DESIGN I DESIGN FOR ENVIRONMENT
risk of corrosion due to their higher oxygen content. Not only will the use of biofuel (e.g. from fatty acid methyl esters, or FAME) in propulsion systems require material changes in the engine to cope with their differing chemistries but also the interaction of biofuels with lubricants will require investigation as this area is not well understood. Furthermore, the tolerance of lubricant systems to the predicted higher soot levels and other chemical consequences will require the development of new lubricant compositions. These issues can be informed by surface characterisation techniques which are able to deliver understanding in terms of both the chemical composition and the physical form of material surfaces.
Surface characterisation techniques
The techniques available include three separate methods for establishing chemical composition and one for quantifying surface topography. Elemental surface spectroscopy: This technique samples the top 10nm of the surface under investigation and is quantitative to an accuracy of 0.1 atomic percent. Surface mass spectrometry: This method samples the top 3nm of the surface and is sensitive to ppm levels. Although highly sensitive, it is a qualitative method which is routinely used to investigate organic material at surfaces and interfaces. Depth profiling mass spectrometry: For multi-layer systems or embedded species this technique continuously sputters the area of interest to generate a crater in the material under investigation. 3D surface profiling: 3D profilometry, a technique for measuring surface topography, generates quantitative information on the physical nature of surfaces and sub-surfaces by using white light interferometry. The introduction of biofuels in the aerospace industry will require significant engine development over the next 5-10 years. Surface characterisation techniques
are already being used to support these developments in the following ways. Surface elemental spectroscopy is
used to investigate tribological samples in order to discern the partitioning of lubricant additive and other species on and off wear scars. In high resolution mode it can distinguish between the different oxidation states of metals and the covalent linkages of, for example, carbon. The technique is equally applicable
to friction (binding) surfaces as it is to lubricated ones and has been used extensively to investigate wear on engine component surfaces. Corrosion is an area where metal oxidation state information can be crucial in establishing the performance of alloys when exposed to the feed of, and the combustion products of, biofuels. Surface mass spectrometry is also
used on tribological samples where it complements the information arising from surface elemental spectroscopy with molecular data; this can be important in generating a complete understanding of lubricant performance. Depth profiling mass spectrometry
is used to establish the chemical changes in surface treatments such as case hardening nitriding, carburising or passivation and also coating/plating integrity following exposure to biofuel operation. 3D surface profiling has been used
to investigate tribological samples to distinguish between indentations (e.g. wear tracks/scars) and deposits and can also measure their depth/height and volume with exceptional accuracy. The need for continued advanced
engine development is a central imperative in the drive towards low
“The aerospace industry is committed to developing and adopting technologies which reduce its environmental impact in both the short- and long-term.” Chris Pickles, principal consultant for aerospace & defence, CERAM
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emissions. Studies on wear or high friction surfaces and the characterisation of the functionality of new lubricant formulations on such surfaces can be extensively informed by comprehensive surface analysis tools such as those described above. Complete chemical and topographical information can be provided on any test material including also on the underlying substrate areas.
An industry commitment In conclusion, the aerospace industry is committed to developing and adopting technologies which reduce its environmental impact in both the short- and long-term. In particular the replacement of kerosene as the fuel of choice is just one area where this policy is already being implemented. In this paper we have considered how techniques for both the chemical and topographical characterisation of surfaces and interfaces can be applied to ensure that these developments benefit significantly from the information they
can provide. y
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