News 34 Tribologie + Schmierungstechnik · volume 71 · issue 4/ 2024 Introduction In order to meet the requirements for the intended use, the properties of steel components and tools are usually adjusted by means of heat treatment. In surface hardening processes, such as induction hardening, only the area near the surface is heated so that no heat conduction into the core is required. In contrast to hardening the entire cross-section, it is therefore much easier to achieve the desired fully martensitic state. Further advantages of induction hardening are the short process durations and high efficiency [1]. However, the achievable hardness is limited by the alloy composition of the material. Another way to improve the surface layer properties are thermochemical processes like nitriding. In this process, the hardness is increased by the diffusion of nitrogen and the resulting nitride precipitates. The compound layer formed at the surface in this process significantly determines the wear and corrosion behavior of the material [2]. As nitriding often requires long treatment times, which can lead to undesirable tempering effects [3], a combination of the two processes nitriding and induction hardening is an interesting option for avoiding high economic costs. However, the high temperatures of shortterm austenitization destabilize the nitride phases of the compound layer, causing them to partially or completely dissolve [3,4,5]. The resulting resistance deficit of the missing compound layer can be compensated for with an amorphous carbon PVD coating. The mechanical, tribological or tribocorrosive properties of such a coating can also be changed by incorporating silicon into the carbon matrix [6]. The aim of the work was to determine the best possible combination of heat treatments (and, if necessary, a coating). The 42CrMo4 heat-treatable steel was nitrided and induction-hardened, nitrided and a-C: H(: Si)-coated, and nitrided, induction-hardened and a-C: H(: Si)-coated. The quenched and tempered or nitrided initial condition was used for comparison. The duplex and triplex treated variants were tested on a two-disk test rig. The wear was determined gravimetrically and optically using confocal laser scanning microscopy. Experimental results The ring-shaped samples (outer diameter = 46 mm) were manufactured from the heat-treatable steel 42CrMo4 (0.44 Ma.-% C, 0.29 Ma.-% Si, 0.82 Ma.-% Mn, 1.09 Ma.-% Cr, 0.23 Ma.-% Mo, Fe bal.) and tempered (850 °C 2 h/ oil, 630 °C 2 h, “QT” condition, 390 HV10). For the further investigations, different surface layer states were produced on the samples by heat treatment and coating in accordance with Table 1. Plasma nitriding (condition “N”) was carried out in a bell furnace (Rübig) at 520 °C for 20 h with a ratio of N 2 : H 2 = 1: 5 (corresponds to variant PN2 in [3]). A com- GfT-Förderpreis 2024 Investigation of combined surface treatments and coatings to increase the wear behavior of 42CrMo4 Rica Baustert, Stefanie Hoja, Rainer Fechte-Heinen* The topic was submitted for the GfT Sponsorship Award 2024 in the category “Bachelor or similar theses”. The award took place at the GfT conference in September 2024. nitriding, compound layer, duplex treatment, induction hardening, surface hardening, PVD, amorphous carbon layers, triplex treatment Keywords * Rica Baustert 1 (corresponding author) Prof. Dr.-Ing. habil. Stefanie Hoja 2 Prof. Dr.-Ing. habil. Rainer-Fechte Heinen 3,4 1 Universität Bremen Bibliothekstraße 1, 28359 Bremen, Deutschland 2 Hochschule Aalen - Technik und Wirtschaft Beethovenstraße 1, 73430 Aalen, Deutschland 3 Leibniz Institut für Werkstofforientierte Technologien - IWT Badgasteiner Str. 3, 28359 Bremen, Deutschland 4 MAPEX Center for Materials and Processes Universität Bremen, Bibliothekstraße 1, 28359 Bremen, Deutschland News 35 Tribologie + Schmierungstechnik · volume 71 · issue 4/ 2024 pound layer of about 2-7 μm thickness and a hardening depth NHD = 0.27 mm was set. In order to remove the compound layer (“N-CL” condition), half of the samples were subsequently trowalized. The induction heat treatment (“IH” condition) was carried out on a VL1000 SINAC 200/ 300 S MFC universal hardening system (EFD Induction) according to [3] with a generator output of 50 kW. The surface hardening depth SHD for a limit hardness of 409 HV1 was 1.10 mm on average. Comprehensive metallographic documentation of all heattreated samples can be found in [7]. The coating (“PVD” condition) consists of an upper a-C: H(: Si) functional layer and an underlying Cr/ CrN x adhesion promoting layer. The coating was applied with the serial PVD system CemeCon CC800/ 9 SinOx using reactive magnetron sputtering. Further information on the coating process and the determination of the coating thickness can be found in [7] and [8]. The total thickness of the coating system was h c = 3.6 µm (± 0.2 µm) [7]. The wear test was carried out on the “Amsler” two-disk test rig. The set normal force was 50 N, resulting in a Hertzian pressure of 1318 MPa, taking into account the specimen geometry. The disks on the test stand run in opposite directions, with the lower disk (specimen) rotating 1.1 times as fast as the upper disk (counter specimen). The samples covered a total distance of 2500 m. Cold work steel X153CrMoV12 (1.51 Ma.-% C, 11.59 Ma.-% Cr, 0.916 Ma.-% Mo, 0.796 Ma.-% V, 0.370 Ma.-% Mn, 0.333 Ma.-% Si, 0.122 Ma.-% W, Fe bal.) was used for the counter specimens (outer diameter = 50 mm) in the quenched and tempered condition (hardness 699 HV10). The crowned surfaces had a radius of curvature of 5 mm. The gravimetric wear of the samples was measured during and after the tests. The optical measurement of the wear track was carried out after 2500 m of wear using confocal laser scanning microscopy (3D Keyence VK-1000). The depth, width and cross-sectional area were used to determine the wear coefficients in accordance with DIN EN 1071-13 [7]. Results and discussion Figure 1 shows the total gravimetric loss of the samples after 2500 m of wear. As expected, the quenched and tempered initial condition (QT) shows by far the highest wear and thus, in contrast to the nitrided initial condition (N), illustrates the effect of a surface layer treatment. In the nitrided sample with missing compound layer (N-CL), however, the wear increased again, as the compound layer has a decisive effect on the wear behavior of the material [2]. Figure 1: Total weight loss of all samples after a total wear distance of 2500 m quenched and tempered nitrided compound layer removed Initial state QT N N-CL Induction-hardened QT+IH N+IH N-CL+IH a-C: H(: Si)-coated QT+PVD N+PVD N-CL+PVD Induction-hardened + a-C: H(: Si)-coated QT+IH+PVD N+IH+PVD N-CL+IH+PVD Table 1: Examined treatment conditions and resulting sample designation News 36 Tribologie + Schmierungstechnik · volume 71 · issue 4/ 2024 In the duplex heat-treated samples that were inductionhardened, the sample quenched and tempered in the initial state (QT+IH) showed a similar weight loss to the sample nitrided in the initial state (N+IH). The nitrided sample whose compound layer was removed (N-CL+IH) is one of the two samples that show an increase in weight after the final 2500 m wear track. The coated duplex samples QT+PVD, N+PVD and N-CL+PVD all show similar wear behavior with very little weight loss compared to the initial state, indicating a positive effect of the coating. When looking at the samples that were both inductionhardened and coated after tempering or nitriding, the QT+IH+PVD and N+IH+PVD samples show a slightly higher weight loss than the duplex samples. The nitrided triplex sample without compound layer (N-CL+IH+PVD), on the other hand, shows an even clearer increase in weight than the uncoated N-CL+IH duplex sample. Table 2 shows the wear coefficients calculated on the basis of the wear track cross-sectional area. The highest wear coefficient by far was observed in the quenched and tempered initial condition QT, which correlates well with the gravimetric wear and is due to the low hardness of the quenched and tempered initial condition. For the samples in the nitrided initial state (N and N-CL), the sample without a compound layer also exhibited higher wear. The lowest optical wear is seen in the induction-hardened specimens, which is probably due to the high hardness of the non-tempered martensite in the area near the surface, which should ensure increased wear resistance. The hardness in the surface layer area of the tempered and induction-hardened specimen (QT+IH) more than doubled from 300 HV1 to 642 HV1 compared to the only tempered initial state (QT). The a-C: H(: Si)-coated duplex specimens also exhibited a low material loss compared to the respective quenched and tempered and nitrided initial states. The fact that the optical wear of the QT+PVD sample is significantly higher is probably due to the low support effect of the comparatively soft base material. In addition, the coating appears to have worn off completely after the entire wear path of 2500 m, as can be seen from the wear depth versus coating thickness. In the triplex treatments, a reduction in material wear is visible in the tempered initial state (QT+IH+PVD) compared to the tempered and coated sample (QT+PVD), which suggests a better supporting effect of the a-C: H(: Si) layer due to the induction-hardened surface layer. The triplex-treated nitrided initial state (N+IH+PVD) shows a significantly higher average wear compared to both duplex samples in the same initial state (N+IH and N+PVD). Summary The wear behavior of the quenched and tempered initial state of the material 42CrMo4 could be significantly improved through combined surface layer treatments and coatings. The best results were achieved by samples that were induction-hardened in the quenched and tempered or nitrided initial state. An a-C: H(: Si) coating was also able to increase the wear resistance compared to the quenched and tempered and nitrided initial states. The triplex treatment did not result in a further increase in wear resistance compared to the duplex-treated samples. References [1] Liedtke, D.: Merkblatt 236 „Wärmebehandlung von Stahl - Randschichthärten“, Wirtschaftsvereinigung Stahl, Ausgabe 2009, ISSN 0175-2006 [2] Hoffmann, F.; Bujak, I.; Mayr, P.; Löffelbein, B.; Gienau, M.; Habig, K.-H.: Verschleißwiderstand nitrierter und nitrocarburierter Stähle, HTM 52 (1997) 6, pp. 376-386 Sample QT N N-CL QT+IH N+IH N-CL+IH Width in µm 906.28 390.09 649.35 321.27 0 97.05 Depth in µm 19.70 3.08 6.04 0.56 0 0.22 Surface in µm 2 10560.80 710.81 2107.74 112.94 0 32.68 Wear coefficient in mm 3 / Nm 1.22*10 -5 8.22*10 -7 2.44*10 -6 1.31*10 -7 0 3.78*10 -8 Sample QT+PVD N+PVD N-CL+PVD QT+IH+PVD N+IH+PVD N-CL+IH+PVD Width in µm 481.11 414.24 315.89 170.25 427.40 152.23 Depth in µm 5.81 2.08 2.59 0.91 3.70 1.25 Surface in µm 2 1711.91 552.44 553.52 489.15 1756.11 651.72 Wear coefficient in mm 3 / Nm 1.98*10 -6 6.39*10 -7 6.40*10 -7 5.66*10 -7 2.03*10 -6 7.53*10 -7 Table 2: Width, depth and area of the wear tracks and the wear coefficients calculated from them News 37 Tribologie + Schmierungstechnik · volume 71 · issue 4/ 2024 [3] Hoja, S.; Haupt, N.; Steinbacher, M.; Fechte-Heinen, R.: Martensitisches Induktionshärten von Nitrierschichten. HTM J. Heat Treatm. Mat. 77 (2022) 6, pp. 393‒408 [4] Bergmann, H. W.; Müller, D.; Amon, M.; Domes, J.: Kombination des Laserstrahlhärtens mit einer Kurzzeitnitrierbehandlung. HTM - Härterei-Techn. Mitt. 48 (1993) 4, pp. 238 -248 [5] Keidel, C.: Zum Einfluß der Verfahrenskombination „Nitrocarburieren plus induktives Randschichthärten“ auf das Schwingfestigkeitsverhalten von Stahl. Dissertation, TU Berlin, 1995 [6] Robertson, J.: Classification of Diamond-Like Carbons, In: Donnet, C., Erdemir, A. (Hrsg.): Tribology of Diamond-Like Carbon Films, Springer-Verlag, 2008, S. 13- 24 [7] Hoja, S.; Baustert, R.; Hasselbruch, H.; Steinbacher, M.; Fechte-Heinen, R.: Investigation of combined surface treatments and coatings to increase the wear behavior of heat treatable steels. Surface and Coatings Technology 472 (2023) 129929 [8] Decho, H.: Entwicklung von wasserstoffhaltigen amorphen Kohlenstoffschichten (a-C: H) für mediengeschmierte Tribosysteme. Forschungsberichte aus dem Leibniz-Institut für Werkstofforientierte Technologien vol. 97, 2023, ISBN 9783844089202. Shaker Verlag, Düren.