Ceramic thermal shock failure

Figure 1. For several glass ceramics, the temperature interval causing thermal shock failure, AT, is approximately inversely proportional to the linear coefficient of thermal expansion of these materials. Glasses and alumina ceramics have less thermal shock resistance than glass ceramics of comparable thermal expansion. ...

Fig. 9. Mean thermal shock failure temperature (burner test, minimum 15 units) for 400 cell ceramic monoliths and catalysts of various types.

Aluminum hydroxide is used in stomach antacids (including Maalox , Mylanta , and Delcid ), as a desiccant powder in antiperspirants and dentifrices in packaging materials as a chemical intermediate as a filler in plastics, rubber, cosmetics, and paper as a soft abrasive for brass and plastics as a glass additive to increase mechanical strength and resistance to thermal shock, weathering, and chemicals and in ceramics (HSDB 1995). Aluminum hydroxide is also used pharmaceutically to lower the plasma phosphorus levels of patients with renal failure (Budavari et al. 1989 Sax and Lewis 1987). 

The description of the thermal shock behaviour of CMCs is given with reference to the thermal shock resistance of monolithic ceramic materials. Monolithic ceramics have greater thermal shock sensitivity than metals and can even suffer catastrophic failure due to thermal shock because of an unfavourable ratio of stiffness and thermal expansion to strength and thermal diffusivity, and their limited plastic deformation. 

Alternative approaches, termed indentation thermal shock tests , with pre-cracks of known sizes have been used by several authors to assess thermal shock damage in monolithic ceramics. Knoop (Hasselmann et al., 1978 Faber etal, 1981) or Vickers (Gong etal., 1992 Osterstock, 1993 Andersson and Rowcliffe, 1996 Tancret and Osterstock, 1997 Collin and Rowcliffe, 1999, 2000 Lee et al., 2002) indentations were made on rectangular bars, which were then heated to pre-determined temperatures and quenched into water. Crack extensions from the indentations were measured as a function of quench temperature differential, and the critical temperature for spontaneous crack growth (failure) was determined for the material. Fracture mechanics analyses, which took into account measured resistance-curve (7 -curve) functions, were then used to account for the data trends. 

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)...

The first and foremost step in failure analysis of ceramics consists of identifying the fracture origin and the type of cracking, which throws light on the type of failure such as failure due to impact, residual stress combined with load, thermal shock, improper machining, oxidation and corrosion. This is aided by micro- and macrofracto-graphy, examination of microstructure by SEM, chemical analysis and metallographic examination. 

Figure 22 plots the failure temperature, measured in a cyclic thermal shock test [29], for ceramic catalyst supports as a function of their axial TSP values, which were controlled by modifying either the substrate, the washcoat, or the substrate/washcoat interaction. There is an excellent correlation between the failure temperature and the TSP value. Most automakers call for a failure temperature in excess of 750 C, although this may depend on the size of the catalyst and inlet pipe. Thus, a TSP value of more than 0.4 is required for the coated substrate. Finally, Fig. 22 shows that the washcoat may reduce the failure temperature of the catalyst support by 100-200 C, a trade-off the automakers are well aware of. 

The introduction of surface compressive layers can strengthen ceramics and is a well-established technique for glasses (see Sec. 13.5 for more details). The underlying principle is to introduce a state of compressive surface residual stress, the presence of which would inhibit failure from surface flaws since these compressive stresses would have to be overcome before a surface crack could propagate. These compressive stresses have also been shown to enhance thermal shock resistance and contact damage resistance. 

In the previous section, the emphasis was on thermal shock, where failure was initiated by a rapid andjor severe temperature change. This is not always the case both single- and multiphase ceramics have been known to spontaneously microcrack upon cooling. Whereas thermal shock can be avoided by slow cooling, the latter phenomenon is unavoidable regardless of the rate at which the temperature is changed. 

Ceramics Ceramic-base ablators constitute another class of heat shielding materials. They generally have high thermal efficiency, but this capability is difficult to realize because of their susceptibility to thermal stress failure. During thermal shock, the material may crack extensively and fail catastrophically. Placing the ceramic in a metal honeycomb tends to alleviate this problem by restricting any cracks to the outer walls of the cell structure. 

Thermal shock resistance is the ability of a material to withstand failure due to rapid changes in temperature. Ceramics and glasses are much more likely to develop thermal stress than metals because... 

Thermal stresses depend on the average temperature because the values of the contributing parameters (a, ) change with temperature. Two figures of merit for comparing thermal shock performance are often used JR = (t(1 — v)/ (Ea) the failure temperature difference at fast shock and R = kR, which gives the heat flow limit and is a better criterion at less severe thermal shock. Table 4.11 lists these parameters with the contributing thermal characteristics of several ceramics. 

Thermal shock (or thermoshock) failure is a very prominent failure mode in ceramics. Indeed, based on the fractographic experience of the present authors, it is apparent that more than one-third of all rejections of ceramic components are caused by thermal shock. 

Some examples of the grain-size effect in ceramics are illustrated below Ti3SiC2 was chosen as one exemplar, since this ternary compound exhibits a unique combination of properties. It is a layered material that is as machinable as graphite. At the same time, CG (100-300 pm) samples of Ti3SiC2 have been observed to be damage-tolerant, not susceptible to thermal shock and oxidation resistant. The specimens are fully dense, bulk, single-phase polycrystalline samples of Ti3SiC2. This material exhibits brittle failure characteristics at RT, but is plastic at 1,300 °C with yield points of 300 and 100 MPa under compression and flexure, respectively. 

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