Vol 40, No 6 (2018)

Using first-principles density-functional theory (DFT) computations, we have predicted a new post-cotunnite (OII) phase of hafnia (HfO₂) at high pressures. Our computations, using the generalized gradient approximation (GGA), predict a phase transition from OII to a Fe₂P-type structure at ~ 120 GPa (~ 140 GPa) with a slight volume collapse at the transition pressure of ~ 0.2% (~ 0.1%) between the two phases using the second- (third-) order Birch-Murnaghan equation of state, respectively. The prediction of the new phase is consistent with recent experiments and computations performed on similar dioxides titania (TiO₂) and zirconia (ZrO₂) at extreme pressure-temperature (p-T) conditions. Importantly, our theoretical prediction for the OII → Fe₂P transition in HfO₂ is experimentally supported by the re-analysis of X-ray diffraction patterns of HfO₂ at extreme pressure-temperature conditions. Additionally, the equation of state and hardness of the predicted phase have been computed and show that Fe₂P-type phase while less compressible than the OII phase is nearly identical in hardness, indicating that none of the HfO₂ phases qualify as superhard.

A new process was developed to synthesize niobium diboride (NbB₂) nanosheets with the dimension of about 500 nm and thickness of about 10 nm by using metal niobium, iodine and sodium borohydride as starting materials in an stainless steel autoclave at 700°C. Iodine was used to facilitate the exothermic reaction between metal niobium and sodium borohydride and the formation of NbB₂. X-ray powder diffraction pattern indicated that the obtained product is hexagonal phase NbB₂ with the calculated lattice constants a = 110 Å and c = 3.2929 Å. The obtained product was also studied by thermogravimetric analysis. It had good oxidation resistance below 400°C in air.

The paper presents some results of an experimental investigation of the influence of machining conditions on the process of diamond grinding of silicon carbide ceramic balls. The process performance parameters are the rate of the ball diameter reduction, the rate of change (decreasing/increasing) of the deviation from spherical form, and the rate of change of the variation of ball lot diameter. To separate the portion due to solely the influence of the machining conditions, the authors have applied a method of graphical approximation of the time behavior of mean value of ball accuracy characteristics per batch. It has been found out that the separated portions can change upwards or downwards depending on the values of the machining conditions, namely: diamond wheel infeed, wheel feed rate, and ball-holder table rotational speed. The full factorial experiments of type 2³ have revealed the most effective (for correcting the ball shape) combination of the factors studied, which is different for each of the three ranges of such deviation: above 300 μm, within 150–300 μm, and below 150 μm.

The paper reviews the present-day trends in the development of new types of wear-resistant coatings, including nanostructured ones, for PCBN tools. The main currently pursued approaches to creation of high-hardness nanostructured coatings are discussed. Based on the analysis of wear behavior of PCBN tools, some new approaches to the development of promising wear-resistant coating systems have been identified, namely: (i) selecting the structural components of a coating, which should act as a hard grease to effectively reduce the tool wear rate; (ii) adding the compounds that inhibit any chemical reactions in the cutting zone; (iii) reducing the friction coefficient and contact loads through the use of solid lubricants and running-in layers of a coating; (iv) providing a coating with the required structural state.

The influence was studied of the addition of n-layer graphene Gn(4) and silicon nanocarbide in the sintering under high pressure and temperature with the silicon impregnation of the mixture on the structure and electrophysical properties of a diamond composite.