Novel coating systems produced by the combined cathodic arc/unbalanced magnetron sputtering for environmental protection of titanium alloys

CrN/NbN, TiAlCrYN and TiAlYN/CrN coatings were deposited onto aerospace titanium alloy TIMETAL 834 using combined cathodic arc/unbalanced magnetron sputtering. Specimens were Nb- and Cr-ion etched before coating deposition, respectively. Isothermal and cyclic oxidation tests were performed in air at 750 degreesC for 1000 h. While CrN/NbN coatings as well as metal ion etching alone did not improve oxidation resistance of the substrate alloy, monolithically grown TiAlCrYN and superlattice TiAlYN/CrN coatings demonstrated enhanced oxidation resistance. TiAlCrYN provided improved oxidation resistance compared with commercial gamma-TiAl alloy Ti-48Al-2Cr-2Nb. Careful post-oxidation microstructural investigations using SEM, EDS, Raman spectroscopy. and TEM revealed that the TiAlCrYN and TiAlYN/CrN formed a thin protective alumina scale which was interrupted by titania agglomerations originating from substrate oxidation that dominated oxidation behaviour after extended exposure times. Despite the presence of coating defects leading to substrate attack, these novel coatings are considered as promising candidates for oxidation protection of titanium alloys. (C) 2002 Elsevier Science B.V All rights reserved.