The baddeleyite-corundum refractories in the bottom of the furnace were distinguished by their excellent glass resistance in both campaigns. The glass in the second campaign was of good quality, and the finished glass had virtually no defects caused by damage refractories. The most stable refractories for melting the $48-3 glass are the Soviet-made corundum95 and Corvishite (made in Hungary). To lower~theconsumption of refractories it is essential to pay attention not only to the correct lining of a furnace and its warm-up after maintenance but also to the total draining away of molten glass from the tank during the cold maintenance shutdown. This will reduce the labor required in the maintenance work and make it possible to retain the lining at the bottom of the tank and in the wall blocks of the working sections, When the furnace is beinglined, the seams between the electrosmelted corundum refractories should not be greater than i mm. The results of the above study were taken into account when building the industrial furnace for melting $48-3 glass (third campaign). At the present time the furnace has been operated for more than a year and the glass produced is of good quality. This indicates the need to organize the industrial output of corundum-95 tiles, 75 and i00 mm thick. LITERATURE CITED i.

O . N . Popov, Corrosion and Service of Refractory Materials in Tank Furnaces in HighTemperature Glass Melting. A Review [in Russian], All-Union Scientific-Research Institute of Enameling and Glass Machinery (1974).

THERMAL-SHOCK P~SISTANCE OF GLASS-CERAMIC COATINGS G. I. Zhuravlev, L. V. Rudenko, G. A. Kudryavtseva, and L. I. Venzel'

UDC 666.291:539.434

Providing thermal-shock resistance to protective coatings, i.e., the capacity to withstand required loads developing during temperature changes, is one of the tasks facing researchers. It should be stated that beside the thermoelastic stresses themselves developing during thermal shock, the coatings form additional stresses connected with differences in the thermal expansion coefficients of the materials of the coatings and the base. The latter always develop because of the impossibility of obtaining two different materials with completely identical coefficients of thermal linear expansion in a wide temperature range. At the present time there is no complete analysis of the stressed state developing in the coatings as a result of differences in the expansion factors, but there are methods of calculating normal stresses acting in planes perpendicular to the planes of bonding [1,2]. Earlier studies [i] showed the satisfactory agreement of theoretical and experimental data. However, the stresses calculated from the proposed equations [i, 2] are correct only at distances adequately removed from the edges of the articles, i.e., the edge effect is not taken into account. It is established [3] that the stresses reach the maximum values on the edges of articles (plates) and become unsubstantial at a distance approximately equal to four thicknesses of the coating. The stresses arising in the center and around the edges of the articles are inherent in all coatings, and are called residual. These stresses vary with changes in temperature, and if the coefficients of thermal linear expansion are less for the coatings than for the metal, then residual compressive stresses develop in the coating. During the service of the coating under thermal-shock conditions, thermoelastic stresses caused by the temperature gradient are aggregated with the residual, which significantly complicates the overall picture of the stressed state.

Leningrad Lensovet Institute of Technology. All-Union Scientific-Research Porcelain. Translated from Steklo i Keramika, No. 7, pp. 13-15, July, 1984.

0361-7610/84/0708- 0293508~50 O 1985 Plenum Publishing Corporation

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Fig. i. Relationship between the ~esidual stresses ~ and thermal shock resistance At of coatings and the weight content W of zircon (a) and a-quartz (b) in the composition at i) 20~ 2) 100~ 3) 200~ 4) 300~ 5) 400~ 6) thermal-shock resistance. The aim of our work was to identify the connection between residual stresses and the thermal-shock resistance of glass-ceramic coatings. We studied the thermal-shock resistance of glass-ceramic coatings consisting of fusible, glassy enamels No. 57 (matrix) having expansion factors equal to 92 x 10 -7 oC-Z ' and inorganic fillers. The fillers consisted of finaly dispersed (grain diameter less than 50~m) crystalline materials: a-quartz with coefficient of thermal linear expansion of 140 • I0-7~ -z. With a combination of enamel and filler we obtained different expansion values in the coating. During all the investigations the substratum consisted of steel 08KP with expansion factors of 140 • 10 -7 to 145 • I0-7~ -z carrying a factory ground coating ~ x pans• factor 94 •176 The compositions and coatings were prepared by the slip-firing method. The coefficient of thermal linear expansion of the composition was m e a s u r e d o n the DKV-4 differential quartz dilatometer with an accuracy of • i • i0-7~ -z. The residual deformations in the coatings were determined by the deflection method with console-fitted steel bands enameled on one side. The accuracy of the measurement was • i • 10-2mm. The thermal-shock resistance of the coatings was tested accordinm to OS$ 26-01-1-70 by sequentially heatin~ and cooling the specimens in water at 20~ The minimal temperature of heating was 150~ rising in steps of 20~ The specimens consisted of enameled plates measuring 75 x50 x 8 mm, and the coating thickness was 1 ~. The level of thermal-shock resistance was judged from the maximum drop in temperature which the coating withstood without damage to its continuity The test results are shown in Fig. 1. The change in the coefficient of thermal linear expansion of the coating as a result of adding fillers led to a change in the residual stresses. With an increase in the content of ~-quartz the expansion factor of the coating increased, and the residual stresses of compression were reduced, since the difference in the factors for coating and steel diminished. With the use of zircon we noted the opposite. During thermal-shock tests the coatings develop tensile stresses which, upon reaction with the residual stresses of compression, are weakened. Thus, the higher the residual com' pressive stresses, the higher the thermal-shock resistance of the coating. Practical results (see Fig. i) do not agree with what seemed to be obvious reasoning. However, the matter here does not rest with erroneous theory, but with its incompleteness. At the present time measurements andcalculations of the stressed state take into account the normal constituent of residual stresses. In connection with this, to compare the stresses under thermal shock 294

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Fig. 2. Relationship between thermal-shock resistance of coating At and the weight content W of zircon (a) and s-quartz (b) in the composition; i) Destruction in central part; 2) edge destruction. only with the normal residual stresses in the coating is not quite correct, since they correspond to those in that part of the coating which is rather removed from the edges. Therefore, during the processing of the data on the destruction of the coating under thermal shock, account was taken of the nature of the damage to the coating at the edges and in the center of the specimen (Fig. 2). Increasing the content of zircon in the enamel increases the residual stresses of compression in the coating (see Fig. ib). As a result this coating is capable of withstanding high tensile loads developing during thermal shock. The destruction of the specimens in the central part occurs with higher values of temperature, i.e., with increase in the coating's zircon content, the thermal-shock resistance, characterised by damage at the central part of the specimen, increases (see Fig. 2a, curve I). At the edges of the plate in accordance with previously published data [I, 3] the residual tensile stresses are higher the lower the coefficient of thermal linear expansion in the coating; therefore, damage at the edges of the plate sets in with lower thermal shocks, i.e., the thermal-shock resistance characterized by edge damage, with rise in zircon concentration, is reduced (see Fig. 2a, curve 2). With increase in the expansion factor of the coating containing s-quartz as filler, and accordingly a reduction in residual stresses of compression, damage at the center of the plate sets in with lower thermal shocks (see Fig. 2b, curve i). A t the edges of the plate with rise in expansion factors the residual stresses of tension were reduced, which leads to a rise in thermal-shock resistance. Thus, if we merely determine the presence of damage without analyzing its nature, i.e., from the position of exploitation, then the relationship between thermal-shock resistance and the concentration of filler in the coating has an extreme character, and is not correlated with the residual stresses in the center of the plate. Since inpractice residual compressive stresses tend to be set up in the plate, this leads to the fact that at the edge of the articles (or on the protruding sections) we shall always have a site of tensile stress. The stronger the compression in the central part of the specimen, the greater the tensile stresses to be expected at the edge. This requires the creation of optimum stresses both in the center and at the edges of the artieles, which may be attained by the combined introduction of s-quartz and zircon in the coating. Analysis of the stressed state of the coatings under investigation with maximum values for the properties showed that the most favorable distribution of stresses at the edge and center is obtained for coefficients of thermal linear expansion in the coating equal to I00• -7 to 105 •176 -I. It should be mentioned that with the recommended expansion factor values it is not possible to obtain a high thermal-shock resistance in glassy coatings. The high level of thermomechanical properties in composite glass-ceramic coatings is a result not only of the optimum distribution of stresses, but also the resulting changes in the macrostructure of the coating. With a rise in the volume fraction of crystalline inclusions in the coating, there is an increase in the total quantity of heterogeneities, i.e., an increase in the porosity and in the number of microcracks arising during the formation of the coating because of the differences in the expansion factors of the phases making up the composition. During thermal

295

Shock the crystalline inclusions, pores, and microcracks may partly or completely dissipate the energy of the destructive mainline crack, which may increase the thermal-shock resistance of the coating. Thus, by adding 10-20% by weight (optimum amount) of cheap crystalline fillers, zircon and marshallite, to glassy enamel, it is possible to control the stressed condition and macrostructure of the glass-ceramic coating, and also to alter the properties over a wide range. LITERATURE CITED i. 2. 3.

296

G . I . Zhuravlev, Chemistry and Technology of Thermal-Shock Resistant Inorganic Coatings [in Russian], Khimiya, Leningrad (1975). G . I . Zhuravlev and M. Kirsh, Tr. Leningr. Tekhnol. Inst. im. Lensovet, No. 3, 40 (1975). S . V . Borovinskii et al., paper deposited at ONIITEKhIM, No. 2658/79.