Mechanical Behavior of
Ceramics and Glasses
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Week 13
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Goals for this unit
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Recognize basic terms related to ceramics
and glasses (Ch. 12)
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Understand the brittle nature and catastrophic
failure in ceramics (Ch. 6.5, 6.6)
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Understand the role of flaws in determining fracture
strength (Ch 8.2, 8.3)
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Explore viscous nature of glasses (Ch. 6.6)
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Classification of Ceramics
More diverse than metals, “ solid solution of two or
more elements”
Classification
– By application (traditional “industry” grouping)
– By chemical composition
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Classification by application
1. Whitewares (pottery, tableware, sanitary ware,
wall tile, etc.)
2. Refractories (materials for lining furnaces and
processing vessels)
3. Structural clay products (brick, pipe, construction
tile, roof tile, etc.)
4. Glass (subcategories of flat glass, container glass,
optical and fiber)
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Classification by application (cont.)
5. Abrasives
6. Cements and plaster
7. Porcelain enamel
8. Technical or fine ceramics
– electronic ceramics
– structural ceramics
– specialized technical applications (bio applications)
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Classification by Chemistry
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Silicate ceramics (based on clays, talc, feldspars and
other natural minerals)
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Simple oxide ceramics (alumina, magnesia, fused
silica, beryllia, zirconia, etc.)
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Complex oxides (ferrites, titanates, zirconates, spinels,
etc.)
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Non-oxides (nitrides, carbides, silicides, graphite, etc.)
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3
Chemical classification (cont.)
Glasses
– silicates
– borosilicates (and borates)
– phosphates
– non-oxide glasses (halides)
– glass-ceramics (formed as glasses and then crystallized)
– glazes (coatings on ceramic)
– enamels (coatings on metals)
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Mechanical Properties of Ceramics
-Very brittle in tension ( brittle fracture –
limited energy absorption)
- Limited load carrying capacity
-The strength of ceramic materials is strongly dependent
on the processing ( because of introduction of strength
limiting flaws )
- Ceramics are usually much stronger in compression
than in tension.
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4
Brittle Fracture of Ceramics
o At room T, both crystalline and amorphous ceramics
fracture before plastic deformation occurs
Stress
Ceramic
Metal
Ceramics don’t “dent”
Strain
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Brittle Fracture of Ceramics
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Fracture is usually transgranular (rather than
intergranular)
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Cracks often grow along high density
crystallographic planes (cleavage planes)
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5
Modulus of Rupture (MOR)
(Ch 6 pp201-203)
o Brittle ceramic materials are usually tested in bending
(not in tension as are most metals), why?
• Sample preparation is easier
• Significant difference in results for testing in
tension, compression and bending
Load
Load
b
Area
d
Area
L
L
Three point bending
Four point bending
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MOR Test
w MOR is calculated as the “maximum fiber stress” on the
tension side at failure (strength parameter)
Load
b
Area
h
L
Three point bending
For a rectangular cross-section:
σ = 3FL2
2bh
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F = load
L = span
h - ||to load
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For a circular cross-section:
σ = 3FL
πr 2
r = radius
ε = 12xr
L2
x=deflection
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6
What limits strength in brittle materials?
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Consider the fracture strength of ordinary window
glass
– Theoretical Strength = 7,000 MPa
(if bonds between individual atoms are broken)
– Actual Strength = 35 MPa
– That is 200x weaker!
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Why the big difference?
- Preexisting flaws-> stress concentration
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Week 13
Strength of Ceramics
o Griffith - 1920’s Proposed fine elliptical flaws exist
that concentrate stress
σ0
σm
ρ
x
x’
a
X
σ0
2a
x
σ0
x’
• Flaws behave as stress magnifiers.
• Applied stress may be fairly low, but effective local stress is very HIGH
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7
σo
Stress magnification by flaws
c
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σο = external stress
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ρ = radius of crack tip
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c = crack length
– half length for internal cracks
σo
w σm = magnified stress at crack tip
σm
σm = 2 σο
c
ρ
ρ
σm
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Stress magnification
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Suppose you have a crack of length (c) 0.5 mm and
radius (r) 1000 angstroms
σm / σo = 70.7
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For the same crack length suppose the radius is
100 angstroms
σm / σo = 224
Crack length
and radius
matters
Best if spherical defect
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8
Principles of Fracture Mechanics
For brittle metals, most crystalline ceramics
and glasses
Stress Concentration!
K IC = Yσ
πa
f
KIC: fracture toughness
Y: geometric constant
a: crack length
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σf
MPa m
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σf
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Fracture toughness
KIC is also sometimes called the “critical stress intensity
factor” for the material, and it can be measured with
an MOR test on a pre-cracked (c) specimen in
3-point bending as
K IC
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3YFL
=
πc
2
2bh
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9
Flaw Populations
All brittle materials contain a certain
population of small cracks with different
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sizes
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orientations
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geometries
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Q. How does this variation affect the strength of the
material?
In designing uses for ceramics, “average strength”
cannot be used. Flaw distribution is critical.
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Ceramic Toughening
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Brittle materials can have their fracture
strength increased by reducing the depth
and sharpness of surface flaws through
careful polishing or etching
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Abrasion of the surface in such a way as to
introduce flaws has just the opposite effect
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Ceramic Toughening (P. 277)
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Transformation toughening
matrix + particles
Cubic Tetragonal
ZrO2
ZrO2
– local stress induces
transformation of dispersed
Monoclinic
ZrO2
second phase
– squeezes crack shut (closure)
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Microcracks
– very fine cracks (much smaller
than critical size) blunt tip of
advancing crack
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Week 13
Static Fatigue (P287)
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low stress crack growth without cycling
– occurs in water containing environments
– occurs at room temperature
– caused by water reacting with oxide network
H2O
O
H
H
- Si - OH
+
OH - Si
- Si - O – Si Crack growth by chemical
breaking of oxide network
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Plastic Deformation in ceramics
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At room T - rarely occurs
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At high T - deformation can occur
– Creep (usually loaded in compression)
– Crystalline ceramics depend on dislocation movement
(difficult)
– Noncrystalline ceramics above Tg exhibit viscous flow
(like any liquid)
– Mixtures creep as glassy materials (viscous flow)
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Glass formation at the liquid-glass transition temperature
Volume (per unit mass)
liquid
Supercooled
liquid
Forming range
s
Glas
stals
Cr y
Tg Tm
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temperature
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12
Glasses
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The mechanical properties of glasses are dual in nature
– Below the glass transition temperature, Tg,
glass is a rigid brittle material
– Above Tg, glass behaves as a viscous liquid
with behavior characterized by continuous deformation
(at a rate inversely related to viscosity) rather than a fixed
elastic strain in response to stress
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Glasses
Annealed: No residual stresses at room temperature
large and sharp segment breakage
Tempered: Surface quenched below Tg
Slow cooled to room temperature
- Surface residual compressive stress,
Core residual tensile stress
- finely fragmented and dull breakage.
Laminated: Two ordinary layer of annealed glass with
a central layer if polymer.
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Summary
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Ceramics fail by brittle fracture
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Brittle materials are sensitive to presence of flaws
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Fracture toughness of a material characterizes
resistance to crack propagation
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Glass is rigid and brittle below Tg and behaves
viscously above Tg
• Read Class notes and relevant portions of
Shackelford, 2001
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