Cracow, 2014
SYNTHESIS AND CONSOLIDATION
OF NANOPOWDERS:
APPROACHES AND METHODS
Michail Alymov
ISMAN

Outline
1. Introduction.
2. Synthesis of nanopowders.
3. Processing of bulk nanostructured materials.
3.1. Consolidation of nanopowders.
3.1.1. Pressing at room temperature.
3.1.2. Sintering without pressure.
3.1.3. Sintering under pressure.
4. Properties of consolidated nanomaterials.
5. Summary.

Classification of nanomaterials
1. Powders.
2. Layers and coatings.
3. Composite materials.
4. Bulk materials.
Powder metallurgy = synthesis of powders + consolidation of powders.
By powder metallurgy methods we can produce all kinds of nanomaterials.
R.W. Siegel, Proc. Of the NATO SAI, 1993,v.233, р.509

METHODS FOR PROCESSING OF BULK
NANOSTRUCTURED MATERIALS
Methods Technologies
Powder metallurgy Consolidation of nanopowders:
Pressing and sintering,
Pressure sintering
Materials
Metals and alloys, ceramic, metal-ceramic, composites, polymers
Metallic materials able to bulk amorphisation.
Crystallization from amorphous state
Severe plastic deformation
Crystallization of amorphous alloys,
Consolidation of amorphous powders with further crystallization
Equal channel angular pressing,
Torsion under high pressure,
Multiple all-round forging.
Nanostructurisation by precision heat treatment and thermomechanical treatment
Heat treatment.
Thermomechanical treatment
Metallic materials
Metallic materials

Powder
Bulk material
Pressure
Temperature
Time
Size of Ni particles = 70 nm
Grain size = 100 nm

Hydroxyapatite ceramics from nanopowders
After pressing
Pressure 3 GPa
Sintering temperature 670°С
After sintering
Grain size 35-50 nm
Microhardness 5,8 GPa
Fomin A.C., Barinov C.M., Ievlev V.М. a.o. 2008.

Methods for synthesis of nanopowders
–
SHS (self-propagating high temperature synthesis),
– chemical – metallurgical method
- plasma-chemical synthesis
– mechanical alloying
- electrical explosion of wires
- vaporization-condensation technique
- flowing gas evaporation technique
- vapor phase synthesis
– cryochemical synthesis
- sol-gel method
- hydrothermal synthesis and others

There are many methods for synthesis have been developed to produce nanopowders. The synthesis routes are diverse and result in nanoparticles with a range of characteristics, such as size, size distribution, morphology, composition, defects, impurities, and agglomeration (“soft” and “hard”). By now, several tens of methods have been developed for the synthesis of metallic, ceramic, cermet, and other nanopowders. Each method is characterized by its own advantages and disadvantages. Some methods are reasonably used for the preparation of metal powders, while other methods are useful for ceramic powders.

The ratio between the average particle size and performance of methods
800
SHS
Chemical and metallurgical
Evaporationcondensation
400
Calcium-hydride method
Plasmachemical
200
EEW
0
4
Levitation-jet method
0 200 400
Size of particles, nm
Alymov M.I. Composites and Nanostructures, 2012, v.3.

METHODS for the NANOPOWDERS CONSOLIDATION
Uniaxial pressing: static, dynamic , vibration
Isostatic pressing
Extrusion
Sintering under pressure
Spark plasma sintering
Sock wave pressing
Severe plastic deformation

Features of the nanopowders consolidation
Impurities play an important role in densification.
Agglomeration of nanoparticles into clusters.
Low dislocation density.
The possibility of new or different mechanisms of densification.
Diffusion-induced grain-boundary migration and boundaryenergy-induced rotations may alter densification mechanisms.

Cold pressing
uniaxial (static, dynamic, vibrational),
- multiaxial (hydrostatic, gasostatic),
- severe plastic deformation,
- cold rolling.

Influence of average iron particle diameter on the density of compacts
100
60
40 mkm
1 mkm
120 nm
60 nm
28 nm
26 nm
23 nm
20
0 0,4 0,8 1,2 1,6
Pressure, GPa
Diameter of dislocation free iron particle is equal to 23 nm
M.I. Alymov, 1990

The friction between the nanoparticles substantially affects the densification of nanopowders.
The contribution of plastic deformation to the densification of nanopowders is insignificant since the nanoparticles are free from dislocations and they cannot be deformed as coarse particles due to the movement of dislocations.

Consolidation process of nanopowders is strongly affected by:
- particle size distribution,
- concentration of impurities,
- surface conditions,
- particle shape,
- pressing technique.

Sintering mechanisms
1 - surface diffusion,
2 - volume diffusion from surface,
3 - vapor transport from surface,
4 - grain boundary diffusion,
5 - volume diffusion,
6 – dislocation diffusion
Alymov M.I., Letters on Materials. 2013.

Sintering of gold nanoparticles

Influence of pressure on sintering
100
90
80
Sintering under pressure
70
Sintering without pressure
Т
2
< Т
1 d
2
< d
1
Sintering temperature
Т
1 d
1

Equipment for the sintering under the pressure
Pressure padding bellows punch yield of gas thermocouple entrance of gas sample heating element anvil vessel

Pressure sintering of iron nanopowder
100
90
80
380 MPa
280 MPa
90 MPa
0 MPa
70
60
400 500 600 700 800
Temperature, °С
М.И. Алымов, ФХОМ, 1997

Influence of the mode of deformation on sintering
HIP
– pressing in dies – forging – extrusion -
ECAP
Hydrostatic component of pressure
Tangential component of pressure

Gas extrusion method chamber sample die block die

Nickel nanopowder green compact after hydrostatic pressing
Compacts of iron and nickel nanopowder after extrusion
Iron
10 cm
Nickel

Grain size near 70 nm

MECHANICAL PROPERTIES OF THE COMPACTS
Method
Hot isostatic pressing
Material Particle size, mkm
Grain size, mkm
Ni
6
0,06
25
1
 в
MPa
440
545
,

,
%
36
7
40 55 350 41
Fe
0,04 1 460 1
Extrusion Ni 0,06 0,1 700 15

Mechanical properties of nanocrystalline and coarse-grained nickel
Nano-grained Coarse-grained

0,2
, MPa

B
, MPa

, %
ψ, %
K c
, MPa ∙m 1/2
Toughness, J/cm 2
530
625
22
19,5
82,3
63-66
80
400
40
-
51,7
198-203
The crack growth resistance for nanocrystalline Ni is on 30% higher the crack growth resistance coarse grained Ni.

Ni
Fe
Cu
Relative elongation , %
Valiev R. 2001

Hardness of WC-8%Co hard alloy depends on the size of WC-grain
26
24
22
20
18
16
14
0 0,5 1,0 1,5 2,0
Size of WC-grain, mkm
Alymov M.I. a.o. Composites and Nanostructures. 2012.

SHS pressure sintering
4
3
4 - mold.
3 - insulating porous medium
(sand);
1 2
1 - tungsten spiral initiating the SHS reaction
2 - tablet from powders of the initial reactants
Sherbakov V.А.

Before SHS extrusion
Ignition system
Initial charge billets
Form of a matrix
The mold assembly
Guide caliber
Stolin A.M.

After SHS extrusion
Material after SHS
(press residue)
Extruded material
(finished product)
Stolin A.M.

Effectiveness for bulk nanopowder materials
Hard alloys
Materials
High strength steels and alloys
Ceramic materials
Nanopowder materials with special properties
Wear resistance coatings
Effectiveness
Increase of hardness by a factor of 5-7
Increase of strength by a factor of 1,5-2
Formability as for titanium alloys
Mechanical, chemical, optical and other properties
Increase of resistance by a factor of 170