Mechanical properties shock resistance

C/C composites find increasing use in industry both at medium and high temperatures in place of asbestos or monolithic graphite. Although much more expensive than the materials they replace, C/C composites display higher mechanical properties, shock resistance and durability. 

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. 

These alloys have corrosion resistance similar to that of copper, with mechanical properties equivalent to mild steel. Because silicon bronzes do not generate sparks under shocks, they can be used in the fabrication of explosion-proof equipment. Compared to tin bronzes, the tinless bronzes have a higher shrinkage (1.7-2.5% against 1.3-1.5% of tin bronzes) and less fluid-flow, which is an important consideration in designing. 

Two other alloys which have been used for their good oxidation- and growth-resistance are Cralfer (Fe-7Al-0-75Cr) and Fe-14-5Si. The production of the former, however, entails considerable difficulties while the latter has poor mechanical properties and poor resistance to thermal shock, with the result that neither is extensively used for this purpose today. 

In inert atmospheres the mechanical properties of RBSN are constant up to 1200-1400 °C because of the absence of a glassy grain boundary phase, which is also the reason for the excellent thermal shock and creep behaviour. The thermal shock resistance, hardness and elastic constants depend on the microstructural parameters but are much lower than for dense Si3N4 ceramics [539]. 

Table 15.1 Values of the thermal shock resistance parameters R, R, R"" for a range of ceramic materials where HPSN is hot pressed silicon nitride and RBSN is reaction bonded silicon nitride (reprinted from Table 11.1 on p 213 of Ceramics Mechanical Properties, Failure Behaviour, Materials Selection by Munz and Fett, 1999, published with permission from Springer-Verlag GmbFI)...

Mechanical properties (stiffness, impact, and shock resistance), electrical properties, clarity, ease of processing, sealing properties, dimensional stability, resistance to hydrocarbons and other organic liquids, resistance to plasticizers, greases, and oils are the characteristic properties desired. In its mechanical properties it approaches polycarbonate most closely. However, since polycarbonate is cheaper than Trogamid T, Trogamid T will be preferred only where polycarbonate cannot be used. 

Diamond electrode — Electrode made from - boron-doped diamond or similar doped diamond materials. Diamond, natural or synthetic, exhibits some exceptional material properties such as low thermal expansion, optical transparency for visible, IR, and microwave radiation, mechanical strength, and shock resistance [i]. 

The characteristic threshold behaviour of conductive composites is an unsatisfactory feature for many applications, both because the poor mechanical properties incurred at concentrations where conduction is obtained cannot be tolerated, and because the high level of conductivity above the threshold is not required or even not desirable. Thus a common requirement is for an antistatic material which has good plastics properties, sufficient conductivity to allow charges to leak away, and sufficient resistivity to prevent dangerous shocks to personnel who may become accidentally connected to mains electrical supplies through it. Unfortunately, the very steep slope of the conductivity versus filler concentration curve in the region of the threshold makes it very difficult to manufacture materials with reliable intermediate conductivities. 

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