CHAPTER 12: mechanical properties of ceramics

Transcription

1 CHAPTER 12: mechanical properties of ceramics ISSUES TO ADDRESS... Mechanical Properties: What special provisions/tests are made for ceramic materials? Chapter 12-1

2 COORDINATION # AND IONIC RADII Coordination # increases with Issue: How many anions can you arrange around a cation? r cation r anion r cation r anion Coord # < Adapted from Table 12.2, Callister 6e. ZnS (zincblende) Adapted from Fig. 12.4, Callister 6e. NaCl (sodium chloride) Adapted from Fig. 12.2, Callister 6e. CsCl (cesium chloride) Adapted from Fig. 12.3, Callister 6e. Chapter 12-4

3 Room T behavior is usually elastic, with brittle failure. 3-Point Bend Testing often used. --tensile tests are difficult for brittle materials -- cannot machine or grip specimens, 0.1% fracture strain, cross section b rect. d R circ. F L/2 L/2 Determine elastic modulus according to: F Measuring elastic modulus x slope = F δ δ linear-elastic behavior E = F δ L 3 4bd 3 = F δ rect. cross section Adapted from Fig , Callister 6e. δ= midpoint deflect ion L 3 12πR 4 circ. cross section Chapter 12-9

4 Measuring strength 3-point bend test to measure room T strength. cross section b rect. d R circ. F L/2 L/2 Adapted from Fig , Callister 6e. location of max tension Flexural strength: σ fs =σ m fail Fmax F = 1.5F maxl bd 2 x δmax rect. δ = F maxl πr 3 Typ. values: Material σfs(mpa) E(GPa) Si nitride Si carbide Al oxide glass (soda) Data from Table 12.5, Callister 6e Chapter 12-10

5 Geometry, load, & material Condition for crack propagation: K K c Stress Intensity Factor: --Depends on load & geometry. Values of K for some standard loads & geometries: σ Fracture Toughness: --Depends on the material, temperature, environment, & rate of loading. σ 2a units of K: MPa m or ksi in Adapted from Fig. 8.8, Callister 6e. a K =σ πa K = 1.1σ πa Chapter 12-10

6 KIc(MPa m 0.5 ) Metals/ Alloys Steels Ti alloys Al alloys Mg alloys Fracture toughness of ceramics Graphite/ Ceramics/ Semicond Diamond Si carbide Al oxide Si nitride <100> Si crystal <111> Glass -soda Concrete Polymers PET PP PC PS PVC Polyester Composites/ fibers C-C( fibers) 1 Al/Al oxide(sf) 2 Y 2 O 3 /ZrO 2 (p) C/C( fibers) σ 4 1 Al oxid/sic(w) 3 Si nitr/sic(w) 5 Al oxid/zro 2 (p) 4 Glass/SiC(w) 6 Glass 6 increasing 2π a K c metals K c comp K c cer K c poly > K IC Fracture strength of ceramics lower than predicted because of the presence of small and omnipresent flaws which are stress raisers (pores, microcracks, etc.) Chapter 12-11

7 Fracture of ceramic materials K The larger the Ic ft Y specimen, the πa crit weaker it is Considerable scatter exists in value of fracture strength σ ft. It depends on probability of existence of a crack with favorable orientation (unstable) σ = Compressive strength σ fc (more complex) is higher because no stress concentration exists. Also caused by flaws; but by linking of cracks parallel to stress axis depends on average crack size, a ave σ = fc Z K π Ic a ave Chapter 12-

8 Silicate glasses Basic SiO 4 4- tetrahedron Cristoballite, (crystalline SiO 2 ) Glass - an inorganic product of fusion that has cooled to a rigid condition without crystallization Na+ Non-bridging oxygen atoms NaO, CaO are network modifiers Sodium-silicate glass Chapter 12-

9 Glass properties Specific volume (1/ρ) vs Temperature (T): Specific volume Supercooled Liquid Glass (amorphous solid) Tg Liquid (disordered) Crystalline (i.e., ordered) solid Tm T Adapted from Fig. 13.5, Callister, 6e. Viscosity: --relates shear stress & velocity gradient: --has units of (Pa-s) Crystalline materials: --crystallize at melting temp, Tm --have abrupt change in spec. vol. at Tm Glasses: --do not crystallize --spec. vol. varies smoothly with T --Glass transition temp, Tg τ=η dv dy τ glass τ dy dv dv dy velocity gradient Chapter 12-9

10 Glass transition temperature, T g Specific volume Liquid Supercooled liquid Crystallization Glass Crystalline solid T g Temperature T m T g, temperature below which disordered structure of liquid is frozen in place giving rise to a supercooled liquid T g is a measure of the rigidity of the glass network Network modifiers decrease Tg Network formers increase Tg Chapter 12-

11 Glass viscosity vs T and impurities Viscosity decreases with T Impurities lower Tdeform glass fused silica Viscosity [Pa s] 96% silica Pyrex soda-lime annealing range T( C) Tdeform : soft enough to deform or work Chapter 12-10

12 Features of a slip Clay is inexpensive Adding water to clay --allows material to shear easily along weak van der Waals bonds --enables extrusion --enables slip casting Shear charge neutral Structure of Kaolinite Clay: Adapted from Fig , Callister 6e. (Fig is adapted from W.E. Hauth, "Crystal Chemistry of Ceramics", American Ceramic Society Bulletin, Vol. 30 (4), 1951, p. 140.) charge neutral Shear weak van der Waals bonding Si 4+ Al 3+ - OH O 2- Chapter 12-13

13 Glass-Ceramics Temperature Melting and forming Nucleation Growth LiO 2 -SiO 2 system (30% LiO 2 typical) Enhanced toughness Reduced thermal expansion coefficient Time - Quartz, feldspar, dolomite, mixed with nucleating agents (TiO 2, ZrO 2 ) - Formed using glass forming techniques - Ceraming heated to temp high enough to form nuclei of crystals 1mm in size - Devitrification - T raised for Growth Chapter 12-

14 Improving the strength of ceramics and Glasses Grain size reduction of ceramics Cracks may be stopped by grain boundaries Make crack path tortuous with whiskers and fibers in ceramics Induce phase transformation (zirconia-containing ceramics) Phase transformation with volume change introduces compressive stresses ahead of advancing crack Crystallization of glasses presence of small crystals limit the size of flaws Close surface defects of ceramics and glasses Use in compression Implant large atoms in the surface Tempering of glass or sandwich glass (Corelle) - + stress Flame polishing to remelt the surface and heals cracks Chapter 12-

15 Toughened ceramics Original metastable zirconia particle (tetragonal structure) Crack Compressive stress field around crack tip Martensite trasnformation in zirconia particle (monoclinic structure) Example of Partially Stabilized (PSZ) ceramics Chapter 12-

16 SUMMARY Room T mechanical response is elastic, but fracture brittle, with negligible ductility Great susceptibility to flaws. Some may be toughened in many ways, including by grain size reduction, martensitic transformation Glass have non-crystalline structure, and are very susceptible to cracking. However, they may be toughened with appropriate thermal treatments Chapter 12-12

17 ANNOUNCEMENTS Reading: Core Problems: Self-help Problems: Chapter 12-0