No abstract

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.

a b s t r a c tTwo 5 vol% Ni/Al 2 O 3 composites with the difference in Al 2 O 3 grain size were fabricated by pulsed electric current sintering technique to investigate the influence of Al 2 O 3 grain size on oxidation behavior of the composites. Average Al 2 O 3 grain sizes of two fabricated composites were 1.1 m and 0.5 m after sintering. Oxidation tests were conducted at temperatures ranging from 1100 to 1350 • C for 1-48 h in air. A thin NiAl 2 O 4 layer was observed in exposed surface of samples after oxidation. An oxidized zone that consisted of Al 2 O 3 matrix and NiAl 2 O 4 grains was defined. Growth of the oxidized zone obeyed the parabolic law. Influences of Al 2 O 3 grain size on high-temperature oxidation of the composites were discussed.

The oxidation kinetics of 5 vol% nano-Ni dispersed Al 2 O 3 with Si-doping (nano-Ni/Al 2 O 3 -Si) was studied in the present paper. The starting powder mixture was prepared by drying aqueous slurry consisting of alumina with nickel nitrate and Si (OCH 3 ) 4 . The powder mixture was reduced at 600 C for 12 h in a stream of Ar-1%H 2 gas mixture. Nano-Ni/Al 2 O 3 -Si was densified by pulsed electric current sintering. The oxidation test was conducted at 1200-1350 C for 1-14 d in air. Oxidized zone consisted of Al 2 O 3 matrix and NiAl 2 O 4 as the oxidation product. Growth of the oxidized zone followed a parabolic manner. A thin NiAl 2 O 4 layer was also observed on the sample surface. Si-doping decreases growth rate of oxidized zone effectively at lower temperatures such 1200 C.

Crack-healing effectiveness was investigated on 5 vol% nano-Ni dispersed Al2O3hybrid 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/Al2O3were 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 NiAl2O4oxidation product which is developed by outward diffusion of cations at grain boundary of Al2O3matrix. Nano-Ni/Al2O3with Y or Si doping or SiC co-dispersion are realized to have crack-healing effectiveness with improved high-temperature oxidation resistance.