Processing of concentrated aqueous fluorapatite suspensions by slip casting

In order to produce stable aqueous fluorapatite (FA) suspensions, its surface reactivity in an aqueous solution having two initial pH values with a concentration of ammonium polyacrylate (NH₄PA) was investigated as a function of time. The rheological behaviour of concentrated aqueous FA slips stabilized with NH₄PA was studied; besides, the effect of poly(vinyl)alcohol (PVA) addition on the relative viscosity of the suspensions was investigated. The influence of the slip rheology on the microstructure of the resultant green slip cast compacts and their sintering behaviour were determined. Upon the FA introduction in the aqueous solutions, an initial release of F anions located at the surface was found, which was not dependent on the pH and the presence of dispersant. The increase in the initial pH of the solution above 7 and/or the addition of NH₄PA markedly reduced the Ca⁺⁺/H⁺ exchange reaction rate. As a result, well-stabilized concentrated aqueous suspensions could be obtained at pH close to 9. The minimum viscosity of 40 vol.% slips at pH 8.9 occurred at 0.6 wt% of NH₄PA added. The addition of 0.5 wt% PVA to a well-stabilized FA slip caused aggregation of particles by a depletion flocculation mechanism, thereby increasing the slip viscosity. The greater permeability of cakes produced from slips with high viscosity values (0.5 wt% PVA) increased the casting rate. The highest sintered densities were obtained for the compacts prepared from the slips without PVA, due to the denser particle packing achieved in the green bodies.     

pH controlled dispersion and slip casting of Si3N4 in aqueous media

The dispersion characteristics of commercial Si₃N₄ powder in aqueous media (deionized water) was studied as a function of pH in the range 2–11. The slip was characterized for its dispersion quality by various experimental techniques like particle size analysis, sedimentation phenomena, viscosity and flow behaviour and zeta potential analysis. The optimum dispersion was found to be in the pH region 9–11 wherein the slurry displayed minimum sedimentation height, minimum viscosity, near Newtonian flow behaviour and maximum zeta potential. The slip is highly agglomerated in the pH range 2–8 as manifested by higher sedimentation height, higher viscosity, lower zeta potential and thixotropic non-Newtonian flow behaviour. The 72 wt% (44 vol.%) Si₃N₄ slips made at pH = 10 resulted in green bodies having 53–59% of theoretical density after casting into plaster molds.     

Slip casting of silicon nitride for pressureless sintering

Properties of aqueous slips of silicon nitride/spinel mixtures were studied. Vanisperse CB was used as deflocculant. It was shown that mixtures previously milled in water form stable slips, while those milled in isopropanol form jelly-like aqueous suspensions unsuitable for casting. A single-stage slip may be prepared by milling and mixing Si₃ N₄ powder with spinel and water without a deflocculant. Viscosity, pH and density of the slips were studied and a set of crucibles was cast and fired in nitrogen at 1650° C for 1 h to bulk density 3.1 g cm^–3.     

Fabrication of a polymer composite with high thermal conductivity based on sintered silicon nitride foam

A novel route was developed to fabricate Si₃N₄/epoxy composite. In this route, the Si₃N₄ particles were constructed into the foamed shape by using protein foaming method, firstly. Then the Si₃N₄ foams were sintered to bond these Si₃N₄ particles together. Finally, the Si₃N₄/epoxy composite was fabricated by infiltrating the epoxy resin solution into the sintered Si₃N₄ foams. This route was proved to be an efficient way in enhancing the thermal conductivity of epoxy matrix at a low loading fraction. For example, the thermal conductivity of the as-prepared Si₃N₄/epoxy composite with a loading fraction of 22.2 vol% was up to 3.89 W m⁻¹ K⁻¹, which was about 17 times higher than that of neat epoxy.     

Parametric study of the plasma synthesis of ultrafine silicon nitride powders

The high-temperature plasma synthesis of ultrafine silicon nitride (Si₃N₄) powders through the vapour-phase reaction between SiCl₄ and NH₃ in an Ar/H₂ radio frequency (r.f.) inductively coupled plasma was investigated. The experiments were carried out at a 25–39 kW plate power level and at atmospheric pressure. Special attention was paid to the influence of the reactor wall temperature and plasma operating conditions on the quality of the powder. With a cold-wall reactor, the powders obtained were white to light brown in colour and were composed of crystalline, amorphous and Si₃N₄ whisker phases. Both and -Si₃N₄ were present in these products. The NH₄Cl, formed as a by-product of the reaction, could be eliminated from the Si₃N₄ by thermal treatment. The BET specific surface area of the powder after thermal treatment was about 60 m²g^–1. The use of the hot-wall reactor resulted in a considerable reduction in the amount of NH₄Cl remaining in the powder (less than 1 wt%) and a considerable increase in the fraction of the powder obtained in crystalline form. These powders were composed of a mixture of amorphous phase and 30 wt% or more of the and -Si₃N₄ crystalline phases. The BET specific surface area of the powder after thermal treatment was found to be 40 m²g^–1. The experimental results are discussed in relation to their use for optimizing reactor design for the vapour-phase synthesis of ultrafine ceramic powders.     

Effects of oxidation on intergranular phases in silicon nitride ceramics

The effects of oxidation on changes in the secondary phases of two Si₃N₄ ceramics were investigated by transmission electron microscopy. The Si₃N₄ materials were oxidized at 1400 °C for 168 h in laboratory air. One material, sintered with 5 vol% Yb₂O₃+0.5 vol% Al₂O₃, containing a Yb₂Si₂O₇ crystalline secondary phase, displayed no gross changes following oxidation. However, the thickness of the amorphous intergranular film was observed to have decreased by 20% from its initial thickness of 1.0 nm. The second Si₃N₄ material, sintered with 5 wt% Y₂O₃+1 wt% MgO, had a completely amorphous secondary phase. Devitrification of the secondary phase at multiple-grain junctions to -Y₂Si₂O₇ accompanied the outward diffusion of additive and impurity cations occurring in the residual amorphous intergranulàr films during oxidation. Substantial cavitation and intergranular phase depletion was observed at both multiple-grain junctions and two-grain boundaries. The equilibrium thickness of the amorphous intergranular film consequently decreased from 1.2 to 0.9 nm following oxidation. Purification of the amorphous intergranular films by diffusion of cations to the surface led to a reduction in impurity concentration, resulting in the observed thinning of grain-boundary films.     

Mechanical properties and microstructure of reaction sintered β′-sialon ceramics prepared by a slip casting method

Reaction sintered β′-sialon ceramics Si_6-zAl_zO_zN_8-z, were prepared by slip casting from α-Si₃N₄, Al₂O₃, and AlN starting powders. The mechanical properties and microstructures of sintered bodies were investigated as a function of composition (varying the z value). The maximum value of the flexural strength, ∼ 600 MPa, and fracture toughness, ∼ 4.1 MPa m^1/2 were observed in the z range of 0.5–1. In the z value range of 2–4, the mechanical properties decreased drastically. This phenomenon is attributed to the variation of fracture energy, which is greatly affected by the sintered crystallite size. This revised version was published online in November 2006 with corrections to the Cover Date.     

Liquid-Phase Sintering and Properties of Si3N4–TiN Composites

Densification during liquid-phase sintering of Si₃N₄–TiN was studied in the presence of Y₂O₃. The content of TiN was varied from 0–50 mass%. During the densification Y-silicate was formed. The amount of silicate increased with both decreasing fraction of TiN and increasing isothermal heating time. Density, fracture toughness, and electrical resistivity were measured as a function of TiN content. It was found that the density and fracture toughness increased with increasing TiN content. The electrical resistivity drops drastically, from 10¹⁰ m for sintered Si₃N₄ to 10^–3 m for sintered Si₃N₄–TiN composite containing 30 vol% TiN.     

Slip casting of mechanochemically synthesized hydroxyapatite

The dispersion behaviour of mechanochemically synthesized hydroxyapatite was investigated. Electrophoretic mobility measurements evidence that pH_i.e.p. = 10.7 and that pH modification has a poor effect on the mobility. Surface charge reverses as anionic polyelectrolytes are added in considerably high amounts (3 wt %). Polyacrylates are effective in stabilizing 10 vol % suspensions while pourable casting slips are obtained at the maximum solid concentration of 21.4 vol %. The mechanical properties of the sintered bodies were tested and resulted significantly improved in the slip cast samples if compared with the cold pressed samples, thus proving the importance of the application of the colloidal approach to the optimization of cold consolidation of fine powders.     

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.     

Fabrication and mechanical properties of silicon carbide–silicon nitride nanocomposites

A powder mixture of ultrafine –SiC–35 wt% –Si₃N₄ containing 6 wt% Al₂O₃ and 4 wt% Y₂O₃ as sintering additives were liquid–phase sintered at 1800°C for 30 min by hot–pressing. The hot–pressed composites were subsequently annealed at 1920°C under nitrogen–gas–pressure to enhance grain growth. The average grain–size of the sintered bodies were ranged from 96 to 251 nm for SiC and from 202 to 407 nm for Si₃N₄, which were much finer than those of ordinary sintered SiC–Si₃N₄ composites. Both strength and fracture toughness of fine–grained SiC–Si₃N₄ composites increased with increasing grain size. Such results suggested that a small amount of grain growth in the fine–grained region (250 nm for SiC and 400 nm for Si₃N₄) was beneficial for mechanical properties of the composites. The room–temperature flexural strength and fracture toughness of the 8–h annealed composites were 698 MPa and 4.7 MPa · m^1/2, respectively.     

Densification behaviour of oxide-coated silicon nitride powders

Powder coating has been explored as a method of incorporating sintering additives into a ceramic powder. This procedure has been explored in the case of Si₃N₄ powders coated with thin layers of MgO.The effectiveness of the powder coating technique has been evaluated by comparing the powder properties, densification behaviour, microstructure and mechanical properties of coated Si₃N₄ powders with identical powders in which the additive oxide has been added in particulate form. It is concluded that the powder coating technique is an excellent method of homogeneously incorporating minor amounts of sintering additive into a powder. The coated powder exhibited improved homogeneity, and gave good green compact density, high green strength, and faster densification rate. Moreover, coated powders densified more easily by pressureless sintering and showed a more homogeneous microstructure, higher strength and faster densification rates, compared with materials prepared using mixed oxide powders. Significant improvements in hardness and fracture toughness were observed for the coated powders.     

Fabrication of hydroxyapatite ceramics with controlled pore characteristics by slip casting

Porous hydroxyapatite (HAp) ceramics with controlled pore characteristics were fabricated using slip casting method by mixing PMMA with HAp powder. The optimum conditions of HAp slip for slip casting was achieved by employing various experimental techniques, zeta potential and sedimentation, as a function of pH of the slips in the pH range of 4–12. HAp suspensions displayed an absolute maximum in zeta potential values and a minimum in sedimentation height at pH 11.5. The optimal amount of dispersant for the HAp suspensions was found at 1.0 wt% according to the viscosity of 25 vol% HAp slurry. The rheological behaviour of HAp slurry displays a shear-thinning behavior without thixotropy, which is needed in slip casting processing. The pore characteristics of sintered porous hydroxyapatite bioceramics can be controlled by added PMMA particle size and volume. The obtained ceramics exhibit higher strength than those obtained by dry pressing.     

The wetting,reaction and bonding of silicon nitride by Cu-Ti alloys

The wettability and reactivity of pressureless sintered Si₃N₄ by powdered Cu-Ti alloy were investigated using sessile drop tests conducted in a vacuum. Bonding of Si₃N₄ to itself was also carried out and joint strength was evaluated by compressive shear testing. The correlation of wetting behaviour with reaction and bond strength was interpreted. The wettability of Cu-Ti alloys on Si₃N₄ was improved greatly by addition of titanium up to 50 wt%. However, the reaction-layer thickness was increased up to 10 wt% and thereafter decreased up to 50 wt%. We propose the dovetail model which describes the reaction-layer growth behaviour with titanium. As the titanium content was increased, it tended to form a continuous thin reaction layer which greatly improved the wettability. From metallographic and XRD analyses, TiN and Ti suicide were found in the reaction layer. The thermodynamic reaction for TiN formation was suggested to be Si₃N₄(s) + 4Ti (1 ? sol) = 4TiN(s) + 3Si(s). Ti-silicide might be formed during cooling by the reaction with Ti and Si which had been decomposed from Si₃N₄, diffused to and dissolved in the liquid Cu-rich alloy. The reaction layer growth was controlled by diffusion of nitrogen or titanium in the reaction layer according to the titanium concentration. The shear strength of Si₃N₄ to Si₃N₄ was affected by the morphology and thickness of the reaction layer rather than the wettability. As the titanium content increased, shear strength also increased rapidly up to 5 wt% and then slowly up to 50 wt%. As the reaction temperature and time were increased, shear strength was lowered due to the greater thickness of the reaction layer despite improved wettability.     

Analysis of coating interlayer between silicon nitride cutting tools and titanium carbide and titanium nitride coatings

Silicon nitride (Si₃N₄) cutting tools exhibit excellent thermal stability and wear resistance in the high-speed machining of cast irons, but show poor chemical wear resistance in the machining of steel. Conventional chemical vapour deposition (CVD) coating of Si₃N₄ tools has not been very successful because of thermal expansion mismatch between coatings and the substrate. This problem was overcome by developing a CVD process to tailor the interface for titanium carbide (TiC) and titanium nitride (TiN) coatings. Computer modelling of the CVD process was done to predict which phases would form at the interface, and the results compared with analyses of the interface. Three Si₃N₄ compositions were considered, including pure Si₃N₄, Si₃N₄ with a glass phase binder, and Si₃N₄ + TiC composite with a glass phase binder. Results of machining tests on coated tools show that the formation of an interlayer provides superior wear resistance and tool life in the machining of steel as compared to uncoated and conventionally coated Si₃N₄ tools.     

BN/Si3N4 nanocomposite with high strength and good machinability

BN/Si₃N₄ nanocomposite was prepared using BN/Si₃N₄ powder obtained by nitriding Si₃N₄/NH₄HB₄O₇ mixture in ammonia gas as the starting powder. Microstructural investigations by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed that BN particles were homogeneously distributed within the matrix grains as well as at the matrix grain boundaries, and the growth of Si₃N₄ matrix grain was significantly retarded by BN particles. The BN/Si₃N₄ nanocomposite showed a higher strength than the conventional BN/Si₃N₄ microcomposite due to the formation of fine and homogeneous microstructure in it. BN/Si₃N₄ nanocomposite with a BN content of 20 vol% and above showed excellent machinability, because of the formation of weak BN/Si₃N₄ interfaces and the cleavage behavior of BN particles.     

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.     

Properties of reaction bonded silicon nitride obtained from slip cast preforms

Fabrication of silicon preforms of high green density (>1·2 g/cm³) by slip casting of silicon (in aqueous medium) has been studied. The nitridation product consists of 59–85% α-Si₃N₄, 7–22%β-Si₃N₄ and 7–23% Si₂N₂O phase. The amounts of un-nitrided silicon were negligible. The microstructure is either granular or consists of needle-like grains (α-Si₃N₄) and whiskers deposited in the large pores. MOR values of the specimens are almost constant up to 1000°C or 1400°C or show slight increase up to 1000°C or 1200°C. In some cases a little dip around 1200°C, then a sharp increase in MOR up to 1400°C was observed.K _ic values are almost constant up to 1000°C, and thereafter increase sharply. Pore size distribution, existence of Si₂N₂O phase and oxidation of RBSN at high temperatures have been considered for the explanation of the observed behaviour.     

Preparation of Si3N4-SiC composites by microwave route

Si₃N₄-SiC composites have been microwave sintered using β-Si₃N₄ and β-SiC as starting materials. Si₃N₄ rich compositions (95 and 90 vol.% Si₃N₄) have been sintered above 96% of theoretical density without using any sintering additives in 40 min. A monotonic decrease in relative density is observed with increase in SiC proportion in the composite. Decrease in relative density has manifested in the reduction of fracture toughness and microhardness values of the composite with increase in SiC content although the good sintering of matrix Si₃N₄ limits the decrease of fracture toughness. Highest value of fracture toughness of 6.1 MPa m^1/2 is observed in 10 vol.% SiC composite. Crack propagation appears to be transgranular in the Si₃N₄ matrix and the toughening of the composites is through crack deflection around hard SiC particles in addition to its debonding from the matrix.     

Pretreatment and sintering of Si3N4 powder synthesized by the high-temperature self-propagation method

Self-propagation high-temperature synthesis (SHS) was applied for the synthesis of low-cost Si₃N₄ powder. The powder was purified and ground until its particle size reached submicron levels and its purity reached 98%. Using this pretreated powder, with α/β = 60/40 content, fully dense Si₃N₄ ceramics, having improved mechanical properties, were obtained by liquid-phase sintering in the presence of (Y, La)₂O₃-AlN. The mechanical properties achieved finally were as follows: strength, 784 MPa; hardness, 15.1 GPa; and fracture toughness, 5.2 MPa m^0.5. The behaviors of the SHS-Si₃N₄ powders before and after the pretreatment were compared. The relation between microstructure and mechanical properties of the sintered specimens and the effect of different β content in the powder on the sintering process of Si₃N₄ were also studied.