Effects of added kaolinite on the strength and porosity of freeze-cast kaolinite–silica nanocomposites

Abstract

This paper examines the effects of kaolinite on the mechanical strength and porosity of freeze-cast kaolinite–silica composites. Measurement of the equibiaxial flexural strength shows that, after sintering at 1250 °C for 1 h, a ceramic containing 10 vol% kaolinite and 8 vol% silica has a strength that is 52 times greater than a ceramic containing only 18 vol% silica and 2.5 times greater than one containing only 18 vol% kaolinite, yet while also being 69 % porous (i.e., the porosity after sintering). It is proposed that this greater strength arises from three mechanisms. First, the kaolinite and silica particles bond together to form a connected network in which the much larger kaolinite platelets distribute the applied stress over a larger region of the sample (this increased strength is even seen in the freeze-cast green bodies before sintering). Second, X-ray diffraction measurements indicate that upon sintering at 1250 °C, new phases are formed when both silica and kaolinite are present, which we postulate contribute greatly to the strength of the composite. Third, the pore walls in samples containing both kaolinite and silica are much more continuous and unbroken than in samples containing only kaolinite.