Transition-metal carbides and nitrides are hard materials widely used for cutting tools and wear-resistant coatings. Their hardness is not yet understood at a fundamental level. A clue may lie in the puzzling fact that transition-metal carbonitrides that have the rock-salt structure (such as TiCxN1−x) have the greatest hardness for a valence-electron concentration of about 8.4 per cell1,2,3, which suggests that the hardness may be determined more by the nature of the bonding than by the conventional microstructural features that determine the hardness of structural metals and alloys. To investigate this possibility, we have evaluated the shear modulus of various transition-metal carbides and nitrides using ab initio pseudopotential calculations. Our results show that the behaviour of these materials can be understood on a fundamental level in terms of their electronic band structure. The unusual hardness originates from a particular band of σ bonding states between the non-metal p orbitals and the metal d orbitals that strongly resists shearing strain or shape change. Filling of these states is completed at a valence-electron concentration of about 8.4, and any additional electrons would go into a higher band which is unstable against shear deformations.

Electronic mechanism of hardness enhancement in transition-metal carbonitrides

Diamond-like carbon (DLC) coatings have been widely recognized as being a wear-resistant solid lubricant with a low friction coefficient. Its tribological behavior strongly depends both on the tribotesting conditions and the nature of the coating, which in turn depends on the technique used for film deposition. Recently, there have been several attempts to improve the tribological behavior of DLC coatings by the addition of elements, such as silicon, nitrogen, fluorine and various metals. The paper will present an updated review of the tribological properties of doped DLC, in comparison with the conventional hydrogenated and non-hydrogenated carbonaceous films.

Recent progress on the tribology of doped diamond-like and carbon alloy coatings: a review

Diamond-like carbon (DLC) films combine several excellent properties like high hardness, low friction coefficients and chemical inertness. The DLC coating material can be further classified in two main groups, the hydrogenated amorphous carbon (a-C:H, ta-C:H) and the hydrogen free amorphous carbon (a-C, ta-C). By adding other elements like metals (a-C:H:Me) or non-metal elements like silicon, oxygen, fluorine or others (a-C:H:X), several modifications of the properties can be adjusted according to application requirements. First reports on hard amorphous carbon films were published in the 1950s and about 20 years later there began worldwide intensive research activities on DLC. In the following years the number of publications increased continuously and the importance for industrial applications became more and more evident. Several deposition techniques were applied to prepare a-C:H, ta-C, metal containing a-C:H:Me and non-metal containing a-C:H:X coatings. In parallel the structure and deposition mechanisms of DLC coatings were extensively studied. An essential obstacle for a broad industrial application was the high compressive stress level in a-C:H films causing delamination and limiting the film thicknesses. With metal based intermediate layer systems most adhesion problems could be solved satisfactorily and thus from the mid-1990s the pre-conditions for a broad application especially in the automotive industry were given. With modified a-C:H:X and a-C:X coatings a considerable friction reduction or surface energy adjustments could be achieved.

History of diamond-like carbon films — From first experiments to worldwide applications

In order to test whether the low-energy excitations explored extensively in amorphous solids are indeed universal, all measurements published on the low-temperature thermal conductivity and acoustic attenuation in these solids have been reviewed, on a total of over 60 different compositions. The ratio of the phonon wavelength \ensuremath{\lambda} to the phonon mean free path l has been found to lie between ${10}^{\ensuremath{-}3}$ and ${10}^{\ensuremath{-}2}$ in almost all cases, independent of chemical composition and frequency (wavelength) of the elastic waves, which varied by more than nine orders of magnitude in the different experiments. When the data were fitted with the tunneling model, which is based on the assumption of atomic or molecular tunneling states with a certain spectral distribution, the tunneling strength C, which describes their coupling to the lattice, was found to range from ${10}^{\ensuremath{-}4}$ to ${10}^{\ensuremath{-}3}$ in almost all cases. The only exceptions reported so far are certain films of amorphous silicon, germanium, and carbon. In these films, low-temperature acoustic attenuations over two orders of magnitude smaller have been observed compared to all other amorphous solids. Another remarkable observation is that a large number of disordered crystals, and even a thermally equilibrated quasicrystal, have low-energy lattice vibrations that are quantitatively indistinguishable from those of amorphous solids. Their tunneling strengths also range from ${10}^{\ensuremath{-}4}$ to ${10}^{\ensuremath{-}3}.$ These measurements have also been reviewed. It is concluded that the absence of long-range order is neither sufficient nor necessary for the existence of the low-energy excitations.

Low-temperature thermal conductivity and acoustic attenuation in amorphous solids

Epitaxial TiN/NbN superlattices with wavelengths/ranging from 1.6 to 450 nm have been grown by reactive magnetron sputtering on MgO(100). Cross-sectional transmission electron microscopy (XTEM) studies showed well-defined superlattice layers. Voided low-angle boundaries, aligned perpendicular to the film plane, were also present. High-resolution images showed misfit dislocations for Λ = 9.4 nm, but not Λ = 4.6 nm. Up to ninth-order superlattice reflections were observed in diffraction, indicating that the interfaces were relatively sharp. Analysis of the first-order x-ray superlattice reflection intensities indicated that the composition modulation amplitude increased and the coherency strains decreased for Λ increased from 2 to 10 nm. Vickers microhardness H was found to increase rapidly with increasing Λ, from 1700 kg/mm2 for a TiN–NbN alloy (i.e., Λ = 0) to a maximum of 4900 kg/mm2 at Λ = 4.6 nm. H decreased gradually for further increases in Λ above 4.6 nm, to H = 2500 kg/mm2 at Λ = 450 nm. The hardness results are compared with theories for strengthening of multilayers.

Growth, structure, and microhardness of epitaxial TiN/NbN superlattices

TiNx films of various composition have been prepared by reactive‐ion‐beam sputtering at a deposition temperature of 50 °C. Young’s modulus E and hardness H of these films were measured by a depth‐sensing nanoindentation technique, whereas the shear modulus G was obtained by a measurement of the velocity of the acoustic surface wave by Brillouin light scattering. The study was extended over a wide range of stoichiometries, 0≤x≤0.8. A proportionality between E and H has been observed.

Elastic constants and hardness of ion-beam-sputtered tinx films measured by brillouin scattering and depth-sensing indentation

Metal-containing amorphous hydrogenated carbon (Me–C: H) films were prepared on silicon substrates. Two kinds of metals (Ti, Ta) were incorporated in the process of reactive rf diode— (13.56 MHz) and DC-magnetron sputtering, respectively. Elastic recoil detection (ERD) and Rutherford backscattering (RBS) of MeV He+ ions were used to determine the hydrogen content and mass density of Me–C: H films. The mechanical properties, i.e., microhardness, Young’s modulus, and adhesion, were measured with the help of a nanoindenter and scratch tester. Results show that (1) the mechanical properties of Me–C: H films depend mainly on metal concentrations. At a certain metal concentration, optimal hardness, Young’s modulus, and critical load were obtained; (2) the M–C: H films with an optimal metal concentration possess similar hardness, Young’s modulus, and higher critical load compared with the corresponding values of diamond-like carbon (a–C: H) films, due to the improvement of the toughness of the films by the incorporation of metals. Therefore, Me–C: H films show high promise of being wear-resistant protective coatings.

Characterization of metal-containing amorphous hydrogenated carbon films

A series of a‐Si:H films has been prepared by rf sputtering in a H2‐Ar gas mixture. To obtain films with different hydrogen content the hydrogen portion of the gas mixture was changed from 0% to 20%. The shear modulus μ was then measured by the frequency of the surface phonon (Rayleigh wave). The stiffness S and the ultramicrohardness H were measured by using a nanoindenter. From the shear modulus μ and the stiffness S, the Young’s modulus E and Poisson’s ratio ν were calculated. The intrinsic mechanical stress was measured by the bending‐beam method. With increasing hydrogen content of the films, the decrease of Young’s modulus, microhardness, and internal stress are observed.

Mechanical properties of a‐Si:H films studied by Brillouin scattering and nanoindenter

Tungsten-containing amorphous hydrogenated carbon (W–C:H) films were prepared on silicon substrates by reactive rf sputtering (13.56 MHz). Elastic recoil detection (ERD) and Rutherford backscattering (RBS) of MeV He+ ions have been performed to determine the hydrogen concentration and mass density of the films, respectively. The mechanical properties, i.e., the microhardness, Young’s modulus, and the adhesion on substrates, have been studied by depth-sensing indentation equipment (nanoindenter) and a scratch tester with an acoustic emission (AE) detector, respectively. The electric conductivity of the films was also measured. The results show that these properties depend mainly on the tungsten concentration. X-ray diffraction suggests that the W–C:H films consist of an a–C: H polymeric matrix with WC1−x(β) particles embedded. With increasing tungsten concentration the films change from polymeric a–C:H dominant to crystalline WC1−x(β) dominant W–C:H, resulting in different film properties.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400016927

K. Schmidt

Papers

Elastic constants and hardness of ion-beam-sputtered tinx films measured by brillouin scattering and depth-sensing indentation

Characterization of metal-containing amorphous hydrogenated carbon films

Mechanical properties of a‐Si:H films studied by Brillouin scattering and nanoindenter

The properties of W–C:H films deposited by reactive rf sputtering

Hardness and Young's modulus of diamond-like carbon films prepared by ion beam methods