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Since the publication of our generic concept for the design of novel superhard
nanocomposites, the preparation and properties of a number of superhard and functional
nanocomposites of different composition were prepared and investigated. The
nc-(Al1-xTix)N/a-Si3N4
nanocopomosite coatings for wear protection of machining tools for
dry and fast cutting as well as the new coating technology needed for their large scale
industrial production were successfully developed and introduced on the market.
In the first part I shall briefly discuss the different approaches to the
preparation of superhard coatings including the hardening by energetic ion bombardment
during the deposition. This includes a variety of ordinary hard coatings, such as TiN,
(Ti,Al,V)N, HfB2 and also the so called "nanocomposites" consisting of a hard transition
metal nitride and a soft, ductile metal which does not form any stable nitride, e. g.
ZrN/Ni, CrxN/Ni, ZrN/Cu, TiN/Cu. As there is no evidence of a contribution to the
hardening by a nanostructure in these coatings, the stability of their hardness is limited
to 400-600 °C.
The second part deals with the present status of our understanding of the
Formation of thermally very stable (?1100 °C), superhard nanostructures by
self-organization as a result of thermodynamically driven spinodal phase segregation. We
discus the recent progress in the understanding of the extraordinary combination of their
mechanical properties, such as high hardness of 40-100 GPa combined with a high elastic
recovery of 80-95 % and a high resistance against brittle failure by catastrophic crack
initiation and propagation. The tensile strength of the super- and ultrahard
nanocomposites prepared in this way reaches 10-40 GPa approaching the ideal cohesive
strength of strong solids. These properties can be relatively easily understood in terms
of conventional fracture physics scaled down to crystallite size of few nanometers and
accounting for the critical activation volume needed for the initiation of plastic
deformation and structural transitions.
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