Development of a Durable and Modified Coating for Braiding Pultrusion
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KeywordsCoating System Diamond Coating Diamond Layer Plastic Application Hybrid Yarn
The braiding pultrusion process, or pulbraiding for short, is a process chain developed by the Institut für Textiltechnik of RWTH Aachen University for continuously manufacturing profiles made of fibre-reinforced thermoplastic material. Working with the company SAM Coating, this approach of modifying the tool coating promises to enhance component quality and reduce the cycle time required.
Fibre-reinforced composites boast exceptional mechanical properties and are also lightweight. This makes them ideal for use in lightweight construction and for aviation and space travel, as well as motor sport. There is also a demand for such materials in mechanical engineering when there is a need to reduce accelerated masses in order to boost production speed and the run time of machines and systems or save energy. In addition, the topic of fibre-reinforced composites has most recently come to the fore in the area of e-mobility against a background of new developments and innovation in this sector.
Friction is generated between the profile to be consolidated and the tool, which leads to warping of the upper braided layers. The key factor dictating the mechanical properties is the braiding angle configured. It is therefore important to minimise the warping of fibres in order to ensure the quality of the profiles.
The project aims to boost the component quality in the braiding pultrusion process. This involves adapting the entire system by applying a tailored coating to the rollers, taking friction, thermal and electrical conductivity and service life into consideration. Currently, release agents are often used and have to be reapplied at regular intervals. This inevitably means downtimes and chemical exposure. Plastic applications feature the use of DLC (diamond-like carbon) layers with lower friction values. Ta-C layers, while not representing the very latest technology available, offer hardness almost equivalent to that of diamonds as well as comparable friction values. What is clear is that, at the time of writing, none of the standard coating systems available can meet all the requirements made. This makes it imperative to develop a more suitable coating system.
The project is being financed as part of the “SME Central Innovation Program” (ZIM) funding initiative of the Federal Ministry of Economics and Technology (BMWi) and conducted in cooperation between the industrial company SAM Coating, Eggolsheim-Neuses, and the Institut für Textiltechnik (ITA) of RWTH Aachen University. A test rig allowing the application to be replicated has also been developed as part of efforts to achieve the project goal. It is intended to use this test rig to conduct initial basic investigations into the DLC and ta-C coating system families in order to ascertain their potential for minimising friction that occurs. This will be followed by a determination of the impact of various dopant elements on the thermal and electrical conductivity of the overall system.
The method of heating using an electrical resistor is only feasible for electrically conductive braidings. The material combination selected is not electrically conductive. This is still possible with other material combinations and so electrical conductivity is still considered. The final step involves coating the pair of rollers of the consolidation unit involved in the braiding pultrusion process with the tailored coating system.
▸ plasma polymer layers
▸ crystalline carbon layers
▸ amorphous carbon layers.
Crystalline layers can be further subdivided into graphite and diamond layers. Graphite layers exhibit anisotropic behaviour. They are not suitable for exposure to shear stresses, which rules out their use for coating tools. Diamond layers, meanwhile, stand out for their exceptional hardness. However, the processing temperature required is very high, which makes applying the diamond coating a very costly and energy-intensive process. Diamond coatings also have high friction [2, 3].
The amorphous carbon coatings can be classified into non-hydrogenous and hydrogenous layers. Hydrogenous amorphous carbon layers are less hard than their non-hydrogenous equivalents. Amorphous carbon layers also exhibit sp3 and sp2 hybridisation. The specific proportion varies based on the type of application involved. The greater the proportion of sp3 hybridisation, the harder the layer involved. A ta-C coating is classed as a non-hydrogenous amorphous carbon layer with a high proportion of sp3 bonds. This helps explain why ta-C coatings have superior hardness qualities. Layers with a greater proportion of sp2 have lower levels of both hardness and friction [4, 5]. These are employed as a state-of-the-art solutions for plastic applications. A ta-C coating modified for plastic applications and with a longer service life does not currently represent the state of the art.
▸ amorphous hydrogenous carbon systems with a low sp3 content (DLC)
▸ amorphous non-hydrogenous carbon systems with a high sp3 content (ta-C).
Surface Checking of the Coating Systems
The required melting temperature of the PP (170 to 200 °C) is achieved using corresponding heating elements installed in the text fixture. A braiding-wrapped plate in the middle of the test rig functioned as the main body of the tribology system. Counterbodies above and below the main body were two blocks to which coated plates could be attached. Both counterbodies are fixed to a frame built into a Zwick Z250 tensile testing machine built by the company Zwick, Ulm. The normal force with which the counterbody plates are pressed against the braid is generated by a pneumatic cylinder. A force transducer measures the tensile force. This all ows force/displacement courses to be illustrated and analysed.
The project aims to boost the component quality in the pulbraiding process.
Selecting a Coating System
Braiding pultrusion is a method which should pave the way to accelerate the manufacture of fibre composite components. The process is intended to boost both component quality and production speeds. Greater focus was therefore given assessing the results achieved at a speed of 100 mm/min than those achieved at lower speeds. For all coatings, the speed of 100 mm/min generated far lower forces than for the reference samples, which underlined their suitability for the braiding pultrusion process.
The measurement results for the required tractive forces of the second ta-C layer were lower, particularly at high speeds. It was assumed that this effect was attributable to the lower friction value of the second ta-C coating. The standard deviation also had little impact. It was tehrefore possible to expect comparatively consistent product quality with the use of the second ta-C layer for the braiding pultrusion rollers. When the electrical conductivity was subsequently examined, it became clear that the electrical resistance of the ta-C layers were lower compared to the DLC layer and the reference sample. When assessing the thermal conductivity, no significant different emerged in terms of the layers and reference samples.
The second ta-C layer only stood out from the first because of its lower layer thickness. It was assumed that when this coating method is used with a higher layer thickness, so-called droplets will form on the surface. These droplets lead to higher frictional values and also explain the variation in results for both ta-C layers. Conversely, since a higher layer thickness extends the service life of the coated tool concerned, the impact of post-processing and removal of droplets from the surface of the first ta-C layer should also be examined. for this purpose, both ta-C layers were selected for doping tests.
Status of the Project and Subsequent Steps
To make a statement concerning the impact of post-processing and doping, the surfaces of both ta-C variants were subjected to a roughness measurement. Ra and Rz values were subsequently compared. What was revealed was that post-processing of the first ta-C layer to remove the droplets increased both Ra and Rz values. It was assumed that the post-processing damaged or even removed the coating.
For the second ta-C layer, an averaged Ra value of 0.08 μm and an averaged Rz value of 0.71 were measured. Based on these results, therefore, the second ta-C layer was selected for further doping tests conducted with chrome and tungsten. Depending on how the doping affected the electrical conductivity, a coating variant was subsequently determined for the roller tool used in a braiding pultrusion machine.
Special thanks go to the SME Central Innovation Program (ZIM) for financing this project from funds from the Federal Ministry of Economics and Technology and to our partner SAM Coating GmbH, Eggolsheim-Neuses, for the excellent collaboration.
- VDI Guideline VDI 2840 Carbon layers — Principles, layer types and propertiesGoogle Scholar
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- Giorgis, F.; Tagliaferro, A.; Fanciulli, M.: “Defects” and their detection in a-C and a-C:H. In: Silva, S.; Robertson, J.; Milne, W.; Amaratunga, G. (eds.): Amorphous carbon: State Of The Art. World Scientific Publishing, Singapore, 1998. ISBN 981-02-3449-X, pp. 143 ffGoogle Scholar