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The effect of reduction on the mechanical properties of CeO2 doped tetragonal zirconia ceramics
Acta Materialia 52 (2004) 1675–1682 www.actamat-journals.com
The effect of reduction on the mechanical properties of CeO2 doped tetragonal zirconia ceramics M. Matsuzawa *, M. Abe, S. Horibe, J. Sakai Department of Materials Science and Engineering, Waseda University, 3-4-1, Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan Received 28 August 2003; received in revised form 2 December 2003; accepted 9 December 2003
Abstract Four types of zirconia ceramic, Ce-TZP, 3Y-TZP, 8Y-FSZ and Mg-PSZ, were heat treated in hydrogen gas. Only Ce-TZP was very effectively reduced after the heat treatment. The drastic weight loss and the increase of internal friction, which were observed in the reduced Ce-TZP, were due to the high number of oxygen vacancies introduced into the matrix. The various mechanical properties of Ce-TZP before and after the heat treatment were investigated. Sufficient reduction effectively increased the hardness but unfortunately also embrittled the sample. The serious increase in embrittlement could not be attributed to the hydrogen introduced into the matrix, but was due principally to the low transformability of the reduced Ce-TZP. On the other hand, the anelastic property of the reduced Ce-TZP was considerably improved by the increase in oxygen vacancies and, furthermore, the tensile fracture strength was influenced due to the fact that anelasticity occurs instead of phase transformation. Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ceramics; Hardness; Toughness; Internal friction; Fracture; Reduction
1. Introduction It is well known that stress-induced transformation from the tetragonal to the monoclinic phase effectively improves the fracture toughness of zirconia based ceramic materials [1–3]. However, the desirable stress-induced phase transformation is inhibited by an increase in temperature. Furthermore, it has been noted that serious strength degradation in zirconia ceramics can occur in certain temperature regions [3–6]. In some of the most important structural materials, these negative factors inhibit the wide application of zirconia ceramics, especially at high temperatures. Ferroelastic domain switching [7–10] and anelasticity [11–18] have recently been observed as significant strengthening or toughening mechanisms. These mechanisms are thermally activated [8,13] and, therefore, could improve mechanical properties even at elevated temperatures [8–10,14–16]. In particular, in a previous study we have found that an*
Corresponding author. Tel./fax: +81-352-863-306. E-mail addresses: [email protected] (M. Matsuzawa), [email protected] (S. Horibe).
elasticity can produce a uniquely recoverable non-elastic strain at lower levels of stress, in contrast to ferroelastic domain switching [16,17]. The effect of anelastic behavior on various mechanical properties has been extensively studied and it has been found that anelasticity should be regarded as one of the most important strengthening– toughening mechanisms [15–18]. Although the exact mechanism of anelastic strain production is still unclear, it is assumed that the effectiveness of anelasticity is closely correlated with oxygen vacancies included in zirconia matrix [16,17]. The number of oxygen vacancies is determined by valency and by the quantity of additive stabilizer dopant [19] and it is well known that migration of the oxygen ions via vacancies plays an important role in major technical applications, such as oxygen sensors, fuel cells, etc. The effect of oxygen vacancies on mechanical properties requires study when considering the application of zirconia-based ceramics as structural materials. Several researchers [20–24] have investigated the change in microstructure and various mechanical properties of a reduced Ce-TZP and reported fascinating results. Tikare and Heuer [21] have carried out indentation testing at
1359-6454/$30.00 Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2003.12.012
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high temperatures in vacuum and found that indentation cracking was more effectively suppressed on a CeTZP sample initially heated to higher temperature. Moreover, Hasegawa and Ozawa [24] have found that hardness or strength of Ce-TZP could be increased by a heat treatment in hydrogen (reducing atmosphere), depending on temperature and exposure time, proposing a reduction-induced strengthening mechanism. It is obvious that oxygen vacancies introduced by reduction could be the origin of the change in mechanical properties of Ce-TZP. Zhu [22] has reported that the crystallographic phase was unchanged but the lattice volume was increased after reduction of Ce-TZP. The effective residual compressive stress seems to be caused by the change in lattice length, resulting in an improvement of the mechanical properties, such as hardness [23,24], strength [20,24] or fracture toughness [21]. However, it is thought that the variation of these mechanical properties depends strongly on the extent of reduction and further study is required. As mentioned above, the mechanism of anelastic strain production is closely associated with oxygen vacancy [16,17], suggesting that the anelastic property also has a high probability of being affected by introduction of oxygen vacancies. Because the various changes of microstructure or mechanical properties in zirconia matrix might be caused by atomic defects associated with oxygen vacancies, it is very important to verify the nature of these vacancies. In this study, the heat treatment in a reducing hydrogen atmosphere was carried out for various engineering zirconia ceramics. The present examination was focused on the following points; (i) to investigate not only general mechanical properties, i.e. hardness, toughness or tensile fracture strength, but also internal friction or anelasticity closely related to oxygen vacancies and furthermore (ii) to survey the role played by hydrogen in modifying mechanical properties during reducing heat treatments.
2. Experimental procedure The material mainly used in this study was Ce-TZP (12 mol% CeO2 doped tetragonal zirconia polycrystals). The material was prepared as a rectangular shaped specimen, with dimensions 4 3 40 mm (Fig. 1(a)) for indentation and bending tests. Similar rectangular specimens of a further three types of zirconia-based ceramics, namely 3Y-TZP (3 mol% Y2 O3 doped tetragonal zirconia polycrystals), 8Y-FSZ (8 mol% Y2 O3 doped fully stabilized zirconia) and Mg-PSZ (9 mol% MgO doped partially stabilized zirconia), were prepared for comparison. In addition, in the case of Ce-TZP, thinsheet specimens with two notches for tensile rupture testing were used, as shown in Fig. 1(b). All the specimens were provided by commercial ceramic manufac-
Fig. 1. The test pieces: (a) rectangular specimen used for the indentation testing and the measurements of internal friction and anelasticity and (b) thin-sheet specimen used for tensile fracture testing.
turers in Japan (Ce-TZP, 8Y-FSZ and Mg-PSZ: Nagano Keiki Co., Ltd., Nagano; 3Y-TZP: Tohoku Ceramic Co., Ltd., Miyagi) and their characteristics are summarized in Table 1. The density measured for each sample by using Archimedes method was extremely close to its theoretical density. Prior to each testing, the surface subjected to be indented on the rectangular specimen and two pinholes of the thin-sheet specimen were carefully polished using 1/4 lm diamond paste. The heat treatment of specimens was carried out in hydrogen gas (reducing atmosphere). Sample weight and fracture toughness were measured and compared before and after the reducing heat treatment. The high temperature heat treatment in hydrogen was carried out as follows. Specimens were heated in a hydrogen atmosphere from room temperature to 1000 °C over a period of 1.5 h in an electric furnace. The samples were maintained at this temperature for 8 h, after which, the furnace was cooled and the samples removed from the hot zone to promote rapid cooling. In order to avoid reoxidization of samples, they were cooled to room temperature with the hydrogen replaced by argon gas. Subsequently, the various mechanical tests were carried out on the processed samples in order to evaluate the change of mechanical properties caused by the heat treatment. 2.1. Fracture toughness and hardness The indentation testing was carried out by pressing the diamond indentor onto the sample surface with dwell time of 10 s. Applied load was varied from 49 to 294 N in order to reduce statistical error. The values of hardness and fracture toughness were calculated by measuring the
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Table 1 Sintering temperature, density, grain size and mechanical properties of the materials Materials
Sintering temperature (°C)
Density (g/cm3 )
Grain size (lm)
Hardness (GPa)
pffiffiffiffi Fracture toughness (MPa m)
Ce-TZP 3Y-TZP 8Y-FSZ Mg-PSZ
1425 1450 1500 1675
6.22 6.10 5.95 5.87
1.0–1.4 0.2–0.4 3.0–5.0 0.4–0.8
9.5 14.2 12.8 9.8
12.2 4.8 1.5 8.4
crack length and the diagonal length of the indent according to the indentation fracture procedure proposed by Niihara et al. [25]; because of the different cracking modes among the materials, the palmqvist crack mode calculation was used for Ce-TZP, 3Y-TZP and Mg-PSZ, while half-penny crack mode calculation was used for 8Y-FSZ and the heat-treated Ce-TZP. 2.2. Internal friction The internal friction testing was carried out with free decay method, where the rectangular sample was horizontally supported at the nodes of oscillation by two thin wires of SUS304. One side of the sample was covered with carbon paint to render it conducting. The flexural vibration was then driven electrostatically by an electrode directly located below the sample, between the wires (JIS G0602). The internal friction was evaluated using the half value breadth method (ASTM E756-93). 2.3. Anelasticity A four-point bending test was carried out, with inner and outer spans of 10 and 30 mm, respectively (Fig. 1(a)). A desired load (300 MPa/s) was applied abruptly to the specimen, maintained for 10 h and then removed. The strain was recorded over the time using a strain gauge attached to the tensile surface of the specimen. The recoverable strain after unloading was defined as the amount of anelastic strain (Refer to our previous paper [17] for the details). 2.4. Tensile fracture strength The uni-axial tensile fracture strength of non-reduced and reduced Ce-TZP was investigated for a wide range of strain rates (3 109 –1 101 s1 ) in air in a manner similar to [18]. The strain rate dependence of tensile strength on both the material was discussed.
3. Results and discussion 3.1. The weight loss caused by reducing heat treatment in hydrogen The normalized weight of each sample, Ce-TZP, 3YTZP, 8Y-FSZ and Mg-PSZ, after the heat treatment at
1000 °C for 8 h in hydrogen is shown in Fig. 2(a). Similarly, Fig. 2(b) shows the change of fracture toughness after the heat treatment. Each data point indicates a mean value obtained by several (more than five) indentation tests. Both figures show an interesting tendency, that the significant weight loss and decrease of fracture toughness are apparently observed only for CeTZP, with almost no change for the other. These results suggest that there is a significant correlation between the weight loss and the decrease of fracture toughness in zirconia-based ceramics. The weight loss during the heat treatment amounted to about 0.74% in Ce-TZP, but was less than 0.1% in the other materials (Fig. 2(a)). The color of the Ce-TZP specimen drastically changed from ivory yellow to black, while the 3Y-TZP and Mg-PSZ specimens, changed from white to gray and the 8Y-FSZ specimen, merely from white to light gray. Other mechanical surface damage such as microcracking or chipping was not observed. Oxygen vacancies produced in the ZrO2 matrix by reduction effectively absorbed light of longer wavelength [26], resulting in a black colored specimen. Therefore, all specimens used in this experiment seem to be more or less reduced, though the extent of reduction in the Ce-TZP overwhelmingly exceeds that in the other specimens. For 3Y-TZP, 8Y-FSZ and Mg-PSZ, therefore, it was not anticipated that the slight reduction observed would have any great effect on fracture toughness. Fig. 2(c) shows the change of internal friction before and after the reducing heat treatment. (Data for MgPSZ could not be obtained because the specimen length was too short for the internal friction testing.) A pronounced change in internal friction, weight change and fracture toughness was detected only for Ce-TZP, as indicated in Fig. 2(a) and (b). The excessive increase in internal friction indicates the introduction of oxygen vacancies in the matrix, agreeing with the results of Ozawa et al. [27] using similarly reduced Ce-TZP samples. They have suggested that the oxygen vacancies significantly contribute to the generation of the internal friction in ZrO2 ceramics [27]. A doping stabilizer with a different valency from that of the Zr4þ cation introduces some oxygen vacancies in the matrix to maintain the electrical valency [19]. Therefore, the large number of oxygen vacancies must already be included in Y2 O3 – ZrO2 and MgO–ZrO2 prior to the heat reducing treatment. In the case of Ce-TZP, on the other hand, the
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*
The change of weight (M/M %)
100.2
The change of fracture toughness (KIC/KIC* %)
(a)
100.0
99.8
99.6
99.4
99.2 0
3Y - TZP
8Y - FSZ
Mg - PSZ
Ce - TZP
3Y - TZP
8Y - FSZ
Mg - PSZ
120
100
80
60
40
20
0
(b) 500
*
The change of internal friction (Q-1/Q-1 %)
Ce - TZP
(c)
400
300
orbits in atomic Ce is [Xe]4f1 5d1 6s2 , the Ce ion tends to be stabilized with a trivalent state ([Xe]4f1 5d0 6s0 ) because of the low shielding effect of the inner electric orbits for 4f orbit, as well as the tetravalent state ([Xe]4f0 5d0 6s0 ). In other words, the energy required for reducing from Ce4þ to Ce3þ is much lower than that of Y3þ ! Y2þ or Mg2þ ! Mgþ . Kiguchi et al. [10] have referred to the valence change form Zr4þ to Zr3þ but not from Y3þ to Y2þ in electrochemically reduced 3 mol% Y2 O3 –ZrO2 , indicating that Y3þ state is more stable than Zr4þ . These results also indicate that Ce ion is more easily reduced than other dopant ions. When all Ce ions in 12 mol% CeO2 –88 mol% ZrO2 are entirely reduced from the tetravalent to the trivalent state, 6% oxygen vacancies are produced in the matrix, corresponding to the weight loss of 0.76%. This value exactly agrees with the above-mentioned experimental result shown in Fig. 2(a). There is a high probability that not only oxygen vacancies but also hydrogen elements might be incorporated into the reduced samples during the heat treatment in flowing hydrogen gas. The amount of hydrogen included in the reduced Ce-TZP samples was investigated using thermal desorption spectrometry (Fig. 3). According to Fig. 3, a significant incorporation of hydrogen into the samples can be clearly observed. It is generally accepted that incorporation of hydrogen into the matrix might result in serious embrittlement of metallic materials. In recent work, the effect of hydrogen in producing significant embrittlement in ceramic materials has been also detected [29,30]. It is most important to understand which of the components caused the serious decrease of fracture toughness after the heat treatment at 1000 °C in hydrogen–oxygen vacancies or hydrogen atoms introduced into matrix. The solution to this problem will be discussed in detail in the next section.
200
3.2. The merits and faults of reduction for Ce-TZP 100
No data 0
Ce - TZP
3Y - TZP
8Y - FSZ
Mg - PSZ
As mentioned previously, the decrease of fracture toughness for Ce-TZP was caused by the heat treatment at 1000 °C in hydrogen and two factors, oxygen vacancy
Fig. 2. The change of (a) weight M, (b) fracture toughness KIC and (c) internal friction Q1 for various zirconia ceramics after the heat treatment at 1000 °C for 8 h in hydrogen. Each value, M, KIC and Q1 after heat treatment is normalized by M , KIC and Q1 before heat treatment, respectively for convenience of comprehension. (Horizontal bars mean the scatter bands).
valencies of the dopant cation, Ce4þ , and the matrix, Zr4þ , are the same, so that few vacancies are originally included in the matrix. However, the drastic weight loss of the reduced Ce-TZP in Fig. 2(a) indicates that a large number of oxygen vacancies was introduced into the matrix, accompanying the valence change from Ce4þ to Ce3þ [28]. Although the electron configuration in outer
Fig. 3. Hydrogen evolution rate as a function of temperature for CeTZP sample after the heat treatment at 1000 °C for 8 h in hydrogen gas.
M. Matsuzawa et al. / Acta Materialia 52 (2004) 1675–1682
and hydrogen incorporation, were considered as major causes of this serious problem. In order to investigate the effect of oxygen vacancy and hydrogen incorporation on the mechanical properties, an additional sequence of heat treatments, (1000 °C in hydrogen ! 950 °C in vacuum ! 1000 °C in air), was conducted, with the change of the weight loss, fracture toughness and hardness being measured after each process. The results are shown in Fig. 4. After the first process of the heating at 1000 °C in hydrogen gas, the large amount of oxygen vacancies was introduced into the matrix and a significant weight loss was recorded, with an accompanyingpdecrease in fracture toughness from 12.2 to 2.3 ffiffiffiffi MPa m. (The fracture toughness values of the samples heat-treated in hydrogen and in vacuum were obtained by using half-penny crack mode calculation [25].) In contrast, it is noted that hardness effectively increases from 9.5 to 12.5 GPa after the reduction. After the second heating process in vacuum, hydrogen was released from the Ce-TZP matrix as shown in Fig. 3. However, Fig. 4(a) and (b) indicate that no significant variation of either weight or mechanical properties was observed. After the final process of the re-heating at 1000 °C in air, the weight and the mechanical properties are all recovered to their original level, suggesting that the sample was re-oxidized. The results obtained in the additional consecutive heating processes lead us to conclude that the serious decrease of fracture toughness
Fig. 4. The change of (a) weight loss and (b) fracture toughness and hardness during consecutive heat treatment.
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of Ce-TZP ceramics after the heat treatment at 1000 °C in hydrogen is caused not by hydrogen incorporation, but, to a great extent, by oxygen vacancies. Considering the wide application of ceramic materials, it is necessary to elucidate the mechanism by which oxygen vacancies seriously decrease the fracture toughness. As one of the most effective approaches to solve the problem, it could be important to evaluate transformability of non-reduced and reduced Ce-TZP samples. In order to evaluate the transformability, the surfaces of both specimens were ground using #100 emery paper and the change of phase composition was investigated using X-ray diffraction (XRD). The data were analyzed using the method proposed by Garvie and Nicholson [31]. Fig. 5 shows the XRD profiles of the specimen before and after grinding. Only tetragonal peaks were observed before grinding for both non-reduced and reduced samples, indicating that the crystallographic phase would not change during the heat treatment in hydrogen. In the case of non-reduced CeTZP, grinding produced monoclinic peaks on the surface, corresponding to about 30% formation of the monoclinic phase. Furthermore, Kitano et al. have confirmed that the asymmetry of the profiles of the tetragonal (1 1 1) peak, which appears at lower angles, could be assigned to the rhombohedral phase [32]. On the other hand, no monoclinic peak was detected for the ground reduced sample, though the rhombohedral phase still seemed to be produced. These results imply that transformability drastically decreased with increase of oxygen vacancies, introduced by reducing heat treatment. This accounted for the serious decrease of fracture toughness. During reduction of Ce-TZP, the increase of oxygen vacancies and the valence change of Ce ions from Ce4þ to Ce3þ , accompanied by the increase of ionic radius occur in the tetragonal crystals [28]. from 0.94 to 1.07 A, As a result, the lattice length is expanded and the lattice volume is increased [21]. Moreover, it is interesting to note that tetragonality decreases after the reduction of Ce-TZP. Comparing the XRD profiles of non-reduced and reduced Ce-TZP (Fig. 5(a) and (b) respectively), it is found that two tetragonal peaks (2 0 0) and (0 0 2) approach each other, corresponding to a decrease of tetragonality c=a. The lattice volume dilatation is considered to produce residual compressive stress in the surface layer, resulting in the effective improvement of various mechanical properties, such as hardness [23,24], fracture toughness [21] or strength [20,24]. However, this concept is not comprehensible on an entirely reduced specimen. Meanwhile, the decrease in tetragonality suggests that the crystallographic phase tends from tetragonal towards cubic. It follows that the tetragonal phase is more stable and, as a result, transformability should be decreased [33]. It is assumed that the undesirable decrease of fracture toughness is attributed to the
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Fig. 5. The change of XRD profiles of non-reduced Ce-TZP after grinding.
fact that the decreasing effect of the low transformability might exceed the increasing effect of the residual compressive stress in the surface layer, resulting from partial reduction. 3.3. The effect of oxygen vacancies on mechanical properties In a previous work [16,17], we have proposed that the incidence of anelastic strain closely correlates with the number of oxygen vacancies. In fact, Y2 O3 -doped zirconia ceramics, which include a large number of oxygen vacancies, tends to exhibit significant anelastic strain, while anelasticity is hardly detected in CeO2 -doped zirconia ceramics including few oxygen vacancies [16,17]. Therefore, Ce-TZP is expected to produce anelastic strain, provided oxygen vacancies are introduced to some extent into the matrix. In this study, we tried to introduce the large amount of oxygen vacancies into the tetragonal matrix in Ce-TZP with the reducing heat treatment and to thus promote anelastic strain. Fig. 6 shows the non-elastic strain behavior in both non-reduced and reduced Ce-TZP at a stress of 100 MPa, with a load-holding time of 10 h. Clearly, the anelastic strain is greatly enhanced in the reduced Ce-TZP, incorporating a large number of oxygen vacancies. Although the figures are not listed, on the other hand, no significant change of anelasticity was observed in any other reduced material, whose weight and mechanical properties were unchanged by reduction. These results confirmed that the basic idea of anelastic strain production mechanism
could be correlated with oxygen vacancies [16,17]. It is assumed that the existence of oxygen vacancies would facilitate a slight shift of the structural ions under an applied stress, resulting in improvement in the effectiveness of anelasticity. The anelastic saturation level of the reduced Ce-TZP at 100 MPa is estimated about 25 le from Fig. 6. Although the anelasticity seems to be considerably improved by the increase of oxygen vacancies, the saturation level of the reduced Ce-TZP is obviously lower than that of 3Y-TZP, about 35 le at 100 MPa from Fig. 3(a) in [17]. Considering the valence and the
Fig. 6. Anelastic behavior for non-reduced Ce-TZP ceramics. Data of the non-reduced Ce-TZP is taken from [17]. (applied stress: 100 MPa, holding time: 10 h).
amount of each additive dopant, it should be noticed that the fully reduced Ce-TZP (12 mol% CeO1:5 ) has twice the level of oxygen vacancies than that of 3Y-TZP (3 mol% Y2 O3 ). In short, Fig. 6 indicates key problem that the reduced Ce-TZP cannot produce anelastic strain more effectively than 3Y-TZP, in spite of the high content of oxygen vacancies. Thus, the extent of anelasticity cannot be determined only by the number of oxygen vacancies in the matrix. According to our previous study [17], Mg-PSZ also does not relate the effectiveness of anelasticity to the number of oxygen vacancies. Therefore, it is presumed that the incidence of anelasticity is also related to the ionic radius of the stabilizers. In a recent work [18], we have investigated fracture tensile strength of various types of zirconia ceramics over a wide range of strain rates. A unique strain rate dependence of tensile strength was observed in Y-TZP and Mg-PSZ, which have anelastic properties, as a result, it has been concluded that the extent of the occurrence or the exhaustion of anelasticity predominantly controls the tensile strength of zirconia ceramics. As mentioned above, Ce-TZP ceramics are easily reduced by heat treatment in hydrogen and the anelastic properties are extremely improved. Therefore, it is supposed that the tensile strength of the reduced Ce-TZP, as well as other zirconia ceramics, might depend on strain rate. The non-reduced and reduced Ce-TZP specimens were ruptured over a wide range of strain rates in air for comparison. Typically, the three strain rates used were 1 101 , 1 106 and 3 109 s1 , which represent a high, a low and a very-low strain rate respectively, as used in previous studies [18]. Fig. 7 shows the relationship between tensile fracture strength and strain rate for the non-reduced and reduced Ce-TZP. Since the number of specimens is limited, we could conduct approximately five testings at every strain rate. However, the data scatter should be considerably reduced by introduction of the ‘‘notch’’ into the smooth specimens. The quite interesting tendency was confirmed from the figure. For reference, the strength data of 3Y-TZP having significant anelastic property, which is taken from [18], are also plotted in Fig. 7. The non-reduced Ce-TZP indicates approximately constant value over the range from high to low strain rate, though slight decrease of strength is observed at very-low strain rate region because of possible sub-critical microcrack growth. On the other hand, it is clear that the tensile strength of the reduced Ce-TZP becomes dependent on strain rate through reduction, which is similar to the result of 3YTZP. This result suggests that strain rate dependence of tensile strength might be mainly caused by anelasticity. Note that the tensile strength of the reduced Ce-TZP appears to be lower overall than that of the non-reduced Ce-TZP. Hasegawa et al. [24] have investigated the bending strength of Ce-TZP with varying extent of re-
Tensile fracture stress (MPa)
M. Matsuzawa et al. / Acta Materialia 52 (2004) 1675–1682
1000 900 800 700 600 500
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3Y-TZP [18] Non-reduced Ce-TZP Reduced Ce-TZP
400 300 200
100 10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
100
Strain rate (s-1)
Fig. 7. The relationship between tensile fracture strength and strain rate. Data plots of 3Y-TZP are taken from [18].
duction and found that the bending strength is improved with the increase in reduction, but on the contrary, degraded by a sufficiently high reduction. In this study, therefore, it is considered that the tensile fracture strength is decreased because the sufficiently high reduction leads to high brittleness (i.e. the low transformability). However, it should be noticed that the strength is improved to some degree in the high strain rate region, though it is not comparable with that of the non-reduced samples. The extremely low transformability of the reduced Ce-TZP suggests that thermally activated anelasticity could be the predominant cause of the strain rate dependence of tensile fracture strength. The effect of ferroelastic domain switching [7–10] should not be ignored as a possible thermally activated mechanism. The activation energy for ferroelastic domain switching is increased with increase in oxygen vacancies in the matrix, but decreased with decrease of tetragonality by reduction [10]. Furthermore, several researchers [34–36] have proposed the existence of an isothermal phase transformation, resulting from fairly large number of oxygen vacancies [34]. In order to investigate the effect of these mechanisms, microfocus XRD was carried out in the neighborhood of the fracture surface of the tensile specimens. However, the traces of ferroelastic domain switching or phase transformation were not detected on any specimens ruptured at any strain rates. It is considered that the ease with which ions can slightly shift should increase under the high oxygen vacancy concentration in the matrix. The results obtained in this study suggest that the anelasticity should be produced by the short-range shift of ions, which also could be the source of internal friction depending on the stress condition. The tensile strength might be enhanced when good balance is attained between the applied strain rate and the anelastic strain producing rate. It is presumed that the peak tensile strength might appear at
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the high strain rate region, in the manner that internal friction peaks occur at a certain resonance frequency.
4. Conclusions Four types of zirconia ceramic, Ce-TZP, 3Y-TZP, 8Y-FSZ and Mg-PSZ, were heated at 1000 °C for 8 h in hydrogen gas. The various mechanical properties before and after the heat treatment for reduction were investigated. The results can be summarized as follows. (1) Ce-TZP was very effectively reduced after the heat treatment in hydrogen but the other samples were not reduced in spite of the severe reducing condition. The drastic weight loss and the increase of internal friction were observed in the reduced Ce-TZP due to the large number of oxygen vacancies introduced into the matrix. (2) The sufficient reduction effectively increased the hardness, but on the other hand, decreased the fracture toughness. It was found that the serious decrease of fracture toughness should be predominantly caused by low transformability in the reduced Ce-TZP. The incorporation of hydrogen into the Ce-TZP matrix was detected after the heat treatment in hydrogen. However, hydrogen-induced embrittlement did not occur for the Ce-TZP ceramics. (3) The anelastic property of Ce-TZP was extremely enhanced by the increase of oxygen vacancies. It is possible that anelasticity is efficient at improving the tensile strength.
Acknowledgements This research was supported by: (i) Grant-in-Aid for Scientific Research and Open Research Center Project by The Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government and (ii) Waseda University Grant for Special Research Projects (2000A-524). The authors wish to acknowledge these supports. They thank Dr. K. Minagawa, NIMS, for materials preparation and discussion.
References [1] Swain MV. Acta Metall 1985;33:2083. [2] Evans AG, Cannon RM. Acta Metall 1986;34:761. [3] Green DJ, Hannink RHJ, Swain MV. Transformation toughening of ceramics. Boca Raton, FL: CRC Press, Inc; 1989. [4] Sato T, Ohtaki S, Endo T, Shimada M. J Am Ceram Soc 1985;68:C-320. [5] Lange FF, Davis BI, Dunlop GL. J Am Ceram Soc 1986;69:237. [6] Kim DJ, Jung HJ, Jang JW, Lee HL. J Am Ceram Soc 1988;81:2309. [7] Virker AV, Matsumoto RLK. J Am Ceram Soc 1986;69:C-244. [8] Foitzik A, Klenke MS, R€ uhle M. Z Metallkd 1993;84:397. [9] Kiguchi T, Urushihara W, Saiki A, Shinozaki K, Mizutani N. J Ceram Soc Jpn 1996;104:529. [10] Kiguchi T, Saiki A, Shinozaki K, Mizutani N. J Ceram Soc Jpn 1996;104:1106. [11] Pan LS, Horibe S. J Mater Sci 1996;31:6523. [12] Pan LS, Horibe S. Acta Mater 1997;45:463. [13] Pan LS, Imai N, Horibe S. Mater Sci Eng A 1997;230:155. [14] Pan LS, Matsuzawa M, Horibe S. Mater Sci Eng A 1998;244:199. [15] Matsuzawa M, Sato F, Horibe S. J Mater Sci 2001;36:2491. [16] Matsuzawa M, Fujimagari E, Horibe S. Mater Sci Eng A 2001;314:105. [17] Matsuzawa M, Horibe S. Mater Sci Eng A 2003;346:75. [18] Matsuzawa M, Abe M, Horibe S. ISIJ Int 2003;43:555. [19] Ando K, Ohishi Y. Ceramics 1982;17:412. [20] Heussner KH, Claussen N. J Am Ceram Soc 1989;72:1044. [21] Tikare V, Heuer H. J Am Ceram Soc 1991;74:593. [22] Zhu HY. J Mater Sci 1994;29:4351. [23] Hasegawa H. J Ceram Soc Jpn 2002;110:731. [24] Hasegawa H, Ozawa M. J Ceram Soc Jpn 2003;111:252. [25] Niihara K, Morena R, Hasselman DPH. J Mater Sci Lett 1982;1:13. [26] Soga N. Shokyu Seramikkusugaku. Tokyo, Japan: Agune Shoufusya Co Ltd; 1993. [27] Ozawa M, Hatanaka T, Hideo H. J Ceram Soc Jpn 1991;99:643. [28] Cartmell E, Fowles GWA. Valency and molecular structure. London, England: Butterworth & Co Ltd; 1977. [29] Horibe S. Reports on Function of Hydrogen in Embrittlement of Advanced Structural Materials: ISIJ, II, 2000. p. 135. [30] Horibe S. Matsuzawa M, Okada Y, Hosokawa T. Reports on Function of Hydrogen in Embrittlement of Advanced Structural Materials: ISIJ, IV, 2002. p. 121. [31] Garvie RC, Nicholson PS. J Am Ceram Soc 1972;55:303. [32] Kitano Y, Mori Y, Ishitani A. J Am Ceram Soc 1988;71:C-34. [33] Kim DJ, Tien TY. J Am Ceram Soc 1991;74:3061. [34] Behrens G, Dransmann GW, Heuer AH. J Am Ceram Soc 1993;76:1025. [35] Nakanishi N, Shigematsu T. Mater Trans JIM 1992;3:318. [36] Rauchs G, Fett T, Munz D, Oberacker R. J Mater Sci Lett 2000;19:1481.
The effect of reduction on the mechanical properties of CeO2 doped tetragonal zirconia ceramics M. Matsuzawa *, M. Abe, S. Horibe, J. Sakai Department of Materials Science and Engineering, Waseda University, 3-4-1, Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan Received 28 August 2003; received in revised form 2 December 2003; accepted 9 December 2003
Abstract Four types of zirconia ceramic, Ce-TZP, 3Y-TZP, 8Y-FSZ and Mg-PSZ, were heat treated in hydrogen gas. Only Ce-TZP was very effectively reduced after the heat treatment. The drastic weight loss and the increase of internal friction, which were observed in the reduced Ce-TZP, were due to the high number of oxygen vacancies introduced into the matrix. The various mechanical properties of Ce-TZP before and after the heat treatment were investigated. Sufficient reduction effectively increased the hardness but unfortunately also embrittled the sample. The serious increase in embrittlement could not be attributed to the hydrogen introduced into the matrix, but was due principally to the low transformability of the reduced Ce-TZP. On the other hand, the anelastic property of the reduced Ce-TZP was considerably improved by the increase in oxygen vacancies and, furthermore, the tensile fracture strength was influenced due to the fact that anelasticity occurs instead of phase transformation. Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ceramics; Hardness; Toughness; Internal friction; Fracture; Reduction
1. Introduction It is well known that stress-induced transformation from the tetragonal to the monoclinic phase effectively improves the fracture toughness of zirconia based ceramic materials [1–3]. However, the desirable stress-induced phase transformation is inhibited by an increase in temperature. Furthermore, it has been noted that serious strength degradation in zirconia ceramics can occur in certain temperature regions [3–6]. In some of the most important structural materials, these negative factors inhibit the wide application of zirconia ceramics, especially at high temperatures. Ferroelastic domain switching [7–10] and anelasticity [11–18] have recently been observed as significant strengthening or toughening mechanisms. These mechanisms are thermally activated [8,13] and, therefore, could improve mechanical properties even at elevated temperatures [8–10,14–16]. In particular, in a previous study we have found that an*
Corresponding author. Tel./fax: +81-352-863-306. E-mail addresses: [email protected] (M. Matsuzawa), [email protected] (S. Horibe).
elasticity can produce a uniquely recoverable non-elastic strain at lower levels of stress, in contrast to ferroelastic domain switching [16,17]. The effect of anelastic behavior on various mechanical properties has been extensively studied and it has been found that anelasticity should be regarded as one of the most important strengthening– toughening mechanisms [15–18]. Although the exact mechanism of anelastic strain production is still unclear, it is assumed that the effectiveness of anelasticity is closely correlated with oxygen vacancies included in zirconia matrix [16,17]. The number of oxygen vacancies is determined by valency and by the quantity of additive stabilizer dopant [19] and it is well known that migration of the oxygen ions via vacancies plays an important role in major technical applications, such as oxygen sensors, fuel cells, etc. The effect of oxygen vacancies on mechanical properties requires study when considering the application of zirconia-based ceramics as structural materials. Several researchers [20–24] have investigated the change in microstructure and various mechanical properties of a reduced Ce-TZP and reported fascinating results. Tikare and Heuer [21] have carried out indentation testing at
1359-6454/$30.00 Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2003.12.012
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high temperatures in vacuum and found that indentation cracking was more effectively suppressed on a CeTZP sample initially heated to higher temperature. Moreover, Hasegawa and Ozawa [24] have found that hardness or strength of Ce-TZP could be increased by a heat treatment in hydrogen (reducing atmosphere), depending on temperature and exposure time, proposing a reduction-induced strengthening mechanism. It is obvious that oxygen vacancies introduced by reduction could be the origin of the change in mechanical properties of Ce-TZP. Zhu [22] has reported that the crystallographic phase was unchanged but the lattice volume was increased after reduction of Ce-TZP. The effective residual compressive stress seems to be caused by the change in lattice length, resulting in an improvement of the mechanical properties, such as hardness [23,24], strength [20,24] or fracture toughness [21]. However, it is thought that the variation of these mechanical properties depends strongly on the extent of reduction and further study is required. As mentioned above, the mechanism of anelastic strain production is closely associated with oxygen vacancy [16,17], suggesting that the anelastic property also has a high probability of being affected by introduction of oxygen vacancies. Because the various changes of microstructure or mechanical properties in zirconia matrix might be caused by atomic defects associated with oxygen vacancies, it is very important to verify the nature of these vacancies. In this study, the heat treatment in a reducing hydrogen atmosphere was carried out for various engineering zirconia ceramics. The present examination was focused on the following points; (i) to investigate not only general mechanical properties, i.e. hardness, toughness or tensile fracture strength, but also internal friction or anelasticity closely related to oxygen vacancies and furthermore (ii) to survey the role played by hydrogen in modifying mechanical properties during reducing heat treatments.
2. Experimental procedure The material mainly used in this study was Ce-TZP (12 mol% CeO2 doped tetragonal zirconia polycrystals). The material was prepared as a rectangular shaped specimen, with dimensions 4 3 40 mm (Fig. 1(a)) for indentation and bending tests. Similar rectangular specimens of a further three types of zirconia-based ceramics, namely 3Y-TZP (3 mol% Y2 O3 doped tetragonal zirconia polycrystals), 8Y-FSZ (8 mol% Y2 O3 doped fully stabilized zirconia) and Mg-PSZ (9 mol% MgO doped partially stabilized zirconia), were prepared for comparison. In addition, in the case of Ce-TZP, thinsheet specimens with two notches for tensile rupture testing were used, as shown in Fig. 1(b). All the specimens were provided by commercial ceramic manufac-
Fig. 1. The test pieces: (a) rectangular specimen used for the indentation testing and the measurements of internal friction and anelasticity and (b) thin-sheet specimen used for tensile fracture testing.
turers in Japan (Ce-TZP, 8Y-FSZ and Mg-PSZ: Nagano Keiki Co., Ltd., Nagano; 3Y-TZP: Tohoku Ceramic Co., Ltd., Miyagi) and their characteristics are summarized in Table 1. The density measured for each sample by using Archimedes method was extremely close to its theoretical density. Prior to each testing, the surface subjected to be indented on the rectangular specimen and two pinholes of the thin-sheet specimen were carefully polished using 1/4 lm diamond paste. The heat treatment of specimens was carried out in hydrogen gas (reducing atmosphere). Sample weight and fracture toughness were measured and compared before and after the reducing heat treatment. The high temperature heat treatment in hydrogen was carried out as follows. Specimens were heated in a hydrogen atmosphere from room temperature to 1000 °C over a period of 1.5 h in an electric furnace. The samples were maintained at this temperature for 8 h, after which, the furnace was cooled and the samples removed from the hot zone to promote rapid cooling. In order to avoid reoxidization of samples, they were cooled to room temperature with the hydrogen replaced by argon gas. Subsequently, the various mechanical tests were carried out on the processed samples in order to evaluate the change of mechanical properties caused by the heat treatment. 2.1. Fracture toughness and hardness The indentation testing was carried out by pressing the diamond indentor onto the sample surface with dwell time of 10 s. Applied load was varied from 49 to 294 N in order to reduce statistical error. The values of hardness and fracture toughness were calculated by measuring the
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Table 1 Sintering temperature, density, grain size and mechanical properties of the materials Materials
Sintering temperature (°C)
Density (g/cm3 )
Grain size (lm)
Hardness (GPa)
pffiffiffiffi Fracture toughness (MPa m)
Ce-TZP 3Y-TZP 8Y-FSZ Mg-PSZ
1425 1450 1500 1675
6.22 6.10 5.95 5.87
1.0–1.4 0.2–0.4 3.0–5.0 0.4–0.8
9.5 14.2 12.8 9.8
12.2 4.8 1.5 8.4
crack length and the diagonal length of the indent according to the indentation fracture procedure proposed by Niihara et al. [25]; because of the different cracking modes among the materials, the palmqvist crack mode calculation was used for Ce-TZP, 3Y-TZP and Mg-PSZ, while half-penny crack mode calculation was used for 8Y-FSZ and the heat-treated Ce-TZP. 2.2. Internal friction The internal friction testing was carried out with free decay method, where the rectangular sample was horizontally supported at the nodes of oscillation by two thin wires of SUS304. One side of the sample was covered with carbon paint to render it conducting. The flexural vibration was then driven electrostatically by an electrode directly located below the sample, between the wires (JIS G0602). The internal friction was evaluated using the half value breadth method (ASTM E756-93). 2.3. Anelasticity A four-point bending test was carried out, with inner and outer spans of 10 and 30 mm, respectively (Fig. 1(a)). A desired load (300 MPa/s) was applied abruptly to the specimen, maintained for 10 h and then removed. The strain was recorded over the time using a strain gauge attached to the tensile surface of the specimen. The recoverable strain after unloading was defined as the amount of anelastic strain (Refer to our previous paper [17] for the details). 2.4. Tensile fracture strength The uni-axial tensile fracture strength of non-reduced and reduced Ce-TZP was investigated for a wide range of strain rates (3 109 –1 101 s1 ) in air in a manner similar to [18]. The strain rate dependence of tensile strength on both the material was discussed.
3. Results and discussion 3.1. The weight loss caused by reducing heat treatment in hydrogen The normalized weight of each sample, Ce-TZP, 3YTZP, 8Y-FSZ and Mg-PSZ, after the heat treatment at
1000 °C for 8 h in hydrogen is shown in Fig. 2(a). Similarly, Fig. 2(b) shows the change of fracture toughness after the heat treatment. Each data point indicates a mean value obtained by several (more than five) indentation tests. Both figures show an interesting tendency, that the significant weight loss and decrease of fracture toughness are apparently observed only for CeTZP, with almost no change for the other. These results suggest that there is a significant correlation between the weight loss and the decrease of fracture toughness in zirconia-based ceramics. The weight loss during the heat treatment amounted to about 0.74% in Ce-TZP, but was less than 0.1% in the other materials (Fig. 2(a)). The color of the Ce-TZP specimen drastically changed from ivory yellow to black, while the 3Y-TZP and Mg-PSZ specimens, changed from white to gray and the 8Y-FSZ specimen, merely from white to light gray. Other mechanical surface damage such as microcracking or chipping was not observed. Oxygen vacancies produced in the ZrO2 matrix by reduction effectively absorbed light of longer wavelength [26], resulting in a black colored specimen. Therefore, all specimens used in this experiment seem to be more or less reduced, though the extent of reduction in the Ce-TZP overwhelmingly exceeds that in the other specimens. For 3Y-TZP, 8Y-FSZ and Mg-PSZ, therefore, it was not anticipated that the slight reduction observed would have any great effect on fracture toughness. Fig. 2(c) shows the change of internal friction before and after the reducing heat treatment. (Data for MgPSZ could not be obtained because the specimen length was too short for the internal friction testing.) A pronounced change in internal friction, weight change and fracture toughness was detected only for Ce-TZP, as indicated in Fig. 2(a) and (b). The excessive increase in internal friction indicates the introduction of oxygen vacancies in the matrix, agreeing with the results of Ozawa et al. [27] using similarly reduced Ce-TZP samples. They have suggested that the oxygen vacancies significantly contribute to the generation of the internal friction in ZrO2 ceramics [27]. A doping stabilizer with a different valency from that of the Zr4þ cation introduces some oxygen vacancies in the matrix to maintain the electrical valency [19]. Therefore, the large number of oxygen vacancies must already be included in Y2 O3 – ZrO2 and MgO–ZrO2 prior to the heat reducing treatment. In the case of Ce-TZP, on the other hand, the
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*
The change of weight (M/M %)
100.2
The change of fracture toughness (KIC/KIC* %)
(a)
100.0
99.8
99.6
99.4
99.2 0
3Y - TZP
8Y - FSZ
Mg - PSZ
Ce - TZP
3Y - TZP
8Y - FSZ
Mg - PSZ
120
100
80
60
40
20
0
(b) 500
*
The change of internal friction (Q-1/Q-1 %)
Ce - TZP
(c)
400
300
orbits in atomic Ce is [Xe]4f1 5d1 6s2 , the Ce ion tends to be stabilized with a trivalent state ([Xe]4f1 5d0 6s0 ) because of the low shielding effect of the inner electric orbits for 4f orbit, as well as the tetravalent state ([Xe]4f0 5d0 6s0 ). In other words, the energy required for reducing from Ce4þ to Ce3þ is much lower than that of Y3þ ! Y2þ or Mg2þ ! Mgþ . Kiguchi et al. [10] have referred to the valence change form Zr4þ to Zr3þ but not from Y3þ to Y2þ in electrochemically reduced 3 mol% Y2 O3 –ZrO2 , indicating that Y3þ state is more stable than Zr4þ . These results also indicate that Ce ion is more easily reduced than other dopant ions. When all Ce ions in 12 mol% CeO2 –88 mol% ZrO2 are entirely reduced from the tetravalent to the trivalent state, 6% oxygen vacancies are produced in the matrix, corresponding to the weight loss of 0.76%. This value exactly agrees with the above-mentioned experimental result shown in Fig. 2(a). There is a high probability that not only oxygen vacancies but also hydrogen elements might be incorporated into the reduced samples during the heat treatment in flowing hydrogen gas. The amount of hydrogen included in the reduced Ce-TZP samples was investigated using thermal desorption spectrometry (Fig. 3). According to Fig. 3, a significant incorporation of hydrogen into the samples can be clearly observed. It is generally accepted that incorporation of hydrogen into the matrix might result in serious embrittlement of metallic materials. In recent work, the effect of hydrogen in producing significant embrittlement in ceramic materials has been also detected [29,30]. It is most important to understand which of the components caused the serious decrease of fracture toughness after the heat treatment at 1000 °C in hydrogen–oxygen vacancies or hydrogen atoms introduced into matrix. The solution to this problem will be discussed in detail in the next section.
200
3.2. The merits and faults of reduction for Ce-TZP 100
No data 0
Ce - TZP
3Y - TZP
8Y - FSZ
Mg - PSZ
As mentioned previously, the decrease of fracture toughness for Ce-TZP was caused by the heat treatment at 1000 °C in hydrogen and two factors, oxygen vacancy
Fig. 2. The change of (a) weight M, (b) fracture toughness KIC and (c) internal friction Q1 for various zirconia ceramics after the heat treatment at 1000 °C for 8 h in hydrogen. Each value, M, KIC and Q1 after heat treatment is normalized by M , KIC and Q1 before heat treatment, respectively for convenience of comprehension. (Horizontal bars mean the scatter bands).
valencies of the dopant cation, Ce4þ , and the matrix, Zr4þ , are the same, so that few vacancies are originally included in the matrix. However, the drastic weight loss of the reduced Ce-TZP in Fig. 2(a) indicates that a large number of oxygen vacancies was introduced into the matrix, accompanying the valence change from Ce4þ to Ce3þ [28]. Although the electron configuration in outer
Fig. 3. Hydrogen evolution rate as a function of temperature for CeTZP sample after the heat treatment at 1000 °C for 8 h in hydrogen gas.
M. Matsuzawa et al. / Acta Materialia 52 (2004) 1675–1682
and hydrogen incorporation, were considered as major causes of this serious problem. In order to investigate the effect of oxygen vacancy and hydrogen incorporation on the mechanical properties, an additional sequence of heat treatments, (1000 °C in hydrogen ! 950 °C in vacuum ! 1000 °C in air), was conducted, with the change of the weight loss, fracture toughness and hardness being measured after each process. The results are shown in Fig. 4. After the first process of the heating at 1000 °C in hydrogen gas, the large amount of oxygen vacancies was introduced into the matrix and a significant weight loss was recorded, with an accompanyingpdecrease in fracture toughness from 12.2 to 2.3 ffiffiffiffi MPa m. (The fracture toughness values of the samples heat-treated in hydrogen and in vacuum were obtained by using half-penny crack mode calculation [25].) In contrast, it is noted that hardness effectively increases from 9.5 to 12.5 GPa after the reduction. After the second heating process in vacuum, hydrogen was released from the Ce-TZP matrix as shown in Fig. 3. However, Fig. 4(a) and (b) indicate that no significant variation of either weight or mechanical properties was observed. After the final process of the re-heating at 1000 °C in air, the weight and the mechanical properties are all recovered to their original level, suggesting that the sample was re-oxidized. The results obtained in the additional consecutive heating processes lead us to conclude that the serious decrease of fracture toughness
Fig. 4. The change of (a) weight loss and (b) fracture toughness and hardness during consecutive heat treatment.
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of Ce-TZP ceramics after the heat treatment at 1000 °C in hydrogen is caused not by hydrogen incorporation, but, to a great extent, by oxygen vacancies. Considering the wide application of ceramic materials, it is necessary to elucidate the mechanism by which oxygen vacancies seriously decrease the fracture toughness. As one of the most effective approaches to solve the problem, it could be important to evaluate transformability of non-reduced and reduced Ce-TZP samples. In order to evaluate the transformability, the surfaces of both specimens were ground using #100 emery paper and the change of phase composition was investigated using X-ray diffraction (XRD). The data were analyzed using the method proposed by Garvie and Nicholson [31]. Fig. 5 shows the XRD profiles of the specimen before and after grinding. Only tetragonal peaks were observed before grinding for both non-reduced and reduced samples, indicating that the crystallographic phase would not change during the heat treatment in hydrogen. In the case of non-reduced CeTZP, grinding produced monoclinic peaks on the surface, corresponding to about 30% formation of the monoclinic phase. Furthermore, Kitano et al. have confirmed that the asymmetry of the profiles of the tetragonal (1 1 1) peak, which appears at lower angles, could be assigned to the rhombohedral phase [32]. On the other hand, no monoclinic peak was detected for the ground reduced sample, though the rhombohedral phase still seemed to be produced. These results imply that transformability drastically decreased with increase of oxygen vacancies, introduced by reducing heat treatment. This accounted for the serious decrease of fracture toughness. During reduction of Ce-TZP, the increase of oxygen vacancies and the valence change of Ce ions from Ce4þ to Ce3þ , accompanied by the increase of ionic radius occur in the tetragonal crystals [28]. from 0.94 to 1.07 A, As a result, the lattice length is expanded and the lattice volume is increased [21]. Moreover, it is interesting to note that tetragonality decreases after the reduction of Ce-TZP. Comparing the XRD profiles of non-reduced and reduced Ce-TZP (Fig. 5(a) and (b) respectively), it is found that two tetragonal peaks (2 0 0) and (0 0 2) approach each other, corresponding to a decrease of tetragonality c=a. The lattice volume dilatation is considered to produce residual compressive stress in the surface layer, resulting in the effective improvement of various mechanical properties, such as hardness [23,24], fracture toughness [21] or strength [20,24]. However, this concept is not comprehensible on an entirely reduced specimen. Meanwhile, the decrease in tetragonality suggests that the crystallographic phase tends from tetragonal towards cubic. It follows that the tetragonal phase is more stable and, as a result, transformability should be decreased [33]. It is assumed that the undesirable decrease of fracture toughness is attributed to the
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Fig. 5. The change of XRD profiles of non-reduced Ce-TZP after grinding.
fact that the decreasing effect of the low transformability might exceed the increasing effect of the residual compressive stress in the surface layer, resulting from partial reduction. 3.3. The effect of oxygen vacancies on mechanical properties In a previous work [16,17], we have proposed that the incidence of anelastic strain closely correlates with the number of oxygen vacancies. In fact, Y2 O3 -doped zirconia ceramics, which include a large number of oxygen vacancies, tends to exhibit significant anelastic strain, while anelasticity is hardly detected in CeO2 -doped zirconia ceramics including few oxygen vacancies [16,17]. Therefore, Ce-TZP is expected to produce anelastic strain, provided oxygen vacancies are introduced to some extent into the matrix. In this study, we tried to introduce the large amount of oxygen vacancies into the tetragonal matrix in Ce-TZP with the reducing heat treatment and to thus promote anelastic strain. Fig. 6 shows the non-elastic strain behavior in both non-reduced and reduced Ce-TZP at a stress of 100 MPa, with a load-holding time of 10 h. Clearly, the anelastic strain is greatly enhanced in the reduced Ce-TZP, incorporating a large number of oxygen vacancies. Although the figures are not listed, on the other hand, no significant change of anelasticity was observed in any other reduced material, whose weight and mechanical properties were unchanged by reduction. These results confirmed that the basic idea of anelastic strain production mechanism
could be correlated with oxygen vacancies [16,17]. It is assumed that the existence of oxygen vacancies would facilitate a slight shift of the structural ions under an applied stress, resulting in improvement in the effectiveness of anelasticity. The anelastic saturation level of the reduced Ce-TZP at 100 MPa is estimated about 25 le from Fig. 6. Although the anelasticity seems to be considerably improved by the increase of oxygen vacancies, the saturation level of the reduced Ce-TZP is obviously lower than that of 3Y-TZP, about 35 le at 100 MPa from Fig. 3(a) in [17]. Considering the valence and the
Fig. 6. Anelastic behavior for non-reduced Ce-TZP ceramics. Data of the non-reduced Ce-TZP is taken from [17]. (applied stress: 100 MPa, holding time: 10 h).
amount of each additive dopant, it should be noticed that the fully reduced Ce-TZP (12 mol% CeO1:5 ) has twice the level of oxygen vacancies than that of 3Y-TZP (3 mol% Y2 O3 ). In short, Fig. 6 indicates key problem that the reduced Ce-TZP cannot produce anelastic strain more effectively than 3Y-TZP, in spite of the high content of oxygen vacancies. Thus, the extent of anelasticity cannot be determined only by the number of oxygen vacancies in the matrix. According to our previous study [17], Mg-PSZ also does not relate the effectiveness of anelasticity to the number of oxygen vacancies. Therefore, it is presumed that the incidence of anelasticity is also related to the ionic radius of the stabilizers. In a recent work [18], we have investigated fracture tensile strength of various types of zirconia ceramics over a wide range of strain rates. A unique strain rate dependence of tensile strength was observed in Y-TZP and Mg-PSZ, which have anelastic properties, as a result, it has been concluded that the extent of the occurrence or the exhaustion of anelasticity predominantly controls the tensile strength of zirconia ceramics. As mentioned above, Ce-TZP ceramics are easily reduced by heat treatment in hydrogen and the anelastic properties are extremely improved. Therefore, it is supposed that the tensile strength of the reduced Ce-TZP, as well as other zirconia ceramics, might depend on strain rate. The non-reduced and reduced Ce-TZP specimens were ruptured over a wide range of strain rates in air for comparison. Typically, the three strain rates used were 1 101 , 1 106 and 3 109 s1 , which represent a high, a low and a very-low strain rate respectively, as used in previous studies [18]. Fig. 7 shows the relationship between tensile fracture strength and strain rate for the non-reduced and reduced Ce-TZP. Since the number of specimens is limited, we could conduct approximately five testings at every strain rate. However, the data scatter should be considerably reduced by introduction of the ‘‘notch’’ into the smooth specimens. The quite interesting tendency was confirmed from the figure. For reference, the strength data of 3Y-TZP having significant anelastic property, which is taken from [18], are also plotted in Fig. 7. The non-reduced Ce-TZP indicates approximately constant value over the range from high to low strain rate, though slight decrease of strength is observed at very-low strain rate region because of possible sub-critical microcrack growth. On the other hand, it is clear that the tensile strength of the reduced Ce-TZP becomes dependent on strain rate through reduction, which is similar to the result of 3YTZP. This result suggests that strain rate dependence of tensile strength might be mainly caused by anelasticity. Note that the tensile strength of the reduced Ce-TZP appears to be lower overall than that of the non-reduced Ce-TZP. Hasegawa et al. [24] have investigated the bending strength of Ce-TZP with varying extent of re-
Tensile fracture stress (MPa)
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1000 900 800 700 600 500
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3Y-TZP [18] Non-reduced Ce-TZP Reduced Ce-TZP
400 300 200
100 10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
100
Strain rate (s-1)
Fig. 7. The relationship between tensile fracture strength and strain rate. Data plots of 3Y-TZP are taken from [18].
duction and found that the bending strength is improved with the increase in reduction, but on the contrary, degraded by a sufficiently high reduction. In this study, therefore, it is considered that the tensile fracture strength is decreased because the sufficiently high reduction leads to high brittleness (i.e. the low transformability). However, it should be noticed that the strength is improved to some degree in the high strain rate region, though it is not comparable with that of the non-reduced samples. The extremely low transformability of the reduced Ce-TZP suggests that thermally activated anelasticity could be the predominant cause of the strain rate dependence of tensile fracture strength. The effect of ferroelastic domain switching [7–10] should not be ignored as a possible thermally activated mechanism. The activation energy for ferroelastic domain switching is increased with increase in oxygen vacancies in the matrix, but decreased with decrease of tetragonality by reduction [10]. Furthermore, several researchers [34–36] have proposed the existence of an isothermal phase transformation, resulting from fairly large number of oxygen vacancies [34]. In order to investigate the effect of these mechanisms, microfocus XRD was carried out in the neighborhood of the fracture surface of the tensile specimens. However, the traces of ferroelastic domain switching or phase transformation were not detected on any specimens ruptured at any strain rates. It is considered that the ease with which ions can slightly shift should increase under the high oxygen vacancy concentration in the matrix. The results obtained in this study suggest that the anelasticity should be produced by the short-range shift of ions, which also could be the source of internal friction depending on the stress condition. The tensile strength might be enhanced when good balance is attained between the applied strain rate and the anelastic strain producing rate. It is presumed that the peak tensile strength might appear at
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the high strain rate region, in the manner that internal friction peaks occur at a certain resonance frequency.
4. Conclusions Four types of zirconia ceramic, Ce-TZP, 3Y-TZP, 8Y-FSZ and Mg-PSZ, were heated at 1000 °C for 8 h in hydrogen gas. The various mechanical properties before and after the heat treatment for reduction were investigated. The results can be summarized as follows. (1) Ce-TZP was very effectively reduced after the heat treatment in hydrogen but the other samples were not reduced in spite of the severe reducing condition. The drastic weight loss and the increase of internal friction were observed in the reduced Ce-TZP due to the large number of oxygen vacancies introduced into the matrix. (2) The sufficient reduction effectively increased the hardness, but on the other hand, decreased the fracture toughness. It was found that the serious decrease of fracture toughness should be predominantly caused by low transformability in the reduced Ce-TZP. The incorporation of hydrogen into the Ce-TZP matrix was detected after the heat treatment in hydrogen. However, hydrogen-induced embrittlement did not occur for the Ce-TZP ceramics. (3) The anelastic property of Ce-TZP was extremely enhanced by the increase of oxygen vacancies. It is possible that anelasticity is efficient at improving the tensile strength.
Acknowledgements This research was supported by: (i) Grant-in-Aid for Scientific Research and Open Research Center Project by The Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government and (ii) Waseda University Grant for Special Research Projects (2000A-524). The authors wish to acknowledge these supports. They thank Dr. K. Minagawa, NIMS, for materials preparation and discussion.
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