CFM Leap

Lightweight, high-temperature ceramic matrix composites will play a key role in the long-term evolution of CFM's Leap-X turbofan engine, ultimately yielding as much as a 1.5% decrease in fuel burn when all planned upgrades are made to the engine hot section.

Upon entry into service (EIS) with the Airbus A320neo in 2016, the CFM Leap-1A will use CMCs for the high pressure turbine (HPT) first stage shroud components, parts that would otherwise be made of nickel super alloy. The Leap-1B for the Boeing 737max and Leap-1C for the Comac C919 will also use CMCs for the shroud.

Sanjay Correa, GE Aviation's CMC programme vice president, says the shroud weight using CMCs is about 1kg (2.2lb), one-third the weight of an equivalent nickel super alloy shroud. GE, a 50/50 partner with Snecma in CFM, builds the engine hot section, or core, for all CFM engines.

Over time, Correa says GE plans to switch out additional nickel components in the hot section of the engine with CMCs, allowing for higher temperature operation with less required cooling air from the compressor. As a result of the lower weight and higher efficiency, the engine will see a reduction in fuel burn of as much as 1.5% due to CMCs alone.

"If I don't give away my precious compressor cooling air, I can use it in the flow path," says Correa. "This brings huge advantages to designers."

Correa says one point of fuel burn equates to $700,000 per aircraft over 10 years at today's fuel prices.

The technology has been in development at GE's Global Research Center in Niskayuna, NY, since 1990, with parts initially tested in the company's ground-based gas turbines for the power industry.

Parts start with 20 micron fibres of silicon carbide (1/5 the diameter of a human hair), produced by Nippon Carbon in Japan through a joint venture with GE and Snecma. The fibres are sent to a facility in Delaware where they are treated with 1 micron coating "to give them the right properties", says Correa. The fibres are made into ply layups that are then cooked at temperatures of 1,400-1,600C for 24h to create the parts.

Out of the oven, the parts are cut to their final shapes and receive dimensional inspections and non-destructive verification testing, including infrared flash photography to determine if there are any inconsistencies in the porosity of the part that may indicate a flaw. Parts are then sprayed with an environmental barrier coating before being shipped to the engine assembly plant.

Correa says GE is approaching 1 million hours of cyclic and endurance testing on the shroud components at "sites all over the US".

Beyond the shroud, GE is investigating a series of other hot section applications for CMCs to offer more fuel burn reductions.

"You always look for where you get the most benefit," says Correa. "Typically it's in the hot gas path, as close as possible to the HPT." He says nozzles and blades are possible candidates. "Stationary parts are easier than rotating parts, but with good engineering and good manufacturing, you can take parts into [the rotating] space."

Following the Farnborough air show this week, CFM is reporting a total of 3,752 orders for the new engine, with 922 orders valued at $12.6 billion received in recent weeks