Silicon nitride (Si 3 N 4 ) based ceramics are gaining more and more attention due to their promising high-temperature thermal and mechanical properties. They have been expected to be the main candidates for applications such as turbocharger rotors and gas turbine engine components which can withstand severe conditions of temperature and heavy loads. Although a big number of studies on silicon nitride are published, a continuous progress in monolithic Si 3 N 4 as well as Si 3 N 4 /Si 3 N 4 composites (seeded materials) leads to new scientific and technological data providing new insight that should be reviewed taking into account their excellent properties at high temperatures. Silicon nitride possesses a bunch variety of interesting properties that can be specifically designed to produce a given behavior profile. That is why the room temperature and high-temperature properties are discussed and described in more detail.

Mechanical properties of silicon nitride-based ceramics and its use in structural applications at high temperatures

Monotonic tension, fatigue and creep behavior of SiC-fiber-reinforced SiC-matrix composites: a review

This paper compares the relative merits of liquid-phase sintered β -Si 3 N 4 with sintered α -SiC for high-temperature applications. These materials represent two extremes of ceramic microstructure: liquid-phase sintered β -Si 3 N 4 contains grains that are coated by a second phase, whereas sintered α -SiC contains grains that are in direct crystalline contact. As will be shown, the mechanical behavior of the two materials differs substantially. At temperatures up to 1500 °C, sintered α -SiC is a creep-resistant solid. At room temperature, however, it is brittle, K Ic =(2–4) MPa·m 1/2 , and has a low bending strength, σ b =(400–500) MPa. By contrast, liquid-phase sintered β -Si 3 N 4 is not as creep resistant since it contains a residual sintering aid at its grain boundaries that deforms at a lower temperature than the silicon nitride grains. Hence, its temperature capability is less than that of sintered α -SiC. Silicon nitride is, however, tougher, K Ic =(6–8) MPa·m 1/2 , and stronger, σ b =(700–1000) MPa, than sintered α -SiC. Deformation of liquid-phase sintered β -Si 3 N 4 , and other ceramics with a second phase at the grain boundaries, depends on the refractoriness of that phase, the more refractory the phase, the more resistant the material is to creep. Experimental results on β -Si 3 N 4 suggest that toughness decreases as creep resistance increases; hence, a trade-off must be made between creep resistance and material toughness to achieve an optimal high temperature microstructure.

Mechanisms of deformation of silicon nitride and silicon carbide at high temperatures

The tensile creep rate of most commercial grades of Si3N4 increases strongly with stress. Although the usual power-law functions can represent the creep data, the data often show curvature and systematic variations of slope with temperature and stress. In this article, we present a new approach to understanding the creep of ceramics, such as Si3N4, where a deformable second phase bonds a deformation-resistant major phase. A review of experimental data suggests that the rate of formation and growth of cavities in the second phase controls creep in these materials. The critical step for deformation is the redistribution of the second phase away from the cavitation site to the surrounding volume. The effective viscosity of the second phase and the density of active cavities determine the creep rate. Assuming that the hydrostatic stresses in pockets of the second phase are normally distributed leads to a model that accurately describes the tensile creep rate of grades of Si3N4. In this model, the creep rate increases exponentially with the applied stress, is independent of Si3N4 grain size, is inversely proportional to the effective viscosity of the deformable phase, and is proportional to the cube of the volume fraction of the deformable phase.

A New Model for Tensile Creep of Silicon Nitride

Metastable Si-B-C-N ceramics and their matrix composites developed by inorganic route based on mechanical alloying: Fabrication, microstructures, properties and their relevant basic scientific issues

Microstructural defects in a commercial self-reinforced silicon nitride (SN 88M), after tensile creep at 1200°-1400°C under stresses that correspond to lifetimes that range from several hours to 10000 h, were investigated by using electron microscopy methods to reveal the type of cavities and mechanisms that control creep deformation. Creep damage is classified in terms of intergranular cavitation in the amorphous boundary phase and intragranular defects that are connected to the Si3N4 grains. Intergranular defects include irregular multigrain-junction cavities, oblate two-grain-junction cavities, microcracks, and creep cracks. Intragranular defects involve broken large Si3N4 grains, small symmetrical cavities of lenticular shape that are formed between two matrix grains, and small asymmetrical cavities and large cracklike cavities that both penetrate into the large Si3N4 grains. Very similar intergranular cavities and broken grains have been observed after both short-term and long-term tests, whereas intragranular cavities have been observed only in long-term tests. Cavitation in multigrain junctions is thought to be controlled by dilatant hydrostatic tensile stresses that develop in dense microstructures after any grain-boundary sliding (GBS). Different sliding rates between large reinforcing grains and small matrix grains have been proposed to generate local tensile stresses that result in cavitation on the facets of large grains. The local tensile stresses that occur at irregularities of two sliding interfaces seem to control the formation of two-grain-junction cavities. Although the controlling stresses differ, GBS and viscous flow are the common mechanisms for all types of intergranular cavities. Intragranular cavitation is fundamentally different and requires local dissolution of Si3N4 in the surrounding glassy phase. Comparison of the apparent density and dimensions of different types of cavities, even without direct measurement of cavitation kinetics, suggests that the multigrain-junction cavities provide the main contribution to the total volume fraction of cavities, whereas two-grain-junction cavities and the lenticular-type cavities have minor importance. The role of intragranular cavities in the large grains in cavitational strain is negligible. Full-facet-size cavities and multigrain-junction cavities can form within a period of <1 h. Numerous very small cavities, which coexist with the large cavities of both types that are observed after very long creep tests, indicate a continuous cavitation during the entire creep deformation. By using a dilatational model, tensile strain has been proposed to result from the continuous addition of multigrain-junction cavities of approximately uniform size, which explains the linear dependence of cavitational strain on the tensile strain that has been reported for this material. GBS and viscous flow, which are considered as being the mechanisms for intergranular cavitation, are controlled by the amount and viscosity of the residual amorphous phase. Therefore, improvement of the creep resistance in silicon nitride ceramics can be expected from the engineering of the residual glass.

Creep Damage in an Advanced Self-Reinforced Silicon Nitride: Part I, Cavitation in the Amorphous Boundary Phase

Hip joint replacements represent the most effective way of treatment for patients suffering from joint diseases. Despite the rapid improvement of implant materials over the last few decades, limited longevity associated with wear-related complications persists as the main drawback. Therefore, improved tribological performance is required in order to extend the service life of replacements. The effect of surface texturing of ultra-high molecular weight polyethylene (UHMWPE) acetabular cup was investigated in the present study. Unique tilling method was utilized for manufacturing the dimples with controlled diameter and depths on the contact surface of the cup. The experiments with four commercial femoral components and two model lubricants were realized. The main attention was paid to a coefficient of friction considering the differences between the original and the dimpled cups. Results showed remarkable lowering of friction, in general. Focusing on the simulated human synovial fluid, friction was reduced by 40% (alumina ceramic), 38.8% (zirconia toughened ceramic), 25.5% (metal), and 9.9% (oxinium). In addition, the dimples helped to keep the friction stable without fluctuations. To conclude, the paper brings a new insight into frictional behaviour of the hip replacements during running-in phase which is essential for overall implant lifespan. It is believed that proper surface texturing may rapidly improve the life quality of millions of patients and may lead to considerable financial savings.

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Running-in friction of hip joint replacements can be significantly reduced: The effect of surface-textured acetabular cup

The present study describes tribological properties of copper based porous alloy containing nano-sized sulfide particles The porous alloy was fabricated with sintering on the steel plate using the developed powders consisted from Cu with dispersed sulfides The micro structure of the alloys consisted from micro sized open pores and sintered particles The measurement of tribological properties were carried out with a ring on disc testing apparatus with a steel ring as the mating specimen First, comparison of friction behavior between the developed material and the lead-bronze alloy used as traditional materials for bearings are evaluated As results, the friction coefficient of the developed material was stable and lower than that of the lead one and larger adhesion resulted in the higher friction resistance was found on the ring surface In order to enhance porous structure on the reduction and stabilization of the friction resistance, graphite powder was penetrated into the pores by rubbing with the graphite rod It was found that the further reduction of friction resistance was obtained

Tribological Properties of Porous Cu Based Alloy Containing Nano Sized Sulfide Particles

Advanced silicon nitride ceramics are candidate materials for high temperature structural parts in the future generation of gas turbines and reciprocal engines with increased efficiency [1]. The applications of silicon nitride in ceramic rotors require high endurance of intensive tensile stresses acting over prolonged periods. Consequently, detailed characterization of creep behavior and creep lifetime data are necessary to achieve longterm reliability of the material at working conditions and for the designing of such parts.

Long-Term Creep Damage Development in a Self-Reinforced Silicon Nitride

Tribological properties of copper based porous alloy with penetrated micro size graphite layer were evaluated using a ring on disc testing apparatus in dry condition. A shot peening treatment was applied to ensure the dimple for the graphite penetration. Specimens with various area fraction of the graphite layer were prepared. It was found that the friction coefficient and the wear loss decreased with the increase in the area fraction. However, the decrease of the friction and the wear loss became smaller at excess graphite area fraction.

Hatsuhiko Usami

Papers

Creep Damage in an Advanced Self-Reinforced Silicon Nitride: Part I, Cavitation in the Amorphous Boundary Phase

Running-in friction of hip joint replacements can be significantly reduced: The effect of surface-textured acetabular cup

Tribological Properties of Porous Cu Based Alloy Containing Nano Sized Sulfide Particles

Long-Term Creep Damage Development in a Self-Reinforced Silicon Nitride

Effects of Penetrated Graphite on Tribological Properties of Copper Based Journal Bearing