Hardness
is an essential but complex property of materials. Although
it is generally accepted that a high hardness is related to a high
covalent bond density, the determinant of hardness is always unclear.
To overcome the restriction of the high density of covalent bonds,
the low-boron content transition metal borides (TMBs) are chosen to
explore a new way to enhance hardness. We fix the density of covalent
bonds and modulate the hardness by designing perpendicular boron zigzag
chain (Bzc) skeletons (pe-Bzcs) and parallel Bzc skeletons (pa-Bzcs)
in α-MoB (I41/amd) and β-MoB
(Cmcm). We utilize pe-Bzcs in α-MoB to enhance
the shear modulus via less slippage than in pa-Bzcs. Pe-Bzcs generate
a higher grain boundary density in α-MoB than in β-MoB
to create a nano-polycrystal morphology under high pressure and high
temperature. Hence, the hardness of α-MoB (18.4 GPa) is greater
than that of β-MoB (12.2 GPa), which can be attributed to the
higher shear modulus and higher density of the grain boundary caused
by pe-Bzcs. This work suggests a new idea that modulating boron covalent
bond substructures is an effective way to enhance hardness even in
low-boron content TMBs. This finding is significant for the design
new hard or superhard functional materials.
Al/TiO2 composite film was successfully deposited on polyester fabrics by using magnetron sputtering techniques. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were used to examine the deposited films on the fabrics, and the structural colors and anti-ultraviolet property of fabrics were also analyzed. The results indicated that polyester fabrics coated with Al/TiO2 composite films achieved structural colors. The reactive sputtering times of TiO2 films in Al/TiO2 composite films were 10 min, 12 min, 18 min, 20 min, 26 min, 27 min, 30 min and 45 min, respectively, the colors of corresponding fabrics were bluish violet, blue, cyan, green, yellow, yellowish red, orange and blue-green, which was consistent with the principle of the thin film interference. The structure of the TiO2 film in Al/TiO2 composite films was non-crystalline, though the fabrics were heated and maintained at the temperature of 200 °C. The anti-ultraviolet property of the fabrics deposited with Al/TiO2 composite films were excellent because of the effect of Al/TiO2 composite films.
Recent
reports exposed an astonishing factor of high hardness that
the connection between transition-metal (TM) atoms could enhance hardness,
which is in contrast to the usual understanding that TM–TM
will weaken hardness as the source of metallicity. It is surprising
that there are two opposite mechanical characteristics in the one
TM–TM bond. To uncover the intrinsic reason, we studied two
appropriate mononitrides, CrN and WN, with the same light-element
(LE) content and valence electron concentration. The two high-quality
compounds were synthesized by a new metathesis under high pressure,
and the Vickers hardness is 13.0 GPa for CrN and 20.0 GPa for WN.
Combined with theoretical calculations, we found that the strong correlation
of d electrons in TM–TM could seriously affect hardness. Thus,
we make the complementary suggestions of the previous hardness factors
that the antibonding d-electron state in TM–TM near the Fermi
level should be avoided and a strong d covalent coupling in TM–TM
is very beneficial for high hardness. Our results are very important
for the further design of high-hardness and multifunctional TM and
LE compounds.
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