Properties of Composites Dependent on: Constituent phases Reinforcement Matrix Relative amounts Geometry of reinforcement Interface properties Processing Methods

Interface Theories of adhesion W a =  SV +  LV -  SL  SV =  SL +  LV Cos  wetting ：  SV >  LV

Types of bonding

Molecular entanglement Interdiffusion Electrostatic attraction Chemical bonding/Reaction bonding Mechanical bond Hydrogen bonding, van der Waals forces

Surface treatments Surface treatments of glass fibre Size To protect the fibre surface from damage (Film former, Lubricant) To bind fibres together (Film former) To lubricate the fibres so that they withstand abrasive tension (Lubricant) To impart anti-electrostatic properties ( Anti-static agent ) To provide a chemical link between the fibre surface and the matrix ( Coupling agent ) Mechanism: chemical reaction and Interpenetrating network model

Surface treatments of carbon fibre Oxidative and Non- oxidative Treatments To remove loose, weak material To increase the surface area To produce various functional groups on the surface

Effects of Interface on Composite Properties Properties enhanced by strong interface Tensile strength Compressive strength Flexural strength (Interlaminar) shear strength Resistance to environmental attack Properties decreased by strong interface Fracture toughness, impact toughness Damage resistance & tolerance (especially under low-energy impact) Notch sensitivity Properties insensitive to interfacial adhesion Longitudinal elastic moduli (tension or compression)

Measurements of bonding strength Schematic representation of measurement of interface bonding strength. Elastic solids with no flaw at interface Elastic solids with a flaw at interface One elastic solids and one plastic solid with no flaw at interface

Adhesive and Cohesive Failure Adhesive failure: separation occurs at interface Cohesive failure: fracture of fibre or matrix Adhesive failure Cohesive failure of matrix Cohesive failure of fibre

Experimental Measurements of Interface Bonding Micro-composites Single fibre compression test Fibre fragmentation test Microbond test Fibre pull-out test Fibre push-out (indentation) test

Experimental Measurements of Interface Bonding Bulk composites Short beam shear test: three point bending Iosipescu shear test (ASTM D2344): Asymmetric four point bend test In-plane lap shear test [±45°] s tensile test (ASTM D3518) [10°] s off-axis tensile test Transverse tensile test Rail shear test Interlaminar fracture tests (mode I, II and mixed mode)

No surface treatment Silane treated Fracture surface observations

Indirect measurement of interface bonding Chemical composition FTIR (Fourier transform infrared) XPS (X-ray photoelectron spectroscopy) TOF-SIMS (Time of flight-secondary ion mass spectroscopy) Physical characteristics Contact angle Morphology Ultrasonic C-scan SAM （ Scanning acoustic microscope ） SEM TEM

Contact angle measurements

Ultrasonic C-scan SAM

SEM Poor interface bonding Strong interface bonding

TEM Before debonding After debonding

Case study Graphite Nanoplatelets reinforced epoxy nanocomposites

Graphite nanoplatelets (GNPs) GNPs are exfoliated from natural graphite by intercalation, high temperature expansion and ultrasonication. Natural graphite Graphite intercalated compound Expanded graphite GNP Basal plane Intercalant Ultrasonication

Graphite nanoplaltelets (GNPs) GIC GNP Rapid heating at 1050 o C for 30s Expanded graphite 4.5nm  The aspect ratio of GNPs is around 1.0 x Natural graphite Acid intercalation Ultrasonication

Nanocomposites fabrication Factors affecting electrical conductivity of composites: Factors affecting electrical conductivity of composites: Aspect ratio and volume fraction of the filler Aspect ratio and volume fraction of the filler Dispersion and interfacial adhesion between the filler and matrix Dispersion and interfacial adhesion between the filler and matrix Inherent electrical conductivities of filler and matrix material Inherent electrical conductivities of filler and matrix material Bromination Surface treatment mPDA Mixture Outgassing Ultrasonication GNP nanocomposite Modified GNP Epoxy Shear mixing Curing Dispersant Mixture Ultrasonication

Morphologies of nanocomposites High aspect ratio, exfoliated GNPs form the electrical conducting network. TEM morphology shows the satisfactory localized dispersion of GNPs. 2% GNP0.5% GNP

Effect of UV/O 3 treatment on GNP surface chemistry Atomic concentrations of GNP surface for varying UV/O 3 exposure time (min) ElementsUV0UV20UV30UV50UV70 C O Si O/C ratio 1.7%5.0%3.3%4.7%3.2% Functional groups of GNP surface for varying UV/O 3 exposure time (min) Functional Groups Position (eV) C %85.8%85.5%84.9%85.5% C-OH or C-O-C %7.7%7.3%8.0%7.7% C=O %2.4%2.9%2.6%2.6% HO-C=O %4.2%4.3%4.5%4.2%

Effect of UV/O 3 treatment on GNP surface morphology Without treatmentWith 20min UV/O 3 treatment The treated graphite exhibited a rougher surface than the untreated one.

Effect of UV/O 3 duration on thermo-mechanical properties of nanocomposites With the increase of UV exposure duration, the storage modulus and Tg increases consistently, suggesting better interaction due to UV/O 3 treatment.

Effect of UV/O 3 treatment on mechanical properties of nanocomposites UV/O 3 treatment increases the flexural properties of nanocomposites.

Morphologies of fracture surface The cracks along the GNP/epoxy interface indicate the interfacial debonding is predominant between the untreated GNPs and epoxy matrix, whereas a stronger bond with the matrix is evident for the treated GNPs. Without treatmentWith 20min UV/O 3 treatment SEM photographs of fracture surface of 2% nanocomposites

Mechanism?