of colonization by bacteria and biofilm formation with reduced anti-infective susceptibility. In 0.5 % to 2 % of the primary endoprosthesis an infection will occur [2]. The risk of infection is influenced by patient, location, surgical and healthcare factors [3] and increases with implant volume. This explains the high infection rate of 20 % in tumor prostheses, which are used for the reconstruction of large bone defects [4, 5]. The risk increases up to 40 % for secondary revision surgeries [6]. An increasing incidence, morbidity and mortality, but also increasing socio-economic costs are predicted [1]. Although different concepts and strategies are known today, they are still in the early stages of basic research or preclinical testing. Clinical applications of antimicrobial implant coatings have been limited to a few market-launched prosthetic systems or coating technologies [7, 8]. To solve the above-mentioned problems, a novel antiinfective multilayer biopolymer implant coating with body-related substances has been developed. For me- Aus Wissenschaft und Forschung 14 Tribologie + Schmierungstechnik · 69. Jahrgang · 2/ 2022 DOI 10.24053/ TuS-2022-0009 1 Introduction Over 1.5 million endoprostheses are implanted in Europe every year [1]. These are constantly exposed to the risk Testing concept for the mechanicaltribological characterization of an antibacterial coating system for implants Julian Hasselmann, Jochen Kurzynski, Manfred Fobker, Georg Gosheger, Martin Schulze* Eingereicht: 21.10.2021 Nach Begutachtung angenommen: 14.3.2022 Dieser Beitrag wurde im Rahmen der 62. Tribologie-Fachtagung 2021 der Gesellschaft für Tribologie (GfT) eingereicht. Um das Problem von Implantat-assoziierten Infektionen zu lösen, wurde ein antibakterielles biokompatibles Mehrschichtsystem entwickelt. Für die Zulassung eines solchen Beschichtungssystems als Medizinprodukt ist die Charakterisierung der mechanischen und tribologischen Eigenschaften erforderlich. Da keine der zurzeit bestehenden Prüfnormen vollständig auf das vorliegende System anwendbar ist, wurde in dieser Arbeit ein Prüfkonzept entwickelt, um die Benetzbarkeit, Topografie, Morphologie, Schichtdicke, Härte, E-Modul, Haftfestigkeit und tribologischen Eigenschaften des Mehrschichtsystems zu untersuchen. Schlüsselwörter Implantatbeschichtung, Antibakterielle Beschichtung, Prüfkonzept, Mechanisch-tribologische Charakterisierung, Bio-Tribokorrosion, Scratch-Test To solve the issue of implant-associated infections, an antibacterial biocompatible multilayer coating was developed. For the approval of such a coating system as a medical device, the characterization of its mechanical and tribological properties is required. Since none of the existing test standards is fully applicable to the present coating system, a testing concept was developed to investigate the wettability, topography, morphology, thickness, hardness, Young’s modulus, adhesion strength and tribological characteristics of the multilayer coating. Keywords Implant coating, Antibacterial coating, Testing concept, Mechanical-tribological testing, Bio-tribocorrosion, Scratch-test Kurzfassung Abstract * Julian Hasselmann, M. Sc. 1,3 , corresponding author Orcid-ID: https: / / orcid.org/ 0000-0002-5334-5171 Dr.-Ing. Jochen Kurzynski 3 Dr. rer. nat. Manfred Fobker 2 Univ.-Prof. Dr. med. Georg Gosheger 1 Orcid-ID: https: / / orcid.org/ 0000-0003-1089-9972 Dr. rer. medic. Dr. med. Dipl.-Ing (FH) Martin Schulze 1 Orcid-ID: https: / / orcid.org/ 0000-0003-2899-1558 1 Clinic for General Orthopedics and Tumororthopedics University Hospital Münster Albert-Schweitzer-Campus 1, 48149 Münster 2 Central Laboratory, University Hospital Münster Albert-Schweitzer-Campus 1, 48149 Münster 3 Materials Engineering Laboratory Department of Mechanical Engineering University of Applied Sciences Münster Stegerwaldstraße 39, 48565 Steinfurt TuS_2_2022.qxp_TuS_2_2022 01.06.22 12: 44 Seite 14 titanium alloys [9]. The selection of these materials results from the requirements regarding corrosion resistance, biocompatibility, bio-adhesion and favorable mechanical properties [10]. Due to its extensive use as an implant material in orthopedics, the titanium alloy Ti6Al4V Grade 23 ELI was chosen as substrate material for the present coating system. The surface was prepared using glass bead blasting (d k = 150 −250 μm, Eisenwerk Würth GmbH, Bad Friedrichshall, Germany) in accordance with the data from literature [11, 12] with a pressure of p = 2.5 bar for 12 s in a distance of 65 mm. The substrate geometry was defined as disc with a diameter of 14 mm, a thickness of 1.5 mm and a centric hole with a diameter of 2 mm. This ensured that all tests could be performed with the same specimen geometry and increased the test efficiency. An illustration of the present coating system is shown in Figure 2. Both biopolymer coating materials are each dissolved in a solvent, mixed with active ingredients, and successively applied to the substrate by a dip coating process. Both coatings are loaded with active ingredients, which are released with their resorption of the human body. While the first biopolymer coating is designed for resorption over several years, the resorption of the second coating takes place within weeks to prevent the initial implant associated infection after transplantation. Both coatings show a definable linear elution kinetics of the active ingredients without a burst effect. Aus Wissenschaft und Forschung 15 Tribologie + Schmierungstechnik · 69. Jahrgang · 2/ 2022 DOI 10.24053/ TuS-2022-0009 Figure 2: Illustration of the antibacterial coating system for implants. Figure 1: Overview of the developed testing concept for biopolymer multilayer implant coatings. chanical characterization relevant standards have been identified with a perspective of a future approval as a medical product: ISO 10993: Biological Evaluation of Medical Devices, ISO 12417: Vascular Medicinal Product-Drug Combination Products and ISO 17327: Nonactive Surgical implants - Implant Coatings. At present, test standards already exist for metallic implant coatings, calcium phosphate coatings and hydroxyapatite coatings on the one hand and for drug eluting coatings of stents and balloon catheters on the other hand. However, none of these standards is fully applicable to the present antibacterial coating system. Especially, the complex mechanical evaluation requires the derivation of numerous methods and partial modification. Therefore, a comprehensive mechanical-tribological testing concept for the identified relevant coating parameters and occurring loads has been developed and conducted to fill this normative gap (Figure 1). On the one hand, this testing concept forms the baseline for the further development of the present coating system and, on the other hand, serves to accelerate the development of similar implant coatings, thus contributing to the improvement of patient care in the future. 2 Materials and Methods 2.1 Coating system The most commonly used metallic biomaterials in medical technology are Co-Cr-Mo alloys, pure titanium and TuS_2_2022.qxp_TuS_2_2022 01.06.22 12: 44 Seite 15 vely analyze the topography of the second coating because of its limited vacuum stability. 2.4 Morphology and thickness The analysis of both parameters was performed microscopically with an SEM and LM using a cross-section polish, according to the ISO 2808 standard. To be able to perceive changes in the layer thickness over the crosssection, 18 measurements were performed along each cross-section polish. The metallographic preparation was done according to Table 1. Additionally, the thickness was determined by an eddycurrent gauge (Fischerscope MMS PC2 ETA3.3, Helmut Fischer GmbH, Sindelfingen, Germany) with 18 measurements randomly distributed along the surface. 2.5 Hardness and Young’s modulus Due to the thin coating layer, hardness and Young’s modulus are investigated with the aid of the nanoindenter ZHN (ZwickRoell GmbH & Co. KG, Ulm, Germany) according to the DIN EN ISO 14577-4 standard [14] with a calibrated spherical diamond indenter (r = 37.7 µm). To test the first coating, indentations were made in a grid of 5 x 15 = 75 indentations with a xand y-spacing of 100 μm and test forces of 0.5 mN, 1 mN, 2 mN, 5 mN and 10 mN, respectively. For testing the second coating, 4 x 15 = 60 indentations with a xand y-spacing of 100 µm and test forces of 0.5 mN, 1 mN, 2 mN and 5 mN were conducted, respectively. The tests were performed in an array with varying test forces to investigate the coatings homogeneity and the substrate effect, respectively. For indentation, the force was increased until the desired testing force in a quadratic manner, hold constant for 5 s and released afterwards while measurands were recorded with a frequency of 16 Hz. Subsequently, the indentation hardness H IT was calculated using the following equation: (1) where F max is the maximum force measured and A p (h c ) is the projected contact area (cross-sectional area) between the indenter and the specimen. The indentation modulus E IT is given by: Aus Wissenschaft und Forschung 16 Tribologie + Schmierungstechnik · 69. Jahrgang · 2/ 2022 DOI 10.24053/ TuS-2022-0009 2.2 Wettability Static contact angle measurements were carried out to investigate the wettability of both the substrate and the coating(s) according to the ISO 19403-2 standard using the DSA100 contact angle measuring instrument with the Drop Shape Analysis software (KRÜSS GmbH, Hamburg, Germany). Two test liquids each were used at a temperature of 23 °C in accordance with the recommendations in the test standard: water as polar medium (4 μl/ drop) and diiodomethane as a nonpolar medium (2 μl/ drop). Immediately after the drop was applied to the sample surface with the dispensing needle, an image of the drop was created with the integrated camera and the contact angle θ was measured using the cone section method (θ = 20°-110°) or the Young-Laplace method (θ = 110°- 180°). For statistical validation of the results, each measurement was conducted in triplicate. Following, the surface free energy was calculated using the Owens-Wendt-Rabel-Kaelble method (OWRK method) [13]. 2.3 Topography The topography of the coating(s) was studied both quantitatively and qualitatively. For a quantitative analysis, contactless surface roughness measurements were conducted to avoid damaging the ‘soft’ coating surfaces using a confocal microscope (MarSurf CM mobile, Mahr GmbH, Göttingen, Germany) with a 20x lens, an evaluation length of 4 mm and a cutoff wavelength of λ c = 0.8 mm in accordance with the ASTM F2791-15 standard in triplicate. The postprocessing was done using the MountainsMap ® software (Digital Surf ® , Besançon, France). For a qualitative analysis regarding surface texture and coating coverage integrity, a scanning electron microscope (SEM) (Zeiss EVO MA10, Carl Zeiss Microscopy GmbH, Jena, Germany) in combination with the energy dispersive X-ray spectroscopy (EDS) (XFLASH ® 6|10 Detektor, Bruker Co., Billerica, USA) was used with an accelerating voltage of EV = 20 kV. To reduce charging of the non-conductive coatings, a thin layer of gold was previously applied on the sample surfaces using DC-sputtering (Polaron E5000, Polaron Equipment Ltd, Watford, UK) for t = 40 s with a current of I = 10 mA and a voltage of U = 1.17 kV. In addition, light microscopy (LM) (Axio Imager, Carl Zeiss Microscopy GmbH, Jena, Germany) was used to qualitati- Table 1: Metallographic preparation method. Grinding 1 Grinding 2 Polishing 1 Polishing 2 Discs / Cloths SIPLAS Re - K400 DIPLAS Re - K600 PT Super Plan PT Chem Abrasive Diamond Diamond Diamond-Susp. P Silica Susp. * Grain size D40 / P400 D60 / P600 9 μm 20 nm Lubricant Water Water Lubricant Green - Rpm 300 min -1 300 min -1 150 min -1 150 min -1 Force/ specimen 30 N 30 N 25 N 15 N Time Until plane 3 min 5 min 8 min TuS_2_2022.qxp_TuS_2_2022 01.06.22 12: 44 Seite 16 (2) with ν s and ν i as Poisson’s ratio of the coating and indenter and E r and E i as reduced modulus according to Oliver and Pharr [15] and Young’s modulus of the indenter, respectively. For the used diamond indenter, the resulting values are ν i = 0.07 and E i = 1140 GPa. The Poisson’s ratio of ν s = 0.4 was assumed for the coatings. 2.6 Adhesion strength One of the most important mechanical properties of a coating is its practical adhesion (adhesion strength) [9, 14, 16-19]. Depending on the material combination, the geometry and the respective industry, more than 200 methods exist for quantitative and qualitative analysis of the adhesion strength [19]. Since the value of the adhesive strength is a decisive criterion for the health of the patients [20] and thus, the approval test of the coating as a medical device, quantitative investigations are necessary. The quantitative analysis depends on coating material, thickness, hardness and substrate material [21]. Furthermore, test methods should be selected that cover different load cases. Therefore, the pull-off tensile test (tensile load) and the scratch-test (shear load) were selected. Pull-off tensile tests were performed using a tensile testing machine with the testXpert software (RKM 100/ 20, ZwickRoell GmbH & Co. KG, Ulm, Germany) according to DIN EN ISO 4624 standard with a custom-built joint mechanism ensuring the application of a tensile stress purely normal to the substrate-coating interface (n = 6). First, the coated substrate is glued on the test rig using the 2-components structural adhesive SikaPowerr- 1277 (Sika Deutschland GmbH, Bad Urach, Germany) and cured for seven days at room temperature. The incorporated glass beads guarantee a constant adhesive layer thickness of 0.3 mm. Furthermore, a PTFE pin and PTFE tape was used to center the specimen and prevent it from sticking to the test rig, respectively. For testing, the assembly was mounted in the testing machine and a uniaxial force was applied with a preload of 1 N and a loading rate of 20 N/ s until failure of the coating. Adhesion strength was calculated using the maximum force measured by the testing machine divided by the adhesive area of the specimen. To determine the mode of failure, the specimens were subsequently analyzed using the digital microscope VHX-600 (KEYENCE DEUTSCH- LAND GmbH, Neu-Isenburg, Germany) in combination with the software ImageJ (National Institute of Health, Bethesda, USA) [22]. The scratch tests with linearly increasing normal load (n = 5) according to the ASTM C162-05 standard were performed by means of nanoindenter with the setup des- cribed in section 2.5 which is perfectly designed for testing thin, multilayer coatings [23, 24]. Prior to the actual scratch tests, the scratch path was pre-scanned with a normal load of 1 mN and the depth was measurement. The resulting values were automatically subtracted from the depths measured during the scratch test to eliminate any misalignment of the specimens. The specimens with the second coating were tested with a normal load F n increasing from 1 mN to 100 mN and a scratch length of 500 µm. The specimens with the first coating were tested with a normal load F n increasing from 1 mN to 1500 mN and a scratch length of 3750 µm, respectively. Since scratch test results are only comparable when constant conditions were applied, the horizontal displacement rate was held constant at 150 µm/ s. By means of discontinuities of the coefficient of friction (COF) (tangential force divided by normal force), the critical forces, i.e. the normal force that leads to the loss of adhesion between the respective layers in multilayer systems or between coating and substrate, can be determined. To precisely determine the value of the critical force, an EDSsupported SEM analysis was carried out in addition to the analysis of the measured value recording. In principle, the scratch test would be suitable for a quantitative measurement of adhesion strength and is also increasingly used. However, the process is of such a complex nature that there is still no generally accepted theory that covers and considers all the variables influencing an absolute measurement. Therefore, the critical forces were evaluated as a measure of the adhesion strength of the coating system. 2.7 Wear characteristics As a specific kind of tribocorrosion, bio-tribocorrosion includes the science of surface transformation due to mechanical stress and electrochemical, biochemical and microbiological reactions occurring between the elements of a tribological system exposed to biological environments [25]. Due to the large number of parameters influencing the tribological system, only an approximation to the in vivo conditions is possible. Two main approaches are distinguished: a mechanism-oriented approach or a good representation of the actual existing system [26]. The latter was chosen to be able to measure the total wear directly in the tribo-test as realistic as possible. The expected type of wear corresponds to soft tissue sliding wear. For the tribological system analysis according to GfT-Arbeitsblatt 7 [27], it was considered that the coating is initially applied to the neck of a hip prosthesis and is thus in direct contact with the surrounding muscle tissue (Figure 3). The analysis of this complex bio-tribological system resulted in the system parameters listed in Table 2. This analysis of the bio-tribological system resulted in the development of the test setup corresponding, regarding load introduction and friction path, to a pin-on-disc test (ASTM G99-05) as shown in Figure 4. Aus Wissenschaft und Forschung 17 Tribologie + Schmierungstechnik · 69. Jahrgang · 2/ 2022 DOI 10.24053/ TuS-2022-0009 TuS_2_2022.qxp_TuS_2_2022 01.06.22 12: 44 Seite 17 the normal force and sliding velocity represent the variable parameter at a constant friction path. Furthermore, the tangential force is measured to calculate the COF. The wear volume is calculated from the difference in weight of the samples (measured by a high-precision scale), considering the density of the coating, and plotted as a function of the normal force. Furthermore, wear surfaces are subjected to an SEM and confocal microscope analysis. In addition, it is possible to add abrasive Aus Wissenschaft und Forschung 18 Tribologie + Schmierungstechnik · 69. Jahrgang · 2/ 2022 DOI 10.24053/ TuS-2022-0009 The counter body is made of porcine tissue. During test, the coated substrates are pressed onto the counter body with variable normal force, creating a conformal contact. The samples perform a rotational movement at a defined speed. To get as close as possible to the real bio-tribological system, the simulated body fluid (SBF, Dulbeccos Modified Eagle Medium, ThermoFisher Scientific Co., Waltham, USA) (pH = 6.8 - 7.2) is heated to 37 °C in the incubator and continuously refreshed. In the experiment, Figure 3: Schematic of the bio-tribological system. Figure 4: CAD-illustration of the developed bio-tribocorrosion test setup. Table 2: Parameters of the bio-tribological system. Motion: Oscillating translational sliding Load: Surface pressure due to muscle tension Body: Coating(s) Counterbody: Muscle facia Interfacial & ambient medium: Mixture of lymph, exudate and serum (37°C) Contact: Conform Friction condition: Boundary friction to mixed friction Wear mechanism: Adhesion, abrasion and tribochemical reactions TuS_2_2022.qxp_TuS_2_2022 01.06.22 12: 44 Seite 18 titanium or osseous particles produced by micromotion at the femoral stem [28]. Since the corresponding tests are still in the conduction phase, the publication of their results will take place at a future date. 3 Results and discussion 3.1 Wettability The wettability of both the substrate and the coating(s) represents a key parameter in developing an implant coating. The results of the contact angle measurements including the calculation of the surface free energy (SFE) are shown in Figure 5. For the titanium surface, this results in a water contact angle of 51° and a surface free energy of 55.7 mN/ m. The coatings show higher water contact angles and therefore lower surface free energies of 91.9° and 116° and 34.7 mN/ m and 18.1 mN/ m for the first and the second coating, respectively. Therefore, the titanium is weakly hydrophilic and the coatings show a hydrophobic behavior according to the definition of Drelich et al. [29]. However, the higher wettability of the titanium surface is favorable regarding the coating process. This is in contrast to the influence of SFE on bacterial adhesion [30]. Jansen, Peters, and Pulverer [31] found a significant correlation between high hydrophobicity of a material and lower bacterial adhesion to its surface. Therefore, the coatings already show a reduction in bacterial adhesion due to their wettability compared to the uncoated titanium surface. This effect is also expected following the Baier curve [32]. 3.2 Topography The surface roughness of the components of the coating system determined by confocal microscope is shown in Table 3. Both coatings have lower roughness values than the titanium surface. A higher roughness value is associated with an enlargement of the implant surface and thus with a larger surface area for bacterial colonization. Therefore, the coatings show preferable properties regarding the surface roughness. In addition, the higher roughness of the titanium substrate is favorable regarding higher coating adhesion due to mechanical interlocking. To avoid incorrect measurements of the semitransparent coatings, the samples were sputter-coated with a thin layer of gold in the following measurements. The qualitative analysis regarding surface texture and coating coverage integrity for both coatings is shown in Figure 6 and Figure 7, respectively. Despite two pores in the second coating, both coatings show a smooth surface without any defects and a homogeneous coverage of the surface, resulting in a complete protection of the titanium surface and a homogeneous elution of the incorporated active ingredients along the implant surface. Aus Wissenschaft und Forschung 19 Tribologie + Schmierungstechnik · 69. Jahrgang · 2/ 2022 DOI 10.24053/ TuS-2022-0009 Figure 5: Contact angle measurements (left) and the resulting surface free energy (SFE) (right). Table 3: Roughness values R a (line) and S a (area) of the coating system with λ c = 0.8 mm. Material Ra [µm] Sa [µm] Ti6Al4V, glass bead blasted 0.67 0.92 First coating 0.33 0.59 Second coating 0.47 1.33 TuS_2_2022.qxp_TuS_2_2022 01.06.22 12: 44 Seite 19 Figure 9). Both coatings and the bilayer coating are fully connected to the titanium surface and the first coating, respectively. Furthermore, no pores are detectable, which again indicates a homogeneous coating. The cross-section polishes were also used to measure Aus Wissenschaft und Forschung 20 Tribologie + Schmierungstechnik · 69. Jahrgang · 2/ 2022 DOI 10.24053/ TuS-2022-0009 3.3 Morphology and Thickness To investigate the coating’s morphology, cross-section polishes were analyzed via LM and SEM (Figure 8 and Figure 6: SEM image of the first coating surface. The images (a)-(c) show a homogeneously coated area at (a) 500x, (b) 2000x and (c) 5000x magnification. The red arrow indicates a residual from preparation, which is not part of the coating. Figure 7: LM bright field images of the second coating surface at magnifications of (a) 100x, (b) 200x and (c) 500x. Red circle indicates two pores. Figure 8: LM bright field cross-section images at 500x magnification of (a) first coating, (b) second coating and (c) bilayer coating. Figure 9: SEM image of the cross-section of the first coating released from the titanium surface. the coating thickness. These results including the eddycurrent gauge measurements are shown in Figure 10. The comparison of the two measurement principles (left) shows a large deviation with large error bars, which already indicates a non-homogeneous thickness distribution along the surface. Furthermore, the thickness distribution along the cross-section polish (right) proofs that observation by only measuring one plane. This leads to the demand of the standardization of the so far manually conducted dip coating process. TuS_2_2022.qxp_TuS_2_2022 01.06.22 12: 44 Seite 20 3.4 Hardness and Young’s modulus As elementary coating parameters, hardness and Young’s modulus were determined via nanoindentation (Figure 11 and Figure 12). Nanoindentation results of the first coating show a hardness of 27 - 45 MPa and a Young’s modulus of 2.04 - 2.8 GPa, respectively. As expected, the second coating shows a lower hardness and Aus Wissenschaft und Forschung 21 Tribologie + Schmierungstechnik · 69. Jahrgang · 2/ 2022 DOI 10.24053/ TuS-2022-0009 Figure 10: Average coating thickness measured by cross-section polish and eddy-current gauge (left) and thickness distribution along the cross-section (right). Figure 11: Hardness and Young’s modulus of the first coating determined by nanoindentation. Figure 12: Hardness and Young’s modulus of the second coating determined by nanoindentation. TuS_2_2022.qxp_TuS_2_2022 01.06.22 12: 44 Seite 21 3.5 Adhesion strength For the first coating, an adhesion strength of σ f = 10 ± 1.74 MPa was measured via pull-off tensile test. Delamination occurred with a proportion of 90 % at the interface of the coating and substrate and 10 % at the interface between coating and adhesive. However, to meet the requirements from ASTM F1147 standard of σ f = 22 MPa, an optimization of the substrate surface is necessary which is part of the ongoing development. Unfortunately, no results were determined testing the second coating and the bilayer coating since the loss of adhesion already took place while the adhesive was Aus Wissenschaft und Forschung 22 Tribologie + Schmierungstechnik · 69. Jahrgang · 2/ 2022 DOI 10.24053/ TuS-2022-0009 Young’s modulus of 10 - 15 MPa and 1.04 - 1.14 GPa, respectively. The values of the first coating are in good accordance to the values in literature [33]. The variance of the single measurements could be an indication of a rather inhomogeneous coating. From the topographical and morphological analyses, which revealed a homogeneous coating, this is unlikely. The LM images of the indenter before and after the nanoindentations, however, revealed a reason for this variance: the coating material adhered to the indenter, especially for the second coating. Therefore, additional measurements shall be conducted including a cleaning of the indenter tip after each measurement. Figure 13: Measurement data from scratch-test and scratch path (SEM) of the first coating. Figure 14: SEM image to precisely determine the critical load of the specimen from figure 13 (top) with an EDS-analysis for the element titanium (bottom). curing because of its shrinkage. Therefore, special attention needs to be paid to the ability of free movement while curing in following experiments. The loss of adhesion induced by shear loading was investigated using the scratch test with linearly increasing normal force. Figure 13 and Figure 14 show representatives of the determination of the critical force for the first coating. Initially, the critical load is determined by means of discontinuities of the COF in the plot. To precisely determine point where first delamination occurred, an EDS-supported SEM analysis was conducted as shown in the figure below for the element titanium. This allowed the precise determination of the critical load. The critical loads of the coating systems second coating on Ti6Al4V, second coating on first coating and first coating on Ti6Al4V were determined with 58.0 ± 3.8 mN, TuS_2_2022.qxp_TuS_2_2022 01.06.22 12: 44 Seite 22 15.4 ± 4.7 mN and 458.6 ± 69.3 mN, respectively. The mode of failure of the second coating applied on the Ti6Al4V substrate first showed ploughing and pilling-up at the edges of the groove with a COF of 0.24. The second coating applied on the first coating showed the same mode of failure but a COF of 0.19 while ploughing. In contrast, the first coating on the Ti6Al4V substrate showed a different mode of failure. At first, conformal cracks arise (cohesive failure) and increase until the loss of adhesion occurs. The COF while plastic deformation was determined with 0.20. In general, scratch tests showed a good reproducibility. Furthermore, the combined analysis of measurements during scratch test, confocal microscope and SEM / EDS proved to be a suitable set of information to precisely determine the critical load and the mode of failure. However, since the complex stress states at the indenter tip cannot be captured analytically so far, the scratch-test acts as a semi-quantitative testing method for the adhesion strength. 4 Conclusion In the present study, a comprehensive testing concept for the mechanical-tribological characterization of an antibacterial coating system was developed and evaluated, since none of the existing test standards could holistically applicated to such an antibacterial implant coating system. It can be concluded that: • The developed testing concept is suitable for evaluating the necessary coating parameters of the present coating system and similar coatings for implants. Pull-off tensile testing has to be adjusted for the multilayer coating. • The improved surface properties due to the coating system indicate an increased antibacterial effect compared to the standard titanium surface. Microbiological investigations are required to confirm this effect. • To fulfill the requirements regarding the adhesion strength according to ASTM F1147 standard of 22 MPa, a modification of the substrate surface is necessary. This is a current aspect of the ongoing development. With the present testing concept, the development of the new antibacterial coating system will be continued. The concept will be enhanced and supplemented for dynamic adhesive strength and mechanical properties testing after aging. Therefore, we conclude that the concept can also be applied to developments of similar implant coatings with an accelerating effect. This can contribute to an improvement of patient care in the future. Acknowledgements This work was financially supported by IMF (Innovative Medizinische Forschung) with the reference number: I-SC122008. References [1] Franceschini M, Sandiford NA, Cerbone V et al. (2020) Defensive antibacterial coating in revision total hip arthroplasty: new concept and early experience. Hip Int 30: 7-11. https: / / doi.org/ 10.1177/ 1120700020917125 [2] Gbejuade HO, Lovering AM, Webb JC (2015) The role of microbial biofilms in prosthetic joint infections. Acta Orthop 86: 147-158. https: / / doi.org/ 10.3109/ 17453674.2014.966290 [3] Lenguerrand E, Whitehouse MR, Beswick AD et al. (2018) Risk factors associated with revision for prosthetic joint infection after hip replacement: a prospective observational cohort study. 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