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Ceramic Matrix Composite (CMC). Purpose of using CMC. Increase the toughness. 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.

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purpose of using cmc
Purpose of using CMC
  • Increase the toughness
difficulties in processing of cmc
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

monolithic ceramic materials
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

ionic covalent mixed bonding
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

types of bonding
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
crystal structures of ceramics
Crystal structures of Ceramics
  • Cesium Chloride (CsCl)
  • Sodium Chloride (NaCl)
crystal structures of ceramics1
Crystal structures of Ceramics
  • Calcium Fluoride (CaF2)
  • Zinc Blende (ZnS)
crystal structures of ceramics2
Crystal structures of Ceramics
  • Corundum (Al2O3)
  • Perovskite (CaTiO3)
processing of ceramics
Processing of Ceramics

Materials preparation forming thermal treatment

Pressing (a)drying

dry pressing (b)sintering

isostatic pressing (c)vitrification

hot pressing

slip casting

extrusion

mechanisms for the deformation of ceramic materials
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

factors affecting the strength of ceramic materials
Factors Affecting the Strength of Ceramic Materials

Structural defects:

(1) surface crack

(2) voids (porosity)

(3) inclusions

(4) grain size

glasses
Glasses
  • A glass is defined as an inorganic product of fusion, which has cooled to a rigid condition without crystallization.
viscous deformation of glasses
Viscous Deformation of Glasses

Annealing range: Stress relief

Flow under its own weight

Glass fabrication

forming methods for glasses
Forming Methods for Glasses
  • Float-glass process (plate glass)
tempered glass
Tempered Glass

 heat the glass softening point,

 rapidly air-cooling of the glass surface

chemically tempered glass
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.

processing of cmcs

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

slide29
Forming by slurries
    • Continuous fiber-reinforcement
slide31
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.

slide33
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

slide34
Vapour deposition processing

Ion plating & sputtering

Chemical vapour deposition (CVD)

Chemical vapour infiltration (CVI) on perform

slide35
Lanxide process and in-situ techniques

Liquid metal + gas reaction ceramic matrix

perform with reinforcement

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

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)
transformation toughening
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.

degradation in transformation toughening
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

glass ceramic matrix composites
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
slide47
Increase toughness, Fig 4.20, Table 4.5

Mechanisms for toughness increase(Fig 4.22)

    • Fiber debonding
    • Fiber pull-out
    • Wake toughening
slide48
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

carbon carbon composites
Carbon-Carbon Composites
  • porous carbon-carbon composites

(carbon bonded carbon fiber (CBCF))

Porosity content 70~90% high temperature insulation

slide51

50% carbon yield

from phenolic

porous & anisotropic

vacuum

low pressure

Discontinuous fibers

(mm in length)

gaseous impurities

(fiber alignment)

Ground recycled CBCF

(rework)

moulding

mixer

slurry

binder

(phenolic resin)

water

High temp heat treatment

Carbonization

(950℃)

drying

water

Product

99.9%℃

  • Processing of CBCF
slide54

Discontinuous fibers

Continuous fibers

  • Impregnation with
  •  thermosetting resins
    • (phenolic, furan polyimide)
  •  pitch
    • (polynuclear aromatic hydrocarbons)

pyrolysis

Carbonization

2500℃

Dense

Thick enough?

Chemical vapour deposition

product

  • Dense carbon-carbon composites
slide56
CVI methods
    • Isothermal method
    • Temperature gradient method (Fig 4.29)
    • Pressure gradient method
slide57
Stress-strain curve

process dependent: Fig 4.30

Form of fiber reinforcement: Fig 4.31

slide60
To prevent oxidation uses protective coating

Good coating should

    • be mechanically, chemically and thermally compatible with composite (Fig 4.34)
    • adhere to composite
    • prevent diffusion of oxygen to composite
    • prevent diffusion of carbon to environment
  • primary oxidation barrier coat ― SiC, Si3N3,…
  • secondary protection system ― filled up crack with a glassy phase, e.g. glass former (Si, or B) or glass coating