Oxidation Behaviour Of Silicon Nitride Research Articles

Effect of rare-earth oxides on properties of silicon nitride obtained by normal sintering and sinter-HIP

This paper presented the microstructure, mechanical properties and oxidation behavior of silicon nitride obtained by both conventional sintering and sintering followed by hot isostatic pressing (HIP). Silicon nitride with additives such as 5 wt.% Al2O3 and 5 wt.% Ln2O3 (Ln= La, La concentrate, Gd or La+Gd) were studied. The results revealed that Gd2O3 additions increased the formation of elongated grains of β-Si3N4, the fracture toughness and oxidation resistance. La2O3 additions led to higher densification and hardness values, while addition of La2O3 concentrate promoted the formation of materials with intermediate properties, compared to the other studied compositions. Hot isostatic pressing increased the hardness but decreased the fracture toughness of the material, mainly because it allowed residual pores to close and also reduced the average aspect ratio of β-Si3N4 grains.

Mechanical Properties and Oxidation Behavior of Si<sub>3</sub>N<sub>4</sub>/BN Laminated Ceramics

Laminated ceramics with high mechanical properties were fabricated in the Si3N4/BN system. The mechanical properties at elevated temperatures were tested, and the oxidation behavior during tested procedure was studied at the same time. The flexure strength of the Si3N4/BN laminated ceramics changed a little below 1000°C. The displacement-load curves appeared non-linear characteristic even at high temperature. During testing procedure at high temperature, oxidation behavior of silicon nitride and silicon carbide happened, and no oxidation product of boron nitride was found. The silicon nitride layers were oxidized to form a protective silicate scale, which prevented oxidation of the boron nitride interlayers. The stability of boron nitride was beneficial to the boron nitride interlayer to partition the silicon nitride matrix layers at high temperature.

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.

Effect of Steam Velocity on the Hydrothermal Oxidation/Volatilization of Silicon Nitride

The hydrothermal oxidation behavior of silicon nitride was studied as a function of vapor velocity in the range of 1–35 cm/s, at atmospheric pressure and ∼100% H2O, using a custom‐designed hydrothermal apparatus. At lower velocities, silicon nitride showed a marginal weight gain during early exposure times followed by a linear weight loss at longer exposure times. At higher velocities only linear weight loss was seen. The linear weight loss rates were determined from the weight change studies and they showed a square root dependence on velocity at higher flow rates. At the low flow rates a threshold velocity of ∼1 cm/s was observed for volatilization to initiate. The weight loss rates obtained in the apparatus at the higher steam velocities were comparable to those obtained in high pressure burner rigs, proving its viability as a good laboratory scale screening method for potential gas turbine materials.

Oxidation of Silicon Nitride in Wet Air and Effect of Lutetium Disilicate Coating

Oxidation behavior of silicon nitride (Si3N4) was investigated in flowing air (2.45 cm/s) containing 10%–50% H2O at a total pressure of 1.8–10 atm at 1300°–1500°C for 100 h. The oxidation of Si3N4 progressed with volatilization of the SiO2 scale; it was more enhanced at a high partial pressure of H2O rather than at high temperature. The total pressure had little effect on the oxidation. In order to avoid the oxidation, Si3N4 substrate was coated with lutetium disilicate (Lu2Si2O7) layer through the intermediate SiO2‐rich phase. While the coating layer well suppressed the oxidation in case of small amount of water vapor, it was not sufficiently effective to suppress the oxidation when the water vapor was rich. SiO2 volatilization was observed between the layer and substrate. The flexural strength of the coated Si3N4 at room temperature was somewhat increased after the oxidation in wet air, while that of the uncoated one was almost unchanged. This increase was attributable to crack healing of the substrate during the oxidation.

Elastic Modulus of Oxidation-formed Silicon Nitride Composite

Silicon nitride ceramics are generally used in high temperature oxidizing environments. The phase composition and microstructure of silicon nitride ceramics change during oxidation resulting in changes in the mechanical properties of materials. The oxidation behaviour of silicon nitride is dealt with in numerous studies [e.g. 1±7]. In these papers the effects of the oxidation on the room-, and the high-temperature strength and fracture toughness were studied [1±5]. The phase composition and the thickness of the surface oxide layer also studied by several authors [6, 7]. The published studies in this ®eld concentrated on the mechanical properties relevant for utilization but little attention was paid to the elastic properties. It is known that the Young's modulus gives valuable information about the structural changes of ceramics during sintering [8], therefore it was assumed that the measurement of the modulus of oxidized samples would contribute to the understanding of the complex process of oxidation. In this paper, the Young's modulus of oxidized silicon nitride ceramics is studied. The sample oxidized at elevated temperatures is considered as a composite material that consists of an internal part and a surface oxide layer. The elastic modulus of the whole sample was measured by the standard four point bending method, that of the internal region and the surface oxide layer was determined by the depth sensing indentation (DSI) method. A formula was derived for the calculation of the modulus of the composite from the moduli of the parts. Mixtures of Si3N4, AlN, Al2O3 and Y2O3 powders were milled in ethanol in a planetary type alumina ball mill. The composition of materials is given in Table I. Samples were compacted by dry pressing at 220 MPa. The materials were sintered in high purity nitrogen by a two-step sinter-HIP method using BN embedding powder. The dimensions of as-sintered specimens were approximately 3:5 mm 3 5 mm 3 50 mm. Oxidation took place in static air at 800 8C, 1000 8C, 1150 8C, 1300 8C and 1400 8C for up to 48 h. The Young's modulus of the samples was determined before and after oxidation by a room temperature four point bending test with spans of 40 and 20 mm. The elastic moduli for different compositions and temperatures of oxidation are shown in Fig. 1, the one standard deviation is less than 5 GPa. The Young's modulus of the samples began to decrease at an oxidation temperature of 1300 8C for materials X, A and F and a drastic reduction occurred at 1400 8C for all the compositions investigated. On the surface of the samples oxidized at 1300 8C and 1400 8C a white, rough oxide layer appeared. The role of this oxide layer in the change of the elastic modulus will be analysed in the following. The oxidized rectangular specimen can be considered as a composite material that consists of an internal part and an oxide layer (Fig. 2a). The Young's modulus of the internal region and the surface oxide layer of the materials oxidized at 1300 8C and 1400 8C was measured by the depth sensing indentation (DSI) method. The surface layer was removed by grinding before the measurement of the modulus of the internal part of the material. The DSI measurements were carried out by a Shimadzu dynamic ultra-micro hardness tester (DUH 202). Each DSI test consisted of a loading±unloading cycle. During the loading period a Vickers indenter penetrated into the surface of the sample at constant loading rate (70 mN sy1) and the same rate was applied in the unloading period when the pyramid moved backwards. The maximum load in each test was 500 mN. The Young's modulus of the samples was calculated from the indentation curves using the method of Oliver and Pharr [9]. The average modulus values of the internal region (Ei) and the surface oxide layer (El) are shown in Table II (the one standard deviation is about 10 GPa). The Young's modulus of the oxide layer of material B oxidized at 1400 8C is much lower than that of other studied materials. The reason for this deviation is under investigation. A simple relationship between the modulus of the

Effects of Aluminum Implantation on the Oxidation Behavior of Silicon Nitride in a Sodium Nitrate-Oxygen Gas Mixture

AbstractThe role of aluminum in negating the adverse effect of alkali species on the oxidation resistance of Si3N4 ceramics was investigated by exposing unimplanted and aluminum-implanted (1 and 5 at.%) Si3N4 samples to a sodium nitrate (95 ppm)-dry oxygen gas mixture at 1. atm and at 900°1100°C. Oxidation of unimplanted Si3N4 was rapid and linear with an activation energy of 57 kJ/mol. In contrast, samples implanted with aluminum exhibited a considerably reduced oxide growth which was parabolic in nature with activation energies of 103–112 kJ/mol. The morphological characteristics of the oxide layer also showed marked improvement as the aluminum concentration increased.

Oxidation behaviour of the sintered Si3N4-Y2O3-Al2O3 system

The oxidation behaviour of silicon nitride composed of Si3N4, Y2O3, Al2O3, AlN and TiO2 was investigated in dry and wet air at 1100–1400 °C. The oxidation rates were confirmed to obey the parabolic law. An activation energy of 255 kJ mol−1 was calculated from the Arrhenius plots of the results of oxidation in an air flow. In still air the oxidation rate was larger than that in an air flow, but the oxidation rate in flowing air was not affected by the air flow rate. α-cristobalite and Y2O3·2SiO2 were formed in oxidized surface layers. These crystal phases increased with increasing oxidation temperature. In particular, a higher content of α-cristobalite was obtained in still air oxidation. The existence of water vapour in flowing air greatly promoted the oxidation.

Oxidation behaviour of silicon nitride under cyclic and static conditions

Silicon nitride, hot-pressed with alumina and yttria additives, was oxidized cyclically at 1200°C in flowing dry air for a total oxidation time of 3000 h. Static oxidation was also carried out for comparison. The cyclic oxidation apparently proceeded in two stages with different reaction rates, but both stages approximately followed the parabolic rate law. This difference of reaction rates may well be related to the change with time of the properties of the oxide layer and sublayer formed during oxidation. The main crystalline phases formed during the gradual oxidation on the surface were cristobalite, yttrium disilicate (Y 2O 3.2SiO 2) and tridymite. The corresponding oxide layers were porous and contained many cracks and channels. The surface roughness increased gradually with time, and reached a maximum value after 750 h oxidation. Then it decreased by degrees up until the end of the runs ( t = 3000 h). A consecutive degradation of the room-temperature flexural strength was observed. The static oxidation resulted in the same weight gain, the formation of different relative quantities of cristobalite, tridymite and Y.2S, approximately the same surface roughness and a slightly higher room-temperature flexural strength compared with the cyclic oxidation.

Hot‐Pressing and Oxidation Behavior of Silicon Nitride with Ceria Additive

Journal of the American Ceramic SocietyVolume 63, Issue 1-2 p. 119-120 Hot-Pressing and Oxidation Behavior of Silicon Nitride with Ceria Additive J. P. GUHA, J. P. GUHA Member, the American Ceramic Society.Search for more papers by this authorP. GOURSAT, P. GOURSATSearch for more papers by this authorM. BILLY, M. BILLY Member, the American Ceramic Society.Search for more papers by this author J. P. GUHA, J. P. GUHA Member, the American Ceramic Society.Search for more papers by this authorP. GOURSAT, P. GOURSATSearch for more papers by this authorM. BILLY, M. BILLY Member, the American Ceramic Society.Search for more papers by this author First published: January 1980 https://doi.org/10.1111/j.1151-2916.1980.tb10673.xCitations: 11 At the time this work was done, the writers were with the Ceramic Research Center, CNRS, ERA 539, University of Limoges, UER des Sciences, 87100 Limoges, France. J. P. Guha is now with the Ceramic Research Laboratory, Institute Jozef-Stefan, University of Ljubljana, 61001 Ljubljana, Yugoslavia. AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article.Citing Literature Volume63, Issue1-2January 1980Pages 119-120 RelatedInformation