Speed and safety can be heightened with the right kind of aircraft coating. Dr Gareth Evans investigates the most cutting-edge techniques.
Aerospace coatings inevitably occupy a particularly high-tech and competitive niche in the marketplace. With all segments of the aviation industry open to new developments that offer tangible benefits and the US sector alone valued at $110m, according to coatings consultants Chemark, the potential for the right product is enormous.
While the market is clearly a lucrative one, the technological challenges are equally demanding.
As a matter of course, any coating needs to be able to withstand temperatures that can fluctuate from -45°C in the air to +50°C on the ground, tolerate high-altitude ultraviolet light, and resist corrosion from fluids and erosion from airborne particles without losing its integrity. It also has to stay looking good, for five or six years.
Unsurprisingly, this means that the aircraft coatings arena is inherently technology driven, with many of the recent developments in the field reflecting some of the hottest emerging technologies.
Never has the expression 'leading edge' been more appropriate than to describe the extension of state-of-the-art biomimetics and nanotechnology to meet the demands of aerospace surface coatings.
Pearl of wisdom
Though oysters seem one of the most unlikely starting points for anything remotely connected to aircraft, the discovery of the true mechanism behind pearl production was to prove the inspiration for a possible new lightweight and durable protective aviation coating.
For years biologists believed that these molluscs made pearls – and their own shells – by precipitating calcium carbonate directly out of sea water, until it was recently shown that they actually relied on blood cells to deposit the crystals.
This breakthrough led scientists at the University of Dayton Research Institute (UDRI) in the US to attempt to use this same mechanism to deposit layers of natural ceramic directly onto metal, without the need for the high temperatures and pressures demanded for conventional ceramic materials.
UDRI senior research scientist Doug Hansen says the transfer of this technique is now beginning to evolve. "We have taken those blood cells and manipulated them to deposit crystals in an ordered manner on a variety of metal surfaces, resulting in a multilayered ceramic coating," Hensen says.
Ceramics already have a well-established role in the aerospace industry but if this US Air Force-funded project results in a method that allows the deposition's thickness and placement to be controlled accurately enough, it could revolutionise their future use.
Corrosion control is one area where this kind of technology could prove invaluable. As Defense Office of Corrosion Policy and Oversight's Airan Perez said before a recent corrosion conference, "coatings are the first defence against corrosion and the most economical way of preventing it".
Biomimetic materials are not the only possibility being considered for military aircraft. Amid the varied technical presentations covering 14 different technology areas, one approach that received particular interest from delegates was the deployment of micro-scale nanotechnology to prevent, control and combat corrosion at the atomic and molecular level.
One for the birds
With the commercial aircraft segment accounting for 75% of the sector's spend, coating innovations are not the exclusive preserve of the military. After the events of 15 January 2009, some of the suggestions have been very innovative indeed.
"Hit birds; we lost thrust in both engines.…" The words that day of Chesley 'Sully' Sulenberger, pilot of US Airways flight 1549, shortly before his heroic landing in the Hudson River are a stark reminder of the ever-present danger of bird strike and may yet form the catalyst for a coating-based solution to the problem.
Successful bird avoidance tests with pulsing lights – a recent one by Qantas yielding a 30% reduction – coupled with the sensitivity different species are known to have to particular wavelengths has led to the potential development of light-emitting paints.
It is something that the US Department of Agriculture (USDA) wildlife biologist and bird strike expert Bradley Blackwell and Precise Flight vice-president of flight safety Scott Philiben have teamed up to examine.
The idea of improving aircraft visibility has been around for decades but the team hope that this research, still very much in its infancy, will let them gain a better understanding of exactly what birds see and how they react.
If successful, it could eventually pave the way for reflective coatings that produce light in wavelengths invisible to human eyes but which birds would find startling – and so choose to avoid.
Economic and environmental convergence
As have many other industries, aviation has found that fuel efficiency is one area where economics and environmental considerations converge head-on.
Between rising operational expenditure and passenger resistance to higher airfares, airlines have increasingly found themselves between the proverbial rock and a hard place, as many recent bottom line figures have shown. Fortunately, against this backdrop, coatings can make a significant contribution to costs in cash and carbon.
Two principal factors affect fuel consumption: weight and drag. The lighter and smoother the coating covering an aircraft, the less of a contribution the airline's layer of painted livery makes to the overall economic equation. It is not simply an ongoing quest for lower-drag and lower-weight paints, however, despite their obvious appeal. The demands of legislation, regulatory approval and the need to protect airframe integrity in a uniquely challenging working environment also need to be met. Reconciling the two forms a major focus for research.
Already there are coating systems available that are 10–12% lighter than existing rival products and some of the next-generation topcoats in development could almost double that saving. For an Airbus A380, that would. More to the point is that estimates from the Aviation Maintenance Technician Society suggest it could save over 35,000l of fuel per aircraft annually and cut nearly 90t of carbon dioxide emissions.
With work also underway on a series of related coating systems, ranging from lighter magnesium-based primers to reduced-thickness, low-VOC films and 'selectively removable' top layers, the whole weight-and-fuel-efficiency issue remains a high priority.
Of all the things expected of coatings, sometimes looking good is the bit that really matters, hence China Southern Airlines' recent five-year deal with AkzoNobel Aerospace Coatings for next-generation systems to improve aircraft appearance. The contract with China's largest airline came after extensive product evaluation and covers the maintenance of the existing fleet and the new builds on order from Airbus and Boeing.
It is an issue that has resonance for the military too. The high gloss and punchy colours of the US Air Force 'Thunderbirds' display squadron's F-16s, for instance, owe their longevity, durability and exceptional definition to an advanced performance primer and topcoat from PPG Aerospace. The lower density of the fast-drying Desothane APC topcoat also shaves 5% off the weight, compared with its predecessor product. At a time when defence budgets are stretched and scrutinised, fuel consumption strikes another common note with the rest of the aviation world.
In the end, development activity within the aerospace coatings arena is always going to reflect the needs of the day and the concerns of the industry; it may be technology driven but it is also intrinsically responsive.
As a result, predicting the route that research will take in the future – or the new technologies that are likely to emerge – with any degree of certainty is near-impossible. One thing is clear: there are more innovations to come.