**4. NIRAM and HSAB (hard and soft acids and bases) and catalytic approaches**

## **4.1. Basic information on NIRAM**

In brief the NIRAM approach comprises the following major steps.


Figure 8 illustrates the NIRAM reaction cycle. Most recent review [11] is on the NIRAM approach and shows its interrelationship with tribocatalysis. The application of the NIRAM approach explains the role of exoelectrons in some relevant tribochemical reactions detailed in [29].

**Figure 8.** Reaction cycle of lubricant components on solid contacts during friction.

In summary, this boundary lubrication model proposes the formation of protective organometallic and inorganic layers on rubbing surfaces. The initiation reaction process is due to mechanical action evolving emission of low-energy electrons (1-4 eV) combined with flash temperature expressed in the thermionic emission (see Figure 1).

#### **4.2. Examples of NIRAM applications**

218 Tribology in Engineering

in [29].

combined with nanoscience [46].

**and catalytic approaches** 

**4.1. Basic information on NIRAM** 

even more entangled. Our present knowledge shows that mechanochemistry is also well

Interaction of the emitted electrons with the lubricant molecules producing negative

 Other reactions, producing organometallic or inorganic film, which protects the rubbing surfaces from wear; if the shear strength is high chemical bonds of organometallic compounds are cleaved resulting in producing inorganic films and further radicals; Eventual destruction of protective layer caused by wear, followed by electron emission

Figure 8 illustrates the NIRAM reaction cycle. Most recent review [11] is on the NIRAM approach and shows its interrelationship with tribocatalysis. The application of the NIRAM approach explains the role of exoelectrons in some relevant tribochemical reactions detailed

**4. NIRAM and HSAB (hard and soft acids and bases)** 

In brief the NIRAM approach comprises the following major steps.

ions and radicals on tops of the rubbing surfaces;

and subsequent formation of a new protective film.

Low-energy electron emission and generation of positively charged spots;

Reaction of negative ions with positively charged sites of friction surfaces;

**Figure 8.** Reaction cycle of lubricant components on solid contacts during friction.

flash temperature expressed in the thermionic emission (see Figure 1).

In summary, this boundary lubrication model proposes the formation of protective organometallic and inorganic layers on rubbing surfaces. The initiation reaction process is due to mechanical action evolving emission of low-energy electrons (1-4 eV) combined with

#### *4.2.1. Traditional approach to mechanisms of AW and EP lubricant additives*

Tribochemistry of lubricating oils is overviewed in Pawlak's book [47]. Important and interesting approach to oil formulations and complex lubrication processes is assigned to inverse micelle. Figure 9 illustrates the inverse micelles involvement to interactions of base oils with major engine oil additives. The nature of the tribochemical film is the key to better understand the mechano-chemical processes that give rise to chemical films separating mating solid elements and thereby reducing wear and seizure.

**Figure 9.** Tribochemical 'TREE' according to reference [47].

The chemistry and tribology of EP additives have been recently well reviewed from the viewpoint of the presently accepted action mechanism [48]. On the other hand, chapters in the same book consider also the NIRAM approach [49-50]. Chapter on *'Tribochemistry'* [51] details the NIRAM approach, presents specific reactive intermediates and mechanisms of selected organic compounds.

#### *4.2.2. Examples of NIRAM controlled tribochemical reactions*

Work [52] investigated tribochemical reactions of carboxylic acids under boundary lubrication conditions and it was found that, apart from regular salt (monodentate carboxylate group), salts with double bond in α, β position and chelating symmetric bidentate carboxylate group are formed. Figure 10 presents the proposed structure of iron salt with double bonding. These types of compounds are not produced under static conditions. It was found that bidentate surface configuration increased after the occurrence of the tribochemical reactions induced by surface rubbing. This proves this finding which was accounted for by the NIRAM approach. The new finding also clearly demonstrates the difference between tribochemical and thermochemical reactions. This finding is evidenced by other research results [53] on tribochemical and thermochemical reactions of stearic acid adsorbed on a copper surface.

General Approach to Mechanochemistry and Its Relation to Tribochemistry 221

) and the free radical R**•**. The second bond cleavage

) and the [R–C=O]**•** free radical that undergoes further

**Figure 11.** Influence of the solution concentration of octadecyl palmitate (Series 1) and equimolar

The ester reactive intermediates produced via the dissociative electron attachment, showing two types of C–O bond cleavage, are presented in Figure 12. The first bond cleavage type

reactions. To produce free radicals and thereby initiate the free radical chain reaction process either heat or catalyst is needed. Therefore, an electron attachment in this case acts as a catalyst. The exoelectron dissociative attachment to an ester molecule yielding two types of negative ions is clearly evidenced by the electron attachment mass spectrographic results [57].

Gilman [58] emphasizes that mechanochemical effects have often been attributed to strain energy assisting thermal energy. When covalent bonds are bent or sheared, the energies of their highest occupied molecular orbitals (HOMO) are raised, whereas the energies of their lowest unoccupied molecular orbitals (LOMO) are lowered and, the gap between levels determining a bond's stability is decreased [59]. If the strain becomes big enough to close the gap, the bonding electrons can move freely, and, the reaction can take place immediately. The simplest way of illustrating that phenomenon is based on a generalized NIRAM–HSAB

mixture of palmitic acid and octadecanol (Series 2) in hexadecane on the ball wear [55].

**Figure 12.** Ester reactive intermediates generated by electron attachment.

**4.3. Basic information on NIRAM-HSAB theory** 

produces carboxylate anion (RCOO**-**

generates the alkoxide anion (RO-

**Figure 10.** Chelating bidentate structure formed from caprylic acid [52].

Work [54], investigated changes of hexadecane under boundary lubrication conditions considering both (i) the chemical transformation of the bulk lubricant and (ii) the chemistry of products generated in wear tracks. Detailed analysis of hexadecane after friction tests showed that during the friction process aldehydes, alcohols and carboxylic acids are produced. This finding clearly indicates that tribochemical reactions cause significant changes of the apparently non-reactive paraffin hydrocarbon. The reaction process initiated by the frictional energy is in line with the NIRAM concept.

Regular monoester ester hydrolysis process produces alcohol and carboxylic acid. Work [55] aimed at checking role of hydrolytic reaction for the soap formation mechanism from esters. It was tried to find if typical conditions under boundary lubrication cause the hydrolysis of esters dissolved in hexadecane. Figure 11 reflects a ball wear reduction versus the additive concentration in hexadecane. These results enabled 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'* is well substantiated. Thus, it is possible to conclude that the soap formation mechanism from esters under boundary lubrication conditions is controlled by the NIRAM approach [56].

adsorbed on a copper surface.

**Figure 10.** Chelating bidentate structure formed from caprylic acid [52].

by the frictional energy is in line with the NIRAM concept.

lubrication conditions is controlled by the NIRAM approach [56].

Work [54], investigated changes of hexadecane under boundary lubrication conditions considering both (i) the chemical transformation of the bulk lubricant and (ii) the chemistry of products generated in wear tracks. Detailed analysis of hexadecane after friction tests showed that during the friction process aldehydes, alcohols and carboxylic acids are produced. This finding clearly indicates that tribochemical reactions cause significant changes of the apparently non-reactive paraffin hydrocarbon. The reaction process initiated

Regular monoester ester hydrolysis process produces alcohol and carboxylic acid. Work [55] aimed at checking role of hydrolytic reaction for the soap formation mechanism from esters. It was tried to find if typical conditions under boundary lubrication cause the hydrolysis of esters dissolved in hexadecane. Figure 11 reflects a ball wear reduction versus the additive concentration in hexadecane. These results enabled 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'* is well substantiated. Thus, it is possible to conclude that the soap formation mechanism from esters under boundary

carboxylate group), salts with double bond in α, β position and chelating symmetric bidentate carboxylate group are formed. Figure 10 presents the proposed structure of iron salt with double bonding. These types of compounds are not produced under static conditions. It was found that bidentate surface configuration increased after the occurrence of the tribochemical reactions induced by surface rubbing. This proves this finding which was accounted for by the NIRAM approach. The new finding also clearly demonstrates the difference between tribochemical and thermochemical reactions. This finding is evidenced by other research results [53] on tribochemical and thermochemical reactions of stearic acid

**Figure 11.** Influence of the solution concentration of octadecyl palmitate (Series 1) and equimolar mixture of palmitic acid and octadecanol (Series 2) in hexadecane on the ball wear [55].

The ester reactive intermediates produced via the dissociative electron attachment, showing two types of C–O bond cleavage, are presented in Figure 12. The first bond cleavage type produces carboxylate anion (RCOO**-** ) and the free radical R**•**. The second bond cleavage generates the alkoxide anion (RO- ) and the [R–C=O]**•** free radical that undergoes further reactions. To produce free radicals and thereby initiate the free radical chain reaction process either heat or catalyst is needed. Therefore, an electron attachment in this case acts as a catalyst. The exoelectron dissociative attachment to an ester molecule yielding two types of negative ions is clearly evidenced by the electron attachment mass spectrographic results [57].

**Figure 12.** Ester reactive intermediates generated by electron attachment.

#### **4.3. Basic information on NIRAM-HSAB theory**

Gilman [58] emphasizes that mechanochemical effects have often been attributed to strain energy assisting thermal energy. When covalent bonds are bent or sheared, the energies of their highest occupied molecular orbitals (HOMO) are raised, whereas the energies of their lowest unoccupied molecular orbitals (LOMO) are lowered and, the gap between levels determining a bond's stability is decreased [59]. If the strain becomes big enough to close the gap, the bonding electrons can move freely, and, the reaction can take place immediately. The simplest way of illustrating that phenomenon is based on a generalized NIRAM–HSAB

theory and provides its possible application for accounting for some tribochemical processes under boundary lubrication conditions. Figure 13 [11] depicts the model.

General Approach to Mechanochemistry and Its Relation to Tribochemistry 223

Applying the NIRAM–HSAB approach, the formation of silicon compounds from Si3N4 lubricated by alcohols is represented in Figure 15, which follows the scheme of Figure 14. The enhanced reactivity of silicon nitride under friction relates to active sites such as dangling bonds, and the action of tribo- and/or thermionic emissions. It should be noted that, according to Figure 16, the intermediate is the anion RO**−**, which is formed also during polymerization of the silicon alkoxide [61-62], thus further increasing the rate of the

**Figure 15.** Interpretation of the tribochemical reaction of alcohols with Si3N4 based on NIRAM–HSAB

**Figure 16.** Free energy Gibbs (G) curve as function of the coordinate of reaction (c.r.). S is an active surface site, LH - lubricant molecule (H is hydrogen, L is the hydrocarbon branch of the lubricant) [60].

In the intermediate depicted in Figure 16, electron and proton are exchanged between the two reagents S and L. This exchange mechanism is based on the subatomic particles involved in chemistry and, it is clear relationship between the intermediate and transition

tribochemical reaction.

theory [60].

**Figure 13.** NIRAM–HSAB lubrication mechanism approach: Sa, tribological microsurface area; **L**m, lubricant molecule; and eΘ is a low-energy electron emitted under boundary lubrication conditions

Presently this model has been applied to account for very complex tribochemistry of silicon nitride (Si3N4) [60]. The tribochemical reaction pathway of Si3N4 was accounted for in terms of the NIRAM–HSAB theory, in which tribo-electrons play an important role to decrease the activation energy. This may explain the reason why some products can be formed only by friction such as the tetrasiliconalkoxide obtained in lubrication with alcohols. Tribochemical wear provides flat surfaces, and decreases stresses since insoluble tribo-products act as lubricants by forming protective films, such as hydrated silicon oxides when water is present, and silicon alkoxide polymers in the case of alcohols. Figure 14 accounts for tribochemistry of Si3N4 with water.

**Figure 14.** Interpretation of the tribochemical reaction of water with silicon nitride based on NIRAM– HSAB theory [60].

Applying the NIRAM–HSAB approach, the formation of silicon compounds from Si3N4 lubricated by alcohols is represented in Figure 15, which follows the scheme of Figure 14. The enhanced reactivity of silicon nitride under friction relates to active sites such as dangling bonds, and the action of tribo- and/or thermionic emissions. It should be noted that, according to Figure 16, the intermediate is the anion RO**−**, which is formed also during polymerization of the silicon alkoxide [61-62], thus further increasing the rate of the tribochemical reaction.

222 Tribology in Engineering

tribochemistry of Si3N4 with water.

HSAB theory [60].

theory and provides its possible application for accounting for some tribochemical processes

**Figure 13.** NIRAM–HSAB lubrication mechanism approach: Sa, tribological microsurface area; **L**m, lubricant molecule; and eΘ is a low-energy electron emitted under boundary lubrication conditions

Presently this model has been applied to account for very complex tribochemistry of silicon nitride (Si3N4) [60]. The tribochemical reaction pathway of Si3N4 was accounted for in terms of the NIRAM–HSAB theory, in which tribo-electrons play an important role to decrease the activation energy. This may explain the reason why some products can be formed only by friction such as the tetrasiliconalkoxide obtained in lubrication with alcohols. Tribochemical wear provides flat surfaces, and decreases stresses since insoluble tribo-products act as lubricants by forming protective films, such as hydrated silicon oxides when water is present, and silicon alkoxide polymers in the case of alcohols. Figure 14 accounts for

**Figure 14.** Interpretation of the tribochemical reaction of water with silicon nitride based on NIRAM–

under boundary lubrication conditions. Figure 13 [11] depicts the model.

**Figure 15.** Interpretation of the tribochemical reaction of alcohols with Si3N4 based on NIRAM–HSAB theory [60].

**Figure 16.** Free energy Gibbs (G) curve as function of the coordinate of reaction (c.r.). S is an active surface site, LH - lubricant molecule (H is hydrogen, L is the hydrocarbon branch of the lubricant) [60].

In the intermediate depicted in Figure 16, electron and proton are exchanged between the two reagents S and L. This exchange mechanism is based on the subatomic particles involved in chemistry and, it is clear relationship between the intermediate and transition

state reflected in the product formed. So that the tribochemical reaction pathway of silicon nitride has been accounted for in terms of the NIRAM–HSAB theory, in which triboelectrons play an important part to decrease the activation energy or increase the reaction rate. This may explain the reason why some products can be formed only by friction such as the tetra silicon alkoxide obtained in lubrication with alcohols.

### **4.4. Catalysis and tribocatalysis**

Catalysis is the phenomenon of a catalyst action and the catalyst is a substance that increases the rate at which a chemical system approaches equilibrium, without being consumed in the process [12]. To initiate thermochemical reactions, the reaction system temperature should be increased to overcome the activation energy barrier (see Figure 16). The same is due to the catalytic process, but the catalyst lowers the reaction activation energy. Usually, it is demonstrated by the difference of the activation energy (Ea). Considering tribocatalytic reaction as the tribochemical one enhanced by the rubbing of catalyst, most recently it was demonstrated that in the tribocatalytic ethylene oxidation, the activation energy (Ea) with friction was less than 2% of the thermochemical reaction [63].

Activation energy lowering is a fundamental principle of catalysis and it applies to all forms of catalysis. For catalytic process to occur, a chemical interaction between catalyst and the reactant-product system is necessary, however this interaction should not change the chemical nature of the catalyst. With a catalyst, the energy required to go into the transition state decreases, thereby decreasing the energy required to trigger the reaction process. The rates of chemical reactions increase as temperature increases. Chemical reactions have rate constants approximated by the Arrhenius equation

$$k = A \exp^{\left(-Ea/RT\right)}\tag{1}$$

General Approach to Mechanochemistry and Its Relation to Tribochemistry 225

of tribochemical reactions of solids with lubricant molecules. Figure 17 demonstrates a general approach to physical and chemical events relating to boundary lubrication

**Figure 17.** Major physical and chemical events in the boundary lubrication contact.

**5.1. Summary of historical background based on Takacs's work** 

*electricity supply energy in the endothermic changes which they bring about'*.

development of mechanochemistry, known for over two centuries.

The common denominator of these reactions is that they are triggered by low-energy electrons. This statement is relevant to hypothesis of the present author saying that the intermediate reactive species of both mechanochemical and tribochemical reactions are produced by the same mechanism. Mechanolysis is very special branch of

mechanochemistry, particularly from the view-point of water splitting technology.

**5. History and present state of the mechanochemistry and its relation to** 

The feasibility that chemical reactions in solids can be initiated by mechanical deformation and/or tribological contact, had been considered for almost 120 years back. At that time M. Carey Lea [1] wrote *'Mechanical force can bring about reactions which require expenditure of energy, which energy is supplied by mechanical force precisely in the same way that light, heat, and* 

Typical historical reviews of mechanochemistry by Takacs [2-3] have presented Matthew Carey Lea as the first systematic researcher on the chemical effects of mechanical action. Mechanical energy triggering chemical reactions had been of particular significance in

Early twentieth century (1919) Ostwald considered mechanical energy influence on chemical reactions and coined the term 'mechanochemistry'. However, at that time Ostwald was not able to say anything on the independent character and the importance of this field of

conditions.

**tribochemistry** 

where **A** is the pre-exponential factor for the reaction. Heterogeneous catalysts provide a surface for the chemical reaction. Most heterogeneous catalysts are solids that act on substrates in a liquid or gaseous reaction mixture.

Reference [64] reviews and discusses the effect of mechanochemical activation on the catalytic properties of different systems. It is noted that the activity of a catalyst with defects is somewhat higher than its activity in the equilibrium state, emphasizing that the degree of increase in activity depends on the amount of the excess energy stored. It is stressed that the energy stored in defects influences the catalytic properties through the variation of thermodynamic potentials.

Practically all types of chemical reactions are accompanied by a change in energy. Some of them release energy to their surroundings mostly in the form of heat and thus are called exothermic. Conversely, some reactions need to consume heat from their surroundings to proceed. These reactions are called endothermic. Reactions that proceed immediately when two substances are mixed together are called spontaneous reactions. The application of mechanical energy associated with friction releases physical processes that can be the cause of tribochemical reactions of solids with lubricant molecules. Figure 17 demonstrates a general approach to physical and chemical events relating to boundary lubrication conditions.

224 Tribology in Engineering

**4.4. Catalysis and tribocatalysis** 

state reflected in the product formed. So that the tribochemical reaction pathway of silicon nitride has been accounted for in terms of the NIRAM–HSAB theory, in which triboelectrons play an important part to decrease the activation energy or increase the reaction rate. This may explain the reason why some products can be formed only

Catalysis is the phenomenon of a catalyst action and the catalyst is a substance that increases the rate at which a chemical system approaches equilibrium, without being consumed in the process [12]. To initiate thermochemical reactions, the reaction system temperature should be increased to overcome the activation energy barrier (see Figure 16). The same is due to the catalytic process, but the catalyst lowers the reaction activation energy. Usually, it is demonstrated by the difference of the activation energy (Ea). Considering tribocatalytic reaction as the tribochemical one enhanced by the rubbing of catalyst, most recently it was demonstrated that in the tribocatalytic ethylene oxidation, the activation energy (Ea) with

Activation energy lowering is a fundamental principle of catalysis and it applies to all forms of catalysis. For catalytic process to occur, a chemical interaction between catalyst and the reactant-product system is necessary, however this interaction should not change the chemical nature of the catalyst. With a catalyst, the energy required to go into the transition state decreases, thereby decreasing the energy required to trigger the reaction process. The rates of chemical reactions increase as temperature increases. Chemical reactions have rate

where **A** is the pre-exponential factor for the reaction. Heterogeneous catalysts provide a surface for the chemical reaction. Most heterogeneous catalysts are solids that act on

Reference [64] reviews and discusses the effect of mechanochemical activation on the catalytic properties of different systems. It is noted that the activity of a catalyst with defects is somewhat higher than its activity in the equilibrium state, emphasizing that the degree of increase in activity depends on the amount of the excess energy stored. It is stressed that the energy stored in defects influences the catalytic properties through the variation of

Practically all types of chemical reactions are accompanied by a change in energy. Some of them release energy to their surroundings mostly in the form of heat and thus are called exothermic. Conversely, some reactions need to consume heat from their surroundings to proceed. These reactions are called endothermic. Reactions that proceed immediately when two substances are mixed together are called spontaneous reactions. The application of mechanical energy associated with friction releases physical processes that can be the cause

( /) exp *Ea RT k A* (1)

by friction such as the tetra silicon alkoxide obtained in lubrication with alcohols.

friction was less than 2% of the thermochemical reaction [63].

constants approximated by the Arrhenius equation

substrates in a liquid or gaseous reaction mixture.

thermodynamic potentials.

**Figure 17.** Major physical and chemical events in the boundary lubrication contact.

The common denominator of these reactions is that they are triggered by low-energy electrons. This statement is relevant to hypothesis of the present author saying that the intermediate reactive species of both mechanochemical and tribochemical reactions are produced by the same mechanism. Mechanolysis is very special branch of mechanochemistry, particularly from the view-point of water splitting technology.
