New kind of coating could revolutionise hypersonic aerospace vehicles

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 by air and gas in the atmosphere is extremely hot and can have a serious impact on an aircraft or projectile’s structural integrity. That is because he temperatures hitting the aircraft can reach anywhere from 2,000 to 3,000°C.

Hot air and gas remove surface layers

The structural problems are primarily caused by processes called oxidation and ablation. This is 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.

Resisting temperatures up to 3,000°C

But, at present, even conventional UHTCs can’t currently satisfy the associated ablation requirements of travelling at such extreme speeds and temperatures. However, the researchers at The University of Manchester’s and the Royce Institute, in collaboration with the Central South University of China, have designed and fabricated a new carbide coating that is vastly superior in resisting temperatures up to 3,000°C, when compared to existing UHTCs.

From London to New York in two hours

Professor Philip Withers, Regius Professor from The University of Manchester, says: “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. 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.”

12 times better than conventional UHTC

So far, the carbide coating 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 and features manufactured at the Powder Metallurgy Institute, Central South University and studied in University of Manchester, School of Materials. This includes extremely good heat resistance and massively improved oxidation resistance.

Process called reactive melt infiltration

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, Professor of Materials Science, explains: “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.”