HUNGARIAN JOURNAL OF INDUSTRY AND CHEMISTRY Vol. 49(2) pp. 77–84 (2021) hjic.mk.uni-pannon.hu DOI: 10.33927/hjic-2021-26 ANALYSIS OF ROUGHNESS PARAMETERS DETERMINING TRIBOLOGI- CAL PROPERTIES IN HARD TURNED SURFACES VIKTOR MOLNÁR*1 AND ISTVÁN SZTANKOVICS1 1Institute of Manufacturing Science, University of Miskolc, Egyetemváros, Miskolc, 3515, HUNGARY Hard-machined components built into automotive industrial products play an important role because they incorporate working surfaces. The machining of them is crucial; the accuracy, surface quality and lifetime have to be ensured. In this paper the tribological properties of hard-turned surfaces are characterized and analyzed based on 3D and 2D sur- face roughness parameters. Functional parameters that provide quantitative information about the wear resistance and fluid retention of the machined surfaces were studied. The aim of the study was to summarize the relevant roughness parameters in terms of the functionality of the surfaces and to collect experimental results for their application. Keywords: wear-resistance, hard turning, surface roughness 1. Introduction Many machined surfaces incorporate working surfaces, which move relative to other surfaces. The analysis and development of such surfaces is important because the surface quality [1, 2] and, within this parameter, the to- pographical characteristics [3, 4] by and large determine the lifetime of parts [5–7]. The characterization of sur- face topography is highlighted in the automotive indus- try, that is, predicting the values of roughness parameters [8] and the effect of technological data on the topography are determining factors in the design of machined parts [9]. Due to the development of superhard materials, the machining of hardened surfaces by single-point tools has become more common in recent decades [10]. In this paper, the topography of internal cylindrical surfaces machined by hard turning is analyzed from a tri- bological point of view. Tribology focuses on the friction, wear and friction-reducing characteristics of surfaces. By analyzing surface topography with the help of several available surface roughness parameters, the wear resis- tance as well as load-bearing and fluid-retention capaci- ties of surfaces can be characterized [11]. These parame- ters have been analyzed in numerous studies which only focus on certain parameter groups, e.g. Rk or volume pa- rameters [12–15]. The aim of the present study is to summarize and compare most of the roughness parameters that describe tribological characteristics. These are mainly the areal (3D) roughness parameters because of their exactness [16–18]. If a parameter has a corresponding line param- eter (2D), it is also analyzed. While less exact than 3D *Correspondence: viktor.molnar@uni-miskolc.hu results, 2D studies can provide useful practical informa- tion and are less time-consuming. Although certain pa- rameters can be considered as more accurate due to their modernity, e.g. volume parameters, most parameters can provide at least directions with regard to the mentioned tribological characteristics. 2. Tribology-oriented roughness parame- ters The simplest information is provided by the maximum peak height (Sp) and maximum valley depth (Sv) within the group of height parameters. A higher peak maximum height could be indicative of a relatively long wear-in phase and a higher maximum valley depth of a higher fluid-retention capacity. The skewness (Ssk) and kurto- sis (Sku) parameters are also regarded as height param- eters. A negative Ssk, e.g,. a burnished surface, means a better fluid-retention capacity comparative to the positive values of highly peaky surfaces. In the case of a zero or negative Ssk value, the load-bearing area of the surface is greater, therefore, its wear resistance is also greater [3]. This effect is enhanced by an Sku of 3 or lower, which is indicative of a relatively filled surface [19]. These char- acteristics are summarized in Fig. 1 based on 2D profile parameters. The Abbott-Firestone curve and the related Sk param- eters (Fig. 2) typically help to analyze the functional and tribological properties of a surface [21]. The shape of the curve itself draws attention to some remarkable topo- graphic characteristics. The curve of a random (isotropic) surface, e.g. the ground, is entirely analogous to that of a normal distribution. In the case of periodic, e.g. hard- https://doi.org/10.33927/hjic-2021-26 mailto:viktor.molnar@uni-miskolc.hu 78 MOLNÁR AND SZTANKOVICS Figure 1: The ranges of the Rsk and Rku parameters for different surface topographies [20] Table 1: Effects of changes in the parameters determining tribological properties Change in roughness parameter Sp: _; Ssk: _ / ≤ 0; Sku: _ / < 3; Spk: _; Sa1: _; Vmp: _; Sbi: ^ Sv: ^; Ssk: _ / ≤ 0; Svk: ^; Sa2: ^; Vvc: ^; Vvv: ^; Sci: ^; Svi: ^ Change in tribological character- istics Wear resistance and/or load- bearing capacity increases Fluid-retention capacity in- creases turned, surfaces, the change in the gradient of the curve is uneven and the curve is asymmetrical. The procedures that reduce or eliminate the surface peaks, e.g. diamond or ball burnishing, result in a plateaued topography and a filled surface [4, 22], yielding a relatively straight mid- dle section with a gradual gradient in the curve. In this case, since the material volume of the surface peaks is relatively low, the wear resistance of the surface is higher [11]. Concerning the Sk parameters, the value of the re- duced peak height (Spk) is low. The increase in the re- duced valley depth (Svk) indicates a higher fluid reten- tion capacity [23]. In the analysis of Sk, volume parame- ters are also applied to the material volume of peaks (Sa1) and the void volume of valleys (Sa2) [24]. Although similar statements are valid for the volume parameters, this parameter group measures the magnitude of the peak and valley zones in a more exact manner. The lower the peak material volume (Vmp) is, the bet- ter its wear resistance capacity, moreover, the higher the core void volume (Vvc) and valley void volume (Vvv), Figure 2: The Abbott-Firestone curve and the determina- tion of the Sk parameters the better its fluid retention capacity [11]. In the volume analysis the peak zone is defined as the top 10% and the valley zone as the bottom 20% of the topography. The so-called functional indices are less well-known or at least less frequently applied. Rather than being de- rived from the Abbott-Firestone curve, they are character- ized by the load-bearing and fluid-retention capacities of the surfaces. The higher the surface bearing index (Sbi), the higher its load-bearing capacity, while the higher the core fluid retention (Sci) and the valley fluid retention (Svi) indices, the better its fluid retention capacity. These tribological properties are summarized in Table 1. 3. Experimental conditions Internal cylindrical surfaces (S1, S2 and S3) of three parts were machined by hard turning. Various feeds resulted in different topographies when the other cutting parameters were fixed. The cutting experiments were carried out by a hard machining tool enter type EMAG VDC 400. The ap- plied insert was of the type CCGW 09T308 NC2 and the tool holder of the type E25T-SCLCR 09-R. The cutting data are summarized in Table 2. The surfaces were bores of gearwheels built into transmission systems. The parts were composed of the steel 20MnCr5. The physical and mechanical properties as well as the chemical composition of this steel are sum- marized in Table 3. The diameters of the machined bores were d = 88 mm and their lengths were L = 34 mm. The surface roughness measurements were carried out on an AltiSurf 520 measuring machine using a CL2- type optical sensor with a nominal measuring range of 0−300µm. The resolution along the z axis was 0.012µm and 5µm along the x and y axes. The scanned area was 4.8 × 2.8 mm2. Gaussian filtering was applied to the evaluation and the cut-off wavelength was λc = 0.8 mm. For the purpose of evaluating the area parameters, a 2×2 mm2 area was taken into account and the evaluation Hungarian Journal of Industry and Chemistry ANALYSIS OF ROUGHNESS PARAMETERS DETERMINING TRIBOLOGICAL PROPERTIES 79 Table 2: Cutting data of the machined surfaces Cutting Machined surfaces data S1 S2 S3 ap [mm] 0.2 n [1/min] 615 f [mm/rev] 0.1 0.2 0.3 Table 3: Physical and mechanical properties as well as the chemical composition of the machined workpieces Yield Strength Tensile Strength Hardness Thermal Conductivity Density Elastic Modulus σs (MPa) σb (MPa) HRC k (W/mK) ρ (g/cm3) E (GPa) 1034 1158 62 − 64 11.7 7.7 − 8.03 190 − 210 C Mn Cr Si Cu S P Al 0.17 − 0.22 1.1 − 1.4 1.0 − −1.3 ≤ 0.4 ≤ 0.4 ≤ 0.035 ≤ 0.025 0.02 − 0.04 Figure 3: Evaluation area and profiles applied to the surface topography analysis length for the line parameters was ln = 4 mm. The anal- ysis of the line parameters was carried out based on the average surface roughness values calculated from three profiles per surface. The line profile was extracted from the 3D area. The measurement setup, evaluation area and location of the 2D profiles are presented in Fig. 3. For the analysis of the 3D parameters, the geometrical product specification (GPS) standard ISO 25178 was used, while the standards ISO 4287 and ISO 13565-2 were used for the 2D parameters. The analyzed functional indices are defined by the standard EUR 15178N. 4. Results and discussion 4.1 Surface characteristics Surfaces machined by hard turning exhibit periodic to- pography in contrast to random surfaces such as ground surfaces. In metrology, this characteristic can be ex- pressed by the degree of isotropy as a percentage or by using the spatial parameter, Str (ranging from 0 to 1). The analyzed surfaces are definitely anisotropic; their values vary between 1.46 and 4.28%. The degree of isotropy in- creases as the feed rate increases. The specific direction of measurement (X) is identical to the direction of the feed, which is perpendicular to the cutting speed vector. In this direction, the roughness height of the turned sur- face is at its maximum. This direction of measurement is important because the direction of the extracted 2D pro- files is X. The dominant texture direction (lay) varies be- tween 90◦ and 90.05◦, which demonstrates the accuracy of the measurements. In Table 4, the isotropy of the ana- lyzed surfaces and the texture directions are summarized. Frequency analysis was performed for additional characterization of the surfaces. In Fig. 4, the Power Spectral Densities (PSD) of the surfaces are presented. It can be observed that the wavelengths are identical to the feed rate values. Additional components appear as peri- odic noises in the analyses. They might result from fur- 49(2) pp. 77–84 (2021) 80 MOLNÁR AND SZTANKOVICS Table 4: Texture direction and isotropy of the analyzed surfaces Surface S1 S2 S3 Isotropy: 1.4576% Isotropy: 1.8917% Isotropy: 4.2799% First direction: 89.9956◦ First direction: 89.9973◦ First direction: 90.0461◦ Figure 4: Power Spectral Density (PSD) analysis of the surfaces ther topographic characteristics of the surfaces or from mechanical circumstances of the machining. In Figs. 5 and 6, the simple height parameters of the surfaces are demonstrated for 3D and 2D measurements, respectively. The arithmetical mean height (Sa and Ra), the maximum height (Sz and Rz), the maximum peak height (Sp and Rp) and the maximum valley depth (Sv and Rv) have similar values for the different surface to- pographies, that is, S1 (f = 0.1 mm/rev) and S2 (f = 0.2 mm/rev). This phenomenon highlights the necessity of including additional parameters in the topography in or- der to characterize it in more detail. By analyzing the deviations in the 2D and 3D height parameters, it can be stated that the maximum peak height (Sp) is 1.2−2.2 times higher than the mean of the Rp val- ues obtained by averaging the three 2D measurements. This multiplier varies between 1.9 and 2.5 in terms of Sv and Rv for the surfaces machined using three different feed rates. The maximum height (Sz) is 1.5 − 2.3 times higher than the average of the three Rz values. By com- paring the arithmetical mean heights (Sa and Ra), it was found that the 3D values are higher than the 2D ones. The Sa values are 8 − 15% higher than the Ra ones and the percentage differences are higher when lower feed rates were applied (Fig. 7). 4.2 Analysis of tribological parameters The parameters characterizing tribological properties are found in the height, Sk and volume parameter groups as well as among the functional indices. In Fig. 8, the 3D parameters that provide information about the wear re- sistance are summarized for the analyzed surfaces. The lower values of Sp, Spk, Sa1 and Vmp indicate better wear resistance. For all four parameters, it can be stated that surface S2 machined at f = 0.2 mm/rev is the most wear-resistant and surface S3 machined at f = 0.3 mm/rev is the least. The same is observed according to the Ssk parameter. However, based on the Sku parame- ter, the most wear-resistant surface is S3, which is ma- chined at f = 0.3 mm/rev. The values of these two pa- rameters indicate that the surfaces that have more filled peak zones and, therefore, whose peaks wear out faster are more wear-resistant. The surface is characterized by the Sbi parameter according to a different method: it is calculated as the ratio of the Sq parameter to the material volume in the top 5% of the surface. As a consequence, the surface machined at a high feed rate (0.3 mm/rev) is ranked first in terms of wear resistance. It should be noted that among the analyzed parameters, the dimensions of Sp and Spk denote length, of Sa1 and Vmp represent vol- ume, while Ssk, Sku and Sbi are non-dimensional. If the volume parameter Vmp is considered to be the base due to its modernity and accuracy, the order of the surfaces in terms of wear resistance is S2, S1 and S3. This is con- firmed by the order of the parameters Sp, Spk, Sa1 and Ssk. The parameters that provide information about the fluid-retention capacity of the surfaces are summarized in Fig. 9. The parameters Sv, Svk, Sa2, Vvv and Svi characterize the valley zone and the fluid-retention ca- pacity increases as their values increase. The core zone Hungarian Journal of Industry and Chemistry ANALYSIS OF ROUGHNESS PARAMETERS DETERMINING TRIBOLOGICAL PROPERTIES 81 Figure 5: 3D height parameters of the analyzed surfaces Figure 6: The profiles extracted from the scanned surface (one profile per surface) and their 2D height parameters (average of the data from three profiles) Figure 7: Comparison of the 2D and 3D arithmetical mean heights is characterized by the parameters Vvc and Sci, more- over, higher values indicate greater fluid-retention capac- ities. This property is better in the case of low or neg- ative Ssk values. Concerning the order of the surfaces, deviations can be observed, which may be derived from the different dimensions of the parameters. If the param- eter Vvv is considered to be the base, the order of the surfaces is S3, S2 then S1. This is not confirmed by any other parameters. Regarding the valley zone, the param- eters Ssk, Sa2 then Svi yield the identical order, that is, S2, S1 then S3. Based on the fluid-retention capacity of the core zone, that is, parameter Vvc, the order is S3, S2 then S1. This order is confirmed by the other parameter of the core zone, Sci. By analyzing the 2D profile parameters (Fig. 10), it was found that all of them provide the same order as their 3D counterparts. By considering the 3D values as bases, the following can be stated based on the 2D pa- 49(2) pp. 77–84 (2021) 82 MOLNÁR AND SZTANKOVICS Figure 8: 3D roughness parameters characterizing wear resistance rameters with regard to the three surfaces. The values of the parameters Rp are 46 to 83% of those of Sp. The rate of the parameter Rv varies between 40 and 52%, while those of Rsk and Rku are 21 − 143% and 71 − 112%, respectively. The differences in terms of Tpk and Rvk are smaller, namely 90 − 112% and 90 − 105%, respectively. In Table 5, the order of the surfaces is summarized based on the findings detailed above concerning wear resistance and fluid-retention capacity. 5. Conclusions From the analysis of the general characteristics of the surfaces, it was found that for the applied cutting data the hard-turned surface is anisotropic and the degree of isotropy varies between 1.5 and 4.3%. The Power Spec- tral Density analysis clearly determined that the wave- lengths are identical to the feed rates. The 3D height pa- rameters (Sv, Sp, Sz, Sa) of the surfaces machined by lower feed rates, namely f = 0.1 and 0.2 mm/rev, show absolute differences of between 5 and 17% and this dif- ference in the case of the 2D parameters (Rv, Rp, Rz, Ra) varies between 11 and 17%. By comparing the 3D and 2D parameters, it was found that the Sa value of the 3D measurement is at most 15% higher than the Ra value of the 2D measurement. The Sv, Sp and Sz values can be up to 2.5 times higher than their 2D counterparts. From the analysis of the surface roughness parameters that indicate tribological properties, the following can be stated: Figure 9: 3D roughness parameters characterizing fluid- retention capacity 1. Based on the parameters that evaluate the roughness peaks and indicate the wear resistance (Sp, Ssk, Spk, Sa1 and Vmp), the most and least wear-resistant are the surfaces hard turned at feed rates of 0.2 and 0.3 mm/rev, respectively. The other parameters yielded different orders, which can be explained by their cal- culation methods. 2. The order of the surfaces with regard to the parame- ters of the core zone that indicate the fluid-retention capacity (Vvc, Sci) is identical: the surfaces ma- chined at feed rates of 0.3 and 0.2 mm/rev exhibit the best and worst fluid-retention capacities, respec- tively. 3. The order of the surfaces with regard to the pa- rameters of the valley zone that indicate the fluid- retention capacity (Ssk, Sa2 and Svi) is identical: the surfaces machined at feed rates of 0.2 and 0.3 mm/rev exhibit the best and worst fluid-retention ca- pacities, respectively. The other parameters yielded different orders, which can be explained by their cal- culation methods. Another useful research direction would be the compari- son of different workpiece materials. In addition to this, based on systematic experimental design, carrying out Hungarian Journal of Industry and Chemistry ANALYSIS OF ROUGHNESS PARAMETERS DETERMINING TRIBOLOGICAL PROPERTIES 83 Figure 10: 2D roughness parameters characterizing wear resistance and fluid-retention capacity Table 5: Order of the surfaces based on the analyzed tribological properties and roughness parameters Peak parameter Order Valley parameter Order Core parameter Order Sp S2 > S1 > S3 Sv S3 > S1 > S2 Vvc S3 > S1 > S2 Ssk S2 > S1 > S3 Ssk S2 > S1 > S3 Sci S3 > S1 > S2 Sku S3 > S2 > S1 Svk S1 > S2 > S3 Spk S2 > S1 > S3 Sa2 S2 > S1 > S3 Sa1 S2 > S1 > S3 Vvv S3 > S2 > S1 Vmp S2 > S1 > S3 Svi S2 > S1 > S3 Sbi S3 > S2 > S1 machining experiments using various cutting parameter setups would lead to generalizable statements. Further- more, why the fluid-retention capacity of the valley zone is characterized by relatively large deviations based on the different roughness parameters could be investigated. 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