Composite Materials Fibre-Matrix Interfaces There is nothing there really except the two of you (or the fiber and matrix).
Composite Parameters Fibre properties Composite Interfaces Matrix properties Fibre content Composite Mechanical Performance Porosity Fibre length Fibre orientation Packing ability
Bonding Mechanism Adsorbsion and wetting and surface tension Chemical bonding Electrostatic interactions Mechanical bonding
Surface tension Surface tension is a measurement of the cohesive energy present at an interface The molecules of a liquid attract each other. The interactions of a molecule in the bulk of a liquid are balanced by an equal attractive force in all directions. Molecules on the surface of a liquid experience an imbalance of forces as indicated below. The net effect of this situation is the presence of free energy at the surface. The excess energy is called surface free energy and can be quantified as a measurement of energy/area. It is also possible to describe this situation as having a line tension or surface tension, which is quantified as a force/length measurement.
Surface tension DEFINITION OF SURFACE TENSION The surface tension γ is the magnitude F of the force exerted parallel to the surface of a liquid divided by the length L of the line over which the force acts γ = F/L SI Unit of Surface Tension: N/m
Surface tension γ = force/length
Definiton of interface and Interphase Interface: It is the boundary separating the distinct phase of fiber, matrix and coating layer Interphase: It is a region where coating and matrix diffused into each other s domain and form a flexible, threedimensional polymer network. The key purpose of the network is to provide a lattice that the matrix molecule can penetrate and come in close proximity to fibers. The interphase is responsible for transferring the load from the matrix to the fibers. Formation of interphase region and the resulting properties are poorly understood.
Interface Fibers reinforce the matrix by taking the stress applied to the matrix, which is transferred across the interface. The adhesion at the interface will determine how effectively the stress is transferred Matrix-interface bonding is of crucial importance to the properties of composites. Because through the interface that stress is transferred from the matrix to the fiber so the fiber can do its job of reinforcement. An interface should: have high load-bearing capacity of the reinforcement should be highly resistant to chemical attack or to influence of the environment
Strong Interface or Weak Interface A strong interface creates a material that displays exemplary strength and stiffness but that is very brittle in nature with easy crack propagation through the matrix and fibre. A weaker interface reduces the efficiency of stress transfer from the matrix to the fibre and consequently the strength and stiffness are not as high, but in contrast toughness is increased. (Matthews and Rawlings, 1999)
Surface, Interface, Interphase Surface and interfaces: two dimensional (no thickness) Interphase: three dimensional (has a certain thickness
Wettability Wettability defines the extent to which a liquid will spread over a solid surface. Good wettability means that the liquid (matrix) will flow over the reinforcement covering the rough surface completely and removing all air. PE drop on two fibres with different wettability
Wettability Intimate contact between a liquid and a solid can be established, providing the liquid is not too viscous and a thermodynamic driving force exists. So, in order for any adhesion to take place, wetting must occur (so it snot like water on wax). A high value for a liquid surface free energy prevents spreading of a liquid droplet lid surface. The wetting or contact angle (q) is obtained by a balance of forces (Young equation). cos θ = (γsg - γsl)/ γlg
Wettability Low contact angles are indicative of good wetting, whereas high contact angles point to unsatisfying wetting Low matrix viscosity and critical surface energy is important for. wetting polar polymers will wet polar reinforcements well, and will hardly wet dispersive reinforcements
Interface bonding Mechanical interlocking may lead to bonds of reasonable stability, depending on the roughness of the surface. Mechanical interlocking may be particularly beneficial to the development of shear strength, whereas it is not very stable with respect to normal forces Electrostatic bonding Interaction between charges at the molecular level act only at very small distances Chemical Bonding : Formation of a covalent bond between matrix and reinforcement (Ionic 600 1100KJ Covalent 60 700KJ/mol) Polymer inter diffusion: Diffusion of the matrix polymer into the reinforcement and formation of an interphase, where matrix properties gradually change to reinforcement properties.
What External Factors Might Affect the Bond? The relationship is going to suffer lots of external pressures. The strength of your bond will determine how much your relationship can stand. Residual stresses result from thermal and mechanical factors or curing reactions
Surface Analysis Methods (study adhesion) Physical bonds can be further explored using a variety of surface energy measurements ( contact angle analysis inverse gas chromatography) Microscopy will help to determine the location of failure and sometimes the cause of the debonding (mechanical adhesion) Spectroscopy can give some chemical information about the chemical make up of the materials and their likelihood for reaction Raman microscopy This method takes advantage of the relationship between the frequency of vibration of molecular groups and the strain applied to the material. Often this is proportional, and therefore we can use the changing frequency as a measure of the strain in the fiber when we apply a certain strain to the matrix. It acts as an internal molecular strain gauge Dynamic Mechanical Analysis
Micromechanical interface measurements Fibre pull out test Microbond test Single fibre fragmentation test
Debonding The process of debonding creates new surfaces in the composite and, therefore, requires energy. It is just like glue unsticking, and a clean break occurs. If the bond fails at the interface, this is called adhesive failure. However, if it fails inside the fiber or resin, this is called cohesive failure. The fiber or matrix itself gets damaged. This is similar to the damage done to one or either partner on the breakdown of the relationship.
Cracks and Interfaces
Pull out This toughening mechanism occurs after debonding and in the case of continuous fiber composites, also fiber fracture. In pull-out, a force is required to overcome frictional forces thatusually originate from residual stresses associated with resin shrinkage during curing and thermal contraction This is like tearing a fiber from a matrix to which it is well bonded. The strong bonds may mean that the debond occurs inside one of the materials (cohesive failure), rather than in between the two (adhesive failure), so either fiber or matrix could get damaged. There are very weak bonds between fiber and matrix
Pull out test The fiber pull-out test uses a straight fiber, a portion of which is embedded in matrix block of different geometries. The test measures the fiber force required to break the interfacial bond as well as to pull the fiber out against the frictional resistance after complete debonding as a function of the embedded length, L. The drawback of the single fibre pull-out test is that it involves only a single fibre. Real composites contain multiple fibres and the pull-out fibre is surrounded by a composite medium
Pull out test the earliest of the single fibre tests. Under load, the fiber will fracture decreasing the segment size until a critical length, lc, is reached which is too short to transfer the stress to break the fiber. The critical fiber length is related to the interfacial shear strength
Micro debond test The procedure involves the deposition of a small amount of resin on to the surface of a fibre in the form of one or more discrete micro-droplets. The droplets form concentrically around the fibre in the shape of ellipsoids and retain their shape after appropriate curing. Once cured, the microdroplet dimensions and the fibre diameter are measured with the aid of an optical microscope. The embedded length is fixed by the diameter of the micro-droplet along the fibre axis, which is dependent on the amount of resin deposited on the fibre. After the resin has cured, it is sheared from the fibre by two parallel plates attached to a microvice.
Microbond test A problem that is associated with the microbond technique is that the maximum debonding force value is influenced by interfacial friction in already debonded regions and, therefore, these parameters are not purely `adhesional' but depend, in an intricate way, on interfacial adhesion and friction. specimen preparation for the micro-droplet test whereby a single fibre is pulled out of a small droplet of resin suffers from several difficulties. the reliability of the data is affected by the shape of the droplet. the size of the droplet is critical. If the length of the droplet exceeds a critical value, the fibre will fracture prior to debonding and pull-out. this test is not applicable to matrices that are soft.
Micro debond test Advantages Eliminates tapered end problems Fiber strength information not required Relatively fast and simple Disadvantages Fiber failure may be dominant for strong interface
Typical micro-debond test results
Fragmentation (single fibre composite) test The fibre fragmentation test was originally developed for use with metalscontaining brittle fibres by Kelly and Tyson (1965) The most important factor is that the stress builds up from the ends of the fiber, and it is assumed that no stress transfers across the ends. Once the fiber stress reaches the breaking stress of the fiber it will fracture, but the matrix will hold it in place. This principle is used in the Fragmentation test.
Fragmentation (single fibre composite) test A single fibre is embedded in a polymer and broken into small pieces. The fibre is neither pushed nor pulled directly, and so fibre Poisson effects are similar to those occurring in a fibre composite Unlike the other methods, it produces only one result for the IFSS, which is the average for the many fragments produced. The failure strain of the matrix must be much larger than the failure strain of the fibre to promote multi-fragmentation of the fibre. This requires the use of matrices, which can undergo large deformations. Therefore the IFSS determined is not directly applicable to the actual composite system. Embedding matrix can inhibit fibre fracture, which initiates from surface flaws.
Fragmentation (single fibre composite) test In this test a dog-bone shaped specimen consisting of an isolated fiber embedded completely in a matrix is loaded in tension. As the applied load increases, the embedded fiber breaks into increasingly smaller segments and load transfer occurs between the broken fibers and the matrix until the segments become too short to be broken. Lc does not take into account the complex distribution of fragment lengths. Secondly, the term f refers to the fibre strength at the critical length and this is not easily measured experimentally.
Critical Fibre Length To achieve the maximum stress in the fibre the fibre length must be greater than the critical length, Lc. A fibre is said to be of the critical length if it is just long enough for the tensile stress to reach its maximum value.
Critical fibre length
Interface measurement For a number of glass and carbon fiber composites, this critical length is in the order of 0.2 to 1mm. Lc = ds/2t Cox used a shear lag analysis, which leads to expressions for the tensile stress in the fiber and the shear stress at the interface. In the region of the fiber ends, the strain in the fiber will be less than in the matrix
DMA Basic Principle
Spectroscopy for chemical analysis X-ray photoelectron spectroscopy (XPS) is an extremely powerful tool for studying solid surfaces. This technique has an information depth of 1±5 nm and therefore it is capable of examining only the outer layers or surfaces of fibres. The development of the laser Raman spectroscopy (LRS) has led to the assessment of the stress field at the interface level. The technique relies on the fact t hat Raman bands corresponding to the vibrational modes of bonds in the fibre, shift towards a lower wavenumber upon the action of strain and stress and this is thought to be due to direct molecular straining/stressing. This has been used to map stresses along fibres embedded in matrix resin to determine the IFSS. Nuclear magnetic resonance spectroscopy very powerful technique to measure and characterise polymer tacticity, helicity, and molecular weight, composition and diffusion coefficient of polymers. Solid-state
Treatments for improving adhesion How do I improve the relationship? What treatments can I use? If you are going to try to improve your relationship
Fibre treatments Electrolytic Chemical treatments esterification-based treatments; silane coupling agents; graft copolymerisation; treatments with various chemicals. Wet and Dry oxidation Mercerisation Plasma and corona treatment Heat treatments maleic anhydride chemical reaction with cellulose fibre constituents.
Sizing Sizing (or primer) Coating (interphase) on fiber or particle surfaces, 0.1 to 10mm thick (1 to 2 w%) Functions: Adhesion promotor (coupling agent) Protect the surface from damage Aid in handling Add strength or stiffness Reduce absorbency
Why sizing Sizings and finishes are a mixture of ingredients (organic or aqueous based) applied at the time of fiber manufacture to: Purposes: Protect the surface (e.g. coupling agent for glass fibers) Hold bundle together Aid in handling (e.g. binder, lubricant, antistatic agent) Sizing is complex mixture of : Coupling agent (silanes) Lubricant Binder Antistat ph control Hardening agent Emulsifying agent
Fibre surface treatments (Finishing) later in processing-life, the size is removed and replaced by a finish, which normally is: organo metallic organo - silane
Fibre surface treatments (Glass fibre) later in processing-life, the size is removed and replaced by a finish, which normally is: organo metallic organo - silane 1 to several monolayer thick film (0.1 w%). The film itself has no contribution to the mechanical properties of the fiber Functions: Adhesion promotor (coupling agent) Lubricate Resin wet-out
Carbon fibre treatment