Materials Full Notes (2) PDF

USEFUL EQUATIONS Below are equations you may need 𝜎 =𝐸∙𝜀 𝜌𝑙 𝑅= 𝐴 𝐹 𝜎= 𝑃 = 𝐼2 𝑅 𝐴...

USEFUL EQUATIONS Below are equations you may need 𝜎 =𝐸∙𝜀 𝜌𝑙 𝑅= 𝐴 𝐹 𝜎= 𝑃 = 𝐼2 𝑅 𝐴 1 = ∆𝐿  𝜀= 𝐿0 𝜌 = 𝜌𝑅𝑇 (1 + 𝛼𝑅 𝛥𝑇) 𝜋 ∙ 𝑑 2 𝑉 𝐴 = 𝜋 ∙ 𝑟2 = 𝐸= 4 𝑙 𝑉 = 𝐼𝑅 𝐴0 − 𝐴𝑑 %𝐶𝑊 = × 100 𝐼 = 𝐽𝐴 𝐴0 𝑄 𝐶= 𝜎𝑐 = 𝜎𝑓 ∙ 𝑉𝑓 + 𝜎𝑚 (1 − 𝑉𝑓 ) 𝑉 𝜀𝐴 𝐶= 𝐸𝑐 = 𝐸𝑓 ∙ 𝑉𝑓 + 𝐸𝑚 (1 − 𝑉𝑓 ) 𝑙 𝜀 = 𝜀𝑟 𝜀0 1 𝜈𝑓 𝜈𝑚 = + 𝐸𝑐 𝐸𝑓 𝐸𝑚 8.85 × 10−12 𝐹 𝜀0 = 𝑚 𝑉 𝐸= 𝑙 Exercise Problems 1. Lecture example problems Practice the problems in the lecture slides (solutions included) 2. Briefly describe how the quenching process for medium carbon steel is performed and what it achieves. 3. Describe tempering and why is it performed after quenching? 4. Explain the full annealing process. 5. Briefly explain the normalising heat treatment and its objective. 6. Briefly explain austenitising. Exercise Problems 1. Discuss the difference between paramagnetic and ferromagnetic materials. 2. Describe the phenomenon of magnetic hysteresis. 3. Hard magnets have a large coercivity to an external magnetic field. Would they make good permanent magnets and why? 4. Name the three types of magnetic response to an electric field. Exercise Problems 1. Lecture example problems Practice the problems in the lecture slides (solutions included) 2. An n-type germanium (Ge) semiconductor is obtained by doping with which element 3. What element can be added to Silicon to produce a p-type semiconductor? Briefly explain how this element produces a p-type behaviour. 4. Briefly define what is dielectric constant? Exercise Problems 1. Lecture example problems 1-5 Practice the problems in the lecture slides (solutions included) 2. What is the effect of temperature on the conductivity of pure metals? 3. What is the effect of (a) cold working and (b) impurities in the metal on the conductivity? 4. Why are metallic materials good conductors? 5. Explain the difference between resistivity and resistance. Exercise Problems 1. What is a composite? 2. Explain briefly in what sporting equipment composite materials are used. What is the main reason why composites are used in these applications? 3. Explain why bonding between carbon fibres and an epoxy matrix should be excellent, whereas bonding between silicon nitride fibres (non-oxide structural ceramic material) and a silicon carbide matrix should be poor. 4. For a polymer-matrix fibre-reinforced composite: a) List three functions of the matrix phase. b) Compare the desired mechanical characteristics of matrix and fibre phases. c) Cite two reasons why there must be a strong bond between fibre and matrix at their interface. 5. A continuous and aligned glass fibre-reinforced composite consists of 40 vol% glass fibre having a modulus of elasticity of 69 GPa and 60 vol% polyester resin that, when hardened, displays a modulus of 3.4 GPa. Compute the modulus of elasticity of this composite in the longitudinal direction. (Answer: 30 GPa) Exercise Problems 1. What are the primary types of atomic bonds in ceramics? 2. What are some typical characteristics of ceramic materials? 3. In terms of bonding, explain why silicate materials have relatively low densities. 4. Briefly answer the following. a) Why may there be significant scatter in the fracture strength for some given ceramic material? b) Why does fracture strength increase with decreasing specimen size? 5. Cite two desirable characteristics of glasses. Exercise Problems 1. (a) Briefly explain the difference between oxidation and reduction electrochemical reactions. (b) Which reaction occurs at the anode and which at the cathode? 2. Provide two reasons as to why steel alone in water with no other metal present will corrode. 3. (a) From the galvanic series below, cite three metals or alloys that may be used to galvanically protect 304 stainless steel in the active site. (b) Galvanic corrosion is prevented by making an electrical contact between the two metals in the couple and a third metal that is anodic to the other two. Using the galvanic series, name one metal that could be used to protect a copper alloy-aluminium alloy galvanic couple. 4. Why does chromium in stainless steels make them more corrosion resistant in many environments than plain carbon steels? ENME502 Engineering Materials I Heat Treatment of Steels Prof. Xiaowen Yuan Heat Treatment of Steels A combination of heating and cooling operations applied to a metal or alloy in the solid state to produce desired properties. The two stages of any heat treatment process: 1. Austenitising – heating to a high temperature in the solid state. 2. Cooling – either rapid, moderate or slow cooling in air, furnace and liquid media. The objective is to produce a structure that will give the desired mechanical properties. Hold Temperature Time 2  Types of Heat-Treatment Annealing(continuous slow cooling ) To produce soft and ductile steel Normalising (continuous moderate cooling) prior to shaping or forming operation Quenching and Tempering (Rapid cooling To produce steel with high strength followed by heating below the critical and hardness temperature) 3  Continuous Cooling Transformation Annealing and Normalising Annealing, or a full anneal, allows the steel to cool slowly in a furnace, producing a coarse grain pearlite structure. (soft and ductile) Improves machineability and formability Normalising allows the steel to cool more rapidly, in air, producing fine grain pearlite structure. Relieves internal stresses and refines the crystal structure 4 Austenitising Temperature For annealing steels below 0.76%C, 30 °C above the A3 line. For annealing steels above 0.76%C, 30 °C above the A1 line. Acm A3 A1 For normalizing steel, austenitising is done at about 55 °C above the A3 and Acm line. 5 Example: Recommend temperatures for the annealing and normalising of 1040 (0.40%C) and 1090 (0.90%C) plain carbon steels. 6 Quenching (Rapid Cooling) and Tempering Quenching – rapid cooling that will produce a totally martensitic structure. Martensite is hard and brittle but can be made more ductile by tempering. Forms the tempered martensite Tempering structure. 7 Recommended Reading Materials Science and Engineering: An Introduction (1st Edition), W. D. Callister and D. G, Rethwisch, Wiley, 2020 Chapter 11 8 Thank you Prof Xiaowen Yuan Office: WZ1003 Email: [email protected] Phone: +64 9 921 9999 ext. 7320 9 ENME502 Engineering Materials I Ferrous and Non-Ferrous Alloys Prof. Xiaowen Yuan Ferrous and Non-Ferrous Alloys 2 Ferrous Alloys Ferrous alloys – those of which iron is the prime constituent (Carbon steel) Their widespread use is accounted for by three factors: iron-containing compounds exist in abundant quantities in the earth’s crust; metallic iron and steel alloys may be produced using relatively economical extraction, refining, alloying, and fabrication techniques; and ferrous alloys are extremely versatile, since they may be tailored to have a wide range of mechanical and physical properties. The principal disadvantage of many ferrous alloys is their susceptibility to corrosion. 3 Carbon Content of Steels and Cast Irons 4  Iron – Iron Carbide (Fe-Fe3C) Phase Diagram  – ferrite - Solid solution of carbon in iron - BCC structure - Limited carbon solubility - Max. solubility of 0.022 % at 723 oC to about 0.008 % at room temperature A4  – austenite - Solid solution of carbon in iron - FCC structure - Better carbon solubility - Max. solubility of 2.14 % at 1147 oC to about 0.76 % at 723 oC ACM  – ferrite - Similar to  – ferrite A3 - Stable only at relatively high temperatures, A1 - have no technological importance - Limited carbon solubility A2 - Max. solubility of 0.09 % at 1495 oC Cementite (Fe3C) - Intermetallic compound of iron and carbon - Fe3C : 6.7 % C and 93.3 % Fe - BCC structure - Metastable phase 5 1010 1050 1095 6 7 8 9 Different Classes of Carbon Steels 10 Thank you Prof Xiaowen Yuan Office: WZ1003 Email: [email protected] Phone: +64 9 921 9999 ext. 7320 11 ENME502 Engineering Materials I Magnetic Properties of Materials Prof. Xiaowen Yuan Magnetic Properties How do we measure magnetic properties? What are the atomic reasons for magnetism? How are magnetic materials classified? 2 Applied Magnetic Field Created by current through a coil: Applied N = total number of turns magnetic field H L = length of each turn current I Relation for the applied magnetic field, H: N I H= L current applied magnetic field units = (ampere/m) 3 Response to a Magnetic Field Magnetic induction results in the material B = Magnetic Induction (tesla) inside the material current I Magnetic susceptibility,  (dimensionless) B  >0  measures the material response vacuum  = 0 relative to a vacuum.  10 GPa and ρ < 2,000 kg/m3. r = 2000 kg/m3 10 Ranking: Using Single Property Material Index Example: Rank according to highest stiffness or lowest density. Rank High Stiffness Low Density 1 CFRP Bamboo 2 Mg alloys CFRP 3 GFRP Mg alloys 4 Bamboo GFRP 11 Ranking: Using group of properties as Material Index Objective: To minimize mass of tie rod. Requires high stiffness-density ratio or high material index Applications: widely used in aerospace and other applications where weight savings are worth the higher material cost. airplane wings bridges masts bicycle frames 12 Ranking: Using Material Indices 13 Modulus-Density Chart 𝑀 = 10−1 𝑀 = 10−2 𝑀 = 10−3 𝑀 = 10−4 𝑀 = 10−5 𝑀 = 10−6 14 Modulus-Density Chart 15 Thank you Prof Xiaowen Yuan Office: WZ1003 Email: [email protected] Phone: +64 9 921 9999 ext. 7320 16 ENME502 Engineering Materials I Dielectric Materials Prof. Xiaowen Yuan Insulators and Dielectric Properties of Materials 2 Insulators Materials used to insulate an electric field from its surroundings are required in a large number of electrical and electronic applications. Electrical insulators obviously must have a very low conductivity, or high resistivity. Insulators must also be able to withstand intense electric fields. At high voltages, a catastrophic breakdown of the insulator may occur, and current may flow. In order to select an insulating material properly, we must understand how the material stores, as well as conducts, electrical charge. Porcelain, alumina, cordierite, mica, and some glasses and plastics are used as insulators. The resistivity of most of these is >1011 W-m, and the breakdown electric fields are 5 to 15 kV/mm 3 Dielectric Materials Dielectrics are electrical insulators which can support electrostatic fields. They are mainly used in capacitors to store electrical energy. Ex: Mica, glass, plastic, ceramics and water Dielectrics have no free charges that can move through the material under the influence of an electric field. 4  Electrical Capacitance Two conductive plates separated by a small gap with a voltage applied Since no Current can flow across the gap Positive charges accumulate on top Negative charges accumulate on bottom The Quantity of the separated charge, Q, is Proportional to V 𝑄 ∝𝑉 The Constant of Proportionality is known as capacitance, C 5 Electrical Capacitance cont. The Value of C can be found from an expression that is analogous to Ohm’s equation Q=CV Where For parallel plates in a vacuum Q ≡ Charge (A-s or Coulombs) C is proportional to the plate V ≡ Electrical Potential (V) area, and the inverse of separation length C ≡ Capacitance (A-s/V or Coul/V or Farads, F) 𝐴 𝐶∝ 𝑙 6 Electrical Capacitance cont. Introducing a constant of proportionality between C & A/ℓ 𝐴 𝐶 = 𝜀0 𝑙 Where: Filling the gap with a non-conductive A  Plate area (m2) dielectric material increases the charge l  Plate distance (m) accumulation 𝜀0  Permittivity of free space (vacuum) = 8.85x10−12 F/m 7 Electrical Capacitance cont. For a dielectric filled capacitor 𝐴 𝐶=𝜀 𝑙 Where 𝜀  Permittivity of the dielectric medium (F/m) 𝜀 𝜀𝑟 = =k Using 𝜀0 as a baseline, defines a material’s 𝜀0 relative dielectric constant Sometimes called “k”, the dielectric constant is always positive with a magnitude greater than unity 8 Electrical Terms Electric Field is the ratio of a voltage drop to distance over which the drop occurs ∆𝑉 𝐸= 𝑙 Units V/m ∆𝑉𝑖−𝑓𝑙𝑜𝑤 𝐸𝑏𝑑 = As V increases toward ∞ at some point 𝑙 the dielectric will “Break Down” and current will flow Units V/m 9 Examples 𝜀𝑟 = 1.00059 For air at room conditions E = 3 x 106 V/m bd 10 Example Consider a parallel-plate capacitor having an area of 6.45 x 10-4 m2 and a plate separation of 2 x 10-3 m across which a potential of 10 V is applied. If a material having a dielectric constant of 6.0 is positioned within the region between the plates, compute the following: (a) The capacitance (b) The magnitude of the charge stored on each plate 11 Example A parallel-plate capacitor with dimensions of 200 mm by 25 mm and a plate separation of 4 mm must have a minimum capacitance of 50 pF (5.0 x 10-11 F) when a potential of 1000 V is applied at a frequency of 1 MHz. a) What is the required dielectric constant or relative permittivity, 𝜀𝑟 ? b) What is the magnitude of the charge, Q stored on each plate? c) What is the electrical field strength, E? d) Which of the materials listed in the table below are possible candidates? Briefly explain why? Material Dielectric Constant, 𝜀𝑟 Dielectric Strength (kV/m) (@ 1MHz) Ceramics 15 20,000 Mica 8 50,000 Glass 7 10,000 Fused Silica 3.8 10,000 12 Solution Given: 𝜀0 = 8.85x10−12 F/m A = 0.2 X 0.025 = 0.005 m2 𝑙 = 4 mm = 4 x 10-3 m C = 5 x 10-11 F V = 1000 V 𝜀𝐴 𝜀𝑟 𝜀0 𝐴 𝐶𝑙 a) 𝐶= 𝑙  𝐶= 𝑙  𝜀𝑟 = 𝜀0 𝐴 𝜀 = 𝜀𝑟 𝜀0 𝜀𝑟 = 4.54 b) Q=? Q = C V = (5 x 10-11 )(1000) = 5 x 10-8 C 𝑉 1000 c) E=? 𝐸= = 4 x 10−3 = 250 𝑘𝑉/𝑚 Ceramics, Mica and Glass 𝑙 are possible candidates 13 Multilayer Capacitors Capacitors in parallel provide added capacitance. This is the reason why multilayer capacitors consist of 100 or more layers connected in parallel. 14 Example A multi-layer capacitor is to be designed using a BaTiO3 formulation containing SrTiO3. The dielectric constant of the material is 3000. Calculate the capacitance of a multi-layer capacitor consisting of 100 layers connected in parallel using nickel electrodes. The area of each layer is 10 mm X 5 mm, and the thickness of each layer is 10 x 10-6 m. 15 Thank you Prof Xiaowen Yuan Office: WZ1003 Email: [email protected] Phone: +64 9 921 9999 ext. 7320 16 ENME502 Engineering Materials I Semiconductors Prof. Xiaowen Yuan What are semiconductors? Semiconductors are materials whose conductivity fall somewhere midway between conductors and insulators. Semiconductor materials, including silicon and germanium, provide the building blocks for many electronic devices such diodes, transistors and integrated circuits. 2 Semiconductor materials 3 Semiconductor Materials Elemental semiconductors – Si and Ge (column IV of periodic table) – compose of single species of atoms Compound semiconductors – combinations of atoms of column III and column V and some atoms from column II and VI (combination of two atoms results in binary compounds) There are also three-element (ternary) compounds (GaAsP) and four-elements (quaternary) compounds such as InGaAsP. 4 Semiconductors Intrinsic semiconductor is one with no impurities. This is not useful for practical applications as it is difficult to control the behaviour because slight variations in temperature can significantly change the conductivity. Extrinsic semiconductor (n- or p-type) is preferred for devices, since its properties are stable with temperature and can be controlled using ion implantation or diffusion of impurities known as dopants. Semiconductor materials, including silicon and germanium, provide the building blocks for many electronic devices. These materials have an easily controlled electrical conductivity, when properly combined, can act as switches, amplifiers, or information storage devices. 5 Conduction Band 6 7 Charge Carriers in Insulators and Semiconductors Fig. 18.6 (b), Callister & Rethwisch 10e. Two types of electronic charge carriers: Free Electron – negative charge – in conduction band Hole – positive charge – vacant electron state in the valence band Move at different speeds - drift velocities 8 Intrinsic Semiconductor - Silicon At T=0 K there are no charge carriers At T> 0 K, increases the probability that an electron in the lattice is knocked loose from its position leaving behind an electron deficiency called a "hole". 9  When a voltage is applied to a semiconductor, the electrons move through the conduction band, while the holes move through valence band in the opposite direction. If a voltage is applied, then both the electron and the hole can contribute to a small current flow. 10 Intrinsic Conductivity – determined by the number of electrons and holes electron mobility n - concentration of electrons in the conduction band Conductivity p - concentration of holes in the valence band Hole mobility No. of holes per cm3 Charge=1.6x10-19 C Intrinsic carrier concentration 11 Increasing conductivity by temperature As temperature increases, the number of free electrons and holes created increases exponentially. Carrie r Concen tration vs Temp (in Si) -Egap /kT ni µ e 17 1 10 16 1 10 15 1 10 14 1 10 13 1 10 Intrins ic Concentration (cm^-3) 12 1 10 11 1 10 ni 1 1010 T 9 1 10 8 1 10 7 1 10 6 1 10 5 1 10 4 1 10 3 1 10 100 150 200 250 300 350 400 450 500 T T emperature (K) 12 Example 13 Extrinsic Semiconductor Another way to increase the number of charge carriers is to add them in from an external source. Doping is the term given to a process whereby one element is injected with atoms of another element in order to change its properties. Semiconductors (Si or Ge) are typically doped with elements such as Boron, Arsenic and Phosphorous to change and enhance their electrical properties. 14 Intrinsic vs Extrinsic Conduction Intrinsic: -- case for pure Si -- # electrons = # holes (n = p) Extrinsic: -- electrical behavior is determined by presence of impurities that introduce excess electrons or holes -- n ≠ p n-type Extrinsic: (n >> p) p-type Extrinsic: (p >> n) Phosphorus atom Boron atom hole 4+ 4+ 4+ 4+ conduction 4+ 4+ 4+ 4+ electron 4+ 5+ 4+ 4+ 4+ 3+ 4+ 4+ valence 4+ 4+ 4+ 4+ electron 4+ 4+ 4+ 4+ Adapted from Figs. 18.11(a) no applied Si atom no applied & 18.13(a), Callister & electric field electric field 15 Rethwisch 10e. Extrinsic Semiconductors: Conductivity vs. Temperature Data for Doped Silicon: -- σ increases with doping doped undoped -- reason: imperfection sites lower the activation energy to 3 concentration (1021/m3) produce mobile electrons. Conduction electron freeze-out 2 extrinsic intrinsic Comparison: intrinsic vs extrinsic conduction... 1 -- extrinsic doping level: 1021/m3 of a n-type donor impurity (such as P). 0 -- for T < 100 K: "freeze-out“, 0 200 400 600 T (K) thermal energy insufficient to excite electrons. Adapted from Fig. 18.16, Callister & Rethwisch 10e. (From S. M. Sze, Semiconductor Devices, Physics and -- for 150 K < T < 450 K: "extrinsic" Technology. Copyright © 1985 by Bell Telephone Laboratories, Inc. Reprinted by permission of John Wiley & Sons, Inc.) -- for T >> 450 K: "intrinsic" 16 Extrinsic Semiconductor (n-Type and p-Type ) n-Type 17 Extrinsic semiconductor conductivity (n-type) 𝑛 ≫≫ 𝑝 18 p-Type 19 Extrinsic semiconductor conductivity (p-type) 𝑝 ≫≫ 𝑛 20 Example Extrinsic Si doped with As at a concentration 1021 atoms/m3. Determine the conductivity at T -300 K. The mobility of charge carrier at T=300 K for silicon n = 0.135 m2/Vs and p = 0.048 m2/Vs;  = 1021 x 0.135 x 1.6 x 10-19 (e/m3 ) (m2 /Vs) (As C) = 0.216 ( cm)-1 21 p-n Rectifying Junction Allows flow of electrons in one direction only (e.g., useful to convert alternating current to direct current). Processing: diffuse P into one side of a B-doped crystal. -- No applied potential: + p-type+ -n-type - Adapted from + Fig. 18.20, no net current flow. + + - - - Callister & Rethwisch 10e. -- Forward bias: carriers flow through p-type and n-type p-type + - n-type + + - regions; holes and ++- - - electrons recombine at +- p-n junction; current flows. -- Reverse bias: carriers n-type - + p-type flow away from p-n junction; - + + - - + junction region depleted of + + - - carriers; little current flow. 22 Properties of Rectifying Junction Fig. 18.21, Callister & Rethwisch 10e. Fig. 18.22, Callister & Rethwisch 10e. 23 Junction Transistor Fig. 18.23, Callister & Rethwisch 10e. 24  MOSFET Transistor Integrated Circuit Device The main working principle of a MOSFET is to control the voltage and the current which is flowing between the source terminal and the drain terminals Fig. 18.25, Callister & Rethwisch 10e. MOSFET (metal oxide semiconductor field effect transistor) Integrated circuits - state of the art ca. 50 nm line width – ~ 1,000,000,000 components on chip – chips formed one layer at a time 25 Recommended Reading Materials Science and Engineering: An Introduction (1st Edition), W. D. Callister and D. G, Rethwisch, Wiley, 2020 Chapter 18 26 Thank you Prof Xiaowen Yuan Office: WZ1003 Email: [email protected] Phone: +64 9 921 9999 ext. 7320 27 ENME502 Engineering Materials I Electrical Properties of Materials Prof. Xiaowen Yuan Electrical Properties of Materials How are electrical conductance and resistance characterised? What are the physical phenomena that distinguish conductors, semiconductors, and insulators? For metals, how is conductivity affected by imperfections, temperature, and deformation? 2  Electrical Conduction Ohm's Law: V=IR voltage drop (volts = J/C) resistance (Ohms) C = Coulomb current (amps = C/s) Resistivity, ρ: -- a material property that is independent of sample size and geometry cross-sectional area of current flow 𝜌𝑙 ⟹ 𝑅= current flow 𝐴 path length Conductivity, σ Note: Passage of electric current through a conductor produces heat known as Joule heating (ohmic or resistive heating). Power (of heating) P = I2 R 3 Electrical Resistivity 4 5 Note: T= 25 oC 6 Electrical Properties Which will have the greater resistance? 2 2 8 D R1 = =  D 2 D 2    2    R1 2D R2 = = =  2D 2 D2 8      2  Analogous to flow of water in a pipe Resistance depends on sample geometry and size. 7 Further Definitions J = E 1.87 mm 9 Example 2: Conductivity Problem An aluminum wire 4 mm in diameter is to offer a resistance of no more than 2.5 Ω. Using the conductivity of aluminium given in the previous table, compute the maximum wire length. What is the maximum length (𝑙) of the wire so that R < 2.5 Ω? ? Al wire - + R = 2.5 Ω 𝑙 𝑅= 𝜋𝐷2 𝜋(0.004)2 𝐴𝜎 = 4 4 3.8 x 107 (Ω m)-1 Solve to get 𝑙 < 1194 𝑚𝑒𝑡𝑟𝑒𝑠 10 Example 3: Design of a Transmission Line Design an electrical transmission line 1500 m long that will carry a current of 50 A with no more than 5 X 105 W loss in power. The electrical conductivity of several materials is included in Table 18-1. Example 3 SOLUTION Electrical power is given by the product of the voltage and current or: 𝑙 𝑙 1500 𝑚 7.5 𝑅= 𝐴= = = 𝐴𝜎 𝑅𝜎 200 Ω 𝜎 𝜎 11 Example 3: SOLUTION (Continued) Let’s consider three metals—aluminium, copper, and silver—that have excellent electrical conductivity. The table below includes appropriate data and some characteristics of the transmission line for each metal.  (ohm-1.cm-1) A (m2) Diameter (m) Aluminium 3.8x107 1.97x10-7 0.00050 Copper 6.0x107 1.25x10-7 0.00040 Silver 6.8x108 1.10x10-7 0.00037 Any of the three metals will work, but cost is a factor as well. Aluminum will likely be the most economical choice, even though the wire has the largest diameter. However, other factors, such as whether the wire can support itself between transmission poles, also contribute to the final choice. 12 What makes materials conductive? Conductivity 𝜎 = 𝑛𝑞𝜇 𝑛 =the number of charge carriers (carriers/m3) 𝑞 =the charge on each carrier (1.6 x 10-19 C) * C, coulombs 𝑣̅ 𝜇 = ( ) = the mobility of the carriers (m2/(volt-s)) 𝐸 The electrical conductivity of materials is controlled by the number of charge carriers in the material or the mobility—or ease of movement—of the charge carriers 13 (a) Charge carriers, such as electrons, are deflected by atoms or defects and take an irregular path through a conductor. The average rate at which the carriers move is the drift velocity v. (b) Valence electrons in the metallic bond move easily. (c) Covalent bonds must be broken in semiconductors and insulators for an electron to be able to move. (d) Entire ions must diffuse to carry charge in many ionically bonded materials. 14 15 Conductivity of Metals and Alloys Movement of an electron through (a) a perfect crystal, (b) a crystal heated to a high temperature, and (c) a crystal containing atomic level defects. Scattering of the electrons reduces the mobility and conductivity. 16 Influence of Temperature For the pure metal and alloys shown in Table 18.1, the resistivity rises linearly with temperature. 𝜌 = 𝜌𝑅𝑇 1 + 𝛼𝑅 Δ𝑇 𝜌𝑅𝑇 = room temperature resistivity 𝛼𝑅 = temperature coefficient of resistivity 17 18 Example 4 - Resistivity of Pure Copper Calculate the electrical resistivity of pure copper at (a) 400oC and (b) -100oC (a) At 400°C: 1 𝜌 = 𝜌𝑅𝑇 1 + 𝛼𝑅 ∆𝑇 = 1 + 𝛼𝑅 ∆𝑇 𝜎𝑅𝑇 = 1.67 × 10−8 (1 + 0.0043 400 − 25 ) = 4.363 × 10−8 Ω 𝑚 (b) At -100°C: 1 𝜌 = 𝜌𝑅𝑇 1 + 𝛼𝑅 ∆𝑇 = 1 + 𝛼𝑅 ∆𝑇 𝜎𝑅𝑇 = 1.67 × 10−8 (1 + 0.0043 −100 − 25 ) = 7.724 × 10−8 Ω 𝑚 19 Influence of Cold Working and Impurities on Resistivity 20 Example 5 - Conductivity of a Metal Alloy Adapted from Fig. 18.9, Callister & Rethwisch 8e. (10 -8 Ohm-m) 50 Resistivity,  40 30 20 10 0 0 10 20 30 40 50 wt% Ni, (Concentration C) 𝜌 = 30 x 10−8 Ω m CNi = 21 wt% Ni 1 𝜎 = = 3.3 x 106 (Ω m)−1 𝜌 21 Recommended Reading Materials Science and Engineering: An Introduction (1st Edition), W. D. Callister and D. G, Rethwisch, Wiley, 2020 Chapter 18 22 Thank you Prof Xiaowen Yuan Office: WZ1003 Email: [email protected] Phone: +64 9 921 9999 ext. 7320 23 ENME502 Engineering Materials I Corrosion and Degradation of Materials Prof. Xiaowen Yuan Corrosion and Degradation of Materials Fundamentals of corrosion Electrochemical reactions EMF and Galvanic Series Corrosion rate and prediction Protection methods Socio-economic-environmental considerations in Materials Science and Engineering 2 Examples of Corrosion 3 Questions How does corrosion occur? Which metals are most likely to corrode? What environmental parameters affect corrosion rate? How do we prevent or control corrosion? The costs…? 4 Corrosion – Fundamental Components Corrosion can be defined as the deterioration of material by reaction to its environment Corrosion occurs because of the natural tendency for most metals to return to their naturel state For e.g., iron in the presence of moist air will revert to its natural state → iron oxide 4 required components in an electrochemical corrosion cell An anode A cathode A conducting environment for ionic movement (electrolyte) An electrical connection between the anode and cathode for the flow of electron current If any of the above components is missing or disabled, the electrochemical corrosion process will be stopped 5 Electrochemical Corrosion Two reactions are necessary: 1. Oxidation reaction 2. Reduction reaction 6 Corrosion – Standard Electromotive Force (EMF) Series 7 Corrosion – Driving Force A driving force is necessary for electrons to flow between the anodes and the cathodes The driving force is the difference in potential between the anodic and cathodic sites This difference exists because each oxidation or reduction reaction has associated with it a potential determined by the tendency for the reaction to take place spontaneously The standard electrode potential is a measure of this tendency 8 Corrosion – Galvanic Series Ranking of the reactivity of metals/alloys in seawater Table 17.2, Callister & Rethwisch 9e. Source is Davis, Joseph R. (senior editor), ASM Handbook, Corrosion, Volume 13, ASM International, 1987, p. 83, Table 2. 9 Corrosion – Galvanic Series 10 Corrosion – Kinetics and Corrosion Rates While it is necessary to determine corrosion tendencies by measuring potentials, it will not be sufficient to determine whether a given metal or alloy will suffer corrosion under a given set of environmental conditions Even though the tendency for corrosion may be high, the rate of corrosion may be very low → corrosion may not be a problem W. D. Callister, D. G. Rethwisch, Materials Science and Engineering: An Introduction 11 Corrosion Prevention Materials selection → use metals that are relatively unreactive in the corrosion environment Reduce temperature → slow kinetics (rates) of oxidation and reduction Passivation → metal oxides form a thin adhering oxide layer that slows corrosion Corrosion inhibitors → substances added to solution that decrease its reactivity Cathodic (or sacrificial) protection → attach a more anodic material to the one to be protected 12 Corrosion Prevention – Passivation The use of a light coat of a protective material, such as metal oxide Usually seen in aluminium, stainless steel, titanium and silicon Apply physical barriers, such as films, coatings, paint, etc. 13 Corrosion Prevention – Corrosion Inhibitor A corrosion inhibitor is a chemical compound that, when added to a liquid or gas, decreases the corrosion rate of a material Slow oxidation/reduction reactions by removing reactants like O2 gas by reacting with it Oxygen is generally removed by reductive inhibitors, such as amines 14 Corrosion Prevention – Cathodic/Sacrificial Protection To control the corrosion of a metal surface by making it the cathode of an electrochemical cell Attach a more anodic material to the one to be protected 15 Degradation of Materials - Metals Cathodic (sacrificial) protection This is where one metal is deliberately sacrificed to protect another Sea water attacks the bronze propellers → a slab of magnesium, aluminium or zinc is attached to the wooden hull near the propeller This becomes the anode and corrodes while the expensive propeller (cathode) is protected → the anode must be replaced regularly 16 Degradation of Materials - Metals Corrosion is the destructive electrochemical attack of a material For e.g., rusting of automobiles and other equipment Cost of corrosion 4-5 % of the Gross National Product (GNP)* In the US, this amounts to over $400 billion/year** * H.H. Uhlig and W.R. Revie, Corrosion and Corrosion Control: An Introduction to Corrosion Science and Engineering, 3rd ed., John Wiley and Sons, Inc., 1985. ** Economic Report of the President (1998). 17 Degradation of Materials – Polymeric Materials Elastomers can cause other plastics to corrode or melt due to prolonged contact For e.g., rubber left on a set square 18 Degradation of Materials – Plastics UV light will weaken certain plastics and produce a chalky faded appearance on the exposed surface → deterioration of plastic materials 19 Degradation of Materials – Plastics Heat will weaken or melt certain plastics even at relatively low temperatures 20 Degradation of Materials – Plastics Cold can cause some plastics to become brittle and fracture under pressure 21 Degradation of Materials – Plastics Mould can grow on plastics in moist humid conditions 22 Degradation of Materials – Plastics Bio-degradation → the chemical breakdown in the body of synthetic solid phase polymers 23 Degradation of Polymers - Mechanisms Mechanism I (top) → breaking crosslinking bonds by absorbing water Most prevalent in amorphous polymers with low molecular weight Mechanism II (middle) → cleavage of side chains leading to formation of hydrophilic groups Mechanism III (bottom) → scission of “mers” in polymer chain 24 Degradation of Polymers – Chemical Degradation Due to the reactive chemicals in the atmosphere Most important cases are Oxygen → leads to oxidation degradation Ozone → leads to ozonolysis Water → leads to hydrolytic degradation Protection A range of antioxidants, coating, etc. 25 Degradation of Polymers – Oxidative Degradation Normally initiated by Radiation, for e.g., UV light Heat Direct O2 attack (not too important with saturated polymer) Initiator residues, for e.g., peroxides 26 Degradation of Polymers – Moisture Absorption Most polymers absorb some moisture through diffusion in high humidity environments Moisture usually act as a plasticiser decreasing the glass transition or softening temperature and yield strength of the polymer The plasticising effects can generally be reversed upon drying the material May act in conjunction with photo-initiated oxidation to produce erosion of the material, which serve to wash away the embrittled surface layer to expose new material to direct sunlight 27 Socio-Economic-Environmental Considerations of Materials Science and Engineering A product must make economic sense Price must be attractive to users Must return a sustainable profit to the company To minimise product cost materials engineers must consider three factors Component design Material selection Manufacturing techniques Other significant factors include labour & fringe benefits, insurance, and profit 28 Total Materials Cycle Fig. 22.1, Callister & Rethwisch 10e. (Adapted from M. Cohen, Advanced Materials & Processes, 147, 1995, p. 70. Copyright © 29 1995 by ASM International. Reprinted by permission of ASM International, Materials Park, OH.) Components of “Green Design” Reduce → redesign the product to use less material For e.g., PET bottles with thinner walls, etc. Reuse → fabricate the product of a material that can be reused For e.g., refillable bottles and shipping containers, grind up old tires for use as mulch, etc. Recycle → reprocess the material into a new product For e.g., convert PET bottles to carpet fibres 30 Recycling Materials Proper product design facilitates recycling Advantages to recycling Reduced pollution emissions Reduced landfill deposits Recycling issues Product must be disassembled or shredded to recover materials Collection and transportation costs are significant factors in recycling economics 31 Summary Metallic corrosion involves electrochemical reactions Electrons are given up by metals in an oxidation reaction The electrons are consumed in a reduction reaction Metals and alloys are ranked according to their corrosiveness in standard EMF and galvanic series Temperature and solution composition affect corrosion rates Forms of corrosion are classified according to the mechanism Corrosion may be prevented or controlled by Materials selection Cathodic protection Reducing the temperature Painting and coating Applying physical barriers Adding inhibitors Socio-economic-environmental considerations of materials design 32 Recommended Reading Materials Science and Engineering: An Introduction (1st Edition), W. D. Callister and D. G, Rethwisch, Wiley, 2020 Chapter 17 33 Thank you Dr Xiaowen Yuan Office: WZ1003 Email: [email protected] Phone: +64 9 921 9999 ext. 7320 34 ENME502 Engineering Materials I Ceramics Prof. Xiaowen Yuan Ceramics Types and classification of ceramics Structure and properties Fabrication process Applications Advantages and limitations of ceramics 2 Nature of Ceramics Keramikos: burnt stuff in Greek, indicating that desirable properties of these materials are normally achieved through a high-temperature heat treatment process called firing (1,500-2,400°C) Two types Traditional ceramics (clays) → based on clay (china, bricks., tiles, porcelain), glasses Engineering ceramics (engineering ceramics) → for electronics, aerospace industries Always composed of more than one element (examples: Al2O3, SiC, SiO2) Bonding? Covalent or ionic 3 Characteristics of Ceramics High temperature resistance Good wear resistance High melting point (up to 5000°C) High electrical insulation Low thermal expansion Good chemical and thermal stability High elastic modulus High stiffness (400-600 GPa): 2-3 times higher than steels (Steel: 200 GPa and Al: 70 GPa) High compressive strength (typically 10 times the tensile strength) Hard and brittle (low tensile strength) 4 Structure of Ceramics Atomic structure Atoms are joined by strong covalent or ionic bonds Different atomic structure → different properties Microstructure Most engineering ceramics have a polycrystalline microstructure (several crystalline “mixed” together) Fine/small grain size → strong More porosity → weak 5 Structure of Ceramics 6 Structure of Ceramics Legends: WC: Tungsten carbide Co: Cobalt 7 Properties of Ceramics Tensile strength Difficult to measure! → Difficult to prepare “dog bone” sample Max tensile strength around 700 MPa Compressive strength 10 times higher than tensile strength (max 4 GPa or 4 x 103 MPa) Stiffness 400-600 GPa 2-3 times higher than steels (Steel: 200 GPa and Al: 70 GPa) Toughness Very poor (brittle fracture) Excellent creep strength Very high melting point (up to 5000°C) 8 Properties of Ceramics – Creep Strength Creep The tendency of a material to slowly deform over a long period of exposure to high levels of stress A time-dependent material deformation under continuous stress below the material’s yield strength Creep resistance Solid material’s ability to resist “creep” Creep strength (limit) Maximum stress required to cause a specified amount of creep in a specified time --> Maximum stress that can be generated in a material at constant temperature under which creep rate decreases with time The stress at specified environmental conditions that produces a steady creep rate, such as 1%, 2%, or 5% Risk: reduces the structural integrity of buildings, Measurement of creep vehicles, and equipment, thus, increasing the risk of a resistance safety incident 9 Mechanical Properties of Ceramics Ceramics are brittle Fracture strength may be enhanced by creating compression in the surface The compressive strength is typically 10 times of the tensile strength 10 Manufacture of Various Ceramics 11 Properties of Ceramics 12 Classification of Ceramics Adapted from Fig. 13.1 and discussion in Section 13.2-6, Callister 7e. 13 Ceramic Types and Characteristics 14 Fabrication of Industrial/Engineering Ceramics Ceramics cannot be simply melted and poured or moulded into shape Vitrification Used for making abrasive wheels and machinable ceramics (mica+glass) Mix ceramics powder with glass powder Dry press into desired shape “Fire” to produce finished product Sintering More common process for engineering ceramics (better mechanical properties compared with those from vitrified process) Compact powdered into desired shape (may involve a binder) → typical “compaction”, such as extrude, injection mould, dry press Sinter (“fire”) to produce finished product → particle diffuse into one another at particle contact point 15 Fabrication of Industrial/Engineering Ceramics Rough machining is done in “green state” → fired ceramic is very hard & brittle Special shapes require moulds (can be expensive) Vitrified ceramics are weaker than sintered ceramics Sintering usually involves a size change (up to 30% shrinkage) Finishing: can be quite an expensive and slow process Finishing may include laser, water jet and diamond cutting, diamond grinding and drilling. EDM (electrical discharge machining) is used if the ceramic is electrically conductive 16 Common Fabrication of ceramics – Compaction & Sintering 17 Types & Applications of Ceramics - Glasses Mixing glass forming oxides with other minerals and melt at high temperature Typical oxides: SiO2 (most common), B2O3, Al2O3, etc. Types of glasses → fused silica, VycorTM, borosilicate, soda lime, etc. Properties: Harder than many metals (very brittle) Low tensile strength Low thermal and electrical conductivity Good resistant to many acids, solvents and chemicals Can be used at high temperature (softening point of silica: 1580°C) Slowly attack by water and some alkali solutions 18 Types & Applications of Ceramics – Glass Products Sheet glass For glazing purposes → made by drawing this film of glass from molten bath, cooling and cutting into sheets. Also called: annealed glass Plate glass High quality glass → used to make toughened glasses (large sheets for shop windows, etc.) Toughened glass Produced by cooling the heated glass plate quickly using air blasts or oil quench. Cannot be cut! Must order to size → it will disintegrate if cut or drilled after manufacture Five times stronger than annealed glass For safety: when breaks, it shatters into many small pieces. For example, car windscreens Laminated glass Produced by sandwiching a resin or plastics between two or more sheets of annealed glass If broken, does not disintegrate 19 Types & Applications of Ceramics – Forming of Glass Blowing → bottles Drawing → sheets (windows) Grinding and polishing → Optical Extrusion → fibres and tube Rolling → patterned glass Pressing → lenses, dishes 20 Types & Applications of Ceramics – Forming Glass Products Blowing of glass bottles Pressing → glass formed by application of pressure Mould is steel with graphite lining For example, plates, cheap glasses Fibre drawing Fig. 13.15, Callister & Rethwisch 10e. (Adapted from C.J. Phillips, Glass: The Miracle Maker. Reproduced by permission of Pittman Publishing Ltd., London.) 21 Types & Applications of Ceramics – Sheet Glass Forming Sheet forming – continuous draw Originally sheet glass was made by “floating” glass on a pool of mercury Fig. 13.16, Callister & Rethwisch 10e. (Courtesy of Pilkington Group Limited.) 22 Types & Applications of Ceramics – Forming of Glass Annealing Removes internal stresses caused by uneven cooling Tempering Puts surface of glass part into compression Suppresses growth of cracks from surface scratches Sequence: Result → surface crack growth is suppressed 23 Types & Applications of Ceramics - Clays Bricks, tiles and refractories Complex mixtures of minerals formed by weathering of rocks Main constituents (minerals) are: Alumina (aluminium oxide: Al2O3) Silica (silicon dioxide: SiO2) Vitrification process Mix clay with water Form to desired shape “Fire” to produce finished product 24 Types & Applications of Ceramics - Refractories Materials to be used at high temperatures (example, in high temperature furnaces) Consider the silica (SiO2) – Alumina (Al2O3) system Silica refractories → silica rich with small additions of alumina that depress melting temperature (phase diagram) Fig. 12.25, Callister & Rethwisch 9e. [Adapted from F. J. Klug, S. Prochazka, and R. H. Doremus, “Alumina–Silica Phase Diagram in the Mullite Region,” J. Am. Ceram. Soc., 70, 758 (1987). Reprinted by permission of the 25 American Ceramic Society.] Types & Applications of Ceramics - Cements Defined as: a chemical binder, in the form of a fine powder, ground from calcium-based compounds. In combination with an appropriate quantity of water, it hardens and adheres to suitable aggregate, thus, binding it into a hard agglomeration called concrete Hydraulic cement can harden under water Non-hydraulic cements (like Portland cement) only harden in air Typical Portland cement → manufactured using either a wet process or a dry process Concrete One of the oldest “composite” materials Full strength achieved in about 28 days Make sure all voids are filled with cement paste Too much water: low strength and durability 26 Types & Applications of Ceramics - Cermets MMC with ceramic contained in a metallic matrix The ceramic often dominates the mixture, sometimes up to 96% by volume Bonding can be enhanced by slight solubility between phases at elevated temperatures used in processing Cermets can be subdivided into: Cemented carbides – most common Oxide-based cermets – less common 27 Types & Applications of Ceramics – Cemented Carbides One or more carbide compounds bonded in a metallic matrix Common cemented carbides are based on tungsten carbide (WC), titanium carbide (TiC), and chromium carbide (Cr3C2) Tantalum carbide (TaC) and others are less common Metallic binders → usually cobalt (Co) or nickel (Ni) 28 Types & Applications of Ceramics – Cemented Carbides Tungsten carbide cermets (Co binder) Cutting tools, wire drawing dies, rock drilling bits, mining tools, powder metal dies, indenters for hardness testers Titanium carbide cermets (Ni binder) Cutting tools, high temperature applications (such as gas-turbine nozzle vanes, thermocouple protection tubes, torch tips, etc.) Chromium carbide cermets (Ni binder) Gage blocks, valves liners, spray nozzles, bearing seal rings 29 Engineering Ceramics - Applications 30 Applications – Drawing Die Blanks Die blanks Ao die Ad tensile Need wear resistant properties! die force Adapted from Fig. 11.8 (d), Callister 7e. Courtesy Martin Deakins, GE Courtesy Martin Deakins, GE Superabrasives, Worthington, OH. Superabrasives, Worthington, Used with permission. OH. Used with permission. Die surface 4 μm polycrystalline diamond particles that are sintered onto a cemented tungsten carbide substrate Polycrystalline diamond helps control fracture and gives uniform hardness in all directions 31 Applications – Cutting Tools Tools For grinding glass, tungsten carbide, ceramics For cutting Si wafers For oil drilling Materials Manufactured single crystal or polycrystalline diamonds in a metal or resin matrix Polycrystalline diamonds resharpen by micro-fracturing along cleavage planes Photos courtesy Martin Deakins, GE Superabrasives, Worthington, OH. Used with permission. 32 Applications – Sensors Example: ZrO2 as an oxygen sensor Principle: increase diffusion rate of oxygen to produce rapid response of sensor signal to change in oxygen concentration Approach Add Ca impurity to ZrO2 to: Increase O2- vacancies Increase O2- diffusion rate Operation Voltage difference produced when O2- ions diffuse from the external surface through the sensor to the reference gas surface Magnitude of voltage difference is proportional to partial pressure of oxygen at the external surface 33 Applications – Advanced Ceramics for Automobile Engines Advantages Disadvantages Operate at high temperatures – Ceramic materials are brittle high efficiencies Difficult to remove internal voids Low frictional losses (that weaken structures) Operate without a cooling system Ceramic parts are difficult to form Lower weights than current and machine engines Potential candidate materials: Si3N4, SiC, & ZrO2 Possible engine parts: engine block & piston coatings 34 Applications – Advanced Ceramics for Ceramic Armour Components Outer facing plates Backing sheet Properties/materials Facing plates → hard and brittle Fracture high-velocity projectile Al2O3, B4C, SiC, TiB2 Backing sheets → soft and ductile Deform and absorb remaining energy Aluminum, synthetic fibre laminates 35 Applications – Ceramic Armour 36 Applications of Ceramics Structural applications Aluminium oxide (Al2O3): rocket nozzles, pump impellers, etc. Silicon carbide (SiC): abrasives, etc. Silicone nitride (Si3N4): furnace parts, cutting tools, etc. Partially stabilized zirconia (PSZ): thermal insulator, etc. Sialon (Si3Al3O3N5): candidate for many structural applications Wear resistance Cemented carbides or cermets Most common: tungsten carbides in cobalt binders Superior abrasion compared with tool steels Used as cutting tools and wear parts (grinder) 37 Applications of Ceramics Environmental Typical: glass and glass-like coating Excellent resistance to most acids excluding HF & hot phosphoric Should not be used for solution with pH >10 at 100°C Insulators Generally good insulators Silicon carbide used for resistance heating elements Magnets Soft magnets: magnetism depending on field Hard magnets: can be made into permanent magnets – blend with polymers and elastomers to make seal/latch for fridge doors Ferrites: can be soft or hard magnets (flexible) 38 Applications – Ceramic Components 39 Applications – Ceramic Components A variety of ceramic components. (a) High-strength alumina for high- temperature applications. (b) Gas-turbine rotors made of silicon nitride. Source: Courtesy of Wesgo Div., GTE. 40 Applications – Ceramic Bearing and Races A selection of ceramic bearings and races. Source: Courtesy of Timken, Inc. 41 Applications – Graphite Components and Electrodes * Diamond and graphite (different forms of carbon) are considered to be ceramics even though they are not composed of inorganic compounds. (a) Various engineering components made of graphite. Source: Courtesy of Poco Graphite, Inc., a Unocal Co. (b) Examples of graphite electrodes for electrical discharge machining. Source: Courtesy of Unicor, Inc. 42 Summary Types and classification of ceramics Structure and properties Advantages and limitations Applications 43 Recommended Reading Materials Science and Engineering: An Introduction (1st Australian and New Zealand, ANZ Edition), W. D. Callister and D. G, Rethwisch, Wiley, 2020 Chapter 12 & 13 44 Thank you Dr Xiaowen Yuan Office: WZ1003 Email: [email protected] Phone: +64 9 921 9999 ext. 7320 45 46 ENME502 Engineering Materials I Composites Prof. Xiaowen Yuan Composite Materials Concept and classification of composite materials Roles of each constituent Reinforcement type and properties Fibre-matrix interactions Critical fibre length Mechanical properties 2 Composite Materials – Introduction Combination of two or more distinct materials, taking advantage of the individual properties of each constituent material, to create synergy in the newly formed material Examples Naturally derived composites → wood Metal matrix composites Polymer composites Ceramic matrix composites Carbon/carbon composites 3 Composite Materials A composite is a material in which two or more distinct materials are combined together but remain uniquely identifiable in the mixture → produce a combination of properties that cannot be achieved with either of the constituents acting alone Three phases Reinforcement phase → fibre Matrix phase Interface/interphase 4 Composite Materials Fibre-reinforced polymer composites Stress-strain relationships for the composite material and its constituents The fibre is stiff but brittle, while the matrix (commonly a polymer) is soft, but ductile 5 Composite Materials – Types of Composites Types of composites Polymer matrix composites (PMCs) Metal matrix composites (MMCs) Ceramic matrix composites (CMCs) Natural fibre composites → “Green composites” Composites reinforced with natural fibres, primarily plant-derived materials Example: wood fibre composites (WPC) 6 Composite Materials – Matrix Types Polymeric matrix Thermoset polymers (resins) Epoxy, polyester, phenolics, polyimide, polybenzimidazoles (PBI), polyphenylquinoxaline (PPQ) Thermoplastic polymers Nylon, thermoplastic polyester (PET, PBT), polycarbonate (PC), polyethylene (PE), polypropylene (PP) Metallic matrix Aluminium and its alloys, titanium alloys, copper-nickel-based superalloys, stainless steel Ceramic matrix Aluminium oxide (Al2O3), silicon carbide (SiC), silicon nitride (Si3N4) 7 Composite Materials – Matrix Roles of the matrix A matrix is required to: Hold reinforcement in correct orientation Protect fibres from damage Transfer loads into and between fibres 8 Composite Materials – Reinforcement Phase Reinforcements May be particulate, short fibres, long fibres or continuous fibres Provide strength and stiffness Influence the formability & machinability of the resulting structure Change the nature of these materials from isotropic to anisotropic 9 Properties of Typical Fibres/Resins 10 Composite Materials – Polymer Composites Composites – 3 phases: reinforcement, matrix, interface Reinforcement Greater mechanical properties The principal load-carrying members Reinforcing effect: interface, aspect ratio, distribution and orientation Matrix A binder to keep the fibres in a desired location/orientation Protects fibre from environmental damage Interface Plays a decisive role on the transformation of load from the matrix to the fibre “Green composites” → composites reinforced with natural fibres, primarily plant derived materials 11 Reinforcement - Fibres Filler and additives in composites Function Costs Fibrous reinforcements The principal constituent in a fibre-reinforced composite material Enhance the mechanical properties of plastics Change the nature of these materials from isotropic to anisotropic 12 Reinforcement – Forms Particulate Micro-balloons (hollow microspheres) Nano particles (sized between 1 and 200 nanometres) Discontinuous Fibre Chopped strand mat Chipped fibres for injection moulding (100 μm long) Continuous fibre and fabric Weaving, non crimp fabrics, knitting, braiding, mats and non-woven, combination fabrics and preforms 13 Reinforcement – Fabric Reinforcements Woven fabrics Braids Knitted fabrics Stitched fabrics Bonded/felted fabrics 14 Reinforcement – Common Fibres for PMCs Glass fibres (SiO2 E-glass fibre) Carbon fibres (Graphite fibres) Aramid fibre (Kevlar) Kevlar fibres (aromatic polyamide) Natural fibres In “Green composite” 15 Reinforcement – Types of Natural Fibres Bast fibres (flax, hemp, jute, kenaf, etc.) Wood core surrounded by stem containing cellulose filaments Leaf fibres (sisal, banana, palm, etc.) Seed fibres (cotton, coconut (coir), kapok, etc.) Wood fibres 16 Types of Reinforcements 17 Types of Matrices 18 Classification of Composite Types (Reproduced from Callister, Materials Science and Engineering - An Introduction, 7th Edition, Wiley) 19 Composite Materials – Applications Where are composites used? Aero industry 20 Composite Materials – Applications Where are composites used? Auto industry 21 Composite Materials – Applications Where are composites used? Sports 22 Reinforcement – Fibres-Matrix Interactions Has a significant effect on: Shear, transverse, flexural, impact and crack propagation properties The bond must have a good shear strength in order to: Transmit load between matrix and fibre Minimise ingress of corrodents Control de-bonding There are a number of factors which affect the bond strength, including: Compatibility of resin and fibre Imperfections on surface of the fibre Finish (or size) applied to the fibre during fibre manufacture Length of the fibre 23 Reinforcement – Fibres-Matrix Interactions The interface between fibre and matrix is crucial to the performance of the composite, in particular fracture toughness, corrosion and moisture resistance Weak interfaces provide a good energy absorption mechanism Composites have low strength and stiffness, but high fracture toughness Strong interface results in a strong and stiff, but brittle composite 24 Reinforcement – Adhesion Mechanisms Adhesion between fibre and matrix is due to one (or more) of the 5 main mechanisms: 1. Adsorption and wetting Depending on the surface energies or surface tensions of the two surfaces. Glass and carbon are readily wetted by epoxy and polyester resins, which have lower surface energies 25 Reinforcement – Adhesion Mechanisms A simple water drop test shows differences in the wettability of a yellow birch veneer surface Three drops were applied to the surface at the same time, then photographed after 30 s. The drop on the left retains a large contact angle on the aged, unsanded surface. The drop in the centre has a smaller contact angle and improved wettability after the surface is renewed by two passes with 320-grit sandpaper The drop on the right shows a small contact angle and good wettability after four passes with the sandpaper 26 Reinforcement – Adhesion Mechanisms Adhesion between fibre and matrix is due to one (or more) of the 5 main mechanisms: 2. Interdiffusion (autoadhesion) Diffusion and entanglement of molecules 27 Reinforcement – Adhesion Mechanisms Adhesion between fibre and matrix is due to one (or more) of the 5 main mechanisms: 3. Electrostatic attraction Important in the application of coupling agents Glass fibre surface may be ionic due to oxide composition 28 Reinforcement – Adhesion Mechanisms Adhesion between fibre and matrix is due to one (or more) of the 5 main mechanisms: 4. Chemical bonding Between chemical group in the matrix and a compatible chemical on the fibre surface 29 Reinforcement – Adhesion Mechanisms Adhesion between fibre and matrix is due to one (or more) of the 5 main mechanisms: 5. Mechanical interlocking Depending on degree of roughness of fibre surface Larger surface area may also increase strength of mechanical bond 30 Reinforcement – Critical Fibre Length ‘Short” fibres do not reinforce as effectively as ‘long’ or continuous fibres The load transfer mechanism results in “end effects” which may reduce the fibre stress Difficult to control the alignment of short fibres Randomly-oriented short fibres cannot be packed at such high- volume fractions as continuous fibres 31 Reinforcement – Critical Fibre Length The effectiveness of the fibre reinforcement depends critically on the degree to which the applied load is transmitted to the fibres by the matrix phase The magnitude of the interface bond between the fibre and the matrix is critical Under an applied load, the fibre-matrix bond ceases at the fibre ends, i.e., there is no load transmittance from the matrix at the fibre extremity The deformation pattern of the matrix surrounding a fibre is illustrated below (Reproduced from Callister, Materials Science and Engineering - An Introduction, 7th Edition, Wiley) 32 Reinforcement – Critical Fibre Length The “end effect” means that some critical fibre length is necessary for effective strengthening and stiffening of the composite This critical length is dependent upon: Fibre length Fibre tensile strength Fibre-matrix bond strength Matrix yield strength 33 Reinforcement – Critical Fibre Length The fibre critical length can be calculated as follows: 34 Reinforcement – Rule of Mixtures Simple linear weighted to determine properties of composite mixture Used for design properties, e.g., Young’s Modulus, Yield Stress, Density, etc. 35 Reinforcement – Rule of Mixtures  c =  f V f +  m( 1 − V f ) Assumptions: Fibres are parallel to the intended loading direction No relative movement at the fibre matrix interface → the displacements of the fibres, matrix and overall composite are identical during loading 36 Reinforcement – Stiffness of Short Fibre Composites For aligned short fibre composites (difficult to achieve in polymers!), the rule of mixtures for modulus in the fibre direction is: E = ηLEfVf + Em(1 − Vf ) The length correction factor (ηL) can be derived theoretically, provided L > 1 mm, ηL > 0.9 37 Reinforcement – Stiffness of Short Fibre Composites For aligned short fibre composites (difficult to achieve in polymers!), the rule of mixtures for modulus in the transverse direction is: 1 Vm Vf = + Ect Em Ef 38 Practice Problem – Rule of Mixtures A polymer composite consists of carbon fibres and phenolic resin and has a fibre volume fraction of 0.37. The composite is completely unidirectional. The material properties are given in the table below: Material Elastic Modulus, E (GPa) Tensile Strength, σ (MPa) Carbon fibre 250 3,200 Phenolic resin (matrix) 10 60 Use the rule of mixtures predict the elastic modulus and tensile strength in the longitudinal direction for the finished composite. (Answers: Ec = 98.8 GPa; σc = 1,221.8 MPa) 39 Summary What is a composite material? Concept Classification Fibre reinforcements Particulate-reinforced Fibre-reinforced Fabric-reinforced Critical fibre length Interfacial bonding Mechanical properties – rule of mixtures 40 Recommended Reading Materials Science and Engineering: An Introduction (1st Australian and New Zealand, ANZ Edition), W. D. Callister and D. G, Rethwisch, Wiley, 2020 Chapter 16 41 References Materials Science and Engineering – An Introduction (10th Edition), W.D. Callister, Jr and D.G. Rethwisch, Wiley, 2019. Manufacturing Engineering and Technology, Chapter 9 – Composite Materials, Kalpakjian and Schmid. 42 Thank you Dr Xiaowen Yuan Office: WZ1003 Email: [email protected] Phone: +64 9 921 9999 ext. 7320 43