Pressureless-sintered ceramics based on the compound Si2N2O

Materials Science and Engineering, 71 (1985) 87-94

87

Pressureless-sintered Ceramics Based on the Compound Si2N20* M. H. LEWIS, C. J. REED and N. D. BUTLER

Centre for Advanced Materials Technology, University of Warwick, Coventry CV4 7AL (Gt. Britain) (Received December 5, 1984)

ABSTRACT

SizN20 is a tetrahedral structure o f SIN30 units which has many o f the required intrinsic properties o f a major phase in high temperature ceramics, similar to the more intensively studied Si3N 4. The rapid decomposition o f Si2N20 above 1 700 °C dictates the use o f ternary eutectics to effect liquid phase siutering below this temperature. In this work the Y203-A1203-SiO 2 ternary eutectic has been used to sinter Si2N20 ceramics to nearly the theoretical density without the application o f pressure. Sintered microstructures are composed o f submicron faceted Si2N20 crystals within a semicontinuous matrix o f Y - S i - A I - O - N eutectic glass o f about 10 vol. %. The crystal morphology is that o f elongated pseudohexagonal prisms, similar to the {J' phase in Syalon ceramics. The prism axis is [001] with facet planes parallel to (100), (i 10) and (1 10) in the orthorhombic unit cell. A post-sintering heat treatment at 1 2 5 0 1350 °C induces a crystallization o f the matrix glass to Y2Si207 with a redistribution o f aluminium and oxygen by substitution in Si2N20. A brief property survey shows that hardness, fracture toughness and oxidation resistance are comparable with the corresponding values for the related ceramics based on ~Si3N4. 1. INTRODUCTION Ceramics based on the ~-Si3N4 crystal structure are approaching a state of microstructural refinement for various engineering applica*Paper presented at the International Symposium on Engineering Ceramics, Jerusalem, Israel, December 16-20, 1984. 0025-5416/85/$3.30

tions [ 1]. However, ceramics based on the oxynitride Si2N20 have received little attention, largely because of an inferior sinterability and hence inferior mechanical properties. A major problem in achieving high temperatures, to enhance liquid phase sintering kinetics, is the decomposition of Si2N20 via a reaction of the type 3Si2N20 ~ Si3N 4 + 3SiO + N2 which is prominent above about 1600 °C. A key factor in the successful pressureless sintering of fi-Si3N4 ceramics is the use of mixed oxides which reduce the liquidus and viscosity of the eutectic silicate sintering medium. The most prominent example is that of mixed Y203-A1203 additions which form a ternary eutectic with SiO 2 at about 1300 °C compared with the binary Y203- SiO2 eutectic at about 1600 °C. This ternary silicate eutectic (with variable amounts of dissolved nitrogen) is the basis for liquid phase sintering in various commercial ceramics in the Si-A1-O-N system [2] (e.g. the Syalon ceramics of LucasCookson-Syalon, Gt. Britain, or the AY6 grade ceramic of GTE Products Corporation, U.S.A.). This eutectic liquid enables good densification in the 1600-1700 °C temperature interval although normal sintering temperatures of 1800-1850 °C are used to achieve theoretical densities in fi-Si3N 4 ceramics. This paper is concerned with the use of a similar Y203-A1203-SiO2-based eutectic in the pressureless sintering of Si2N20 ceramics. In previous work concerning Si2N20 ceramics, single oxide additions have been used and hence hot pressing is required [3]. 2. CRYSTALLOGRAPHYAND PHASE RELATIONS The Si2N20 crystal structure has many of the necessary intrinsic properties of a potenQ Elsevier Sequoia/Printed in The Netherlands

88 tial high temperature engineering ceramic which derive from the high proportion of covalent bonding. Whereas Si3N4 is based on an open network of SiN4 linkage, in Si2N20 one of the nitrogen atoms in the coordination tetrahedra is replaced by an oxygen atom [4]. These are selected in an ordered manner such that pseudo-planar arrays of SiN3 units are linked by Si--O--Si bonds which form a singular (100) plane in the orthorhombic structure (see Fig. 6(a)). There is a possibility that this singular plane of reduced covalency may impair the elastic moduli and the resistance to dislocation motion (and hence the hardness) or indirectly influence ceramic properties via its effect on anisotropy in Si2N20 crystal growth morphology. The relationship between Si2N20, the eutectic sintering liquid and the various stoichiometric terminal phases (Si3N4 etc.) is illustrated via the Janecke prism, shown in perspective in Fig. 1. Si2N20 may be presynthesized from equimolar quantities of Si3 N4 and SiO2 but the reaction is sluggish at temperatures below that for decomposition of the oxynitride. In this work, both presynthesized powders and the terminal Si3N4-SiO2 mixtures have been used in conjunction with a sintering liquid of near-ternary eutectic (Y203-A1203-SiO2) composition. Such liquids

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Fig. I. The three-dimensional Janecke prism representation illustrating the relation between Si2N20, terminal components Si3N4 etc. and the eutectic liquid zone: YAG, yttrium aluminium garnet; YAM, yttrium aluminate ; N-YAM, nitrogen-doped yttrium aluminate ; Y. 2S, Y2Si207; 3A.2S, mullite ; M, melilite ; ~-W, ~ wollastonite. Y2Si207 is formed on liquid crystallization with increased substitution of aluminium and oxygen in Si2N20.

and those containing nitrogen in solution are glass forming within volume of composition indicated by broken lines (Fig. 1) when furnace cooled from the sintering temperature. In this work the sintering liquid is normally formed by powder additions of Y203 + A1203 + SiO2 to a-Si3N4-SiO2 mixtures or presynthesized Si2 N20.

3. DENSIFICATION AND MICROSTRUCTURE The weighed powder mixtures were milled in propanol, using A1203 balls, for about 6 h. The particle mixture was reclaimed from the slurry by evaporation of propanol and reweighed to calculate the A1203 pick-up during milling (usually about 2 wt.%). Particle sizes after milling were all submicron. Green compacts were obtained by die pressing, the resulting tablets being cut to convenient size for pressureless sintering within BN powder contained in a graphite crucible. Sintering was conducted in the temperature range 16001800 °C for times between 1 and 4 h, using r.f. heating within a nitrogen atmosphere. A typical archimedean density-temperature plot (Fig. 2) for one of the optimum mixtures illustrates the problem of decomposition above about 1650 °C. Above 1700 °C, compacts are severely gas bloated with whisker crystal growth of condensed decomposition products (probably a-Si3N4) within the large cavities. With a reduction in temperature to below 1700 °C there is a progressive decrease in the width of a surface zone of gas porosity. Densities within a few per cent of the theoretical value (calculated on the basis of the Si2N20 crystal density (2.82 g c m -3) together with about 10 vol.% of a glass of composition defined below) are obtained after removal of surface layers from tablets sintered between 1650 and 1670 °C. The best densities in this temperature range were obtained with additions of 10 wt.% Y203 + 6 wt.% A1203 to either premixed Si3N4-SiO 2 or presynthesized Si2NuO. Additions of Y203, A1203 and SiO2 of eutectic ratio were too SiO 2 rich, resulting in impaired densification kinetics because of an increased liquid viscosity. There appears to be enough surface or unreacted SiO 2 in presynthesized Si2N20 to produce eutectic sintering liquids. A similar effect which is

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due to surface SiO 2 occurs in the sintering of /3-Si3N4 and derivative ceramics. The microstructure of ceramics of nearly theoretical density in the as-sintered state consists of Si 2N20 crystals within a ternary eutectic liquid (Figs. 3 and 4). The SEM and TEM images are markedly similar to those from/3' ceramics of the low nitrogen Syalon type [2] with faceted crystals of pseudo-hexagonal prismatic morphology in a semicontinuous glass matrix of about 10% volume fraction. X-ray diffraction (Fig. 3(b)) shows that some /J-Si3N 4 is also formed, probably as a result of the imbalance of Si3N4 and SiO2 in the sintering mixtures. The glass, which is the residue of a solution-reprecipitation liquid phase sintering mechanism, has an analysed composition near to the Y203-A1203-SiO2 eutectic (Fig. 4). The presence of all added Y203 within this glass results in its strong contrast in backscattered SEM images {Fig. 3(a)). The nitrogen-to-oxygen ratio for this glass cannot be determined from "windowless" energydispersive X-ray analysis (or wavelengthdispersive techniques) because of lack of spatial resolution, but the products of crystal-

lization (described in Section 4) indicate a negligible nitrogen content. Although Si2N20 exists with limited substitution of aluminium and oxygen for silicon and nitrogen respectively, the crystals analysed for a range of sintering additions exhibit a negligible solid solution (Fig. 3(b)). These substitution levels are always less than for/3'Si3N4 crystals dispersed within the sintered microstructure. Si2N20 crystals may also be distinguished from/3'-Si3N4 in TEM images because of their high stacking fault density, similar to most silicate crystal structures. However, their visibility is sensitive to diffracting conditions (Fig. 4(b)). The faulting and crystal morphology are explicable via the crystal structure, shown in projection with an appropriately oriented crystal and diffraction patterns in Fig. 5. Stacking faults are, as expected, parallel to the singular (100) planes linked by the relatively weak Si--O--Si bonds in the orthorhombic crystal. This plane has the lowest stacking fault energy and the faults are introduced randomly during the addition of successive atomic planes during crystal growth from the sintering liquid. The high

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Fig. 3. (a) Backscattered scanning electron microscopy (SEM) image of a polished section of the sintered ceramic; (b) X-ray powder diffraction traces from the sintered ceramic (lower trace) and after matrix glass crystallization (upper trace) (peaks ~, ~-Si3N4; peaks O, Si2N20; peaks Y, 0~-Y2Si207).

density b u t irregular fault distribution is d e m o n s t r a t e d by the c o n t i n u o u s streaking o f diffraction, representing a r e l a x a t i o n o f the Laue d i f f r a c t i o n c o n d i t i o n for the fault planes in certain o r i e n t a t i o n s (Fig. 5). T h e (100) fault planes are also those with the lowest

s o l i d - l i q u i d surface energy and h e n c e are amplified during growth. H o w e v e r , a hexagonal prism, r a t h e r than plate-like, m o r p h o l o g y is d e v e l o p e d as a result o f faceting o f the (110) and (110) planes. These m a y be selected to provide s e c o n d a r y planes o f low solid-

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Fig. 4. (a) Transmission electron microscopy (TEM) image and (b), (c) associated energy-dispersive X-ray analysis spectra from component phases of the sintered ceramic; (d) diffraction contrast image of stacking faults in Si2N20 parallel to (100) planes in the orthorhombic structure.

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liquid energy by breaking relatively few Si--N bonds per unit area of surface (such as the broken planar projections in Fig. 5). The (110) and (210) planes enclose an angle of

about 116 ° which, in combination with (100) planes, results in the elongated pseudo-hexagonal prism morphology with the maximum dimension along [001].

93 4. MICROSTRUCTURAL STABILITY AND PROPERTIES Sintered microstructures are metastable and, as for ~' ceramics [2, 5], are expected to undergo a matrix glass crystallization when reannealed at temperatures above the glass transition ( 800- 900 °C for Y - S i - A 1 - O - N glasses). An interdiffusive reaction with surface oxidation layers is also anticipated [6, 7]. The heat treatment of specimens (which had been cut from the centre of an as-sintered tablet) at 1 2 5 0 - 1 3 0 0 °C results in crystallization of the matrix glass as yttrium disilicate (Y2Si207) (Fig. 3(b)). This is typical of a low nitrogen glass of near-ternary eutectic composition. For bulk glasses there is a simultaneous crystallization of aluminium-rich phases, such as mullite (Al~Si2013) [8]. However, in composite Si2N20-glass microstructures, excess aluminium and oxygen may be a c c o m m o d a t e d within the existing major phase consistent with a detectable rise in its substitution level (the substituted form of Si2N20 is known as the O' phase [9]). This is similar to the accommodation of excess silicon and nitrogen within the ~' phase during the crystallization of high nitrogen glasses as yttrium aluminium garnet (Y3Al~O12) [5]. The nucleation of Y2Si207 in low nitrogen glass matrices of fi' ceramics is normally heterogeneous; oriented columnar crystals grow inwards from the free surface [10]. A similar phenomenon is observed in Si2N20 ceramics via amplification of particular X-ray reflections in diffractometer traces recorded from outer layers of heat-treated ceramic about 100 pm thick. The surface crystallization is a different Y2Si207 polymorph (~) from that nucleated within the bulk (~). Y2Si207 is also detected on oxidized surfaces and, as in the related/3' ceramics, is crystallized from the SiO2-rich surface film after the out-diffusion of yttrium from the intergranular glass. However, oxidation is restricted to an extremely thin ( 5 - 1 0 pm) and mainly glassy film to temperatures of about 1350 °C. This excellent oxidation resistance is due to the rapid suppression of yttrium out-diffusion by crystallization of the near-surface matrix glass. Oxidation rates are similar to those of ~'-Y2Si207 composite surfaces and hence are not intrinsic to the Si2N20 phase.

A brief exploration of the low temperature properties shows them to be similar to those of the fi'-Si3N4-glass composites which have related microstructures. The indentation hardness, measured with a Vickers pyramid diamond, is about 15 GPa compared with about 14 GPa for a ~' ceramic. Thus the relative ease of dislocation motion for the expected singular slip system (100)[010] does not impair hardness. The fracture toughness K~c measured by indent crack initiation is again of similar magnitude (about 6 MPa m 1/2) to those of fi' ceramics.

5. CONCLUSIONS (i) Using the ternary Y203-A1203-SiO2 eutectic it is possible to carry out liquid phase sintering on Si2 N20 ceramics to attain a nearly theoretical density in the absence of applied pressure. (ii) The sintering temperature interval is restricted by the decomposition of Si2N20 and by the kinetics of reaction in high viscosity liquids. These upper and lower limits are approximately 1670 °C and 1600 aC respectively. (iii) Sintered microstructures consist of faceted Si2N20 crystals of micron size with negligible aluminium-plus-oxygen substitutions, within a matrix glass of about 10 vol.% which has a ternary eutectic Y203-A1203SiO2 composition. The crystal structure, although of orthorhombic symmetry, results in a pseudo-hexagonal prism morphology because of the development of facets on (110)(110) and (100) planes. The singular (100) planes which are linked by Si--O--Si bonds are not strongly developed in preference to (110). (iv) Heat treatment between 1250 and 1350 °C results in matrix glass crystallization as Y2Si207 with a solid solution of excess aluminium and oxygen within Si2N20. (V) A sample measurement of low temperature properties shows them to be similar to those of ~'-Si3N4-glass ceramic composites. In the crystallized state the high temperature stability in an oxidizing environment is excellent up to about 1350 °C and comparable with the crystallized derivatives of ~'-SinN 4glass composites.

94 ACKNOWLEDGMENT

This research forms part of a broad programme on oxynitride ceramic alloys, which is generously supported by the Wolfson Foundation.

REFERENCES 1 M. H. Lewis and G. E. Syers, Proc. Parsons Int. Turbine Conf., Institution of Mechanical Engineers, London, 1984, p. 181. 2 M. H. Lewis and R. J. Lumby, Powder Metall., 26 (1983) 73. 3 P. Boch and J. C. Glandus, J. Mater. Sci., 14 (1979) 379.

4 I. Idrestedt and C. Brosset, Acta Chem. Scand., 18 (1964) 1879. 5 M. H. Lewis, A. R. Bhatti, R. J. Lumby and B. North, J. Mater. Sci., 15 (1980) 103. 6 M. H. Lewis, B. S. B. Karunaratne, J. Meredith and C. Pickering, in B. Wilshire and D. R. J. Owen (eds.), Proc. Int. Conf. on the Creep and Fracture o f Engineering Materials and Structures, Swansea, March 24-27, 1981, Pineridge, Swansea, 1981, p. 365. 7 M. H. Lewis, G. R. Heath, S. M. Winder and R. J. Lumby, in R. E. Tressler and R. C. Bradt (eds.), Deformation o f Ceramic Materials II, Plenum, New York, 1983, p. 605. 8 G. Leng-Ward and M. H. Lewis, Proc. Int. Syrup. on Engineering Ceramics, Jerusalem, December 16-20, 1984, inMater. Sci. Eng., 71 (1985) 101. 9 K. H. Jack, J. Mater. Sci., 11 (1976) 1135. 10 G. R. Heath and M. H. Lewis, University of Warwick, unpublished work, 1983.