On Developing Coatings

On and off, I have been developing coatings for over 30 years. It has been one of my favorite occupations. Coatings are very interesting from a scientific point of view (they have lots of surface area) and from a technical point of view. Coatings are used mostly to protect and preserve products, but they can also be used to impart properties to the base material of the product that the product would not otherwise have, such as color and other optical properties such as emissivity. Properties such as easy release of ice, water replency, and antifouling can also be imparted. In one project, I was assigned the task of developing a non-catalytic coating. I was asked to do this by NASA. They were working on hypersonic vehicle designs.

Hypersonic vehicles go faster than the speed of sound. Much faster, say somewhere between 5 and 25 times the speed of sound. At those speeds, the skin of the hypersonic vehicle gets hot, very hot. How hot, well, that depends on a lot of things such as the shape of the vehicle and properties of the skin of the plane such as emissivity. That's the second time I have mentioned emissivity so I should explain what it is. Emissivity is an optical property of a surface that controls how efficiently the surface radiates heat: the higher the emissivity, the more efficiently the surface radiates heat and, in an environment such as that formed in hypersonic travel, emissivity controls how hot or cool a surface becomes. Emissivity is thus an important property to control, and it can be controlled with coatings. For the environments created by hypersonic travel, though, it is not the only important surface property that needs to be controlled. Catalicity, or catalytic efficiency, is an important surface property that can have dramatic effects on the transfer of heat from the air flowing around the vehicle to the surface of the vehicle.

So why do hypersonic vehicles get hot? It's for the same reasons that meteoroids burn up on entering the earth's atmosphere. The usual explanation given for this is atmospheric friction. This explanation is not quite right. There is some pretty complex physics and chemistry going on. As a meteoroid or hypersonic vehicle travels through the atmosphere, a shock wave forms in front of it. That shock wave contains some very energetic gas, which is one of the reasons that meteors are visible streaks of light in the night sky. The gas in the shock layer has enough energy to make the gas glow and even to melt the surface of the meteoroid. For hypersonic vehicles, melting the surface would seem to be something that we want to avoid. That's not always true. The coatings on the surface of the Apollo space capsules were designed to melt and "ablate" during reentry. Ablation was used to absorb the heat and then evaporate taking the heat away and keeping the space capsule cool. For reusable hypersonic vehicles, ablation is not a great strategy. Something more permanent is needed and you need a different strategy. To find that strategy, looking at the details of what's going on is crucial. I said that some pretty complex physics and chemistry is going on in that shock layer. One thing that is happening is that the gas is dissociating and ionizing. The gas in our atmosphere primarily comprises two gases: nitrogen and oxygen. Other gases are present in very small quantities such as argon, water vapor and carbon dioxide. Nitrogen and oxygen, in the earth's atmosphere, are present as the molecules dinitrogen and dioxygen. In other words, nitrogen is present as a molecule comprising two nitrogen atoms, dinitrogen, and oxygen is present as a molecule comprising two oxygen atoms, dioxygen. This is important because, in the shock wave, the molecules of nitrogen and oxygen can dissociate into their atomic forms. At first, this is a good thing. Dissociation of these molecules into their atomic forms uses up some of the energy in the shock wave so there is less energy available for heating the meteor or hypersonic vehicle. Oxygen tends to dissociate first and then nitrogen at higher energies. So far, so good. But, here's the down side: once you have atomic oxygen and nitrogen present in the gas surrounding the meteoroid or atmospheric vehicle, they are chemically very active. That's where the properties of the surfaces becomes important. If the surface is a good catalyst, and the surface of most meteoroids are, the transfer of heat to the surface is greatly enhanced by the recombination of the atomic oxygen and nitrogen. If recombination can be avoided, you get the upside of dissociation, less sensible heat in the surrounding gas, without the downside of more efficient transfer of heat. Hence, the request by NASA to develop non-catalytic coatings for its hypersonic vehicle designs.

As you can see, developing coatings is a highly interesting field and one that takes you into many areas of science and technology that you would not initially expect. And that's what I love about developing coatings: there's so much to learn.