Alumina Zirconia Fibers
This fiber in which 20% wt of partially stabilized tetragonal
zirconia was added to increase the elongation to failure of the fiber and
easier to weave. The dispersion of zirconia intergranular particles of
(0.15μm) limited grain growth of the alumina grains which had a mean
diameter of (0.3μm). These particles underwent a martensitic reaction in
the vicinity of the crack tips, which in a similar bulk ceramic results in
the partial closure of cracks and in an increase of the fiber strength. This
fiber has a failure strength 1.8 GPa and E = 344 GPa,
Whiskers
Whiskers are normally obtained by vapor phase growth. They are
mono-crystalline, short fibers with extremely high strength because of
their high aspect ratio (50 to 10000). 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 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. The fibers are too fine to be tested by conventional techniques but
can be tested as bundles. At present this technology is still at the
laboratory stage so that few data are available however the nano-metric
diameter could be expected to confer on the filaments perhaps
exceptional properties which are not obtained with larger diameter fibers.
This is primarily because dislocation movement should be restricted so
that high strengths and low creep rates could be expected. At the
laboratory scale oxide fibers such as Al2O3, ZrO2 and TiO2 as well as
carbon fibers have been made but they are far from having being fully
evaluated.
Non oxide fibers
A) 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.
The characteristic commercial fiber of this type is that known as 'Nicalon'
. It has a rough surface, making for good fiber/matrix adhesion, but is
somewhat reactive towards oxygen. It is well wetted by molten metals
and is reasonably stable as a reinforcement for MMCs based on
aluminum and copper although it lacks long-term thermal stability. There
have been various attempts to improve this feature of the fiber, for
example by reducing the oxygen content and by adding titanium.
Methods of manufacturing SiC fibers
1) SiC fibers via polymers
The SiC fiber obtained via CVD is very thick and not very flexible. By an
alternative route, very fine, continuous and flexible fiber was obtained by
using a process of controlled pyrolysis of polymeric precursor. The
ceramic fibers produced by this process good mechanical properties;
good thermal stability and oxidation resistance have enormous potential
for the development of ceramic matrix composites.
2) SiC fibers via CVD
Silicon carbide (SiC) fibers are produced by using (methel trichloro siliane)
where it is deposited on fine grain shape with diameter (100nm). This
fibers have strength (6 Gpa) but very stiff and difficult to forming .
B) 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.
C) Carbon fibers
By oxidizing and pyrolysing a highly drawn textile fiber preventing
it from shrinking in the early stages of the degradation process, and
subsequently hot-stretching it, it is possible to convert it to a carbon
filament with an elastic modulus that approaches the value we would
predict from a consideration of the crystal structure of graphite, although
the final strength is usually well below the theoretical strength of the
carbon-carbon chain. The influence of strength-limiting defects is
considerable, and clean-room methods of production can result in
substantial increases in the tensile strength of commercial materials. Prior
to sale, fibers are usually surface-treated by chemical or electrolytic
oxidation methods in order to improve the quality of adhesion between
the fiber and the matrix in a composite. Depending on processing
conditions, a wide range of mechanical properties (controlled by
structural variation) can be obtained, and fibers can therefore be chosen
from this range so as to give the desired composite properties. Although
the fiber is highly organized and graphite-like, the structure is not
identical with that of graphite and the fibers should not, strictly speaking,
be referred to by that name.
Recent developments in this field have led to the use of pitch as a
precursor in place of textile fibers, and these newer materials have
extremely high stiffness, but rather lower strengths .
Properties of carbon fibers
1. High modulus.
2. Good strength.
3. Good thermal stability in the absence of O2
4. High thermal conductivity, assisting good fatigue properties
5. Low thermal expansion coefficient
6. Excellent creep resistance
7. Good chemical resistance .
8. Low electrical resistivity
The disadvantages are:
1. Relatively high cost, but prices have been falling and more emphasis is
now placed on using large tows
2. Low strain to failure with attendant handling problems
3. Compressive strength is lower than tensile strength and larger diameter
fiber does not give improved compression properties
4. Poor impact strength of composites
5. Care required during handling carbon fiber, since it is electrically
conducting and can cause havoc with electrical systems
6. Oxidizes in air at temperatures above 450 C˚.
7. Exhibits anisotropy in the axial and transverse directions.