Thermal shock resistance parameters

It is useful to define parameters that indicate the resistance of a material to thermal shock. The difficulty with this approach is that the thermal stresses 

For extremely rapid cooling, the maximum stress from Eq. (9.21) can be set equal to the strength cr of the material and a TSR parameter 2ft can be defined as the critical temperature difference that causes failure, i.e.. 

It is not immediately clear which type of testing geometry should be associated with the strength value but the biaxial nature of the stress field would suggest that biaxial flexure values might be the most appropriate estimate. 

The very rapid cooling rates needed to define dt values do not occur often and, thus, a second TSR parameter 2ft is sometimes used. This approach introduces the other important material parameter, thermal conductivity. For slow cooling rates, it is found that the stress reduction factor is 0.31 jS and is, thus, inversely dependent on thermal conductivity. If this condition is introduced, 2ft is defined 

F ure9.13 The variation in the thermal conductivity of ceramics with temperature can be an important factor in the determination of thermal shock resistance. 

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Chaklader, A.C.D. and Bradley, F., Thermal shock resistance parameters and their application to refractories , UNITECR 89, 1989. 

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)...

Table 9.2 Typical thermal shock resistance parameters for engineering ceramics (Adapted from Davidge, 1979.)...

Table II. Thermal shock resistance parameters of pressureless sintered SiC-AiN-YaOj composites (this study ) and GPS SiC-AlN-Y203 ceramics (data extracted from Rixecker et al. ).

The capacity of a material to withstand this kind of failure is termed its thermal shock resistance. For a ceramic body that is cooled rapidly, the resistance to thermal shock depends not only on the magnitude of the temperature change, but also on the mechanical and thermal properties of the material. The thermal shock resistance is best for ceramics with high fracture strengths and high thermal conductivities, as well as low moduli of elasticity and low coefficients of thermal expansion. The resistance of many materials to this type of failure may be approximated by a thermal shock resistance parameter TSR ... 

D3 (a) What are the units for the thermal shock resistance parameter (TSR) ... 

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]. 

For material initially undamaged, the appropriate parameter expressing the tendency for cracks to be developed, and therefore strength to be lost, can be considered to be that for crack initiation. This has been expressed in terms of thermal stress resistance parameters.25,30,52,86-88 Kingery used the infinite slab symmetrically heated or cooled with a constant heat transfer coefficient to derive thermal shock fracture resistance parameters R, R and fusing the equations ... 

Before embarking on a detailed consideration of the application of dielectrics and insulators, it is opportune to focus attention briefly on dielectric strength and thermal shock resistance . Both properties demand careful consideration in certain applications of dielectrics and insulators. They are by no means simple to define and, generally speaking, it is necessary only to develop some appreciation of how component and operational parameters determine them. 

Chemical vapor deposition (CVD) is an atomistic surface modification process where a thin solid coating is deposited on an underlying heated substrate via a chemical reaction from the vapor or gas phase. The occurrence of this chemical reaction is an essential characteristic of the CVD method. The chemical reaction is generally activated thermally by resistance heat, RF, plasma and laser. Furthermore, the effects of the process variables such as temperature, pressure, flow rates, and input concentrations on these reactions must be understood. With proper selection of process parameters, the coating structure/properties such as hardness, toughness, elastic modulus, adhesion, thermal shock resistance and corrosion, wear and oxidation resistance can be controlled or tailored for a variety of applications. The optimum experimental parameters and the level to which... 

The development of SAN was triggered by the idea of building a polar comonomer into polystyrene to improve its resistance to chemicals and to stress cracking. The relatively polar acrylonitrile presented itself as a suitable comonomer in this case. Styrene-acrylonitrile copolymers are further characterized by high rigidity and thermal shock resistance. Two parameters substantially determine the properties of SAN molecular weight and the proportion of acrylonitrile. 

One parameter which is not included in either model, and which clearly must have an important effect on thermal shock resistance, is the thermal conductivity of the ceramic (see Sec. 13.6). Given that thermal gradients are ultimately responsible for the buildup of stress, it stands to reason that a highly thermally conductive material would not develop large gradients and would thus be thermal shock resistant. For the same reason, the heat capacity and the heat-transfer coefficient between the solid and the environment must also play a role. Thus an even better indicator of thermal shock... 

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