Tensile Creep and Creep Rupture Behavior of Monolithic and SiC-Whisker-Reinforced Silicon Nitride Ceramics

The tensile creep and creep rupture behavior of silicon nitride was investigated at 1200° to 1350°C using hotpressed materials with and without SiC whiskers. Stable steady-state creep was observed under low applied stresses at 1200°C. Accelerated creep regimes, which were absent below 1300°C, were identified above that temperature. The appearance of accelerated creep at the higher temperatures is attributable to formation of microcracks throughout a specimen. The whisker-reinforced material exhibited better creep resistance than the monolith at 1200°C; however, the superiority disappeared above 1300°C. Considerably high values, 3 to 5, were obtained for the creep exponent in the overall temperature range. The exponent tended to decrease with decreasing applied stress at 1200°C. The primary creep mechanism was considered cavitationenhanced creep. Specimen lifetimes followed the Monkman–Grant relationship except for fractures with large accelerated creep regimes. The creep rupture behavior is discussed in association with cavity formation and crack coalescence.     

Tensile Creep and Rupture of Silicon Nitride

We have characterized the tensile creep, rupture lifetime, and cavitation behavior of a commercial, gas-pressure-sintered silicon nitride in the temperature range 1150° to 1400°C and stress range 70 to 400 MPa. Individual creep curves generally show primary, secondary, and tertiary creep. The majority of the primary creep is not recoverable. The best representation of the data is one where the creep rate depends exponentially on stress, rather than on the traditional power law. This representation also removes the need to break the data into high and low stress regimes. Cavitation of the interstitial silicate phase accompanies creep under all conditions, and accounts for nearly all of the measured strain. These observations are consistent with a model where creep proceeds by the redistribution of silicate phase from cavitating interstitial pockets, accommodated by grain-boundary sliding of silicon nitride.     

Creep and Creep Rupture of an Advanced Silicon Nitride Ceramic

Creep and creep rupture behavior of an advanced silicon nitride ceramic were systematically characterized in the temperature range 1150° to 1300°C using uniaxial tensile creep tests. Absence of tertiary creep and the order-of-magnitude breaks in both creep rate and rupture lifetime at certain threshold combinations of stress and temperature were two characteristic features of the creep behavior observed. Thermal annealing was found to have enhanced both subsequent creep resistance and creep rupture life. The stress exponent (n) and the activation energy (Q) defined in the Norton relation were found to be 12.6 and 1645 kJ/mol for the material investigated. Both values appear to fall in the general range of those reported for other but similar types of Si₃N₄ ceramic materials. The stress exponent, m , equivalent to the slope of the Larson–Miller equation was found to be in the range 13 to 14.4, and that defined as p in the Monkman–Grant relation to be 0.91, based on the available experimental data. The values of m , n , and p obtained above approximately support the interrelationship of the three exponents given by p = m/n.     

Results of an International Round-Robin for Tensile Creep Rupture of Silicon Nitride

Fourteen laboratories participated in an interlaboratory study to establish the within- and between-laboratory repeatability of tensile creep rupture of silicon nitride. In air at 1375°C at 200 MPa, the times to failure ranged over a factor of 50, and the minimum creep rates ranged over a factor of 20. Despite these large ranges, taken individually, no one laboratory stands out from any other; all produced equally acceptable data. Consumers of silicon nitride tensile creep data must accept this magnitude of variability in reported creep data. The wide variety of specimen shapes and sizes, gripping systems, extensometry techniques, and temperature measurement strategies makes it impossible to assign definitively the root cause of the variability. However, there was a significant specimen size effect. As a group, the small-diameter specimens lasted roughly five times longer and crept three times more slowly than the large-diameter buttonhead specimens. A possible interpretation of the origin of this difference is that the oxidizing conditions affected more of the volume of the small specimens during the test.     

Comparison of Tensile and Compressive Creep Behavior in Silicon Nitride

The creep behavior of a commercial grade of Si₃N₄ was studied at 1350° and 1400°C. Stresses ranged from 10 to 200 MPa in tension and from 30 to 300 MPa in compression. In tension, the creep rate increased linearly with stress at low stresses and exponentially at high stresses. By contrast, the creep rate in compression increased linearly with stress over the entire stress range. Although compressive and tensile data exhibited an Arrhenius dependence on temperature, the activation energies for creep in tension, 715.3 ± 22.9 kJ/mol, and compression, 489.2 ± 62.0 kJ/mol, were not the same. These differences in creep behavior suggests that mechanisms of creep in tension and compression are different. Creep in tension is controlled by the formation of cavities. The cavity volume fraction increased linearly with increased tensile creep strain with a slope of unity. A cavitation model of creep, developed for materials that contain a triple-junction network of second phase, rationalizes the observed creep behavior at high and low stresses. In compression, cavitation plays a less important role in the creep process. The volume fraction of cavities in compression was ∼18% of that in tension at 1.8% axial strain and approached zero at strains <1%. The linear dependence of creep rate on applied stress is consistent with a model for compressive creep involving solution–precipitation of Si₃N₄. Although the tensile and compressive creep rates overlapped at the lowest stresses, cavity volume fraction measurements showed that solution–precipitation creep of Si₃N₄ did not contribute substantially to the tensile creep rate. Instead, cavitation creep dominated at high and low stresses.     

Elevated-Temperature Creep of Silicon Carbide-Aluminum Nitride Ceramics: Role of Grain Size

Samples containing 50 mol% SiC and 50 mol% AIN were fabricated to neartheoretical density by hot-pressing in graphite dies in N₂ atmosphere. Grain size was varied by varying the hot-pressing conditions. Bar-shaped samples cut from the billets were subjected to creep deformation in four-point bending. Creep was found to depend upon the grain size with coarse-grained material exhibiting lower creep rate. The stress exponent was ∼2.0.     

Creep and Stress Rupture Behavior of an Advanced Silicon Nitride: Part II, Creep Rate Behavior

In Part I of this paper, experimental observations on creep testing of 74 tensile specimens of an advanced silicon nitride were presented. In this part, equations are developed for predicting creep rates in the primary and secondary regimes in the temperature range 1477–1673 K. The resulting model predicts creep strain rates to within a factor of two. The underlying phenomenological basis, which employs an activation energy approach, is discussed. The mechanisms that are likely to be responsible for the transiency of the primary creep regime and for the unique stress and temperature dependencies of the creep rates are explored.     

Si3N4及其复合材料强韧化研究进展

简述了氮化硅陶瓷的结构、性能和制备工艺，并分别通过自增韧补强、纤维／晶须强韧化、层状结构强韧化、相变强韧化以及颗粒弥散强韧化等方法对氮化硅陶瓷的强韧化研究进行了分类叙述。     

Seamless Joining of Silicon Nitride Ceramics

High-strength joining of Si₃N₄ ceramics has been achieved by developing a process that effectively eliminates the seam, and may allow for fabrication of large or complex silicon nitride bodies. This approach to joining is based on the concept that when sintering aids are effective in bonding individual grains, they could be equally effective in joining bulk pieces of Si₃N₄. Optimization of the process led to Si₃N₄/Si₃N₄ joints with room-temperature bend strengths as high as 950 MPa, corresponding to more than 90% of the bulk strength of the Si₃N₄. At elevated temperatures of 1000° and 1200°C joint strengths of 666 and 330 MPa, respectively, were obtained, which are the highest values reported to date for these temperatures. These bend strengths are also more that 90% of the strength of bulk Si₃N₄ measured at these temperatures.     

Liquid-Phase Bonding of Silicon Nitride Ceramics

Mg-Si-O-N glasses were used to bond dense Si₃N₄ ceramic pieces by a liquid-phase diffusion bonding mechanism. In this case, it was difficult to achieve defect-free bonding because, at low nitrogen content in the glass, a large mismatch in thermal expansion coefficient produced cracks perpendicular to the bonding glass layer. With an increase in nitrogen content, the glass layer became frothy and contained "bubbles." However, when a small amount of elemental silicon was added to the glass, volatile reaction was suppressed and intimate bonding was achieved without thermal cracks.     

Electrical Joining of Silicon Nitride Ceramics

The electrical joining of sintered Si₃N₄ ceramics by Joule heating was studied. A mixture of CaF₂/kaolinite (70/30 wt%) with excellent electroheating characteristics and reactivity with Si₃N₄ ceramics was selected as a joining agent. The optimum conditions for electrical joining were determined using this joining agent. Analysis of the joint obtained under optimum conditions revealed that joining was accomplished by the formation of reaction zones and a joining layer through the mutual diffusion of the components in the joining agent and the sintering aids in the Si₃N₄. The joint layer was composed of a glassy substance consisting of Ca─Al─Si─Y─O─(F)─(N) and contained a few particles of β─Si₃N₄. Four-point bend tests indicated that joined bodies could be obtained which maintained a strength of about 300 MPa up to 800°C. Finally, a comparative study was made with a joint obtained using furnace heating. These results indicated that the joints obtained using electrical joining were superior to those produced in the furnace.     

Stress-Enhanced Oxidation of Silicon Nitride Ceramics

Silicon nitride ceramics show an accelerated oxidation rate under load in air. This phenomenon was observed for porous and dense ceramics with and without additives in a wide temperature range (700°–1450°C) and can be interpreted as stress corrosion in oxygen-containing environments. Stresses cause an alteration of the amount and composition of oxidation products, formation of pits and cracks on stressed parts of specimens, and changes of the surface coloration and oxide scale morphology. Both tensile and compressive stresses can affect the oxidation process. An exponential dependence of mass gain on stress was found. On the other hand, oxidation of silicon nitride-based ceramics can affect the material response to mechanical stresses as, for example, deformation, cavitation and cracking. Stress-assisted chemical reaction at lower temperatures and stress-affected diffusion at higher temperatures seem to be the main reasons for the susceptibility of Si₃N₄ ceramics to stress corrosion. The effect of stress corrosion on mechanical properties is discussed.     

氮化硅陶瓷在现代制造业中的应用

本文论述了氮化硅结构陶瓷材料的性能,着重介绍了此种材料在国内外制造业中的应用现状,也讨论了氮化硅陶瓷的缺陷及研究方向。     

Creep Behavior of a Sintered Silicon Nitride

A commercial sintered silicon nitride has been crept in bending and compression at temperatures of 1100°C to 1400°C. In the as-sintered condition the material contains an amorphous intergranular phase. This phase undergoes partial devitrification as a result of high temperature exposure. Preannealing the material to a stable microstructure has very little effect on the creep properties. Deformation behavior compares well with that predicted from a model for creep due to viscous flow of a non-Newtonian grain boundary phase. In bending, the model predicts an initial constant strain rate at low strains as the intergranular phase is squeezed out from between grains under compression. Samples crept in compression are not expected to have this same initial constant strain rate regime. The model also predicts a strong initial strain rate dependence (in bending) on the initial thickness of the amorphous grain boundary layer. Experimentally this strain rate is not affected by partial grain boundary crystallization, suggesting that partial devitrification does not alter the intergranular film thickness or viscosity. This is supported by transmission electron microscopy, which has shown that crystallization of the intergranular phase occurs largely in the pockets between grains, leaving amorphous films between grains.     

Creep and Stress Rupture Behavior of an Advanced Silicon Nitride: Part I, Experimental Observations

Cylindrical buttuohead specimens of an advanced silicon nitride were tested in uniaxial tension at temperatures between 1422 and 1673 K. In the range 1477 to 1673 K, creep deformation was reliably measured using high-temperature contact probe extensometry. Extensive scanning and transmission electron microscopy has revealed the formation of lenticular cavities at two-grain junctions at all temperatures (1422–1673 K) and extensive triple-junction cavitation occurring at the higher temperatures (1644–1673 K). Cavitation is believed to be part of the net creep process. The stress rupture data show stratification of the Monkman–Grant lines with respect to temperature. Failure strain increased with increase in rupture time or temperature, or decrease in stress. Fractography showed that final failure occurred by subcritical crack growth in all specimens.     

Sintering Silicon Nitride Ceramics in Air

Silicon nitride (Si₃N₄) ceramics, prepared with Y₂O₃ and Al₂O₃ sintering additives, have been densified in air at temperatures of up to 1750°C using a conventional MoSi₂ element furnace. At the highest sintering temperatures, densities in excess of 98% of theoretical have been achieved for materials prepared with a combined sintering addition of 12 wt% Y₂O₃ and 3 wt% Al₂O₃. Densification is accompanied by a small weight gain (typically <1–2 wt%), because of limited passive oxidation of the sample. Complete α- to β-Si₃N₄ transformation can be achieved at temperatures above 1650°C, although a low volume fraction of Si₂N₂O is also observed to form below 1750°C. Partial crystallization of the residual grain-boundary glassy phase was also apparent, with β-Y₂Si₂O₇ being noted in the majority of samples. The microstructures of the sintered materials exhibited typical β-Si₃N₄ elongated grain morphologies, indicating potential for low-cost processing of in situ toughened Si₃N₄-based ceramics.     

Electrically Conductive CNT-Dispersed Silicon Nitride Ceramics

Carbon nanotube (CNT)-dispersed Si₃N₄ ceramics with electrical conductivity were developed based on the lower temperature densification technique, in which the key point is the addition of both TiO₂ and AlN as well as Y₂O₃ and Al₂O₃ as sintering aids. This new ceramic with a small amount of CNTs exhibits very high electrical conductivity in addition to high strength and toughness. Since Si₃N₄ ceramics with Y₂O₃–Al₂O₃–TiO₂–AlN were originally used as a wear material, electrically conductive Si₃N₄ ceramics are expected to be applied for high-performance static-electricity-free bearings for aerospace and other high-performance components.     

Creep and Stress Rupture Behavior of an Advanced Silicon Nitride: Part III, Stress Rupture and the Monkman–Grant Relationship

The applicability of the Monkman–Grant relationship to predict the stress rupture life of NT154 silicon nitride is examined. The data show that the Monkman–Grant lines relating rupture life to minimum creep rate are stratified with respect to temperature. A modification to the current expression for the Monkman–Grant relation is proposed to accommodate this temperature dependence. A phenomenological approach based on crack growth as the failure mechanism is presented to explain this temperature dependence. The results can be interpreted as suggesting that the stress dependence of the failure process is greater than that for creep.     

Shear Thickening Creep in Superplastic Silicon Nitride

A novel shear-thickening phenomenon has been observed in superplastic silicon nitrides compression tested between 1500° and 1600°C. Liquid-enhanced creep of SiAlONs undergoes a transition from Newtonian behavior to shear-thickening behavior at a characteristic stress, with the strain rate sensitivity increasing from unity to around 2. The transition stress is always around 20 MPa, even though the Newtonian flow stress is very sensitive to temperature, grain size, and phase composition. Rheopexic hysteresis, manifested as a slow stress relaxation to a steady-state value after a strain rate decrease, was also observed in the shear-thickening regime. We attribute the cause for shear thickening to a repulsive force between initially wetted SiAlON grains, which form a "dry" and "rigid" bridge in between when pressed above a characteristic stress, possibly due to the contact of the residue Stern layers on the opposing grain/liquid interfaces. A micromechanical model, which takes into account the stress variation among differently oriented grain boundaries, has been formulated to assess the effect of "rigid" grain boundaries. A continual stochastic rearrangement of grain configurations and a relatively thick Stern layer are suggested as the necessary prerequisites for shear thickening in liquid-enhanced creep.     

Fracture Behavior of Multilayer Silicon Nitride/Boron Nitride Ceramics

The fracture behavior of multilayer Si₃N₄/BN ceramics in bending has been studied. The materials were prepared by a process of tape casting, coating, laminating, and hot pressing. The Si₃N₄ layers were separated by thin, weak BN interlayers. Crack patterns in bending bars were examined with a scanning electron microscope. The weak layers deflected cracks in bending and thus prevented catastrophic failure. In one well-aligned multilayer ceramic A, a main crack propagated through the specimen although along a zigzag path. A second multilayer ceramic B was made to simulate a wood grain structure. Its failure was dominated by shear cracking along the weak BN layers. Besides crack deflection, interlock bridging between toothlike layers in the wake of the main crack appeared also to contribute to toughening.