Abstract: Superhard metals are of interest as possible replacements with enhanced properties over the metal carbides commonly used in cutting, drilling, and wear-resistant tooling. Of the superhard metals, the highest boride of tungsten—often referred to as WB4 and sometimes as W1–xB3—is one of the most promising candidates. The structure of this boride, however, has never been fully resolved, despite the fact that it was discovered in 1961—a fact that severely limits our understanding of its structure–property relationships and has generated increasing controversy in the literature. Here, we present a new crystallographic model of this compound based on refinement against time-of-flight neutron diffraction data. Contrary to previous X-ray–only structural refinements, there is strong evidence for the presence of interstitial arrangements of boron atoms and polyhedral bonding. The formation of these polyhedra—slightly distorted boron cuboctahedra—appears to be dependent upon the defective nature of the tungsten-deficient metal sublattice. This previously unidentified structure type has an intermediary relationship between MB2 and MB12 type boride polymorphs. Manipulation of the fractionally occupied metal and boron sites may provide insight for the rational design of new superhard metals.

Abstract: Boriding is a thermochemical surface treatment in which boron diffuses into and combines with a substrate material to form a single- or double-phase metal boride layer at the substrate surface. This chapter discusses the importance of boriding for ferrous materials in which the formation of boride layers, such as FeB and Fe2B, improves hardness, wear resistance, temperature resistance and corrosion resistance at the surface of steels. The chapter includes a discussion of the growth kinetics of boride layers associated with traditional diffusion models. To understand the intended industrial applications, the chapter then reviews the physicochemical and mechanical characterisations of the different steels exposed to the boriding process.

Abstract: NiCrBSi/WC composite coatings containing various amounts of WC/W2C particles were laser cladded on low carbon steel substrate S235JR. Coatings were processed using two different laser systems, a 1 kW Nd:YAG and a 3.8 kW high power diode laser (HPDL). Coatings obtained with the Nd:YAG source demonstrate significant changes in the matrix microstructure with WC/W 2 C particle addition. Specific analysis shows the formation of new carbides (W,Cr) x C y and boride phases (W,Cr) x B y resulting from a partial dissolution of the WC/W 2 C particles within the metal matrix. The Brinell macrohardness of the coatings reveals surprisingly low values for coatings containing 10 vol.% and 20 vol.% WC/W 2 C. Through nanoindentation measurements, it is suggested that the low hardness of these new carbide and boride phases most likely counteracts the WC/W 2 C addition and may explain this unexpected behavior. On the contrary, the same coatings deposited using the HPDL source exhibits no change in the microstructure of the NiCrBSi matrix and display an expected monotonic increase of composite hardness with WC/W 2 C amount. It is suggested that the microstructural appearance of new carbide and boride phases may not be related to the type of laser used but to the specific laser energy during the coating process. Contrarily to hardness, measurements show that the erosive wear is marginally affected by the microstructural differences of the coatings. These results demonstrate that evaluating the quality of laser cladded coatings by simply assessing their density and the absence of a crack (as usually done) is insufficient as it does not automatically guarantee reaching optimal mechanical performance.

Abstract: The effect of the element cerium on the microstructure and wear behavior of Fe–B cast alloy was investigated by scanning electron microscope, transmission electron microscope, X-ray diffraction analysis, Leica digital image analysis, hardness tester and abrasion tester. The microstructures of as-cast Fe–B alloy are composed of the phase ferrite, pearlite and eutectic boride. Moreover, the as-cast eutectic boride structures in Fe–B alloy containing cerium are finer than that in the alloy having no cerium. After heat treatment, the average boride area and wear weight loss of the alloy containing cerium are lower than these of the alloy having no cerium. Before the formation of primary austenite, cerium can combine with oxygen to form Ce2O3. Ce2O3 can act as nuclei of primary austenite, promoting the refinement of austenite and borides during solidification, and improve the wear property of Fe–B alloy.

Abstract: Immobilized Pt clusters are interesting catalysts for dehydrogenation of alkanes. However, surface-deposited Pt clusters deactivate rapidly via sintering and coke deposition. The results reported here suggest that adding boron to oxide-supported Pt clusters could be a “magic bullet” against both means of deactivation. The model systems studied herein are pure and B-doped Pt clusters deposited on MgO(100). The nonstoichiometric boride cluster obtained via such alloying is found to anchor to the support via a covalent B–O bond, and the cluster-surface binding is much stronger than in the case of pure Pt clusters. Additionally, B introduces covalency to the intracluster bonding, leading to structural distortion and stabilization. The energy required to dissociate a Pt atom from a boride cluster is significantly larger than that of pure Pt clusters. These energetic arguments lead to the proposal that sintering via both Ostwald ripening and particle coalescence would be discouraged relative to pure Pt clusters...

Abstract: In the present study, CpTi and Ti6Al4V alloy were successfully plasma paste borided using borax paste. The process was carried out in a dc plasma system at temperatures of 973, 1023 and 1073 K for 3, 5 and 7 h in a gas mixture of 70% H2–30% Ar under working pressure of 5 mbar. The properties of the boride layers were evaluated by scanning electron microscopy, X-ray diffraction and the micro-Knoop hardness tester. Growth kinetics of the titanium boride layers on the Cp–Ti and Ti6Al4V alloy were obtained by analyzing the experimental results and parameters including the thickness of boride layer, correspondent diffusion temperature and holding time. The presence of both TiB2 top-layer and TiB whisker sub-layer was confirmed by the X-ray diffraction and scanning electron microscopy techniques. The formation rate of the dual titanium boride layers was found to have a parabolic character at all applied process temperatures. The surface hardness of TiB2 layer ranged between 2077 and 2373 HK0.025. The activation energies of boron in Cp–Ti and Ti6Al4V alloys were calculated as 93.61 and 99.79 kJ·mol− 1, respectively depending on the temperature and layer thickness.

Abstract: In this study, adhesion properties of boride layers formed on the surface of AISI 52100, AISI 5140, AISI 440C, AISI 420 and AISI 304 steels were investigated. Boronizing treatment was carried out in Ekabor-II powders at the temperatures of 850 and 950 °C for 4 h. The properties of boride layers were evaluated by optical microscopy, SEM, X-ray diffraction and micro-Vickers hardness tester. The Daimler-Benz Rockwell-C adhesion test was used to assess the adhesion of boride layers. Test result showed that adhesion of boride layers depended on the dual-phase structure. The stresses at the FeB/Fe2B interphase caused delamination failure and poor interphase adhesion with increase in the depth of hard and brittle FeB-based layer.

Abstract: Ni-based single superalloys play a crucial role in the hottest parts of jet engines. However, due to the complex geometry and macro-segregation during the solidification process, the cast defect such as stray grains is inevitable. Therefore, the transient liquid phase (TLP) bonding which can join several small single crystalline castings together is gradually believed to be an effective method for improving the yields of production of the complex components. The melting point depressant element B is always added into the interlayer filler material. Consequently, borides including the M3B2 and M5B3 phase usually precipitate during the TLP bonding process. So a comprehensive knowledge of the fine structural characteristics of the borides is very critical for an accurate evaluation of the TLP bonding process. In this work, by means of the aberration-corrected transmission electron microscopy, we show, at an atomic scale, the Wyckoff positional order phenomenon of the metal atoms in the unit cell of M3B2- and M5B3-type boride. Meanwhile, the defect along the (001) plane of the above two types of boride are determined to be the polyhedral intergrowth with complex configurations.

Abstract: We have deposited Cr-B and Cr-B-C thin films by co-sputtering from chromium boride and carbon targets. The binary Cr-B films consist of nanocrystalline and substoichiometric CrB2 - x grains (B/Cr a ...

Abstract: Ni-based alloys are characterized by great resistance to corrosion and high temperature oxidation. Diffusion boriding is well-known as a process which can improve the wear protection of these alloys. In this paper, continuous gas boronizing in N 2 –H 2 –BCl 3 atmosphere is proposed for the production of the boride layer on Nimonic®80A-alloy. Microstructural characterization of this layer is studied with the use of an optical microscope, scanning electron microscope, energy-dispersive X-ray microanalysis and X-ray diffraction. The diffusion zone mainly consists of a mixture of nickel and chromium borides, occurring in the compact boride zone. Beneath this zone, there are also some areas in which borides appeared at grain boundaries. However, it is an exceptional situation. In most cases this zone is invisible. The relatively high chromium content in this alloy results in diminished depth of the boride layer in comparison with Inconel®600-alloy containing less chromium. The increase in concentration of chromium causes that grain-boundary diffusion of boron more difficult. Hence, during boriding of Nimonic®80A-alloy volume diffusion plays the more important role, which requires the high activation energy. As a consequence, the zone with borides at grain boundaries can obtain a limited depth or cannot occur. Chromium content also influences some mechanical properties of the layer. Its higher content causes an increase in hardness because of the higher percentage of harder chromium borides in the compact boride zone. The limited iron concentration in this alloy improves the quality of the layer. The microstructure is free of any porosity. It improves the tribological properties of the layer in comparison with Inconel®600-alloy.

Abstract: The present study reports on the kinetics of borided Nickel 201 alloy. The thermochemical treatment of boronizing was carried out in a solid medium consisting of B4C and KBF4 powders mixture at 1123, 1173 and 1223 K for 2, 4 and 6 h, respectively. The boride layer was characterized by optical microscopy, X-ray diffraction technique and micro-Vickers hardness tester. X-ray diffraction analysis revealed the existence of NiB, Ni2B, Ni3B and Ni4B3 compounds at the surface of borided Nickel 201 alloy. The thickness of the boride layer increased with an increase in the boriding time and the temperature. The hardness of the nickel borides formed on the surface of the nickel substrate ranged from 1642 to 1854 HV0.05, whereas the Vickers hardness value of the untreated nickel was 185 HV 0.05. The growth kinetics of boride layers forming on the borided Nickel 201 alloy was also analysed. The boron activation energy (Q) was estimated as equal to 203.87 kJ mol-1 for the borided Nickel 201 alloy.

Abstract: Precipitates in the diffusion-affected zone (DAZ) during transient liquid phase bonding (TLP) single-crystal superalloys were observed and investigated. Small size and dendritic-shaped precipitates were identified to be M3B2 borides and intergrowth of M3B2/M5B3 borides. The orientation relationships among M3B2, M5B3, and matrix were determined using transmission electron microscope (TEM). Composition characteristics of these borides were also analyzed by TEM energy-dispersive spectrometer. Because this precipitating phenomenon deviates from the traditional parabolic transient liquid phase bonding model which assumed a precipitates free DAZ during TLP bonding, some correlations between the deviation of the isothermal solidification kinetics and these newly observed precipitating behaviors were discussed and rationalized when bonding the interlayer containing the high diffusivity melting point depressant elements and substrates of low solubility.

Abstract: The title compound is synthesized by arc-melting of the elements and characterized by powder and single crystal XRD, electrical resistivity and magnetic susceptibility measurements, and by TB-LMTO-ASA calculations.

Abstract: Boriding has attracted great interests in improving mechanical performance of materials. The high processing temperature and the relatively thin boride layer, however, have limited its applications. Here we report a method to boronize high-carbon steels at a relatively low temperature (780 °C). Using the arc melting process, a thick (> 400 μm) boronized coating is obtained. Two iron-boride phases (FeB and Fe2B) coexist in the borided layer. The surface hardness of the steel is increased by ~ 57%, while the friction is reduced by ~ 17% and ~ 65% under steel-steel and steel-diamond contacts, respectively. In a 40,000 cycle wear test, the wear track on the steel surface becomes invisible after the boriding treatment. The low-temperature process developed in this study is applicable to other materials and is effective in making thick boride coatings with less energy consumption.

Abstract: The structure, phase composition, and wear mechanisms of plasma-sprayed NKhTB20 coating (NiCrSiB–20 wt.% TiB2) are studied. To produce NKhTB20 composite powder, commercial PR-NKh16SR3 (NiCrSiB) powder was mixed with 20 wt.% TiB2 and the charge was pressed and sintered in vacuum at 1100°C for 30 min. During sintering, the components react to form chromium borides. The sinters were ground and classified into the particle size fraction –100+60 nm for plasma spraying. The plasma-sprayed NKhTB20 coating consists of a nickel-based matrix reinforced with titanium diboride and chromium boride grains. The friction and wear behavior of the NKhTB20 coating in dry friction against plasma-sprayed NiCrSiB and NKhTB20 coatings is examined. It is revealed that the NKhTB20/NiCrSiB friction pair has higher wear resistance than NKhTB20/NKhTB20. The contact surfaces of the NKhTB20/NKhTB20 friction pair are damaged under oxidative and abrasive wear mechanisms. Oxidative wear is the dominant mechanism for the NKhTB20/NiCrSiB friction surface. Complex oxide films form on the NKhTB20/NiCrSiB sliding surface and prevent it from damage.

Abstract: We trapped an unknown metastable boride by applying low-temperature solution synthesis. Single-phase nickel boride, Ni7B3, was obtained as bulk samples of microcrystalline powders when annealing the amorphous, nanoscale precipitate that is formed in aqueous solution of nickel chloride upon reaction with sodium tetrahydridoborate. Its crystal structure was solved based on a disordered Th7Fe3-type model (hexagonal crystal system, space group P63mc, no. 186, a = 696.836(4) pm, c = 439.402(4) pm), using synchrotron X-ray powder data. Magnetic measurements reveal paramagnetism, which is in accordance with quantum chemical calculations. According to high-temperature X-ray diffraction and differential scanning calorimetry this nickel boride phase has a narrow stability window between 300 and 424 °C. It crystallizes at ca. 350 °C, then starts decomposing to form Ni3B and Ni2B above 375 °C, and shows an exothermic effect at 424 °C.

Abstract: In this work, a new model was suggested to simulate the paste boriding process. This model was based on the numerical solving of the diffusion equations in both boride layer and substrate by taking into account the displacement of (Fe2B/substrate) interface. This diffusion problem with moving boundary was solved via a front tracking method. The numerical resolution was achieved by the finite difference method using an implicit scheme. The suggested model was able to predict the boride layer growth kinetics and the boron concentration profiles. The present model was validated by comparing the simulation results with the experimental data available in the literature. The obtained results were also compared to the models previously reported.

Abstract: In the present study, an aluminum alloy AA6351 was reinforced with different percentages (1, 3 and 5 wt %) of TiB2 particles and they were successfully fabricated by in situ reaction of halide salts, potassium hexafluoro-titanate and potassium tetrafluoro-borate, with aluminium melt. Tensile strength, yield strength and hardness of the composite were investigated. In situ reaction between the inorganic salts K2TiF6 and KBF4 to molten aluminum leads to the formation of TiB2 particles. The prepared aluminum matrix composites were characterized using X-ray diffraction and scanning electron microscope. Scanning electron micrographs revealed a uniform dispersal of TiB2 particles in the aluminum matrix. The results obtained indicate that the hardness and tensile strength were increased with an increase in weight percentages of TiB2 contents.

Abstract: Boride layers have potential industrial applications as abrasive and corrosion-resistant materials due to their high hardness values and chemical characteristics. In this study, boride layers are formed on the surface of St 14 steel samples using the plasma electrolyte method. The process was performed for a holding time of 10 min at the preset temperature of around 900 °C. The microstructure, hardness, and corrosion resistance characteristics of the boride layer are investigated by X-ray diffraction, hardness indentations and Tafel polarization. XRD results showed that boride layers are detectable on the surface of steel when the borax content of the solution is above 20 wt.%. Moreover, there are no peaks of the boride phase in X-ray patterns when the borax content is below 20 wt.%. Corrosion resistance characterization showed that layer created in 25% borax provides the best corrosion resistance. Maximum hardness values of samples processed with 10, 15, 20 and 25 wt.% borax were 750, 915, 1100 and 1250 HV, respectively.

Abstract: Some mechanical properties for cobalt boride (CoB and Co2B) coatings were obtained using the Vickers depth-sensing indentation technique. The coatings were developed on the surface of a CoCrMo alloy using the powder-pack boriding process at temperatures between 1223 and 1273 K using different exposure times for each temperature. Vickers indentations were conducted at constant distances from the surface using loads ranging from 15 to 450 mN. For the entire set of experimental conditions, the behavior of the indentation properties was examined as a function of the indentation loads. Universal expressions were used to determine the apparent or real hardness, the indentation Young’s modulus, and fracture toughness of the CoB and Co2B coatings. The results indicated that the CoB and Co2B coatings exhibited an apparent hardness of 20 and 17 GPa, respectively, in which the fracture toughness of the cobalt boride coatings only varied slightly in the set of experimental conditions proposed in this work.

Abstract: Composite Fe n B-Fe-B boride coatings were obtained on the surface of agricultural machine details, such as arrow-shaped pads made of 65G steel with a velocity RF-borating process. Four embodiments of the reaction mixtures were tested that form the coatings of different types, compositions, and structures during boriding. Tribotechnical tests of the isolated coatings in the laboratory showed a complex mechanism of wear by friction on the flexibly attached abrasive with a formation of transfer areas at the boundary between the coating and the abrasive. The wear of arrow-shaped pads strengthened with composite boride coatings during real running-in is 1.5–3 times smaller than for the control samples.