NASA researchers have developed an innovation to promote the adhesion and improve the durability of high atomic number metal coatings on oxygen- or hydroxyl-rich surfaces. This new approach is useful for building radiation shields via the Z-grading method, the process of layering metal materials with different atomic numbers to provide radiation protection for protons, electrons, and x-rays.
High atomic number materials, such as tantalum, do not bond well to oxygen-and hydroxyl-rich surfaces, such as glass fibers. These metals often form surface oxides when layered on glass fabric, resulting in flaking of the high atomic number material off the fabric during cutting, folding, and/or handling.
To improve coating durability, this technique applies a lower atomic number metal as a tie down layer first before applying the high atomic number metal layer. The tie down layer reduces oxide formation between the substrate and the high atomic number material, promoting adhesion.
Titanium has shown strong adhesion with different metals and is effective at reducing oxide formation when diffusion bonded to itself or other materials. It has been shown to be effective at improving durability when thermally sprayed onto a glass fiber fabric as a tie down layer for a subsequent tantalum layer (also applied via RF plasma spray). The titanium layer is only approximately 1 mil thick but results in strong adhesion of the tantalum layer by inter-metallic or diffusion bonding. A thermal spray process may be used, as well.
This technology enables Z-graded coatings to be applied to lower cost and lighter weight substrates, such as glass fiber fabrics, while maintaining the durability and flexibility needed for spacecraft electronics, radiation protective clothing, and other applications.