Physica 139 & 140B (1986) 183-186 North-Holland, Amsterdam

EFFECT OF PRESSURE ON THE ELASTIC CONSTANTS OF YTTRIA-STABILISED ZIRCONIA

S. HART, J. WALLIS and I. SIGALAS National Institute for Material Research, Council for Scientific and Industrial Research, P.O. Box 395, Pretoria 0001, Republic of South Africa

The pressure derivatives of the elastic constants of yttria-stabilised zirconia were measured as a function of yttria concentration. For all concentrations between 9% and 24% yttria the elastic behaviour is normal showing a stiffening with pressure. The ambient pressure values of the elastic constants are in good agreement with previously published data while the pressure derivatives are considerably smaller in magnitude than the only other data reported for a single composition. The present pressure derivatives are numerically similar to those of other fluorite type materials. An unusual feature is the decrease to small value of (0/9p){ ½(C n - Clz)} at high yttria concentration and may indicate a decrease in this oxide's ability to stabilise the cubic crystal structure.

1. Introduction

Pure zirconia (ZrO2) undergoes two phase transformations, from monoclinic to tetragonal at 1170°C and from tetragonal to cubic at 2370°C. Because of the volume changes associated with these phase transformations, it is impossible to use pure ZrO 2 in structural refractory applic- ations. YzO3, among other oxides, is used to stabilise the cubic fluorite structure at room temp- erature.

The single crystal elastic constants of yttria stabilised zirconia (YSZ) have been determined in the past by several investigators at atmospheric pressure. The reader is referred to Kandil et al. [1] for a literature survey on the subject. Hailing and Saunders [2] measured the pressure depen- dence of the elastic constants of 8 tool% YSZ. These authors found that the crystal's resistance to shear increases with hydrostatic pressure.

In this work we investigated this effect with Y 2 0 3 content, in an effort to better understand the mechanism by which Y203 stabilizes the cubic fluorite structure in ZrO 2.

2. Experimental

Single crystals of YSZ containing 9.4, 15, 18, 21

and 24 mol% of Y203 were obtained from Ceres Corp. Waltham, Ma. USA. The samples were oriented by means of X-ray Laue back-reflection photographs. On each sample a number of pairs of flat and parallel faces corresponding to crystal- line planes of high symmetry were cut and lapped. The pressure derivatives were determined by measuring the change in the velocity of ultrasonic longitudinal and shear waves of 10 MHz frequen- cy, propagating along directions of high symmetry under hydrostatic pressure in the range 0-1 GPa.

A detailed description of the ultrasonic and high pressure techniques has been published elsewhere [3].

The experiments yielded the natural modulus (p0W2)0 and its pressure derivative (p0W2)0 at p = 0. Here P0 is the atmospheric pressure density and W is the natural velocity [4]. We obtained the pressure derivatives (pV2)0 of pV 2 for p = 0 by means of the equation

(pV )o = (poW%o + - - (Oo W2)o

3Br

where B r is the isothermal bulk modulus and p and V are the density and velocity at pressure p. Table I summarises the results obtained for the elastic constants of YSZ and their pressure de- rivatives.

0378-4363/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

184

Table I Values in GPa

S. Hart et al. / Elastic constants o f yttria-stabilised zirconia

Yttria content

Constant 9.4 15 18 21 24

C~ 401.443 387.916 380.43 367.745 357.156 C,4 57.457 64.254 68.867 71.245 72.164 C12 105.583 104.336 102.263 105.805 104.796 1 (C u _ C12) 147.93 141.79 139.29 130.97 126.18 B = ~ (Cll + 2C12 ) 204.20 198.86 194.99 193.12 188.92 a C u /ap 8.362 5.966 5.214 5.698 4.232 0C44/0 p 1.464 1.038 1.2142 1.155 1.155 (a/ap)[~ (ell - - C12)] 1.395 1.001 0.835 0.276 0.329 aB/Op 6.50 4.63 4.10 5.32 3.79

Fig. 1 shows the variation of the elastic con- stants of YSZ with Y 2 0 3 c o n t e n t . We have included for comparison Kandil et al's [1] results. Good agreement between our data and those of Kandil et al. is observed.

420

4O5

3 r , 0 390

375

36O

~9

C'J -- 144 I

D ~38

*oJ 152

.3 7o £1_ L.9

,,a'- 65

(D

60

55

j .

/ , f /

,; ,'~ 2'o MOLE % YTTRIA

I 25

Fig. 1. Variation of the elastic constants of yttria stabilised zirconia with Y203 content, x, this work; +, Kandil et al. [1].

Figs. 2 and 3 show the Y 2 0 3 c o n t e n t depen- dence of the pressure derivatives of YSZ. Within the experimental accuracy (typically a few % for pressure derivatives) the variation is smooth with respect to yttria content.

3. Discussion

All the samples examined by us displayed a normal behaviour in that the pressure derivatives of their elastic moduli were positive. There is no mode softening in these materials, irrespective of yttria content.

The positive derivatives of the shear stiffnesses C44 and C ' = ½(C11- C 1 2 ) show that the resis- tance to shear increases with pressure. The tetra- gonal to monoclinic phase change in ZrO 2 is diffusionless and could be associated with shear mode softening. The particular modes of shear associated with this transition are related to C44 in the stabilized cubic phase [2]. If the transition were elastic it would be driven by a soft shear mode and the associated elastic constant would be small and its pressure derivative would be nega- tive. Hailing and Saunders [2] observed that for 8 mol% YSZ C 4 4 w a s much smaller than ½(Cll - C12 ) and dC44/d p was much smaller than either (a /ap)[½(Cl l -- C12)] or ac,,/ap. This might be an indication of the stabilizing role of Y203 at low Y203 concentration.

Although an 8% YSZ was not examined here a 9.4% sample was, and considerable differences in the pressure derivatives between our data and

S. Hart et al. / Elastic constants of yttria-stabilised zirconia 185

7

0- "0

6

C.)

5

+

I I I

I0 15 2 0

MOLE % YTTRIA

Fig. 2. Y 2 0 3 c o n t e n t dependence of the pressure derivative of C H.

2 5

those of ref. [2] should be noted. Our data is numerically much like those of other similar crystalline materials while Hailing and Saunders [2] reported unusually high data with dB/dp being 12.6 for example. It may be that odd behaviour is possible for samples close to the low

yttria composition phase stable region, however no such anomalies occur at the higher compos- itions (24%) where a mixed oxide is the stable phase.

Our data also shows that the pressure deriva- tives of the two shear modes are similar in

1.5 ' ~ ' X i i i

1 . 2 5

I

.5 + = d C ' / d P

X - dC 4 4 / d P

2 5

0 L i L

5 I0 15 2 0 25

MOLE/MOL % Y203

Fig. 3. Y203 content dependence of the pressure derivative of C44 and C' = ~(C H - C~2 ).

rl .-r5

" o

186 S. Hart et al. ! Elastic constants o f yttria-stabilised zirconia

magnitude - about unity - at the low yttria com- positions.

When the Y203 content is increased, however, dCaJd p shows little change, as can be seen in fig. 3, and it is dC'/dp that decreases quite drastically and would extrapolate to negative value at con- centrations greater than 25%. It is possible that such mode softening would be associated with some other phase transformation which might be brought about by pressure, at higher Y203 con- centrations. The phase boundary in the ZrO 2- Y203 phase diagram at that Y203 concentration is rather uncertain. The indications are however that one enters a region of Z r O 2 - Y 2 0 3 solid solution and YaZr3012 [5]. This implies that the Y203 concentration in the solid solution compo-

nent cannot increase any further. It appears therefore, that although Y 2 0 3 stabilizes the cubic ZrO 3 structure up to 24%, its stabilizing ability decreases with increasing Y203 concentration.

References [1] H.M. Kandil, J.D. Greiner and J.F. Smith, J. Amer.

Ceram. Soc. 67 (1984) 341. [2] T. Hailing and G.A. Saunders, J. Mat. Sci. Letter 1 (1982)

416. [3] M.H. Manghnani, L.C. Ming and T. Matsui, in: High

Pressure Science and Technology, eds. B. Vodar and Ph. Marteau (Pergamon, Oxford, 1980) p. 1092.

[4] R.N. Thurston and K. Bruger, Phys, Rev. 133 (1964) A1604.

[5] C. Pascual and P. Duran, J. Amer. Ceram. Soc. 66 (1983) 23.