Composition, nanostructure and origin of the ultrahardness in nc-TiN/a-Si₃N₄/a- and nc-TiSi₂ nanocomposites with H_v = 80 to ≥105 GPa

Multiphase nanocomposite coatings (3-20 μm thick) consisting of nanocrystalline TiN, amorphous Si₃N₄, and amorphous and nanocrystalline TiSi₂, nc-TiN/a-SiN_x/a- and nc-TiSi₂ were deposited on steel substrates by means of plasma CVD. The load-independent Vickers microhardness from 80 to >105 GPa was measured by the load-depth sensing technique for applied loads between 30 and 200 mN and verified by measuring the size of the remaining plastic indentation using SEM. The results of a complex analysis provide a consistent picture of the nature of the grain boundaries which determines the hardness in the whole range of silicon content between approximately 3 and 22 at.%. At a high discharge current density of ≥2.5 mA/cm² the a-Si₃N₄ forms the grain boundaries and the nanocomposites are superhard (40-50 GPa) as we reported earlier. At a lower current density of ≤1 mA/cm² a mixture of TiSi₂ and Si₃N₄ is formed. With increasing Si-content the amount of a-TiSi₂ in the grain boundaries of the TiN nanocrystals increases, and above 10 at.% of Si approximately 3 nm small TiSi₂ nanocrystals precipitate. The hardness depends critically and in a complex way on the Si₃N₄ content and the TiSi₂/Si₃N₄ ratio. The ultrahardness of <&ge;80 GPa is achieved when the surface of the TiN nanocrystals is covered with approximately one monolayer of Si₃N₄. Under these conditions the ultrahardness of 80-100 GPa depends on the amount of a- and nc-TiSi₂.