Sintering Of Silicon Nitride Research Articles

Colour of gas-pressure-sintered silicon nitride ceramicsPart II. Thermodynamic considerations

Abstract Gas-pressure sintering is the most widely used process for manufacturing high-performance silicon nitride ceramics. Generally gas-pressure sintering of silicon nitride is conducted under reducing atmospheric conditions due to the use of graphite heaters. The interaction between the sintering atmosphere and the silicon nitride ceramics is often featured by an inhomogeneous grey coloration of the near surface area and a grey coloration of the bulk material. In the first part of this paper [Herrmann, M. and Goeb, O., Colour of gas-pressure-sintered silicon nitride ceramics Part I. Experimental data J. Eur. Ceram. Soc. , 2001, 21 (3), 303–314] the dependence of the colour of the material on the sintering parameters was investigated experimentally. It was shown that silicon inclusions, having sizes ranging from a few nm up to several μm, cause the coloration of the material. Gas pressure, temperature and sintering time influence the formation of the inhomogeneous grey coloration. In this second part, the dependence of the coloration in silicon nitride ceramics on the sintering parameters will be explained on the bases of thermodynamic considerations. The model developed explains the possibilities of governing the colour formation of silicon nitride materials.

Role of Specimen Insulation on Densification and Transformation during Microwave Sintering of Silicon Nitride.

Silicon nitride ceramics, with Y2O3, Al2O3 and MgO as sintering aids, have been sintered in a 28GHz microwave applicator using a number of sample insulation techniques. The sintering characteristics in terms of densification, α→β transformation, microstructural development and power requirement were studied and compared to identical samples sintered conventionally. All of the microwave sintered materials could be sintered to near theoretical density and a full α→β transformation obtained at temperatures around 200°C lower than the materials sintered conventionally. In addition, the microstructural development showed important differences, with the selective development of elongated β-grains being observed from the very early stage of transformation. Samples sintered using a powder bed insulation technique, achieved full α→β transformation at the lowest temperature, but a tendency to thermal runaway using this method of insulation meant the samples were susceptible to cracking. Samples sintered using silicon carbide plates as a low temperature microwave absorber reached full density and onset of transformation earlier than the other materials. In addition, the maximum power requirement for these samples was around 1/3 of that required for other insulation techniques, and more uniform heating meant that these samples were free from cracking.

Kinetics of Densification and Phase Transformation at Microwave Sintering of Silicon Nitride with Alumina and Yttria or Ytterbia as Additives.

Kinetics of Densification and Phase Transformation at Microwave Sintering of Silicon Nitride with Alumina and Yttria or Ytterbia as Additives.

放電焼結の基礎と応用 ２８ＧＨｚミリ波加熱法による窒化珪素の焼結とＹｂ２Ｏ３‐Ａｌ２Ｏ３系新助剤の開発

New sintering aids based on ytterbia-alumina system were developed for the sintering of silicon nitride with 28GHz millimeter-wave (mm-wave) heating. Sinterability was examined in silicon nitrides with 5wt%Yb2O3-3wt%Al2O3 (5Yb-3Al) and 5wt%Y2O3-3wt%Al2O3 (5Y-3Al). It was found that fully densified silicon nitride with 5Yb-3Al aid was obtained at 1600°C in mm-wave heating, and 1850°C in conventional heating. On the other hand, nearly same densified silicon nitride with 5Y-3Al aid was obtained at 1700°C in mm-wave heating, and 1750°C in conventional heating. For the silicon nitride with ytterbia-alumina system, it was indicated that their densification temperatures strongly depended on the ratio of Yb2O3 to Al2O3. In the mm-wave sintering, the temperature necessary for densifying up to 96% theoretical density showed a quite similar tendency to the liquidus temperature in the ytterbia-alumina system. While, the densification temperatures for the same silicon nitrides in the conventional sintering were rather higher than those in the mm-wave sintering and monotonously decreased with increasing the content of alumina in the aid. Because of lower sintering temperatures in the mm-wave heating, finer microstructures were obtained in the mm-wave sintered silicon nitrides, compared with those sintered by the conventional heating.

Gas pressure sintering of silicon nitride — current status

Gas pressure sintering of silicon nitride is now common for obtaining a dense as well as tough product. A review of earlier works has revealed that there are still controversies in prediction of mechanism of the sintering process. Analysis of the kinetics of the process obtained by integration of a dilatometer inside the furnace has been presented. The mechanism of sintering silicon nitride powder compacts in presence of hyperbaric nitrogen atmosphere has been discussed. It has been observed that intermediate stage densification kinetics is very much susceptible to sintering atmosphere and time of pressurisation. The microstructure of the final product has been found to be dependent primarily on the method of incorporation of additive in the starting silicon nitride powders.

Uso da técnica de dilatometria no estudo do efeito de diferentes aditivos na sinterização de Si3N4

Nos últimos anos vem sendo testados óxidos de terras raras pesadas como aditivos de sinterização, via fase líquida, do nitreto de silício (Si3N4). Este trabalho visou o estudo do comportamento da sinterização do Si3N4 aditivado com um concentrado de óxidos de ítrio e terras raras pesadas e sílica, nas quantidades de 14 e 21%. Os resultados obtidos foram comparados com os apresentados quando do uso de óxido de ítrio e sílica, amplamente conhecidos como aditivos de sinterização de cerâmicas covalentes. Foi comparada também a influência da forma em que os aditivos foram usados, óxidos e dissilicatos. As experiências foram realizadas num dilatômetro e os resultados da sinterização são dados em termos de retração linear e velocidade de retração. Verificou-se que as amostras com maior quantidade de aditivos apresentaram maiores retrações, bem como velocidades de retração superiores. Para as amostras aditivadas com 14 e 21% de Y2O3/SiO2, os resultados foram de 13,1 e 15,5%, respectivamente, e para as aditivadas com concentrado/SiO2, 13,7 e 15,7%, respectivamente. Além disso, maiores retrações e razão de retração foram observadas para amostras com dissilicatos como aditivos, em comparação às amostras com a correspondente mistura de óxidos. Em todos os casos, as amostras aditivadas com o concentrado/SiO2, apresentaram retrações e velocidades de retração superiores às apresentadas pelas aditivadas com Y2O3/SiO2, mostrando ser mais eficiente como aditivo de sinterização do Si3N4.

Crystallization of MgO during sintering of silicon nitride with magnesia and ceria

This paper deals with the crystallization of MgO during sintering of Si3N4-MgO-CeO2 ceramics. At the initial stage of sintering (1450–1550°C), MgO-CeO2 reacts with free silica to form sufficient liquid phase to densify Si3N4 into nearly full density ceramics; at final stage (1600–1800°C), the crystallization of MgO reduces the amount of glassy phase which is harmful to the high-temperature properties. MgO-CeO2 is found to be an excellent sintering aid for Si3N4.

Pressureless sintering of silicon nitride with magnesia and ceria

An investigation of the pressureless sintering of Si 3N 4 with MgO–CeO 2 revealed that MgO–CeO 2 is a much more effective sintering aid for silicon nitride than either MgO or CeO 2 alone. The concentration of MgO–CeO 2 and the MgO/CeO 2 weight ratio have a strong influence on the densification behavior and mechanical properties of the sintered materials. The pressureless sintered silicon nitride with MgO–CeO 2 achieved a relative density of 98.5% and a bending strength of 948 MPa.

Densification and α/β phase transformation of Si 3N 4 containing MgO and CeO 2 during sintering

This paper deals with the densification and phase transformation during sintering of silicon nitride with MgO-CeO 2 as sintering aids. For Si 3N 4-MgO-CeO 2 ceramics, the densification occurs rapidly from 1500°C and nearly ends at 1550°C; the phase transformation from α-Si 3N 4 to β-Si 3N 4 occurs rapidly from 1550 to 1600°C and nearly ends at 1600°C. The phase-transformation process lags behind the densification process, showing that the solution and reprecipitation do not play a major role in the densification of the Si 3N 4-MgO-CeO 2 system.

Transient Liquid Phase Sintering of Silicon Nitride by Hafnia Addition

Transient Liquid Phase Sintering of Silicon Nitride by Hafnia Addition

Effect of iron and silicon addition on the densification, microstructure and mechanical properties of silicon nitride

In this work, attention was concentrated on the microstructure optimization of normal sintering of silicon nitride by interference on the α → β-Si 3N 4 transformation and the morphology and spatial distribution of β-Si 3N 4 grains, aiming for improvement of the mechanical properties in these non-oxide based ceramics. Iron has a significant influence on the α → β-Si 3N 4 transformation, allowing the formation of a great number of β-phase nuclei. Under appropriate conditions of temperature and time, a more homogeneous microstructure results, in which the grains have a high aspect ratio and exhibit a low degree of abnormal grain growth, achieving an in situ reinforcement. The refinement of the microstructure was accompanied by an increase of fracture toughness values ( K IC) for samples sintered under the best densification conditions.

Advanced Ceramics Sintering Using High-Power Millimeter-Wave Radiation

AbstractThe results of ceramics sintering experiments using high-power millimeter-wave radiation are reported. Sintering of silicon nitride with 5%Al2O3 and 5%Y2O3 was performed in a multi-mode applicator using a 10-kW 28-GHz gyrotron in CW operation. It was found that the silicon nitride samples sintered with 28 GHz radiation at 1650'C for 30 min reached to as high as theoretical density (TD), while the conventionally sintered samples at 1700°C for 60 min resulted in the density as low as 90% TD. Focusing experiments of millimeter-wave radiation from the high-power pulsed 60-GHz gyrotron have been performed using a quasi-optical antenna system (two-dimensional ellipso-parabolic focusing antenna system) to demonstrate the feasibility of the power density of as high as 100 kW/cm2. Typical heating characteristics using the focused beam were made clear for this system. It was found that the densification of yttria-stabilized zirconia (ZrO2-8mol%Y2O3) samples to as high as 97% TD was obtained from the sintering with focused 60 GHz beam in pulse operation with a 10-ms pulse duration at a 0.5Hz repetition. The densification temperature for the zirconia could be lowered by 200°C than that expected conventionally.

The effect of process atmosphere on the intergranular phase in silicon nitride ceramics

The influence of the process atmosphere in the gas pressure sintering of silicon nitride has been investigated, particularly with regard to formation of the intergranular phase. The intergranular phases identified after different gas pressure sintering cycles were compared with that present in a material fabricated by the hot isostatic pressing of an identical powder composition, allowing comparison of the two processing methods. The results suggested infiltration of the nitrogen processing gas into the sintering body, which adversely affected the intergranular phase formation in a composition that had been tailored for optimal mechanical behaviour. The duration of exposure to the processing gas and the post-sintering cooling cycle were identified as being critical factors in the intergranular phase crystallisation.

Low-Temperature Oxidation of silicon nitride by water in supercritical condition

Oxidation tests carried out in supercritical water (400–500 °C) revealed a noticeable corrosion attack on the silicon nitride surface in spite of the low temperatures. Increasing water pressure generally caused an increase in the oxidation phenomena. Oxidation scale evolution depended strictly on the silicon nitride sintering process, sintering aids, porosity and impurities, which influenced oxidation kinetics and surface morphology. In such test conditions the solubility of silica in water seemed to have a great influence on the stability of the oxidation scale, mainly in reaction-bonded silicon nitride samples.

Activated sintering of silicon nitride powder made by SHS

A combination of traditional and novel technological methods has been used to provide effective sintering of silicon nitride synthesized by SHS: adding yttrium oxide, adding finely-divided SHS materials as activators, combining the operations of mixing and dry planetary grinding, and the use of activating additives. This can produce a density ≥96% of theoretical and a strength of 690 MPa.

Early Stages of Sintering of Si3N4 Nanoclusters Via Parallel Molecular Dynamics

AbstractWe investigate early stages of sintering of silicon nitride (Si3N4) nanoclusters by molecular-dynamics (MD) simulations on parallel computers. Within 100 pico seconds, an asymmetric neck is formed between nanocrystals at 2,000K. In the neck region, there are more four-fold than three-fold coordinated Si atoms. In contrast, amorphous nanoclusters develop a symmetric neck, which has nearly the same number of three-fold and four-fold coordinated Si atoms. In the case of sintering among three nanoclusters, a chain-like structure forms in 200 pico seconds. The present study shows that sintering is driven by rapid diffusion of surface atoms and cluster rearrangement.

Role of additives in the sintering of silicon nitride: A29Si, 27Al, 25Mg and 89Y MAS NMR and X-ray diffraction study

Multinuclear MAS NMR in conjunction with X-ray diffraction (XRD) has been used to study the role of Al2O3, Y2O3 and MgO, both singly and in combination, in the sintering of silicon nitride at 1500–1800 °C. Under the present experimental conditions, Al2O3 enters the silicon nitride to form a low-z β′-SiAlON, whereas MgO reacts both with the oxidised surface SiO2 layer to form forsterite (Mg2SiO4), and with the Si3N4 to form an X-ray amorphous Mg–Si–0–N phase characterised by a broad 25Mg signal at about –50 ppm and a fast 29Si relaxation time. Y2O3 forms an yttrium-rich Y–Si–O–N phase at 1500 °C which progressively becomes silicon-rich at higher temperatures. The 89Y spectra of these phases are broad and could be detected only in samples containing added Yb2O3 to shorten the relaxation time. When used in combination, the Al2O3/MgO and Y2O3/MgO pairs behave similarly to the separate components, in terms of intergranular phase formation, but Al2O3/Y2O3 forms Y10Al2Si3O18N4, for which the 29Si and 27Al MAS NMR spectra are reported.

Electron probe microanalysis of chemical gradients in gas pressure sintered silicon nitride

During gas pressure sintering of silicon nitride (Si3N4) — which normally contains oxide additives such as SiO2, Al2O3 and Y2O3 — in a resistance heated graphite furnace, a reduction of the Si3N4 sample takes place. At high temperatures (>1800°C) this effect is accompanied by decomposition reactions of Si3N4. Both lead to chemical gradients in larger components which influence the strength of the sintered article. Electron probe microanalysis (EPMA) has been carried out in order to study the influence of the crucible material [graphite (C), boron nitride (BN)] and the quantity of filling on the gradient formation.

Sintering of Silicon Nitride with YAIO3 Additive

Sintering of Si3N4 powder with the addition of a Y2O3+ Al2O3 mixture or YAlO3 as sintering aids was investigated. Sintering was improved in the case of YAlO3 additive compared to that for the Y2O3+ Al2O3 mixture. An initial delay in densification was most likely caused by heterogeneity of the liquid phase formed in the case of the separate oxide additions at temperatures above 1700°C.

Some New Aspects of Microstructural Development During Sintering of Silicon Nitride

Some New Aspects of Microstructural Development During Sintering of Silicon Nitride