Sintering and Microstructure of Ceramics.

1. Sintering and Microstructure of
Ceramics
By
Name :- Dibyadyuti Kar
Roll :-120CR0384

2. Types of sintering :-
• Solid state sintering:-
• Solid state sintering occurs when the powder compact is densified wholly in a solid state at the sintering
temperature(heated to a temperature just below its melting point)
• Solid state sintering, on the other hand, is commonly used for ceramics like zirconia and alumina
• Solid state sintering, on the other hand, is more suitable for materials that can be densified through diffusion
alone.
• Liquid phase sintering;-
• while liquid phase sintering occurs when a liquid phase is present in the powder compact during sintering.
• Liquid phase sintering is often used for ceramics that are difficult to densify, such as silicon nitride and silicon
carbide
• One advantage of liquid phase sintering is the lower sintering time required compared to solid state sintering.
This is because the presence of the liquid phase facilitates particle rearrangement and densification

3. • In addition to solid state and liquid phase sintering, other types of sintering,
for example, transient liquid phase sintering and viscous flow sintering.
• Viscous flow sintering occurs when the volume fraction of liquid
is sufficiently high, so that the full densification of the compact can
be achieved by a viscous flow of grain–liquid mixture without having
any grain shape change during densification.
• Transient liquid phase sintering is a combination of liquid phase sintering
and solid state sintering. In this sintering technique a liquid phase forms in
the compact at an early stage of sintering, but the liquid disappears as
sintering proceeds and densification is completed in the solid state.

4. DRIVING FORCE OF SINTERING
The driving force of sintering is the reduction of the total interfacial energy. The reduction of the total
energy can be expressed as
Here, the change in interfacial energy is due to densification and the change in interfacial area is due to
grain coarsening. For solid state sintering, is related to the replacement of solid/vapor interfaces (surface)
by solid/solid interfaces.

5. Solid Phase Sintering (SPS)
• Solid-state sintering is the bonding and densification of particles by the application of
heat below the melting point of a material. During sintering, the free surface area of the
compact decreases, and this is normally accompanied by an increase in the density,
• There will be changes in other properties of the compact, such as increased strength, an
increase in electrical and thermal conductivity.
• For most applications, an increase in density is the desired result of sintering. As with
any physical process, there must be a lowering of the total internal energy of the system
to drive the process
• In sintering this reduction in energy is the reduction in the total surface energy of the
powder particles due to the replacement of surfaces by grain boundaries and due to
coarsening.

7. SINTERING VARIABLES :
Variables related to raw materials
(material variables )
• Powder:
shape, size, size distribution, agglomeration,
mixedness, etc.
• Chemistry:
composition, impurity, non-stoichiometry,
homogeneity, etc.
Variables related to sintering condition
(process variables)
• Temperature, time, pressure, atmosphere,
heating and cooling rate, etc.

8. Examples :
Materials Conditions/Type Applicaitons
Al203 Solid state (MO additive) Structural Ceramics
MgO Silicate bonded Refractories
SI3N4 LPS with Y2O/Al2O3 , as
additives
Structural
SiC LPS with Al/B/C, SSS with B/C Structural
ZnO2 BiO3 as additives in LPS Varistor

9. Sintering stages (solid phase)
Sintering Stage Microstructural
Features
Relative Density Idealized Model
• Initial:-Initial stage (a) Rapid
interparticle growth (various
mechanisms), neck formation,
linear shrinkage of 3-5%.
Interparticle
neck growth
Up to 0.65 Spheres in contact
• Intermediate:-(b) Continuous
pores, porosity is along grain
edges, pore cross section
reduces, finally pores pinch off.
Up to 0.9 of TD.
Equilibrium pore
shape with
continuous porosity
0.65 - 0.9 Tetrakaidecahedron
with
cylindrical
pores of the same radius along
edges
Final:-(c) Isolated pores at grain
corners, pores gradually shrink
and disappear. From 0.9 to TD.
Equilibrium pore
shape with isolated
porosity
>0.9 Tetrakaidecahedron with
spherical pores
at grain corners

11. Sintering Mechanisms and Routes (solid phase)
Mechanisms Source Sink Densifying
Surface Diffusion Surface Neck No
Lattice
Diffusion(from the
surface
Surface Neck No
Vapor Transport Surface Neck No
Lattice
Diffusion(grain
boundary)
GB Neck Yes
Vapor Transport Surface Neck No
Plastic Flow Dislocations Neck Yes

12. Liquid Phase Sintering (LPS)
• Liquid phase sintering is a process where a secondary phase (liquid at
elevated/fabrication temperatures) is used in the body to enhanced densification.
• Typically the liquid phase volume is of a few percent(vol).
• Particularly useful for the densification of carbide and nitride ceramics(SiC,Si3N4)
where diffusion OS difficult owing to the strong covalent bonding.
• If the liquid phase present is sufficient volume (25-30%) the process is called
vitrification ,common for clay based ceramics.

13. Examples :
Ceramic Sistem Additive (weight%) Applications
Al2O3 (talc) ~5 Electrical Insulators
MgO (CaO-Sio2) <=5 Refractories
MgO (LiF) <3 Refractories
ZnO (Bi2O3) 2-3 Electrical Varistors
BaTio3 (Tio3) ~5 Capacitors
Si3N4 (Mg0) 5-10 Structural Ceramics
UO2 (TiOs) ~1 Nuclear Fuels

14. STAGES OF LIQUID-PHASE SINTERING
Stage-1 :-Redistribution of the liquid and rearrangement of the particulate solid under the influence
of capillary stress gradients.
• The additive phase by itself melts at the fabrication temperature (a eutectic liquid) or reacts
with the matrix to form a low temp phase/
• Capillary exerted by the liquid holds the solid phase together
• If the liquid phase persists throughout the fabrication process it is called persistent LPS .The
liquid throughout the fabrication process , it is called persistent LPS .The liquid cools to form
intergranular glassy phase.
• On the contrary ,the liquid phase disappears before sintering is complete in transient
LPS Example :BaTiO3 with LiF additive
Stage-2 :-Densification and grain shape accommodation by solution-precipitation.
Final-stage sintering driven by the residual porosity in the liquid.

16. Advantages of Sintering :-
1. The parts produced have an excellent surface finish , and good dimensional
accuracy.
2. The porosity inherent in sintered components is useful for specialized application
such as filters and bearings.
3. Refractory materials which are impossible to shape using other methods can be
fabricated by sintering with metals of lower melting points.
4. A wide range of parts with special electrical and magnetic properties can be
produced.

17. REFERENCES :-
• Ceramic Processes and Sintering by MN. Rehman
• Suk-Joong L. Kang Sintering Densification Grain Growth &
Microstructure
• Ceramic Matrix Composites by Richard Warren
• Ceramic Fabrication Technology by Roy. W. Rice