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 mum thick) consisting of nanocrysta lline TiN, amorphous Si3N4, and amorphous and nanocrystalline TiSi2, nc-TiN /a-SiNx/a- and nc-TiSi2 were deposited on steel substrates by means of plas ma 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 inde ntation using SEM. The results of a complex analysis provide a consistent p icture 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-Si3N4 forms the grain boundaries and the nanocomposites are superhard (40-50 GPa) as vie r eported earlier. At a lower current density of < 1 mA/cm(2) a mixture of Ti Si2 and Si3N4 is formed. With increasing Si-content the amount of a-TiSi2 i n the grain boundaries of the TiN nanocrystals increases, and above 10 at.% of Si approximately 3 nm small TiSi2 nanocrystals precipitate. The hardnes s depends critically and in a complex way on the Si3N4 content and the TiSi 2/Si3N4 ratio. The ultrahardness((1)) of 80 GPa is achieved when the surfac e of the TiN nanocrystals is covered with approximately one monolayer of Si 3N4. Under these conditions the ultrahardness of 80-100 GPa depends on the amount of a- and nc-TiSi2. (C) 2000 Elsevier Science B.V. All rights reserv ed.