Oxidation behavior of silicon nitride sintered with Lu2O3 additive

Kyocera SN282 silicon nitride ceramics sintered with 5.35 wt% Lu₂O₃ were oxidized in dry oxygen at 930–1,200 °C. Oxidation of SN282 follows a parabolic rate law. SN282 exhibits significantly lower parabolic rate constants and better oxide morphological stability than silicon nitride containing other sintering additives under similar conditions. The activation energy for oxidation of SN282 is 107 ± 5 kJ/mol K, suggesting inward diffusion of molecular oxygen in the oxide layer as the rate-limiting mechanism.     

Reaction sintered silicon nitride

The strength, composition and structure of reaction sintered silicon nitride formed in the presence of oxygen or water vapour have been investigated. It is shown that chemical contamination of the nitriding gas by oxygen and water vapour to relatively high concentrations is not the cause of abnormally low strength silicon nitride. The coefficient of variation in strength and apparent crystallite size for a particular batch of samples are related to the water vapour concentration in the nitriding gas. The formation of - and -silicon nitride is discussed and it is suggested that compositional variations in -silicon nitride must be small.     

Oxidation of sintered silicon nitride

The influence of oxidation at 1200 °C in air for up to 1000 h on the mechanical properties of two Si₃N₄-Y₂O₃-Al₂O₃ materials with different Y₂O₃/Al₂O₃ ratios, Material A (Si₃N₄-13.9 wt% Y₂O₃-4.5 wt% Al₂O₃) and Material B (Si₃N₄-6.0 wt% Y₂O₃-12.4 wt% Al₂O₃), was investigated. The oxidation significantly improves the high-temperature strength and fracture toughness of both materials, but more for Material A. After oxidation, Material A at 1300 °C retains 93% of its room-temperature strength and 87% higher than that before the oxidation. The oxidation has a different effect on the room-temperature K _IC for the two materials. The room-temperature Weibull modulus of Material A decreased by more than half while the 1200 °C Weibull modulus decreased slightly after oxidation. The annealing treatment prior to oxidation had no effect on the high-temperature strengths of the materials after oxidation. The effect of oxidation on mechanical properties is discussed in terms of the microstructure change of the materials.     

R-curve behaviour of sintered silicon nitride

R-curves for two in situ reinforced silicon nitrides A and B of different microstructures were determined by three different characterization methods. The saturated crack growth resistance was found to be 5.2, 7.2 and 9.2 MPa m^1/2 for silicon nitride A and 5.8, 8.0 and 10.0 MPam^1/2 for silicon nitride B, respectively, by indentation, indentation-crack growth, and indentation-strength methods. The rising behaviour of R-curves was also found to depend on the characterization method. These results indicate that care should be taken in interpreting and utilizing R-curves evaluated from different characterization methods and an R-curve characterization method with appropriate qualifiers is needed for rising R-curve materials.Visiting scientist, on leave from Korea Institute of Science and Technology, Seoul, Korea.     

Strength characterization of yttria-doped sintered silicon nitride

The flexural strength of yttria-doped sintered silicon nitride was evaluated as a function of temperature (20 to 1300 C in air environment), applied stress and time. Two mechanistic regimes were manifest in the temperature dependence of the fracture stress. A temperature-independent region of fast fracture (catastrophic crack extension) existed up to 900 C, in which the mode of crack propagation was primarily transgranular. Above 1000 C, the strength (fracture stress) decreased considerably due to the presence of subcritical or slow crack growth which occurred intergranularly. This material did not show a static oxidation problem in short-term (100 h) tests in the low-temperature regime (600 to 1000 C) as has been observed in other yttria-doped silicon nitrides. Flexural-stress rupture testing in the temperature range 800 to 1200 C in air indicated the material's susceptibility to time-dependent failure, and outlines safe applied stress levels for a given temperature.     

R-curve behaviour and microstructure of sintered silicon nitride

R-curves for two in situ reinforced silicon nitrides A and B, of different microstructures, were characterized using indentation-crack growth measurements. Silicon nitride B, with its coarser microstructure and 8 MPa m^1/2 toughness, showed higher resistance to crack growth and more damage tolerance than silicon nitride A, with its finer microstructure and 7 MPa m^1/2 toughness. However, silicon nitride A showed a higher Weibull modulus than that of silicon nitride B due to the relatively narrow critical grain-size distribution. These results suggest that a coarse microstructure with narrow flaw-size distribution is beneficial to toughening, damage tolerance, and reliability.     

Characterization of silicon nitride single crystals and polycrystalline reaction sintered silicon nitride by microhardness measurements

Identification of - and -phases of Si₃N₄ single crystals grown from Si melt could be made with the help of Vickers microhardness measurements. The effect of chemical additives, e.g. metallic Fe and BaF₂, on the microhardness of Si₃N₄ was also determined. Different constants involved in the empirical Meyer relationship between load and indentation diameters could be correlated with the porosity and microhardness of Si₃N₄ single crystals and polycrystalline, reaction sintered Si₃N₄.     

Interfacial structure in silicon nitride sintered with lanthanide oxide

Three independent research groups present a comparison of their structural analyses of prismatic interfaces in silicon nitride densified with the aid of lanthanide oxide Ln₂O₃. All three groups obtained scanning transmission electron microscope images which clearly reveal the presence of well-defined Ln segregation sites at the interfaces, and, moreover, reveal that these segregation sites are element-specific. While some results differ across the three research groups, the vast majority exhibits good reproducibility.     

Oxidation behavior of hot isostatically pressed silicon nitride containing Y2O3

Oxidation in the presence of air and water vapor at high temperatures wasstudied for Si₃N₄ ceramics containing Y₂O₃ and Al₂O₃ as sintering aids.The test environments for this study consisted of air with 0, 1.2,and 6.4 v/o H₂O at temperatures from 1000°C to 1350°C. Theoxidation exposure times were up to 500 hours. The presence of water vaporenhances oxidation and crystallization of the oxidation phases. Weight losswas observed for the oxidation in air or dry air because of Na contaminationduring the fabrication processing. The effect of applied stress on the growthof oxide scale is minimal, however, the applied stress resulted in deeperpenetration of oxygen and pit formation in the oxide phase.     

Oxidation of silicon nitride sintered with ceria and alumina

Abstract

Sinter-hipped silicon nitride with additions of ceria and alumina has been subjected to oxidation for up to 72 h at various temperatures. Within the experimental temperature range (1250–1425°C), a parabolic weight gain was observed with an activation energy of 350 kJ mol^?1 for the process. On oxidation there formed an oxide scale whose morphology depended on temperature and cooling rate, and a subscale, at the interface between the substrate and the oxide scale, consisting of β3-Si₃MN₄ grains, a Ce-rich silicate glass, and Si₂N₂0. The morphology and phase content of the oxide products were characterised by X-ray diffractometry and electron microscopy. Detailed analyses by SEM showed that the oxide scales were not homogeneous, but exhibited several layers with important microstructural and compositional variations through their thickness.

MST/1107     

The development of strength in reaction sintered silicon nitride

The development of strength in reaction sintered silicon nitride has been investigated by determining the elastic moduli, fracture mechanics parameters, strengths and critical defect sizes of silicon compacts reacted to various degrees of conversion using static or flowing nitrogen. The relationship between each property and the nitrided density is shown to be independent of the green silicon compact density but is influenced by the nitriding conditions employed. Young's moduli, rigidity moduli and strengths vary linearly with the nitrided density. After an initial period when increases may occur, the critical defect sizes in both static and flow materials decrease continuously with increasing nitrided density, although at any particular density they are larger in material produced under flow conditions. A model is suggested for the development of the structure of reaction sintered silicon nitride involving the development of a continuous silicon nitride network within the pore space of the original silicon compact. The experimental data are discussed in terms of the proportion of silicon nitride which contributes effectively to the continuous network.     

High-temperature deformation and fracture processes in sintered reaction-bonded silicon nitride

Studies of the high-temperature deformation behaviour of sintered reaction-bonded silicon nitride (SRBSN) materials were conducted at 1200 °C in air under selected stress levels, which were applied at a single stress or as a sequence of stepwise increasing stresses. The objective was to evaluate the effects of the fabrication methods (conventional versus microwave heating process), microstructure, and precursor silicon powder purity on the deformation and fracture processes during creep loading of SRBSN materials containing a mixture of 3 wt% Al₂O₃ and 9 wt% Y₂O₃ sintering additives. Results indicated that all of the SRBSN materials exhibited a threshold stress above which the dominant process underwent transition from creep to extensive creep-assisted crack growth (CACG) from existing pores. In addition, the microwave SRBSN materials exhibited a better resistance (higher threshold stress) to CACG process, compared with those fabricated by conventional heating with the same metallurgical grade of silicon powder. The higher threshold stress observed in microwave SRBSN is mainly associated with the increased number density of elongated grains and the related higher fracture toughness. However, the minimum creep rates and stress exponents obtained in the creep regime were independent of the heating method. The microwave SRBSN material fabricated with lower purity silicon also exhibited a higher threshold stress for multiple crack formation and growth as compared with that processed with higher purity silicon. Conversely, the creep rate of microwave SRBSN materials was decreased by decreasing the impurity level (i.e. iron) in silicon powder.     

Dissolution of sintered silicon nitride bulk specimens for elemental analysis

Heating bulk, sintered silicon nitride samples in an aqueous hydrofluoric-hydrochloric acid mixture is shown to decompose the silicon nitride. Subsequent addition of sulphuric acid and volatilization of fluorides permits total dissolution of the bulk specimens for analysis. The elemental compositions that were determined by inductively coupled plasma atomic emission and atomic absorption spectrometries agreed with the nominal sample composition and confirmed analyses performed by scanning transmission electron microscopy. Neutron activation determinations on the same samples are not believed to be as accurate as the spectrometric determinations. Furthermore, the precision of the neutron activation measurements were less satisfactory, especially for key elements such as yttrium.     

Fracture of flash oxidized,yttria-doped sintered reaction-bonded silicon nitride

The oxidation behaviour of a slip cast, yttria-doped, sintered reaction-bonded silicon nitride after flash oxidation was investigated. It was found that both the static oxidation resistance and flexural stress rupture life (creep deformation) were improved at 1000° C in air compared to those of the same material without flash oxidation. Stress rupture data at high temperatures (1000 to 1200° C) are presented to indicate applied stress levels for oxidation-dependent and independent failures.     

Fracture phenomenology of a sintered silicon nitride containing oxide additives

Crack propagation mechanisms in a sintered silicon nitride containing various oxide additives (ceria, magnesia, zirconia and strontium oxide) were studied as a function of initial flaw size, temperature, applied stress and time. Surface cracks of controlled size were introduced using the microhardness indentation-induced-flaw technique. At 20° C, the fracture stress was found to depend on initial crack size according to the Griffith relationship and extrapolation of the data indicated that processing flaws of 20 to 35 were strength-controlling. The flexural strength was found to be independent of temperature from 20 to 800° C and the mode of crack propagation was primarily transgranular. At temperatures above 800° C the flexural strength decreased significantly, due to viscous flow of the glassy phase present in the material and resulting in sub-critical crack growth (SCG). The mode of crack propagation during SCG was essentially intergranular. Flexural stress-rupture evaluation in the temperature range 800 to 1000° C has identified the stress levels for time-dependent and time-independent failures.     

Fracture mechanics approach to evaluation of strength in sintered silicon nitride

The evaluation of strength of ceramics is an important problem in their application to engineering components. Although fracture initiating from inherent flaws in ceramics is supposed to be analyzed through linear elastic fracture mechanics, the applicability of a plane-strain fracture toughness criterion is in dispute.In this investigation, fracture tests for two grades of sintered silicon nitride were conducted under plane-bending and ring-compression. As the size of inherent flaw observed at the fracture origin was larger, the critical stress intensity factor for the flaw increased and then almost coincided with the fracture toughness. This suggests that ordinary fracture mechanics data should not always be applied directly for predicting the strength of engineering ceramic components. The flaw-size effect was discussed from the point of the applicability of fracture mechanics and the relation with material microstructure.

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Résumé L'évaluation de la résistance des céramiques est un problème important pour leur application à des composants mécaniques. Bien que l'amorçage d'une fissure dans une céramique au départ de défauts inhérents soit normalement analysable par la mécanique de rupture linéaire et élastique, on n'est pas d'accord sur la pertinence d'un critère de ténacité à la rupture en état plan de déformation.Dans cette recherche, des essais de rupture de deux nuances de nitrure de silicium frittées ont été exécutés en flexion plane et en compression d'anneaux. On a observé que lorsque augmente la taille d'un défaut initial repérable comme origine de la rupture, le facteur critique d'intensité de contrainte pour le défaut considéré augmente et coïncide presque avec la ténacité à la rupture. Ceci suggère que les données habituelles de la mécanique de la rupture ne peuvent pas être toujours appliquées directement pour prédire la résistance de composants mécaniques en céramique.L'effet de la taille du défaut est discuté des points de vue de l'application possible de la mécanique de rupture et de la relation avec la microstructure du matériau.