Ceramic inertia

SiHcon nitride (see Nitrides) is a key material for stmctural ceramic appHcations 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, exceUent abrasion and corrosion resistance, low density, and, consequently, a low moment of inertia. Additionally, siHcon nitride is made from abundant raw materials. 

Figure 13 Schematic of ceramic catalyst support subjected to inertia, back pressure, and frictional forces. (From Ref. 44, courtesy of SAE.)...

This ceramic fiber-lined furnace has a low thermal inertia and can operate at a wall temperature up to 1600°C. The combustion chamber is 6.0 m long, and has a rectangular cross section of 1.5 by 2.0 m. These internal dimensions are large enough to minimize flame confinement effects and allow investigating the interaction between the flame aerodynamics and the furnace... 

The mechanical properties of polycrystalline ZnO ceramics are of special importance for their applications as varistors (see Section 1.4.4.1). When a varistor experiences a high-current pulse, the electrical energy is quickly converted to heat. The inertia of the material, which resists its thermal expansion, and the resonances of the resultant elastic waves in the block, may lead to microcracks and finally to mechanical failure [107]. Characteristic values of mechanical properties of ZnO ceramics are 1.2-1.4 MPa m for fracture toughness, and 100-125 MPa for flexural strength. 

The great chemical inertia of the ceramics, their high compression strength and then-aesthetic appearance, caused that these materials were begun to use in dentistry, mainly in dental crowns. Later their use extended to orthopaedic applications [4-6]. 

Due to their chemical nature, MIP present some attractive properties, compared to metals and organic polymers chemical inertia, temperature resistance, low energy input, incombustibility. On the other hand they present, just as other ceramics, an important lack of toughness and thus a brittle behaviour and limited tensile strength. 

Silicate ceramics are generally alumino-silicate based materials obtained from natural raw materials. They exhibit a set of fundamental properties, such as chemical inertia, thermal stability and mechanical strength, which explain why they are widely used in construction products (sanitary articles, floor and wall tiles, bricks, tiles) and domestic articles (crockery, decorative objects, pottery). They are often complex materials, whose usage properties depend at least as much on microstructure and aesthetics as on composition. Silicate products with an exclusively technical application (refractory materials, insulators or certain dental implants) will not be explicitly discussed in this chapter. 

Silicate ceramics are very old materials that are very widely used in society thanks to their undeniable assets, namely their high mechanical strength, excellent chemical inertia and esthetic qualities. A common feature is that they are obtained... 

For many applications, surface properties, such as hardness, coupled with high chemical inertia are required. Non-oxide ceramics have the required characteristics, but their sintering necessitates temperatures that are often incompatible with the thermal stability of the support to be protected. Methods for synthesizing these materials in the form of depositions or coatings have been developed considerably during the last few decades [WEI 92]. They can be divided into two groups ... 

High performance ceramics (AI2O3, SiC, Si3N4), pure or reinforced by fiber, have a lower density, a higher chemical inertia and often greater hot hardness than sintered hard metals. However, in the majority of cases, a minimum of toughness must be guaranteed and it is always the sintered hard metal that is finally chosen. 

Alumina is one of the most widely used multipurpose ceramics. It is essentially used in orthopedics for its good tribological properties and its outstanding chemical inertia. One of the advantages of alumina is that it is a very bad substrate for the crystalline growth of calcium phosphates, which can alter other friction couples [ROY 93]. It constitutes the heads of femoral prostheses and is used also in the development of the acetabulum. Early applications raised some problems of... 

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