OXYNITRIDE GLASSES AS GRAIN BOUNDARY PHASES IN SILICON NITRIDE: CORRELATIONS OF CHEMISTRY AND PROPERTIES

Stuart Hampshire

Materials and Surface Science Institute,
University of Limerick, Limerick, Ireland

ABSTRACT

Silicon nitride is recognized as a high performance material for both wear resistant and high temperature structural applications. Oxide sintering additives, such as yttrium or rare earth oxides plus alumina or magnesia, are used in processing the ceramic to provide conditions for liquid phase sintering. The oxynitride liquid promotes densification and on cooling remains as an oxynitride glass at triple point junctions and also as intergranular films between the elongated hexagonal ß-Si3N4 grains. The properties of silicon nitride, especially fracture behavior and creep resistance at high temperatures are influenced by the glass chemistry, particularly the concentration of modifier, and the volume fraction within the ceramic.

This paper provides an overview of liquid phase sintering of silicon nitride ceramics, grain boundary oxynitride glasses and the effects of chemistry and structure on properties. As nitrogen substitutes for oxygen in bulk oxynitride glasses, increases are observed in glass transition and softening temperatures, viscosities, elastic moduli and thermal expansion coefficient. These property changes are compared with known effects of grain boundary glass chemistry on properties of silicon nitride ceramics.

INTRODUCTION - SINTERING AND MICROSTRUCTURAL DEVELOPMENT IN SILICON NITRIDE CERAMICS

Silicon nitride has been the subject of major programmes of research for the last four decades, principally in response to the challenge to develop a suitable ceramic for high temperature structural applications in gas turbine engines1, 2. The search for improved materials has led to a better understanding of the role of sintering additives in the densification and microstructural development of silicon nitride-based ceramics and the consequences for final properties2, 3. Improvements in powder manufacture and ceramic forming techniques and the development of alternative firing processes has led to a complete "family" of silicon nitride materials1, 2 including Reaction-bonded Silicon Nitride RBSN, Hot-pressed Silicon Nitride HPSN, Sintered Silicon Nitride (SSN), Gas-pressure Sintered Silicon Nitride (GPSSN), Sintered Reaction-bonded Silicon Nitride (SRBSN), Hot-Isostatically-Pressed Silicon Nitride (HIPSN) and solid solutions known as SiAlONs, after their major elemental components4.

Oxynitride glasses were first discovered in silicon nitride based ceramics as intergranular phases which are formed because the sintering additives, usually mixed oxides such as yttria plus alumina, promote liquid phase densification during high temperature processing1-6. At ~1750-1900°C, the additives react with silicon nitride and silica present on the nitride particle surfaces to form a Y-Si-Al-O-N liquid phase which promotes densification and transformation of a- to ß-Si3N4 through a solution-diffusion-precipitation process, according to the following reaction2:

M = Y or RE (other rare earths) the oxides of which may be used in place of yttria7, 8 and some silicon nitrides are sintered with RE (incl. Y) oxides plus MgO9, 10 in place of alumina. Growth of elongated prismatic hexagonal ß-Si3N4 crystals occurs along their c-axes to form an interlocking microstructure and, following sintering, the liquid cools as an intergranular glass, at triple points or as vitreous films between grains with film thickness in the range 0.5-1.5 nm, depending strongly on chemical composition7.

Figure 1 shows a scanning electron micrograph of silicon nitride densified with 6 wt.% yttria and 2 wt.% alumina11. The microstructure consists of ß-Si3N4 grains, with high aspect (length to diameter) ratios, surrounded by a Y-Si-Al-O-N glass phase (white). Figure 2 shows a high resolution transmission electron microscope (HRTEM) image of two ß-Si3N4 grains separated by an amorphous intergranular film (IGF) leading to an oxynitride glass triple pocket (TP)11. The grain boundary chemistry (RE, Al or Mg content and 0:N ratio) and volume fraction of these glass phases control the microstructural development which determines the overall properties of the ceramic, especially fracture resistance, ambient and high temperature strengths, creep resistance and oxidation resistance2, 5, 6, 8. Essentially, the elongated grains canfunction as reinforcements similar to whiskers or fibers in reinforced ceramics. As an example, with Y2O3/Al2O3 additives, as the Y:A1 ratio of the intergranular glass increases, fracture toughness of the ceramic also increases which is indicative of easier debonding at the silicon nitride grain interfaces6, favouring the activation of toughening mechanisms such as crack-deflection and crack-bridging6, 8, 10. The evolution of ß-Si3N4 microstructures during sintering is influenced by the adsorption of RE cations at silicon nitride grain surfaces and by the viscosity of the intergranular liquid. Theoretical and scanning transmission electron microscopy show12 that RE atoms exhibit different tendencies to segregate from the liquid to grain surfaces and have different binding strengths at these surfaces.

Figure 1 Scanning electron micrograph of silicon nitride (6 wt.% Y2O3 + 2 wt.% Al2O3) showing dark ß-Si3N4 grains and bright YSiAlON glass11.

Figure 2 TEM micrograph of silicon nitride showing two ß-Si3N4 grains, a triple point (TP) glass pocket and intergranular film (IGF)11.

The desire to understand more fully the nature of these grain boundary phases resulted in many further investigations of oxynitride glass formation and properties13-28. In the following sections, an overview is given of oxynitride glass synthesis and the effects of composition on properties.

With a knowledge of silicon nitride additive compositions and quantities and also properties of the bulk glasses, the residual stresses in the interfacial glasses can be calculated. This allows correlations of mechanical behaviour of the ceramic with grain boundary glass chemistry.

SYNTHESIS OF OXYNITRIDE GLASSES AND REPRESENTATION OF SYSTEMS

Oxynitride glasses can be formed when a nitrogen containing compound, such as Si3N4 (or A1N), dissolves at high temperatures in either a silicate or alumino-silicate liquid which then cools to form a M-Si-O-N or M-Si-Al-O-N glass (M is usually a di-valent [Mg, Ca] or tri-valent [Y, RE] cation). The extent of the glass forming regions in various M-Si-Al-O-N systems has been studied previously and represented using the Jänecke prism14, 18, 19 with compositions expressed in equivalent percent (eq.%) of cations and anions instead of atoms or gram-atoms. One equivalent of any element always reacts with one equivalent of any other element or species. For a system containing three types of cations, M1, M2 and M3 with valencies of vM1, vM2, and VM3, respectively, then:

where [M1], [M2] and [M3] are, respectively, the atomic concentrations of M1, M2 and M3, in this case, SiIV, AlIII and, for example YIII, with its normal valency of 3.

If the system also contains two types of anions, X1 and X2 with valencies vx1 and vx2, respectively, then:

where [X1] and [X2] are, respectively, the atomic concentrations of X1 and X2, i.e. OII and NIII.

Figure 3 shows the glass forming region in the Y-Si-Al-O-N system which was studied by exploring glass formation as a function of Y:Si:Al ratio on vertical planes in the Janecke prism representing different 0:N ratios. The region is seen to expand initially as nitrogen is introduced and then diminishes when greater than ~10 eq.% N is incorporated until the solubility limit for nitrogen is exceeded at approximately 28 eq.% N.

Figure 3 Glass forming region of the Y-Si-Al-O-N system on cooling from 1700°C19

Preparation of glasses involves weighing the appropriate quantities of silica, alumina, the modifying oxide and silicon nitride powders and ball milling in isopropanol for 24 hours, using sialon milling media, followed by evaporation of the alcohol before pressing into pellets.

Batches of 50-60 g are melted in boron nitride lined graphite crucibles at 1650-1725°C for 1 h under 0.1 MPa nitrogen pressure in a vertical tube furnace, after which the melt is poured into a preheated graphite mould at ~850-900°C to form test bars (4 cm × 1 cm × 0.8 cm). The glass is annealed at a temperature close to the glass transition temperature (Tg) for one hour to remove stresses and then slowly cooled. Standard techniques are utilized for measurement of thermal, mechanical and physical properties of the glasses.

EFFECTS OF COMPOSITION ON PROPERTIES OF OXYNITRIDE GLASSES

The first systematic studies on the effect of replacing oxygen by nitrogen on properties of oxynitride glasses with fixed cation compositions were reported by Hampshire, Drew and Jack18. They reported that incorporation of nitrogen resulted in increases in glass transition temperature (Tg), microhardness, viscosity, refractive index, dielectric constant, a.c. conductivity and resistance to devitrification. In a more extensive study of the Y-Si-Al-O-N system19, it was confirmed that glass transition temperature (Tg), viscosity, microhardness and elastic moduli all...