**1. Introduction**

Lubricant formulations used to control friction and wear in metallic forming processes typically contain mixtures of molecular additives in the base oil. Common lubricant additives in the metal-rolling processes consist of one or more aliphatic alcohols, acids or esters such as vinyl-phosphonic acid, acetic acid, butanoic acid and butanol alcohol etc. which have hydroxyl (OH) or carboxyl (O═C─OH) functional group [named oxygen-rich base (O-base)] to behave like a cationic anchor with electron-rich charges [1–3]. Initially, these molecules are

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thought to anchor onto metal (e.g., hydroxide-alumina) surfaces through their O-bases. And then, such O-bases are believed to decompose on fresh (pure aluminum) surfaces occurring in the actual rolling work. As a result, these adsorbed and decomposed molecular pieces will contribute to the boundary thin-film lubrication and protection, i.e., their residual fragments (e.g., molecular chain tails) adhered on fresh surfaces with several molecular layers thick may serve as corrosion inhibitors of surface.

For adsorbate examples, **Figure 1** shows an illustration of two molecular structures: a vinylphosphonic acid [VPA, H3 C2 P(O)(OH)2 ] with a tri-podal O-base [2(OH)─P═O] plus a vinylhydrocarbon tail [4] as shown in **Figure 1(a)**, and an acetic acid [EA, H3 C2 (O)(OH)] with an alkyl-chain plus a bi-podal O-base [O═C─OH] as shown in **Figure 1(b)**.

In **Figure 1(a)**, a study of the inelastic tunneling spectroscopy (IETS) for a VPA adsorption on a hydroxide-Al2 O3 (0001) surface implied that the vinyl-tail on VPA did not participate in bonding to the oxide surface [5], but left itself accessible to react with other general lubricant molecules, which may serve as a molecular cap on the surface to inhibit migrations of corrosive species into the oxide surface. This reaction usually resulted in a tri-dentate coordination for the VPA on surface through a tri-podal O-base even if such a tri-dentate coordination was not unique on surface [6]. Crowell et al. utilized the electron energy loss spectroscopy (EELS) to observe an EA adsorption on an Al (111) surface starting at a very low temperature of 120 K [7]. They observed that a symmetrically bi-bridged geometry of main piece decomposed from the EA was more likely to bond to the surface than other configurations. In brief, both of above studies found that O-bases on acid molecules may usually bond to aluminum/alumina surfaces and oxidize them prior to other twigs, and then had molecular residual fragments (molecular chain tails) form thin-film inhibitors on the surfaces.

In **Figure 2**, two lubricant additives, butanol alcohol [H3

**Figure 2.** Schematics of (a) a butanol molecule; (b) a butanoic molecule.

which is very similar to the EA as shown in **Figure 1(b)**.

noic acid [H3

C─(CH2 ) 2 C─(CH2 ) 2 ─H2

Ab-Initio Modeling of Lubricant Reactions with a Metal Al (111) Surface

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5

─C(O)(OH)], are shown schematically: the alcohol in **Figure 2(a)** has

a C─OH functional group; while the acid in **Figure 2(b)** has an O═C─OH functional group

In this chapter, we carry out series of density functional (DFT) analyses on how two commonly used boundary-layer lubricant additives, VPA and EA, bond statically to a clean (pure) Al (111) surface in their optimal geometries. During the actual rolling (forming) processes to hydroxide-alumina surfaces, since top textures (layers) of the surfaces are usually peeled off so that fresh (pure) aluminum surfaces (below top layers) with nascent islands can be subsequently exposed in the air, without loss of generality, such the DFT static outputs may help powerfully determine the favorably bonding mechanisms of additives to the highly reactive islands on fresh surfaces, so as to make clear the formation of protective thin-film on alumina surfaces. Then next, we examine dynamic decomposition pathways on the clean Al (111) surface using ab-initio molecular dynamics (AIMD) for two other important aliphatic boundary-layer lubricant additives: butanol alcohol and butanoic acid, to determine their thermal mechanisms of monolayer formation on the surface. During the AIMD simulation, each molecule is orientated to collide with the clean surface through its reactive O-base. Initial approaching speeds of molecules toward the surface are taken from the actual Al-rolling process. Decomposition pieces of additive molecules on the Al (111) surface are explored in details throughout the whole simulation. Simulation outputs are qualitatively compared with experimental observations using the EELS and the XPS for similar molecules [7, 8], which may

reveal some unknown decomposed configurations in a previous DFT static study [9].

**Figure 3** shows a side view of a VPA structure, along with isosurfaces of charge density with

) localization function (ELF) representing the probability of finding a second

**2. Configurations of adsorbates and adsorbents**

with the same spin in the neighboring region of the first (reference) e−

**2.1. Adsorbate configurations**

the electron (e<sup>−</sup>

e−

C─(OH)] and buta-

within [0, 1]. In other

In addition, Underhill and Timsit [8] applied the X-ray photoelectron spectroscopy (XPS) to the study of dynamic decomposition pathways for 1-butanol and propanoic acid through molecular collisions with a clean Al (111) surface. At room temperature (e.g., 300 K), their results suggested that acid molecules break up on the surface, leading to attachments of aliphatic chains via their O-bases on surface. Alternatively, aliphatic alcohols were found to react with Al ions in the surface via their O-bases alone. At elevated temperatures (about 400 K), both acid and alcohol were found to dissociate on the clean surface, leading to attachments of aliphatic chains via their end C ions and pieces of decomposed O-base on surface, to form molecular boundary thin-films of surface. However, such the dynamic decomposition has received minimal attention in the literature.

**Figure 1.** Two acid molecules: (a) a VPA molecule; (b) an EA molecule.

Ab-Initio Modeling of Lubricant Reactions with a Metal Al (111) Surface http://dx.doi.org/10.5772/intechopen.72512 5

**Figure 2.** Schematics of (a) a butanol molecule; (b) a butanoic molecule.

thought to anchor onto metal (e.g., hydroxide-alumina) surfaces through their O-bases. And then, such O-bases are believed to decompose on fresh (pure aluminum) surfaces occurring in the actual rolling work. As a result, these adsorbed and decomposed molecular pieces will contribute to the boundary thin-film lubrication and protection, i.e., their residual fragments (e.g., molecular chain tails) adhered on fresh surfaces with several molecular layers thick may

For adsorbate examples, **Figure 1** shows an illustration of two molecular structures: a vinyl-

In **Figure 1(a)**, a study of the inelastic tunneling spectroscopy (IETS) for a VPA adsorption

bonding to the oxide surface [5], but left itself accessible to react with other general lubricant molecules, which may serve as a molecular cap on the surface to inhibit migrations of corrosive species into the oxide surface. This reaction usually resulted in a tri-dentate coordination for the VPA on surface through a tri-podal O-base even if such a tri-dentate coordination was not unique on surface [6]. Crowell et al. utilized the electron energy loss spectroscopy (EELS) to observe an EA adsorption on an Al (111) surface starting at a very low temperature of 120 K [7]. They observed that a symmetrically bi-bridged geometry of main piece decomposed from the EA was more likely to bond to the surface than other configurations. In brief, both of above studies found that O-bases on acid molecules may usually bond to aluminum/alumina surfaces and oxidize them prior to other twigs, and then had molecular residual fragments

In addition, Underhill and Timsit [8] applied the X-ray photoelectron spectroscopy (XPS) to the study of dynamic decomposition pathways for 1-butanol and propanoic acid through molecular collisions with a clean Al (111) surface. At room temperature (e.g., 300 K), their results suggested that acid molecules break up on the surface, leading to attachments of aliphatic chains via their O-bases on surface. Alternatively, aliphatic alcohols were found to react with Al ions in the surface via their O-bases alone. At elevated temperatures (about 400 K), both acid and alcohol were found to dissociate on the clean surface, leading to attachments of aliphatic chains via their end C ions and pieces of decomposed O-base on surface, to form molecular boundary thin-films of surface. However, such the dynamic decomposition

] with a tri-podal O-base [2(OH)─P═O] plus a vinyl-

(0001) surface implied that the vinyl-tail on VPA did not participate in

C2

(O)(OH)] with an

serve as corrosion inhibitors of surface.

4 Lubrication - Tribology, Lubricants and Additives

O3

C2

P(O)(OH)2

(molecular chain tails) form thin-film inhibitors on the surfaces.

has received minimal attention in the literature.

**Figure 1.** Two acid molecules: (a) a VPA molecule; (b) an EA molecule.

hydrocarbon tail [4] as shown in **Figure 1(a)**, and an acetic acid [EA, H3

alkyl-chain plus a bi-podal O-base [O═C─OH] as shown in **Figure 1(b)**.

phosphonic acid [VPA, H3

on a hydroxide-Al2

In **Figure 2**, two lubricant additives, butanol alcohol [H3 C─(CH2 ) 2 ─H2 C─(OH)] and butanoic acid [H3 C─(CH2 ) 2 ─C(O)(OH)], are shown schematically: the alcohol in **Figure 2(a)** has a C─OH functional group; while the acid in **Figure 2(b)** has an O═C─OH functional group which is very similar to the EA as shown in **Figure 1(b)**.

In this chapter, we carry out series of density functional (DFT) analyses on how two commonly used boundary-layer lubricant additives, VPA and EA, bond statically to a clean (pure) Al (111) surface in their optimal geometries. During the actual rolling (forming) processes to hydroxide-alumina surfaces, since top textures (layers) of the surfaces are usually peeled off so that fresh (pure) aluminum surfaces (below top layers) with nascent islands can be subsequently exposed in the air, without loss of generality, such the DFT static outputs may help powerfully determine the favorably bonding mechanisms of additives to the highly reactive islands on fresh surfaces, so as to make clear the formation of protective thin-film on alumina surfaces. Then next, we examine dynamic decomposition pathways on the clean Al (111) surface using ab-initio molecular dynamics (AIMD) for two other important aliphatic boundary-layer lubricant additives: butanol alcohol and butanoic acid, to determine their thermal mechanisms of monolayer formation on the surface. During the AIMD simulation, each molecule is orientated to collide with the clean surface through its reactive O-base. Initial approaching speeds of molecules toward the surface are taken from the actual Al-rolling process. Decomposition pieces of additive molecules on the Al (111) surface are explored in details throughout the whole simulation. Simulation outputs are qualitatively compared with experimental observations using the EELS and the XPS for similar molecules [7, 8], which may reveal some unknown decomposed configurations in a previous DFT static study [9].

## **2. Configurations of adsorbates and adsorbents**

#### **2.1. Adsorbate configurations**

**Figure 3** shows a side view of a VPA structure, along with isosurfaces of charge density with the electron (e<sup>−</sup> ) localization function (ELF) representing the probability of finding a second e− with the same spin in the neighboring region of the first (reference) e− within [0, 1]. In other

**Figure 3.** Side view of a VPA structure: (a) a vinyl-group and a tri-podal based P─O bonds on the VPA; (b) isosurfaces of charge density at the ELF = 0.81 for the VPA.

words, a high ELF value means a highly localized behavior for the first (reference) e− [10–13]. According to this definition, high ELF values are typically associated with covalent bonds, e− lone pairs, or inert cores [14]. In **Figure 3(b)**, ELF = 0.81 is the best visual difference of isosurfaces for each of atomic bonds according to comments in ref. [15]. The VASP (Vienna Ab-initio Simulation Package) calculations for the VPA indicated that two e<sup>−</sup> lone pairs aggregated to O2 (and O3) nearby (a circle-lunar lobe) because of its *aniso-sp3* hybrid bonding to H2 and P neighbors. However, O1 formed a weak double bond with P and had slightly more e− lone pairs (a hemisphere lobe) than O2 and O3, see **Figure 5(a)**. In **Figure 3(a)**, such a VPA

conformation was the most favorably energetic for the vinyl-group coplanar with the P═O1 double bond, and was consistent with that predicted in ref. [15]. Also, C1═C2 double bond

**Figure 4** shows a side view of an EA structure, along with isosurfaces of charge density at the ELF = 0.67 to provide the best visual difference for each of atomic bonds. VASP analyses for

hybrid type) resulted in a lower charge density in e<sup>−</sup>

lone pairs aggregated to O2 nearby (a hemisphere lobe) because

Ab-Initio Modeling of Lubricant Reactions with a Metal Al (111) Surface

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lone pairs around O1

hybrid bonding to H4 and C2 neighbors. However, a strong C2═O1 double

hybrid bonding type.

**Figure 5.** Charge density of states (DOS) for O ions on (a) the VPA and (b) the EA.

(a cashew type) than around O2, see **Figure 5(b)**.

showed an *aniso-sp2*

of its *aniso-sp3*

bond (an *aniso-sp2*

the EA indicated that two e<sup>−</sup>

**Figure 4.** Side view of an EA structure: (a) a methyl group and a bi-podal-based C─O bonds on the EA; (b) isosurfaces of charge density at the ELF = 0.67 for the EA.

Ab-Initio Modeling of Lubricant Reactions with a Metal Al (111) Surface http://dx.doi.org/10.5772/intechopen.72512 7

**Figure 5.** Charge density of states (DOS) for O ions on (a) the VPA and (b) the EA.

words, a high ELF value means a highly localized behavior for the first (reference) e−

Ab-initio Simulation Package) calculations for the VPA indicated that two e<sup>−</sup>

gated to O2 (and O3) nearby (a circle-lunar lobe) because of its *aniso-sp3*

e−

charge density at the ELF = 0.81 for the VPA.

6 Lubrication - Tribology, Lubricants and Additives

of charge density at the ELF = 0.67 for the EA.

e−

According to this definition, high ELF values are typically associated with covalent bonds,

**Figure 3.** Side view of a VPA structure: (a) a vinyl-group and a tri-podal based P─O bonds on the VPA; (b) isosurfaces of

H2 and P neighbors. However, O1 formed a weak double bond with P and had slightly more

**Figure 4.** Side view of an EA structure: (a) a methyl group and a bi-podal-based C─O bonds on the EA; (b) isosurfaces

lone pairs (a hemisphere lobe) than O2 and O3, see **Figure 5(a)**. In **Figure 3(a)**, such a VPA

 lone pairs, or inert cores [14]. In **Figure 3(b)**, ELF = 0.81 is the best visual difference of isosurfaces for each of atomic bonds according to comments in ref. [15]. The VASP (Vienna

[10–13].

lone pairs aggre-

hybrid bonding to

conformation was the most favorably energetic for the vinyl-group coplanar with the P═O1 double bond, and was consistent with that predicted in ref. [15]. Also, C1═C2 double bond showed an *aniso-sp2* hybrid bonding type.

**Figure 4** shows a side view of an EA structure, along with isosurfaces of charge density at the ELF = 0.67 to provide the best visual difference for each of atomic bonds. VASP analyses for the EA indicated that two e<sup>−</sup> lone pairs aggregated to O2 nearby (a hemisphere lobe) because of its *aniso-sp3* hybrid bonding to H4 and C2 neighbors. However, a strong C2═O1 double bond (an *aniso-sp2* hybrid type) resulted in a lower charge density in e<sup>−</sup> lone pairs around O1 (a cashew type) than around O2, see **Figure 5(b)**.

[15]. Prior to these calculations, lattice constant, bulk modulus and cohesive energy for a pure aluminum bulk by fitting data of energy versus volume to the Murnaghan Eq. [22] was calculated. A regular Monkhorst-Pack grid [23] of 17 × 17 × 17 was chosen as the best *k*-point sampling, so that the total energy of system was converged within 1–2 meV/atom. The computed

Then next, calculations of acid adsorptions on the Al (111) surface slab were conducted in a

plane-wave cutoff energy, 400 eV, which was primarily required by the "hardest ions (C and O)", was chosen. A regular Monkhorst-Pack grid of a 5×5×1 *k*-point sampling was selected

10.0 Å in *c* direction and with one bottom layer of the Al (111) slab fixed along *c* direction in the supercell. The atomic geometry was optimized through minimizing the Hellman-Feyman forces using a conjugate gradient algorithm [11], until the total force on each ion reduced to

In this study, all ab-initio molecular dynamics (AIMD) simulations were also based upon the DFT as implemented in the VASP. A Vanderbilt-type ultrasoft pseudopotentials (USP) were utilized for elemental constituents by means of the GGA [24]. In real practice, the GGA usually yielded inaccurate reaction barriers [25–27], while a semilocal-BLYP and hybrid-B3LYP functionals seemed to predict adsorption energies accurately and distinguish adsorption sites correctly. However, for the study of dynamic decomposition, we believed that the GGA was also a reasonable compromise since a highly colliding velocity acting on molecules toward the

temperature (300 K) plus an ambient pressure of 1.0 bar for about 1500 time steps, a simulation time step of 0.001 ps was selected. A regular gamma-centered grid of 5 × 5 × 5 was chosen as the best *k*-point sampling for the unit cell. Total energy of the system was converged within 1–2 meV/atom. A plane-wave cutoff energy, 400 eV, as dictated by the hardest oxygen pseudopotential, was adopted in all simulations. The computed lattice constant, *a*<sup>0</sup> = 4.05(7) Å, was

Besides, for modeling interactions between additive molecules and an Al (111) slab, a Monkhorst-Pack grid of 5 × 5 × 1 was selected for the best *k*-point sampling. A supercell with the entire Al (111) slab consisted of four Al layers (36 ions per layer) of 144 ions. This

24.0 Å in the *c* direction to preclude interactions with periodic images. The bottom layer of the Al (111) slab was fixed along the *c* direction to prevent the whole slab motion during impacts

along the Y axis, and *c* [111] = 40.0 Å along the Z axis plus a vacuum distance of

\_ 10] = \_\_3 2√ \_\_ <sup>6</sup> *a*<sup>0</sup>

Al surface slab would likely overwhelm any barrier to the decomposition.

using the NPT ensemble, which thermally equilibrated one 2*a*<sup>0</sup> × 2*a*<sup>0</sup> × 2*a*<sup>0</sup>

favorably fitting to other calculations and experimental observations [28].

During the AIMD simulation, first of all, lattice constant (*a*<sup>0</sup>

orthorhombic geometry had three definite orientations: *a*[1

of additive molecules onto the Al slab surface.

' along the Y axis, and *c* [111] = 26.0 Å along the Z axis, plus a vacuum distance of

'units (16 Al ions per layer) in XY directions. A

' along the X axis,

9

) of pure Al bulk was calculated

unit cell at a room

along the X axis,

\_ 10] = 2√ \_\_ 2*a*0

Ab-Initio Modeling of Lubricant Reactions with a Metal Al (111) Surface

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'× 3*a*<sup>0</sup>

'= 4.05(3) Å, see ref. [9].

for this orthogonal supercell with three definite orientations: *a*[1

supercell with a periodic four-layer 3*a*<sup>0</sup>

lattice constant, *a*<sup>0</sup>

0.05 eV/Å or less.

**3.2. Dynamic simulations**

*b*[11 \_ 2] = √ \_\_ 6*a*0

*b*[11 \_ 2] = 3√ \_\_ 2*a*0

**Figure 6.** Three most favorable adsorption sites in top layer of Al (111) surface slab.

**Figure 5** shows distributions of charge density of states (DOS) for three O ions on the VPA and two O ions on the EA, respectively. In **Figure 5**, the *E*lumo represented the energy level corresponding to the lowest unoccupied molecular orbital, i.e., above the *E*lumo the DOS for O ions represented unoccupied states. In **Figure 5**, more portions of the DOS for O ions on the VPA appeared in orbitals above the *E*lumo than those on the EA, implying that VPA may be more reactive with the Al surface than EA by feat of their O ions.

### **2.2. Adsorbent configurations**

Since Al (111) surface has the lowest surface energy among all surface geometries in aluminum bulk, it is likely to expose in the air during the actual rolling process. So here it is selected as an adsorption slab surface (adsorbent) to react with above adsorbates. **Figure 6** shows three distinguishable adsorption sites in the top layer of Al (111) surface slab. We refer to them as site-1, -2, and -3, respectively. Among these sites, site-1 (S-1) had corners at three Al ions; site-2 (S-2) had corners at three cave points (cross signs); and site-3 (S-3) had corners at three saddle points (ice-star signs). According to past experiences [14], these three sites were more likely to react with adsorbates than others because they allowed O-bases on adsorbates to bond to surface stronger than other ones.
