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Thirteen glasses of the general formula (M1, M2)9.33Si14Al5.33O41.5N5.67 where M1=La or Nd and M2=Y or Er have been prepared with M1/(M1+M2) fractions of 1, 0.75, 0.5, 0.25, and 0. Data for molar volume (MV), glass compactness (C), Young's modulus (E), microhardness (H), glass transition temperatures (Tg), and dilatometric softening temperatures (Td) have been recorded. In addition, temperatures at which crystallization exotherms arise have also been determined as well as crystalline phases present after the glasses had been heat treated to 1300°C in nitrogen. The results clearly demonstrate that glass properties vary linearly with effective cation field strength (CFS) of the combined modifiers (M1, M2), which is calculated from the atomic fractions of M1 and M2 and their associated CFSs. Glass stability in both the La–Y and La–Er systems reaches a maximum at M1 and M2 fractions of 0.5 because of the relative stability of different oxynitride and disilicate phases with changes in ionic radius. Furthermore, La appears to stabilize the α polymorph of yttrium disilicate because of combined La–Y ionic radius effects.

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°C and T g + 100°C for 1 h and crystal-growth temperatures in the range of 1170°-1310°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°C (1025°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°C (1020°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.

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