Hypersonic aircraft move closer to reality with ‘revolutionary’ ceramic coating

A new kind of ceramic coating has been developed which is touted as having the ability to revolutionise hypersonic travel for the air, space and defence industries.

Hypersonic travel means moving at Mach five or above, which is at least five times faster than the speed of sound.

When moving at such velocity, the heat generated can have a serious impact on an aircraft or projectile’s structural integrity. That is because the aircraft is typically subjected to temperatures up to 3,000°C.

The structural problems are primarily caused by processes called oxidation and ablation. This is the when extremely hot air and gas remove surface layers from the metallic materials of the aircraft or object travelling at such high speeds.

To combat the problem materials called ultra-high temperature ceramics (UHTCs) are needed in aero-engines and hypersonic vehicles such as rockets, re-entry spacecraft and defence projectiles.

But, at present, even conventional UHTCs can’t satisfy the associated ablation requirements of travelling at such extreme speeds and temperatures.

NASA developed a prototype aircraft dubbed the X-43 (pictured above), which was flight tested in 2004 and was designed to test various aspects of hyper-sonic flight.

The plane set several airspeed records for jet-propelled aircraft and is currently the fastest aircraft on record reaching approximately Mach 9.6 (11,850 km/h).

However, the aircraft was never designed to be reused and the prototypes were destroyed during testing, ultimately incapable of withstanding the immense heat generated during the flight.

Now, researchers at The University of Manchester and the Royce Institute, in collaboration with the Central South University of China, have designed and fabricated a carbide coating that is vastly superior in resisting temperatures up to 3,000°C, when compared to existing UHTCs.

“Future hypersonic aerospace vehicles offer the potential of a step jump in transit speeds. A hypersonic plane could fly from London to New York in just two hours and would revolutionise both commercial and commuter travel,” said professor Philip Withers from the University of Manchester.

“But at present one of the biggest challenges is how to protect critical components such as leading edges, combustors and nose tips so that they survive the severe oxidation and extreme scouring of heat fluxes at such temperatures cause to excess during flight.”

Future hypersonic aerospace vehicles offer the potential of a step jump in transit speeds. A hypersonic plane could fly from London to New York in just two hours and would revolutionise both commercial and commuter travel.

So far, the carbide coating developed by teams in both University of Manchester and Central South University is proving to be 12 times better than the conventional UHTC, zirconium carbide (ZrC). ZrC is an extremely hard refractory ceramic material commercially used in tool bits for cutting tools.

The much improved performance of the coating is due to its unique structural make-up that allows it to withstand incredibly high temperatures while combating oxidation.

What makes this coating unique is it has been made using a process called reactive melt infiltration (RMI), which dramatically reduces the time needed to make such materials, and has been in reinforced with carbon–carbon composite (C/C composite). This makes it not only strong but extremely resistant to the usual surface degradation.

Professor Ping Xiao who led the study said: “Current candidate UHTCs for use in extreme environments are limited and it is worthwhile exploring the potential of new single-phase ceramics in terms of reduced evaporation and better oxidation resistance. In addition, it has been shown that introducing such ceramics into carbon fibre- reinforced carbon matrix composites may be an effective way of improving thermal-shock resistance.”

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