The conventional vacuum  arc  coating  technology  yields  Al–Ti– Si–N coatings where the “metallic” silicon is dissolved within the (TiAl)N crystals forming a metastable (Al,Ti,Si)N solution。(11) The  hardness  of these coatings of 34 GPa is somewhat lower than that achieved with the

5

nc-(Al1−x Tix )N/a-Si3N4  nanocomposites of ≥40 Gpa。

Nevertheless, also

these coatings showed a better cutting performance than the conventional

(11)

(Al1−x Tix )N ones when applied to ball-nosed carbide end-mills。

The development of the nc-(Al1−x Tix )N/a-Si3N4 industrial  coatings in  1994  was  motivated  by  the  work  of  Li  Shizhi  et al。(12)   In    parallel

to the development of the materials, also  the  new  coating  technology based on rotating arc cathodes was developed because of the requirement for uniform erosion  of  that  cathode  and  high-density  plasma。  Accord- ing to the generic design  principle(9, 10)  these  conditions  are  needed for the  formation  of  the  nanocomposites  with  a  high  thermal  stability   and

5The value of the hardness reported  in  [11]  might  be  somewhat  underestimating  the real one because of  the  relatively  small  thickness  of  the  coatings  of  about  4  µm。  However, in an earlier patent DE 195 17 119 Al of these researchers similar hardness of about 32 GPa  was  reported  for  10  µm  thick coatings。

also to evaporate pure aluminium that is not possible with conventional cathodes。 This technology was recently included into a new advanced coating system π 80 that utilizes lateral rotating ARC cathodes LARC® technology developed by SHM together with the company PLATIT (Switzerland)。 The asymmetric arrangement of the cathodes with respect to the coated tools provide automatically a nano-layered structure of the coatings and enables a pre-cleaning of the cathodes by means of Virtual Shutter® that significantly improves the performance of the coatings。 This enabled us to significantly improve the cutting performance。 Examples will be presented in order to demonstrate that the nano-layered-nanocomposite nc-(Al1−x Tix )N/a-Si3N4 coatings display excellent cutting performance that is superior to the state-of-the-art (Ti1−x Alx )N   coatings。

2。THE  DEVELOPMENT  OF  THE  COATING  TECHNOLOGY

According to the generic principle for their  design,(9, 10)  the  for- mation of the nanocomposites with a high thermal stability requires a sufficiently high chemical activity of nitrogen in order to assure the seg- regation of stoichiometric phases, such as nc-(Ti1−x Alx )N and a-Si3N4。 This  is  achieved  in  an  intense  plasma  at   a  sufficiently  high     nitrogen

pressure。(9, 10)  A deposition temperature of ≥500◦C is required in order    to

make the thermodynamically driven and diffusion rate controlled    segrega-

tion to proceed fast。 Because such conditions are rather difficult to achieve in conventional PVD coating systems with planar electrodes, SHM devel- oped a new vacuum arc technology based on a central cathode。

Figure 1 shows schematics  of  the  axially  symmetric  central  cath- ode consisting of two independent segments, one made of pure titanium and the other one of AlSi alloy of  the  eutectic  composition  of  11。8 wt (11。3 at%) of Si。 The axially symmetric magnetic field together with the perpendicular electric field at the cathode provides the Lorenzian  force which results in a fast rotational movement of the cathodic spot on the surface around that axis。 By an appropriate change of the axially symmet- ric magnetic field in the axial direction, the arc can be moved from one segment to the other one with a frequency of the order of kHz。 In such a way,  atomically mixed (Ti1−x Alx )N/a-Si3N4  coatings of a chosen  stoichi-