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Lasers to address composite surface modification

Surface modification of carbon fibre composites in the aerospace industry will be one of the first applications the laser will address in this field, say Dr Paul French, Dr Martin Sharp and Sobhi Mahmoud at Liverpool John Moores University

From 22 to 25 June, Laser World of Photonics will take place in Munich, Germany. This is a biennial meeting of the laser community, manufacturers, users and research institutes around the world exposing their latest products and research. For those of us who have been attending regularly we have seen increasing interest in laser material processing of carbon fibre reinforced polymers (CFRP) with whole sections of stands dedicated to this particular application. German research and technology organisations such as Fraunhofer, Laser Zentrum Hannover, Bayerisches Laserzentrum, etc, are very active in this area of research, particularly supporting German industrial OEMs such as BMW in understanding the potential and importantly the limitations of laser machining of CFRP.

In the UK we have generally been a bit slow in getting off the mark in this important area of research, but now with the establishment of the UK’s Catapult Centres, such as the National Composites Centre in Bristol, the Advanced Manufacturing Research Centre at the University of Sheffield, and the Manufacturing Technology Centre in Coventry, this important area of manufacturing of composites with laser technology is coming to the fore.

The laser group at Liverpool John Moores University (LJMU) has been taking a different approach to machining CFRP. The group has been investigating composites since 2005, recognising early that if lasers were to be part of the future of UK aerospace manufacturing, the UK laser community would have to come up with the laser systems and processing strategies for laser processing technology to be successfully employed on the shop floor and be attractive to companies such as BAE Systems, GKN Aerospace, Boeing, and General Electric.

Lasers are being looked upon as an alternative to the traditional composite processing technologies. One of the first applications that the laser will address in the aerospace industry is surface modification of CFRP prior to adhesive bonding. At present this is accomplished by a human operator who will typically use a Teflon abrasive disk to roughen the surface of a CFRP component before the component is joined to a structure with adhesive. The potential problems with this method are that the control over the surface quality is poor, the quality of the resulting surface finish is down to the skill of the human operator, and it is very easy to damage the surface. Laser micromachining has the potential to offer an automated solution that will produce a superior product. 

The lasers that the LJMU group has been investigating for processing CFRP are based on fibre laser technology, such as those from Fianium, SPI Lasers and JK Lasers. The work horse is a 20W pulsed fibre laser from SPI Lasers, on which the group has developed the machining strategies that make the combination of fibre laser and CFRP possible.

Figure 1 (left) Figure 2 (right)
Figure 1: Mechanically abraded (left) and laser abraded (right) carbon fibre reinforced polymers, with the laser abraded surface showing regular structures

Figure 1 shows the surface finish of both mechanical and laser machined surfaces. The mechanically abraded surface shows signs of broken fibres and uneven erosion. By comparison, the laser abraded surface shows regular structures, either by controlling the hatch spacing or the machined depth of the structures, or by exposing the fibres or just removing micron layers from the top surface of the resin. 

LJMU has investigated surface micromachining of CFRP for enhanced adhesive joint strength using an infrared nanosecond SPI fibre laser system. The study was undertaken in conjunction with the National Composites Centre and Spectrum Technologies, specialists in laser wire marking and stripping technology. A later study by LJMU supported the findings of the initial trials.

In the LJMU study, 100 x 25mm CFRP coupons were laser surface textured to a depth of 12.5mm on the end of each coupon. An adhesive was then applied and cured in an oven as prescribed by the adhesive manufacturer. The samples were then tested in a shear lap joint configuration on a tensile testing machine. The results show that laser and mechanical surface texturing shear lap strengths were equivalent, and that both processing methods produce a joint that was stronger than a surface prepared by sand blasting. This would seem to suggest that when processing with a nanosecond pulse in the infrared region of the spectrum, the laser only produces mechanical adhesive attributes and does not enhance the surface of the CFRP chemically by producing any surface activation. Surface activation is what happens when a carbon fibre reinforced polymer surface is plasma treated prior to adhesive bonding.

When a CFRP surface is placed in the discharge path of a plasma, the electrons generated in a discharge impact the surface with energies two to three times that necessary to break the molecular bonds on the surface of most substrates. This creates very reactive free radicals and it is these free radicals in the presence of oxygen that react rapidly to form various chemical functional groups on the substrate surface. Functional groups resulting from this oxidation reaction are the most effective at increasing surface energy and enhancing chemical bonding to the resin matrix. This complex chemistry is not seen when an infrared laser structures a composite surface. 

The main advantage of using an infrared nanosecond laser over mechanical abrading is the laser process offers better control over the properties of the processed surface, and as such makes the joints’ adhesive strength more predictable. The laser group at Liverpool John Moores University  is now working on a hybrid approach to the question of enhanced surface chemistry, developing technology that will give the best of both worlds of both laser and plasma, but at equitable capital cost.

CFRP surface enhancement prior to adhesive bonding is just only one area of laser composite processing that the laser group at LJMU is investigating. Another important aspect of machining of composite components is process monitoring. LJMU are working with sensor manufacturer Micro-Epsilon, based in Birkenhead, near Liverpool. Working together, both hope to bring more control to the laser material processing. Process monitoring is important with respect to aerospace applications due to the fact that modern sensors these days can supply a complete process history for a component. This gives confidence to an industry that deals in such mission critical components, especially when exploiting a new processing technology. 

Laser processing of composites in the aerospace industry will be seen in the near future. In Germany, processing of composites on the shop floor may already be well established due to the fact that laser manufacturers, end users and Fraunhofer centres have worked together in developing the technology. Once the possibilities of laser processing of composites are realised the result will be that other sectors will take up composite processing, mainly in transport where the use of lightweight composite material is becoming increasingly important.  

Dr Paul French and Dr Martin Sharp founded the Photonics in Engineering Group at Liverpool John Moores University in 2008, which focuses on laser processing of carbon fibre composites. Sobhi Mahmoud is an MSC graduate at the group

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