Rod Macleod brought up the subject of the Bloodhound SSC project. The subject was new to me, but there is an excellent entry in Wikipedia about it that explains what the project is about (building a supersonic car) and speaks to why it is being done, how, and what is hoped to be gained.
While thinking about the Bloodhound project, a lot of interesting memories came to mind. In the course of developing coatings, I have had the pleasure of meeting and working with many top-notch scientists and engineers. As a group, they sometimes get the reputation for being dull and rather boring; they’re just not the ones that you expect to be the life of the party. I think that this reputation is unfortunate and undeserved. But, I also think that this reputation is understandable when you consider how focused these people are. Their attention is tightly focused on their work. They don’t just plot-up their data and call it a day. They study their data, memorize it, agonize over it and reflect on it. Obviously, this is not something that everyone does or can do or would choose to do if they could. It takes a certain passion. To an outsider, one who did not understand what they were going through, or one who was not similarly afflicted, their demeanor could come off as distant, detached and disinterested.
Actually, I have found that, as a generality, these people are very approachable, especially after regular office hours when the hubbub of the day’s activities have subsided and the quiet of the evening has set in. Then, in that setting, you can have some very interesting conversations and, if you are so inclined, you can learn some very interesting things. In the space of 40 minutes or maybe a few hours, you can learn more than you could have learned in your whole career. And, it’s not things that you could ever learn from a text book; it’s much better than that. They tell you about their frustrations and the challenges that they are up against. If they have a complaint, their complaint is usually directed toward the basic cussedness of nature. Nature has a way of staying a few steps ahead of us and is always more complex than we had hoped. Text books don’t reflect this and the popular view of science is that science provides all the answers. It doesn’t. The best science can usually do is to provide some of the questions, a few at a time.
That’s one of the things that make the Bloodhound SSC project so interesting. What the scientists and engineers are doing here is that they are putting something to a test. What they are putting to the test is not the Bloodhound craft itself or even their knowledge and understanding of the underlying science and technology. What they are putting to the test is their curiosity and their ability to learn.
So, what does all of this have to do with being a coating developer? Quite a lot, actually. Rod limited his question to the outer surfaces of the Bloodhound craft. On something that complex, the outer surfaces, well – please forgive me, they hardly scratch the surface of the potential for coating applications. Wherever you have a surface, an interface between two media, you have a potential application for coatings, especially if one of the media is abrasive, erosive, corrosive, or chemically or energetically aggressive. The Bloodhound craft is loaded with these, but I think that, for the most part, that these surfaces are well characterized and provided for. Those surfaces really don’t worry me. There is the matter of the bespoke hybrid rocket engine. That’s not something I know much about. A number of my co-workers worked on the NASA shuttle engines right up until their recent retirement. They will know much more than I do.
The thing that captures my attention about this project is the wheels. The wheels are to be made of aluminum. Now, aluminum is inherently a soft, ductile, malleable material with a very low specific gravity and a fairly high specific strength. When we say that a material is soft, we are referring to how it responds to pressing a hard indenter into it. It is a standard test. The way that this test works is that you place a hard indenter on the surface of the material that you would like to test. Then, you apply some pressure. What happens is that the hard indenter sinks into the softer material that you are testing. When you remove that pressure, there is some rebound. The surface of the softer material tends to return back to its original shape. That sounds all well and good, but if you carefully examine the test point under a microscope you will see something very interesting. Provided that you applied enough pressure while making your test, you will find a mark on the surface of the softer material and you can use the size of that mark to determine some rather fundamental properties of the material you are testing. This test is actually a rather common quality control test, and it is used in many industries. (What happens, you ask, if the hardness of the indenter and the material you are testing are equal or very close. Ah, well, you would have to ask. But we don’t have time for that right now.)
The thing is, aluminum is inherently very soft. If you push something into it, it tends to leave a mark. As the Bloodhound craft travels over the 12 mile test track, it is going to encounter a lot of very hard materials, and they will leave a mark. Actually, I don’t think trying to coat the wheels is a good idea. No coating would adhere or last at those speeds. No. The thing that I think should be done is to coat the test track to hold down the dust and to ensure that the hardness of the material that the wheels roll over is less than the hardness of the aluminum wheels.
Thursday, September 19, 2013
Wednesday, September 11, 2013
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.
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.
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