1. Composite Materials Krishan K. Chawla Chapter 3. Matrix Materials

2. A brief description of the various matrix materials – Polymers, Metals, Ceramics 3.1 Polymers - Complex, cheap, easily processible. - lower strength and modulus and lower temperature use limits. - Structure : Giant chainlike molecules. → Polymerization : Condensation polymerization – stepwise reaction of molecules, simple compound Addition polymerization – monomers join to form a polymer - 1. Linear polymers 2. Branched polymers 3. Crosslinked polymers 4. Ladder polymers Glass transition temperature - Specific volume(volume/unit mass) versus temperature cruves for amorphous and semicrystalline polymers. - When a polymer liquid is cooled, it contracts. - In the case of amorphous polymers, → Contraction continues belowT m, T g, the glass transition temperature Where the supercooled liquid polymer becomes extremely rigid. - T g is akin to the melting point for the crystalline solids. - Many physical properties change abruptly at T g -T g is function of the chemical structure of the polymer.

3. Thermiplastics and Thermosets -Most linear polymers soften or melt on heating. → Thermoplastic polymers(polyethylene, polystyrene, PMMA) -When the molecules in a polymer are cross linked in the form of a network, they do not soften on heating → thermosetting polymers Copolymers - polymer chains having two different monomers 1. Random 2. Block 3. Graft Molecular Weighr - Many mechanical properties increase with increasing molecular weight(MW). ex. Resistance to deformation - The degree of polymerization(DP) indicates the number of basic units in a polmer. MW = DP x (MW)μ, (MW)μ : Molecular weight of the repeating unit Degree of Crystallinity - Polymers can be amorphous or partially crystalline. - The amount of crystallinity in a polymer can vary from 30 to 90% - The inability to attain a fully crystalline structure is mainly due to the long chain structure of polymers. - Linear milecules: Crystallize easily(Polyethylene-90%) Branched chain molecules: Do not crystallized as easily(Polyethylene-65%) - Generally, the stiffness and strength of a polymer increase with the degree of crystallinity.

4. Stress-Strain Behavior - Amorphous, elastomer polymer(Fig. 3. 5a,b) - elastomer does not show a Hookean behavior. → non linear elastic. → large elastic range : easy reorganization of the tangled chains under the action of an applied stress. -Variation of the elastic modulus of an amorphous polymer with temerature.(Fig. 3. 6) Thermal ezpansiom - Polymers generally have higher thermal ezpansivities than metals and ceramics. - Their thermal expansion coefficients are mot truly constnts. 3.1.1 Common polymeric matrix materials - The common polymer matrices : polyester and epoxy resins. - Polyester : resistance to water and a variety of chemical, weathering, aging and very low cost. can withstand up to about 80 ℃, combine easily with glass fibers. Shrink between 4 and 8% on curing. A majority of common glass fiber reinforced composites gas polyester as the matrix. - Thermosetting epoxy resins : more expensive, better moisture resistance, lower shrinkage - Polyimides(Thermosetting polymers) : high temperature, 250-300 ℃, brittle. - Bismaleimides(BMI, Thermosetting polymers) : 180-200 ℃, good resistance to hygrothermal effect - In the presence of fibers bonded to the matrix, these hygrothermal effects can lead to severe internal stresses in the composite.

5. Matrix toughness - Thermosetting resins : highly cross linked, adepuate modulus, strength, creep resistance, But brittleness, very low fracture toughness - Amorphous thermoplastic polymers : higher fracture energy values 3.2 Metals - Strong, tough, can be plastically deformed, can be strengthened 3.2.1 Structure - The vast majority of metals 1. FCC 2. BCC 3.HCP - Metals contain variety of crystal imperfections. 1. Point defect(0-dimensional) 2. Line defect (1-dimensional) 3. Planar or interfacial defects (2-dimensional) 4. Volume defects (3-dimensional) - 1. Point defect(0-dimensional) - A vacancy, An interstitial, A substitutional - Point defects can have a marked effect on the mechanical properties. - 2. Linear imperfections(Dislocations) - Critically important structural imperfections in the area of physical and mechanical metallurgy, diffusion, and corrosion - Defined by two vectors : A dislocation line vector & Burgers vector - Make it easy to deform metals plastically (movement of these dislocations) - 3. Interfacial or planar defects - occupy an area or surface of the crystal(grain boundaries, twin boundaries, antiphase boundaries - 4. The volumetric or tridimensional defects - gas porosity

6. 3.2.2 Conventional Strengthening Methods - Work hardening(or strain hardening), which is the ability of a metal to become more resistant to deformation as it is deformed is related in a singular way to the dislocation density(Ρ) after deformation. - Linear relationship between the flow stress τ and Ρ ½ : τ = τ 0 + αGB Ρ ½ - basically, work hardening results form the interactions among dislocations moving on different slip planes. - Relationship between the flow stress τ and the mean grain size τ = τ 0 + α ’ Gb/D ½ (Hall-Petch realtionship) - Another easy way of strengthening metals by impeding dislocation motion is that of introducing heterogeneities such as solute atoms or precipitates or hard particles in a ductile matrix - Interstitial are much more efficient strengthening agents than substitutional solutes - Precipitation hardening of a metal is obtained by decomposing a supersaturated solid solution to form a finely distributed second phase - Quenching a steel to produce a martensitic phase has been a time honored strengthening mechanism for steels - most important being the amount of carbon - Carbon saturation and he lattice distortion that accompanies the transformation lead to the high hardness and strength of martensite - Rapid solidification processing - By cooling metals at rates in the 10 4 – 10 9 Ks -1 range, it is possible to produce microstructures that are unique

7. 3.2.3 Properties of Metals - Typical values of elastic modulus yield strength, and ultimate strength in tension, as well as those of fracture toughness of some common metals and their alloys, are listed in Table 3.3, whil typical engineering stress-strain curves in tension are shown in Fig.3.12. 3.2.4 Why Fiber Reinforcement of Metals? - Precipitation or dispersion hardening → Dramatic increase in the yield stress and/ or the work hardening rate. → Only function is to impede dislocation mivement in the metal - The improvement in stiffness can be profitably obtained by incorporating so-called advanced high-modulus fibers in a metal matrix -

8. 3.3 Ceramic Matrix Materials - Ceramic materials are very hard and brittle. - They consist of metals with a non metal - They have strong ionic bonds and very few slip systems → Low failure strains & low toughness or fracture energies - Brittle → Lack uniformity in properties, low thermal and mechanical shock resistance, low tensile strength - Very high elastic moduli, low densities, withstand very high temperatures - Major disadvantage of ceramics is their extreme brittleness. 3.3.1 Bonding and Structure - Ceramic materials are crystalline - mostly ionic bonding, some covalent bonding - Generally, ceramic compounds are stoichimetric(Al 2 O 3, BeO, MgAl 2 O 4, SiC..) - Crystalline ceramics generally exhibit simple cubic, close-packed cubic, and hexagonal close-packed structures - Simple cubic structure : Cesium chloride strucrure(CsCl), not very common - Cubic cliose packed : variation of the fcc structrure, many ceramic materials(NaCl) - Hexagonal close-packed structure : ZnS, Al 2 O 3 - Glass ceramic materials : Composite material(95-98% Crystalline, the rest is glassy) - Li 2 O-Al 2 O 3 -SiO 2 - MgO-Al 2 O 3 -SiO 2 - Ceramic materials can also form solid solutions