Evaluation of Si3N4 joints: bond strength and microstructure

Joining of pressurelessly sintered silicon nitride ceramics was carried out using adhesive slurries in the system Y-Si-Al-O-N in a nitriding atmosphere. The effects of bonding parameters, such as joining temperature (1450–1650°C), applied pressure (0– MPa) and holding time (10–60 min), on the bond strength of joint were evaluated. A typical microstructure of the joint bonded with the optimum adhesive was investigated. The three point bend testing of joined samples with 3 × 4 × 36 mm³ in dimension was employed to study the bond strength of joints. The results show that an optimum joining process was achieved by holding at 1600°C for 30 min under an external pressure of 5 MPa and the maximum bond strength was 550 MPa, compared to 700 MPa of unbonded Si₃N₄ ceramic, using the adhesive having the Si₃N₄/(Y₂O₃ + SiO₂ + Al₂O₃) ratio of 0.39. The good bond strength is attributed to the similarity in microstructure and chemical composition between joint zone and ceramic substrate. The fracture modes were classified into two types according to the values of bond strength. This revised version was published online in September 2006 with corrections to the Cover Date.     

Si<Subscript>3</Subscript>N<Subscript>4</Subscript>/TiN composites produced from TiO<Subscript>2</Subscript>-Modified Si<Subscript>3</Subscript>N<Subscript>4</Subscript> powders

Si₃N₄/TiN composites have been produced by hot pressing at temperatures from 1600 to 1800°C in a nitrogen atmosphere, using silicon nitride powders prepared by self-propagating high-temperature synthesis and surface-modified with titanium dioxide nanoparticles. We examined the effect of TiO₂ content on the microstructure, phase composition, and mechanical strength of the ceramics. It is shown that titanium nitride can be formed by the reaction Si₃N₄ + TiO₂ → TiN + NO + N₂O + 3Si. The Si₃N₄/TiN composites containing 5–20% TiN have a low density, high porosity, and a bending strength of 60 MPa or lower. In Si₃N₄/TiN ceramics produced using calcium aluminates as sintering aids, the silicon nitride grains are densely packed, which ensures an increase in strength to 650 MPa.     

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.     

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.     

Sintering and characterization of fully dense aluminium nitride ceramics

Aluminium nitride ceramics with no sintering additives could be densified to close to theoretical density (99.6% theoretical) by pressureless sintering of tape-cast green sheets at 1900 °C for 8 h. The thermal conductivity and bending strength of the specimens were 114 Wm^–1 K^–1 and 240 MPa, respectively. The effect of Y₂O₃ additive on sinterability, thermal conductivity and microstructure of aluminium nitride ceramics was investigated. Thermal conductivity increased with increasing amount of Y₂O₃ additive, sintering temperature and holding time at the sintering temperature. Samples with a thermal conductivity up to 258 Wm^–1 K^–1 were fabricated by elimination of the grain-boundary phase.     

Joining of silicon nitride to silicon nitride and to Invar alloy using an aluminium interlayer

Si₃N₄ has been bonded to Si₃N₄ and to the Invar alloy using an aluminium interlayer at temperatures above the melting point of aluminium. Reaction was hardly observed at the interface between Si₃N₄ and aluminium up to 1223 K. The highest strength of the Si₃N₄-Al-Si₃N₄ joints was beyond 500 M Pa. In the Si₃N₄-Al-Invar joint, two main intermetallic compound layers were formed at the AI-Invar interface. The strength of the joints was between 150 and 200 MPa. It is expected that the aluminium layer and the reaction layer with the fine cracks growing perpendicular to the interface play an important role to compensate for the thermal expansion mismatch.     

Friction and wear behaviour of β-silicon nitride–steel couples under unlubricated conditions

Abstract

The friction and wear behaviour of colloidally processed and pressureless sintered β-silicon nitride (Si₃N₄) ceramics against steel DIN Ck45K under unlubricated condition were investigated using a pin on disk tribometer. β-silicon nitride (Si₃N₄) ceramics consolidated by slip casting from suspensions with different solid loading have been studied, aiming at increasing the use of β-Si₃N₄ as cutting tools in industrial applications. The morphology of the worn surfaces of β-silicon nitride ceramics was analysed by scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS). Under the conditions used, it was found that β-Si₃N₄ ceramics exhibited a low wear rate (10^?6 mm³ N^?1 m^?1) and the frictional behaviour of β-Si₃N₄ ceramics–steel couples depended on a metallic layer transferred from the steel disk to the β-Si₃N₄ ceramic pin.     

Effects of organic additives on microstructure and mechanical properties of porous Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics

Green bodies of porous Si₃N₄ ceramics were shaped by extrusion technique using different organic additives as binder during extrusion molding. Different porosity, microstructures and mechanical properties after the extrusion, drying, debinding and sintering stages were investigated. The solid slurry content of 70–75% and extrusion pressure of 0.5–1.0 MPa had played a decisive role in the smooth realization of extrusion molding. The porous Si₃N₄ ceramics were obtained with excellent properties using 4% hydroxypropyl methyl cellulose (HPMC) as binder and polyethylene glycol (PEG) of molecular weight, 1000, as plasticizer with a density of 1.91 g cm⁻³, porosity of 41.70%, three-point bending strength of 166.53 ± 20 MPa, fracture toughness of 2.45 ± 0.2 MPa m^1/2 and Weibull modulus (m) of 20.75.     

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.     

The nature of sialon joints between silicon nitride based bodies

Quite strong joints between silicon nitride based bodies have been made by incorporating a layer of aluminium and oxides between the bodies and heating in a nitriding atmosphere. The joints are resistant to thermal shock and maintain their strength at 1200° C. Microscopic, DTA and X-ray diffraction studies indicated that sialon phases are present in the joints, and that the bonding reaction involves the reduction of Si₃N₄ by aluminium and the subsequent renitriding of the resultant silicon, as well as the simultaneous nitriding of a portion of the aluminium. Transmission electron microscopy of a joint between hot pressed and reaction bonded silicon nitrides showed that 15R aluminium nitride polytype sialon was present on the reaction bonded side of the joint and ß-sialon on the hot pressed side.     

Preparation and interface modification of Si_3N_(4f)/SiO_2 composites

In order to modify the interface, SiON coating was introduced on the surface of silicon nitride fiber by perhydropolysilazane conversion method. Si₃N_4f/SiO₂ and Si₃N_4f/SiON_c/SiO₂ composites were prepared by sol-gel method to explore the influence of SiON coating on the mechanical properties of composites. The results show that with the protection of SiON coating, Si₃N₄ fiber enjoys a strength increase of up to 24.1% and Si₃N_4f/SiON_c/SiO₂ composites have a tensile strength of 170.5 MPa and a modulus of 26.9 GPa, respectively. After 1000 °C annealing in air for 1 h, Si₃N_4f/SiON_c/SiO₂ composites retain 65.0% of their original strength and show a better toughness than Si₃N_4f/SiO₂ composites. The improvement of mechanical properties is attributing to the healing effect of SiON coating as well as its intermediate coefficient of thermal expansion between Si₃N₄ fiber and SiO₂ matrix.     

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.     

Corrosion of silicon nitride ceramics in aqueous HF solutions

The leaching behaviours of hot-pressed Si₃N₄ ceramics containing Y₂O₃, Al₂O₃ and AIN as additives and hot isostatically pressed Si₃N₄ without additives were studied in 0.1 to 10 M HF aqueous solutions at 50 to 80° C. Silicon and aluminium ions were dissolved into the HF solutions, but yttrium ion did not dissolve at all and formed insoluble YF₃. The dissolution of silicon and aluminium ions was controlled by the surface chemical reaction and the apparent activation energies were 70.5 to 87.6 kJ mol^–1, respectively. The corrosion rate increased with increasing degree of crystallization of the grain boundary phases. The corrosion resulted in roughness of the surface and degradation of the fracture strength. Si₃N₄ ceramics containing an amorphous phase at the grain boundaries showed the most excellent resistance to corrosion with HF solution, and kept a fracture strength of above 400 M Pa even after leaching 40% of the silicon ions.     

Development and cytotoxicity evaluation of SiAlONs ceramics

SiAlONs are ceramics with high potential as biomaterials due to their chemical stability, associated with suitable mechanical properties, such as high fracture toughness and fracture resistance. The objective of this work was to investigate the mechanical properties and the cytotoxicity of these ceramic materials. Three different compositions were prepared, using silicon nitride, aluminum nitride and a rare earth oxide mixture as starting powders, yielding Si₃N₄–SiAlON composites or pure SiAlON ceramics, after hot-pressing at 1750 °C, for 30 min. The sintered samples were characterized by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). Furthermore, hardness and fracture toughness were determined using the Vicker's indentation method. The biological compatibility was evaluated by in vitro cytotoxicity tests. Ceramic with elevated hardness, ranging between 17 and 21 GPa, and high fracture toughness of 5 to 6 MPa m^1/2 were obtained. Since a nontoxic behavior was observed in the cytotoxicity tests, it may be assumed that SiAlON-based ceramics are viable materials for clinical applications.     

Microstructural control of green bodies prepared from Si-based multi-component non-aqueous slurries and their effects on fabrication of Si3N4 ceramics through post-reaction sintering

A Si-based slurry containing Si particles covered with Y₂O₃ and MgO nanoparticles (NPs) has been successfully prepared and then applied to shape Si-based green compacts for the fabrication of silicon nitride (Si₃N₄) ceramics via post-reaction sintering. It was found that Y₂O₃ and MgO NPs modified with polyethyleneimine-oleic acid complex (PEI-OA) could be effectively attached to Si particles by simple mixing in dense toluene slurry. Field emission scanning electron microscopy observations confirmed the attachment of PEI-OA-modified sintering aids to Si particles without forming large NP agglomerates. The adsorption of the PEI-OA-modified sintering aids and PEI-OA on the surface of Si particles drastically improved the stability of the Si-based toluene slurry, which was subsequently molded through wet vacuum casting and dewaxed to fabricate a Si-based green body. The obtained green body was nitrided at 1375?°C for 4?h at a N₂ pressure of 0.15?MPa and further sintered at 1850?°C for 2?h at a N₂ pressure of 0.9?MPa. The adsorption of sintering aid particles on the Si surface reduced the number of contact points between Si particles in the green body, which effectively suppressed the Si melting process during nitriding and improved the characteristics of the produced nitride body such as the degree of nitriding and α/(α+β) ratio of Si₃N₄, leading to the successful fabrication of high-density Si₃N₄ ceramics during the subsequent densification step.     

Preparation of zirconium pyrophosphate bonded silicon nitride porous ceramics

Abstract

A new method for preparing high bending strength porous silicon nitride ceramics with controlled porosity was developed using a pressureless sintering technique, using zirconium pyrophosphate as a binder. The fabrication process was described in detail and the sintering mechanism of porous ceramics was analysed by an X-ray diffraction method. The microstructure and mechanical properties of the porous Si₃N₄ ceramics were investigated, as a function of the content of ZrP₂O₇. The resultant porous silicon nitride ceramics sintered at low temperature (1000 and 1100°C) showed fine micropore structure and a high bending strength. Porous silicon nitride ceramics with porosity of 34–47%, a bending strength of 40–114 MPa and a Young's modulus of 20–50 GPa were obtained.     

Preparation and characterization of porous Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics prepared by compression molding and slip casting methods

Porous silicon nitride (Si₃N₄) ceramics were fabricated by compression molding and slip casting methods using petroleum coke as pore forming agent, and Y₂O₃-Al₂O₃ as sintering additives. Microstructure, mechanical properties and gas permeability of porous Si₃N₄ ceramics were investigated. The mechanical properties and microstructure of porous Si₃N₄ ceramics prepared by compression molding were better than those which were prepared by slip casting method, whereas slip casting method is suitable for the preparation of porous Si₃N₄ ceramics with higher porosity and excellent gas permeability.     

Effect of starting PMMA content on microstructure and properties of gel casting BN/Si3N4 ceramics with spherical-shaped pore structures

By gel casting with polymethylmethacrylate microbeads (PMMA) as pore-forming agent, porous boron nitride/silicon nitride (BN/Si₃N₄) composite ceramics were successfully prepared. The obtained ceramic shows bimodal hierarchical structures that composed of spherical-shaped micro pores depending on PMMA content and irregular sub-micro pores formed by the stacking of ceramic particles. Porosity of the porous BN/Si₃N₄ ceramics can be well controlled from 53.0 to 60.6 % by the PMMA content from 10 to 40 wt%, as well as the mechanical and dielectric properties. Effect of PMMA content on phase composition and the relationship between microstructure and the basic properties of the porous BN/Si₃N₄ ceramics was discussed in detail. Microstructure analysis reveals that the sub-micro pores acted as channels between micro pores. BN particles have a relatively denser distribution on the wall of spherical-shaped micro pores with a window between micro and sub-micro pores, and resulting in a half-closed micro pore structure, which is meaningful for material design with concentration of BN particles on the wall of pore structure.     

Microstructure characterization of hot-pressed β-silicon nitride containing β-Si3N4 seeds

Self-reinforced silicon nitride ceramics were synthesized by introducing 5 wt% previously prepared β-Si₃N₄ seeds into a mixture of α-Si₃N₄ and 8 wt.% of sintering additive. The microscopic evidence indicates that β-Si₃N₄ seeds play an important role in microstructural development and mechanical properties for silicon nitride ceramics. The growth of β-Si₃N₄ grains is initiated from the seeds, resulting in a core/shell microstructure observed in the abnormal grain growth. The internal dislocation, π-boundary, and Moire fringes are present in Si₃N₄ grains, which were formed to compensate the mismatch and compositional differences between the two grains. Coalescence can occur at the final stage of sintering.     

High-strength silicon nitride ceramics obtained by grain-boundary crystallization

A monolithic type of toughened silicon nitride ceramics has been developed from Si₃N₄-Y₂O₃ systems. However, because of the existence of a second phase, the fracture strength decreases at elevated temperatures. To improve the high-temperature strength of silicon nitride, some additional components were investigated. It was found that the addition of hafnia to the Si₃N₄-Y₂O₃-AIN system gave a greater high-temperature strength based on the promotion of grain-boundary phase crystallization: namely, 126 kg mm⁻² in 3-point bend strength at 1300 °C for the hot-pressed specimen, and 90 kg mm⁻² at 1300 °C for the pressureless sintered specimen. The role of the hafnia in crystallization is not yet clear, and is being characterized by electron microscopy and microanalysis.