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1 Knives and Steel 1 Knives and Steel Observations about Knives and Steel Knives and Steel 2 Some knives can t keep their cutting edges Some knives bend while others break Making good knives involves heat treatment Some steel is stainless and doesn t rust Some stainless steel is poorly suited to knives 4 Questions about Knives and Steel Knives and Steel 3 1. Why do some knives bend and others break? 2. Why does a good knife require heat treatment? 3. Why is some stainless steel unsuitable for knives? 4. Why are some good knives made of alloy steels? Knives and Steel 4 Q: Why do some knives bend and others break? A: Their steel respond differently to stress. Iron and soft steels contain ferrite crystals (bcc) Layers of atoms can slip across one another These low-strength materials bend when overstressed Hardened steels contain martensite crystals Distorted crystals are resistant to slip These high-strength steels break when overstressed. Knives and Steel 5 Knives and Steel 6 Q: Why does a good knife require heat treatment? A: Hardening steel often involves thermal effects. Hot iron and steel contain austenite crystals (fcc) Carbon is much more soluble in austenite than ferrite Heating and cooling redistributes the carbon in steel Slow cooling lets carbon precipitate ferrite Fast cooling traps carbon martinsite Q: Why is some stainless steel unsuitable for knives? A: Those steels are austenitic at room temperature. Stainless steels contain chromium and nickel Together, these additions stabilize austenite Basic stainless is austenitic at room temp Cannot be hardened by carbon and heat treatment Austenitic iron, steel, or stainless steel is non-magnetic Special stainless can be martensitic at room temp 1

2 Knives and Steel 7 Knives and Steel 8 Summary about Knives and Steel Q: Why are some good knives made of alloy steels? A: Alloying can produce hard, rust-proof stainless Martensitic stainless isn t completely rust-proof Alloying can precipitation-harden rust-proof steel Deforming the steel can work-harden the steel Slip in its crystals allows steel to deform Limiting slip hardens the steel Ordinary steel is hardened by carbon and heat Ordinary stainless steel is rust-proof but soft Alloy stainless steel are hard and rust-proof Knives and Steel 9 Windows and Observations about Windows and Knives and Steel 10 is made from sand somehow is typically transparent can be formed into any shape Molten glass is a viscous liquid As molten glass cools, it thickens into a solid Broken glass has sharp, irregular edges 5 Questions about Windows and Knives and Steel Why does molten sand freeze into glass? 2. Why isn t ordinary glass made only from sand? 3. How is flat window glass made? 4. How is cooking glass different from window glass? 5. Why does tempered glass break into tiny pieces? Knives and Steel 12 Q: Why does molten sand freeze into glass? A: Crystallization of molten quartz takes too long. Quartz sand (silicon dioxide) is a network former Silicon and oxygen atoms attach via covalent bonds Liquid silica freezes into a disordered, glassy solid 2

3 Knives and Steel 13 Knives and Steel 14 Q: Why isn t ordinary glass made only from sand? A: Quartz melts at too high a temperature. Pure quartz melts at 1723 C and is ultraviscous Soda (sodium oxide) and lime (calcium oxide) act as fluxes, lowering the melting temperature weaken the network and lower its viscosity Soda-lime-silica glass is more practical than quartz glass Q: How is flat window glass made? A: Liquid glass solidifies on a pool of molten tin. is less dense than liquid tin and tin don t mix Tin is liquid over a very broad temperature range Liquid glass can solidify on liquid tin Knives and Steel 15 Knives and Steel 16 Question 5 Q: How is cooking glass different? A: Cooking glass exhibits less thermal expansion Cooking gg glass contains boron oxide Borosilicate glass expands less with temperature It is less likely to break during temperature changes Specialty glasses include other oxides Chemically resistant glass contains aluminum oxide Crystal glass contains lead oxide X-ray absorbing glass contains barium oxide Q: Why does tempered glass break into tiny pieces? A: It s core is under tension and can shred itself Tempered glass has a compressed surface layer breaks by tearing A compressed surface layer is hard to tear Tempered glass is very difficult to break The core of tempered glass is under tension When tempered glass breaks, its core shreds itself Summary about Windows and Knives and Steel 17 es are made from network formers Network formers can freeze into glassy solids Practical glasses are often mixtures of oxides Flat window glass is made by floating it on tin Knives and Steel 18 Plastics 3

4 Knives and Steel 19 Observations about Plastics Knives and Steel 20 6 Questions about Plastics They can take almost any shape They can be clear, translucent, or opaque They can tear or shatter They can be hard, soft, elastic, fiberous They can form by mixing chemicals They can form by evaporating solvents 1. How do plastics differ from ordinary molecules? 2. How does temperature affect plastics? 3. Why are some plastics clear, others translucent? 4. Why are some plastics unable to melt? 5. How do plastics form from simpler chemicals? 6. Why are some plastics so strong? Knives and Steel 21 Knives and Steel 22 Q: How do plastics differ from ordinary molecules? A: Plastics consist of giant molecules Plastic molecules are enormous Many are long linear chains Others are branched or networked They can become entangled Q: How does temperature affect plastics? A: Thermal energy allows local and distant mobility. Plastics can exhibit five distinct mobility regimes With increasing temperature, plastics go through: y solid: not even local mobility Leathery solid: some local mobility Elastic solid: local mobility, but not long-range mobility Rubbery flow: some long-range mobility Liquid flow: extensive long-range mobility Knives and Steel 23 Local Mobility Knives and Steel 24 Long-Range Mobility Local mobility is governed by molecular adhesion Some plastic molecules cling together tightly Acrylic plastics (Plexiglas, Lucite) Polystyrene (Styrofoam, plastic cups) PET and PETE (Mylar, soda bottles, plastic cups) Others cling weakly Polyethylenes (milk jugs, grocery store bags) Natural rubber Silicones Long-range mobility is governed by reptation Thermal energy causes chain motion Chain motion is called reptation Reptation allows chains to disentangle themselves Some plastics stay tangled Polyethylenes (jugs, bags) Other plastics disentangle Chicle (chewing gum) Silicones 4

5 Knives and Steel 25 Plasticizers Knives and Steel 26 Plastics can be softened by chemical plasticizers small molecules that are compatible with the plastics go into solution in the plastics (or vice versa) increase local l and long-range rn mobilities Examples of plasticized plastics: Solvent-based glues and paints Wet hair, fabrics, paper, noodles, bread Vinyl upholstery fabrics Q: Why are some plastics clear, others translucent? A: Some are partly crystalline, others all amorphous Some plastics are all amorphous They are homogenous throughout Light is undisturbed; they re clear Other plastics are partly crystalline They are inhomogenous Light scatters at boundaries; translucent Knives and Steel 27 Knives and Steel 28 Question 5 Q: Why are some plastics unable to melt? A: Their molecules are crosslinked in one network Crosslinks tack polymer chains to one another Reptation cannot disconnect or disentangle them They remain in the elastic regime They can t flow, so they don t melt They are thermosets thermosets (set shapes at all temperatures) Meltable plastics are thermoplastics (variable shapes) Q: How do plastics form from simpler chemicals? A: Molecular chain reactions assemble them. Most plastics begin as monomer molecules Monomers are small building block molecules Monomers bind together in chains to form polymers Plastics can have one monomer or several Plastics can be linear or branched Plastics can be orderly or more complicated Knives and Steel 29 Question 6 Knives and Steel 30 Summary about Plastics Q: Why are some plastics so strong? A: If all the molecules work together, they re strong Aligning g polymer chains into fiber gives strength Organizing those chains can yield extreme strength Liquid crystal fibers are naturally organized Aramids, Kevlar Melt-drawn fibers are organized during formation Spectra Plastics consist of giant molecules Temperature affects local and long-range mobility Entanglements limit long-range mobility Crosslinks can prevent long-range mobility 5