UDC 666.3.017:620.173.26

EFFECT OF AQUEOUS SOLUTIONSON THE STRENGTH OF 22KhS CERAMICS

S. M. Barinov,1 N. V. Ivanov,1 A. V. Kurepin,1 B. Ballokova,1 P. Hvizdosh,1 and . Rudnajova1

Translated from Ogneupory i Tekhnicheskaya Keramika, No. 12, pp. 10 14, December, 2000.

Ceramics based on alumina undergo delayed failure in aqueous solutions as a result of stress corrosion. The ef-

fect of the loading rate on the strength of the 22KhS ceramics in aqueous solutions with pH ranging from 1 to

12 is studied. It is shown that acid and alkaline media diminish the resistance of the ceramics to delayed failure

substantially. The parameters of statistical distribution of the strength also depend on the characteristics of the

media. It is shown that the resistance of the 22KhS ceramics to delayed failure can be enhanced by preliminary

chemical treatment.

INTRODUCTION

The 22KhS ceramics is widely used as a structural mate-

rial for the production of parts of machines and mechanisms

(gaskets, sockets, seals of various pumps, cocks, etc.) that

work in water and aqueous solutions [1]. In order to form a

grain-boundary phase in such ceramics, the compositions are

enriched with additives based on sodium, manganese, chro-

mium, and silicon oxides that interact with matrix Al2O3crystals in sintering. It is known that materials based on

Al2O3 a with grain-boundary silicate phase undergo chemical

corrosion, which mainly occurs over the silicate component

[2]. It is assumed that protons and H3O+ ions of the acid sub-

stitute for the metal (alkaline) ions of the silicon phase and

cause breakage of the Si O Me and Si O Si bonds,

yielding a silica gel on the surface [2]. Hydrochloric acid is

one of the most corrosion-active media with respect to co-

rundum ceramics with a glass phase. The decrease in the

mass of such ceramics held in 35% HCl attains 1.0 gm2 aday [2]. It can be assumed that ceramics with a silicate

grain-boundary phase will be sensitive to stress corrosion,

which causes growth of the structural defects existing in the

material, i.e., microcracks and pores, and is responsible for

the dependence of the strength on the loading rate (a phe-

nomenon known as dynamic fatigue) [3].

The subcritical propagation of cracks in ceramics occurs

in three stages. In the first stage, the dependence of the rate

of crack growth v on the coefficient of stress intensity K is

described by an empirical formula:

v = AK n, (1)

where A is a coefficient and n is an exponent that quantita-

tively describes the sensitivity of the material to the process

of delayed failure; the higher the value of n the more resis-

tant to delayed failure the material is. The duration of this

stage determines the time before failure under load.

The value of n is determined by testing the material un-

der a constant load with measurement of the time before fail-

ure (the method of static fatigue) or by performing strength

tests in a wide range of loading rates (the method of dynamic

fatigue). In the latter case, the value of n is found from the

proportion of the measured ultimate bending strength b tothe specified deformation rate , i.e.,

log b = C +1

1 nlog , (2)

where C is a constant that depends on the properties of the

ceramics [3, 4].

It has been shown earlier that the value of the exponent n

for a ceramics tested in the given medium depends on the

composition of the grain-boundary phase [5 7]. We found it

interesting to evaluate it for the 22KhS ceramics widely used

in industry. In addition, we expected that a preliminary

chemical treatment that causes modification of the grain-

boundary phase should affect the dynamic fatigue of the ce-

ramics. This became the aim of the present work.

Refractories and Industrial Ceramics Vol. 41, Nos. 11 12, 2000

4221083-4877/00/1112-0422$25.00 2001 Plenum Publishing Corporation

1 Institute of Physicochemical Problems of Ceramic Materials of

the Russian Academy of Sciences, Moscow, Russia; Institute for

Materials Research of the Slovak Academy of Sciences, Brati-

slava, Slovak Republic.

MATERIALS AND METHODS OF STUDY

We studied 22KhS ceramics obtained by hot casting with

subsequent roasting in vacuum. The composition included

(in wt.%) 94.5 Al2O3, 2.5 SiO2, 0.03 Fe2O3, 0.48 Cr2O3,

1.96 MnO, 0.2 Na2O plus impurities (the remainder). The

content of corundum grains in the material was

88 89 vol.% and that of the grain boundary phase was

11 12 vol.%. The microstructure of the ceramics was repre-

sented by elongate corundum grains up to 30 m long and upto 15 m thick. The density of the material was 3.78 gcm3,and the content of the open pores did not exceed 2%.

The tests were performed by the method of three-point

flexure of specimens with a cross section of 5 5 mm at adistance between the supports of the loading device equal to

32 mm with the help of a UTS-100 universal testing machine

(UTS Testsysteme GmbH, Germany). The speed of the

crosspiece of the loading device was changed from 0.01 to

15 mmmin. The dependence of b on was measured inthe following media: water (pH = 7), 0.1 moleliter HCl

(pH = 1), and 0.1 moleliter NaOH + a buffer solution(pH = 12). In each series of tests, we tested 14 16 speci-

mens. We also studied the statistical distribution of the

strength in the mentioned media at a constant loading rate of

0.5 mmmin (controlled by the acting standards ASTMC 1161, JIS R 1601, DIN 51 110, and AFNOR B41-104 [4]).

The data were processed statistically with the help of an esti-

mator of the type

Pi =i

N

0 5., (3)

where Pi is the probability of failure, i is the number of the

specimen, and N is the number of specimens in the series; the

Weibull distribution function is used in a biparameter repre-

sentation:

P = 1 exp

V

V

m

0 0

, (4)

where P is the cumulative probability of failure, V is the

stressed volume, V0 is the scale factor, is the stress, 0 is anormalizing factor, and m is the modulus of the Weibull

function that characterized the uniformity of the distribution

of defects responsible for the strength of the specimens

[4, 8]. In order to find the Weibull function we tested

15 specimens at a constant loading rate.

We determined the values of crack resistance by standard

methods with the use of specimens in the form of bars with a

single side notch (SENB specimens) [4]. The notch was de-

posited by a diamond cutter about 100 m thick. The depthof the notch was 0.5 of the height of the section of the speci-

men. The crack resistance was calculated for a maximum

load, which corresponded to the beginning of uncontrolled

propagation of the crack in the specimen, by known formulas

and values of calibration functions [4].

In addition, we studied the influence of a 60-day hold of

specimens in 0.1- and 1.0-moleliter solutions of hydrochlo-

ric acid on their dynamic fatigue in subsequent tests in water.

Mechanical tests were performed in a UTS-100 machine

under the conditions of three-point flexure and hard loading.

A special device was created for tests in corrosion-active

media.

Fracture surfaces were studied under a TESLA electron

microscope. The changes in the chemical composition were

determined by the method of energy dispersive x-ray spectral

analysis.

RESULTS AND THEIR DISCUSSION

The strength of the ceramics increases with the deforma-

tion rate in tests performed in different water-containing me-

dia (Fig. 1). The dependence of the parameter n calculated by

Eq. (2) on the pH of the solution is presented in Fig. 2. It can

be seen that n decreases both in acid and alkaline media.

The value of n in tests performed in water is about the

same as that of other alumina materials with a glass phase,

Effect of Aqueous Solutions on the Strength of 22KhS Ceramics 423

log (MPa)

log (MPa)

log (MPa)

log (sec ) 1.

log (sec ) 1.

log (sec ) 1.

2.6

2.4

2.2

2.6

2.4

2.2

2.6

2.4

2.2

6 5 4 3 2 1

6 5 4 3 2 1

6 5 4 3 2 1

pH = 1

pH = 7

pH = 12

n = 28.4

n = 32.3

n = 24.6

Fig. 1. The strength of the ceramics as a function of the loading ratein different media.

i.e., for a ceramics with yttrium-alumosilicate glass phase

n = 36, for a material with calcium-alumoborosilicate grain-

boundary phase n = 32, and for the GB-7 commercial cera-

mics n = 30 [5 7]. However, the value of n for the 22KhS

ceramics is much lower than for a microlite-type material

with 99.5% Al2O3 and 0.4% MgO (n = 60) [5, 6]. Electron

microscopic studies of fracture surfaces in the ceramics

tested in different media show that the crack chiefly propa-

gates by an intercrystalline mechanism over grain-boundary

phases. Thus, we can assume that the composition and the

properties of the glass phase in the 22KhS ceramics are the

main factors responsible for its behavior when it is loaded in

aqueous solutions.

The main component of grain-boundary phases in all the

studied materials with a glass phase was silica, which ac-

tively interacted with water in the process of stress corrosion.

A model of such an interaction mechanism is suggested in

[9, 10] within the theory of dissociative chemisorption. The

model allows for the following stages of the process: adsorp-

tion of a water molecule to a Si O Si bridge bond, redis-

tribution of the electron density in the region of the stressed

and strained bridge bond, and breakage of this bond with for-

mation of OH radicals with uncompensated charge. The

model predicts the values of n for silica, which exceed 50.

It can be assumed that the presence of modifiers and

other elements in the grain-boundary phase substantially

complicates the mechanism of stress corrosion. Sodium oxi-

de leaches out of the glass phase in the loading process,

changing the pH of the medium right at the tip of the crack,

and reduces the resistance to stress corrosion. A similar ef-

fect has been observed for alkaline-silicate glasses [11] in

which n decreases considerably as a result of leaching in the

loading process. Chromium and manganese form chemical

bonds with oxygen in the glass phase. It is known that chro-

mium and manganese oxides are characterized by a high

solubility in alkalis and that chromium oxide dissolves in

mineral acids. It is obvious that this should decrease the re-

sistance of the glass phase to dynamic fatigue.

The components of the composition used for the forma-

tion of the grain-boundary phase are introduced in the fol-

lowing proportion: SiO2 : MnO : Cr2O3 = 2.5 : 1.96 : 0.48.

Chromium and manganese oxides do not possess a glass-

forming capacity. Their content in the composition in ques-

tion is such that it can hinder the formation of a continuous

structural glass net, causing the formation of microinhomo-

geneities. In addition, the dissolution of alumina crystals in

this flux in the sintering process can be accompanied by fur-

ther intensification of the inhomogeneity as a result of the

segregation of alumosilicate phases, for example, mullite.

Mullite is known to possess a low resistance to dynamic fa-

tigue (n = 27) [12]. An electron microscopic study has shown

the presence of needle-like crystals in the structure of grain

boundary phases. The crystals dissolve both in the acid and

in the alkali directly in the process of the dynamic fatigue

tests. Unfortunately, the components of grain-boundary

phases could not be identified by the method of x-ray phase

analysis because of the low volume of the latter.

Thus, we can assume that the low resistance of the

22KhS ceramics to delayed failure can be a result of the

chemical composition of the grain-boundary phase contain-

ing soluble oxides of sodium, chromium, and manganese and

by the inhomogeneity of the structure of this phase.

The ultimate bending strength measured under standard

loading conditions [4] was 230, 224, and 219 MPa in tests in

water (pH = 7), in acid (pH = 1), and in alkalis (pH = 12) re-

spectively. We see that the composition of the medium influ-

ences the strength. The crack resistance KIc also decreased

from 4.7 to 4.2 MPa m12 when we passed from tests in wa-ter to tests in an acid. In accordance with the Griffith crite-

rion, the decline of the strength and crack resistance deter-

mined from the conditions of critical equilibrium for the re-

sistance of a fractured specimen can be explained on the ba-

sis of the Rebinder effect, i.e., the decrease in the surface

424 S. M. Barinov et al.

n

pH of the solution

32

30

28

26

24

0 3 6 9 12

Fig. 2. Effect of the pH of the solution on the parameter n.

ln ln [1 (1 )] P2

1

0

1

2

3

45.5 5.6 5.7 5.8 5.9 6.0 6.1

m1 = 8.9

m2 = 11.7

ln (MPa)

Fig. 3. Statistical distribution of the ultimate bending strength forceramics tested in water (1 ) and in a 0.1 M solution of HCl (2 ).

energy of fracture of a solid body upon its interaction with

the ambient [13, 14]. Another cause can be the subcritical

growth of the defects, i.e., the microcracks responsible for

the strength and the macronotch responsible for the crack re-

sistance of the specimen, due to the stress corrosion. Figu-

res 3 and 4 present the results of statistical tests of ceramics

in water and in acid. The strength distributions in these me-

dia differ substantially; the tests in acid result in a bimodal

distribution. The mean modulus of the Weibull function m

increases from 8.9 to 11.7 upon the transition from water

tests to acid tests. Two segments of the distribution function

in acid tests correspond to a modulus of 33.9 (the domain of

low strength) and 5.8 (the domain of high strength). All these

facts indicate that in acid tests the microcracks controlling

the strength of the ceramics grow to a subcritical size, which

agrees with the data of tests for dynamic fatigue.

It is known that the treatment of many silicate glasses in

acids increases their water resistance [15]. Since the studied

ceramics fracture over the grain-boundary phase, we studied

the possibility of increasing the fracture resistance of the ce-

ramics by its preliminary treatment in acids. Ceramic speci-

mens were held in 0.1- and 1.0-moleliter solutions of hydro-chloric acid for 60 days at room temperature and then tested

for dynamic strength. The decrease in the mass as a result of

the treatment did not exceed 0.08%, which, as recalculated

for the content of the grain-boundary phase and under the as-

sumption that only this phase dissolves, corresponds to a

0.8 1.0% decrease in the mass of the grain-boundary phase.

The results of an energy dispersive x-ray spectral analysis

of regions of ceramic surfaces performed on an area

600 450 m in size are presented in Table 1. It followsfrom these data that the acid treatment decreases the concen-

tration of SiO2 and Cr2O3; Na2O completely leaches from

the ceramics; Al2O3 and MnO virtually do not dissolve in

the acid.

Figure 5 presents data on the dynamic fatigue of speci-

mens. The values of n range from 93 to 85 after treatment in

0.1- and 1.0-moleliter solutions, which is much higher thanin the initial state of the ceramics. It can be assumed that the

increase in the resistance to stress corrosion is caused by the

change in the chemical composition of the grain-boundary

phase. At the same time, the values of the ultimate bending

strength of the ceramics measured under standard conditions

were 209 and 214 MPa after the treatment in 0.1- and

1.0-moleliter solutions of the acid respectively. It seems thatthis is caused by the formation of new pores or the volume

growth of the existing pores due to the acid treatment, which

is confirmed by the change in the mass of the specimens.

Thus, the preliminary acid treatment increases the resistance

of the 22KhS ceramics to delayed failure in water media at a

certain decrease in its short-term strength.

CONCLUSIONS

We studied the influence of aqueous solutions with pH

changed from 1 to 12 on the mechanical properties of the

22KhS ceramics. It turned out that the ceramics possesses a

low resistance to delayed failure in water due to the pro-

cesses of stress corrosion occurring in the grain-boundary

phase. Acid and, especially, alkali solutions reduce this resis-

Effect of Aqueous Solutions on the Strength of 22KhS Ceramics 425

log (MPa)

log (MPa)

log (sec ) 1.

log (sec ) 1.

2.4

2.2

2.4

2.2

6 5 4 3 2

6 5 4 3 2

pH = 7

pH = 7

n = 93.3

n = 84.5

b

Fig. 5. Dependence of the strength of the ceramics on the defor-mation rate in water tests after a 60-day hold in solutions of 0.1 M

HCl (a) and 1.0 M HCl (b ).

TABLE 1. Results of X-Ray Spectral Analysis of Specimens

State of ceramicsContent of oxides, %

Al2O3 SiO2 Cr2O3 MnO Na2O

Initial 92.036 4.483 0,738 2.599 0.166

After treatment in HCl:

0.1 M 92.635 3.993 0.608 2.740 0.020

1.0 M 92.712 3.732 0.530 2.902 0

ln ln [1 (1 )] P2

1

0

1

2

3

45.6 5.7 5.8 5.9

m = 5.8

m = 33.9

ln (MPa)

Fig. 4. Bimodal statistical distribution of the strength of the speci-mens tested in 0.1 M HCl.

tance substantially, as well as the strength and the crack re-

sistance of the ceramics. The latter can be caused by the

Rebinder effect and by the processes of dissociative

chemisorption. The pH of the solution determines the param-

eters of the function of statistical distribution of the strength.

Specifically, when the tests are performed in an acid the

strength distribution becomes bimodal, which is a sign of

subcritical growth of the existing defects of the micro-

structure. The resistance of the 22KhS ceramics to delayed

failure in a water medium can be increased substantially by

preliminary chemical treatment in an acid. Such a treatment

diminishes the contents of silica and chromium oxide and

completely leaches sodium oxide from the grain-boundary

phase. The decrease in the mass of the phase during the treat-

ment attains 1.0%, which leads to a certain loss in the

short-term strength of the ceramics.

This work has been performed within the program of

academic cooperation between the Russian and Slovak

Academies of Science and with support from the Russian

Fund for Fundamental Research, Grant No. 00-03-32601.

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