sintering additives
the additives help to control the microstructure of the sintered materials
they can be classified under two categories:
– additives that react with the basic compound to give a liquid phase, for
example silicon nitride si3n4 ceramics are an example of where some
sintering additives are selected to react with the silica layer (sio2) that
covers the nitride grains. thus, magnesia mgo reacts with sio2 to form
mgsio3, from which we have liquid phase at about 1550°c.
– additives that do not lead to the formation of a liquid phase and which
consequently enable the sintering to take place in solid phase. this is the
case of the doping of al2o3 with a few of mgo , because the lowest
temperature at which a liquid can appear in the al2o3-mgo system
exceeds the sintering temperature (which, for alumina, does not go
beyond 1700°c).
the chronothermic effect (“a long duration heat treatment at lower
temperature is equivalent to a short duration heat treatment at higher
temperature”).
sintering with liquid phase
parameters of the liquid phase
various technical ceramics, most metals and cermets are all sintered in
the presence of a liquid phase. it is rare that sintering with liquid phase
does not imply any chemical reactions, but in the simple case where these
reactions do not have a marked influence, surface effects are
predominant. the main parameters of the liquid phase:
1) quantity of liquid phase: as the compact stacking of spheres leaves
a porosity of approximately 26% this value is the order of
magnitude of the volume of liquid phase necessary to fill all the
interstices and allow the rearrangement of the grains observed at the
beginning of the vitrification. however, the presence of a small
quantity of liquid (a few volumes percent) does not make it possible
to fill the interstices.
p=
damb
d3
??
????
(tf-t)
where p porosity , damb is the ambipolar diffusivity , d is the average
diameter of the sintering particles, k constant , t temperature , tf final
time and t the initial time for sintering , l volume fraction of liquid phase.
sintering -lecture (3)
9
2) viscosity of the liquid: this decreases rapidly when the temperature
increases. pure silica melts only at a very high temperature to
produce a very viscous liquid. the presence of lkaline earths quickly
decreases the softening temperature and the viscosity of the liquid.
? = ?o exp(q/rt)
where q is the apparent activation energy of the process, r is the ideal
gas constant, t the temperature ,? viscous flow of vitreous phases , ?o
the viscosity .
the viscosity of the liquid should be neither too low – because then the
sintered part becomes deformed in an unacceptable way – nor too high –
because then the viscous flow is too limited, making grain rearrangement
difficult.
3) wettability: wettability is quantifiable by the experiment of the
liquid droping placed on a solid, because the equilibrium shape of the droping
minimizes the interfacial energies. if ?lv is the liquid-vapor energy,
?sv the solid-vapor energy and ?sl the solid-liquid energy, the angle
of contact (?) is such that
?lvcos? = ?sv – ?sl
when ?sl is high, the droping minimizes its interface with the solid, hence a
high value of ?: ? > 90° corresponds to non-wetting (depression of the
liquid in a capillary). on the contrary, when ?sl << ?sv, the liquid
spreads on the surface of the solid: ? < 90° corresponds to wetting (rise of
the liquid in a capillary) and for ? = 0, the wetting is perfect.
4) the respective solubilities of the solid in the liquid and the liquid in
the solid.
the stages in liquid phase sintering
the shrinkage curve recorded during an isothermal treatment of liquid
phases shows three stages:
– viscous flow and grain rearrangement: when the liquid is formed, the
limiting process consists of a viscous flow, which allows the
rearrangement of the grains. the liquid dissolves the surface asperities
and also dissolves the small grains . the granular rearrangement is
limited to the liquid phase sintering itself, but it can be enough to allow
complete densification if the liquid phase is in sufficient quantity,
as is the case in the vitrification of silicate ceramics.
sintering -lecture (3)
10
– solution-reprecipitation: the solubility of the solid in the liquid increases
at the inter-particle points of contact. the transfer of matter followed by
reprecipitation in the low energy areas results in densification.
– development of the solid skeleton: the liquid phase is eliminated
gradually bythe formation of new crystals or solid solutions and the last
stage of the elimination of porosity .
the disintegration of the grains attacked by the liquid results in the
ripening (coalescence of small particles to give a larger particle) and
changes in the shape of the particles, with flattening of the areas of
contact. as the crystalline growth is less hampered when a crystal grows
in a liquid than when it remains in contact with solid obstacles, we
sometimes observe grains whose morphology reflects these anisotropy
effects: for instance, they are elongated and faceted.
r2
0.av – r2
av = (2 ?sv pflat kr /kt ) t
where r0.av is the average particle size at t = 0. r2
av is the average particle
size after rearrangement , kr is a proportionality constant related to the
mobility of the interface , ?sv surface –vapor energy , k constant , t
temperature , t sintering time , pflat the partial pressure over any particle of
radius of flattening of the particles in the areas of contact .
– weak reaction between liquid and solid: the liquid has the primary role,
after cooling, of forming the matrix in which the grains that have not
reacted have been glued. this is the case of abrasive materials where the
grains (silicon carbide sic or alumina al2o3) are bound by a solidified
vitreous phase.
pore evolution during sintering
microstructural coarsening during sintering is commonly considered in
terms of how fast the grains grow. however, a more realistic view of the
sintering compact may be that of a network of contacting grains
interpenetrated by a network of porosity, microstructure evolution in
porous system. the compact formed from fully dense particles, the
porosity is initially all connected. as sintering proceeds, more and more
of the open porosity is converted to closed porosity. the conversion is,
however, dependent on the packing uniformity of the structure.
initially, the total porosity of the zno compact was 0.37, made up of an
open porosity of 0.36 and closed porosity of 0.01. during sintering at
1400°c, the open porosity decreased continuously, whereas the closed
porosity increased initially to 0.04 but returned to the value of 0.01. this
sintering -lecture (3)
11
initial variation in the closed porosity may be due to the sintering of fairly
dense agglomerates. when the open porosity had decreased to 0.15, the
closed porosity began to increase again and reached a maximum value of
0.05, after which it decreased with further sintering.
pf - p0= -(const) damb ?sv/d3kt
where p0 is the initial size of the pores , pf is the final size of the
pores , damb is the ambipolar diffusivity , d is the average diameter of
the sintering particles, k constant , t temperature , ?sv surface –vapor
energy .
controlled use of abnormal grain growth
whereas the suppression of abnormal grain growth is commonly a key
goal in sintering, the ability to exploit abnormal grain growth in a
controlled manner can provide significant benefits for several ceramic
systems. for example, seeding a fine-grained polycrystalline ceramic
with a very large grain does have the effect that this grain will become
very large before the slower growing matrix will have developed an
average size even comparable to the seed grain.
the procedure proved successful for batio3, al2o3, and. the in situ
growth of a controlled distribution of anisotropic abnormal grains in a
fine-grained matrix has been used to enhance the fracture toughness of
si3n4, sic, and mullite.
a more elegant procedure developed, referred to as templated grain
growth, involves the use of large, elongated seed crystals that are mixed
with a fine equiaxial powder and aligned during the forming of the green
body by, for example, tape casting. the aligned seed crystals act as
templates to pattern the growth of the matrix during sintering, leading to
microstructural texturing.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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