Chapter 13: Ceramics Materials - Applications and Processing

1. Chapter 13: Ceramics Materials - Applications and Processing ISSUES TO ADDRESS... • How do we classify ceramics? • What are some applications of ceramics? • How is processing different than for metals?

2. Taxonomy of Ceramics Glasses Clay Refractories Abrasives Cements Advanced products ceramics -bricks for -optical -whiteware -sandpaper -composites engine high T - composite - bricks - cutting - structural - rotors (furnaces) reinforce - polishing - valves - containers/ - bearings household -sensors • Properties: -- Tm for glass is moderate, but large for other ceramics. -- Small toughness, ductility; large moduli & creep resist. • Applications: -- High T, wear resistant, novel uses from charge neutrality. • Fabrication -- some glasses can be easily formed -- other ceramics can not be formed or cast.

3. Application: Refractories 2200 3Al2O3-2SiO2 T(°C) mullite 2000 Liquid alumina + L (L) 1800 mullite alumina crystobalite + L + + L 1600 mullite mullite + crystobalite 1400 0 20 40 60 80 100 Composition (wt% alumina) • Need a material to use in high temperature furnaces. • Consider the Silica (SiO2) - Alumina (Al2O3) system. • Phase diagram shows: mullite, alumina, and crystobalite as candidate refractories.

4. Application: Die Blanks die A d tensile A o force die • Die blanks: -- Need wear resistant properties! • Die surface: -- 4 mm polycrystalline diamond particles that are sintered onto a cemented tungsten carbide substrate. -- polycrystalline diamond helps control fracture and gives uniform hardness in all directions. Courtesy Martin Deakins, GE Superabrasives, Worthington, OH. Used with permission.

5. Application: Cutting Tools • Tools: -- for grinding glass, tungsten, carbide, ceramics -- for cutting Si wafers -- for oil drilling • Solutions: blades oil drill bits -- manufactured single crystal or polycrystalline diamonds in a metal or resin matrix. coated single crystal diamonds -- optional coatings (e.g., Ti to help diamonds bond to a Co matrix via alloying) polycrystalline diamonds in a resin matrix. -- polycrystalline diamonds resharpen by microfracturing along crystalline planes. Photos courtesy Martin Deakins, GE Superabrasives, Worthington, OH. Used with permission.

6. Application: Sensors 2+ Ca • Approach: Add Ca impurity to ZrO2: -- increases O2- vacancies -- increases O2- diffusion rate 2+ A Ca impurity 4+ removes a Zr and a 2 - O ion. • Operation: -- voltage difference produced when O2- ions diffuse from the external surface of the sensor to the reference gas. sensor gas with an reference unknown, higher gas at fixed 2- O oxygen content oxygen content diffusion - + voltage difference produced! • Example: Oxygen sensor ZrO2 • Principle: Make diffusion of ions fast for rapid response.

7. Ceramic Fabrication Methods-I Pressing Gob operation Parison mold • Fiber drawing: Compressed • Blowing: air suspended Parison Finishing mold wind up PARTICULATEFORMING CEMENTATION GLASS FORMING • Pressing: plates, dishes, cheap glasses --mold is steel with graphite lining

8. Sheet Glass Forming • Sheet forming – continuous draw • originally sheet glass was made by “floating” glass on a pool of mercury

9. Glass Structure 4- Si0 tetrahedron 4 4+ Si 2 - O + Na 4+ Si 2 - O • Basic Unit: • Glass is amorphous • Amorphous structure occurs by adding impurities (Na+,Mg2+,Ca2+, Al3+) • Impurities: interfere with formation of crystalline structure. • Quartz is crystalline SiO2: (soda glass)

10. Glass Properties Liquid Supercooled (disordered) Liquid Glass (amorphous solid) Crystalline (i.e., ordered) • Specific volume (1/r) vs Temperature (T): • Crystalline materials: -- crystallize at melting temp, Tm -- have abrupt change in spec. vol. at Tm Specific volume • Glasses: -- do not crystallize -- change in slope in spec. vol. curve at glass transition temperature, Tg -- transparent - no crystals to scatter light solid T T T g m

11. t dy dv glass dv dy t velocity gradient Glass Properties: Viscosity • Viscosity, h: -- relates shear stress and velocity gradient: hhas units of (Pa-s)

12. Glass Viscosity vs. T and Impurities fused silica glass 96% silica soda-lime Pyrex s] × 14 10 strain point annealing range 10 10 Viscosity [Pa 6 T : soft enough 10 deform to deform or “work” 2 10 Tmelt 1 T(°C) 200 600 1000 1400 1800 • soda-lime glass: 70% SiO2 balance Na2O (soda) & CaO (lime) • Viscosity decreases with T • Impurities lower Tdeform • borosilicate (Pyrex): 13% B2O3, 3.5% Na2O, 2.5% Al2O3 • Vycor: 96% SiO2, 4% B2O3 • fused silica: > 99.5 wt% SiO2 Tg Softening Point Working Point

13. Heat Treating Glass before cooling surface cooling further cooled compression cooler hot hot tension compression cooler • Annealing: --removes internal stress 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.

14. Ceramic Fabrication Methods-IIA GLASSFORMING PARTICULATE FORMING CEMENTATION drain pour slip pour slip “green absorb water mold into mold A into mold into mold ceramic” o “green container die holder ceramic” force ram A billet extrusion d die container hollow component • Milling and screening: desired particle size • Mixing particles & water: produces a "slip" • Form a "green" component --Hydroplastic forming: extrude the slip (e.g., into a pipe) --Slip casting: solid component • Dry and fire the component

15. Clay Composition A mixture of components used (50%) 1. Clay (25%) 2. Filler – e.g. quartz (finely ground) (25%) 3. Fluxing agent (Feldspar) binds it together aluminosilicates + K+, Na+, Ca+

16. Features of a Slip Shear charge neutral weak van der Waals bonding 4+ charge Si 3 + neutral Al - OH 2- O Shear • Clay is inexpensive • Adding water to clay -- allows material to shear easily along weak van der Waals bonds -- enables extrusion -- enables slip casting • Structure of Kaolinite Clay:

17. Drying and Firing wet slip partially dry “green” ceramic • Firing: --T raised to (900-1400°C) --vitrification: liquid glass forms from clay and flows between SiO2 particles. Flux melts at lower T. Si02 particle (quartz) micrograph of glass formed porcelain around the particle 70mm • Drying: layer size and spacing decrease. Drying too fast causes sample to warp or crack due to non-uniform shrinkage

18. Ceramic Fabrication Methods-IIB GLASSFORMING PARTICULATE FORMING CEMENTATION 15m Sintering: useful for both clay and non-clay compositions. • Procedure: -- produce ceramic and/or glass particles by grinding -- place particles in mold -- press at elevated T to reduce pore size. • Aluminum oxide powder: -- sintered at 1700°C for 6 minutes.

19. Powder Pressing Sintering • powder touches & forms neck & gradually neck thickens • add processing aids to help form neck • little or no plastic deformation Uniaxial compression - compacted in single direction Isostatic(hydrostatic) compression - pressure applied by fluid - powder in rubber envelope Hot pressing - pressure + heat

20. Tape Casting • thin sheets of green ceramic cast as flexible tape • used for integrated circuits and capacitors • cast from liquid slip (ceramic + organic solvent)

21. Ceramic Fabrication Methods-III GLASSFORMING PARTICULATE FORMING CEMENTATION • Produced in extremely large quantities. • Portland cement: -- mix clay and lime bearing materials -- calcinate (heat to 1400°C) -- primary constituents: tri-calcium silicate di-calcium silicate • Adding water -- produces a paste which hardens -- hardening occurs due to hydration (chemical reactions with the water). • Forming: done usually minutes after hydration begins.

22. Applications: Advanced Ceramics • Disadvantages: • Brittle • Too easy to have voids- weaken the engine • Difficult to machine Heat Engines • Advantages: • Run at higher temperature • Excellent wear & corrosion resistance • Low frictional losses • Ability to operate without a cooling system • Low density • Possible parts – engine block, piston coatings, jet engines • Ex: Si3N4, SiC, & ZrO2

23. Applications: Advanced Ceramics Electronic Packaging • Chosen to securely hold microelectronics & provide heat transfer • Must match the thermal expansion coefficient of the microelectronic chip & the electronic packaging material. Additional requirements include: • good heat transfer coefficient • poor electrical conductivity • Materials currently used include: • Boron nitride (BN) • Silicon Carbide (SiC) • Aluminum nitride (AlN) • thermal conductivity 10x that for Alumina • good expansion match with Si

24. Applications: Advanced Ceramics • Ceramic Armor • MEMs (MicroElectronicMechanical Systems) • Optical Fiber • Ceramics Ball Bearing • Piezoelectric Ceramics

25. Summary • Basic categories of ceramics: -- glasses -- clay products -- refractories -- cements -- advanced ceramics • Fabrication Techniques: -- glass forming (impurities affect forming temp). -- particulate forming (needed if ductility is limited) -- cementation (large volume, room T process) • Heat treating: Used to -- alleviate residual stress from cooling, -- produce fracture resistant components by putting surface into compression.