Thermal shocks

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 

Rapid cooling of a ceramic or a glass is more likely to inflict thermal shock than heating, since the induced surface stresses are tensile. And, as you know, crack formation and propagation from surface flaws are more probable when the imposed stress is tensile. 

The thermal shock resistance (/ ts) of a material can be estimated using 

Values to calculate Rjs for several ceramics are given in Table 34.11. As an example, the Rjs of SiC is 2.3 x 10 W/m, while that of AI2O3 is 740W/m Ceramics such as SiC and Si3N4, which also have a high Rjs. are the most useful for components that are loaded at high temperature. 

The thermal properties of ceramics, like many of their other physical properties, vary over a very wide range. A good example is that of thermal conductivity. Diamond, a ceramic material, has the highest known thermal conductivity, whereas the thermal conductivity of a multiphase ceramic such as partially stabilized zirconia is three orders of magnitude lower. Thermal properties of ceramics are dominated primarily by the nature of the interatomic bonding (bond strength and ionicity). In practical ceramics we, of course, have to consider the presence of defects, impurities, and porosity as these all affect thermal properties. 

Generally speaking, thermal stresses are to be avoided since they can significantly weaken a component. In extreme cases, a part can spontaneously crumble during cooling. As noted earlier, rapid heating or cooling of a 

Experimental Details Measuring Thermal Shock Resistance 

From a practical point of view, it is important to be able to predict AT. . Furthermore, it is only by understanding the various parameters that affect thermal shock that successful design of solids which are resistant to it can be carried out. In the remainder of this section, a methodology is outlined for doing just that, an exercise that will by necessity highlight the important parameters that render a ceramic resistant to thermal shock. 

The material contains N identical, uniformly distributed, Griffith flaws per unit volume. 

The body is uniformly cooled with the external surfaces rigidly constrained to give a well-defined triaxial tensile state of stress given by  

According to Eq. (1.26), the rapid cooling of a cathode carbon block from 900 to 100 °C gives thermal stress that is sufficiently higher than the bending strength. The same is true for sdicon carbide material. 

Thermal stresses in materials can introduce cracks. The critical temperature drop — after which the thermal stresses in the material will equal or exceed the 

Strength — may be considered the critical quench temperature and may be used for the ranking of the material to withstand the rapid change of the temperature, or thermal shock  

This equation was introduced by Hasselman [109,110], who was the first to give the quantitative criteria for thermal shock resistance, or R factor. 

Thermal shock resistance is the ability of the material to withstand temperature drops and increases, that is, to withstand thermal shock. Thermal shock resistance is one of the most important characteristics of refractory materials. According to rough estimations, approximately one third of refractories are damaged due to 

Although the stresses formed during cooling are temporary, failure can occur due to the high stress that occurs when the surface and bulk temperatures differ. The maximum possible stress will be generated if the surface is instantaneously cooled, while the bulk is still at the original temperature. Under these conditions, the stress is given by the simple expression  

In practical situations, one is usually more interested in the maximum possible AT that can exist without failure of the glass. By rearranging Eq. 9.13, we can write the expression  

In reality, an instantaneous cooling rate cannot be obtained for a sample of finite size. If we consider the case of a plate cooled at a constant rate, in K s , we will still generate a parabolic thermal gradient through the thickness of the plate. If the material has a thermal 

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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. 

Uses. Lithium fluoride is used primarily in the ceramic industry to reduce firing temperatures and improve resistance to thermal shock, abrasion, and acid attack (see Ceramics). Another use of LiF is in flux compositions with other fluorides, chlorides, and borates for metal joining (17) (see Solders). 

The upper use temperature for annealed ware is below the temperature at which the glass begins to soften and flow (about Pa-s or 10 P). The maximum use temperature of tempered ware is even lower, because of the phenomenon of stress release through viscous flow. Glass used to its extreme limit is vulnerable to thermal shock, and tests should be made before adapting final designs to any use. Table 4 Hsts the normal and extreme temperature limits for annealed and tempered glass. These data ate approximate and assume that the product is not subject to stresses from thermal shock. 

Thermal Properties. Many commercial glass-ceramics have capitalized on thek superior thermal properties, particularly low or zero thermal expansion coupled with high thermal stabiUty and thermal shock resistance properties that are not readily achievable in glasses or ceramics. Linear thermal expansion coefficients ranging from —60 to 200 x 10 j° C can be obtained. Near-zero expansion materials are used in apphcations such as telescope mirror blanks, cookware, and stove cooktops, while high expansion frits are used for sealing metals. 

The interelectrode insulators, an integral part of the electrode wall stmcture, are required to stand off interelectrode voltages and resist attack by slag. Well cooled, by contact with neighboring copper electrodes, thin insulators have proven to be very effective, particularly those made of alumina or boron nitride. Alumina is cheaper and also provides good anchoring points for the slag layer. Boron nitride has superior thermal conductivity and thermal shock resistance. 

Because of its high modulus of elasticity, molybdenum is used in machine-tool accessories such as boring bars and grinding quills. Molybdenum metal also has good thermal-shock resistance because of its low coefficient of thermal expansion combined with high thermal conductivity. This combination accounts for its use in casting dies and in some electrical and electronic appHcations. 

Glass offers good resistance to strong acid at high temperatures. However, it is subject to thermal shock and a gradual loss in integrity as materials such as iron and siUca are leached out into the acid. Nonmetallic materials such as PTFE, PVDC, PVDF, and furan can be used for nitric acid to a limited degree, but are mainly restricted to weak acid service at ambient to moderate temperatures. 

Sihcones (qv) have an advantage over organic resias ia their superior thermal stabiUty and low dielectric constants. Polyurethanes, when cured, are tough and possess outstanding abrasion and thermal shock resistance. They also have favorable electrical properties and good adhesion to most surfaces. However, polyurethanes are extremely sensitive to and can degrade after prolonged contact with moisture as a result, they are not as commonly used as epoxies and sihcones (see Urethane polymers). 

Tips of platinum, platinum—nickel alloy, or iridium can be resistance-welded to spark-plug electrodes for improved reHabiHty and increased lifetime. These electrodes are exposed to extremely hostile environments involving spark erosion, high temperature corrosion, thermal shock, and thermal fatigue. 

Silicon carbide has very high thermal conductivity and can withstand thermal shock cycling without damage. It also is an electrical conductor and is used for electrical heating elements. Other carbides have relatively poor oxidation resistance. Under neutral or reducing conditions, several carbides have potential usehilness as technical ceramics in aerospace appHcation, eg, the carbides (qv) of B, Nb, Hf, Ta, Zr, Ti, V, Mo, and Cr. Ba, Be, Ca, and Sr carbides are hydrolyzed by water vapor. 

Sihcon nitride has good strength retention at high temperature and is the most oxidation resistant nitride. Boron nitride [10043-11 -5] has excellent thermal shock resistance and is in many ways similar to graphite, except that it is not an electrical conductor. 

The modulus of elasticity (MOE) is related to the strength and can be used as a nondestmctive quaUty control test on high cost special refractory shapes such as sHde gate valves employed in the pouring of steel (qv). The sHde gate type must be selected to ensure chemical compatibiUty and it must be used in a way to reduce thermal shock. The performance of a properly selected and used sHde gate is direcdy related to its strength and therefore predicted by its MOE. 

The resistance against thermal spalling of fireclay and high alumina brick is indicated in Table 5. No standard test has been adopted for basic brick. Refractories composed of 100% magnesia exhibit poor thermal shock resistance, which is improved by addition of chrome ore. So-called direct bonded basic brick, composed of magnesia and chrome additions, exhibits good thermal shock resistance. 

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