ENGR 151 Materials of Engineering LECTURE 19

Transcription

1 ENGR 151 Materials of Engineering LECTURE 19

2 CHAPTER 13: APPLICATIONS AND PROCESSING OF CERAMICS TOPICS TO ADDRESS... How do we classify ceramics? What are some applications of ceramics? How is processing of ceramics different than for metals? 2

3 CLASSIFICATION OF CERAMICS 3

4 CLASSIFICATION OF CERAMICS Ceramic Materials Glasses Clay products Refractories Abrasives Cements Advanced ceramics -optical - composite reinforce - containers/ household -whiteware - structural -bricks for high T (furnaces) -sandpaper - cutting - polishing Adapted from Fig and discussion in Section , Callister & Rethwisch 9e. -composites - structural -engine rotors valves bearings -sensors 4

5 CERAMICS APPLICATION: DIE BLANKS Die blanks: -- Need wear resistant properties! Die surface: -- 4 μm polycrystalline diamond particles that are sintered onto a cemented tungsten carbide substrate. -- polycrystalline diamond gives uniform hardness in all directions to reduce wear. A o die die A d tensile force Adapted from Fig. 11.9(d), Callister & Rethwisch 9e. Courtesy Martin Deakins, GE Superabrasives, Worthington, OH. Used with permission. 5

6 CERAMICS APPLICATION: 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 microfracturing along cleavage planes. oil drill bits blades Single crystal diamonds polycrystalline diamonds in a resin matrix. Photos courtesy Martin Deakins, GE Superabrasives, Worthington, OH. Used with permission. 6

7 CERAMICS APPLICATION: SENSORS Example: ZrO 2 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: -- increases O 2- vacancies -- increases O 2- diffusion rate Operation: -- voltage difference produced when O 2- ions diffuse from the external surface through the sensor to the reference gas surface. -- magnitude of voltage difference partial pressure of oxygen at the external surface Ca 2+ gas with an unknown, higher oxygen content A substituting Ca 2+ ion removes a Zr 4+ ion and an O 2- ion. + sensor O 2- diffusion voltage difference produced! - reference gas at fixed oxygen content 7

8 GLASSES

9 GLASS-CERAMICS Transformation from amorphous state to crystalline state via a process called crystallization. Fine-grained polycrystalline material glassceramic. Grain formation is via a phase transformation. Properties: High mechanical strength Low coefficients of thermal expansion Relatively high temperature capabilities Good dielectric properties (electrical insulators) Good biological properties Can be made optically transparent E.g. Pyroceram, CorningWare, Cercor, Vision

10 REFRACTORIES Materials that withstand high temperatures without melting or decomposing (chemically, for example). Unreactive and inert when exposed to severe environments. Ability to provide thermal insulation. Applications: Furnace linings for metal refining Glass manufacturing Metallurgical heat treatment Power generation 10

11 REFRACTORIES 11

12 REFRACTORIES Materials to be used at high temperatures (e.g., in high temperature furnaces). Consider the Silica (SiO 2 ) - Alumina (Al 2 O 3 ) system. Silica refractories - silica rich - small additions of alumina depress melting temperature (phase diagram) and must be minimized: 2200 T(ºC) 2000 Liquid (L) 3Al 2 O 3-2SiO 2 mullite alumina + L 1800 crystobalite + L mullite + L mullite + crystobalite alumina + mullite Composition (wt% alumina) Fig , 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[10], 758 (1987). Reprinted by permission of the American Ceramic Society.] 12

13 REFRACTORIES ABRASIVES AND CEMENTS Abrasives: Used to wear, grind or cut away softer material. E.g.: Diamond (natural and synthetic) expensive Silicon carbide, tungsten carbide (WC), aluminum oxide (corundum), silica sand Cements: when mixed with water, form a paste that eventually sets and hardens. E.g. Portland cement 13

14 ADVANCED CERAMICS: MATERIALS FOR AUTOMOBILE ENGINES Advantages: Operate at high temperatures high efficiencies Low frictional losses Operate without a cooling system Lower weights than current engines Disadvantages: Ceramic materials are brittle Difficult to remove internal voids (that weaken structures) Ceramic parts are difficult to form and machine Potential candidate materials: Si 3 N 4, SiC, & ZrO 2 Possible engine parts: engine block & piston coatings 14

15 ADVANCED CERAMICS: MATERIALS FOR CERAMIC ARMOR Components: -- Outer facing plates -- Backing sheet Properties/Materials: -- Facing plates -- hard and brittle fracture high-velocity projectile Al 2 O 3, B 4 C, SiC, TiB 2 -- Backing sheets -- soft and ductile deform and absorb remaining energy aluminum, synthetic fiber laminates 15

16 ADVANCED CERAMICS: MEMS Microelectromechanical Systems Miniaturized smart systems highly integrated Fabrication techniques similar to integrated circuits 16

17 NANOCARBONS Fullerenes spherical cluster of 60 carbon atoms, C 60 Like a soccer ball Carbon nanotubes sheet of graphite rolled into a tube Ends capped with fullerene hemispheres Fig , Callister & Rethwisch 8e. Fig. 13.7, Callister & Rethwisch 9e. 17

18 NANOCARBONS (CONT.) Graphene single-atomic-layer of graphite composed of hexagonally sp 2 bonded carbon atoms Fig. 13.9, Callister & Rethwisch 9e. 18

19 NANOCARBONS (CONT.) Applications: Medicine, textiles, alloys, springs, coatings and films, radar absorption material, electronics 19

20 CERAMIC FABRICATION METHODS 20

21 CERAMIC FABRICATION METHODS (I) GLASS FORMING Blowing of Glass Bottles: Parison mold Suspended parison Gob PARTICULATE FORMING Pressing operation Compressed air CEMENTATION Pressing: plates, cheap glasses -- glass formed by application of pressure -- mold is steel with graphite lining Fiber drawing: Finishing mold Fig , Callister & Rethwisch 9e. (Adapted from C.J. Phillips, Glass: The Miracle Maker. Reproduced by permission of Pittman Publishing Ltd., London.) wind up 21

22 SHEET GLASS FORMING Sheet forming continuous casting sheets are formed by floating the molten glass on a pool of molten tin Fig , Callister & Rethwisch 9e. (Courtesy of Pilkington Group Limited.) 22

23 GLASS STRUCTURE Basic Unit: 4- Si0 4 tetrahedron Si 4+ O 2 - Glass is noncrystalline (amorphous) Fused silica is SiO 2 to which no impurities have been added Other common glasses contain impurity ions such as Na +, Ca 2+, Al 3+, and B 3+ Quartz is crystalline SiO2: Na + Si 4+ O 2 - (soda glass) Adapted from Fig , Callister & Rethwisch 9e. 23

24 GLASS PROPERTIES Specific volume (1/ρ) vs Temperature (T ): Specific volume Supercooled Liquid Liquid (disordered) Crystalline materials: -- crystallize at melting temp, T m -- have abrupt change in spec. vol. at T m Glass (amorphous solid) Crystalline (i.e., ordered) T g T m Adapted from Fig , Callister & Rethwisch 9e. solid T Glasses: -- do not crystallize -- change in slope in spec. vol. curve at glass transition temperature, T g -- transparent - no grain boundaries to scatter light 24

25 GLASS PROPERTIES: VISCOSITY Viscosity, η: -- relates shear stress (τ) and velocity gradient (dv/dy): τ τ glass dy dv dv dy velocity gradient η has units of (Pa-s) 25

26 Viscosity [Pa-s] LOG GLASS VISCOSITY VS. TEMPERATURE Viscosity decreases with T soda-lime glass: 70% SiO 2 balance Na 2 O (soda) & CaO (lime) borosilicate (Pyrex): 13% B 2 O 3, 3.5% Na 2 O, 2.5% Al 2 O 3 Vycor: 96% SiO 2, 4% B 2 O 3 fused silica: > 99.5 wt% SiO strain point annealing point T melt T(ºC) Working range: glass-forming carried out Fig , Callister & Rethwisch 9e. (From E.B. Shand, Engineering Glass, Modern Materials, Vol. 6, Academic Press, New York, 1968, p. 262.) 26

27 HEAT TREATING GLASS Annealing: -- removes internal stresses caused by uneven cooling (thermal stresses due to poor thermal conductivity). Tempering: -- puts surface of glass part into compression -- suppresses growth of cracks from surface scratches. -- sequence: before cooling hot initial cooling cooler hot cooler -- Result: surface crack growth is suppressed. at room temp. compression tension compression 27

28 CERAMIC FABRICATION METHODS (IIA) GLASS FORMING Hydroplastic forming: Mill (grind) and screen constituents: desired particle size Mix with water Extrude this mass (e.g., into a brick) force A o PARTICULATE FORMING ram container billet container die holder extrusion die A d CEMENTATION Fig (c), Callister & Rethwisch 9e. Dry and fire the formed piece E.g. Brick, tiles, pipes, ceramic blocks 28

29 CERAMIC FABRICATION METHODS (IIA) GLASS FORMING Slip casting: Mill (grind) and screen constituents: desired particle size Mix with water and other constituents to form slip Slip casting operation pour slip into mold absorb water into mold green ceramic solid component PARTICULATE FORMING pour slip into mold drain mold hollow component CEMENTATION green ceramic Fig , Callister & Rethwisch 9e. (From W.D. Kingery, Introduction to Ceramics, Copyright 1960 by John Wiley & Sons, New York. Reprinted by permission of John Wiley & Sons, Inc.) Dry and fire the cast piece 29

30 TYPICAL PORCELAIN COMPOSITION (50%) 1. Clay (25%) 2. Filler e.g. quartz (finely ground) (25%) 3. Fluxing agent (Feldspar) -- aluminosilicates plus K +, Na +, Ca + -- upon firing - forms low-melting-temp. glass 30

31 HYDROPLASTICITY OF CLAY Clay is inexpensive When water is added to clay -- water molecules fit in between layered sheets -- reduces degree of van der Waals bonding -- when external forces applied clay particles free to move past one another becomes hydroplastic Structure of Kaolinite Clay: Fig , Callister & Rethwisch 9e. [Adapted from W.E. Hauth, "Crystal Chemistry of Ceramics", American Ceramic Society Bulletin, Vol. 30 (4), 1951, p. 140.] charge neutral Shear charge neutral weak van der Waals bonding Si 4+ Al 3+ - OH O 2- Shear 31

32 micrograph of porcelain DRYING AND FIRING Drying: as water is removed - interparticle spacings decrease shrinkage. wet body partially dry completely dry Fig , Callister & Rethwisch 9e. (From W.D. Kingery, Introduction to Ceramics, Copyright 1960 by John Wiley & Sons, New York. Reprinted by permission of John Wiley & Sons, Inc.) Drying too fast causes sample to warp or crack due to non-uniform shrinkage Firing: -- heat treatment between C -- vitrification: liquid glass forms from clay and flux flows between SiO2 particles. (Flux 70 μm lowers melting temperature). Fig , Callister & Rethwisch 9e. (Courtesy H.G. Brinkies, Swinburne University of Technology, Hawthorn Campus, Hawthorn, Victoria, Australia.) Si0 2 particle (quartz) glass formed around the particle 32

33 CERAMIC FABRICATION METHODS (IIB) GLASS FORMING PARTICULATE FORMING CEMENTATION Powder Pressing: used for both clay and non-clay compositions. Powder (plus binder) compacted by pressure in a mold -- Uniaxial compression - compacted in single direction -- Isostatic (hydrostatic) compression - pressure applied by fluid - powder in rubber envelope -- Hot pressing - pressure + heat 33

34 SINTERING Sintering occurs during firing of a piece that has been powder pressed -- powder particles coalesce and reduction of pore size Aluminum oxide powder: -- sintered at 1700 C for 6 minutes. Fig , Callister & Rethwisch 9e. Fig , Callister & Rethwisch 9e. (From W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceramics, 2nd edition, p Copyright 1976 by John Wiley & Sons, New York. Reprinted by permission of John Wiley & Sons, Inc.) 15 μm 34

35 TAPE CASTING Thin sheets of green ceramic cast as flexible tape Used for integrated circuits and capacitors Slip = suspended ceramic particles + organic liquid (contains binders, plasticizers) Fig , Callister & Rethwisch 9e. 35

36 CERAMIC FABRICATION METHODS (III) GLASS FORMING PARTICULATE FORMING CEMENTATION Hardening of a paste paste formed by mixing cement material with water Formation of rigid structures having varied and complex shapes Hardening process hydration (complex chemical reactions involving water and cement particles) Portland cement production of: -- mix clay and lime-bearing minerals -- calcine (heat to 1400 C) -- grind into fine powder 36

37 SUMMARY Categories of ceramics: -- glasses -- clay products -- refractories -- cements -- advanced ceramics Ceramic Fabrication techniques: -- glass forming (pressing, blowing, fiber drawing). -- particulate forming (hydroplastic forming, slip casting, powder pressing, tape casting) -- cementation Heat treating procedures -- glasses annealing, tempering -- particulate formed pieces drying, firing (sintering) 37