This report discusses a demonstration which is suitable for students in materials science at the high school or college levels. The demonstrations presents the concept of thermal shock in ceramic materials, and illustrates the pertinent factors of materials selection

Bunnell, L. R.

UNT Digital Library

PNL-SA---20165DE92 006472THERMAL SHOCK OF CERAMICMATERIALS "&1 ,i I., 0 --, !L R. Bunnell _o _'_-o,,-• _,.__=u_ _ _._.--__-KS -"-_C,.., s., >..,_''_ 6 I= "G-"'_November 1991 "- - - -== _o_ _.., _,_ _ U ,_ _Presentedat the -.- '-.- =National Educators' Workshop: . = _. _.- _-.= _ _Update '91 _= .o _%o= _ =NovemberII-13, 1991 . == _-- oOak Ridge Tennessee _°>__,.-'_ o o 8' o'" = _'=t,o_o=__=__= _. ._Work supportedby .=._-__ ____ _ =the U.S. Department of Energy _-_._-''_ -_., R,_.=_,__-_.under ContractDE-ACO6-76RLO1830 _ _ _ ° .__- =.___ o _._ _ "_PacificNorthwestLaboratory _ .=..N_ = ."o _R E_._ ,..:_'._-,= -=._-_Richland,Washington 99352 _-_=__,_:_"° --.,,-'B."_''-" _ _THERMALSHOCK OF CERAMICMATERIALSL. Roy BunnellPacificNorthwestLaboratory',Richland,WAKEY WORDS: thermalshock,Young'smodulus,thermalexpansioncoefficientPREREQUISITEKNOWLEDGE: This demonstrationis suitable for students inmaterials science at the high school or collegelevels. The students shouldunderstandelementarymechanicsconcepts,such as Young'sModulus of Elasticity.OBJECTIVES: To demonstratethe concept of thermal shock in ceramicmaterials,and to illustratethe pertinentfactorsof materials selection.EQUIPMENTAND SUPPLIES: Furnacecapableof controlledtemperaturesup to500oC;aluminumoxide rodor 2-holethermocoupleinsulator,approx.3 mm diameterx 10 cm long; water; tongs; black or blue ink.PROCEDURE: Thermal shock is a mechanismoften leadingto the failureofceramicmaterials. Many uses for ceramicsinvolvehightemperatures,and if thetemperatureof a ceramic is rapidlychanged failuremay occur. Thermal shockfailuresmay occur duringrapid coolingor duringrapid heating. As an example,considerrapid cooling,which is easier to visualize. If a ceramicmaterial iscooledsuddenly,the surfacematerialwill approachthe temperatureof the coolerenvironment. In doing so, it will experience thermal contraction. Since theunderlying material is still hot, the skin material is stretched and soexperiencestensile stress. If the resultingstrain is high enough (0.01%to0.1% for most ceramics),the ceramicwill fail from the surface and cracks willpropagate inward° Even if these cracks do not cause immediate failure, theceramicwill be severelyweakenedand may failfrommechanicaloverloadof forcesit would normally withstand.After distributing aluminum oxide rods to the students urge them tomanipulate and bend the rods with their hands. Soon, popping sounds will beheard in the classroom. These sounds demonsratehow a brittle material failswithout warning, with no permanentstrain, and at a low-strain value. Next,place about ten of the rods into a stainlesssteel beaker or a small metal panand heat to 500°C in a suitablefurnace. Remove the containerfrom the furnaceand quickly quench the rods in a bucket of water, then dry them overnightatabout I00°C. The followingday, dip the rods in ink, which acts as a crude dyepenetrantto make any crackingvisible. Wipe excessink from the rods, handlingcarefullyto avoid breaking. Again,urge the studentsto bend the rods and notethat the rods are much weaker than before. Also, note from the partialpenetrationof the ink that the cracks do not extend into the centers of therods. This is becausethe cracksstart at the surfacein a tensile stress area,but propagate into regionsof lower stress until they stop. When a quench isperformedat lower temperatures(downto about 300"C)crack density is lower andcrack length is shorter. A quenchtemperaturethat is lower still wi'llnot" Pacific NorthwestLaboratory is operated by BattelleMemorialInstitutefor the U. S. DepartmentofEnergyunder ContractDE-AC06-76RLO1830.result in any detectabledamage. This temperatureis not a constant,but is afunctionof both configurationand material.When comparing differentceramicsfor thermal shock applications,it iscommon to use a figure of merit or indexof thermalshock performance. This isa number (ratio)that is usefulfor both choosingmaterialsand for visualizingthe thermal shock process. Since the index should be high for a thermal shockresistantceramic, its numeratorshouldcontainpropertiesthat are numericallylarge when good thermal shock performanceis exhibitedby a material. Tensilestrength(S) and thermalconductivity(K)are thereforeplaced in the numerator,the former for obvious reasonsand the latter becausea high value of thermalconductivitytends to decreasethermalgradients,other factorsbeingequal. Thedenominatorof the thermal shock index is composed of the thermal expansioncoefficient(A)and the Young'smodulusof elasticity(E). The thermalexpansionis responsiblefor the failure-causingstrain, so it should be as small aspossible. The elasticmodulusis a measureof the stress resultingfrom a givenstrain, so it should be a low value for good thermal shock performance.Combiningthese factors,ThermalShock Index (TSl) = S_KKAEwhere the units of measurementshould be consistentwithin a given comparison.In the case of common glasses, all of the propertiesexcept thermal expansionfall into a relatively narrow range. By choosing a glass with low thermalexpansion,thermalshock failurecan be avoidedinmost cases. See, for example,the index values for soda-limeglass, borosilicateglass, and fused silica inTable I. Note the large differencebetweenthe thermalshock indicesof aluminumoxide and graphite. This differenceis backed by experience;it is extremelydifficultto cause graphite to fail by thermal shock, principallybecause itsYoung'smodulus is so low and its thermal conductivityis high."IAMPLE DATA SHEETS" If students quench specimens from a series oftemperatures'data sheets might be constructedto record crack frequency as afunctionof quench temperature. Alternately,ceramicrods might be broken in 3-point bending on a testing machine, and the bending strength plotted as afunctionof quench temperature.INSTRUCTOR NOTES" Provided above in Procedure. Follow usual safetyprecautionsto avoid burns.REFERENCES" Kingery,W. D., H. K. Bowen, D. R. Uhlmann, IntroductiontoCeramics,Second Edition, john Wiley & Sons, New York, 1979, Ch. 16.SOURCESOF SUPPLIES" The aluminashoul_be >95% dense, but can be of anypurity greaterthan 95%. One supplieris Coors Ceramics,Golden,COTable I. ThermalShock Index for Some Common CeramicMaterialsK,{I) A, °C"I,Material Wjcm.oC _ ___ E_ GPa TS___LSoda'lime-silica 2E-2 6B(z) 9.2 69 2.1glassBorosilicateglass 2E-2 6B 3.3 63 6.5Fused SiO2 6E-2 6B 0.6 72 94Aluminum Oxide 3E-I 204 5.4 344 _3Graphite(3) 1.4 8.7 3.B 7.7 416(I)Thermal conductivity and expansion coefficientsfrom ThermophysicalPr___ertiesof Matter, Y. S. Touloukian,ed., Plenum Press, New York, 1970.(2)Becauseglass tensile strength is so dependenton surfacecondition,asingle "reasonable"value was chosen for all glass strengths.(3)Values are typicalof nuclear-gradegraphite,from Industrial GraphiteEnqineerinq,Union CarbideCorp., 1959.

Thermal shock of ceramic materials

https://digital.library.unt.edu/ark:/67531/metadc1280899/m2/1/high_res_d/10114826.pdf

Thermal shock of ceramic materials

Abstract

Similar works

Full text

Available Versions

UNT Digital Library