Composites i Green manufacturinG
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 multi-
layer 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
Turbine blade: 3D topographical profile
that an insufficient adhesion promoter had been used in the
pre-treatment of the glass fibres prior to curing. often of silicone-based materials. This can be as relevant
The surface characterisation of both carbon fibre and to coated composites as it is to coated metals and alloys.
glass fibre prepregs is an important application of all three Aerospace manufacturers routinely operate multi-stage cleaning
analytical techniques whereby fibre surfaces can be diagnosed procedures to ensure substrates are adequately cleaned prior
for both inorganic and organic residues as well as for chemical to being presented to the coating process. The quality of
modification and surface irregularities. Gel coats can similarly cleaning is commonly monitored by a rinse/filter/particle
be measured for surface smoothness and/or imperfection with count procedure with various limit values. Crucially, however,
quantitative statistical data allied to nanometre resolved 3D this procedure does not reveal what potential contamination
images. Surface chemical characterisation has also been applied may still be remaining on the cleaned component. Surface
successfully to the analysis of release papers to investigate characterisation can measure part cleanliness directly and
potential material transfer. In an exercise to study the surface quantitatively. Cleaning processes can be routinely monitored
profiles of a series of treated release papers, valuable data on using pre-qualified coupons which are passed through the
the variation in surface smoothness and treatment distribution process with the components. These can then be analysed by
has been gathered. surface elemental spectroscopy and, by the use of a bespoke
The adhesion of coatings to composite structures is chemical combinatorial algorithm, a percentage cleanliness
paramount in delivering in-service performance. The causes measure can be generated. This capability allows for both an
of delamination or the adhesive failure of coatings can be unequivocal cleanliness specification to be set and an ongoing
investigated using surface characterisation techniques. In a quality control procedure to be implemented that relates
typical example, a two layer coating was exhibiting adhesive directly to the cleanliness of the substrate surface to be coated.
failure. Surface molecular mass spectrometry together with
quantitative surface elemental spectroscopy investigations Future developments
of the substrate and coating layers showed that ingredient The aerospace industry is committed to developing and
segregation from one of the layers was affecting inter-layer adopting technologies which reduce its environmental impact
bond strength. in both the short and long term and the use of composite
The consequential costs materials for lightweighting to reduce fuel burn is just one area
of these post-mortem where this policy is already being implemented. We expect
investigations are significantly to see an ever widening range of composite materials being
higher than those that would developed, all of which will benefit from development and
have been incurred when manufacturing processes that are strongly linked to robust
establishing material surface surface characterisation procedures. Future developments may
properties during the design also see an increase in the use of recycled fibres, the validation
and development process, of which will be achieved using similar characterisation
where the same techniques can techniques.
be applied to ensure quality With the combined incentives of enforced environmental
standards are met. targets and potential operational cost savings, the development
of composite materials will continue to be a focus for the
Cleaning validation aerospace manufacturing industry. In line with this, surface and
The most common cause interface characterisation will continue to play an important
of adhesive failure is the role in every stage of the process – from initial design through
Dr Chris Pickles: Principal consultant
occurrence of extraneous to development and manufacture. ❙
for aerospace & defence at CERAM
surface contamination – www.ceram.com
AEROSPACEMANUFACTURING | NOvEMbER 2009 17
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