Scholarly article on topic 'Enhancing the Surface Integrity of Tribologically Stressed Contacting Surfaces by an Adjusted Surface Topography'

Enhancing the Surface Integrity of Tribologically Stressed Contacting Surfaces by an Adjusted Surface Topography Academic research paper on "Materials engineering"

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Procedia CIRP
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{"Surface integrity" / Tribology / "Hard machining"}

Abstract of research paper on Materials engineering, author of scientific article — Sebastian Goeke, Dirk Biermann, Daniel Stickel, Priska Stemmer, Alfons Fischer, et al.

Abstract Tribological systems are more and more used under mixed and boundary friction conditions. Typically the running-in phase of a tribological system shows a wear rate which is significantly higher than the wear rate during steady state. Commonly it is assumed that smoother surfaces would lead to lower wear rates. A new approach aims at the shortening of the running-in phase by adjusting the surface topography. Specific topographies resulting from milling, grinding, and honing processes were generated and tribologically tested under lubricated sliding wear conditions. Wear tests showed a distinct influence of the surface topography regarding wear rate and running-in time.

Academic research paper on topic "Enhancing the Surface Integrity of Tribologically Stressed Contacting Surfaces by an Adjusted Surface Topography"

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Procedia CIRP 13 (2014) 214 - 218

2nd CIRP Conference on Surface Integrity (CSI)

Enhancing the Surface Integrity of Tribologically Stressed Contacting Surfaces by an Adjusted Surface Topography

Sebastian Goekea*, Dirk Biermanna, Daniel Stickelb, Priska Stemmerb,c, Alfons Fischerb,c, Karina Geenend, Stephan Huthd, Werner Theisend

aTUDortmund University, Institute of Machining Technology, 44227 Dortmund, Germany bUniversity of Duisburg-Essen, Materials Science and Engineering, itm, 47057 Duisburg, Germany cCENIDE Center for Nanointegration Duisburg-Essen, 47057 Duisburg, Germany dRuhr-Universität Bochum, Chair of Materials Technology, 44780 Bochum, Germany * Corresponding author. Tel.: +49-(0)231755-2528; fax: +49-(0)231755-5141. E-mail address:


Tribological systems are more and more used under mixed and boundary friction conditions. Typically the running-in phase of a tribological system shows a wear rate which is significantly higher than the wear rate during steady state. Commonly it is assumed that smoother surfaces would lead to lower wear rates. A new approach aims at the shortening of the running-in phase by adjusting the surface topography. Specific topographies resulting from milling, grinding, and honing processes were generated and tribologically tested under lubricated sliding wear conditions. Wear tests showed a distinct influence of the surface topography regarding wear rate and running-in time.

© 2014TheAuthors.PublishedbyElsevierB.V.This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Selection and peer-review under responsibility of The International Scientific Committee of the "2nd Conference on Surface Integrity" inthepersonoftheConferenceChair Prof DragosAxinte

Keywords: Surface integrity; Tribology; Hard machining

1. Introduction

Surface texturing of tribologically stressed contacting surfaces in order to shorten the running-in time and decrease the wear rate has been subject to many studies [1-5]. A wide range of machining processes, especially the finishing processes grinding, honing, and polishing, were analyzed regarding their potential to generate a surface layer which fits the needs of tribotechnical systems [6-8]. Beside the surface roughness there are several further aspects concerning the surface integrity. Characteristic values like bearing area ratio, residual stresses, and crystal structure have a major influence on the resulting surface integrity, too [9-14].

Concerning the bearing area ratio, most of the studies concluded that a high surface roughness with distinct profile peaks is inappropriate for a loaded surface under boundary lubrication conditions [1-4, 6-7, 15-16]. In order to decrease the surface roughness, finishing

processes were used subsequently to a premachining process like milling or turning. The most widely used finishing step is a grinding process. By using a grinding process as final machining step, low surface roughness can be generated with a high material removal ratio. The surface layer resulting from the premachining is completely removed and the grinding process generates new characteristics. Typical ground surfaces are characterized by a low surface roughness and a smooth surface structure with randomly distributed profile peaks and valleys [8].

In addition, regarding the contact situation in a tribotechnical system, further characteristic values need to be considered. Beside the surface roughness, a high bearing area ratio is favorable to decrease the specific area load in a contact situation. Those surfaces are traditionally generated with several honing steps by using a subsequently decreasing grain size. Within the first step, the rough honing, profile peaks as well as profile valleys are generated. The following steps, called

2212-8271 © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (

Selection and peer-review under responsibility of The International Scientific Committee of the "2nd Conference on Surface Integrity" in the person

of the Conference Chair Prof Dragos Axinte


finishing honing steps or fine honing, are only made to cut the profile peaks and retain the profile valleys. Furthermore, the remaining profile valleys can act as an oil retaining volume and prevent a dry contact situation between the mating surfaces. As a result, a plateau surface structure occurs with a low surface roughness and a high bearing area ratio [6, 8]. Commonly, within these distinguished honing steps the initial surface layer with all its features concerning surface roughness, deformation gradient, and residual stresses is removed and a completely new layer will be generated.

Although these processes have been intensively studied regarding their specific advantages concerning the generation of a high surface integrity, there are less studies, that consider the surface integrity in combination with a general view on the process chain [17, 18]. The aim of this study is to analyze the surface integrity especially the influence of the surface structure in combination with a process chain consisting of milling, grinding, and honing.

2. Experimental Setup and Methods

The achieved experimental results were obtained by using the case hardening steel 18CrNiMo7-6. Due to its high hardness of 63 HRC, this material is widely used for tribologically highly stressed contacting surfaces. Specific applications are, for example, gears and transmissions which are used for wind power stations or in naval engineering. These gears have to grant an exceedingly long lifetime because of their high repairing charges respectively the costs for their replacement. Therefore, the processes for machining these gears are continuously improved [19-22].

Figure 1 shows an illustration of the chosen processes as well as the shape which was used as test-workpiece. All three machining processes milling, grinding, and finishing or a combination of them were conducted on these specimens.

(a) (b) (c)

(d) Indexable insert: Grains: 46 |jm CBN Grains: 2 |jm AI;Oa

TiAIN Bond: Synthetic resiroid Bond; Synthetic resinoid

vt = 640 m/min vs = 30 m/s F = 600 N

f2 = 0.05 mm/Z v„ = 2 m/min v„ = 5 m/min

ap = 0,2 mm a, = 50 ym fos = 4.2 Hz

Fig. 1. (a) milling process; (b) grinding process; (c) honing process; (d) process parameter

2.1. Machining processes

All workpieces were milled at first in order to guarantee the form and shape accuracy needed for the final machining steps grinding and honing. The milling process was conducted on the five-axis machining center Deckel Maho DMU 50 eVolution. For the subsequent machining steps, the surface grinding machine Geibel & Hotz FS 635-Z CNC was used, while the honing process was realized with the finishing system Supfina 202.

2.2. Characterization of the surface structure

The generated surface topographies were characterized by tactile measurement methods as well as optical measurement methods. The tactile measurements were made with the tactile measurement system Mahr XR20. For the evaluation of the optical measurements the confocal whitelight-microscope Nanofocus [isurf was used.

Both measurement systems were compared by extracting profiles of the measured area data retained from the optical measurement. The surface roughness of the workpieces was characterized according to DIN EN ISO 4287 orthogonal to the feed direction as well as along the feed direction. This additional characterization was made because the sliding direction of the conducted tribological test was the same as the feed direction.

2.3. Methods to analyze the tribological behavior

The tribological behavior of the generated surface topographies was evaluated with the self-mating reciprocation ball on plane system MTS Tytron 250. The synthetic lubricant Mobil Gear SHC XMP 320 was used, in order to map the specific tribological conditions of an application as wind turbine. The specific test parameters for this wear test are listed in Table 1. During the tests the acting normal force and the tangential force were measured with the 3-component-dynamometer Kistler type 9257A.

Table 1. Test parameters for the conducted reciprocating sliding wear test

Parameter Specific value Unit

Test frequency 5 Hz

Stroke 6 mm

Normal force 30 N

Radius Counter Body 5 Mm

Lubricant Mobil Gear SHC XMP 320

Based on the measured forces the occurring friction coefficient was evaluated and analyzed to characterize the sliding behavior. Additionally, the worn surface topographies were characterized after two million sliding cycles with a light-microscope to get a visual impression of the changes within the surface topography.

3. Results

The generated surface topographies were measured directly after the machining process. They are characterized by roughness values, characteristic values from the bearing curve as well as by 3D visualizations gained from the optical measurements. After the sliding test a further characterization of the surface topography was made to illustrate the changes concerning the surface topography.

3.1. Surface topography after machining

As shown in Figure 2, all three machining processes lead to different surface structures concerning the roughness values as well as the bearing ratio. The roughness values were evaluated with tactile as well as optical measurements. In addition to the mean value, the minimum and the maximum of the measured values are shown in the form of error bars.

The milled specimens are characterized by a comparatively high surface roughness in sliding direction with a maximum height of profile between Rz = 1.4-1.6 ^m. Both, the ground surface and the honed surface have a much lower roughness of Rz = 0.1-0.8 ^m for ground specimens and Rz = 0.2-0.4 ^m for honed specimens. Though the mean maximum height of profile Rz for honed workpieces is higher than for the ground one, the variation between the measured values is much smaller for the honed structure than for the ground one. This proves that the honed structure is characterized by a more regular structure than the ground surface topography.

Regarding the characteristic values of the bearing area ratio, the surfaces are characterized by a specific

Fig. 3. Roughness profile and bearing curve after machining by (a) milling; (b) milling and grinding; (c) milling and honing

topography, too. Considering the high valley depth of Rvk = 0.4-0.6 ^m for the milled structure, the peak height of Rpk < 0.1 ^m is comparatively low. This relation between peak height and valley depth is also given for the honed structure with the exception that the absolute values are much lower. An extremely low peak height of Rpk < 0.02 ^m and a valley depth of Rvk = 0.1-0.2 ^m for the honed specimen lead to the same relation of Rpk/Rvk < 0.2 as for the milled surface.

In contrast to this, the ground surface is characterized by an irregular surface profile with a big variety for all three values peak height, core roughness depth and valley depth. Nevertheless, the absolute values are much lower, compared to the milled structure. Except of the higher variation of the specific values for the ground surface, both the honed as well as the ground surface are characterized by a high bearing area ratio in low profile depths; as shown in Figure 3.

Based on these values it could be assumed that the ground surface as well as the honed surface will have a comparable behavior under the influence of a sliding

Fig. 2. Surface quality depending on the specific process respective

process chain (a) maximum height of profile Rz; (b) reduced peak height Rpk, core roughness depth Rk, reduced valley depths Rvk

Fig. 4. Surface topography generated by (a) milling (b) milling and grinding (c) milling and honing

motion. Taking into consideration further aspects like roughness profiles and 3D visualizations of the surface topographies, the differences between these two topographies become visible. As shown in Figure 4, the milled surface topography is characterized by a periodic roughness profile with distinct profile valleys. By honing this structure with a subsequent honing process the remaining profile peaks can be removed and a plateau structure occurs, which is characterized by periodic profile valleys. When comparing these results to the ground surface, the irregular distribution between peaks and valleys has to be emphasized. Although the ground surface is really smooth, there is no specific structure. The measured values for the profile valleys would not act as beneficial oil retaining volumes because they are connected in sliding direction. Thus the oil is able to avoid the contact between the two mating specimens and there is a risk of partially dry friction [5].

3.2. Tribological behavior

To prove the theoretical assumptions made on the tribological behavior of these topographies, reciprocating sliding tests were carried out to test the influence of the topography on the sliding wear resistance and the occurring changes in surface topography. Additionally, a polished specimen was tested in order to have a comparison to a surface with an extraordinary low roughness without any distinct profile peaks or valleys. As shown in Figure 5, the friction coefficients are evaluated as mean values for different ranges of sliding cycles in order to give an overview of the tribological behavior.

The initial friction coefficient of the tested surfaces varies between ^ = 0.08-0.12. Except the polished specimen, for all tested surface structures a running-in wear behavior with a continuously decreasing friction coefficient over the tested sliding cycles can be determined. The ground surface exhibited the highest friction coefficient over the whole test cycle. This stands in contrast to the assumed wear behavior expected from

• •---- —------ —j*_____


— ...,r-m—

t ----- — r— --------

0 75k 150k 300k 600k 1200k 2000k Number of tested sliding cycles • Milled • Milled and Ground • Milled and Honed • Pofished

Fig. 5. Friction coefficients occurring during reciprocating sliding tests for all surface topographies

the roughness values and the characteristic values gained from the bearing curve. Starting with a friction coefficient of ^ = 0.12, a friction coefficient of ^ = 0.11 after 2 million cycles occurs.

Comparing these results to the milled and honed structure a much lower friction coefficient even in the running-in stage occurs. From the beginning the friction coefficient of ^ = 0.1 decreases to ^ = 0.085 after 2 million cycles for the milled surface structure. In comparison, the friction coefficient for the honed surface structure starts at a value of ^ = 0.087 and decreases to ^ = 0.085 after 2 million sliding cycles. With regard to the variety of the measurement, a significant change concerning the sliding behavior cannot be determined. Considering the evaluated friction coefficients for the polished specimen, an increase over the tested sliding cycles occurs. This proves that a completely flat surface without any specific structure is inappropriate for a tribotechnical application [16].

Regarding the photographies, gained from light-microscope measurements of the worn surfaces, shown in Figure 6, a distinct change of the milled and ground surface layer can be determined, whereas the honed specimen shows only little changes of the initial surface topography. In case of the milled and the ground structure the contacting bodies seem to adapt to each other's surface topography. Therefore the profile peaks were cut in the running-in stage and a plateau structure was created. With regard to the ground surface, the direction of the grooves has a major influence on the tribological behavior. Grooves along the sliding direction provide the worst conditions for a tribological contact situation [5]. Nevertheless, this specific loading condition has to be considered in case of using a grinding process as final machining step.

This proves that the exclusive analysis of roughness values is not sufficient to characterize a surface structure with regard to its potential for a tribologically stressed surface. Rather, the whole structure has to be analyzed and characterized by surface profiles as well as area measurements. Additionally, the running-in behavior of

Fig. 6. Initial surface topography resulting from (a) milling; (b) milling and grinding; (c) milling and honing. Surface topography after 2 million sliding cycles (d) milled; (e) milled and ground; (f) milled and honed

the contacting surfaces can be significantly influenced not only by the final machining step. Instead an overall analysis of the whole manufacturing sequence can be beneficial in order to improve the surface integrity.


Machining processes like milling, grinding, and honing are commonly used for the generation of highly stressed workpiece surfaces. In this study, it has been shown that considering and adjusting all manufacturing processes can lead to beneficial surface structures concerning their wear behavior. By the adjustment of the pre and final machining realized by a milling and a honing process, a more favorable surface structure, compared to a ground structure with grooves along the sliding direction occurs.

Thereby, the running-in phase can be significantly shortened and a lower friction coefficient occurs. For the used tribosystem, which was based on a sliding motion between two mating 18CrNiMo7-6 specimens, the initial friction coefficient could be reduced from 0.12 to 0.8 by adjusting the processes not only for their own efficiency but in regard to the preconditions for the subsequent machining step.

Although this first approach was successful, much work has to be done in order to get sufficient knowledge of all influencing parameters of a complex tribosystem. Specific issues, which have to be analyzed, are the influence of a probably deformed surface layer, the influence of stresses beneath the surface, and the influence of different lubrication conditions during the sliding test, only to mention a few. In order to analyze the long-time wear behavior of the contacting specimens, more sliding cycles can prove the determined benefits for the running-in.


The scientific results presented in this paper are based on the research project "Increasing Power Density and Durability of Highly Stressed Slide Faces by an Improved Surface Integrity Generated with Adapted Machining Processes", which is funded by the Mercator Research Center Ruhr Initiative.


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