Dmitrij Morozow

This paper contributes to the knowledge of binderless tungsten carbide (WC), which attracts the attention of many engineers and scientists for its superior properties, but its application is limited due to difficulties with the consolidation of initial powders. In the present study, the microstructure and mechanical properties of binderless WC, sintered with the electroconsolidation technique from the initial powder of a grain size of 100–200 nm, were investigated. The material was compared with nWC sintered with the same method from a nanopowder with particles of size ca. 70 nm. The binderless μWC demonstrated hardness of HV = 30.06 ± 0.09 GPa, which is almost 14% higher than that of nWC, but its fracture toughness was lower (K_IC = 6.59 ± 0.46 MPa·m^1/2 under 1 kg load). These differences can be attributed to the improved homogeneity of the μWC microstructure, where no large agglomerates appeared to be present in nWC. The measured plastic properties, with no signs of brittle fracture, further confirm the applicability of the binderless WC under contact stress conditions.

The repeatability of the material properties is required to ensure the proper performance of the engineered systems that are constructed using these materials. In this paper, an analysis of the sintered ceria-stabilized zirconia is presented. This material exhibited high mechanical properties, due to the mechanism of strengthening via phase transition. The reproducibility was assessed for the material made out of a starting powder produced by fluoride salt precipitation. To fabricate specimens, a novel electroconsolidation method was used, ensuring a high heating rate, relatively low sintering temperatures, and short holding time. Weibull analysis was performed considering the bending strength of specimens and their microhardness. The obtained values of both shape parameter m and scale parameter σ₀ indicated that the ZrO₂ stabilized with 5 wt.% CeO₂ samples exhibited low variability of strength and hardness. The experimental evidence and statistical analysis reveal an influence of the m-phase, which has lower symmetry and therefore its addition makes ceramic weaker and softer. Furthermore, its progressive replacement by the t-phase, which has higher symmetry, makes ceramic both harder and stronger. Reducing the mol% increases the risk of the appearance of the highest addition of the monoclinic phase; increasing it is unfavorable from the point of view of the sintering process. Statistical and manufacturing evidence suggests that the choice of 5%/mol is optimal.

This paper presents the results of the experimental research on diamond-reinforced composites with WC–Co matrices enhanced with a ZrO₂ additive. The samples were prepared using a modified spark plasma sintering method with a directly applied alternating current. The structure and performance of the basic composite 94 wt.%WC–6 wt.%Co was compared with the ones with ZrO₂ added in proportions up to 10 wt.%. It was demonstrated that an increase in zirconia content contributed to the intense refinement of the phase components. The composite 25 wt.%C_diamond–70.5 wt.%WC–4.5 wt.%Co consisted of a hexagonal WC phase with lattice parameters a = 0.2906 nm and c = 0.2837 nm, a cubic phase (a = 1.1112 nm), hexagonal graphite phase (a = 0.2464 nm, c = 0.6711 nm), as well as diamond grits. After the addition of zirconia nanopowder, the sintered composite contained structural WC and Co₃W₃C phases, amorphous carbon, tetragonal phase t-ZrO₂ (a = 0.36019 nm, c = 0.5174 nm), and diamond grits—these structural changes, after an addition of 6 wt.% ZrO₂ contributed to an increase in the fracture toughness by more than 20%, up to K_Ic = 16.9 ± 0.76 MPa·m^0.5, with a negligible decrease in the hardness. Moreover, the composite exhibited an alteration of the destruction mechanism after the addition of zirconia, as well as enhanced forces holding the diamond grits in the matrix.

The paper presents and discusses questions on structure formation during the sintering process of Cr₂O₃-based composites using the hot pressing method, when a chemical reaction between the components takes place. The task was difficult because Cr₂O₃ decomposes when sintered at temperatures above 1300 °C. The proposed novel method allowed for interaction between aluminum and chromia, thus avoiding the decomposition of the latter. Here, ultrafine aluminum powder played the role of the active agent forming a liquid phase and reacting with Cr₂O₃. The appearance of the solid solutions of (Cr,Al)₂O₃ with different stoichiometry of Cr and Al depended on the aluminum content in the initial mixture. The solid solution significantly strengthened boundaries between composite phases, resulting in the composite material of high fracture toughness between 5 and 7 MPa m^½ and bending strength of ca. 500 MPa. The best mechanical properties exhibited the cermet with 22 wt.% of the restored chromium.

The study is devoted to structure and mechanical properties of a diamond composite used for manufacturing of cutting tools applied in a wide range of technological fields. The sample tools were fabricated by cold-pressing technology followed by hot-pressing in vacuum of the 51Fe–32Cu–9Ni–8Sn matrix mixture with diamond bits, both in absence and presence of nano-VN additives. It was demonstrated that without VN addition, the diamond–matrix interface contained voids and discontinuities. Nanodispersed VN added to the matrix resulted in the formation of a more fine-grained structure consisting of solid solutions composed of iron, copper, nickel, vanadium and tin in different ratios and the formation of a tight diamond–matrix zone with no visible voids, discontinuities and other defects. Optimal concentrations of VN in the CDM matrix were found achieving the maximum values of nanohardness H = 7.8 GPa, elastic modulus E = 213 GPa, resistance to elastic deformation expressed by ratio H/E = 0.0366, plastic deformation resistance H³/E² = 10.46 MPa, ultimate flexural strength R_bm = 1110 MPa, and compressive strength R_cm = 1410 MPa. As-prepared Fe–Cu–Ni–Sn–VN composites with enhanced physical and mechanical properties are suitable for cutting tools of increased durability and improved performance.

Negative Poisson’s ratio materials (called auxetics) reshape our centuries-long understanding of the elastic properties of materials. Their vast set of potential applications drives us to search for auxetic properties in real systems and to create new materials with those properties. One of the ways to achieve the latter is to modify the elastic properties of existing materials. Studying the impact of inclusions in a crystalline lattice on macroscopic elastic properties is one of such possibilities. This article presents computer studies of elastic properties of f.c.c. hard sphere crystals with structural modifications. The studies were performed with numerical methods, using Monte Carlo simulations. Inclusions take the form of periodic arrays of nanochannels filled by hard spheres of another diameter. The resulting system is made up of two types of particles that differ in size. Two different layouts of mutually orthogonal nanochannels are considered. It is shown that with careful choice of inclusions, not only can one impact elastic properties by eliminating auxetic properties while maintaining the effective cubic symmetry, but also one can control the anisotropy of the cubic system.

Fabrication of alumina–tungsten carbide nanocomposite was investigated. Characteristics of the densification and sintering were analyzed considering both the nano-size particle starting powders and the processing stages. Different heating rates were generated during densification and consolidation with a maximal load was applied only after a temperature of 1000 °C was reached. Due to the varying dominance of different physical processes affecting the grains, appropriate heating rates and pressure at different stages ensured that a structure with submicron grains was obtained. With directly applied alternating current, it was found that the proportion Al₂O₃ (50 wt.%)–WC provided the highest fracture toughness, and a sintering temperature above 1600 °C was found to be disadvantageous. High heating rates and a short sintering time enabled the process to be completed in 12 min, saving energy and time.

In this work, simulations of friction at the atomic level were performed to evaluate the influence of inclusions coming from metallic nanoadditives in the friction pair. The simple 2D model was applied considering appropriate values of Lennard–Jones potential parameters for given sets of interacting atoms. The real sliding pairs were replaced by effective equivalents consisting of several atoms. The calculations were based on the pseudo-static approximation. The simplicity of the model enabled to repeat the fast calculations in a very wide range of local pressures and for several types of atomic tribopairs. The performed simulations demonstrated a strong dependence of the coefficient of friction (COF) on the atomic environment of the atoms constituting a tribopair. It was confirmed theoretically that the Mo-Fe pair is characterized by lower atomic COF than Fe-Fe, Cu-Fe, and Ag-Fe pairs. This points to the great applicational potential of metallic molybdenum coating applications in tribological systems. Moreover, it was demonstrated that, although Cu-Cu and Ag-Ag pairs are characterized by relatively high COF, they lower the friction as inclusions in Fe surfaces.

The paper is dedicated to the life prolongation of the tools designed for deep-hole drilling. Among available methods, an ion implantation process was used to improve the durability of tungsten carbide (WC)-Co guide pads. Nitrogen fluencies of 3 × 10¹⁷ cm⁻², 4 × 10¹⁷ cm⁻² and 5 × 10¹⁷ cm⁻² were applied, and scanning electron microscope (SEM) observations, energy dispersive spectroscopy (EDS) analyses, X-ray photoelectron spectroscopy (XPS) and Secondary Ion Mass Spectrometry (SIMS) measurements were performed for both nonimplanted and implanted tools. The durability tests of nonimplanted and the modified tools were performed in industrial conditions. The durability of implanted guide pads was above 2.5 times greater than nonimplanted ones in the best case, presumably due to the presence of a carbon-rich layer and extremely hard tungsten nitrides. The achieved effect may be attributed to the dissociation of tungsten carbide phase and to the lubrication effect. The latter was due to the presence of pure carbon layer with a thickness of a few dozen nanometers. Notably, this layer was formed at a temperature of 200 °C, much smaller than in previously reported research, which makes the findings even more valuable from economic and environmental perspectives.