Atomic Layer Deposition (ALD) is being used as a means of coating various substrate materials with a variety of metallic and ceramic oxides for corrosion and thermal protection. The technology necessary to develop a state-of-the-art, low-cost method of polishing and coating a one-piece combustion device using electro-polishing (EP) and ALD was demonstrated in this work. By combining material components made using Selective Laser Melting (SLM) with the process of EP and the application of uniform thin-film coatings using ALD, a complete, scalable manufacturing process can be developed by which high-precision, complex components can be produced at a fraction of their current cost. SLM technology has shown the potential to reduce production costs by 70% or more for complex propulsion component fabrication compared to traditional manufacturing techniques.
EP is an extremely simple and mature technology by which some amount of surface can be removed from a material via an electrochemical process. Because of the nature of this process, the roughness of a surface can be greatly reduced. Under certain circumstances, EP can attain the same degree of surface smoothness obtained by other methods such as lapping and super-finishing. EP works in an almost reverse process as compared with electroplating. Rather than adding material to a surface, the material on the surface is oxidized and dissolved in electrolyte.
ALD is considered a modification of a chemical vapor deposition (CVD) method, where special chemical ALD precursors are pulsed separately into a reactor chamber one at a time. In ALD, chemical reactions proceed in a surface-saturative, self-limiting manner. Repeatability, conformity, uniformity, and precise Angstrom thickness controllability of the deposited thin films are all advantageous characteristics of ALD. The ALD technology deposits thin films (approximately 0.1 – 0.2 nm per layer) using pulses of chemical precursor vapors to chemisorb at the target surface one atomic layer at a time, and to be reacted with the second precursor. ALD is based on the sequential deposition of individual monolayers or fractions of a monolayer in a controlled fashion. In ALD, the growth substrate surface is alternately exposed to the vapors of only one of two chemical reactants (complementary chemical ALD precursors). ALD coatings are typically synthesized up to a thickness of about 0.1 micron. Beyond that thickness, the time needed to coat increases significantly, and cost benefits begin to decrease compared to other coating technologies.