1.05 COMPOSITES
(From Notes on SURFACE FINISHES http//www.gautamshah.in)
DIFFERENTIATING COMPOSITES and STRUCTURAL COMPOSITIONS
Natural and manufactured raw materials are invariably compounds, made of many materials. Many such compounded materials are in the form of Composites. The composite materials come into being, by putting together natural and manufactured materials in such a special way that the strength and other qualities are different from the constituents, individually and cumulatively.
The term -different, is considered here as an improved quality, because man-made composites are designed and created towards specific performance requirements only.
Natural materials, manufactured materials and composites, all are further shaped, re-formed and geometrically integrated to create components as well as Structural Compositions.
A composite is a designed material entity with potential utility, but has no operational functionality. A component, on the other hand is a configuration of many materials into a utilitarian product. Component manufacturing employs processes that are many times similar to a composite formation. As a matter of fact for component manufacturing, the ‘composite formation’ and the ‘component creation’ both occur simultaneously. Structural compositions (trusses, bridges, buildings) are geometric-configuration of materials, often assisted by components (nuts, rivets, pins, bearings, etc.). Structural compositions use composites to form the constituent elements.
DEFINITIONS of COMPOSITES
1 Consisting of two or more physically distinct and conceptually separable or visually identifiable materials.
2 Products that can be made by mixing separate materials so the dispersion of one material in the other can be done in a controlled way to achieve optimum properties.
3 Products with properties that are superior and possibly unique in some specific respects compared to the properties of their individual components.
CLASSES of COMPOSITES
Natural composites Wood, Bamboo
Bone, Muscle and other tissues
Macro composites Galvanized steel
(engineering products) Reinforced cement concrete (beams, etc.)
Helicopter blades
Skis, Tennis rackets
Microscopic composites Metallic alloys
Toughened plastic (impact polystyrene, ABS)
Sheet moulding compounds
Reinforced plastics
Nano composites Electronics circuits, diodes, transistors
Natural composites are easy to identify, such as: wood, bamboo, bones, muscles, etc. First man-made composites were related to the bronze, as man tried to fix natural stones and ceramic pieces by hammering into the bronze. Layered wood composites have been used by Egyptians. Mud bricks reinforced with hay, hair, and rice husk have been used. Cow-dung is also reinforced with granular sand particles for wall plaster. Gypsum (Plaster of Paris) has been applied on a lattice of jute, papyrus and such other fibres.
Macro, Micro and Nano Composites: Composites can be categorized in terms of the size of constituent particulate matter. Ingredients of macro composites can be distinguished by naked eye, whereas one may need an electron microscope to understand the constituents of micro composites and nano composites. Nano composites are created by introducing nano-particulate, which drastically add to the electrical, thermal, and mechanical properties of the original material.
CATEGORIZATION of COMPOSITES on the basis of STRENGTHENING MECHANISMS
Composite materials can be distinguished into three categories based on the strengthening mechanism. These categories are: Dispersion strengthened, Particle reinforced and Fibre reinforced.
● Dispersion Strengthened Composites have a fine distribution of secondary particles (filler) in the matrix of the material. These particles impede the mechanisms that allow a material to deform. Many Metal-matrix composites would fall into the dispersion strengthened composites’ category.
● Particle reinforced composites have a large volume of particles dispersed in the matrix. The load is shared by the particles and the matrix. Most commercial ceramics and many filled polymers are particle-reinforced composites.
● Fibre-reinforced composites have fibre as the main load-bearing component. Fibreglass and carbon fibre composites are examples of fibre-reinforced composites.
MATRIX FILLER and INTERFACE
The constituents of a composite are ordinarily classified as Matrix and Filler. It is the nature of relationship between the filler and matrix, or the Interface that defines the composite. Fillers serve to resist stresses, mainly tension, and the matrix serves to resist the shear, and all materials present including any aggregates, serve to resist the compression.
Matrix and filler both can be of THREE types: Metals, Ceramics and Polymers. This set provides nine possible combinations.
Composite materials' combinations: Possibilities of combinations and type-examples.
Matrix + Filler = Composite Type-Examples
● Metal matrix composites MMC
Metal + Metal Aluminium-Tin are non miscible metals,
yet can be alloyed as a composite
Metal + Ceramic Electrical semi conductors, Carbide cutting
tool tips, Scissors, knives
Metal + Polymer Not feasible, Metals become soft at veryhigh
temperature -unsuitable for polymer filler
● Ceramic matrix composites CMC
Ceramic + Ceramic Carbon-carbon composites
Ceramic + Metal Metal sprayed optic glass fibre cables
Ceramic + Polymer Not feasible, Ceramics require high
temperature for formation -unsuitable
for polymer filler
● Polymer matrix composites PMC
Polymer + Polymer Polyester or rayon fibre reinforced plastics
Polymer + Metal Grinding and polishing abrasives
Polymer + Ceramic Fibreglass, Fibre reinforced plastic FRP
Asphalt roads, imitation granite,
cultured marble sinks and counter tops
INTERFACE of MATRIX and FILLER
In a composite material the Filler in the form of particles, fibres and sheets, is expected to take up the stresses in unison with the matrix because of the strong interface provided by the later. Composite materials with weak interfaces have low strength and stiffness, but high resistance to fracture, On the other hand materials with strong interfaces have high strength and stiffness but are brittle. The bonding between the matrix and the filler is dependent on the atomic arrangement and chemical properties of filler and on the molecular conformation and chemical constitution of the matrix. A crack that starts in a monolithic material generally continues to propagate until that material fails, whereas the filler-matrix combination reduces the potential for a complete fracture.
Bonding at the interface: In a simple system the bonding is due to adhesion between filler and the matrix. Adhesion can be attributed to following five main mechanisms:
1 Adsorption and wetting: When two electrically neutral surfaces are brought sufficiently close, there is a physical attraction. Most solids have surfaces that are rarely perfectly in level and without any contamination. So a wetting agent that substantially covers every hill and valley displaces all air and overcome effects of contamination, is required.
2 Inter-diffusion: It is possible to form a bond between two polymer surfaces by the diffusion of the polymer molecules on one surface into a molecular network of the other surface. The bond strength will depend on the amount of molecular entanglement and the number of molecules involved. Inter-diffusion may be promoted by the presence of solvents and plasticizing agents.
3 Electrostatic attraction: When one surface carries a net positive charge and the other surface, a net negative charge, electrostatic attraction occurs (as in acid+base reaction). Electrostatic attraction does not play any major role in contribution of bond strength, but has importance on how things initially begin to get mixed.
4 Chemical bonding: It is formed between a chemical group of filler and a chemical group of a matrix. The bond formation or breakage usually involves thermal activity.
5 Mechanical adhesion: Some bonding occurs by the mechanical interlocking of two surfaces (e.g. fibre shape-section).
Shocks, impact, loadings or repeated cyclic stresses can cause the Individual fibres to separate from the matrix, e.g. a fibre pull-out. In case of laminated or layered construction there could be a separation at the interface between two layers, a condition known as de-lamination.
MATRICES
A matrix is an environment or material within which something develops. A matrix surrounds a Filler material while creating a bond with it. A matrix thus creates a network within which the filler components are supported by maintaining or reinforcing their intended positions. For a matrix to be affective, it must at some stage have a lower phase than the filler material. The lower phase may occur before or while the filler material is being formed or introduced. The matrix material may turn to a higher phase by evaporation of the solvent, removal of the heat or pressure, and polymerization or action of a catalyst. Polymer matrices are most common, followed by metals and ceramics. However, paper pulp, mud, wax, etc. are some matrix materials that do not fit into any of the above-mentioned categories. Ceramic matrix composites though difficult to form, show greatest promise in material sciences.
Portland cement, Gypsum plaster, mud (clay), and Bitumens are widely used matrix materials. Polymer matrix materials are thermosetting resins such as polymers, poly-amides, epoxies, or thermoplastic resins such as polycarbonate or polysulphones. Typically a polymer matrix composite of Epoxy and carbon fibres is of two thirds the weight of aluminium, and two and a half times as stiff.
For metal matrices most commonly used metals are aluminum, titanium, magnesium, and copper. Composites with metal matrices generally have metal or ceramic as filler materials. Aluminum reinforced with fibres of the ceramic silicon carbide is a classic example of a metal matrix with ceramic filler. The composite material combines the strength and stiffness of a silicon carbide with the ductility of aluminum. Metal to metal composites consist of two immiscible metals (metals that do not form alloys), such as magnesium and titanium. Such metal-metal composites with bronze matrixes have been in use since Bronze Age to create many useful materials.
FILLERS
Fillers are inevitable constituents of composites. Fillers, besides providing the reinforcement, also impart special properties to the new material. Fillers have many forms, such as fine particulate, staple fibre (whiskers or short fibres), filaments (long or continuous fibres), unwoven (felt) and woven fabrics, knit textiles, aggregates, and sheets. Filler materials are natural (wood, plant, hair), minerals (asbestos, sand, stones, powders), and man-made (polymers, metals, ceramics).
Straw, hair, coir, hemp, jute, papyrus, rice-husk etc., have been mixed with clay to form bricks. Sand, ash, and mineral dust were added to mud to reduce the plasticity for plaster work. Wood planks were glued together to form block board or plywood like construction in 15th C BC.
Man-made materials include: Fibreglass, quartz, Kevlar, Dyneema or carbon fibre, graphite, carbon-graphite, silicon carbide, titanium carbide, aluminium oxide, boron, coated boron, boron carbide, alumina, alumina-silica, niobium-titanium, niobium-tin, etc.
Fillers Particles (of 10 to 250 μm in diameter) help block the movement of dislocations in the composites and provide distinctive strength properties. Staple fibres used as fillers have high length to a diameter ratio and are generally in their random orientations. Whereas filaments are used for high performance structural applications and are prearranged (for a particular structural use) before introduction of matrix, or in certain cases a fixing compound. Depending on the load conditions the reinforcement is random, unidirectional (align ed in a single direction), or multi-directional (oriented in two or three directions). Continuous fibres are more efficient at resisting loads than are short ones, but it is more difficult to fabricate complex shapes from materials containing continuous fibres than from short-fibre or particle-reinforced materials.
Particles (fillers) of one material are dispersed in another material (matrix) in many different ways. Particles are mixed in a liquid phase of the matrix and allowed to harden to a solid phase, the particles are allowed to grow in the matrix or particles are pressed into the matrix and inter-diffusion is encouraged by mechanical working or other energy input. Particulate fillers in ceramic matrices enhance characteristics such as electrical conductivity, thermal conductivity, thermal expansion, and hardness. Particles of Alumina, Silicon carbide and Boron nitride embedded in a polymer matrix formed abrasives are used for grinding and polishing stone floors, tools etc. Carbon black (as powder) added to vulcanised rubber provides hardness and toughness for automobile tyres. The rubber is further reinforced with metal, rayon, polyester and other threads as continuous fibre filler.
High-performance ceramic composites are strengthened with filaments that are bundled into yarns. Each yarn, strand or tow may contain thousands of filaments, each of which with a diameter of approximately 10 micrometers (0.01 millimetres).
Often components are formed that are strong in all directions, by creating a three-dimensional lattice of filler component. The filler component itself could be a composite material.
Fillers affect the quality of a composite. Fillers are usually combined with ductile matrix materials, such as metals and polymers, to make them stiffer. Fillers are added to brittle-matrix materials like ceramics to increase toughness. The length-to diameter ratio of the fibre, the strength of the bond between the fibre and the matrix, and the amounts of fibre are variables that affect the mechanical properties. It is important to have a high length-to-diameter aspect ratio so that the applied load is effectively transferred from the matrix to the fibre.
A variety of reinforcements can be used, including particles, whiskers (very fine single crystals), discontinuous (short) fibres, continuous fibres, and textiles perform (made by braiding, weaving, or knitting fibres together in specified designs).
Glass: Glass is the most common and inexpensive fibre and is usually use for the reinforcement of polymer matrices. Glass has a high tensile strength and fairly low density (2.5 g/cc).
Carbon-graphite: In advance composites, carbon fibres are the material of choice. Carbon is a very light element, with a density of about 2.3 g/cc and its stiffness is considerable higher than glass. Carbon fibres can have up to 3 times the stiffness of steel and up to 15 times the strength of construction steel. The graphitic structure is preferred to the diamond-like crystalline forms for making carbon fibre because the graphitic structure is made of densely packed hexagonal layers, stacked in a lamellar style. This structure results in mechanical and thermal properties are highly anisotropic and this gives component designers the ability to control the strength and stiffness of components by varying the orientation of the fibre.
Polymers: A variety of polymer materials are used as filler material for composites. The strong covalent bonds of polymers offer tailor-made properties in the form of bristles, whiskers, staple fibres, filaments, yarns or tows, spun yarns, threads, ropes, unwoven and woven fabrics, knitted compositions. Nylons, polyesters, rayon, acrylic, Kevlar and many other fibres are used for composite formation.
Ceramics: Ceramic fibres made from materials such as Alumina and Silicon carbides are used in very high temperature applications, and also where environmental attacks are severe. Tungsten-boron filaments, Ceramics have poor properties in tension and shear, so most applications as reinforcement are in the particulate form.
Metallic: Metallic fibres have high strengths but since their density is very high they are of little use in weight critical applications. Drawing very thin metallic fibers (less than 100 microns) is also very expensive.
● MMC -Metal Matrix Composites:
Metal matrix composites have either Metal or Ceramic as the filler material. Polymer fillers are nominally not feasible, because at processing temperature of metal matrix material, most polymers cannot survive. Majority of MMCs are aluminum matrix composites, but a growing number of composites are produced with the matrix of superalloys, titanium, copper, magnesium, or iron. Lightest metal-matrix composite is heavier than any other polymer or polymer matrix composite and are comparatively complex to process.
Metal matrix composites are as old as the Bronze age. However, work on modern MMC began in the late 1950s. A copper-clad 316 stainless steel shell was manufactured by electroforming, possessing an outer skin of nickel and a reflective platinum final surface, for aerospace industry in USA. Today, metal cladding (a layered metal composite) is very common, as seen in coins, cooking ware, armour, and other items. Metal-matrix composites are used for the space shuttles, commercial airliners, electronic substrates, bicycles, auto-mobiles, golf clubs, and a variety of other applications. Super-alloy composites reinforced with tungsten alloy fibres are used for components of jet turbine engines that operate at temperatures above 1000 °C. Under sea cables for communication and cables for shipping and elevators are invariably of composite materials.
Compared to monolithic metals, MMCs have higher strength-to-density ratios, higher strength and stiffness, better wear resistance, fatigue resistance and elevated temperature properties, a lower creep rate and coefficients of thermal expansion. MMCs have higher cost, involve difficult to use technologies but provide wonderful super alloys and super-conductive materials.
Carbide drill bits are a metal matrix composite. Tungsten carbide powder is mixed with cobalt powder, and then pressed and sintered together, the tungsten carbide retains its identity. The resulting material has a soft cobalt matrix with tough tungsten carbide particles inside.
Metal Matrix composites in comparison to other materials: Compared to monolithic metals and polymer matrix composites, MMCs are high cost systems with relatively newer and immature technology. However, MMCs have better temperature capability, superior fire resistance, higher electrical and thermal conductivities.
MMC reinforcements are metal wires, and ceramic materials like particulate, whiskers, filaments, fibres. Key continuous fibres include boron, graphite (carbon), alumina, alumina-silica, boron carbide and silicon carbide.
o Aluminium Matrix composites have the advantage of low cost over most other MMCs. Aluminium matrix composites have excellent thermal conductivity, high shear strength, excellent abrasion resistance, high-temperature operation, non flame-ability, minimal attack by fuels and solvents, and the ability to be formed and treated on conventional equipment. Aluminum MMCs are produced by casting, powder metallurgy, in situ development of reinforcements, and foil-and-fibre pressing techniques. Aluminium matrix composites are used in brake rotors, pistons, and other automotive components, golf clubs, bicycles, machinery components, electronic substrates, extruded angles and channels, and many structural and electronic applications.
o Copper Matrix composites have metal wires and filaments of tungsten, beryllium, titanium, and molybdenum for reinforcement. Ductile superconductors are fabricated with a matrix of copper and superconducting filaments of niobium-titanium and niobium-tin. Copper matrices are also fortified with fine tungsten wires. Copper matrix reinforced with tungsten particles or aluminum oxide particles, is used in heat sinks and electronic packaging. Copper-Graphite composites can be designed to have very specific qualities such as high temperatures in air, excellent mechanical characteristics, as well as high electrical and thermal conductivity. Copper matrix materials are easier to process compared to titanium. Copper matrix composites have lower density compared with steel.
o Titanium Matrix composites are new age composites. Titanium reinforced with silicon carbide fibres is under development as skin material for the National Aerospace Plane. Stainless steels, tool steels, and Inconel are among the matrix materials reinforced with titanium carbide particles.
● PMC -Polymer-Matrix Composites:
These are most widely used types of composite materials. PMC consists of a polymer matrix of thermosetting and thermoplastic materials with nearly all types of fillers. Most popular fillers are wood, glass and carbon fibres.
o Thermosetting Materials for Polymer Matrix: Thermosetting polymer molecules develop interlinks during the curing process which usually occurs at room temperature, but can have temperature input to achieve the optimum results. Shrinkage during curing and thermal contraction on cooling, can lead to inbuilt stresses in the composite material. Thermoset materials have a shelf life problem, often requiring freezing to retard the changes that continuously occur in the resin. Thermoset materials are recyclable so are considered to be environment friendly. Epoxies and polyester resins cover a broad range of thermoplastic resins for composites.
Thermosetting composites are manufactured through many different processes. In all these processes wetting of fibre with the resin material is an important aspect. Fibres in the form of a mat are impregnated with resin, tows or bundles of fibres are wetted and laid to shape a component, Paper-covered sheets or narrow stripes (pre-preg) of resin impregnated fibres are partially cured, and pressed to form the final shape of the component, or resin and fibres, are mixed and co-extruded, or hot pressed.
o Thermoplastic Materials for Polymer Matrix: Thermoplastics can be repeatedly melted and solidified for reprocessing. Thermoplastics as a result are considered recyclable and preferred from the environment point of view. Manufacturing technologies for thermoplastics are not as advanced as those for thermoset.
Thermoplastics derive their strength and stiffness from the inherent properties of the monomer units and very high molecular weight. In amorphous thermoplastics there is high concentration of molecular entanglements, which acts like cross links, and in crystalline materials there is high degree of molecular order and alignment. In amorphous materials heating reduces entanglement and the material turn from solid to viscous liquid. With crystalline materials heating melts the crystalline phase, to give an amorphous viscous liquid.
Thermoplastics are structurally very sensitive to temperature conditions, as under constant load conditions these materials show an increase in strain with time, i.e. materials creep under loads. This means that in composite material with thermoplastics a redistribution of the load between the resin (matrix) and the fibres occur during the deformation.
Thermoplastics are more expensive, and they generally do not resist heat as well as thermoset, however, some thermoplastics with higher melting temperatures are available. Three common thermoplastics matrix polymers are polypropylene, nylon and polycarbonate.
● CMC -Ceramic Matrix Composites:
Ceramic matrix composites have evolved from needs that certain components must maintain their structural integrity at very high temperatures, yet remain operative. Temperatures in boilers, chimneys, exhausts, heat sinks, combustion engines, furnaces, etc. are such that metal materials become soft, whereas polymer materials get degraded. Here traditional ceramic materials work well with their superior heat resistance, low abrasive and non corrosive properties, but brittleness is a major drawback. Ceramic Matrix Composites reinforced with ceramic or metal fibres provide better structural properties against not only traditional ceramics but also metals and alloys. Ceramics by themselves are often not conductive of heat, electricity, etc. but this can be altered by suitable addition of a filler material. The desirable characteristics of CMCs include: high-temperature stability, high thermal-shock resistance, high hardness, high corrosion resistance, light weight, non-magnetic and non-conductive properties.
Unlike polymers and metals, which are processed by techniques that involve melting (or softening) followed by solidification, constituents of high-temperature ceramics cannot be melted. They are generally produced by some variation of sintering, a technique that renders a combination of materials into a coherent mass by heating to high temperatures without complete melting. In some instances a polymer-filler mixture, on pyrolysis, is converted to a ceramic matrix (consisting of silicon carbide, boron carbide, boron nitride, and a silicon oxy-carbide -SiOC glass), without reacting with the carbon fibre. Where continuous fibres or textile weaves (as opposed to short fibres or whiskers) are involved, sintering is preceded by impregnating the assembly of fibres with a slurry of ceramic particles dispersed in a liquid.
Technologies for CMC are still new compared to PMC, but technological achievements are very remarkable. Major barriers to wide use of ceramic matrix composites are: lack of detailed specifications, information, problems of fixing other materials to ceramic matrix components, in-service repair methodology, high cost.
Ceramic matrices can be categorized as either oxides or non oxides and in some cases may contain residual metal after processing. Common oxide matrices include alumina, silica, mullite, barium aluminosilicate, lithium aluminosilicate and calcium aluminosilicate. Common non oxide ceramics include, SiC, Si3N4, boron carbide, and AlN. Oxide matrices are considered environmentally stable yet non oxide ceramics, with their superior structural properties, hardness, and corrosion resistance are considered commercially better.
Ceramic reinforcements in the form of discontinuous fibre include whiskers, platelets, and particulate having compositions of Si3N4, SiC, AlN, titanium di boride, boron carbide, and boron nitride are commonly used. Of these, SiC is most prominent because of its stability with a broad range of ceramic oxide and non oxide matrices. Discontinuous oxide ceramic reinforcements are less prevalent because of their incompatibility with many common ceramic matrices.
Some of the more common continuous reinforcements of ceramics include: glass, mullite, alumina, carbon, and SiC. Of these, SiC-fibres are commonly used because of their high strength, stiffness and thermal stability.
Common trade names for silicon carbide fibre include Nicalon™, Hi-Nicalon™, SCS, Sylramic™, and Tyranno. For applications where temperature is lower (< 1100°C) or exposure times limited, mullite fibres are used because of their lower cost. Nextel is a common trade name for both mullite and alumina fibre. Continuous ceramic fibres are favoured because of their ability to provide pseudo-ductile characteristics to otherwise brittle ceramic materials.
ADVANCED COMPOSITES
‘Advanced-composites’ is a misleading term. In case of composites it could mean different concepts. It could mean composites requiring: high tuned multiple processing, composites of uncommon materials, or composites with very specific material properties.
Advanced composites had come of age in the early 1960s with the development of high-modulus whiskers and filaments. While whiskers were easily made, their composites were of poor quality. But the 60 million modulus strength boron filaments with reinforcing epoxy, were very successful and were used in fighter aircraft and, later, in golf-club shafts, fly-rods, and tennis rackets.
But it was the improvement of graphite fibre that led to the major jump in mechanical advantages for military and sports applications. A sprinkling of boron now turned up but, just like the early gasoline with boron, its contribution was minor; still, it sold. The large modulus differences for fibre and matrix were also accompanied by large differences in expansion coefficients and consequent residual thermal stresses.
In the First category, as an example, one can include, Carbon-carbon composites which are closely related to Ceramic Matrix Composites, but differ in the methods by which they are produced. Carbon-carbon composites consist of semi-crystalline carbon fibres embedded in a matrix of amorphous carbon. The composite begins as a PMC, with semi-crystalline carbon fibres impregnated with a polymeric phenolic resin. The resin-soaked system is heated in an inert atmosphere to pyrolyze, or char the polymer to a carbon residue. The composite is re-impregnated with a polymer, and the pyrolysis is repeated. Continued repetition of this impregnation/pyrolysis process yields a structure with least voids. Carbon-carbon composites retain their strength at 2500 C and are used in the nose cones of re-entry vehicles. However, because they are vulnerable to oxidation at such high temperatures, they must be protected by a thin layer of ceramic.
In the Second group, as an example, one can include composites of piezoelectric materials which generate an electrical current when they are bent, conversely, when an electrical current is passed through these materials, they stiffen. This property can be used to suppress vibration, the electrical current generated during vibration could be detected, amplified, and sent back, causing the material to stiffen and stop vibrating. Other examples are efforts to develop smart or responsive material systems, that mimic a living organism by reacting to stimuli generates a desired response.
In the third group, for example, materials are needed with a near-zero coefficient of thermal expansion can be included. These materials have to be thermally stable and should not expand and contract when exposed to extreme changes in temperature.
MANUFACTURING METHODS for COMPOSITES
Composites to be extra efficient are engineered to form or shape of the component. This involves strategically placing the reinforcements while manipulating the matrix properties to achieve a perfect combination. These fabrication methods are commonly named moulding, casting, injection and extrusion processes. Other processes include curing (through catalysts, radiation, exothermic reactions and heat), heat treatments like baking, melting, sintering, forging, pressing and rolling.
Each matrix system requires specific manufacturing method. Polymer matrices, need no or low heat input, followed by metal matrices. Ceramic matrices need very high temperatures for composite formation.
The most common form of material used for the fabrication of composite structures is the pre-impregnated tape, or Pre-Preg. There are two types commercial pre-preg materials, Tapes (of 75 ml width) and Broad goods (of several metres widths) intended for hand lay-up and large sheet applications. To make a composite pre-preg with surface treated fibres are passed through a resin bath and laid on to a component form or sheet form. For thermosetting polymers, the resin coated structure of tape or broad goods are autoclaved or baked. Thermoplastic systems do not require heat input, so prove to be less costly and easier to handle.
Pultrusion is a continuous process where fibres and the resin are pushed through a heated die for manufacturing long shapes. Resin transfer moulding is generally limited to low viscosity materials. Fibres are laid in a mould and resin is injected in the closed mould.
To make a low temperature super conductor, Niobium-tin alloys (Nb3 Sn), are found to be brittle. However, Copper and Niobium nominally immiscible, when melted together provides a ductile material. The new material has Niobium solidified within the copper into structures called dendrites. When drawn into a thin wire, the Niobium dendrites form small filaments within the copper. After these stage the wire is passed through molten tin. The tin combines with the copper to provide a wind able wire of super-conducting properties.
SHORT COMINGS of COMPOSITES
Although composite materials have certain advantages over conventional materials, composites also have some disadvantages. For example, PMC and other composite materials tend to be highly anisotropic, that is, their strength, stiffness, and other engineering properties are different depending on the orientation of the filler material. Joining composite components is difficult. Advanced composites have not only high manufacturing costs but require very exact quality controls. The composite material must result into a definite component, which makes the processes very much labour-intensive, and not easily amenable to automation.
Further reading:
An Introduction to Composite Materials: Hull Derek / Cambridge University Press
Selection and use of Engineering Materials : Charles, Crane, Furness / Butterworth-Heinemann
Engineering Materials : Budinski, Budinski / Prentice Hall
Engineering Materials and their Application : Flinn, Trojan
