ALD Coating of Selective Laser Melted 3-D Printed Stainless Steels

Abstract

Selective laser melting (SLM), also known as 3-D printing or additive manufacturing process, has been developed for over 2 decades. This technique exhibits the capability to fabricate three-dimensional metal objects of complex geometries & structures from metal powders used in aerospace or medical orthopedics. This additive manufacturing technology has received a lot of attention, because various metals and metal alloys can now be processed to manufacture metal parts or structures, which includes the technologically important metals of stainless steel, cobalt chrome, titanium, tungsten and various other alloys. Typically SLM selectively melts thin layers of fine metal powder above each other by using a focused laser beam with high thermal energy on a substrate plate under argon atmosphere, resulting in production of almost full density metal or alloy parts. In this fashion successive layers of material are formed according to the computer edited 3-D-CAD design. Thereby, SLM could be applied to make functional end-use and geometrically complex products with cost-effect process especially in industry. Due to the geometrically complex form of the SLM manufactured structures, atomic layer deposition (ALD) is uniquely suitable to control the composition, thickness and conformability of thin films in the large scale and complex structures. In this study we report ALD coatings of Al2O3 of SLM stainless steel with the objective of providing corrosion protection by an alumina diffusion barrier and ALD ZrO2 coatings. During the course of this work we found the parts fabricated by 3-D printers tend to be porous and exhibit a rough texture. Figures 1 (a) show FE-SEM micrographs at progressively larger magnification of the surface of the 3-D printed stainless steel samples. As can be seen, the surface is very rough and contain many beads or spheres produced during the SLM manufacturing process, which is a direct result of the laser melting of the metal powder and rapid quenching. In order to reduce the surface roughness of the 3-D printed samples, electropolishing was used to smoothen and streamline the rough surface of the 3-D samples. After electropolishing, the surface of the 3-D printed samples is bright, clean and microscopically smooth and featureless. There are no beads or spheres remaining on the samples as shown in Figures 1 (b). We used SLM 316 stainless steel samples in a cross-flow ALD reactor with TMA and H2O vapor to attempt ALD coatings, while TDMA-Zr and H2O were used as precursors for ALD ZrO2. The analysis of the ALD Al2O3 and ZrO2 films revealed satisfactory chemisorption on the 3-D printed stainless steel surface. The ALD Al2O3 and ZrO2 films were uniformly grown on the 3-D printed stainless steel surface as shown in Figure 2. Figure 1