Going green with surface characterisation

The green agenda continues to dominate aerospace developments both from the regulatory perspective and from economic operating imperatives. The use of biofuels as a means of reducing carbon footprint, whilst at the same time achieving fuel economies and performance improvements is a major part of the combined industry response to meeting the challenge of reduced dependency on hydrocarbon fuels.

Engine component materials evaluation and lubricant developments are two important aspects of the successful adoption of biofuels. One feature of these efforts is the need to understand surface and interface functionality from both a chemical and physical standpoint. In this paper we give examples of where surface characterisation techniques are continuing to make a major contribution to the endeavours to reduce the environmental impact of the aerospace industry in the future.

According to the UK Committee on Climate Change, by using biofuels, aircraft could become 40% to 50% more fuel efficient by 2025, compared to 2006 models. This would equate to a saving of 60 million tons of CO2. Air Transport Action Group executive director, Paul Steele says it's now time for the aviation industry to alter its view towards emissions, especially when it demands the right to grow. His group is a strong advocate for the use of biofuels, given the fuel's recent performance indicators.

Rolls-Royce vice-president for strategic marketing Robert Nuttall has previously remarked: “Jet engine technology, coupled with the use of biofuels, will actually be the biggest single contributor in the next 20 years to reducing our environmental footprint. There is no other technology that can reduce fuel burn, CO2 and noise like it.” Ian Dawkins, senior vice-president and head of future programmes, Airbus, in his presentation at the 2009 Aero Engineering Strategic Business and Skills Summit confirmed that the number one contributor to reducing the industry's carbon footprint was the adoption of biofuels.

These operational and regulatory pressures have only been intensified by the recent increases in the price of oil and the impending ‘peak oil' scenario. Over the last two years, airlines, engine manufacturers and oil technology companies have collaborated to conduct flight trials on a range of biofuel types and substitution rates. The advent of biofuel powered air transport is upon us. The development of non-competing crops (jatropha, camelina, halophytes, algae) and other sources (gas to liquid) have effectively surmounted the objections regarding food displacement.

The introduction of alternative (lower carbon) fuels is, however, recognised as a challenge for engine components in terms of the potential for increased wear due to soot formation and the increased 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.

Applications in aerospace materials

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 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.

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.

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