Fig。 6。 Transmission electron micrograph of a nc-(Al1−x Tix )N/a-Si3N4 nanocomposite showing the equiaxial nanocrystals of an almost uniform size of 3–4 nm。

Only the Bragg reflections of the face-centred cubic lattice of the TiN are observed, the hexagonal wu¨ rtzite reflections of h-AIN are com- pletely absent (see Fig。 7)。 Thus, the (Al1−x Tix )N nanocrystals form the hard metastable solid solution keeping the crystal structure of TiN。 This is also supported by the selected area electron diffraction that shows only the maxima from fcc TiN (not shown here)。 Because the XPS shows the binding energy of Si 2p corresponding to Si3N4, i。e。 Si bonded to four N-atoms, it is clear from these results that the (Al1−x Tix )N nanocrystals are separated by a thin Si3N4      tissue。

Fig。 7。 Glancing incidence X-ray diffraction pattern of the nc-(Al1−x Tix )N/a-Si3N4 coating。 (Incidence angle 1。5◦) after the annealing to 1200 ◦C。 Notice the absence of Bragg reflections from h-AIN。

The importance of the silicon segregated out of the (Al1−x Tix )N nanocrystals is the formation of a thin amorphous Si3N4 tissue that sepa- rates those nanocrystals。 This is illustrated by Fig。 8 that shows the depen- dence of the hardness of nc-TiN/a-Si3N4 nanocomposite coatings on the coverage of the TiN nanocrystals with Si3N4 In order to estimate the cov- erage, the specific interface area was determined from the crystallite size assuming—in agreement with studies by means of high resolution trans- mission electron microscopy(20)—a regular shape of the nanocrystals (see also Fig。 6)。 The volume fraction of Si3N4was determined by  a combi- nation of elastic recoil detection spectroscopy, ERD, and X-ray photo- electron spectroscopy。 It is seen from Fig。 8, that the maximum hardness is achieved at about one monolayer coverage。 The Si3N4 acts as “glue” between the TiN nanocrystals decreasing the interface grain boundary energy and thus stabilizing it  against  the  grain  boundary  sliding。 With- out this stabilization, softening is observed at a crystallite size decreasing below 10–20 nm (“reverse Hall-Petch”)。(21, 22) At a larger coverage, when the interfacial Si3N4 matrix becomes thicker, the incoherency strain desta- bilizes the nanostructure and the hardness decreases     again。

Fig。 8。 Dependence of the hardness of the nc-TiN/a-Si3N4 coatings on the coverage of the surface of the TiN nanocrystals with    Si3N4。

Similar results were found in all other nanocomposites, such as nc-W2N/a-Si3N4, nc-VN/a-Si3N4, nc-TiN/a-Si3N4/a- and nc-TiSi2, nc-TiN/a-BN  and  nc-TiN/a-BN/a-TiB2,  which  were  investigated  in detail

so  far。(15, 23)   In  the  case  of  the nc-(Al1

x Ti  )N/a-Si N

produced  in an

− x 3    4

industrial  unit  such  systematic  studies  could  not  be  done  because      of

the high costs associated with the fabrication of Al/Si cathodes  of  dif- ferent Si-content。 However, because the standard composition of about (Al0。65Ti0。35)N/a-Si3N4  that  is   used,   the   surface   coverage   of the (Al1−x Tix )N nanocrystals is close to the optimum。 Thus the conclusions obtained with TiN/Si3N4 and other similar systems apply also to the pres- ent coatings。

3。3。Morphology and Surface Roughness

Already in our first  papers  we  have  shown  that  upon  the  forma- tion of the nanocomposite during their deposition by plasma CVD, the columnar structure, which is typical of refractory transition metal nitrides deposited at relatively  low  temperature,  vanished  and  perfectly  isotro- pic nanostructure was formed when the optimum composition and max- imum hardness was achieved。(10, 24) Recently the same was demonstrated for  PVD nc-TiN/a-Si3N4  nanocomposites deposited by reactive   magnetron