Design of Amorphous Carbon Coatings Using Gaussian Processes and Advanced Data Visualization

In recent years, an increasing number of machine learning applications in tribology and coating design have been reported. Motivated by this, this contribution highlights the use of Gaussian processes for the prediction of the resulting coating characteristics to enhance the design of amorphous carbon coatings. In this regard, by using Gaussian process regression (GPR) models, a visualization of the process map of available coating design is created. The training of the GPR models is based on the experimental results of a centrally composed full factorial 23 experimental design for the deposition of a-C:H coatings on medical UHMWPE. In addition, different supervised machine learning (ML) models, such as Polynomial Regression (PR), Support Vector Machines (SVM) and Neural Networks (NN) are trained. All models are then used to predict the resulting indentation hardness of a complete statistical experimental design using the Box–Behnken design. The results are finally compared, with the GPR being of superior performance. The performance of the overall approach, in terms of quality and quantity of predictions as well as in terms of usage in visualization, is demonstrated using an initial dataset of 10 characterized amorphous carbon coatings on UHMWPE

Design of Amorphous Carbon Coatings Using Gaussian Processes and Advanced Data Visualization

In recent years, an increasing number of machine learning applications in tribology and coating design have been reported. Motivated by this, this contribution highlights the use of Gaussian processes for the prediction of the resulting coating characteristics to enhance the design of amorphous carbon coatings. In this regard, by using Gaussian process regression (GPR) models, a visualization of the process map of available coating design is created. The training of the GPR models is based on the experimental results of a centrally composed full factorial 23 experimental design for the deposition of a-C:H coatings on medical UHMWPE. In addition, different supervised machine learning (ML) models, such as Polynomial Regression (PR), Support Vector Machines (SVM) and Neural Networks (NN) are trained. All models are then used to predict the resulting indentation hardness of a complete statistical experimental design using the Box–Behnken design. The results are finally compared, with the GPR being of superior performance. The performance of the overall approach, in terms of quality and quantity of predictions as well as in terms of usage in visualization, is demonstrated using an initial dataset of 10 characterized amorphous carbon coatings on UHMWPE

Adaption of tribological behavior of a-C:H coatings for application in dry deep drawing

Nowadays the sheet metal forming industry faces challenges regarding efficient usage of resources and sustainability. One strategy to increase the environmental friendliness is to abandon the application of lubricants. The direct contact between tool and workpiece leads to an intensive interaction which increases friction. Especially for deep drawing processes with long sliding distances, this causes distinctive wear. The tool sided application of carbon based coatings is a well-known approach to reduce friction and wear. Former studies have shown a beneficial behavior of hydrogenated amorphous carbon based coatings (a-C:H) to improve the tribological conditions in contact with steel sheets and aluminium alloys under dry conditions. Within this study the coating process and the resulting coating properties will be analyzed. Afterwards mechanical and laser based surface treatment processes prior and after the deposition process will be investigated to reduce the coating roughness. Different roughness values were achieved by varying the surface treatment processes. The laser based finishing enables a reduction of the Spk values by removing single roughness asperities. In order to identify the necessary process parameters for the laser treatment, an analytical model of the material removal was applied. The laser surface treatment achieved similar roughness characteristics compared to mechanical treatment. In this study the tribological behavior of a-C:H coated tools was analyzed under dry conditions within strip drawing tests. The tribological investigations revealed that for dry deep drawing of zinc coated DC04 a broader range of Spk values leads to acceptable tribological conditions whereas for AA5182 a smoother tool surface has to be ensured to prevent adhesion and utilize the full potential of a-C:H coatings

Adaption of tribological behavior of a-C:H coatings for application in dry deep drawing

Nowadays the sheet metal forming industry faces challenges regarding efficient usage of resources and sustainability. One strategy to increase the environmental friendliness is to abandon the application of lubricants. The direct contact between tool and workpiece leads to an intensive interaction which increases friction. Especially for deep drawing processes with long sliding distances, this causes distinctive wear. The tool sided application of carbon based coatings is a well-known approach to reduce friction and wear. Former studies have shown a beneficial behavior of hydrogenated amorphous carbon based coatings (a-C:H) to improve the tribological conditions in contact with steel sheets and aluminium alloys under dry conditions. Within this study the coating process and the resulting coating properties will be analyzed. Afterwards mechanical and laser based surface treatment processes prior and after the deposition process will be investigated to reduce the coating roughness. Different roughness values were achieved by varying the surface treatment processes. The laser based finishing enables a reduction of the Spk values by removing single roughness asperities. In order to identify the necessary process parameters for the laser treatment, an analytical model of the material removal was applied. The laser surface treatment achieved similar roughness characteristics compared to mechanical treatment. In this study the tribological behavior of a-C:H coated tools was analyzed under dry conditions within strip drawing tests. The tribological investigations revealed that for dry deep drawing of zinc coated DC04 a broader range of Spk values leads to acceptable tribological conditions whereas for AA5182 a smoother tool surface has to be ensured to prevent adhesion and utilize the full potential of a-C:H coatings

Amorphous Carbon Coatings for Total Knee Replacements—Part I: Deposition, Cytocompatibility, Chemical and Mechanical Properties

Diamond-like carbon (DLC) coatings have the potential to reduce implant wear and thusto contribute to avoiding premature failure and increase service life of total knee replacements(TKAs). This two-part study addresses the development of such coatings for ultrahigh molecularweight polyethylene (UHMWPE) tibial inlays as well as cobalt–chromium–molybdenum (CoCr)and titanium (Ti64) alloy femoral components. While a detailed characterization of the tribologicalbehavior is the subject of part II, part I focusses on the deposition of pure (a-C:H) and tungstendopedhydrogen-containing amorphous carbon coatings (a-C:H:W) and the detailed characterizationof their chemical, cytological, mechanical and adhesion behavior. The coatings are fabricated byphysical vapor deposition (PVD) and display typical DLC morphology and composition, as verifiedby focused ion beam scanning electron microscopy and Raman spectroscopy. Their roughness ishigher than that of the plain substrates. Initial screening with contact angle and surface tension aswell as in vitro testing by indirect and direct application indicate favorable cytocompatibility. TheDLC coatings feature excellent mechanical properties with a substantial enhancement of indentationhardness and elastic modulus ratios. The adhesion of the coatings as determined in modified scratchtests can be considered as sufficient for the use in TKAs

Lubricant free forming with tailored tribological conditions

AbschlussberichtChanged ecological and economic situations motivate research into environmentally friendly and efficient manufacturing processes. Forming without lubricant has the potential to meet both requirements by avoiding the usage of environmentally harmful lubricants and shortening the process chain by omitting lubricant application and component cleaning. Within the scope of the project, an increase in friction and adhesive wear were identified as major challenges, resulting in failure of components due to cracking. Therefore, this project focused on the investigation of measures to meet these challenges. Amorphous carbon coatings, the reduction of roughness and the application of discrete microtextures were considered as potential measures. Hydrogen-containing amorphous carbon coating systems (a-C:H) fabricated by reactive physical vapor deposition (PVD), plasma-enhanced chemical vapor deposition (PECVD) and PVD/PECVD hybrid techniques, as well as a PVD-generated tetrahedral hydrogen-free amorphous carbon coating (ta-C) were investigated with respect to their properties and tribological performance Three specific profile requirements –a dopant free carbon network, smooth and defect-free surfaces and a high coating adhesion to substrate –are identified as requirements, in order to prolonger the service life of the coated tools. Moreover, to enable the steering of the material flow ultrafast laser based micro texturing for locally tailored tribological conditions were investigated. Thereby two wavelength dependent ablation regimes which differ in ablation mechanism and freedom of form were identified. Using these approaches the friction coefficient could either be reduced by up to 20 % or selectively increased. To improve the efficiency of the process, several beam shaping approaches were evaluated to provide a homogeneous beam profile for uniform modification. By applying ta-C and a-C:H coatings in forming tests, the findings of the laboratory tests were validated and the feasibility of lubricant free deep drawing was proven. In order to benefit from the forming-process-specific advantages, high quantities and therefore high durability of the measures are required. An application-oriented wear test rig has been designed to investigate their durability. By this, it was proven that 3 000 components can be produced from DC04 without wear with both a-C:H and ta-C coatings, and thus increasing tool life by a factor of 15 compared to unmodified tools. Even in the case of wear-critical AA5182, 3 000 parts were produced without wear using ta-C. Within the scope of the project, a fundamental understanding of lubricant free deep drawing processes and measures was created and proof of feasibility in form of a high number of components was achieved.22826

Amorphous Carbon Coatings for Total Knee Replacements—Part II: Tribological Behavior

Diamond-like carbon coatings may decrease implant wear, therefore, they are helping toreduce aseptic loosening and increase service life of total knee arthroplasties (TKAs). This two-partstudy addresses the development of such coatings for ultrahigh molecular weight polyethylene(UHMWPE) tibial inlays as well as cobalt-chromium-molybdenum (CoCr) and titanium (Ti64) alloyfemoral components. While the deposition of a pure (a-C:H) and tungsten-doped hydrogencontainingamorphous carbon coating (a-C:H:W) as well as the detailed characterization of mechanicaland adhesion properties were the subject of Part I, the tribological behavior is studied in Part II.Pin-on-disk tests are performed under artificial synovial fluid lubrication. Numerical elastohydrodynamiclubrication modeling is used to show the representability of contact conditions for TKAs andto assess the influence of coatings on lubrication conditions. The wear behavior is characterized bymeans of light and laser scanning microscopy, Raman spectroscopy, scanning electron microscopyand particle analyses. Although the coating leads to an increase in friction due to the considerablyhigher roughness, especially the UHMWPE wear is significantly reduced up to a factor of 49% (CoCr)and 77% (Ti64). Thereby, the coating shows continuous wear and no sudden failure or spallationof larger wear particles. This demonstrated the great potential of amorphous carbon coatings forknee replacements

Objasnění mechanismů mazání v náhradě kolenního kloubu – Část I: Experimentální studium

This contribution is aimed at the detailed understanding of lubrication mechanisms within total knee replacement. While Part I is focused on the experimental investigation, Part II deals with the development of a predictive numerical model. Here, a knee simulator was used for direct optical observation of the contacts between a metal femoral and a transparent polymer components. Transient dynamic conditions were applied. Mimicked synovial fluids with fluorescently labelled constituents were used as the test lubricants. The results showed that -globulin forms thin boundary lubricating film, being reinforced by the interaction of phospholipids and hyaluronic acid. Further development of lubricating film is attributed to albumin layering. Based on the results, a novel lubrication model of the knee implant is proposed.Tato studie je zaměřena na detailní porozumění mechanismů mazání v náhradě kolenního kloubu. Zatímco část I představuje experimentální pozorování, část II se zabývá vývojem prediktivního numerického modelu. V této části studie byl využit kolenní simulátor pro přímé optické pozorování kontaktu kovové femorální komponenty a vložky zhotovené z průhledného polymeru. Byly aplikovány proměnné dynamické podmínky. Jako mazivo byly využity modelové synoviální kapaliny s fluorescenčně označenými složkami. Výsledky ukázaly, že -globulin má tendenci formovat tenkou meznou vrstvu, která je vyztužená fosfolipidy v interakci s kyselinou hyaluronovou. Další nárůst mazacího filmu je připisován vrstvení albuminu. Na základě výsledků je představen nový teoretický model mazací vrstvy v kolenní náhradě

Wear Mechanism of Superhard Tetrahedral Amorphous Carbon (ta‐C) Coatings for Biomedical Applications

Abstract Tetrahedral amorphous carbon (ta‐C) coatings have the potential to protect biomedical implants from wear and increase their service life. This study elucidates the biocompatibility, mechanical properties, adhesion, and wear resistance of ta‐C coatings fabricated by physical vapor deposition on cobalt‐chromium‐molybdenum (CoCr) and titanium (Ti64) alloys as well as ultrahigh molecular weight polyethylene (UHMWPE). Satisfactory cytocompatibility is verified using contact angle and surface tension measurements as well as indirect and direct cell testing. Scratch testing demonstrates excellent adhesion to the substrates and as confirmed by nanoindentation, the coatings represent an up to 13‐fold and 182‐fold increase in hardness on the hard and soft materials. In metal pin‐on‐UHMWPE disk sliding experiments under simulated body fluid lubrication, the wear rates of the disk are reduced by 48% (against CoCr) and 73% (against Ti64) while the pin wear rates are reduced by factors of 20 (CoCr) and 116 (Ti64) compared to uncoated pairings. From optical and laser scanning microscopy, Raman measurements, and particle analyses, it is shown that the underlying substrates remain well protected. Nonetheless, focused ion beam scanning electron microscopy revealed coating process‐related and thermally driven subductions as well as tribologically induced near‐surface fatigue, which can potentially constitute critical wear mechanisms