RESUMO
Si3N4 ceramic as a tool material shows promising application prospects in high-speed machining fields; however, the required high mechanical properties and low-cost preparation of Si3N4 ceramic tool materials restrict its application. Herein, synergistic reinforced Si3N4 ceramic tool materials were fabricated by adding ß-Si3N4 seeds, inexpensive Si3N4 whiskers and TiC particles into coarse commercial Si3N4 powder (D50 = 1.5 µm), then sintering by hot-pressing with low temperature and short holding time (1600 °C-30 min-40 MPa). The phase assemblage, microstructure evolution and toughening mechanisms were investigated. The results reveal that the sintered Si3N4 ceramics with synergistic reinforcement, compared to those with individual reinforcement, present an enhancement in relative density (from 94.92% to 97.15%), flexural strength (from 467.56 ± 36.48 to 809.10 ± 45.59 MPa), and fracture toughness (from 8.38 ± 0.19 to 10.67 ± 0.16 MPa·m1/2), as well as a fine Vickers hardness of 16.86 ± 0.19 GPa. Additionally, the various reinforcement modes of Si3N4 ceramics including intergranular fracture, crack deflection, crack bridging and whiskers extraction were observed in crack propagation, arising from the contributions of the added ß-Si3N4 seeds, Si3N4 whiskers and TiC particles. This work is expected to serve as a reference for the production of ceramic cutting tools.
RESUMO
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RESUMO
A nanostructured laminar W-La2O3 alloy (WL10) with improved ductility was prepared using a surface mechanical attrition treatment (SMAT). φ1.5 mm ZrO2 WL10 balls subjected to SMAT (called φ1.5 mm ZrO2 ball SMATed WL10) samples possess the best surface profile and excellent integrated mechanical properties (the ductile-brittle transition temperature (DBTT) value decreases by approximately 200 °C, and the bending strength decreases by 100 Mpa). A highly dense group of laminates was detected near the surface of the φ1.5 mm ZrO2 ball SMATed WL10 sample. The SMATed WL10 laminates were composed of a micro-grain layer, an ultrafine-grain layer and a nanosized-grain layer. The nanostructured laminar surface layer of the φ1.5 mm ZrO2 ball SMATed WL10 sample is approximately 1-2 µm. The top surface of the WL10 plates with and without the SMAT process possesses residual compressive stress of approximately -883 MPa and -241 MPa, respectively, in the y direction and -859 MPa and -854 MPa, respectively, in the x direction. The SMAT process could be a complementary method to further improve the toughness of tungsten-based materials.
RESUMO
The frontal polymerization process was used to produce superabsorbent hydrogels based on acrylic acid monomers grafted onto starch. Using a simple test tube which was nonadiabatic and permitted contact with air, the effects of initial temperature and tube size on the propagating front of grafting copolymerization and the properties of hydrogels were explored. The unrestricted access of the reaction mixture to oxygen delayed the formation of self-propagating polymerization front. The ignition time was markedly lengthened with the increasing of tube size attributed to the formation of large amounts of peroxy radicals. The front velocity dependence on initial temperature could be fit to an Arrhenius function with the average apparent activation energy of 24 kJ mol(-1), and on tube size to a function of higher order. The increase of the initial temperature increased the front temperature, which lead to more soluble oligomers and higher degree of crosslinking. The interplay of two opposite effects of oligomer and crosslinking determined the sol and gel content. An increase in tube size had two effects on the propagating front. One was to reduce heat loss. The other effect was to increase the number of escaping gas bubbles. The combined action of the two effects resulted in a maximum value of front temperature, an increase in sol content and a reduction in gel content with tube size. The highest swelling capacity of hydrogels was obtained when the initial temperature or tube size favored a formation of porous microstructure of hydrogels.
RESUMO
Recently, a considerable amount of research has centered on uniquely structured polymers synthesized through self-propagating frontal polymerization. The obtained polymer materials have better features than those obtained by using the classical batch route. The additional advantages are short reaction times and low cost. This work describes the first frontal polymerization synthesis of a graft copolymer superabsorbent hydrogel of acrylic acid onto starch at high monomer and initiator concentration. The effects of varying the relative amounts of the reaction components on the most relevant parameters relating to frontal polymerization were explored. The front velocity dependence on initiator concentration could be fit to a power function. The temperature profiles were found to be very sharp with a maximum temperature below 150 degrees C, which was responsible for high monomer conversion. The ultimate properties of the product appear to depend on the polymerization front velocity and the temperature. The high-temperature and rapid temperature increase at the polymerization front led to products with interconnected porous structures caused by the evaporation of water. So, a fast-swelling, highly absorbing hydrogel with respect to batch polymerization was obtained.