Sintered Silicon Nitride Research Articles

Dislocation Sub-Boundaries in As-Grown ?-Silicon Nitride

Dislocation arrangements resulting from the elaboration process of gas pressure sintered silicon nitride have been investigated by transmission electron microscopy. Dislocations with Burgers vector b = 1/3〈110〉 are found to be dissociated in a {110} prismatic plane. Structural features of the induced stacking faults are discussed. Furthermore, reactions between dislocations with Burgers vector b = 1/3[110], [0001], and 1/3[113] are observed. Stability calculations considering elastic anisotropy show that a reaction between dislocations with Burgers vectors b = [0001] and b = 1/3〈110〉 leading to a junction dislocation with Burgers vector b = 1/3〈113〉 is always possible.

Joining of silicon nitride ceramic composites with Y 2O 3–Al 2O 3–SiO 2 mixtures

Hot-pressure sintered silicon nitride ceramic composites were joined to themselves using Y 2O 3–Al 2O 3–SiO 2 mixtures. Reaction behavior at interface between Si 3N 4 and Y 2O 3–Al 2O 3–SiO 2 mixtures during silicon nitride composite joining was studied by means of SEM, EPMA and XRD. The joint strength under different bonding conditions was measured by four-point bending tests. The results of EPMA and XRD analyses show that the liquid glass solders react with silicon nitride at interface, forming the Si 3N 4/Si 2N 2O/Y–Si–Al–O–N glass/Y–Si–Al–O glass gradient interface. From the results of four-point bending tests, it is known that with increasing joining temperature and holding time, the joint strength first increased, reaching a peak, and then decreased. The maximum joining strength of 315 MPa measured by the four-point bending test is achieved for the Si 3N 4 composite–Si 3N 4 composite joint bonded at 1823 K for 15 min. Destructed part is considered to start at the diffusion layer, indicating that the joining strength will be much higher than that value.

Mechanical properties of gas pressure sintered Si3N4/SiC nanocomposites

Abstract The mechanical properties of gas pressure sintered Si3N4/SiC nanocomposite materials were studied at room temperature and in the range of 1400–1600 °C. Fracture strength at room temperature was found to be between 640 and 840 MPa and thus within the range of data reported in the literature for other gas pressure sintered silicon nitride ceramics. At elevated temperature the creep and stress rupture behaviour were determined in four-point bending. The creep behaviour is characterised by primary and pronounced secondary creep, very high activation energies for creep in the range of 1050–1450 kJ/mol, and two different stress dependencies. At stresses below a characteristic transition stress in the range 170–200 MPa the stress dependency is described by a stress exponent of around two. Above the transition stress, stress rupture was observed to be controlled by subcritical crack growth with stress rupture exponents of 30–40. Annealing of the as-sintered materials promotes crystallisation of the intergranular glassy phase resulting in an increase in creep strength and resistance against subcritical crack growth. An extensive analysis of the available literature data revealed that the materials tested in this study belong to the most creep resistant silicon nitride ceramics prepared by gas pressure sintering.

Creep behaviour of two sintered silicon nitride ceramics

Abstract The high temperature mechanical properties of two sintered silicon nitride—based ceramic materials with different microstructural features, labelled N7202 and N3208, are presented. The mechanical behaviour was examined by creep tests in compression, at temperatures between 1450 and 1700 °C in argon atmosphere. The results, analysed in terms of the creep equation, yield n =0.6±0.1 and Q =470±20 kJ/mol for N7202, and n =0.6±0.1 and Q =530±10 kJ/mol for N3208. The microstructural observations allow identify a possible microscopic deformation mechanism compatible with the set of creep parameters obtained experimentally.

Effects of α/β ratio in starting powder on microstructure and mechanical properties of silicon nitride ceramics

The effects of α/β ratio in the starting powder on microstructure development and mechanical properties of sintered silicon nitride ceramics were investigated. With increasing β-phase content, the microstructure became finer, the finest-grained microstructure was developed from the powder with pure β-phase, whereas a duplex microstructure was developed from high α-phase content powder. The results show that the microstructure of silicon nitride ceramics can be tailored by changing the α/β ratio in the starting powder. The mechanical properties have an optimum region where the α/β ratio equals 1. Both flexural strength and fracture toughness can be improved by controlling size, number, aspect ratio and size distribution of the rod-like β-Si 3N 4 grains

Importance of chemistry in high-tech ceramics design

Abstract This paper deals with the role of chemistry in the design of high-tech ceramic materials. Grain boundary composition of polycrystalline ceramics dictates the hardness fracture toughness and creep resistance of liquid-phase sintered silicon nitride and silicon carbide materials.

Grain coarsening in gas pressure sintered silicon nitride

The grain growth of silicon nitride sintered by GPS has been studied in the case of an α-rich initial powder. As time increases, the microstructure becomes bimodal. A few large grains grow abnormally at the expense of the smallest ones. The growth rate of the major portion of grains is slow and can be assimilated to a normal growth with interface-reaction control. This rate limiting mechanism has been determined by three different methods: a method based on the Kingery’rate laws, a more direct method based on the variation of the grain size vs. the amount of the liquid phase and considerations on grain morphology according to Lee.

Mechanical spectroscopy connected to creep and stress relaxation in a high resistant silicon nitride

Abstract Silicon nitride processed by gas pressure sintering contains a very small amount of glassy phase and consequently exhibits a strong resistance to deformation until 1450 °C. Above this temperature, both relaxation kinetics and creep rate rapidly increase. To explain such a behaviour, the formation of a liquid phase by dissolution of YSiAlON phases was proposed. The present paper shows that mechanical spectroscopy argues for the existence of such a liquid phase at high temperature. The mechanical loss is very low in the as-sintered material. Nevertheless, the internal friction peak generally observed in silicon nitride, and attributed to the glass transition in the glassy pockets, is also observed in the gas pressure sintered silicon nitride. Moreover, the peak is much higher in annealed and “quenched” specimens and it increases with annealing time. These results show that the annealed and “quenched” material contains much more glassy phase and so argues for the dissolution of crystalline phases at high temperature.

806 窒化けい素における 2 段繰返し変動荷重下の疲労き裂進展挙動に及ぼす頻度比の影響

In order to investigate effects of frequency ratio on fatigue crack growth behavior in a gas-pressure sintered silicon nitride, fatigue crack growth tests under two kinds of repeated two-step loading were carried out using compact tension specimens. Fatigue crack growth rate and macroscopic crack closure were measured by means of the unloading elastic compliance method. Crack growth rate at low level loading, (da/dn)L, was found to be accelerated in comparison with that under constant amplitude loading having same ΔK value. Effect of frequency ratio on crack growth behavior under repeated two-step loading holding K_<min> constant is remarkable, but under repeated two-step loading holding K_<max> constant is small.

Microstructural designing of silicon nitride related to toughness

Abstract Two ways of microstructural control were investigated with the intention to improve fracture toughness of gas pressure sintered silicon nitride : seeding and optimisation of sintering time. In order to study the effect of seeding, different amounts of large β -Si 3 N 4 whiskers were added to different silicon nitride powders. Seeding can lead to a low toughening improvement, providing that the initial powder is free from large pre-existing β nuclei. If not, seeding has a negligible effect on the microstructure because of nuclei interaction and an unfavourable effect on densification, hence on toughness. The effect of the sintering time was studied in a case of abnormal nuclei growth. Toughness can be improved by increasing the sintering time, providing that the composition of the intergranular phase remains stable. Correlations between microstructural parameters and toughness are consistent with the reinforcing mechanisms in composites (crack bridging and crack deviation): toughness is proportional to the grain diameter square root and increases as the fraction of the large grains increases.

Microstructural Development in Milli Wave Sintered Silicon Nitride Based Ceramics

Microstructural Development in Milli Wave Sintered Silicon Nitride Based Ceramics

Creep and stress-rupture behavior of Y2O3–Nd2O3-doped silicon nitrides with different additive contents

Abstract Tensile creep tests were performed on two grades of gas-pressure sintered silicon nitride. Silicon nitride SN4 contained 2.0 mol% Nd 2 O 3 and 2.0 mol% Y 2 O 3 as sintering aids, and the additives for silicon nitride SN1 were 0.5 mol% Nd 2 O 3 and 0.5 mol% Y 2 O 3 . The delayed failure of SN4 was thought to result from creep rupture because considerable creep deformation was found with a high stress exponent of n =10.7 in the stress range of 137 to 220 MPa at 1300°C, and the stress-rupture parameter was determined to be N =8.4. The delayed failure of SN1 was thought to result from subcritical crack growth since no significant creep deformation was found and a high stress-rupture parameter of N =20.7 was determined. The creep and stress-rupture mechanisms are discussed based on the creep test results and microstructure observation.

Improvement of high-temperature strength of hot-pressed sintering silicon nitride with Lu 2O 3 addition

The flexural strength of Lu 2O 3-doped hot-pressed sintering Si 3N 4 was studied in N 2 at elevated temperatures. The high-temperature strength above 1400°C was significantly improved due to Lu 2O 3 addition. This was attributed to the formation of a grain boundary phase, with a higher melting point, which was extensively crystallized during the sintering process.

Grinding-Induced Residual Stress Estimation by Indentation-Fracture Method in Ground Silicon Nitrides

An estimating procedure for grinding-induced residual stress based on the indentation-fracture (IF) method was proposed by considering a nonuniform distribution of residual stress in the specimen depth. The proposed procedure was applied to gas pressure sintered and pressureless sintered silicon nitride ceramics, which were ground under different grinding conditions. The estimated residual stress was found to be compressive for both materials. The residual stress was dominantly affected by the grit size of the grinding wheel rather than the cutting depth. Although the dependency of the estimated residual stress on the grit size was different between the two materials, it was revealed that the estimated residual stress in both materials qualitatively corresponded with the stress measured by the x-ray diffraction method. In both materials, the bending strength was reasonably correlated with the estimated residual stress. It was elucidated that the proposed procedure was applicable to a relative evaluation of the grinding-induced residual stress in machined silicon nitride ceramics.

Tensile behaviour of gas pressure sintered silicon nitride in the 1600–1700°C temperature range

The tensile behaviour of a silicon nitride ceramic has been studied between 1600 and 1680°C under strain rates from 6×10−6 to 1.2×10−5 s−1. At low temperature and/or high strain rate, the behaviour was essentially brittle: failure occurred by cavitation along boundaries of acicular grains lying normal to the tensile axis and linking of these cavities by interfacial debonding. At 1650°C, the deformation started to be ductile: a stress peak was observed in the tensile curve and cavities formed at multigrain junctions while grain boundary sliding began to occur. As the temperature increased, the contribution of grain boundary sliding to deformation increased at the expense of the cavitational component. The stress peak is interpreted in terms of a relaxation effect that governs the competition between cavitation and grain boundary sliding.

セラミックス／金属接合体の破壊機構と残留応力挙動の相関性

In order to clarify the correlation of the fracture mechanism and the behavior of the residual stress in the ceramic/metal joint, the residual stress was measured by X-ray diffraction method during applied bending stress. Sintered silicon nitride (Si3N4) and stainless steel (SUS304) were used as specimen materials. The stress analysis by finite element method (FEM) was carried out, and was compared with X-ray measurement results. The obtained results were as follows: (1) The measured stress was larger with the applied stress. (2) As the applied stress was larger, intensity of the stress singularity, K was larger and λ which shows the order of the stress singularity was smaller and then became constant. (3) The behavior of K and λ agreed in the experimental results and the analytical results qualitatively well. (4) At the low-loaded region, there was a difference for the behavior of K and λ depending on the shape and dimension of specimen. But, when the applied stress was larger, K and λ become constant respectively. (5) To observe the change of stress singularity at the interface, it is possible to estimate the stress of fracture initiation.

Inelastic Analysis of Notched Components in Silicon Nitride Ceramics at Elevated Temperature.

The displacement rate-controlled tensile tests were performed for smooth and circumferentially notched specimens of the sintered silicon nitride ceramics in argon-gas atmosphere at 1300°C, and then the axial load-displacement response was measured. It was shown from the tests that inelastic deformation was greatly dependent on displacement rate and easier to generate at the lower displacement rate. The constitutive equation was constructed on the basis of tensile stress-strain curves in the smooth specimen. Then Inelastic FEM analysis was performed for the smooth and notched specimens, using this equation. The calculated load-displacement responses quantitatively agreed quite well with those in the smooth and notched specimens, showing the validity of the proposed analysis. The development of the Inelastic deformation in loading process was shown from the calculation around the notch root.

Effect of Oxygen Content on Thermal Conductivity of Sintered Silicon Nitride.

Silicon nitride powders with concurrent addition of Yb2O3 and MgO were sintered at 1900°C for 2-48 h under 0.9 MPa nitrogen pressure. Microstructure, lattice oxygen content and thermal conductivity of the sintered specimens were evaluated. All specimens had a duplex microstructure composed of small matrix grains and elongated grains. Thermal conductivities of sintered bodies were remarkably increased with sintering time and reached a maximum value of about 120W·m-1·K-1. The lattice oxygen content of dense silicon nitride sintered body decreased with sintering time. The thermal conductivity increased with decreasing the lattice oxygen content. So far, it was reported that the thermal conductivity of Si3N4 was affected by grain size and morphology, composition and thickness of grain boundary layer. It was revealed that thermal conductivity of sintered specimen was closely related to the oxygen content present in the silicon nitride lattice.

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.

117 超高温下における窒化けい素セラミックスの応力 : ひずみ応答と疲労寿命の関係(材料力学VI)

117 超高温下における窒化けい素セラミックスの応力 : ひずみ応答と疲労寿命の関係(材料力学VI)