1) The document discusses the tribological study of metal matrix composites (MMCs), which are engineered materials made by embedding reinforcements into metal matrices. 2) It outlines several intrinsic and extrinsic factors that influence the wear and friction properties of MMCs, including the type, size, shape, and volume fraction of reinforcements used, as well as external parameters like normal load, sliding speed, and temperature. 3) The optimal properties of MMCs depend on selecting reinforcements and processing conditions that improve the composite's strength, hardness, and load-bearing ability while lowering friction and wear rates under different tribological stresses.

1. TRIBOLOGICAL STUDY OF
Metal Matrix Composite
Course- CR-4102 – TRIBOLOGY OF MATERIALS
Presented by Nambari Ghanshyam (118CR0676)
Course Mentor: Prof. Debasish Sarkar
Department of Ceramic Engineering

2. Introduction
 Metal matrix composites (MMC) are an engineered blend of two or more materials in which
specialized characteristics are generated by taking the advantages of both reinforcement and
metallic matrix, offering us a high degree of material design flexibility.
 Metal matrix composites (MMCs) are gradually replacing conventional materials in the
automotive, aircraft, marine, and sports sectors owing to its low weight and excellent mechanical
properties.
 Nonmetallic materials are inserted as reinforcements into metals or alloys to create a unique
material with desirable technical qualities such increased ultimate tensile strength, ductility,
toughness, and Tribological behaviour.
 Three critical parameters that determine MMC performance -
1. The composition and the microstructure of MMCs.
2. The size, volume fraction, and distribution of particles in metal matrices.
3. The properties of the interface between the metal matrices and the reinforcements.
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3. Basic Tribological parameters that
causes Wear and Friction in MMCs
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Physical factors
Material factors
Type of Reinforcement
Mechanical factors
Size of Reinforcement
Shape of Reinforcement
Volume fraction of
Reinforcement
Microstructure of matrix
Normal Load
Sliding velocity
Sliding distance
Temperature
Other Environmental conditions

4. Intrinsic Factors
Let’s start with the first set of slides

5. Type of Reinforcement
 To enhance tribological behaviour of metals suitable reinforcements are embedded into
metal matrices to produce composites. Some typical reinforcements, such as Al2O3, SiC, and
B4C, increase the composite's friction coefficient and wear resistance. Other solid lubricants,
such as graphite, molybdenum disulfide, hexagonal boron nitride and others, are used as
reinforcements. Soft particles in the matrix lower the friction coefficient of MMCs and
increase wear resistance, whereas hard particles in the matrix increase composite strength
and wear resistance.
 Engineered materials reinforced using a mix of two or more distinct types of reinforcements
are known as hybrid metal matrix composites (HMMCs). The wear rate of hybrid composites
is lower than that of composites strengthened by only one form of reinforcement.
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6. Reinforcement size
 The strength enhancement of the composites induced by embedding the particles into the
matrix causes MMC materials to have a lower wear rate than unreinforced alloys. In
compared to unreinforced alloys, the particles operate as load-bearing elements in
composite materials, increasing the wear resistance of metal matrices reinforced by particles
with different sizes.
 Metal matrix nano-composites (MMNCs) exhibit better mechanical and tribological
characteristics than micron-sized particle-reinforced MMCs. When compared to micron-sized
particles, nanoparticles have greater surface area than volume.
 The characteristics of MMNCs are significantly influenced by two elements. They are
(a) nanoparticle dispersion in the metal matrix
(b) nanoparticle deagglomeration.
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7.  The wear rate of composites changes dramatically when the particle size of reinforcements
reaches a threshold level. The wear rate of aluminium matrix composites enhanced by SiC
particles (2–167 µm) is shown in the graph above. The composites' wear rate reduces with
increasing particle size up to 20 µm, then increases with increasing particle size up to 167 m. A
crucial particle size has been shown to be significant in determining the wear behaviour of
composites. The 20 µm SiC particle size is the key size for this composite in order to get the
lowest wear rate.
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8.  The friction coefficient of the micro-composite reinforced by micron-size particles of varying
sizes increases with increasing particle size of the reinforcements, as shown in the above
figure. However, it is difficult to predict the wear behaviour of the micro-composite reinforced
by micron-size particles of varying sizes.
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9. Shape of Reinforcement
 Consider the following scenario: Aluminum matrix composites reinforced with alumina fibres
only prevent adhesion and seizure at higher pressures, such as those over 1 MPa. When
compared to Al/Saffil composites, the wear rate of aluminium matrix composites reinforced
with alumina fibres and graphite (hybrid composites) was reduced. The wear rate of hybrid
composites is also affected by the form of the graphite. The graphite fibres were shown to
be more effective than the graphite flakes in the experiments. At low volume fractions of
alumina fibre in the composites and at high pressure, this difference is very noticeable.
Furthermore, the orientation of the alumina fibres has an impact on the composites' wear
behaviour, particularly at higher pressures. The wear rate of parallel fibres to the friction
surface is lower than that of perpendicular fibres. The wear rate of parallel fibres to the
friction surface is lower than that of perpendicular fibres.
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11. Volume fraction of Reinforcement
 In general, increasing the reinforcing content of composite materials improves their wear
resistance. This is owing to the fact that the mechanical qualities of MMCs, such as hardness
and strength, improve as the volume percentage of reinforcements increases. The hard
second phase particles protect the metal matrix from wear during sliding. These second
phase particles can get dislodged from the matrix at times, and the wear rate of the matrix
material rises as a result of the abrasion of the second phase particles. The metal matrix will
be protected to the greatest degree possible at a particular critical volume percent of the
reinforcement.
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 The wear rate of aluminium MMC reinforced with
micron-sized SiC particles as a function of SiC
particle volume % under various normal loads. It
can be seen that when the volume proportion of
SiC particles rises, the composite's wear rate
drops linearly.

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 Weight loss is reduced by increasing the volume fraction of SiC particles in the
aluminium alloy (Al-1080) reinforced by 10–50% volume fraction of SiC with
particle size of 20 µm. The weight loss is greatly reduced till 20 percent of the
reinforcement is applied. However, as the volume percent of particles increases
over 20%, the weight loss becomes insignificant.

13. Extrinsic Factors
Let’s start with the next set of slides

14. Normal Load
 Fracture of the reinforcing particles in the matrix may occur as the applied normal load
increases, especially at higher normal loads. As a result, the composite's mechanical
characteristics deteriorate, and the composite's wear rate rises to values equivalent to those
of an unreinforced metallic matrix. The contacting asperities are plastically deformed when
the surfaces are subjected to a normal load, and the RAC grows with increasing normal
stress and approaches the amount of AAC.
 When the RAC between two surfaces is increased, the wear rate and temperature between
the two surfaces normally increases when sliding. Because of the rise in the RAC, there is a
critical normal load at which significant wear develops. It becomes extremely difficult for two
touching surfaces to glide over each other when the RAC rises and approaches AAC. Seizure
is the term used in tribology to describe this scenario. Both unreinforced and reinforced
composites experience seizure. A rapid rise in wear rate, significant loudness, and vibration
are typically observed during a seizure.
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15. Sliding speed
 The wear rate of the MMCs increases as the sliding speed increases, and the wear
resistance of the MMCs decreases. The interface temperature rises as the sliding speed
rises, resulting in -
(a) micro-thermal softening of the matrix material,
(b) oxide development on the contact region,
(c) a decrease in the material's flow stress.
 In addition, changes in the microstructure, such as precipitate dissolution, may occur,
which will be reflected in the wear behaviour. Microthermal softening, oxidation, and
flow stress for various materials occur at different temperatures because the physical
and mechanical characteristics of different matrices in different composites are not the
same. As a result, the sliding speed of composites with various matrices may have
distinct impacts.
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 The wear rate increases as the sliding speed increases, especially at higher
sliding speed ranges. For example, at lower sliding speeds, Al–Fe and Al–SiC
composites exhibit a decrease in wear rate; yet, at higher sliding speeds, these
composites show a rapid rise in wear rate.

17. Effect of Temperature
 MMCs' wear rate lowers at first as the temperature rises, but rises again as the
temperature rises. The wear resistance of composites can be improved by increasing the
thermal conductivity of the reinforcements in the composite. The wear rate and coefficient
of friction of MMCs are affected by a critical temperature. The wear rate and coefficient of
friction are more or less constant below this crucial temperature, or decrease
insignificantly with rising temperature in some circumstances. With increasing temperature
above this critical temperature, the wear rate and coefficient of friction increase
significantly. At high temperatures, mechanical characteristics of materials, such as
hardness, alter, resulting in these variances.
 When compared to monolithic alloys, the critical temperature of MMCs is often greater.
Temperature has a different influence on the friction coefficient of self-lubricating MMCs
than it does on the wear rate of these composites. In general, when the temperature rises,
the coefficient of friction of the MMCs decreases.
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18. THANK YOU !
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