Chihiro Kawashima

Studies on TiC-Co Cermets Sintered in a Hydrogen Molybdenum Furnace

The cermets of TIC-Ni system containing 5 to 30% metallic nickel as a bond were studied. As the technical processes of producing TiC-Co cermets apply almost unmodified to the present case, the authors have limited their attention mainly to the effects of the forming process and the sintering temperature. The compacting pressure, however, was increased to 6ton/cm2 as the higher pressure has proved to give the better results in the sintering shrinkage, and the transverse rupture strength of the sintered psecimens.The general trends which the TiC-Ni system showed in the process of production as well as in the properties of the finished products were found to be the same as in the case of the TiC-Co cermets. Although the former is easier to handle it is less resistive to aerial oxydation than the latter. The study of the micro-photographs of the specimens of TiC-Ni cermets revealed that the grain growth of TiC was accelerated considerably with the increasing Ni content, which is the cause that brings to the lower transverse rupture strength of the finished products.

On the Apparatus for Measuring the Warpage of Refractory Coated Steel Plate at high Temperatures

A new method for measuring the glost-warpage of refractory coating caused by the internal stress developing during the heating was described. The apparatus was designed so that it satisfies the following three conditions:(1) Easy and stable setting of the specimen, (2) Preventive measure against the oxidation during the heating, (3) Accurate measurement of slight strain.The specimen was prepared by applying the coating to one side of a piece of a Steel plate (0.8×20×80mm). It was inserted into the silica tube of the heating furnace to rest horizontally on the saddles of silica glass (Fig. 3). To avoid the oxidation nitrogen or argon was introduced from one end of the silica tube to which a small hole was provided at a proper position as a gas-outlet. For easy setting of the specimen the upper part of the furnace was cut off to form a longitudinal groove of 5mm in width. The deflection was measured by the displacement of the thin silica tube standing vertically at the middle point of the specimen.Transition point, Pt, and softening point, Ps, of the thin coated layer could easily be evaluated from the deflection-temperature curve. A linear relation between Pt, Ps, and the content of alumina in mill addition was confirmed when the specimen was fired in nitrogen atmosphere. Moreover, the possibilities of evaluating the expansivity of coating layer and the internal stress developed in it were discussed making use of the examples of the actual deflection curves.Deflection of warpage, δ, can be expressed byδ=K⋅Δα⋅Δθ⋅l2/hK=3pq(1+2p)/(4pq+1), p2<<1, q<1, where, Δα stands for the difference of the thermal expansion coefficients between the coating layer and the base metal. Δθ the temperature difference, l the span, and h the total thickness of the specimen, p=hc/hs and q=Ec/Es, where hc, Es and Ec are respectively, the thickness and the elastic constant of the base metal, that of the coating layer as indicated in Fig. 8.It was established experimentally that the relation between the coefficients of glost-warpage and the thickness of coating is expressed by a non-linear function.

Effects on the Thermal Shock Resistance of Ceramic Coating of Nickel-flash and Mill Addition of Cobalt Oxide Fired in Nitrogen Stream

The effects of nickel flashing on thermal-shock resistance of ceramic coating and mill additions of cobalt oxide on enamelling in nitrogen stream were studied from the points of interfacial microstructure proposed and adherence strength.The new method for measuring thermal-shock resistance of ceramic coating is that the coated specimens are heated for 5 min. at the specified constant temperature which is taken either as the maximum saftey-temperature for actual use, or (0.7-0.8)×(enamelling temperature 3°C), and rapidly cooled in water. These heating and cooling operations are repeated until the gain in weight by oxydation of base metal becomes comparable with loss by peeling off. Then the relation between the amounts of peeling of coated layer and times of repeating will be expressed by a curve which has asymptotic constant value of saturation. From this curve thermal-shock resistance, R(%), can be calculated by following formula.R(%)=Maxium amounts of peeling/Initial weights of coating layer×100%The resistance, is thought to be nearly proportional to the adherence strength at room temperature. However, the existence of ferrosilicate layers which develope at 850°C for 30 sec. on interface by prefiring of nickel flashed base metal. Also increases thermal-shock resistance. From this reason, it is considered the ferro-silicate layer whose thickness is about 2-3μ, acts as to decrease the maximum amount of stress occured in coating layer.The catalytic action of metallic nickel deposited by nickel flashing, being expressed by Fe3O4+Fe+Ni→4FeO+Ni, is promoted by acid etching after nickel flashing. Therefore, the kinds of acid and etching methods are important procedures for good enamelling.For firing in nitrogen stream, mill additions of cobalt oxide are effective to increase adherence strength and thermal-shock resistance. Most effective addition is 3% CoO. This is due to the fact that selective galvanic corrosion of CoO remarkably occurs at this content and this corrosion state can be seen by the micro-photographs of interface. (Fig. 12)When the galvanic corrosion of CoO and reducing action of nickel flashing simultaneously act during enamelling, these actions cancel each other so that both of thermal-shock resistance and adherence strength of specimen greatly decrease, in general.

Fundamental Studies on the Spray Drying and Semi-Dry Press of Ceramics, (III)

The free-flowing phenomenon of spherical fine particles (48-88μ) obtained by spray drying of ceramic clay slips was studied. As a result, we concluded that the following expression is established between particle size dp, nozzle diameter d0 and equilibrium flow velocity Ve.Ve=Kd2(dp-d0)where, K is a constant defined only by the physical properties of particles and is so-called the coefficient of free flowing of spherical fine particle, and d0 is a diameter of the smallest particle that is necessary to do free flowing.In the case of irregular particles and mixing powders, the above relationship is approximately satisfied. However, according to the nozzle diameter becoming larger, the equilibrium velocity has a tendency to become unstable.7 Figs, 6 References.