OXIDATION-RESISTANT COATINGS ON TITANIUM-ALLOYS BY ION-BEAM ASSISTED DEPOSITION

The high strength and creep resistance of modern titanium alloys makes them potentially attractive for aero engine compressor blades operating at temperatures above 500-degrees-C, to improve thermal efficiency and performance. However, all these alloys suffer from oxidation at such elevated temperatures and the in-diffusing oxygen stabilizes the more brittle alpha-phase of the alloy making it prone to crack under stress. There is thus a need for protective highly adherent coatings. In the present work, chromium was deposited with simultaneous ion bombardment using 60 keV nitrogen ions, to a thickness of 1-mu-m. Ion assisted deposition provides excellent adhesion and a very compact, pore-free coating with a small grain size. However, at high temperatures titanium diffuses into the chromium and impairs its protectiveness. To combat this, a diffusion barrier of silicon nitride was deposited first, by evaporating silicon and matching the rate of arrival of Si atoms with a 0.5 keV nitrogen ion beam from a Kaufman source to produce Si3N4 to thickness from 0.1 to 0.3-mu-m. As an added protection, in some experiments an overlay coating of about 0.3-mu-m of silicon nitride was deposited after the chromium layer. Oxidation was carried out in air at temperatures between 600 and 800-degrees-C and for periods up to 100 h. Subsequent analysis by SIMS showed excellent protection up to at least 700-degrees-C but diffusion across the barrier occurred at 800-degrees-C. Titanium nitride was less successful than silicon nitride for barrier purposes.