Traditional and engineering ceramics

Transcription

1 Traditional and engineering ceramics Traditional ceramics Clay Silica Feldspar + + Al2O3.2SiO2. 2H2O Structural clay products : bricks, sewer pipe, roofing tile EX: Triaxial bodies: Whiteware, porcelain, chinaware, sanitary ware. SiO 2 K 2 Na OAl. 2 2 OAl. O.6SiO O.6SiO 2 Reactions of a triaxial body

2 Traditional and engineering ceramics Traditional ceramics Triaxial whiteware chemical composition

3 Traditional and engineering ceramics Traditional ceramics

4 Traditional and engineering ceramics Traditional ceramics quartz Mullite needles High silica glass Electron micrograph of an electrical insulator porcelain (etched 10 s, 0 o C, 40% HF, silica replica)

5 Traditional and engineering ceramics Slip casting process Pottery Master and plaster moulds Slip casting Fire Colour paint Dry Fresh cast

6 Traditional and engineering ceramics Slip casting process Sanitaryware Hemihydrate plaster produced from gymsum CaSO o 150 C 1. 2H O CaSO H O H 2 O Slip preparation in ball mill Slip casting in plaster moulds and demoulding

7 Traditional and engineering ceramics Engineering ceramics Contain more of pure compounds of oxides, carbides, nitrides. Ex: Al 2 O 3, Si 3 N 4, SiC, ZrO 2, refractory oxides Mechanical properties of engineering ceramics

8 Traditional and engineering ceramics Engineering ceramics Alumina Refractory tubing High purity crucibles for high temp High quality electrical applications (low dielectric loss and high resistivity) Spark plug insulator Microstructure of sintered, powdered aluminium oxide doped with magnesium oxide Alumina tubes

9 Traditional and engineering ceramics Engineering ceramics Silicon nitride (Si 3 N 4 ) N 2 flow Dissociate at T > 1800 o C. Cannot be directly sintered reaction bonding. Silicon powder nitriding Microporous Si 3 N 4 High strength nonporous Si 3 N 4 Hot pressing with 1-5%MgO Silicon nitride for engineering applications

10 Traditional and engineering ceramics Engineering ceramics Silicon carbide (SiC) Hard refractory carbide. Form skin of SiO 2 at high temp. Resistance to oxidation at high temp. Can be sintered 2100 o C with 0.5-1%B. Fibrous reinforcement in ceramicmatrix composite material. SiC fibre reinforced Titanium matrix

11 Traditional and engineering ceramics Engineering ceramics Zirconia (ZrO 2 ) o C Polymorphic: tetragonal monoclinic. Volume expansion Zirconia Heat treatment Cubic structure Mixed with CaO, MgO and Y 2 O 3 Partially stabilized zirconia (PSZ).

12 Mechanical properties of ceramics Brittle High strength (varying from MPa) Better compressive strength than tensile (5-10 times) Level of strength (MPa) Materials > polycrystalline long ceramic fibres (Al 2 O 3, SiC): 1-2 GPa, single crystal short ceramic fibres (Al 2 O 3, SiC whiskers): 5-20 GPa, Hot Pressed structural ceramics such as silicon nitride, silicon carbide, alumina; sintered tetragonal zirconia and sialon; cemented carbides <50 sintered pure alumina and SiC; tempered glass impure and/or porous alumina; mullite; high-alumina porcelains; reaction bonded silicon nitride and carbide; glass ceramics porcelains; steatite, cordierite; magnesia, polished glasses; refractory; porous ceramics; glasses

13 Mechanical properties of ceramics Deformation mechanisms Lack of plasticity due to ionic and covalent bonding (directional). Stressing of covalent crystal separation of electron-pair bonds without subsequent reformation brittle Deforming of ionic single crystal (MgO or NaCl) shows considering amount of plastic deformation under compressive force. However ionic polycrystals are brittle due to crack formation at grain boundaries. NaCl structure showing slip on the (110) plane [110] direction or AA and on the (100) plane [010] direction BB

14 Mechanical properties of ceramics Factors affecting strength of ceramics Depending on amount of defects giving stress concentration Surface cracks Porosity Inclusions Excessive grain sizes Fabrication Should control chemical composition microstructure surface condition temperature environment Note: No plastic deformation during crack propagation from defects very brittle.

15 Mechanical properties of ceramics Toughness of ceramics Low toughness due to covalent-ionic bonding. Using hot pressing, reaction bonding to improve toughness. Fibre-reinforced ceramic matrix composites. Fracture toughness of ceramics

16 Mechanical properties of ceramics Chapter 1 Toughness of ceramics Example A reaction-bonded silicon nitride has a strength of 300 MPa and a fracture toughness of 3.6 MPa.m 1/2, What is the largest-size internal crack that this material can support without fracturing? Given Y = 1 K a a IC = Yσ 2 KIC = 2 πσ f f = πa ( 3.6MPa. m) π = ( 300MPa) m 2 2 = 45.8µ m Therefore the largest internal crack 2a = 91.6 µm

17 Mechanical properties of ceramics Chapter 1 Transformation toughening of Partially Stabilized Zirconia (PSZ) Sintering at 1800 o C+rapid cooling to RT+ reheating at 1400 o C to give fine precipitates Zirconia + (CaO, MgO or Y 2 O 3 ) PSZ (metal stable) Volume expansion Tetragonal monoclinic under stressing

18 Mechanical properties of ceramics Chapter 1 Fatigue failure of ceramics Fatigue failure in ceramics is rare due to lack of plastic deformation during cyclic loading. Fatigue cracking of polycrystalline alumina under cyclic loading

19 Mechanical properties of ceramics Chapter 1 Abrasive property of ceramics Hard and brittle Used as cutting, grinding and polishing tools. Aluminium oxide Silicon carbide Titanium nitride Tungsten carbide Boron nitride Ceramic cutting tools Ceramic grinding wheels

20 Thermal properties of ceramics Low thermal conductivity due to ionic-covalent bonding insulator. Also used as refractories in metal, chemical and glass industries. Thermal conductivity of ceramic materials

21 Thermal properties of ceramics Ceramic refractory materials A mixture of ceramic compounds Low-high temperature strength Low bulk density ( g.cm -3 ) Porosity insulating img.alibaba.com Acidic refractory Mainly based on SiO 2 and Al 2 O 3 Basic refractory Mainly based on magnesia (MgO), lime (CaO) and Cr 2 O 3 Refractory bricks (60% Al 2 O 3 ) for hot blast furnace

22 Thermal properties of ceramics

23 Thermal properties of ceramics Acidic refractory Basic refractory Silica refractory has high refractoriness, high mechanical strength and rigidity at high temperature. Fireclays (fine plastic clays + flint + coarse clay or grog) High alumina refractories contains 50-99% alumina, giving higher fusion temperature (more expensive than fireclay). Basic refractory consists of mixtures of MgO, CaO and Cr 2 O 3. High bulk density High melting point Good resistance to chemical attack (basic slag, oxides) Ex 92-95% MgO used for lining in basic-oxygen steelmaking process

24 Thermal properties of ceramics Ceramic tile insulation for the space shuttle orbiter About 24,000 ceramic tiles (70%) of silica-fibre compound are used for insulating external surface of space shuttle.

25 Thermal properties of ceramics Ceramic tile insulation for the space shuttle orbiter media.nasaexplores.com High temperature reusable surface (HTRS) made from 90% silica fibres and 10% empty space. Density = g.cm -3 Temp ~ 1260 o C Borosilicate coating upload.wikimedia.org Microstructure of LI900 high-temperature reusable surface insulation (HTRS)

26 Glass Definition of glass An inorganic and noncrystalline material which maintains its amorphous microstructure below its glass transition temperature. Properties of glass Blown glass Transparency Hardness and strength Corrosion/chemical resistance Vacuumtight enclosure Insulator Tinted or heat-absorbed glass

27 Glass Glass transition temperature (T g ) Unlike solidified metal, a glass liquid does not crystallize but follow an AD path. Temp (decrease) Viscous Plastic Glassy The faster cooling rate, the higher values of T g. Solidification of crystalline and amorphous materials showing a change in specific volume

28 Glass Structure of glass Glass forming oxide - SiO 2 Si-O tetrahedron Ideal crystalline silica (crystobalite) Simple silica glass with no-long range order

29 Glass Structure of glass Glass modifying oxides - Na 2 O, K 2 O, CaO, MgO Oxygen from Na 2 O breaks up silica network, leaving oxygen atoms with an unshared electron. Na + or K + ions fits into interstices of network. Network modified glass (soda-lime glass)

30 Glass Structure of glass Intermediate oxides in glass - Al 2 O 3, Pb 2 O 3 Oxides such as Al 2 O 3 or Pb 2 O 3 cannot form glass network but join into an existing network. Aluminosilicate glass provides higher temperature than common glass.

31 Glass Glass composition Silica glass No radiation damage Soda-lime glass Reduced T m ~ 730 o C Borosilicate glass (Pyrex glass) Low thermal expansion Lead glass Shielding from high energy radiation

32 Glass Viscous deformation of glasses Glass remains its viscous (supercooled) liquid above T g. Temp > T g Viscosity η =η o e +Q RT η = viscosity of the glass η o = pre-exponential constant Q = molar activation energy for viscous flow R = gas constant T = absolute temperature

33 Glass Viscosity reference points Working point Viscosity = 10 4 poise (10 3 Pa.s) fabrication Softening point Viscosity = 10 8 poise glass flows at an appreciate rate under its own weight (and surface tension). Annealing point Viscosity = poise relieving internal stresses Strain point Viscosity = poise glass is rigid with slow rate of stress relaxation. Note: glass are usually melt at temp relating to viscosity = 10 2 poise

34 Glass Example A 96 % silica glass has a viscosity of P at its annealing point of 940 o C and a viscosity of 10 8 P at its softening point of 1470 o C. Calculate the activation energy in kj/mol for the viscous flow of this glass in this temperature range. T anneal = = 1213 K, η ap =10 13 P T softening = = 1743 K, η ap =10 8 P η η ap sp η 13 Q = exp = = 10 8 R Tap Tsp 10 5 =η o e+q RT Q 10 5 = exp Q= 382kJ/ mol K K

35 Glass Fabrications of glass Forming sheet and plate glass Float glass process molten glass ribbon moves on the top of molten tin in a reducing atmosphere. Remove glass sheet when the glass surface is hard enough then pass to annealing furnace called lehr to remove residual stresses. Blowing, pressing and casting of glass For deep, hallow shapes like bottles, jars, light bulbs envelops. Blowing air to force molten glass into moulds. Pressing a plunger into a mold containing molten glass. Casting into open moulds.

36 Glass Float glass process

37 Glass a) Reheat, b) final blow stage of a glass blowing machine process

38 Glass Pyrex glass Borosilicate glass Low thermal expansion Inert to almost all materials with the exception of hydrofluoric acid, hot phosphoric acid and hot alkalies. Approximate composition SiO 2 Na 2 O 81% 4.0% K 2 O 0.5 B 2 O % Al 2 O 3 2.0%

39 Glass Tempered glass The surface cools first (by rapid air cooling) and contract while the interior is warm, developing compressive on the surface and tensile in the middle. a) After surface has cooled from high temperature near glass-softening temperature. b) after centre has cooled.

40 Glass Tempered glass Tempering effect increases the strength (4 x stronger than annealed glass. Has higher impact resistance than annealed glass. Ex: Auto side window, safety glass for doors. Distribution of residual stresses across the sections of glass thermally tempered and chemically strengthend

41 Glass Laminated glass Plastic interlayer (PVB-poly vinyle butyral) is sandwiched with floated/annealed glass. Safety glass: Breaking like a spider web. Laminated glass Spider web breaking pattern

42 Glass Laminated glass

43 Glass Chemical strengthened glass Used in supersonic aircraft glazing, ophthalmic lenses. Submerging sodium aluminosilicate glass in a bath containing a potassium salt at T~ o C for 6-10 h. Replacing Na ions with larger K ions on the glass surface. Producing thin compressive stresses at the surface and tensile stresses in the centre. Distribution of residual stresses across the section of glass thermally tempered and chemically strengthened.