Transient creep behaviour of hot isostatically pressed silicon nitride

Transient creep is shown to dominate the high-temperature behaviour of a grade of hot isostatically pressed silicon nitride containing only 4 wt% Y₂O₃ as a sintering aid. Contributing factors to transient creep are discussed and it is concluded that the most likely cause of longterm transient creep in the present study is intergranular sliding and interlocking of silicon nitride grains. In early stages of creep, devitrification of the intergranular phase, and intergranular flow of that phase may also contribute to the transient creep process. The occurrence of transient creep precluded the determination of an activation energy on the as-received material. However, after creep in the temperature range 1330–1430°C for times exceeding approximately 1100 h, an apparent activation energy of 1260 kJ mol^–1 was measured. It is suggested that the apparent activation energy for creep is determined by the mobility and concentration of diffusing species in the intergranular glassy phase. The time-to-rupture was found to be a power function of the minimum strain rate, independent of applied stress or temperature. Hence, creep-rupture behaviour followed a Monkman-Grant relation. A strain rate exponent of – 1.12 was determined.     

An analysis of the creep of hot pressed silicon nitride in bending

The creep behaviour of hot pressed silicon nitride is investigated in four-point-bend tests at temperatures of about 1200° C. By use of appropriate creep laws for the primary creep range as well as for the secondary range the experimental results can be well described analytically.     

Residual strength of ground hot isostatically pressed silicon nitride

Surface flaws are introduced during grinding of most high strength ceramics. These flaws reduce the strength and it is therefore important to choose grinding parameters such that surface damage is minimized. The assumption that it is the same mechanism that causes cracking beneath both an indenter and a diamond tool made it possible to propose a grinding model. According to this model high wheel speeds, low workpiece velocities and low depths of cut would reduce the grinding forces and thus be beneficial to the strength after grinding. Grinding experiments on hot isostatically pressed silicon nitride showed that this was the case. The experiments also showed that the grinding direction had the strongest influence on the strength, and if possible the direction ought to be parallel with the expected principal stress. Even what can be considered to be mild machine parameters introduce flaws and residual compressive stresses in the surface of the workpiece.     

Mechanical properties of hot isostatically pressed zirconium nitride materials

The influence of powder properties on the sintering behaviour, the microstructural development and the mechanical properties of hot isostatically pressed (HIPed) zirconium nitrides were investigated. The results show that the densification behaviour is dependent on the powder characteristics, more precisely the grain morphology and size, and oxygen, carbon and metallic impurity contents. The mechanical properties are controlled mainly by the amount of porosity and the presence of a complex intergranular phase in direct relation to the purity of the starting powder. The differences in the fracture strength and toughness between the two grades are perceptible. On the other hand, the thermal shock resistance to fracture initiation, as well as the elastic parameters of both dense materials, are similar.     

Cyclic fatigue of hot isostatically pressed silicon nitride at elevated temperatures

Cyclic fatigue properties of a hot isostatically pressed silicon nitride were investigated at 1150, 1260 and 1370 °C in ambient air. The uniaxial tensile tests were conducted under various cyclic loading wave forms and frequencies. The correlation of stress-life relations between cyclic and static fatigue results was evaluated. At 1150–1370 °C, cyclic loading caused less damage than static loading, as evidenced by the longer failure time under cyclic loading versus static loading with the same maximum applied stresses. The cyclic loading effect was more pronounced in high frequency tests at 1260 and 1370 °C and might be related to the viscoelastic behaviour of the intergranular phase. Microstructural analyses and macroscopic cyclic stress-strain and strain-time relations indicated that cyclic loading/unloading may inhibit the normal accumulation of creep damage.     

Influence of powder characteristics and powder processing routes on microstructure and properties of hot pressed silicon nitride materials

Four different commercial Si₃N₄ powders were hot pressed with the addition of La₂O₃ and Y₂O₃ as sintering aids. Two powder processing routes were set up: addition of sintering aid powders by ultrasonic dispersion, addition of nanodispersed amorphous additive species by chemical coprecipitation. The following aspects were analyzed: characteristics of starting powders and powder mixtures, with reference to surface modification (electrokinetic behaviour and surface properties) induced by the powder treatment; sintering behaviour of the powder mixtures; influence of raw powders characteristics and processing route on microstructure and properties of dense materials. The microstructural characteristics of hot pressed materials (grain size, aspect ratio, grain boundary phases) were found to be dependent on powder characteristics and its process history. Significant variation of the mechanical properties (Young modulus, hardness, toughness and strength) were related to microstructural features. Strength, for example, ranges from 600 to 1200 MPa at room temperature and from 400 to 1000 MPa at 1200°C; toughness ranges from about 4 to about 6 MPam^1/2.     

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.     

Microstructures and mechanical properties of silicon nitride bonded silicon carbide ceramic foams

Silicon nitride bonded silicon carbide foams have been produced by nitridation of the foamed compacts containing silicon carbide and silicon powders. When no nitridation additive was used the ceramic foams nitrided at all temperatures studied contained a significant amount of whisker phase α-Si₃N₄ formed both inside and outside the cell walls leading to a loose microstructure and a low mechanical strength. When the Al₂O₃ and Y₂O₃ were used as nitridation additives, the ceramic foams nitrided at temperatures of 1360 and 1395 °C containing certain amount of Si₂N₂O and whisker α-Si₃N₄ phases that are bonded by a glassy phase and behave as reinforcements for the ceramic foams exhibited a much higher mechanical strength. At nitridation temperature of 1430 °C, the ceramic foam showed the locally formed β-Si₃N₄ as the main nitrided phase that caused no increase in bonding area between the nitrided phase and the silicon carbide particles. Thus, a relatively lower mechanical strength was observed for the ceramic foam.     

Tensile creep of whisker-reinforced silicon nitride

This paper presents a study of the creep and creep rupture behaviour of hot-pressed silicon nitride reinforced with 30 vol% SiC whiskers. The material was tested in both tension and compression at temperatures ranging from 1100 to 1250°C for periods as long as 1000 h. A comparison was made between the creep behaviour of whisker-reinforced and whisker-free silicon nitride. Principal findings were: (i) transient creep due to devitrification of the intergranular phase dominates high-temperature creep behaviour; (ii) at high temperatures and stresses, cavitation at the whisker-silicon nitride interface enhances the creep rate and reduces the lifetime of the silicon nitride composite; (iii) resistance to creep deformation is greater in compression than in tension; (iv) the time to rupture is a power function of the creep rate, so that the temperature and stress dependence of the failure time is determined solely by the temperature and stress dependence of the creep rate; (v) as a consequence of differences in grain morphology and glass composition between whisker-free and whisker-reinforced material, little effect of whisker additions on the creep rate was observed.     

Oxidation resistance and creep behaviour of a silicon nitride ceramic densified with Y2O3

The corrosion resistance and creep behaviour in air of hot-pressed materials with a composition of 70 Si₃N₄-25 SiO₂-5 Y₂O₃ (mol%) has been studied. Kinetics data and microstructural changes in the 1180 to 1650° C temperature range indicate the presence of two oxidation mechanisms. Between 1180 and 1420° C, the preferential oxidation of the intergranular phase containing nitrogen is interpreted in terms of an inward diffusion of ionic oxygen. At temperatures higher than 1420° C, the degradation of the material is the sum of many processes (solution of silicon nitride, migration of oxygen and yttrium and release of nitrogren) but the diffusion to the nitride-oxide interface of a complex combination of yttrium and nitrogen in the boundary phase seems to be the limiting step. The three-point bending creep is discussed in relation to the evolution of the secondary intergranular phases in an oxidizing environment. The creep deformation is the sum of a viscoelastic component and a diffusional component characterized by the same activation energy (720 kJ mol^–1).     

Mechanical behaviour of a hot pressed aluminum nitride under uniaxial compression

Failure strength of a hot pressed aluminum nitride (AlN) is measured as a function of strain rate under uniaxial compression. At low strain rates (10-6–10-2s-1), the material is found to exhibit a weak strain rate sensitivity and at higher strain rates (102–103s-1), a strongly strain rate sensitive behaviour is observed. The quasi-static failure strength is found to be around 2.81 GPa and it increases to 5.25 GPa at a strain rate of 2200 s-1. During high strain rate testing, the specimen fractured into columnar fragments by axial splitting. Microscopic examination of the fractured surfaces revealed a typical brittle fracture with a combination of inter and intragranular failure modes. Based on the experimental results and microscopic observations, a micromechanical model has been developed to predict the constitutive behaviour of these ceramics under uniaxial compression. The model predictions of failure strength are shown to to be in agreement with the experimental observations. © 1998 Chapman & Hall     

Effect of water vapor on the mechanical behaviors of hot isostatically pressed silicon nitride containing Y2O3

The effect of water vapor on the mechanical behavior of Si₃N₄ ceramics was studied. Strength measurements by flexural dynamic fatigue tests were made at temperatures from 1038°C to 1350°C and at actuator speeds of 8.4 × 10^–2 and 8.4 × 10^–5 mm/s (200 and 0.2 MPa/s). Step stress rupture tests were also performed at 1288°C and 1150°C. Water vapor had a beneficial effect on the flexural strength due to flaw healing, and/or blunting mechanisms. Dynamic fatigue results demonstrated that the beneficial effects of water vapor on the strength increases as temperature increases and/or loading rate decreases. Time-to-failure was always longer in wet air during step stress rupture testing. Creep crack growth by formation and coalescence of cavities ahead of the crack tip generated from the oxidation pits or subsurface pores were the primary mechanism for slow crack growth for NT 164 Si₃N₄.     

氮化硅陶瓷的研究进展

氮化硅陶瓷是一种有广阔发展前景的高温高强度结构陶瓷.其具有高性能(如强度高、抗热震稳定性好、疲劳韧性高、室温抗弯强度高、耐磨、抗氧化、耐腐蚀性好等).已广泛应用于各行各业.氮化硅的制备方法主要有反应烧结法(RS)、热压烧结法(HPS)、常压烧结法(PLS)和气压烧结法(GPS)等.目前存在的主要问题是氮化硅陶瓷产品韧性低、成本较高.今后应改善制粉、成型和烧结工艺及氮化硅与碳化硅的复合化,研制出更加优良的氮化硅陶瓷.     

A fractographic criterion for subcritical crack growth boundaries at internal fracture origins in hot pressed silicon nitride

Using the elliptic integral method, stress intensity factors (K _I) were estimated at boundaries defined by fracture features observed at various distances from internal fracture origins in H.P. silicon nitride. The fracture origins are surrounded by regions of transgranular fracture. At the outer boundaries of these regionsK _I is less thanK _IC showing that these are regions of subcritical crack growth. Regions of hummocks and depressions were observed surrounding the regions of transgranular fracture.K _I was calculated at the elliptical boundary determined by the outer edge of the nearest of these features to the fracture origin. At this boundary,K _I K _IC. Therefore, these features can be used to locate the subcritical crack growth boundary.     

The compression creep behaviour of silicon nitride ceramics

A comparison has been made of the compression creep characteristics of samples of reaction-bonded and hot-pressed silicon nitride, a sialon and silicon carbide. In addition, the effects of factors such as oxide additions and fabrication variables on the creep resistance of reaction-bonded material and the influence of dispersions of SiC particles on the creep properties of hot-pressed silicon nitride have been considered. For the entire range of materials examined, the creep behaviour appears to be determined primarily by the rate at which the development of grain boundary microcracks allows relative movement of the crystals to take place. Now with the BNF Metals Technology Centre, Wantage.     

氮化硅晶须对反应烧结氮化硅多孔陶瓷介电性能的影响

以硅粉和氮化硅晶须为原料,通过添加30%(质量分数)成孔剂球形颗粒,以聚乙烯醇作粘结剂,采用干压成型工艺,反应烧结制备了多孔氮化硅陶瓷,分析对比了氮化硅晶须对反应烧结氮化硅多孔陶瓷介电性能的影响.实验结果表明,随着氮化硅晶须加入量的升高,氮化硅多孔陶瓷的介电常数和介电损耗都升高,介电性能恶化.