Closure to “Discussion of ‘Specific Heat of Tribological Wear Debris Material’” (2015, ASME J. Tribol., 137(3), p. 031601)

Abstract

A variety of models are proposed to explore the complex nature of friction and its effects. Therefore, my conclusions from the discussion on specific heat of debris “in statu nascendi” must be treated as open to discussion [1]. I initially assumed in Ref. [2] (in line with your comments) that specific heat of a refined solid is not essentially different than its specific heat before being refined. I took the formula for the first law of thermodynamic for open stationary systems and balanced fluxes of different forms of energy and mass within the system and on its boundaries with the environment. I went on to introduce two systemic quantities, C and D, and provided their physical interpretations. C—specific heat times maximum temperature of the friction zone—was very high for selected experimental examples. Given the limited value of this temperature, the postulate that specific heat of debris is very high seemed the only possibility. That persuaded me to modify the original thermal balance. In the circumstances, absorption of friction heat is not accompanied by a marked rise in the temperature of particles being generated. This interpretation is corroborated by the recently discovered “cooling effect” associated with friction and wear of solids [3]. The high value of specific heat in the process of material dispersion may also be explicated by analogy to the phase transition of melting, where specific heat of a substance tends toward infinity. The unconventional properties of matter accompanying tribological wear have been explained, inter alia, by means of the so-called magma–plasma model [4]. Other interpretations of unconventional effects associated with friction are possible, proof that we are still a long way from a full understanding of the nature of friction.The enthalpy change h° as per Eq. (1) characterizes reversible elastic strain of a certain quantity of a substance (a closed thermodynamic system) and its concomitant thermal effect. Equations (1) and (2) also include specific heat of the substance, which remains continuous. They do not apply to debris detachment from a material involved in friction. In my work, I use opportunities offered by phenomenological thermodynamics for open systems. In open systems, enthalpy is the energy released out of a system into its environment together with debris (the latter's further role is not a subject of my discussion). Density of its fluxes is described by Eq. (12)J[adyss+cp'(Θo-Θ)]Increment of specific enthalpy has two components—growth of internal energy caused by a change in temperature of a unit of debris mass and unit work of mechanical dissipation causing tribological wear. This is in line with the definition of enthalpy as the sum total of internal energy and work performed by a substance. The work includes two components: superficial and volumetric [3]. It is required for a debris particle to detach. This kind of energetic interaction is not taken into consideration by Eq. (1).Application of Fourier's equation to description of thermal processes in the area of direct energy dissipation at the time of solid friction is not reasonable in physical terms as physical processes other than conduction occur and properties of the substance filling this area are not uniquely defined. That is the reason why I did not use this equation in my discussion. Thank you for your contributions and suggestions.