Coating Thickness Key to Space Use
In Huntsville, AL, a few miles from NASA’s Marshall Space Flight Center, sits a 22,000-square-foot facility that houses AZ Technology Inc. One of the specialties at AZ Technology is the precise application of coatings designed to meet the harsh demands of space environments, including those that will be faced by the International Space Station Project.
One such concern is the use of coatings for thermal control. Electronic parts, whether in the Space Station’s low-earth orbit or a living room, cause heat buildup. Heat can cause parts to malfunction.
Coatings in space
AZ Technology has a long history in creating and applying materials and coatings for the space environment. In environmentally controlled laboratories, more than 20 coatings are produced which adhere to aerospace standards, are low-outgassing and space stable, and are designed to meet toxicity and flammability requirements. Coatings that have been qualified or tested by NASA for in-flight applications are subjected to the extreme temperatures of space.
“Primarily, the way to control temperature in orbit is by radiating the heat into space,” says Jim Zwiener, a material scientist at AZ Technology. “In space, there is no convection, no conduction, no air or water, so the coatings dissipate the heat into the cold of space.”
To run cool, Zwiener adds, a white paint is used to reflect light and absorb 10% to 15% of the sun’s visible energy. “In the infrared, the white paint is actually black and radiates about 90% of what a black body would,” he says. “In this way, it can run cool.”
Zwiener is not talking about the same paint that is applied to the side of a house. “It’s sprayed on using a paint spray system,” he says. “But it’s modified, because many of the coatings we use are ceramic. It’s like a concrete.”
One of the most often used coatings has a pigment of zinc oxide, with a potassium silicate binder. This coating has proven to be stable in a space environment for up to 20 years. But, thickness is critical.
“After we get a coating a few mils thick, it’s opaque and the optical properties are pretty well fixed,” Swiener says. “You just don’t want to get it too thick; if it’s too thick, then it’ll crack.”
The thickness of the coating varies depending on the coating used, says Steve Jones, a senior technician in the AZ Technology materials department. “White coatings are anywhere from a minimum of five mils up to 12 mils, and have to be applied within a thickness range of about 4 to 6.5 mils throughout the part,” he says.
AZ Technology measures coating thickness on both ferrous and nonferrous metals, as well as coatings on nonmetallic surfaces with a benchtop nondestructive testing (NDT) instrument. The coating thickness-measuring device allows AZ Technology to combine and configure different test methods to suit virtually any of its coating and substrate combinations. Using different types of probes, the company performs nondestructive measurements using the magnetic induction, eddy current, beta backscatter or a combination of these test methods.
For ferrous and nonferrous material, the measurement probe is placed on the test specimen. The probe generates a magnetic field and detects resulting eddy currents or changes in the magnetic field, which are related to the thickness of the coating, he says. The probe supplies a signal, proportional to the coating thickness, to the NDT instrument. Calibration using Mylar foils, traceable to NIST, take the
application-specific measurement conditions into account.
Measuring nonmetallic parts
AZ Technology can measure the thickness of coatings that were applied to nonmetallic surfaces. The company uses the beta backscatter method to measure these surfaces. With the beta backscatter technique, beta particles are directed from an isotope onto the test specimen. Some of the beta particles are “backscattered” and then counted by a Geiger M?ller tube. Because the substrate and base materials have different densities, they have different backscatter rates. The resultant backscatter rate is proportional to the coating thickness.
The ability to measure coatings on nonmetallic parts is particularly useful to AZ Technology. “We do some work with beta cloth, which is a
Teflon-coated glass fiber cloth, and we’re working with more composites all the time. For instance, we recently worked with an antennae housing that’s made of a cyanate ester composite honeycomb,” says Jones.
Recording data is a requirement for AZ Technology. “We log everything so we can enter it into the final report that goes back to the customer,” says Jones. Ultimately, the end customer is NASA. “We may get parts from plating companies, for instance. But they’re working for Boeing, Lockheed or somebody else. We’ll report the data to them, which they’ll in turn report to Boeing, which will report it all to NASA.”
In the past, AZ Technology could not directly verify the coating thickness on a thin substrate material, which is of particular importance to the coating of “targets” that are used on the Space Station.
“By watching any video of the docking, or mating, of the shuttle to the Space Station, it looks like it has measles; it has all these black and white dots all over it. Those are the targets, and they range in size, anywhere from 26 inches down to about 3 inches,” says Jones.
The targets are a coated foil made by AZ Technology for the Canadian Space Agency, as part of its contribution to the International Space Station. The targets are used as a critical component of the Advanced Space Vision System, which helps to orient the Space Station’s modules and their positioning relative to other modules. By use of carefully positioned television cameras, the targets and some complex computer software, the system can determine an object’s exact location and orientation.
“And this coating’s thickness is critical,” Jones says. “Too thick and it might crack up and leave a lot of debris. If it’s too thin, it’s not going to give you the reflectance you need.”
Sidebar: Tech Tips
1. Temperature in orbit is controlled by radiating the heat into space. Coatings help dissipate the heat into the cold of space.
2. To cool the parts, a white paint is used because it reflects a lot of light, and absorbs about 10 to 15% of the sun’s visible energy.
3. Coatings are sprayed on and the coating thickness can vary. If the thickness of the coating goes beyond tolerance, then it is susceptible to cracking.
4. The benchtop instrument that is used can do a number of NDT tests including eddy current, backscatter, magnetic induction or a combination of methods.