Masahiro Nawa

Fabrication and mechanical behaviour of Al2O3/Mo nanocomposites

Two types of Al2O3/Mo composites were fabricated by hot-pressing a mixture of γ- or α-Al2O3 powder and a fine molybdenum powder. For Al2O3/5 vol% Mo composite using γ-Al2O3 as a starting powder, the elongated molybdenum layers were observed to surround a part of the Al2O3 grains, which resulted in an apparent high value of fracture toughness (7.1 Mpa m1/2). In the system using α-Al2O3 as a starting powder, nanometre sized molybdenum particles were dispersed within the Al2O3 grains and at the grain boundaries. Thus, it was confirmed that ceramic/metal nanocomposite was successfully fabricated in the Al2O3/Mo composite system. With increasing molybdenum content, the elongated molybdenum particles were formed at Al2O3 grain boundaries. Considerable improvements of mechanical properties were observed, such as hardness of 19.2 GPa, fracture strength of 884 MPa and toughness of 7.6 MPa m1/2 in the composites containing 5, 7.5, 20 vol% Mo, respectively; however, they were not enhanced simultaneously. The relationships between microstructure and mechanical properties are also discussed.

Biaxial flexure strength and low temperature degradation of Ce‐TZP/Al2O3 nanocomposite and Y‐TZP as dental restoratives

The purpose of the present study was to evaluate the mechanical durability of a zirconia/alumina nanocomposite stabilized with cerium oxide (Ce-TZP/Al(2)O(3) nanocomposite) in comparison to yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) and discuss its application on ceramic dental restorations. The disk-shaped specimens of both materials were stored in physiological saline solution at 80 degrees C for 30 days, in 4% acetic acid at 80 degrees C for 30 days, and in an autoclave at 121 degrees C for 10 days. Before and after storage, specimens were subjected to the biaxial flexure test and to the determination of the monoclinic zirconia content. After autoclaving, Y-TZP showed remarkable increasing of the content of monoclinic zirconia: 0.3 vol % before and 49.9 vol % after, and slight decreasing of biaxial flexure strength: 1046 MPa before and 892 MPa after; whereas Ce-TZP/Al(2)O(3) nanocomposite showed no significant difference in the monoclinic content (4.8-5.5 vol %) and the biaxial flexure strength (1371-1422 MPa) after storage in any conditions. It is concluded that, compared to Y-TZP, the Ce-TZP/Al(2)O(3) nanocomposite has a high biaxial flexure strength along with a satisfactory durability in terms of low-temperature aging degradation in above conditions. This study indicates that the Ce-TZP/Al(2)O(3) nanocomposite demonstrates excellent mechanical durability for dental restorations such as all-ceramic bridges.

The effect of TiO2 addition on strengthening and toughening in intragranular type of 12Ce-TZP/Al2O3 nanocomposites

Abstract To compensate the definite disadvantages of lower strength and lower hardness in Ce-TZP, an intragranular type of 12Ce-TZP/0–30 vol% Al2O3 nanocomposite was investigated. Furthermore, the effect of TiO2 addition on strengthening and toughening was also examined for the optimum component of 30 vol% Al2O3, which showed a maximum strength of 866 MPa. For these composites, a novel interpenetrated intragranular microstructure was presented, in which either 200-nm-sized Al2O3 particles or 30–50-nm-sized ZrO2 particles were trapped within the ZrO2 grains or Al2O3 grains, respectively. The TiO2 was confirmed to dissolve into the tetragonal ZrO2 lattice, and contributed to the phase stability of the tetragonal phase and the promotion of intragranular nano-dispersion. For the small amount (0.2 mol%) of TiO2 doped 12Ce-TZP/30 vol% Al2O3 composite, simultaneous improvements in strength (1012 MPa) and toughness ( 10.2 MPa · m 1 2 for the IF method, 5.7 MPa · m 1 2 for the SEVNB method) were achieved through tailoring the sintering condition.

Microstructure and mechanical behaviour of 3Y-TZP/Mo nanocomposites possessing a novel interpenetrated intragranular microstructure

Yttria stabilized tetragonal zirconia polycrystal (Y-TZP)/0-100 vol % molybdenum (Mo) composites were fabricated by hot-pressing a mixture of Y-TZP powder containing 3 mol % yttria (Y2O3) and a fine Mo powder in vacuum. This composite system possessed a novel microstructural feature composed of an interpenetrated intragranular nanostructure, in which either nanometer sized Mo particles or equivalent sized zirconia (ZrO2) particles located within the ZrO2 grains or Mo grains, respectively. The strength and toughness were both greatly enhanced with increasing Mo content for the 3Y-TZP/Mo composites thus breaking through the strength-toughness tradeoff relation in transformation toughened ZrO2 and its composite materials. They exhibited a maximum strength of 2100 MPa and a toughness of 11.4 MPa·m1/2 for the composite containing 70 vol % Mo. These simultaneous improvements in strength and toughness were determined to be the result of a decrease in flaw size associated with the interpenetrated intragranular nanostructure, and a stress shielding effect created in the crack tip by the elongated Mo polycrystals bridging the crack tip in addition to the stress induced phase transformation.

A new type of nanocomposite in tetragonal zirconia polycrystal-molybdenum system

Abstract A ceramic/metal nanocomposite in tetragonal zirconia-molybdenum system possesses a novel microstructural feature composed of a mutual intragranular nanostructure, in which either nanometer sized Mo particles or equivalent sized zirconia particles are located within the zirconia grains or Mo grains, respectively. It achieves a simultaneous improvement in strength and toughness that overcomes the strength-toughness tradeoff relation.

Effect of Sandblasting and Heat Treatment on Biaxial Flexure Strength of the Zirconia/Alumina Nanocomposite

The effect of sandblasting and heat treatment on biaxial flexure strengths of the zirconia/alumina nanocomposite stabilized with cerium oxide (Ce-TZP/Al2O3 nanocomposite) was evaluated in comparison to that of yttria stabilized tetragonal zirconia polycrystals (Y-TZP). The disc-shaped specimens of the nanocomposite and Y-TZP were sandblasted with 70)m alumina powder. After sandblasting, half of the specimens were heated at 1000°C for 5 min. The biaxial flexure strengths of Y-TZP were independent on the sandblasting, but decreased with the heat treatment. On the other hand, the biaxial flexure strength of the nanocomposite increased with the sandblasting and significantly decreased with the heat treatment. The content of monoclinic ZrO2 of Y-TZP and the nanocomposite increased with the sandblasting pressure and dramatically decreased with the heat treatment. These results suggest that the stress-induced transformation from tetragonal to monoclinic of the nanocomposite occurs more easily than Y-TZP.

Mechanical Properties of Zirconia/Alumina Nano-Composite after Soaking in Various Water-Based Conditions

Yttria stabilized tetragonal zirconia polycrystals (Y-TZP) have been applied to dental crown and bridges. Whereas, to further improve its mechanical strength, the zirconia/alumina nano-composite stabilized with cerium oxide (Ce-TZP/Al2O3 nano-composite) was developed. In the present study, biaxial flexure strength, fracture toughness and hardness were determined before and after soaking in water-based conditions and the possibility of application to all ceramic dental restorations was discussed. In comparison to Y-TZP, Ce-TZP/Al2O3 nano-composite has quite high flexure strength and fracture toughness along with satisfied durability for LTAD in various water-based conditions encountered in dentistry. Therefore, it is concluded that the nano-composite can be safely applied to dental restoratives such as all-ceramic bridges.

Effect of Grinding, Sandblasting and Heat Treatment on the Phase Transformation of Zirconia Surface

This study was aimed to investigate the effect of grinding, sandblasting by alumina and SiC, and heat treatment on the phase transformation from tetragonal to monoclinic zirconia on the surface of yttria stabilized tetragonal zirconia (Y-TZP) and zirconia/alumina nanocomposite stabilized with cerium oxide (Ce-TZP/Al2O3 nanocomposite). The monoclinic phase content of both materials increased with the grinding and the sandblasting, while it decreased with the heat treatment. The monoclinic content sequentially increased with the sandblasting and decreased with the heat treatment to each specific value. The SiC-sandblasting produced the larger monoclinic content than alumina-sandblasting. Furthermore, the content changes of the nanocomposite were larger than Y-TZP.

Effect of Sintering Condition, Sandblasting and Heat Treatment on Biaxial Flexure Strength of Zirconia

The effect of sintering condition, sandblasting and heat treatment on biaxial flexure strengths of the zirconia/ alumina nanocomposite stabilized with cerium oxide (Ce-TZP/Al2O3 nanocomposite, referred to NANOZR) was evaluated in comparison to that of yttria stabilized tetragonal zirconia polycrystals (Y-TZP). The disc-shaped specimens of NANOZR and Y-TZP were cut from the full-sintered or middle-sintered ones. The discs cut from the middle-sintered ones were finally sintered at the same temperature for the full-sintered one. These four kinds of disc were treated in various conditions combined with the sandblasting, the heat treatment, and the storage. The biaxial flexure strength of both middle- and full-sintered Y-TZP decreased with the autoclaving, whereas those of both NANOZR did not change with it. The monoclinic content of both the materials increased with the sandblasting and decreased with the heat treatment. Regardless of the sintering condition, the monoclinic content of the Y-TZP dramatically increased with the autoclaving and those of NANOZR remarkably increased with the sandblasting. Regardless of the different surface roughness, the biaxial flexure strengths of both materials strongly depended on the content of monoclinic ZrO2 on the surface.

Fracture Mechanics and Toughening Mechanisms Analysis of Ce-TZP/Al2O3 Nanocomposite for Biomedical Applications

Zirconia ceramics were introduced in the seventhies for use as structural biomaterials after laboratory tests and simulator studies. However, nowadays concerns remain about their reliability in vivo, despite published clinical studies have already established the safety and the good tribological performance of these materials. It is still unclear what level of reliability can be achieved in ceramic biomaterials and how much their toughness level can be enhanced by microstructural design. The polycrystalline nature of ceramic materials may make both the observed properties and performance very scattered. In particular, the grain size and other microstructural features likely play a fundamental role in the mechanical behavior of the material. In this paper, we propose a set of fracture mechanics assessments, aimed to establish the quantitative amount of toughness achievable in a zirconia/alumina nanocomposite stabilized with cerium oxide (Ce-TZP/Al2O3 nanocomposite), and in situ confocal Raman spectroscopy to visualize toughening mechanisms, including polymorph transformation and residual stress fields stored around the crack path.