Papers by Author: J.H. Ko

Abstract: The Ni-19at.%W coating was electodeposited to protect the stainless steel substrate, and then corroded between 500 and 700oC in Ar-0.2% SO2 atmosphere for 5 hr. The coating effectively protected the substrate from corrosion at 500oC, but failed at 700oC owing to serious corrosion. The corrosion products were mainly NiO, WO3 and NiWO4, together with some NiS, FeS, Fe2O3 and FeWO4. Detailed scale morphologies and corrosion mechanism are presented.

Abstract: Fine-grained and coarse-grained Ti3SiC2 materials were synthesized by hot pressing using TiC0.67 and Si powders as starting materials. The oxidation between 900 and 1100oC in air resulted in the formation of an outer TiO2 (rutile) layer and an inner mixed layer of TiO2 and amorphous SiO2, accompanying the liberation of carbon into the air. Fine-grained Ti3SiC2 displayed better oxidation resistance than coarse-grained one, with activation energies of 376.5~380 kJ/mol for oxidation.

Abstract: The oxidation behavior of the equiatomic TiNi alloy was investigated at 600-900oC. The oxidation resistance of the TiNi alloy was better than the pure Ti alloy, but worse than the pure Ni alloy. The oxide scales consisted of TiO2, with and without embedded particles of TiNiO3 and Ni. The subscale matrix phase was (TiNi3+TiNi) at 600oC, TiNi3 at 700oC, (Ni+TiNi3) at 800oC, and Ni at 900oC. Oxidation was mainly governed by the inward diffusion of oxygen, and the outward diffusion of Ti and a bit of Ni. Detailed microstructures and the oxidation mechanism are proposed.

Abstract: Stainless steel and Ti metal fibers having a diameter of 3 µm were produced from wires by multiple extrusions. The suitable sheath coating for stainless steel to extrude the core wires to fibers was the Cu coating having ~30 µm thickness. Zinc was not a suitable sheath coating, because Zn of the low melting point had diffused into the stainless steel wires during extrusion. The oxidation of stainless steel fibers produced using the Cu sheath coating oxidized rapidly above 750°C due to the high surface area of fibers. The utilization of the Cu coating as a sheath material to extrude the core Ti wires to fibers was not possible, because the highly reactive Ti wires resisted deforming to fibers.