Composition, nanostructure and origin of the ultrahardness in nc-TiN/a-Si 3N 4/a- and nc-TiSi 2 nanocomposites with HV=80 to ≥105 GPa

Abstract

Multiphase nanocomposite coatings (3–20 μm thick) consisting of nanocrystalline TiN, amorphous Si 3N 4, and amorphous and nanocrystalline TiSi 2, nc-TiN/a-SiN x /a- and nc-TiSi 2 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/cm 2 the a-Si 3N 4 forms the grain boundaries and the nanocomposites are superhard (40–50 GPa) as we reported earlier. At a lower current density of ≤1 mA/cm 2 a mixture of TiSi 2 and Si 3N 4 is formed. With increasing Si-content the amount of a-TiSi 2 in the grain boundaries of the TiN nanocrystals increases, and above 10 at.% of Si approximately 3 nm small TiSi 2 nanocrystals precipitate. The hardness depends critically and in a complex way on the Si 3N 4 content and the TiSi 2/Si 3N 4 ratio. The ultrahardness 1 In 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. 1 of ≥80 GPa is achieved when the surface of the TiN nanocrystals is covered with approximately one monolayer of Si 3N 4. Under these conditions the ultrahardness of 80–100 GPa depends on the amount of a- and nc-TiSi 2.