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Interfaces
Interfaces
1
Tribology and Interfaces
Tribology deals with materials in contact. The contacts can be solid-solid or solid-fluid. In
a typical tribological system, at a contact region, materials with different properties and
characteristics meet. Therefore, the contact region is an interface between phases, and
interphase interface. The interfacial region is essentially heterogeneous and its properties
vary sharply in the normal direction.
As the free surfaces of two materials come in close proximity of each other, atoms in one
material begin to experience the electronic environment of the other material. The result is
often adhesive interaction. Adhesion is somewhat analogous to internal cohesion of atoms
in the bulk, except that it is the result of interatomic interactions between atoms at or near
the free surfaces of separate materials. Adhesion between solids can result in the absence of
intervening medium (e.g. solid surfaces in a vacuum environment) or through the agency of
an intervening fluid phase (e.g. liquid mediated adhesion). Liquids can adhere and spread
onto solid surfaces (like water on a hydrophilic surface) or be repelled by them (like water
on a hydrophobic surface).
2
Atomic Theory
The atomic model of matter assumes all matter is made of a limited number of atomic
species that can combine in a large number of different arrangements. In the simplest
picture, atoms consist of relatively large nuclei and small electrons. Nuclei in turn consist of
positively charged nuclei and uncharged neutrons. Electrons carry a unit of negative charge
and are said to orbit around the nucleus. The outermost, highest energy (valence) electrons
in atoms are the ones directly involved in chemical reactions.
According to quantum mechanics, electrons posses dual particle-wave characteristics and
their energies are solutions of the Schroedinger equation.
The periodic table of the elements summarizes nicely key aspects of the electronic structure of all known atomic species.
1
Most elements can be exist in the pure, uncombined state under standard conditions of
temperature and pressure. Elemental substances can then be gaseous, liquid or solid under
such conditions. A characteristic feature of solid materials is that in many cases, their atoms
or molecules arrange in space in the form of relatively simple repetitive patterns. When that
is the case one refers to the material as a crystalline solid. However, sometimes the atomic
arrangement lacks long range periodicity and it becomes more liquid-like. Solids with these
characteristics are called amorphous.
A large number of elements have metallic character. A simple model of the electronic
structure of metals envisions that the valence electrons are collectively shared by the cores
formed by the atomic nuclei and the lower energy electrons. Because of the existence of
relatively free high energy electrons, metals are generally good conductors of electricity.
The chemical reactivity of individual atomic species is described by their electronic affinity. Electronegative atoms have strong affinity for additional electrons (e.g. Cl) while electropositive atoms are electron donors (e.g. Na). Molecules are easily formed by combination
of electropositive and electronegative atoms through electron exchange. The materials produced by aggregation of these molecules are generally electrically insulating. Most ceramic
materials belong to this group.
Some elements and compounds are said to posses semi-conducting character. This means
that under normal conditions, they act as insulators but under suitable excitation, their most
energetic electrons can become free enough to conduct electricity.
3
Atomic Structure of Surfaces
A solid surface represent an relatively abrupt interruption of the atomic arrangements prevailing in the bulk. In the case of crystalline solids, this interruption creates distortions in
the arrangement of atoms at the surface but some of the characteristic periodicity of the
bulk material is still preserved. However, the electronic structure of atoms in the surface
region is quite different to that of atoms in the bulk since a whole set of atomic neighbors is
missing on one side of the surface.
Two key atomic rearrangement processes are surface relaxation and surface reconstruction. In surface relaxation, there is a slight contraction of the interatomic spacing between
the first (surface) and second (subsurface) layer of atoms. In surface reconstruction there
are changes in the interatomic spacing of atoms at the surface
Since materials that form stable bulk solids under standard conditions do not spontaneously subdivide into smaller particles, it is obvious that energy is required to create surfaces
(i.e. to convert bulk atoms into surface atoms). The energy required to create a unit area
of surface from bulk matter is called the surface energy. In liquids, the surface energy is
entirely associated with the process of transporting atoms from the bulk towards surface
sites. In solids, besides the energy required to transport atoms from the bulk to the surface,
2
additional energy may be required to stretch the atomic bonds of surface atoms. However,
the surface energy of a solid is directly related to the number and strength of interatomic
bonds that are broken when the surface is created.
Density Functional Theory (DFT) is a modern computational technique specially useful
for the determination of the electronic structure of solids and their surfaces.
A useful model of metal surfaces is the so called Jellium model that considers the metal as
abruptly terminated at the surface and the free electrons spilling somewhat out of the surface.
Simple metals and transition metals. Cohesion. Relationship between surface energy and
bulk cohesion.
4
Solid-Gas Interfaces
Because of its importance in many areas of science and industry, a great deal of research
effort has been devoted to the investigation of the chemical processes that take place on
solid surfaces. Fundamental studies have focused on the investigation of processes taking
place on relatively clean and well characterized (low index, single crystal) surfaces. Two key
phenomena that have been the subject of study are the physical adsorption of atomic species
on surface sites (predominantly by weak, van der Waals type bonding and the formation of
more stable surface layers produced by stronger chemical bonding (chemisorption).
A third, very important area of study is the formation of macroscopic layers on surfaces.
A good examples of this is the formation of oxide layers by high temperature reaction with
oxygen.
Clean solid surfaces exhibit enhanced chemical reactivity and it is therefore very difficult
to maintain the surface in the clean state once formed. The chemical composition of most
real solid surfaces differs sometimes significantly from that in the bulk.
5
Metal-Electrolyte Interfaces
An electrolyte is a liquid containing positively and negatively charges ions. Electrolytes are
widely found in nature as many natural substances readily dissolve in water forming ions in
solution (e.g. NaCl in H2 O).
When metals are exposed to an electrolyte concurrent electron and ion exchange reactions
with the electrolyte are common. Such reactions may involve the conversion of metal atoms
into metallic ions that are then transported into the electrolyte (leaving behind electrons
in the bulk metal) and the conversion of ions from the electrolyte into other species by
consumption of the electrons generated in the ion formation reaction.
Ion formation reactions are called oxidation reactions (since the metal atom gains positive
charge) while electron consumption reactions are called reduction reactions.
3
A metal-electrolyte interface is usually called an electrode. At any electrode, if oxidation
and reduction reactions take place at the same rate the system is said to be in electrochemical
equilibrium.
If the electrical potential of such an electrode is measured with the help of another,
suitably designed reference electrode, the resulting quantity is called the equilibrium electrochemical potential of the electrode. By constructing a sequence of electrodes using different
metals in the same electrolyte, the electrochemical series of equilibrium electrochemical potentials can be constructed.
When two distinct electrodes are electrically connected a phenomenon know as electrode
polarization takes place. Polarization consists in the shifting of the electrochemical potential
at both electrodes. The potential of the electrode with the lowest value of equilibrium
potential shifts upward while that of the other electrode shifts downward.
The positivitely polarized electrode is called the anode and the negatively polarized
electrode is called the cathode. At the anode, the ion formation (oxidation) reaction become
dominant and at the cathode the electron consumption process dominates. The net result
of such process is often the dissolution of the anode material and the formation of reduction
species at the cathode. This phenomenon is called corrosion.
6
Capillarity Phenomena
As mentioned before, the electronic state of atoms on material surfaces is drastically different
from that of atoms in the bulk. An atom on a surface has fewer nearest neighbors and excess
unsaturated bonding compared to one in the bulk. The net result is free surface energy.
The effect of this free surface energy is readily observed at liquid-gas interfaces as a broad
collection of capillary phenomena. Specifically, atoms at the surface experience a pull towards
the interior and this creates a tendency for the interface to contract. Atoms in fluids have
relatively high mobilities. Hence when such an interface is stretched, atoms from the bulk
move to the surface and when the interface contracts, atoms move from the surface into the
bulk. This process takes place rather quickly such that a state of mechanical equilibrium is
reached practically instantly. L-G interfaces spontaneously contract until they contain the
maximum volume of bulk material possible per unit area of interface. Work must be done
in order to extend a L-G interface. The work done per unit area of new interface is the free
surface energy.
The free surface energy in L-G interfaces is also known as the surface tension. The reason
for this is readily understood from the results of a few simple experiments. Consider a thin
layer of fluid (e.g. a soap film) in the shape of a square membrane of side L surrounded by
a gas. If a force F is applied normal to one edge of the square membrane in order to stretch
it by an amount dx, the energy spent in stretching the L-G interface is given by
dG = γdA = γ = Ldx
4
where γ = Gs = dG/dA is the work done in stretching the interface a unit area (units J/m2).
The quantity γ can also be interpreted as the force per unit length that must be applied to
stretch the L-G interface, i.e.
F
L
γ=
in units of N/m.
As a second example, consider now the thin liquid film has the shape of a closed surface
(e.g. a soap bubble). The equilibrium radius r of the bubble is the result of a balance
between surface tension and the pressure drop across the film. The total free energy of the
surface (G = 4πr2 γ) would be reduced by the amount dG = 8πrγdr if the bubble radius
were to decrease by dr. However, this would require the performance of work against the
pressure difference across the film ∆P in the amount ∆P dV = ∆P 4πr2dr, therefore
∆P =
2γ
r
This important result is known as the Young-Laplace formula and it indicates that rather
large pressure differences can be expected at equilibrium across films in tiny soap bubbles.
A simple method for the approximate determination of the value of γ consists in measuring the weight of a drop ejected from a capillary tube. If the radius of the tube is r and
the measured mass of one drop is m, the surface tension is approximately given by
γ=
mg
2πr
The phenomenon of adhesion is intimately connected to surface energy. Consider two
different liquids 1 and 2 exposed to a gas phase. The corresponding surface tensions of the
L-G interfaces are γ1 and γ2 , respectively. Imagine now that the free surfaces of the two
liquids are brought in contact so as to form a liquid-liquid interface with surface tension γ12
(and thus replacing the interfaces formed between the individual liquids and the gas phase).
If one were then to separate the two liquids, the amount of energy involved would be given
by
W = γ1 + γ2 − γ12
This is known as the work of adhesion and is known as Dupre’s formula. Note that if the
value of γ12 is relatively small (i.e. atoms at the 1−2 interface experience only a weak inward
pull into the bulk phases 1 and 2), W12 would be positive, i.e. work will have to be spent
in separating the phases since they adhere to each other. On the other hand, if the value
of γ12 is relatively large, the liquid phases repel and useful work could be obtained from the
system as they do so.
5
7
Surface Energy in Solids and Solid-Liquid Interfaces
As is the case with liquids, atoms at the surface of a solid have fewer neighbors than those in
the interior. As a result, there is an excess energy at the surface, namely, the surface energy
γ . Surface energies of solids in contact with their vapor are of the order of 0.5 to 2 J/m2
while those of liquids range between 0.05 and 0.2 J/m2 .
In solids, the mobilities of atoms are significantly less. As a result, when an interface is
pulled in the tangential direction, is more difficult for atoms from the bulk to move rapidly
to the surface. As a result, the interatomic bonds of the surface atoms are stretched and the
surface energy of a solid surface will vary with the amount of deformation of the surface and
will be given by
Gs = γ + (
∂G
)
∂A
An important situation is what happens at the line where a liquid, a gas, and a solid
phase meet. Such is the case obtained when a liquid droplet is deposited onto a solid surface
exposed to air. Depending on the materials involved, the droplet will spread for some distance
until it acquires an equilibrium shape determined by the angle of contact θ (sessile drop).
Denoting vapor, liquid and solid by V , L and S, and regarding the corresponding interfacial
free energies as surface tensions acting tangentially to the corresponding interfaces, a balance
of force along the horizontal direction yields
γSV = γSL + γLV cos θ
If this is combined with Dupre’s equation expressing the solid-liquid work of adhesion WSL
one obtains
WSL = γSV + γLV − γSL = γLV (1 + cos θ)
This indicates that the contact angle is directly related to the strength of the adhesion
between liquid and solid and suggests that contact angle measurements can be used to
estimate adhesion energies.
8
Adhesion Coefficient
When two solids are pushed together by a force Fp often they tend to remain attached to
each other when the load is removed and a separating force, the adhesive force Fa, is required
to pull them apart despite elastic recovery. The coefficient of adhesion fa between the two
bodies is defined as
6
fa =
Fa
Fp
Factors affecting the strength of the adhesive bond include
• the chemical compatibility of the materials
• the presence of intervening film(s) between the two solids
• the crystallographic orientation of the surfaces
• the temperature of the system
• the roughness of the surfaces
9
Material Compatibilities and Energy of Adhesion
If two materials, 1 and 2 with surface energies γ1 and γ2 , respectively, are pushed together
to form an interface 1 − 2 with interfacial energy γ12, the work of adhesion Wad is defined as
Wad = ∆γ = γ1 + γ2 − γ12
Note that if the value of γ12 is relatively small (i.e. atoms at the 1 − 2 interface experience
only a weak inward pull into the bulk phases 1 and 2), W12 would be positive.
Since interfacial energies γ12 are often difficult to estimate or measure it is convenient to
write the adhesion energy as
Wad = cm (γ1 + γ2 )
where cm is the compatibility parameter describing the extent of the affinity between the two
materials.
Values of the compatibility parameter depend on the extent of the affinity between the
two materials involved. Specifically, in the case of metals, greater compatibility is associated
with increased solubility as indicated by the phase diagram. A semiquantitative classification
is given in the following table
7
Contact
Identical Metals
Identical Nonmetals
Compatible Metals
Compatible Nonmetals
Partially Compatible Metals
Partially Incompatible Metals
Incompatible Metals
Incompatible Nonmetals
Other Solids
Compatibility Parameter cm
1.0
1.0
0.5
0.6
0.32
0.20
0.12
0.36
0.22
Compatibility coefficients are often found to be directly proportional to the ratio of the
adhesion energy over the hardness of the softer material Wad /H.
10
Adhesion and Solid-Solid Contact
Adhesion forces influence the geometry of contacts leading to non-Hertzian behavior even in
the elastic case. Consider a smooth sphere of with radius R (material 1) which is brought into
elastic contact with a smooth, flat, rigid surface (material 2) under the action of a load L.
If there were no adhesion, the sphere would initially touch the flat surface at a single point.
However, adhesion forces pull the surface atoms of the sphere that are in close proximity to
the contact point towards the flat surface and a contact area (radius a) is established even
if the applied normal load is zero.
Johnson-Kendall-Roberts developed a model of contact with adhesion (JKR model).
They first assumed that the contact pressure p was expressible as the sum of a pressure
distribution producing uniform displacement and the Hertzian distribution, i.e.
p = p0,1(1 −
r2 1/2
r2 −1/2
)
+
p
(1
−
)
0,2
a2
a2
Assuming linear elastic behavior, the principle of superposition yields the resulting displacement due to p as
1
r2
πa
uz = ∗ [p0,1 + p0,2 (1 − 2 )]
E
2
2a
The surface displacement is also related to the far field displacement (or penetration distance)
δ by
uz = δ −
8
r2
2R
Solving the above for p0 and p1 yields
2aE ∗
s πR
∗
a
γE ∗
E δ
( − )=2
=
π a R
πa
p0,1 =
p0,2
where γ is the energy of the contact interface and, as before
1 − ν12 1 − ν22
1
=
+
E∗
E1
E2
is the contact modulus and
1
1
1
=
+
R
R1 R2
is the relative or reduced radius
Now, the total energy of the system UT , consists of an elastic and an adhesive component,
i.e.
UT = Uel + Uad =
1
2
Z Z
puz dxdy + γπa2
The equilibrium contact radius is the one that minimizes the total energy, i.e. ∂Ut/∂a = 0.
This yields the following relationship between δ and a:
a2
±
δ=
R
s
2γπa
E∗
Furthermore, the force on the sphere is given as
F =−
∂UT
2 a3
dUT
=−
= E ∗ [2δa −
]
dδ
∂δ
3R
since ∂UT /∂a = 0. Introducing the expression obtained above for δ finally yields
F =
4E ∗ a3 q
− 8γπE ∗a3
3R
Note that this reduces to the Hertz formula when γ = 0. This force has a minimum for the
critical contact radius ac , given by
ac = (
9 γπR2 1/3
)
8 E∗
9
and the minimum value is
3
Fmin = − πRγ
2
This is called the adhesive force since it is the force required to pull the sphere apart from
the flat surface.
Alternatively, in the Deryagin-Muller-Toporov (DMT) model the contact is assumed to
remain Hertzian and that the adhesive forces act just outside the contact area. According to
the DMT model the force of interaction of the non-deformed sphere which is making contact
at the surface just at one point is
F0 = 2πRψa
where ψa is the specific energy of adhesion, obtained when the sphere is replaced by a half
space of the same material and is then allowed to come to equilibrium with the flat surface
at an equilibrium separation ǫ of a fraction of a nanometer. Under load, a contact area forms
with radius
√
a = αR
where α is the approach from the center of the sphere to the surface under loading. The
resulting pressing force Fh is the sum of the externally applied load L and the molecular
interaction of the particles of the sphere with the substrate, the adhesion force of molecular
interaction Fa which is in turn given by
dW
= πRψa + 2π
Fa =
dα
Z
0
∞
dψ ∂H(x, α)
xdx
dH
∂α
where W is the total energy of molecular interaction, x = (r2 − α2 )1/2, and H is the distance
between elements of the surfaces of the sphere and the substrate, i.e.
v
u
Moreover,
q
u r2
1
H(r, α) =
[ra r2 − ra2 − (2ra2 − r2 ) tan−1 (t 2 − 1)] + ǫ
πR
ra
8 ψa ǫ 3
ǫ
dψ
=
[( ) − ( )9 ]
dH
3 ǫ H
H
inspired on the well known Lennard-Jones interaction potential. With the above one can
then calculate the adhesion force.
The DMT is most appropriate when dealing with hard solids (small deformations) while
the JKR model is best for soft solids.
10
11
Liquid Mediated Adhesion
Thin intervening liquid films often increase adhesion of solids because of the effect of capillary forces. The basis for this behavior lies on Laplace’s equation which gives the pressure
difference ∆P across a curved liquid-vapor interface as
∆P =
2γ
r
where γ is the surface tension of the liquid and r is the radius of curvature of the interface.
Consider now two large solid cylinders of materials 1 and 2 and radii R. The cylinders
are joined at their bases with an intervening liquid phase of thickness h and surface energy
γl . The liquid film forms a meniscus at the outer rim of the gap between the cylinders. Let
the radius of the meniscus be r and the contact angles of the liquid film with solids 1 and 2
be α and β, respectively. The resulting adhesive (meniscus) force is
Fa ≈
πR2 γ
πR2γ
=
r
h/(cos(α) + cos(β))
The term stiction is sometimes used to describe the adhesive effect associated with thin
intervening liquid films. A common example is the interaction between head and disk in
magnetic recording systems. At high ambient humidities thin water films (up to 50 nm in
thickness) form on the surface of magnetic media. If the film is discontinuous, it will form
localized islands (toe-dipping or pillbox regimes) and the stiction will be low. When the gap
is immersed in liquid, the radius of curvature of the meniscus is low and the stiction is low
again. The stiction is maximum when the gap is just entirely filled with a liquid film having
a small radius of curvature at the rim.
When one of the contacting surfaces is rough and there are N peaks of mean radius Rp in
the nominal contact area with an average gap d and a liquid film of thickness h < d, surface
tension γ and contact angle θ the meniscus force Fa is given by
Fa = 2πRp γ(1 + cos θ)N
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