Control of grain morphology in Ca-α sialon ceramics by changing the heating rate

Abstract

A Ca α-sialon (α′) ceramic with an overall composition of Ca 0.8Si 8.8Al 3.2O 1.6N 14.4 was fabricated from α-Si 3N 4, AlN, Al 2O 3 and CaCO 3 starting powders using pressureless sintering (PLS) at 1800°C for 1 h. The microstructural morphology of the resultant Ca-α′ depended greatly on firing patterns, i.e., heating rate. Typical fine and equiaxed α′ grains could only be found in samples fired at a regular heating rate of 20°C/min. In contrast, aciculate grains with a high aspect ratio of 5 to 10 dominated the Ca-α′ sample sintered at a heating rate of up to 60°C/min. The number of α′ nuclei in the Ca-α′ sample during heating was found to be significantly reduced by rapidly increasing the temperature because the yield of α′ through reactions of Si 3N 4, AlN, Al 2O 3 and CaO was somewhat retarded. Fewer nuclei could then grow into elongated grains during the subsequent isothermal holding process at higher temperature. Furthermore, much liquid phase existed in the system until higher temperatures were reached due to rapid heating, and this was another key factor in enhancing the elongation of α′ grain.