Large caliber gun barrels for military tanks have been electroplated with Chromium (Cr) for many years. Sponsored by the U.S. Army a few years ago, SwRI developed a Cylindrical Magnetron Sputtering (CMS) technology and demonstrated that Tantalum (Ta) could be sputter-deposited onto the inner surface of the barrels. Since the CMS process only uses solid Ta, it eliminates the toxic Cr plating waste. The Israel Army has been using the U.S. Army Cr electroplating process. They are very interested in the SwRI CMS process. Based on an earlier target IR project (18-R8993), in which SwRI reestablished the CMS process and demonstrated the Ta sputtering process, SwRI submitted a preliminary proposal to the Israeli Army. Since the Ta material is very expensive, the Israeli Army requested that other materials be looked into. SwRI executed this IR to study a few alternative coatings.
In this short project, the SwRI CMS vacuum deposition system and the cylindrical magnetron established the previous IR were used to deposit several coatings including Zirconium (Zr) and Niobium (Nb) that are believed to have the potential to replace Ta. It is understood that the melting point for Zr and Nb is 2125K and 2740K, respectively. Although it is lower than that of Ta (3287K), it is equivalent to or higher than that of Cr (2130K) that is currently used. But the lower material cost for both Zr and Nb is very attractive. The cost factor for Zr, Nb and Ta is 1X, 1.8X and 5.5X respectively, showing the potential significant savings over Ta. Another commonly used material, Hastelloy C-276 (a Ni-based alloy), was also studied since it is a heat resistant alloy and its cost is even lower than Zr.
In this project, using the CMS technology, thick coatings of C-276, Zr and Nb were deposited on the inner surface of short sections of simulated gun barrel sections. The coated tubes were sectioned and studied using scanning electron microscopy to determine the coating thickness and microstructure. Very thick coatings of 200-250 µm that were needed for the gun barrel protection were achieved. The coating adhesion was also determined by scratch testing. All three coatings exhibited excellent properties in terms of adhesion morphology and density. The coated tubes were sectioned into two halves and one half of each was sent to the Israeli Army for evaluation. A revised proposal was also submitted to them.