classification of rtm

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


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


Flow chart showing classification of composites

Ref: engineering materials polymers, ceramics, composites;

A. K. Bhargava


Some classes of composites

Ref: engineering materials polymers, ceramics, composites;

A. K. Bhargava


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


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


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


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.


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.


Schematic of composite molding press


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.


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.


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.


- 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


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.


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


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).


Helical winding

Polar winding


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


Layout of computer controlled

filament winding machine

Layout of numerical controlled

filament winding machine


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


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.


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


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.


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


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


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.


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.


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


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


Powder metallurgy technique

Processing route for continuous fiber reinforced MMCs


Processing route for discontinuous fiber, whisker,

particulate reinforced MMCs


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.


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


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.


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


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.


Processing routes for CMCs


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.


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.


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


Fe2O3 + 2Al → 2Fe + Al2O3 => Metal-ceramic composite product

10Al + 3TiO2 + 3B2O3 → 5Al2O3 + 3TiB2 => Ceramic composite product

4Al + 3TiO2 + 3C → 2Al2O3 + 3TiC


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


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


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


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


The following cases are discussed in class.

-Thermoplastics vs thermosetting plastics

-Conductive polymers

-High temperature polymers

-Liquid crystals & LCD

classification of rtm

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