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

## **2.1. Frictional thermal energy**

210 Tribology in Engineering

discussed in [6].

nowadays.

*prevention*.

*reactions and processes*.

*mechanical energy'*, has been selected [7-8].

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

**1.3. Definition of tribochemistry 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

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

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* 

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* 

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*  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*) points on the origin of enhanced reactivity of solids.

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 engineering aspects.

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

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

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 thermionic emission [4].

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

#### **2.3. Thermionic emission**

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 3 ms. Recent publication [29] stresses that the maximum temperature reached at a single asperity contacts corresponds to the flash temperature. As there may be many asperity contacts of different size interacting at once, there should be a distribution of such temperatures. Reference [28] also demonstrates that the high flash temperatures occur even while the overall temperature rise of the surface may be much lower. This information leads to a linkage of triboemission with flash temperature.

General Approach to Mechanochemistry and Its Relation to Tribochemistry 213

This is a result of the strong exponential nonlinear relationship between absolute temperature and current density. It is evidenced by results shown in Figure 2, displaying the effect of increasing velocity or load over the narrow range of parameters. This effect is due to the strong nonlinearity and sensitivity between current density and

**Figure 2.** Effect of velocity and load on the maximum temperature divided by friction coefficient and

Two decades back tribo-stimulated and photo-stimulated exoelectron emission (TSEE, PSEE) from reactor graphite was investigated, because an increase in TSEE from graphite was due to high friction. Selective deformation of graphite fibrils was identified with transmission electron microscopy [32] and it was found that the exothermic process during the preferred orientation caused emission of tribo-stimulated electrons. PSEE from reactor graphite was also detected. New surface analysis method using TSEE has been developed to estimate the ability for metals in practical use to emit electrons. This method uses an exoelectron emission phenomenon observed only while metal surfaces are mechanically rubbed with a PTFE (polytetrafluoroethylene) rotator. Number of electrons emitted from copper and iron metals during rubbing was measured using a newly hand-made electron counting device [33]. The relationship of emitted electrons to the XPS (x-ray photoelectron spectroscopy) results of the metal surfaces also was

Thermionic emission due to frictionally generated temperatures in sliding contact can have a number of important consequences, including activation of tribochemical reactions according to the NIRAM approach and enhancement of surface reactivity [4]. Therefore similarly to the typical TSEE, generated in the sliding contact, thermionic emission (eth), can be combined with the NIRAM approach in terms of enhancement of surface activity as

Accordding to the NIRAM approach tribochemical reactions are initiated by both tribo (**e)** 

temperature.

investigated.

demonstrated in Figure 3 [29].

and thermionic (eth) electrons of low-energy.

total current from the surface of iron [4].

Now we need to come back to the present author's suggestion that flash temperature, expressed as the maximum computed friction temperature, can also be considered in terms of the thermionic emission [29]. Thereafter, flash temperatures might be expressed in the form of electronic energy. Most recently, that assumption was confirmed by examining of thermionic emission due to frictionally generated heat. The emission of electrons from a surface due to heating was investigated theoretically for sliding contacts [4]. A thermal model previously developed by Vick and Furey [30-31] for sliding contact was used to predict the temperature rise over the surface and the Richardson–Dushman equation for thermionic emission was then used to estimate the corresponding current density from the surface. The computed results demonstrate that high local temperatures generated by friction of the contacts between rubbing surfaces can activate the emission of electrons (see Figure 1). The temperature increase in Figure 1 is observable over both the actual contact area and a region immediately downstream of the contact due to energy convected by the motion of the material. The maximum temperature rise is 1003 K and the thermionic current density increase has a severe peak in the neighborhood of the maximum temperature and is almost unnoticeable elsewhere.

**Figure 1.** Normalized temperature distribution and corresponding current density from the surface of iron for an applied load of *F =* 1N, sliding speed of *V* =10 m/s and coefficient of friction of 1 [4].

This is a result of the strong exponential nonlinear relationship between absolute temperature and current density. It is evidenced by results shown in Figure 2, displaying the effect of increasing velocity or load over the narrow range of parameters. This effect is due to the strong nonlinearity and sensitivity between current density and temperature.

212 Tribology in Engineering

to a linkage of triboemission with flash temperature.

3 ms. Recent publication [29] stresses that the maximum temperature reached at a single asperity contacts corresponds to the flash temperature. As there may be many asperity contacts of different size interacting at once, there should be a distribution of such temperatures. Reference [28] also demonstrates that the high flash temperatures occur even while the overall temperature rise of the surface may be much lower. This information leads

Now we need to come back to the present author's suggestion that flash temperature, expressed as the maximum computed friction temperature, can also be considered in terms of the thermionic emission [29]. Thereafter, flash temperatures might be expressed in the form of electronic energy. Most recently, that assumption was confirmed by examining of thermionic emission due to frictionally generated heat. The emission of electrons from a surface due to heating was investigated theoretically for sliding contacts [4]. A thermal model previously developed by Vick and Furey [30-31] for sliding contact was used to predict the temperature rise over the surface and the Richardson–Dushman equation for thermionic emission was then used to estimate the corresponding current density from the surface. The computed results demonstrate that high local temperatures generated by friction of the contacts between rubbing surfaces can activate the emission of electrons (see Figure 1). The temperature increase in Figure 1 is observable over both the actual contact area and a region immediately downstream of the contact due to energy convected by the motion of the material. The maximum temperature rise is 1003 K and the thermionic current density increase has a severe peak in the neighborhood of the maximum temperature and is almost unnoticeable elsewhere.

**Figure 1.** Normalized temperature distribution and corresponding current density from the surface of

iron for an applied load of *F =* 1N, sliding speed of *V* =10 m/s and coefficient of friction of 1 [4].

**Figure 2.** Effect of velocity and load on the maximum temperature divided by friction coefficient and total current from the surface of iron [4].

Two decades back tribo-stimulated and photo-stimulated exoelectron emission (TSEE, PSEE) from reactor graphite was investigated, because an increase in TSEE from graphite was due to high friction. Selective deformation of graphite fibrils was identified with transmission electron microscopy [32] and it was found that the exothermic process during the preferred orientation caused emission of tribo-stimulated electrons. PSEE from reactor graphite was also detected. New surface analysis method using TSEE has been developed to estimate the ability for metals in practical use to emit electrons. This method uses an exoelectron emission phenomenon observed only while metal surfaces are mechanically rubbed with a PTFE (polytetrafluoroethylene) rotator. Number of electrons emitted from copper and iron metals during rubbing was measured using a newly hand-made electron counting device [33]. The relationship of emitted electrons to the XPS (x-ray photoelectron spectroscopy) results of the metal surfaces also was investigated.

Thermionic emission due to frictionally generated temperatures in sliding contact can have a number of important consequences, including activation of tribochemical reactions according to the NIRAM approach and enhancement of surface reactivity [4]. Therefore similarly to the typical TSEE, generated in the sliding contact, thermionic emission (eth), can be combined with the NIRAM approach in terms of enhancement of surface activity as demonstrated in Figure 3 [29].

Accordding to the NIRAM approach tribochemical reactions are initiated by both tribo (**e)**  and thermionic (eth) electrons of low-energy.

General Approach to Mechanochemistry and Its Relation to Tribochemistry 215

**Figure 5.** Physical processes evolved by friction.

and acoustic emission, (h) heat evolvement.

**Figure 6.** Classification of triboemission phenomena by physical nature.

as shown in Figure 6 [29,34].

Details are in reference [34], which distinguishes the following main types of triboemission phenomena: (a) emission of gas atoms and molecules, including emission of radicals and molecular clusters, (b) emission of electromagnetic radiation, (c) emission of electrons, (d) emission of ions, (e) emission of magnetic field, (f) emission of electric field including emission of electric charges and generation of tribocurrents, (g) emission of noise, vibration

Emission of gas atoms and molecules at friction results from the competition gas release and gas adsorption processes. When at certain sliding conditions, the rate of gas adsorption exceeds the rate of gas release, total emission rate becomes negative. Such emission of negative rate has been called *anti-emission.* The triboemission phenomena are classified into two classes by physical nature: emission of particles ('corpuscular'), and emission of energy,

**Figure 3.** Initiation process of tribochemical reactions by the mechanical action.
