


Vol 40, No 6 (2018)
- Year: 2018
- Articles: 10
- URL: https://journals.rcsi.science/1063-4576/issue/view/11665
Production, Structure, Properties
The Influence of in situ Formed TiB2–VB2 Borides on the Structure and Properties of Hot-Pressed B4C–(TiH2–VC) Ceramic System
Abstract
A method of the reaction hot pressing was used to produce composites from the B4C–(5–15%)(TiH2–VC) system, established the peculiarities of their structures and measured physico-mechanical parameters. The dense dispersion-hardened materials having a high bending strength (Rbm = 420–580 MPa) and increased fracture toughness (KIc = 4.1–4.8 MPa·m1/2) on retention of the HKN = 19–20 GPa are promising for the use in the conditions of the severe abrasive wear.


Theoretical and Experimental Evidence for a Post-Cotunnite Phase Transition in Hafnia at High Pressures
Abstract
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.


The Impact of Hydrogen-Free Ion Nitriding on Physicomechanical and Performance Characteristics of Hard Alloys T5K10 and T15K6
Abstract
It has been shown that ion nitriding of hard alloys in hydrogen-free medium leads to an increase of the average value of microhardness to 20.1 GPa and a rise of the breaking load by 15% at cantilevered bending. The turning of steels using nitrided inserts results in the reduction of the friction coefficient and the cutting force to 10% of their corresponding values before nitriding, and the decrease of wear rate by half. The efficiency of nitriding is confirmed by production tests during the finish and rough machining.


Chemical Synthesis of Niobium Diboride Nanosheets by a Solid-State Reaction Route
Abstract
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.


Investigation of Machining Processes
On the Wear Comparative Analysis of Cutting Tools Made of Composite Materials Based on Polycrystalline Cubic Boron Nitride when Finish Turning of AISI D2 (EN X153CrMoV12) Steel
Abstract
This study presents the results of research on the condition of low content polycrystalline cubic boron nitride cutting tools (cBN-L) when finish turning of hardened AISI D2 tool steel. The least changes in the condition of wedges throughout the investigated turning feeds were observed for the composite cBN020 whereas the highest wear intensity was found for the composite cBN200. For all researched cutting materials, abrasive wear, as well intense adhesion were observed.



The Influence of Machining Conditions on Performance of Diamond Grinding of Silicon Carbide Ceramic Balls
Abstract
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.



Tools, Powders, Pastes
A Study of the Tendency and Extent of Variation of Morphometric Characteristics of Standard Synthetic Diamond Grits in a Continuous Series of Their Grades and Sizes
Abstract
The authors have performed analytical processing of great body of experimental data (obtained by them) on static strength and morphometric characteristics of standard (to DSTU 3292–95) synthetic diamond grits. This has revealed tendencies and an extent of variation of morphometric characteristics of the grits within a wide range of grades and grit sizes. A review of the relevant publications has been done, which has demonstrated the originality of the proposed problem statement. The paper defines the lines of further development of such investigations and shows their practical significance.



Promising Types of Coatings for PCBN Tools
Abstract
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.



Letters to the Editor



Structure and Electrophysical Properties of the Diamond–Graphen–Silicon Carbide Composite
Abstract
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.


