Silicon nitride matrix composites with unidirectional silicon carbide whisker reinforcement

SiC whisker reinforced Si₃N₄ was fabricated by fiber extrusion and hot pressing. SiC whiskers were unidirectionally oriented in a carrier fiber. The fibers containing the oriented whiskers were hot pressed in Si₃N₄ powder to form a SiC_w/Si₃N₄ composite with approximately 5 volume% whiskers. SEM micrographs were image processed to quantify whisker orientations in the extruded fiber and the composite. Oriented whiskers contributed to nominal increase in fracture strength over monolithic samples before and after thermal shock testing from 500, 600 and 700°C.     

Synthesis of Si3N4 whiskers in porous SiC bodies

Si₃N₄ whiskers were synthesized by the carbothermal reduction process in porous SiC bodies. The SiC bodies had a sponge microstructure with pore sizes of approximately 600 μm. The raw materials for the Si₃N₄ whiskers were powder mixtures of Si₃N₄, SiO₂ and Si for silicon and phenolic resin for carbon. Cobalt was used as a metal catalyst. The carbothermal reaction was performed at 1400 °C or 1500 °C for 1 or 2 h. The α-Si₃N₄ whiskers grew inside the SiC pores by the VLS process, and their diameters ranged from 0.1 to 1.0 μm. The length of the grown Si₃N₄ whiskers was over 100 μm and their growth direction was [100].     

NMR and X-ray diffraction studies of amorphous and crystallized pyrolysis residues from pre-ceramic polymers

²⁹Si MAS NMR and X-ray diffraction studies are presented of black and white pyrolysis residues obtained by initial 1100°C pyrolyses in N₂ and NH₃ atmospheres followed by 1550°C pyrolyses in Ar, N₂ or vacuum atmospheres of a polycarbosilane and four polysilazane precursors to SiC and Si₃N₄ ceramics. Amorphous white pyrolysis residues crystallized under the various conditions to give not only Si₃N₄ but also Si₂N₂O, SiC, SiO₂ and Si, while black amorphous pyrolysis residues crystallized to form only Si₃N₄ or SiC. In general, the crystalline ceramic products observed depended on a variety of factors, i.e. moisture sensitivity of polymer, the initial 1100°C pyrolysis gas (N₂/NH₃), the dryness of the 1100°C-NH₃ pyrolysis gas and the 1550°C pyrolysis atmosphere (N₂, Ar, vacuum). An additional factor of interest affecting product distribution was the choice of crucible (alumina/graphite) employed in the 1550°C pyrolysis. The combined studies suggest that the white amorphous pyrolysis residues are complex silicon oxycarbonitrides (Si_xN_yO_zC_a), while the amorphous black residues are silicon carbonitrides (Si_xN_yC_z). This revised version was published online in November 2006 with corrections to the Cover Date.     

Effects of amorphous silica coatings on the sintering behaviours of SiC whisker-reinforced Al2O3 composites

In this study, pressureless sintering of silicon carbide whisker (SiC_w)-reinforced alumina composites was investigated. SiC whiskers or Al₂O₃ powders were coated with amorphous silica, and sintering behaviour was analysed according to the powder characteristics of the composite. It was found that amorphous silica coatings improved densification as compared with uncoated powders, because the viscous flow allows the release of any tensile stress due to differential shrinkage between the matrix and the silicon carbide whiskers. Mullite occurred when amorphous silica coatings reacted with alumina at 1500 °C, which resisted the viscous sintering of the amorphous silica coatings.     

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.     

A high-strength SiCw/SiC–Si composite derived from pyrolyzed rice husks by liquid silicon infiltration

A high-strength SiC composite with SiC whiskers (SiC_w) as reinforcement has been fabricated by liquid silicon infiltration (LSI) using pyrolyzed rice husks (RHs) as raw material. RHs were coked and pyrolyzed subsequently at high temperature to obtain a mixture containing SiC whiskers, particles, and amorphous carbon. The pyrolyzed RHs were then milled and modeled to preforms, which were then used to fabricate biomorphic SiC_w/SiC–Si composites by liquid silicon infiltration at 1,450, 1,550, and 1,600 °C, respectively. Dense composite with a density of 3.0 g cm⁻³ was obtained at the infiltration temperature of 1,550 °C, which possesses superior mechanical properties compared with commercial reaction-sintered SiC (RS-SiC). The Vickers hardness, flexure strength, elastic modulus, and fracture toughness of the biomorphic SiC_w/SiC–Si composite were 18.8 ± 0.6 GPa, 354 ± 2 GPa, 450 ± 40 MPa, and 3.5 ± 0.3 MPa m^1/2, respectively. Whereas the composites obtained at the other two infiltration temperatures contain unreacted carbon and show lower mechanical properties. The high flexure strength of the biomorphic composite infiltrated at 1,550 °C is attributed to the dense structure and the reinforcement of the SiC whiskers. In addition, the fracture mechanism of the composite is also discussed.     

Densification and microstructural developments during the sintering of aluminium silicon carbide

Densification and microstructural developments during the sintering of aluminum silicon carbide (Al₄SiC₄) were examined. Two types of Al₄SiC₄ powders were prepared by the solid-state reactions between: (i) Al, Si, and C at 1600°C for 10 h (designated as Al₄SiC₄(SSR)), and (ii) chemically-vapour deposited ultrafine Al₄C₃ and SiC powders at 1500°C for 4 h (Al₄SiC₄(CVD/SSR)). The specific surface areas of the Al₄SiC₄(SSR) and Al₄SiC₄(CVD/SSR) powders were 2.7 and 15.5 m² · g^–1, respectively. Relative densities of the pressureless-sintered Al₄SiC₄(SSR) and Al₄SiC₄(CVD/SSR) compacts were as low as 60–70% for firing temperatures between 1700°C and 2000°C. The relative densities of Al₄SiC₄(SSR) and Al₄SiC₄(CVD/SSR) compacts could be enhanced using the hot-pressing technique; the relative density of the Al₄SiC₄(SSR) compact hot-pressed at 1900°C for 3 h was 97.0% whereas that of the Al₄SiC₄(CVD/SSR) compact hot-pressed at 1900°C for 1 h attained 99.0%. The former microstructure was composed of plate-like grains of width 10–30 m and thickness 10 m whilst the latter microstructure was comprised of equiaxed grains with a typical diameter of 10 m. Densification of the Al₄SiC₄(CVD/SSR) compacts appeared to be promoted compared to the Al₄SiC₄(SSR) compact and this was attributed to the higher surface area, reduced agglomeration of the starting primary particles, and more homogeneous chemical composition.     

Preparation of ceramic coatings from pre-ceramic precursors

Scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) has indicated that adherent crack-free coatings of amorphous SiC and Si₃N₄/Si₂N₂O can be built up on planar alumina substrates by pyrolysis of layers of polycarbosilane (PCS) and poly(diphenyl)silazane (PDPS) precursors applied by spin- or dip-coating methods. In general, multilayers of black SiC can be prepared by pyrolysis of PCS layers at 1100°C in a nitrogen atmosphere while transparent coatings consisting of multiple layers of Si₃N₄ are prepared by pyrolysis of either PCS or PDPS layers in a flowing atmosphere of ammonia at 1100°C. The Si₃N₄/Si₂N₂O layers prepared by pyrolysing spin-coated layers of PDPS layers are found to be superior in quality (with respect to blemishes and embedded debris) than those prepared from spin-coated layers of PCS. Microhardness tests reveal that the coatings derived from PCS and PDPS are significantly softer than would be expected for SiC and Si₃N₄. X-ray photoelectron studies reveal that the surface of the PCS-derived SiC coatings consists of an SiO₂ layer while the surface of the PDPS-derived Si₃N₄/Si₂N₂O coating consists of an oxygen-rich silicon oxycarbonitride. These results are also generally supported by Rutherford backscattering spectra which also indicate considerable phase mixing of silicon, carbon, oxygen and nitrogen components within the bulk of the SiC and Si₃N₄/Si₂N₂O coatings on alumina.     

Effect of whisker aspect ratio on the density and fracture toughness of SiC whisker-reinforced Si3N4

Controlling the suspension properties prior to slip casting optimizes the homogeneity, density and fracture toughness of silicon carbide whisker reinforced silicon nitride (SiC_w/Si₃N₄). Further improvements in the mechanical properties are realized by combining ball milling with ultrasonic dispersion of the composite suspension. Ball milling reduces the SiCw aspect ratio from 25 to 15 which in turn increases the dispersion of the whiskers within the suspension, resulting in increases in the green and sintered density, along with the fracture toughness. In a binderless process, 20 volume% reduced aspect ratio (r = 15) SiC_w/Si₃N₄ can be densified to 95% theoretical density by pressureless sintering using 8% Y₂O₃ and 2% Al₂O₃ by weight as sintering aids. These composites had measured values of fracture toughness from 9–10.5 MPa · m^1/2, representing an average increase of approximately 30% over the fracture toughness for monolithic Si₃N₄ processed under identical conditions.     

Formation of SiC whiskers from compacts of raw rice husks

The formation of SiC whiskers from compacts of raw rice husks without coking and catalyst has been studied. A pyrolysis temperature of 1600°C has yielded a considerable quantity of SiC whiskers. The formation of spherical particles of silica is observed. Whisker formation occurs by reaction between SiO_(g) and CO_(g).     

Magnetic resonance characterization of solid-state intermediates in the generation of ceramics by pyrolysis of hydridopolysilazane

Chemical intermediates produced from the pyrolysis of hydridopolysilazane (HPZ) were studied in the solid state by multinuclear nuclear magnetic resonance and electron spin resonance. When pyrolysed at temperatures of 1200°C, uncured HPZ forms a ceramic material with a composition of Si_2.2N_2.2C_1.0. A series of HPZ-derived ceramics was produced using a number of different heat-treatment temperatures, varying between 300 and 1200°C. Solid-state magnetic resonance data generated from this set of HPZ-derived ceramics elucidate important features of this complex transformation. Silicon atoms initially exist in two types of sites in the polymer,Si(Me)₃ and ()₃SiH sites. Upon pyrolysis between 300 and 400°C, the silazane cyclizes and cross-links, forming an intractable, insoluble solid. Increasing the pyrolysis temperature to between 400 and 600°C creates a matrix that is partially inorganic; at heat-treatment temperatures in this range, many of the C-H bonds of the starting polymer are cleaved. Elevating the heat-treatment temperature to between 600 and 1200°C generates a series of chemical structures with silicon in a tetrahedral site of the general form SiN_4–xC_x, where x=0, 1, 2, 3, 4. No crystalline forms of Si₃N₄ or SiC were detected in the material prepared at even the highest heat-treatment temperature of 1200 °C.     

Sintering anisotropy in slip-cast SiC-whisker/Si3N4-powder compacts

Sintering anisotropy in slip-cast SiC-whisker/Si₃N₄-powder compacts was studied at 1750°C in 0.1 MPa N₂ or at 1825°C in 1.0 MPa N₂. It was shown that whiskers oriented parallel to the mould surface and nearly 1.5-dimensionally along the slip flow direction when the whisker content was 10 wt%. Linear shrinkage was largest perpendicular to the mould surface and smallest perpendicular to the whisker alignment. It was shown that the retardation of densification by whiskers is due to the formation of a rigid network along the whisker alignment, which is in accordance with percolation theory. The addition of up to 20 wt% whisker did not affect sintering kinetics but lowered sinterability by 2-dimensional alignment of the whiskers. The anisotropy in fracture toughness is related to the orientation of the whiskers.     

Characterization of porous silicon nitride/silicon oxynitride composite ceramics produced by sol infiltration

Porous silicon nitride/silicon oxynitride composite ceramics were fabricated by silica sol infiltration of aqueous gelcasting prefabricated Si₃N₄ green compact. Silica was introduced by infiltration to increase the green density of specimens, so suitable properties with low shrinkage of ceramics were achieved during sintering at low temperature. Si₂N₂O was formed through reaction between Si₃N₄ and silica sol at a temperature above 1550 °C. Si₃N₄/Si₂N₂O composite ceramics with a low linear shrinkage of 1.3–5.7%, a superior strength of 95–180 MPa and a moderate dielectric constant of 4.0–5.0 (at 21–39 GHz) were obtained by varying infiltration cycle and sintering temperature.     

The direct bonding between copper and MgO-doped Si3N4

An application of direct bonding method for copper to silicon nitride (Si₃N₄) joining was investigated. Si₃N₄ was sintered with 5wt% MgO at 1700 ° C for 30 min in nitrogen atmosphere, and oxidized at various temperatures. The bonding was performed at 1075 ° C in nitrogen atmosphere with low oxygen partial pressure. The direct bonding was not achieved for the Si₃N₄ oxidized below 1200 ° C or nonoxidized. During oxidation, magnesium ion added as sintering aids, diffused out to the surface of Si₃N₄ and formed MgSiO₃, which seemed to have an important role in the bonding. Fracture of the bonded specimen under tensile stress took place within the oxide layer of Si₃N₄. The bonding strength was decreased with oxidation temperature and time. Maximum strength was found to be 106 kg cm^–2 for the Si₃N₄ oxidized at 1200 ° C for 1 h.     

Thermal mismatch stress in SiC whisker reinforced aluminium composite: new measurement method by X-ray diffraction

Abstract

A new X-ray diffraction method for characterising thermal mismatch stress (TMS) in SiC_w–Al composite has been developed. The TMS and thermal mismatch strain (TMSN) in SiC whiskers are considered to be axis symmetrical, and can be calculated by measuring the lattice distortion of the whiskers. Not only the average TMS in whiskers and matrix can be obtained, but the TMS components along longitudinal and radial directions in the SiC whiskers can also be deduced. Experimental results indicate that the TMS in SiC whiskers is compressive, and tensile in the aluminium matrix. The TMS and TMSN components along the longitudinal direction in the SiC whiskers are greater than those along the radial direction for a SiC_w–Al composite quenched at 500°C.     

Preparation of Ti<Subscript>3</Subscript>SiC<Subscript>2</Subscript> through reduction of titanium dioxide with silicon carbide

This paper describes the reduction of titanium dioxide with a mixture of silicon carbide and silicon powders at a temperature of 1550°C under vacuum. It has been shown that the use of the combined reductant enables the preparation of the ternary phase Ti₃SiC₂ through concurrent carboand silicothermic processes. The optimal compositions for Ti₃SiC₂ formation are TiO₂ + (1.5–x)SiC + 2xSi with x = 0.4–0.5. The Ti₃SiC₂ yield then reaches 96 wt %.     

Microstructures of ultrafine Si3N4 powder compacts induced by rapid heating under controlled thermograms

The rapid heating of ultrafine Si₃N₄ powder compacts with relatively high oxygen content was investigated with particular attention to their microstructures. The specimens were heated without resorting to additives and pressure under controlled thermograms attained by an Xe image heating apparatus. The effects of particle size and oxygen contents, as well as heating conditions, were investigated. When fired to 1700°C within 15 s and then immediately held at 1350°C for 10 min in N₂ atmosphere, significant densification took place in the limited region, in addition to decreasing the oxygen content to less than 0.3 wt%. This decrease of oxygen content was drastic and found to be a prominent feature of this heating process; especially, weakly oxidized ultrafine powder less than 30 nm was found to be advantageous to obtain uniform and homogeneous Si₃N₄ microstructures.     

Continuous synthesis of silicon carbide whiskers

A two-step reaction scheme has been employed for the synthesis of SiC whiskers at 1450 °C under an argon or hydrogen flow. First, SiO vapour was generated via the carbothermal reduction of silica in a controlled manner. Second, the generated SiO vapour was reacted with carbon-carrying vapours such as CO and CH₄, which resulted in the growth of SiC whiskers on a substrate away from the batch. A higher growth rate was observed in the hydrogen atmosphere due to the formation of CH₄ which provides a more favourable reaction route. By the use of thermodynamic calculations, the preferred reaction routes have been selected for an efficient synthesis of SiC whiskers, and a continuous reactor has been designed. The system consists of a boat-train loaded with the silica-carbon mixture and iron-coated graphite substrate above it in an alumina-tube reactor. By pushing the boat-train into the hot zone at a fixed speed, SiO vapour is constantly generated. High-quality SiC whiskers have been grown on the substrate with diameters of 1–3 m. The yield was about 30% based on the silicon input as SiO₂ and silicon output as SiC whiskers. This demonstrates the feasibility of continuous production of high-quality SiC whiskers which does not require additional processes such as purification and classification.     

Mechanical properties of Si3N4/BN fibrous monolithic ceramics at elevated-temperature

The mechanical properties at high temperature of Si₃N₄/BN fibrous monolithic ceramics were tested. The flexural strength of SiC whisker reinforced Si₃N₄/BN fibrous monolithic ceramics from 25°C to 1200°C were investigated. The strength degraded slowly from 1000°C to 1200°C which was different to Si₃N₄ monolithic ceramics. The creep behaviors of the material at different temperatures were characterized. Si₃N₄/BN fibrous monolithic ceramics possess high creep resistance. The chemical composition and microstructure of the composites were analyzed by XRD and SEM.     

Si3N4-ZrO2 composites with small Al2O3 and Y2O3 additions prepared by HIP

Si₃N₄-ZrO₂ composites have been prepared by hot isostatic pressing at 1550 and 1750 °C, using both unstabilized ZrO₂ and ZrO₂ stabilized with 3 mol% Y₂O₃. The composites were formed with a zirconia addition of 0, 5, 10, 15 and 20 wt%, with respect to the silicon nitride, together with 0–4 wt% Al₂O₃ and 0–6 wt% Y₂O₃. Composites prepared at 1550 °C contained substantial amounts of unreacted -Si₃N₄, and full density was achieved only when 1 wt% Al₂O₃ or 4 wt % Y₂O₃ had been added. These materials were generally harder and more brittle than those densified at the higher temperature. When the ZrO₂ starting powder was stabilized by Y₂O₃, fully dense Si₃N₄-ZrO₂ composites could be prepared at 1750 °C even without other oxide additives. Densification at 1750 °C resulted in the highest fracture toughness values. Several groups of materials densified at 1750 °C showed a good combination of Vickers hardness (HV10) and indentation fracture toughness; around 1450 kg mm^–2 and 4.5 MPam^1/2, respectively. Examples of such materials were either Si₃N₄ formed with an addition of 2–6 wt% Y₂O₃ or Si₃N₄-ZrO₂ composites with a simultaneous addition of 2–6 wt%Y₂O₃ and 2–4 wt% Al₂O₃.