This presentation explores the critical properties that define the performance of ceramics: Hardness, Fracture Toughness, and Strength.

1. HARDNESS, FRACTURE
TOUGHNESS AND
STRENGTH OF CERAMICS
Akumalla Rajkumar
120CR0396
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SEMINAR AND TECHNICAL WRITING

2. CONTENTS
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INTRODUCTION​
HARDNESS​
FRACTURE TOUGHNESS​
STRENGTH​
SUMMARY​

3. INTRODUCTION
• Ceramics are a diverse class of materials
valued for their unique properties. They are
generally known for their high hardness, making
them ideal for wear-resistant applications. They
are strong, durable, and often resistant to high
temperatures. But how do we quantify these
strengths and weaknesses?
• This introduction explores three key mechanical
properties that define a ceramic's capacity to
withstand stress and deformation:
• Hardness
• Fracture toughness
• Strength.
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4. HARDNESS
• Hardness, itself, is not uniquely defined. In the most general sense it
refers to a materials resistance to penetration by a sharp object.
• Hardness is a measure of the material’s resistance to localized plastic
deformation (e.g. dent or scratch)
• A qualitative Moh’s scale, determined by the ability of a material to
scratch another material: from 1 (softest = talc) to 10 (hardest =
diamond).
• Different types of quantitative hardness test has been designed
(Rockwell, Brinell, Vickers, etc.). Usually a small indenter (sphere, cone,
or pyramid) is forced into the surface of a material under conditions of
controlled magnitude and rate of loading. The depth or size of indentation
is measured.
• The tests somewhat approximate, but popular because they are easy
and non-destructive (except for the small dent).
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5. METHODS OF HARDNESS MEASUREMENT
• Two types of sharp-pointed hardness indenters:
Vickers and Knoop.
Vickers Hardness:
• For most ceramics, the hardness value is obtained with the
Vickers indentation technique using the following expression:
Hv=1.8544P/(2a)^2, Hv is Vickers hardness, P is the applied
load, and d is the average value of two diagonals.
• Like many hardness tests, Vickers relies on the concept of
indentation. A standardized indenter, typically a pyramid-
shaped diamond, is pressed onto the material's surface with
a specific load. The resulting indentation size reflects the
material's resistance to deformation. Harder materials will
produce smaller indentations under the same load.
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6. • Knoop Hardness:
• Only the long diagonal of the Knoop indentation is
measured to determine the Knoop hardness (HK).
• Knoop hardness measurements are the preferred
ones for very brittle ceramics such as SiC and
loads, P, not exceeding 2kg to avoid cracking.
• Knoop indenters are also used to measure
hardness when a small penetration depth is
desired, for instance, in measuring the hardness of
a coating.
• Vickers and Knoop indenters normally form a chisel
edge of approximately 1mm in length.
• The measure of depth of penetration to determine
hardness has the advantage that the depth can be
measured continually to produce a load
displacement (depth) curve such as the one shown
in Figure
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Typical load–displacement curve for instrumented hardness

7. FACTORS AFFECTING HARDNESS
Mineral Composition
Ceramics composed of minerals with
strong interatomic bonds tend to be
harder.
Minerals like alumina (aluminum oxide)
and silicon carbide are naturally very
hard, contributing significantly to the
overall hardness of the ceramic.
Microstructure
The microstructure refers to the
arrangement and size of grains within
the ceramic.
Finer grain sizes generally result in
higher hardness.
Additionally, lower porosity leads to
increased hardness. Pores act as weak
points and can reduce the material's
resistance to deformation.
Processing Techniques
The manufacturing methods used to
create the ceramic can influence the
microstructure and, consequently, the
hardness.
Techniques like hot pressing or
sintering can influence the grain size
and porosity, impacting the final
hardness of the ceramic.
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8. APPLICATIONS OF HARDNESS OF CERAMICS
CUTTING TOOLS
Ceramic cutting
tools, like blades
and inserts, are
significantly harder
than traditional
steel tools.
WEAR PLATES
AND LINERS
Ceramic wear plates
and liners are used
to protect
equipment surfaces
from abrasion and
impact.
DENTAL
IMPLANTS
Some dental
implants utilize
advanced ceramics
due to their
biocompatibility and
exceptional
hardness
ARMOR
Ceramic materials
are sometimes
incorporated into
ballistic armor due
to their ability to
stop projectiles.
SANDBLASTING
NOZZLES
Ceramic nozzles
used in sandblasting
applications utilize
their hardness to
propel abrasive
particles at high
velocities for
cleaning or surface
preparation.
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9. FRACTURE TOUGHNESS
• Fracture toughness is a critical property for
ceramics, especially considering their reputation for
brittleness. It goes beyond hardness to assess a
material's ability to resist crack propagation.
• A parameter, used to describe fracture toughness is
known as critical stress Intensity, K = σ (πC)^1/2
• The stress intensity factor K describes the stress
distribution around a flaw.
• In brittle materials, lack of plastic deformation or
yielding at the crack front, causes sufficient stress
concentration to cause the crack to propagate
without very little expended energy. Crack tip stress
intensity >> fracture strength of the matrix in the
immediate vicinity of the crack tip.
• The rapid propagation of the crack from the crack
tip, especially in a monolithic material results in low
fracture toughness or resistance to crack
propagation.
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Typical stress-strain curves for (a) brittle solids and (b) ductile
materials.

10. • It is also recommended that the toughness of ceramics
with new compositions be measured at different loads to
check whether toughness increases with crack length,
leading to “ R - curve ” behavior.
• It should be mentioned here that the indentation method is
now routinely used for convenience to compute the
fracture toughness of small and relatively brittle
specimens
• Once cracking starts, it is therefore rather easy for cracks
to grow in ceramics.
• if the interaction between a growing crack and the
• microstructure can absorb a fraction of the energy
available at the crack tip stress field, then the driving force
for crack propagation will be lowered.
• Mechanisms to improve crack growth resistance, that is,
toughening mechanisms, Various toughening mechanisms in ceramic - based materials.
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11. FACTORS AFFECTING FRACTURE TOUGNESS
Flaws and Defects
• Pre-existing cracks, voids, or
other imperfections within the
ceramic act as starting points
for crack growth.
• These flaws can significantly
reduce the fracture toughness
of the material.
• Minimizing these defects
during processing is crucial for
achieving good toughness.
Microstructure
• Finer grain sizes generally lead
to higher fracture toughness.
This is because smaller grains
create a more tortuous path for
cracks to propagate through
the material.
• The presence of pores within
the ceramic can act as stress
concentrators, making it easier
for cracks to initiate and
propagate.
Residual Stresses
• Internal stresses within the
ceramic, introduced during
processing or thermal cycling,
can also affect fracture
toughness.
• Minimizing these stresses can
improve overall toughness.
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12. APPLICATIONS OF FRACTURE TOUGHNESS OF CERAMICS
TURBOCHARGERS
AND ENGINE
PARTS
The fracture toughness
of ceramics allows them
to withstand these
demanding conditions
without succumbing to
cracks that could cause
catastrophic failures.
CUTTING TOOLS
FOR HIGH-
TEMPERATURE
APPLICATIONS
Certain advanced
ceramics with good
fracture toughness can
be used as cutting tools
for machining high-
temperature materials
like superalloys or
titanium.
CUTTING-EDGE
AEROSPACE
APPLICATIONS
Advanced ceramics
with exceptional
fracture toughness are
being explored for use
in aerospace
components like
engine parts and heat
shields.
JOINT
REPLACEMENTS
Advanced ceramic
materials are being
used in artificial joints
due to their
biocompatibility, good
wear resistance and
improved fracture
toughness compared to
traditional materials
NUCLEAR WASTE
STORAGE
Ceramics with high
fracture toughness are
being investigated for
use in nuclear waste
storage containers.
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13. STRENGTH OF CERAMICS
• Strength is a broad term encompassing a material's ability to
withstand various forces without breaking.
• It's important to consider different types of strength, such as
flexural strength (resistance to bending) and tensile strength
(resistance to pulling forces).
• The tensile strength as measured in a tensile test is simply
the stress P/A at the instant of fracture. The most common
experiment is to program a testing machine to increase the
displacement (typically for a screw-driven testing machine) or
the load (typically for a hydraulic testing machine) at a
constant rate. Strength measured in this way at a rapid strain
rate is often termed the instantaneous strength. 13
Schematic of tensile test.

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o The relative simplicity of flexural (bend) testing
and the low cost of the required specimens have
made this test very popular for ceramics despite
several drawbacks in interpretation of the
resulting data.
o These drawbacks include a stress distribution that
is both nonuniform and changes with increasing
strain if some plastic deformation takes place.
o Bars of rectangular or circular cross section are
generally used for bend tests on ceramics.
o Flexure testing in four-point loading is the most
widely used technique for strength measurements
of ceramics over the temperature range in which
creep is not appreciable.
Flexure of a beam
Four-point bending
FLEXURE TESTS

15. FACTORS AFFECTING STRENGTH OF CERAMICS
Mineral composition
• The inherent strength of the
individual minerals used in the
ceramic plays a vital role.
• Materials with stronger mineral
constituents will naturally result
in a stronger overall ceramic.
Temperature
• Strength can be temperature
dependent.
• Some ceramics might exhibit
good strength at room
temperature but lose strength
at elevated temperatures.
Chemical Environment
• Exposure to corrosive
chemicals can weaken the
ceramic over time, affecting its
long-term strength.
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16. APPLICATIONS OF STRENGTH OF CERAMICS
STRUCTURAL
COMPONENTS
Ceramics with high
flexural strength, or the
ability to resist bending
forces, are used in
building materials like
floor tiles, roof tiles,
and even some
architectural panels.
HEAT
EXCHANGERS
Ceramic heat
exchangers, used in
various industries, rely
on their flexural
strength to withstand
the pressure
differences between
hot and cold fluids
flowing through them.
BUILDING
MATERIALS
Concrete, a composite
material containing
ceramic aggregates,
utilizes the
compressive strength
of the ceramic
particles to bear the
weight of structures.
PIPING AND
PRESSURE VESSELS
Advanced ceramics
with good tensile
strength are being
explored for use in
high-pressure
applications like piping
for chemical processing
or pressure vessels for
gas storage.
DENTAL IMPLANTS
AND BONE REPAIR
MATERIALS
Some ceramics used in
dental implants and
bone repair materials
benefit from good
tensile strength to
withstand the biting
forces in the mouth or
the forces experienced
during bone healing.
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17. HARDNESS VS FRACTUTRE TOUGHNESS VS STRENGTH OF CERAMICS
Hardness
 Focuses on resistance to permanent indentation
or scratching.
 Higher hardness indicates a material that can
better withstand wear and tear on the surface.
 Think of it as the difficulty of making a mark on
the ceramic.
 Measured using techniques like Vickers
hardness test.
Fracture Toughness
•Focuses on a material's ability to resist crack
propagation.
•Higher fracture toughness indicates a material that can
tolerate existing cracks or flaws without breaking easily.
•Think of it as the ceramic's ability to absorb impact or
resist a crack from growing into a complete fracture.
•Measured using techniques like Single-Edge Precracked
Beam (SEPB) or Vickers Indentation Fracture (VIF).
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18. Strength of Ceramics
•Broader term encompassing a material's ability to withstand various
forces (tension, compression, bending) without breaking.
•There are different types of strength, such as flexural strength
(resistance to bending) and tensile strength (resistance to pulling
forces).
•Strength depends on multiple factors, including hardness, fracture
toughness, and the specific type of stress applied.
•Measured using various techniques depending on the type of strength
being evaluated.
HARDNESS VS FRACTUTRE TOUGHNESS VS STRENGTH OF CERAMICS
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PERFETCT CERAMIC?
Hardness: Extremely high. The perfect ceramic should be
virtually scratch-proof and resistant to abrasive wear. Imagine a
material that can grind other materials without showing any signs
of wear itself.
Fracture Toughness: Moderately high. While high hardness is
crucial, some fracture toughness is still necessary to prevent
catastrophic failure from unexpected impacts or internal stresses
during grinding operations.
Strength: High overall strength, particularly compressive
strength to handle the crushing forces involved in grinding.
The above mentioned properties may not match. It's important to
remember that perfect materials don't exist. However, we can
explore ideal properties for hardness, fracture toughness, and
strength depending on the desired application:

20. SUMMARY
• This presentation explored the critical
properties that define the performance of
ceramics: hardness, fracture toughness, and
strength.
• Each property plays a vital role, and the
ideal combination depends on the specific
application.
• Understanding these properties allows us to
select the right ceramic material for optimal
performance.​
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21. THANK
YOU
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