24th International Colloquium Tribology - January 2024 91 Modification of Surface Properties on Various Mg-Based Alloys for Tribological Applications via Plasma Electrolytic Oxidation Process Ashutosh Tiwari 1* , Jörg Zerrer 1 , Anna Buling 1 1 ELB - Eloxalwerk Ludwigsburg Helmut Zerrer GmbH, Ludwigsburg, Germany * Corresponding author: buling@ceranod.de 1. Introduction Magnesium (Mg) and its alloys have become popular for material selection in different fields such as aerospace, automotive, biomedical, and robotics. Due to their low density and high weight-to-strength ratio, they enable better energy efficacy and, thus, reduction in CO 2 emission which supports overcoming recent ecological challenges [1]. However, poor wear resistance and rapid corrosion in challenging conditions greatly limit the application in the transportation and medical sectors [2]. To overcome such issues, a ceramic surface modification on various Mg-based alloys by the PEO (plasma electrolytic oxidation) process is developed, which ensures superior, robust, and durable CERANOD ® surfaces under harsh tribological and corrosive conditions. Moreover, the PEO process is an environmentally friendly technique in which alkaline electrolytes and no toxic byproducts or waste occur during the process as well as while the recycling of the Mg material. The current research exemplifies the quality and versatility of CERANOD ® surfaces created on differently manufactured Mg alloys such as 3D printing, casting, or extruding processes, irrespective of their dimensions and geometries. A comparative investigation of surface properties between differently manufactured Mg alloys treated with our PEO process was performed. Additionally, hybrid surfaces in the combination of PEO and a polymer (doped PEEK: poly-ether-ether-ketone) were fabricated on Mg-based alloys to further reduce the coefficient of friction of the surfaces without using any lubricants or oils. 2. Materials and Methods As substrate material various Mg-based alloys were used to modify their surfaces via the PEO process. All the treated samples have coupon geometry with 31 mm diameter and 6-mm height. Alloys, their manufacturing process, and surface treatment conditions are described in Table 1 below: Table 1: description of various Mg-based alloys Alloy Manufacturing process Surfaces treatment condition/ Alias WE43 Additive Manufacturing PEO/ (A) AZ31 wrought PEO/ (B) ZK60 wrought PEO/ (C) AZ31 wrought PEO + PEEK/ (D) AZ31 wrought PEO + doped-PEEK/ (E) All the substrates mentioned were refined with our CERAN- OD ® surfaces. Surfaces D & E were modified by the PEO process and further treated by in-house produced PEEK and doped-PEEK dispersion on the CERANOD ® surface by laser melting technique. The PEO process is optimized in alkaline-based electrolyte under high pulsed voltage which leads to plasma discharging on the Mg alloy surface and forms a dense ceramic-like layer on the substrate. A schematic setup of the PEO process is shown in Figure 1: Figure 1: illustration of PEO process on Mg substrate Surface roughness parameters R a (mean thematic roughness) and R p (maximum profile peak) after surface modification and wear tracks (or: wear volume) were analysed with the LSM (Laser Scanning Microscope; Keyence, VK-X100) of all the Mg-based alloys. The tribological performance of modified CERANOD ® surfaces (A, B, and C) was analysed using a pin-on-disc tribometer (TRB MZKO; Anton Paar) in linear reciprocal mode with a 6-mm WC (tungsten carbide) ball as a static partner. The testing parameters were set as: v max -=-4 cm/ s, F N (normal force) = 5 N, l (path length) = 10-mm, and total sliding distance = 70 m. The tribological tests of CERANOD ® surfaces D & E were performed with 100Cr6 ball with 6 mm diameter under test conditions: v max -=-0.64 and 29.7 cm/ s, F N- = 4 & 10 N, l = 3 and 40 mm, and total distance = 80 and 1000 m, respectively. The specific wear coefficient after tribological analysis was calculated using the equation 1: (1) where K n is the specific wear coefficient, F N is the applied normal force and d is the total distance. The tribological traces on the surfaces (refined and unrefined) were investigated using SEM (Scanning Electron Microscope; FlexSEM 1000, Hitachi) and EDS (electron dispersive spectroscopy; with Aztec 4.2 software). The wear depth and surface pro- 92 24th International Colloquium Tribology - January 2024 Modification of Surface Properties on Various Mg-Based Alloys for Tribological Applications via Plasma Electrolytic Oxidation Process file from BSE images of different angles after tribological analysis were constructed using Hitachi 3D Map software. Hardness measurements were performed using a universal hardness tester (HM2000; Helmut Fischer). The HV results shown are an average value of 10 random points across all the CERANOD ® surfaces. Results and Discussion Surface roughness parameters R a / R p of reference WE43 surface (polished) and surface A were recorded as 1.2 ± 0.06/ 5.33 ± 0.3 μm and 0.8 ± 0.06/ 5.2 ± 0.9 μm respectively. The developments of friction over testing time are shown in Figure 2. The unmodified additively manufactured WE43 reference surface shows high deviation and unstable μ (coefficient of friction, COF). Nevertheless, CERANOD ® surfaces exhibit slightly increased but stable μ after running-in. It was observed after tribological testing that the reference WE43 surface exhibits a high rate of wear under 5 N load, whereas the modified surface A exhibits no wear and damage as shown in Figure 3. Surfaces B and C show similar behavior with no sign of wear on the surface. However, a sudden increase in μ after 20 m in the case of surface C is due to the presence of asperities on the surface. These asperities break when they come into contact with the WC ball and redeposit on the surface. Hence, an increase in the contact area between the WC ball and the surface leads to an increased coefficient of friction. Figure 2: coefficient of friction ( μ ) of various modified CERANOD ® surfaces (A, B, C, and reference WE43 surface) with WC ball Figure 3: surface overview after tribological test with WC ball: a) reference WE43 b) modified CERANOD ® surface A on WE43 substate Furthermore, hybrid surfaces such as D and E were analyzed by pin-on-disc test with 100Cr6 ball under different testing conditions. The obtained results are shown in Figure 4. A clear difference in μ occurs between modified Mg PEO surfaces and hybrid surfaces D & E. Evidently, the COF further improved with doped PEEK due to minimized inhomogeneity on the surface. It can be inferred from Figure 4 that hybrid PEEK surfaces provide solid lubrication by exhibiting a low coefficient of friction. Figure 4: coefficient of friction of surfaces D & E during the tribological test with 100Cr6 ball: a) slow b) fast testing regimes [3] The measured hardness value of Mg-PEO at 50 mN was 518-±-90 HV 0.05 . Surfaces D & and E were measured with 5-mN force. They exhibit a value of 40 ± 3 HV 0.005 due to their polymeric properties [3]. Conclusion Preliminary outcomes show great improvements in the wear resistance of various Mg-based alloys after surface modification with CERANOD ® surfaces via the PEO process. Further, surface modification lowers the coefficient of friction with hybrid coatings which can act as a solid lubricant and open numerous potential applications. References [1] Dong, H. (2010). Surface Engineering of Light Alloys: Aluminium.-Magnesium and Titanium Alloys, 243-247. [2] Vignesh Kumar, M., Padmanaban, G., & Balasubramanian, V. (2018). Sliding wear characteristics of friction stir processed CAST ZK60 magnesium alloy under different applied loads.- Transactions of the Indian Institute of Metals,-71, 1223-1230. [3] Buling, A., & Zerrer, J. (2020). Lifting lightweight metals to a new level-tribological improvement by hybrid surface solutions on aluminium and magnesium.- Lubricants,-8(6), 65.