- Email: [email protected]
High-temperature crack growth in Y-TZP
MATERIALS SCIENCE & ENGINEERING
ELSEVIER
A
Materials Science and Engineering A232 (1997) 103-109
High-temperature
crack growth in Y-TZP
Jorge Alcalk at*, Marc Anglada b ’ Department of Muterials Science alld Engineering, Massachusetts Imtitute of Technology, Cambridge, MA 02139, USA ’ Department of Materials Science and Metallurgical Engineering, Uniuersitat Politknica de Catabmya, Barcelma 08028, Spain
Received 3 February 1997; received in revised form 28 March 1997
Abstract
Fatigue and static crack growth is studied in Y-TZP at intermediatetemperatures.Fractographic observationsindicate that a reduction
in interfacial
fracture
energy, which
induces intergranular
fracture
at intermediate
temperatures,
is responsible
for a
decreasein crack propagation thresholdsand fracture toughness.Static and cyclic growth rates increasewith temperature. Although cyclic loading in the near-thresholdregimepromotescrack propagation, cyclic crack growth velocitiesfor higher values of I\ are slower than predictions basedupon static crack growth results.0 1997Elsevier ScienceS.A. Keywords:
Y-TZP; Fatigue;Crackgrowth;Zirconia ceramics; Fractography; Fracture
1. Introduction
Zirconia ceramics such as Y-TZP are candidate materials for high temperature applications in thermal barrier coatings [l] and solid-oxide fuel cells [2]. YTZP is known to exhibit a high tensile strength ( - 1 GPa) and a modest fracture toughness ( - 5 MPa,/&). Its fracture behavior is susceptible to a stress-induced tetragonal-to-monoclinic (t-m) phase transformation which vanishes at elevated temperatures. Hence, fracture toughness is expected to decrease with temperature increase [3]. An important aspect controlling the crack growth behavior of YTZP is its sensitivity to environment. As a result of this, cracks propagate under static loads at values of K which are much lower than the fracture toughness of the material [4]. Although cyclic fatigue effects accelerate crack growth at ambient temperature [S], only a modest difference between static and fatigue crack growth rates has been reported [6]. In ceramic materials, crack growth studies were usually conducted either at ambient or at very high temperatures ( > 1OOO’C). At ambient temperature, fa-
* Corresponding author. Tel: + 1 617 2539825; fax: + 1 617 2530868; e-mail: [email protected],edu 0921-5093/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved. PIISO921-5093(97)00106-S
tigue (cyclic) degradation mechanisms play a significant role, leading to higher cyclic crack growth rates in relation to static crack growth rates. In contrast, at very high temperatures, viscous deformation of glassy phases is shown to play a deterministic role resulting in crack growth retardation under fatigue [7,8]. While there is a significant wealth of information in ceramic materials regarding crack growth at the extremities of the temperature range, little work has been conducted so far at intermediate temperatures. As temperature is increased, a transition in the micromechanisms of crack growth can be expected. For example, at intermediate temperatures, while creep deformation at the crack tip might still be insignificant, environmentally assisted crack growth could become dominant. In Y-TZP, an enhancement of environmental interactions with temperature may occur to the extent that fatigue growth rates are only a manifestation of static mechanisms. The present work is motivated by the lack of crack growth results for Y-TZP at temperatures other than room temperature. This work aims to compare the cyclic and static crack propagation behavior of this material at intermediate temperatures. Attempts are also made to rationalize the temperature dependence of the fracture behavior (i.e., cyclic and static crack growth rates, crack growth thresholds and fracture
toughness) in terms of relevant micromechanisms.
104
2. Material
J. Alcaki,
and experimental
M.
Anglaa’a/ldaterials
Science
methods
The zirconia studied is a fully tetragonal material stabilized by 2.8 mol.% Y,O,. Transmission electron microscopy (TEM) observations indicate a grain size ranging from 0.3-0.7 pm. Grain boundary glassy phases are not detected by TEM observations. Specific microstructural characteristics were discussed in a previous paper [6]. Long through-thickness cracks were introduced into Y-TZP bars having dimensions of 5 x 8 x 50 mm using the load-bridge indentation technique [6]. The crack length-to-specimen width ratio was 0.25. All specimens were heat treated at 1100°C before testing to revert the t-m transformation induced during precracking [6]. High-temperature crack growth experiments were conducted in a Sic four-point bending fixture with inner and outer spans of 20 mm and 40 mm, respectively. All experiments were conducted in an air environment which had a room temperature relative humidity of m 60% using a MTS 810 servohydraulic testing machine. The crack tip location was monitored in-situ with a Questar long-range telescope. To ensure that crack growth was uniform through the thickness, the specimens were periodically removed from the testing machine to monitor crack extension on both side surfaces. In crack growth experiments under static loads, s ecimens were initially loaded to K= 1.5 MPa ,-” m. If crack growth was not detected after 30 min, K was incremented by 0.1 MPafi until crack growth commenced. Since crack arrest developed after short crack extensions, K was increased in increments of 0.1 MPafi to restart crack growth. This produced a pattern of intermittent crack growth or fluctuations (i.e., a series of fast crack propagation events after each load increase followed by gradual crack growth arrest). Fatigue crack propagation experiments were conducted with a minimum-to-maximum load ratio, R = 0.15 using a sinusoidal wave of frequencies, v = 0.2, 2.0, and 20 Hz. Crack arrest was also detected under cyclic conditions in the initial (near-threshold) regime. Crack growth was restarted by increasing the maximum K level (K,,.J in increments of 0.1 MPafi. Additional experiments were carried out in the near-threshold crack growth regime by periodically changing loading conditions from cyclic to static and vice versa. The fracture toughness was determined under monotonic loading with k 5 1 MPafi s - l i. Crack extension was monitored in-situ with the Questar telescope and recorded in video format. Such a set-up enabled precise measurements of the maximum stable crack extension which, along with the maximum ’ ri- increased by 30% due to stable crack growth during testing.
and Engineering
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recorded load, were used in the calculation of the fracture toughness of the material. The selected testing temperatures were 450 and 900°C. At 450°C the tetragonal phase still retains some of its room temperature tendency to transform to the stable monoclinic phase [9]. At 9OO”C, however, the tetragonal phase is fully stable and transformation toughening effects are not present. An important factor that might influence present results is a degradation of the material at temperatures between k 100 and 400°C [lo]. This is commonly referred to as low-temperature aging and is known to drastically decrease strength and fracture toughness. The mechanism underlying such degradation is a spontaneous t-m transformation which embrittles the bulk material after exposure to the above temperature range over a period of time. In order to establish if low-temperature aging effects occurred during crack growth experiments at 450°C a sample was maintained at this temperature for 7 h. Since the phase content as measured by X-ray diffraction remained unchanged, the development of low-temperature aging was ruled out.
3. Results The growth rates are plotted here in terms of K,,,,, and not of the stress intensity range, AK, to facilitate the comparison with room temperature results where K maxdominated the fatigue crack growth behavior in Y-TZP [5,6]. Fig. 1 shows the crack growth fluctuations which developed at 450°C. This behavior is similar to that described in prior work at room temperature and is induced by load-bridge precracking rather than by short-crack effects [6]. It was shown that the actual crack growth threshold of this material, Ko, corresponds to the value of the stress intensity factor at which the discontinuous crack growth behavior ends and growth rates steadily increase with crack extension [6]. Although the development of crack growth discontinuities is artificial because of its association with load-bridge precracking, it provides a means to maintain the near-threshold regime throughout a relatively wide range of applied K values. This is because any small increment in K is followed by gradual crack growth arrest and thus, by a decrease in the effective K to a value below K,. In the near-threshold regime, a comparison of fatigue and static crack growth rates was attempted by subsequently changing the type of loading after a crack arrest under cyclic or static conditions, Section 2. The results show that after a static crack growth arrest (point A in Fig. I(a) and l(b)), crack propagation can be restarted under cyclic conditions with a value of Km,, equal to the loading level previously applied under
J. Alcald,
M. Anglada
/Materials
Science
static loads. Crack growth is not induced when a static load is applied after a cyclic crack growth arrest at a level equal to the previously applied K,,, (see point B on Fig. l(b)). These observations indicate that at 450°C cyclic loads can reactivate crack propagation in the near-threshold regime. However, the difference between the actual cyclic and static crack growth thresholds, K,, lies within experimental scatter. Crack growth results beyond the crack growth discontinuities are plotted in Fig. 2. Although cracks propagated in stable manner at 450°C no such behavior was detected either under static or cyclic loads at 900°C. Fig. 2 shows that (i) the crack growth rates are higher at 450°C than at room temperature and (ii) at 450” and for K > 2.5 MPa&, static velocities are higher than those measured under cyclic conditions. Table 1 and Fig. 3 show the sharp decrease with temperature detected in the range of K values at which stable crack growth occurs. Fractographic observations show that at ambient temperature the fracture morphology is mixed (i.e., both trans- and inter-granular features are detected),
‘Om6
lo-’
! 0 Static
Loads
- o Cyclic
Loads
z .w ? -f:
3 3
10-s
(stati !
1 o-g
Ji I I
$
lo-lo 1.5
2.0 K, Km,,
(4
A232
1 o-3
[
(1997)
-
IO5
A0
Curve Fit Cyclic (Prediction)
---
3
103-109
A
1 d5
z
+J 9 0” 0 Static
Static
l n
A 2 Hz
0.2 Hz
A
2Hz
+
20 Hz
1O-g 2
3
4
2.2
Wadm)
o Static Loads
5
K, K,, (MPafl) Eig. 2. Comparison of fatigue and static crack growth rates at 450°C and ambient temperature. Fatigue results at ambient temperature are from Ref. [6].
Table 1 Crack growth thresholds (K,) and fracture toughness (K,) 1 .a 0))
2
2.1
2.3
1
Fig. 4(a). In contrast, at 450°C the fracture mode is completely intergranular, Fig. 4(b). If kinetic effects such as creep or environmental interactions are present in the material, it may be possible that the fracture morphology created by stable (slow) crack growth may differ from that developed by unstable (fast) crack growth. This is not supported by present results since scanning electron microscopy (SEM) observations showed that the fracture mode remained constant regardless of crack growth conditions. Fatigue ‘striations’ (alternating light and dark bands) were observed in specimens tested at room temperature in areas of stable crack growth where the applied K was close to the fracture toughness of the material [5,6]. However, in specimens tested at 450°C such features were not as clear as in those tested at room temperature. Grain boundary cavitation was not detected by SEM observations on the side surfaces of the specimens. This indicates that a substantial deformation at the crack tip region which is reported in other ceramics tested above 1000°C is not present in Y-TZP at intermediate temperatures.
(cyclic) A’
5
‘;;
and Engineering
2.6
Temperature (“C)
K, Kmax(MPafi)
Fig. 1. (a) and (b) Crack propagation at 450°C in the initial nearthreshold regime. Arrows indicate the abrupt crack propagation induced by an increase in applied load after a prior crack arrest. Loading conditions are changed periodically from cyclic to static.
K, (h@afi) KC
(MPa&)
22
450
900
2.7 (static) 2.5 (cyclic) 5.0
2.2 (static) (cyclic)
1.7
2.1 3.8
1.7
106
J. Alcaici,
1
r
"0
M.
dnglada
/Materials
Science
and
Enginceriug
A232
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c
+Crack Growth Threshold Fracture Toughness
200
400
Temperature
600
800
1000
(“C)
Fig. 3. Range of stress intensity factors at which stable crack growth occurs at different temperatures.
X-ray diffraction spectra of the fracture surfaces of specimens tested at 450°C (in areas created by stable or unstable crack propagation) were similar to those determined at ambient temperature. This indicates that the t-m ratio on the fracture surfaces remains constant up
Fig. 5. Crack wake at room temperature. (a) Transgranular fracture showing t-m transformation twins. (b) Extensive transformation within an intergranular fracture feature marked ‘i’.
to 450°C. Also, observations of cracks induced on TEM samples at ambient temperature indicate that both inter- and trans-granular fracture induce transformation, Fig. 5. Thus, preferential development of t-m transformation was not associated with either type of fracture.
4 Discussion
Fig. 4. Fracture surface morphologies at (a) ambient temperature and (b) 450°C.
The transition from the mixed fracture morphology at room temperature to fully intergranular fracture at intermediate temperatures is a manifestation of a lowering in grain boundary fracture energy with increasing temperature. This factor, in addition to accelerated environmental interactions at the crack tip with temperature, may also account for the decrease in crack growth thresholds and fracture toughness. On the other
3. Alcal6,
M. Anglada
/Marerids
Science
hand, a reduction of transformation toughening effects with increasing temperature due to stabilization of the monoclinic phase may not play a key role in the decrease of fracture toughness from ambient temperature up to 450°C. This is supported by X-ray measurements which showed that the monoclinic content at the fracture surface remained constant up to 450°C. The stable crack growth regime narrows with increasing temperature to a point where it is fully suppressed at 900°C Table 1 and Fig. 3. A similar trend was observed in a prior work on Mg-PSZ [l I]. These results are in contradiction to the expectation that environmental interactions promote crack growth. The occurrence of stable crack growth in glasses and monoclinic zirconia [12], where toughening mechanisms are not active, indicate that the suppression of stable crack growth in Y-TZP may not be related to the lack of an active transformation toughening mechanism at 900°C. Hence, present suppression of stable crack growth seems to be more likely related to the aforementioned decrease in fracture energy at the grain boundaries. As discussed in Section 1, prior studies in ceramics have shown that the viscous flow of intergranular glassy phases at the crack tip region may promote stable crack growth above 1000°C. These investigations suggest that a ductile behavior of the material at the crack tip is required for the development of stable crack growth at very high temperatures. Since glassy phases were not detected in the studied Y-TZP, crack growth mechanisms associated with their viscous flow are not present here. On the other hand, an increase in ductility at very high temperatures may occur in Y-TZP as a result of its tendency to undergo superplastic flow [13]. Compression tests conducted above 1200°C evidenced that such a tendency developed in the present material (L. Iturgoyen et al., unpublished results). Hence, stable crack growth might be reactivated in Y-TZP at temperatures higher than 900°C. A complementary investigation is needed to verify this possibility. In a prior study conducted at room temperature it was proposed that the mixed fracture morphology of Y-TZP is a result of intergranular fracture of tetragonal grains and transgranular fracture of cubic grains [14]. By contrast, the fracture features at room temperature of the present material show that considerable transgranular fracture may also occur within tetragonal grains. An explanation for this discrepancy in fracture morphology may lie in the chemistry of the grain boundaries. This could induce intergranular fracture in some Y-TZP materials while promoting transgranular separation in others. Support for this assumption is provided in a recent study on Y-TZP-A&O, composites and monolithic Y-TZP which suggests that the ratio between inter- and trans-granular fracture varies due to impurity segregation at the grain boundaries [ 151. Since the chemistry of the grain boundary affects its strength,
and Engineering
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this may also be a key factor controlling the fracture behavior, and particularly the suppression of stable crack growth, in Y-TZP at intermediate temperatures. 4.2. Cock
growth
velocities under static rind cyclic
lOll&
The following are relevant characteristics of present crack growth results, Fig. 2. (i) For a given K, static crack growth rates increase with temperature, (ii) indications of a plateau-like crack growth region under static loads are detected for values of K - 3 MPafi at 450°C and (iii) the difference between crack growth velocities at ambient temperature and 450°C decreases with K. These observations are consistent with the presence of environmentally assisted crack growth mechanisms in the material. Hence, the above plateaulike crack growth regime may be indicative of a limited migration of reactive species to the crack tip. This could also bridge the gap between crack growth rates at different temperatures. At 450°C cyclic loading enhanced crack propagation in the near-threshold regime, Section 3. Possible mechanisms enhancing cyclic crack growth rates can be related to (i) a reduction in the level of crack tip shielding and (ii) damage accumulation at the crack tip region [6]. Since room temperature observations showed that a transformation zone is only active at relatively high values of K, a reduction in crack tip shielding due to cyclic loading is not likely to affect the near-threshold regime. Thus, damage accumulation mechanisms might be proposed to account for fatigue crack growth in this regime. Static and fatigue crack growth rates can be compared by integration of the static results over a fatigue cycle. In such a comparison it is assumed that the crack propagation mechanisms are independent of loading conditions (cyclic or static) [8]. Cyclic growth rates, (da/dt),, are calculated based upon static crack growth results, (da/dt),, as
(1) If the static crack growth behavior following relation
0 da 5,
is fitted to the
=CK”
(2)
where K for a sinusoidal wave is given by RAK K(t) = ~(1 L R) + F
(1 - cos 27-m)
(3)
and C and rz are constants for a given material, temperature and environmental conditions, cyclic growth rates can be estimated by
108
J. Alcalir,
M. Anglan’ct i Materials
Science
and Engineering
‘4232 (1997)
103-109
Table 2 Static and calculated fatigue growth rates K, Km,,
22°C 450°C 900°C
range (MPa&)
X-3.5 3.5-4.5 2.5-3.1 -
Static
Fatigue
C (m s-’ (MPa&)-“)
n
A (tn s-’ (MPavG)-P)
P
1.9x 1o-‘5 5.5 x 1O-8 6.3 x 10-l’
19 5 13 -
2.7 x 10-l” 1.5 x 10-S 1.1 x lo-” -
19 5 13 -
Fatigue calculations fitted to dajdt = AK”,,,.
1
5. Summary
+ ; (1 - CDS27~1~)‘*dt (4)
In these calculations fatigue crack growth occurs at values of K(t) which are lower than the actual static crack growth threshold of the material, K, (i.e., Eq. (2) is extrapolated throughout a complete fatigue cycle without considering that crack growth arrests below KO). Although this assumption yields an overestimation of the fatigue rates, this is only the case for values of K(t) which are close to the threshold. This is because the static da/dt-K curve is so steep that, below K,, calculated cyclic crack extensions are negligible. The estimations of fatigue crack growth rates based on the static behavior at 450°C are included in Fig. 2 and Table 2. As discussed above, cyclic loading enhances crack propagation in the near-threshold regime. For higher values of K, however, the predicted cyclic behavior, Eq. (4), is gradually shifted to higher growth rates than those measured in actual fatigue experiments. While the latter observation contrasts with findings at room temperature, where cyclic loading enhances crack growth rates over static predictions, Fig. 2, it is in good agreement with the results found in other ceramics tested above 1000°C. In these materials, the rate-dependent deformation of glassy phases was considered to retard crack propagation under creep-fatigue conditions [S]. It is important to notice that in these cases, crack growth occurred under creep mechanisms at temperatures which are significantly higher than 450°C and that glassy phases were abundant in the materials. Hence, present retardation of fatigue growth rates are not related to such creep mechanisms. A possible explanation for the fatigue retardation detected in Y-TZP may lie in a reduction of environmental interactions due to cyclic loading. This may occur if reaction rates at the crack tip are not immediately reestablished upon reloading. Although the influence of loading rate on crack velocities was not detected here between 0.2 and 20 Hz, a word of caution is necessary as changes in growth rates due to variations in frequency may lie within present experimental scatter.
Crack growth thresholds and fracture toughness decrease with increasing temperature due to a reduction in the interfacial fracture energy of Y-TZP. The range of K where stable crack growth occurs decreases with temperature to the point where at 900°C stable crack growth is completely suppressed. This finding is inherently related to the aforementioned decrease of interfacial strength which induces a fully intergranular fracture morphology at intermediate temperatures. In addition, present results seem to indicate that the transformation toughening capabilities of the material are not affected by temperature up to 450°C. For a given value of K, cyclic and static crack velocities increased with temperature. Although cyclic loading enhanced crack growth in the near-threshold regime at intermediate temperatures, crack growth was retarded by the application of cyclic loads for higher values of K. Such retardation of fatigue crack growth, however, is not related to the viscous flow of glassy phases described in prior investigations of ceramics tested at very high temperatures. Thus, fatigue retardation in Y-TZP at intermediate temperatures seems to indicate that environmental interactions at the crack tip are sensitive to the nature of the loading (static or cyclic), Acknowledgements The authors are indebted to I-J. Ramamurty, C. Bull and M. Marsal for their help during this work. This research was supported by the Spanish agency CICYT under grant MAT93-0328. Additional financial support was provided by the the Spanish Ministry of Education and Science and the Research Agency of Catalonia (DGR). References [I] A. Mortensen, S. Suresh, Functionally graded metals and metalceramic composites. Part 1: processing,Int. Mater. Rev. 40 (6) (1995) 239-265.
J. Alcalh,
M.
AngIada
/ MatehIs
Science
[Z] N.Q. Minh, Ceramic fuel cells, J. Am. Ceram. Sot. 76 (3) (1993) 563-588. [3] D.J. Green, R.H.J. Hannink, M.V. Green, Transformation Toughening of Ceramics, CRC Press, Boca Raton, 1989. [4] H. Yin, M. Gao, R.P. Wei, Phase transformation and sustained load crack growth in ZrO, t 3 mol.% Y,O,: experiments and kinetic modeling, Acta Metall. Mater. 43 (1) (1995) 371-382. [5] S.-Y. Liu, I.-W. Chen, Fatigue of yttria-stabilized zirconia: II, crack propagation, fatigue striations, and short-crack behavior, J. Am. Ceram. Sot. 74 (6) (1991) 1197-1205. [6] J. Alcala, M. Anglada, Fatigue and static crack propagation in Y-TZP: crack growth micromechanisms and pre-cracking effects, J. Am. Ceram. Sot., accepted for publication. [7] U. Ramamurty, T. Hansson, S. Suresh, High-temperature crack growth in monolithic and Sic,-reinforced silicon nitride under static and cyclic loads, J. Am. Ceram. Sot. 77 (11) (1994) 2985-2999. [8] U. Ramamurty, Retardation of fatigue crack growth in ceramics by glassy ligaments: a rationalization, J. Am. Ceram. Sot. 79 (4) (1996) 945-952.
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[9] H.G. Scott, Phase relationships in the zirconia-yttria system, J. Mater. Sci. 10 (1975) 1527-1535. [lo] J.J. Swab, Low temperature degradation of Y-TZP materials, J. Mater. Sci. 26 (1991) 6706-6714. [ll] D.L. Davidson, J.B. Campbell, J.A. Lankford, Fatigue crack growth through partially stabilized zirconia at ambient and elevated temperatures, Acta Metall. Mater. 39 (6) (1991) 13191330. 1121 L.-S. Li, R.F. Pabst, Subcritical crack growth in partially stabilized zirconia (PSZ), J. Mater. Sci. 15 (1980) 2861-2866. 1131 I.W. Chen, L. Xue, Development of superplastic structural cerainics, J. Am. Ceram. Sot. 73 (9) (1990) 2585-2609. [14] D.B. Marshall, G.W. Dransmann, R.W. Steinbrech, A. Pajares, F. Guiberteau, F.L. Cumbrera, A. Dominguez-Rodriguez, Indentation studies on Y,O,-stabilized ZrO,: I, development of indentation induced cracks, J. Am. Ceram. Sot. 77 (5) (1994) 1185m-1193. [15] S.J. Glass, D.J. Green, Mechanical properties of infiltrated alumina-Y-TZP composites, J. Am. Ceram. Sot. 79 (9) (1996) 2227-2239.
ELSEVIER
A
Materials Science and Engineering A232 (1997) 103-109
High-temperature
crack growth in Y-TZP
Jorge Alcalk at*, Marc Anglada b ’ Department of Muterials Science alld Engineering, Massachusetts Imtitute of Technology, Cambridge, MA 02139, USA ’ Department of Materials Science and Metallurgical Engineering, Uniuersitat Politknica de Catabmya, Barcelma 08028, Spain
Received 3 February 1997; received in revised form 28 March 1997
Abstract
Fatigue and static crack growth is studied in Y-TZP at intermediatetemperatures.Fractographic observationsindicate that a reduction
in interfacial
fracture
energy, which
induces intergranular
fracture
at intermediate
temperatures,
is responsible
for a
decreasein crack propagation thresholdsand fracture toughness.Static and cyclic growth rates increasewith temperature. Although cyclic loading in the near-thresholdregimepromotescrack propagation, cyclic crack growth velocitiesfor higher values of I\ are slower than predictions basedupon static crack growth results.0 1997Elsevier ScienceS.A. Keywords:
Y-TZP; Fatigue;Crackgrowth;Zirconia ceramics; Fractography; Fracture
1. Introduction
Zirconia ceramics such as Y-TZP are candidate materials for high temperature applications in thermal barrier coatings [l] and solid-oxide fuel cells [2]. YTZP is known to exhibit a high tensile strength ( - 1 GPa) and a modest fracture toughness ( - 5 MPa,/&). Its fracture behavior is susceptible to a stress-induced tetragonal-to-monoclinic (t-m) phase transformation which vanishes at elevated temperatures. Hence, fracture toughness is expected to decrease with temperature increase [3]. An important aspect controlling the crack growth behavior of YTZP is its sensitivity to environment. As a result of this, cracks propagate under static loads at values of K which are much lower than the fracture toughness of the material [4]. Although cyclic fatigue effects accelerate crack growth at ambient temperature [S], only a modest difference between static and fatigue crack growth rates has been reported [6]. In ceramic materials, crack growth studies were usually conducted either at ambient or at very high temperatures ( > 1OOO’C). At ambient temperature, fa-
* Corresponding author. Tel: + 1 617 2539825; fax: + 1 617 2530868; e-mail: [email protected],edu 0921-5093/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved. PIISO921-5093(97)00106-S
tigue (cyclic) degradation mechanisms play a significant role, leading to higher cyclic crack growth rates in relation to static crack growth rates. In contrast, at very high temperatures, viscous deformation of glassy phases is shown to play a deterministic role resulting in crack growth retardation under fatigue [7,8]. While there is a significant wealth of information in ceramic materials regarding crack growth at the extremities of the temperature range, little work has been conducted so far at intermediate temperatures. As temperature is increased, a transition in the micromechanisms of crack growth can be expected. For example, at intermediate temperatures, while creep deformation at the crack tip might still be insignificant, environmentally assisted crack growth could become dominant. In Y-TZP, an enhancement of environmental interactions with temperature may occur to the extent that fatigue growth rates are only a manifestation of static mechanisms. The present work is motivated by the lack of crack growth results for Y-TZP at temperatures other than room temperature. This work aims to compare the cyclic and static crack propagation behavior of this material at intermediate temperatures. Attempts are also made to rationalize the temperature dependence of the fracture behavior (i.e., cyclic and static crack growth rates, crack growth thresholds and fracture
toughness) in terms of relevant micromechanisms.
104
2. Material
J. Alcaki,
and experimental
M.
Anglaa’a/ldaterials
Science
methods
The zirconia studied is a fully tetragonal material stabilized by 2.8 mol.% Y,O,. Transmission electron microscopy (TEM) observations indicate a grain size ranging from 0.3-0.7 pm. Grain boundary glassy phases are not detected by TEM observations. Specific microstructural characteristics were discussed in a previous paper [6]. Long through-thickness cracks were introduced into Y-TZP bars having dimensions of 5 x 8 x 50 mm using the load-bridge indentation technique [6]. The crack length-to-specimen width ratio was 0.25. All specimens were heat treated at 1100°C before testing to revert the t-m transformation induced during precracking [6]. High-temperature crack growth experiments were conducted in a Sic four-point bending fixture with inner and outer spans of 20 mm and 40 mm, respectively. All experiments were conducted in an air environment which had a room temperature relative humidity of m 60% using a MTS 810 servohydraulic testing machine. The crack tip location was monitored in-situ with a Questar long-range telescope. To ensure that crack growth was uniform through the thickness, the specimens were periodically removed from the testing machine to monitor crack extension on both side surfaces. In crack growth experiments under static loads, s ecimens were initially loaded to K= 1.5 MPa ,-” m. If crack growth was not detected after 30 min, K was incremented by 0.1 MPafi until crack growth commenced. Since crack arrest developed after short crack extensions, K was increased in increments of 0.1 MPafi to restart crack growth. This produced a pattern of intermittent crack growth or fluctuations (i.e., a series of fast crack propagation events after each load increase followed by gradual crack growth arrest). Fatigue crack propagation experiments were conducted with a minimum-to-maximum load ratio, R = 0.15 using a sinusoidal wave of frequencies, v = 0.2, 2.0, and 20 Hz. Crack arrest was also detected under cyclic conditions in the initial (near-threshold) regime. Crack growth was restarted by increasing the maximum K level (K,,.J in increments of 0.1 MPafi. Additional experiments were carried out in the near-threshold crack growth regime by periodically changing loading conditions from cyclic to static and vice versa. The fracture toughness was determined under monotonic loading with k 5 1 MPafi s - l i. Crack extension was monitored in-situ with the Questar telescope and recorded in video format. Such a set-up enabled precise measurements of the maximum stable crack extension which, along with the maximum ’ ri- increased by 30% due to stable crack growth during testing.
and Engineering
A232
(1997)
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recorded load, were used in the calculation of the fracture toughness of the material. The selected testing temperatures were 450 and 900°C. At 450°C the tetragonal phase still retains some of its room temperature tendency to transform to the stable monoclinic phase [9]. At 9OO”C, however, the tetragonal phase is fully stable and transformation toughening effects are not present. An important factor that might influence present results is a degradation of the material at temperatures between k 100 and 400°C [lo]. This is commonly referred to as low-temperature aging and is known to drastically decrease strength and fracture toughness. The mechanism underlying such degradation is a spontaneous t-m transformation which embrittles the bulk material after exposure to the above temperature range over a period of time. In order to establish if low-temperature aging effects occurred during crack growth experiments at 450°C a sample was maintained at this temperature for 7 h. Since the phase content as measured by X-ray diffraction remained unchanged, the development of low-temperature aging was ruled out.
3. Results The growth rates are plotted here in terms of K,,,,, and not of the stress intensity range, AK, to facilitate the comparison with room temperature results where K maxdominated the fatigue crack growth behavior in Y-TZP [5,6]. Fig. 1 shows the crack growth fluctuations which developed at 450°C. This behavior is similar to that described in prior work at room temperature and is induced by load-bridge precracking rather than by short-crack effects [6]. It was shown that the actual crack growth threshold of this material, Ko, corresponds to the value of the stress intensity factor at which the discontinuous crack growth behavior ends and growth rates steadily increase with crack extension [6]. Although the development of crack growth discontinuities is artificial because of its association with load-bridge precracking, it provides a means to maintain the near-threshold regime throughout a relatively wide range of applied K values. This is because any small increment in K is followed by gradual crack growth arrest and thus, by a decrease in the effective K to a value below K,. In the near-threshold regime, a comparison of fatigue and static crack growth rates was attempted by subsequently changing the type of loading after a crack arrest under cyclic or static conditions, Section 2. The results show that after a static crack growth arrest (point A in Fig. I(a) and l(b)), crack propagation can be restarted under cyclic conditions with a value of Km,, equal to the loading level previously applied under
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static loads. Crack growth is not induced when a static load is applied after a cyclic crack growth arrest at a level equal to the previously applied K,,, (see point B on Fig. l(b)). These observations indicate that at 450°C cyclic loads can reactivate crack propagation in the near-threshold regime. However, the difference between the actual cyclic and static crack growth thresholds, K,, lies within experimental scatter. Crack growth results beyond the crack growth discontinuities are plotted in Fig. 2. Although cracks propagated in stable manner at 450°C no such behavior was detected either under static or cyclic loads at 900°C. Fig. 2 shows that (i) the crack growth rates are higher at 450°C than at room temperature and (ii) at 450” and for K > 2.5 MPa&, static velocities are higher than those measured under cyclic conditions. Table 1 and Fig. 3 show the sharp decrease with temperature detected in the range of K values at which stable crack growth occurs. Fractographic observations show that at ambient temperature the fracture morphology is mixed (i.e., both trans- and inter-granular features are detected),
‘Om6
lo-’
! 0 Static
Loads
- o Cyclic
Loads
z .w ? -f:
3 3
10-s
(stati !
1 o-g
Ji I I
$
lo-lo 1.5
2.0 K, Km,,
(4
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-
IO5
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Curve Fit Cyclic (Prediction)
---
3
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A
1 d5
z
+J 9 0” 0 Static
Static
l n
A 2 Hz
0.2 Hz
A
2Hz
+
20 Hz
1O-g 2
3
4
2.2
Wadm)
o Static Loads
5
K, K,, (MPafl) Eig. 2. Comparison of fatigue and static crack growth rates at 450°C and ambient temperature. Fatigue results at ambient temperature are from Ref. [6].
Table 1 Crack growth thresholds (K,) and fracture toughness (K,) 1 .a 0))
2
2.1
2.3
1
Fig. 4(a). In contrast, at 450°C the fracture mode is completely intergranular, Fig. 4(b). If kinetic effects such as creep or environmental interactions are present in the material, it may be possible that the fracture morphology created by stable (slow) crack growth may differ from that developed by unstable (fast) crack growth. This is not supported by present results since scanning electron microscopy (SEM) observations showed that the fracture mode remained constant regardless of crack growth conditions. Fatigue ‘striations’ (alternating light and dark bands) were observed in specimens tested at room temperature in areas of stable crack growth where the applied K was close to the fracture toughness of the material [5,6]. However, in specimens tested at 450°C such features were not as clear as in those tested at room temperature. Grain boundary cavitation was not detected by SEM observations on the side surfaces of the specimens. This indicates that a substantial deformation at the crack tip region which is reported in other ceramics tested above 1000°C is not present in Y-TZP at intermediate temperatures.
(cyclic) A’
5
‘;;
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Temperature (“C)
K, Kmax(MPafi)
Fig. 1. (a) and (b) Crack propagation at 450°C in the initial nearthreshold regime. Arrows indicate the abrupt crack propagation induced by an increase in applied load after a prior crack arrest. Loading conditions are changed periodically from cyclic to static.
K, (h@afi) KC
(MPa&)
22
450
900
2.7 (static) 2.5 (cyclic) 5.0
2.2 (static) (cyclic)
1.7
2.1 3.8
1.7
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c
+Crack Growth Threshold Fracture Toughness
200
400
Temperature
600
800
1000
(“C)
Fig. 3. Range of stress intensity factors at which stable crack growth occurs at different temperatures.
X-ray diffraction spectra of the fracture surfaces of specimens tested at 450°C (in areas created by stable or unstable crack propagation) were similar to those determined at ambient temperature. This indicates that the t-m ratio on the fracture surfaces remains constant up
Fig. 5. Crack wake at room temperature. (a) Transgranular fracture showing t-m transformation twins. (b) Extensive transformation within an intergranular fracture feature marked ‘i’.
to 450°C. Also, observations of cracks induced on TEM samples at ambient temperature indicate that both inter- and trans-granular fracture induce transformation, Fig. 5. Thus, preferential development of t-m transformation was not associated with either type of fracture.
4 Discussion
Fig. 4. Fracture surface morphologies at (a) ambient temperature and (b) 450°C.
The transition from the mixed fracture morphology at room temperature to fully intergranular fracture at intermediate temperatures is a manifestation of a lowering in grain boundary fracture energy with increasing temperature. This factor, in addition to accelerated environmental interactions at the crack tip with temperature, may also account for the decrease in crack growth thresholds and fracture toughness. On the other
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hand, a reduction of transformation toughening effects with increasing temperature due to stabilization of the monoclinic phase may not play a key role in the decrease of fracture toughness from ambient temperature up to 450°C. This is supported by X-ray measurements which showed that the monoclinic content at the fracture surface remained constant up to 450°C. The stable crack growth regime narrows with increasing temperature to a point where it is fully suppressed at 900°C Table 1 and Fig. 3. A similar trend was observed in a prior work on Mg-PSZ [l I]. These results are in contradiction to the expectation that environmental interactions promote crack growth. The occurrence of stable crack growth in glasses and monoclinic zirconia [12], where toughening mechanisms are not active, indicate that the suppression of stable crack growth in Y-TZP may not be related to the lack of an active transformation toughening mechanism at 900°C. Hence, present suppression of stable crack growth seems to be more likely related to the aforementioned decrease in fracture energy at the grain boundaries. As discussed in Section 1, prior studies in ceramics have shown that the viscous flow of intergranular glassy phases at the crack tip region may promote stable crack growth above 1000°C. These investigations suggest that a ductile behavior of the material at the crack tip is required for the development of stable crack growth at very high temperatures. Since glassy phases were not detected in the studied Y-TZP, crack growth mechanisms associated with their viscous flow are not present here. On the other hand, an increase in ductility at very high temperatures may occur in Y-TZP as a result of its tendency to undergo superplastic flow [13]. Compression tests conducted above 1200°C evidenced that such a tendency developed in the present material (L. Iturgoyen et al., unpublished results). Hence, stable crack growth might be reactivated in Y-TZP at temperatures higher than 900°C. A complementary investigation is needed to verify this possibility. In a prior study conducted at room temperature it was proposed that the mixed fracture morphology of Y-TZP is a result of intergranular fracture of tetragonal grains and transgranular fracture of cubic grains [14]. By contrast, the fracture features at room temperature of the present material show that considerable transgranular fracture may also occur within tetragonal grains. An explanation for this discrepancy in fracture morphology may lie in the chemistry of the grain boundaries. This could induce intergranular fracture in some Y-TZP materials while promoting transgranular separation in others. Support for this assumption is provided in a recent study on Y-TZP-A&O, composites and monolithic Y-TZP which suggests that the ratio between inter- and trans-granular fracture varies due to impurity segregation at the grain boundaries [ 151. Since the chemistry of the grain boundary affects its strength,
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this may also be a key factor controlling the fracture behavior, and particularly the suppression of stable crack growth, in Y-TZP at intermediate temperatures. 4.2. Cock
growth
velocities under static rind cyclic
lOll&
The following are relevant characteristics of present crack growth results, Fig. 2. (i) For a given K, static crack growth rates increase with temperature, (ii) indications of a plateau-like crack growth region under static loads are detected for values of K - 3 MPafi at 450°C and (iii) the difference between crack growth velocities at ambient temperature and 450°C decreases with K. These observations are consistent with the presence of environmentally assisted crack growth mechanisms in the material. Hence, the above plateaulike crack growth regime may be indicative of a limited migration of reactive species to the crack tip. This could also bridge the gap between crack growth rates at different temperatures. At 450°C cyclic loading enhanced crack propagation in the near-threshold regime, Section 3. Possible mechanisms enhancing cyclic crack growth rates can be related to (i) a reduction in the level of crack tip shielding and (ii) damage accumulation at the crack tip region [6]. Since room temperature observations showed that a transformation zone is only active at relatively high values of K, a reduction in crack tip shielding due to cyclic loading is not likely to affect the near-threshold regime. Thus, damage accumulation mechanisms might be proposed to account for fatigue crack growth in this regime. Static and fatigue crack growth rates can be compared by integration of the static results over a fatigue cycle. In such a comparison it is assumed that the crack propagation mechanisms are independent of loading conditions (cyclic or static) [8]. Cyclic growth rates, (da/dt),, are calculated based upon static crack growth results, (da/dt),, as
(1) If the static crack growth behavior following relation
0 da 5,
is fitted to the
=CK”
(2)
where K for a sinusoidal wave is given by RAK K(t) = ~(1 L R) + F
(1 - cos 27-m)
(3)
and C and rz are constants for a given material, temperature and environmental conditions, cyclic growth rates can be estimated by
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Table 2 Static and calculated fatigue growth rates K, Km,,
22°C 450°C 900°C
range (MPa&)
X-3.5 3.5-4.5 2.5-3.1 -
Static
Fatigue
C (m s-’ (MPa&)-“)
n
A (tn s-’ (MPavG)-P)
P
1.9x 1o-‘5 5.5 x 1O-8 6.3 x 10-l’
19 5 13 -
2.7 x 10-l” 1.5 x 10-S 1.1 x lo-” -
19 5 13 -
Fatigue calculations fitted to dajdt = AK”,,,.
1
5. Summary
+ ; (1 - CDS27~1~)‘*dt (4)
In these calculations fatigue crack growth occurs at values of K(t) which are lower than the actual static crack growth threshold of the material, K, (i.e., Eq. (2) is extrapolated throughout a complete fatigue cycle without considering that crack growth arrests below KO). Although this assumption yields an overestimation of the fatigue rates, this is only the case for values of K(t) which are close to the threshold. This is because the static da/dt-K curve is so steep that, below K,, calculated cyclic crack extensions are negligible. The estimations of fatigue crack growth rates based on the static behavior at 450°C are included in Fig. 2 and Table 2. As discussed above, cyclic loading enhances crack propagation in the near-threshold regime. For higher values of K, however, the predicted cyclic behavior, Eq. (4), is gradually shifted to higher growth rates than those measured in actual fatigue experiments. While the latter observation contrasts with findings at room temperature, where cyclic loading enhances crack growth rates over static predictions, Fig. 2, it is in good agreement with the results found in other ceramics tested above 1000°C. In these materials, the rate-dependent deformation of glassy phases was considered to retard crack propagation under creep-fatigue conditions [S]. It is important to notice that in these cases, crack growth occurred under creep mechanisms at temperatures which are significantly higher than 450°C and that glassy phases were abundant in the materials. Hence, present retardation of fatigue growth rates are not related to such creep mechanisms. A possible explanation for the fatigue retardation detected in Y-TZP may lie in a reduction of environmental interactions due to cyclic loading. This may occur if reaction rates at the crack tip are not immediately reestablished upon reloading. Although the influence of loading rate on crack velocities was not detected here between 0.2 and 20 Hz, a word of caution is necessary as changes in growth rates due to variations in frequency may lie within present experimental scatter.
Crack growth thresholds and fracture toughness decrease with increasing temperature due to a reduction in the interfacial fracture energy of Y-TZP. The range of K where stable crack growth occurs decreases with temperature to the point where at 900°C stable crack growth is completely suppressed. This finding is inherently related to the aforementioned decrease of interfacial strength which induces a fully intergranular fracture morphology at intermediate temperatures. In addition, present results seem to indicate that the transformation toughening capabilities of the material are not affected by temperature up to 450°C. For a given value of K, cyclic and static crack velocities increased with temperature. Although cyclic loading enhanced crack growth in the near-threshold regime at intermediate temperatures, crack growth was retarded by the application of cyclic loads for higher values of K. Such retardation of fatigue crack growth, however, is not related to the viscous flow of glassy phases described in prior investigations of ceramics tested at very high temperatures. Thus, fatigue retardation in Y-TZP at intermediate temperatures seems to indicate that environmental interactions at the crack tip are sensitive to the nature of the loading (static or cyclic), Acknowledgements The authors are indebted to I-J. Ramamurty, C. Bull and M. Marsal for their help during this work. This research was supported by the Spanish agency CICYT under grant MAT93-0328. Additional financial support was provided by the the Spanish Ministry of Education and Science and the Research Agency of Catalonia (DGR). References [I] A. Mortensen, S. Suresh, Functionally graded metals and metalceramic composites. Part 1: processing,Int. Mater. Rev. 40 (6) (1995) 239-265.
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[Z] N.Q. Minh, Ceramic fuel cells, J. Am. Ceram. Sot. 76 (3) (1993) 563-588. [3] D.J. Green, R.H.J. Hannink, M.V. Green, Transformation Toughening of Ceramics, CRC Press, Boca Raton, 1989. [4] H. Yin, M. Gao, R.P. Wei, Phase transformation and sustained load crack growth in ZrO, t 3 mol.% Y,O,: experiments and kinetic modeling, Acta Metall. Mater. 43 (1) (1995) 371-382. [5] S.-Y. Liu, I.-W. Chen, Fatigue of yttria-stabilized zirconia: II, crack propagation, fatigue striations, and short-crack behavior, J. Am. Ceram. Sot. 74 (6) (1991) 1197-1205. [6] J. Alcala, M. Anglada, Fatigue and static crack propagation in Y-TZP: crack growth micromechanisms and pre-cracking effects, J. Am. Ceram. Sot., accepted for publication. [7] U. Ramamurty, T. Hansson, S. Suresh, High-temperature crack growth in monolithic and Sic,-reinforced silicon nitride under static and cyclic loads, J. Am. Ceram. Sot. 77 (11) (1994) 2985-2999. [8] U. Ramamurty, Retardation of fatigue crack growth in ceramics by glassy ligaments: a rationalization, J. Am. Ceram. Sot. 79 (4) (1996) 945-952.
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[9] H.G. Scott, Phase relationships in the zirconia-yttria system, J. Mater. Sci. 10 (1975) 1527-1535. [lo] J.J. Swab, Low temperature degradation of Y-TZP materials, J. Mater. Sci. 26 (1991) 6706-6714. [ll] D.L. Davidson, J.B. Campbell, J.A. Lankford, Fatigue crack growth through partially stabilized zirconia at ambient and elevated temperatures, Acta Metall. Mater. 39 (6) (1991) 13191330. 1121 L.-S. Li, R.F. Pabst, Subcritical crack growth in partially stabilized zirconia (PSZ), J. Mater. Sci. 15 (1980) 2861-2866. 1131 I.W. Chen, L. Xue, Development of superplastic structural cerainics, J. Am. Ceram. Sot. 73 (9) (1990) 2585-2609. [14] D.B. Marshall, G.W. Dransmann, R.W. Steinbrech, A. Pajares, F. Guiberteau, F.L. Cumbrera, A. Dominguez-Rodriguez, Indentation studies on Y,O,-stabilized ZrO,: I, development of indentation induced cracks, J. Am. Ceram. Sot. 77 (5) (1994) 1185m-1193. [15] S.J. Glass, D.J. Green, Mechanical properties of infiltrated alumina-Y-TZP composites, J. Am. Ceram. Sot. 79 (9) (1996) 2227-2239.