Testing and Characteristic of Ceramic Materials/mechanical properties
‫فرع السيراميك ومواد البناء‬/‫المرحلة الثالثة‬
1-4. Friction, wear and abrasion
The relative displacement, or sliding, of two parts maintained in contact does
not take place without resistance. The tangential force T needed for ensuring the
movement depends on the normal force N at contact: this is the phenomenon of
friction. This phenomenon is well defined by Coulomb formula T = μN, where μ is
the coefficient of friction. Ceramics are excellent materials for applications where
friction is present by virtue of their high hardness, their chemical inertia and their
temperature stability. Their applications include friction parts and seal components
for the automobile industry (sealing items for water pump), gears, valve heads,
precision ball bearings, cutting tools, conduits for casting of refractory metals,
abrasives, etc.
1-4.1 Wear
Wear resistance is the ability of a material to resist mechanical (or chemicalmechanical) abrasion. Two main mechanisms lead to removal of material from the
surface of a ceramic:
- Grain pullout: In polycrystalline ceramics with weak grain boundaries
- Cracking: Fracture due to abrasion, gouging, or erosion
Wear resistance is usually closely associated with hardness and corrosion
resistance. Erosion usually specifies wear of a material by an abrasive in a fluid, the
type of situation encountered when ceramics are polished.
Although ceramics generally characterized as being wear resistant, their
commercial use as wear parts is less than 10% of the overall market. Alumina is the
most commonly used wear-resistant ceramic. One of the early applications was in
seal faces for rotary water pumps for automobiles. Alumina is particularly suitable
because it is resistant to engine cooling fluids. Another application for alumina,
which we mentioned, is in total hip prosthesis. In terms of wear rates at the contact
surfaces between the ball and socket, alumina is far superior to alternative metal and
polymer systems.
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Testing and Characteristic of Ceramic Materials/mechanical properties
1-4-2 Friction and wear
Wear measured at the depth of the friction track by the volume of material
removed per unit length of friction or by volume of unit length and normal force
(specific wear rate). The volume of wear V is proportional to the contact load N and
to the friction distance x. We may therefore account for the wear by using the specific
wear rate defined by:
Vs = V/ (xN)
(1.22)
It is difficult to measure the quantity of wear. It done indirectly by measuring the
decrease in the mass of the specimen, which would also include all mass changes due
to physico-chemical processes or the adherence of debris, or by measuring the
volume of material removed along the friction path by optical or electronic
microscope. The first method gives an overall result with accuracy increasing, as the
specimen size gets smaller.
Following the classic model of friction and wear in the presence of adhesion, it is
possible to relate Vs to the hardness and to the geometry of areas of contact, which
gives a specific wear:
Vs = k/ (3H)
(1.23)
Where H is the hardness of the softer material, the factor k known as the coefficient
of wear. For a conical indenter with half- angle at apex θ, k will be 3cotanθ/π :
Vs = cotanθ/ (πH)
This relationship confirms the observation that harder materials are more wear
resistant. The wear resistance also increases with toughness and, in fact, it appears
that the ratio KIc/H governs it. A brittleness index B has in fact been defined as the
inverse of the ratio: B = H/KIc .It also varies with the sliding speed: in most cases, the
wear rate goes through a maximum for speeds between (0.1 and 10) ms-1.
The atmosphere also plays an important role on friction and wear. In particular,
friction is higher under vacuum and humidity leads to physico-chemical
modifications of the contact surfaces, which clearly reflected in the tribological
behavior (in general μ and Vs increase with humidity).
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Testing and Characteristic of Ceramic Materials/mechanical properties
‫فرع السيراميك ومواد البناء‬/‫المرحلة الثالثة‬
1-4.3. Abrasion
With the increase in load or contact speed between a hard object (diamond
indenter, for example) and a ceramic material, we see the transition from a relatively
soft regime of scratching without removal of material to a plowing regime with
removal of material (mechanical polishing) and finally to a rapid wear regime with
peeling corresponding to mechanical abrasion. Materials with the highest abrasion
resistance used as abrasives and are meant for application in polishing surface
finishing, or in machining (grinding). We then find that diamond, boron nitride
silicon and aluminum carbide are high on the list.
Diamond tools are widely used for shaping grinding wheels. Single tip diamond
tools used to cut Al mirror (high precision). Polycrystalline diamond may be used to
cut aluminum alloys (small business though).
In order to understand the mechanisms, which govern machining by abrasion, we
should analyze the different interactions between the tool and the material being
machined. For a grinding operation, we refer to the following four interactions:
abrasive/material, binder (grinding wheel)/material, abrasive/debris and binder/
debris. This difficult problem is further complicated by physico-chemical processes
linked particularly to the use of “cutting” fluids, which not only acts as a lubricant,
but also condition thermodynamic machining parameters (contact temperature) and
the kinetics of removal of debris.
1-5 Superplasticity:
There are two basic types of superplasticity, termed transformation and
structural superplasticity respectively. (A third type of superplasticity, termed
temperature-cycling superplasticity, refers to the temperature cycling under a small
load of a material, such as uranium or zinc, where there is a high degree of anisotropy
in the coefficients of thermal expansion). Transformation superplasticity refers to the
temperature cycling of a material through a phase change; structural superplasticity
refers to attaining superplastic tensile elongations without a phase change and under
conditions of constant temperature. Although both types may be important in selected
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Testing and Characteristic of Ceramic Materials/mechanical properties
‫فرع السيراميك ومواد البناء‬/‫المرحلة الثالثة‬
ceramics under certain conditions, technological requirements dictate a major
emphasis on the development of ceramics capable of exhibiting structural
superplasticity.
Superplasticity is the ability of a material to sustain very large strains. From our
discussions so far on the mechanical properties of ceramics, the general requirements
are that the grains should be small (typically < 1μm) and Equiaxed.
The mechanism for superplasticity in ceramics must clearly be different from that
in metal alloys because there is no appreciable change in grain shape. Several models
have been proposed. The one illustrated in Figure 1.23 involves grain switching and
accounts for the constancy of grain shape during deformation, but cannot account for
the increase in surface area resulting from plastic deformation. Other models involve
grain boundary sliding, but again do not appear fully account for the process.
Although superplasticity is a useful forming process for metals, it tends not to work
for ceramics because of the problem of cavitation and the requirement of high
temperatures.
FIGURE 1.23 Model showing how grain switching can produce a shape change.
Requirements for superplasticity in ceramic materials: As noted in a recent review
of the high-temperature creep of ceramics the grain sizes of ceramic materials are
invariably substantially smaller than in metals. This is due both to the processing
procedures adopted for the production of polycrystalline ceramics, and to their low
grain boundary mobilities, so it is generally fairly easy to stabilize, and to maintain, a
very fine grain size. The presence of a very small grain size, often < 10μm, and the
difficulties of promoting significant grain growth even at high temperatures, indicate
the potential for superplastic deformation in ceramics. However, this potential
invariably not realized in practice because an additional requirement for
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Testing and Characteristic of Ceramic Materials/mechanical properties
‫فرع السيراميك ومواد البناء‬/‫المرحلة الثالثة‬
superplasticity, not generally important in metals, is a lack of intergranular
brittleness. This latter requirement hinders and usually prevents the attainment of true
superplastic elongations in ceramic materials.
1-5.1 Transformation superplasticity
The occurrence of transformation superplasticity well documented in metals
where it arises from thermally cycling a material through an allotropic phase
transition. In ceramics, evidence for superplastic like behavior provided during the
monoclinic-tetragonal transformation of ZrO2. The results illustrated schematically in
Fig1.24 for Zirconia heating rate of 5°C min-1. Although the precise curve depended
on the specimen density, it observed that the creep deflection increased rapidly at
temperatures above ~ 1160°C in the range of the monoclinic to tetragonal
transformation. When all of the zirconia was in the tetragonal state, at a temperature
of 1205°C there was a short temperature range of ~20°C wherein no further creep
detected.
Figure 1.24 Deflection versus temperature for
ZrO2 tested in three point bending at a
constant heating rate of 5 ° C min-1
1-5.2 Structural Superplasticity
Crystallinity maintained up to the interfaces in metals, but in ceramics, there is
the additional possibility of an intergranular glassy phase. Structural superplasticity
has been reported in ceramics both with and without a glassy phase: With
* With an intergranular glassy phase
There are early indications of superplastic-like behavior in the presence of an
intergranular liquid phase in sintering experiments with ceramics. Liquid-phase
sintering is often used in ceramics because it permits densification at lower
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Testing and Characteristic of Ceramic Materials/mechanical properties
temperatures, thereby minimizing the effect of grain growth, and it is usually easy to
obtain a liquid phase by the addition of alkaline and alumina silicates. The
segregation of a liquid phase to the grain interfaces may enhance the grain boundary
diffusivity by up to several orders of magnitude. In addition, the glassy state is
expected to show ideal Newtonian viscous flow, with m = 1 in Eq. (1.23):
(1.23)
So that the superplastic effect should be optimized. One of the earliest reports of a
superplastic-like behavior is related to the use of a LiF additive for the hot-pressing
of MgO. It well known that LiF acts as a densification aid for magnesia, initially by
acting as a lubricant for the rearrangement of MgO particles and subsequently by
forming a thin liquid film between the grains for pressure-enhanced liquid phase
sintering. Glass ceramics are ideal materials for the attainment of Superplasticity
because they consist of fine-grained crystals in a glassy matrix.
* Without a significant intergranular glassy phase
Metals generally do not contain an intergranular glassy phase, but the situation for
ceramics less clearly defined, it is appropriate to refer to a wide range of ceramics as
nominally single phase even though, due to very low solubility limits, they may
contain small pockets of residual impurities or glassy layers at the grain interfaces. In
this section, either structural superplasticity examined in ceramics where there is no
intergranular glassy phase or, if a glassy phase is present; it tends to be isolated
primarily in pockets at the triple junctions. The very small grain sizes inherent in
many systems suggest that they may be ideal materials for superplastic deformation
but the overall ductilities are generally restricted because of the occurrence of
intergranular brittleness through the nucleation and growth of grain boundary
cavities, in an attempt to identify specific microstructures and strain-rate regimes
where superplastic deformation may be favored. Their analysis shows that cavity
nucleation suppressed in the absence of a grain boundary amorphous phase. It noted
that it would be helpful to identify specific solid-solution additives that lead to rapid
grain-boundary.
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