Dr Ralph Delmdahl, Coherent, and Dr Thomas Lukasczyk, Markus Veltrup and Dr Ralf Ihde, Fraunhofer IFAM Bremen
Adhesive bonding is used to join surfaces in a myriad of applications and industries. One of the important factors in adhesive bond strength is surface cleanliness. For polymer parts that are produced via moulding, the presence of residual mould release agents negatively influences bond strength. Excimer laser ablation to remove contaminants and unstable interface layers prior to bonding has emerged as an attractive alternative to wet chemical and mechanical methods for surface cleaning in the production of polymer parts as well as other applications. This article reviews the use of excimer lasers for the cleaning of carbon fibre reinforced plastics (CFRPs) in preparation for bonding. This class of polymer materials is of increasing importance in the aerospace, aviation, automotive and marine industries.
About CFRPs
CFRPs are composite materials that offer a highly desirable combination of physical strength and low mass. A CFRP comprises a reinforcement and matrix. The reinforcement, which provides load bearing strength and rigidity, is carbon fibres, usually woven like a fabric. The matrix, which surrounds the reinforcement and binds it together, is most commonly epoxy or some other polymer resin. There are several fabrication methods in use for applying the matrix, and most of them involve pouring the liquid resin into a mould or form that already contains the reinforcement. A release agent is frequently used to ensure easy separation of the hardened resin from the mould after the curing process. Also, depending on the mould geometry, a release agent is often used to ensure good resin infiltration in the mould (flow of resin over the mould surface).
Creating CFRP structures sometimes requires joining the CFRP with metals to produce a lightweight multi-material construction. Also, individually fabricated CFRP pieces are sometimes joined to create larger assemblies. However, traditional mechanical fasteners (screws, rivets, etc.) have several drawbacks for this application. For example, drilled through-holes can concentrate internal stress and damage the load carrying fibres, which may then require additional reinforcement. Plus, the metal fasteners may significantly increase the weight of the assembly.
Adhesive bonding is an alternative solution that avoids these problems. Specifically, it does not require puncturing of the CFRP, it spreads the mechanical loading evenly over the entire bonded surface and it does not add significant weight to the finished assembly.
However, achieving a high-strength adhesive bond requires the removal of any mould release agents, other trace contaminants and unstable interface layers remaining on the surface from previous manufacturing steps. This surface pre-treatment must be accomplished without causing any damage to the underlying CFRP and its load carrying fibres.
Techniques for surface pre-treatment
Several techniques are currently used for cleaning and preparation of CFRP parts prior to adhesive bonding, including mechanical abrading and grit blasting. Unfortunately, each of these methods has its own drawbacks. Most mechanical abrading processes suffer from low-throughput speed and are usually performed wet, thus necessitating subsequent rinsing and drying. Grit blasting leaves residues and dust on the surface, which makes subsequent cleaning necessary. Furthermore, mechanical methods introduce the risk of damage to the carbon fibres.
In the aerospace industry, peel-plies are also used for CFRP surface preparation. Peel-plies are sheets that are laminated onto the CFRP surface prior to curing the matrix resin. They are removed before adhesive bonding, leaving a clean surface. The main drawback of peel-plies is that they increase CFRP manufacturing complexity. Also, they can lead to an unstable interface layer, for example, some transfer of wax into the topmost resin layer, resulting in low reproducibility. Lastly, peel-plies are not suitable for CFRP repair work.
Advantages of laser ablation
Laser ablation involves removing a thin layer of material from the surface of the CFRP resin, which can effectively remove virtually all contaminant residues. Unlike mechanical techniques, laser ablation requires practically no surface preparation, is performed dry and does not require the surface to be cleaned of debris afterwards. Also, it is suitable for the preparation of large surface areas, can be readily automated, delivers highly consistent results because it is a wear- and contact-free process, and is applicable to CFRP repair applications.
However, it is essential that laser ablation does not cause damage to the bulk resin or load carrying fibres. This makes excimer lasers a particularly attractive option for CFRP cleaning (figure 1). Their high-energy, deep ultraviolet pulses remove material primarily through photo-ablation, rather than thermal means, resulting in essentially zero heat affected zone (HAZ). Also, the ability to regulate delivered excimer dosage by controlling pulse count enables highly precise ablation depth control.
The energy density range used at the excimer laser wavelength is far from what would be necessary to ablate the fibres. From a practical standpoint, the large rectangular beam produced by excimer lasers can be easily shaped and homogenised to match the requirements of typical CFRP surface preparation applications. This results in rapid material removal and high throughput.
Figure 1: Excimer lasers enable precise, controllable material removal on CFRPs. These are laser scanning confocal microscopy (LSCM) images of a CFRP surface: a) untreated, with no exposed fibres, b) after two laser pulses per area, with fibres becoming exposed, and c) after twenty laser pulses per area, with fibres clearly exposed but not damaged. (Images taken from a PhD thesis by Markus Veltrup of Fraunhofer IFAM, in Bremen, Germany, yet to be published).
Implementation
There are two basic ways to implement excimer laser CFRP pre-treatment, namely the line scan method and the step-and-repeat method (figure 2). In the line scan method, the laser beam is shaped into a line.
The laser line is swept continuously across the surface to be cleaned. The number of pulses that a given spot on the material is exposed to is determined by a combination of line width, line travel speed and laser repetition rate. If the line length is shorter than the width of the area to be cleaned, several adjacent passes of the area are made.
In the step-and-repeat method, the laser beam is shaped into a rectangular field. The laser field is positioned at a fixed point on the CFRP surface, and an exposure is made (consisting of one or more laser pulses). The field is then moved and the process repeated. The entire area to be cleaned is sequentially exposed in this manner.
Figure 2: A schematic comparison of line scan and step-and-repeat methods for excimer laser ablation.
Current developments
Ongoing research to optimise the results, throughput and cost characteristics of excimer laser CFRP cleaning is being performed by several groups, including one at the Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM) in Bremen, Germany. Markus Veltrup of Fraunhofer IFAM explained: “Our research investigates laser-induced fibre exposure effects during CFRP pre-treatment and also studies the redeposition of ablated material back on to the CFRP surface after laser exposure. This redeposited material lowers subsequent bonding strength. We’ve examined and created mathematical models and practical tests for how redeposition is influenced by laser fluence and pulse overlap.”
The laser technology group at Fraunhofer IFAM employed a Coherent COMPex excimer laser with a wavelength of 248 nm to determine that, for a given material, there is a set of optimum laser fluence and pulse overlap parameters for delivering an entirely debris-free surface in a minimum processing time. In particular, the group discovered that a specific combination of laser fluence and pulse overlap generates a shock front that consistently blows off all redeposited material from previous exposures, creating a pristine, debris-free surface for subsequent bonding1.
In conclusion, adhesive bonding is more widely used than ever. In the case of CFRPs, it offers several advantages over other joining techniques. However, adhesive bonding requires that surfaces be properly pre-prepared. Excimer laser ablation is a single-step, dry process that delivers superior results over other methods, and it has proven to be highly reproducible and therefore ideal for volume production applications and even repair work. Furthermore, it is applicable to the cleaning of delicate surfaces in many other applications.
Coherent
Reference
1Veltrup, M., Lukasczyk, T., Ihde, J., and Mayer, B. (2018). Distribution and avoidance of debris on epoxy resin during UV ns-laser scanning processes. Applied Surface Science; volume 440, May 15, pp.1107–15.