Fabricating the Cermet of the System Alumina-Chromium by Hot Pressing

Alumina-chromium cermet bodies containing 20 to 70 vol. % Cr were hot-pressed at 1400°-1470°C and 150kg/cm2 for 20-30 min., in argon atmosphere using graphite molds having a circular hole of 8mm in diameter, or a rectangular hole of 4×15mm.From the results of this study it was concluded that:(1) In hot pressing, increase in the chromium content decreased the press-temperature, for instance, 1400°-1430°C for 60 vol. % of chromium, being about 300°C lower than the hot-pressing temperature in hydrogen atomosphere (Table 1).(2) Measuring the hardness distribution along the center line of hot-pressed bodies whose hight to dia. ratio was 2:1 the method of pressing was studied, and the best result was obtained when the pressure was applied with a plunger moving downwards in a floating die. (Table 2 and Fig. 3).(3) The distribution of hardness in hot-pressed bodies of complex shape was studied with a result that there are some regions giving lower hardness values caused by the lack of the flowability of the mix, which suggests the necessity of three directional pressing (Photo. 2).(4) The transverse rupture strength of hot-pressed bodies of the size 4×4×15mm were 55-72kg/mm2 at room temperature.(5) The microstructure of hot-pressed bodies was characterized by the occurence of smaller corundum crystals of the order 1-3μ, and by the more rough boundaries between the alumina and chromium phase than those in the bodies sintered in hydrogen atmosphere. Continuous metallic phase was observed in the hot-pressed bodies containing more chromium than 60-70 vol. % (Photo. 3).(6) Fractures in the tension side of the specimens of bending rupture test passed through the crystals. of corundum, chromium as well as their phase boundary, and consequently, both chromium and corundum phases were appeared in the electron microphotographs (Photo. 4-5).