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

A Ca alpha-sialon (alpha') ceramic with an overall composition of Ca0.8Si8. 8Al3.2O1.6N14.4 was fabricated from alpha-Si3N4, AlN, Al2O3 and CaCO3 start ing powders using pressureless sintering (PLS) at 1800 degrees C for 1 h. T he microstructural morphology of the resultant Ca-alpha' depended greatly o n firing patterns, i.e., heating rate. Typical fine and equiaxed alpha' gra ins could only be found in samples fired at a regular heating rate of 20 de grees C/min. In contrast, aciculate grains with a high aspect ratio of 5 to 10 dominated the Ca-alpha' sample sintered at a heating rate of up to 60 d egrees C/min. The number of alpha' nuclei in the Ca-alpha' sample during he ating was found to be significantly reduced by rapidly increasing the tempe rature because the yield of alpha' through reactions of Si3N4, AlN, Al2O3, and CaO was somewhat retarded. Fewer nuclei could then grow into elongated grains during the subsequent isothermal holding process at higher temperatu re. Furthermore, much liquid phase existed in the system until higher tempe ratures were reached due to rapid heating, and this was another key factor in enhancing the elongation of alpha' grain. (C) 2000 Elsevier Science B.V. All rights reserved.