Fabrication of high entropy boride ceramics using high-purity Hf, Ti, Zr, Mo, W, Mn, Cr boride powders produced from cost-effective oxides via mechanochemical synthesis method

Abstract

High melting temperature, amazing mechanical strength, incredible hardness, and remarkable resistance to wear, corrosion, and thermal shock are just a few of the exceptional features that transition metal borides display. They also provide excellent transmission stability and retain chemical and thermal stability in harsh environments. A wide range of industries, including catalysis, refractory components, high-resolution sensors, protective and decorative coatings, abrasive products, cathode coatings, polishing and grinding tools, and crucial industries like the aerospace use these materials because of their special set of properties. Over time, a range of synthetic methods have been used to create transition metal borides. These include molten salt electrolysis, autoclave-assisted synthesis, metallothermic reduction, carbothermic/borothermic reduction technique, thermal plasma processing, self-propagating high-temperature synthesis, and traditional solid-state processes. Furthermore, in the last several decades, mechanochemical synthesis (MCS) has become a viable option. In contrast to conventional high-temperature techniques, MCS is a powder metallurgy method that allows for the production of fine-grained, composite metal boride powders with regulated microstructures at room temperature. The need for advanced materials has increased dramatically in recent years in a variety of industries, including electronics, biomedical engineering, energy, aerospace, automotive, and medicine. High-entropy alloys (HEAs) have become a potential class of materials to meet these changing demands. HEAs are made up of four or more principal elements combined in about equimolar ratios, as opposed to traditional alloys, which usually consist of one primary element with minor additions. Surprisingly, they frequently solidify into straightforward solid-solution phases rather than the anticipated complex intermetallic phases. HEAs often crystallize into single-phase structures, most frequently displaying body-centered cubic (BCC) or face-centered cubic (FCC) configurations, despite their complicated constituents. The existence of hexagonal close-packed (HCP) structures in certain compositions has also been documented in more recent studies. Four fundamental core effects govern the unique behavior of HEAs: the severe lattice distortion effect, which affects the mechanical and crystal lattice behavior; the sluggish diffusion effect, which is linked to slower atomic mobility; the high-entropy effect, which influences thermodynamic stability; and the cocktail effect, which describes the synergistic interactions among the various elements involved. These processes enable HEAs to have a variety of remarkable features, such as extraordinary strength and fracture toughness, high hardness, superior wear, corrosion, and oxidation resistance, and great thermal and chemical stability. Because of these qualities, HEAs may be used in challenging fields such sophisticated cutting tools, refractory materials, maritime engineering, and aerospace components. Recent years have seen an increase in study into the creation of high-entropy ceramics, which are based on the idea of HEAs. A wide range of compounds, such as oxides, carbides, nitrides, silicides, and borides, are included in this new class of sophisticated materials. Of these, high-entropy metal borides, which were initially suggested in 2016, constitute a new class of ultra-high-temperature ceramics that hold great promise for demanding applications. High-entropy diborides (HEBs), which belong to the P6/mmm space group, usually crystallize in a hexagonal close-packed form. The unusual combination of metallic, ionic, and covalent bonding produced by the presence of metal–metal, metal–boron, and boron–boron connections inside this structure helps to balance structural stability and exceptional material performance. These ceramics have the appealing features that are frequently linked to advanced ceramics: high hardness, great resistance to wear and corrosion, low/moderate density, and excellent mechanical strength at high temperatures. High-entropy borides are being explored for use in cutting tools, aircraft and solar energy systems, and microelectronic devices because of this remarkable combination of properties. HEB ceramics have been made using a variety of synthesis methods. These include of pressureless sintering and pressure-assisted techniques including hot pressing and spark plasma sintering. A two-step method integrating several approaches may be used in situations when obtaining a single-phase high-entropy diboride is difficult. Because it uses inexpensive raw materials and very basic equipment to create powders with fine microstructures at room temperature, mechanical alloying stands out among these techniques. Moreover, under high temperature and pressure, spark plasma sintering makes it possible to consolidate these powders into dense, single-phase ceramics, which makes it a crucial method for attaining the required mechanical integrity and phase purity. The current study involved the synthesis of HfB2, ZrB2, TiB2, TaB, Cr-boride, Mn-boride, Mo-boride, and W-boride powders using a mechanochemical process, followed by leaching for purification. The powders of the synthesized metal borides were mixed in an equimolar ratio with five to six constituents. A high-energy ball mill (HEBM) was used to grind the compositions for six hours at a ball-to-powder weight ratio of 10:1. Using X-ray diffractometry, scanning electron microscopy, transmission electron microscopy, particle size measurement, and density measurement with a pycnometer, metal boride powders and hybridized samples were characterized. After mechanical alloying, single-phase high-entropy diboride was not achievable in the powders' microstructure. The composition of (Hf0.2Ti0.2Zr0.2W0.2Ta0.2)B2 had the maximum density at 7.3942±0.0033 g/cm3, whereas the composition of (Hf0.2Ti0.2Zr0.2Mo0.2Cr0.2)B2 had the lowest density at 6.185±0.008 g/cm3. After milling, a single phase high-entropy structure was produced using spark plasma sintering (SPS). Low intensity oxide phases (Hf, Zr) were also detected. Low-intensity secondary phases developed in compositions with five and six components. The following methods were employed to characterize the sintered samples: X-ray diffractometer, scanning electron microscope/energy dispersive spectrometer, Vickers hardness measurement, dry-sliding wear test, and Archimedes density measurement. The composition relative densities vary from 90.52% for (Hf0.2Ti0.2Zr0.2Mo0.2Ta0.2)B2 to 99.24% for (Hf0.2Ti0.2Zr0.2Mo0.2W0.2)B2, with HEB 4 < HEB 10 < HEB 5 < HEB 11 < HEB 2 < HEB 8 < HEB 1 < HEB 6 < HEB 9 < HEB 7 < HEB 3. The hardness values varied from 13.54 ± 2.33 GPa for (Hf0.2Ti0.2Zr0.2Mn0.2W0.2)B2 to 30.09 ± 4.79 GPa for (Hf0.2Ti0.2Zr0.2W0.2Ta0.2) B2, in the order of HEB 2 < HEB 1 < HEB 4 = HEB 6 < HEB 8 < HEB 10 < HEB 7 < HEB 11 < HEB 3 < HEB 5. All of the samples' average hardness values were determined to be around 21.94 GPa. The compositions' wear resistance rankings are as follows: HEB 3 < HEB 11 < HEB 4 < HEB 8 < HEB 7 < HEB 1 < HEB 5 < HEB 10 < HEB 9 < HEB 2. The compositions with the lowest and highest wear resistances, respectively, are (Hf0.2Ti0.2Zr0.2Mn0.2W0.2)B2 and (Hf0.2Ti0.2Zr0.2Mo0.2W0.2)B2.