Science and Research 59 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0021 2 Materials and Methods In order to systematically investigate the friction and wear properties of all material-oil-combinations tribological tests were conducted using a pin-on-disc tribometer. Coated steel pins manufactured from tool steel 1.2379 were brought into contact with stainless steel sheets (1.4404) to simulate realistic conditions. All combinations were tested under identical conditions. The following section outlines the tribological test setup, the coatings applied, and the lubricants and evaluation parameters considered. 2.1 Tribological Testing The pin-on-disc tests were carried out on a BRUKER TriboLab universal tribometer. Evaluated parameters in- 1 Introduction Tool and workpiece materials are subject to extraordinary load in the field of sheet metal forming. Lubricants deliver friction reduction, wear protection and overall process stability. Conventional lubricants, however, are increasingly criticised due to their fossile origin and low decomposability. Bio-based lubricants originating, especially coming from residual material, are a sustainable alternative. Oil from Hermetia Illucens (the black soldier fly) is a byproduct of waste treatment ad can be transformed into technical ester. In addition, bio-based additives from various agricultural or plant byproducts can also be utilized. These components must prove equal lubrication capabilities compared to conventional lubricants in order to show eligibility for industrial production environments. Furthermore, there is a high interest into the interaction with plasma vapor deposited and diffusion treated surfaces. This work investigates and compares the applicability of lubricants based on oil of the black soldier fly Hermetia Illucens and plants and the effect of various added conventional and bio-based additives. Consideration is laid on the interaction of different surface modifications of tool steel 1.2379 with fine steel 1.4404. Comparing studies inspect the eligibility of Hermetia based oil as well as the interplay of various plant-based and synthetic additives with different tool surface modification. Aim of this work is to describe the influence of these surfaces on the specific additives. The different plasma vacuum deposited coatings and modifications differ in chemical bonding, surface energy and reactivity. These properties highly influence the additive effects and suitable adjustment may optimize the lubrication effect. Influence of tool coatings on the tribological effectiveness of biolubricants for sheet metal forming Hoang Viet Le, Kai Weigel* Presented at GfT Conference 2025 Environmentally friendly lubricants are essential for sustainable sheet metal forming. Their performance depends on interactions between additives and tool surfaces. This study evaluates bio-based TMP-esters, conventional and bio-based additives, and plasma vacuum deposited coatings. Lubricants and coatings were tested in a pin-on-disk setup at different temperatures in oscillating and rotating mode. Results indicate that bio-based lubricants from Hermetia illucens and plants are suitable for sheet metal forming. Biobased additives further show promising potential. The investigated coatings demonstrated excellent tribological properties, both independently and in combination with bio-based lubricants, supporting their use in sustainable forming processes. Keywords sheet metal forming, lubricant additives, tool coatings, friction and wear, bio-based lubricants, pin-ondisk test, additive-surface interaction, resource-efficient tribsystems Abstract * Hoang Viet Le, M.Sc. Dipl.-Ing. Kai Weigel Fraunhofer-Institute for Surface Engineering and Thin Films IST Riedenkamp 2, 38108 Braunschweig, Germany clude the coefficient of friction, friction track width on the sheet surface, stick-slip behavior, and wear on the pin surface. The influence of lubricants, additives, and tool coatings was analysed under conditions closely resembling industrial practice. The test setup and parameters were derived from preliminary experiments that showed good agreement with real forming processes (see Table 1). 2.2 Tool coatings and Counter body An austenitic stainless steel of type 1.4404 was used as the counter body. This material is widely applied in forming technology and commonly used in the automotive industry. It is characterised by high strength and good corrosion resistance. Typical tribological challenges, however, include a pronounced tendency for adhesion and strain hardening during forming, which can lead to increased tool wear. Science and Research 60 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0021 Parameters Unit Test values Normal force [N] 25 Temperature [°C] 30 / 110 Motion type Oscillating, rotating Stroke length [mm] 20 Stroke frequency [Hz] 1,25 Duration per load step [s] 600 Pin Ø 10 / tool steel 1.2379 58 HRC Sheet 50x40 x1 mm, Nirosta (1.4404) Table 1: Test parameters of the pin-on-disc tribometer tests performed Classification Lubricant Additives Description Reference KTL N16 Unknown Mineral oil-based fully formulated lubricant Bio-based base oils TMP-ester 1 None Based on Hermetia illucens TMP-ester 2 None Plant-based TMP-ester 3 None Bio-based base oils with 5% additive TMP-ester EP additive 1 EP additives with different sulfur contents TMP-ester EP additive 2 TMP-ester EP additive 2 TMP-ester Additive package 1 Commercial additive packages TMP-ester Additive package 2 TMP-ester Bio additive 1 Bio-based additives TMP-ester Bio additive 2 TMP-ester Bio additive 2 Bio-based base oils with 10 % additive TMP-ester EP additive 1 EP additives with different sulfur contents TMP-ester EP additive 2 TMP-ester Bio additive 1 Bio-based additive Table 3: Investigated lubricants, additives and description Type of coatings/ surface treatment Selected variants Dominant attachment type Diamond-like carbon (DLC) coatings a-C: H Covalent, weakly polar a-C: H: Si Covalent, weakly polar a-C: H: W Covalent, moderately polar Carbide and nitride coatings WC Covalent, moderately polar CrN Covalent, strongly polar Oxide coatings ZrO 2 Weakly ionic Diffusion treatment with nitrogen Fe X N Y Covalent, strongly polar Table 2: Investigated Coatings and Surface Treatments Science and Research 61 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0021 The test pins, representing the tool material, were made of tool steel 1.2379 (X153CrMoV12) with a hardness of 60-63 HRC. Prior to coating deposition, the pins underwent a plasma nitriding process followed by dry electropolishing in order to enhance coating adhesion and optimise tribological performance. A series of hard coatings with different chemical bonding characteristics was applied. Table 2 provides an overview of the coating systems and their predominant bonding types. 2.3 Lubricants and Additives A fully formulated mineral oil (Raziol KTL N16) was used as the reference lubricant. The exact composition of this oil is not disclosed by the manufacturer. KTL N16 is well established in sheet metal forming and therefore serves as an industry relevant lubricant. In addition, Hermetiaand plant-based TMP-esters of different origins were used as base oils for additive formulation. Additives were blended into the base oils at concentrations of 5 wt.% and 10 wt.% (see Table 3). The following types of additives were investigated: • Extreme pressure additives • Commercial additive packages • Bio-based additives derived from plant residues 3 Results To evaluate the tribological behavior, both friction and wear parameters were analysed. Friction was assessed based on the average coefficient of friction. The wear analysis included the friction track width on the sheet surface, the occurrence of stick-slip effects in the form of chatter marks, as well as the size and extent of the wear zone on the spherical pin surface representing the forming tool. The results are presented below. The comparison (oscillation tests) with the fully formulated reference lubricant KTL N16 shows that bio-based base oils achieve comparable friction levels at 30 °C. At higher temperatures, particularly at 110 °C, the coefficient of friction of the reference decreases due to additive activation, whereas the unadditivated bio-based base oils maintain higher friction levels (see Fig. 1). This results in an increasing deviation from the reference, highlighting the necessity of targeted additive formulation. Only the combination of a bio-based base oil with the EP additive 1 (TMP-EP) achieved friction values on the same level as the reference lubricant. A higher additive concentration did not yield further improvement. All other bio-based additives exhibited lower tribological performance across all temperature levels. The choice of tool coating also had a pronounced effect on the tribological behaviour (see Fig. 1, right). Carbon-based coatings, in particular a-C: H: Si, led to a significant reduction in friction regardless of the lubricant applied and thus confirmed the effectiveness of these coatings. Other coatings achieved only selective improvements in separate tests, depending on the additive type. In addition to the oscillating tests, initial rotating tribometer experiments were performed. In these tests, the bio additive 1 in the bio-based base oil achieved the best friction performance of all lubricants tested, including the reference. This indicates a high tribological potential of plant-based additives under rotating load conditions. Figure 2 shows the friction track widths on the sheet surface at 30 °C, 70 °C and 110 °C after completing all load stages with both uncoated and coated pins. The results for friction track widths largely follow the same trends as the friction values. Notably, differences Figure 1: Average coefficient of friction for various lubricant-surface systems tely avoided with a-C: H. This highlights the effective role of carbon-based coatings in preventing stick-slip effects. The size and extent of the wear zone on the spherical pin surface provide an indication of the expected tool wear. As a result of the groove-shaped friction track on the sheet surface, an oval contact zone develops on the pin surface, the width of which generally corresponds to the friction track width. The wear zone predominantly shows abrasive marks and is often superimposed by a linear zone with adhesion of sheet material. Under effective lubrication, both areas are only weakly pronounced. While the uncoated variant exhibits a clearly developed wear zone with visible material adhesions, no adhesions are observed with a-C: H. 4 Discussion The key findings regarding friction behaviour, wear mechanisms and the interaction between lubricants and tool coatings are summarised below: Science and Research 62 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0021 between the fully formulated reference lubricant and the bio-based base oils are already evident at 30 °C, whereas for the friction values these differences only became pronounced at elevated temperatures. Apart from this, the friction track widths confirm the same tendencies as the friction values: only the combination of a bio-based base oil with an EP additive reached levels comparable to the reference, while all other bio-based additives showed lower performance across all temperature stages. The positive effect of the carbon-based coating a-C: H: Si was also clearly confirmed by the reduced friction track widths. The supplementary rotating tests revealed a similar overall picture, although the differences were less pronounced compared to the friction values. Stick-slip effects occur when the lubricant provides insufficient separation between the contact partners and manifest in the form of chatter marks. These effects can be influenced by suitable coatings. Figure 3 illustrates exemplary friction tracks of one lubricant in combination with an uncoated pin and with an a-C: H coating. While pronounced chatter marks appear in all temperature stages with the uncoated variant, they are comple- Figure 2: Friction track widths on the sheet metal surface for different lubricant-surface systems Figure 3: Scratch marks and pen wear - uncoated vs. a-C: H • The addition of EP1 led to a significant reduction in the coefficient of friction and the friction track width. Without additive, the tribological parameters were clearly higher than those of the fully formulated reference lubricant (KTL N16). • Carbon-based coatings such as a-C: H and a-C: H: Si showed a significant reduction in friction and wear, independent of the lubricant applied. These layers apparently act as friction-reducing boundary films, even without specific chemical interaction with the additive. • The comparison with a-C: H: W and WC revealed that a higher tungsten content in the coating can increase the tendency for chatter mark formation and track wear, particularly at elevated temperatures. • Stick-slip effects occurred primarily on uncoated pins. Their occurrence was suppressed by suitable coatings, indicating more stable interfacial friction. • The wear zones on the pin surfaces demonstrated that the combination of carbon-based coatings and additives not only reduces friction but also leads to lower adhesion and abrasive attack. • The investigated bio-based additives did not show a consistent tribological effect in the initial tests. Further studies are required to assess their reactivity and protective performance under different conditions. 5. Conclusions The results demonstrate that bio-based lubricating greases derived from Hermetia illucens can provide a lubrication performance comparable to conventional lubricants. Bio-based lubricants also allow for highly effective additive formulation. Amorphous carbon coatings reduce friction and tool wear regardless of the lubricant type. Future investigations should include additional materials to evaluate the transferability to different forming scenarios. Furthermore, the targeted optimisation of additive combinations as well as tests under more applicationoriented conditions - such as elevated temperatures, continuous sliding motion, or forming trials - will be essential. The described work was accomplished in coorperation of the Fraunhofer Institutes IST, IVV, IWU, IME and UMSICHT. Reference [1] Murrenhoff, H.: Umweltverträgliche Tribosysteme - Die Vision einer umweltfreundlichen Werkzeugmaschine. Berlin: Springer, 2010. [2] Fachagentur Nachwachsende Rohstoffe; Peterek, G. (Mitarb.): Bioschmierstoff-Kongress: 12.-13. November 2014, Hagen. Gülzow-Prüzen: Fachagentur Nachwachsende Rohstoffe e.V., 2015 (Gülzower Fachgespräche, Band 50). [3] Wang, Y.-S.; Shelomi, M.: Review of Black Soldier Fly (Hermetia illucens) as Animal Feed and Human Food. Foods (Basel, Switzerland), 6 (2017) 10. [4] Holweger, W.: Wechselwirkung von Additiven mit Metalloberflächen. 2., überarb. u. erw. Auflage. Renningen: expert-Verlag, 2022 (Tribologie - Schmierung, Reibung, Verschleiß). [5] Maßmann, T. Ch.: Wirkmechanismen additivierter Schmierstoffe in der Kaltumformung. 1. Auflage. Aachen: Shaker, 2007 (Berichte aus der Produktionstechnik, 2007/ 18) [6] Le, H. V.: Einfluss von Werkzeugbeschichtungen auf die tribologische Wirksamkeit von Schmierstoffadditiven für die Blechumformung. Masterarbeit, Technische Universität Braunschweig, 2025. Science and Research 63 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0021