MODULE FIVE COMPOSITE MATERIALS 10. COMPOSITE MATERIALS Introduction Classification of composites Types of matrix materials and reinforcements Production of FRP s and MMC s Advantages and applications of composites 10.1 INTRODUCTION A composite material or a composite may be defined as a materials system composed of two or more physically or chemically distinct phases that are insoluble in each other and when combined have improved properties over the individual materials. Example: Wood (combination of cellulose fiber and lignin) naturally occurring composite Reinforced cement concrete artificial composite Components of a composite material: Most of the composite materials are composed of just two phases; a continuous phase known as matrix in which the second phase is dispersed without being soluble in it. The dispersed phase acts as reinforcement and can be of different shape and sizes. Primary phase (continuous) matrix Secondary phase (dispersed) reinforcement Figure 10.1: Composite In reinforced cement concrete the steel bars along with the gravel and sand act as the reinforcement which are bound together by cement matrix. Typically, reinforcing materials are strong whereas the matrix is usually tough or ductile. Thus a composite material combines the Pruthvi Loy, Chiranth B. P. 1 SJEC, Mangaluru
strength of the reinforcement with the toughness of the matrix to achieve superior properties than the individual components. Even though the high strength of the composite is largely due to the reinforcement, the matrix provides support for the reinforcement and assists them in carrying the loads. The properties of the composite depend on the following factors; The relative amount of the phases Geometry of the dispersed phase (size, shape, distribution & orientation of reinforcement) Bonding between the phases Functions of matrix: It holds the reinforcing material together and distributes the load evenly between them. Protects the reinforcing phase from severe environment. Helps to avoid the propagation of crack through the reinforcement by providing alternate failure path along the interface. Improves impact and fracture resistance of a composite. It helps in alignment of reinforcement phase in a pre-determined direction. Functions of reinforcement: It gives strength and stiffness to the composite. It helps in achieving directional properties in a composite. Provides flexibility in designing a composite for specific applications. 10.2 CLASSIFICATION OF COMPOSITES They are classified at two distinct levels. I. Classification with respect to matrix constituent a. Polymer matrix composites (PMC) b. Metal matrix composites (MMC) c. Ceramic matrix composites (CMC) II. Classification based on reinforcement form a. Fiber reinforced composites. b. Particle reinforced composites. c. Structural/laminar composites Pruthvi Loy, Chiranth B. P. 2 SJEC, Mangaluru
10.3 MATRIX AND REINFORCEMENT MATERIALS 10.3.1 Matrix materials The most commonly used matrix materials are metals, ceramics and polymers which are discussed further. 10.3.1(a) Polymers Polymer matrix material are generally called as resins and are classified as a. Thermosetting material b. Thermoplastic material Thermosetting material A thermosetting material is one which when cured by heat or chemical reaction changes into an infusible and insoluble material. i.e., they undergo irreversible chemical reaction upon application of heat Example: polysters, epoxies, vinyl esters, phenolics etc Thermoplastic material Thermoplastic don t undergo a chemical reaction upon application of heat, they simply melt on application of heat and pressure to form a component. They can be repeatedly softened by heating and hardened by cooling. Example: polypylene, polymide etc Table 10.1: Characteristics of commonly used polymers SL No. Polymers Characteristics Low cost Good mechanical properties, electrical properties and heat 1. Polyesters resistance Curing temperature is 120 C High shrinkage and low chemical resistance Improved strength and stiffness Excellent corrosion resistance 2. Epoxies No by products during curing and low curing shrinkage Curing temperature at 120-175 C High toughness 3. Vinyl ester Outstanding heat and chemical resistance Corrosion and moisture resistance Pruthvi Loy, Chiranth B. P. 3 SJEC, Mangaluru
4. Phenolics Low cost Excellent high temperature resistance Curing temperature is 175 C Good mechanical properties Dimensional and thermal stability High shrinkage and by products Table 10.2: Comparison of thermosetting and thermoplastic polymers Thermosets Resin cost is low High processing times for curing Most of them are toxic and requires storage in refrigerator Cannot be reused Moderate shrinkage Fracture toughness is low Relatively hard and brittle Thermoplastic Resin cost is slightly higher Fast and economic process Non-toxic as there are no reactive chemical Re melted and reused. Low shrinkage High fracture toughness Soft and ductile 10.3.1(b) Metal matrix materials Characteristics: o Metal or alloys are used as matrix materials o Generally used for structural materials at high temperature. o They are advantages over PMC in applications requiring a long term resistance to serve environment. o Metal matrix can be plastically deformed and strengthened by various thermal and mechanical treatments. o But metals have high melting temperature and hence needs high processing temperature. Examples: Aluminium and Titanium composites reinforced with Silicon carbide (SiC), boron or carbon fibers. Pruthvi Loy, Chiranth B. P. 4 SJEC, Mangaluru
Applications: For developing lighter MMC s aluminium and titanium alloys are used as matrix materials for aerospace and automobile applications (Aluminium alloy matrix composites are suited to applications below the temperature of 400 C for high temperature titanium based alloys are used for applications in temperature of 900 C ) o Missile body casings o Wing structural elements o Engine shaft o Disk for turbine engine o Hollow fan blades SiC reinforced Aluminium Composites SiC reinforced Titanium Composites 10.3.1(c) Ceramic matrix materials Ceramic materials are well known for their high temperature properties as well as their resistance to oxidation, but they are very brittle which limits their application. Ceramics make a better reinforcement material than the matrix. Most commonly used ceramic materials are; Silicon nitride (Si 3 N 4 ), Silicon carbide (SiC) and Alumina (Al 2 O 3 ) 10.3.2 Reinforcement materials 10.3.2(a) Fiber reinforcement material This is the most commonly used reinforcement in which the dispersed phase is in the form of a fiber. They can be further grouped into three categories on the basis of fiber diameter; o whiskers - extremely large length to diameter ratio (very small diameter) o fibers - moderate length to diameter ratio (small diameter) o wires small length to diameter ratio (large diameter) Examples: Graphite, silicon carbide, aluminum oxide. Polymers, glass, carbon, boron. Steel, molybdenum, tungsten-automobile tyres Pruthvi Loy, Chiranth B. P. 5 SJEC, Mangaluru
Influence of fiber orientation on properties of composites: There are two possible arrangements a) Uniformly orientated (parallel alignment) b) Randomly oriented (a) Longitudinal loading (b) Transverse loading Figure 10.2: Fiber orientation The matrix phase of fibrous components may be metal, polymer or ceramic, in general metals and polymers are used as matrix materials because some ductility is desirable. There must be strong adhesive bonding forces between fiber and matrix to minimize fiber pull out. 10.3.2(b) Particle reinforced composites Particles of different shape and size are dispersed in a matrix of second material to form a particulate composite. It can be sub classified as o Large particle composites o Fine particle / Dispersion strengthened composites For large particle composites the particulate phase is harder and stiffer than matrix, for dispersion strengthened composites, particles are normally much smaller (having diameter between 10 to 100 nm). The strengthening in dispersion composites is due to particle-matrix interaction, where major portion of the load is taken up by the matrix whereas the dispersion only acts as barrier for dislocation motion. 10.3.2(c) Structural composites-laminates A uniformly aligned fiber reinforced composites show anisotropic properties (i.e., properties varies along different directions) but if layers of such composites are stacked and bonded together in such a way that successive layers have their fibers aligned in different directions, the Pruthvi Loy, Chiranth B. P. 6 SJEC, Mangaluru
composite on the whole will have high strength and uniform properties all direction, such composites are called laminates. Example: plywood. Figure 10.3: Laminated composites 10.4 PRODUCTION OF COMPOSITES The production of the following composites is discussed further; Fiber Reinforced Plastics (FRP) Metal Matrix Composites (MMC) 10.4.1 Production of FRP s FRP s can be manufactured by several methods; I. Open mould process: a. Hand lay-up process b. Spray-up process c. Filament winding d. Compression moulding II. III. Closed mould process: a. Pressure bag moulding b. Vacuum bag moulding Bag moulding c. Autoclave process Continuous process: a. Pultrusion process b. Sheet moulding compound Pruthvi Loy, Chiranth B. P. 7 SJEC, Mangaluru
10.4.1(a) Hand lay-up process: This is the simplest and conventional method of producing fiber reinforced composites wherein glass fibers are used as reinforcement and a polyester resin is used. The process details are as follows; a. A gel coat is applied to the mould (wax, clay or wood) to facilitate easy removal of the product from the mould. b. A fiber glass reinforcement normally in the form of cloth or mat is placed on it. c. The plastic resin mixed with catalyst and accelerator is applied on it by pouring. d. A roller is used to thoroughly wet the reinforcement with resin and to remove the entrapped air. e. To increase the thickness of the composite more layers of fiberglass and resin may be added. Advantages: Figure 10.4: Hand lay-up process a. Simple and low cost process. b. Ideal for large size components. c. Design flexibility and longer fiber layup possible. Disadvantages: 1. Only one side good surface finish. 2. Skilled labor is required for good quality. 3. Longer cure time is required. Applications: Wind turbine blades, boats, etc. Pruthvi Loy, Chiranth B. P. 8 SJEC, Mangaluru
10.4.1(b) Spray-up process: In this process, continuous fibers are cut into desired length by a chopper, this is then mixed with resin, catalyst and promoter as they are sprayed on to a gel coated mould through a spray gun. The density of the composite can be increased by rolling to remove the entrapped air. Multiple layers are added to produce composite of desired thickness. Advantages: Figure 10.5: Spray-up process 1. Low cost technique 2. Faster production as compared to hand layup. Disadvantages: 1. Laminates tend to be resin rich and requires low viscosity resin to facilitate spraying. 2. Difficult to maintain uniform distribution of chopped fibers. Applications: Bathtubs, shower trays, etc. 10.4.1(c) Filament winding: In this process, strands of fiber reinforcement are fed through a plastic resin bath and then wound on a suitable rotating mandrel in a pre-determined orientation. When sufficient layers have been taken up on the mandrel it is cured at room temperature or at elevated temperature in an oven. The molded composite is then stripped from the mandrel. Pruthvi Loy, Chiranth B. P. 9 SJEC, Mangaluru
Figure 10.6: Filament winding Advantages: 1. Excellent mechanical properties due to continuous fibers. 2. Fast and economical production. Disadvantages: 1. Difficult to wind complex shapes. 2. Mandrel cost for larger components is high. Applications: Tubular sections, chemical and fuel tanks, pressure vessels. 10.4.1(d) Compression moulding: In this process, the part to be cured is placed between two matched steel dies under high pressure and high temperature. The exertion of pressure eliminates the problem of development of voids. Disadvantage: 1. High tooling cost 2. Need for large heated presses Advantages: 1. Good dimensional accuracy. Pruthvi Loy, Chiranth B. P. 10 SJEC, Mangaluru
Figure 10.7: Compression moulding 10.4.1(e) Bag moulding: Bag moulding process is an improvement over most of the open mould processes (where compaction is done using rollers) in that it further removes the entrapped air. The different bag moulding techniques are pressure bag, vacuum bag and autotoclave process. In all these techniques, plastic resins are first pored over a fiber reinforcement mat placed in a mould. This mixture of fiber and resin is covered by a flexible sheet and then pressure is applied so as to compact the composite. In case of pressure bag moulding pressurized air is used for compaction whereas in vacuum bag moulding pressure is applied by vacuum principle. Autoclave process is similar to vacuum bag process except that compaction is done in a furnace at elevated temperature and higher pressures are applied. Figure 10.8: Pressure bag moulding Pruthvi Loy, Chiranth B. P. 11 SJEC, Mangaluru
10.4.1(f) Pultrusion process: Figure 10.9: Vacuum bag moulding Pultrusion is generally used for producing glass fiber reinforced polyester. In this process resin impregnated fibers are first fed into a preforming box which gives appropriate orientation to the fibers. Then it is passed through a heated die to get required cross section such as beams, tubes and sections. Advantages: 1. It is continuous and automated 2. Low material scrap 3. Dimensional stability Disadvantages: 1. Heated die cost 2. Complex geometry Figure 10.10: Pultrusion process Pruthvi Loy, Chiranth B. P. 12 SJEC, Mangaluru
10.4.1(g) Sheet moulding compound: In this process, plastic resin is first deposited over travelling polyethylene sheet with the help of filler. On the top of this resin paste, continuous strand fiber glass roving cut to length of about 5 cm is deposited. Another layer of resin filler paste is added over this combination to form continuous sandwich of fiber glass and resin. This sandwich is compacted using rollers. Advantages: 1. High volume production. 2. Minimum material scarp. Figure 10.11: Sheet moulding compound 10.4.2 Production of MMC s MMC s can be manufactured by the following two techniques; Liquid state processing Stir casting Solid state processing Powder metallurgy 10.4.2(a) Stir casting A high ductility metal that forms the matrix phase is first melted in a crucible placed in a furnace; the reinforcement phase is then added and is mixed thoroughly by a mechanical stirrer. This composite mixture is then poured into a sand mould of desired shape and size and allowed to solidify. Pruthvi Loy, Chiranth B. P. 13 SJEC, Mangaluru
10.4.2(b) Powder metallurgy Metal matrix composites are extensively produced by powder metallurgy techniques. Metallic materials such as copper, nickel, aluminium, cobalt and steel are used in their powder form as matrix material. The metal matrix in the form of powders are mixed with fibers, whiskers or particles and then fed into a mould of desired shape. Pressure is then applied to compact the powder which is followed by sintering in order to strongly bind the particles. 10.5 ADVANTAGES AND APPLICATIONS OF COMPOSITES 10.5.1 Advantages The advantages of composites are; Light Weight - Composites are light in weight, compared to most woods and metals. Their lightness is important in automobiles and aircraft, to enhance the fuel efficiency. High Strength - Composites can be designed to be far stronger than aluminum or steel. Metals are equally strong in all directions. But composites can be engineered and designed to be strong in a specific direction. High Strength to Weight ratio - Composite materials can be designed to be both strong and light; they have highest strength-to-weight ratios in structures today. Good Corrosion Resistance - Composites resist damage from the weather and from harsh chemicals that can eat away at other materials. Composites are good choices where chemicals are handled or stored. Outdoors, they stand up to severe weather and wide changes in temperature. High-Impact Strength - Composites can be made to absorb impacts the sudden force of a bullet, for instance, or the blast from an explosion. Because of this property, composites are used in bulletproof vests and panels, and to shield airplanes, buildings, and military vehicles from explosions. Design Flexibility - Composites can be molded into complicated shapes more easily than most other materials. This gives designers the freedom to create almost any shape or form. Dimensional Stability - Composites retain their shape and size when they are hot or cool, wet or dry. Wood, on the other hand, swells and shrinks as the humidity changes. Composites can be a better choice in situations demanding tight fits that do not vary. Nonconductive - Composites are nonconductive, meaning they do not conduct electricity. This property makes them suitable for such items as electrical utility poles and the circuit boards in electronics. If electrical conductivity is needed, it is possible to make some composites conductive. Pruthvi Loy, Chiranth B. P. 14 SJEC, Mangaluru
Low Thermal Conductivity - Composites are good insulators they do not easily conduct heat or cold. They are used in buildings for doors, panels, and windows where extra protection is needed from severe weather. Durable - Structures made of composites have a long life and need little maintenance. 10.5.2 Applications The various application areas of composites are; Aircraft/Military - Commercial, pleasure and military aircrafts, including components for aerospace and related applications. Appliance/Business - Composite applications for the household and office including appliances, power tools, business equipment, etc. Automotive/Transportation - The largest of the markets, products include parts for automobiles, trucks, rail and farm applications. Civil Infrastructure - A relatively new market for composites, these applications include the repair and replacement of civil infrastructure including buildings, roads, bridges, piling, etc. Construction - Includes materials for the building of homes, offices, and architectural components. Products include swimming pools, bathroom fixtures, wall panels, roofing, architectural cladding. Consumer - Products include sports and recreational equipment such as golf clubs, tennis rackets, snowmobiles, mobile campers, furniture, microwave cookware. Corrosion-resistant equipment - Products for chemical-resistant service such as tanks, ducts and hoods, pumps, fans, grating, chemical processing, pulp and paper, oil and gas, and water/wastewater treatment markets. Electrical - This encompassing market includes components for both electrical and electronic applications such as pole line hardware, substation equipment, microwave antennas, printed wiring boards, etc. Marine - Products for commercial, pleasure and naval boats and ships. References: 1. Fundamentals of Materials Science & Engineering William D. Callister 2. Material Science and Metallurgy K.R.Phaneesh 3. Material Science and Metallurgy Kesthoor Praveen Pruthvi Loy, Chiranth B. P. 15 SJEC, Mangaluru