Classical and differential thermal analysis studies of the glass-ceramic transformation in a YSiAlON glass

Nucleation and crystallization studies were conducted on a YSiAlON glass that contained 17 equiv.% nitrogen (7.5 at.%) by using a two-stage nucleation-and-growth treatment. Classical and differential thermal analysis (DTA) techniques were both used to study the crystallization process, to ensure that the optimum heat-treatment schedule that yielded a fine microstructure and minimum residual glass was applied. The optimum nucleation and crystallization temperatures were determined from DTA traces that were recorded from isothermal heat treatments at different nucleating temperatures, ranging between T-g - 40 degrees C and T-g + 100 degrees C for Ih and crystal-growth temperatures in the range of 1170 degrees - 1310 degrees C for 0.5 h, respectively. The activation energy for the crystallization process was determined, based on the analysis of the variation of peak temperature at five different heating rates. Specimens heat treated in a tube furnace under nitrogen gas were subjected to microscopical investigation, and results showed variations in the volume fraction of crystalline phases and crystal size with nucleation temperature. The nucleation temperature, T-g + 40 degrees C (1025 degrees C), which corresponded to the maximum volume fraction of crystalline phases and minimum crystal size, was consistent with the optimum nucleation temperature, T-g + 35 degrees C (1020 degrees C), as determined from DTA, The time and temperature of nucleation and crystal growth dictated the nature and size of the crystalline phases. Properties such as hardness and density were assessed and correlated to the nucleation temperature. The influence of sample specific surface on the devitrification mechanisms was estimated, and bulk nucleation was observed to be the dominant nucleation mechanism.