Grain boundary phase in AlN ceramics fired under reducing N2 atmosphere with carbon

An AlN ceramic was prepared with a dopant Y₂O₃ under a reducing nitrogen atmosphere with carbon at 1900 °C for 20 h. The AlN ceramic had thermal conductivity, 220 W/m°C, which contained crystalline Y₂O₃ and an amorphous intergranular film. The intergranular phase decreased during the isothermal hold period by the migration of a liquid phase that consisted of Y₂O₃, Al₂O₃, and AlN. The liquid phase composition was maintained during the firing process. Comparison of the microstructures of the ceramics prepared with different isothermal hold times revealed that the lower the quantity of intergranular phase, the higher the thermal conductivity attained.     

Phase formation in porous liquid phase sintered silicon carbide: Part I:: Interaction between Al2O3 and SiC

The structure and phase formation of porous liquid phase sintered silicon carbide (porous LPS-SiC), containing yttria and alumina additives have been studied. The present paper is focused on the system Al–Si–C–O, which is part of the system describing the interactions with sintering additives.The influence of different sintering atmospheres, namely argon and Ar/CO, and different temperatures on structure and composition was investigated by XRD and SEM. Additionally, reaction products were calculated from thermodynamic data and correlated with experimentally determined reaction products. Alumina and SiC reacted at 1950 °C in an argon atmosphere, forming a metal melt of aluminium and silicon. No reduction of Al₂O₃ was observed in a CO-containing argon sintering atmosphere.In the second and third parts of this paper the interactions between Y₂O₃–SiC and Y₂O₃–Al₂O₃–SiC are analysed [J. Eur. Ceram. Soc. (in press), parts II and III].     

Phase formation in porous liquid phase sintered silicon carbide:: Part II Interaction between Y2O3 and SiC

During the sintering of porous liquid phase sintered silicon carbide (porous LPS-SiC) a strong interaction with the atmosphere takes place, influencing the composition and stability of porous LPS-SiC components. The present paper is focused on the interaction of Y₂O₃ with SiC, which is part of the common used sintering additives for LPS-SiC (Y₂O₃–Al₂O₃–SiC). The interaction of Al₂O₃ and SiC has been studied in a previous paper [J. Eur. Ceram. Soc. (in press)].The reaction products of the interaction of Y₂O₃ with SiC and the resulting microstructures were analysed using model experiments. The effects of the influence of different sintering atmospheres, namely Argon and Ar/CO, as well as vacuum and different temperatures have been investigated. The phase formation was determined by X-ray diffraction (XRD) and can be explained on the basis of thermodynamic calculations. Depending on the sintering conditions, silicides or yttrium carbides can be formed in addition to stable oxides, which can result in the decomposition of the samples after sintering. Reactions between SiC and Y₂O₃ during sintering can be suppressed successfully if an Ar/CO atmosphere is used.     

Inhibition of grain growth in liquid-phase sintered SiC ceramics by AlN additive and spark plasma sintering

SiC ceramics were prepared from nanosized β-SiC powder with different compositions of AlN and Y₂O₃ sintering additives by spark plasma sintering (SPS) at 1900 °C for 600 s in N₂. The relative density of the sintered SiC specimens increased with increasing amount of AlN, reaching a relative density higher than 99%, while at the same time grain size decreased significantly. The smallest average grain size of 150 nm was observed for SiC sample sintered with 10 vol% of additives consisting of 90 mol% AlN and 10 mol% Y₂O₃. Fully dense nanostructured SiC ceramics with inhibited grain growth were obtained by the AlN additive and SPS technique. The flexural strength of the SiC body containing 70 mol% AlN and 30 mol% Y₂O₃ additives reached the maximum value of 1000 MPa. The SiC bodies prepared with AlN and Y₂O₃ additives had the fracture toughness of around 2.5 MPam^1/2.     

Hydrolysis protection and sintering of aluminum nitride powders with yttria nanofilms

Aluminum nitride (AlN) is a promising material for electronic substrates and heat sinks. However, AlN powders react with water that adversely affects final part properties and necessitates processing in organic solvents, increasing the cost of AlN parts. Small quantities of yttrium oxide (Y₂O₃) are commonly added to AlN particles to enable liquid phase sintering. To mitigate the reaction of AlN particles with water, particle atomic layer deposition (ALD) was used to coat AlN powders with conformal films of Y₂O₃ prior to densification and powder processing. When AlN particles were coated with 6 nm thick films of amorphous Y₂O₃, the hydrolysis reaction was significantly suppressed over 48 h, demonstrating that Y₂O₃ nanofilms on AlN powders act as a barrier coating in an aqueous solution. AlN powders with Y₂O₃ addition by particle ALD sintered to high relative densities (≥90% theoretical) after sintering at 1800°C for 50 min.     

Influence of intergranular phases on edge integrity of Si3N4/SiC wood cutting tools

Si₃N₄/SiC composites used for industrial wood cutting were processed by a near net shape route involving gas pressure sintering with sintering additives such as Al₂O₃, La₂O₃, Y₂O₃ and MgO. The cutting edge integrity of these knives was tested in a cutting trial and compared to knives made by a hot pressing route. It was found that the intergranular phase has a crucial influence on the cutting edge integrity. The boundary phase was analysed by EFTEM and EDX mapping on TEM samples: in gas pressure sintered composites the crystallisation of the apatite Y₅Si₃O₁₂N phase was identified. In the hot pressed composite the boundary phase consisted only of silicates. These composites showed better edge stability than cutting tools with a Y-N-apatite phase. The formation of the type of intergranular phase was found to be determined by the amount of MgO sintering aid and the temperature of the post sintering heat treatment.     

Control of Electrical Resistivity in Silicon Carbide Ceramics Sintered with Aluminum Nitride and Yttria

The electrical properties of β‐SiC ceramics were found to be adjustable through appropriate AlN–Y₂O₃ codoping. Polycrystalline β‐SiC specimens were obtained by hot pressing silicon carbide (SiC) powder mixtures containing AlN and Y₂O₃ as sintering additives in a nitrogen atmosphere. The electrical resistivity of the SiC specimens, which exhibited n‐type character, increased with AlN doping and decreased with Y₂O₃ doping. The increase in resistivity is attributed to Al‐derived acceptors trapping carriers excited from the N‐derived donors. The results suggest that the electrical resistivity of the β‐SiC ceramics may be varied in the 10⁴–10^?3 Ω·cm range by manipulating the compensation of the two impurity states. The photoluminescence (PL) spectrum of the specimens was found to evolve with the addition of dopants. The presence of N‐donor and Al‐acceptor states within the band gap of 3C–SiC could be identified by analyzing the PL data.     

Effect of sintering atmosphere on the grain growth and hardness of SiC/polysilazane ceramic composites

Silicon carbide (SiC) monoliths were synthesised using nano-size SiC powder mixed with/without polysilazane by hot pressing at 1750°C for 1?h under an applied pressure of 20?MPa in N₂ or Ar atmosphere. The effects of polysilazane and sintering atmosphere on the microstructure and hardness of SiC were examined. The grain sizes of the SiC ceramics sintered in N₂ atmosphere with and without the polysilazane were 161 and 605?nm, while the density for those samples were 96.5 and 98.1%, respectively. It was shown that Si₂N₂O was formed for the SiC/polysilazane composite and sintered in N₂. In addition, the sample mixed with polysilazane followed by sintering in N₂ atmosphere revealed a quite high hardness in spite of its relatively low density. It was suggested that Si₂N₂O phase played an important role for the inhibition of grain and subsequent high hardness.     

Development of spark plasma sintered conductive SiC–TiB2 composites for electrical discharge machining applications

Highly dense electrically conductive silicon carbide (SiC)–(0, 10, 20, and 30 vol%) titanium boride (TiB₂) composites with 10 vol% of Y₂O₃–AlN additives were fabricated at a relatively low temperature of 1800°C by spark plasma sintering in nitrogen atmosphere. Phase analysis of sintered composites reveals suppressed β→α phase transformation due to low sintering temperature, nitride additives, and nitrogen sintering atmosphere. With increase in TiB₂ content, hardness increased from 20.6 to 23.7 GPa and fracture toughness increased from 3.6 to 5.5 MPa m^1/2. The electrical conductivity increased to a remarkable 2.72 × 10³ (Ω cm)^–1 for SiC–30 vol% TiB₂ composites due to large amount of conductive reinforcement, additive composition, and sintering in nitrogen atmosphere. The successful electrical discharge machining illustrates potential of the sintered SiC–TiB₂ composites toward extending the application regime of conventional SiC-based ceramics.     

Fabrication and thermal conductivity of AlN/BN ceramics by spark plasma sintering

Aluminum nitride/boron nitride (AlN/BN) ceramics with 15–30 vol.% BN as secondary phase were fabricated by spark plasma sintering (SPS), using Yttrium oxide (Y₂O₃) as sintering aid. Effects of Y₂O₃ content and the SPS temperature on the density, phase composition, microstructure and thermal conductivity of the ceramics were investigated. The results revealed that with increasing the amount of starting Y₂O₃ in AlN/BN, Yttrium-contained compounds were significantly removed after SPS process, which caused decreasing of the residual grain boundary phase in the sintered samples. As a result, thermal conductivity of AlN/BN ceramics was remarkably improved. By addition of Y₂O₃ content from 3 wt.% to 8 wt.% into AlN/15 vol.% BN ceramics, the thermal conductivity increased from 110 W/m K to 141 W/m K.     

Enhanced mechanical and thermal properties of AlN ceramics via a chemical precipitation process

A uniform dispersion of sintering additives is crucial to improve the thermal and mechanical properties of AlN ceramics. In this study, the Y₂O₃-coated AlN composite powder was successfully prepared by the chemical precipitation (CP) process, thereby improving the homogenization of Y₂O₃ in AlN green compacts. The precipitation coating behavior of Y₂O₃ precursor was investigated by FTIR and TG-DSC, and the corresponding reaction equation was proposed. The results of TEM, XRD, and XPS for the CP processed AlN powder indicated that a uniform amorphous Y₂O₃ layer was fully wrapped on the surface of AlN powder. The microstructures and phases of the sintered AlN samples prepared via the CP and conventional ball-milling (BM) processes, respectively, were compared. The CP process can result in decreasing oxygen content in AlN grains, facilitating the formation of the desirable isolated second phases, and strengthening the grain and grain boundary of AlN ceramic. As a result, the thermal conductivity, bending strength and fracture toughness of the CP processed AlN ceramic are 9.43%, 10.56%, and 18.50% higher than those of the BM processed sample, respectively, illustrating the CP process is a pretty effective way to simultaneously improve the thermal and mechanical properties of AlN ceramics.     

Microstructural,electrical, and mechanical properties of conductive SiC ceramics fabricated by spark plasma sintering

Nitrogen (N)-doped conductive silicon carbide (SiC) of various electrical resistivity grades can satisfy diverse requirements in engineering applications. To understand the mechanisms that determine the electrical resistivity of N-doped conductive SiC ceramics during the fast spark plasma sintering (SPS) process, SiC ceramics were synthesized using SPS in an N₂ atmosphere with SiC powder and traditional Al₂O₃–Y₂O₃ additive as raw materials at a sintering temperature of 1850–2000°C for 1–10 min. The electrical resistivity was successfully varied over a wide range of 10⁻³–10¹ Ω cm by modifying the sintering conditions. The SPS-SiC ceramics consisted of mainly Y–Al–Si–O–C–N glass phase and N-doped SiC. The Y–Al–Si–O–C–N glass phase decomposed to an Si-rich phase and N-doped Y_xSi_yC_z at 2000°C. The Vickers hardness, elastic modulus, and fracture toughness of the SPS-SiC ceramics varied within the ranges of 14.35–25.12 GPa, 310.97–400.12 GPa, and 2.46–5.39 MPa m^1/2, respectively. The electrical resistivity of the obtained SPS-SiC ceramics was primarily determined by their carrier mobility.     

Effects of mechanically alloying Al2O3 and Y2O3 additives on the liquid phase sintering behavior and properties of SiC

In order to improve the sintering of SiC, mixtures of Al₂O₃ and Y₂O₃ powders are commonly included as sintering additives. The aim of this work was to use mechanically alloyed Al₂O₃–Y₂O₃ mixtures as sintering additives to promote liquid phase sintering of SiC using spark plasma sintering. The results showed that milling reduced the particle size of the powders and led to the formation of complex oxide phases (YAP, YAM, and YAG) at low temperatures. As the ball milling time increased, the mass loss of specimens sintered with mechanically alloyed Al₂O₃–Y₂O₃ mixtures decreased, and accordingly the relative density increased. However, the hardness and flexural strength of sintered SiC specimens first increased and then decreased. Because the specimens prepared with oxides milled for a long time contained too much YAG/YAP and accordingly too much liquid at sintering temperature. This negatively affected the mechanical properties of the SiC specimens because of the increased volume of the complex oxide phases, which have inferior mechanical properties to SiC, in the sintered specimens. When the ball milling time was 6 h, the hardness (24.02 GPa) and flexural strength (655.61 MPa) of the SiC specimens reached maximum values.     

Influence of Y2O3 addition on electrical properties of β-SiC ceramics sintered in nitrogen atmosphere

The influence of Y₂O₃ addition on electrical properties of β-SiC ceramics has been investigated. Polycrystalline SiC samples obtained by hot-pressing SiC–Y₂O₃ powder mixtures in nitrogen (N) atmosphere contain Y₂O₃ clusters segregated between SiC grains. Y₂O₃ forms a Y–Si-oxycarbonitride phase during sintering by reacting with SiO₂ and SiC and by dissolution of N from the atmosphere; this induces N doping into the SiC grains during the process of grain growth. The SiC samples exhibit an electrical resistivity of ～10^?3 Ω cm and a carrier density of ～10²⁰ cm^?3, which are ascribed to donor states derived from N impurities. The increase in defect density with increasing Y₂O₃ content is likely to be a main limiting factor of the electrical conductivity of SiC ceramics.     

Low temperature pressureless sintering of α-SiC with Al2O3 and CeO2 as additives

The combination of Al₂O₃ and CeO₂ was testified as suitable sintering additive for liquid phase sintering of SiC ceramics, which has lower sintering temperature than that sintered with Al₂O₃ and Y₂O₃ as sintering aids. However, the mechanical properties including flexural strength, Vickers’ hardness and fracture toughness of this system were similar to those of the samples sintered with Al₂O₃ and Y₂O₃ as sintering aids. The good wettability of the eutectic liquid phase on SiC plate, the high solubility of SiC particles into the liquid phase and the penetration of the liquid phase along the SiC–SiC grain boundaries all confirmed the suitability of the combination of Al₂O₃ and CeO₂ as liquid phase sintering additive for SiC.     

Production of porous SiC by liquid phase sintering using graphite as sacrificial phase: Influence of SiO2 and graphite on the sintering mechanisms

Porous silicon carbide (SiC) is a promising ceramic for high-temperature applications due to its unique combination of properties. In the present work, a fabrication route for porous SiC is presented using graphite spherical powder as sacrificial phase to introduce porosity. By varying the initial amount of sacrificial phase, high-performance SiC materials with porosities in the range 30–50% were manufactured and characterized in terms of microstructure, density, thermal conductivity and flexural strength. The materials were fabricated by liquid phase sintering in presence of 2.5 wt.% Al₂O₃ and Y₂O₃ as sintering additives. The results indicate that the SiO₂ present in the starting SiC powders interacts with the sintering additives forming liquid phases that promote densification and weight loss. Besides, an Al-Si liquid phase is formed at higher sintering temperatures, whose contribution to densification is inhibited in presence of graphite due to the formation of Al-rich carbides.     

Role of WC additive on reaction,solid-solution and densification in HfB2–SiC ceramics

A comparative study of phase components and compositions was performed for the pressureless sintered HfB₂–SiC–WC composites by various analytical methods. The relative decrease of HfB₂ phase leads to a new reaction of HfO₂ removal by WC to create B₂O₃. By using SiC instead of Si₃N₄ as milling medium, the WB phase was suppressed to the trace level while the W solid-solution in HfB₂ phase was favored. The W solution in both the primary HfB₂ and resultant HfC phases indicates that the WC additive was involved throughout the sintering process by dissolving into sintering liquid, which remains at the intergranular regions to form amorphous oxides as well as trace W-rich phases. This is effectively a reactive liquid-phase sintering to realize the reaction, solid-solution and densification collectively to achieve a designable HfB₂–SiC–HfC composite by pressureless sintering, which may also be extended to other sintering methods.     

Nano- versus macro-hardness of liquid phase sintered SiC

Silicon carbide polycrystalline materials were prepared by liquid phase sintering. Different rare-earth oxides (Y₂O₃, Yb₂O₃, Sm₂O₃) and AlN were used as sintering additives. The final microstructure consists of core–rim structure owing to the incorporation of AlN into the rim of SiC grains by solid solution. Nano- versus macro-hardness of polycrystalline SiC materials were investigated in more details. The nano-hardness of SiC grains was in the range of 32–34 GPa and it depends on the chemical compositions of grains. The harness followed the core–rim chemistry of grains, showing lower values for the rim consisting of SiC–AlN solid solution. The comparison of nano- and macro-hardness showed that nano-hardness is significantly higher, generally by 5–7 GPa. The macro-hardness of tested samples had a larger scatter due to the influence of several factors: hardness of grains (nano-hardness), indentation size effect (ISE), microstructure, porosity, and grain boundary phase. The influence of grain boundary phase on macro-hardness is also discussed.     

Preparation of O′-Sialon-based ceramics by two-stage liquid phase sintering and study on the toughening mechanism of ultrafine-grained sintered clusters

Ultrafine-grained O′-Sialon-based ceramics were prepared by two-stage sintering at 1250 °C, with large particle GH4169 superalloy powder and nano Al₂O₃–Y₂O₃ as composite sintering aids. The effects of these aids on the densification, microstructure, and mechanical properties of O′-Sialon-based ceramics during two-stage sintering were also studied. Studies have shown that the densification process of O′-Sialon-based ceramics promoted by composite sintering additives, presents with the characteristics of two-stage liquid-phase sintering. In the first stage, GH4169 formed ultrafine-grained sintered clusters in the sintered material through liquid phase diffusion. In the second stage, the uniformly dispersed nano Al₂O₃–Y₂O₃ realized the uniform sintering of the material. In the fracture process, the ultrafine-grained sintered clusters hindered the crack propagation and promoted multiple deflections of the crack around the edge of the clusters, achieving the effect of crack deflection toughening. This effect, dominated by ultrafine-grained sintered clusters, significantly improved the fracture toughness of O′-Sialon-based ceramics up to 8.52 MPa m^1/2.     

Low‐Temperature Preparation of β‐Si3N4 Porous Ceramics with a Small Amount of Li2O–Y2O3

Porous β‐Si₃N₄ ceramics are sintered at 1600°C in N₂ and postheat treated at 1500°C under vacuum using Li₂O and Y₂O₃ as the sintering additives. The partial sintering and phase transformation are promoted at low temperature by the addition of Li₂O. The addition of Y₂O₃ is advantageous for the formation of high aspect ratio β‐Si₃N₄ grains. After postheat treatment, a large amount of intergranular glassy phase is removed, and the Li content in the samples is decreased. By this method, the β‐Si₃N₄ porous ceramic with a porosity of 54.1% and high flexural strength of 110 ± 8.1 MPa can be prepared with a small amount of sintering additives, 0.66 wt% Li₂O and 0.33 wt% Y₂O₃, and it is suitable for high‐temperature applications.