Composition, nanostructure and origin of the ultrahardness in nc-TiN/a-Si3N4/a- and nc-TiSi2 nanocomposites with HV=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-SiNx/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 ultrahardness1In order to differentiate we suggest to use the term ‘superhard’ as usual for coatings with hardness of ≥40 GPa and the term ‘ultrahard’ for those with hardness exceeding 80 GPa, i.e. in the range of diamond or more. 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.