PROPERTIES OF MATERIALS PART HARDNESS

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Nathan Ellis
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CHAPTER 3 PROPERTIES OF MATERIALS PART 2 30.07.2007 3.1.

1 CHAPTER 3 PROPERTIES OF MATERIALS PART HARDNESS A Resistance to permanently indenting the surface Large hardness means resistance to plastic deformation or cracking In compression, better wear properties

3.2 ELECTRICAL PROPERTIES Electrical Conductivity is a function of a material s ability to carry an electrical current. Conduction requires electron transport.

2 3.2 ELECTRICAL PROPERTIES Electrical Conductivity is a function of a material s ability to carry an electrical current. Conduction requires electron transport. The conductivity (σ) of a material is determined by taking the reciprocal of the measured electrical resistance (R) to the flow of electricity in a length (L) of material divided by the cross-sectional area (A). σ = 1 R A L ELECTRICAL CONDUCTIVITY

3 EXAMPLES Electrical conductivity order of the first four metals: Silver >Copper>Gold >Aluminum The electrical properties of ceramic materials vary greatly, with characteristic measures spanning over many orders of magnitude.ceramics are probably best known as electrical insulators. Some ceramic insulators (such as BaTiO3) can be polarized and used as capacitors. Other ceramics conduct electrons when threshold energy is reached, and are thus called semiconductors. In 1986, a new class of ceramics was discovered, the high Tc superconductors. These materials conduct electricity with essentially zero resistance TEMPERATURE DEPENDENCE Conductivity is temperature dependent. As the temperature increase, the conductivity of a metal decrease. In contrast, the conductivity of pure semiconductors and insulators increases as the temperature increases

10 10 10 10 8 Conductivity ohm -1 cm -1 ( ) 30.07.20

4 3.2.2 TEMPERATURE DEPENDENCE Electrical conductivities of some common materials insulators semiconductors metals Conductivity ohm -1 cm -1 ( ) TEMPERATURE DEPENDENCE The energy bands of metals, semiconductors and insulators

5 3.3 OPTICAL PROPERTIES An optical property describes the way a material reacts to exposure to light. Visible light is a form of electromagnetic radiation with wavelengths in the range of 400 to 700 nm corresponding to an energy range of 3.1 to 1.8 electron volts (ev) (From E = hc/λ, where c = 3 x nm/s and h = 4.13 x ev.s) LIGHT INTERACTION WITH SOLIDS When light strikes an object it may be transmitted, absorbed, or reflected

3.3.2 OPTICAL CLASSIFICATION Materials vary in their ability to transmit light, and are usually described as transparent, translucent, or opaque.

6 3.3.2 OPTICAL CLASSIFICATION Materials vary in their ability to transmit light, and are usually described as transparent, translucent, or opaque. Transparent materials, such as glass, transmit light with little absorption or reflection. Materials that transmit light diffusely, such as frosted glass, are translucent. Opaque materials do not transmit light Applications: Fiber Optics

7 PHOTOCONDUCTIVITY MAGNETIC PROPERTIES

8 3.4.1 MAGNETIC STORAGE THERMAL PROPERTIES HEAT CAPACITY In general heat capacity is the ability of a material to absorb heat SPECIFIC HEAT CAPACITY Specific heat capacity is the amount of heat energy required to raise the temperature of a unit mass by 1 unit

3.5.2 SPECIFIC HEAT CAPACITY Comparison: Heat Capacity Values 30.07.2007 17 3.5 THERMAL PROPERTIES 3.5.3 SPECIFIC LATENT HEAT Specific latent heat is the energy per unit mass absorbed or evolved when a substance changes phase.

9 3.5.2 SPECIFIC HEAT CAPACITY Comparison: Heat Capacity Values THERMAL PROPERTIES SPECIFIC LATENT HEAT Specific latent heat is the energy per unit mass absorbed or evolved when a substance changes phase. There are 2 types: Latent heat of fusion the heat given When a liquid changes into a solid. Latent heat of vaporization the heat absorbed when a liquid turns into a gas THERMAL CONDUCTIVITY The ability of a material to transfer heat

5 THERMAL EXPANSION Materials change size when

10 3.5.4 THERMAL CONDUCTIVITY Comparison: Thermal Conductivities THERMAL EXPANSION Materials change size when heating. Comparison: Thermal Expansion Values

3.5.6 THERMAL PROTECTION SYSTEM 30.07.2007 21 Comparison of thermal properties of different ceramic materials Material Melting Temp. ( o C) Heat Capacity (J/kgK) Coef.

11 3.5.6 THERMAL PROTECTION SYSTEM Comparison of thermal properties of different ceramic materials Material Melting Temp. ( o C) Heat Capacity (J/kgK) Coef. of Linear Expansion 1/ o Cx10-6 Thermal Conductivity (W/mK) Aluminum Metal Copper Metal Alumina Fused Silica Soda-lime glass Polyethylene Polystyrene

12 3.6 CORROSION PROPERTIES Most materials experience some type of interaction with a large number of diverse environments. Such interactions impair a material s usefulness as a result of decrease in mechanical property qualities (ductility and strength), other physical properties, or appearance. In metals there is actual material loss by dissolution : Corrosion Oxidation of metals by exposure to air/moisture/salts Ceramic materials are relatively resistant to deterioration, usually occurs at elevated temperatures or in rather extreme environments : Corrosion CORROSION PROPERTIES For polymers mechanisms are different from metals and ceramics: Degradation Fe OH Fe(OH) 2 (s) Fe 2 O 3 O 2 + 2H 2 O + 4e 4OH Fe 2+ cathode anode e Iron Iron Fe Fe e

13 3.6.1 CORROSION ENVIRONMENTS Atmospheric corrosion accounts for the greatest losses. Moisture containing dissolved oxygen is the primary corrosive agent and also sulfur compounds and NaCl. Marine atmospheres are highly corrosive because of NaCl. Acid rain (dilute sulfuric acid solutions) in industrial environments can also cause corrosion problems PREVENTING CORROSION For protection, alloys of Al and Cu, galvanized steel are applied. Coatings or Cathodic Protection Coat with Zn to galvanize it. Zn reacts immediately with oxygen to form a hard protective coating. The coating is ZnO and completely seals the surface. If the surface is scratched, any exposed Zn will react to seal it again. Only if the steel core is exposed will the sample oxidize. ZnO ZnO Zn Zn Steel

14 3.6.3 CATHODIC PROTECTION Use a piece of metal that is more reactive and unimportant. Here, the item cannot be physically galvanized due to size. Zn will oxidize before the ship s hull will. Zn

Chapter 16 Corrosion and Degradation of Materials

Chapter 16 Corrosion and Degradation of Materials Concept Check 16.1 Question: Would you expect iron to corrode in water of high purity? Why or why not? Answer: Iron would not corrode in water of high