Key Engineering Materials Vols. 317-318 (2006) pp. 419-424 online at http://www.scientific.net © (2006) Trans Tech Publications, Switzerland
Oxynitride Glasses and Their Properties – Implications for High Temperature Performance of Silicon Nitride-based Ceramics Stuart Hampshirea and Michael J. Pomeroy Materials and Surface Science Institute, University of Limerick, Limerick, Ireland. a
Keywords: Oxynitride Glass, Silicon Nitride, SiAlON Glass, Viscosity, Rare Earth cation, Creep
Abstract. Oxynitirde glasses are found at triple point junctions and as intergranular films in silicon nitride based ceramics. The glass chemistry, particularly the content of modifyer,usually Y or a rare earth (RE) ion, and the volume fractions of these oxynitride glass phases within the ceramic control the properties of silicon nitride, in particular, creep at high temperature. It is known that, as nitrogen substitutes for oxygen in silicate and aluminosilicate glass networks, increases are observed in glass transition and softening temperatures, viscosities (by two to three orders of magnitude), elastic moduli and microhardness. If changes are made to the RE:Si:Al ratios or different rare earth cation are substituted, properties such as viscosity can be increased by a further two to three orders of magnitude. These effects have implications for the high temperature properties of silicon nitride based ceramics, especially creep resistance. This paper provides an overview of oxynitride glasses and outlines the effect of composition on properties such as glass transition temperature and viscosity and discusses the effects on high temperature behaviour of silicon nitride ceramics. Introduction Silicon nitride based ceramics  are densified by a process of liquid phase sintering. Additives such as yttrium or rare earth oxides (with alumina) react with surface oxides on the silicon nitride particles and some of the nitride itself to form a M-Si-Al-O-N liquid (M=Y or Ln) which on cooling remains as a glass [2-5], located both at grain triple points and as intergranular films. These glasses are effectively silicate or alumino-silicate glasses in which oxygen atoms in the glass network are partially replaced by nitrogen atoms [6-8]. The high temperature mechanical properties, especially creep, of silicon nitride depend on the types and amounts of sintering additives used which determine the volume and chemistry of the grain boundary glasses [2-4, 6-7]. At temperatures in the range 1200-1500°C, the intergranular glass phase softens and creep is controlled by the rate of formation and growth of cavities in this second phase and the creep rate is determined by the effective viscosity and volume fraction of the intergranular glass phase [6-7]. It is important, therefore, to understand the nature of these intergranular glasses in silicon nitride. A number of studies [5, 8-13] on oxynitride glass formation, structure and properties have shown that oxynitride glasses exhibit higher glass transition temperatures, elastic moduli, viscosities and values of hardness compared with the equivalent silicate glasses due to extra cross-linking within the glass network as a result of substitution of oxygen by nitrogen. For RE-SiAlON (RE = La, Nd, etc.) glasses with constant O:N and Si:Al ratios, properties increase with increasing rare earth cation field strength (CFS) [12-13]. This paper outlines the effect of compositional changes, especially nitrogen content, on properties of oxynitride glasses, such as glass transition temperature and viscosity, and discusses the implications for high temperature behaviour of silicon nitride ceramics.
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______ (a) (b) 10nm Figure 1: (a) Scanning electron micrograph of silicon nitride sintered with yttria and alumina showing grain boundary glass phase (white), (b) transmission electron micrograph showing glass film between two silicon nitride grains and at triple point. Experimental Procedure Glass compositions are expressed as equivalent percentages of cations and anions [5, 10]. Appropriate quantities of silica, alumina, the modifying oxide and silicon nitride powders are wet ball milled in isopropanol for 24 hours, using sialon milling media, followed by evaporation of the alcohol. 50g batches are melted in boron nitride lined graphite crucibles at 1700-1715oC for 1h under 0.1MPa nitrogen pressure in a vertical tube furnace, after which the crucible is removed from the hot zone and the melt is poured into a preheated graphite mould. The glass is annealed at ~850-900oC and then slowly cooled. Bulk densities were measured by a technique based on the Archimedes principle using distilled water. X-ray diffraction confirmed that the glasses were totally amorphous. Differential thermal analysis (DTA) was carried out to detect Tg, observed as the onset point of the endothermic drift, which corresponds to the beginning of the transition range. Viscosity was deduced from creep tests performed in air between 750 and 1000°C in 3-point bending with a span of 21 mm on bars of dimensions: 25 mm x 4 mm x 3 mm. The expression for the viscosity, η is based on the analogy existing between the stress/strain relations in an elastic solid and those governing a viscous fluid: η = σ / [2(1+υ)έ] (1) where σ and έ are the applied stress and the creep rate on the outer tensile fibre and υ is Poisson's ratio. Results from compressive creep tests show good agreement. Results and Discussion Fig. 2 shows the increase in viscosity with nitrogen at 950oC and 1020oC for glasses with a fixed cation composition (in eq.%) of 28Y: 56Si: 16Al (standard composition). Increases in viscosity of more than 2 orders of magnitude are observed as 18 e/o oxygen is replaced by nitrogen at 950oC. This is known to be due to the increased cross-linking within the glass structure as 2-coordinated bridging oxygen atoms are replaced by 3-coordinated nitrogen atoms [5, 10-11]. The glass network contains SiO4, SiO3N and possibly also SiO2N2 tetrahedra as structural groups . The SiO3N group requires the presence of an extra cationic charge locally to balance an extra negative charge and therefore the situation is very similar to an AlO4 tetrahedron within the network. Therefore, oxynitride glasses containing SiO3N
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groups can accommodate more modifiers in “charge accommodating” sites than the equivalent oxide glasses. Fig. 3 shows the effect of Al:Y ratio on viscosity of glasses with a fixed N content of 17 e/o. As Y content decreases, there is a reduction in viscosity of over 1 order of magnitude at 950oC from ~1013 Pa.s at 4 e/o Al to ~1012 Pa.s at 16 e/o Al and then with further increase in Al, viscosity increases again. Overall, these effects can be assumed to be related to changes in the density of the glass network and the numbers of non-bridging oxygens as Al changes from a network ion (AlO4) to a modifying role (AlO6). groups can accommodate more modifiers in “charge accommodating” sites than the equivalent oxide glasses.
14 13 950 °C
10 9 8 0
Nitrogen content (e/o) Figure 2: Effect of nitrogen content on viscosity of Y-Si-Al-O-N glass at 950oC and 1020oC.
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log(viscosity (Pa. s))
14 const. Si and N Y replaced by Al
13 12 11 10 9 0
Al content (eq. %) 950ºC
Figure 3: Effect of replacement of Y by Al on viscosity at 950 and 1000°C for (44-x) eq. % Y: 56 eq. % Si: x eq. % Al: 83 eq. % O: 17eq. % N.
log (viscosity (Pa. s))
temperature (ºC) Eu
Figure 4: Effect of rare earth (RE) cation on viscosity-temperature relationship of RE-Si-Al-O-N glasses. Fig. 4 shows the effects of rare earth cations (Ce, Sm, Dy, Y, Ho, and Er) on viscosity of RE-SiAlON glasses with the standard cation composition. At any particular temperature, viscosity decreases by some 3 orders of magnitude in the order: Er>Ho≥Dy>Y>Sm>Ce>Eu. Viscosities of some Ln-Si-Al-O-N liquids (Sm, Ce, Eu) for a given level of nitrogen are less than those of the equivalent Y-Si-Al-O-N liquids and this should promote easier densification
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of silicon nitride. However, this will have consequences for high temperature behaviour, particularly creep resistance. Modification of the sintering additives results in changes in the composition of the grain boundary glass phases in silicon nitride. When 17 e/o N is substituted for oxygen, viscosity increases by >2 orders of magnitude. Increasing the Y:Al ratio of the glass results in a further slight increase in viscosity. Changing the rare earth cation from a larger ion such as Ce to a smaller cation such as Er, increases viscosity by a further 3 orders of magnitude. Overall, a change of almost 6 orders of magnitude in viscosity can be achieved by modification of N and rare earth cation content as shown schematically in Fig.5. Observations of intergranular films in silicon nitride ceramics  show that the thickness of these films decreases with decrease in RE ion radius. Viscous flow of the intergranular films contributes to the initial stage of tensile creep deformation. Smaller RE ions prefer an oxygen environment in the glass structure so, as N is concentrated in the IG films, smaller RE ions (Lu, Er) diffuse out to the triple points . Larger RE ions have a preference for N and remain concentrated in the IG films, with less in the triple points. Creep behaviour is thus dependent on both the intergranular film and triple point viscosities and glasses with more N, less Al and smaller RE cations will provide enhanced creep resistance.
950 °C increasing nitrogen
12 17 eq. % N 10
8 0 eq. % N 6 2.5
cation field strength (Å-2)
Figure 5: Combined effects of cation field strength and nitrogen content on viscosity of RE-Si-Al-O-N glasses. Summary Oxynitride glasses which occur as intergranular amorphous phases in silicon nitride based ceramics have been studied to assess the effects of composition on viscosity which increases by more than two orders of magnitude as 18 e/o N is substituted for oxygen. Viscosity generally increases as more Si or Y is substituted for Al but this is a smaller effect than that of nitrogen. A further increase in viscosity of three orders of magnitude is achieved by substituting smaller rare earth cations for the larger ones. The implications for silicon nitride and sialon ceramics are that intergranular glasses containing more N and less Al and smaller RE cations will provide enhanced creep resistance.
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Acknowledgement Studies of Oxynitride Glasses and Glass-ceramics at the University of Limerick, Ireland, have been supported by the Fine Ceramics Research Association, Japan under the NEDO Synergy Ceramics Programme and by the European Commission. References  
             
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