Nanocomposites nc-TiN/a-Si 3 N 4 /a- and nc-TiSi 2 with Hardness Exceeding 100 GPa and High Fracture Toughness
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Mat. Res. Soc. Symp. Proc. Vol. 581 © 2000 Materials Research Society
paramagnetic centers the strength of synthetic diamond crystals increases to a maximum at a concentration of about (2 - 3).10 9 N-Atoms/cm 3 [22]. EXPERIMENTAL Because reliable measurement of the hardness in the range of > 60 GPa is a difficult task [2], we have carefully compared the measurements on our nc-TiN/a- and nc-TiSi 2 coatings with the measurements on various diamond films. In this paper we shall present a comparison with the hardest diamond coatings which we have received from various laboratories, namely with pure, single phase nanocrystalline diamond films [23]. In order to avoid possible artifacts which may be associated with the nanoindentation we deposited 3 to 11 jtm thick films, used relatively large loads and performed at least six to ten indentation measurements at each applied load. The measurements (indentometer Fischerscope 100) were done repetitively on many deposited coatings with the standard calibration (Si, Sapphire and 3C-SiC) and measurements on the nc-diamond in-between the measurements on our coatings. Finally, the size of the remaining "pseudoplastic" indentation was measured in scanning electron microscope (SEM) and the hardness was calculated according to the Vickers formula [24]. Measurements of the Vickers hardness in the range of > 80 GPa are possible because the diamond indentor is loaded mainly in compression whereas the "plastic" deformation of the measured sample is to a large extent in shear. Under compression, diamond is about 8 to 10 times stronger than under shear [6]. The coatings were prepared by plasma chemical vapor deposition (CVD) in abnormal direct current glow discharge with the substrate connected as cathode from TiCI4 , Sill4 in a large excess of H2 and N2 at a temperature of 550 °C [16]. In our earlier work [1,15] we used discharge operating at a high frequency, power density and pressure in order to minimize the energy of ions bombarding the surface of the growing film. These films had the composition of nc-TiN/a-Si 3N 4 [1] and their hardness showed a pronounced increase with decreasing crystallite size (which was controlled by the silicon content [1,25]) in the range of 3-10 nm. The minimum crystallite size and maximum hardness of > 50 GPa was achieved at a silicon content of 7-9 at.% which corresponds to a content of Si3N4 of 16-21 mol.%, close to the percolation threshold [26]. This behavior indicates that the segregated non-polar Si 3N 4 is wetting the surfaces of the TiN nanocrystals which decreases the total Gibbs free energy of the system [4]. In our more recent work we used the abnormal DC discharge [16] in order to increase the ion bombardment of the film during the deposition. The recent studies have shown, that the hardness correlates with the Si3N 4 content in the same way as found in the HF discharge, but not with the crystallite size when this is decreased by the energetic ion bombardment at a relatively large discharge current density of 2.5-3 mA/cm 2 [27]. At such a high
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