The authors present a well executed investigation of the temperature rise in the sliding of rough surfaces. However, this discussant has some comments regarding the technical aspects of this interesting work.
The paper represents the common practice, which dominates the tribology community, of addressing frictional heating problems with the aid of the Blok-Jaeger model, that is, analyzing the friction temperature rise by applying these models even out of the scope of their validity. It is recognized that, as of yet, alternative practical models are virtually non-existent. However, it is necessary to correct conventional results to match practical physical situations. This may be achieved by addressing the important factors that affect the transfer of heat within a sliding pair. In particular, there is a need to investigate the effect of thermal storage in transient conditions, the influence of energy release due to plastic deformation, and the effect of the rate of deformation on the heat transfer properties of the rubbing pair (thermo-mechanical coupling effect).
The authors mention that the heat partition function was evaluated; however, no graph or a comparison between the expression obtained from the FFT [Eq. (18)] and the simple expressions of Jaeger or Blok was presented. This is important on two counts:
1 The marked difference between the temperatures obtained by the authors’ approach and those obtained using Blok’s model. Note that the discrepancy in the results was attributed to the approximative nature of Blok’s heat partition function.
2 The authors mention that an examination of the partition function showed a greater rate of heat dissipation through the faster moving surface. This result is interestingly in contrast to the foundation of the flash temperature theory that postulates that the faster moving solid will dissipate less heat. It is worth noting here that such a finding is hard to envision from the mathematical treatment of the authors. Note that a source term, which may cause the partition function to indicate more heat for the faster moving contact, is not included and that heat generation is considered independent of the temperature. Moreover, it is not clear whether the bulk temperatures of both solids entrain a high difference.
