5690
J. Phys. Chem. 1992, 96, 5690-5701
FEATURE ARTICLE Chemical Aspects of Ceramic Tribology
T.E.Fischer* Department of Materials Science and Engineering, Stevens Institute of Technology, Hoboken, New Jersey 07030
and W. M. Mullins School of Materials Engineering, Purdue University, West Lufayette, Indiana 47907 (Received: February I, 1992)
Chemical interactions with ambient gases and with lubricant liquids have been found to influence friction forces and wear rates of ceramics and to determine the mechanisms by which these materials wear. In the presence of water vapor, for example, tribochemical reactions (which are accelerated by simultaneousfriction) produce smooth surfaces and decrease wear by several orders of magnitude in silicon nitride and chemisorption embrittlementincreases wear rates of zirconia. Hydrocarbon lubricants produce similar effects, decreasing wear by surface reaction in some cases and increasing it by grain boundary attack in others. These interactions can be understood in terms of the surface chemistry of ceramics, which is dominated by charge transfer with ambients through acid-base reactions. The surface chemistry of ceramics is reviewed and related to their electronic structure. Tribochemical maps are proposed for the interaction with water and selected hydrocarbons by comparing the electronic structure of the ceramics and fluids. The ability to generalize the behavior of lubricants on ceramics by the use of these maps indicates an improved understanding of the lubricant-surface interaction and provides a tool for the development of new lubricant systems.
1. Introduction The development of high-performance ceramics is stimulating major advances in a large spectrum of technologies that include machine design, integrated circuits, manufacturing techniques, and process engineering. In contrast to traditional ceramics, which are natural and complex mixtures of diverse oxides and carbides, the high-performance ceramics are synthesized as relatively pure materials. Representative examples are alumina (AI*O3),silicon nitride (Si3N4),silicon carbide (Sic), and zirconia (ZrOz). The technological interest of these materials resides in their unique mechanical, electrical, and chemical properties. The covalent-ionic bonding, for instance,gives the materials their well-known hardness which is maintained to elevated temperatures. The chemical properties of ceramics show equally interesting differences from those of metals: oxide ceramics are an obvious choice for hightemperature service, where metals deteriorate by oxidation; in addition, most ceramics are remarkably resistant to corrosion by acids. The present paper concerns itself with the tribological properties of ceramics. Tribology is the science of friction, lubrication, and wear. The hardness, low density, and high-temperature mechanical stability of ceramics makes them natural candidates for applications where solids rub against each other in situations inaccessible to present lubrication technology. We can cite as examples the cutting tools for fast machining, magnetic storage where the distance between magnetic head and storage medium must be a fraction of a micrometer, bearings or engine parts operating at very high or low temperatures as in minimally cooled diesel engines or rocket fuel pumps, and valve lifter cams in engines that experience high wear because the relative motions of the rubbing surfaces exclude, rather than pull in, the lubricant. Other mechanical applications utilize the low density or the high elastic stiffness of ceramics; this is the case in ball bearings that operate at very high speeds where centrifugal forces would destroy the much heavier steel elements or in high-precision machinery that must minimize elastic deformation. The chemical properties of
0022-365419212096-5690$03.00/0
ceramics are of advantage in bearings or seals that operate in water, steam, or chemically aggressive environments or under conditions that prevent the use of lubricants, such as in food processors. Quite early in the exploration of ceramics for tribological service, it was found that the chemical environment has a major influence on the friction of these materials in lubricated and unlubricated contact and on their wear resistance.'-3 An increase in atmospheric humidity, for instance, can decrease the wear rate of one ceramic by as much as 2 orders of magnitude2s4or increase the wear of another by similar amounts.5 Obviously, this behavior is related to the chemical properties of ceramics and represents an interaction of mechanical and chemical processes. This interaction takes several forms, depending on the material, the environment, and the mechanical parameters. It can consist in the formation of surface coatings that decrease wear: in a purely chemical form of wear (by dissolution in the liquid environment6,'), in chemisorption and boundary lubrication effectiveness of hydrocarbons and other molecule^,^^^ and in chemically induced fracture that increases wear rates5 The rates of these chemical reactions are very much influenced by simultaneous friction; usually, the reactions proceed at room temperature with rates that, in the absence of friction, exist only above 800 K. Because of this interaction of friction and chemistry, these reactions are called "tribochemi~al".~~'~ They have been observed in all materials subjected to friction,' ' - I 7 but they are particularly pronounced in the tribological behavior of ceramics. Their understanding is a prerequisite for the successful application of these materials in tribological service and can form a rational basis for the development of synthetic lubricants or lubricant additives suited for them. In section 2, we will describe the observed chemical effects on the friction and wear of ceramic, section 3 is a broad review of the physical chemistry of ceramic surfaces, and in section 4 we shall, where it is already possible, discuss the connection between the two. In order to facilitate the reading, we shall first review 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, NO. 14, 1992 5691
Feature Article
a’
w
F m 1. F’rincipleof friction. Atom A of the stationary body is attached to its neighbors by chemical bonds. Atoms B and C belong to the moving body. (a) This is assumed to be the position of minimum energy with closest approach of atoms A and B. (b) After a short distance of sliding, bond A-B is stretched, atom A is pulled away from its equilibrium position by elastic deformation, and the energy increases. (c) Atom A, in its displaced position, is midway between atoms B and C; bond A-B is about to be ruptured. (d) Atom A vibrates about is equilibrium position in regard to atom C by virtue of A-C attraction and release of elastic stretch energy. This vibration is dissipated into body by phonon emission. the principles of tribology and the properties of ceramics that will be used in the later text. Principles of Tribology.’* During the relative motion of two solids pressed against each other by a normal force F,, one observes a force Fd that tends to oppose the movement. Associated with this force of friction is the observed breakdown or wear of the two solids. The measurement of friction and wear is quite simple. The tribometer consists of a moving and a stationary surface pressed against each other. One measures the friction force, and after a chosen time or distance of sliding, one measures the amount of material removed, either by weight change or by volume loss. The latter method is preferred in ceramics because water adsorption on porous materials causes appreciable changes in total mass. The wear rate is usually expressed in cubic millimeters removed per newton load and meter sliding (mm3/Nm). The linear relationship between these quantities implied by this definition is not generally obeyed by ceramics for which this wear rate is not a constant and should be used only for purposes of comparison. Information about the operating wear mechanisms is obtained most often by observation of the wear scars in the scanning electron microscope. Scanning transmission mimosc~py and optical or electronic spectroscopies can be used to observe defects, dislocations, crystal structures, or chemical reaction products. With few exceptions, the friction force does not depend on the apparent contact area A, nor on the sliding velocity but is proportional to the normal force F, usually called “the load”. For this reason, one defines a friction coefficient IJ
= FdFn
(1)
In the absence of lubricant, the friction coefficient varies between 0.2 and 1 in most practical cases. It is lower in the case of diamond and usually higher than one for metals sliding in vacuum. The origin of the friction coefficient is complex and still quite poorly understood. For rough surfaces, plowing of one surface by the hard asperities of the other and other wear processes can make a contribution, but closer examination shows that the wear processes account for less than 10%of the friction energy spent; the latter is almost completely transformed into heat. Systems that approach ideally flat and parallel surfaces do not exhibit lower friction. The real origin of friction lies in the second law of thermodynamics. Consider an atom A subjected to the moving periodic potential describing its interaction with the other, sliding body (Figure 1). As the closest atom B of the counterface moves away, atom A is pulled along and causes a stretch of its bonding to its own substrate. With further sliding, the bond to atom B breaks and
atom A is pulled in the reverse direction by its attraction to atom C. The accumulated elastic energy and the attraction to atom C combine to accelerate a atom backward, With its considerable energy, the atom vibrates in the potential minimum and emits phonons which propagate in the body and add to the thermal vibration energy. The connection of the sliding atoms to their environment provides for the transformation of translational to thermal energy. Obviously, a lowering of the adhesive energy decreases the friction force. Such a decrease, caused by adsorption of molecules from the ambient, explains why friction is higher in vacuo than in air. A reduction of the friction coefficient to values as low as 0.05 can be achieved by the adsorption of selected surfactants called “boundary lubricants”. We define wear as the progressive removal of material from the sliding surfaces. It consists of the deformations, cutting, and fracture caused by the contact stresses that are due to the normal and frictional forces. In order to understand how wear depends on the material’s properties, let us review briefly the mechanical degradation processes of solids, At moderate stresses (tensile, compressive, or shear), all the atoms are displaced from their equilibrium positions. This is elastic deformation. When the shear stress exceeds a threshold value, planes of atoms in the crystals glide on each other by means of dislocation motion; this is plastic deformation. (Note that a pure (i.e., hydrostatic) compressive or tensile stress does not cause plastic deformation.) Hardness is measured by pressing a diamond pyramid into the surface of the solid and observing the size of the indent: hardness is a measure of the resistance to plastic deformation. Pure metals are soft: the metallic bond is nondirectional; atoms slide on each other easily as long as their distance does not increase. Covalent bonds are strongly directional: in the sp3 bond of diamond, for instance, the four neighbors attached to an atom must be located at precise positions with respect to each other, and the bending of a bond required to move a dislocation requires large amounts of energy. In ionic crystals, the alternation of positive and negative charges prevents the gliding of atomic planes in all but very few crystal planes. Ionic ceramics are hard; covalently bonded solids are the hardest known. Fracture always occurs where the tensile or shear stresses are much higher than the average or design stress. This occurs at sharp edges, crack tips, or nonspherical voids. At these sites, the local stress can exceed the maximum cohesive force between atoms. Soft solids do not fracture easily: plastic deformation causes a rounding of the edge and a decrease in the maximum stress. Hard materials are brittle because crack tips cannot be blunted by plastic deformation. The resistance to fracture is called toughness; it is related to the strength of the interatomic bond and the hardness. The interatomicbonds at a crack tip can be weakened by strongly adsorbed molecules. Thii causes a decrease of the stresses required for crack propagation; this effect is called “chemisorption embrittlement”. With this short primer, we are ready to understand wear. There are several types of wear that can be classified and understood in terms of the material’s response to local stresses. Abrasive wear is caused by the penetration of hard particles into the surface and the cutting of grooves. Since the phenomenon is similar to a hardness test, the resistance to abrasive wear is proportional to the hardness in metals; in the case of ceramics, the indent also causes cracks, so that the wear resistance is related to hardness and toughness in a complex manner. Adhesive wear is caused by the friction forces described above. In metals, it occurs mostly by plastic deformation and ultimately by crack propagation. According to Archard,19 the resistance to adhesive wear is proportional to the hardness in metals, but contributions from fatigue resistance and toughness have been claimed by others. In the case of ceramics, plastic deformation, although observed in the softer and tougher materials, is less important than fracture, but the relative contributions from the material properties to wear resistance are not yet established. Tribochemistry is the modifcation of the chemical reaction rates of solids by simultaneousfriction. The chemical reaction rate of solids is often governed by reaction products that accumulate on
Fischer and Mullins
5692. The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 the surface and through which the reagent molecules must diffuse. We can see several mechanisms by which such reactions are accelerated by friction: (1) wear removes the protective product layer and exposes fresh surface; (2) frictional stresses cause deformation or microfracture in the layer or the solid and produce diffusion paths or high-energy sites of increased reactivity; (3) frictional heat increases the local temperature; (4) the making and breaking of bonds that accompany friction create highly reactive chemical intermediate states. Since tribochemical reactions occur only where friction takes place, they can modify the surface geometry and lead to surface polishing as we shall see below; this modifies the local contact stresses and decreases mechanical wear. On the other hand, this interaction can accelerate the chemical attack, in which case we speak of corrosive wear.
t
2. Chemical Effects on Ceramic Tribology
Wear Reduction by the Tribochemical Oxidation of Silicon Nitride. Silicon nitride wears rapidly when sliding in dry argon. If the environment contains various amounts of water vapor, the wear rate decreases by as much as 2 orders of magnit~de.~ Minimum wear rate is reached at 90% relative humidity in argon and 5wo relative humidity in air. Under these conditions, the wear scar is much smoother than after sliding in dry gases, and it is covered with an amorphous silicon oxide which is probably strongly hydrated (Figure 2). Thus, under the action of friction, enough silicon nitride is oxidized to form a several micrometer thick layer ahead of the wear scar of the sliding pin and a thinner but easily detectable layer on the larger wear track of the plate. Oxidation of silicon nitride has been studied extensively and will be described below. It is known that the oxidation rate of this material is increased up to 1000-fold by the presence of humidity in the air.20 However, oxidation of silicon nitride proceeds measurably only above 1000 KZ1Figure 2 shows that the rate of oxidation of this material is accelerated by the simultaneous action of friction; how this occurs exactly has not been determined vet. In the oxidative wear of ~ t e e I , ' ~the * ' ~tribochemical mechanism has been identified." The change in the oxidation kinetics of steel from a logarithmic to parabolic law during friction and the increase in its rate by 4 orders of magnitude result from a large increase in the density of diffusion paths that is caused by the extensive plastic deformation due to wear. In the case of silicon nitride: the experimental evidence is not as easy to interpret, and the introduction of diffusion paths by plastic deformation is unlikely since wear occurs mostly by microfracture. One can speculate that the reaction is accelerated because the hydroxide formed on the surface is continuously removed and fresh surface is exposed by friction, but clear experimental evidence for any one mechanism is still lacking. The friction coefficient of silicon nitride is very nearly the same in dry and in humid en~ironment.~ Thus, the total mechanical forces acting on the contacting surfaces are the same. Why, then, is wear not increased but decreased when chemical attack is added to the friction force? The answer lies in the different topographies of the surfaces (Figure 2). In a humid environment, removal of matter by tribochemical-oxidationoccurs at the asperities; consequently, the contacting surfaces are smoother and the load and friction forces are distributed over wider areas than in the absence of tribochemistry. In other words, the local stresses responsible for microcracking and the mechanical wear of the material are reduced by the tribochemical oxidation.22 Tribocbemical Dissolution of Sicon Nitride and Silicon Carbide in Water. The dissolution of silicon oxide in water is a well-known phenomenon. It is also known that silicon nitride dissolves in water slowly but measurably above 400 KZ3When silicon nitride is sliding in water at room temperature, even at velocities and loads for which the temperature at contacting asperities is less than 300 K, wear produces ultraflat surfaces by molecular dissolution of the material (Figure 2C). Although measurable quantities of material have been removed, no wear particles are found. Analysis of the water reveals an amount of dissolved silicon that corresponds
Figure 2. Wear scars of silicon nitride spheres after sliding in different environments: (a, top) dry argon; rough surface causes stress concentrations. (b, middle) Tribochemistry in humid air produces smooth surface. (c, bottom) Tribochemical wear in liquid argon produces ultraflat surface. Reprinted with permission from refs 4 and 7. Copyrights 1985 Elsevier Sequoia and 1987 American Society of Lubrication Engineers, respectively.
to the material removed from the sliding surfaces. It is thought that the dissolution occurs by the oxidation of the materialZ3 Si3N4+ 6 H 2 0
--
3sio2 + 4NH3 3sio2 + 2N2 + 6H2
(2)
(through oxynitride intermediates) to form water-soluble silicic acid: 3sio2 6 H 2 0 3Si(OH)4 (3)
+
-
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 5693
Feature Article
1o-'L
FRICTION COEFFICIENT OF OXIDIZED S13N4
,
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0
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oxldatlon tlme (mln) FQure 3. Self-lubrication of silicon nitride by oxide formed in laboratory air at room temperature. The figure shows the friction coefficient in unlubricated sliding as a function of oxidation time at room temperature. Reprinted with permission from ref 25. Copyright 1989 Materials Re-
search Society. The tribochemical nature of this reaction is responsible for the very smooth surfaces on the rubbing surfaces: the reaction occurs only at contacting asperities. The smoothness of the surfaces is such that hydrodynamic lubrication (with attendant near-zero friction) is obtained when the silicon nitride surfaces slide in water at a velocity of only 6 cm/s despite an average bearing load pressure of 30 MPa. By assuming that the viscosity of the water is not changed by the small amount of silicic acid dissolved, we estimate that the hydrodynamic water film, and with it the roughness of the surfaces it separates, has a thickness of less than 10 nm. Similar experiments with silicon carbide also produce dissolution of the material, but hydrodynamic lubrication is not obtained because the material is mechanically weaker than silicon nitride and suffers local fracture as well as di~solution.~ Formation of a Lubricious Surface. At elevated temperature, sliding in humid air reduces friction as well as wear in a limited range of load and sliding ~elocities?~ A combination of scanning Auger spectroscopy and scanning electron microscopy revealed that the surfaces are covered by a smooth layer of silicon oxide. One also observes the presence of very fme debris outside the wear contact. As the product of load and sliding velocity is increased beyond a certain value, friction and wear are high and the wear surfaces are rough. Apparently, the passage from low to high friction is the result of a competition between the kinetics of formation of the lubricious oxide layer and its removal by wear. Lubricious oxides have recently been formed on silicon nitride by preo~idation.~~ In order to achieve low friction (friction coefficients as low as p = 0.05 have been obtained), it is necessary to prepare a very smooth and flat surface to avoid friction and wear by ploughing. This is achieved by friction in water as described above. Subsequent oxidation in air for a few minutes produces a surface which presents a low friction coefficient (Figure 3) * Gates, Hsu, and Klaus%have shown that sliding in water causes the formation of stable aluminum hydroxides which are modified to a layered structure (i.e., bayerite) by the frictional shear stresses. Easy shear in the layered structure results in lower friction coefficients and wear rate for the surface. Boundary Lubrication by Paraffins. Boundary lubrication is the decrease in friction caused by adsorbed organic molecules. For metallic surfaces, classic boundary layer lubricants are polar species such as fatty acids, alcohols, and esters. These layers decrease the adhesion between the rubbing surfaces and produce a friction coefficient p S 0.1 at low velocities where hydrodynamic lift is not capable of separating the surfaces. Nonpolar hydrocarbons such as paraffins and the molecules of lubricant basestocks do not act as boundary lubricants: at low enough velocities ( u 1 mm/s), the friction coefficient is as high as p = 0.6 in the presence of these fluids. For many ceramic^^,^ paraffins act as boundary lubricants, with friction coefficients in the neighborhood of p = 0.12. It is easy to verify that this is not due to contamination of the paraffin by polar impurities: one first observes a low friction coefficient with
-
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.Tetragonal
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Figure 4. Wear rates of brittle (cubic) and tough (tetragonal) zirconium oxide in various environments and lubricants. The numbers labeling the curves are the friction coefficientsf. Reprinted with permission from ref 5 . Copyright 1988 Elsevier Sequoia.
a ceramic and then high friction with a metal using the same sample of paraffin liquid. There is no proven explanation for this difference, but the facts suggest that ceramics can be considered as either acids or bases. The acid sites on the surface of ceramics, strong enough to break carbon-carbon bonds at elevated temperatures, are capable of adsorbing the molecules at room temperature. This phenomenon deserves further study. Chemically Induced Fracture in Oxide Ceramics. Oxide ceramics, such as alumina and zirconia, are obviously incapable of undergoing the oxidation reactions that are responsible for the decrease of wear in silicon nitride. This different chemistry manifests itself during friction by a different reaction to the environment and to lubricants. Wallbridge, Dowson, and have reported that wear of alumina sliding in water is higher than in air. With zirconia? water increases the wear rate by an order of magnitude over wear in dry nitrogen (Figure 4), and humid air causes an increase in the wear rate that is almost as large. Observation of the wear tracks by electron microscopy reveals that this increase in wear is due to intergranularfracture. This strongly indicates the presence of chemisorption embrittlement which is well-known for silica and glass where humidity increases the propagation rate of subcritical cracks. According to Widerhom** and MichalskeZ9and their co-workers, this phenomenon occurs by the attack of the bonds between neighboring metal and oxygen ions at a crack tip by water according to M-0-M + H20 2MOH (4) This reaction is chemically related to the well-known tendency of oxide ceramics to form hydroxylic surfaces; it is accelerated by tensile stresses at the crack tip. Adsorption-induced fracture is also observed in the presence of hydrocarbon lubricants. Pure paraffin decreases friction of zirconia to p = 0.1 1 as described above. With metals, such a decrease in friction decreases wear by several orders of magnitude. In the case of zirconia5 (Figure 4), paraffin causes an increase in the wear rate of about 50% over the wear rate in dry nitrogen despite the large reduction in friction from p = 0.7 to 0.1. When sliding occurs in paraffin with 0.5% stearic acid, which is a classic boundary lubricant, the friction coefficient decreases further to 0.09 but wear increases by another factor of 3. Electron microscopy of the wear scar reveals chemical attack of the grain +
5694
Fischer and Mullins
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992
boundaries and intergranular fracture as the cause of this increase in wear. We shall see below that stress corrosion cracking of zirconia presents a problem: simple hydroxides of zirconia are not known, and consequently, the weakening of the Zr-O-Zr bond by direct action of the water molecule is not likely. We refer the reader to the discussion in section 3. Friction of Diamond and Graphite. Diamond exhibits a very low friction coefficient ( p < 0.1) in unlubricated sliding against itself and most other substances as long as this occurs in humid ambient^.^,^^ In vacuum, the friction coefficient is high. Graphite is a well-known solid lubricant, but it lubricates in humid ambients only; it is not usable in space because it depends, for its low shear strength, on the intercalation of water molecules. The extreme hardness of diamond makes it a natural candidate as a cutting tool. It is used as such in rock drilling but cannot be used for the cutting of steels and other ferrous alloys because of a tribochemical reaction in which carbon dissolves in iron. A similar chemical reaction makes alumina the cutting tool of choice for titanium-containing steels at very high speeds; the reaction of the ceramic with the titanium contained in the steel produces a very hard and wear-resistant coating on the surface of the tool.3’ The above survey has shown that chemical interaction of ceramics with a variety of molecules plays an important role in the performance of these materials in all aspects involving friction and wear. A better understanding of these phenomena and their technological exploitation must be based on the knowledge of the surface chemistry of these materials. In what follows, we shall review the chemical properties of ceramics and relate them to the tribochemical behavior.
3. Surface Chemistry of Ceramic Materials The chemical properties of ceramics are determined by their electronic structure; these materials are insulators with a large energy gap between the fully occupied valence band and the empty conduction band. The Fermi level, or electrochemical potential of the electrons, lies somewhere inside this band gap. As a consequence, chemical reactions of ceramics are usually described as predominantly polar with little electron transfer to or from adsorbates . Lewis Acid-Base Theory and Solid Surfaces. According to the Lewis theory, acid-base reactions involve the loss or donation of electrons by the base and the acceptance of these electrons by the acid.32v33The reaction product, or adduct, consists of the acid and base functionals bound together by either electrostatic forces or the formation of new orbitals by the transfer of charge. The adduct formation reaction can be quantified by examining the energy change of the reaction from a quantum chemistry point of view. Using Mulliken’s approach,32 the adduct orbitals are described as a linear combination of the acid and base molecular orbitals. The adduct bonding orbital energy is described as the sum of an electrostatic term due to the charges on the species and a covalent term related to the energies of the new adduct orbitals f~rmed.~~,~~ If the electrostatic term dominates, the adduct is charge-cont r ~ l l e d . ~Little ~ , ~charge ~ is transferred in the reaction, and it consists of little more than the attraction of oppositely charged ions. This occurs when the atomic orbitals are small (tight binding) or when the energy difference of the two overlapping orbitals is large. This is termed a ”hard” acid-base reaction.35 If the covalent term dominates, the involved orbitals are diffuse, with significant overlap, and their energy difference is small. Their overlap forms an orbital between the two reacting species, with little net charge transfer from one species to the other. This is a ‘frontier-controlled” reaction, or soft acid-base reaction, which occurs between aromatics for example. In between are combinations of the two, with both frontier- and charge-controlled constituents. These are the borderline acids and bases, which tend to have both charge-transfer and orbital overlap interactions with both hard and soft species. Most of these concepts are useful for defining a hard or soft acid or base, but the strength of the interaction and the acid and
base selection remain to be described. In the “frontier”-controlled reactions, the highest energy occupied molecular orbitals (HOMO) of each species modulate the lowest energy unoccupied molecular orbitals (LUMO) of the other. In an ideal soft acid-base reaction, the adduct bonding orbitals are occupied equally by electrons from both species so that the “acid” and “base” cannot be distinguished; the HOMO and LUMO involved have nearly identical free energies in both reagents. The larger the H O M G L U M O overlap integral, the larger and more diffuse the frontier orbital and the more exothermic the adduct reaction will be. The energy difference between the empty orbitals of the acid and the full orbitals of the base results in an unequal makeup of the hybrid adduct orbitals so that the adduct bonding orbitals are composed more of the acid than the base. The reverse is true for the antibonding orbitals. This results in a significant effective charge on each species; the resultant electrostatic force further binds the two species together, making this a charge-transfercontrolled reaction. Since this reaction is controlled by the relative energies of the involved orbitals, the selection of base and acid is based on the electronic structure of the individual species. Using density-functional theory36is convenient for quantifying the acid-base reaction. The energy change of a moiety associated with a reaction can be expressed as 6E = p06N
+ 2q(6W2 + l p ( r ) 6v(r) dr
(5)
where p = (aE/aN), is the chemical potential of electrons, q = ‘ / z ( a p / a N ) v is the hardness, N is the number of transferred electrons, p(r) is the electron charge density function, and v(r) is the potential function. The chemical potential for a moiety is p
= po
+ 2q6N + l f ( r ) 6v(r) dr
(6)
where
(7) is the Fukui f~nction.~’ When no charge is transferred in the reaction, the energy change is determined entirely by the response of the electron charge density to the perturbation in the local potential38
This is an ideal “hard” or charge-controlled reaction. The geometry of the interaction is determined by the shapes of the potential and charge density functions and the requirement to maximize the overlaps. Reaction sites are those with the largest charge densities and largest (negative) potentials. For “soft” or frontier-controlled reactions, the two species interact by the transfer of electrons. Both species, in local q u i librium, have the same chemical potential of electrons, pA = p9.39 As the two species approach, the amount of charge transferred from the base to the acid is given by the ‘Clapeyron-type” equation S_fB(I)
6N =
6vB(r) 2(TA
-fA(r)
+ qB)
6vA(r)
dr
(9)
High hardness interactions, large q, have very little charge transfer. Soft-species reaction sites are those with the largest Fukui function densities and largest potential^.^'*^^ Figure 5 shows the valence charge density, and Figure 6 shows the Fukui functions calculated for an isolated water molecule. These functions were calculated using an ab initio molecular orbital program, written by one of the authors. The program was based on the overlapping atomic potential (OAP) linear combination of Gaussian-type atomic orbitals (LCGTO) method4 but utilized only a minimal basis set. The OAP-LCGTO program used curve-fitted Herman-Skillman41 atomic potential functions and wave functions along with atomic positions and trial values for atomic charge as input. The energy eigenvalues and Mulliken occupations along with plots of the molecular orbital charge
Feature Article
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 5695 spectrophotometer. The number reflects a measure of the valence charge density surrounding the oxygen ion in the network. The larger the optical basicity index, the higher the charge density around the oxygen and the greater the tendency to act as a Lewis base. The optical basicity can be related directly to the electronic structure of the oxide network through a Mulliken type of a p proach. Assuming the mixing of two orbitals32of energies Eo and EMthe occupation number of the metal orbital is
Figure 5. Calculated valence charge density for the free water molecule.
densities, the total valence charge density, and the potential function are output. As can be seen in Figure 5 , most of the valence charge density is concentrated on the oxygen. This results in the formation of a long-range charge dipole. The Fukui functions (Figure 6) for gain and loss of electrons are the LUMO and HOMO densities, respectively. A completely "hard" interaction will involve oxygen, with the direction determined be the charge of the attacking species. A completely "soft" interaction will be through the 0 2p orbitals for an electrophilic (acid) species (Figure 6) or the 0-H antibonding orbital for a nucleophilic (base) species. Iigat Metal Oxides, Magnesia, alumina, and silica are probably the most widely studied ceramics. Molecular orbital calculations have shown that these three oxides have similar electronic structures42consisting of a split-valence band; the lower band is in the nature of atomic 2s orbitals of oxygen, and the upper band consists of 2p oxygen orbitals with some cationic characteristics. The very upper portion of the valence band is generally considered atomic 0 2p, with no bonding characteristics at all. The conduction band is cationic 3s and 3p in nature with some mixture of 0 2p. The band gaps of these materials are among the largest measured, from 7.5 to 9.5 eV.43 These characteristics classify these oxides as very hard acids and bases.33 To describe the solvent chemistry of these systems for metal ions, the optical basicity index"" was developed. In principle, this is a one-parameter empirical model that relates reactions to the charge density associated with the oxygen ions in the network. It is easily measured using a heavy-metal probe and a UV
Figure 6. Calculated Fukui function for the free water molecule.
a % Ax/(EM - E O ) (10) where X is determined by the overlap and resonance integrals for the system and x is the metal-oxygen ratio. The antibonding orbital energy is Eanti z
EM+ (EM- Eo)(a2/x2)
( 1 1)
If the HOMO is the 0 2p orbital and the LUMO is one of the antibonding orbitals, the Mulliken occupation numbers can be estimated from the free atomic valence energies and band gap by
(and neglecting differences in overlap, etc.). Figure 7 shows the observed relationship between optical b a ~ i c i t and p ~ ~the Mulliken occupations calculated using the Herman-Skillman4' atomic eigenvalues and experimental values for band gap.43 The basic oxides are those with the least cation character in the bonding orbitals (valence band) which is evident from the empirical relationship A = 1.34 - 1.97a (13) The intrinsic Fermi energy should be halfway between the HOMO and LUMO. Using the relations above, this is Ef
Y2(EM
+ Eo)
(14)
This can be substituted into the above relations to show a parabolic relationship between the optical basicity an Fermi energy as shown in Figure 8. As is expected, the higher the Fermi energy, the more basic the oxide.
Fischer and Mullins
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992
5696
0 0
0.2 0.3 0.4 Mulliken Occupation Number
0.1
0.5
0.6
Optical basicity as a function of calculated Mulliken occupation of cation for several oxides. The greatest uncertainty is associated with the Na, which is very sensitive to selected values of Em,. Figure 7.
I
I 1
.
2
I
-8
-7
-6
.
'
"
-b
'
Na ~
'
_
.
-4
-3
Calculated Fermi Energy
F i p e 8. Optical basicity vs calculated Fermi energy for simple oxides.
Chemical shifts in the XPS measured 0 1s photopeak can be directly related to shifts in the 0 1s eigenvalue, changes in the Madelung potential of oxygen, and shifts in the reference level of the measurement.& The separation interval between the 1s and 2p eigenvalues was calculated as a function of 2p state occupation using the Herman-Skillman program!' The results were fitted to obtain the empirical relationship
AE2p-ls = - 4 . 0 9 5 5 ~-~ 501.1991
(15)
in electronvolts, where a is the Mulliken occupation number calculated above. This represents a relaxation in the 0 1s state due to valence level screening. The measured XPS photopeak energy is referenced to the Fermi level of the sample?' which is calculated above. The Madelung potential for the oxygen ion in the lattice has the form
where qat is the calculated charge on the cation, ro is the equilibrium cationanion separation distance, and a is the Madelung constant for the structure. Values of the Madelung constant were obtained4*for NaCl, j3-quartz, corundum, and other structures. These were then fitted as a function of stoichiometry to obtain the approximate relation a = 0.468 + 1.272% (17) still assuming full ionization of the cations. This relationship should be within 108 of the true value for simple oxides. Setting r, to the sum of the aluminum and oxygen ioni radii in bob units, 3.66, including a term of (1 - a2) to account for the change in cation charge and converting from hartrees to electronvolts, gives EMag
=-
54.4(0.468 + 1.272~)( 1 - a2) 3.66~
(18)
0.2
0.4
0.6 0.8 Optical Basisity
1
1.2
1.4
Figure 9. Optical basicity vs 0 1s binding energy; squares are calculated values, and circles are measured in ref 57.
Combining the three energy shift terms gives the estimate for the 0 1s binding energy EOI, = 54.4(0.468 + 1.272~)(1- a') Egap 1 -501.1991 - 4.0955~'- -2 3.66~ Values for this function, along with experimental mea~urements~~ for comparison, are shown in Figure 9. Based on these calculations, the 0 1s binding energy can be used to estimate the optical basicity of a simple oxide. For heavy-metal oxides and mixed oxides, the relationships are more complicated, but the general trends for optical basicity should be observable. The interactions of the surfaces of these oxides with water do not agree with the optical basicity predictions. Water has an optical basicity4 of A = 0.40, which is considerably more acidic than silica and most of the other metal oxides. On immersion in water, silica will develop a negative surface charge due to the adsorption of hydroxyl radicals. The silica is apparently acting as an Arrhenius acid while the water acts as the corresponding Arrhenius base. Adjusting the pH of the water changes the interaction equilibrium and changes the surface charge. The pH at which the net charge on the silica is zero is 1.2-4.50*51This is termed the point of zero charge (pzc) and indicates that silica is considerably more acidic than water. Alumina has a pzc of 8-8.550,51 which makes it amphoteric to slightly basic while magnesia is very basic with a pzc of l Z 5 0 F'arks suggested modelsMfor the interaction of water with oxides based on Pauling's electrostatic model of oxide bonding. He introduced corrections for ion coordination, crystal field effects, and state of hydration into the electrostatic interaction model which accurately predicts the pzc of pure oxides. Changing the composition was predicted to change the average charge on the surface oxygen, which shifted the surface acidity. The numerical predictions made by Park's model for mixed oxides do not appear as accurate as for single oxides. To describe the individual effects of different surface sites, Knowzinger and R a t n a s a m ~related ~ ~ the activity of surface anions to the local coordination and charge using Pauling's electrostatic valence rule.53 Each surface site has a different charge density and reactivity. The preaence of both acidic and basic sites of the same surface is used to explain the catalytic activity of the oxide surface. This is borne out by IR and Raman studies of adsorbed hydroxyls on y-alumina which show distinct bonding states. Dopants introduce defect states in the band gap, along with increased electron or hole concentrations. These carriers are attracted to the surface anions to shift the average charge density on the surface oxygenss4and shift the surface equilibrium. Directly opposed to this view is the description made by Henrichs5that the oxide surface consists of nearly unrelaxed ions. Surface reactivity and adsorption are determined by the presence of cations and anions on the surface and local coordination of the surface site. This description explicitly ignores the dissociative adsorption of oxygen and water onto the surface in ambient
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 5697
Feature Article
Figure 10. Valence charge density and Fukui functions for an oxygen-terminated A1203(0001)surface viewed from ( 1 120).
conditions to leave the oxide surface oxygen of hydroxyl terminated. The techniques sited by Henrich to investigate the surface structure (LEED, XPS, etc.) are ultrahigh-vacuum techniques that would desorb many of these species. The fundamental model for the adsorption reaction, though, is similar to the Parks and Knowzinger descriptions and is electrostatic in nature. These electrostatic models are perfect “hard-hard” interactions and ignore any possible effect of charge transfer between the surface and the water molecule. Considering the system using a Mulliken-type analysis,34the states involved in the bonding are the acceptor state of the acid and the donor state of the base, I$A) and I$B), respectively. The final state for the surface adduct is a linear combination of these states. From Mulliken,32the energy of the final bonding state is El$) = ~EAI~CIA) + ~ E B ~ $+BasvAl$A> ) + bGvBl$B) (20) assuming simple overlapping potentials for the system. For an ideal hard-hard interaction (no net electron transfer) MI$)= El$) - ~EAWA) - ~EBWB) = ~ ~ ~ A I ~+C~ISA~ )B I $ B ) (21) the energy change is
AE = a2(rtA16vAItCIA) + b2(rClB16v~IrcI~)-.
(22) These inner products can be replaced by the appropriate Fukui functions to give = a 2 I f A ( r )6 V A ( r ) dr m
+ b 2 1 f B ( r )8 V B ( r ) dr m
(23)
to leading order. Summing this energy change for each combination of electronic states in each species gives the integral of this equation, which reduces to eq 8. Notice that eq 23 produces no change in the chemical potential of the adduct system, which only appear in the higher order terms in eq 22. Using the density functional descriptions for the evolution of a closed system as described for eq 9, let the potential G be G = pN (24) with the variation being given by bG=p6N+NBp (25) A change in the system that lowers p at constant total N reduces the potential of the system. For the system described for eq 9, the chemical potential change is qAlfB(r)
6vB(r)
dr + 7 B l f A ( r )
8vA(r)
dr
(26) + OB which can be large for systems with large 7’s. Note also that the hardnesses and chemical potentials here are not the free atom reference values.39 6p =
7A
The system will evolve along a path to minimize G, which will be determined by the overlap of the Fukui functions with the potential variations for the subsystems. This is very similar in form to the relation in eq 23 but does not explicitly assume zero charge transfer, only conservation of charge in the adduct. Unlike eq 23, it correctly accounts for the lowering of the electronic chemical potential in the system in the leading order approximation. The density functional approach will show the transfer of charge during an adsorption reaction, even for an interaction between very hard species. Fundamentally, the predicted energy changes and reaction sites for adsorption should be little different than the predictions for the electrostatic models since the two are similar in leading order. Figure 10 shows the valence charge density and Fukui functions calculated for an oxygen-terminated A 1 2 0 3 (0001 surface viewed from (OOO1) using the program described above. Figure 11 shows the same functions as viewed from ( 1120). Thep’(r) is from the 0 2p nonbonding orbitals at the top of the valence band. This, and the potential function, are localized around the surface oxygen layer with little cation component. Thef(r) is more cationic in nature but highly delocalized, as would be suggested by the high electron mobility in this oxide.56 As a result, the surface oxygen dominates the chemistry of the oxide surface. The charges calculated for the cations near the surface are nearly identical to the results of the calculations made above for Figure 7. The charges on the surface oxygen layer were significantly smaller than the “bulk” values. This is probably due to the reduced Madelung potential at this surface, which would result in a shift in the charge density away from the surface. The Madelung potential at a stoichiometric, flat surface is nearly identical to that of the For a kink site in a surface ledge, the Madelung potential is half the bulk value (half the crystal surrounding). Lower coordination, atomically rough surfaces such as a stoichiometric alumina {OOOl]surface should have an even lower Madelung potential. The presence of dissociatively adsorbed O2to saturate the surface would lower the average expected charge density on the sbrface. The near stoichiometric surface expected in vacuo could be produced from the saturated surface by desorption of oxygen with the formation of surface vacancies. This would remove surface oxygens and slightly increase the charge density on the remaining oxygens. Since the surface oxygens have a much lower effective charge than the bulk, the charge density on the remaining oxygens is not expected to be strongly affected. The potential and Fukui functions will be simple modifications of the full coverage calculated functions, surface reactivity will be similar, and the surface oxygen will dominate in reactions. The reduced charge density on the surface oxygen layer decreases the local optical basicity. This makes the surface more acidic than the bulk and may provide a driving force for the
5698
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992
Fischer and Mullins
Figure 11. Valence charge density and Fukui functions for an oxygen-terminated A1203(0001)surface viewed from ( 1 120).
IO0 n \
-
dE
I
I
1
I
I
I
I
I
I I Ibrieity
1
yttrla
'"
nn-X)
T
m effect
zirmnia
I
W
\
I 'b
\
x -v e-
-
0-
a
I
-I
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tltmla
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-
t3
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4 2
Band Gap (eV) Figure 12. Relationship between the band gap separating HOMO and LUMO states of ceramics and their reactivity with water. The righthand scale and the solid line show the calculated magnitude (squared) of the charge dipole formed at the adduct (ref 35); the left-hand scale and the points show the solubilities of the ceramics in water (after ref 61).
segregation of acidic species to the surface of glasses and slags.s8 By this model, the effect of the cation on surface properties is through the moderation of the charge density on the surface oxygens. Further work is clearly needed in this area. An empirical relationship between optical basicity and miscibility has been observed for molten oxides.4s This relationship is also observed for aqueous solubility. Figure 12 shows a plot of the reported aqueous solubilitiesof various oxidess9as a function of band gap43at room temperature. The band gap was used above to estimate the Mulliken occupation and the optical basicity of the oxide. During the surface reaction, the net charge transferred from the base to the acid forms a charge dipole at the adduct, which can be estimateda using a Mulliken analysis. Also shown in Figure 12 is a plot of the calculated6' charge dipole (squared) formed in the reaction with water as a function of the substrate band gap. The calculation assumed a constant conformation for the water molecule and no effect of defect states in the substrate. The curve shows a relative minimum of the dipole corresponding to zero charge transfer, or a neutral reaction, at a gap of -8 eV. A maximum interaction is expected at this point; little electrostatic hindrance to reactions is expected at surface sites. There is an
"3t 1
0 8 6 4 Band Gcp (eV>
2
Figure 13. Calculated adduct dipole charge (squared) vs oxide ceramic band gap for adsorbed C-H functional.
equal likelihood for species dissociation and little complexation is required, so that dissolution reaction is not hindered in the polar solvent.62 Changing the band gap of the substrate in either direction changes the optical basicity of the surface, which increases the magnitude of the dipole formed by adduct formation. As mentioned above, acidic complex ions such as PO4', SO4"-, and Cr042-are observed to be surface active for most ~ x i d e s ? ~ * ~ ~ They are also known to specifically adsorb onto surfaces from aqueous s0lutions.6~Other ions such as SO4*-and NO3-adsorb also but reach equilibrium more slowly.63 The organofunctional derivatives of these ions are also known to react strongly with the oxide surface through the complex ion and form strong bonds with the surfaces. Such materials as organosilanes and organophosphonic acids are commonly used as coupling agents for paints and adhesives and as surface hydration inhibitors. The effect these would have on mechanical properties is not presently known. The model interaction for a CH3 moiety as a function of band gap can be described in the same manner as 'for the water molecule. Assuming a similar wave function overlap configuration to the water molecule, we calculate the relationship between band gap and surface adduct dipole shown in Figure 13. The relationship is similar to that of Figure 12, but it is shifted to a lower band gap because of the lower energy gap of the CH3 functional. In analogy to the results for water, significant adsorption is expected for yttria, but solubility in a nonpolar medium will be small because of the low dielectric constant. For two species adsorbing simultaneously, their different electronic structures will allow for different relative minima in the charge transfer and maxima for the interactions. Between
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 5699
Feature Article
.;;.It V
I
1
0 8 6 4 2 Band Gap (eV) Figure 14. Predicted combined interaction of water and CH3 moieties as a function of band gap of ceramics.
the individual extrema, opposite polarities of dipoles are formed on alternating sites and compensate each other. The effective interaction curve for the two species is shown in Figure 14; it is much wider than for the individual environments, Figures 12 and 13. Correspondingly, chemisorption is expected to extend through the entire region. Reactions with highly polar functional groups such as OH, COOH, and NH2, which tend to dissociate or form polar 'hydrogen" bonds,@are similar to those one would expect to find with water. The surfaces of these oxides are known to catalyze many ring-opening reactions when the ring is small and highly polar.66 The general tendencies of aluminosilicates to catalyze alkene production, cracking, and isomerizations are attributed to the products of these reactions. Nonpolar organic compounds, such as paraffin, are built up from the CH2 and other groups. The C-H bond is localized and polar but not as strongly polar as the 0-H bond. Weak, hard acid-base interactions are possible between the C-H bond and the oxide surface, and this would result in weak chemisorption. When the carbon species is bonded to a highly electronegative species or is,involved in a multiple bond, the C-H bond becomes more polar. The resulting surface adduct reaction is much stronger and can result in bond cleavage. Olefins would be expected to adsorb fairly well onto these oxides, as described in section 2.4. Both saturated and especially unsaturated hydrocarbons should adsorb well enough to provide some boundary layer lubrication to these oxides. Sic aod Si&. Silicon carbide and silicon nitride are narrow band gap materials (2.8 eV for S i c and 5.0 eV for Si3N4).43In addition, both materials have strong tendencies toward nonstoichiometry, which further reduces the effective band gap. Because of this, we would initially expect these compounds to be much "softer" than the above-mentioned oxides. Significant substoichiometry would further soften their activity. Both of these compounds react slowly with air or water to form the chemically more stable oxide or oxynitride surface coatings, which have the chemical properties mentioned in the previous ~ection.~'Also, oxides are commonly added as tougheners and sintering aides to bulk silicon carbide and nitride, which makes the grain boundary oxide phases even more prevalent.68 Consequently, these materials have surface chemistries in air that are nearly identical to that of silica, with identical pzc, organic reactions, and surface layer solubilities in water. Za2. The band gap of zirconia is 5.0 eV,43much smaller than that of alumina and water, but the structure of the bands is expected to be similar. Because of this, Zr02 is a much softer acid-base than alumina or the oxide formations on Si3N4. The aqueous surface chemistry of zirconia is very well known;s1~69370 ZrO2 is considered very weakly acidic in water. Measurements are complicated by the speclfic adsorption of softer complex anions onto the surface, such as NO3- and Clod-, which produce considerable surface charge double layers even though no extensive reaction with water has taken place.51 Using Pauling electronegativity values, the optical basicities of zirconia and yttria are estimated44to be r = 0.75 and 0.86, respectively. Both of these values are at least as basic as magnesia. Using the parameters suggested by Parks,soand neglecting crystal
field effects, the isoelectric point (IEP)for zirconia in 8-fold coordination is 12. This value is near that predicted by the optical basicity and agrees with the value of 11.3 reported from the literature by Parks.so The calculated value for the 7-fold coordinated oxide is closer to 8.5-9, which agrees better with the reported pzc values of 6-6.7 but is still not very close. The IEP predicted for yttria by the Parks data is 9.8, which is much closer to alumina than what is predicted from the optical basicity estimates. Models of the surface reactiona indicate that much more charge transfer is involved in surface adduct formation than for alumina and silica (Figure 12). The magnitude of this charge transfer limits the maximum solubility of the species by electrostatic repulsion in a dielectric As dissolved species concentrations exceed the dielectric capacity of the solvent, at 10-8-10-1', complexation to (Zr4(0H)8(H20)16)8+ and recrystallization of the oxide occur.69 No zirconium hydroxides are known to occur. These reactions are typical of the weak interaction with water and would seem to preclude any possibility for environmentally stimulated fracture. But wear testing indicates interaction between water and yttria-stabilized zirconia leads to increased fracture. This behavior is presently not well understood and is not predicted by the interaction with pure Zr02. One possible mechanism" is the interaction of water with point defects in the stabilized zirconia. Yttrium is included in the lattice by the reaction Y203--c 2Y'zI
+P
o
The yttrium-vacancy pair react with water by 00
+ V"0 + H20
-*
2(0H)',
which is a solution of Zr02 and YO(OH)2. The addition of yttrium fluoride consumes oxygen vacancies and can reduce the incorporation of water. The exact mechanism of this type of reaction requires further study. Unlike the aqueous chemistry, the organic reactions with zirconia are very rich. Since zirconia is "softer" than alumina, acid-base reactions with the C-H functionals in olefins and more complex organics involve much less charge transfer. The extent of the reactions is increased (Figure 13) along with increased probability of bond scission and oxygen group grafting reactions. In tribological conditions, the potential exists for similar types of oxidation and catalysis reactions of organic lubricants with the surfaces or with wear debris to form complex oxidized oligomers. The attack of the zirconia surface by stearic acid,s observable outside of the frictional surface but vastly accelerated by the friction in the wear scar, is an example of such a reaction. 4. Discussion As shown above, chemical interactions between ceramics and their environment are quite active during friction, even at room temperature, and these reactions influence not only the dissolution of the materials or the growth of superficial films but also the mechanical response of the materials to the contact stresses. The preceding sections have examined how these tribochemical interactions modify the friction forces and the mechanisms of wear and how they are related to the known chemical properties and the electronic structure of the materials. Understanding of these relationships is still tentative, and in most cases, evidence of the precise mechanisms by which friction increases the rate of chemical reactions is lacking. A picture, still largely speculative, is emerging and can be drawn by describing the interactions with water first. The solubility of silicon nitride in water is low. Interaction with water takes the form of oxidation, which is normally slow but is tribochemically accelerated. Thus, oxidation reactions produce smooth surfaces (Figure 2b,c). The reaction occurs at contacting asperities, which are removed. Because of the low reactivity of silicon nitride with water, chemisorption embrittlement does not occur. Silicon oxide is the most reactive of the ceramics we studied. This explains the dissolution of the oxide in water (Figure 2c). It also explains why lubrication of silicon nitride by oxide films
5700 The Journal of Physical Chemistry, Vol. 96, No. 14, 1992
(Figure 3) occurs in air but not in water: the films are dissolved. According to Figure 12, the reactivity of alumina with water is intermediate between those of silicon oxide and nitride. This reactivity is sufficient for the occurrence of chemisorption embrittlement, which is a mutual stimulation of reaction and fracture: the very large stresses and strains occurring at a crack tip modify the local electronic structure and provide mechanical activation for the reaction, and the latter weakens the Al-0 bond. This lower reactivity also allows the tribochemical formation of aluminum by hydroxides that are stable in water and provide lubrication. Zirconia reacts much less with water than the two other ceramics (Figure 12). The water adsorption involves so much charge transfer that the extent of the reaction is negligible. In fact, the reactivity of zirconia is so low that adsorption-induced fracture is surprising on these grounds. Since pure zirconia undergoes several destructive phase transformations during cooling from high temperature, most structural zirconias are doped with several percent yttria to stabilize the high-temperaturephase. A fraction of this stabilizer segregates to the grain boundaries of the material. It is possible that water reacts with the Y203stabilizer, which has a slightly wider band gap (5.6 eV) and is much more reactive with water than Z r 0 2 (Figure 12). It remains to be verified whether the bulk zirconia or an impurity-enriched grain boundary region is responsible for the observed embrittleme~~t.~ In the case of hydrocarbon lubricants, our knowledge is much more anecdotal and our understanding is very limited. Yet precisely the tribochemical effects of hydrocarbon lubricants are the phenomena that require the most pressing mastery since they determine the performance of ceramic elements in machinery that is lubricated by conventional oils. Based on what has been deduced for water, regions of lubricity, chemisorption embrittlement, and boundary layer lubrication can be predicted as in Figure 13 for saturated hydrocarbons. Additional considerations such as lubricant scission and grafting must be taken into consideration. A knowledge of ceramic tribochemistry with hydrocarbons is also necessary as a theoretical basis for the development of synthetic lubricants for these materials, especially for high-temperature service. Our measurements with zirconia have shown the importance of these phenomena: certain hydrocarbons increase its wear rate over that of unlubricated sliding despite the fact that they provide low friction.
5. Conclusions The tribochemical reactions of ceramics with lubricants and the environment are much more complex than those of metals. The varying surface chemistries of the different ceramics dominate the reactions and change the observed wear properties. Because of this, little consistent lubrication and wear properties have been observed for the various ceramics. By understanding the nature of the surface reactions, the extent of these surface reactions can be related directly to their electronic structure. The differences in the electronic structure of the ceramics can then be shown to cause different surface reaction, which are directly related to the lubrication and wear properties. Criteria can then be developed for surface lubricity, chemisorption embrittlement, and boundary layer lubrication based on the structure of the ceramic and the chemistry of the lubricant.
Acknowledgment. This research was made possible by grants from the Dow Chemical Corp., the Charles Schaefer Fund for Excellence, and the Purdue Research Foundation. The computer graphics were produced using software from the National Center for Supercomputing Applications, University of Illinois.
References and Notes (1) Frisch, B. Ber. Drsch. Keram. Ges. 1967, 44, 584. (2) Shimura, H.; Tsuya, Y. In Wear of Materials; Ludema, V ., Ed.; ASME: New York, 1977; p 452. (3) Suzuki, K.; Sugita, T. In Wear of Materials; Rhee, S. K., Ruff, A. S., Ludema, K. C., Eds.;ASME: New York, 1981; p 518. (4) Fischer, T. E.; Tomizawa, H. Wear 1985, 105, 29. ( 5 ) Fischer, T. E.; Anderson, M. P.; Jahanmir, S.; Salher, R. Wear 1988, 124, 133. (6) Sugita, T.; Ueda, K.; Kanemura, Y. Wear 1984, 97, 1.
Fischer and Mullins (7) Tomizawa, H.; Fischer, T. E. ASLE Trans. 1987, 30, 41. (8) Jahanmir, S.; Fischer, T. E. STLE Trans. 1988, 31, 32. (9) Heinicke, G. Tribochemistry; Carl Hanser Verlag: Munich, 1984. (10) Thiessen, P. A.; Meyer, K.; Heinicke, G. Grundlagen der Tribochemie; Akademie Verlag: Berlin, 1967; p 15. (11) Sakurai, T.; Sato, K. ASLE Trans. 1966, 9, 77. (12) Sakurai, T. J. Lubr. Technol. 1981, 103, 73. (13) Habeeb, J. J.; Stover, W. H. ASLE Trans. 1987, 30, 419. (14) Willermette, P. A.; Kandah, S. K.; Siege], W. P.; Chase, R. E. ASLE Trans. 1983, 26, 523. (15) Spedding, H.; Watkins, R. C. Tribology 1982, 15, 9; 1983, 15, 15. (16) Quinn, T. F. Tribology Int. 1983, 16, 257, 305. (17) Sexton, M. D.; Fischer, T. E. Wear 1984,96, 17-30. Fischer, T. E.; Sexton, M. D. In Physical Chemistry of the Solid Stare: Applications to Metals and Their Compounds;Lacombe, P., Ed.; Elsevier: Amsterdam, 1984; p 97. (18) See for instance: Czichos, H. Tribology; Elsevier: Amsterdam, 1978. Dorinson, A,; Ludema, K. C. Mechanics and Chemistry in Lubrication; Elsevier: Amsterdam, 1985. Rigney, D. A., Ed. Fundumentals of Friction and Wear of Materials; American Society for Metals: Metals Park, OH, 1981. (19) Amhard, J. F. Proc. R . Soc. London 1956, A236, 297. (20) Singhal, S. C. J . Am. Ceram. Soc. 1976, 59, 81. (21) Kiehle, A. J.; Heung, L. K.; Gielisse, P. J.; Rockett, T. J. J. Am. Chem. SOC.1975, 58, 17. (22) Fischer, T. E. Annu. Rev. Mater. Sei. 1988, 18, 303. (23) Iler, R. K. The Chemistry of Silica; John Wiley & Sons: New York, 1979. (241 Tomizawa. H.: Fischer. T. E. ASLE Trans. 1986. 29. 481. (25) Fischer, T:E.;.Liang, H.; Mullins, W. M. Mater.-Rei. Soc. Symp. Proc. 1989, 140, 339. (26) Gates. R. S.; Hsu. S. M.; Klaus. E. E. Tribolonv Trans. 1989.32.357. (27) Wallbridge, N.; Dowson, D.; Roberts, E. Wear of Materials; Ludema, K. C., Ed.; ASME New York, 1983. (28) Wiederhorn, S. M.; Freiman, S. W.; Fuller, E. R.; Simmons, C. J. J . Mater. Sci. 1982, 17, 3460. (29) Michalske, T. A.; Bunker, B. C. J. Appl. Phys. 1984, 56, 2686. (30) Bowden, F. P.; Tabor, D. Friction and Lubrication of Solids; Clarendon Press: Oxford, 1964. (31) Brandt, G.; Mikus, M. Wear 1987, 118, 99. (32) Mulliken, R. S. J. Am. Chem. SOC.1952, 74, 811. (33) Jensen, W. B. Chem. Rev. 1978, 78, 1. (34) Klopman, G. J. Am. Chem. SOC.1968, 90, 223. (35) Pearson, R. G. J. Am. Chem. Soc. 1963,85, 3533. (36) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and Molecules; Oxford University Press: New York, 1989. (37) Parr, R. G.; Yang, W. J. Am. Chem. SOC.1984, 106,4049. (38) Nalewajski, R. F. J. Am. Chem. Soc. 1984, 106, 944. (39) Berkowitz, M. J. Am. Chem. Soc. 1987, 109, 4823. (40) Ching, W. Y.; Lin, C. C.; H u h , C. C. Phys. Rev. B 1976,14,620. Ching, W. Y.; Lin, C. C.; Guttman, L. Phys. Rev. B 1977,15,5488. Chaney, R. C.; Tung, T. K.; Lin, C. C.; Lafon, E. E. J. Chem. Phys. 1970, 52, 361. Simmons, J. E.; Lin, C. C.; Fouquett, D. F.; Lafon, D. F.; Chaney, R. C. J . Phys. C Solid Stare Phys. 1975, 8, 1549. Ching, W. Y.; Lam, D. J.; Lin, C. C. Phys. Rev. B 1980, 21, 2378. (41) Herman, F.; Skillman, S. Atomic Structure Calculations; PrenticeHall: Englewood Cliffs, NJ, 1963. (42) Tossel, J. A. J. Am. Chem. SOC.1975, 19, 4840. Ciraci, S.; Batra, I. P. Phys. Rev. B 1983,28,982. Batra, I. P. J. Phys. Solid State Phys. 1982, 15, 5399. Ching, W. Y . Phys. Rev. B 1982, 26, 6610. Ching, W. Y. Phys. Rev. B 1982, 26, 6622. Ching, W. Y. Phys. Rev. B 1982, 26, 6633. (43) Strehlow, W. H.; Cook, E. L. J. Phys. Chem. ReJ Data 1973,2, 163. (44) Duffy, J. A.; Ingram, M. D. J. Am. Chem. SOC.1971, 93, 6948. Duffy, J. A.; Ingram, M. D. J. Inorg. Nucl. Chem. 1975, 37, 1203. (45) Gaskell, D. R. Met. Trans. B 1989, 20B, 113. (46) Egelhoff, W. F. Surf.Sci. Rep. 1986, 6, 253. (47) Mullins, W. M.; Averbach, B. L. Surf.Sei. 1988, 206, 29. (48) Sherman, J. Chem. Rev. 1932,11, 93. (49) Mullins, W. M.;Averbach, B. L. Surf.Sci. 1988, 206, 41. (50) Parks, G. A.; de Bruyn, P. L. J. Phys. Chem. 1962,66,967. Parks, G. A. Chem. Rev. 1%5,65,177. Parks, G. A. Ado. Chem. Ser. 1%7,67,121. (51) Hasz, W. C. Surface Reactions and Electrical Double Layer Prop erties of Ceramic Oxides in Aqueous Solutions. SM Thesis, Massachusetts Institute of Technology, 1983. ( 5 2 ) Knowzinger, H.; Ratnasamy, P. Caral. Rev.-Sei. Eng. 1978, 17, 31. (53) Pauling, L. The Narure of the Chemical Bond, 3rd ed.; Cornell University Press: Ithaca, NY, 1960. (54) Roy, P.; Fuerstenau, Surf. Sei. 1972,30, 487. Aplan, F. F.; Spearin, E. Y. Colloids Surf.1980, 1, 361. ( 5 5 ) Henrich, V. E. Rep. Prog. Phys. 1985, 48, 1481. (56) Kroger, F. A. Electrical Properties of a-A1203. In Advances in Ceramics, 9; Kingery, W. D., Ed.; American Ceramic Society: Columbus, OH, 1984; p 1. Kroger, F. A. J. Amer. Cerum. SOC.1982, 63, 162. (57) Watson, R. E.; Davenport, J. W.; Perlman, M. L.; Sham, T. K . Phys. Rev. 1981, B24, 1791. (58) Cooper, C. F.; Kitchener, J. A. JISI 1959, 193, 48. (59) Linke, W. F. Solubilities: Inorganic and Metal-Organic Compounds, 4th ed.; American Chemical Society: Washington, DC, 1958; Vol. 1. LInke, W. F. Solubilities: Inorganic and Metal-Organic Compounds, 4th ed.; American Chemical Society: Washington, DC, 1965; Vol. 2.
w.
J. Phys. Chem. 1992,96, 5701-5704 (60) Mullins, W. M.Surf. Sci. 1989, 21 7, 459. (61) Mullins, W. M.;Fischer, T. E. Adsorption and Tribochemical Reactions. Materials Research Society Symposium Proceedings/fB 159,/fF' 257, 1990. (62) Bagchi, B. Annu. Reo. Phys. Chem. 1989,40, 115. Maroncelli, M.; MacInnis, J.; Fleming, G. R. Science 1989, 243, 1674. Arnett, E. M.; Amamath. K.;Harvey, N. G.; Cheng, J. P. Science 1990, 247, 423. (63) Katsanis, E. P.; Matijevec, E.Colloid Polym. Sci. 1983, 261, 255. (64) For examples see: Sung, N. H.; Kaul, A.; Ni, S.;Sung, C. S.P.; Chin, I. J. SPI Proc. 1981.2-B. Sung, N. H.; Sung, C. S.P. SPZ Proc. 1980,23-8. Adhem, J. S.; Davis, G. D.; Desai, A.; Venbles, J. D. US. Army Report AD-A106222, Oct 1, 1981. (65) Tormey, E. S.The Adsorption of Glyceryl Esters at the Alumina/ Toluene Interface. Ph.D. Thesis. Massachusetts Institute of Technology, 1982. (66) Posner, G. H.; Rogers, D. Z . J. Am. Chem. Soc. 1977, 99, 8208. Posner, G. H.; Rogers, D. Z . J . Am. Chem. Soc. 1977,99, 8214. Joshi, V.
5701
S.;Damodaran, N. P.; Dev, S. Tetrahedron 1968, 24, 5817. Joshi, V. S.; Damodaran, N. P.; Dev; S.Tetrahedron 1971,27, 459. Joshi, V. S.;Damodaran, N. p.; Dew, S.Tetrahedron 1971,27, 475. Dev, S.J. Sci. Ind. 1972, 31, 60. (67) Feke, D. L. Annual Report for NASA Grant NAG 3-468, 1985. Crimp, M.J.; Johnson, R. E., Jr.; Halloran, J. W.; Feke, D. L. Presented to the S m n d International Conference on Ultrastructure Procwsing of Ceramics, Glasses, and Composites, Sheraton Palm Coast, FL, 1985. (68) Mieskowski, D. M.; Sanders, W. A. J. Am. Ceram. Soc. 1985,68, C160. Rahaman. M. N.; Boiteux, Y.; De Jonghe, L. C. Am. Ceram. SOC. Bull. 1986, 65, 1171. (69) Cotton, F. A.; Wilkinson, A. Advanced Inorganic Chemistry; John Wiley & Sons: New York, 1980. (70) Regauoni, A. E.; Blesa, M. A.; Maroto, A. J. G. J . Colloid Interface Sci. 1983, 91, 560. (71) Yoshimura, M. Ceram. Bull. 1988, 67, 1951.
ARTICLES Viscosity Dependence of Chemically Induced Dynamic Electron Spin Polarization Generated by the Radical-Triplet Pair Mechanism Akio Kawai and Kinichi Obi* Department of Chemistry, Tokyo Institute of Technology, Ohokayama, Meguro, Tokyo 152, Japan (Received: December 6, 1991; In Final Form: March 24, 1992)
Chemically induced dynamic electron spin polarization generated through the interaction between the excited singlet and triplet molecules with nitroxide radicals is measured in several solvents and micellar solutions. The relative intensity of net polarization is more enhanced than hyperfine-dependentpolarization in highly viscous solvents and micellar solutions. The enhancement of net polarization is interpreted by the mixing of )Q3/2) and ID1/2) states in the radical-triplet pair mechanism (RTPM) due to the slack motion of radical-triplet pairs. These results verify that net and multiplet polarizations are independently generated in RTPM.
Introduction Quenching of excited molecules with free radicals is an interesting subject in photochemistry, but the details of interaction in the excited molecule-radical pair have not been cleared yet. Kinetic studies have been done on this subject by flash photolysis, and quenching rates have been measured in many systems.' The quenching is explained by several mechanisms such as energy transfer from excited molecule to radical,' contact charge transfer? and dipoledipole interaction? Theoretical studies have also been carried out on this theme? However, few ESR s t u d i e ~ ~have -~ been reported on the quenching of excited molecules with free radicals even though the paramagnetic species like free radicals and triplet molecules are concerned in these systems. Recent discovery of the radical-triplet pair mechanism (RTPM)S-7gives us an opportunity to study this subject with the timeresolved (TR) ESR method. Extensive studies have been carried out on radical reactions by TR-ESR, and a clear account for radical-radical interaction is given as the radical pair mechanism (RPM).9 Recently developed RTPM is expected to give us new insight into the quenching process of excited molecules with free radicals. In RPM, Trifunac reportedlo the solvent viscosity effects on chemically induced dynamic electron spin polarization (CIDEP) signal pattern and interpreted them through the contribution of S-T-' mixing in highly viscous solvents. Interaction between radicals is strongly afkctcd by the circumstances, which is reflected in the CIDEP signal pattern. In analogy with the RPM, such a solvent effect is a b expected to be reflected in the RTPM signal. 0022-3654/92/2096-5701$03.00/0
In RTPM, net polarization and hyperfine-dependent polarization are generated with mechanisms similar to the S-T-' and S-To mixing in the RPM, respectively.6 The former mixing generates CIDEP at a short distance and the latter at a long distance. Actually the S-T-' mixing is known to be effective in viscous solvents. In this paper, we have investigated the solvent viscosity effect on RTPM signals. Results obtained indicated the significant viscosity effects: Net polarization is enhanced in viscous solvents compared to hyperfine-dependent polarization. Such tendency was also observed in micellar solutions. These results give an important evidence for net and hyperfine-dependent polarization sources in RTPM and are interpreted in terms of these two polarizations.
Experimental Section Transient ESR signals of free radicals were obtained with a conventional X-band ESR spectrometer (Varian E-1 12) without field modulation and integrated with a boxcar integrator (Stanford SR-250). A XeCl excimer laser (Lambda Physik LPX 100) was used as an excitation light source. TR-ESR measurements were carried out at room temperature, and sample solutions were degassed by passage of nitrogen gas. Normally, the gate was opened from 1.O to 1.5 I.CS after the laser pulse. Details of the equipment were described previously." 4,4,6,6-Tetramethylpiperidinyloxy (TEMPO) (Aldrich) and other chemicals (Tokyo Kasei) were used as received. 0 1992 American Chemical Society
