Modeling Sliding Friction of a Multiscale Wavy Surface over a Viscoelastic Foundation Taking into Account Adhesion

Round 1

Reviewer 1 Report

• Although it is not explicitly stated in the present paper, but with respect to the fact that the coefficient of friction is stated as dimensionless (Figure 6), it is shear friction. The coefficient of shear friction is the material coefficient and its value at a certain interval and under certain conditions can only be obtained by measurement. The coefficient of shear friction is the sum of many different influences, but at the same time only the comparative value is comparative, ie for comparing the friction of a combination of other materials.

• A key question for the present paper is: HOW was the friction force measured? How and how were the devices measured in order to interpret the results of shear friction coefficients and model the results of other indirect measurements based on the results of their measurements?

 

• In general, in the case of friction with lubricants, the tendency of the shear friction coefficient to velocity is increasing, see fig .: 0 static friction, 1 dry friction, 2 mixed friction, 3 fluid friction.

• In general, however, it is only one of the three parameters on which the shear friction coefficient is primarily dependent: the speed of movement, the viscosity of the lubricant and the pressure exerted on the lubricant. With regard to pressure as normal force acting on the surface, nobody in the world has yet shown that the shear friction alone would depend on the size of the contact surfaces.

• Furthermore, the coefficient of shear friction with lubricants is temperature-dependent because the viscosity of the fluids depends on temperature and pressure, with increasing temperature the viscosity decreases, with increasing pressure increasing, but the pressure effect is usually negligible as difficult to measure. Temperature not resolved?

• Ad Stribeck curve - according to the declared value of the shear friction coefficient (Figure 6), it is a hydrodynamic mode only at relatively high speeds (5 lubrication modes: hydrostatic, hydrodynamic, elastohydrodynamic, limiting, lubrication with solid lubricants).

 

• There is nothing surprising about "resonance" baking, because that is the nature of the friction force trend: its continuous values are ALWAYS oscillating, especially on the wavy surface.

• In the past, nobody in the world has yet demonstrated that the frictional force, respectively, the coefficient of shear friction, whether dry or dynamic, depends on the radius of curvature of the friction surface.

• Ripening as the difference between "surface elevations" (ad discussion of the results of the presented paper) is of the order of micro-level, 1 micron and 100 nanometer, ie 10 times the micrometer, thus it is an examination at the level of surface roughness, the roughness of the surface being one of the parameters is projected to the mean value of the coefficient of shear friction. Individual micro-particles and micro-phenomena only produce synergies with macroscopic measurable phenomena. Again, the key question arises as to how the results of the friction force measurements have been achieved and what were they specifically?

• The problem of hysteresis: the authors apparently see it inversely in that the surface is undulating and there is something similar to rolling friction. "Hysteretic friction force of the viscoelastic bodies is caused by the energy dissipation occurring due to the cyclic deformation of the underlying layers of material in the sliding." This is true for rolling friction with lubricants.

• The so-called " Hertz pressure is also present in the case of rolling friction only. It is historically known from tribology that at the point of contact between the elastic full cylindrical body and the elastic flat washer, due to their deformation, the force is formed by a vertical force to form a contact surface on which the contact pressure of the Hertz pressure is parabolic. At rest, this course is symmetrical and the resulting reaction acts against the loading force. If the body is subjected to a horizontal force (or torque force), it starts to roll over the front of the contact surface and the back begins to relieve it. As a result of hysteresis, relief is slower than compression. This is manifested by a deformation of the contact pressure, the resultant of which is advanced forward by the so-called rolling resistance arm. The size of this arm is determined by the material properties, such as, in particular, internal friction. This causes internal friction that causes the hysteresis, which in turn causes rolling resistance. It is a mixture of friction types, rolling resistance is NOT associated with shear friction!

• The peak, ie the resonance energy output, may theoretically occur in the surface of the studied material during friction, but the phenomenon could be examined only on the basis of the energy balance! When you insert energy into the system, the resonance will either resonate or not.

• It is not an isolated problem within the overall energy balance, the energy that results from friction is dissipative energy.

• The presented publication lacks conclusions, conclusions based on measurement results. Discussion of results is not a clear conclusion of the work the authors have come to. This sentence is not transparent to the reader: "This allows one to calculate the total friction force by direct summation of contributions of all scale levels."

Author Response

Thank you very much for reading my manuscript and your helpful comments. My answers to your comments are in the attached file, they typed in red color. The numbers of equations and references are the same as in the manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments on “Modeling friction of a multiscale wavy surface over a viscoelastic foundation taking into account adhesion” by Yulia Makhovskaya

In this work, the author has developed an analytical solution for modeling the friction of a multiscale wavy surface over a viscoelastic function. The friction force is attributed to two terms: one is due to the hysteretic losses occurring when the asperities of this scale level cyclically deform the viscoelastic foundation during sliding; the other terms is given by the law of friction determined from the solution of the contract problem at the inferior scale level. In addition, the strip method has been applied to numerically solve the 3D contact problem by reducing it into a 2D formulation. The author has applied the proposed method to study the two-level wavy surface, focusing on the influence of sliding velocity and specific energy of adhesion between contact surfaces. Before this work can be considered for publication, there are several issues to be addressed:

1)      For the modeling parameters, it is not clear what the rationales are behind these parameters are. It will be great if the author could provide references for these parameters.

2)      The author only provides the solution to two-level wavy surface according to the present method. It is not clear how the other methods can be applied to solve the same problem. It is difficult for the reviewer to compare the proposed method with previous well-established methods.

3)      Could the author also provide some numerical solutions to the two-level wavy surface problem, such as finite element simulations?

4)      Is there any experimental data comparable with present problem?

Author Response

Than you very much for reading my manuscript and for your valuable comments. My answers to your comments are in the attached file. They are given in red color. The numbers of references are those in the manuscript.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Paper is after improved OK.