Sung R. Choi

Ohio Aerospace Institute, Brook Park, Ohio

Narottam P. Bansal

Glenn Research Center, Cleveland, Ohio

Strength, Fracture Toughness, and Slow Crack

Growth of Zirconia/Alumina Composites at

Elevated Temperature

NASA/TM—2003-212108

February 2003
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Sung R. Choi

Ohio Aerospace Institute, Brook Park, Ohio

Narottam P. Bansal

Glenn Research Center, Cleveland, Ohio

Strength, Fracture Toughness, and Slow Crack

Growth of Zirconia/Alumina Composites at

Elevated Temperature

NASA/TM—2003-212108

February 2003

National Aeronautics and

Space Administration

Glenn Research Center
Acknowledgments

The authors are grateful to Ralph Pawlik, for mechanical testing during the course of this research. This work was

supported by the Zero CO2 Emission Technology (ZCET) Project, Aerospace Propulsion and Power Program, and

NASA Glenn Research Center.

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7121 Standard Drive

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paper, intended to solicit comments and

ideas from a technical peer group.

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identification only. This usage does not constitute an official

endorsement, either expressed or implied, by the National

Aeronautics and Space Administration.

The Propulsion and Power Program at

NASA Glenn Research Center sponsored this work.

Available electronically at http://gltrs.grc.nasa.gov
NASA/TMó2003-212108 1

STRENGTH, FRACTURE TOUGHNESS, AND SLOW CRACK GROWTH OF 

ZIRCONIA/ALUMINA COMPOSITES AT ELEVATED TEMPERATURE 

Sung R. ChoiÜ

Ohio Aerospace Institute 

Brook Park, Ohio 44142 

Narottam P. Bansal 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Various electrolyte materials for solid oxide fuel cells were fabricated by hot pressing 10 mol % yttriastabilized zirconia (10-YSZ) reinforced with two different forms of aluminaóparticulates and 

plateletsóeach containing 0 to 30 mol % alumina. Flexure strength and fracture toughness of platelet 

composites were determined as a function of alumina content at 1000 °C in air and compared with those 

of particulate composites determined previously. In general, elevated-temperature strength and fracture 

toughness of both composite systems increased with increasing alumina content. For a given alumina 

content, flexure strength of particulate composites was greater than that of platelet composites at higher 

alumina contents (≥20 mol %), whereas, fracture toughness was greater in platelet composites than in 

particulate composites, regardless of alumina content. The results of slow crack growth (SCG) testing, 

determined at 1000 °C via dynamic fatigue testing for three different composites including 0 mol % 

(10-YSZ matrix), 30 mol % particulate and 30 mol % platelet composites, showed that susceptibility to 

SCG was greatest with SCG parameter n = 6 to 8 for both 0 mol and 30 mol % particulate composites 

and was least with n = 33 for the 30 mol % platelet composite. 

 

INTRODUCTION 

Solid oxide fuel cells (SOFC) are currently being developed for various power generation applications. 

The major components of a SOFC are the electrolyte, the anode, the cathode, and the interconnect. The 

two porous electrodes, anode and cathode, are separated by a fully dense solid electrolyte. Currently, 

yttria-stabilized zirconia (YSZ) is the most commonly used electrolyte in SOFC because of its high 

oxygen ion conductivity, stability in both oxidizing and reducing environments, availability, and low 

cost [1]. However, similar to other ceramics, YSZ is brittle and susceptible to fracture due to the 

existence of flaws, which are introduced during fabrication and use of the SOFC. In addition, the 

properties of YSZ such as low thermal conductivity and relatively high thermal-expansion coefficient 

make this material thermal-shock sensitive. Fracture in the solid oxide electrolyte will allow the fuel and 

oxidant to come in contact with each other resulting in reduced cell efficiency or in some cases 

malfunction of the SOFC. Therefore, YSZ solid electrolyte with high fracture toughness as well as 

enhanced strength is required from a performance and structural reliability point of view. Furthermore, 

because of high operating temperature (around 1000 °C) in SOFCs, it is certainly expected that slow 

crack growth would take place, which limits the life of electrolyte or SOFC system. Therefore, it is 

important to determine slow crack growth or life prediction parameters of the material to ensure 

accurate life prediction of SOFC components at elevated temperatures. 

 

Ü NASA Senior Resident Research Scientist at Glenn Research Center
NASA/TMó2003-212108 2

The main objective of this work was to improve the strength and fracture toughness of the YSZ 

electrolyte for SOFC applications without adversely affecting its high-temperature ionic conductivity to 

an appreciable extent. The 10 mol % yttria-stabilized zirconia (10-YSZ) was reinforced with two 

different forms of alumina, particulates and platelets, each containing 0, 5, 10, 20, and 30 mol % 

alumina through mixing, milling, and hot pressing to full density [2,3]. Flexure strength and fracture 

toughness of YSZ/alumina composites were determined as a function of alumina content at ambient 

temperature for both particulate [2] and platelet [3] composites and at 1000 °C for the particulate 

composites [2], together with elastic modulus, density, and microhardness. Ambient-temperature flexure 

strength and fracture toughness of both particulate and platelet composites increased with increasing 

alumina content, reaching a maximum at 30 mol % alumina. For a given alumina content, flexure 

strength of the particulate composites was greater than that of the platelet composites, whereas, the 

difference in fracture toughness between the two composite systems was negligible. No virtual 

difference in elastic modulus and density was observed for a given alumina content between particulate 

and platelet composites. 

This paper, as an extension of the previous work [2,3], describes behavior of flexure strength and 

fracture toughness of the platelets composites as a function of alumina content at 1000 °C, which is 

close to typical operating temperature of SOFC systems. The high-temperature flexure strength and 

fracture toughness of the platelet composites were compared with those of particulate composite 

determined previously [2]. Slow crack growth required for component design and life prediction was 

determined in flexure at 1000 °C in air using dynamic fatigue testing for selected materials including 

0 mol % (10-YSZ matrix), 30 mol % particulate and 30 mol % platelet composites. 

EXPERIMENTAL PROCEDURES 

Processing and Test Specimen Preparations 

Material processing was essentially the same as described elsewhere [2,3]. Briefly, the starting materials 

used were alumina powder [2] (high purity BAILALOX CR-30) from Baikowski International 

Corporation, Charlotte, NC, 10-mol % yttria fully stabilized zirconia powder (HSY-10) from Daiichi 

Kigenso Kagaku Kogyo Co., Japan, and alpha alumina hexagonal platelets (Pyrofine Plat Grade T2) [3] 

from Elf Atochem, France. Appropriate quantities of alumina and zirconia powders were slurry mixed 

in acetone and mixed for ~24 h using zirconia media. Acetone was than evaporated and the powder 

dried in an electric oven. The resulting powder was loaded into a graphite die and hot pressed at 

1500 °C in vacuum under 30 MPa pressure into 152 by 152 mm billets using a large hot press. Grafoil 

was used as spacers between the specimen and the punches. Various hot pressing cycles were tried in 

order to optimize the hot pressing parameters that would result in dense and crack free ceramic samples. 

The processing flow diagram is shown in Figure 1. Five different YSZ/alumina composites containing 

0 to 30 alumina mol % were fabricated for each of particulate and platelet composite systems. 

The billets were machined into flexure test specimens with nominal depth, width and length of 

3.0 by 4.0 by 50 mm, respectively, in accordance with ASTM test standard C 1211 [4]. Machining 

direction was longitudinal along the 50 mm-length direction. It should be noted that unlike transformationtoughened (from tetragonal to monoclinic) zirconias, the cubic yttria-stabilized zirconia is very unlikely to 

induce any transformation-associated residual stresses on the surfaces of test specimens due to machining. 

The sharp edges of test specimens were chamfered to reduce any spurious premature failure emanating 

from those sharp edges. 
NASA/TMó2003-212108 3

 

 

Figure 1. Processing flow diagram for 10-mol % yttria-stabilized zirconia/alumina composites, 

applied to both particular and platelet composites [2,3]. 

Flexure Strength and Fracture Toughness Testing 

Flexure strength was determined as a function of alumina content for the platelet composites at 1000 °C in 

air. Note that both flexure strength and fracture toughness of the particulate composites were determined 

previously at 1000 °C in air as a function of alumina content [2]. A SiC four-point flexure fixture with 

20 mm-inner and 40 mm-outer spans was used in conjunction with an electromechanical test frame 

(Model 8562, Instron, Canton, MA). A fast test rate of 50 MPa/s was applied in load control to reduce 

slow crack growth effect of the materials.á

 A total of 10 test specimens were tested for each alumina 

content. Testing was followed in accordance with ASTM test standards C 1211 [4]. 

Facture toughness of the platelet composites using flexure bar specimens measuring 3 by 4 by 25 or 

50 mm was determined at 1000 °C in air using single edge v-notched beam (SEVNB) method [6]. This 

method was verified as an appropriate technique at elevated temperature free from crack healing due to 

oxidation [2]. This method utilizes a razor blade with diamond paste with a grain size of 9 μm to introduce 

a final sharp notch with a root radius ranging 10 to 20 μm by tapering a saw notch [2,3,6]. The sharp 

v-notched specimens with a notch depth of 0.9 mm were fractured in a four-point flexure fixture with 

20 (or 10) mm-inner and 40 (or 20) mm-outer spans using the electromechanical test frame (Model 8562, 

Instron) at an actuator speed of 0.5 mm/min. A total of five specimens were tested for each alumina 

content. Fracture toughness KIC was calculated based on the formula by Srawley and Gross [7] as follows: 

( ) 2(1 )

( ) 3

3/ 2

1/ 2

3/ 2 α

α

α f BW

P L L

K f o i

IC −

− = (1) 

where Pf, Lo, Li, B, W are fracture load, outer span, inner span, specimen width, and specimen depth, 

respectively, α = a/W with a being precrack size, and f(α) is expressed 

2

2

(1 )

(1 )(3.49 0.68 1.35 ) ( ) 1.9887 1.326

α

α α α α

α α

+

− − + f = − −

 

á

 Elevated temperature strength of many advanced ceramics depends on test rate due to slow crack growth during testing. It has been shown 

that elevated-temperature strength of advanced ceramics increases with increasing test rate and converges to ambient-temperature strength (or 

inert strength) at ìultraî-fast test rates ≥105

 MPa/s [5]. Although the test rate of 50 MPa/s used in this work was not sufficient to obtain an 

appropriate ìinertî strength whereby no slow crack growth occurs, the test rate of 50 MPa/s was chosen to determine the conventional, 

so-called ìfast-fractureî strength of the material in which a test rate of around 30 to 100 MPa/s is typically employed. 

Hot Pressing 

(30 MPa;2h;vacuum;1500 °C)

Billets 

Drying 

Machining into Test Specimens 

Ball Milling (10-YSZ+alumina) 

(zirconia media;acetone;24h) 
NASA/TMó2003-212108 4

Slow Crack Growth (Dynamic Fatigue) Testing 

Slow crack growth (SCG) behavior of the chosen composites was determined at 1000 °C in air using 

dynamic fatigue (or called ëconstant stress-rateí) testing in accordance with ASTM test method C 1465 

[8]. Three different composites including 0 and 30 mol % alumina particulate and 30 mol % alumina 

platelet composites were chosen based on the results of optimum strength and fracture toughness 

properties exhibited by the 30 mol % particulate and platelet composites [2,3]. The additional 0 mol % 

composite, 10-YSZ matrix, was used to generate the baseline data. Dynamic fatigue testing was 

performed in flexure using flexure test specimens (3 by 4 by 50 mm) at two different test rates of 50 and 

0.005 MPa/s for a given composite under load control of the electromechanical test frame (Model 8562, 

Instron). A SiC four-point flexure fixture with 20/40 mm spans was used. A total of 10 test specimens 

were used at each test rate for a given composite. 

RESULTS AND DISCUSSION 

Flexure Strength 

The results of elevated-temperature strength testing for the platelet composites are presented and 

compared with the particulate composite strength [2] in Figure 2, where average strength was plotted for 

clarity as a function of alumina mol % for both composite systems. The strength of the platelet 

composites, except for 10 mol %, remained almost unchanged or a little decreased from 10 mol % with 

increasing alumina content. The maximum strength occurring at 10 mol % was 17% greater than the 

ëzeroí alumina-content strength. The strength of the platelet composites was greater at 5 and 10 mol % 

but lower at 20 and 30 mol % than the particulate composites counterpart. The trend in strength increase 

with alumina content was more significant in the particulate composites than in the platelet composites. 

The strength of the particulate composites with respect to the ëzeroí-alumina content (10-YSZ) strength 

decreased initially at 5 mol % and increased thereafter with increasing alumina content, reaching a 

maximum at 30 mol %. The maximum strength of the particulate composites occurring at 30 mol % 

alumina content was 40% greater than the ëzeroí-alumna content strength. Also, the particulate strength 

at 30 mol % was 30% greater than the respective platelet strength. 

As also can be seen from Figure 2, the strength scatter was less significant in the platelet composites 

than in the particulate composites. Weibull modulus (m), estimated despite a limited number (10) of 

specimens tested at each alumina content, was in the range of m = 8 to 19 and m= 5 to 10, respectively, 

for the platelet and particulate composites. Fracture originated distinctly from surface-connected defects 

(ìsurface flawsî) for both composites. Platelets, mostly consistent in size, were frequently associated as 

primary failure origins. Hence, alumina platelets acted as strength-controlling flaws rather than as 

reinforcing media, typical of many platelets-reinforced composites, resulting in less scatter in strength 

or higher Weibull modulus. The reason why the particulate composites exhibited improved strength 

over the platelet composites particularly at 30 mol % content is primarily due to the fact that alumina 

particulates might have acted as a major source of reinforcement. The effect of residual stress on overall 

resulting strength of the platelet composites, due to CTE mismatches between matrix and platelets, was 

shown negligible from the previous thermal cycling experiment [3]. 
NASA/TMó2003-212108 5

ALUMINA CONTENT, mol %

0 10 20 30 40

FLEXURE STRENGTH, 

σf [MPa]

0

100

200

300

400

500

Particulates

Platelets

STRENGTH/1000oC

Figure 2. Flexure strength of 10-YSZ/alumina platelet composites as a function of alumina content 

at 1000 °C in air. Flexure strength of 10-YSZ/alumina particulate composites [2] is 

included for comparison. Error bars indicate ±1.0 standard deviations. 

ALUMINA CONTENT, mol %

0 10 20 30 40

FLEXURE STRENGTH, 

σf [MPa]

0

100

200

300

400

500

Particulates

Platelets

STRENGTH/RT

Figure 3. Flexure strength of 10-YSZ/alumina particulate and platelet composites as a function 

of alumina content at ambient temperature in air [2,3]. Error bars indicate ±1.0 standard deviations. 

For comparison, ambient-temperature flexure strength of both platelet and particulate composites 

determined previously [2,3] are presented in Figure 3. The particulate composite strength increased 

almost linearly with increasing alumina content with a maximum 40% increase at 30 mol %, while the 

platelet composite strength was not significantly dependent on alumina content with a maximum 11% 

increase at 20 mol %. Flexure strength of the particulate composites for a given alumina content was 

approximately 15% greater than that of the platelet composites counterpart. Similar to the case of 

elevated-temperature strength, the scatter of ambient-temperature strength was all greater in the 

particulate composites than in the platelet composites, as seen in Figure 3. More exactly, Weibull 

modulus was found as m ≈ 13 and 7, respectively, for the particulate and platelet composites. The 

overall ambient-temperature strength of the two composite systems for a given alumina content was 

greater (20 to 50% and 10 to 30%, respectively, for the particulate and platelet composites) than their 

elevated-temperature counterpart, attributed to strength degradation at elevated temperature presumably 

by slow crack growth phenomenon. 
NASA/TMó2003-212108 6

Fracture Toughness 

A summary of fracture toughness of the platelet composites determined at 1000 °C is presented in 

Figure 4, in which the values of fracture toughness by the SEVNB method were plotted as a function of 

alumina mol %. Fracture toughness of the particulate composites determined at 1000 °C in air [2] was 

also included for comparison. Fracture toughness of the platelet composites increased monotonically 

with increasing alumina content with an initial jump from 0 to 5 mol %, reaching a maximum at 

30 mol %. Fracture toughness increased significantly by 74% when alumina content increased from 

0 to 30 mol %. In a similar way, fracture toughness of the particulate composites, in general, increased 

with alumina content, resulting in a maximum increase of 50% at 30 mol % alumina. Unlike the 

elevated-temperature strength, the difference in fracture toughness between the particulate and platelet 

composites was distinct for all of alumina contents. Fracture toughness was greater (15 to 80%) in the 

platelet composites than in the particulate composite, irrespective of alumina content, indicating that 

toughening by addition of platelets to 10-YSZ matrix might have been in effect through possible 

mechanisms such as bridging and/or crack deflection. At 30 mol % alumina content where maximum 

fracture toughness was attained for both composite systems, fracture toughness of the platelet 

composites was approximately 16% greater than that of the particulate composites. 

It has been observed that an incompatibility is generally operative for many advanced monolithic 

ceramic composites between strength and fracture toughness in such a manner that one property 

increases while the other decreases. This seemed to be the case for the platelet composites that showed 

not only insensitive or somewhat decreasing strength but also fracture-toughness increase with 

increasing alumina content. This confirms again a notion that platelets were significantly effective in 

toughening but sacrificial in strengthening. 

ALUMINA CONTENT, mol %

0 10 20 30 40

FRACTURE TOUGHNESS, KIC [MPa m1/2]

0

1

2

3

4

5

Particulates

Platelets

SEVNB/1000oC

Figure 4. Fracture toughness of 10-YSZ/alumina platelet composites as a function of alumina content at 1000 oC 

in air. Fracture toughness of 10-YSZ/alumina particulate composites [2] is included for comparison. 

Error bars indicate ±1.0 standard deviations. The lines represent the best fit. 
NASA/TMó2003-212108 7

ALUMINA CONTENT, mol %

0 10 20 30 40

FRACTURE TOUGHNESS, KIC [MPa m1/2]

0

1

2

3

4

5

Particulates

Platelets

SEVNB/25oC

Figure 5. Fracture toughness of 10-YSZ/alumina platelet and particulate composites as a function of 

alumina content at ambient temperature (25 o

C) in air [2,3]. Error bars indicate ±1.0 standard deviations. 

The line represents the best fit for both particulate and platelet composites. 

Fracture toughness of the two composite systems determined at ambient temperature [2,3] is presented 

for comparison in Figure 5. Fracture toughness increased with increasing alumina content for both 

composites, resulting in a maximum increase of about 60% at 30 mol % [2,3]. Contrast to the elevatedtemperature fracture toughness, ambient-temperature fracture toughness exhibited little difference 

between the particulate and platelet composites. It is also noted that the difference in fracture toughness 

of the particulate composites between 25 and 1000 °C was negligible, as can be seen by comparing 

Figures 4 and 5. Therefore, fracture toughness behavior of the platelet composites at 1000 °C is unique 

and therefore imposes a difficulty in terms of understanding governing mechanisms, since the platelet 

composites yielded much higher fracture toughness at elevated temperature than at ambient temperature. 

The only plausible explanation is that toughening mechanism(s) in the platelet composites might have 

been more operative at elevated temperature together with occurrence of possible crack healing and/or 

crack-tip plasticity. A further detailed fractographic analysis is a prerequisite to pinpoint governing 

mechanism(s) associated. 

Slow Crack Growth: Dynamic Fatigue 

The results of slow crack growth (ëdynamic fatigueí or ëconstant stress-rateí) testing at 1000 °C in air 

for three different composites including 0 mol % (10-YSZ matrix) and 30 mol % particulate and 

30 mol % platelet composites are shown in Figure 6, where fracture stress of each composite was 

plotted as a function of applied stress rate. The decrease in fracture stress with decreasing stress rate, 

which represents susceptibility to slow crack growth (SCG), was evident for all three composites with 

its degree of strength degradation with decreasing stress rate being dependent on material. The basic 

underlying formulation of slow crack growth for many advanced monolithic ceramics and composites 

(reinforced with particulates, platelets or whiskers) at elevated temperatures follows the following 

power-law form [9] n A KI KIC v = [ / ] (2) 

where v, KI and KIC are crack velocity, mode I stress intensity factor, and mode I fracture toughness, 

respectively. A and n are material/environment dependent SCG parameters. The responsible mechanism 

of SCG at elevated temperatures has been known as grain boundary sliding. In case of dynamic fatigue 
NASA/TMó2003-212108 8

loading, a constant stress rate (σ& ) is applied to a test specimen until the test specimen fails. The 

corresponding fracture stress (σf) can be derived from Eq. (2) and related stress intensity factor with 

some mathematical manipulations to give [8, 10,11] 

D

n f log log 1

1 log +

+

σ = σ& (3) 

where D is another SCG parameter associated with A, n, KIC, inert strength, and crack geometry factor 

[8,11]. The SCG parameters n and D can be determined from the slope and intercept by a linear 

regression analysis when log (fracture stress) is plotted as a function of log (applied stress rate). 

Equation (2) is the basis commonly used in dynamic fatigue testing, which has been adopted to 

determine SCG parameters of advanced ceramics in ASTM test standards at both ambient and elevated 

temperatures as well [8,11]. 

The results shown in Figure 6 were plotted according to Eq. (2) with units of MPa for σf and MPa/s 

forσ& . The SCG parameters n and D were found to be n = 8 and D = 126, n = 6 and D = 151, n = 33, 

and D = 179, respectively, for 0 mol % (10-YSZ), 30 mol % particulate and 30 mol % platelet 

composites, as summarized in Table 1. The respective correlation coefficients (rcoef) of the linear 

regression lines were rcoef = 0.8226, 0.9558, and 0.8860. Both 0 mol % and 30 mol % particulate 

composites exhibited very high susceptibility to SCG with relatively low SCG parameter n = 6 to 8 and 

the 30 mol % platelet composite exhibited intermediate susceptibility with n = 33.ß

 Hence, the 30 mol % 

platelet composite exhibited higher resistance to SCG as compared with both 0 and 30 mol % particulate 

composites. The addition of 30 mol % alumina platelets into 10-YSZ matrix might have resulted in 

increased resistance to grain boundary sliding, while the addition of 30 mol % fine alumina particulates 

would not have had any positive effect on reducing or minimizing such grain boundary sliding. Note 

that the value of SCG parameter n determined at 1000 °C in air for typical aluminas with 96 to 99% 

purity is in a range of n = 7 to 13 [12,13]. Significant improvement in SCG resistance was obtained by 

the composite approach with two specific materialsóYSZ and alumina (platelets)óin which each of 

YSZ and alumina exhibits a significantly high SCG susceptibility (n = 6 to 13) if they are used 

individually in a form of monolith. 

Table 1. Summary of slow crack growth parameters determined for different composites 

via dynamic fatigue testing in flexure at 1000 °C 

Slow crack growth 

parameters 

Materials Applied 

stress rate 

(MPa/s) 

No. of test 

specimens 

Mean flexure 

strength (MPa) 

n D 

10-YSZ (matrix) 50 

0.005 

10 

10 

 204.5(64.5)* 

 74.8(25.4) 

8.3 126.3 

10-YSZ/30 mol %-

alumina particulate 

composite 

50 

0.005 

10 

10 

 287.0(77.0) 

 66.7(7.7) 

5.5 150.6 

10-YSZ/30 mol %-

alumina platelet 

composite 

50 

0.005 

10 

10 

 201.1(17.1) 

152.9(8.0) 

32.9 178.5 

Notes: Tests conducted in four-point flexure with 20/40 mm spans, ASTM C1465 [8] in ambient air; 

The number in the parenthesis indicates ±1.0 standard deviation. 

 

ß Note that susceptibility to slow crack growth is typically categorized in advanced ceramics such that SCG susceptibility is very high for n

<20, intermediate for n = 30 to 50, and very low for n > 50. 
NASA/TMó2003-212108 9

Col 28 vs Col 29 

APPLIED STRESS RATE, [MPa/s]

10-4 10-3 10-2 10-1 100 101 102 103

FLEXURE STRENGTH, 

σf [MPa]

30

40

50

60

70

8090

200

300

400

500

600

100

Dynamic Fatigue/1000oC

(10 mol% YSZ)

σ&

n=8

(a)

APPLIED STRESS RATE, [MPa/s]

10-4 10-3 10-2 10-1 100 101 102 103

FLEXURE STRENGTH, 

σf [MPa]

30

40

50

60

70

8090

200

300

400

500

600

100

Dynamic Fatigue/1000oC

(10YSZ-30mol% Al2O3 particulate)

σ&

n=6

(b)

Col 28 vs Col 29 

APPLIED STRESS RATE, [MPa/s]

10-4 10-3 10-2 10-1 100 101 102 103

FLEXURE STRENGTH, 

σf [MPa]

30

40

50

60

70

8090

200

300

400

500

600

100

Dynamic Fatigue/1000oC

(10YSZ-30mol% Al2O3 platelet)

σ&

n=33

(c)

Figure 6. Results of dynamic fatigue (ëslow crack growthí or ëconstant stress-rateí) testing in flexure for 

three different 10-YSZ/alumina composites at 1000 o

C in air: (a) 10-YSZ (matrix); (b) 10-YSZ/30 mol % alumina 

particulate composite; (c) 10-YSZ/30 mol % alumina platelet composite. 

The line indicates the best-fit based on Eq. (3). 
NASA/TMó2003-212108 10

Col 28 vs Col 29 

TIME TO FAILURE, t

f [s]

100 101 102 103 104 105 106 107

APPLIED STRESS, 

σ[MPa]

20

30

40

50

60

70

8090

200

300

400

100

Flexure /1000oC

n=8

30 mol % platelet

10-YSZ

30 mol % particulate

n=33

n=6

Figure 7. Life prediction diagram constructed from the dynamic fatigue results at 1000 °C for three 

different 10-YSZ/alumina composites including 10-YSZ (matrix), 10-YSZ/30 mol % alumina particulate 

composite, and 10-YSZ/30 mol % alumina platelet composite. The prediction represents at a 

failure probability of approximately 50%. 

A life prediction diagram would give a better interpretation of SCG behavior among the three 

composites, which was constructed in Figure 7 under the same specimen configuration based on the 

following relation [14] 

n

n

f n

D

t −

+

+ = ]σ

1 [

1

 (4) 

where tf and σ are time to failure (in sec) and constant applied stress (in MPa), respectively. Of course, 

the prediction is valid when the same failure mechanism(s) is operative, irrespective of loading 

condition, either dynamic or static. As can be seen from the figure, for a given applied stress lifetime is 

greatest and least, respectively, for the 30 mol % platelet composite and the 10-YSZ and 30 mol % 

particulate composite. As a consequence, the 30 mol % platelet composite would be a most reasonable 

choice among the three materials in conjunction with component life. A detailed life prediction and 

reliability of actual, complex fuel cell components can be made using analytical (finite element 

modeling) and reliability tool such as CARES/Life integrated computer code [15] with known SCG 

parameters of a material. 

Choice of Material in View of Structural Reliability/Life of SOFC 

As seen in the forgoing results, elevated-temperature flexure strength increased with increasing alumina 

content particularly at 30 mol % alumina for the particulate composites, but increased a little or 

remained almost unchanged with alumina content for the platelet composites. Fracture toughness 

increased with alumina content for both composite systems reaching a maximum at 30 mol %. At 

30 mol % alumina content where both particulate and platelet composites exhibited both maximum 

flexure strength and fracture toughness, flexure strength of the particulate composites was 30% greater 

than that of the platelet composites, while fracture toughness of the platelets composites was 

16% greater than the particulate composites. The resistance to SCG susceptibility was greater in the 

30 mol % platelet composite with a higher SCG parameter of n = 33 than in the 30 mol % particulate 

composites with a lower SCG parameter of n = 6. 
NASA/TMó2003-212108 11

From the structural reliability/life point of view, a composite which gives long life and is strongest (in 

strength), toughest (in fracture toughness), stiffest (in elastic modulus) and lightest (in weight) is 

certainly of the best choice for fuel cells material. Elastic modulus was found increased with increasing 

alumina and density, by contrast, decreased with increasing alumina content [2,3]. It has been shown 

that the 30 mol % particulate composite was the best choice of material based on strength, fracture 

toughness, elastic modulus and density that were evaluated at ambient temperature [2,3]. Since 

operating temperatures of typical SOFCs are within or close to 1000 °C, a choice of a candidate material 

should not be solely based on ambient temperature properties but based on elevated temperature 

properties, particularly including slow crack growth which controls life of fuel cell components. 

With respect to elevated-temperature strength, the particulate composite is better than the platelet 

counterpart. By contrast, with regard to fracture toughness and SCG resistance, the platelet composite is 

better than the particulate counterpart. Hence, a unified choice to satisfy all the important 

requirementsóstrong, tough, long lifeócan hardly be made. A case-by-case selection, depending on 

the types of operation/service conditions (temperature, loading (continuous or intermittent), 

environment, and components configurations, etc.), is needed. For example, if the components are 

subjected to a continuous, isothermal type of operations without frequent interruption (thus 

encountering little thermal shock loading, etc.), then the 30 mol % platelet composite would be a good 

candidate since the material would give longer service life of components. This structural consideration 

should not neglect the SOFCsí important electrical performance, that is, oxygen (O2

)-ion conductivity. 

Preliminary results, however, have shown that electrical conductivity of both composite systems was 

almost independent of alumina content [3]. 

SUMMARY AND CONCLUSIONS 

1. The 10-mol % yttria-stabilized zirconia (10-YSZ)/alumina composites reinforced with alumina 

platelets containing 0 to 30 mol % alumina were tested at 1000 °C in air to determine flexure 

strength and fracture toughness. Slow crack growth was also determined in flexure at 1000 °C in air 

using dynamic fatigue testing for selected composites including 0 mol % (10-YSZ matrix) and 

30 mol % particulate and 30 mol % platelet composites. 

2. Elevated-temperature flexure strength of the platelet composites, except for 10 mol % alumina 

content, remained virtually unchanged with respect to the 0 mol % (10-YSZ matrix) strength. The 

strength of 10 mol % alumina composite was increased by 17%. However, flexure strength was 

approximately 30% lower in the platelet than in the particulate composites at 30 mol % where a 

maximum strength yielded for the particulate composites. 

3. Elevated-temperature fracture toughness of the platelet composites, determined by SEVNB method, 

increased with increasing alumina content, reaching a maximum at 30 mol %. A 74% increase in 

fracture toughness was achieved when alumina content increased from 0 to 30 mol %. Fracture 

toughness was approximately 16% greater in the platelet composites than in the particulate 

composites. 

4. The susceptibility to slow crack growth was high for both 0 and 30 mol % particulate composites 

with lower SCG parameters of n = 6 to 8, whereas the susceptibility to SCG was low for the 

30 mol % platelet composite with its higher SCG parameter of n = 33. The resistance to SCG was 

thus significantly improved with the addition of 30 mol % platelet alumina into 10-YSZ matrix, 

which is very desirable for loner service lives of SOFC components. 
NASA/TMó2003-212108 12

REFERENCES 

1. N.Q. Minh, ìCeramic Fuel Cells,î J. Am. Ceram. Soc., 76[3] 563ñ588 (1993). 

2. S.R. Choi and N.P. Bansal, ìStrength and Fracture Toughness of Zirconia/Alumina Composites for 

Solid Oxide Fuel Cells,î Ceram. Eng. Sci. Proc., 23[3] 741ñ750 (2002). 

3. S.R. Choi and N.P. Bansal, ìProcessing and Mechanical Properties of Various Zirconia/Alumina 

Composites for Fuel Cells Applications,î NASA/TMó2002-211580, National Aeronautics and Space 

Administration, Glenn Research Center, Cleveland, OH (2002); also presented at CIMTEC 2002 

Conference, paper # G1:P03 (to be published in the proceedings), June 14ñ18, 2002, Florence, Italy. 

4. ASTM C 1211 ìTest Method for Flexural Strength of Advanced Ceramics at Elevated Temperatures,î 

Annual Book of ASTM Standards, Vol. 15.01, American Society for Testing & Materials, West 

Conshohocken, PA (2001). 

5. S.R. Choi and J.P. Gyekenyesi, ìëUltraí-Fast Fracture Strength of Advanced Structural Ceramics at 

Elevated Temperatures: An Approach to High-Temperature ëInertí Strength,î pp. 27ñ46 in Fracture 

Mechanics of Ceramics, Vol. 13, Edited by R.C. Bradt, D. Munz, M. Sakai, V. Ya. Shevchenko, and 

K.W. White, Kluwer Academic/Plenum Publishers, New York (2002). 

6. J. K ̧bler, (a) ìFracture Toughness of Ceramics Using the SEVNB Method: Preliminary Results,î 

Ceram. Eng. Sci. Proc., 18[4] 155-162 (1997); (b) ìFracture Toughness of Ceramics Using the 

SEVNB Method; Round Robin,î VAMAS Report No. 37, EMPA, Swiss Federal Laboratories for 

Materials Testing & Research, D ̧bendorf, Switzerland (1999). 

7. J.E. Srawley and B. Gross, ìSide-Cracked Plates Subjected to Combined Direct and Bending Forces,î 

pp. 559ñ579 in Cracks and Fracture, ASTM STP 601, American Society for Testing and Materials, 

Philadelphia (1976). 

8. ASTM C 1465, ìTest Method for Determination of Slow Crack Growth Parameters of Advanced 

Ceramics by Constant Stress-Rate Flexural Testing at Elevated Temperatures,î Annual Book of ASTM 

Standards, Vol. 15.01, American Society for Testing and Materials, West Conshohocken, PA (2001). 

9. S.M. Wiederhorn, ìSubcritical Crack Growth in Ceramics,î pp. 613ñ646 in Fracture Mechanics of 

Ceramics, Vol. 2, Edited by R.C. Bradt, D.P.H. Hasselman, and F.F. Lange, Plenum Press, New York 

(1974). 

10. A.G. Evans, ìSlow Crack Growth in Brittle Materials under Dynamic Loading Condition,î Int. J. 

Fracture, 10 [2] 251ñ259 (1974). 

11. ASTM C 1368, ìTest Method for Determination of Slow Crack Growth Parameters of Advanced 

Ceramics by Constant Stress-Rate Flexural Testing at Ambient Temperature,î Annual Book of ASTM 

Standards, Vol. 15.01, American Society for Testing and Materials, West Conshohocken, PA (2001). 

12. K. Jakus, T. Service, and J.E. Ritter, ìHigh-Temperature Fatigue Behavior of Polycrystalline 

Alumina,î J. Am. Ceram. Soc., 63[1ñ2] 4ñ7 (1980). 

13. S.R. Choi and J.P. Gyekenyesi, ìSpecimen Geometry Effect on the Determination of Slow Crack 

Growth Parameters of Advanced Ceramics in Constant Flexural Stress-Rate Testing at Elevated 

Temperatures,î Ceram. Eng. Sci. Proc., 20[3] 525ñ534 (1999). 

14. J.E. Ritter, ìEngineering Design and Fatigue of Brittle Materials,î pp. 667ñ686 in Fracture Mechanics 

of Ceramics, Vol. 4, Edited by R.C. Bradt, D.P.H. Hasselman, and F.F. Lange, Plenum Publishing 

Co., New York (1978). 

15. N.N. Nemeth, L.M. Powers, L.A. Janosik, and J.P. Gyekenyesi, ìTime Dependent Reliability Analysis 

of Monolithic ceramic Components using the CARES/LIFE Integrated Design program,î pp. 390ñ408 

in Life Prediction Methodologies and Data for Ceramic Materials, ASTM STP 1201, Edited by 

C.R. Brinkman and S.F. Duffy, American Society for Testing and Materials, Philadelphia (1994). 

This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

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18

Strength, Fracture Toughness, and Slow Crack Growth of Zirconia/Alumina

Composites at Elevated Temperature

Sung R. Choi and Narottam P. Bansal

Ceramic composites; Zirconia-alumina composites; Elevated-temperature mechanical

properties; Strength; Fracture toughness; Slow crack growth

Unclassified -Unlimited

Subject Category: 07 Distribution: Nonstandard

Sung R. Choi, Ohio Aerospace Institute, Brook Park, Ohio 44142 and Resident Research Scientist at Glenn Research

Center; and Narottam P. Bansal, NASA Glenn Research Center. Responsible person, Narottam P. Bansal, organization

code 5130, 216–433–3855.

Various electrolyte materials for solid oxide fuel cells were fabricated by hot pressing 10 mol % yttria-stabilized zirconia

(10-YSZ) reinforced with two different forms of alumina—particulates and platelets—each containing 0 to 30 mol %

alumina. Flexure strength and fracture toughness of platelet composites were determined as a function of alumina content

at 1000 °C in air and compared with those of particulate composites determined previously. In general, elevated-temperature strength and fracture toughness of both composite systems increased with increasing alumina content. For a given

alumina content, flexure strength of particulate composites was greater than that of platelet composites at higher alumina

contents (≥20 mol %), whereas, fracture toughness was greater in platelet composites than in particulate composites,

regardless of alumina content. The results of slow crack growth (SCG) testing, determined at 1000 °C via dynamic fatigue

testing for three different composites including 0 mol % (10-YSZ matrix), 30 mol % particulate and 30 mol % platelet

composites, showed that susceptibility to SCG was greatest with SCG parameter n = 6 to 8 for both 0 and 30 mol %

particulate composites and was least with n = 33 for the 30 mol % platelet composite.