Contents

TRIBOCHEMISTRY

SUMMARY

Keywords: tribochemistry, boundary lubrication, lubrication mechanism, NIRAM, catalysis

INTRODUCTION

Figure 1: Major physical and chemical events in the boundary lubrication contact

the influence of boundary and/or mixed lubrication operating conditions. From the view-point of chemical reactions at surfaces, a very comprehensive review of mechanically initiated reactions is described in [1]. It is stressed that many tribological problems are concerned with various tribochemical reactions that cannot be accounted for by frictional heat alone. Another review [2] emphasizes the technological importance of the mechanical initiation of chemical reactions in such processes as grinding, drilling, crushing and cutting which are often facilitated by chemical compound formation at the worked interface. Tribochemical reactions are distinct from those of thermochemical ones because activation energy (E_a) for the latter is much higher. For example, E_a for iron oxide formation from iron and oxygen differs very significantly if the thermochemical reaction (E_a=54 kJ/mol) is compared with tribochemical reaction (E_a=0.7 kJ/mol) [1]. Similar dependence may be observed for other reactions. The occurrence of reaction velocity independent of temperature shows clearly that the energy for releasing the reactions is applied to the solid through the mechanically activated regions only. The same is due to catalysis and tribocatalysis. It is to note at this point that chemically stimulated exoelectron emission occurs continuosly from silver catalyst during partial oxidation of ethylene and the emission rate is proportional to the rate of ethylene oxide formation [3]. The central thrust of this paper is to review and discuss a wide variety of tribological experimental results relating to metals, ceramics as well as diamond-like carbon (DLC). It aims mostly at answering some specific questions concerning both tribochemistry and catalysis. Additional objective of the present paper is to look for links between tribochemistry and tribocatalysis in terms of the NIRAM concept.

2 BOUNDARY LUBRICATION AND TRIBOCHEMISTRY

When oxides of the contact area are thick and porous they are ruptured easily. That results in both plastic deformation of the sliding surfaces and a wide variety of physical phenomena, particularly those related to the triboemission process. Disruption of the surface oxide layer generally results in some physical processes and usually in higher friction and severe wear. The wear debris produced are entrapped at the interface and plowing and microcutting of the surfaces occurs. During these interactions (a) forces are transmitted, (b) energy is consumed, (c) physical and chemical natures of these materials are changed, and (d) the surface topography is altered. Understanding the nature of these interactions is addressed to tribophysics [7] which describes the basic mechanisms that govern interfacial behavior and illustrate how the basic theories can be applied to provide practical solutions to important friction and wear problems. Physics and chemistry of tribological wear has been discussed in [8]. The formation of fresh surface sites forms a real bridge between physics and chemistry of the wear processes leading to microscopic aspects of a wide variety of wear mechanisms. These mechanisms are often connected with tribochemical reactions that are initiated by the surface enlargement effects. Typical physical phenomena evolved by wearing processes encompass mostly high temperature and triboemission. Detailed information concerning the triboemission process and related phenomena are described in [9]. Triboemission is defined as emission of electrons, charged particles, lattice components, photons, etc., under conditions of boundary friction conditions and/or surface damage caused by fracture processes. Figure 2 illustrates the triboemission process connected with the surface enlargement during friction.

Figure 2: Physical processes evolved by friction [9]

One of the most important component of triboemitted particles encompasses electrons. Accordigly, the triboemission process is of particular significance for both the boundary friction process as such and tribochemistry of the boundary lubrication process.

3 BASIC INFROMATION ON THE NIRAM APPROACH

3.1 General Description

Figure 3: Schematic of the electron emission process during rubbing

Most recent research [11] provided clear evidence that the major part of negatively charged particles emitted from diamond, alumina, and sapphire is low-energy from zero to 5 eV. This finding allows to consider the NIRAM approach as an important factor initiating typical tribochemical reactions of the environment compounds with mechanically treated solids or more specifically, being under boundary lubrication conditions. Actually, the chemical reactions under friction conditions include several types of reaction processes (i) "pure" tribochemistry, i.e. specific chemical reactions initiated only by the mechanical action of the system (mechanochemistry), (ii) reactions of thermochemical processes (e.g., oxidation, degradation), and (iii) catalytic processes related to both catalysis and tribocatalysis. The specific tribochemical reactions proceed according to both the free radical mechanism and ionic mechanism. The shearing process generates free radicals and other tribochemical reactions comprise ionic and free radical reaction types. Negative-ion-radical intermediate species are of particular interest. Therefore, some 15 years ago the negative-ion-radical action mechanism (NIRAM) approach was proposed [12-14]. Recently, this concept has been reviewed and its importance for lubricant compound reactions with the friction surfaces was presented [15]. In brief, the NIRAM approach considers the following major phases:

1. Low-energy electron emission process and creation of positively-charged spots, generally on tops of asperities;
2. Action of the emitted electrons with the lubricant causing the formation of reactive intermediates -- negative ions and free radicals -- on the rubbing surfaces;
3. Reaction of negative ions with metal surface, and other reactions, e.g., free radical reactions, forming mostly an organometallic film, which protects the rubbing surfaces from wear;
4. If shear strength is high, it can cause cracking of chemical bonds producing inorganic films and further radicals.
5. Eventual destruction of the wear protecting layer, followed by the electron emission process and subsequent formation of a new protective film according to stages (a) through (d).

A + :B ® A:B

The species A:B is called a coordination compound or an acid-base complex. Since the HSAB principle is a pragmatic one and has its application as a predictive and correlative principle it seems to be important to apply this approach for getting a better understanding of lubrication mechanisms.

3.2 Example of Specific Reactive Intermediates

Figure 4: Negative ions emitted from the Demnum S200 lubricant during irradiation with low-energy electrons [18]

negative-ion-radical action mechanism for the PFPE lubricant degradation process. Interestingly, the NIRAM approach can also be applied in accounting for experimental results concerning chemical bonding of PFPE lubricant films with DLC under sliding conditions. Earlier paper [20] demonstrated how low-energy electrons may attach to the PFPE lubricant molecules and allow the chemical reactions between lubricant molecules and a DLC surface. All the discussed results clearly suggest that anionic intermediates play an important role in the electron-induced degradation process of PFPE lubricants. Review paper [21] discusses and analyses the literature concerning the interaction and degradation mechanisms of perfluoropolyether lubricants with carbon protective overcoats used for magnetic media.

Formation mechanism of the negative species (m/e) ^- = 163 generated from the Demnum S200 lubricant can be interpreted as follows. After splitting off the (m/e) ^- = 185 anion (CF₃CF₂CF₂O^-) from the Demnum S200 molecule, the left free radical ^oO-CF₂-CF₂-CF₂-O-CF₂- CF₂-CF₂ ....... interacting with a low-energy electron can form the following negative-ion-radical species:

oO-CF2-CF2-CF2-O-

with the m/e ratio of 182. This reactive intermediate splits off one free radical (Fo) generating the negative ion (m/e) ^- = 163 having the following structure:

O=CF-CF2-CF2-O-

which corresponds to the strong signal found in the negative-ion mass spectrum of the Demnum S200 lubricant (Figure 4). Although the catalytic degradation mechanism is widely accepted in the literature -- basing on a wide variety of discussed experimental data -- it is reasonable to emphasize that anionic intermediates (negative ions and/or negative-ion-radical species) produced by low-energy electrons play an important part in the electron mediated degradation process of PFPE lubricants. This is in line with the negative-ion-radical action mechanism (NIRAM) approach. This approach is evidenced somehow by elucidating the formation mechanism of the negative ion (m/e) = 163 from the negative-ion-radical species ?(m/e)^.- = 182.

4 TRIBOCHEMISTRY OF ALIPHATIC HYDROCARBONS

4.1 General Remarks

4.2 Reaction Mechanism

It has been found that n-hexadecane under boundary friction conditions forms very specific complex compounds with aluminum [23]. FTIRMA spectra taken from the wear track clearly demonstrated two new significant absorption bands at 1547 cm ^-1 and 1657 cm ^-1. The most specific situation arises from the fact that similar peaks are observed in hydroxyl group containing compounds [24]. The similarity of spectra taken from n-hexadecane lubricated wear tracks with those lubricated with alcohols clearly shows that n-hexadecane during the friction process is oxidized either to alcohols or just transformed to alkoxide anions. The latter might be generated from (C_nH_2n+2OH) ^- ions. Figure 5 depicts the general fragmentation mechanism of negative ions produced from apliphatic hydrocarbons. The reactions in Figure 5 are based on previous experimental data concerning the negative ionization and fragmentation of n-paraffin hydrocarbons [25] in the electron attachment mass spectrography.
To obtain negative ions of paraffins, HO^- ions have to be attached [10]. This mechanism of ionization can be tested with small amount of heavy water vapour in the ion source. Another mechanism of ionization [(M - 1) ^-]

Figure 5: Fragmentation of (CnH2n+2.OH) - ions

was found in work [25]. Under the conditions of the investigation only (M - 1) ^- ions were formed by n-hexane molecules. The stability of (M + HO^-) ions decreased with a decrease in the molecular weight of the n-paraffins. The intensity of M+17 peaks increase from n-heptane to n-nonane in the ratio 1:2.5:6.

It is to note that major ions formed closely relate to those that are generated from alcohols, as described in [24]. This clearly evidences the similarity of FTIR spectra taken from wear scars lubricated with alcohols and those taken from wear scars generated under boundary friction conditions lubricated with n-hexadecane.

5 TRIBOCHEMISTRY OF ESTERS

It is well known that esters under boundary lubrication conditions form soap with the friction metal surface. The lubrication related literature suggests that the soap formation by the ester under boundary lubrication is due to the hydrolysis process of the ester. This leads to the formation of a very small amount of the fatty acid which when adsorbed attach the metal to form the corresponding soap film. On the other hand, it is supposed that the soap formation proceeds according to the NIRAM approach. It is assumed that the soap formation is due to the ester C-O bond cleavage leading to the formation of the RCOO^- species which reacts with the positively charged surface spot to form the soap. Recent study [26] aimed at elucidating the actual reaction mechanism by which a series of palmitic esters, used as antiwear additives to n-hexadecane, reduce the wear of the steel-on-steel system. It has been assumed [27] that alcohols are physically adsorbed on the reactive metal surfaces; esters with reactive metals such as zinc and cadmium, however, cause chemical attack. It was emphasised that the reaction with the ester is due to hydrolysis. The hydrolysis reaction is known to be catalysed by acids or bases. A carboxylic ester is hydrolysed to a carboxylic acid and on alcohol with aqueous acid or aqueous base. Base promotes hydrolysis of esters by providing the strongly nucleophilic reagent OH^-. It is self evident that under alkaline conditions the carboxylic acid is obtained as its salt, for example the sodium salt, from which it can be liberated by addition of mineral acid. The alkali hydrolised reaction is essentially irreversible due to a resonance - stabilized carboxylate anion which shows little tendency to react with an alcohol. On the other hand, acidic hydrolysis is reversible,

RCOOR1 + H2O

¬H+®

RCOOH + R1OH

thus the mechanism for acidic hydrolysis is also the mechanism for esterefication. Mineral acid speeds up both processes by promoting carbonyl oxygen and this rendering carbonyl carbon move susceptible to nucleophilic attack. In hydrolysis, the nucleophile is a water molecule and leaving group is an alcohol; in esterifcation, the roles are exactly reversed. Based on the above information it is difficult to realise that under typical boundary lubrication conditions mineral acid might be present to catalyse the ester hydrolysis process. To check this we have carried out a set of experiments aiming at finding if typical conditions existing under boundary lubrication could cause the hydrolysis process. Under there conditions, but without a catalyst, we have not found any evidence for the acid formation from aliphatic esters. Experiments performed in that work were designed in a way aiming at checking what might be the effect of a possible breaking of the ester bond by hydrolysis on the lubricant anti-wear property. Thus, two series of lubricants have been selected. The first lubricant series includes n-hexadecane with ester additives at the concentrations ranging from 0,03% to 1,5% wt. The second series of lubricants relates to mixtures of these ester substrates (acid and alcohol), added to n-hexadecane at the same concentrations as for the esters. Equimolar ratio of the acid to alcohol was used for all the mixtures applied. If under the testing conditions the ester hydrolysis actually involves the breaking of the ester bond to produce an acid and an alcohol, the anti-wear behaviour of the esters should correspond to the anti-wear behaviour of their substrates. In other words, the equimolar acid/alcohol mixture at very low concentration in n-hexadecane should not provide higher relative wear reduction of the ball than the ester at much higher concentration. Figure 6 shows examples of the ball wear reduction versus the additive concentration in n - hexadecane.

The most striking finding, clearly demonstrated in Figure 6, relates to the fact that the acid/alcohol mixtures investigated have the same effect on the steel-on-steel wear process. Their anti-wear property does not change with the concentration in n-hexadecane. Additionally, the mixtures affect dramatically the wear reduction of steel under boundary lubrication conditions. Results obtained in this work enable to state that the hypothesis saying "during the friction process under boundary lubrication conditions, lubricated by aliphatic esters, the ester hydrolysis process cannot proceed without an adequate catalyst" was proved for aliphatic esters dissolved in n-hexadecane.
Another interesting finding relates to the best wear reduction ability of n-hexadecyl palmitate and the equimolar mixture of palmitic acid and 1-hexadecanol

Figure 6: Influence of the extreme concentrations (300 ppm and 1.5%) of two palmitates and equimolar mixture of palmitic acid and C14 alcohol in n-hexadecane on the pin-on-disk ball wear [28]

[27]. This finding is interpreted in terms of the chain matching effect with n-hexadecane. More research is needed to clarify these interactions.

6 TRIBOCHEMISTRY OF CERAMICS

6.1 Silicon Nitride and Silicon Carbide

As a possible application of the NIRAM - HSAB approach for lubrication of ceramics, detailed account for tribochemical wear reactions of silicon nitride with water and alcohols was recently discussed in a review paper on tribochemistry of ceramics [28]. Usually, tribochemical reaction between silicon nitride and water to form silica is postulated via termochemical reaction process as follows:

Si3N4    +    6H2O ® 3SiO2    +    4NH3	(1)
SiO2    +    2H2O ® Si(OH)4	(2)

Results presented in work [29] clearly show that ammonia originates from the mechanical grinding of any kind of silicon nitride, irrespective of the preparation method adopted. The likely reaction is mechanically activated hydrolysis by water vapour, i.e., a reaction caused by direct contact during the milling procedure between disturbed Si-N bonds and water, yielding silica and ammonia. Paper [30] describes a mechanistic model of ammonia formation from silicon nitride in the presence of water. To find out the relationship between adsorption and/or reaction of water with silicon nitride and generation of uncondensable gases, the moles of evolved gas have been reported as a function of adsorbed amounts of water [29]. Figure 7 demonstrates that linear relationship was found between the two quantities which indicates that a definite fraction of uptaken water is involved in a chemical reaction.

Figure 7: Relationship between moles of water adsorbed on silicon nitride powder and moles of ammonia evolved under grinding conditions [29]

The ratio of gases evolved n_gas (in moles) to moles of the water vapour adsorbed nads, is around 0.11. On the other hand, the average ratio ngas/nads for Equation (1) is 0.666 which is several times higher than the ratio determined experimentally. The difference can be explained in terms of the NIRAM-HSAB approach as presented in Figure 8.

Figure 8: NIRAM - HSAB Model for reaction of water with silicon nitride under friction conditions [28]

Very similar approach seems to be applicable to silicon carbide. The suggested tribochemical reaction process of silicon carbide with water under rubbing conditions is depicted in Figure 9.

Figure 9: NIRAM - HSAB Model for reaction of water with silicon carbide under friction conditions

6.2. Tribochemistry of Alumina and Silica

The well known fact of hydroxide formation from alumina under friction conditions in the presence of water molecules can also be explained in terms of the NIRAM - HSAB approach. Under rubbing conditions the emitted electrons produce OH^- and H^• species from water and positively charged sites ( Al⁺ ) and free radicals ( Al-O^• ) from the alumina substrate. HSAB interaction and radical recombination produce the hydroxyl groups (Al-OH ). Similar tribochemical process can be considered for silica. Figure 10 depicts that situation.

Figure 10: NIRAM - HSAB Model for reaction of water with silica under friction conditions

7 LINKS BETWEEN TRIBOCHEMISTRY AND CATALYSIS

Numerous chemical reactions relating also to catalytic processes can proceed with the same velocity at significantly reduced temperatures in comparison to the thermal reference procedure [1]. This situation is very similar to the non-catalytic thermochemical reactions and their tribochemical analogues. Therefore from the view-point of the reaction mechanism both catalytic processes and typical tribochemical reactions might relate to the same driving force -- governed by the affect of exoelectrons. Heterogeneous catalytic processes can be initiated by thermally emitted electrons. Tribocatalytic process is the typical catalytic reaction enhanced by the action of triboelectrons produced in the test system. This hypothesis can be evidenced by earlier experiments described in [3]. That work studied electron emission from silver catalyst during partial oxidation of ethylene by measuring simultaneously the emission rate and the rate of the ethylen oxide formation. Fig. 11 demostrates the effect of temperature on the emission rate of exoelectrons and on the formation rate of ethylen oxide, for which the temperature of the silver catalyst was raised step-wise from 25-210°C followed by the descent to 25°C.

Figure 11: Effect of temperature on the exoelectron emission rate and the formation rate of ethylene oxide [3]

Simultaneous measurements of the exoelectron emission rate and the formation of ethylene oxide at each level of temperature were made as a function of time. In work [3] the exoelectron emission from silver catalyst during partial oxidation of ethylene was accounted for as thermo-electron emission in a thin semiconducting oxide on silver. Considering the discussed results it is hypothesized that some heterogeneous catalytic processes proceed according to the NIRAM-HSAB approach similarly to tribochemical reactions. However, typical pure tribochemical reactions are initiated by triboelectrons and heterogeneous catalytic processes can be initiated by thermally emitted electrons. On the other hand, a tribocatalytic process can be considered as the catalytic process enhanced by the action of low-energy electrons emitted from the friction contact.

8 CONCLUSIONS

The present paper is an interdisciplinary review of extremely fast developing research on tribochemistry. This review demonstrates the importance of a pathway combining the negative-ion-radical action mechanism (NIRAM) approach with the Hard and Soft Acids and Bases (HSAB) concept in initiating tribochemical reactions. Major features of the lubrication model which presents a specific reaction cycle of lubricant components with friction solid contact spots is depicted in Figure 12.

Figure 12: Reaction cycle of lubricant components on solid contacts during friction

The most important conclusion is that the NIRAM approach is associated with both tribochemistry and heterogeneous catalysis.Characteristic IR common feature of n-hexadecane reflects in very specific absorption bands of wear products appearing at around 1550 cm ^-1, 1650 cm ^-1 and 3300 cm ^-1 for different metals. Similar IR spectra relate also to tribochemical products of alcohols and esters. These specific spectra are also typical for both tribochemistry of ethers [31] and tribochemistry of fatty acids [32]. Finaly, it seems that the most recent research [33] provides a good deal of additional evidence for the importance of NIRAM approach in initiating tribochemical reactions.

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Contents

• C. Kajdas; Tribochemistry
• Summary
• Introduction
• 2 Boundary Lubrication and Tribochemistry
• 3 Basic Infromation on the Niram Approach
  • 3.1 General Description
  • 3.2 Example of Specific Reactive Intermediates
• 4 Tribochemistry Of Aliphatic Hydrocarbons
  • 4.1 General Remarks
  • 4.2 Reaction Mechanism
• 5 Tribochemistry of Esters
• 6 Tribochemistry of Ceramics
  • 6.1 Silicon Nitride and Silicon Carbide
  • 6.2. Tribochemistry of Alumina and Silica
• 7 Links Between Tribochemistry And Catalysis
• 8 Conclusions
• 9 References

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