AVS 52nd International Symposium
    Advanced Surface Engineering Monday Sessions
       Session SE-MoM

Invited Paper SE-MoM1
John Thornton Memorial Award Lecture:  Understanding of the Preparation and Properties of Superhard Nanocomposites

Monday, October 31, 2005, 8:20 am, Room 201

Session: Nanocomposites and Coatings with Enhanced Thermal Stability
Presenter: S. Veprek, Technical University Munich, Germany
Correspondent: Click to Email
Since the first report on the strong enhancement of hardness to 60-70 GPa in the Ti-Si-N system by Li Shizhi et al.,@footnote 1@ followed by the publication of the generic design concept,@footnote 2,3@ a large number of papers were published on this and similar systems which have led to some misunderstanding and controversies regarding the suitable deposition techniques and conditions, the maximum achievable value of hardness, the mechanism of the hardness enhancement, thermal stability and others. So far, plasma induced deposition techniques, such as glow discharge CVD, reactive sputtering and vacuum arc evaporation were used to prepare thin films of such materials. In my lecture I shall try to answer some of the open questions. First it will be shown how to differentiate between the superhard nanocomposites and coatings in which the hardness enhancement is due either to energetic ion bombardment during deposition or to solution hardening. The focus will be on the recent results regarding the spinodal nature of the phase segregation in this system, and on the thermodynamic and kinetic conditions needed to complete it during the deposition to obtain superhard nanocomposites with high thermal stability. The high hardness enhancement achieved in these materials is associated with the formation of a nanostructure where few nanometer small crystals of a hard transition metal nitride are "glued" together by about one monolayer of silicon nitride, as reported in our earlier HYPERLINK "mailto:work.@footnote"work.@footnote 1,2@ This finding was recently confirmed by the work of Oden et HYPERLINK "mailto:al.@footnote"al.@footnote 5@ on the preparation of high-quality heterostructures where the highest hardness enhancement was achieved for one monolayer of Si@sub 3@N@sub 4@. The first principle DFT calculations by C. Stampfl et HYPERLINK "mailto:al.@footnote"al.@footnote 6@ lend further support to this concept and show that, as predicted (see references quoted in@footnote 4@), such nanostructure reaches the ideal strength. The unusual combination of a high hardness, high resistance against crack formation and high elastic limit is a simple consequence of the almost flaw-free nature of these nanocomposites@footnote 4@ and of a finite activation volume for the initiation of plastic deformation within an amorphous HYPERLINK "mailto:phase.@footnote"phase.@footnote 7@) In order to correctly describe the mechanical properties of these materials, a new constitutive material's model was elaborated that accounts for the pressure enhancement of elastic moduli and of the flow stress. To be able to quantitatively describe the plastic deformation under conditions of a pressure dependent flow stress, the von Mises yield criterion had to be expressed in terms of a critical deviatoric strain. This model was then implemented into an advanced Finite Element Method code and the behavior of the materials upon indentation was studied in some HYPERLINK "mailto:detail.@footnote"detail.@footnote 8@ An important result of this work is the strong stiffening for both, elastic and plastic deformation due to the increase of the elastic moduli and of the flow stress by the high pressure under the indenter which is not found in conventional materials. The lecture will conclude with a brief overview of the recent industrial applications. @FootnoteText@ @footnote 1@ S. Z. Li, Y. Shi and H. Peng, Plasma Chem. Plasma Process. 12 (1992) 287. @footnote 2@ S. Veprek and S. Reiprich, Thin Solid Films 268 (1995) 64. @footnote 3@ S. Veprek, S. Reiprich and S. Z. Li, Appl. Phys. Lett. 66 (1995) 2640. @footnote 4@ S. Veprek, M. G. J. Veprek-Heijman, P. Karvankova and J. Prochazka, Invited Review, Thin Solid Films 476 (2005) 1. @footnote 5@ M. Oden, invited paper at the 51st Int. Symp. of the American Vacuum Society, Anaheim, November 14 – 19, 2004. @footnote 8@ S. Hao, B. Delley, and C. Stampfl, School of Physics, The University of Sydney, to be published. @footnote 7@ M. J. Demkowicz and A. S. Argon, Phys. Rev. Lett. 93 (2004) 025505-1. @footnote 8@ R. G. Veprek, D. M. Parks, A. S. Argon, and S. Veprek, to be published.