  More brittle

Strength and Hardness with grain size  Strength and Hardness of nanostructured material increases with decreasing size  Grain boundaries deformation

Comparison of Young modulus Material Rubber Al Fe SiC Fe nanoparticles (100 nm) C nanotubes Diamond Young modulus (GPa) 0.1

70 200 440 800 1000 1200

Elongation nanostructured materials  Elongation decreased  Lower density of mobile dislocations  Short distance of dislocation movement

III. Conclusions

Mechanical properties  Mechanical properties: Strength, toughness, hardness increased  Materials more brittle  Due to increased grain boundaries density and less dislocations density

Important factors on mechanical properties 

History

of the material:

Temperature, strain

: influence on amount of dislocations, grain size 

Impurities

: segregate at high temperature and affect mechanical properties

Applications  Biomedical: bones, implants, etc.

 High strength, strong, long-lasting materials: automotives, electronics, aerospace, etc.

 Composites materials