Applied Chemistry-Sem-II
Dr. Payal Joshi
Composite Materials Modern technologies demand materials with unusual and extraordinary combinations of properties that cannot be provided by conventional metal alloys, ceramic and polymeric materials required for aerospace, under water & transportation applications--Structural materials having low density, stiffness, high strength, abrasion resistance, impact resistance & corrosion resistance. Such combination of properties is difficult to achieve in conventional materials. New class materials are called “Composites”. Composite materials may be defined as a multiphase material system consisting of a mixture of two or more macro-constituents, which are mutually insoluble, differing in form &/or composition & forming different phases. Composite materials process such combination of properties, which were not within the easy reach of energies either because – such basic materials were not available or were too costly. Stiffness (ability of materials to resist elastic deformation or deflection on loading) is increased, without the disadvantages of brittleness. By using composites; it is possible to have combination of properties like high specific strength, lower specific gravity, higher stiffness, maintain strength up to higher temperatures, better toughness, impact and thermal shock resistance, easy to fabricate, better corrosion and oxidation resistance. Examples: Wood: Composite of cellulose fibers & lignin; Bone: Composite of protein, collagen & apatite; Rain- proof cloth (cloth impregnated with water proof material). Combining two or more distinct materials one can engineer a new material with desired combination of properties. Better combination of properties can be achieved by Principle of combined constitution action. Mixture gives “averaged properties.” Composite material comprises of two phases namely matrix which continuously surrounds the other phase called the dispersed phase. Composite is an artificially prepared multiphase material in which chemically dissimilar phases are separated by a distinct inter phase e.g., wood consist of strong flexible cellulose fibers surrounded & held together by different material called Lignin. Properties of composites are determined by three factors namely; materials used as component phases in the composite, geometric shapes of the constituents and resulting structure of the composite system and manner in which phases interact with one another. Matrix Phase: Continuous body constituents that encloses the composite & give its bulk form. It may be metal, ceramics or polymers. Composites using these matrix materials are known as, 1. Metal Matrix Composites (MMCs) - Mixtures of ceramics and metals, such as cemented carbides and other cermets. 2. Ceramic Matrix Composites (CMCs) - Al2O3 and SiC imbedded with fibers to improve properties, especially in high temperature applications. 3. Polymer Matrix Composites (PMCs) - Thermosetting resins are widely used in PMCs; Examples: epoxy and polyester with fiber reinforcement, and phenolic with powders. 1
Applied Chemistry-Sem-II
Dr. Payal Joshi
Functions of Matrix: binds the dispersed phase together; acts as a medium to transmit & distribute externally applied load to the dispersed phase; protects dispersed phase from chemical action & keep in proper position & orientation during application of load; Prevents propagation of brittle cracks due to its plasticity & softness. Requirements of a good matrix phase: Ductile, corrosion resistant, possess high bonding strength between matrix and dispersed phase. Metals like Al, Cu, & certain commercial thermoplastic & thermosetting polymers are used as matrix materials. Dispersed phase: It is a structural constituent determines internal structure of the composite. Important dispersed phase properties are, thin single crystal, possess extremely large length / diameter ratio, flame free (critical surface flaw free which otherwise leads to fracture), high tensile strength. Important Dispersed phases are, 1. Fiber: Long thin filament of any polymer, metal, ceramic having high length to diameter ratio. Characteristics: high length to diameter ratio, high tensile strength & stiffness. a. Glass fibers: obtained by forcing glass melt through number of small orifices/openings of a spinneret, rapidly pulling followed by cooling the emerging continuous filaments. Individual fiber (diameter of about 10 µm)—“Monofilament”. Glass fibers are high performance materials:- Ready availability, low cost, easy to draw from molten phase, can be coupled with polymer matrix to achieve chemical inertness. b. Carbon fibers: obtained as continuous filament by pyrolysis in an inert atmosphere of organic fibers like cellulose or polyamylonitrile. Monofilaments (diameter of 8 µm.): Used as reinforcing material with epoxy/polyester resins to form composites, high performance fiber but costly, Sensitive to moisture, acids, bases & number of solvents. c. Aramid fibers: made by spinning liquid crystal aramid oligomer (Kevlar), Use along with polyster/epoxides. High performance, high tensile strength, stability at high temperatures, resistant to creep & fatigue failure, excellent loughness & impact resistance. But they are susceptible to degradation by acids & strong bases. 2. Particulates /particles: Small pieces of hard solid material (metallic/nonmetallic) – metal powders, metal oxides, carbon black, metal carbides, silica, mica, intimately mixed with polymer matrix to get a composite. Effect of adding particulates to matrix materials: Surface hardness is increased, performance at elevated temperature improved, cost of composite is reduced, thermal & electrical conductivity modified. 3. Flakes: Thin solids with 2-D geometry.eg. Mica flakes. 4. Whiskers: Thin strong filaments/fibers made by growing a crystal. Special types of fibers – Very thin single crystals possessing extremely large length- to- diameter ratio. Due to small size they exhibit high degree of crystalline perfection (free of flakes), extraordinarily high strength. Strongest known material – Extreme use as reinforcement medium is prohibited due to high cost. However, one faces difficulty of incorporating them in to matrix. Graphite, Al Oxide , SiC , Silicon nitride are generally used. 2
Applied Chemistry-Sem-II
Dr. Payal Joshi
Role of interface in Composites: Transmission of stress from matrix phase to reinforcing dispersed phase depends on interfacial bond strength – failure is resisted if this bond is strong. Interface can serve as: locus for chemical reaction, site for nucleation, site for preferential adsorption. This is due to the differences between thermal coefficients of two constituents of composites and cure shrinkage (in thermoset) crystallization (in thermoplastics) when polymer is used in matrix. Composite Classification
Particle – reinforced Composites: It is made by dispersing particles of varying size & shape of one material in a matrix of another material. It is made by adding particles to a liquid matrix material, which later solidifies or may be pressed together. Large particle Composites: In these composites, particulate phase is harder & stiffer than the matrix. These reinforcing particles help to restrain the movement of the matrix phase in the vicinity of each particle. Matrix transfers some of the applied stress to the particles, which bears a fraction of the load. Strength of bonding at matrix – particle interface dictates the degree of reinforcement & the mechanical properties. Examples of large particle composites (LPC): Cement matrix + particulate of sand gravel forms concrete. LPC are used with all three major types of materials namely metals, polymers & ceramics. Ceramic metal composites is known as Cermets .Commonly used cermets: cermented carbide composed of extremely hard particles of refractory carbide ceramic such as tungsten carbide WC or TiC embedded in matrix of a metal like Cobalt or Nickel. Dispersion strengthened composites: In this case, the particle size is smaller (10-100nm). Metals and alloys are made into extremely small particle size in a given range and are dispersed in the matrix phase. This is achieved by appropriate heat treatment called as, ‘precipitation hardening’ or ‘Age hardening.’ Alloys like Cu-Sn, Mg-Al, Cu-Be, Al-Cu, ferrous alloys are hardened and made into composites with ceramics.
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Applied Chemistry-Sem-II
Cermets Classification: (Ceramic bonded with metals) Oxide based Carbide based eg. Al2O3 dispersed in WC & Co matrix cermet Cr matrix. Good strength, wire drawing Good thermal shock values & machine parts -resistance.
Dr. Payal Joshi
CrC & Co matrix cermets Resistant to abrasion corrosion Application involves spray nozzle pump parts.
Fiber-Reinforced composites: Dispersed phase is in the form of fibers. These composites provide high strength & stiffness on a weight basis. It involves three components namely, filament, a polymer matrix (encapsulating agent for the filament) & a bonding agent (that ties fiber filament to polymer). Glass fiber & metallic fibers are commonly employed for this purpose.
Continuous & Aligned fiber components: Properties of such components are highly anisotropic i.e. they depend on the direction in which they are measured. Maximum strength & reinforcement are achieved along the alignment (longitudinal) direction. However in transverse direction, fiber reinforcement is virtually non-existent – hence fracture usually occurs at relatively low tensile stress.
Fig: Schematic representation of different types of fiber-reinforced composites. (a) Continuous and aligned; (b) discontinuous and aligned; (c) discontinuous and randomly oriented 4
Applied Chemistry-Sem-II
Dr. Payal Joshi
Discontinuous Fiber Composites: Reinforcement efficiency of discontinuous fibers is lower than that of continuous fibers, but still they find wider applications. Chopped glass fibers, carbon & aramid discontinuous fibers are employed. Important types of FRP are as follows: 1. Glass Fiber-Reinforced polymer composites: It employs glass fibers as the dispersed phase and polymer is the matrix phase. These composites possess lower density, higher tensile strength, high impact resistance and excellent resistance to corrosion and chemicals. They are used in fabricating automobile parts, pipes and transportation industries. 2. Carbon Fiber-Reinforced polymer composites: Also known as advance polymer matrix composites providing excellent resistance to corrosion, lighter density and retention of desired properties. It is generally used in structural components of aircrafts and sport goods like golf clubs, fishing rods, etc. 3. Aramid Fiber-Reinforced polymer composites: They are generally of two types:
Short (discontinuous) aramid FRP composites: They have high aspect ratio, high surface area, inherent toughness and high wear resistance. It is used in fabricating automobile parts like clutches and brakes. Long (continuous) FRP composites: They are metal ductile and respond non-catastrophically to compressive stress. They are excellent structural and advanced engineering materials used in aircrafts.
4. Alumina and/or carbon FRP composites: They possess improved strength, stiffness, abrasion resistance, creep resistance and dimensional stability. Eg.Al2O3 (dispersed) or carbon fibres + Al alloy (matrix): Used to fabricate engine components. Structural composites / Layered Composites: These composites comprise of both homogenous & composite materials, the properties of which depend not only on properties of constituent materials but also on their geometrical design.
Laminar Composite: It consists of 2-D sheets/panels that have preferred high strength direction as found in wood & continuous & aligned fiber – reinforced plastics. Successive oriented fiberreinforced layers are stretched & then cemented together in such a way that orientation of high strength varies with each successive layer. e.g. Plywood is a laminated composite of thin layers of wood with alternate layers glued together so that the grain of each layer is at right angles of 5
Applied Chemistry-Sem-II
Dr. Payal Joshi
its neighbors. Layered composites possess high strength in both directions of reinforcement, but their strength is comparatively low. Sandwich panel consists of two strong outer sheets (faces) & an intervening layer of comparatively less dense “core” material. These layers are joined by an adhesive. Typical “face” materials include fiber reinforced plastics, plywood, Ti, steel & Al alloys. Typical “Core” materials include synthetic rubbers, formed polymers & inorganic cement. The “faces” bear most of the in – plane loading as well as any transverse bending stresses. The “core” serves the following two structural properties: a. It separated “faces” & resists any deformations perpendicular to face plane. b. Provides certain degree of shear rigidity along planes which are perpendicular to the faces. Popular core material: Honeycomb structure: Thin foils formed in to interlocking hexagonal cells with their axes oriented in perpendicular to face planes. Applications of Composite Materials: Composites are one of the most widely used materials because of their adaptability to different situations and the relative ease of combination with other materials to serve specific purposes and exhibit desirable properties. In surface transportation, reinforced plastics are the kind of composites used because of their huge size. They provide ample scope and receptiveness to design changes, materials and processes. The strength-weight ratio is higher than other materials. Their stiffness and cost effectiveness offered, apart from easy availability of raw materials, make them the obvious choice for applications in surface transportation. Polyester resin with suitable fillers and reinforcements were the first applications of composites in road transportation. The choice was dictated by properties like low cost, ease in designing and production of functional parts. Commercial aircraft applications are the most important uses of composites. Aircraft, unlike other vehicles, need to lay greater stress on safety and weight. A modern civil aircraft must be so designed as to meet the numerous criteria of power and safety. Use of fibre-reinforced composites has become increasingly attractive alternative to conventional metals for many aircraft components mainly due to their increased strength, durability, corrosion resistance, resistance to fatigue and damage tolerance characteristics. Composite materials used in aircraft industry are generally reinforced fibers or filaments embedded in a resin matrix. The most common fibers are carbon, aramid, glass and their hybrid. Composites have long been used in the construction industry. Applications range from non-structural gratings and claddings to full structural systems for industrial supports, buildings, long span roof structures, tanks, bridge components and complete bridge systems. Composites are an alternate material replacing timber, steel, aluminium and concrete in buildings. Construction holds priority for the adaptation of composites in place of conventional materials being used like tanks, bridge systems, etc. Components made of composite materials find extensive applications in shuttering supports, special architectural structures imparting aesthetic appearance, large signages etc. with the 6
Applied Chemistry-Sem-II
Dr. Payal Joshi
advantages like corrosion resistance, longer life, low maintenance, easy workability, fire retardancy etc. Usage of composites for damage repairing, seismic retrofitting and upgrading of concrete bridges finds increased adoption as a way to extend the service life of existing structures, they are also being considered as an economic solution for new bridge structures. FRP has been found quite suitable for repair and upgrading of concrete bridges as a way to extend the service life of existing structures. Bamboo is one of the fastest renewable plants with a maturity cycle of 3-4 years, thus making it a highly attractive natural resource compared to forest hardwoods. Bamboo offers good potential for processing it into composites as a wood substitute. Bamboo laminates could replace timber in many applications such as furniture, doors and windows and their frames, partitions, wardrobes, cabinets, flooring etc. A unique idea in mass transit: the ‘Sky Bus’, which is all set to bring in a revolution in mass transportation system was conceptualized by Konkan Railway that would run on overhead rails along the length and supported over vertical columns at regular intervals in the road median. Most of the coach (exterior & interior) is made of composites & plastics. Composite materials are increasingly being used in the railway industry especially passenger coaches worldwide for excellent structural properties and improved aesthetics. For mass transit systems, lighter bodied coaches are instrumental for achieving higher speed. Apart from these, composite material has been identified as the new class of synthetic bio-materials. An important development has been the usage of carbon-fiber reinforced polymer-matrix for composite limb. Prosthetics and Orthotics help people who acquire disability or were born with physical defects, by fitting them with artificial supports. Bio-medical prosthetic devices are artificial replacements that are used in the human body to function as original parts. Materials used for such prosthetic aids must non-toxic, biologically and chemically stable with sufficient mechanical integrity and strength to withstand physiological loads. The classic example of prosthesis or artificial limb is the Jaipur foot. It is made of rubber, wood and aluminium and can be assembled locally. The foot is light in weight, economical and comfortable to walk. Composite materials for microelectronics: Organically modified resins retain important roles in electrical component coatings such as resistors and molding compounds, as well as spin-on dielectrics in microelectronic interlayer and multilayer dielectric and planarization applications. Conducting polymer composites in energy storage applications: The electrochemical activity of the inorganic metal oxide cluster can be harness by integrating them with conducting organic polymer matrix to form a composite material. Molecular hybrid materials formed from polyoxometalates dispersed in conducting polymers represent an innovative concept in energy storage. Energy storage devices such as fuel cells and renewable energy devices, such as photovoltaic or dye sensitized solar cells, will need the support of efficient energy storage technologies. Rechargeable lithium batteries and electrochemical super capacitors are two of the most prominent alternatives in this respect.
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