Mikio Iwasa

Centrifugal casting of Al2O3–15 wt.%ZrO2 ceramic composites

Abstract Aqueous ceramic slurries with different solid contents (60, 70 and 78 wt.%) were prepared by mixing 85 wt.% low-temperature-sinterable high purity α-alumina and 15 wt.% ZrO 2 (3 mol% Y 2 O 3 ) in distilled water and a small amount of poly-carboxylic ammonium as the dispersion agent. Consolidation was performed by using a high-speed centrifugal casting process at 4000 rpm for 1 h. The result showed that the dispersion agent had a great influence on the viscosities of the slurries. It was also observed that the green and sintered densities of the Al 2 O 3 –15 wt.%ZrO 2 composites increased with the solid contents. Phase segregation could be effectively eliminated when the solid content of the slurry was 78 wt.%.

Fabrication of three-dimensional tyranno fibre reinforced sic composite by the polymer precursor method

A three-dimensional Tyranno fibre reinforced SiC composite was fabricated by infiltration of molten polycarbosilane into three-dimensional textile preform (volume fraction; 39%) in an autoclave followed by pyrolysis in argon up to 1200 °C. An attempt to improve the density of the composite was done by repeated infiltration and pyrolysis. As a result of 10 times repetition, the density of the composite was increased to 2.4 g cm−3 (relative density; 83%, open porosity; 5%). Bending strength and fracture toughness were 420 MPa and 11.5 MPa m12, respectively. The composite showed semistable fracture behaviour because of bridging of fibre parallel with the tensile direction.

Self-reinforced Y-α-sialon ceramics with barium.aluminosilicate as an additive

Y-α-sialons (Y 0 . 3 3 3 Si 1 0 Al 2 ON 1 5 ) were prepared by hot pressing using 5 wt.% BaAl 2 Si 2 O 8 as an additive. The results showed that barium aluminosilicate not only served as a liquid-phase sintering aid to promote densification, but also facilitated the development of elongated α-sialon grains. The obtained self-toughened α-sialon was both hard and tough. The Vickers hardness, flexural strength, and fracture toughness are 18.9 GPa, 802 MPa, and 6.0 MPam 1 / 2 , respectively. Post heat treatment could promote the growth of elongated α-sialon grains, hence further increasing its toughness.

Increased cell response to zirconia ceramics by surface modification

The authors present a method to enhance the cell response to zirconia materials without sacrificing the mechanical properties. Zirconia specimens prepared by slip casting and pressureless sintering have excellent mechanical properties. After hydrothermal treatment in phosphoric solutions at 150°C for 2h, enhanced cell proliferation and high alkaline phosphatase activity were determined. This can be explained by the development of a more biocompatible surface phase (zirconium phosphate), which is believed to have high affinity for proteins. Results showed that it was possible to increase the cell response to zirconia ceramics by the proposed surface modification method.

Dispersion of ZrO2 Particles in the Al2O3/ZrO2 Ceramics by the Partial Chemical Dispersion Processes

To improve the mechanical properties of Al2O3/ZrO2 composites, the homogeneous dispersion of ultra low size ZrO2 particles in Al2O3 ceramics have been controlled by partial dispersion of ZrO2 by chemical processes such as coprecipitation or polymeric precursor method(Pechini process). So nanosized Zr/Y hydroxide were coprecipitated or polymerized directly to the surfaces of commercial sub-micron size α-alumina powder(Sumitomo: AES-11(0.4 μm)) using ZrOCl2 /Y(NO3)3 solution. By the partial coprecipitation method, dispersion of relatively small sized ZrO2 in Al2O3/ZrO2 composites could be achieved at 1500~1600° C of sintering temperature. In case of the polyesterization of Zr/Y(NO3)3-citric acid solution in ethylene glycol directly to the commercial sub-micron size α-alumina powder, more homogeneous dispersion of relatively low sized ZrO2 in Al2O3/ZrO2 composites could be obtained at 1450~1600°C of sintering temperature range and their mechanical strength was more enhanced.

Corrosion Resistance of Commercial Si3N4 Bearing Balls

The corrosion rate (Cr) of commercially available ball bearings made of silicon nitride (Si3N4) of high degree (HD) and standard quality (S) was measured at hydrothermal conditions in different aqueous concentrations of sulphuric acid (H2SO4) and sodium hydroxide (NaOH). The Cr magnitude for the materials diminished when increasing the acid concentration. At higher concentrations of H2SO4 (98%) the formation of SiO2 on the surface of the balls inhibited the progress of corrosion. The corrosion rate of the materials was increased when the sodium hydroxide solution concentration was increased. The surface of the materials was analyzed by means of scanning electron microscopy (SEM), while the corrosion rate was determined based on weight losses.

Size-Controlled Hydroxyapatite Nanoparticles as Self-Organized Organic-Inorganic Composite Materials

Sodium salt carboxymethyl cellulose (CMC) was used to prepare HAp-CMC composites through co-precipitation process. HAp nanorods with well controlled particle size were welll aligned along the c axis in the final composites. TEM, XRD, FTIR analysis were used to characterized the samples. It was found that the carboxyl groups in cellulose might be the main guiding site for the precipitation and growth of HAp and the formation of the resulting composites.

Synthesis and mechanical properties of 40 wt%BAS/Si3N4 ceramic composites

Advanced glass-ceramics, such as barium aluminosilicate (BaO · Al2O3 · 2SiO2) have found applications as matrices in composites, as heat exchanger parts and substrate materials [1]. Of the glass-ceramics being considered for high-temperature application, barium aluminosilicate (BAS) is one of the most promising because of its high melting temperature (1760 ◦C) [2], low thermal expansion (2.29 × 10−6 ◦C−1 from 22 ◦C to 1000 ◦C for the monoclinic form) [3], good oxidation resistance and low dielectric constant. However, the monolithic BAS glass-ceramic exhibits relatively poor mechanical properties, which limits its use in many structural applications. Whiskers [4], platelets [5] and continuous fibers [6] have been used to improve the mechanical properties of BAS. While improvements in properties have been demonstrated, ease of production and low cast have been sacrificed. In situ whisker growth can alleviate these problems. Although many candidate whisker materials are difficult to grow in situ, the formation of elongated, whisker-like β-Si3N4 grains has been observed in multiple studies [7–9]. Preparation of dense Si3N4 usually requires the use of additives, which react with the SiO2 present on the Si3N4 surface to form a liquid phase that facilitates densification. Although many oxide additives have been successfully demonstrated to produce dense Si3N4 materials, in most cases, the additives in Si3N4 end up as a grain-boundary glass phase, which softens and degrades the properties at high temperature. Therefore, a good additive system should form the liquid phase at a low temperature (a low liquid eutectic temperature) and later the liquid phase should crystallize completely into a compound with a high melting point. BAS fulfils these requirements. In this study, BAS composites reinforced in situ with Si3N4 whiskers were fabricated to investigate the effects of the BAS additive and sintering condition on

Dynamic hardness of thin films and its thickness dependence

Thin films of indium tin oxide, silica and chromium were deposited on the soda-lime-silica glass substrates with the thickness ranging from 50 to 2000 nano-meter. Their dynamic hardness were measured by the nano-indentation method with the indentation depth from 50 to 200 nano-meter. The hardness of indium tin oxide film increased and that of silica decreased with increasing film thickness. The hardness of chromium was almost constant regardless of film thickness. When the hardness of thin film was higher than that of substrate, the influence of substrate was greater compared with the reverse case. The dynamic hardness of engineering ceramics were about fifty percent higher than the ordinary Vickers hardness, but the ranking order was in good agreement. The Young`s modulus from nano-indentation were a little higher than those by resonance method.

Wear of hot-pressed aluminum nitride

窒化アルミニウム焼結体を1700°-2000℃の温度範囲でホットプレス法により作製した. その摩擦摩耗特性をピンオンディスク法で測定し, また摩耗の摺動速度依存性をサバン式試験法により調べた.焼結体の特性としては, ホットプレス温度1700℃では気孔が多く, 破壊靱性, ビッカース硬度はかなり低かった. 1800℃以上ではほぼ一定値に達するが, ホットプレス温度とともに粒子成長が激しくなっている.ピンオンディスク法により測定されたSiCディスクに対する摩擦係数, 比摩耗量はホットプレス温度とともに上昇した. マイルドな摺動条件では微小な結晶粒子よりなる摩耗粉が固体潤滑作用を示すのではないかと考えられる.サバン式試験法によれば各焼結体の比摩耗量はある摺動速度より急速に増加し, 明確なピークを示す. このピークの高さはホットプレス温度とともに低下する. 高速での摩耗は連続的な微小破壊によると考えられ, それゆえ, 破壊靱性やビッカース硬度の高い焼結体ほど摩耗しにくいと考えられる.