1-4. Friction, wear and abrasion

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‫فرع السيراميك ومواد البناء‬/‫المرحلة الثالثة‬
Characteristic of Ceramic Materials/mechanical properties
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. The friction causes wear of the two parts at the level of the
contact surfaces. For a couple of given materials, the wear, as well as the value of μ,
depends on the temperature, the atmosphere or even on the sliding speed. This wear
is generally harmful to the in-service behavior of the part and we will look for
operating conditions and surface treatments which will minimize it. It is, however,
desirable in machining by abrasion. 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. There are two main mechanisms that 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 are 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
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‫فرع السيراميك ومواد البناء‬/‫المرحلة الثالثة‬
Characteristic of Ceramic Materials/mechanical properties
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.
1-4-2 Friction and wear
Wear can be 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). Ceramic wear is more of the abrasive type and is
characterized by the generation of debris giving rise to propagation of cracks in type I
and II mixed (opening and straight sliding). It increases drastically above the critical
load. 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 can be 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.
Friction and wear are complex phenomena, whose mechanisms are not well
known. 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 is known as the
coefficient of wear. For a conical indenter with half- angle at apex θ, k will be
3cotanθ/π and Vs = cotanθ/ (πH). This relationship confirms the observation that
harder materials are more wear resistant. The wear resistance also increases with
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‫فرع السيراميك ومواد البناء‬/‫المرحلة الثالثة‬
Characteristic of Ceramic Materials/mechanical properties
toughness and, in fact, it appears that the ratio K Ic/H governs it. A brittleness index B
has in fact been defined as the inverse of the ratio: B = H/KIc .It also varies very
appreciably 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 is clearly reflected in the tribological
behavior (in general μ and Vs increase with humidity). Under friction at high speed
and under small load, and in ambient air, silicon nitride is presently the ceramic
which offers best wear resistance.
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 are used as abrasives and are meant for application in polishing surface
finishing, or in machining (grinding). We then find that diamond, boron nitride (cBN)
silicon and aluminum carbide are high on the list.
Diamond tools are widely used for shaping grinding wheels. Single tip diamond
tools are used to cut Al mirror (high precision). Polycrystalline diamond may be used
to cut aluminum alloys (small business though). Major technical drawbacks of
diamond are its reaction with many metals and its oxidation above 600°C. Problems
occur in particular with cast-iron, titanium, nickel, etc. The grinding of light metals
such as aluminum and Ni-based alloys is difficult. Local burning may occur. As a
matter of fact, although cBN is less hard than diamond, it is more suitable as far as
ferrous alloys are concerned. Three families of abrasive are defined, namely:
– Coated abrasives (suitable for plastics and metals);
– Bonded abrasives (suitable for hard metals and ceramics);
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‫فرع السيراميك ومواد البناء‬/‫المرحلة الثالثة‬
Characteristic of Ceramic Materials/mechanical properties
– Super abrasives (suitable for hard ceramics and crystals).
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.
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