L4 version 2 (Lec Notes + Teachers notes)
Lecture 4 Notes: Composite Reinforcements
1. Introduction to Reinforcements
In composite materials, the reinforcement phase is added to improve the properties of the matrix material. Reinforcements are responsible for carrying most of the applied load and improving the overall performance of the composite.
Reinforcements are mainly used to improve:
strength
stiffness
toughness
thermal conductivity
electrical conductivity
wear resistance
impact resistance
There are two main forms of reinforcements used in composites:
Particle reinforcements.
Fibre reinforcements.
The type of reinforcement selected depends entirely on the application and the design requirements of the final structure.
2. Ideal Requirements for Reinforcements
For a reinforcement to work effectively in a composite, it must satisfy several important requirements.
2.1 Reinforcements must provide the required properties
The reinforcement must provide the specific property that the designer wants to improve.
For example, if the goal is to improve stiffness, then the reinforcement must itself have high stiffness. If the goal is to improve electrical conductivity, then the reinforcement must conduct electricity well.
The reinforcement is selected based on the performance requirements of the composite.
2.2 Reinforcements must bond well with the matrix
Good bonding between the reinforcement and the matrix is extremely important.
The bonding occurs at the interface between the reinforcement and the matrix. This interface controls how effectively stress can transfer from the matrix into the reinforcement.
If the bonding is weak, then the reinforcement cannot carry load properly, and the composite will fail more easily.
Particles and fibres with rough surfaces usually bond better because the roughness creates mechanical interlocking with the matrix material.
Smooth surfaces generally bond poorly because there is less mechanical interaction between the reinforcement and the matrix.
2.3 Smaller particles usually bond better
Smaller particles usually create better bonding because they have a larger surface area relative to their size.
A larger surface area means there is more contact between the reinforcement and the matrix. This increases the possibility of strong bonding and better stress transfer.
This is why nanoparticles often produce better reinforcement effects than larger particles.
2.4 Surface coatings can improve bonding
Some reinforcements need coatings or surface treatments to improve bonding.
Examples of surface treatments include:
stearic acid coatings
silane coupling agents
These treatments improve compatibility between the reinforcement and the matrix material.
2.5 Reinforcements must be chemically stable
The reinforcement should not chemically react with the matrix material.
If a chemical reaction occurs, a new chemical product may form. This damages the composite structure and reduces performance.
2.6 Reinforcements must be thermally stable
The reinforcement should remain stable when temperature changes occur.
If the reinforcement expands or contracts too much compared with the matrix, the interface may weaken. This can reduce the strength and durability of the composite.
2.7 Reinforcements should have low cost where possible
Manufacturers generally prefer low-cost materials.
However, cost should never compromise safety or structural performance. If a cheaper material gives poor mechanical properties or poor bonding, it should not be selected for critical applications.
2.8 Reinforcements should have low health and environmental risks
Some reinforcements, especially nanoparticles, can create health risks.
For example, graphene nanoparticles and carbon nanotubes can potentially be inhaled or absorbed into the body.
Environmental effects must also be considered when selecting reinforcement materials.
3. Particle Reinforcements
Particle reinforcements are commonly used in composite materials to improve mechanical properties, reduce cost, improve UV resistance, or modify electrical and thermal behaviour.
4. Ceramic Particle Fillers
4.1 Calcium Carbonate (CaCO₃)
Calcium carbonate, including chalk, is one of the most widely used fillers in composite materials.
Calcium carbonate is mainly used as a low-cost, general-purpose filler.
It provides:
reasonable stiffness
whiteness
UV protection
The white colour helps reflect ultraviolet light, which reduces degradation of the matrix material.
4.2 Silica (SiO₂)
Silica particles provide good stiffness and bond well with matrix materials.
Because silica bonds effectively with the matrix, it can improve composite strength.
Silica can also improve whiteness in the final component.
4.3 Calcium Hydroxide (Talc)
Talc particles have a plate-like structure.
Talc provides reasonable stiffness and also allows easier colouring of the composite compared with silica or calcium carbonate.
4.4 Mica
Mica particles are also plate-like in structure.
However, mica fractures easily along its plates. This can reduce strength, which is why mica is less commonly used compared with other fillers.
4.5 Kaolins
Kaolins improve crack resistance in composites.
However, kaolins tend to make the material darker in colour.
4.6 Wollastonite
Wollastonite has a needle-like structure.
Because of this structure, wollastonite behaves similarly to short fibre reinforcement.
Wollastonite can therefore provide good strength improvements.
4.7 Glass Spheres
Glass spheres are small spherical particles that are used in composites.
Glass spheres can provide:
smooth surface finishes
reflectivity
However, their smooth surfaces reduce bonding with the matrix material.
Some glass spheres are hollow. Hollow spheres are produced by including a gas-forming agent inside the glass particle. When heated, the gas expands and creates a hollow structure.
Hollow glass spheres help reduce density and overall weight.
4.8 Titanium Dioxide (TiO₂)
Titanium dioxide is widely used as a filler because of its colouring properties.
It is mainly used to produce a strong white colour in the final material.
Titanium dioxide does not significantly improve mechanical properties.
5. Graphene Reinforcements
5.1 Structure of Graphene
Graphene is a one-atom-thick sheet of carbon atoms arranged in a two-dimensional honeycomb lattice.
The carbon atoms are bonded using strong sp² covalent bonds.
Graphene is considered the parent structure of:
graphite
carbon nanotubes
fullerenes
5.2 Properties of Graphene
Graphene has exceptional properties.
Graphene possesses:
extremely high stiffness
very high strength
excellent electrical conductivity
excellent thermal conductivity
good flexibility
optical transparency
low coefficient of thermal expansion
Graphene is considered one of the strongest materials currently known.
5.3 Graphene vs Graphite
Graphene usually consists of approximately 1–10 carbon layers.
If many more layers are present, the material behaves more like graphite instead of graphene.
This distinction is important because some manufacturers may incorrectly describe graphite as graphene.
5.4 Problems with Graphene
Graphene sheets tend to agglomerate and restack together because of strong van der Waals forces.
Graphene sheets can also curl.
Agglomeration and curling reduce the effectiveness of graphene as a reinforcement material because they reduce bonding and stress transfer.
5.5 Functionalisation of Graphene
Functionalisation is used to improve graphene performance in composites.
Functionalisation involves adding chemical groups onto the graphene surface.
These groups may include:
oxygen groups
amine groups
carboxyl groups
fluoride groups
Functionalisation improves:
bonding with the matrix
dispersion in the matrix
resistance to agglomeration
6. Carbon Nanotubes (CNTs)
6.1 Structure of Carbon Nanotubes
Carbon nanotubes are graphene sheets rolled into cylindrical tubes.
Carbon nanotubes can exist as:
single-walled nanotubes (SWNTs)
multi-walled nanotubes (MWNTs)
Single-walled nanotubes usually have diameters around 1 nm, while multi-walled nanotubes can reach around 10 nm.
6.2 Properties of Carbon Nanotubes
Carbon nanotubes possess:
extremely high strength
extremely high stiffness
excellent electrical conductivity
excellent thermal conductivity
These properties make carbon nanotubes attractive for advanced composite materials.
6.3 Production of Carbon Nanotubes
Carbon nanotubes are commonly produced using chemical vapour deposition (CVD).
During this process, carbon monoxide gas decomposes on metal catalysts such as:
nickel
iron
cobalt
Multi-walled nanotubes are easier and cheaper to produce than single-walled nanotubes.
6.4 Challenges with Carbon Nanotubes
The use of carbon nanotubes in composites is still limited because:
they are expensive
they are difficult to disperse evenly
they tend to agglomerate
controlling alignment is difficult
7. Glass Fibres
7.1 Structure of Glass Fibres
Glass fibres are based on silica tetrahedra structures.
Each silicon atom bonds to four oxygen atoms.
Pure silica would be extremely stiff and difficult to process. Therefore, other atoms are added to modify the structure.
These additional atoms include:
sodium
magnesium
calcium
potassium
aluminium
boron
These additions make glass fibres easier to process and more flexible.
7.2 Types of Glass Fibres
E-glass
E-glass stands for electrical glass.
E-glass has the best processability and is the most widely used general-purpose glass fibre.
C-glass
C-glass provides improved chemical resistance.
S-glass
S-glass provides higher strength compared with E-glass.
ECR and AR glass
ECR and AR glass provide improved acid and alkali resistance.
7.3 Properties of Glass Fibres
Glass fibres possess:
good stiffness
good strength
relatively low cost
However, glass fibres are brittle, which limits their toughness.
7.4 Processing of Glass Fibres
Glass fibres are produced by melting glass and passing it through circular dies.
The fibres are then pulled into thin filaments, usually around 10 μm in diameter.
The fibres are coated with a “size” or coupling agent.
This coating protects the fibre surface and improves bonding with the matrix material.
8. Carbon Fibres
8.1 Why Graphite Cannot Be Processed Directly
Graphite cannot easily be processed directly into fibres because it is brittle and has an extremely high melting temperature of approximately 3675°C.
Instead, carbon fibres are produced using precursor fibres.
8.2 Production of Carbon Fibres
The most common precursor material is polyacrylonitrile (PAN).
Pitch can also be used as a precursor.
The precursor fibres undergo:
stretching
oxidation
heat treatment
graphitisation
This converts the polymer into a graphite-like carbon structure.
8.3 Structure of Carbon Fibres
Carbon fibres contain graphite-like layers where strong covalent bonds are aligned mainly along the fibre direction.
This alignment is the main reason carbon fibres are stronger and stiffer than glass fibres.
In glass fibres, the covalent bonds are randomly distributed. In carbon fibres, the bonds are highly aligned along the loading direction.
8.4 Types of Carbon Fibres
High-Modulus (HM) Fibres
High-modulus fibres undergo greater heat treatment.
This improves alignment of the graphite structure and increases stiffness.
However, greater heat treatment also introduces more defects, reducing strength and increasing brittleness.
High-Strength (HS) Fibres
High-strength fibres undergo less heat treatment.
These fibres provide higher strength and fewer defects, although stiffness is lower than HM fibres.
8.5 Properties of Carbon Fibres
Carbon fibres possess:
very high stiffness
very high strength
low density
smaller diameter than glass fibres
Carbon fibres are therefore widely used in aerospace and high-performance engineering applications.
9. Polyaramid Fibres (Kevlar)
Kevlar is the most well-known polyaramid fibre.
Kevlar fibres are produced from rigid polymer chains that form liquid crystalline structures.
9.1 Processing of Kevlar
Kevlar is produced by:
extruding fibres from solution
stretching the fibres
heating the fibres
This process creates highly crystalline fibres.
9.2 Properties of Kevlar
Kevlar possesses:
high toughness
high strength
excellent impact resistance
low density
low flammability
Kevlar does not melt easily.
However, Kevlar degrades under ultraviolet light and is generally limited to temperatures below 300°C.
Kevlar is widely used in protection and impact-resistant applications.
10. Polyethylene Fibres
Highly oriented polyethylene fibres can achieve extremely high stiffness.
Commercial examples include:
Spectra
Dyneema
10.1 Properties of Polyethylene Fibres
Polyethylene fibres possess:
high toughness
high strength
high stiffness
low density
However, polyethylene fibres:
melt around 130°C
are usually limited to temperatures below 100°C
bond poorly with matrices
are relatively expensive
11. Silicon Carbide Fibres
Silicon carbide fibres are ceramic fibres with strong covalent bonding.
These fibres possess:
high stiffness
high strength
high temperature resistance
However, they also possess low toughness because of their ceramic nature.
11.1 Production by Chemical Vapour Deposition
One method of producing silicon carbide fibres is chemical vapour deposition.
In this process:
CH3SiCl3→SiC+3HClCH_3SiCl_3 \rightarrow SiC + 3HClCH3SiCl3→SiC+3HCl
Silicon carbide crystals grow on tungsten or carbon cores.
This method produces large-diameter monofilaments.
11.2 Production Using Polymer Precursors
Silicon carbide fibres can also be produced using polymer precursors such as polycarbosilane (PCS).
Heat treatment converts the polymer into silicon carbide.
A common commercial version is called Nicalon fibre.
11.3 Silicon Carbide Whiskers
Silicon carbide whiskers are single-crystal reinforcements.
Because they are single crystals, they possess extremely high strength and stiffness.
Silicon carbide whiskers can be produced using:
gas-phase reactions
rice husks
Typical whiskers have diameters below 1 μm and lengths around 50 μm.
12. Boron Fibres
Boron fibres are produced using chemical vapour deposition onto tungsten cores.
The reaction is:
2BCl3+H2→2B+6HCl2BCl_3 + H_2 \rightarrow 2B + 6HCl2BCl3+H2→2B+6HCl
Boron fibres are similar to silicon carbide monofilaments in structure and behaviour.
13. Fibre Bending and Curvature
Fibre flexibility is important during processing because fibres often need to bend around curved shapes.
The maximum tensile stress during bending is:
σ=dE2R\sigma = \frac{dE}{2R}σ=2RdE
Where:
ddd is fibre diameter
EEE is modulus
RRR is radius of curvature
When tensile stress reaches the fibre failure stress:
R=dE2σfR = \frac{dE}{2\sigma_f}R=2σfdE
Where:
σf\sigma_fσf is fibre failure stress
A smaller radius of curvature means the fibre can bend more tightly.
Carbon nanotubes are extremely flexible and can bend significantly without failure.
Silicon carbide monofilaments are very brittle and difficult to bend.
14. Key Revision Points
Reinforcements are added to composites to improve mechanical and functional properties.
Good bonding between the reinforcement and the matrix is essential for effective stress transfer.
Rough reinforcement surfaces generally improve bonding.
Smaller particles usually provide better bonding because they have higher surface area.
Graphene is a one-atom-thick sheet of carbon atoms with exceptional properties.
Graphene tends to agglomerate and curl, which limits its effectiveness.
Functionalisation improves graphene dispersion and bonding.
Carbon nanotubes are rolled graphene sheets with excellent conductivity and strength.
Glass fibres are cheap and widely used but relatively brittle.
Carbon fibres are stronger, stiffer, and lighter than glass fibres.
Kevlar provides excellent toughness and impact resistance.
Polyethylene fibres have very high toughness but poor temperature resistance.
Silicon carbide fibres provide excellent high-temperature performance but low toughness.
Silicon carbide whiskers are single crystals with extremely high strength and stiffness.
High-modulus carbon fibres are stiffer but more brittle than high-strength carbon fibres.