Tribological properties of high-entropy high-speed steels under conditions of friction on stainless steel 12H18N9 Т

. This paper is devoted to the study of tribological characteristics of standard grades of high-speed steels (HSS) under conditions of friction on stainless steel 12H18N9Т without the use of a lubricant. The grades of HSS selected for research differ in the value of thermal entropy, which is considered as an integral characteristic of their chemical composition. The tests were carried out according to the "pin-disk" friction scheme. The following differences in the characteristics of the friction process have been experimentally established, depending on the value of the thermal entropy of the HSS. Frictional interaction of high-entropy HSS was characterized by increasing the thickness of dissipative structures over time. The forming intermediate layer has a shielding effect, protecting the surfaces of rubbing bodies from destruction, but at the same time it has a large shear resistance, due to which higher coefficients of friction were recorded. The process of friction of HSS grades with low values of thermal entropy is characterized by the convergence of contacting bodies over time. For this group of materials, lower coefficients of friction were recorded against the background of a significant change in the surface micro relief relative to the initial state.


Introduction
Currently, the performance characteristics of tools made of high-speed steels (HSS) are increased by chemical-thermal treatment, deposition of coatings, laser hardening methods and other methods related to the modification of the surface or volume properties of the material, as well as through the development of new grades of HSS [1][2][3][4][5].The ability to predict a priori some operational characteristics of experimental compositions of tool cutting materials (TCM) is provided by the study of thermodynamic aspects of processes of wear during friction (cutting).One of the most important factors affecting the service life of blade tools is its wear resistance, largely determined by the tribological properties of the TCM [6,7].A significant reduction in the wear intensity during friction and cutting can be achieved by providing a thermodynamic state in the TCM characterized by minimal density of accumulated thermal entropy through the use of materials with high entropy values [8].High-entropy TCMs are characterized by low values of absolute (relative) thermo-EMFs, thereby increasing their resistance to gas corrosion.Thermal entropy, as well as absolute (relative) thermo-EMF of the material depends on their chemical composition, and can be used as its integral characteristic.The thermal entropy of the TCM, like all thermodynamic potentials, can be calculated according to the rule of additivity for a known chemical composition of the material.Austenitic stainless steels are widely used in various modern industries, the materials have high resistance to atmospheric and intergranular corrosion, heat resistance, strength, good weldability, and parts made of them can function in a wide temperature range.Steels of this group are characterized by low thermal conductivity and strong surface hardening during the cutting processes, due to which they are considered difficult to process materials [9][10][11][12].Widely used in the production of food industry devices, nuclear reactors, chemical production, aircraft construction and prosthetics from this group of materials is the brand 12H18N9T (analogue of AISI 321).The manufacture of parts in these industries is often associated with the use of small-sized tools, with a working part made of high-speed steels.Thus, the study and identification of the features of the frictional interaction of HSS with various thermodynamic characteristics with this structural material is an urgent task.This study is devoted to the study and comparison of the features of the friction process of highand low-entropy grades of HSS on stainless steel 12H18N9T without the use of a lubricant.

Materials and methods
Studies of cylindrical samples d = 5 mm from HSS of various levels of thermal entropy (Table 1) with a roughness of friction surfaces Ra 0.14-0.17μm were carried out.Tribological tests were carried out on the tribometer T-11 (Poland), which implements the friction scheme "pindisk".The studied and compared parameters were the coefficients of friction f and the displacement of the indenter relative to the rotating counter body (disk) Y over time T. Experiments were carried out at a constant sliding speed velocity v = 0.5 m/s and a normal load P = 40 N for various values of the friction path L = 200 ... 800 m.Counter body material -stainless steel 12H18N9Т, Ra 0,15-0,17 μm, HRC 39...42.Studies of the micro-relief of friction surfaces and the determination of roughness were carried out on the BRUKER Contour GT scanning interference microscope.

Results and discussion
Among the HSS samples studied, two groups can be distinguished depending on the kinetics of the formation of the intermediate layer, described by Y(T) curves.Characteristic for the first group curves Y(T) and f(T) are presented in Fig. 1, a, b.At the initial stage of the friction process (Stage I), there is a fairly rapid increase in the thickness of the intermediate layer, accompanied by an increase in the friction force due to an increase in the actual area of tribocontact.However, the intermediate layer formed in this case has less resistance to frictional action than was observed during similar experiments carried out under conditions of friction on structural steels [14,15].After the thickness of the layer increases to a value of δ, its abrasion is observed (Stage II), which leads to the convergence of the indenter with the counter body to a value of δ800, corresponding to the thickness of the layer for the maximum friction path L = 800 m.The development of the friction process according to the scenario described above is typical for HSS with high entropy values, namely R18, EP658 and EP657 grades.The coefficient of friction for this group of HSS was the largest, which is explained primarily by the rheological properties of the intermediate layer, its high shear resistance.
For the second group of samples, the processes of destruction of the intermediate layer significantly prevailed over the processes contributing to its growth.During the experiment, either a slight increase in the thickness of the intermediate layer was recorded in stage I, followed by rapid abrasion and displacement of the Y(T) curve to the region of negative values, or from the very beginning there was a convergence of the indenter and counter body (Fig. 2).The materials of this group include HSS grades R6M3, R6M5, R6M4F4, R8M3F4.During the friction of samples made of these compositions, a slight increase in the coefficient of friction over time was recorded, and the values of this parameter in comparison with the HSS grades of the first group were significantly lower.2. If no growth in layer thickness was observed, the parameter δ in Table 2 is not indicated.In the process of friction, the micro-relief of the contacting surfaces changes, resulting in a mutually perpendicular system of irregularities -longitudinal and transverse roughness [13].For the HSS grades of the first group, roughness will be formed mainly due to the interaction of surfaces with the intermediate layer, and depends on the chemical composition and physical and mechanical properties of materials.On the one hand, there may be damage to the surfaces by solid particles of oxides separated due to fatigue phenomena, on the other hand, the intermediate layer can act as a solid lubricant, reducing the likelihood of adhesion interaction of the surfaces.Surface micro-relief for HSS grades of the second group, when a stable formation of the intermediate layer was not observed, is formed mainly due to the adhesive (cohesive) interaction of the samples surfaces.In this case, the roughness will be a characteristic of the intensity of the adhesive setting process and the scale of the destruction of the surface integrity caused by it [13].
Measurements were made at a section perpendicular to the direction of sliding (Fig. 3).The length of the roughness measurement path was 1 mm.The path for measuring the roughness was chosen adaptively in the middle of the contact patch on the surfaces of the indenters and in the middle of the circular friction track on the counterbody.   .2) contributes to the separation of contact surfaces and the preservation of the surface integrity of the samples (Fig. 4).At the same time, due to the shear resistance of the intermediate layer itself, the friction coefficients recorded in the tribosystem are slightly higher than for HSS grades with low thermal entropy values.For low-entropy HSS grades there is a significant change in the surface micro relief relative to the initial state, which is a consequence of the processes of formation / destruction of adhesive seams.Also, the adhesive seams formed during friction of low-entropy HSS on stainless steel have a low shear resistance (Table 2).The established features of the process of friction of high-entropy HSS on stainless steel 12H18N9T are consistent with previous studies of their tribotechnical characteristics.Thus, the process of frictional interaction with structural steel 30HGSA for these materials in similar tests is also characterized by the fastest increase in the thickness of the intermediate layer and the smallest change in the micro relief both on the surface of the indenter and on the friction tracks on the disk [14].The process of friction of high-entropy experimental hard alloys on various structural materials proceeds according to a similar scenario [16,17].The formation of a stable intermediate layer for this group of materials is facilitated, in particular, by their surface mechanical characteristics at the microlevel, i.e., higher values of microhardness and modulus of elasticity [13][14][15][16][17].

Conclusion
As a result of tribological tests of standard HSS grades, the following features of the friction process were established, depending on the value of the thermal entropy of the composition.As a result of the frictional interaction of high-entropy HSS with stainless steel 12H18N9T, the thickness of the intermediate layer (dissipative structures) increases over time.The presence of an intermediate in the gap helps to maintain the integrity of the rubbing surfaces; however, due to the high shear resistance of this layer, it leads to an increase in the frictional force.The friction process of low-entropy materials is characterized by the convergence of contacting bodies (wear) and a significant change in the initial surface micro relief.Due to the low shear resistance of the adhesive seams in this case, the lowest values of friction coefficients (f<0.5)among the HSS grades under study were recorded.Thus, the thermal entropy of the CTM, determined with a known chemical composition according to the additivity rule, can be used to predict the relative level of some tribotechnical characteristics.When developing new grades of HSS, preference should be given to those compositions that, having the required performance properties, are also characterized by a higher thermal entropy.

E3SFig. 1 .
Fig. 1.Change of parameters Y(a) and f(b) over time T characteristic of high-entropy HSS grades.

E3SFig. 2 .
Fig. 2. Change of parameters Y and f over time T characteristic of low-entropy HSS grades.The values of the maximum δ and the final δ800 for the friction path of 800 m thicknesses of the intermediate layer are shown in Table2.If no growth in layer thickness was observed, the parameter δ in Table2is not indicated.

Fig. 4
Fig.4shows the dependence of the average values of the roughness of the friction surfaces Ra of indenters made of various HSS grades and of counter body on the value of their thermal entropy S. The increase in the thickness of the intermediate layer observed during friction of high-entropy grades of HSS (Table.2) contributes to the separation of contact surfaces and the preservation of the surface integrity of the samples (Fig.4).At the same time, due to the shear resistance of the intermediate layer itself, the friction coefficients recorded in the tribosystem are slightly higher than for HSS grades with low thermal entropy values.For low-entropy HSS grades there is a significant change in the surface micro relief relative to the initial state, which is a consequence of the processes of formation / destruction of adhesive seams.Also, the adhesive seams formed during friction of low-entropy HSS on stainless steel have a low shear resistance (Table2).The established features of the process of friction of high-entropy HSS on stainless steel 12H18N9T are consistent with previous studies of their tribotechnical characteristics.Thus, the process of frictional interaction with structural steel 30HGSA for these materials in similar tests is also characterized by the fastest increase in the thickness of the intermediate layer and the smallest change in the micro relief both on the surface of the indenter and on the friction tracks on the disk[14].

Fig. 4 .
Fig. 4. The dependence of the average values of the roughness of the friction surfaces Ra of indenters made of various HSS grades (a) and of counter body (b) on the value of their thermal entropy S.

Table 1 .
Composition and thermal entropy of HSS grades.

Table 2 .
Friction coefficient and characteristics of the intermediate layer during friction of HSS of various grades.