Grain growth anisotropy of β-silicon nitride in rare-earth doped oxynitride glasses

https://doi.org/10.1016/j.jeurceramsoc.2003.10.034Get rights and content

Abstract

Grain growth experiments in oversaturated Me–Si–Mg–O–N glasses were carried out for six different metallic elements (Me=Sc, Lu, Yb, Y, Sm and La). These elements all form a cation with a +3 valence but have increasing ionic radius. Ostwald ripening was observed for all compositions and showed that anisotropy of growth increases with increasing Me3+ radius. Cyclic heat treatments of the samples revealed furthermore that growth anisotropy is controlled by the adsorption behaviour of the cation at the interface between grain and intergranular film. The use of elements of the same valence (Sc, Y) but different electronic structures than the rare-earth elements (Lu, Yb, Sm and La) showed that the electronic structure has a major role on the adsorption behaviour of the Me3+ cation.

Introduction

The main reason for the outstanding mechanical properties of silicon nitride is related to the formation of a so-called in-situ reinforced microstructure upon phase transformation and densification.1, 2, 3, 4, 5, 6, 7 Best toughness is achieved when a bimodal distribution of grain size exists and the large grains are elongated with an aspect ratio over 4. It has been showed that the composition of the sintering additives have a significant influence on microstructure development. Sanders and Mieskowski8 found for rare-earth doped silicon nitrides with RE=Y, Ce, Sm or La the largest grains in the Y-based composition. Goto and Thomas9 reported largest grains when Yb or Dy are used. Pyzik and Beaman10 reported microstructure variations when MgO, CaO and Y2O3 content changed, from coarse in MgO doped ceramics, to bimodal in samples doped with Y2O3 and CaO, to fine monomodal if simultaneously MgO, CaO and Y2O3 were used. Though, there exist only few quantitative data and simulations on grain growth of silicon nitride, and to our knowledge only one systematic investigation of the impact of rare-earth sintering additives.11 Most grain growth models reported in the literature refer to the Ostwald ripening theory, where grain growth as a function of time is described bydn−d0n=K·twith d0 the mean grain size at t=0, d the mean grain size at each time t and K a constant. If the rate limiting step is the attachment reaction at the surface, n take the value 2 and if it take the value 3, growth is controlled by diffusion. Measurements were made on loose powders or seeded materials and reveal fluctuations for n between 3 and 5. The main reason for divergence of the results from the theoretically predicted growth exponents might be that the major conditions for applying the Lifshitz–Slyozow–Wagner (LSW) theory,12, 13 i.e. a dilute system and isotropic growth, are not fulfilled in the case of the silicon nitride ceramic. Models exist that made some attempts to the description of anisotropic Ostwald ripening. Bernard-Granger et al.14 presented a modified equation of Eq. (1) that takes into account different exponents and activation energies for growth in length and in diameter. They showed exponents of 3 for the length and 5 for the diameter growth and an abrupt change in the activation energies at 1780 °C, but did not explain it. Kitayama et al.15 investigated grain growth of loose silicon nitride powder sintered with SiO2 and Y2O3 at different temperatures. Their simulations16 are in good agreement with the experimental results they present, and predict again growth exponents of 3 for attachment controlled and 5 for diffusion controlled growth. Still, the volume fraction condition of the LSW-theory they used is again not fulfilled for these loose powders, and the diffusion fields of the particles influence each other. Therefore oversaturated oxynitride glasses were designed17 in which a low volume fraction of Si3N4 grains is dispersed in a glass matrix. In this case, the diffusion fields of the particles do not interact and Ostwald ripening can be assumed. Further advantage of these experiments is that, on the contrary of the ceramic, no steric hindrance18 takes place.

The equilibrium shape of silicon nitride in vacuum has been calculated by Kraemer et al.19 using the Periodic-Bond-Chain theory. The atomic structure of the prominent faces revealed to be partly responsible for the anisotropic grain growth in silicon nitride. The atomic roughness of the basal planes, observed for grains dispersed in an oxynitride glass, allow a diffusion controlled growth whereas the prism planes are atomically smooth so that the rate limiting step for their growth is the attachment reaction. Since the equilibrium shape is a function of the surface energy, it necessarily changes with the composition of the surrounding material. In silicon nitride ceramics, grains are separated by a continuous amorphous film, that results from the sintering additives needed for liquid phase sintering. Rare-earth oxides or Y2O3 are commonly used sintering aids. They are necessary not only for easier sintering,20, 21 but also because they induce intergranular crack growth22 and thus increased toughness. Therefore, it proved relevant to investigate grain growth anisotropy of β-Si3N4 in rare-earth based oxynitride glasses.

The rare-earth elements all form a cation with a +3 valence. It has been showed for glass compositions containing a rare-earth oxide and Al2O3 that the properties of the intergranular film, like composition and thickness, scales with the radius of the Me3+ cation.23 The intergranular film that forms between two grains in the polycrystalline material also forms between flocculated particles in the glass. The film thickness, and therefore its composition, were showed to be equivalent in both the ceramic and the glass, provided the same elements were used. Therefore, one can assume that the reaction at the interface between grain and surrounding glass are the same in both the ceramic and the oversaturated glass and grain growth experiments on glasses with low volume fraction of free growing grains are a reliable model for grain growth in bulk ceramics. Moreover, thermo-mechanical properties of the rare-earth based oxynitride glasses, as bulk material, also proved to directly depend on the cation size of Me3+.24, 25, 26, 27 In all studies,23 a merely different behavior was found for the Y-based, compared to the rare-earth based compositions. Yttrium also forms a cation with +3 valence but does not belong to the rare-earth elements. The present study focussed on the glass compositions with six different Me3+ cations: Sc3+, Lu3+, Yb3+, Y3+, Sm3+, La3+. We will address the impact on grain growth of cation size and of different electronic configuration by comparing elements from the 3rd column of the periodic table with rare-earth elements. Moreover, existing investigations were all carried out on compositions containing Al2O3,17, 15 in which SiAlON formation that influence grain growth depends on the cation size. In this study we used, instead of Al2O3, MgO that does not form a solid solution with Si3N4, so that a more accurate description of the role of the cation size on grain growth anisotropy will arise.

Section snippets

Experimental procedures

In order to investigate the impact of the radius of different ions on grain growth six elements were chosen. They all form a cation with a +3 valence and are all considered to present a coordination number of six in oxynitride glasses. In Table 1 the corresponding cation sizes are listed. These six elements can be furthermore partitioned in two groups: Sc, Y and La belongs to the 3rd column of the periodic table of the elements, La, Sm, Yb and Lu are rare-earth elements. They show what is

Results

The following section describes the variations in grain size and aspect ratio distribution with increasing annealing time as the radius of the Me3+ cation changes. The results are sustained by definition and measurements of a growth ratio, that depends on the composition of the oxynitride glass. It has to be preliminary stated, that the Si3N4 powder used for the preparation of the glasses consists up to 95% of the α-modification of the crystal. Therefore, during any heat treatment first a phase

Anisotropic Ostwald ripening

Kraemer described at the end of the α/β phase transformation, in a Y-based oxynitride glass containing Al, an aspect ratio–diameter distribution that decreases with increasing diameter of the grains and matches the distributions shown in Fig. 4, Fig. 5 for the Lu- and La-based glasses. Although the reasons for the development of such a grain size distribution is not clear, the state assumed in this set of experiments as starting state correlates well with literature data from the end of the α/β

Conclusions

Systematic investigations of the Ostwald ripening behavior of β-silicon nitride grains dispersed in Me–Mg–Si–O–N glasses have brought some insights on the impact of Me3+ cations (Me=La, Lu, Sc, Sm, Y, Yb) on grain growth anisotropy of silicon nitride.

Typical aspect ratio–diameter distributions at the end of the α-β transformation, known from the literature, were confirmed for all compositions. Only the range of the aspect ratio is a function of Me3+ radius. Flattening of the distribution during

Acknowledgements

Financial support from the European Community “Growth”-Programm, NANOAM project contract Nr.GRD2-200-3030351, is gratefully acknowledged.

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