GE definitely seems to be leading the way right now with adoption of CMC components on commercial engines. But other companies like P&W have been evaluating the material for use on military engines for over a decade.
The
NASA GRC paper I linked was published in January 2013, so I'm sure the information is mostly up to date. The paper states that the
CMC material itself provides an increase in operating temperature of 200-300degF over current superalloy materials and permits a reduction in cooling airflow, but does not totally eliminate the need for cooling airflow. Here's a quote from page 2 of the paper:
"A. CMC Combustor
As a combustor liner material, SiC/SiC composites are an enabling material that can help meet the NOx reduction goals of ERA. Current superalloys require high cooling air flows to keep them below their maximum allowable operating temperatures (up to about 80% of their melting temperature). CMC materials offer operating temperatures that are 200º-300ºF higher than for superalloys. The higher temperature capability and less component cooling requirements allow for a wider combustor design space so that it can be run more efficiently. Less cooling flow to the component allows for more air to be put into the combustion process."
Here's an article from March of this year discussing GE's testing of CMC components on their GE9X rig. The first build, which began ground test in late January, uses CMC for combustor liner panels, HPT stage 1 nozzle shrouds, and HPT stage 2 nozzles. The second build of the test rig will have CMC HPT stage 1 nozzles, but since GE plans to lock down their design configuration by the middle of this year I doubt the CMC HPT stage 1 or stage 2 nozzles will make it into the first production model of the engine for the 777X. The commercial aircraft industry is very conservative when it comes to introducing new technologies like CMC materials in very demanding applications, and they have very legitimate reasons for being this way. If a CMC component were to experience unforeseen problems in service, the cost to the OEM for replacement and compensation paid to the operator for lost revenue could be devastating.
Lastly, about mid way through the article linked there is this quote about cooling air requirements for CMCs versus superalloys:
"More durable than metal, CMC components contain one-third the density of typical metallic parts, making them lighter and longer-lasting. Also more heat-resistant than metals, the material requires 20 percent less cooling air, which improves overall engine efficiency."
Interesting topic of discussion.
