Potential of Nd-Si-Al-O-N glasses for crystallisation to glass-ceramics

U-phase Ln₃Si_3-xAl_3+xO_12+xN_2-x (Ln = La, Ce, Nd) occurs as a major devitrification product in rare earth sialon systems. Heat treatment studies have been carried out at temperatures corresponding to optimum nucleation and crystal growth on Nd-U-phase compositions. Two starting glass compositions for U-phase with varying cation ratios, Nd:Si:Al = = 1:1:2 and Nd:Si:Al = 1:1:1, containing 20 eq.% N were prepared and their densities, hardnesses and glass transition temperatures measured. Detailed nucleation and crystallisation measurements were carried out using a two-stage heat treatment process employing differential scanning calorimetry and a tube furnace. Optimum nucleation temperatures and the activation energies for crystal growth were determined and the influence of sample-specific surface on the devitrification mechanisms assessed. The composition with 1:1:2 atomic ratio was further investigated to assess the effect of varying nitrogen contents in the mixtures. This investigation revealed the existence of a small range of unit cell dimensions for U-phase with variation in nitrogen content. The refractoriness and microstructure of the resulting phases are described and their potential as grain boundary phases in silicon nitride based ceramics discussed.