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, Volume 10, Issue 1, pp 18–23 | Cite as

Development of a Durable and Modified Coating for Braiding Pultrusion

  • Pia Münch
  • Thomas Gries
Materials Tool Coating

Keywords

Coating System Diamond Coating Diamond Layer Plastic Application Hybrid Yarn 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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.

Currently, however, the fact that such fibre-reinforced plastics remain far costlier than components made from more traditional materials is proving a hindrance to their market penetration. When considering the cost structure of a composite component, there are no medium-term prospects for notable cost savings on the raw materials behind such composites. Where scope does exist for such savings, it is in the ability to shorten cycle times required. Accelerating this process is possible thanks to the braiding pultrusion method. This involves hybrid yarns comprising reinforcement and thermoplastic fibres being braided around a mandrel. The next process step sees the plastic melted and then reconsolidated into the final moulded profile. Figure 1 shows the process sequence.
Figure 1

Process chain of pulbraiding (© ITA)

The process of consolidation takes place in a tool based around rollers. The rollers are shaped to match the form of the product and, as well as being useful for the shaping and consolidation stages, they also facilitate the melting of the thermoplastics. Figure 2 shows a pair of rollers. Here, the thermal energy required can be applied in two different ways. One possibility is to heat the rollers using a heat transfer oil. The other is heating by using the electrical resistance of the braiding.
Figure 2

Roller-tool involved in the pulbraiding process (© ITA)

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.

Coating Systems

Carbon-based coating systems are appropriate for the braiding pultrusion application described here. In accordance with VDI 2840, these systems can be classified into three groups [1]:
  • ▸ plasma polymer layers

  • ▸ crystalline carbon layers

  • ▸ amorphous carbon layers.

From a material technology perspective, plasma polymer layers are structured like polymers and are only suitable for low-temperature applications. With this in mind, these coating systems are unsuitable for braiding pultrusion applications.

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.

The properties of amorphous carbon layers can be significantly modified by doping with a range of elements (tungsten, chrome, etc.). Depending on the underlying applications involved, this may make it possible to manufacture optimised layer systems in a targeted manner. The ta-C and DLC layer systems generated in this project are manufactured using PVD methods. In summary, two basic coating systems can be considered the preferred choices:
  • ▸ amorphous hydrogenous carbon systems with a low sp3 content (DLC)

  • ▸ amorphous non-hydrogenous carbon systems with a high sp3 content (ta-C).

Compared to the DLC layer, the ta-C layers are far less sensitive to temperature. DLC layers can be used at temperatures of up to 350 °C, while the application temperature for ta-C layers is up to 550 °C [1]. This demonstrates a further advantage of ta-C layers in future circumstances where the braiding pultrusion system may also have to be used with thermoplastics, which have a higher melting point than polypropylene. In other words, using the ta-C layer is likely to expand the scope of application of the braiding pultrusion machine.

Surface Checking of the Coating Systems

The first work package involved assessing the processability of the hybrid yarn and the manufacture of the braided structures as part of a series of friction tests. The project involved a commercially available hybrid yarn made of glass and polypropylene fibres and supplied by the company Comfil, Denmark. The structures involved were manufactured on a radial braiding machine supplied by the company Herzog, Oldenburg. The ITA had access to a RF1/144-100 type machine, which is shown in Figure 3. With this method, for example, foam or steel functions as the mandrel.
Figure 3

RF 1/144-100 radial braiding machine made by the Herzog company (© ITA)

The key material parameter that was varied in this project is fibre orientation; which in a braiding context is known as the braiding angle. The production speed of the braiding process is configured such that the desired braiding angle can be adjusted. Samples with braiding angles of 30°, 45° and 60° were manufactured for the purpose of friction testing. Each braided area was 1.5 m long and was then cut into smaller samples for the purpose of friction testing, Figure 4. The braiding angle was then monitored using an image-processing camera system (AVS) supplied by the company Apodius, Aachen.
Figure 4

Braided samples for the friction tests (© ITA)

The second work package focused on developing and constructing a test rig to determine friction ratios during the melting and pressing processes. This test rig was designed to allow the procedural parameters of braiding angle and haul-off speed to be reproduced on the test device together with temperature. Figure 5 is a diagram of the test rig developed.
Figure 5

Schematic illustration of the test rig (© ITA)

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

Four pairs of contact surface elements were produced and coated for the selection of the coating systems (DLC, ta-C). One pair remained uncoated as the reference sample, while a further pair was coated with DLC. Two additional pairs each had a ta-C layer applied which differed in terms of roughness, hardness and layer thickness, Figure 6.
Figure 6

Coating systems: reference sample uncoated (a), DLC (b), Ta-C1 (c), Ta-C2 (d) (© ITA)

A designs of experiments (DoE) approach was used to identify the coating variants offering the greatest potential for braiding pultrusion rollers by varying the braiding angle, the pull-out speed and the plate coating in the above-mentioned test rig. Figure 7 shows the results of the respective coating system as dictated by the frictional force, the braiding angle and the haul-off speed.
Figure 7

Evalution of the friction tests (© ITA)

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.

Notes

Thanks

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.

References

  1. [1]
    VDI Guideline VDI 2840 Carbon layers — Principles, layer types and propertiesGoogle Scholar
  2. [2]
    Ferrari, A.; Robertson, J.: Interpretation of Raman spectra of disordered and amorphous carbon. In: Physical Review B 61 (2000) 20, pp. 14095 ffCrossRefGoogle Scholar
  3. [3]
    Robertson, J.: Deposition mechanism of diamond-like carbon. 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. 32 ffGoogle Scholar
  4. [4]
    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
  5. [5]
    Robertson, J.: Diamond-like amorphous carbon. In: Materials Science and Engineering R 37 (2002), pp. 129–281CrossRefGoogle Scholar

Copyright information

© Springer Fachmedien Wiesbaden 2017

Authors and Affiliations

  • Pia Münch
    • 1
  • Thomas Gries
    • 1
  1. 1.RWTH Aachen UniversityAachenGermany

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