Sintering Shrinkage
The starting compact has a porous volume (P) of about 40% of the total
volume. However, for most applications, we want relatively non-porous,
even dense, ceramics (P ? 0%). In the absence of reactions leading to an
increase in the specific volume, densification must be accompanied by an
overall contraction of the part:
characterized by linear withdrawal (dl/l0), this contraction usually
exceeds 10%. The control of the shrinkage is of vital importance for the
industrialist: on the one hand, the shrinkage should not result in
distortions of the shape and on the other hand, it must yield final
dimensions as close as possible to the desired dimensions.
In fact, an excessive shrinkage would make the part too small, which
cannot be corrected, and an insufficient shrinkage would make the part
too large; in this case machining for achieving the desired dimension
must be done by rectification, often by means of diamond grinding wheel.
A finishing treatment all the more expensive as the volume of matter to
be abraded is large. It is difficult to control shrinkage with a relative
accuracy higher than 0.5%.
Because of the phenomenon of shrinkage, dilatometry tests are widely
used for the in situ follow-up of sintering: starting with the “green”
compact to arrive at the fired product, a heating at constant speed
typically comprises three stages:
i) thermal expansion, accompanied by a vaporization of the starting water
and a pyrolysis of the organic binders introduced to support the pressing
of the powder.
ii) a marked contraction, due to particle rearrangement, the development
of sintering necks and granular changes.
iii) a resumption of the thermal expansion of the sintered product.
Porosity is open as long as it is inter-connected and communicating: the
material is then permeable to fluids.
Porosity is closed when it is not inter-connected: even if it is not yet
dense, the material can then be impermeable. The porosity level
corresponding to the transformation of open pores to closed pores is
about P ? 10%.
???
??
=
3??????
4???
??
Where ? is the viscosity of the liquid phase ,
?L change in length of the fired product , L intial length of the fired
product.
Sintering -------------------Lecture (5)
15
Applying a pressure during sintering
In most cases, ceramics are sintered by pressure less sintering and it is
only for very special applications that we use “pressure sintering” or “hot
pressing”, which consists of applying a pressure during the heat treatment
itself. The characteristic of pressure sintering is that the pressures brought
into play which are usually about 10 to 70 MPa, but can exceed 100 MPa
have considerable effects compared to capillary actions, thus offering
advantages:
1) rapid densification at appreciably lower temperatures (several hundred
degrees sometimes) than those demanded by pressureless sintering.
2) possibility of reaching the theoretical density (zero porosity).
3) possibility of limiting the grain growth.
Furthermore, it can be possible to obtain the sintered part with its exact
dimensions (net shape), without the need for a machine finishing in
applications that require high dimensional accuracy.
We must have pressurization devices manufactured in materials that resist
the temperatures required by sintering – and even if these temperatures
are lower compared to those required by pressureless sintering, they are
still high – and the chemical reactions between these materials and the
environment (for example, oxidation of refractory metals), like the
reactions between the mould and the ceramic powder, must be limited.
One last difficulty: if the manufacture of parts with simple geometry
(pellets) can be done in a piston + cylinder mould (“uniaxial pressure
pressing”), obtaining more complex shapes, in particular undercut parts,
cannot be done by pressure sintering.
The powders to be sintered are generally very fine (< 1 ?m) and it is not
always necessary for them to contain additives required by pressureless
sintering (for example, MgO for the sintering of Al2O3). The justifiable
applications of pressure sintering are, for example, cutting tools (ceramics
or cermets) or optical parts, with the essential objectives of achieving a
100% densification and/or very fine grains – but the microstructure and
the crystallographic texture can present anisotropy effects because of the
uniaxiality of the pressing.
Alumina for cutting tools, carbides (B4C, for instance) or cermets are
examples of materials that can benefit from pressure sintering and HIP
Functional ceramics (BaTiO3 or, especially, magnetic ferrites) can gain
from very fine grains and the absence of residual porosity made possible
by pressure sintering.
As regards the mechanisms, pressure sintering implies:
i) rearrangement of the particles.
ii) lattice diffusion.
Sintering -------------------Lecture (5)
16
iii) grain boundary diffusion, and finally.
iv) plastic deformation and a viscous flow.
Surface Curvature
In the absence of an external stress and a chemical reaction, surface
curvature provides the driving force for sintering consider, for example,
one mole of powder consisting of spherical particles with a radius a. The
number of particles is:
N =
????
??????????
=
??????
????????
where ?? is the density of the particles, which are assumed to contain no
internal porosity, M is the molecular weight, and Vm is the molar volume.
The surface area of the system of particles is
S =4? a2 N =
3Vm
a
If ??sv is the specific surface energy (i.e., the surface energy per unit area)
of the particles, then the surface free energy associated with the system of
particles is
ES =
???????? ????
??
Es represents the decrease in surface free energy of the system if a fully
dense body were to be formed from the mole of particles and provides a
motivation for sintering.
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 .
الرجوع الى لوحة التحكم
|