Superhard Materials

Hard materials can be divided into three groups (Holeck, 1986) depending on chemical bonding character: (a) metallic hard materials (borides, carbides, and nitrides of the transition metals, such as TiN, TiC, TiB₂, etc); (b) covalent hard materials (borides, carbides, and nitrides of Al, Si, and B, as well as diamond), and (c) ionic (ceramic) hard materials (oxides of Al, Zr, Ti, and Be).
    Materials and coatings consisting of elements from the B-C-N triangle are among the hardest materials. They include diamond, cubic boron nitride (cBN), C₃N₄ , BC₂N and B₄C.

High Hardness and Elasticity of Nanomaterials

      We demonstrated that superhard nanomaterials deviate significantly from this linear trend and exhibit anomalously high H_o values (see Fig. below). This can be attributed to the nanostructure of nanomaterials. For metals, the hardness increases with decreasing grain size (a) as (Hall-Petch effect). Simple extension of the Hall-Petch effect in the nanoscale range gives extremely high hardness and appears not to be valid for nanostructured materials.

Vickers hardness vs. shear modulus for various hard and superhard materials. Data for BC₂N were taken from Refs. (Tkachev, Solozhenko, Zinin et al., Phys. Rev. B, 68, 052104, 2003) for nc-TiN/-a Si3N4 from Refs. (Manghnani,  Tkachev, Zinin, et al. J. Appl. Phys. 97, 2005), and the rest from Ref. (Teter. Mater. Res.. Soc. Bull. 23, 22-27, 1998). The experimental data in the review paper (Teter, 1998) can be best-fitted by a linear trend:

where H_o is the hardness of the crystalline material. Based on the measured shear G moduli of the c-BC₂N (238 GPa) the hardness H_o from equation should be 39.2 GPa in contrast to the H_o value of 76 GPa reported by Solozhenko. Thus the SBS measurements demonstrate that superhard nanomaterials deviate sufficiently from the linear trend (see Fig.). We attribute the higher hardness values of the nanocomposite films to the nanostructure.

Characterization of Hard and Superhard Films by Surface Acoustic Waves

    The use of hard and superhard coatings is nowadays widely spread in surface engineering. The system keeps its bulk properties while the film ensures protection against the environment and achieves properties unattainable in the substrate alone. Among these layers, one finds hard coatings, which represent a major improvement in the resistance to wear or abrasion of numerous industrial components. Their range of applications is large, extending from intermittent lubricants to more demanding systems such as coatings on cutting, drilling tools and parts of engine. The refractory nature of some of these films is used for high temperature applications, on engine blades for example.
    Elastic properties of the superhard films need to be determined experimentally because they strongly depend on the synthesis conditions, and they can not be derived from theoretical considerations alone. The thickness of such films ranges commonly from a few tens of nanometers up to an order of a micrometer. The nanoindentation technique widely used for mechanical characterization of thin micron films does not allow measurements of the hardness and elastic moduli of the submicron coating and substrate separately. Conventional methods usually employ surface acoustic waves (SAW). The surface wave displacements are concentrated within a wavelength from the surface and can thus probe the samples within a depth inversely proportional to the frequency used. For submicron films, frequencies in the range 1 GHz to 50 GHz are needed. The surface Brillouin scattering technique offers the unique opportunity to cover this range of frequencies.

Although the use of surface waves is well established for measuring the properties of thin layers, they have rarely been used to investigate hard films. To fill the gap in the theory together with my colleagues Dr. Odile Lefevre (O. Lefeuvre. Ph. D. Thesis. 1998. University of Oxford, Prof. Arthur Every, Prof. Andrew Briggs we have developed a theoretical framework for treating surface wave propagation on such films and demonstrated its application to the characterization of thin hard films (Lefeuvre, Zinin, Briggs, Every.  Appl. Phys. Lett. 1998. 72. 856). We are able to distinguish several types of dispersion behaviour, depending on the relative properties of the layer and substrate. When the elastic properties of the layer and the substrate are not very different, the velocity of the pseudo surface wave beyond cut-off increases up to the Rayleigh wave velocity of the layer. When the elastic properties of the two materials are quite dissimilar, the pseudosurface wave tends towards an attenuated interfacial mode. In addition, another mode appears evolving into the Rayleigh wave of the layer material. We show that with this new understanding of surface wave propagation, the characterisation of thin hard films by surface waves is indeed possible.

Left: Two dimensional grey image of the calculated Brillouin spectra of TiN/steel at free TiN surface.

    This approach has been experimentally applied to hard films belonging to the various dispersion types, including a very fast layer on slow substrate: Si₃N₄ on GaAs; cBN on Si, films with similar properties to those of the substrate porous oxide layer on aluminium and intermediate systems (barrier films on aluminium (see review M. G. Beghi, A. G. Every, and P. V. Zinin. "Brillouin Scattering Measurement of SAW Velocities for Determining Near-Surface Elastic Properties", in T. Kundu ed., Ultrasonic Nondestructive Evaluation: Engineering and Biological Material Characterizationn, CRC Press, Boca Raton, chapter 10, 581-651, 2003).

Diamonds from C₆₀

We used Surface Brillouin Scattering (SBS) technique to study the elastic properties of superhard amorphous carbon, synthesized from C₆₀ under high pressure (13 - 13.5 GPa) and temperature (900+100). This is the first reliable determination of bulk and shear elastic moduli for this type of amorphous carbon material. The measured moduli are found to be close to those of diamond in agreement with the general trend for elasticity, hardness, and density among the carbon phases, demonstrating as well the efficiency of high-pressure fullerene technique for synthesis of sp³-based amorphous carbon materials.