COMPOSITIONS FOR STRENGTHENING PORCELAIN BODIES IN ALUMINA FELDSPAR KAOLIN SYSTEM

The effect of composition on the bend strength of porcelain bodies in the alumina-feldspar-kaolin system has been investigated and the chara cteristics of the materials examined by X-ray diffraction, thermal exp ansion, and scanning electron microscopy. Porcelain bodies containing 45 wt-% feldspar and 55 wt-% kaolin showed full densification between 1150 and 1350-degrees-C. Without the addition of alumina, the mean str ength increased from 150 to 200 MN m-2 with increase in firing tempera ture from 1200 to 1350-degrees-C, and is attributed to the dissolution of coarse quartz grains in the feldspathic melt at the higher tempera ture to give a reduction in the effective size of fracture flaws. The strength of bodies fired at 1350-degrees-C increased proportionately w ith alumina additions up to 50 wt-%. A high strength of 371 MN m-2 was obtained for a body containing 50 wt-% alumina which is principally a ttributed to the achievement of full densification, in spite of the la rge amount of alumina, through the use of an optimum feldspar/kaolin r atio of 45:55. In addition, a dense layer free of pores larger than se veral micrometres formed close to the surface and is considered a cont ributing factor in the achievement of high strength.