Synthesis of Mg-α-sialon from the mixture of silicon, aluminum and magnesia powders in a flowing nitrogen atmosphere

Synthesis of Mg-α-Sialon has been investigated by the mixture of silicon, aluminum and magnesia powders in a flowing nitrogen atmosphere in the range of 1300–1600 °C, when Mg-α-Sialon is designed with a chemical formulation of Mg _x Si_12−3x Al_3x O _x N_16−x in present work. The results showed that Mg-α-sialon initially occurred at 1400 °C and basically increased with elevated temperatures. For the samples of x = 0.6, 0.8 and 1.0 the products mainly consisted of Mg-α-Sialon with small amounts of Si, AlN and 21R AlN-polytypoid phases at 1600° C. However, in final products of x = 1.2, 1.4 and 1.6 only a little of Mg-α-Sialon formed and a great amount of Si remained in these samples at all the fired temperatures. Fortunately, the content of Mg-α-Sialon in these samples were obviously increased by adding a small amount of α-Si₃N₄ as seeds before nitridation.     

Optimization of time-temperature schedule for nitridation of silicon compact on the basis of silicon and nitrogen reaction kinetics

A time-temperature schedule for formation of silicon-nitride by direct nitridation of silicon compact was optimized by kinetic study of the reaction, 3Si + 2N₂ = Si₃N₄ at four different temperatures (1250°C, 1300°C, 1350°C and 1400°C). From kinetic study, three different temperature schedules were selected each of duration 20 h in the temperature range 1250°-1450°C, for complete nitridation. Theoretically full nitridation (100% i.e. 66.7% weight gain) was not achieved in the product having no unreacted silicon in the matrix, because impurities in Si powder and loss of material during nitridation would result in 5–10% reduction of weight gain. Green compact of density < 66% was fully nitrided by any one of the three schedules. For compact of density > 66%, the nitridation schedule was maneuvered for complete nitridation. Iron promotes nitridation reaction. Higher weight loss during nitridation of iron doped compact is the main cause of lower nitridation gain compared to undoped compact in the same firing schedule. Iron also enhances the amount of Β-Si₃N₄ phase by formation of low melting FeSi_x phase.     

Chemical vapour deposition of Si3N4 from a gas mixture of Si2Cl6, NH3 and H2

Si₃N₄ layers were obtained on a quartz substrate from a gas mixture of Si₂Cl₆, NH₃ and H₂ under a reduced pressure in a temperature range of 800 to 1300‡ C. Amorphous Si₃N₄ layers that were dense and adherent to the substrate were obtained in a temperature range of 800 to 1100‡ C. On the other hand,α-Si₃N₄ layers were obtained at 1200‡ C and a source-gas ratio (N/Si) of 1.33 to 1.77. The lowest deposition temperature of amorphous Si₃N₄ was considered to be about 700‡ C. The microhardness of amorphous Si₃N₄ obtained in a temperature range of 800 to 1100‡ C was 2400 to 2600 kg mm⁻² (load: 50 g), and that ofα-Si₃N₄ obtained at 1200‡ C was 3400 kg mm⁻². Chlorine contents in the Si₃N₄ layer decreased with increasing deposition temperature and source-gas ratio (N/Si), and with decreasing total pressure.     

Preparation and characterization of silicon nitride-silicon carbide composites

Silicon nitride-silicon carbide (Si₃N₄-SiC) composites were prepared by varying the percentage of silicon nitride at temperatures of 1350 to 1450°C. The mechanical and thermal properties of these composites were determined. The modulus of rupture of the composites increases with increase of temperature whereas the thermal expansion decreases. Composites with 10% and 50% Si₃N₄ have modulus of rupture of 49 and 86 MPa at 1400°C and thermal expansion coefficients (25°–1000°C) of 4·4 × 10⁻⁶ and 3·2 × 10⁻⁶°C⁻¹ respectively.     

Microstructure and property enhancement of silicon nitride-barium aluminum silicate composites with β-Si3N4 seed addition

Si₃N₄-barium aluminum silicate (BAS) self-reinforced composites have been prepared by pressureless sintering at 1800 °C for 2 h. The β-Si₃N₄ seeds incorporated in the starting α-Si₃N₄ powders encouraged the α- to β-Si₃N₄ phase transformation, and the final bimodal microstructure with large grains, consequently, led to the improvement of the fracture toughness, from 7.74 to 8.34 MPa m^1/2. The almost-complete crystallized BAS benefited the high-temperature mechanical properties. The residual stress, crack deflection, grain bridging, and pullout were considered as the major toughening mechanisms in this composite.     

Compressive creep behaviour of Yb4Si2O7N2 containing silicon nitride ceramic between 1400 and 1500°C

Abstract

A Yb₂O₃-SiO₂ doped silicon nitride ceramic, prepared such that the composition was placed directly on the Yb₄Si₂O₇N₂-Si₃N₄ tie line, was hot pressed sintered. The compressive creep behaviour of the sintered Yb₄Si₂O₇N₂-Si₃N₄ material was examined at 1400, 1450 and 1500°C under a stress range of 250-400 MPa in a nitorgen atmosphere. The sintered material exhibited high resistance to creep. The stress exponents were found to be ~1.9 at 1400°C, ~2.1 at 1450°C and ~2.1 at 1500°C. The activation energy obtained was 510 ± 25 kJ mol^-1. The values of the stress exponents and the activation energy suggest a cavitational process, accommodated by grain boundary sliding, viscous flow and solution-reprecipitation, as the most probable dominant creep mechanism.     

Direct bonding of Cu to oxidized silicon nitride by wetting of molten Cu and Cu(O)

When pressureless sintered silicon nitride with the main additives Y₂O₃ and Al₂O₃, having a thermal conductivity K = 20 W/m K, was oxidized at 1240–1360 °C in still air, the resulting surface oxide layer easily bonded to a copper plate in the temperature region between 1065 and 1083 °C, and in the oxygen concentration range of 0.008–0.39 wt%, as shown in a Cu–O phase diagram. The oxide on the silicon nitride was characterized as Y₂O₃·2SiO₂ and mixed silicate glass with additives and impurities that diffused through the grain boundary. The bonding strength of Cu/Si₃N₄ depends on the amount or layer thickness of silicate glass and reaches as high as 100 MPa by shear at room temperature. Detailed analysis of the oxidation layer and the peeled-off surfaces of directly bonded Si₃N₄/Cu reveal that the main mechanism of bonding is wetting to glassy silicate phase by mixtures of molten Cu and α-solid solution Cu(O), which solidify to α + Cu₂O below 1065 °C by a eutectic reaction. The direct reactive wetting of molten Cu, supplied from the grain boundary of a Cu plate, on the glassy phase enables very tight chemical bonding via oxygen atoms.     

Synthesis and properties of in situ Si3N4-reinforced BaO·Al2O3·2SiO2 ceramic matrix composites

Silicon nitride (94.5% α, 5.5% β), BaCO₃, Al₂O₃, and SiO₂ powders were mixed and pressureless sintered to produce a ceramic matrix composite consisting of 30 vol% barium aluminosilicate (BaO·Al₂O₃·2SiO₂ or BAS) matrix reinforced with in situ grown whiskers of β-Si3N4. In situ X-ray studies of the reactions indicated that BaCO₃ decomposes first to yield BaO which reacts with SiO₂ to yield a series of barium silicates which then react with Al₂O₃ between 950 and 1300°C to yield hexacelsian BAS. The sintering times were varied in order to develop a material system that combines the favourable properties of BAS with the high strength of Si₃N₄. In situ high-temperature X-ray studies after composite processing did not reveal any changes in the BAS or Si3N4 up to temperatures of 1300°C. Dilatometry studies of the sintered composite indicated a low-temperature transformation between 230 and 260°C with the temperature of transformation and volume change associated with the hexagonal to orthorhombic transformation decreasing with an increase of sintering time. Room- and high-temperature (1400°C) strengths were evaluated using four-point bend flexural tests. Composites exhibited near theoretical densities and an increase in flexural strength that was primarily dependent on the higher α- to β-Si₃N₄ transformation. This revised version was published online in November 2006 with corrections to the Cover Date.     

Hot-pressed β-Si3N4 containing small amounts of Be and O in solid solution

Single phase, hot-pressed Si₃N₄ ceramics with relative densities >95% and equiaxed grain structures have been prepared from high purity Si₃N₄ powders having specific surface areas of 8 to 20 m² g⁻¹ and oxygen contents ⩾2 wt % using a small amount of Be₃N₂ or BeSiN₂ as a densification aid. Densification depended sensitively on the concentration of Be and O in a given Si₃N₄ powder and on the usual hot-pressing parameters of pressure, temperature and time. A close association was found between densification and the conversion ofα- toβ-Si₃N₄ during hot-pressing. Based on the data presented, chemical reactions that occur during hot-pressing involve: (1) reaction of the densification aid with SiO₂ on the Si₃N₄ particle surfaces to form BeO and Si₂N₂O; (2) the further reaction of these two reaction products to give probable formation of a transient liquid phase (TLP); and (3) the reaction between TLP andα-Si₃N₄ particles to cause densification, probably by a solution-reprecipitation process, and conversion ofα-Si₃N₄ into aβ-Si₃N₄ solid solution. The chemical composition of a single phaseβ-Si₃N₄ solid solution prepared in this study by hot-pressing was approximately Si_2.9Be_0.1N_3.8O_0.2.     

Synthesis of monodisperse and high-purity α-Si3N4 powder by carbothermal reduction and nitridation

Carbothermal reduction-nitridation method is an effective means for synthesizing Si₃N₄ powder. Herein, spherical monodisperse silica was used as silicon source. The effects of reaction temperature, nitrogen flow rate and Si₃N₄ seeds content on the products were studied. It was found that high-purity α-Si₃N₄ (>99.0 wt%) was synthesized from C/SiO₂ = 3:1 at 1400 °C, reaction time of 6 h and nitrogen flow rate of 800 ml/min. The powder, with an average size of 0.5 μm, showed good dispersity and regular morphology because spherical monodisperse silica could be completely coated with carbon. The more contact sites between SiO₂ and C, the higher concentration of SiO(g) would be produced in the initial stage. It also indicated that the nucleation rate of α-Si₃N₄ increased, thereby inhibiting the formation of an agglomerate phase and suppressing the grain growth of α-Si₃N₄. Furthermore, higher nitriding temperature and Si₃N₄ seeds content both decreased the grain size and increased β-Si₃N₄ content. The forming mechanism of non-agglomerated and submicron-sized α-Si₃N₄ was clarified.     

Effects of talc and clay addition on pressureless sintering of porous Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics

Porous Si₃N₄ ceramics were successfully synthesized using cheaper talc and clay as sintering additives by pressureless sintering technology and the microstructure and mechanical properties of the ceramics were also investigated. The results indicated that the ceramics consisted of elongated β-Si₃N₄ and small Si₂N₂O grains. Fibrous β-Si₃N₄ grains developed in the porous microstructure, and the grain morphology and size were affected by different sintering conditions. Adding 20% talc and clay sintered at 1700°C for 2 h, the porous Si₃N₄ ceramics were obtained with excellent properties. The final mechanical properties of the Si₃N₄ ceramics were as follows: porosity, P ₀ = 45·39%; density, ρ = 1·663·g·cm⁻³; flexural strength, σ _b (average) = 131·59 MPa; Weibull modulus, m = 16·20.     

Manufacture of fibrous β-Si3N4-reinforced biomorphic SiC matrix composites for bioceramic scaffold applications

The manufacturing of the Si₃N₄ reinforced biomorphic microcellular SiC composites for potential medical implants for bone substitutions with good biocompatibility and physicochemical properties was performed in a two step process. First, wood-derived porous Si/SiC ceramics with various porosities were produced by liquid silicon infiltration (LSI) at 1550 °C with static nitrogen atmosphere protection (0.1 MPa), followed by subsequent partial removing of the Si in vacuo at 1700 °C for different periods of time. Secondly, the final porous Si₃N₄ fiber/SiC composite was obtained by further chemical reaction of nitrogen with the infiltrated residual silicon at 1400 °C for 4 h under high concentration flowing nitrogen atmospheres (0.5 MPa). The bending strengths of the porous Si₃N₄ fiber/SiC composite at axial and radial direction were measured as 180.03 MPa and 90 MPa respectively. The improvement in bending strength was primarily attributed to grain pull-out and bridging enhanced by the elongated β-Si₃N₄ grains cross-linked in the depth of the pore channels. The TG analysis showed an obvious improvement in oxidation resistance of the nitride specimens.     

Effect of microstructure on the high temperature strength of nitride bonded silicon carbide composite

Four compositions of nitride bonded SiC were fabricated with varying particle size of SiC of ∼ 9.67, ∼ 13.79, ∼ 60 μ and their mixture with Si of ∼ 4.83 μ particle size. The green density and hence the open porosity of the shapes were varied between 1.83 to 2.09 g/cc and 33.3 to 26.8 vol.%, respectively. The effect of these parameters on room temperature and high temperature strength of the composite up to 1300°C in ambient condition were studied. The high temperature flexural strength of the composite of all compositions increased at 1200 and 1300°C because of oxidation of Si₃N₄ phase and blunting crack front. Formation of Si₃N₄ whisker was also observed. The strength of the mixture composition was maximum.     

Oxidation bonding of porous silicon nitride ceramics with high strength and low dielectric constant

Silica (SiO₂) bonded porous silicon nitride (Si₃N₄) ceramics were fabricated from α-Si₃N₄ powder in air at 1200–1500 °C by the oxidation bonding process. Si₃N₄ particles are bonded by the oxidation-derive SiO₂ and the pores derived from the stack of Si₃N₄ particles and the release of N₂ and SiO gas during sintering. The influence of the sintering temperature and holding time on the Si₃N₄ oxidation degree, porosity, flexural strength and dielectric properties of porous Si₃N₄ ceramics was investigated. A high flexural strength of 136.9 MPa was obtained by avoiding the crystallization of silica and forming the well-developed necks between Si₃N₄ particles. Due to the high porosity and Si₃N₄ oxidation degree, the dielectric constant (at 1 GHz) reaches as low as 3.1.     

The preparation and characterization of SiC-AlN-Fe2Si composites with high porosity from allophane

The reaction products of an allophane heated with carbon at 850–1600 °C in the stream of nitrogen for a given time were characterized by X-ray diffractometry. As a result, it was found that cristobalite and mullite were stable phases at 850–1300 °C, β-Si₃N₄ and α-Al₂O₃ at 1300–1500 °C, and SiC-AlN-Fe₂Si at temperatures higher than 1500 °C. SiC-AlN-Fe₂Si composites with high porosity of about 50% were easily prepared by a heat treatment at a temperature higher than 1500 °C with carbon in a stream of nitrogen. The formation mechanism of the composites is kinetically discussed from a viewpoint of small-pore shrinkage and large-pore expansion by volume diffusion during heating. The resultant microstructure of the composites obtained is also discussed.     

Initial nucleation of amorphous Si–B–C–N ceramics derived from polymer-precursors

Nucleation behavior of amorphous Si–B–C–N ceramics derived from boron-modified polyvinylsilazane procusors was systematically investigated by transmission electron microscopy(TEM) combined with spatially-resolved electron energy-loss spectroscopy(EELS) analysis. The ceramics were pyrolyzed at1000?C followed by further annealing in N2, and SiC nano-crystallites start to emerge at 1200?C and dominate at 1500?C. Observed by high-angle annular dark-field imaging, bright and dark clusters were revealed as universal nano-structured features in ceramic matrices before and after nucleation, and the growth of cluster size saturated before reaching 5 nm at 1400?C. EELS analysis demonstrated the gradual development of bonding structures successively into SiC, graphetic BNCxand Si_3N_4 phases, as well as a constant presence of unexpected oxygen in the matrices. Furthermore, EELS profiling revealed the bright SiC clusters and less bright Si_3N_4-like clusters at 1200–1400?C. Since the amorphous matrix has already phase separated into SiCN and carbon clusters, another phase separation of SiCN into SiC and Si_3N_4-like clusters might occur by annealing to accompany their nucleation and growth, albeit one crystallized and another remained in amorphous structure. Hinderance of the cluster growth and further crystallization was owing to the formation of BNCxlayers that developed between SiC and Si_3N_4-like clusters as well as from the excessive oxygen to form the stable SiO_2.     

Effect of annealing on the microstructure, room-temperature strength, and fracture toughness of Al2O3−ZrO2−TiN ceramics

The microstructure, phase composition, room-temperature flexural strength, and fracture toughness of Al₂O₃−ZrO₂−TiN (AZT) ceramics were studied on specimens annealed in air at 1000, 1200, and 1400°C. The strength of the ceramics decreased with annealing temperature. The degradation in strength was caused by defects formed on or near the surface of the ceramics during oxidation of TiN which started at 600–700°C. The surface defects after annealing are influenced by the formation of rutile (TiO₂) at 1000 and 1200°C, aluminum titanate (Al₂TiO₅), and titanium suboxide Ti₅O₉ at 1400°C as well as by diffusion processes associated with ZrO₂. If the annealing of smooth AZT specimens in air resulted in lower strength, specimens in the form of single-edge notched beam (SENB) exhibited a considerable increase in fracture toughness (K _Ic) with annealing temperature. Such behavior was caused by the formation of an oxide layer which hindered the propagation of the main crack from the notch base. Thermal treatment of the smooth AZT specimens and further edge notching and testing did not result in a change of K _Ic values. The Al₂O₃ and Al₂O₃−ZrO₂ ceramics were also tested for comparison. Translated from Problemy Prochnosti, No. 1, pp. 132–138, January–February, 1999.     

Correlation between fracture toughness and microstructure of seeded silicon nitride ceramics

Microstructure development and fracture toughness of Si₃N₄ composites were studied in the presence of seeds and Al₂O₃ + Y₂O₃ as sintering aids. The elongated β-Si₃N₄ seeds were introduced into two different α-Si₃N₄ matrix powders; one was the ultra fine powder matrix and the other was the coarse powder matrix. The amount of seeds varied from 0 to 6 wt%. The grain growth inhibition and the mechanism of toughening were discussed and correlated with microstructure. The maximum fracture toughness of 9.0 MPa m^1/2 was obtained for ultra fine powder with 5 wt% seeds hot pressed at 1,700 °C for 6 h.     

Creep behavior in SiC-whisker reinforced silicon nitride composite

Tensile and flexural creep tests of 20 vol % SiC whiskers reinforced Si₃N₄ composite processed by gas pressure sintering have been carried out in air in the temperature range of 1000–1300°C. The stress exponent for flexural creep is 16 at 1000°C. However, at 1200 and 1250°C the stress exponents for both tensile and flexural creep vary with increasing stress. In the low stress region, the activation energy for creep is 1000 kJ/mol. In the high stress region, it is 680 kJ/mol. The different creep mechanisms dominate in the low and high stress regions, respectively. This revised version was published online in September 2006 with corrections to the Cover Date.     

Low temperature preparation ofβ-alumina by an alkoxide route

β-alumina has been prepared by the thermal decomposition of a mixture of sodium and aluminium isopropoxides followed by heating up to 1000°C. It has been found necessary to use sodium isopropoxide in excess (25–30%) for the complete formation ofβ-alumina at 1000°C. On the other hand one obtains a mixture ofβ-alumina andα-alumina when the starting materials are taken in the stoichiometric ratio Na₂O:11Al₂O₃. DTA, TG and DTG studies of a mixture of sodium and aluminium isopropoxide showedβ-alumina formation at 1000°C. NCL Communication No. 4446.