Ceramic matrix composites (CMCs) have improved fracture toughness over conventional structural ceramics through the addition of fibers that increase crack resistance. This document discusses the tribological properties and wear mechanisms of various CMCs, including those reinforced with silicon carbide (SiC) fibers in a silicon nitride (Si3N4) matrix or carbon fibers in a silicon carbide (SiC) matrix. The lowest wear rates were found for zirconium diboride (ZrB2) composites containing 8-32% aluminum oxide (Al2O3). Proper material selection and microstructure optimization can improve CMC reliability and performance in tribological applications.

1. TRIBOLOGICAL STUDY OF CMCS (CERAMIC
MATRIX COMPOSITES)
Institute : NATIONAL INSTITUTE OF TECHNOLOGY ,ROURKELA
Department: Ceramic Engineering
Course name: CR4102 Tribology Of Materials
Course Instructor : Dr Debasish Sarkar
Assignment Submitted by: Shiba Sankar Dash
Roll no: 118CR0675
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2. Ceramic wear mechanisms
 Brittle solids such as ceramics wear by fracture. When the
applied mechanical stresses exceed the fracture strength of
the solid surface, fracture occurs. The fracture strength is a
complex function of composition, grain boundary energy
release rate, grain fracture energy release rate (the G ratio),
and the presence of defects and residual stresses on the
surface either from machining or sintering. Under mild contact
conditions (defined generally as the mild wear regime) where
the macroscopic contact stresses are below the fracture
strength, the asperity (surface roughness) contact may
exceed the fracture criteria and localized fracture takes place.
This often results in grain boundary cracking and grain pull-
outs. Subsurface cracks can take place at the grain boundary
or through the grains depending on the relative G ratios
(energy release rate in the grain boundary versus the energy
release rate through the grain). Therefore, one of the key
parameters for ceramic wear is surface roughness or third
body dimensions in that they affect directly the mechanical
stress intensity surfaces encountered during wear.
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3.  Since ceramics are insulators, thermal diffusitivity is
relatively low. Under high speed dry sliding conditions,
localized thermal heating can be substantial. T Thermal
shock accelerated wear was observed
 Abrasion and micro fracture are the two dominant wear
mechanisms in the mild wear regime, brittle fracture in
the form of intergranular cracking is observed as the
dominant wear mechanism in the severe wear regime.
Gross fracture in the form of intragranular cracking and
delaminational cracks (large flakes) are seen in the
ultra-severe wear regime. The accelerated cracking can
be attributed to thermal shock stresses under high
speed high load conditions
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4. PROPERTIES GOVERNING WEAR RESISTANCE
 Mechanical – Elastic Modulus(Y) , Hardness(H),
Fracture Toughness(K1C), Coefficient of Friction(µ)
 Thermal- Thermal conductivity(K), Coefficient of
Linear thermal expansion(CTE)
 Tribo Chemical- Flash temperature(Tf)
 Hence before analyzing a tribo pair these set of
data needs to be known so that by tuning these
parameters we can optimize our desired result
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5.  The reason to develop CMCs was to overcome the
problems associated with the conventional structural
ceramics like alumina, silicon carbide, aluminum
nitride, silicon nitride or zirconia – they fracture easily
under mechanical or thermo-mechanical loads because
of cracks initiated by small defects or scratches
 To increase the crack resistance or fracture toughness,
particles were embedded into the matrix phase.
 Ceramic Matrix Composite (CMC)
 They consist of ceramic fibers embedded in a
ceramic matrix
 The fibers unites into a single whole, and distributes
load between fibers, and on the other hand it does not
prevent pull-out of fibers during failure of this material,
increasing the toughness of its breakdown.
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6.  The integration of long multi-strand fibers has drastically
increased the crack resistance, thus increasing the
fracture toughness of the ceramic matrix composite
 The primary function of ceramic fiber is to –
1. Increase the initial stress required for the micro cracks
to propagate through the CMC
2. Also acts as crack bridging by providing compressive
stress at the crack tip, thus increasing the K1C.
 Ceramic wear is a complex function of microstructure,
grain size and shape, grain boundary toughness, and
the operating conditions
 . Wear prediction, therefore, needs to address not only
the amount of wear for a given range of operating
conditions, it also needs to address the location of wear
transitions, and the onset of different dominant wear
mechanisms 6

7.  Advanced ceramics possess a unique combination of
light weight, hardness, and chemical inertness. Thus
they are often used in wear resistant applications. Their
inherently brittle nature, however, creates a concern for
potential premature catastrophic failures. Recent field
success of ceramics in engines (water pumps seals,
cam roller lifters, and wear pads), industrial pumps, and
seals have alleviated many of these concerns. The
strength and fatigue resistance of these materials have
also increased substantially over the last decade. Wear
life prediction, therefore becomes more important in
providing much needed design guidelines for further
application of ceramics.
 Wear is a complex system function. As such, wear of a
material is dependent on contact geometry, surface
roughness, microstructural features, grain sizes, fracture
toughness, speed, load, temperature, duration,
environment, and lubrication. 7

8. ZRB2 WITH AL2O3 AS A TRIBO SUBJECT
 Zirconium diboride (ZrB2) is a transition metal boride
compound that belongs to the UHTC family. It has
higher melting point than carbide or nitride ceramics, as
well as higher thermal conductivities and lower electrical
resistance at ambient temperature.
 The nanohardness of the ZrB2 andAl2O3 are very
similar with mean values of 34.2 GPa and 32.6 GPa,
respectively The Young modulus for ZrB2 is higher with
a mean value of 555.6 GPa in comparison to the Young
modulus of alumina with mean value of473.8 Tribolayer
formation connected with debris origin, oxidation and
tribochemical reactions were characteristic for both
composites with similar chemical composition but
different size and thickness of tribolayers.
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9. Zirconium diboride gains its high-temperature
mechanical stability from the high atomic
defect energies (i.e. the atoms do not deviate easily
from their lattice sites). This means that the
concentration of defects will remain low, even at
high temperatures, preventing failure of the
material.
The lowest coefficient of friction was measured
for ZrB2 + 8%Al2O3with the value of approximately
0.5 during the test at 5 N load
The lowest wear rate, 5.4 × 10−7 was found for the
ZrB2 + 32 %Al2O3:system at 5 N load, while at a
load of 50 N the systems show similar wear rate
with a value approximately1.0 × 10-5 mm3/Nm
.
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10. SILICON CARBIDE MATRIX, REINFORCED WITH
CARBON FIBERS (CF/SIC)
 Due to the exceptionally high hardness( SiC
microhardness is 33.4 Gpa), and abrasion
resistance of silicon carbide, Cf/SiC composites are
one of the most promising contemporary materials
for tribological objects, and mainly for frictional
purposes.
 They can be used as braking disk for heavily-
loaded high-speed automobiles ,aviation
technology ,high speed railway transport etc.
 CMC brake shoe that can withstand extreme
service conditions (heating above 1000°C and
abrasive friction), have good tribochechnical,
physico-mechanical, and thermophysical
characteristics. 10

11.  SiC based CMCs have high thermal conductivity
due to which ,low flash temp ,low oxidation ,low
wear volume . It also facilitates more uniform
distribution of heat fluxes between the surface
elements of a tribo pair. This leads to a reduction in
CMC thermal stresses, which takes care of the
friction operating capacity.
 They have high coefficient of friction with low wear
volume ,low density ,low coefficient of thermal
expansion
 It can be economically prepared through liquid
silicon infiltration technique(LSI process).
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12. DIFFERENT THEORY ON CMC CERMET
INTEREACTION
 The first version assumes that ceramic particles do not
break down during friction. In this case they are
attracted to the CMC surface until deformation
resistance exceeds the adhesive force of surfaces. As a
result of this there is breakdown in the contact plane and
marked weight loss. These particles provide a high
friction coefficient for friction material, although they also
cause relatively high wear both for the friction CMC and
the cermet counterbody itself
 The second version assumes that counterbody hard
inclusions break down during operation. In this case
they are also attracted by the CMC, although they break
down during contact with CMC with fine fragment
formation. As a rule these ceramic particle fragments
are introduced into the fragmented layer of counterbody
material, providing its strength properties and reducing
the degree of wear
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13. SIC FIBERS REINFORCED IN SI3N4 MATRIX
 There is always a fear of abrupt failure of ceramics in
service and to mitigate this the tribological study based
on their mechanical properties needed to be done
 By elimination of pore (process dependant) and
increasing fracture toughness to control crack
growth the reliability can be increased.
 It is observed with increase in incorporated whiskers of
SiC the wear was decreased.
 If the adhesive force at the interface is low the materials
do not wear easily
 Even a material with high hardness will wear off when it
tends to react and adhere at the interface which will
increase both resistance and amount of wear
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14. TRIBO ANALYSIS OF SI3N4 CMC ON STEEL
DISC
 Ceramic pin on metallic disc ,the wear was
decreased with incorporation of SiC whiskers also
the adhesion tendency was selectively towards
Si3N4 matrix.
 The major reason behind decreasing of wear was
not only improved mechanical properties but
reduced adhesion which was initiating fracture at
the junction.
 Tribological anisotropy exist when composite
materials are applied as sliding parts(i.e. the tribo
properties in longitudnal direction is different from
transverse) 14

15.  Silicon carbide due to its high thermal conductivity
diffuses out the thermal flux generated which
reduces the chances of oxidation or thermal stress
at the mating point.
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16. CONCLUSION
 There is always a fear of abrupt failure of ceramics
in service and to mitigate this the tribological study
based on their mechanical properties needed to be
done
 By elimination of pore (process dependant) and
increasing fracture toughness (Fiber reinforcing) to
control crack growth the reliability can be increased.
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