Making TBCs that can survive 1800 deg C is only part of the problem. There are other challenges like how low the thermal conductivity is, how resistant is the TBC to ablation etc.
Missiles offer more freedom on the materials side, after all they are meant to be used once. The layer thickness of TBC can be significantly higher in case of scramjet combustors than on TF engine blades & vanes.
Scramjets are also relatively simpler in geometry, so easier to fabricate. The only real challenge in scramjet engines are the fuel injector struts. Those are a pain to design, fabricate, validate.
Been reading about GTRE & NAL's forays into SiC-SiC CMC material development recently. Some papers cite stress testing at ~1420 deg C. From what I understand they now have a reliable way of producing SiC-SiC CMCs that can withstand ~1420 deg C. We can get an additional 240 deg C advantage with active cooling & our current generation YSCZ-LZ TBC on the CMC. Overall, we can get sustained 1900+ K with this.
Of course there are problems. There is little to no domestic manufacturing capacity of SiC fibers. We buy them from Japan. Attempt at lab scale production of high quality SiC fibers have been unsuccessful so far. They have found a way to stabilize the CMC matrix, but not yet a way to shape them. Making SiC-SiC CMC of the correct geometry is still some time away. Machining coolant holes on the CMC has proven notoriously difficult. As with any ceramic material, SiC-SiC CMCs are incredibly tough but also brittle. We don't have the machines needed to drill the right sized holes.
I think material wise we will reach 5th gen levels way before we have a 5th gen TF engine design.
