|
Mechanical Behaviour of Yttria- and Ferric Oxide-Doped Zirconia at Different Temperatures
George A. Gogotsi
Pisarenko Institute for Problems of Strength, 2, Timiryazevskaya Str., 01014 Kiev, Ukraine
Received 18 October 1996; accepted 8 July 1997
|
CERAMICS
INTERNATIONAL
|
Abstract
The variation of strengh, deformability, fracture toughness and other characteristics of partially stabilized zirconia ceramics (Y-Fe-PSZ) doped with 3% yttrium and 3% ferric oxide over a temperature range from -140 to 1400 °C were investigated. Fracture toughness (
K
1c
) values obtained by the methods such as SENB, SEPB, IS and IF were compared. Lower temperatures resulted in an increase in fracture toughness by approximately 29%. Using the Vickers indents as stress concentrators for the IS tests we derived the relationship between
K
1c
values and mean values of radial crack length described by a second-degree polynomial. The data for zirconia (Y-PSZ and Mg-PSZ) and silicon-nitride ceramics, as well as the fractographic data, were used to analyse the results. It was established that the addition of ferric oxide exerted a positive efect on the strengh and fracture toughness of zirconia ceramics.
© 1998 Elsevier Science Limited and Techna S.r.l.
1 INTRODUCTION
As single crystals transformation-toughened zirconia-based ceramics stabilised with yttria are usually linear elastic and those stabilised with magnia and ceria are usually inelastic at room temperature
[1]
. This gives rise to other diferences in their behaviour on loading. However, the mechanical behaviour was mainly studied on the materials characterised with considerable inelasticity
[3-5]
while the materials with the limit of proportionality not greatly differing from the ultimate strength were given little attention. Since any possible inelasticity can affect the evaluation of real performance of ceramics, Y-PSZ with the addition of ferric oxide somewhat changing their mechanical behaviour were used for the investigation.
2 MATERIALS AND METHODS
The ceramics under study were produced by slipcast molding followed by isostatic pressing at 2 kbars and sintering. Besides ZrO
2
of technical purity, they contained 3% Y
2
O
2
and 3% F
2
O
3
(see
Ref. 6
for more detailed information).
To compile additional information for the data analysis, we also studied other materials (
Table 1
). In particular we tested conventional partially stabilised ceramics Y-PSZ produced almost in the same way as the above
[6]
, tetragonal polycrystalline ceramics Y-TZP prepared by EMPA (Switzerland), their fracture toughness being tested room temperature (RT)
[7]
, and partially stabilised ceramics Mg-PSZ of TS-grade made by Nilcra Ceramics (Australia), being investigated at low temperatures (LT)
[8]
. We also used Y-PSZ crystals (of the same composition as Y-PSZ-C) prepared by the Institute of General Physics of Russian Academy of Sciences
[9]
, comparatively uniform reaction-bonded (Si
3
N
4
-RB
[10]
) and hot-isostatically-pressed (Si
3
N
4
-HIP
[11]
) silicon nitride ceramics.
Table 1. Materials for additional tests
|
|
Materials |
Index |
Additives (%) |
Brittleness measure, χ |
Flexural strength,
S
(MPa) |
Modulus of elasticity,
E
(GPa) |
|
|
Y-TZP |
TZP |
Y
2
O
3
(3) |
1 |
774 |
211 |
|
Y-PSZ |
PSZ |
Y
2
O
3
(3) |
1 |
425 |
197 |
|
Y-PSZ-C |
C |
Y
2
O
3
(3) |
1 |
506 |
297 |
|
Mg-PSZ |
Mg |
MgO (9) |
0.46 |
641 |
207 |
|
Si
3
N
4
-RB |
S-RB |
SiC (30) |
1 |
165 |
178 |
|
Si
3
N
4
-IP |
S-HP |
Y
2
O
3
(4) |
1 |
658 |
314 |
Rectangular beam specimens were prepared from the blanks and ground with diamond tools, their edges being rounded. To prepare the specimens for flexural fracture toughness tests, three stress concentrators were applied on to the surface along their axis. Therefore, in these cases the experimental points possess double designation the first part of which is the specimen number and the second one is the index of stress concentrator location. For indentation tests the fragments of these specimens were used. To study the indenter impressions and to observe the crack development, the sides of the specimen were polished with diamond paste.
The investigations were mainly based on our procedures for specimen testing at different temperatures
[12]
. In accordance with them the displacement of specimens (δ) and the applied loads (P) were measured on 4-point flexure (
Fig. 1
). For this purpose we used experimental loading modules mounted on conventional test machines. The δ values were measured by LVTD with a sensitivity of ±0.1 μm, the specimens were cooled under liquid nitrogen vapors. The loading module for RT indentation tests was equipped with the Vickers indenter and the 2-co-ordinate support table.
|
|
Fig. 1.
(a) Ceramtest-type loading module for flexural tests (strength, fracture toughness, deformability, acoustic-emission and elasticity) of ceramics at RT and (b) schematic drawing of a specimen for the strength test.
|
The inelasticity of ceramics was estimated by their brittleness measure χ
2
which is equal to the ratio of the specific elastic energy accumulated in the material by the moment of its fracture to the total specific energy spent for its deformation by the same moment:
|
(1)
|
where
σ
u
is the ultimate strength,
E
is the elasticity modulus,
ε
u
is the ultimate strain and
σ
is the stress at current strain values
ε
. For elastic materials the flexural strengths were calculated by the conventional equation of applied mechanics
[13]
:
|
(2)
|
For inelastic materials Nadai's equation
[14]
was used (for designations see
Fig. 1(b)
:
|
(3)
|
The
ε
values were determined as
|
(4)
|
To calculate the ultimate strength
σ
u
and the ultimate strain
ε
u
the ultimate values of
P
and
δ
were substituted into (
3
) and (
4
), respectively.
The fracture toughness results were obtained by several competing procedures (
Table 2
): by the simplest and widely used method (SENB); by the method which appears to be the most reliable (SEPB), by the method characterised by a simple preparation of stress concentrators (IS); and by the method requiring a small amount of test materials (IF).
Table 2. Test methods
|
Method
|
Speciment
|
|
Single edge notched beam SENB
|
|
|
Single edge precracted beam SEPB
|
|
|
Indentional strength IS
|
|
|
Indentional fracture IF
|
|
SENB and SEPB test results were processed as usual by
[15]
:
|
(5)
|
Here
IS fracture toughness was determined according to
[16]
|
(6)
|
Here
η
=0.59,
H
V
is the Vickers hardness,
Q
is the indentation load, and
S
i
is the flexural strength of the specimen with the impression.
In the IF method the
K
V
values (
K
V
is defined as an approximation of
K
1c
for the IF-method ) were determined by
[17]
:
|
(7)
|
But since
[17]
is designed for the cases of median cracks formed under the Vickers indenter (until now we revealed them in ZrO
2
materials only in cubic single crystals), the equation
[18]
for the Palmqvist crack was also applied:
|
(8)
|
3 RESULTS
Fe
2
O
3
-doped ceramics sintered at different temperatures (
Table 3
) were tested on 3-point flexure (40mm span). It was established that at 1200 °C or higher they exhibited considerable creep (crosshead speed was 0.5mm min
-1
). Proceeding from the data obtained, ceramics sintered at 1340 °C were chosen as optimum ones, they were designated Y-Fe-PSZ (``Fe'' is the index for
Fig. 2
) and investigated at RT and LT. The results of their strength determinations (eqns (2) and (3)) at RT on 4-point flexure (20-40mm span) are summarised in
Table 4
. Fracture toughness results (eqns (5) and (6)) on 3-point flexure (20mm span) are depicted in
Fig. 2
, the data for other materials are also presented for comparison. The
K
v
values (eqns (7) and (8)) as a function of the load applied to the indenter are demonstrated in
Fig. 3
.
Table 3. Test results for ZrO
2
-ceramics with Y
2
O
3
and Fe
2
O
3
|
|
Sintered temperature (°C) |
Test temperature (°C) |
|
|
|
|
|
|
AT |
800 |
1200 |
|
|
χ |
S (MPa) |
K
1c
(MPam
1/2
) |
S (MPa) |
K
1c
(MPam
1/2
) |
S (MPa) |
K
1c
(MPam
1/2
) |
|
|
1320 |
0.99 |
812 |
10.9 |
308 |
4.3 |
- |
- |
|
1330 |
0.94 |
859 |
8.8 |
237 |
3.5 |
- |
- |
|
1340 |
0.88 |
977 |
13.3 |
303 |
5.5 |
207 |
2.3 |
|
1350 |
0.95 |
786 |
12.0 |
217 |
3.3 |
120 |
2.9 |
|
|
|
Fig. 2.
Results of comparative fracture toughness tests at RT and -1400 °C.
|
|
|
Fig. 3.
Fracture toughness and
c/a
relation for Y-Fe-PSZ on the Vickers indentation.
|
4 DISCUSSION
Data in
Table 4
show that Y-Fe-PSZ displays not only the scatter of strength values which is usually taken into account, but also that of the brittleness measure.
The analysis of
Fig. 2
reveals a growth of fracture toughness with a fall of test temperature both for the ceramics under study and for other ceramics and single crystals with Y2O3. In this case the S value for Y-Fe-PSZ ceramics increased up to 917 MPa. On the whole these data do not contra- dict other results of Refs
1
,
8
,
19
. At the same time the fracture toughness of values for Si
3
N
4
ceramics (S-HP and S-RB in
Fig. 2
) remained practically unchanged under such conditions.
Table 4. Test results for Y-Fe-PSZ ceramics at RT (4-point flexure)
|
|
|
35 |
41 |
37 |
45 |
39 |
44 |
40 |
46 |
Average |
SD |
|
|
χ |
0.7 |
0.82 |
0.86 |
0.92 |
0.93 |
0.96 |
1 |
1 |
0.88 |
0.1 |
|
σ
u
(MPa) |
760 |
767 |
673 |
637 |
717 |
791 |
638 |
765 |
711 |
62 |
|
ε
u
10
-4
(mm
-1
) |
45.9 |
42.0 |
41.1 |
35.7 |
38.0 |
42.9 |
33.1 |
40.2 |
39.9 |
4.1 |
|
S
(MPa) |
866 |
811 |
698 |
658 |
732 |
797 |
638 |
765 |
746 |
79 |
|
E
(GPa) |
203 |
196 |
181 |
188 |
196 |
189 |
193 |
190 |
192 |
7 |
Inelastic ceramics Mg-PSZ exhibited an increase and then a decrease of their
K
1c
-values (Fig. 4) with lower test temperatures. This e€ect (see also
Ref. 8) was observed in the temperature range of a considerable inelasticity decrease (in
Fig. 4
it is shown as a change of the brittleness measure χ) for these ceramics in the zone of their first low temperature elastic-inelastic transition.
During the
K
1c
calculations of values based on SEPB data (eqn (4)) one could pay attention to a considerable dependence (
Table 5
) of the results on the methods of determining the crack length which were as follows: (i) compliance data for the specimen with a crack; (ii) mean values of crack lengths on both sides of the specimen or (iii) measurements (ASTM E-399
20
) on the fracture surface of the specimen (
Fig. 5
). If we compare IS and SEPB fracture toughness data (
Fig. 2
and
Table 5
), the result corresponding to the third method appear to be the closest ones. It should be noticed that just this method is recommended, e.g. in Refs
21
,
22
. Unfortunately, we could hardly overcome (and not always) technical problems associated with measurements on fracture surfaces which also took place in
[21]
.
|
|
Fig. 4.
Fracture toughness variation for Mg-PSZ ceramics in the temperature range of inelastic-elastic transition.
|
|
|
Fig. 5.
Fracture surface of the specimen (SEPB method): 1 - saw notch, 2 - precrack front, 3 - fast fracture region.
|
Table 5. Effect of crack length measurement procedure on
K
1c
values (SEPB)
|
|
(i) Method |
(ii) Method |
(iii) Method |
|
I
(mm) |
K
Ic
(MPam
1/2
) |
I
(mm) |
K
Ic
(MPam
1/2
) |
I
(mm) |
K
Ic
(MPam
1/2
) |
|
|
Y-Fe-PSZ |
|
2.31 |
2.1 |
2.79 |
3.2 |
3.09 |
4.5 |
|
3.01 |
4.7 |
3.0 |
5.0 |
3.34 |
7.4 |
|
3.24 |
4.7 |
3.0 |
3.2 |
3.41 |
6.1 |
|
3.52 |
5.4 |
- |
- |
3.71 |
6.9 |
|
Y-PSZ |
|
2.91 |
3.8 |
2.96 |
4.0 |
3.17 |
5.0 |
|
- |
- |
2.77 |
5.3 |
2.9 |
6.6 |
|
- |
- |
2.77 5.3 3.0 5.8 |
5.3 |
3.0 |
5.8 |
|
Mg-PSZ |
|
2.22 |
9.5 |
2.33 |
10.2 |
2.60 |
12.0 |
|
2.23 |
9.8 |
2.51 |
11.6 |
2.55 |
11.9 |
The data of Table 5 are also interesting because they, for instance, explain the difference in the shape of R-curves
[23]
plotted by methods (i) and (ii) and generally point to the signi®cance of such curves obtained by the method (i) on flexure.
A detailed consideration of IS fracture toughness data for Y-Fe-PSZ ceramics (
Table 6
) can hardly reveal any unambiguous dependence between K1c values and crack lengths
c
formed on the indentation of the specimen. Taking into account the observation of the authors of this method
[16]
that a certain change in the length of such cracks did not exert a noticeable influence on the values of
K
Ic
and remaining within the Griffth theory, we assumed the following: if in our case the geometric parameters of loading are constant and the distribution of acting stresses in the specimen is the same, the beginning of uncontrolled fracture of every material is determined by its own critical crack length
l
cr
. Thus, an initial crack c of any length should develop up to the
l
cr
value during the flexural loading of the specimen. Therefore, there are no grounds to consider (see in
Ref. 21
for observations of different authors on this problem) that such heterogeneous ceramics as Y-Fe-PSZ should display an increase of 2.52 times, as it was shown in
Ref. 16
and confirmed in
Ref. 24
. The fact that every material is characterised by its own behaviour of subcritical crack (c) growth is also con®rmed by the differences in prefracture zones on load-displacement curves for the IS specimens (
Fig. 6
).
Table 6. Test results for Y-Fe-PSZ ceramics (IS method)
|
|
Specimen |
Test temperature (°C) |
|
|
|
|
Specimen |
Test temperature (°C) |
|
(no./index) |
AO |
-140 |
|
|
|
|
K
Ic
(MPam
1/2
) |
c
(mm) |
K
Ic
(MPam
1/2
) |
c
(mm) |
c
av
(mm) |
|
|
23/1 |
7.9 |
338 |
- |
- |
|
|
/2 |
- |
- |
10.4 |
337 |
348 |
|
/3 |
- |
- |
10.5 |
339 |
|
|
24/2 |
- |
- |
14.9 |
262 |
|
|
/3 |
11.6 |
260 |
- |
- |
266 |
|
25/2 |
- |
- |
15.2 |
278 |
|
|
/3 |
- |
- |
13.7 |
241 |
278 |
|
64/2 |
- |
- |
12.2 |
295 |
|
|
/3 |
8.7 |
305 |
- |
- |
298 |
|
66/1 |
- |
- |
10.2 |
334 |
|
|
/2 |
8.3 |
331 |
- |
- |
312 |
|
/3 |
- |
- |
11.3 |
289 |
|
|
69/1 |
8.6 |
258 |
- |
- |
|
|
/2 |
- |
- |
11.5 |
264 |
297 |
|
70/1 |
9.4 |
271 |
- |
- |
|
|
/2 |
- |
- |
12.2 |
267 |
283 |
Having no possibility to make direct measurements confirming the above, we tried to find a characteristic associated with
l
cr
. As the first approach, we used an averaged crack length near the indenter impressions (
c
av
in
Table 6
) specific for each specimen. For its determination on each fragment of IS specimens tested at a load of 500 N, 12 impressions were made. The curves were plotted with cav value (
Fig. 7
); they are described by second-degree polynomial the coeficients of which coincide in rough approximation for RT and -140°C.
As opposed to the above, the results of similar Y-PSZ tests (
Table 7
) did not reveal any specific features.
The tests by the IS method show that Y-Fe-PSZ ceramics (data were obtained on other series of specimens) exhibit lower
K
Ic
values with an increase in indentation loads. Thus at 100, 250 and 500N loads they were equal to 11.07, 11.1 and 9.95 MPam
1/2
, respectively. For Y-PSZ the picture is approximately the same, their
K
Ic
values were 6.1, 5.8 and 5.8 MPam
1/2
, respectively.
Considering these unusual results, we tried to verify the reliability of the test method used. For this purpose Y-TZP ceramics and the test data of
Ref. 7
were employed. A certain agreement between our data and the data by other authors was established (
Table 8
).
Table 7. Test results for Y-PSZ ceramic (IS method)
|
Specimens
(no./index) |
8/3 |
6/3 |
3/3 |
7/3 |
5/3 |
13/3 |
|
|
K
1c
(MPam
1/2
) |
5.95 |
5.95 |
5.81 |
5.77 |
5.75 |
5.62 |
|
c
(μm) |
4.00 |
4.11 |
4.35 |
4.24 |
4.17 |
4.28 |
|
c
av
(mm) |
4.10 |
4.12 |
4.35 |
4.11 |
4.08 |
4.24 |
|
Table 8. Fracture toughness results for Y-TZP ceramics
|
|
Test method |
IS |
Chevron notched beam method |
|
|
No. of specimens |
4 |
3 |
4 |
4 |
|
K
1c
(MPam
1/2
) |
5.67 |
5.82 |
5.61 |
5 |
|
SD (MPam
1/2
) |
0.38 |
0.1 |
0.09 |
0.27 |
|
Indentation load (Q) |
100 |
500 |
- |
- |
|
Investigators |
IPP |
IPP |
EMPA
[7]
|
ESN
[7]
|
|
|
|
Fig. 6.
Final sections of prefracture zones on load-displacement curves for the IS-tested specimens of different ceramics.
|
The analysis of IF data (
Fig. 3
) has demonstrated that higher indentation loads result in lower Kv values. They also differ considerably depending on the equation applied for calculations. It is typical that in all the cases of indentation only Palmqvist cracks were formed in the specimens (
Fig. 8(a)
). For Y-Fe-PSZ ceramics the ratio of
c/a
achieved 2.3 (usual condition of validity of applying
eqn (7)
at loads close to those inducing the failure of indenter impressions (
Fig. 8(b)
). It is interesting that similar failure of indenter impressions was earlier observed also in the tests of ZrO
2
PSZ crystals
[19]
. It appears, e.g. that the ceramics under study do not meet the requirements set forth in
Ref. 22
, only when they meet them, their fracture toughness can reliably be evaluated by the IF method. It should be noted that the use of
eqn (8)
does not introduce any specific features in the unambiguity of data obtained by different test methods.
|
|
Fig. 7.
K
1c
values for Y-Fe-PSZ ceramics as a function of average crack lengths near the indenter impression (IS method).
|
|
|
Fig. 8. Indentation results for Y-Fe-PSZ specimens: (a) radial crack pattern near the impression (surface layer is ground-off at 500 N; (b) fracture zone of the impression at 800N (* surface of impression).
|
5 CONCLUSIONS
We investigated the variation of strength, deformability, fracture toughness and other characteristics of partially stabilised zirconia ceramics (Y-Fe-PSZ) doped with 3% yttrium and 3% ferric oxide over a temperature range from -140 to 1400°C. Fracture toughened (
K
1c
) values obtained by such methods as SENB, SEPB, IS and IF were compared. Lower temperatures resulted in an increase in fracture toughness by 29% approximately. For the IS test the Vickers indents used as stress concentrators we derived the relationship between
K
1c
values and mean value of radial cracks length described by a second-degree polynomial. It was demonstrated that for the SEPB-method
K
1c
values were strongly dependent on a procedure of measuring the crack initial length. The data for zirconia (Y-PSZ and Mg-PSZ) and silicone-nitride ceramics as well as fractographic ones were used to analyse the results. It was established that the addition of ferric oxide exerted a positive effect on the strength and fracture toughness of Y-Fe-PSZ ceramics. This is associated with a decrease of brittleness caused by their relative inelastisity.
ACKNOWLEDGEMENTS
Some of the materials studied were kindly supplied by S. Yu. Pliner, G. D. Quinn and M. V. Swain.
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