Introduction to Composites

Transcription

1 Section 1 Introduction to Composites By definition, composite materials are formed from two or more materials that have quite different properties. The resultant material has a heterogeneous microstructure with extraordinary performance that displays a combination of the best characteristics of the component materials. Composites are widely used because their overall properties can be engineered through microstructural design to become superior to those of the individual monolithic counterparts. Nature has provided some of the best-performing composites such as seashell, bones, macadamia nutshells, wood and bamboo. These natural composites have superior mechanical efficiency in strength, hardness and toughness compared to many man-made composite materials. These biological composites display graded structures at several levels of hierarchy with length scales that range from micro- to nanometres. For instance, seashells have two to three orders of lamellar structure whilst bone has seven orders of hierarchy. 1

2 A composite material is a material that has a chemically and/or physically distinct phases distributed within a continuous phase. The composite generally has characteristics better than or different from those of either component. The matrix phase is the continuous phase, while the distributed phase, commonly called the reinforcement phase, can be in the form of particles, whiskers or short fibers, continuous fibers or sheet. Figure 1 shows the types of composites based on the form of reinforcement. Oftentimes it is convenient to classify different types of composites as per the matrix material characteristics, viz., polymer matrix composites (PMCs), metal matrix composites (MMCs), and ceramic matrix composites (CMCs). Fig. 1. Types of composites based on the form of reinforcement. History of composite materials The earliest man-made composite materials were straw and mud combined to form bricks (unfired) for building construction. The history of modern composites probably began in 1937 when salesman from the Owens Coring Fiberglass Company began to sell fiberglass (had been made, almost by accident in 1930) to interested parties around the United State. 2

3 Fig..2 Performance of a composite is linked to some important factors : composition of components, their mechanical behavior, processing, and the characteristics of the interface between matrix and reinforcement. 3

4 Why study composites? With a knowledge of the various types of composites, as well as an understanding of the dependence of their behaviors on the Characteristics Relative amounts Geometry / distribution And properties of the constituent phases. It is possible to design materials having property combinations that are better than those found in metals alloy, ceramics, and polymeric materials alone. Disadvantages and Limitations of Composite Materials Properties of many important composites are anisotropic - the properties differ depending on the direction in which they are measured this may be an advantage or a disadvantage Many of the polymer-based composites are subject to attack by chemicals or solvents, just as the polymers themselves are susceptible to attack Composite materials are generally expensive Manufacturing methods for shaping composite materials are often slow and costly. 4

5 Section 2 Functions of Materials Nearly all composite materials consist of two phases: (Primary Phase) Functions Matrix Material Protect phases from environment Transfer Stresses to phases Holds the imbedded phase in place, usually enclosing and often concealing it When a load is applied, the matrix shares the load with the secondary phase, in some cases deforming so that the stress is essentially born by the reinforcing agent (Secondary Phase) Reinforcing ( imbedded phase) sometimes referred to as a reinforcing agent, because it usually serves to strengthen the composite. The reinforcing phase may be in the form of fibers, particles, or various other geometries. 5

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15 Section3 Components of reinforcing phase Fibre ( Oxide and Non-oxide Fibers). Oxide fibers find uses both as insulation and as reinforcements. Glass fibers, based on silica, possess a variety of compositions in accordance with the characteristics desired. They represent the biggest market for oxide fibers. Unlike other oxide fibers, glass fibers are continuously spun from the melt and are not used at temperatures above 250 C. Short oxide fibers can be melt blown whilst other aluminasilicate and alumina based continuous fibers are made by sol-gel processes. Initial uses for these fibers were as refractory insulation, up to 1600 C, but they are now also produced as reinforcements for metal matrix composites. Continuous oxide fibers are candidates as reinforcements for use up to and above 1000 C. Glass Fibers, Alumina Fibers, Aluminosilicate Fibers Non-oxide fibers are being considered for many applications, but are currently being developed and produced primarily as continuous-length structural reinforcement for ceramic matrix composites (CMC). Since only those fiber types with compositions based on silicon carbide (SiC) have demonstrated their general applicability for this application, this chapter focuses on commercially available SiCbased ceramic fiber types of current interest for CMC and on our current state of experimental and mechanistic knowledge concerning their production methods, microstructures, physical properties, and mechanical properties at room and high 15

16 temperatures. Particular emphasis is placed on those properties required for successful implementation of the SiC fibers in high-temperature CMC components. It is shown that significant advances have been made in recent years concerning SiC fiber production methods, thereby resulting in pure near-stoichiometric small-diameter fibers that provide most of the CMC fiber property requirements, except for low cost. Filaments of reinforcing material, usually circular in cross-section Diameters range from less than mm to about 0.13 mm, depending on material Filaments provide greatest opportunity for strength enhancement of composites.the filament form of most materials is significantly stronger than the bulk form.as diameter is reduced, the material becomes oriented in the fiber axis direction and probability of defects in the structure decreases significantly Whiskers whisker reinforced composites exhibit significant improvements in mechanical properties, such as strength and fracture toughness. These composites are typically densified by pressureassisted sintering (i.e. hot-pressing) with SiC whisker contents ranging from 10 to 30 vol.%. Cutting tools for high nickel alloys are the major application, but other wear and structural uses are also being developed. Micro-mechanical modeling and available experimental evidence indicates that the composite toughness,k (composite), can be described as the sum of the matrix toughness, (matrix) K, and a contribution due to whisker toughening, K(whisker reinforcement). In other words, 16

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18 Particles and Flakes ranging in size from microscopic to macroscopic Flakes are basically two dimensional particles small flat platelets The distribution of particles in the composite matrix is random, and therefore strength and other properties of the composite material are usually isotropic Strengthening mechanism depends on particle size Laminates Ceramic,metal, polymer Flat, Hollow 18

19 Section4 Classification of composites-matrix polymer-matrix composites (PMC) Polymer Matrix Composite (PMC) is the material consisting of a polymer (resin) matrix combined with a reinforcing dispersed phase. Polymer Matrix Composites are very popular due to their low cost and simple fabrication methods. Use of non-reinforced polymers as structure materials is limited by low level of their mechanical properties: tensile strength of one of the strongest polymers - epoxy resin is psi (140 MPa). In addition to relatively low strength, polymer materials possess low impact resistance. (advantages of PMC) Reinforcement of polymers by strong fibrous network permits fabrication of Polymer Matrix Composites (PMC) characterized by the following properties: High tensile strength; High stiffness; High Fracture Toughness; Good abrasion resistance; Good puncture resistance; Good corrosion resistance; Low cost. The main disadvantages of Polymer Matrix Composites (PMC) are: Low thermal resistance; High coefficient of thermal expansion. 19

20 Two types of polymers are used as matrix materials for fabrication composites: 1-Thermosets (epoxy, phenolic) 2-Thermoplastics (Low Density Poly-ethylene (LDPE), High Density Poly-ethylene (HDPE), poly-propylene, nylon, acrylics). Metal Matrix Composites (MMCs) A metal matrix reinforced by a second phase Reinforcing phases: Particles of ceramic (these MMCs are commonly called cermets) Fibers of various materials: other metals, ceramics, carbon, and boron Examples of matrices in such composites include aluminum, magnesium, and titanium. Metals are mainly reinforced to increase or decrease their properties to suit the needs of design. For example, the elastic stiffness and strength of metals can be increased, and large coefficient of thermal expansion and thermal and electric conductivities of metals can be reduced, by the addition of fibers such as silicon carbide. Advantages of MMC s Advantages over polymer matrix composites. These include higher elastic properties; higher service temperature; insensitivity to moisture; higher electric and thermal conductivities; and better wear, fatigue, and flaw resistances. The drawbacks of MMCs over PMCs include higher processing temperatures and higher densities. 20

21 Ceramic Matrix Composites (CMC) Ceramic matrix composites (CMCs) have a ceramic matrix such as alumina calcium alumino silicate reinforced by fibers such as carbon or silicon carbide. Advantages of CMC s High strength, Hardness, High service temperature limits for ceramics, Chemical inertness, and Low density. However, ceramics by themselves have low fracture toughness. Under tensile or impact loading, they fail catastrophically. Types of ceramic composite Non oxide- Non oxide Composites Oxide- Oxide Composites; Non-oxide- oxide Composites; Glass - Ceramic Composites; Manufacturing Method of CMC The most common methods to manufacture ceramic matrix composites are: Processing : Integration Powders methodchemically methods: Melting process; Heat Pressure Process; Slip casting low-pressure sintering; Reaction sintering; Pressure-less sintering; Chemical vapor infiltration; Directed melt oxidation; Sol-gel processing; combustion synthesis 21

22 5-Advanced CMC: SiC-SiC Composites, Carbon-Fiber Reinforced SiC Composite, Carbon Fiber Reinforced SiN Composite, SiC Whisker Reinforced Alumina, Alumina Reinforced Zirconia Composites, Cermet, and other types 22