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 ultrahardness of <≥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.