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ABSTRACT: Bioactive tricalcium phosphate/titania ceramic composites were synthesized by pressureless air sintering of mixed hydroxyapatite and titania (TiO(2)) powders. The sintering process was optimized to achieve dense ceramic bodies consisting in a bioactive/bioresorbable matrix (β-tricalcium phosphate) reinforced with defined amounts of sub-micron sized titania particles. Extensive chemico-physical and mechanical characterization was carried out on the resulting composites, which displayed values of flexural strength, fracture toughness and elastic modulus in the range or above the typical ranges of values manifested by human cortical bone. It was shown that titania particles provided a toughening effect to the calcium-phosphate matrix and a reinforcement in fracture strength, in comparison with sintered hydroxyapatite bodies characterized by similar relative density. The characteristics of the resulting composites, i.e. bioactivity/bioresorbability and ability of manifesting biomimetic mechanical behavior, are features that can promote processes of bone regeneration in load-bearing sites. Hence, in the perspective of developing porous bone scaffolds with high bioactivity and improved biomechanical behavior, TCP/TiO(2) composites with controlled composition can be considered as very promising biomaterials for application in a field of orthopedics where no acceptable clinical solutions still exist.
Journal of the mechanical behavior of biomedical materials. 10/2012;
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ABSTRACT: The purpose of this study was to evaluate the effect of three popular soft drinks on the Young's modulus, hardness, surface topography and chemical composition of widely used nickel-titanium-based orthodontic wires.
Thirty-two specimens (20 mm in length) were cut from the straight portion of pre-formed 0.019 × 0.025 inch Nitinol Heat-Activated archwires and randomly divided into four groups of eight specimens each: Group A1 (Coca Cola(®) regular); Group A2 (Santal(®) orange juice); Group A3 (Gatorade(®)); Group B (distilled, deionized water; dH(2)O). Each specimen was immersed in 10 ml of one of the soft drinks or dH(2)O, control, for 60 min, at 37°C. At the end of the soaking time, the Young's modulus and hardness were determined using a nanoindenter. Scanning Electron Microscope-Energy Dispersive Spectroscopy (SEM-EDS) was used to characterize the effects on the topography and chemical composition of the wires.
No statistically significant differences were found between the groups either in the Young's modulus or in hardness after the selected soaking protocol. Besides some surface colour changes, the topography and the chemical composition of the wires were not affected by the immersion in any of the chosen soft drinks.
These in-vitro results suggest that the consumption of soft drinks cannot be acknowledged as one possible reason for the degradation of the physical and chemical properties of heat activated nickel titanium orthodontic wires in patients undergoing fixed orthodontic treatment.
Acta odontologica Scandinavica 04/2011; 70(1):49-55. · 1.41 Impact Factor
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ABSTRACT: The densification behavior along with the microstructure evolution and some mechanical properties of four ultrafine SiC particle-dispersed
ZrB2 matrix composites were studied. The SiC–ZrB2 composites, with a SiC content of 5, 10, 15 and 20 vol%, were densified to near full density by vacuum hot pressing at 1,900°C
under a maximum uniaxial pressure of 45 MPa. The presence of SiC greatly improved the sinterability of ZrB2. Grain growth of the diboride matrix was increasingly inhibited for larger amounts of SiC added. Elastic modulus, Poisson
ratio, microhardness, flexural strength and fracture toughness were measured at room temperature. Unexpectedly, no obvious
effect of the increasing amount of SiC on flexural strength and fracture toughness was found. The former property ranged from
650 to 715 MPa but was actually affected by the exaggerated size of several tenths of micrometers of sintered SiC clusters
which acted as dominant critical defects. Also fracture toughness did not receive a marked contribution from the increase
of the SiC content. As for the matrix, the prevailing fracture mode of the composites was intragranular, regardless of the
SiC content.
KeywordsZrB2
-fracture toughness-X-ray diffraction-residual stresses
01/2010: pages 261-272;
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ABSTRACT: Fully dense fine-grained ZrB2- and HfB2-based composites contaning 15 vol% TaSi2 were produced by hot pressing at 1850°–1900°C. Gas formation and mass loss, which occurred during sintering in both systems, were in agreement with thermodynamic predictions. In both composites, the presence of a solid solution formed by the diffusion of tantalum into the boride matrix was observed. The HfB2-based composite was harder (22 GPa), stiffer (528 GPa), and tougher (4.1M Pa·m1/2) than the ZrB2-based composite. Although the room-temperature flexural strength of the ZrB2-based composite (830 MPa) was higher than that of the HfB2-based composite (700 MPa), the opposite was true at 1200° and 1500°C. Contrary to the significant strength decrease observed for the ZrB2-based materials at elevated temperature, the HfB2 composite retained ∼86% of its room temperature strength up to 1500°C (∼600 MPa).
Journal of the American Ceramic Society 09/2008; 91(10):3285 - 3291. · 2.27 Impact Factor
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ABSTRACT: Submicro- and nano-sized liquid-phase-sintered SiC ceramics were mechanically tested by nanoindentation in the peak load range 5–400 mN. The submicro-sized sample showed a marked indentation size effect which the nano-sized samples did not exhibit. The relevance of indentation depth with respect to the microstructural scale has been outlined. In the investigated grain-size range, the hardness dependence on the grain size could be described by a load-dependent inverse Hall–Petch relation. Young's modulus was less microstructure- and load-dependent. Because of the very fine microstructure, the nano-sized SiC materials gave lower elastic values than the submicro-sized SiC ceramic.
Journal of the American Ceramic Society 01/2005; 87(11):2101 - 2107. · 2.27 Impact Factor
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ABSTRACT: Two fully dense composites containing AlN, SiC, and MoSi2 in different amounts were produced by hot pressing. Young modulus, hardness, electrical conductivity, and thermal expansion coefficient were measured at room temperature. Strength and toughness were evaluated up to 1300°C in air. According to statistical analysis, the temperature has little or no effect on high-temperature fracture behavior of the composites. These electroconductive materials are suitable for thermomechanical applications at high temperature, the strength being about 600 MPa at 1300°C.
Journal of the American Ceramic Society 12/2004; 86(10):1720 - 1726. · 2.27 Impact Factor
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ABSTRACT: Depth-sensing indentation tests were carried out on several SiC-based liquid-phase-sintered SiC ceramics characterized by different mean grain size spanning from 78 to 540 nm. The indentation tests were performed with peak loads ranging from 5 to 400 mN in order to investigate the property variation with the variation of the peak load and material microstructure. The values of indentation hardness and Young's modulus were calculated according to the models developed by Oliver and Pharr (O&P) and by Cheng and Cheng (C&C). According to the O&P model, the finest grained SiC ceramics did not show the indentation size effect (ISE) which was observed in the largest grained SiC ceramics. The grain-size dependence of the indentation hardness can be described by an inverse Hall–Petch relation. With the C&C model, the correlation of the indentation hardness with the peak load and the grain size was less evident. The indentation hardness calculated by the O&P model was lower than that calculated by the C&C model and in better agreement with the values of Vickers microhardness. The O&P indentation Young's modulus was higher than the C&C indentation Young's modulus but the latter was in very good agreement with the values measured by resonant frequency. For both models, the indentation Young's modulus was almost load-independent even if a dependence on the microstructure was observed.
Journal of the European Ceramic Society 27:1399-1404. · 2.35 Impact Factor
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ABSTRACT: The wear behaviour of two Al2O3–Mo and two Al2O3–Nb particulate composites with different Mo particle size and Nb contents, respectively, obtained by hot-pressing, was evaluated. The inverted pin-on-disc tests were performed by making ceramic pins slide against WC discs at different loads with a constant sliding speed and sliding distance. When the load increased the specific wear of alumina–molybdenum composites increased dramatically while the wear of alumina–niobium material remained almost constant inspite of the increase of the applied load. On the other hand, the wear of the WC discs was very low against Al2O3–Mo but extremely high against Al2O3–Nb composites. It was also found that the friction coefficient strongly depends on the applied load but weakly on the pin material. The lower the load the higher the friction. A possible explanation of the wear mechanisms is proposed.
Wear.
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ABSTRACT: In order to improve the fracture toughness, SiC whiskers or SiC chopped fibers were added to a ZrB2 matrix in volumetric fraction of 10 and 20 vol.%. The composites were hot-pressed between 1650 and 1730 °C and their final relative densities were higher than 95%. Even at the lowest sintering temperature, the whiskers showed an evident degradation. On the other hand, the fibers maintained their initial shape and a strong interface formed between matrix and reinforcement. The fracture toughness of the composites increased from 30 to 50% compared to the baseline material, with the fibers showing a slightly higher toughening effect. In the whiskers-reinforced composites, the room-temperature strength increased when 10 vol.% whiskers were added. In the fibers-reinforced composites, the room-temperature strength decreased regardless the amount of fibers added. The high-temperature strength of the composites was higher than that of the baseline material for both types of reinforcement.
Journal of the European Ceramic Society.
