News 47 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 GfT award Disseration 2025 1 Introduction Despite the optimal adaptation of the natural biotribological system knee joint to its physiological environment, progressive joint wear may necessitate the implantation of an artificial knee joint. On the one hand, such endoprosthetic procedures are increasingly common due to the aging population worldwide. On the other hand, younger, active individuals also require endoprosthetic treatment at an early stage due to above-average intense sporting activity. Especially in younger, active patients, the service life of an artificial knee joint is insufficient when subjected to increased stress. Accordingly, implant-associated wear, together with the particles worn off, can be made responsible for premature prosthesis failure due to aseptic prosthesis loosening as a result of osteolysis. Reducing wear can be achieved, for example, by using low-wear material combinations or surface coatings. The application of biotribologically effective amorphous carbon (diamond-like carbon, DLC) to both articulating surfaces of a metallic-polymeric total knee replacement (TKR) is a promising approach to enhancing wear resistance. [I 1 ] Despite the significant progress that has been made with regard to endoprostheses and implant materials, challenges remain with respect to the interaction between articulating surfaces and the interaction of released particles and wear products with the human body, thus requiring further development. In the following section 2, the need for action, the objectives within the research questions, and the corresponding approach for successfully clarifying the research questions are shown. [I] Amorphous carbon coatings for extending the service life of total knee replacements Benedict Rothammer* The topic was submitted for the GfT Sponsorship Award 2025 in the category “dissertation or similar theses”. The award took place at the GfT conference in September 2025. The durability of a total knee endoprosthesis - especially in younger, active patients - is due to increased higher stresses insufficient. Accordingly, implantassociated wear particles lead to aseptic endoprosthesis loosening as a result of osteolysis and thus to premature failure of the endoprosthesis. Amorphous carbon coatings can meet the high requirements for implants. They combine tribologically effective behavior with high biocompatibility and excellent mechanical properties. The focus was on the application and the mechanical-biotribological preclinical in vitro investigation of amorphous carbon coatings on metallic/ polymeric articulation partners of endoprostheses. Using rheological investigations and numerical simulations, an application-related, biotribological test chain consisting of a ball-on-three-pin tribometer, pin-on-disk tribometer and fully kinematic knee simulator was established. This test chain allowed a comprehensive wear analysis of numerous contact combinations in a reasonable time with respect to dominant wear mechanisms, typical particle sizes and morphologies, as well as their correlation across the test links. In all test links, the biotribological effectiveness of the amorphous carbon coatings was shown, indicating a significant increase in the service life of endoprostheses. Keywords Service Life, TKRs (Total Knee Replacements), Amorphous Carbon Coatings, Coating Hardness, Biotribological Performance, Knee Simulation, Wear Protection Abstract * Dr. Benedict Rothammer Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) Lehrstuhl für Konstruktionstechnik KTmfk Martensstraße 9 | 91058 Erlangen 1 Since this contribution is a presentation of the author’s dissertation for the GfT Award in Category 1, this contribution refers to the original literature, which is clearly cited in the dissertation at the relevant points. News 48 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 2. Need for action, objectives, methodology There are no sufficiently detailed descriptions of coating processes or follow-up medical studies in literature. Nevertheless, from the literature already published, it is clear that amorphous carbon coatings have been used successfully in preclinical studies for load-bearing implants. This is mainly due to the excellent physical, chemical, and structural properties of amorphous carbon coatings, which enable biocompatibility and wear protection when sufficient adhesion to the substrate is ensured. [I] Currently, femoral components made of cobalt-chromium-molybdenum alloys such as Co28Cr6Mo (CoCr) are the standard, although titanium alloys such as Ti6Al4V (Ti64) in combination with tribologically effective amorphous carbon coatings may replace CoCr as a substrate material in the intermediate-term. However, understanding the biotribological system better than in the past is crucial to the reliable use of these coatings in systems subjected to high mechanical and biotribological stress, such as artificial knee joints in particular. Based on this, the coating adhesion needs to be improved to avoid the cell biological consequences of coating delamination. It’s also vital to do a comprehensive investigation of the mechanical properties, cell biological behavior, and biotribological behavior of the amorphous carbon coatings on metallic and polymeric specimens and on components of TKRs. In my research, I intend to transfer the progress made in recent years at the model level to more application-relevant test conditions and subsequently to real components of TKRs. [I] The long-term goal is to significantly extend the in vivo service life of knee implants made of Ti64 and ultrahigh molecular weight polyethylene (UHMWPE) compared to the state of the art by using tribologically effective, biocompatible amorphous carbon coating systems, thus creating an advantageous alternative to existing CoCr 2 implants. Furthermore, a coating on the polymeric tibial inlay should minimize wear-related, aseptic TKR loosening while providing high biocompatibility. In particular, the main weakness of existing carbon coatings, i.e., insufficient coating adhesion for load-bearing implants, should be remedied. For this reason, amorphous carbon coatings are applied to metallic and polymeric implant surfaces - i.e., CoCr, Ti64, and UHMWPE - using plasma-enhanced chemical vapor deposition (PECVD) and physical vapor deposition (PVD) technologies. Requirements regarding biocompatibility, mechanical properties, good coating adhesion, and desired tribological behavior must be met. Additionally, the key factors influencing biocompatibility, coating adhesion, and tribological behavior need to be identified and their interactions understood. [I] With regard to a framework for future coating development processes for implants, a systematic investigation should ultimately be conducted to determine the extent to which, under what stresses, and under what environmental conditions during preclinical in vitro testing, simpler model tests and more complex component tests can provide reliable information on wear behavior. Consequently, the following key research question: [I] Do polymeric and metallic contacts coated with amorphous carbon in TKRs significantly improve biotribological effectiveness compared to conventional (uncoated) metallic-polymeric contacts? [I] and derived sub-research questions regarding the following topics had to be answered: 1. Substitute medium: Which rheological properties are required for a suitable artificial synovial fluid that is as similar as possible to natural synovial fluid in order to shorten application-oriented or person-specific, biotribological investigations of implant materials at various levels of abstraction, such as the model level, under peri-endoprosthetic boundary conditions? [I] 2. Loading and stresses: Can application-oriented or person-specific stresses be predicted with the aid of a musculoskeletal biomechanic simulation and an elastohydrodynamic simulation and can these be transferred to biotribological investigations of implant materials at different levels of abstraction, such as the model level, under peri-endoprosthetic boundary conditions? [I] 3. Coating development: Can amorphous carbon coatings be designed in such a way that they meet the requirements for biocompatibility, mechanical properties, coating-substrate adhesion and thus tribological behavior and can be applied to polymeric and metallic implant materials by means of plasma coating? [I] 4. Biotriobology: For which contact partners can an improvement in the biotribological effectiveness of amorphous carbon coatings compared to conventional implant materials be already demonstrated in model tests and can this be transferred to more complex knee wear simulator tests? [I] The approach in the scope of this work was characterized by the close interlocking between coating development of biocompatible, low-defect, mechanically excellent and adherent amorphous carbon coatings and biotribological preclinical in vitro testing. Prior to establishing a 2 Ti64 is considerably preferable to CoCr in terms of high biocompatibility, but the unfavorable tribological behavior of Ti64 excludes its use as a direct sliding partner. News 49 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 tribological test chain, the periendoprosthetic environmental conditions as well as the in vivo contact conditions and stresses had to be adequately represented. Therefore, in this work, the artificial synovial fluid had to be primarily investigated for its rheological suitability and proper load collectives had to be derived by means of a musculoskeletal simulation coupled with an elastohydrodynamic simulation for an application-like biotriobological testing. The tribological testing followed a stringent chain of tribological testing procedures, combining a ball-on-three-pins tribometer, a pin-on-disk tribometer and a fully kinematic knee simulator. This allowed for a comprehensive wear analysis of numerous contact pairings in a reasonable time with respect to prevailing wear mechanisms, typical particle sizes and morphologies as well as their correlation across the links of the test chain. Thus, the biotribological interactions could be described integrally. The approach pursued in this PhD thesis to answer the research questions and thus to increase the service life of TKRs by coating them with amorphous carbon is illustrated in Figure 1. [I] Initially, amorphous carbon coatings were applied to metallic and polymeric implant surfaces by PVD and PECVD processes. The biotribologically effective amorphous carbon coatings were developed to exhibit good mechanical properties, in particular sufficiently good adhesion, as a fundamental requirement for further examinations with regard to their biocompatibility and tribological behavior. If, additionally, the coating proved suffi- Figure 1: Procedure for extending the service life of TKRs by coating with amorphous carbon and classification of the corresponding publications. [I] © B. Rothammer News 50 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 ciently good biocompatibility and tribologically favorable behavior in ball-on-three-pins tests, it could be used for further tribological tests in the pin-on-disk tribometer and also in the knee simulator. The comparison with the uncoated references allowed for a classification of the mechanical properties, the biocompatibility and the biotribological effectiveness of the coating. The ability to test a large number of coatings with regard to their biotribological behavior in the first two links of the test chain also enables the extent to which and the factors influencing this behavior to be determined. The sequence of different stages - from coating to mechanical characterization to biotribological testing - initially shown linearly in Figure 1 allows for iterations at the individual “gates” (blue boxes). For instance, testing the biocompatibility or adhesion of a first series of coatings can provide insights that result in the adjustment of coating parameters to produce a beneficial second series of coatings. [I] 3 Summary of results and discussion Amorphous carbon coatings can make a significant contribution to increasing the service life of TKRs. These coatings meet the requirements for implant materials and surfaces as well as for their cell biological environment. Meaning, they combine tribologically effective behavior with high biocompatibility and excellent mechanical properties. Due to the close link between the mechanical properties, cell biological and tribological behavior, and the characteristic chemical-structural properties, these properties of amorphous carbon coatings can be influenced by selectively adjusting the coating process parameters. Therefore, the overarching, longterm goal was to significantly extend the in vivo service life of Ti64 and UHMWPE knee implants compared to the state of the art by using tribologically effective, biocompatible amorphous carbon coating systems. This should create an attractive alternative to existing CoCr implants. In particular, research groups dealing with issues relating to the specific influencing of friction and wear reduction of amorphous carbon coatings for TKRs should be provided with an understanding of the biotribological mechanisms involved. An efficient framework for the design of coated metallic femoral shields and polymeric tibial inlays was made available for this purpose. The focus was on the application and mechanical-biotribological investigation of amorphous carbon coatings on endoprosthetic materials and TKRs. [I] The approach within the scope of my work was characterized by the close interlinking of coating development of biocompatible, defect-free, mechanically excellent, and adherent amorphous carbon coatings and biotribological preclinical in vitro testing. Prior to establishing a tribological testing chain, it was necessary to adequately simulate the periendoprosthetic environmental conditions as well as the in vivo contact conditions and stresses. [I] Therefore, in this work, the artificial synovial fluid had to be primarily investigated for its rheological suitability and proper load collectives had to be derived by means of a musculoskeletal simulation coupled with an elastohydrodynamic simulation for an application-like biotribological testing. The tribological testing followed a stringent chain of tribological testing procedures, combining a ball-on-three-pins tribometer, a pin-on-disk tribometer and a fully kinematic knee simulator. This allowed for a comprehensive wear analysis of numerous contact pairings in a reasonable time with respect to prevailing wear mechanisms, typical particle sizes and morphologies as well as their correlation across the links of the test chain. Thus, the biotribological interactions could be described integrally. [I] Initially, amorphous carbon coatings were applied to metallic and polymeric implant surfaces by PVD and PECVD processes. The biotribologically effective amorphous carbon coatings were developed to exhibit good mechanical properties, in particular sufficiently good adhesion, as a fundamental requirement for further examinations with regard to their biocompatibility and tribological behavior. If, additionally, the coating proved sufficiently good biocompatibility and tribologically favorable behavior in ball-on-three-pins tests, it could be used for further tribological tests in the pin-on-disk tribometer and also in the knee simulator. The comparison with the uncoated references allowed for a classification of the mechanical properties, the biocompatibility and the biotribological effectiveness of the coating. [I] Derived from the first sub-research question concerning a suitable substitute medium for tribological testing, the influence of temperature, shear rate and pressure on the density and viscosity of an artificial synovial fluid was investigated in Pub I by means of rheological measurements. Thereby, a temperature dependence of both the density and the viscosity could be observed with both values decreasing at higher temperatures. The temperature dependence of the viscosity within the range from room to human core temperature could be adequately approximated by an ARRHENIUS model. Furthermore, shear-thinning properties could be demonstrated, allowing for the synovial fluid to be simulated and fitted well to a CROSS model, which has also been described in studies on human synovial fluid in the literature. However, the non-NEWTONian behavior was more pronounced at lower temperatures and less pronounced at higher temperatures, the latter of which being more relevant to the human body. An anomaly in the pressure dependence of the viscosity was also present, which correlates with the behavior of pure water as the main constituent. At lower temperatures, viscosity first decreased to a minimum and then increased again at higher pressures. Although further research is needed on the influences of the individual components of the artificial synovial fluid, it can be concluded that the examined synovial fluid can be used for wear tests of total joint replace- News 51 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 ments and for comparing different implant materials, since the most important properties of the human synovial fluid are adequately mimicked. The rheological data as well as the adjustments to the model provide useful formulas for numerical modeling of similar fluids or for studies in different environments. Since most rheological tests on synovial fluid behavior and many tribological experiments on the performance of total joint replacements are performed under ambient conditions, Pub I emphasized the importance of realistic test conditions in terms of temperatures, pressures and shear rates to ensure or even shorten transferability and comparability. [I] Fundamental knowledge about the in vivo contact conditions at the articulating surfaces of TKRs for predicting and optimizing the behavior of implant systems could be obtained in Pub II, answering the second subresearch question. The contact stresses prevailing in TKRs cannot be accurately determined by conventional in vivo measurement methods. In turn, in silico modeling allowed the prediction of loads, velocities, deformations, stresses, and lubrication conditions across the scales during an entire gait cycle. Thus, in this work, a combination of musculoskeletal and tribocontact modeling was performed. In a first step, reliable contact forces were calculated, with low kinematic errors and very low residual forces and torques, which showed sufficient correlation with literature data, even though some of the values were higher. This could be explained by the approach of this investigation. The aim was to determine the contact forces during healthy/ physiological gait of young subjects rather than the best possible reproduction of the forces measured in vivo in much older subjects performing a non-physiological gait. In a second step, the derived data were used as input data for an elastohydrodynamic model to predict subject-specific lubrication conditions, contact pressures, deformations, and stresses. Thus, Pub II has the potential to further stimulate and accelerate research and optimization of the biomechanical and biotribological behavior of artificial synovial joints, such as TKRs. [I] In the context of the third sub-research question, several investigations on the coating topography and structure, cell-biological interaction, mechanical properties and coating-substrate adhesion of (tetrahedral) amorphous carbon coatings on Ti64, CoCr and UHMWPE substrates were conducted in Pub III - V. The coatings exhibited a morphology and composition typical of (tetrahedral) amorphous carbon coatings. The roughness of the coatings was higher than that of the substrates, especially on UHMWPE. Initial studies with contact angles and surface tensions as well as indirect and direct in vitro biocompatibility tests on (tetrahedral) amorphous carbon coatings showed comparable behavior to the substrates. The surface modifications exhibited no cytotoxic effects, confirming the potential for biomedical application. The developed coatings proved excellent mechanical properties with a substantial improvement in indentation hardness/ indentation modulus (H ITx / E ITy ) ratios, indicating favorable biotribological wear behavior. The adhesion of the coatings to the substrates Ti64, CoCr and UHMWPE could be considered at least sufficient - to very good - for use in TKRs. The findings lead to the assumption that the coatings presented in this work are capable of outperforming uncoated metallic-polymeric reference pairings under biotribological loading. [I] The assumption derived from the third sub-research question was being tested in Pub IV - VI and PrePub to answer the fourth sub-research question regarding the biotribological effectiveness of amorphous carbon coatings. In this context, different test categories were used to confirm the biotribological effectiveness of amorphous carbon coatings in terms of a significant increase in the service life of TKRs. In general, the favorable biotribologically effective behavior of amorphous carbon coatings could be successfully demonstrated across the links of the tribological test chain, from simple model tests to complex knee simulator tests, and transferred between the links individually. The possibility of testing numerous coatings with respect to their biotribological behavior in the first two links of the test chain (ball-onthree-pins and pin-on-disk tribometer tests) also allowed for determining to which extent and by what means this behavior can be influenced. In addition, the last link, the knee simulator, already represented an operational test (category III) and thus a considerable extension of the test chain in this work. In this near-application test the first, current results on the excellent biotribologically effective behavior of TKRs coated with amorphous carbon were confirmed repeatedly by the evaluation of the wear mass of the polymeric tibia inlay. Such an audit trail could possibly be taken up as a proposal for legal requirements for conformity and validation of medical devices. [I] In Pub IV, screening tests were performed on the biotribological behavior of amorphous carbon coatings in a ball-on-three-pins configuration, mimicking the conditions of TKRs during gait. Thus, the influence on complete and single-sided coating of TKR components (metallic and/ or polymeric) and the variation thereof as well as the differences between a higher and a moderate load case with uncrosslinked and conventionally crosslinked UHMWPE, respectively, were investigated. Although the coatings predominantly resulted in a friction increase due to the considerably higher roughness, the wear was substantially reduced. In particular, the coating of the polymeric component is of crucial importance for improving the wear behavior and increasing the service life of load-bearing implants. In addition, a one-sided coating led to higher wear of the uncoated counterpart. Accordingly, coating systems that are applied to both joint surfaces should be pursued further and specifically modified. [I] News 52 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 In Pub V and Pub VI, the wear behavior and mechanisms of (tetrahedral) amorphous carbon coatings were investigated for the evaluation of the biotribological behavior of TKRs using pin-on-disk tests. A numerical elastohydrodynamic contact simulation was used in Pub VI to assess the influence of coatings on contact and lubrication conditions for TKRs. The wear behavior was analyzed using LM, LSM, SEM and Raman spectroscopy. This allowed for the targeted selection of a suitable coating combination and the isolated observation of wear phenomena. Nevertheless, the subsequent transfer to component tests remained essential. The macrogeometric elastic deformation behavior and the contact area of TKRs remained almost unchanged due to the coating (Pub VI). However, the higher roughness of the coated specimens resulted in the increase of friction compared to the uncoated references. Despite the higher friction, all coating combinations contributed to a substantial wear reduction on the metallic pins and especially on the UHMWPE disks (Pub V and Pub VI). Thus, the UHMWPE disk was still protected by an intact ta-C layer without signs of adhesive or abrasive wear (Pub V). However, crack networks and first signs of near-surface fatigue appeared after the completion of the entire test duration (Pub V). In contrast, the a-C: H layer showed no delamination or spalling of larger wear particles, but rather continuous and slow wear (Pub VI). At the same time, the UHMWPE disk was still protected by a largely intact coating with no signs of cracking or fatigue after the entire test period (Pub VI). The particle analysis revealed that nanometer-sized particles were released by all tested groups, their sizes being comparable. In general, it can be assumed that the particles of the coated groups exhibit a more favorable cell biological behavior than pure PE particles (Pub V and Pub VI). This suggests that the service life of the femoral shield and in particular of the tibial inlay is significantly prolonged by adherent amorphous carbon coatings (Pub V and Pub VI). However, the time of failure of the ta-C coating can barely be predicted due to the prevailing failure mechanisms (Pub V). [I] To protect TKRs from continuous abrasive and adhesive wear, a sufficiently high coating hardness is preferred. At the same time, the coating hardness has to be sufficiently low to allow for the deformation of the soft substrate without near-surface fatigue of the coating. [I] The current results of the experimental in vitro testing of both uncoated and coated TKRs presented in PrePub showed that a substantial reduction in the polymeric wear mass of about 57 % compared to CoCr/ CPE and about 62 % compared to Ti64/ CPE could be achieved by coating both articulating counterparts, so that the service life of TKRs can in general be significantly extended by amorphous carbon coatings. The wear results obtained initially were in accordance with the findings of Pub IV - Pub VI and confirmed the biotribological effectiveness of amorphous carbon coatings. Due to the updating of ISO 14243-1, a quantitative comparison with previously published results with regard to wear rates is not yet expedient. In addition, the limitations of a component test rig under ideal conditions must be considered in order to derive accurate, realistic wear predictions. Nevertheless, based on the investigations carried out within the scope of this work, it can be stated that the service life of TKRs could be significantly increased by biotribologically effective amorphous carbon coatings. However, the current investigations from the knee simulation must be fully continued and consolidated in order to make a holistic statement regarding the biotribological performance of coated TKRs. [I] 4 Prospect The biotribological findings obtained from this work can be used in the future for a targeted development of advanced amorphous carbon coatings for TKRs. Thereby, modifications of the coating system can be made, for instance in the form of a multilayer functional coating, in order to adjust the occurrence of wear, the particle size and to control possible cell biological consequences. Likewise, the influence of a medical functionalization of the top layer, such as SiO or Ag, on the antibacterial and tribological behavior can be investigated. Furthermore, the addition of clinically relevant occurances, such as knee flexion or three-body wear tests to the proceedings of the knee simulator is rendered useful for evaluating the biotribological behavior of the endoprosthesis under the account of extreme conditions. The interaction of an amorphous carbon coating exhibiting high wear resistance even under extreme test conditions with a medical functionalization can minimize infection-related and wear-associated failure causes of TKRs and thus lead to a significant holistic extension of the service life. Additionally, this approach can be investigated on the next experimental level - in vivo small animal experiments - in order to enable a transfer from preclinical to clinical testing and finally to clinical usage. [I] Literature [I] ROTHAMMER, B., Dissertation: Amorphous carbon coatings for extending the service life of total knee replacements. 2024.