アルミナ系サーメットにおける配合金属の選択について

The factors for the selections of metals to be added to alumina were discussed after the fabrication experiments were described. Since thirteen metals given at Table 2 could be added as metal components in the cermets, all of these metals, their mixtures and industrial ferro-alloys were used in experiments. The mixing of metal and alumina components were made in small steel mill using tungsten carbide balls and methanol for 15-20 hours. The mixed batches were pressed under 1000Kg/cm² to 4×12×68mm. bars, and were sintered for 1 hour in the molybdenum furnace in the atomosphere of hydrogen dried by P₂O₅.<br>The results of sintering experiments were summerized in Table 4 and 5 where composition, sintering temperature, shrinkage, bulk density, water absorption, and transverse strength at room temperature and at 1000°C in air are listed. Contact angles of molten metals on alumina supports were measured at room temperature and listed in Table 6. From these, experiments, the following results were obtained; The cermets containing Sn, Al, Ag, Cu, Mn, and Si were impossible to sinter. Al, Mn, Si, and Ti metals were partially oxidized in this atmosphere. Strengths at room themperature increased in the order of Ni, Co, Fe, Cr, this order being also that of the decrease of contact angle values (except of Co), while the 1000°C-strength was supposed to be related to the oxidation resistance of metals. Tungsten and molybdenum were oxidized and sublimed at the 1000°C-tests. The strength of alumina-chromium or alumina-chromium alloy cermets were generally high, and this might be related to small contact angles of Cr or Cr-alloy.<br>Two groups of factors are considered to raise the strength of cermet. These are (1) the factors to promote sintering and (2) the factors concerning to the good texture of sintered body. The former factors are; particle size and crystal grade of original materials, the slight oxidation of metal by oxygen component in the sintering atmosphere and by reaction with alumina, and contact angle and surface tension of metals; and the latter factors are the strength and the microstructure of oxide phase, metal phase, and boundary phase. The strength of boundary phase between metal and oxide is related to physical adherence and the difference in thermal expansion coefficients between oxide and metal phase. It is apparent from the above two factors that iron or chromium metal and these alloys have suitable qualities to be added to alumina.<br>It is considered that the different sintering atmospheres, especially some changes in oxygen content in hydrogen atmosphere, may be required for the respective metals, and these contents may be approximately the same as those in the oxidation and reduction equilibrium of metals calculated from their free energy values. The abrupt adsorption and desorption of hydrogen at temperatures of phase transformation of metals may have also an important effect on the boundary adherence. The presence of carbon in a hydrogen furnace causes the carbide formation in Cr, Mo and W metals through CO gas, and the formation of Al metal, carbide and oxycarbide by contact with alumina, finally the formation of the AlO and Al₂O vapors. The reactivity of metals with alumina to produce AlO and Al₂O increases in the order of Ni, Co, Fe, Cr with relation to the smaller contact angles. Ti has a great reactivity with alumina. The water vapor pressure inhydrogen, calculated by using the oxidation and reduction equilibrium of metal-alumina compound (for instance, spinell) is generally lower than those calculated similarly with metals alone. It is, therefore, thaught from these results that the vapor pressure of water actually present in hydrogen is perhaps less than the calculated value (Table 7) from metals only. (Rf. This Journal, 64, 183 (1956)