Key Engineering Materials Vols. 317-318

Abstract: Ceramic nanocomposites became nowadays an important ingredient of many structural and electronic ceramics, as well as ceramic coatings. The same applies to chemically processed and environmental related ceramics. The performance and characteristics of ceramic components are considerably influenced by the characteristics of precursor powder. The outstanding properties possessed by advanced nanoceramics are achieved through exceptional composition and microstructure that require very careful control throughout the successive stages of the applied processing.

Abstract: Based on the FGM concept, laminated alumina tube with a tailored porosity gradient along the radial direction has been successfully fabricated by the lamination method in the centrifugal molding technique. Experiments were performed with colloidally processed alumina powder and pore-former agent. The powder mixture was made into slurry in water media and the tubes were cast using a stainless steel mold. The porosity profile was designed with the presence of PMMA particles as pore-former agent and the nature of porosity was investigated by microstructure observations. The bimodal pore structure of the tubes was constructed from large spherical pores about 10 μm formed by burning-out the pore-former agent and small sub-micron pores caused by the lower sintering temperature. The fracture behavior of porous tubes with tailored porosity gradient was investigated using the O-ring compression testing. The failure behavior was divided into two types depending on the pore-former agent. The air permeability and fracture behavior were independent of the lamination process. The influence of the porosity gradient on the fracture strength was investigated and the results showed there was a reduction in fracture strength with increase in the laminate number, and the minimum strength was found in continuous graded tubes.

Abstract: Nickel dispersed alumina matrix nanocomposites were fabricated using a novel soaking method. Secondary particles were introduced into the nano-pores of a porous matrix grains by a soaking method, such that γ-alumina powder was soaked in nickel nitrate solution under vacuum. During pre-calcination, nickel oxide particles were created inside of the nano-pores of γ-alumina. The alumina powders were then reduced under hydrogen atmosphere to obtain nano-sized metallic nickel embedded in γ-alumina grains. The alumina-nickel composite powders were sintered by pulse electric current sintering (PECS) technique with α-alumina seeds. The maximum strength of the alumina-nickel nanocomposites was 984 MPa after sintering at 1,450 °C with α-alumina seeds, where the specimen size was 2
2
10 mm3. The maximum fracture toughness was 5.5 MPa·m1/2 after sintering at 1,350 °C with seeds measured by the single edge V-notched beam (SEVNB) method.

Abstract: Mullite-based iron nanocomposites were prepared by the reduction of a mullite-iron oxide solid solution and successive hot pressing. The solid solution was obtained from the heat treatment of diphasic gel by sol-gel method. Some of the α-iron nanoparticles have an intra-granular structure just after reduction. Mechanical properties are strongly affected by the content of iron. Low iron content is beneficial to strengthening while high iron content can improve the fracture toughness. Furthermore, the nanocomposites also behave ferromagnetic properties at room temperature.

Abstract: The 0.75 to 3 mol% Y2O3-stabilized tetragonal ZrO2 and Al2O3/Y-TZP nano-composite ceramics with 0.2 to 0.7 wt% of alumina were produced by a colloidal technique and low-temperature sintering. The influence of the resulting density, microstructure, the yttria-stabilizer and the alumina content on toughness was determined. The bulk 2.7Y-TZP ceramic with an average grain size of 110 nm reached fracture toughness of 11.2 MPa·m1/2. A nano-grained alumina/zirconia composite with an average grain size of 92 nm was obtained. Y-TZP ceramics with a reduced yttria-stabilizer content were shown to reach fracture toughness of 13.8 MPa·m1/2 (2Y-TZP), and 14.5 MPa·m1/2 (1.5Y-TZP). Y-TZP/alumina composites with 0.35 wt% of Al2O3 were shown to reach fracture toughness of 15.7 MPa·m1/2 (2Y), 15.3 MPa·m1/2 (1.5Y).

Abstract: A bundle of feedrod composed of ordinary arranged alumina and zirconia green rods was co-extruded through a 6:1 reduction die. The volume fraction of zirconia phase was varied from 10 to 88 vol%. After the first co-extrusion, the individual pieces were bundled and co-extruded again, reducing the lateral size of each phase and multiplying the number of continuous monofilaments. After a 3rd extrusion step and sintering at 1600oC, crack-free composites with a fiber diameter of ~50 μm were obtained for all compositions. The fracture toughness of the composites was improved by introducing fine second phase filaments into the matrix. The maximum fracture toughness of 6.2 MPa
m1/2 was attained in the 3rd co-extruded composite which consisted of 53 vol% alumina and 47 vol% zirconia. Bending strength of the composites was almost the same as that of the monolithic alumina regardless of the composition.

Abstract: Nanocomposite formation of metal-metal oxide systems by mechanical alloying (MA) has been investigated at room temperature. The systems we chose are the Fe3O4-M (M=Al, Ti), where pure metals are used as a reducing agent. It is found that nanocomposite powders in which Al2O3 and TiO2 are dispersed in a α-Fe matrix with nano-sized grains are obtained by MA of Fe3O4 with Al and Ti for 25 and 75 hours, respectively. It is suggested that the shorter MA time for the nanocomposite formation in Fe3O4-Al is due to a large negative heat associated with the chemical reduction of magnetite by aluminum. X-ray diffraction results show that the average grain size of α-Fe in Fe-TiO2 nanocomposite powders is in the range of 30 nm. From magnetic measurement, we can also obtain indirect information about the details of the solid-state reduction process during MA.

Abstract: Y- α-sialon (m=1.35, n=0.675) ceramics were prepared by high-energy mechanical milling followed by spark plasma sintering. The milling promoted not only liquid-phase sintering, but also phase transformation from β-Si3N4 to α-sialon. Under the same holding time of 5 min, milled powder could be completely densified at 1500oC, which is about 250oC lower than that required for as-received powder. The temperature where the phase transformation finished was 1600oC and 1750oC for milled and as-received powder, respectively. The grain size of obtained dense ceramics from milled powder was significantly decreased. Nano-sized dense ceramics have been obtained by sintering the milled powder at 1500oC for 5 min. Although 100 % α-sialon has not been achieved, the nano-sized ceramics can be used for superplastic deformation, taking advantage of small grain size and large amount of transient liquid phase.

Abstract: β-SiAlON nanoceramics were fabricated from β-SiAlON nano powder using the spark-plasma sintering (SPS) technique. The β-SiAlON nanopowder (Si4Al2O2N6) was synthesized from a mixture of SiO2 (QS-102, Tokuyama Co., Japan), AlOOH (Tomita, Japan) and C (Mitsubishi Chemical, Japan) using the carbothermal reduction nitridation (CRN) method. The heating rate for SPS was 50
/min. The β-SiAlON nanoceramics had high strength (500 MPa). TEM observation showed that the intergranular glassy phase was scarcely present at the grain boundary of the β-SiAlON nanoceramics. Aqueous corrosion resistance was evaluated by measuring the weight loss after soaking in 5 and 35 wt.% H2SO4aq. and 5 wt.% HNO3aq. at 80
for 100 h. It was found that β-SiAlON nanoceramics have much higher corrosion resistance than commercialized silicon nitride ceramics in acid solutions. Commercialized Si3N4 ceramics have an intergranular glassy phase created as a result of the sintering aids in them. Thus, they are easily corroded by acid solutions because the intergranular glassy phase is easily corroded under such conditions. The excellent corrosion resistance of the β-SiAlON nanoceramics stems from their glass-free grain boundaries, since the β-SiAlON nanoceramics were produced without using a sintering aid.

Abstract: The AlN/h-BN nanocomposite powders were synthesized through the reaction of AlN powder, boric acid (H3BO3) and urea (CO(NH2)2) in a nitrogen atmosphere, and the machinable AlN/BN ceramic nanocomposite s were fabricated by hot-pressing in N2 atmosphere. The existing and distribution of h-BN phase are revealed by X-ray diffraction (XRD), TEM and SEM. For the existing of weak interface between h-BN and AlN grains, the machinability of AlN/BN composites is improved obviously. For the finer microstructures, the mechanical properties and the machinability of the composites with micrometer sized AlN coated with nano-sized BN are better than the AlN/h-BN composite of mechanical mixing type.