S. N. Dub

Mechanical properties of cubic BC2N, a new superhard phase

Abstract A new superhard phase, cubic BC 2 N, has very recently been synthesized by direct conversion of graphite-like BN–C solid solutions at 25 GPa and 2100 K. The hardness, Youngs modulus, fracture toughness and structure of this phase have been examined using micro- and nanoindentation and transmission electron microscopy. The hardness and elastic modulus values ( E , G ) of the c-BC 2 N are intermediate between diamond and cubic boron nitride, which makes this new phase the hardest known solid after diamond.

Diamond-like carbon films in multilayered interference coatings for IR optical elements

Abstract In the majority of modern IR interference multilayer coatings (MLC), conventional film-forming materials (FFM) of fluoride and chalcogenide types are used. Such coatings are characterized by relatively low mechanical strength and stability against enhanced humidity and, therefore, require surface protection. Our present results support the view that mechanical strength of these MLCs can be improved by applying a diamond-like carbon (DLC) film as an external layer. Nanoindentation measurements show that the addition of a DLC film to ZnSe/BaF 2 /Y 2 O 3 IR antireflection MLC increases the combined hardness of the coatings from 0.5 to 3.6 GPa. The formation of an indent on the upper and subsequent layers of MLC has been studied by SEM and X-ray spectrum microanalysis. The resistance of DLC films applied onto MLC against light irradiation, organic solvents as well as against environmental factors was also studied. Atomic force microscopy (AFM) was used to study variations of the surface morphology of the initial MLC components before and after DLC film deposition.

Hardness and fracture toughness of CVD diamond film

Abstract The hardness and fracture toughness of a chemically vapor deposited (CVD) diamond film were measured by the Vickers indentation method. A free-standing 90 μm thick diamond film with a grain size of approximately 25 μm was tested. For comparison, the (011) plane of a natural type Ia diamond single crystal was also tested. The indentation testing of the diamond crystal and CVD diamond film was performed using a microhardness tester under a load of 5 N. The indent size was used to measure the hardness. From the length of radial cracks emanating from the corners of the indent, the fracture toughness was evaluated. To reveal the indents and radial cracks, the samples were etched in KNO 3 melt at about 600 °C. The hardness of natural diamond varied from 64 to 67 GPa. In natural diamond, long radial cracks emanate from the indent corners. The length and direction of propagation of these cracks depend on the orientation of the indent. After prolonged etching we observed the emergence of internal cracks under the indent. There was evidence of stress-corrosion cracking owing to a high level of residual stresses around the indents. The hardness value of 75 GPa was obtained for CVD diamond film. In contrast to natural diamond, the CVD diamond film, which we tested does not show as a rule any radial cracks emanating from the corners of the Vickers indent (at 5 N load). This is likely to be due to a high level of internal compression stresses in the diamond film. After etching, stress-corrosion cracking around indents in CVD diamond was observed. Stress-corrosion cracking at fine grain groups, which was not associated with the hardness indents, was also observed. This fact supports the presence of high internal stresses in CVD diamond film.

The transition from elastic to plastic behaviour in an Al-Cu-Fe quasicrystal studied by cyclic nanoindentation

Abstract Comparative mechanical tests of an Al-Cu-Fe quasicrystal and a tungsten (001) single crystal have been carried out using cyclic nanoindentation by a Berkovich indenter. The first loading-unloading cycle up to 30 mN formed the initial indent. On reloading of the indenter up to 50 mN, a transition from elastic to plastic deformation of the material was observed. To reveal the region of the transition better, the reloading curve was differentiated with respect to time. It was found that, for tungsten, the transition from elastic to plastic behaviour was smooth. The transition began at 25 mN and terminated at 34mN only. During the transition the strain rate in the indent trebled. In the Al-Cu-Fe quasicrystal, the transition from elastic to plastic deformation was very abrupt. No evidence of plastic deformation in the indent was observed up to 32 mN. Only a further increase in the load by 0.35mN caused the onset of plastic deformation. A pop-in was observed in the reloading curve at this point. The displacement increased by about 10 nm and the pressure in the indent dropped by 360 MPa. This was probably due to the destruction of the quasicrystalline structure and the formation of the crystalline structure. A plastic crystalline phase appeared to be pressed between the indenter and the hard quasicrystal substrate and then to be extruded from the indent. Because of this, the pressure in the quasicrystal drops during the transition from elastic to plastic deformation.

High-pressure synthesis of a bulk superconductive MgB2-based material

Abstract The addition of Ta (2–10 wt.%) to a starting mixture of Mg and B (taken in the MgB 2 stoichiometry) and application of high pressure (2 GPa) during the synthesis process (800–900 °C for 1 h) allow us to produce bulk MgB 2 -based materials with the critical current densities ( j c ) of: 630 kA/cm 2 at 10 K, 425 kA/cm 2 at 20 K, 165 kA/cm 2 at 30 K in the 0 T field; 570 kA/cm 2 at 10 K, 350 kA/cm 2 at 20 K and 40 kA/cm 2 at 30 K in the 1 T field and 650 A/cm 2 at 10 K in the 10 T field. X-ray and SEM studies have shown that Ta did not react with B or Mg, but absorbed the impurity gases to form Ta 2 H, TaH, TaN 0.1 , etc. The samples with highest superconductive characteristics exhibited a reduced amount or absence of MgH 2 in the Mg–B–O-matrix phase, as well as, the impurity nitrogen and oxygen in MgB 2 single crystals distributed over the matrix. Samples with a higher level of critical currents included some amount of unreacted Mg. The Vickers microhardness of the matrix material was H v =12.54±0.86 GPa (at 0.496-N load). The nanohardness (at 60 mN load) of MgB 2 single crystals located in the matrix was 35.6±0.9 GPa, i.e. higher than the nanohardness of sapphire (31.1±2.0 GPa), and that means that MgB 2 belongs to superhard materials.

Features of the structural state and mechanical properties of ZrN and Zr(Ti)-Si-N coatings obtained by ion-plasma deposition technique

The possibility of obtaining hard nanocrystalline coatings using vacuum-arc deposition in a high-frequency discharge stimulated regime has been studied. Condensates of the Zr(Ti)-Si-N system obtained by this method contain ZrN and TiN crystalline grains. These grains exhibit a compressive strain of about −1.1% in the film growth plane, which corresponds to compressive stresses up to about 3.5 GPa in the film-substrate system. The results of nanoindentation show evidence for a strong inhomogeneity of the coatings, in which regions possessing a hardness of 29–30 GPa alternate with those where the hardness exceeds 45–47 GPa. This pattern agrees with a two-phase model based on the structural data, according to which the condensed material consists of less hard ZrN grains and harder TiN grains.

Transition from polymer-like to diamond-like a-C:H films structure and mechanical properties

Abstract Amorphous hydrogenated carbon (a-C:H) films were deposited on polymeric substrates by the glow-discharge decomposition of butane. IR spectroscopy and electron paramagnetic resonance were used for the structural characterization of the films. For a set of samples, produced at different pressures, mechanical properties such as nanohardness and wear resistance were investigated. It was found that the mechanical properties of a-C:H films are strongly dependent on the pressure in the reactor. At a reactor pressure of about 10 Torr, the deposition of polymer-like a-C:H films occurs. Such films are characterized by low hardness and wear resistance and visco-elastic behaviour at indent reloading. On reducing the pressure in the reactor, an improvement of the mechanical properties of polymer-like a-C:H films was observed due to ion bombardment. Ion bombardment of the growing a-C:H film surface caused drastic increases of its hardness, the value of elastic recovery of the indent depth after unloading and wear resistance. With respect to the mechanical properties, polymer-like a-C:H films hardened by ion bombardment occupy a position between polymer-like and diamond-like a-C:H films. A high wear resistance and adhesion to a polymeric substrate, as well as the possibility of being deposited on substrates with a large area at temperatures below 80°C, make polymer-like a-C:H films, hardened by ion bombardment, a promising protective coating for polymeric materials.

Structure, composition, and physicomechanical characteristics of HfB2 and Hf-B-N films

The paper deals with the study of the effect of the deposition conditions (the bias potential and substrate temperature) on the structure, composition, and physicomechanical characteristics of nanocrystalline films of hafnium diboride and boridonitride formed by the method of nonreactive (in Ar) and reactive (in Ar + N2) HF magnetron sputtering, respectively. The optimal conditions for the deposition of the hafnium diboride coatings with growth texture in plane (00.1) and the best physicomechanical characteristics are deter-mined. It is shown that at a bias potential of ±50 V and a substrate temperature of ∼500°C superstoichiometric highly textured films are formed with a nanohardness of 44 GPa and an elastic modulus of 396 ± 11 GPa. A relation between the composition, structure, and physicomechanical characteristics of the films is found. Reactive sputtering in (Ar + N2) makes it possible to produce amorphous-crystalline films of the composite (HfB2 + BN) that consists of grains of the HfB2 nanocrystalline phase, the spaces between which are filled with the amorphous phase of graphite-like BN.

Structure and peculiarities of nanodeformation in Ti–Zr–Ni quasi-crystals

Ti–Zr–Ni samples with a substantial predominance of icosahedral quasicrystalline phase were produced by the melt-spinning technique. Their structure and mechanical properties were studied by X-ray diffraction and nanoindentation methods. The quasicrystalline phase was found to have a primitive lattice with the quasicrystallinity parameter a q = 0.5200–0.5210 nm. Quasicrystalline deformation behaviour under nanoindentation versus phase composition and structure is discussed in comparison with single crystal W–12 wt% Ta. The estimated elastic modulus E of the quasicrystalline phase shows no correlation with the element composition. The nanohardness was shown to increase with increasing quasicrystalline-phase perfection. Load–displacement curves of Ti–Zr–Ni quasicrystals (QCs) show stepwise character with alternation of elastic and plastic sections. Such non-uniform plastic flow in QCs might be caused by the localization of plastic deformation in shear bands. The non-uniformity of the plastic deformation increases with the increasing quasicrystalline phase perfection.

The analysis of microhardness measurement approach for characterization of hard coatings

Abstract The analysis of problems concerning thin film (1–5 μ) microhardness measurement by means of different techniques is conducted in case of diamond-like carbon coatings. It is shown experimentally that the extrapolation of the lg( H m − H s ) vs. d dependence to zero d values, where H s is microhardness of the film substrate, H m is formal microhardness of the substrate with coating and d is Berkovich pyramid depth, leads to reliable coating microhardness values H f independent on substrate properties and characterizing the properties of the coating itself. This approach is used to obtain the average H f values of multilayer coatings. It is found that the surface roughness of the coatings impedes the correct determination of the coating microhardness by means of nano-indentor techniques. The coating surface smoothening (mechanical or by ion etching) provides the conditions for correct microhardness measurement. It is concluded that the real H f values of diamond-like carbon coatings obtained by pulse arc graphite sputtering exceeds 100 GPa.