DEBOND COATING REQUIREMENTS FOR BRITTLE-MATRIX COMPOSITES

The cause of improved fracture toughness in Y2O3-coated niobium-toughened TiAl relative to either uncoated niobium or Al2O3-coated niobium was examined. Reactively sputtered Y2O3 coatings, 1-2 mum thick, were deposited on to rock salt (NaCl), polished single-crystal (0001) Al2O3, and polished polycrystalline niobium. Sputtered niobium coatings, 1-2 mum thick, were also deposited on to polished single-crystal Y2O3 substrates for comparison. The oxide coating was characterized and consisted of stoichiometric bcc Y2O3 with a(o) = 1.0602 nm. Indentation tests were performed to correlate the fracture toughness and debond characteristics of as-deposited Y2O3 Coatings on Al2O3 and polycrystalline niobium, and niobium coatings on single-crystal Y2O3, to that found in TiAl/Nb and Al2O3/Al2O3 laminates. The calculated fracture toughness of sputtered Y2O3 on sapphire was similar to reported values for bulk Y2O3. However, a wide variation in interfacial fracture toughness was obtained by indentation methods, and is attributed to the microstructure of as-deposited coatings and to weak bonding between as-deposited yttria and the sapphire substrate. These results are related to factors that affect debonding and fracture toughness of brittle matrix composites. Reactive and non-reactive metal/ceramic systems were reviewed in an effort to understand why Y2O3 coatings perform well. It is postulated that yttrium oxide coatings applied to niobium have an atomically sharp interface that has a lower fracture energy compared to Nb/Al2O3, resulting in improved interfacial debonding and composite fracture toughness.