[Sintering]
Introduction
 The word "sinter" comes from the German
Sinter, a cognate of English “cinder”,
which according to Concise Dictionary
means, “the refuse of burned coals”
 In plain English “solid piece of matter
remaining after having been subjected to
combustion”
Introduction
The ISO definition of the term ‘sintering’ reads:“The thermal treatment of a powder or compact
at a temperature below the melting point of the
main constituent, for the purpose of increasing its
strength by bonding together of the particles”.
Introduction
In other words
The bonding of powders by solid-state diffusion,
resulting in the absence of a separate bonding phase.
OR
Sintering is a method for making objects from
powder, by heating the material (below its melting
point) until its particles adhere to each other.
Introduction
The definition by Thummler from the point of
view of physical chemistry is:
‘Sintering is a thermally activated mass transport
process which leads to strengthening of particle
contacts and/or a change in porosity and pore
geometry accompanied by a reduction of the free
energy. A liquid phase can take part in the process.’
Introduction
Sintering is traditionally used for manufacturing
ceramic objects, and has also found uses in such
fields as powder metallurgy.
Almost all ceramic bodies and metal powder
compacts must be sintered to produce a
microstructure with the required properties. The
widespread use of the sintering process has led to
a variety of approaches to the subject.
Background
In metals as well as in ceramics, we are
concerned with two types of structure, both
of which have a profound effect on
properties.
 The first type of structure is at the atomic
scale: the type of bonding and the crystal
structure (for a crystalline material) or the
amorphous structure (if it is glassy).
Background
 The second type of structure is at larger
scale: the microstructure, which refers to the
nature, quantity, and distribution of the
structural elements or phases in the
metals/alloys or ceramics (e.g., crystals, glass
and porosity)
Types of Sintering
1. Solid state sintering
Only solid phases are present at the
sintering temperature.
2. Liquid phase sintering
Small amounts of liquid phase are
present during sintering.
3. Reactive sintering
Particles react with each other to form
new product phases.
Important Parameters in Sintering
These parameters can be divided into four
broad categories:
1. Powder preparation:
-- Particle size
-- Shape
-- Size distribution
Important Parameters in Sintering
2. Distribution of:
-- Second phases
3. Powder Consolidation:
-- Green density
-- Pore size distribution
Important Parameters in Sintering
4. Firing/Sintering:
-- Heating rate
-- Temperature
-- Time
-- Applied pressure
-- Atmosphere
Important Parameters in Sintering
 Some parameters, such as the sintering
temperature, sintering time, applied pressure,
average particle size and atmosphere can be
controlled with sufficient accuracy.
 Others, such as the powder characteristics and
particle packing are more difficult to control but
have a significant effect on sintering.
What Happens During Sintering?
 Atomic diffusion takes place and the
welded areas formed during compaction
grow until eventually they may be lost
completely.
 Recrystallisation and grain growth may
follow, and the pores tend to become
rounded and the total porosity, as a
percentage of the whole volume tends to
decrease.
What Happens During Sintering?
 In the pressing operation the powder
particles are brought together and deformed
at the points of contact.
 At elevated temperature - the sintering
temperature - the atoms can move more
easily and quickly migrate along the
particle surfaces (the technical term is
Diffusion).
What Happens During Sintering?
Metals consist of crystallites
 At the sintering temperature new crystallites
form at the points of contact so that the
original inter-particle boundaries disappear,
or become recognizable merely as grain
boundaries (This process is called
Recrystallisation).
What Happens During Sintering?
 The total internal surface
area of the pressed body is
reduced by sintering.
 Neck-like junctions are
formed between adjacent
particles as can be seen on
the adjoining scanning
electron micrograph.
Driving Force for Sintering
As with all processes, sintering is accompanied
by an increase in the free energy of the system.
The sources that give rise to the amount of free
energy are commonly referred to as the driving
forces for sintering. The main possible driving
forces are:
The curvature of the particle surfaces
An externally applied pressure
A chemical reaction
Driving Force for Sintering
Schematically it can be shown as
SINTERING
A MODEL SKETCH
Stages of Sintering
Three stages are distinguished in sintering

First Stage
After burn out of any organic additives,
two things happen to the powder particles
when the mobility of the surface atoms
has become high enough; initially rough
surface of the particles is smoothed and
neck formation occurs.
INITIAL
STAGE
OF
BONDING
Stages of Sintering

Second Stage
Densification and pore shrinkage. If grain
boundaries are formed after the first stage, these
are new source of atoms for filling up the
concave areas which diminishes the outer
surface of the particle.

Third Stage
Grain growth takes place, the pores break up
and form closed spherical bubbles.
Stages of Sintering
The three stages in the dry sintering can be
shown as
Mechanisms of Sintering
Six distinct mechanisms can contribute to the
sintering of a consolidated mass of crystalline
particles:
1.
2.
3.
4.
5.
6.
Surface diffusion
Lattice diffusion from the surface
Vapor transport
Grain boundary diffusion
Lattice diffusion from the grain boundary
Plastic flow
Mechanisms of Sintering
Liquid Phase Sintering
 During liquid phase sintering a liquid phase
coexists with a particulate solid at the
sintering temperature
 The wetting liquid provides a capillary
force that pulls the solid particles together
and induces particle re-arrangement.
Liquid Phase Sintering
 The second phase chosen has lower
melting temperature than the main
constituent.
 The sintering temperature is set just above
the melting point of the added phase so that
during sintering it forms a liquid phase that
wets the solid particles.
Liquid Phase Sintering
 The pores in the compact are largely
surrounded by the liquid phase and the
driving force for sintering is liquid surface
energy.
 With high liquid fractions, full density can be
achieved almost entirely by rearrangement.
Liquid Phase Sintering
Grain boundary ‘wetting’ breaks the
polycrystalline particle into single crystal
particles in the initial stages of liquid phase
sintering. These single crystal particles then
spheroidize and coarsen.
Polycrystalline
particle
Liquid
Liquid Phase Sintering
Polycrystalline
particle
Liquid
Liquid Phase Sintering
Wetting is a very important phenomena which
is happening during LPS.
Figure represents the surface tensions of a
multi-phase junction as vectors drawn parallel
to the respective surfaces.
γlv
γsv
θ
γsl
Liquid Phase Sintering
The surface energies for different interfaces is given by
γsl = solid/liquid
γsv = solid/vapor
γlv = liquid/vapor
The vectors representing these surface energies must
balance at the three phase triple junction. The equation
representing this balance is known as “Youngs’ equation”;
sv  sl  cos( )lv
Liquid Phase Sintering
γlv
γsv
θ
γsl
γlv
γsv
θ
γsl
Large γsl, non-wetting
θ Large
Large γsv, wetting
θ small
Liquid Phase Sintering
Grain boundary wetting
during LPS occurs
when θA and θB
approach zero
Grain B
θB
γAB
Grain A
AB  BL cos(B)  AL cos(A)
θA
γBL
Liquid
γAL
Examples of LPS
 Powder Metallurgy parts
-- Copper/Tin alloys
-- Iron/Copper structural parts
--Tungsten Carbide/Cobalt cemented carbides
 Ceramics
-- Silicon Nitride with a glassy liquid phase
(2wt% alumina + 6wt% yttria)
-- SiC with Silicon liquid phase
Disadvantages of LPS
 Compact slumping (shape distortion) which
occurs when too much liquid is formed during
sintering.
 The same parameters which control the sintered
microstructure often control the final properties.
 Useful application temperature of the material is
sometimes limited by the presence of too much
low melting point material.
Reactive Sintering
 Two or more constituents in a compact react
during sintering to form a new phase or phases.
 The reaction is normally exothermic and can
contribute to an enhancement of sintering.
 In some cases the reaction is so exothermic
that it can generate sufficient heat to cause
self-sintering without external heating except
that required for initiating the reaction.
Reactive Sintering
 This is the basis of combustion synthesis
which if properly controlled can produce a
relatively dense compact of the synthesized
reaction product.
Example of reaction sintering is
3TiO2 + 4AlN → 2Al2O3 + 2TiN + N2
Sintering Procedure
Ancient sintering techniques for the making
of pottery and ceramic art objects remain in
wide use to this day but research has also led
to more advanced techniques which work for
a wider array of ceramics and metals.
Sintering Procedure
In a typical sintering procedure
-- Most ceramic materials have a lower
affinity for water and a lower
plasticity index than clay, requiring
organic additives in the stages before
sintering.
-- A mixture of binder, water and
ceramic powder is pressed into a mold
a green body (un sintered item).
to form
Sintering Procedure
-- The green compact is placed on a
mesh belt and moved slowly through
the sintering furnace.
-- In the preheat zone, the lubricant
volatilizes, leaves the part as a vapor,
and is carried away by the dynamic
atmosphere flow.
Sintering Procedure
-- The temperature within the furnace
rises slowly in the preheat zone until
reaching the actual sintering
temperature.
-- It remains essentially constant during
the time at that temperature, and
proceeds into the cooling zone where
the drop in part temperature is
controlled.
Sintering Procedure
Schematically
Sintering Procedure
 As the parts travel through the furnace, the
temperature cycle results in changes in
composition and microstructure.
 In the hot zone metallurgical bonds
develop between particles and solid state
alloying takes place.
 The microstructure developed during
sintering determines the properties of the
part.
Sintering Atmospheres
 The operation is almost invariably carried
out under a protective atmosphere, because
of the large surface areas involved, and at
temperatures between 60 and 90% of the
melting-point.
 These are essential for almost all sintering
processes, to prevent oxidation and to
promote the reduction of surface oxides.
Sintering Atmospheres
 In practice dry hydrogen, cracked
ammonia, and partially combusted
hydrocarbons are mainly used.
 Although the first named is often precluded
because of cost. It is however, used for
sintering carbides and magnetic materials
of the Alnico type.
Sintering Atmospheres
 It can replace pure hydrogen for many
applications at approximately one-third the
cost, with the obvious exceptions where
reaction with nitrogen cannot be tolerated.
 It is particularly useful for sintering iron,
steel, stainless steel, and copper-base
components.
Sintering Atmospheres
The most widely used atmospheres
primarily because of their lower cost, are
produced by partial combustion of hydrocarbons.
Post Sintering Operations
1)Re-Pressing
Even with the best control that is feasible
in practice, there will inevitably be some
variation in the dimensions of parts
produced from a given material in a given
die set
Post Sintering Operations
Typically, it is possible for parts 'assintered' to be accurate to a tolerance of 0.0508mm per mm, in the direction at right
angles to the pressing -direction, and
0.1016mm per mm parallel to the pressing
direction
Post Sintering Operations
Dimensional accuracy can be greatly
improved by re-pressing the part after
sintering. This operation is called “sizing”
Post Sintering Operations
2) Hot Re-Press
Hot Repressing will give even greater
densification, with consequent greater
improvement in the mechanical properties,
but less accurate control of the final
dimensions is to be expected
Post Sintering Operations
3) Hot Isostatic Pressing
HIP is used as a post-sintering operation to
eliminate flaws and micro-porosity in
cemented carbides
Post Sintering Operations
4) Forging
Forging is a comparatively recent
technique in which a blank is hot repressed in a closed die which significantly
changes the shape of the part, and at the
same time can give almost complete
density and hence mechanical properties
approaching or even surpassing those of
traditional wrought parts
Post Sintering Operations
5) Infiltration
An alternative method of improving the
strength of inherently porous sintered parts
is to fill the surface connected pores with a
liquid metal having a lower melting point.
Pressure is not required, capillary action is
sufficient
Post Sintering Operations
6) Impregnation
This term is used for a process analogous
to infiltration except that the pores are
filled with an organic as opposed to a
metallic material
Post Sintering Operations
7) Heat Treatment
Although many, perhaps the bulk of
sintered structural parts are used in the assintered or sintered and sized condition,
large quantities of iron-based parts, are
supplied in the hardened and tempered
conditions. Heating should be in a gas
atmosphere followed by oil-quenching
Post Sintering Operations
8) Surface-Hardening
Carburizing and carbonitriding of PM parts
is extensively used, and again gaseous
media are indicated
Post Sintering Operations
9) Steam Treatment
A process peculiar to PM parts is steamtreatment which involves exposing the part
at a temperature around 500°C to high
pressure steam. This leads to the formation
of a layer of magnetite.
Post Sintering Operations
10) Blueing
Heating in air at a lower temperature (200250°C) can also be used to provide a thin
magnetite layer that gives some increase in
corrosion resistance, but it is much less
effective than steam treatment
Post Sintering Operations
11) Plating
Sintered parts may be plated in much the
same way as wrought or cast metals, and
copper, nickel, cadmium, zinc, and
chromium plating are all used.
Post Sintering Operations
12) Coatings
A large percentage of hard metal cutting
tool inserts are now coated using chemical
vapor deposition (CVD) or physical vapor
deposition (PVD)
Post Sintering Operations
13) Mechanical Treatments
Although a major attraction of PM parts is
that they can be produced accurately to the
required dimensions, there are limitations
to the geometry that can be pressed in rigid
dies, and subsequent machining, for
example of transverse holes or re-entrants
at an angle to the pressing direction is not
uncommon
Post Sintering Operations
14) De-burring
De-burring is done with sintered parts, and
is used to remove any 'rag' on edges,
resulting from the compacting operation or
a machining step
Advantages of Sintering
Particular advantages of this powder
technology include:
1. the possibility of very high purity for the
starting materials and their great uniformity
2. preservation of purity due to the restricted
nature of subsequent fabrication steps
Advantages of Sintering
3. stabilization of the details of repetitive
operations by control of grain size in the
input stages
4. absence of stringering of segregated
particles and inclusions (as often occurs in
melt processes)
5. no requirement for deformation to produce
directional elongation of grains
References
 Ceramic Processing and Sintering by M.N.
Rahaman
 Ceramic Matrix Composites by Richard
Warren
 Ceramic Fabrication Technology by Roy.
W. Rice
 The Inorganic Chemistry of Materials- How
to Make Things Out of Elements by Paul J.
Van Der Put
References
 Liquid Phase Sintering by Randell M.
German-Technology
 Blake Concise Dictionary
 Wikepedia-the Free Encyclopedia
 www.azom.com
 www.mse.mtu.edu
 www.epma.com