Showing papers on "Sialon published in 1999"

New developments in ? -SiAlON ceramics

Abstract: Two different α -sialon compositions have been densified by either pressureless sintering or capsul-free sinter HIPing using either single cation namely strontium, lanthanum, cerium, neodymium or their equimolar mixtures with calcium/ytterbium as a principal cation. The resulting materials have been heat-treated at 1450 °C for a maximum of 720 h to observe the α → β sialon transformation. Effect of starting compositions on the phase stabilities, microstructure, room temperature mechanical properties and optical transparent nature of sintered materials have been investigated. SEM observations for oxygen rich starting compositions revealed that α -sialon grains exist in needle like morphology in all the systems studied after both pressureless sintering and HIPing. Suprisingly, the nitrogen rich starting compositions give rise to coloured, optically transparent α -sialon ceramics after capsul-free HIPing. Clear evidence of α -sialon formation with incorporation of large cations namely, La3+, Ce3+, Nd3+ and Sr2+, has been established by XRD, SEM and EDX analysis. The observed phenomenon could possibly extend the usefulness of α -sialons ceramics to both structural and functional applications.

High temperature syntheses of novel nitrido- and oxonitrido-silicates and sialons using rf furnaces

Abstract: The high-temperature reaction of pure metals with silicon diimide Si(NH) 2 in a specially developed radiofrequency furnace has been successfully applied to the synthesis of novel highly condensed nitridosilicates. With SrCO 3 as an additional starting material this procedure has now been extended to the synthesis of oxonitridosilicates and oxonitridoaluminosilicates (sialons). Two novel sialons SrSiAl 2 O 3 N 2 [space group P2 1 2 1 2 1 (no. 19), a=491.89(6), b=789.73(7), c=1134.94(18) pm, Z=4] and SrErSiAl 3 O 3 N 4 [space group P6 3 mc (no. 186), a=606.53(3), c=985.90(8) pm, Z=2] have been obtained as coarsely crystalline materials. According to single crystal X-ray diffraction both compounds adopt structure types which are unprecedented for sialons. They derive from the nitridosilicates LnSi 3 N 5 (Ln=La, Ce, Pr, Nd) and MYbSi 4 N 7 (M=Sr, Ba, Eu), respectively, by partial substitution of Si by Al and N by O. For SrSiAl 2 O 3 N 2 and SrErSiAl 3 O 3 N 4 an unambiguous crystallographic differentiation between the tetrahedral centres (Al and Si) as well as the bridging atoms of the framework structures (N and O) seems reasonable.

Phase relationships and stability of α'-SiAlON

Abstract: Phase diagrams on the α′-SiAlON plane have been investigated for ytterbium-, yttrium-, and neodymium-containing systems at different temperatures. Phase relationships between α′-SiAlON, β′-SiAlON, AlN polytypoids, and liquid/grain-boundary phases have been determined. The stability region of the α′-SiAlON phase increases as the temperature increases and the ionic size of the rare-earth cation decreases. Meanwhile, the solubility of aluminum and oxygen in β′-SiAlON varies with the solubility of aluminum and oxygen in α′-SiAlON, which causes migration of all the phase lines on the α′-plane toward the nitrogen-rich side at lower temperatures and with larger modifying cations. These systematic variations of the phase relationships fully explain the occurrence of α′→β′ reverse transformations that are sometimes observed in this system.

Kinetics of phase transformations in SiAlON ceramics: I. effects of cation size, composition and temperature

Abstract: Kinetics of direct α / β -Si 3 N 4 → α -SiAlON transformations, reverse α ′-SiAlON→ β ′-SiAlON transformation, and the formation of intermediate phases were investigated for SiAlON ceramics with rare earth stabilizing cations. It was determined that smaller cations (Yb), and α -Si 3 N 4 starting powders lead to faster Si 3 N 4 → α ′-SiAlON transformations. Using the knowledge of phase stability of α ′-SiAlON, these observations have been correlated to the overall driving force for the transformation, and the correlation is further extended to the reverse α ′-SiAlON→ β ′-SiAlON transformation. Formation of intermediate phases during Si 3 N 4 → α ′-SiAlON transformation is shown to depend on the α ′-SiAlON formation and to be inversely correlated to the stability of α ′-SiAlON. A thermodynamic interpretation thus emerges to account for the kinetics of SiAlON transformation that encompass the effects of the size of the stabilizing cations, type of starting powder, overall composition, and the reaction temperature.

Characteristics of Ca-α-sialon : Phase formation, microstructure and mechanical properties

Abstract: Ca-α-Sialon (Ca x Si 12 -( m+n ) Al m+n O n N 16-n ,) ceramics with extensive compositions on the line of Si 3 N 4 –CaO:3AlN (where m =2 n ) ranged from x =0·3 up to x =2·0 were fabricated by hot-pressing. An exploration for Ca-α-Sialon involving reaction sequences, phase compositions, cell dimensions, microstructure and mechanical properties was carried out in the present work.

Kinetics of phase transformations in SiAlON Ceramics: II. Reaction Paths

Abstract: Kinetics of various α/β-Si3N4→α′/β′-SiAlON transformations were investigated for SiAlON ceramics with additions of rare-earth cations. It was determined that α-Si3N4 starting powders lead to faster Si3N4→α′/β′-SiAlON transformations than β-Si3N4 powders. In reactions with a two-phase α′/β′-SiAlON product, the formation of a SiAlON that is structurally similar to the majority starting powders is initially favored, and such SiAlON has a solute composition leaner than the final equilibrium composition. These results suggest that the transformation kinetics is largely governed by the heterogeneous nucleation that is strongly sensitive to the driving force, dictated by the majority phase of the starting powders, and that coherent nucleation is often favored when abundent isostructural nucleation sites are available.

Preparation and crystal structure of a new Sr containing sialon phase Sr2AlxSi12–xN16–xO2+x (x≈2)

Abstract: A nitrogen-rich sialon phase containing Sr or Eu, named the S-phase, has been reported to form in the M′-Si-Al-O-N systems with M′=Sr and Eu. A sample with overall composition Sr 2 Al 2.5 Si 10 O 4 N 14.5 hot-pressed at 1800°C for 2 h contained approximately 85 vol% of the S-phase, and in addition the α- and β-sialon phases and an amorphous phase. Its structure was solved from X-ray synchrotron powder data (λ=1.1608 A), using direct methods, and was refined by the Rietveld method from 131 reflections in the 2θ range 10-59° to R F =2.7%, with the assumed composition Sr 2 Al 2 Si 10 O 4 N 14 , space group Imm2, a=8.2788(9), b=9.5757(9), c=4.9158(4) A, V=389.7 A 3 . The structure model was confirmed by its electron diffraction pattern and by high-resolution electron microscopy studies. The structure exhibits a tetrahedral network with high connectivity, each tetrahedron sharing corners with seven surrounding tetrahedra, and the Sr atoms, irregularly coordinated by eight O/N atoms, are found in tunnels extending along [001].

Carbothermal Preparation of β-SiAlON Powder at Elevated Nitrogen Pressures

Abstract: β-SiAlON powders have been prepared by carbothermal nitridation of kaolin or SiO 2 plus Al 2 O 3 mixtures at 1450-1800°C and elevated nitrogen pressures (1-50 bar). Powders with widely variable particle size and morphology were obtained depending on temperature, pressure, gas-flow rate, and raw-material type. An elevated nitrogen pressure effectively reduced SiO loss and suppressed SiC formation. Pure β-SiAlON powder of composition Si 2·8 Al 3·2 O 3·2 N 4·8 and with a fine particle size (∼1 μm) was produced at 1650°C, 50 bar N 2 and a high gas-flow rate for 2 h. The nitridation rate was hampered by an increase of the CO concentration in the system.

Silicon nitride and sialon ceramics

Abstract: The most intriguing recent development in the field of silicon nitride ceramics has undoubtedly been the discovery of a cubic form of silicon nitride. Major advances were made in α-SiAlON ceramics, including the development of thermally stable, in situ reinforced grades. Significant achievements were reported in tailoring the mechanical properties of silicon nitride ceramics through control of secondary phase chemistry and grain morphology.

high-resolution electron microscopy of aSr-containing sialon polytypoid phase

Abstract: A new type of Sr-containing sialon polytypoid phase with the structural formula SrSi10-xAl18+xN32-xOx (x approximate to l) has been found in the Sr-Si-Al-O-N system. The phase was characterised by ...

Sialon Transformation in Calcium-containing α-SiAlON Ceramics

Abstract: Recent studies on rare earth densified α-sialon ceramics have shown that the resulting α-sialon product, present either as a single phase or in conjunction with β-sialon, is unstable when heat treated at lower (1350–1600°C) temperatures, and transforms to a mixture of β-sialon plus other crystalline or liquid metal sialon phases. The present paper describes similar studies carried out on calcium-densified α-sialon compositions, and shows that for a wide range of starting compositions, with calcium as the sole sintering additive or present with other (Nd, Sr) cations, the resulting calcium stabilised a-sialon products are fully resistant towards αβ transformation when heat treated in the temperature range 1450–1550°C. Whereas αβ transformation in rare earth stabilised α-sialons is influenced by the nature of the rare earth cation, the α-sialon composition, the composition and melting behaviour of the liquid phase and the presence or absence of β-sialon nuclei, transformation in calcium a-sialons appears to be influenced by none of these parameters. Clearly if αβ sialon transformation occurs in this system, the transformation temperature for calcium α-sialons must be below 1450°C, the heat-treatment temperature which has been most frequently used in current research on rare earth densified α-sialons.

Optimizing mechanical properties and thermal stability of Ln-α-β-sialon by using duplex Ln elements (Dy and Sm)

Abstract: The densification behavior, phase transformation, microstructure and mechanical properties of α-β-sialon ceramics have been compared among the counterpart Ln-α-β-sialon compositions where Ln represents Dy, Sm and (Dy+Sm). Sm-sialons have a rather finer microstructure, higher flexural strength and fracture toughness, but their hardness are relatively lower. Dy-sialons possess a higher hardness. Unlike Sm-sialon, Dy-sialons have kinetically priority in the formation of α-sialon over the other Dy-containing phases (such as melilite and garnet phases) thus giving a higher content of α-sialon than the designed value. Dy-α-sialon is much stable than Sm-α-sialon during heat treatment. The higher flexural strength and fracture toughness of Sm-sialons and the higher hardness and good stability of Dy-sialons can remain in the (Dy+Sm)-sialons.

Novel Developments in α-SiAION Ceramics

Abstract: Advanced Research Workshop on Engineering Ceramics 99: Multifunctional Properties - New Perspectives -- MAY 11-15, 1999 -- SMOLENICE CASTLE, SMOLENICE, SLOVAKIA

Phonon densities of states and related thermodynamic properties of high temperature ceramics.

Abstract: Structural components and semiconductor devices based on silicon nitride, aluminum nitride and gallium nitride are expected to function more reliably at elevated temperatures and at higher levels of performance because of the strong atomic bonding in these materials. The degree of covalency, lattice specific heat, and thermal conductivity are important design factors for the realization of advanced applications. We have determined the phonon densities of states of these ceramics by the method of neutron scattering. The results provide a microscopic interpretation of the mechanical and thermal properties. Moreover, experimental data of the static structures and dynamic excitations of atoms are essential to the validation of interparticle potentials employed for molecular-dynamics simulations of high-temperature properties of multi-component ceramic systems. We present an overview of neutron-scattering investigations of the atomic organization, phonon excitations, as well as calculations of related thermodynamic properties of Si 3 N 4 , β -sialon, AlN and GaN. The results are compared with those of the oxide analogs such as SiO 2 and Al 2 O 3 .

Stability of α-Sialons in Low Temperature Annealing

Abstract: Phase and microstructural change due to post-sintering annealing of Nd, Dy, Y and Yb-α-sialon ceramics have been investigated for the composition of α-sialon-rare earth aluminum garnet (RAG) systems. For R= Dy, Y and Yb, only α-sialon was observed as the crystalline phase after sintering at 1750°C. After subsequent post-sintering annealing at 1450°C for 72 h, no phase transformation from α-to β-sialon was observed, although the crystallization of garnet phase at grain boundaries was detected. However, for R=Nd, α-,β-sialon and melilite phase (M’) were observed after sintering at 1750°C. The β-sialon and melilite phases increased with low-temperature heat treatment, implying that the transformation from α- to β-sialon occurred. It was concluded that the α-sialon phase stabilized by small ions like Y, Dy or Yb has high thermal stability and does not transform to β-sialon when there is no chemical reaction between α-sialon grains and the grain boundary phase.

Hard sialon ceramics reinforced with SiC nanoparticles

Abstract: SiC nanoparticles were added to a range of rare-earth doped Ln-sialon (Ln=Nd, Dy, Yb) ceramics to prepare a series of increased hardness sialon/SiC p composites. The composites, of overall composition Ln 0.33 Si 9.3 Al 2.7 O 1.7 N 14.3 and containing 10 and 20 wt.% of SiC particles, were sintered by hot-pressing at 1800°C and then heat-treated at 1450°C to modify the microstructure and properties by α⇔β sialon transformation. Enhanced hardness was found in proportion to the content of SiC particles and with increasing α:β sialon phase ratio in the matrix. The kind of grain boundary phases which occurred also affected the results. In Nd-densified samples with or without SiC particles, the α-sialon phase was unstable under the heat-treatment conditions used and transformed substantially to β-sialon in elongated grain morphology, with Nd-melilite phase as the dominant grain boundary crystalline phase. These materials were tough with a highest indentation fracture toughness K 1C of 7.0 MPa m 1/2 . In contrast, Dy- and Yb-densified samples with or without SiC addition showed good stability for the α-sialon phase under the same conditions, with only a small amount of β-sialon phase produced and Dy- or Yb-garnet as the main grain boundary phase. These composites were hard and brittle with a maximum Vickers hardness HV 10 of 20 GPa.

Influence of the Interfacial Properties on the Microstructural Development and Properties of Silicon Nitride Ceramics

Abstract: The influence of the sintering additive system on the grain growth anisotropy of silicon nitride has been studied in the two supersaturated oxynitride glass systems M-Si-Al-O-N (where M = Yb, Gd, Nd, La, and Y) and M-Si-Mg-O-N (where M = Lu, Sm, La, Sc and Y). The glasses contained a low volume fraction of grains and offer the possibility to study grain growth without impingement. Both systems show an increase of the aspect ratio with an increasing cation radius of the rare earth elements. The aspect ratios for the Sc- and Y-based glasses are higher than expected from the ionic radius. Analytical transmission electron microscopy of the bulk glass and grain boundary film between flocculated particles reveals a difference in chemical composition. The larger rare earth cations are enriched in the film and the smaller ones are depleted compared to the bulk composition. The enrichment of the larger cations can be considered as an adsorption layer which reduces the growth rate of the prism planes. Glass systems with smaller cations (Yb, Gd) reveal a pronounced growth of the prism planes and sialon is formed in the outer region of the grains. The interfacial strength which determines the fracture toughness increases with a decreasing cation radius, but the effect is mainly attributed to the sialon formation rather than by the cation itself.

Alpha sialon particle

Abstract: PROBLEM TO BE SOLVED: To obtain high strength, high hardness and dense sialon consisting of micro-particles of nanometer level by incorporating as a principal component alpha sialon having specific composition consisting of an alkaline earth metal or rare earth element, Si, Al, O and N and making the alpha sialon into hollow ball-shaped particles having a specific diameter. SOLUTION: The objective sialon is obtained by incorporating the alpha sialon shown by the general formula MxSi12-m-nAlm+nOnN16-n (wherein M is a alkaline earth metal or rare earth element; 0<(x)<=2;(m)=νX(x) (ν is electric charge of M); 0<(n)<12-(m)) by 20 wt.% or more, preferably 70% or more, more preferably 85% or more, most preferably around 100% and pulverizing the hollow ball-shaped particles having 50-1,000 nm diameter, preferably 200-500 nm, of the alpha sialon into the nanometer particles having 2-50 nm diameter. The oxides of respective elements of the alpha sialon or their compounds to become the oxides by firing and carbon are mixed by specific ratios in a ball mill or the like. The mixture is fired at 1,400-1,600 deg.C in a nitrogen current and then subjected to heat treatment at around 70 deg.C in the air to remove the residual carbon so that the alpha sialon can be obtained.

Role of Intergranular Films in Toughened Ceramics

Abstract: Self-reinforced silicon nitride ceramics rely the generation of elongated grains that act as reinforcing elements to gain increases in fracture toughness. However, the size and number of the reinforcing grains must be controlled, along with the matrix grain size, to optimize the fracture toughness and strength. Furthermore, the toughening processes of crack bridging are dependent upon retention of these reinforcing grains during crack extension by an interfacial debonding process. Both the debonding process and the resultant toughening effects are found to be influenced by the composition of the sintering aids which typical are incorporated into the amorphous intergranular films found in these ceramics. Specifically, it is shown that the interface between the intergranular glass and the reinforcing grains is strengthened in the presence of an epitaxial SiAlON layer. In addition, the interface strength increases with the Al and 0 content of the SiAlON layer. Micromechanics modeling indicates that stresses associated with thermal expansion mismatch are a secondary factor in interfacial debonding in these specific systems. On the other hand, first principles atomic cluster calculations reveal that the debonding behavior is consistent with the formation of strong Si-0 and Al-O bonds across the glass-crystalline interface.

Silicothermal synthesis and densification of x-sialon in the presence of metal oxide additives

Abstract: The effect is reported of seven inorganic oxide additives on both the formation mechanism and the densification of X-sialon prepared by a silicothermal process The oxides were added to the starting mixture of halloysite clay, alumina and elemental silicon at a level of 1 wt% of the calculated final product, and fired in nitrogen at 1200–1500°C The formation of X-sialon was monitored by thermal analysis, powder XRD and 27 Al and 29 Si solid state MAS NMR The effects of the additives are temperature dependent, and influence the various stages of the reaction by differing degrees The oxides which best promote the formation of crystalline X-sialon (Y 2 O 3 , CaO and MgO) are also those which facilitate the conversion of initially-formed Si 3 N 4 to SiO 2 N 2 and SiO 3 N units, the latter being particularly enhanced by Y 2 O 3 , Fe 2 O 3 enhances the initial nitridation of Si but suppresses X-sialon formation by stabilising the preceding mullite phase Densification is most enhanced by Y 2 O 3 , CaO and CeO 2 ; MgO exerts its maximum effect on sintering at lower temperatures The beneficial influence of MgO and Y 2 O 3 on both X-sialon formation and sintering is due to the formation of liquid phases

Aqueous processing of carbothermally prepared Ca-α-SiAlON and β-SiAlON Powders: powder and suspension characterisation

Abstract: Properties of carbothermally prepared Ca-α-SiAlON and β-SiAlON powders and aqueous suspensions thereof were determined. The isoelectric points of Ca-α-SiAlON and β-SiAlON were 3·4 and 4·6. After addition of deflocculant, Dolapix CE64, the behaviour of both suspensions is nearly identical. The isoelectric points become 5·5 and 5·3, respectively. Despite differences in bulk composition, grain size distribution, grain size and shape, both SiAlON suspensions show a similar dependence of a zeta potential on pH. Optimum slip casting properties, i.e. lowest viscosity values (below 10 mPa s), the highest absolute zeta potential values, the smallest floc size and sediment volume were found between pH 10-11 for both powders. The potentials of the different suspension characterisation techniques were compared and zeta potential and viscosity measurements were found the most convenient. © 1999 Elsevier Science Ltd.

Fabrication and microstructure of sialon-bonded silicon carbide

Abstract: The sintering behaviour of ceramic-matrix composites consisting of a sialon matrix and silicon carbide particulate reinforcement has been studied in order to elucidate the densification behaviour and the formation of the microstructure. The materials are produced from a mixture of clay, silicon nitride and yttria sintering additive mixed with 60% of silicon carbide grit. The results lead to a description of the mechanism of phase formation which consists of the following steps: 1. Decomposition of china clay and formation of mullite and silica on heating. 2. Formation of liquid phase from these reaction products, together with excess silica from silicon carbide and yttria sintering additive. 3. Dissolution of silicon nitride in the liquid and precipitation of sialon X and O′ phases. 4. Formation of a high-yttrium, nitrogen glass on cooling. Composites of this kind with porosity of about 14% and zero linear shrinkage on firing, due to the rigid skeleton of carbide particles, are promising candidate materials for high-temperature applications where the microstructure offers the prospect of superior high-temperature strength through the continuous interlocking texture of the X-phase and O′-silicon oxynitride crystalline phases, the carbide skeleton and the high viscosity yttrium-nitrogen glass matrix. ©