Scratching the surface of green technologies

The green agenda continues to dominate aerospace developments both from a regulatory perspective and from economic operating imperatives. CERAM's principal consultant for aerospace and defence, Dr Chris Pickles looks at the applications of surface characterisation technology in low carbon developments. The use of composite materials as a means of ‘lightweighting' aircraft structures and components is a direct response to the need to reduce carbon footprint whilst at the same time achieving fuel economies. With all airlines that land in Europe gearing up to be included in the European Emissions Trading Scheme (EU-ETS) from 2012 - and with a baseline reporting year of 2010 - improving performance efficiency has become a huge priority. New material developments hold the key to the successful achievement of green objectives whilst maintaining, or improving, other essential product performance requirements, and surface characterisation techniques that continue to make a major contribution to these endeavours. The picture is changing continuously, but a reasonable estimate of the current use of carbon fibre in aerospace applications is in the region of 30,000 tonnes per annum. According to informed sources this will rise to 170,000 tonnes per annum by 2020. Fuel represents over half of total airline operating costs and a 30% weight loss would deliver a 6% fuel cost saving. Maintenance costs are also reduced significantly by the use of composite structures. Developments continue apace with thermoset resins coming under the influence of the REACH directive in terms of the need to phase out the curing agent MDA. Beyond this, the attraction of thermoplastic resins in terms of their recyclability and the lower drag available from thinner wings are just two of the future developments that will maintain the inexorable demand for surface characterisation as an essential component of composite material development programmes. Composite surface analysis Composite materials depend critically for their operational performance on the strength of the bond between the fibre and the matrix polymer, which must withstand wide ranging temperature and humidity conditions. Any disruption to this interface will affect the adhesion and can lead to resin-fibre disbondment. Similarly, coating adhesion to composite structures is dependent on both the physical and chemical nature of the substrate and of the coating. Whilst the substrate surface roughness has an optimum value related to the keying of the coating, in the aerospace industry the coating outer surface needs to be as smooth as possible to reduce drag. The machining of composites is also an area of potential concern in terms of fibre break-out and edge finish, whether the structure is machined or waterjet cut. In all of these areas, surface characterisation techniques are able to deliver quantitative and highly spatially resolved chemical and physical information. The most common surface characterisation techniques used for the testing of composite materials include the analysis of the chemical composition of materials (using elemental surface spectrometry, surface mass spectrometry and depth profiling spectrometry) and the analysis of the physical nature of the material (using 3D surface profiling). A typical example of the application of surface analysis techniques to composite materials was the analysis of a multilayer composite board in which the glass fibre filled epoxy prepreg layer was delaminating. Chemical analysis using surface mass spectrometry of both the fibre surfaces and the resin residues, when compared with conforming material, showed that an insufficient adhesion promoter had been used in the pre-treatment of the glass fibres prior to curing. The surface characterisation of both carbon fibre and glass fibre prepregs is an important application of all three analytical techniques whereby fibre surfaces can be diagnosed for both inorganic and organic residues as well as for chemical modification and surface irregularities. Gel coats can similarly be measured for surface smoothness and/or imperfection with quantitative statistical data allied to nanometre resolved 3D images. Surface chemical characterisation has also been applied successfully to the analysis of release papers to investigate potential material transfer. In an exercise to study the surface profiles of a series of treated release papers valuable data on the variation in surface smoothness and treatment distribution was gathered. The adhesion of coatings to composite structures is paramount in delivering in-service performance. The causes of delamination or the adhesive failure of coatings can be investigated using surface characterisation techniques. In a typical example, a two layer coating was exhibiting adhesive failure. Surface molecular mass spectrometry together with quantitative surface elemental spectroscopy investigations of the substrate and coating layers showed that ingredient segregation from one of the layers was affecting inter-layer bond strength. The consequential costs of these post-mortem investigations are significantly higher than those that would have been incurred when establishing material surface properties during the design and development process, where the same techniques can be applied to ensure quality standards are met. Cleaning validation The most common cause of adhesive failure is the occurrence of extraneous surface contamination - often of silicone-based materials. This can be as relevant to coated composites as it is to coated metals and alloys. Aerospace manufacturers routinely operate multi-stage cleaning procedures to ensure substrates are adequately cleaned prior to being presented to the coating process. The quality of cleaning is commonly monitored by a rinse/filter/particle count procedure with various limit values. Crucially, however, this procedure does not reveal what potential contamination may still be remaining on the cleaned component. Surface characterisation can measure part cleanliness directly and quantitatively. Cleaning processes can be routinely monitored using pre-qualified coupons which are passed through the process with the components. These can then be analysed by surface elemental spectroscopy and, by the use of a bespoke chemical combinatorial algorithm, a % cleanliness measure can be generated. This capability allows for both an unequivocal cleanliness specification to be set and an ongoing quality control procedure to be implemented that relates directly to the cleanliness of the substrate surface to be coated. AMNov09Com - Ceram 3 Future developments The aerospace industry is committed to developing and adopting technologies which reduce its environmental impact in both the short- and long-term and the use of composite materials for lightweighting to reduce fuel burn is just one area where this policy is already being implemented. We expect to see an ever widening range of composite materials being developed, all of which will benefit from development and manufacturing processes that are strongly linked to robust surface characterisation procedures. Future developments may also see an increase in the use of recycled fibres, the validation of which will be achieved using similar characterisation techniques. With the combined incentives of enforced environmental targets and potential operational cost savings, the development of composite materials will continue to be a focus for the aerospace manufacturing industry. In line with this, surface and interface characterisation will continue to play an important role in every stage of the process – from initial design through development and manufacture. www.ceram.com

Related Articles

Good vibrations!

Providing custom-designed cutting tool solutions for composite components, Walter AG explains how vibration drilling can help guarantee high quality for stack machining.
7 years ago Features

It’s a material whirl!

Bindatex – Europe's only independent specialist in the precision cutting of advanced materials – is working with BAE Systems to create an agile UK supply chain. Aerospace Manufacturing reports.
8 years ago Features

Six of the best

Mike Richardson meets Velocity Composites' director Gerry Johnson and managing director, Darren Ingram to hear how the company's total kit provision of engineered kits for the aerospace industry is knocking material waste for six!
8 years ago Features
Most recent Articles

Carter enjoys success at HAI HELI-EXPO event

Acknowledged as the World’s largest vertical aviation conference and trade show, HAI HELI-EXPO 2024 provided precision bearing specialists Carter Manufacturing a perfect opportunity to exhibit its extensive range of aerospace bearing installation, removal and testing tools, along with PMA bearing options.
1 day ago News

Login / Sign up