The document discusses the functions and desired properties of matrices in composite materials. Matrices hold fibers together, protect them from the environment, distribute loads evenly between fibers, and improve properties like impact resistance. Desired matrix properties include low moisture absorption, shrinkage, and thermal expansion. The document also discusses polymer matrix composites, classifications of polymers, factors affecting composite properties, and manufacturing processes for composites with thermoplastic and thermosetting matrices.

1. Advanced materials
Unit-2
Functions of Matrix
• Holds the fibres together.
• Protects the fibres from environment.
• Distributes the loads evenly between fibres so that all fibres are subjected to the same amount
of strain.
• Enhances transverse properties of a laminate.
• Improves impact and fracture resistance of a component.
• Helps to avoid propagation of crack growth through the fibres by providing alternate failure
path along the interface between the fibres and the matrix.
• Carry inter-laminar shear
Desired Properties of a Matrix
• Reduced moisture absorption.
• Low shrinkage.
• Low coefficient of thermal expansion.
• Good flow characteristics so that it penetrates the fibre bundles completely and eliminates
voids during the compacting/curing process.
• Must be elastic to transfer load to fibres. Reasonable strength, modulus and elongation
(elongation should be greater than fibre).
• Strength at elevated temperature (depending on application).
• Low temperature capability (depending on application).
• Excellent chemical resistance (depending on application).
• Should be easily processable into the final composite shape.
• Dimensional stability (maintains its shape).
Polymer composites: is the material consisting of a polymer (resin) matrix combined with a
fibrous reinforcing dispersed phase.
Polymer Matrix Composites are very popular due to their low cost and simple fabrication
methods.
Classification of Polymers
Linear polymer - Any polymer in which molecules are in the form of chains.
Thermoplastic polymers - Linear or branched polymers in which chains of molecules are
not interconnected to one another.
Thermosetting polymers - Polymers that are heavily cross-linked to produce a strong three
dimensional network structure.
Elastomers - These are polymers (thermoplastics or lightly cross-linked thermosets) that have
an elastic deformation > 200%.
Molecular chain configurations:
a. Linear
b. Branched
c. Crossed linked, d. Ladder

2. Factors affecting properties of polymer composites
1. Interfacial adhesion
2. Shape and Orientation of Dispersed Phase
3. Matrix properties
4. Concentration of dispersed phase
5. Size of dispersed phase
Factors affecting properties of polymer composites
1. Interfacial adhesion
• To attain superior mechanical properties the interfacial adhesion should be strong.
• Matrix molecules can be anchored to the fiber surface by chemical reaction or adsorption
2. Shape and Orientation of Dispersed Phase
The shape of the reinforcing particles can be:
• Spherical,
• Cubic,
• Platelet,
• Regular or irregular geometry.
3. Polymer matrix
Varieties of polymers for composites are :
• Thermoplastic polymers,
• Thermosetting polymers,
• Elastomers,
• Blends.
Polymer composites interface characterization
• The characterization of interface gives relevant information on interactions between fiber and
matrix.
• The mechanical properties of fiber-reinforced composites are dependent upon the stability of
interfacial region.
1. Micromechanical: fiber-pullout & micro-deboned test
2. Spectroscopic: XRD & EDX
3. Microscopic: Opitcal microscope, SEM, TEM & AFM
4. Thermodynamics: wettability & contact angle.
Limitations of PMC
– Low maximum working temperature.
– High coefficient of thermal expansion- dimensional instability

3. – Sensitivity to radiation and moisture.
– Processing temperature is generally higher than those with thermosets.
Thermoplastic polymers:
• Thermoplastics consist of linear or branched chain molecule shaving strong intermolecular
bonds but weak intermolecular bonds.
• They can be reshaped by application of heat and pressure and are either semi crystalline or
amorphous in structure.
• E.g. polyethylene, polypropylene, polystyrene.
Some thermoplastics normally do not crystallize, they are termed as “amorphous" plastics and
are useful at temperatures below the Tg.
• Generally, amorphous thermoplastics are less chemically resistant.
Reasons for the use of thermoplastic matrix composites
• Refrigeration is not necessary with a thermoplastic matrix.
• Parts can be made and joined by heating.
• Parts can be remolded, and any scrap can be recycled.
• Thermoplastics have better toughness and impact resistance than thermosets.
• Shorter fabrication time.
• Can be recycled
Thermosets (thermosetting plastics): Thermosets have cross-linked or network
structures with covalent bonds with all molecules.
• They do not soften but decompose on heating.
• Once solidified by cross-linking process they cannot be reshaped.
• E.g. epoxies, polyesters, phenolics, ureas.
Thermoset materials are usually liquid or malleable prior to curing, and designed to be molded
into their final form.
• Has the property of undergoing a chemical reaction by the action of heat, catalyst, ultraviolet
light, etc., to become a relatively insoluble and infusible substance.
• They develop a well-bonded three-dimensional structure upon curing. Once hardened or cross-
linked, they will decompose rather than melt.
• Thermoset materials are generally stronger than thermoplastic materials due to this 3-D
network of bonds, and are also better suited to high-temperature applications up to the
decomposition temperature of the material.
Thermosets are made by mixing two components (a resin and a hardener) which react and
harden, either at room temperature or on heating.
• The resulting polymer is usually heavily cross-linked, so thermosets are also called as network
polymers.
• The cross-links form during the polymerization of the liquid resin and hardener, so the structure
is almost always amorphous.
• On reheating the crosslink prevent true melting or viscous flow so the polymer cannot be hot-
worked. Further heating just causes it to decompose.

4. Manufacturing of polymer matrix composites:
Forming Processes for Thermosetting matrix composites:
➢ Hand layup and spray up techniques.
➢ Filament winding.
➢ Pultrusion.
➢ Resin transfer molding.
➢ Autoclave molding.
Manufacturing Processes for Thermoplastic matrix composites:
➢ Injection molding.
➢ Film stacking.
➢ Diaphragm forming.
➢ Thermoplastic tape lying.
Reinforcements for Metal Matrix Composites
Metal matrix composites materials can be produced by many different techniques.
➢The focus of the selection of suitable process engineering is the desired kind, quantity and
distribution of the reinforcement components (particles and fibers), the matrix alloy and the
application.
➢By altering the manufacturing method, the processing and the finishing, as well as by the form
of the reinforcement components it is possible to obtain different characteristic.
➢These demands can be achieved only by using non-metal inorganic reinforcement components.
For metal reinforcement ceramic particles or, rather, fibers or carbon fibers are often used.
Interfaces (Bonding)
➢There is always an interface between constituent phases in a composite material
➢For the composite to operate effectively, the phases must bond where they join at the
interface.
Interfaces between phases in a composite material: (a) direct bonding between primary and
secondary phases; (b) addition of a third ingredient to bond the primary phases and form an
interphase

5. ➢Interphase consisting of a solution of primary and secondary phases
➢Interfaces and interphases between phases in a composite material: (c) formation of an
interphase by solution of the primary and secondary phases at their boundary.
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Why are Reinforcement matrix interfaces important?
➢Such large differences are shared through the interface. Stresses acting on the matrix are
transmitted to the fiber across the interface.
➢The interfacial bond can influence
• Composite strength
• Modes of failure
• Young’s modulus
• Interlaminar shear strength
• Compressive strength
• Environmental resistance
• Structural stability at elevate temperatures, • Fracture and fatigue behavior
Wettability
➢Is defined the extent where a liquid will spread over a solid surface
➢During the manufacturing process, the matrix is often in the condition where it is capable of
flowing or its behavior is like a liquid
➢Good wettability means that the liquid (matrix) will flow over the reinforcement, covering
every ‘bump’ and ‘dip’ of the rough surface of reinforcement and displacing all air.
➢Wetting will only occur if the viscosity of the matrix is not too high.

6. ➢Interfacial bonding exists due to the adhesion between the reinforcement and the matrix
(wetting is good) All surfaces have an associated energy and the free energy per unit area of the
solid-gas, liquid-gas and solid-liquid are γSG, γLG and γSL,
➢θ is called the contact angle. May be used as a measure of the degree of the wettability
Types of interfacial bonding at interface
▪ Mechanical bonding
▪ Electrostatic bonding
▪ Chemical bonding
▪ Reaction or inter-diffusion bonding
Mechanical bonding
Depending on degree of roughness of fiber surface †
➢Larger surface area may also increase strength of chemical bond.
Chemical bonding
The bond formed between chemical groups on the reinforcement surfaces and compatible groups
in the matrix
➢Strength of chemical bonding depends on the number of bonds per unit area and the type of
bond Reaction or interdiffusion bonding Diffusion and entanglement of molecules

7. Electrostatic bonding: Occur when one surface is positively charged and the other is negatively
charge (refer to the figure)
➢Interactions are short range and only effective over small distances of the order of atomic
dimensions
➢Surface contamination and entrapped gases will decrease the effectiveness of this bonding
profiles, although the same composition and amounts of the components are involved.
Manufacturing of metal matrix composites.
➢ Solid state methods
➢ Semi-solid state methods
➢ Liquid state methods
➢ Vapor Deposition
➢ In-situ fabrication technique
Liquid state fabrication of Metal Matrix Composites:
Liquid state fabrication of Metal Matrix Composites involves incorporation of dispersed
phase into a molten matrix metal, followed by its Solidification.
In order to provide high level of mechanical properties of the composite, good interfacial
bonding (wetting) between the dispersed phase and the liquid matrix should be obtained.
Wetting improvement may be achieved by coating the dispersed phase particles (fibers).
Proper coating not only reduces interfacial energy, but also prevents chemical interaction
between the dispersed phase and the matrix

8. The methods of liquid state fabrication of Metal Matrix Composites:
1. Stir Casting
2. Infiltration
I. Gas Pressure Infiltration
II. Squeeze Casting Infiltration
III. Pressure Die Infiltration
Stir Casting is a liquid state method of composite materials fabrication, in which a dispersed
phase (ceramic particles, short fibers) is mixed with a molten matrix metal by means of
mechanical stirring.
Stir Casting is the simplest and the most cost effective method of liquid state fabrication.
The liquid composite material is then cast by conventional casting methods and may also be
processed by conventional Metal forming technologies.
Stir Casting is characterized by the following features:
▪ Content of dispersed phase is limited (usually not more than 30 vol.%).
▪ Distribution of dispersed phase throughout the matrix is not perfectly homogeneous:
1. There are local clouds (clusters) of the dispersed particles (fibers);
2. There may be gravity segregation of the dispersed phase due to a difference in the densities of
the dispersed and matrix phase.
▪ The technology is relatively simple and low cost.
Distribution of dispersed phase may be improved if the matrix is in semi-solid condition.
The method using stirring metal composite materials in semi-solid state is called Rheocasting.
High viscosity of the semi-solid matrix material enables better mixing of the dispersed phase.

9. Infiltration: is a liquid state method of composite materials fabrication, in which a preformed
dispersed phase (ceramic particles, fibers, woven) is soaked in a molten matrix metal, which fills
the space between the dispersed phase inclusions.
The motive force of an infiltration process may be either capillary force of the dispersed phase
(spontaneous infiltration) or an external pressure (gaseous, mechanical, electromagnetic,
centrifugal or ultrasonic) applied to the liquid matrix phase (forced infiltration).
Infiltration is one of the methods of preparation of tungsten-copper composites.
The principal steps of the technology are as follows:
▪ Tungsten Powder preparation with average particle size of about 1-5 mkm.
▪ Optional step: Coating the powder with nickel. Total nickel content is about 0.04%.
▪ Mixing the tungsten powder with a polymer binder.
▪ Compacting the powder by a molding method (Metal injection molding, die
pressing, isostatic pressing). Compaction should provide the predetermined porosity level
(apparent density) of the tungsten structure.
▪ Solvent debinding.
▪ Sintering the green compact at 2200-2400F (1204-1315C) in Hydrogen atmosphere for 2 hrs.
▪ Placing the sintered part on a copper plate (powder) in the infiltration/sintering furnace.
▪ Infiltration of the sintered tungsten sceleton porous structure with copper at 2100-2300F
(110-1260C) in either hydrogen atmosphere or vacuum for 1 hour.
Gas Pressure Infiltration: is a forced infiltration method of liquid phase fabrication of
Metal Matrix Composites, using a pressurized gas for applying pressure on the molten metal and
forcing it to penetrate into a preformed dispersed phase.
Gas Pressure Infiltration method is used for manufacturing large composite parts. The method
allows using non-coated fibers due to short contact time of the fibers with the hot metal. In
contrast to the methods using mechanical force, Gas Pressure Infiltration results in low damage
of the fibers.

10. Squeeze Casting Infiltration: is a forced infiltration method of liquid phase fabrication of
Metal Matrix Composites, using a movable mold part (ram) for applying pressure on the molten
metal and forcing it to penetrate into a performed dispersed phase, placed into the lower fixed
mold part. Squeeze Casting Infiltration method is similar to the Squeeze casting technique used
for metal alloys casting.
Squeeze Casting Infiltration process has the following steps:
▪ A preform of dispersed phase (particles, fibers) is placed into the lower fixed mold half.
▪ A molten metal in a predetermined amount is poured into the lower mold half.
▪ The upper movable mold half (ram) moves downwards and forces the liquid metal to
infiltrate the preform.
▪ The infiltrated material solidifies under the pressure.
▪ The part is removed from the mold by means of the ejector pin.
The method is used for manufacturing simple small parts (automotive engine pistons from
aluminum alloy reinforced by alumina short fibers).
Pressure Die Infiltration: is a forced infiltration method of liquid phase fabrication of Metal
Matrix Composites, using a Die casting technology, when a preformed dispersed phase
(particles, fibers) is placed into a die (mold) which is then filled with a molten metal entering the
die through a sprue and penetrating into the preform under the pressure of a movable piston
(plunger).

11. Solid state fabrication of Metal Matrix Composites is the process, in which Metal
Matrix Composites are formed as a result of bonding matrix metal and dispersed phase due to
mutual diffusion occurring between them in solid states at elevated temperature and under
pressure. Low temperature of solid state fabrication process (as compared to Liquid state
fabrication of Metal Matrix Composites) depresses undesirable reactions on the boundary
between the matrix and dispersed (reinforcing) phases.
Metal Matrix Composites may be deformed also after sintering operation by rolling, Forging,
pressing, Drawing or Extrusion. The deformation operation may be either cold (below
the recrystallization temperature) or hot (above the recrystallyzation temperature).
Deformation of sintered composite materials with dispersed phase in form of short fibers results
in a preferred orientation of the fibers and anisotropy of the material properties There are two
principal groups of solid state fabrication of Metal Matrix Composites:
▪ Diffusion bonding
▪ Sintering
Diffusion Bonding is a solid state fabrication method, in which a matrix in form of foils and
a dispersed phase in form of long fibersare stacked in a particular order and then pressed at
elevated temperature.
The finished laminate composite material has a multilayer structure.
Diffusion Bonding is used for fabrication of simple shape parts (plates, tubes).
Variants of diffusion bonding are roll bonding and wire/fiber winding:
Roll Bonding is a process of combined Rolling (hot or cold) strips of two different metals
(e.g. steel and aluminum alloy) resulted in formation of a laminated composite material with a
metallurgical bonding between the two layers.
Wire/fiber Winding is a process of combined winding continuous ceramic fibers and metallic
wires followed by pressing at elevated temperature.
Sintering
Sintering fabrication of Metal Matrix Composites is a process, in which a powder of
a matrix metal is mixed with a powder of dispersed phase in form of particles or short fibers for
subsequent compacting and sintering in solid state (sometimes with some presence of liquid).
Sintering is the method involving consolidation of powder grains by heating the “green”
compact part to a high temperature below the melting point, when the material of the separate
particles diffuse to the neighboring powder particles.

12. In contrast to the liquid state fabrication of Metal Matrix Composites, sintering method allows
obtaining materials containing up to 50% of dispersed phase
.When sintering is combined with a deformation operation, the fabrication methods are called:
▪ Hot Pressing Fabrication of Metal Matrix Composites
▪ Hot Isostatic Pressing Fabrication of Metal Matrix Composites
▪ Hot Powder Extrusion Fabrication of Metal Matrix Composites
▪ Hot Pressing Fabrication of Metal Matrix Composites
▪ Hot Pressing Fabrication of Metal Matrix Composites
Hot Pressing Fabrication of Metal Matrix Composites
Hot Pressing Fabrication of Metal Matrix Composites – sintering under a unidirectional
pressure applied by a hot press;

13. Hot Isostatic Pressing Fabrication of Metal Matrix Composites – sintering under a
pressure applied from multiple directions through a liquid or gaseous medium surrounding the
compacted part and at elevated temperature;
Hot Powder Extrusion Fabrication of Metal Matrix Composites – sintering under
a pressure applied by an extruder at elevated temperature.
In-situ fabrication of Metal Matrix Composites
In situ fabrication of Metal Matrix Composite is a process, in which dispersed (reinforcing)
phase is formed in the matrix as a result of precipitation from the melt during its cooling
and Solidification.
Different types of Metal Matrix Composites may be prepared by in situ fabrication method:
1. Particulate in situ MMC – Particulate composite reinforced by in situ synthesized dispersed
phase in form of particles.
Examples: Aluminum matrix reinforced by titanium boride (TiB2) particles, magnesium matrix
reinforced by Mg2Si particles.

14. 2. Short-fiber reinforced in situ MMC – Short-fiber composite reinforced by in situ
synthesized dispersed phase in form of short fibers or whiskers (single crystals grown in form of
short fibers).
Examples: Titanium matrix reinforced by titanium boride (TiB2) whiskers,
Aluminum matrix reinforced by titanium aluminide (TiAl3) whiskers.
3. Long-fiber reinforced in situ MMC – Long-fiber composite reinforced by in situ synthesized
dispersed phase in form of continuous fibers.
Example: Nickel-aluminum (NiAl) matrix reinforced by long continuous fibers of Mo (NiAl-
9Mo alloy).
Solid-State Processes Powder Processing: Science of producing metal powders and making
finished /semi finished objects from mixed or alloyed powders with or without the addition of nonmetallic
constituents
• Steps in powder metallurgy: Powder production, Compaction, Sintering, Secondary operations
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15. 6Production of powders
• Cheapest of the powder production methods.
• These methods involve using mechanical forces such as compressive forces, shear or impact to facilitate
particle size reduction of bulk materials. Ex: Milling
• Milling: During milling, impact, attrition, shear and compression forces are acted upon particles
• Impact: Striking of one powder particle against another occurs.
• Attrition: The production of wear debris due to the rubbing action between two particles.
• Shear: Cutting of particles resulting in fracture.
• Compression: The particles are broken into fine particles by squeezing action in compression
Main objective of milling: Particle size reduction (main purpose), Particle size growth, shape change,
agglomeration (joining of particles together), solid state alloying, mechanical or solid state mixing,
modification of material properties.
• Mechanism of milling: Changes in the morphology of powder particles during milling results in the
following events.
• Micro forging
• Fracture
• Agglomeration
• Deagglomeration
Micro forging: Individual particles or group of particles are impacted repeatedly so that they flatten with
very less change in mass
• Fracture: Individual particles deform and cracks initiate and propagate resulting in fracture
• Agglomeration: Mechanical interlocking due to atomic bonding or vandeWaals forces
• Deagglomeration: Breaking of agglomerates
• The different powder characteristics influenced by milling are shape, size, texture, particle size
distribution, crystalline size, chemical composition, hardness, density, flowability, compressibility,
sinterability, sintered density
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1Blending: mixing powder of the same chemical composition but different sizes.
• Mixing: combining powders of different chemistries.
• Blending and mixing are accomplished by mechanical means:
Except for powders, some other ingredients are usually added:
❖Lubricants: to reduce the particles-die friction
❖Binders: to achieve enough strength before sintering
❖Deflocculants: to improve the flow characteristics during feeding

16. Compaction Blended powers are pressed in dies under high pressure to form them into the required
shape. The work part after compaction is called a green compact or simply a greenAs a result of
compaction, the density of the part, called the green density is much greater than the starting material
density, but is not uniform in the green. The density and therefore mechanical properties vary across the
part volume and depend on pressure in compaction.
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FINISHING / SECONDARY OPERATIONS
1. Sizing: cold pressing to improve dimensional accuracy
2. Coining or pressing:
• Cold working process, to press details into surface
• Condensation of sintered product
3. Impregnation:
• Impregnation with heated oil (self lubricated bearings)
• Oil fills the pores of the part
4. Infiltration:
• Placement of slug of a lower melting point metal against the sintered part
• Infiltration of molten metal by capillary action
• Pores are filled with a molten metal
5. Heat treatment, Plating, Painting
Advantages
•Elimination/reduction of machining
•High production rates
•Complex shapes can be produced
•Wide composition variations are possible
•Wide property variations are possible
•Scrap is eliminated or reduced
Disadvantages
• High cost of powder material & tooling
• Les strong parts than wrought ones
• Less well known process

17. Vapor deposition is a group of various methods, utilizing materials in vapor state: Physical
Vapor Deposition (PVD), Chemical Vapor Deposition (CVD), and Direct Vapor Deposition
(DVD).
In these methods coating of solid material is formed as a result of vapor condensation or
chemical reaction on a substrate surface.
Vapor deposition is used for coating fibers, creating multilayer depositions, fabricating
nanostructure composite materials.