Lec. No. 2
Processing of glass fibers
All glass are derived from compositions containing sand, but it also
includes varying quantities of feldspar, sodium sulfate, boric acid, and
many other materials. In the glass melting furnace, the raw materials are
heated to temperatures ranging from 1500 to 1700°C and are transformed
through a sequence of chemical reactions to molten glass. In operation, raw
materials are introduced continuously on top of a bed of molten glass,
where they slowly mix and dissolve. Mixing is effected by natural
convection, gases rising from chemical reactions, and, in some operations,
by air injection into the bottom of the bed.
the molten glass is forced through heated platinum bushings containing
numerous very small openings(d=(1-2) mm). The continuous fibers
emerging from the openings are drawn over a roller applicator to produce
diameter of fiber in the range (5-15µm . the choice degree of glass molten
is depending on viscosity and temperature , the best temperature is upper
from 100C°from liquid line according to phase diagram.
Furnace for glass melting
Lec . No . 3
Oxide fibers
Ceramic oxide fibers, both continuous and discontinuous, have been
commercially available since the 1970s, and processing and microstructure
control are very important in obtaining the desired properties. Among the
desirable characteristics in any ceramic fiber for structural applications are:
1)
2 ) Small grain size for low-temperature applications
3 ) Large grain size for high-temperature applications
4) High purity.
1)Alumina fibers
Alumina fibers have γ, δ and α
α -Alumina is the
thermodynamically stable form. In practice, it is very difficult to control
the time and temperature conditions to proceed from γ
α . At low
firing, the fibers will give a smaller grain size and therefore an
unacceptable level of porosity. At higher processing temperatures, porosity
can be eliminated but excessive grain growth will result. This dilemma can
be avoided by introducing a second phase that restricts grain boundary
mobility while the porosity is removed at high temperature. It is possible to
select the type and amount of the second phase that inhibits the grain
growth at the service temperature.
Lec. No.4
Whiskers
Whiskers are normally obtained by vapor phase growth. They are
monocrystalline, short fibers with extremely high strength because
of their high aspect ratio (50 to 10 000). They have a diameter of a
few microns, but they do not have uniform dimensions and
properties. ceramic Whiskers produce from oxides : Al2O3 , MgO ,
MgO- Al2O3 , BeO , NiO , ZnO , Cr2O3.
ceramic Whiskers are produced from molten metal at air or wet
hydrogen inert gas then pull and crystal growth at one direction .
more Whiskers are used (Al2O3 , SiC).
table : properties of oxide whiskers
Material
Al2O3
BeO
B2O3
MgO
S.G
3.9
1.8
2.5
3.6
Melting point
2082
2549
2449
2799
T.S(Gpa)
14-28
14-21
7
7-14
E(Gpa)
550
700
450
310
Nano oxide fibers
An emerging technology is the production of fibers of very small diameter,
of the order of 50nm. These fibers are produced by the spinning of a
precursor organic fiber from a pipette to a collecting plate. A high voltage
(tens of kilovolts) is passed between the pipette and the plate and the
polymer is drawn from the pipette to the plate. The fibers are generally
collected on the plate to form a random array although work is proceeding
to align the fibers.
Lec. No.5
Non-oxide fibers
Commercially available non-oxide ceramic reinforcements are in three
categories: continuous, discontinuous, and whiskers. non-oxide fibers are
used in different application at high and low temperature . Silicon carbide
fiber is a major development in the field of ceramic reinforcements.
1. Silicon carbide
Continuous SiC fibers were produced by thermal degradation of a polymer
precursor such as a polycarbosilane . to from a continuous fiber is made
by melt-spinning. The fiber is then converted by pyrolysis at 1300°C into
a fiber consisting mainly of( β-SiC) of about 15µm diameter.
Methods of manufacturing SiC fibers
1) SiC fibers via polymers
2) SiC fibers via CVD
2)boron carbide and boron nitride
There are other promising ceramic fibers, e.g. boron carbide and boron
nitride. Boron nitride fiber has the same density (2.2 g /cm3) as carbon
fiber, but has a greater oxidation resistance and excellent dielectric
properties .Boron carbide fiber is a very light and strong material.
Lec. No.6
Fibrous monolithic ceramics (FMs)
Fibrous monolithic ceramics (FMs) consist of a hexagonal arrangement of
submillimeter ‘cells’ of strong polycrystalline ceramic and a network of
crack-deflecting weak ‘cell boundaries’. These composites are sintered or
hot-pressed monolithic ceramics with a distinct fibrous texture. This
unique architecture opened new avenues for ceramic composites, in which
they fail in a nonbrittle manner because of crack interactions with weak
cell boundaries such as crack deflection or crack delamination . This
approach provides simple and versatile method for manufacturing
nonbrittle ceramic composites from a variety of different material
combinations that include oxide ceramicsAl2O3/Al2O3–ZrO2 and nonoxide ceramics (SiC/graphite), (SiC/ BN ) and( Si3N4/BN).
three processing methods for producing fibrous monolithic ceramics, i.e.
coextrusion , microfabrication by coextrusion , and hybrid extrusion and
dip-coating .
Lec. No.7
Structures of Fibrous monolithic ceramics
Various material combinations Fibrous monolithic ceramics consist of
dense cells separated by a continuous cell boundary, in which the cells
provide most of the strength of the FM and the cell boundary provides the
toughness by isolating the cells from each other and promoting dissipation
of fracture energy by mechanisms such as pullout of the cells or deflection
of a crack through the cell boundary . The cell boundaries must be either
weak themselves or poorly bonded to the cells to dissipate fracture energy
and exhibit minimal or no reaction with the cells for long-term use at
elevated temperatures. To date, many kinds of structural ceramics have
been examined for a strong cell phase. They are in the forms of either
oxides (Al2O3, ZrO2 and ZrSiO4 ) or nonoxides( SiC , Si3N4 , and
borides ). Oxides have the advantage of stability in oxidizing
environments, while non-oxides have the advantage of substantially higher
strength and superior creep resistance.
In all-oxide FMs, porous cell boundaries are generally employed because
Lec. No.8
SiC/SiC Composites
CMCs exhibit high mechanical properties at high or very high
temperatures (400– 3000°C), and in severe environments. They were
developed initially for military and aerospace applications. Now they are
being introduced into new fields and their range of applications will grow
when their cost is lowered drastically. CMCs can be fabricated by different
processing techniques, using either liquid or gaseous precursors. The CVI
SiC/SiC composites consist of a SiC-based matrix reinforced
by SiC fibers. They are produced by Chemical Vapour Infiltration (CVI).
This technique derives directly from Chemical Vapour Deposition. In very
simple terms, the SiC-based matrix is deposited from gaseous reactants on
to a heated substrate of fibrous preforms (SiC). CVI is a slow process, and
the obtained composite materials possess some residual porosity and
density gradients. Despite these drawbacks, the CVI process presents a few
advantages:
(i) the strength of reinforcing fibers is not affected during composite
manufacture.
(ii) the nature of the deposited material can be changed easily, simply by
introducing the appropriate gaseous precursors into the infiltration
chamber.
(iii) a large number of components.
(iv) large complex shapes can be produced in a near net shape.
Lec. No.9
Interface properties – influence on the mechanical behavior
The fiber-matrix interfacial domain is a critical part of composites because
load transfers from the matrix to the fiber and occur through the interface.
Most authors promote the concept of weak interfaces to increase fracture
toughness. The major contribution to toughness is attributed to crack
bridging and fiber pull-out. Weak interfaces are detrimental to composite
strength. A high strength requires efficient load transfers from fibers to the
matrix. This is obtained with strong interfaces. These latter requirements,
to be met for strong composites, are therefore incompatible with the former
ones for tough composites, if toughening is based solely upon the above
mentioned weak interface-based mechanisms. Fiber/matrix interfaces exert
a profound influence on the mechanical behavior and the lifetime of
composites. Efforts have been directed towards optimization of interface
properties.
( FIGURE. Schematic diagram showing crack deflection when the fiber
coating/interface is strong (a) or weak (b))
Lec. No.10
Silicon Melt Infiltrated Ceramic Composites(MI-CMCs)
Silicon melt infiltrated, SiC-based ceramic matrix composites (MI-CMCs)
have been developed for use in gas turbine engines. These materials are
particularly suited to use in gas turbines due to their:
1) low porosity.
2) high thermal conductivity.
3) low thermal
expansion.
4) high toughness. 5)high matrix cracking stress.
PROCESSING of (MI-CMC)
the term “melt infiltrated ceramic matrix composite” (MI-CMC) will refer
only to continuous fiber composites whose matrices are formed by molten
silicon (or silicon alloy) infiltration into a porous SiC- and/or C-containing
preform. The process of (MI-CMC) includes :
1) As with most other ceramic composite systems, a coating is applied to
the fibers to serve as the fiber-matrix interphase.
2) fibrous perform
a) In the prepreg process
b)In the slurry cast process
Some important thermal properties of Prepreg and Slurry Cast measured at
room temperature and at 1200°C, are listed in Table 3. Overall, the thermal
properties
Table 3
Lec. No.11
Fracture Strength
The in-plane tensile fracture response of materials are typically
characterized by a stress-strain curve as shown in Figure 2 when measured
in a simple displacement controlled method. In general, the curve can be
divided into four sections (shown by the dotted lines in Figure 2), with the
first section representing the simple linear elastic loading of the composite.
FIGURE 2. Typical in-plane tensile stress-strain behavior for a continuous
fiber reinforced ceramic composite.
Lec. No.12
Carbon Fiber Reinforced Silicon Carbide Composites
(C/SiC, C/C-SiC)
Ceramic matrix composites (CMC), based on reinforcements of carbon
fibers and matrices of silicon carbide (called C/SiC or C/C-SiC
composites) represent a relatively new class of structural materials. In the
last few years new manufacturing processes and materials have been
developed.
PROCESSING of (C/SiC, C/C-SiC)
Three different techniques are currently used in an industrial scale for the
production of C/SiC and C/C-SiC composites, each of them leading to
specific microstructures and properties :
1)Chemical Vapour Infiltration (CVI).
2)Liquid Polymer Infiltration (LPI) or Polymer Infiltration and Pyrolysis
(PIP).
3)Liquid Silicon Infiltration (LSI).
Lec. No.13
Carbon fiber / carbon matrix composite (c/c composite)
Carbon fiber-reinforced carbon is a composite material consisting of
carbon fiber reinforcement in a matrix of graphite . Carbon–carbon is wellsuited to structural applications at high temperatures, or where thermal
shock resistance and/or a low coefficient of thermal expansion is needed.
While it is less brittle than many other ceramics, it lacks impact resistance
.carbon/Carbon (C/C) is a lightweight, high-strength composite material
capable of withstanding temperatures over 3000°C in many environments.
Processing of (c/c composite)
The material is made in three stages:
First, material is laid up in its intended final shape, with carbon filament
and/or cloth surrounded by an organic binder such as plastic. Often, coke
or some other fine carbon aggregate is added to the binder mixture.
Second, the lay-up is heated, so that pyrolysis transforms the binder to
relatively pure carbon.
Third, the voids are gradually filled by forcing a carbon-forming gas such
as acetylene through the material at a high temperature, over the course of
several days.