Ceramic Matrix Composite (CMC)

1. Ceramic Matrix Composite (CMC)

2. Purpose of using CMC • Increase the toughness

3. Difficulties in processing of CMC • development of high temperature reinforcement • induced residual stress due to the differences in the coefficients of thermal expansion () particulate reinforcement fiber reinforcement

4. Monolithic Ceramic Materials • Ceramic materials are inorganic, nonmetallic materials which consist of metallic and nonmetallic elements bonded together primarily by ionic and/or covalent bonds. Ceramic Materials: traditional ceramic materials engineering ceramic materials

5. Ionic-Covalent Mixed Bonding Pauling’s equation % ionic character = Where xA and xB are the electro negativities of the atoms A and B in the compound

6. Types of Bonding • Simple Ionic Arrangements in Ionically Bonded Solids determined by following factors • The relative size of the ions in the ionic solid • Balance of electrical neutrality in the ionic solid

7. Stable Configurations

8. Crystal structures of Ceramics • Cesium Chloride (CsCl) • Sodium Chloride (NaCl)

9. Crystal structures of Ceramics • Calcium Fluoride (CaF2) • Zinc Blende (ZnS)

10. Crystal structures of Ceramics • Corundum (Al2O3) • Perovskite (CaTiO3)

11. Crystal structures of Ceramics • Silicate structures

12. Processing of Ceramics Materials preparation forming thermal treatment Pressing (a)drying dry pressing (b)sintering isostatic pressing (c)vitrification hot pressing slip casting extrusion

13. Dry pressing

14. Slip casting

15. Extrusion

16. Sintering

17. Mechanical Properties of Ceramics

18. Mechanisms for the deformation of ceramic materials Covalently bonded ceramics: single-crystal: brittle fracture polycrystalline: brittle fracture Ionically bonded ceramics: single-crystal: lattice slip, considerable plastic deformation polycrystalline: brittle fracture, limited slip system in the lattice grain boundary, crack

19. Factors Affecting the Strength of Ceramic Materials Structural defects: (1) surface crack (2) voids (porosity) (3) inclusions (4) grain size

20. Glasses • A glass is defined as an inorganic product of fusion, which has cooled to a rigid condition without crystallization.

21. Glass Transition Temperature, Tg

23. Viscous Deformation of Glasses Annealing range: Stress relief Flow under its own weight Glass fabrication

24. Forming Methods for Glasses • Float-glass process (plate glass)

25. Forming Methods for Glasses • Blowing

26. Tempered Glass  heat the glass softening point,  rapidly air-cooling of the glass surface

27. Chemically Tempered Glass  place the glass in a bath at temperature slightly lower than its strain point  soak for long duration (5~10hrs) The larger ions in the bath diffuse into the surface by replacing smaller glass ions. Thus, it introduces compression, stresses near the surfaces.

28. Powder of matrix Particulate or whisker reinforcement mixer pressed fired Binder Processing of CMCs • conventional mixing and pressing Problems: 1. nonuniform mixing 2. low volume fraction of reinforcement 3. damage of whiskers during mixing and pressing

29. Forming by slurries • Continuous fiber-reinforcement

30. -- Particulate, whisker or discontinuous fiber

31. Liquid State Processing  melt infiltration techniques ― not suitable for CMCs due to • reaction between reinforcement and matrix at high temperature • high viscosity of the melt • Matrix transfer moulding  pyrolysis of polymer in liquid impregnation of a perform ― process for carbon-carbon composite.

32. Matrix transfer molding

33. Sol-gel processing sol = a dispersion of small particles of less than l00 nm in a liquid gel = a sol that has lost some liquid, hence has increased viscosity

34. Vapour deposition processing Ion plating & sputtering Chemical vapour deposition (CVD) Chemical vapour infiltration (CVI) on perform

35. Lanxide process and in-situ techniques Liquid metal + gas reaction ceramic matrix perform with reinforcement

36. Alumina matrix composites (discontinuous-fiber reinforced) • SiC whisker reinforced alumina • Processing: slurry method • Mechanical properties: increase in strength & toughness

38. --Toughness is maintained at elevated temperature

39. Lower CTE, higher toughness  increase thermal shock resistance.

40. Increase creep resistance

41. Zirconia-toughened alumina (ZTA)

42. Monoclinic at low temperature Tetragonal at elevated temperature Athermal transformation 3% volume change Microcracking in alumina matrix Toughness increased but strength degraded Micro cracking toughening • Zirconia(ZrO2)

43. Transformation toughening ZTA+3% stabilizing oxide (Y2O3)  The t-m transformation during cooling can be suppressed.  Meta stable phase retained at low temperature stressmeta stable phase  t-m transformation ：both toughness and strength are increased.

44. Degradation in transformation toughening HCL+(ZTA+Y2O3)  promote t-m transformation by leaching out Y2O3  Microcrack on surface  HCL penetrates further  Microcrack linking  Larger crack  Degradation in strength

45. Glass-ceramic matrix composites • SiC yarns (Tyranno, Nicalon) reinforced LAS (Lithium Alumino Silicate) system • Processing: slurry based method (Fig 4.4) • Mechanical properties: Increase Young’s modulus

46. Increase strength, Fig 4.20, Table 4.6

47. Increase toughness, Fig 4.20, Table 4.5 Mechanisms for toughness increase(Fig 4.22) • Fiber debonding • Fiber pull-out • Wake toughening

48. At elevated temperature In inert gas: no degradation up to 1000℃ In air: O2 penetrates thru microcrack, reacts with carbon rich layer, Degradation Fig 4.23, Table 4.7

49. Fatigue strength, Table 4.8

50. Carbon-Carbon Composites • porous carbon-carbon composites (carbon bonded carbon fiber (CBCF)) Porosity content 70~90% high temperature insulation