classification of rtm
TRANSCRIPT
R. Ganesh Narayanan, IITG
Composite materials
The term composite can be defined in few ways. A composite is the combination of
two or more dissimilar materials having a distinct interface between them such that the
properties of the resulting material are superior to the individual constituting
components.
Composites can be defined as two or more dissimilar materials that are intimately
bonded to form integrated structure.
In general, two phases –Matrix which is continuous and surrounds the discontinuous
second phase – reinforcement are present. An advanced composite material is defined
as a resin, metal or ceramic matrix reinforced with the high strength and high stiffness
material in continuous fiber or filament form. Example for this is Glass Fiber
Reinforced Plastics (GRP) which combines the advantages of both plastics (less
strong & stiff) and glass fibers (less load bearing ability & ductility).
Function: i) The reinforcing phase is of low density, strong, stiff and thermally stable.
The major load on the composite is born by the reinforcing phase; ii) The matrix
performs the following functions. It takes the load and transfers it to the
reinforcement, it binds or holds the reinforcement and protects them from mechanical
and chemical damage, it also separates the individual fibers and prevents brittle cracks
from passing completely across the composite section.
Ref: Composite materials processing, fabrication
and applications Vol II, Mel M. Schwartz
R. Ganesh Narayanan, IITG
General requirements of composite materials
1. The second phase (fibers or particles) must be uniformly distributed throughout the
matrix and not in contact with one another
2. The constituents should not react with one another at high temperatures, otherwise
the interfacial bond will become weak leading to premature failure of the composite
3. In no case the second phase loose its strength, it should be well bonded with matrix
4. Lower modulus of elasticity is expected in matrix when compared to fiber
5. Both matrix and fiber should not have different coefficient of linear expansion
R. Ganesh Narayanan, IITG
Flow chart showing classification of composites
Ref: engineering materials polymers, ceramics, composites;
A. K. Bhargava
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Some classes of composites
Ref: engineering materials polymers, ceramics, composites;
A. K. Bhargava
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Reinforcements
• Reinforcements need not necessarily be in the form of long fibers. One can have them
in the form of particles, flakes, whiskers, short fibers, continuous fibers, sheets. It turns
out that most reinforcements used in composites have a fibrous form because materials
are stronger and stiffer in the fibrous form than in any other form.
• The use of fibers as high performance engineering materials is based on three
important characteristics => 1) smaller the fiber size, lower is the probability of having
imperfections in the material. The strength of the carbon fiber decreases as its diameter
increases; 2) a high aspect ratio (l/d), which allows a very large fraction of the load
applied to be transferred via matrix to the stiff and strong fiber; 3) a very high degree of
flexibility, which is really a characteristic of a material that has a high modulus and a
small diameter. This flexibility permits the use of variety of techniques for making
composites with the fibers.
• glass fibers, Boron fibers, carbon fibers, organic fibers, ceramic fibers, Nonoxide
fibers, whiskers are generally used reinforcement material
R. Ganesh Narayanan, IITG
Matrices
Polymer matrices: A polymer is defined as a long chain molecule containing one or more
repeating units of atoms joined together by strong covalent bonds. A polymeric materials is
a collection of a large number of polymer molecules of similar chemical structure. IN the
solid state, these molecules are frozen in space either in a random fashion or in a mixture
of random and orderly fashion.
Thermoplastic polymers – individual molecules are linear in structure with no chemical
linking between them. They are held in place by week secondary bonds such as vander
waals forces. With the application of heat and pressure, these intermolecular bonds in a
solid thermoplastic polymer can be temporarily broken and the molecules can be moved
relative to each other to flow into new positions. Upon cooling, the molecules freeze in
their new positions, restoring the secondary bonds between them and resulting in a new
solid shape.
Thermoset polymer – in this case, the molecules are chemically joined together by cross-
links, forming a rigid, three dimensional network structure. Once these cross-links are
formed during the polymerization reaction, the thermo set polymer can not be melted and
reshaped by the application of heat and pressure.
Thermoplastic polymers => high impact strength and fracture resistance, imparting
excellent damage tolerance to the composite material; higher failure strains than thermoset
polymers that provide a better resistance to matrix micro-cracking in the composite
laminate
R. Ganesh Narayanan, IITG
Fabrication techniques for polymeric matrices
Leaky mold technique using carbon fiber and a cold-setting resin: - The apparatus has
an open ended metal trough with a loose fitting top force that is T-shaped in cross
section. The mold is lightly coated with stearate grease parting agent, a quantity of
freshly catalyzed resin (like epoxy, polyester) is poured into the bottom of the mold and
weighted amount of fiber is dropped in to it.
-The resin wets the fiber bundle. After 10 mts the top force of the mold is placed over
the array and heavy weight is placed on top. The excess resin is removed through top-
bottom face clearance.
-After the resin is hardened, the mold may be opened and the smooth parallel-sided
rectangular specimen is removed. The sample can be trimmed to required size.
Leaky mold technique
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High pressure compression molding
- Used mainly for molding thermosetting (like phenolic, alkyd) powders and rubber
compounds. This method has some advantages over injection molding process. This
method is performed using relatively simple tools with no sprues, runners, gates.
Very little material is wasted in this. High fiber volume fractions and long fiber
lengths can be made using compression molding.
- The compression molding process can be divided into three basic steps:
1) Charge preparation: The stack of SMC plies placed in the preheated mold (called
charge). The plies are die cut in the desired shape and size from a properly matured
SMC roll. Rectangular ply patterns are commonly used in the charge, however,
circular, elliptical or other ply patterns can also be used
2) Mold closing: After placing the charge in the bottom half, the top mold is quickly
moved to touch the top surface of the charge. The top mold is closed at a slower rate
of 5-10 mm/s. As the molding pressure increases with continued mold closure, the
SMC flows toward the extremities, forcing the air in the cavity to escape through the
shear edges or other vents. The mold pressure ranges from 1-40 MPa. The common
mold temperature is 150°C. Both top and bottom are externally heated to maintain
the mold surface temperature within ±5°C of desired value
SMCs are thin sheets of fiber pre-compounded with a thermoset resin and are employed primarily in
compression molding processes.
R. Ganesh Narayanan, IITG
3) Curing: After the cavity has been filled, the mold remains closed for a fixed
period of time to ensure curing and ply consolidation. The curing time will
depend on factors like mold temperature, part thickness etc. At the end of curing,
the part is removed and allowed to cool outside the mold. As the part cools
outside the mold, it continues to cure and shrink.
The location of charge placement in the mold, amount of flow in the compression
molding process, temperature distribution during cooling are important
parameters in compression molding process.
Applications: Computer enclosures, dishwasher inner doors, light truck tailgate,
automotive road wheels etc.
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Schematic of composite molding press
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Autoclave molding
- This is well suited for large components where double curvature and the highest
quality molding are specified.
- The autoclave is a pressure vessel that can generate a pressure of several
atmospheres. It is also equipped with a means of producing a vacuum within any
airtight membranes placed within the vessel so that volatile matter such as solvents or
water vapor can be removed. Heating is closely controlled by electric heaters that
warm the atmosphere (usually nitrogen), and this transfers heat to the composite
layup by convection and conduction.
-This method produces denser, void free moldings because of higher heat and
pressure are used in the cure. Curing pressures are generally in the range 3.4 to 7x105
Pa.
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Pressure bag molding
- Air pressure, usually 2.04-3.4x105 Pa, is applied to a rubber bag or sheet that covers
the laid composite on the mold. Excess resin and entrapped air are removed during
this process. Pressurized steam can also be used to accelerate the cure. Only female
molds can be employed.
Vacuum bag molding
- This method uses vacuum to eliminate entrapped air and excess resin. A non-
adhering file of polyvinyl alcohol or nylon is placed over the layup and sealed at the
edges. A vacuum is drawn on the bag formed by the film and the composite is cured
at room temperature.
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Injection molding
- Widely used for high volume production of thermoplastic resin parts, reinforced and
also thermoset resins.
- Pellets of resin containing fiber reinforcement are fed into a hopper and then into a
heated barrel containing a rotating screw that mixes and heats the material. The heated
resin in then forced at high pressure through sprues and runners into a matched metal
mold. Precise and complex parts can be made.
- Process parameters - Melt temperature: controlled by the temperature control system
of the injection unit but may be affected by injection speed and back pressure; Injection
speed: Speed profile is used instead of single constant value; Injection pressure: This is
not constant during mold filling stage. Injection pressure builds up during mold filling
stage as the resistance to flow increases. When the mold is full, transfer from speed
control to pressure control takes place.
- Thermoplastic materials: Every thermoplastic resin is injection molded. They are
present in filled and reinforced forms. Filled and reinforced indicate that a second,
discontinuous, usually rigid phase has been blended into the polymer. Aspect ratio
(largest to smallest dimension ratio) is close to 1, the second phase is referred as filler.
If the aspect ratio is much larger than 1 (like in fibers), the term reinforcement is used.
- Glass fibers provide higher room and high-temperature rigidity than unfilled PP.
R. Ganesh Narayanan, IITG
- Reinforcing material can be either fibrous or planar shape. In practice fibrous
reinforcements are almost exclusively used with glass fibers dominating the market.
Carbon or aramid fibers are also used but expensive comparatively. Planar
reinforcements => talc, mica, glass flake can be used where stiffness and isotropy are
required.
- Orientation and redistribution of the reinforcing fibers occurs during injection
molding and can exert a strong influence on the mechanical properties of the composite
part.
- Applications: housing for electrical tools, automotive applications, plastic drawers,
metal inserts
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Filament winding
- This is a process in which a filamentary yarn or tow is first wetted by a resin and
then uniformly and regularly wound about a rotating mandrel. The finished pattern is
cured and the mandrel removed. The result can be as simple as a piece of pipe or as
complex as an aircraft fuselage or an automobile frame.
- Advantages: low material and labor costs, reproducibility due to robotic motions
- disadvantages: tooling limitations for removable mandrels and inability to wind on
negatively curved surfaces
Materials: fibers => fiberglass, carbon, aramid; resins => thermoset polyesters, vinyl
esters, epoxies, phenolics
Fibers: fiber glass – either single end or multi strand roving. Single end roving is one
strand of glass filament collected into a discrete bundle during the spinning
operation.
Aramid fiber – High strength to weight ratio compared to fiber glass, good abrasive
wear resistance hence used as an external layer for structures that receive
considerable wear and abrasion.
Carbon fiber – It is brittle comparatively and hence has tendency to break. The no.
of turns and twists must be kept low when using this.
R. Ganesh Narayanan, IITG
Resin
Filament winding can utilize resin in three distinct forms. The predominant one is as a
liquid, where fiber is wet as it passes through a resin bath. Another form is prepreg
tow, where the fiber is impregnated in an early step, and wound on a bobbin. A third
form utilizes thermoplastic resins, which may be in the form of a dry bobbin, a
powdered coating.
Filament winding process
R. Ganesh Narayanan, IITG
Basics of process
-A large number of fiber rovings are pulled from a series of creels into a liquid resin
bath containing liquid resin, catalyst and other ingredients such as pigments and UV
absorbers. Fiber tension is controlled by fiber guides or scissor bars located between
each creel and the resin bath.
- At the end of the resin tank, the resin impregnated rovings are pulled through a
wiping device that removes excess resin from the rovings and controls the resin
coating thickness around each roving. The most commonly used wiping devices are
squeeze rollers and orifice (like wire drawing). Pulling through orifice provides
better control of resin content.
- Once the rovings are thoroughly impregnated and wiped, they are gathered together
in a flat band and positioned on the mandrel. Typical winding speed range from 90-
110 linear m/min.
- The filament winding can be either helical or polar winding (in figure).
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Helical winding
Polar winding
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Equipment
- Winders, mandrel & curing systems; Mandrel: metal mandrels, expandable mandrels,
single use mandrels; curing system: ovens, hot oil, lamps, steam, autoclave, microwave
Process parameters
- Fiber tension: adequate fiber tension is required to maintain fiber alignment on the
mandrel as well as to control resin content in the wound part. Excessive fiber tension can
cause differences in resin content in the inner & outer layers, undesirable residual stresses
in the finished part and large mandrel deflections.
- Good fiber wet out is needed for reducing voids in a filament wound part. The following
material and process parameters control fiber wet out – 1) viscosity of the catalyzed resin,
2) no. of strands in a roving, which determines the accessibility of resin in each strand, 3)
fiber tension, 4) speed of winding and duration of resin bath.
- Proper resin content and uniform resin distribution
Defects: voids, delaminations, fiber wrinkles are predominantly occuring; voids => poor
fiber wet out, presence of air bubbles in the resin bath, improper band width resulting in
gapping or overlapping, excessive resin squeeze out from the interior layers;
delaminations => reducing the time lapse and brushing the wound layer with fresh resin
just before starting the next winding are recommended for reduced delamination; wrinkles
=> improper winding tension & misalinged rovings
R. Ganesh Narayanan, IITG
Layout of computer controlled
filament winding machine
Layout of numerical controlled
filament winding machine
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Resin transfer molding: - RTM has the potential of becoming a dominant low cost
process for the fabrication of large, integrated, high performance products
Process definition: - Used to make wide variety of articles from small armrests to large
water treatment plant components
-A dry reinforcement material that has been shaped into a preform piece, generally
called as a preform, is placed in a prepared mold cavity. The mold is closed and sealed
properly. Then resin is injected into the mold cavity where it flows through the
reinforcement preform, expelling the air in the cavity and wetting out or impregnating
the reinforcement.
-The optimum range for the low viscosity premixed resin is 200-300 cps. Important
resins used are polyester, vinyl ester, epoxy.
-Once curing is completed, it is removed from the mold and the process can begin again
to form additional parts.
Structural reaction injection molding: - Preform and mold preparation are similar in
RTM and SRIM. Some changes in mold release and reinforcement sizings are
incorporated. After mold closing is done, the resin is rapidly introduced into the mold
and reacts with the reinforcement. Curing is completed shortly after the resin reaches
extremities of the components. The part is removed from the mold after curing.
R. Ganesh Narayanan, IITGRTM process
SRIM process schematic
R. Ganesh Narayanan, IITG
Difference between RTM & SRIM
- RTM resins are typically low viscosity liquids in the range 100-1000cP. Resin has two
components and required preinjection mixing ratio of 100:1. The liquid parts can be
mixed at low pressure. SRIM also has two part, low viscosity liquids in the viscosity
range of 10-100cP. They are very reactive in comparison to RTM resins and require
very fast, high pressure impingement mixing to achieve thorough mixing before entering
the mold. Mix ratios of 1:1 are desirable.
- In RTM there are possibilities to position the preform in the mold that provides control
of the fiber content and the mechanical properties. RTM provides minimum movement of
reinforcement during filling and curing process, that allows optimum performance at
minimum weight. But in SRIM there is the tendency for fiber particles to move during
the filling process as a result of rapid flow.
- RTM process occurs within the mold and hence offers limited chemical exposure and
limits the release of emissions during the process.
- RTM disadvantages: difficult to automate the process, long cycle times possible, lack
of reinforcement at the edges of the preform inside the mold, filling large parts
containing a high glass content at low injection pressures and with the undeveloped
nature of higher speed versions of the process.
R. Ganesh Narayanan, IITG
Process variations in RTM
Pressure injection: The method previously described utilize resin that was initially placed
in reservoir and flowed by pressure differential between the reservoir and the mold
outlet. The pressure differential could be caused by gravity, vacuum applied to the mold
outlet, pressure applied to the reservoir or a combination of all. This process is termed as
pressure injection.
Resin film infusion: In this flow through thickness of the preform is seen. A mold is
required on only one side of the preform. Resin is placed on the mold surface in film
form so that the preform may be placed and vacuum bagging material applied without
uncontrolled flow of resin. To infiltrate the preform, air is evacuated from the vacuum
bag and heat is applied. The resin flows through preform. If a pressure higher than
atmospheric pressure is required, an autoclave is used.
Pressure injection Resin film infusion
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Recently developed processes
Seeman composite resin infusion molding process (SCRIMP)
This technique for comolding composite skins and core in one piece without the need
for an oven or autoclave has been used to fabricate glass fiber-vinyl ester arc segments
and onoinskin that has been glued to concrete columns. The parts made by this method
is 30% less expensive to manufacture that produced by other methods.
SCRIMP process
Only a one sided tight vacuum surface is required. IN one infusion step, resin eliminates air
voids and wets out both skins and core. The use of thick materials can speed up layup.
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Thermoformed thermoplastic materials
The laminate is loaded into the clamp frame and placed in the oven for heating stage of
the process. Once the forming temperature is reached, the laminate is rapidly transferred
via the clamp frame to the forming station, at which point the tool is closed and pressure
is applied. The clamp frame is released just before the upper and lower tools close,
allowing the laminate to slip the mold as required. Vacuum forming is also applicable
for thermoplastic composites.
Thermo forming
Vacuum forming
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Metal matrix composite processing
-The critical need of high strength, light weight, high stiffness materials has in recent
years resurrected much interest in continuous and discontinuous reinforced MMCs.
-MMCs consists of two components at least, 1. metal matrix, 2. reinforcement. Matrix is
generally an alloy. In the production of composite, the matrix and reinforcement are
mixed together unlike in any alloy with two or more phases.
-MMC reinforcement is divided into five types, i) continuous fibers, ii) discontinuous
fibers, iii) whiskers, iv) wires, v) particulates. Reinforcements are generally ceramics like
oxides, carbides, nitrides. They have excellent combinations of specific strength, stiffness
at ambient temperature and elevated temperature. The typical reinforcement used are
given in table.
Typical reinforcements used in MMCs
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Interfaces in MMCs
- The interface region in a composite is extremely important in determining the
ultimate properties of the composite. An interface is a bi-dimensional region through
which there occurs a discontinuity in one or more material parameters. In practice,
there is always some volume associated with the interface region over which a gradual
transition in one or more material parameters occurs.
-Important discontinuities are elastic moduli, thermodynamic parameters such as
chemical potential, leading to chemical compound formation, thermal expansion
coefficient.
-The applied load is transferred from the matrix to the reinforcement via a well-bonded
interface. There is a chemical potential gradient across the fiber matrix interface. The
interface region thus formed generally have characteristics different from those of
either of the components.
-Ceramic metal interfaces are generally formed at high temperatures. Diffusion and
chemical reaction kinetics are faster at elevated temperatures. Various parameters like
time, temperature, pressure combined with the thermodynamic, kinetic and thermal
data can be used to obtain an optimum set of interface characteristics in a given MMC.
-Mechanical and chemical bonding can contribute to the bond strength.
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Processing
The MMCs can be manufactured by processes in any of the three categories, liquid phase
processes, solid-phase processes, two phase (solid-liquid) processes
1) Liquid phase processes: The ceramic particulates are incorporated into a molten metallic
matrix using various techniques. This is followed by mixing and eventual casting of the
resulting composite mixture into shaped components or billets for further fabrication. The
selection criteria for ceramic reinforcement includes, 1. elastic modulus, 2. tensile strength,
3. density, 4. melting temperature, 5. thermal stability, 6. size and shape of the reinforcing
particles.
Pressure infiltration or squeeze casting: IN this process, liquid metal is forced into a fibrous
preform. Pressure is applied until solidification is complete. By forcing the molten metal
through small pores of a fibrous preform, this method requires good wettability of the
reinforcement by the molten metal. Composites fabricated by this method involves minimal
reaction of reinforcement with molten metal and free of common casting defects such as
porosity and shrinkage cavities. Inexpensive for making near net shaped parts.
When the infiltration of fiber preform occurs readily, reactions between the fiber and the
molten metal can significantly degrade fiber properties. Fiber coatings applied prior to
infiltration, which improve wetting and control reactions, have been developed and can
produce impressive results. In this case, caution should be taken such that fiber coatings
must not be exposed to air prior to infiltration because surface oxidation alters the positive
effects of coating.
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squeeze casting
Melt infiltration: In this process, a molten alloy is introduced into a porous ceramic preform,
utilizing either inert gas or a mechanical device as a pressurizing medium. The pressure
required to combine matrix and reinforcement is a function of the friction effects due to the
viscosity of the molten matrix as it fills the ceramic preform. Wetting of the ceramic preform
by the liquid alloy depends on alloy composition, ceramic preform material and surface
morphology, temperature, time. This method is used to make toyota diesel piston. Drawbacks
include reinforcement damage, preform compression, micro-structural non-uniformity, coarse
grain size, undesirable interfacial reactions
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2) Solid phase processes
The fabrication of particulate reinforced MMCs from blended elemental powders involves
a number of steps prior to final consolidation. Two methods viz., powder metallurgy
method and high energy rate processing are generally used.
Powder metallurgy technique: This includes blending of rapidly solidified powders with
particulates, platelets, whiskers using number of steps. They are, sieving of rapidly
solidified powders, blending with the reinforcement phase, pressing to 75% density,
degassing, final consolidation by extrusion, forging, rolling or other hot working methods.
PM methods involving cold pressing, sintering or hot pressing produce MMCs. The
matrix and the reinforcement powders are blended to produce a homogeneous
distribution. The blending stage is followed by cold pressing to produce green compact
that is app. 80% dense. The green compact is degassed to remove any absorbed moisture
from the particle surfaces. The final step is hot pressing to make fully dense composite.
PM hot pressing method produces properties superior to those obtained by casting and by
liquid metal infiltration methods. This process produces homogeneous distribution of
whiskers when compared to that obtained with melt infiltration.
Limitations: limited availability of appropriate prealloyed metal powders, high cost of
metal powders, high cost of hot pressing
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Powder metallurgy technique
Processing route for continuous fiber reinforced MMCs
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Processing route for discontinuous fiber, whisker,
particulate reinforced MMCs
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High energy, high rate processes
This approach has been used successfully to consolidate rapidly quenched powders
containing a fine distribution of ceramic particulates. In this approach, consolidation
is done by applying high energy over a shot time period. Both mechanical and high
electrical energy can be used for this.
For eg., Al/SiC MMCs can be made by heating a customized powder blend through a
fast electric discharge obtained from a generator. The high energy, high rate pulse
facilitates rapid heating of the conducting powder in a die with cold walls. The rapid
energy controls phase transformation, microstructural aspects that are not possible by
other methods.
Diffusion bonding
Common solid state welding technique for joining similar or dissimilar metals.
Interdiffusion of atoms at elevated temperature leads to welding.
Advantages: ability to process a wide variety of matrix materials, control of fiber
orientation and volume; disadvantages: processing times of several hours, cost of
high processing temperature and pressure, objects of limited size can be made.
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DB process: - Two materials are pressed together (typically in a vacuum) at a specific
bonding pressure with a bonding temperature for a specific holding time.
-Typically 50-70% of the melting temperature of the most fusible metal in the
composition. Raising the temperature aids in the inter-diffusion of atoms across the face
of the joint.
Sequence for diffusion bonding a ceramic to
a metal
a) Hard ceramic and soft metal edges
come into contact.
b) Metal surface begins to yield under
high local stresses.
c) Deformation continues mainly in the
metal, leading to void shrinkage.
d) The bond is formed
Making MMCs: Here primarily the metal or alloys in the form of sheets and the
reinforcement material in the form of fiber are chemically surface treated for the
effectiveness of interdiffusion. The fibers are placed on the metal foil in predetermined
orientation and bonding takes place by press forming. However, the fibers are sometimes
coated by plasma spraying or ion plating to enhance the bonding strength before diffusion
bonding. DB can be done under vacuum conditions also.
R. Ganesh Narayanan, IITGComposite fabrication by diffusion bonding
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3) Two phase processes
This involves mixing of ceramic and matrix with matrix containing both solid and
liquid phases. Applicable two phase processes include the Osprey, rheocasting and
variable codeposition of multiphase materials (VCM).
Osprey deposition: In this process, the reinforcement particulates are introduced into
a stream of molten alloy which is subsequently atomised by jets of inert gas. The
sprayed mixture is collected on a substrate in the form of a reinforced metal matrix
billet. Similar to blending and consolidation steps in PM processes for making
MMCs.
Rheocasting: fine ceramic particulates are added to a metallic alloy matrix at a
temperature with the solid-liquid range of the alloy. This is followed by agitation of
the mixture to form a low viscosity slurry. The ceramic particles are mechanically
entrapped initially and are prevented from agglomeration. The ceramic particles
interact with liquid matrix to effect bonding.
Compocasting is an application of the rheocasting process in which particulate or
fibrous materials are added to the semisolid slurry. The particles or short fibers are
mechanically entrapped and prevented from settling or agglomerating because the
alloy is already partially solid. This is one of the most economical methods of
fabricating a composite with discontinuous fibers.
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Advantages: - performed at temperatures lower than those conventionally employed in
foundry practice during pouring, resulting in reduced thermochemical degradation of
the reinforced surface;
- Can be carried out by conventional foundry methods
Disadvantages: residual pores between fibers cannot be eliminated completely,
method cannot be used for fabricating fiber reinforced composites.
Variable codeposition of multiphase materials:
Deposition methods like spray forming, electroplating, CVD, PVD etc. are also used
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Ceramic matrix composite processing
- Occupies 40% of the total market
- continuous fiber composites => fiber orientation, architecture is important
- Typical fiber architecture is obtained with fine fibers is the use of fiber tows.
Handling of fiber tows becomes a major factor in fiber composite processing. The
most common approach in introducing the matrix is to put fiber tows through a bath
that is the source of the matrix. Matrix can be a slurry, but can be a sol or a
preceramic polymer too. Distribution of matrix in the tows is important.
Fiber-matrix interface:
-Fiber-matrix interface region is important like in the cases of MMCs and PMCs
-For high toughness in fiber reinforced CMCs it is essential to produce and maintain
a desirable level of interfacial shear stress to permit fiber debonding during the
fracture process
-Exhibit low inter-laminar and transverse tensile and shear strengths (say 2-3 MPa)
-The interface must serve the various functions like controlling interfacial strength
and prevent fiber matrix reactions not only during processing and fabrication but
also during service at high temperatures and in aggressive environments.
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Processing routes for CMCs
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Processing methods
This can be classified under two broad groups like powder consolidation and
chemically based methods.
Powder based methods: Powder consolidation methods, like hot pressing, is
extensively used for both glass based and crystalline matrices. The most common way
of introducing such powders is to draw fibers (or tows) through a slurry or a sol.
Hot pressing allows achievement of low to zero porosity levels is applicable to all
ceramic materials. Limitations of hot pressing are two fold – 1) applicable to simple
shapes like plates, blocks, cylinders and not a low cost process; 2) other limitation
include temperatures commonly required for low level of porosity. This temperature is
of the order of 100-200 °C higher than required for matrix alone, present limitations
with regard to both reaction between fibers and matrices and degradation of the fibers.
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Hot pressing produces the highest densities at higher processing temperatures from
1400-1600 °C, which seriously limits the type of fiber used as well as the various fiber-
matrix combinations.
Hot pressing is applied predominantly to 2D composites like cloth laminates assuming
adequate infiltration of matrix between the fibers within the cloth can be obtained. Its
applicability to three and higher dimensional composites will be limited by fiber
damage via buckling and from interference with densification caused by the fibers in
the axial direction.
HIP Method:
-High densities can be achieved at low temperatures than required for hot pressing
-Applicable for broader range of shapes than is hot pressing
Sintering method:
-Green ceramic fiber composite compact is much higher than that of a conventionally
green ceramic compact because of higher fiber costs and higher composite body
formation costs.
- inherent problems in densifying ceramic fiber composites and temperature limitations
based on fiber-matrix interactions and fiber temperature limitations.
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Chemical based methods
Our interest here is to know the reactions involving a powder compact that yields a
composite product in conjunction with heating, possibly with pressure, such as in
HIP.
Some of the examples of the processes are,
• Autocatalytic reaction processes (Al2O3/B4C)
• Displacement reactions
• Reactions of organo-metallic compounds
• Sol-gel and polymer reactions techniques
• Melting phase infiltration techniques
• Direct melt oxidation
• Gas phase infiltration/deposition
Self propagating high temperature synthesis (SHS) => exothermic reactions results
in a high temperature reaction front that actually sweeps through a compact of the
reactants once the reaction is ignited at some point
Single phase compounds (TiC, TiB2) can be made & also composites directly
High temperature
reaction front
Compact of
reactants
R. Ganesh Narayanan, IITG
Fe2O3 + 2Al → 2Fe + Al2O3 => Metal-ceramic composite product
10Al + 3TiO2 + 3B2O3 → 5Al2O3 + 3TiB2 => Ceramic composite product
4Al + 3TiO2 + 3C → 2Al2O3 + 3TiC
R. Ganesh Narayanan, IITG
Polymer materials
Polymers => complex giant molecules of higher molecular weight (104-107). These
big molecules are also called as macromolecules. They are basically hydrocarbons
and frequently contain atoms of oxygen, chlorine, fluorine, nitrogen or sulphur.
Polymer can be categorized into plastic, fiber, resin
Plastics => solid substances in their final state are made plastic at some stage during
their fabrication, enabling them to be moulded under the application of heat and
pressure is called plastic. Examples => PVC, polyethylene, nylon, perspex, teflon,
bakelite.
Fibers => Polymers drawn into long thread like molecules (length atleast hundred
times its diameter). Examples => nylon, polyester, cellulose etc.
Liquid resins => Polymers used in liquid form like adhesives, sealants
Polymerization => process by which monomers are joined together to form large,
chain like molecules. The chemical reactions in the process can be induced by
application of heat and pressure or by using catalyst.
Mechanism – two types => 1. addition or chain polymerization, 2. condensation or
step polymerization
R. Ganesh Narayanan, IITG
Addition or chain polymerization
Long chain macromolecules are formed by chemical reaction of one or more types of
monomer units having one double bond prior to polymerization. The chemical
reaction is initiated by a substance called initiator (I). This has one unpaired
electron called as free radical (R*).
Monomer (M) combines
with free radical
Propagation
Termination
Propagation
C* => unpaired
electron at right end
Monomer combines
with free radical
Single bond
R. Ganesh Narayanan, IITG
Termination
reaction
Finally two growing chains may react to terminate growth activity of each other
and result in macromolecule
Examples of additional polymers include polyethylene, polypropylene, PVC, poly
vinyl alcohol, poly vinyl acetate, polystyrene, poly methyl methacrylate
R. Ganesh Narayanan, IITG
Condensation polymerization
A polymer is produced by the chemical reaction of at least two bi- or poly-functional
monomer units with the production of a non-polymerizable molecule with water as by
product. The reaction continues until almost all the monomeric reagent of one type is
used up. Examples for condensation polymerization include polyester, phenol
formaldehyde, polyurethanes, epoxies.
A molecule of water is given off as by product and the nylon is formed. The
properties are determined by the R and R' groups in the monomers.
dicarboxylic acids polyamines polyamide
Condensation reaction
R. Ganesh Narayanan, IITG
The following cases are discussed in class.
-Thermoplastics vs thermosetting plastics
-Conductive polymers
-High temperature polymers
-Liquid crystals & LCD