Effect of composition on viscosities of rare earth oxynitride glasses

Journal of Non-Crystalline Solids 344 (2004) 1–7 www.elsevier.com/locate/jnoncrysol

Effect of composition on viscosities of rare earth oxynitride glasses Stuart Hampshire *, Michael J. Pomeroy Materials and Surface Science Institute, University of Limerick, Limerick, Ireland Available online 27 August 2004

Abstract MASiAAlAOAN glasses (where M = Mg, Y or rare earth cation) are intergranular phases in silicon nitride based ceramics in which the composition and volume fraction of such oxynitride glass phases determine the properties of the material, in particular, high temperature creep phenomena. A number of investigations on oxynitride glass formation and properties have shown that nitrogen increases glass transition and softening temperatures, viscosity, elastic modulus and hardness. By changing the cation ratios or the type of rare earth cation, properties such as viscosity can be increased further. This paper provides an overview of oxynitride glasses and outlines the effect of composition on properties such as glass transition temperature and viscosity. These effects have important implications for creep in silicon nitride based ceramics where amorphous intergranular films control the creep resistance.
2004 Published by Elsevier B.V. PACS: 81.05.K; 66.20; 61.72.M

1. Introduction Silicon nitride and ÔsialonÕ ceramics have been developed for high temperature structural applications [1–3]. During the sintering process, oxide additives, such as yttria and alumina, are used to facilitate densification. These additives react with a surface layer of silica on the Si3N4 powder particles and some of the nitride to form a yttrium SiAAlAOAN liquid phase, which on cooling remains as an intergranular glass [1–5] either as grain boundary films or at the triple points between grains. A number of investigations on oxynitride glass formation, structure and properties [6–23] have sought to understand the nature of these intergranular phases and how they affect the properties of silicon nitride and sialon ceramics, particularly the high temperature mechanical behavior [24–28].

*

Corresponding author. Tel.: +353 61 202 640; fax: +353 61 202 967. E-mail address: [email protected] (S. Hampshire). 0022-3093/$ - see front matter
2004 Published by Elsevier B.V. doi:10.1016/j.jnoncrysol.2004.07.027

Reviews of oxynitride glasses were published in the period 1990–1995 [11,13,16] and more recently [23]. These glasses are basically silicate or alumino-silicate glasses in which oxygen atoms in the SiO4 tetrahedra within the silicate network are partially replaced by nitrogen atoms [6–9,11,13–16]. This leads to a higher than average coordination of non-metal atoms as in

The overall result is increased crosslinking and, hence, a stiffer and more rigid glass network and this explains the observed increases in properties such as elastic modulus, viscosity and hardness compared to the corresponding oxide glasses. The first systematic studies on the effect of replacing oxygen by nitrogen in oxynitride glasses (fixed cation compositions) [8,9] showed that, for MgA, CaA, NdA and YASiAAlAOAN glasses with

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constant cation ratios, nitrogen increased glass transition temperature (Tg), viscosity, resistance to devitrification, refractive index, dielectric constant and ac conductivity. The use of rare-earth oxides as densification additives for silicon nitride and, particularly, SiAlON ceramics has been explored by a number of authors [29,30]. In many ways, rare-earth oxides behave in a similar way to Y2O3, but with some important differences. Firstly, nitrogen has a higher solubility in LnASiAAlAOAN liquids (>25a/o) than in the other MASiAAlAOAN liquids [18] and the glassy grain boundary phases in silicon nitride densified with these oxides often have higher softening temperatures. Secondly, for a given level of nitrogen, viscosities of some LnASiAAlAOAN liquids are less than those of similar YASiAAlAOAN liquids [18] and this allows easier densification of silicon nitride and sialon ceramics when using these oxides for sintering. However, this will also affect the characteristics of the residual grain boundary glass phase. Various reports on LnSiAlON and LnSiON glasses [17–19] indicate that properties such as density, hardness, Tg, elastic modulus and viscosity increase with increasing lanthanide cation field strength (CFS) or decreasing cation radius. The changes observed are correlated with either the cationic field strength or the ionic radius of the rare earth cation. This paper reviews data on oxynitride glasses and the effect of composition (nitrogen content, cation ratios) on properties, such as glass transition temperature and viscosity, that control the creep resistance of these nitride ceramics.

2. Experimental procedure 2.1. Glass preparation In SiAAlAOAN and MASiAAlAOAN systems, the concentrations of all components are expressed in equivalents instead of atoms or gram-atoms [9,13]. One equivalent of any element always reacts with one equivalent of any other element or species. For a system containing three types of cations, A, B and C with valencies of vA, vB and vC, respectively, then:

equivalent concentration of A ¼ ðvA ½AÞ=ðvA ½A þ vB ½B þ vC ½CÞ; where [A], [B] and [C] are, respectively, the atomic concentrations of A, B and C, in this case, SiIV, AlIII and the metal cation, M, with its normal valency. If the system also contains two types of anions, C and D with valencies vC and vD, respectively, then: equivalent concentration of C ¼ ðvC ½CÞ=ðvC ½C þ vD ½DÞ; where [C] and [D] are, respectively, the atomic concentrations of C and D, in this case OII and NIII. The following oxynitride glass compositions in equivalent % (e/o) were prepared for property investigation: 1. A series of ÔstandardÕ compositions with cation ratios of 28 e/o Y, 56 e/o Si and 16 e/o Al with varying nitrogen contents (Table 1). 2. A series of compositions with constant 17 e/o N and 28 e/o Y content, and the Si:Al ratio varying from 64:8 to 48:24 (Table 2). 3. A series of compositions with constant 17 e/o N and 56 e/o Si content and the Y:A1 ratio varying from 44:0 to 0:44 (Table 3). 4. A series of rare earth sialon compositions with 10 and 17 e/o N contents with a fixed cation ratio of 28 e/o RE, 56 e/o Si, 16 e/o Al, where RE = Ce, Nd, Sm, Dy, Ho, Er. Preparation of oxynitride glasses involves mixing appropriate quantities of silica, alumina, the modifying oxide and silicon nitride powders. Y2O3, Nd2O3, Sm2O3, Eu2O3, Dy2O3, Ce2O3, Ho2O3 and Er2O3 powders with 99.9% purity (Rare Earth Products Ltd.) were used. Ca, Si and Al (1–20 ppm level) are the common metal impurities present in the rare earth oxides. Al2O3 was analar grade (BDH) with the maximum limit of impurities given as water soluble matter (0.2%), chloride, sulphate and Fe (0.005% each). SiO2 was from Fluka Chemicals Ltd and contained Ca (0.02%), Fe (0.02%), K (0.05%) and Na (0.01%) as the major impurities. All oxides were calcined at 900 C to remove any volatiles and/or chemically absorbed water and stored in a drying oven at a temperature of 120 C. The silicon

Table 1 Standard compositions with varying N contents Y e/o

Si e/o

Al e/o

O e/o

N e/o

N Calc.

Weight % Exp.

28 28 28 28 28

56 56 56 56 56

16 16 16 16 16

100 90 83 80 77

0 10 17 20 23

0 – 3.77 – –

0.0 – 3.43 ± 0.19 – –

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Table 2 Compositions with Y constant and fixed O:N = 83:17 with nitrogen analysis Y e/o

Si e/o

Al e/o

O e/o

N e/o

N Calc.

Weight % Exp.

28 28 28 28 28 28

64 60 56 52 50 48

8 12 16 20 22 24

83 83 83 83 83 83

17 17 17 17 17 17

– 3.76 3.77 3.78 3.78 –

– 3.53 ± 0.10 3.43 ± 0.19 3.62 ± 0.14 3.61 ± 0.12 –

Table 3 Compositions with Si constant and fixed O:N = 83:17 with nitrogen analysis Y e/o

Si e/o

Al e/o

O e/o

N e/o

N Calc.

Weight % Exp.

44 40 36 32 28 25 22 16 0

56 56 56 56 56 56 56 56 56

0 4 8 12 16 19 22 28 44

83 83 83 83 83 83 83 83 83

17 17 17 17 17 17 17 17 17

– 3.38 3.50 3.64 3.77 3.89 4.01 – –

– 3.32 ± 0.08 3.42 ± 0.1 3.59 ± 0.11 3.43 ± 0.19 3.64 ± 0.13 3.75 ± 0.13 – –

nitride (LC12SX) powder was obtained from Hermann C. Starck, Berlin with oxygen level of 2.06%, C of 0.15% and negligible traces of Fe, Ca and Al (total less than 0.015%). The powders are wet ball milled in isopropanol for 24 h, using sialon milling media, followed by evaporation of the alcohol before pressing into pellets. Typically, large batches (50–60 g) are melted in boron nitride lined graphite crucibles under 0.1 MPa nitrogen pressure at 1700–1750 C for 1 h in a vertical tube furnace, after which the melt is poured into a preheated graphite mould at 850 C. The glass is annealed at this temperature for 1 h to remove stresses and slowly cooled.

crucibles in a flowing nitrogen atmosphere at a heating rate of 10 C min1, using A12O3 as a reference material. The onset point of an endothermic drift on the DTA curve corresponding to the beginning of the glass transition range is reported as Tg while the peak of the exotherm is taken as Tc. Errors in measurement are ±3 C. Viscosity was deduced from creep tests performed in air between 750 and 1000 C in three point bending with a span of 21 mm on bars with the following dimensions: 25 mm · 4 mm (width) · 3 mm (height). The strain transducer had an accuracy of ±50 lm. The expression for the viscosity, g is based on the strong analogy existing between the stress/strain relationships in an elastic solid and those governing a viscous fluid:

2.2. Glass characterization and properties

g ¼ r=½2ð1 þ tÞe0 ;

X-ray analysis was carried out using a Philips X-ray powder diffractometer (Cu-Ka radiation) in order to confirm that the glasses were totally amorphous. Scanning electron microscopy was used to confirm this and to assess homogeneity. Nitrogen content was also determined on a number of glasses using a Carlo Erba 1106 nitrogen analyzer. The bulk densities were measured by the Archimedes principle using distilled water as working fluid. Differential thermal analysis (DTA) was carried out in order to detect the glass transition temperature (Tg) and crystallization temperature (Tc). The instrument used was a Stanton Redcroft 1640 series simultaneous thermo-gravimetric differential analyzer. Small samples (30 mg) were analyzed in boron nitride lined platinum

ð1Þ

0

where r and e are the applied stress and the creep rate on the outer tensile fiber and t is PoissonÕs ratio for which a value of 0.5 was assumed in the calculations.

3. Results Tables 1–3 show the calculated and measured values of nitrogen content for a number of the glasses investigated. As can be seen, a small insignificant amount of nitrogen was lost during melting and casting of the glasses equivalent to <0.3% of the total weight of samples. Fig. 1 shows increases in viscosity (measured by compressive creep tests) with nitrogen for the standard

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Fig. 1. Variation with nitrogen content of viscosity of YASiA AlAOAN glasses (open symbols) in the range 875–950 C (after [31]) and of LaASiAAlAOAN glasses (filled symbols) at 925–950 C (after [32]).

cation ratio YASiAAlAOAN glasses in the range 875– 950 C [31] compared with data from Becher et al. for LaASiAAlAOAN glasses at 925–950 C [32]. The change in viscosities with nitrogen observed by the two different studies shows good agreement with the gradient of lines for YASiAAlAOAN glasses of 0.102 (eq.% N)1, while that for LaASiAAlAOAN glasses is of the order of 0.11 (eq.% N)1. For both types of glasses, viscosity increases by almost three orders of magnitude as 18 e/o oxygen is replaced by nitrogen. For example, the viscosity of the YASiAAlAOAN glass series increases from 1010.2 Pa s for the oxide glass to 1013.0 Pa s at 18 e/o N. These results have also been validated by the current authors using the three point bending tests as described in Section 2. Fig. 2 shows changes in viscosity of YASiAAlAOAN glasses with temperature for two different Al:Si ratios (constant 28 e/o Y) at fixed N content (17 e/o). As can be seen, at the lower temperatures, there is very little change in viscosity with increasing Al content while, at 950 C, viscosity decreases from 1012.5 Pa s at 12 e/o

Fig. 2. Change in viscosity of YASiAAlAOAN glasses with temperature showing effect of Al:Si ratio (constant 28 e/o Y) at fixed N content of 17 e/o.

Fig. 3. Effect of Al:Y ratio on viscosity of YASiAAlAOAN glasses with constant Si (56 e/o) and fixed N content of 17 e/o at 950 and 1000 C.

Al to 1012.0 Pa s at 24 e/o Al. These differences are much less than observed with similar changes in N content. Fig. 3 shows the effect of A1:Y ratio on viscosity of YASiAAlAOAN glasses with constant Si (56 e/o) and 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 950 C from 1013.0 Pa s at 4 e/o Al to 1011.8 Pa s at 16 e/o Al and then with further increase in Al, viscosity increases again to 1012.3 Pa s at 22 e/o Al. These effects are related to changes in the compactness of the glass network and the numbers of non-bridging oxygens as Al changes from a network ion to that with a modifying role. Fig. 4 shows a plot of the glass transition temperatures, Tg, of different LnASiAlON glasses (Ln = Ce, Sm, Dy, Y, Ho and Er) against cationic field strength (CFS) of the Ln ion [18], calculated from ionic radii given by Shannon and Prewitt [33]. All glasses have

Fig. 4. Glass transition temperatures, Tg, of various LnASiAlON glasses as a function of cation field strength – data in increasing order: Ce, Sm, Dy, Y, Ho, Er (after [18]).

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Fig. 5. Viscosity vs. temperature relationships for 28LnA56SiA 16AlA83OA17N glasses (Ln = Y, Ce, Sm, Eu, Dy, Ho, Er) (after [18]).

the same cation ratios of Ln:Si:Al = 28:56:16. A fairly linear increase of Tg with CFS is observed from 900 C for the Ce-sialon glass to 965 C for the Er-sialon glass. The linear plot has been extrapolated to the CFS for Lu in order to estimate the expected Tg for LuASiAAlAOAN glasses ( 975 C). Fig. 5 shows the variation of viscosity with temperature for the same Y and lanthanide sialon glass compositions [18]. Viscosity is seen to decrease in the order: Er > Ho P Dy > Y > Sm > Ce > Eu. The variations in viscosity observed between Er, Ho and Dy SiAlON glasses are quite small with a reduction at 950 C from 1013.2 Pa s for the Er glass to 1012.3 Pa s for the Dy glass. Y glasses exhibit only slightly lower viscosities and this can be ascribed to similar ionic radii for this group of rare earth cations. These observations also suggest that Er, Ho and Dy oxides may form liquids with similar characteristics to that formed when Y2O3 is used as the densifying additive with silicon nitride and sialons. Further reductions in viscosities of about two orders of magnitude are observed for the glasses with larger cations (Ce and Eu). The Eu glass is known [34] to contain Eu in the +2 oxidation state rather than +3 for all the other cations. Overall, increases in viscosity of three orders of magnitude are possible by substituting the smaller rare earth cations for the larger ones.

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Activation energies for viscous flow as well as glass transition temperatures can be deduced from the relationships between viscosity and temperature. Table 4 shows the values of Tg from DTA and from the viscosity measurements and also the activation energies for viscous flow for the series of Ln-sialon glasses as a function of their cation field strengths. Values for Lu have been extrapolated assuming a linear trend in Tg and viscosity across the lanthanides. It can be seen that for the larger cations (Ce and Sm), Tg values from DTA are much higher than those from viscosity measurements. For the medium sized ions (Dy, Ho, Y), values of Tg are similar from both techniques while for the smaller cations (Er and the extrapolated Lu data) values from viscosity measurements are higher than those from DTA. The activation energy for viscous flow increases as cation field strength increases so, at any temperature, not only do glasses with smaller cations have higher viscosities but also the activation energies for viscous flow are higher, reaching values of >1200 kJ/mol for the Er-sialon glasses.

4. Discussion 4.1. Structure – property relationships in MASiAAlA OAN glasses For all oxynitride glasses, it is clear that N has a very marked effect on properties with changes in viscosity of nearly three orders of magnitude observed when 18 e/o oxygen is substituted by nitrogen. This is known to be due to the increased cross-linking within the glass structure as two-coordinated bridging oxygen atoms are replaced by three-coordinated nitrogen atoms or indeed by two-coordinated partially bridging N atoms, as in BSiAN ASiB It is also possible that non-bridging nitrogen atoms may also be present as in BSiAN2

Table 4 Cation field strengths, glass transition temperatures and activation energies for viscous flow for LnASiAAlAOAN glasses with 17 e/o N and fixed cation ratios: 28Ln:56Si:16Al ˚ 2) Cation CFS (A Tg (C) (from DTA, Activation energy for Tg range (C) heating rate 10 C min1) (from viscosity measurements) viscous flow (kJ/mol) Ce Sm Y Dy Ho Er Lu (estimated values)

2.80 3.23 3.76 3.64 3.79 3.87 4.15

900 920 943 945 957 965 975

869–877 887–899 930–945 954–966 960–974 975–990 1010–1020

847 860 892 1020 1153 1220 1300–1400

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The local charge is balanced by the presence of interstitial modifying cations in their vicinity. In the case of silicate glasses, non-bridging oxygen atoms replace bridging oxygen atoms as modifier contents increase. It is also known that the glass network contains Si(O4), Si(O3N) and possibly also Si(O2N2) tetrahedral structural groups. It should be noted that the Si(O3N) group requires the presence of a cation locally to balance an extra negative charge and therefore the situation is very similar to that for an Al(O4) tetrahedron within the network. Therefore, oxynitride glasses containing Si(O3N) groups can accommodate more modifiers in Ônetwork dwellingÕ sites than the equivalent oxide glasses. As noted from the studies of YSiAlON glasses with varying Al:Si and Al:Y ratios, non-linear variation in properties are observed and these have been attributed to changes in the overall glass structure with composition. From Raman spectroscopy studies [35], at constant Y content, an increase in Al:Si ratio results in the replacement of BSiAOASiB by @AlAOASiB and so the Y will act more as a network dwelling ion for local charge balance. For compositions with constant silicon content, an increase in Al:Y ratio results in the replacement of non-bridging oxygens by @AlAOASi„ linkages. The relative number of differently coordinated (4, 5 or 6) Al ions increases with Al:Si ratios and decreases with Al:Y ratios in these glasses. For the LnSiAlON glasses, properties such as glass transition temperature and viscosity increase with increasing cationic field strength (decreasing ionic radius). All properties, in general, appear to vary linearly with ionic size of the rare earth cation [18]. It is known that in some complex oxides, trivalent rare earth ions take a higher coordination state compared with other trivalent cations such as Al3+. This has been attributed to the strong ionicity of the LnAO bond and an ionic size large enough to allow the cation to adopt a coordination larger than or equal to 8. The ionicity of the LnAO bond does not change appreciably with the size or field strength of the rare earth cation and so, depending on their size, they can adopt different coordinations. However, as a linear trend in properties is observed, it may be concluded that the overall structures of these LnSiAlON glasses remain the same and the property changes appear to solely depend on the modifier cation field strength. As the field strength of the Ln cation increases, the attractive forces between the Ln ion and the surrounding network structural units increases. This explains the increases observed in Tg and viscosity (and other properties) with cation field strength.

These changes in properties with composition can be applied to grain boundary glassy phases in silicon nitride. By modifying the composition of glasses which make up the intergranular films in these ceramics, the overall creep resistance of the ceramic can be increased. 4.2. Implications for grain boundary glass phases in silicon nitride ceramics As noted above, when 18 e/o N is substituted for oxygen in MASiAAIAOAN glasses, there is an increase in viscosity of nearly three orders of magnitude. Furthermore, by decreasing the Al content of the glass, a further increase in viscosity of one order of magnitude can be effected. Finally, by changing the rare earth cation from a larger ion such as Ce to a smaller cation such as Er, a further increase in viscosity of three orders of magnitude is possible. Overall, a change of over six orders of magnitude in viscosity can be achieved by careful modification of glass compositions. This has also been reported for other rare earth SiA(AlA)OAN glass compositions [32]. From previous studies of intergranular films in silicon nitride ceramics, it is known that: 1. The thickness of IG glass films decreases as RE ion radius decreases [27]. 2. Viscous flow of the intergranular films contributes to the initial stage of the tensile creep deformation of silicon nitride ceramics [27,28]. 3. The effective viscosity, g, of IG glass films has been estimated to be six orders of magnitude greater than g for the bulk glass [25]. 4. Smaller RE ions prefer to bond to O with the result that, as N is concentrated in the IG films, smaller RE ions diffuse out to the triple points [30]. 5. Larger RE ions have a preference for N and so remain concentrated in IG glass films, with less in the triple points [30]. Taking 4 and 5 together, with smaller RE cations, a lower concentration of these ions in the IG films will result in much higher viscosities than for the larger cations. In addition, in the case of the smaller ions, the glass at the triple points would also have higher viscosities (from a consideration of the intrinsic effects on viscosity reported above). Thus creep rates are dependent on the nature of both the intergranular film and the glass at the triple points. In terms of overall composition of the intergranular glass phase in silicon nitride, higher viscosities (increasing by >6 orders of magnitude) are achieved by increasing the N content and by reducing the Al content of the grain boundary glass and by substituting smaller rare earth cations for larger cations.

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5. Conclusions • Viscosity of YA and rare earth (LnA) SiAAlAOAN glasses, increase with increasing N content. • Viscosity increases by three 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. • Tg and viscosities of LnASiAAlAOAN glasses increase with increasing cation field strength, CFS (decreasing ionic radius). • The implications for silicon nitride ceramics are that intergranular glasses containing more N and less Al and smaller RE cations will provide enhanced creep resistance.

Acknowledgments We wish to acknowledge the contributions of Dr Herve Lemercier, Dr Elizabeth Nestor and Dr Ragavendra Ramesh to the work on Oxynitride Glasses at the University of Limerick, Ireland, which has been financially supported by the European Commission and by the Fine Ceramics Research Association, Japan under the NEDO Synergy Ceramics Programme. We wish to also thank our collaborators, Professor Derek Thompson, University of Newcastle upon Tyne, Professor Jean-Louis Besson and Professor Paul Goursat, Limoges, Professor Lena Falk, Chalmers University, Gothenburg, Professor Jean-Claude Descamps and Dr Francis Cambier, Faculte Polytechnic de Mons, and Professor Tanguy Rouxel, Rennes, for their collaboration and useful discussions.

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