1 Introduction Nitrocarburising is a thermochemical process in which nitrogen and carbon are introduced into the surface of ferrous materials by diffusion. As a result, the hardness of the treated workpieces can be increased, and their wear resistance, fatigue strength and adhesion tendency can be improved [1-3], whereby the life of machine components can be considerably extended [4-7]. In addition, the formation of a compound layer on the surface of the treated material increases the corrosion resistance of the workpieces [8-10]. These nitrocarburised layers can be further post-oxidised, producing a supplementary thin iron oxide film (oxide layer) on the surface. Since this oxide layer also closes the pores in the compound layer, an additional improvement in the corrosion resistance of the components can be achieved [5, 11]. Through this post-oxidation, an improvement in the friction and anti-galling properties of dry steel contacts can also be observed [5, 12]. Figure 1 shows a typical layer structure which is formed on steel during nitrocarburising combined with post-oxidation. The diffusion layer is 200 to 500 μm thick and has a higher hardness than the base steel material. It is Aus Wissenschaft und Forschung 25 Tribologie + Schmierungstechnik · 66. Jahrgang · 1/ 2019 DOI 10.30419/ TuS-2019-0003 Influence of surface properties of nitrocarburised and oxidised steel on its tribological behaviour Igor Velkavrh, Florian Ausserer, Stefan Klien, Joel Voyer, Klaus Lingenhöle, Fevzi Kafexhiu, Djordje Mandrino, Bojan Podgornik, Johannes Rattenberger, Hartmuth Schröttner, Ferdinand Hofer, Alexander Diem* Nitrocarburierte Schichten können mit unterschiedlichen Verfahren und/ oder Prozessparametern erzeugt werden. So können eine Vielzahl von unterschiedlichen Schichten hergestellt werden, die sich in den mechanischen Eigenschaften, in der Oberflächenmorphologie, in der chemischen Zusammensetzung und somit auch im Reibungs- und Verschleißverhalten unter tribologischer Beanspruchung unterscheiden. In diesem Artikel werden Beispiele vorgestellt, die zeigen, wie das tribologische Verhalten von verschiedenen nitrocarburierten Schichten durch unterschiedliche Produktionsverfahren beeinflusst wird. Schlüsselwörter Nitrocarburierung, Oxidation, Oberflächencharakterisierung, Reibung, Verschleiß Nitrocarburised coatings may be produced using different processing methods and/ or process parameters, allowing a variety of possible mechanical properties, surface morphologies and chemical compositions, and thus, as a result, their friction and wear behaviour under tribological loading can be very different. In the present work, various examples are highlighted which show how the tribological behaviour of different nitrocarburised layers is affected by different coating properties. Keywords Nitrocarburising, Oxidation, Surface Characterisation, Friction, Wear Kurzfassung Abstract * DI Dr. Igor Velkavrh 1 , Orcid-ID: https: / / orcid.org/ 0000-0002-4293-9978 DI (FH) Florian Ausserer MSc. 1 , Orcid-ID: https: / / orcid.org/ 0000-0001-8541-3063 DI (FH) Stefan Klien 1 , Orcid-ID: https: / / orcid.org/ 0000-0001-8727-4909 Dr. Joel Voyer 1 , Orcid-ID: http: / / orcid.org/ 0000-0002-0649-8140 DI (FH) Klaus Lingenhöle 2 , Orcid-ID: https: / / orcid.org/ 0000-0001-7803-7151 DI Dr. Fevzi Kafexhiu 3 , Orcid-ID: https: / / orcid.org/ 0000-0001-6848-9293 DI Dr. Djordje Mandrino 3 , Orcid-ID: https: / / orcid.org/ 0000-0002-1402-1625 Prof. DI Dr. Bojan Podgornik 3 , Orcid-ID: https: / / orcid.org/ 0000-0002-3224-6449 DI Dr. Johannes Rattenberger 4 , Orcid-ID: https: / / orcid.org/ 0000-0002-9337-2465 Ing. Hartmuth Schröttner 4 , Orcid-ID: https: / / orcid.org/ 0000-0001-5981-0890 Univ.-Prof. DI. Dr. Ferdinand Hofer 4,5 , Orcid-ID: https: / / orcid.org/ 0000-0001-9986-2193 DI Alexander Diem 1 Orcid-ID: https: / / orcid.org/ 0000-0002-6757-6966 1 V-Research GmbH, 6850 Dornbirn, Austria 2 Lingenhöle Technologie GmbH, 6800 Feldkirch, Austria 3 Institute of Metals and Technology, 1000 Ljubljana, Slovenia 4 Graz Centre for Electron Microscopy (ZFE), 8010 Graz, Austria 5 Institute of Electron Microscopy and Nanoanalysis (FELMI), Graz University of Technology, 8010 Graz, Austria T+S_1_2019.qxp_T+S_2018 29.01.19 09: 11 Seite 25 Furthermore, it has been found elsewhere that a thick compound layer can, due to its brittleness, increase the wear rate due to the formation of abrasive wear particles during tribological testing [2, 16, 17]. In the present work, the influence of nitrocarburising and oxidation processes on the surface properties of steel samples as well as on their tribological behaviour under dry sliding conditions is investigated. The focus is based on the nitrocarburising process and the formation of the oxide layers. 2 Experimental investigations 2.1 Nitrocarburising and post-oxidation Disc shaped untreated test samples were made of nitriding steel 1.8519 (DIN 31CrMoV9) having a surface roughness R a of 0.5 μm and a hardness of 385 ± 20 HV 0.05. Table 1 lists the process parameters of the nitrocarburising and oxidising processes applied for the production of the samples. 2.2 Materials characterisation Sample hardness was determined using a commercially available hardness tester (Tukon TM 2100B Wolpert Wilson Instruments, USA). The surface topography and its roughness parameters were measured with a laser scanning microscope (VK-X250/ 260, Keyence, Japan). The morphology of the sample surfaces was determined by a field emission scanning electron microscope (FE-SEM) (JEOL JSM-6500F, JEOL, Japan). The chemical properties of the sample surfaces were determined by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) (Microlab 310F, VG-Scientific, USA). Sample cross sections were prepared using the Broad Ion-Beam (BIB) method (Ilion™, Gatan Inc., USA). The BIB cross sections were analysed by field emission scanning electron microscopy (Ultra 55, Zeiss, Germany) and by energy dispersive X-ray spectroscopy, EDX (Octane Elite, EDAX AMETEK Inc., USA). 2.3 Tribological investigations The tribological investigations were carried out on a SRV®4 tribometer Aus Wissenschaft und Forschung 26 Tribologie + Schmierungstechnik · 66. Jahrgang · 1/ 2019 DOI 10.30419/ TuS-2019-0003 covered with a very hard compound layer having a thickness of 0.5 to 10 μm, which consists mainly of ε-nitrides (Fe 2-3 N) and γ'-nitrides (Fe 4 N) and usually possess a porous structure. The formed oxide layer on top of the compound layer typically consists of Fe 2 O 3 and/ or Fe 3 O 4 and is up to 1 μm thick. Based on the process medium used, nitrocarburising can be distinguished in plasma, gas or salt bath nitrocarburising. Depending on the process, nitrocarburised layers with similar mechanical properties can be produced, which, however, differ significantly in their chemical composition and surface morphology, which can have a significant influence on their friction and wear behaviour under tribological loading. For example, in previous studies performed by the authors it was shown that for different nitrocarburising processes or by applying different process parameters, layers with different tribological behaviours were produced [13-15]. It has been observed that the tribological behaviour of the nitrocarburised layers under specific conditions correlates well with the surface microstructure and the mechanical properties of the compound layer. Figure 1: Schematic representation of a cross section of a nitrocarburised and post-oxidised steel Process Designation Duration (h) Temperature (°C) Oxidising agent Salt bath nitrocarburising SNC 1.5 580 Oxidation solution consisting of alkali metal hydroxide, alkali metal nitrate and alkali metal carbonate Gas nitrocarburising GNC 12 540 Water vapour Plasma nitrocarburising PNC 22 540 Water vapour 30 540 Water vapour Table 1: Process parameters of the applied nitrocarburising processes T+S_1_2019.qxp_T+S_2018 29.01.19 09: 11 Seite 26 (Optimol Instruments Prüftechnik GmbH, Germany). Table 2 lists the applied experimental parameters. The wear depths on the nitrocarburised disc samples were measured after the tribological tests with a 3D confocal microscope (μSurf, NanoFocus AG, Germany). 3 Results and discussion 3.1. Topography and microstructure After the nitrocarburising process, the SNC and GNC samples had a hardness of 1310 ± 300 HV 0.05 and 1085 ± 140 HV 0.05, respectively, while for the PNC samples with different process times of 22 h and 30 h, hardness values of 1080 ± 45 HV 0.05 and 950 ± 135 HV 0.05 were measured, respectively. The R a value of the SNC samples increased to 0.9 μm, while for GNC and PNC samples the increase of roughness was less pronounced and R a values of up to 0.6 μm were measured. Figure 2 shows SEM micrographs of the nitrocarburised surfaces of SNC, GNC and PNC (22 h and 30 h) samples. On the SNC surfaces, agglomerations of iron nitride particles with a size of ~ 3 μm can be seen. Similar particle agglomerates were found on the GNC surfaces, but the iron nitride particles were smaller (~ 1 μm) and more densely distributed. The smallest iron nitride particles (< 1 μm) were found on the PNC surfaces with the shortest process time of 22 h. On the PNC surfaces with the longest process time of 30 h, iron nitride crystal clusters with a diameter of up to ~ 5 μm could be observed. Figure 3 shows SEM micrographs of the cross sections of the nitrocarburised samples. The measured compound layer thickness was 4.5 ± 0.7 μm for the SNC samples, 4.6 ± 0.1 μm for the GNC samples, 2.8 ± 0.5 μm and 7.4 Aus Wissenschaft und Forschung 27 Tribologie + Schmierungstechnik · 66. Jahrgang · 1/ 2019 DOI 10.30419/ TuS-2019-0003 Lubricating medium Dry (unlubricated) Atmosphere Air (relative humidity 45 to 55%) Temperature 20°C Contact geometry Cylinder / flat Contact shape Line contact Normal force 143 N Hertzian contact pressure (initial) p mean 140 MPa, p max 180 MPa Motion dynamics linear oscillating Frequency 8 Hz Stroke 4 mm Sliding velocity v mean 64 mm/ s, v max 100 mm/ s Test duration 120 min Sliding distance 460.8 m Table 2: Parameters applied for the tribological investigations. Figure 2: SEM micrographs of the nitrocarburised surfaces (a) SNC, (b) GNC, (c) PNC (22 h), (d) PNC (30 h) T+S_1_2019.qxp_T+S_2018 29.01.19 09: 11 Seite 27 ples. From Figure 4a, it can be seen that the SNC sample exhibited the lowest wear depth (~ 3 µm) of all samples tested. For GNC, a wear depth of around 15 μm was measured and for both PNC samples, the wear depth was around 40 μm. The wear on the 100Cr6 cylinder counter body showed a trend in the opposite direction. For SNC, the wear on the cylindrical counter body was ~ 70 μm, for GNC around 45 μm and for the PNC samples between 10 µm and 15 μm. For the SNC samples, the wear depth was lower than the compound layer thickness, while for the GNC and both Aus Wissenschaft und Forschung 28 Tribologie + Schmierungstechnik · 66. Jahrgang · 1/ 2019 DOI 10.30419/ TuS-2019-0003 ± 0.1 μm for the PNC samples (22 h and 30 h), respectively. The SNC sample showed a quite uniform thick oxide layer (> 1 μm) on top of the compound layer. On the GNC sample, a very thin (< 1 μm) and partially interrupted oxide layer could be observed. On the PCN samples (22 h and 30 h) no visually distinct oxide layer could be observed. 3.2 Friction and wear behaviour Figure 4 shows the measured wear depths and dynamic friction curves of the investigated nitrocarburised sam- Figure 3: SEM micrographs of cross sections of nitrocarburised samples: (a) SNC, (b) GNC, (c) PNC (22 h), (d) PNC (30 h) Figure 4: (a) Average wear depths and (b) dynamic friction curves of investigated nitrocarburised 1.8519 steel samples T+S_1_2019.qxp_T+S_2018 29.01.19 09: 11 Seite 28 PNC samples, it was higher than the compound layer thickness. Due to the high hardness and low adhesion of the compound layer, its presence on the steel surface can ensure very high wear resistance over long periods of operation, while its removal and the exposure of the less hard diffusion zone, can on the other hand result in an increase of the wear rate. From Figure 4b, it can be seen that the dynamic coefficients of friction of the SNC and GNC samples were between 0.8 and 0.95, which were slightly higher than for the PNC samples, for which, especially in the first half of the tests, average dynamic friction coefficients of 0.75 were measured. Figure 5 shows optical micrographs of the wear tracks present on the nitrocarburised steel samples. It can be observed that the surface of the SNC samples was only slightly smoothened and that the original machining lines are still visible after the tribological tests, while the GNC sample showed a more pronounced smoothening along with plastic deformation since the original machining lines are no longer visible. On the PNC samples, abrasive grooves oriented in the sliding direction were observed (Figures 5c and 5d) indicating that for these samples abrasive wear was the predominant wear mechanism. The dark areas on both GNC and PNC samples are actually iron oxides, as supplementary EDX analysis have revealed. These iron oxides are most likely wear particles coming from the contacting bodies which were generated during the tribological tests and finally accumulated on the friction surfaces. The low wear of the SNC samples observed previously is probably related to the porous structure of their compound layer and to the presence of large crystal clusters observed on these surfaces (Figure 2a). The valleys between the clusters could also have served as reservoirs for the abrasive particles which may promote an efficient running-in by preventing a possible three-body abrasion mechanism. However, it cannot be excluded that the low wear of SNC samples is associated with its layer structural composition, since its oxide layer is compacter and thicker (Figure 3a) as compared to other nitrocarburised samples (Figures 3b to 3d). For the GNC layers, higher abrasive wear was observed than for the SNC layers and for PNC layers, the highest abrasive wear was observed. It is assumed that the compound layer of the GNC layers was less brittle than that of the PNC layers and a correlation between the brittleness of the bonding layer and the degree of abrasive wear may exist. Namely, as reported elsewhere [2, 16], a thick compound layer can have an adverse effect on the wear behaviour. During tribological loading, the brittle compound layer may be damaged and hard abrasive particles could be formed and end up damaging the contacting surfaces. It is believed that a similar wear mechanism may have occurred for the investigated GNC and PNC samples. 3.3 Morphology of the oxide layer Figure 6 shows AES elemental concentration depth profiles for SNC, GNC and PNC (22 h and 30 h) samples. The elements C, Fe, O and N were measured Aus Wissenschaft und Forschung 29 Tribologie + Schmierungstechnik · 66. Jahrgang · 1/ 2019 DOI 10.30419/ TuS-2019-0003 Figure 5: Optical micrographs of wear tracks on nitrocarburised steel samples (a) SNC, (b) GNC, (c) PNC (22 h), (d) PNC (30 h). The arrows indicate the direction of movement during the tribological investigations T+S_1_2019.qxp_T+S_2018 29.01.19 09: 11 Seite 29 and the concentration values were quantified, so that for each measuring point the sum of the concentrations equals 100 at. %. From Figure 6, it can be seen that in the edge, near the surface region, i.e. within the first ~ 1 μm, the O concentration in the SNC and GNC samples was significantly higher than in both PNC samples. Inversely, the Fe concentration in the edge region Aus Wissenschaft und Forschung 30 Tribologie + Schmierungstechnik · 66. Jahrgang · 1/ 2019 DOI 10.30419/ TuS-2019-0003 Figure 6: AES elemental concentration depth profiles for SNC, GNC and PNC (22 h and 30 h) samples: (a) oxygen (O), (b) iron (Fe) Figure 7: XPS spectra of: (a) O 1s and (b) Fe 2p peaks for SNC, GNC and PNC (22 h and 30 h) samples Component Binding energy (eV) SNC GNC PNC (22 h) PNC (30 h) Metallic Fe 706.7 and 707.1 - - - 14% FeO 708.2 / 708.7 17% - - - Fe-Oxide* 709.2 - 709.3 - - 21% 86% Fe 3 O 4 710.2 38% - - - Fe 2 O 3 710.8 - 711.4 45% 100%** 79% - *various FeO x oxides (1 < x < 1.5) **For GNC samples, a peak at 710.8 eV was observed, which corresponds to the lower energy limit of Fe 2 O 3 . It may also be possible that this energy level is due to a combination of different Fe oxides (see footnote * above). Table 3: Ratio of various Fe bonds from the evaluation (fitting) of the XPS-measured Fe 2p spectra oxide* T+S_1_2019.qxp_T+S_2018 29.01.19 09: 11 Seite 30 was higher in both PNC samples than in SNC and GNC samples. Based on these results, it may be concluded that on the SNC and GNC samples a thicker iron oxide layer was formed than on the PNC samples, as already observed on the micrographs shown in Figure 3. Figure 7 shows XPS spectra of O 1s and Fe 2p peaks while Table 3 lists the quantification results of the corresponding peaks. The quantification of the Fe 2p peak shows that, in the SNC sample, a large number of different Fe oxides (FeO, Fe 2 O 3 and Fe 3 O 4 ) were present. It should be noted that FeO is not directly related to the oxidation process itself, but is probably a bath residue (e.g. rust). For GNC samples, a dominant peak at 710.8 eV was observed, indicating the presence of Fe 2 O 3 . For the PNC sample (22 h), predominantly Fe 2 O 3 (~ 80 %) and around 20 % of mixed oxides were found, while for the PNC sample (30 h), an accumulation of FeO x oxides (1 < x < 1.5) was observed. Figure 8 shows EDX elemental mappings of C, Fe, O and N measured on BIB cross sections, before and after Aus Wissenschaft und Forschung 31 Tribologie + Schmierungstechnik · 66. Jahrgang · 1/ 2019 DOI 10.30419/ TuS-2019-0003 Figure 8: EDX elemental maps of C, Fe, O, and N distributions on BIB cross sections of nitrocarburised samples before (unworn) and after (worn) tribological investigations T+S_1_2019.qxp_T+S_2018 29.01.19 09: 11 Seite 31 References [1] Mittemeijer, E. J.: Fundamentals of Nitriding and Nitrocarburizing. Dossett, J. (Ed.); Totten, G. E. (Ed.).: ASM Handbook, Volume 4A, Steel Heat Treating Fundamentals and Processes. Ohio, USA: ASM International, 2013. [2] Podgornik, B.; Vižintin, J.; Leskovšek, V.: Tribological properties of plasma and pulse plasma nitrided AISI 4140 steel. Surface & Coatings Technology, vol. 108-109, p. 454-460, 1998. DOI: 10.1016/ S0257-8972(98)00571-4 [3] Karamboiki, C.-M.; Mourlas, A.; Psyllaki, P.; Sideris, J.: Influence of microstructure on the sliding wear behavior of nitrocarburized tool steels. Wear, vol. 303, no. 1-2, p. 560-568, 2013. DOI: 10.1016/ j.wear.2013.04.002 [4] Marušić, K.; Otmačić, H.; Landek, D.; Cajner, F.; Stupnišek-Lisac, E.: Modification of carbon steel surface by the Tenifer® process of nitrocarburizing and post-oxidation. Surface and Coatings Technology, vol. 201, no. 6, p. 3415-3421, 2006. DOI: 10.1016/ j.surfcoat.2006.07.231 [5] Cajner, F.; Landek, D.; Stupnišek Lisac, E.: Improvement of properties of steels applying salt bath nitrocarburizing with post-oxidation. Materiali in tehnologije/ Materials and Technology, vol. 37, no. 6, p. 333-339, 2003. ISSN 1580-2949 [6] Zhang, J. W.; Lu, L. T.; Shiozawa, K.; Zhou, W. N.; Zhang, W. H.: Effects of nitrocarburizing on fatigue property of medium carbon steel in very high cycle regime. Materials Science and Engineering: A, vol. 528, no. 22-23, p. 7060- 7067, 2011. DOI: 10.1016/ j.msea.2011.05.029 [7] Yang, W. J.; Zhang, M; Zhao, Y. H.; Shen, M. L.; Lei, H.; Xu, L.; Xiao, J. Q.; Gong, J.; Yu, B. H.; Sun, C.: Enhancement of mechanical property and corrosion resistance of 316L stainless steels by low temperature arc plasma nitriding. Surface & Coatings Technology, vol. 298, p. 64- 72, 2016. DOI: 10.1016/ j.surfcoat.2016.04.045 [8] Alphonsa, J.; Raja, V. S.; Mukherjee, S.: Development of highly hard and corrosion resistant A286 stainless steel through plasma nitrocarburizing process. Surface & Coatings Technology, vol. 280, p. 268-276, 2015. DOI: 10.1016/ j.surfcoat.2015.09.017 [9] Alphonsa, J.; Raja, V. S.; Mukherjee, S.: Study of plasma nitriding and nitrocarburizing for higher corrosion resistance and hardness of 2205 duplex stainless steel. Corrosion Science, vol. 100, p. 121-132, 2015. DOI: 10.1016/ j.corsci.2015.07.014 [10] Basso, R. L. O.; Candal, R. J.; Figueroa, C. A.; Wisnivesky, D.; Alvarez, F.: Influence of microstructure on the corrosion behavior of nitrocarburized AISI H13 tool steel obtained by pulsed DC plasma. Surface & Coatings Technology, vol. 203, no. 10-11, p. 1293-1297, 2009. DOI: 10.1016/ j.surfcoat.2008.10.006 [11] Li, G.; Peng, Q.; Wang, J.; Li, C.; Wang, Y.; Gao, J.; Chen, S.; Shen, B.: Surface microstructure of 316L austenitic stainless steel by the salt bath nitrocarburizing and postoxidation process known as QPQ. Surface & Coatings Technology, vol. 202, no. 13, p. 2865-2870, 2008. DOI: 10.1016/ j.surfcoat.2007.10.032 Aus Wissenschaft und Forschung 32 Tribologie + Schmierungstechnik · 66. Jahrgang · 1/ 2019 DOI 10.30419/ TuS-2019-0003 the tribological tests, i.e. the BIB cross sections of the unworn and the worn samples are compared. Figures 8a and 8b show that the diffusion zone, the compound and the oxide layers of the SNC sample are clearly visible before and still after the tribological test. On the worn SNC surface, parts of the oxide layer are still present, which can be seen by the presence of an oxygen-rich layer on the worn samples. From Figures 8c to 8h, it can be seen that after the tribological tests, little to no oxide layers were present on the GNC and PNC (22 h and 30 h) samples, since the O content on the worn samples significantly decreased in comparison to the unworn samples. 4 Summary Based on the results presented in the present work, the following conclusions can be drawn: 1. Under applied tribological loading, SNC samples were more resistant to wear than GNC and PNC samples. 2. SNC samples differed from GNC and PNC samples in terms of the structure of their nitrocarburised layer (compound layer structure, porosity and morphology of the oxide layer). This is believed to be the reason for the better tribological behaviour of the SNC layers compared to GNC and PNC samples. 3. A correlation between the presence or properties of iron oxides (Fe 2 O 3 , Fe 3 O 4 and FeO x , where 1 < x < 1.5) and the tribological behaviour may exist. 4. The nitrocarburising time has an influence on the compound layer properties, foremost on the layer thickness and its hardness (or brittleness), which subsequently has an effect on the wear behaviour of the nitrocarburised samples. Acknowledgements The work presented herein was funded by the Austrian COMET Programme (Project XTribology, no. 849109) and carried out at the „Excellence Centre of Tribology“ (AC2T research GmbH) in cooperation with V-Research GmbH, Lingenhöle Technologie GmbH, Institute of Metals and Technology (IMT), Graz Centre for Electron Microscopy (ZFE) and Institute of Electron Microscopy and Nanoanalysis (FELMI). The project „Innovative Material Characterization“, with the project number SP2016-002-006, was supported by the Austrian Federal Ministry for Digital and Economic Affairs in the framework of the ACR - Austrian Cooperative Research - Strategic Project Program 2016. T+S_1_2019.qxp_T+S_2018 29.01.19 09: 11 Seite 32 [12] Sun, Y.: Unlubricated sliding frictional behaviour of oxide films on plasma nitrocarburised steel. Tribology International, vol. 40, no. 2, p. 208-215, 2007. DOI: 10.1016/ j.triboint.2005.09.028 [13] Badisch, E.; Trausmuth A.; Rodríguez Ripoll, M.; Diem, A.; Kunze, W.; Glück, J.; Lingenhöle, K.; Orth, P.: Influence of nitrocarburizing process parameters on the development of surface roughness and layer formation. Key Engineering Materials, vol. 674, p. 325-330, 2016. DOI: 10.4028/ www.scientific.net/ KEM.674.325 [14] Velkavrh, I.; Trausmuth, A.; Rodríguez Ripoll, M.; Kunze, W.; Glück, J.; Lingenhöle, K.; Orth, P.; Diem, A.; Badisch, E.: Damage Mechanisms of Plasma, Gas and Salt Bath Nitrocarburized Steel in Lab-Scale Sliding Test. Key Engineering Materials, vol. 674, p. 152-158, 2016. DOI: 10.4028/ www.scientific.net/ KEM.674.152 [15] Velkavrh, I.; Ausserer, F.; Klien, S.; Voyer, J.; Diem, A.; Trausmuth, A.; Rodríguez Ripoll, M.; Badisch, E.; Kunze, W.; Glück, J.; Lingenhöle, K.: Optimisation of Plasma Nitrocarburising for Reducing Wear in Dry Sliding Contacts. Key Engineering Materials, vol. 721, p. 389-393, 2016. DOI: 10.4028/ www.scientific.net/ KEM.721.389 [16] Podgornik, B.; Vižintin, J.; Leskovšek, V.: Wear properties of induction hardened, conventional plasma nitrided and pulse plasma nitrided AISI 4140 steel in dry sliding conditions. Wear, vol. 232, no. 2, p. 231-242, 1999. DOI: 10.1016/ S0043-1648(99)00151-9 [17] Karaoğlu, S.: Structural characterization and wear behavior of plasma-nitrided AISI 5140 low-alloy steel. Materials Characterization, vol. 49, no. 4, p. 349-357, 2002. DOI: 10.1016/ S1044-5803(03)00031-7 Aus Wissenschaft und Forschung 33 Tribologie + Schmierungstechnik · 66. Jahrgang · 1/ 2019 DOI 10.30419/ TuS-2019-0003 T+S_1_2019.qxp_T+S_2018 29.01.19 09: 11 Seite 33