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Titanium diboride (TiB2) is a hard, refractory material, attractive for a number of applications, including wear-resistant machine parts and tools, but it is difficult to densify. The spark plasma sintering (SPS) method allows producing TiB2-based composites of high density with different sintering aids, among them titanium silicides. In this paper, Ti5Si3 is used as a sintering aid for the sintering of TiB2/10 wt % Ti5Si3 and TiB2/20 wt % Ti5Si3 composites at 1600 °C and 1700 °C for 10 min. The phase composition of the initial powders and produced composites was analyzed by the X-ray diffraction method using CuKα radiation. The microstructure was examined using scanning electron microscopy, accompanied by energy-dispersive spectroscopy (EDS). The hardness was determined using a diamond indenter of Vickers geometry loaded at 9.81 N. Friction–wear properties were tested in the dry sliding test in a ball-on-disc configuration, using WC as a counterpart material. The major phases present in the TiB2/Ti5Si3 composites were TiB2 and Ti5Si3. Traces of TiC were also identified. The hardness of the TiB2/Ti5Si3 composites was in the range of 1860–2056 HV1 and decreased with Ti5Si3 content, as well as the specific wear rate Wv. The coefficient of friction for the composites was in the range of 0.5–0.54, almost the same as for TiB2 sinters. The main mechanism of wear was abrasive. [ABSTRACT FROM AUTHOR]

Partial hydration of organic compounds can be achieved by high-pressure crystallization. This has been demonstrated for the high-nitro-gen-content compound 6-chloro-1,2,3,4-tetrazolo[1,5- b ]pyridazine (C 4 H 2 N 5 Cl), which becomes partly hydrated by isochoric crystallizations below 0.15 GPa. This hydrate, C 4 H 2 N 5 Cl· x H 2 O, is isostructural with the ambient-pressure phase α of C 4 H 2 N 5 Cl, but the crystal volume is somewhat larger than that of the anhydrate. At 0.20 GPa, the α-C 4 H 2 N 5 Cl anhydrate phase transforms abruptly into a new higher-symmetry phase, α'; the transformation is clearly visible due to a strong contraction of the crystals. The hydrate α-C 4 H 2 N 5 Cl· x H 2 O can also be isothermally compressed up to 0.30 GPa before transforming to the α'-C 4 H 2 N 5 Cl· x H 2 O phase. The isochoric recrystallization of C 4 H 2 N 5 Cl above 0.18 GPa yields a new anhydrous phase β, which, on releasing pressure, transforms back to the α phase below 0.15 GPa. The structural transition from the α to the β phase is destructive for the single crystal and involves a large volume drop and significant elongation of all the shortest intermolecular distances which are the CH⋯N and CH⋯Cl hydrogen bonds, as well as the N⋯N contacts. The α-to-α' phase transition increases the crystal symmetry in the subgroup relation; however, there are no structural nor symmetry relations between phases α and β., (© Anna Olejniczak et al. 2022.)

High-nitrogen-content compounds 6-azido-1,2,4-triazolo[4,3-b]pyridazine (C 5 H 3 N 7 ) and its 3-methyl derivative (C 6 H 5 N 7 ) have been in situ crystallized in a diamond-anvil cell and their structures determined by single-crystal X-ray diffraction. Under ambient and high-pressure conditions the crystallizations yield the same phases: the C 5 H 3 N 7 anhydrate and C 6 H 5 N 7 hydrated clathrate. In both the structures there are clearly distinguished regions of short CH...N and N...N intermolecular contacts, the latter involving exclusively the azide groups. High pressure initially increases the contents of water in the channel pores of the clathrate.