Daisuke Maruoka

Crack Disappearance by High-Temperature Oxidation of Alumina Toughened by Ni Nano-Particles

Crack disappearance by high-temperature oxidation was studied in alumina (Al2O3) composites toughened by Ni nanoparticles. This process is performed in air at temperature ranging from 1000 to 1300°C for 1 to 48 h. The results showed that crack disappearance depends on both annealing temperature and time. Complete crack disappearance in this composite was confirmed at lower temperatures for long oxidation period, 1100oC for 48 h, and higher temperature for shorter time, 1300oC for 1 h in air. The crack disappearance mechanism was explained on the basis of the formation of NiAl2O4 spinel on sample surfaces produced by the oxidation reaction during the heat treatment.

Crack-Healing Effectiveness of Nano Ni + SiC Co-Dispersed Alumina Hybrid Materials

Crack-healing effectiveness of nano-Ni+SiC co-dispersed Al2O3 hybrid materials by heat treatment was studied as well as oxidation behavior. The starting powder mixture was prepared by drying slurry consisting of distilled water, Al2O3, SiC powder and nickel nitrate. After reduction, the powder mixture was densified by pulsed electric current sintering. Cracks were introduced by a Vickers indenter. Specimens were exposed into air at temperature ranging from 1000 and 1300°C for 1 and 48 h. Thickness of oxidized zone in SiC+nano-Ni/Al2O3 thinner than nano-Ni/Al2O3. Cracks of SiC+nano-Ni/Al2O3 disappeared completely, for example, by oxidation at 1200°C for 6 h, as same as nano-Ni/Al2O3. Bending strength of crack-disappeared SiC+nano-Ni/Al2O3 showed 519 MPa and was comparable with that of as-sintered one. SiC+nano-Ni/Al2O3 pocesses crack-healing effectiveness with improved high-temperature oxidation resistance.

Influences of Al2O3 grain size on high-temperature oxidation of nano-Ni/Al2O3 composites

Abstract Two 5 vol% Ni/Al2O3 composites with the difference in Al2O3 grain size were fabricated by pulsed electric current sintering technique to investigate the influence of Al2O3 grain size on oxidation behavior of the composites. Average Al2O3 grain sizes of two fabricated composites were 1.1 μm and 0.5 μm after sintering. Oxidation tests were conducted at temperatures ranging from 1100 to 1350 °C for 1–48 h in air. A thin NiAl2O4 layer was observed in exposed surface of samples after oxidation. An oxidized zone that consisted of Al2O3 matrix and NiAl2O4 grains was defined. Growth of the oxidized zone obeyed the parabolic law. Influences of Al2O3 grain size on high-temperature oxidation of the composites were discussed.

Crack Healing of Nano-Ni/Al2O3 Hybrid Materials via High-Temperature Oxidation

Crack-healing effectiveness was investigated on 5 vol% nano-Ni dispersed Al2O3 hybrid materials. Influence of the Y or Si doping or SiC co-dispersion was also studied on the crack healing behavior. Cracks were introduced by a Vickers indentation to be a crack length of approximately 60 μm. Cracks of nano-Ni/Al2O3 were completely disappeared, for example, by oxidation at 1200°C for 6 h in air, Y/Si doped one and SiC co-dispersed one have similar performance of crack disappearance. Bending strength of crack-disappeared samples showed about 550 MPa and was comparable or improved with that of as-sintered one. Mechanism of crack healing was considered as filling up of cracks by NiAl2O4 oxidation product which is developed by outward diffusion of cations at grain boundary of Al2O3 matrix. Nano-Ni/Al2O3 with Y or Si doping or SiC co-dispersion are realized to have crack-healing effectiveness with improved high-temperature oxidation resistance.

Crack-healing function of metal/Al2O3 hybrid materials

Nano-Ni/Al2O3 hybrid materials have the crack-healing function by thermal oxidation process such as 1200°C for 6 h in air. In this hybrid material system, crack was filled up by an oxidation product, NiAl2O4, via outward diffusion of cations along grain boundaries of Al2O3 matrix. Ni/Al2O3 with Y2O3 doping and SiC+Ni/Al2O3 nano-hybrid materials have similar crack-healing performance with better oxidation resistance at high temperatures than Ni/Al2O3 nano-hybrid materials. Mo/Al2O3 hybrid materials were studied on a candidate with crack-healing function via thermal oxidation process at temperatures as low as 700°C.

Multi-Functional Metal/Ceramic Hybrid Composites for Structural Applications

Some ceramic based hybrid composites with metallic particles have excellent mechanical properties. In particular, metal/ceramic nanocomposites has high mechanical strength and improved fracture toughness. Magnetized hybrid composites can be produced by dispersing Fe, Co or Ni nanoparticle in the structural ceramics. Crack healing of metal/ceramic hybrid composites was investigated for structural applications. Cracks of nanoNi/Al2O3 introduced by the Vickers indentation of 49 N could be repaired completely, for example, by air oxidation at 1200°C for 6 h. Even larger cracks introduced by 490 N indentation was healed up to a comparable level of mechanical strength by oxidation at 1200°C for 24 h in air. Outward diffusion of cations during high-temperature oxidation would cause crack repair by filling with an oxidation product. Faster crack disappearance, 900°C for 1 h, was realized on Mo/Al2O3. Surface crack healing effectiveness promoted by outward diffusion of cations is potentially applied into various Al2O3 based hybrid material systems, as well as excellent mechanical properties.

Oxidation of Mo/Al2O3 hybrid materials at high temperatures

In the present work, the oxidation behaviour of 5 vol% Mo/Al2O3 hybrid materials was investigated at high temperatures in air. At oxidation temperature ranging from 600 to 800?C, Al2(MoO4)3 and MoO3 were developed. The growth of the oxidized zone, at which metallic Mo was not observed, obeyed the parabolic manner. The apparent activation energy of the parabolic rate constant was equal to 95 kJmol?1. The microstructures showed that there were many voids in the oxidized zone. Significant cracks, which would be formed due to volume expansion of Mo particles via inward diffusion of oxygen, were not observed in oxidized zone. They must be the evidence of outward diffusion of cations, most likely Mo6+ at grain boundary. As the temperature over 800?C, the evaporation of MoO3 occurred dramatically.