H. Lemercier

Yttrium oxynitride glasses: Properties and potential for crystallisation to glass-ceramics

Abstract Silicon nitride-based ceramics contain oxynitride glass phases at the grain boundaries which can impair subsequent high temperature properties. Studies of bulk glasses in the Y-Si-Al-O-N system have been carried out and it has been shown that up to 10 atomic % N can be incorporated into these oxynitride glasses. Nitrogen increases the viscosity, hardness and glass transition temperature of the glasses. Heat treatments of Y-Si-Al-O-N glasses have been carried out and the crystalline phases formed are reported. Further improvements are possible if glass-ceramic processes using two-stage heat treatments are introduced. This paper reviews the development of oxynitride glasses, the effects of nitrogen on properties and reports on the glassceramic heat treatments.

The crystallisation of the yttrium–sialon glass: Y15.2Si14.7Al8.7O54.1N7.4

Abstract The development of microstructure during crystallisation of a glass with composition Y15.2Si14.7Al8.7O54.1N7.4 has been studied by analytical and high resolution transmission electron microscopy. Crystal nucleation at temperatures in the range 965–1050°C occurs by the heterogeneous nucleation of lenticular-shaped yttrium, silicon and aluminium containing crystals on silicon-rich clusters that formed during glass preparation. The lenticular crystals have a wide range of composition after heat treatment at 1050°C; the yttrium cation percentage varies around that of the expected B-phase composition Y2SiAlO5N but the aluminium content is lower and the silicon content generally significantly higher than that. The crystals display the hexagonal crystal structure of B-phase, although the results from EDX analysis imply that the atomic arrangement of the lattice is not the previously proposed B-phase structure. Crystal growth during prolonged heat treatment at 1050°C occurs to a significant extent by coalescence.

Preparation of Pure B-Phase Glass-Ceramic in the Yttrium SiAION System

The present study aims to determine controlled crystallisation heat treatments and thermal stability for pure B phase glass-ceramic in the Y-Si-Al-O-N system. Experiments using differential thermal analysis and the ultrasonic techniques were performed on a YSiAlON glass to define the optimum nucleation temperature of B phase according to Marottas method. Changes in elastic properties allowed to determine the optimum nucleation time and to follow the glass to glass-ceramic transformation on massive glass samples. Maximum bulk nucleation occurs after one hour in the range 960-970°C and the grain growth ends after 10 hours at the crystallisation temperature of 1050°C. XRD analyses confirm B phase as the unique crystalline phase thermally stable up to 1150°C and SEM observations show a very fine and homogeneous microstructure with a high crystal volume fraction. By determining optimum conditions for nucleation and crystal growth of B phase, good quality nitrogen glass-ceramics have been produced.

On the glass transition domain in some M-SiAlON (M = Y or Ln) oxynitride glasses

The behaviour in the glass transition domain of some oxynitride glasses has been studied by thermoanalytical methods (dilatometry and differential thermal analysis) and mechanical techniques (creep and ultrasonic measurements of Youngs modulus). The thermoanalytical data are in good agreement with the glass transition domain defined from viscosity data. The sharp decrease of Youngs modulus, that starts at the temperature of the strain point, is compared to results obtained from mechanical spectroscopy by other authors. The difference in the apparent activation energies for viscous flow above and below the temperature of the strain point is used to separate the contribution of the thermal and structural components. The high temperature apparent activation energy of viscosity is in fair agreement with that of the α-relaxation peak described by the formalism of hierarchically correlated molecular mobility.