Structural ceramics nitride

The very hard structural ceramics silicon carbide, SiC, and silicon nitride, Si3N4 (used for load-bearing components such as high-temperature bearings and engine... 

A.1 Fundamentals of Vapor Phase Synthesis. In this section we will concentrate on the vapor phase synthesis of some structural ceramics, such as carbides and nitrides. The principles described here apply equally well to the production of oxide ceramics, but we reserve some of this description for later sections, particularly with respect to the formation of optical fibers. 

G. D. Quinn and W. R. Braue, Fracture Mechanism Maps for Advanced Structural Ceramics, Part 2, Sintered Silicon Nitride, /. Mater. Sci., 25, 4377-4392 (1990). 

Properties. Properties of structural silicon nitride ceramics are given in Table 2. These values represent available, well-tested materials. However, test methodology7 and the quality of the specimens, particularly7 their surface finish, can affect the measured values. Another important material property is tensile strength. Values obtained on Norton s NT154 material are 750 MPa at RT, 500 MPa at 1200°C, and 350 MPa (50,000 psi) at 1400°C (62). 

The industrial ceramics produced by conventional sintering of 8-phase Si3N4 powders synthesized by SHS (Petrovskii et al, 1981) can be used as high-temperature articles with attractive dielectric properties (tan6=4.4 10 at/= 10 Hz, and dielectric strength, Ea=9.2 kV/mm). Also, silicon nitride powders with a relatively high a-phase content ( 80%) have been used for the production of advanced structural ceramics with good mechanical properties ([Pg.109]

Non-oxide ceramic materials such as silicon carbide has been used commercially as a membrane support material and studied as a potential membrane material. Silicon nitride has also the potential of being a ceramic membrane material. In fact, both materials have been used in other high-temperature structural ceramic applications. Oxidation resistance of these non-oxide ceramics as membrane materials for membrane reactor applications is obviously very important. The oxidation rate is related to the reactive surface area thus oxidation of porous non-oxide ceramics depends on their open porosity. The generally accepted oxidation mechanism of porous silicon nitride materials consists of two... 

For many traditional ceramics such as structural elements (tiles, bricks, etc.), white-wares, (tableware, sanitaryware, etc.), and common refractories, the raw materials are naturally occurring minerals, and moderate levels of impurities are tolerated. More specialized technical ceramics such as electronic ceramics (substrates, electronic packages, capacitors, inductors, etc.) or high performance structural ceramics (silicon carbide, silicon nitride, etc.) demand low or controlled levels of impurities and make use of higher purity powders often made by more specialized techniques. 

Silicon nitride (see Nitrides) is a key material for structural ceramic applications in environments of high mechanical and thermal stress such as in vehicular propulsion engines. Properties which make this material uniquely suitable are high mechanical strength at room and elevated temperatures, good oxidation and creep resistance at high temperatures, high thermal shock resistance, excellent abrasion and corrosion resistance, low density, and, consequendy, alow moment of inertia. Additionally, silicon nitride is made from abundant raw materials. 

The tensile fracture strengths of three different structural ceramics are listed below hot-pressed silicon nitride (HPSN), reaction-bonded silicon nitride (RBSN), and chemical vapor-deposited silicon carbide (CVDSC), measured at room temperature. 

For reactive metals, sintering is performed under vacuum or under inert atmospheres containing sufficiently low partial pressures of gases containing oxygen, carbon and nitrogen in order to minimize formation of brittle bulk oxides, carbides and nitrides. In some cases, however, residual thin adherent oxides of the reactive metals may be beneficial and act as intermediary layers to aid wetting with the structural ceramic oxide in the cermet. 

Silicon nitride is receiving considerable attention as a structural ceramic, which requires a fully dense material, but it is also of interest to consider whether it could be prepared in high surface area forms. Our work in this direction has led to a synthesis of a high surface area silicon oxynitride. We summarise here the synthetic work and then report data on the stability of the surface. 

Now HlPing is used for a wide variety of ceramic (and metallic) components, such as alumina-based tool bits and the silicon nitride nozzles used in flue-gas desulfurization plants by the utility industry. The advantages of the HlPing process are becoming more important as interest in structural ceramics such as Si3N4 grows. 

Hard materials available for the production of bulk components fall into two major groups hard metals , used primarily in the manufacture of cutting tools and related applications, and structural ceramics, primarily the oxides, carbides, borides, or nitrides of the low atomic number cations. Relatively few materials are of engineering importance, and we will list these, explaining briefly why similar compositions are less useful. 

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