Vol 41, No 2 (2019)

In this work molecular dynamics simulations are employed to compare the mechanical properties and hardness of three polytypes of cubic BC₃. Firstly, two interatomic Tersoff potentials, with different parameterizations, were identified from the literature for the boron-carbon system. Based on cohesive energies and structural properties from existing ab-initio studies, the suitability of the two potentials for predicting the properties of BC₃ was analyzed. Secondly, using the better interatomic potential, more detailed molecular dynamics simulations were conducted to estimate and compare the elastic, yield, post-yield behavior and hardness of the three polytypes. The elastic constants compare well with existing abinitio values and vary by at most by 15% amongst the three polytypes. Response to indentation showed considerable qualitative differences in yield and post-yield response. One of the polytypes showed lower yield strength and seemed more ductile than the other two. The hardness also showed a complex dependence on both the material and the indentation depths. A peculiar, indenter-size dependent pile-up behavior was also seen. Specifically, for lower radii, pile-up was seen on indentation. As the radius of the indenter was increased, pile-up was seen only on retracting the indenter. The higher volume occupied by the indentation-amorphized material was found to be the reason for pile-up on retracting the indenter.

The effect of the gas mixture composition on the structure, chemical bond character and hardness of Si-B-C-N films was systematically studied. A series of Si-B-C-N films was deposited by reactive dc magnetron sputtering of the target composed of Si disc with B₄C chips placed in the sputtering zone of disc. The films were deposited with nitrogen fractions 30-to-70% in Ar/N₂ gas mixture and annealed in a vacuum at temperatures up to 1200°C. The films were characterized by X-ray diffraction, X-ray photo-electron spectroscopy, Fourier transform infrared spectroscopy, indentation tests. Addition of nitrogen in the gas mixture up to 60% led to an increase of hardness from 13.4 up to 17.8 GPa. With further increase in nitrogen content in gas mixture the film hardness decreased. The latter is caused by formation of the weak B-N bonds as well as C-C and C-C bonds that are characteristic of h-BN-like phase and graphite phase, respectively.

The results of a study of the influence of the environment during sintering of fine-grained WC—10Co cemented carbide on its structural characteristics and physico-mechanical properties, as well as the operational durability and wear mechanisms of the cutting plates of this alloy in impact cyclic turning process of 40Kh steel parts are presented. It is shown that the sintering of a cemented carbide in vacuum as compared with sintering in a methane-hydrogen environment reduces the residual microporosity of its structure, improves the physico-mechanical properties and operational durability of the cutting plates made from it. A new phenomenon was found to be the formation of continuous chip in the conditions of discontinuous turning of 40Kh steel parts and the welding it to a working part of a cutting plate from WC-10Co cemented carbide sintered in vacuum, and the absence of such a phenomenon for an alloy sintered in a methane-hydrogen environment.

The paper addresses the variation of cutting temperatures and forces and machined surface roughness versus the cutting speed in fine turning of VT1-0 titanium alloy by a PCD tool. Optimal cutting conditions have been substantiated.

The conditions for making use of porosity of superabrasive composites in the process of water absorption have been defined for the first time. It is shown how physical-mechanical properties (hardness, strength, conductivity) of the composites vary under the influence of water absorption, the duration of this effect has been determined, and the conditions for increasing wear resistance of the composites in grinding tools have been clarified.

Microstructure of boron subphosphide B₁₂P₂ produced by self-propagated high-temperature synthesis has been studied by high-resolution transmission electron microscopy. Two systems of twins have been found i.e. conventional twins on the (0003)_h plane and nanotwins resulting from duplication of the rhombohedral unit cell of B₁₂P₂ along one of the basic vectors.