**1. Introduction**

## **1.1. Compact information on the considered disciplines**

Mechanochemistry and tribochemistry disciplines are of particular importance for fundamental research and tribology engineering practice. They relate to specific coupling of physical and chemical phenomena leading to initiation of heterogeneous chemical reactions due to mechanical action.

Chemical reactions in solids initiated by mechanical action had been considered for a long period of time [1]. From the viewpoint of terms, mechanochemistry and tribochemistry may be compared with terms: physical chemistry and chemical physics. In the latter term physics is first and chemistry second. In tribochemistry, friction (tribos) is the first. Since mechanics includes friction, tribochemistry should be included in mechanochemistry. Reviews on mechanochemistry [2,3] show Matthew Carey Lea as the first systematic researcher on the chemical effects of mechanical action.

Mechanochemical reactions are clearly distinct from those of thermochemical ones. To initiate thermochemical reactions an adequate heat amount has to be supplied to overcome the activation energy.

Mechanical interaction of a solid-solid enables chemical reactions to be initiated by lower activation energy than regular thermochemical reactions.

Even a very high calculated 'flash temperature' is short-lived, thus, it rather cannot initiate chemical reactions by heat. Reference [4] demonstrates that high local temperatures generated by friction at the contacts between rubbing surfaces can evolve thermionic activity, likely to be short lived and random. These localized, dynamic regions of intense thermionic emission can act as catalytic sites for chemical activity.

## **1.2. Definition of mechanochemistry for the present book chapter**

Mechanochemistry is the science field dealing with ultra-fast chemical reactions between solids or solids and surrounding gaseous or liquid molecules under mechanical forces. There are many detailed definitions focused on selected branches of mechanochemistry. Reference [5] defines mechanochemistry as the branch of solid state chemistry where bonds are mechanically broken The bond breakage can induce electron transfers, triboelectricity (known as mechanoelectricity), and triboluminescence. These phenomena are in a branch of mechanophysics. Similarly, thermal expansion, piezoelectric effects, or compression by pressurizing might also be related to mechanophysics; details are discussed in [6].

General Approach to Mechanochemistry and Its Relation to Tribochemistry 211

**2. Flash temperature vs. thermionic emission** 

points on the origin of enhanced reactivity of solids.

**2.2. Brief information on the** *'flash temperature'* **term** 

Details related to flash temperature till 1990 are summarized in [18].

The input of thermal energy generated in a tribological system of boundary friction is lower than the output. Thus, it seems convenient to consider heat evolution also in electronic terms. Not considering any heat loss, the difference is controlled by *energy stored* in the system. Looking at the mechanical work plane proposed in [15], various portions of the work (power) include: input power, use-output power, loss-output energy rate, and a stored energy (*thermal energy transformed from mechanical work*). The energy stored (*excess energy*)

Mechanochemical reactions constitute a complex multistage process, which include stages involving mechanical deformation of a substance (*the supply and absorption of mechanical energy*), the primary chemical reactions, and different secondary processes [16]. It should be noted that higher reactivity of solids is of particular importance from the view-point of

The temperature rise at the peaks of the contacting asperities can be high but their duration is very short. The high order of magnitude and very short duration is due to the tiny area of contact [17]. Flash temperature is a means of accounting for the local frictional heat flux.

Reference [19] reviewing the present literature on flash temperature, demonstrates that for low-speed sliding, thermal effects on tribochemical reactions are negligible. Attempts to measure the contact temperature at very low sliding velocities in fretting [20], show low temperature increases and, they are well corroborated by results of theoretical calculations.

An early publication by Archard [21] deals with temperatures evolved by friction. The second part of reference [22] is on temperature distributions of friction bodies. Critical assessment of the flash-temperature concept has been presented in work [23]. It is of note at this point that frictionally generated high local temperatures can also be reflected as the

Summarizing this information, it can be said that the flash temperature term relates to the maximum local temperature generated in a sliding contact. It occurs at areas of real contact due to the frictional heat dissipated at these spots. Theoretical study on frictional temperature rises and the flash temperature concept are assigned to Blok [24, 25] and

Work [28] shows that when the friction contact takes place at the tip of the contacting asperities, local temperatures reach significant values even in the very small time interval of

**2.1. Frictional thermal energy** 

engineering aspects.

thermionic emission [4].

**2.3. Thermionic emission** 

continued by other researchers, eg. [21,23,26-27].

In this chapter the following general definition, taken from [7-8] has been selected '*Mechanochemistry is a branch of chemistry which is concerned with chemical and physicochemical transformations of substances in all states of aggregation produced by the effect of mechanical energy*'. This definition was formulated 50 years ago and is accepted nowadays.

### **1.3. Definition of tribochemistry for the present book chapter**

Similarly to mechanochemistry, also tribochemistry relates to mechanically initiated chemistry. The activation energies of tribochemical reactions are lower than those of thermochemical ones. Chemical reactions of tribological additives proceeding during the boundary lubrication (BL) process involve the formation of a film on the contact surface. BL is the condition of lubrication, in which the friction and wear between two surfaces in relative motion are determined by the properties of the surfaces and by the properties of the lubricant other than viscosity. This definition is closely related to the Hardy's first approach to the boundary lubrication process [9]. Campbell [10] emphasizes that *BL is perhaps the most confusing and complex aspect of the subject of friction and wear prevention*.

Tribochemical reactions are also distinct from those of thermochemical reactions. The same is due to heterogeneous catalysis (HetCat) and tribocatalysis [11]. Principles and applications of HetCat are compiled in [12]. To initiate thermochemical reactions heat should be supplied. There are many definitions of the tribochemistry term. Reference [13] defines tribochemistry as the chemical reactions that occur between the lubricant and the surfaces under BL conditions and stresses that the precise nature of the chemical reactions is not well understood. Book [14] just states that *tribochemistry concerns interacting chemical reactions and processes*.

The present author proposes to consider tribochemistry as a subset of mechanochemistry and thus, for this book chapter the following general definition *'Tribochemistry is a branch of chemistry dealing with the chemical and physico-chemical changes of solids due to the influence of mechanical energy'*, has been selected [7-8].
