Dr Susan Krumdieck from the University of Canterbury is helping to develop new materials for possible future hypersonic vehicles that travel 10 times the speed of sound (Mach 10). She is part of collaboration with the US National Hypersonic Science Centre (NHSC) formed by NASA and the US Air Force.
A hypersonic vehicle will be made of woven silicon carbide (SiC) ceramic composites (similar to carbon fibre). At Mach 10, the temperature of the structure will be 2000 °C.
Silicon carbide can withstand very high temperatures, but the SiC composite would be attacked by oxygen under the intensely hot conditions of hypersonic flight, so an oxygen-resistant ceramic coating for the structure is required. Dr Krumdieck’s role in the research collaboration is to develop this protective ceramic coating.
Growing ceramic layers
Ceramic materials are stable compounds of metals and non-metals such as oxygen, nitrogen and carbon. Man-made ceramics are important in everyday life as they can be made with a very hard and dense surface that is easy to clean and doesn’t wear out. Unlike metal, once a ceramic is formed at high temperature, it is very hard to change its shape.
Dr Krumdieck is using a process she invented when she was a PhD student called pulsed pressure chemical vapour deposition (PPCVD) to grow ceramic layers on complex-shaped parts. A coating of less than a millimetre of ceramic can protect a structure from heaterosion and chemical attack.
Dr Krumdieck and her research team begin with a liquid chemical (a precursor) that contains metal oxides bound to hydrocarbon molecules. This precursor is turned into a vapour, as it is injected into a vacuum chamber that has had nearly all of the air removed.
The object to be coated is inside this vacuum chamber and is heated to about 500 °C. When the precursor hits the hot surface, it undergoes a chemical reaction and ‘unzips’, leaving behind the solid metal oxide particles that grow as a ceramic layer on the object.
The remaining hydrocarbon molecules are sucked out of the chamber before more precursor vapour is injected. This repeating cycle is the ‘pulsed pressure’ part of the chemical vapour deposition.
Changing the variables
Several things affect the way that the ceramic crystal grows on the material being coated. Each of these can be carefully changed, one variable at a time, in a series of tests to see what conditions produce the ideal size and shape of the ceramic crystals that form.
Variables that can be changed include:
- the temperature at which the crystals grow
- the rate at which the precursor liquid is injected
- the amount of precursor injected at each pulse
- adding in other materials (such as alumina
- the number of repeated cycles.
Dr Krumdieck describes her work in material science as a lot like a test kitchen. She and her team have to adjust all of the ingredients and cooking processes and see what they get, but they are going to do it in a scientific way. Each variable is changed in small steps. Each resulting sample is then carefully analysed on a scanning electron microscope (SEM) to see if the results are heading in the right direction.
Analysis methods include high-powered microscopes like the scanning electron microscope (SEM) and X-ray diffraction (XRD) to analyse the crystal structures. When the team has some good layers made on NASA’s SiC composite material, the samples will be sent to the University of California at Santa Barbara for high-temperature testing and to Berkeley for micro-cat-scan analysis.
‘What if?’ science
Dr Krumdieck describes this kind of future thinking research as ‘What if?’ science.
‘What if?’ science is about solving challenges for things that don’t exist yet, such as what would we need if we could build a hypersonic vehicle?Dr Susan Krumdieck, University of Canterbury
There are currently no operational hypersonic vehicles that can fly for more than a few seconds, but what if we could build them to be able to cruise for longer periods in the Earth’s atmosphere? Many challenges would need to be solved before that could happen, and there are several groups of scientists and engineers working together to solve each of these challenge