S C I E N C E F O R THE C E R A M I C S I N D U S T R Y .

PREDICTION OF CRACK RESISTANCE OF CERAMICS USING THE METHOD OF ACOUSTIC EMISSION I. I. Nemets, V. B. Zlatkovskii, N. S. Bel'maz, and M. A. Trubitsyn Ceramics are heterogeneous multicomponent conglomerates having a complex structure. In view of this, the fracture mechanics of ceramic materials significantly differs from the relatively well known fracture mechanics of homogeneous materials such as glass, metals etc. At the present time, there are three main directions of research on the fracture processes of inhomogeneous materials, viz., phenomenological, statistical, and structural studies. The phenomenological theories treat fracture as an instantaneous process coinciding with discontinuity (defect formation) in the material and do not analyze the physical reasons (phenomena) leading to fracture. The statistical theories are based on the fact that the strength of a body is determined by the strength of the weakest primary element (the "weak link" theory), i.e., the interaction of the defects is taken into account. In this case, most frequently, the actual structure of the material and the related features of the stress state (stress concentration in the vicinity of pores and the filler grains and the possibility of formation of initial cracks between the filler and the binding matrix) are not considered. The structural theories provide for a fairly deep analysis of the physical essence of the material behavior under load. They show that cracks must develop mainly in the vicinity of pores, inclusions, and other structural defects. However, the progress of the structural theories is restricted by the necessity of determining the stresses leading to the formation of the first cracks and the theories are not capable of describing the entire fracture process of the material. When examining the behavior of a ceramic under the action of thermal stresses, one must consider its brittleness and strength loss and the significant volumetric changes occurring in the structure of the material during polymorphic transformations and thermal expansion. All these factors lead to a reduced crack resistance of the ceramic in that their contribution to the entire fracture process is not constant in different materials over different temperature ranges. The behavior of certain ceramics (having compositional and structural differences) was studied under the conditions of thermal loading. For this purpose, we employed the method [I] based on recording the signals of acoustic emission (AE) accompanying crack initiation and growth in the ceramic materials during thermal loading (1000-20~ The required measurements were carried out using an AF-II apparatus within the frequency band 0.1-2.0 MHz. The modulus of elasticity was determined according to the dynamic method using a UK-10P device. We studied coarse-grained aluminosilicate unfired materials whose structure is characterized by the presence of a large number of microcracks at the contact surface between the binding material and the filler (these microcracks formed during shrinkage of the binder when heating the material). Finely dispersed amorphous silica and a finely dispersed mixture with the aluminosilicate material [2] were used as binders. Figure i shows the AE diagrams obtained during the thermal shock tests. It can be seen that destruction of the materials occurs in the 800-700~ (the region B) and 600-450~ (the region C) ranges. The first temperature range corresponds to the stresses developed in the material because of the differences in the values of CLTE of the matrix and the filler (~ of the I matrix amounts to 16.0.I0-6~ -l, = of the II matrix was found to be 8.0.10-60C -I, and ~ of the filler is 6.5.10-6~ The second temperature range of the AE signals corresponds to the stresses developed because of the large volumetric changes occuring during the polymorphic transformations of free quartz contained in the grains of the filler. I. A. Grishmanov BTISM.

Translated from Steklo i Keramika, No. ii, pp. 16-18, November,

1988.

412

0361-7610/88/1112-04125'12.50

9 1989 Plenum Publishing Corporation

I

E

.&

12;

'

:

I

~A

IB

u~8 0

0

,J .r-t

i

i

!

L

~4 r e-

/.I

~0~o 900

800

700 6OO

5O0 ~00

Thermal cycles

Temperature, ~ Fig. i

Fig. 2

Fig. i. Variation of the intensity of the AE signals during thermal shock testing (I000-20~ of the specimens of coarse-grained aluminosilicate refractories: i) material having a uniform matrix; 2) material having a two-phase matrix; cooling rate 100~ Fig. 2. Variation of the relative dynamic modulus of elasticity E~y n during thermal cycling (1300-20~ air) of the specimens of the coarsegrained aluminosilicate refractories: l) material having a singlephase matrix; 2) material having a two-phase matrix; E~y n = E AT /E ~ dyn" dyn" The absence of recordable AE signals during the initial period of cooling (the region A) indicates high strength levels (indices) of the material that exceed the indices of the thermal stresses during this period. The I order strsses accumulated in the material during this period add up subsequently with the II order microstructural stresses and this is accompanied by the additional effects (phenomena) influencing the fracture process and is reflected by the AE signals (the regions B and C). Thermal shock resistance of the aforementioned materials was also evaluated on the basis .~ of the variation of the relatmve dynamlc modulus of elastlclty Eay n that is defined as the ratio of the modulus of elasticity observed after thermal shock (1300-20~ air) E AT and the dyn modulus of elasticity of the specimen prior to this thermal shock E~ (Fig. 2). The data presented in this diagram confirm the results of AE with regard to t~ne high thermal shock resistance of the material having a two-phase matrix. .

.

~

~

One of the methods of improving the thermal shock resistance of structural materials is to produce composites that are strengthened by dispersed particles or fibers [3]. However, the absence of suitable methods for producing such fibers, damage of the reinforcing crystals, and loosening of the structure of the material during reinforcing (that lead to strength loss) do not make it possible to use the externally reinforced composite materials extensively [4]. We developed composite ceramics that are reinforced not by introducing the reinforcing component into the matrix, but by its synthesis during the sintering process. Such a method of self-reinforcement of ceramics was described in detail by Timashev et el. [5] (applicable to the aluminosilicate systems). This method makes it possible to obtain a material exhibiting a combination of high strength and thermal shock resistance after firing (see Table i) [6]. When such aluminosilicate materials are subjected to thermal shock, their fracture process is characterized by three successive stages of variation of the AE signals in the following temperature ranges: 1000-850, 850-700, and 750-500~ (Fig. 3). It can be seen that at I000-8500C, the intensity of the AE signals reflecting the fracture process of the material in this range decreases from the nonreinforced material having the minimum strength up to the strongest self-reinforcing material whose AE signals are almost not identified. This owes to its structural features (the presence of a strong contact between the grains of the thinner and the reinforced matrix) and strengthening of the pore walls due to reinforcement of their surface by 'paling' of acicular mullite crystals. Reactivation of the AE signals at 750-500~ is due to the destructive effect of the microstructural stresses that are developed during the polymorphic transformations of free

413

TABLE 1

Aluminosilicate ceramic Nonreinforced Externally reinforced SelF-reinforced

I0 14

3 6

18

10

45

12

,

.5N

~3 o 4~

~2

fA

F-

1

!000

900

800

700

600

500 tlc

Fig. 3

9L70

800

700

6OO

50D t,~

Fig. 4

Fig. 3. Variation of the intensity of AE signals during thermal shock testing (I000-20=C) of the aluminosilicate materials: i) nonreinforced material; 2) material having external reinforcement; 3) self-reinforced material; cooling rate 100~ Fig. 4. Variation of the intensity of AE signals during thermal shock testing (I000-20~ of the specimens of a periclase ceramic: i) nonreinforced material; 2) reinforced material. quartz. The intensity of these signals is minimum in the externally reinforced ceramic. This is related to the presence of strong acicular crystals of mullite in the matrix and the formation of a less rigid structure due to its cracking in the 1000-850~ range. The increased activity of the AE signals due to polymorphism of quartz (at 700-500~ in the self-reinforcing and nonreinforced ceramics ows to the high 'rigidity' of the structure of the former material and the absence of a reinforcing phase in the structure of the latter material. In the materials having high values of CLTE (for example, periclase ceramics), considerable thermal stresses develop immediately after applying thermal loads and the fracture processes occur in the entire temperature range (Fig. 4). The crack resistance of such materials increases due to the increased strength of the regions of the matrix phase adjoining the filler grains and due to the evolution of a structure having a network of channel-type (interconnected) pores in the binding matrix. In this case, the level of the thermal stresses decreases and there is a corresponding decrease in the intensity of the AE signals [7]. Thus, when studying the fracture processes of ceramics, it is necessary to consider not only the mechanical models of crack nucleation and propagation, but also the structural models (in particular, those delineating the reasons underlying crack formation or the factors facilitating the growth of the existing cracks). The acoustic emission method is the most suitable technique for studying the fracture kinetics of such a class of ceramic materials. 414

LITERATURE CITED i. 2. 3. 4. 5. 6. 7.

I . I . Nemets, V. B. Zlatkovskii, and G. A. Fokin, "Applicability of acoustic emission for studying the thermal shock resistance of refractories," Ogneupory, No. 3, 47-49 (1982). I . I . Nemets and M. A. Trubitsyn, "Ceramic binders and ceramic concretes (keramobeton) having quartz-chamotte composition," ibid., No. 5, 5-9 (1986). G . P . Cherepanov, Mechanics of Brittle Fracture of Composite Materials [in Russian], Nauka, Moscow (1983). A . S . Vlasov, "Ceramic composite materials," Zh. Vses. Khim. Obshch. im. D. I. Mendeleeva., 27, No. 4, 530-537 (1982). V . V . Timashev, L. I. Sycheva, and N. S. Novikova, "Self-reinforcement of cement stone," Tr. Inst. Mosk. Khim-Tekhnol. Inst. im. D. I. Mendeleeva., No. 92, 155-156 (1976). I . I . Nemets, N. S. Bel'maz, and A. I. Nestertsov, "Development of a ceramic material that is self-reinforced with acicular mullite crystals," ibid., No. 123, 30-33 (1982). I . I . Nemets, "Effect of the structure and the nature of ceramic materials on their crack resistance during thermal loading," Abstracts of the Reports of II All-Union Symposium on Fracture Mechanics [in Russian], Kiev (1985), pp. 85-86.

PRODUCTION OF FLAT PORCELAIN PARTS FROM LOW-TEMPERATURE BODIES ACCORDING TO FAYENCE TECHNOLOGY V. L. Zakusilo, V. D. Boguslavskii, L. L. Oleinikova, Yu. G. Kovalenko, and A. E. Polishchuk

UDC 666.51

At the present time, the requirements of the "Obshchepit" enterprises (public catering organizations) of the Ukrainian Republic with regard to flat dishes are being met mainly by fayence products. Porcelain products account for an insignificant proportion of the consignments suplied to these enterprises because of their higher cost as compared to that of the fayence products. The future plans of developing the national economy include expansion of the 'Obshchepit' system, modernizing the equipment of the enterprises, and improving the service culture. This requires improvement of the service characteristics and the artistic and esthetic finish of the domestic ware. Fayence products do not meet the service conditions (specifications) of the "Obshchepit" enterprises. Their service life amounts to 1-1.5 months which is 2-2.5 times less that that of the porcelain products. Besides this, within 2-3 weeks of service under severe conditions, fayence dishes exhibit penetration of food and the washing solutions into the porous fayence sherd at the damaged regions of the glaze layer and become unusable because of their poor sanitary and hygienic parameters. In view of this, in recent years, fayence products are not being preferred in spite of their lower cost as compared to the cost of porcelain products. We note that when manufacturing fayence products, the productivity of the furnaces (thermal units) is higher (as compared to their productivity during the production of porcelain products) and this makes it. possible to achieve fuel economy and energy conservation owing to the decreased consumption of the refractory provisions per unit weight of the products. One of the methods of solving the problem of supplying dishes having high physical, technological, and service characteristics to the enterprises is to develop and introduce an industrial production process of porcelain products according to the technological scheme (flowsheet) of fayence. This makes it possible to obtain procelain products having virtually zero water absorption and high physical, technological, and quality parameters of the glaze layer. In this case, the cost of firing the products decreases and the productivity is retained at the same level as that obtained during the production of flat fayence products. In the international practice, enough experience has been gained in the production of porcelain according to the flowsheet (process sheet) of fayence. The so-called "restaurant Ukrainian Scientific-Research Institute of Porcelain Products (UkrNIIFP). from Steklo i Keramika, No. ii, pp. 18-19, November, 1988.

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9 1989 Plenum Publishing Corporation

Translated

415