Secondary hardness enhancement in large period TiN/TaN superlattices

Monolithic and superlattice films composed of TaN and TiN layers were synthesized using pulsed laser deposition on a variety of substrates, and were characterized using X-ray diffraction, transmission electron microscopy, and nanoindentation. Films deposited on square-symmetry substrate surfaces, e.g., (100)-oriented MgO, TiN, or TaN, were epitaxial (100)-oriented rock-salt (rs-)polymorphs at both 750 °C and 950 °C. On hexagonal-symmetry substrate surfaces, e.g., Al2O3(001), both TiN and TaN were epitaxial (111)-oriented rs-polymorphs at 750 °C, as were TiN films deposited at 950 °C. Similar TaN films deposited at 950 °C, however, were (001)-oriented hexagonal (h-)TaN on Al2O3(001) and > 2.5 nm away from rs-TiN(111) buffer layers, including in superlattices with thick TaN layers. Superlattices (all with a 3:7 TiN:TaN ratio, > 500 nm thick, which took > 13 h to complete) deposited at 750 °C behaved as expected, exhibiting clear superlattice peaks and a maximum hardness when the superlattice period was between 5 and 10 nm. No superlattice peaks were observed in the (100)-oriented isostructural rs-superlattices deposited at 950 °C, indicating that a significant degree of interdiffusion occurred, but the amount was orientation dependent, as (111)-oriented isostructural rs-layers were similar at the two temperatures. Non-isostructural superlattices deposited at 950 °C that had (111)-oriented rs-layers and (001) h-TaN layers also exhibited little interdiffusion and, importantly, had a secondary hardness maximum near a chemical repeat period of 15 nm, rendering it the hardest superlattice for these longer chemical repeat periods. These composite results indicate that multi-functionality could be built into polycrystalline coatings because the phases, chemical stabilities, and hardness properties of TiN/TaN superlattices can be strongly dependent on local orientation and chemical periodicity.