Study On Tungsten Borides Synthesized At High Pressure And High Temperature

Superhard materials are a series of materials with high Vickershardness (Hv>40GPa). One of the most important research directions iscompounds formed by transition metals and light elements (B, C, N, O),and potential superhard materials must meet the following three criteria:high valence electron density, short and strong covalent bond,three-dimensional net-work covalent bonded structure. We studiedtungsten-boron system compounds–one potential superhard materials,because of high valence electron density of metal tungsten, and boronatoms can provide short and strong covalent bonds and B-B dimensionalform3D net-work structures. Such materials are widely used asanti-abrasion materials, cutting tools, corrosion resistant coating, etc. Andthey can make up some shortcomings of diamond and other traditionalsuperhard materials. If tungsten-boron system compounds also have goodelectrical, mechanical and thermal properties, it would greatly expand thescope of its applications. We synthesized a series of tungsten-boron compounds underhigh-temperature and high-pressure (HTHP) with a six anvilsSPD6×600T press, and determine formation conditions of each phase.The raw materials are tungsten powder of purity99.9%and amorphousboron powder of purity99.9%. Tungsten powder and boron powder aremixed evenly according to a certain molar ratio, then we find properpressure and temperature conditions, and successfully synthesizedW2B(I4/mcm), WB(I41/amd), WB2(P63/mmc), possible WB3(P63/mmc).Additionally, we calculated lattice constants of tungsten-boroncompounds, and relating physical parameters by means of first principlecalculation with the VASP package, and draw their lattice structure. Themain results are as follows:a) Theoretically calculate lattice constants bulk modulus, shearmodulus, Young’s modulus, Poisson’s ratio, Vickers hardness and densityof the five tungsten-boron compounds: W2B(I4/mcm), WB(I41/amd),WB2(P63/mmc), possible WB3(P63/mmc) and WB4(P63/mmc) with firstprinciple simulation software package VASP, with a cut-off of600eV.Our results are well according with the experimental results withdeviations about1%, or even less. Meanwhile, we also calculatedabove-mentioned physical parameters of WB4(P63/mmc). Compared withexperimental results, the maximum deviation is7%. Otherwise, weproved that WB4(P63/mmc) is mechanically instable. On many aspects, our results support those of Liang’s that WB4obtained in the experimentsbefore is in fact WB3with a similar structure and lacks one boron atom.b) We obtain above-mentioned four tungsten-boron compounds byHPHT synthetic method with SPD6×600T cubic anvil apparatus, thesynthesis conditions are as follows: pressure-4.0~5.6GPa,temperature-1300K~2300K, dwell time-15~60minutes. Besideslooking for the synthesis conditions of each compound, we also find thatwhen tungsten and boron molar ratio in mixture is certain, thecrystallinity of the products increases with rising temperature. If otherconditions are fixed, and only the boron content in the mixture is within acertain range, we can obtain the corresponding stoichiometric tungstenboron compounds. Elongating the time of heat preservation can promotecrystallization of the interior grains of samples, and increases theircrystallinity. Instead, increasing pressure will suppress smallre-crystallization of the small grains in samples, but it will increase thehardness of the material.c) Theoretical hardness of the material is calculated with the methodproposed by Chen et al.2011, which is based on calculated shearmodulus. Besides, we measured Vickers hardness of WB and WB2underdifferent loads with micro-hardness tester. With load increasing, thehardness of samples reduce continuously, and tend to a certain value.Under load of0.98N, the hardness of WB is18.9GPa. While under0.98N, the hardness of WB2is28.2GPa, slightly larger than the theoretical value27.6GPa.d) With Vanderbilt methods, we measure resistivity of WB and WB2,which are equal to13.52×10-8Ω m and5.54~6.84×10-8Ω m(associated with the temperature conditions). And theirs can be comparedwith the resistivity of elemental tungsten, which suggests thattungsten-boron compounds are potential superhard materials with goodconductive properties.e) We prove that HTHP method is an efficient way for high densitymaterials synthesis. In our experiments, the highest density of WB andWB2can reach96.43%and96.78%of theoretical prediction respectively.We can get ratios the experimental results to the theoretical results withthe Archimedes method. Observation of the morphology of samplesurface also determine their density.f) By studying the DG curves, we find that oxidation temperaturesof WB and WB2are about450°C and460°C respectively, after that, themass of samples starts increasing, which show that tungsten borides havegood oxidizing resistance. Additionally, in the DTA curve of WB, we canfind two exothermic peaks at490°C and590°C exist; in the DTA curve of WB2, there are4exothermic peaks at490°C,610°C,700°C and900°C, and at880°C, there is a small endothermic peak.