2018 Volume 59 Issue 1 Pages 23-26
Nano-indentation tests with a Vickers-type diamond tip were carried out on the {100} surface of a single crystal of 10 mol% yttria-stabilized zirconia (10YSZ), (Y2O3)0.1(ZrO2)0.9. By changing the contact angle of the indentation tip against the single crystal sample, the fracture toughness of 10YSZ was investigated as a function of the crystal orientation from <100> to <110> on the {100} surface. It was empirically shown that the fracture toughness of 10YSZ is anisotropic. The fracture toughness of 10YSZ was lowest in the direction of <100> among investigated orientations. This result suggests that mechanical failures in 10YSZ may occur preferentially along the direction of <100>.
Solid oxide fuel cells (SOFCs) attract attentions as an environment-friendly energy conversion system with many advantages such as high energy conversion efficiency, high fuel flexibility, efficient use of high-quality exhaust-heat, nonuse of expensive precious-metal catalysis, and so on. In the last couple of years, SOFCs have been commercialized by several companies. For further widespread commercialization of SOFCs, it is highly required to improve their reliability and durability. Mechanical damages of cell components are considered as one of main causes for the performance deterioration of SOFCs. In order to reduce risks for mechanical failures of SOFCs, it is important to design the cell/stack properly and to understand the operational margin based on sufficient understandings of mechanical properties of the components.
In this paper, we report mechanical properties of yttria-stabilized zirconia (YSZ). Since YSZ having the cubic fluorite structure is a typical SOFC electrolyte material as well as a conventional constructional material, its mechanical properties have been extensively investigated so far1–14). However most of the data are for the polycrystalline bulk YSZ, and the information on the anisotropy of mechanical properties is rather limited. In our previous work14), the anisotropy of the Young's modulus of YSZ was investigated by using nano-indentation tests combined with SEM observation and EBSD (electron back scattering diffraction) analysis. As results, it was demonstrated that the Young's modulus of YSZ significantly depends on the crystal orientation. The Young's modulus had the highest value on the {100} surface, approximately 270 GPa, while gradually decreased to approximately 250 GPa with approaching the {111} or {110} surface. A similar crystal orientation dependence of the Young's modulus of YSZ was reported by Kurosaki et al.2) and Fujikane et al.3) using nano-indentation tests, although their data was limited for only three specific surfaces, {100}, {110} and {111}, of the single crystals. Such an anisotropy of the Young's modulus indicates that the mechanical stress is largest in the direction of <100> when YSZ is isotropically deformed. However, for precisely evaluating risks of mechanical failures of YSZ, the anisotropy of the fracture toughness should be also understood.
Considering the background mentioned above, in this work, we examine the anisotropy of the fracture toughness of (Y2O3)0.1(ZrO2)0.9 (10YSZ). Nano-identation tests using a quadrilateral Vickers-type indentation tip were carried out on the {100} surface of a 10YSZ single crystal. In this method, the fracture toughness along the diagonal direction of the indentation tip can be evaluated from the length of the radial cracks at the edges of the indent. By changing the contact angle of the indentation tip relative to the single crystal sample, the anisotropy of the fracture toughness was investigated.
A single crystal of (Y2O3)0.1(ZrO2)0.9 (10YSZ) exposing a {100} surface (Crystal Base Co., Ltd.) was used in this work. The specimen was annealed at 1273 K for 3 hrs before nano-identation tests to remove residual stresses.
Measurements of fracture toughness were performed by using a load-control type nano-indentation tester (ENT-1100a, ELIONIX Co., Ltd.) having a quadrilateral-shaped Vickers tip. The fused silica was used for calibration of the tip shape. The calibration method was followed by Oliver and Pharr15), assuming 66.77 GPa of the Young's modulus for the fused silica. In the nano-indentation tests, the indentation tip was first placed with its diagonal parallel to the <100> direction of 10YSZ. Thereafter, similar tests were repeated, while the contact angle of the indentation tip relative to the sample was varied from 0 to 45° with a 11.25° step. Such a variation in the contact angle corresponds to a variation in the crystal orientation on the {100} surface from <100> to <110>. To avoid interaction, the distance between the indentation points was set longer than 200 μm, which was longer than a tenfold length of the impression size. The applied indentation load was 600 or 700 mN. All measurements were carried out at room temperature.
The impressions after indentation tests were traced by scanning electron microscope (SEM, JSM-7001F, JEOL Ltd.) observations. The length of cracks around the impression was measured, and the fracture toughness, Kc, was then evaluated according to the following relation;
| \[K_{\rm c} = \chi (E/H)^{1/2} P/L^{3/2}\] | (1) |
Figures 1 and 2 show SEM images of the impressions after the nano-indentation tests with a load of 600 and 700 mN, respectively. As seen in these images, when the applied load was constant, the impression had an almost same size. This indicated that the hardness could be assumed to be constant regardless of the crystal orientation, as described in the experimental section. This assumption was also supported by the conclusion in our previous work using 8YSZ.14) In the case of 8YSZ, the observed hardness was almost constant, around 19–19.5 GPa, within the range of 5%.
SEM images of the impressions after nano-indentation tests on the {100} surface of 10YSZ with a load of 600 mN. The contact angle of the impression diagonal relative to the <100> direction of 10YSZ was (a) 0, (b) 11.25, (c) 22.5, (d) 33.75, and (e) 45°.
SEM images of the impressions after nano-indentation tests on the {100} surface of 10YSZ with a load of 700 mN. The contact angle of the impression diagonal relative the <100> direction of 10YSZ was (a) 0, (b) 11.25, (c) 22.5, (d) 33.75, and (e) 45°.
In all the tests shown in Figs. 1 and 2, radial cracks from the indent edges were observed. The observed length of the radial cracks was given in Fig. 3 as a function of the contact angle of the indentation tip relative to the <100> direction. The testing angles were 0, 11.25, 22.5, 33.75, and 45°, and changing the angle means changing the crystal orientation examined. The crack length showed a similar orientation dependence regardless of the indentation load, although its absolute value was larger, as a matter of course, for 700 mN than 600 mN of the indentation load. It seems that the radial cracks along the direction parallel to the <100> direction were slightly longer than those to the other directions, suggesting that the fracture toughness of 10YSZ showed an anisotropy.
Observed length of the radial cracks around the impression after nano-indentation tests on the {100} surface of 10YSZ. The horizontal axis expresses the contact angle of the indentation tip relative to the <100> direction. The applied indentation loads were 600 and 700 mN.
Fracture toughness of 10YSZ was evaluated from eq. (1) and is presented in Fig. 4 as a function of the contact angle of the indentation tip relative to the <100> direction. The fracture toughness obtained from the nano-indentation tests with 600 and 700 mN fairly agreed with each other within the experimental error, and showed a similar dependence on the crystal orientation. Among the five investigated orientations, the fracture toughness in the direction of <100> was lowest, while those in the other directions were comparable. When 700 mN of the indentation laod was applied for the nano-indentation tests, the fracture toughness along the <100> and <110> directions were 0.85 ± 0.06 and 1.00 ± 0.10 MPa･m1/2, respectively.
Fracture toughness of 10YSZ evaluated from nano-indentation tests on the {100} surface. The horizontal axis expresses the contact angle of the indentation tip relative to the <100> direction. The applied indentation loads were 600 and 700 mN in this work while 2 N in Ref. 7).
According to nano-indentation tests in our previous work14), it was shown that the Young's modulus of YSZ had the highest value on the {100} surface, i.e. in the direction of <100>. This indicates that the mechanical stress is largest in the direction of <100> when a same amount of deformation is introduced into YSZ. On the other hand, it was empirically demonstrated in this work that the fracture toughness of YSZ had a relatively lower value along the <100> direction. These suggest that mechanical cleavages in 10YSZ may occur preferentially in the direction of <100>.
Pajares et al. investigated the anisotropy of the fracture toughness of 9.4 mol% Y2O3 stabilized ZrO2 by means of the indentation method and the single-edge notched-beam (SENB) method, although they reported the fracture toughness only for two orientations, <100> and <110>7). From the indentation tests with the indentation load of 2 N, they obtained the fracture toughness of 1.9 ± 0.1 and 1.1 ± 0.1 MPa･m1/2 along the respective directions. By using the SENB method, fracture toughness was evaluated as 1.9 ± 0.1 and 1.48 ± 0.04 MPa･m1/2 along the <100> and <110> directions, respectively. These values were also given in Fig. 4 for comparison. For the direction of <110>, the fracture toughness in the literature is reasonably close to that in this work. On the other hand, there is a large discrepancy between the fracture toughness along the <100> direction in Ref. 7) and in this work. In particular, the orientation dependence of the fracture toughness is opposite; the fracture toughness is higher for the <110> than the <100> direction in this work, while lower in the results by Pajares et al.
In Ref. 7), loads from 500 mN to 30 N, which were comparable or larger compared with those in this work, were applied for indentation tests. It was reported that lateral cracks in addition to radial cracks were found for the <100> direction, while only radial cracks were observed for the <110> direction7,8). It was mentioned that formation of lateral cracks was more frequent with increasing the indentation load and that lateral cracks were found at all indents with the loads of 2 N and above. On the other hand, radial cracks were mainly observed in this work possibly because the applied indentation load was relatively low. Lateral cracks were detected at some indents also in this work, but such results were not counted when evaluating the fracture toughness. In indentation tests, the fracture toughness is evaluated from the length of radial cracks, assuming that indentation creates only radial cracks with a symmetric “half-penny” geometry along the indentation diagonal. It is considered that formation of lateral cracks reduces the length of radial cracks. Thus, the fracture toughness is overestimated if lateral cracks are formed. This is probably a reason why the fracture toughness in Ref. 7) was larger in the <100> direction while was comparable in the <110> direction, when comparing to the fracture toughness in this work.
This work was made as a part of the research project on development of systems and elemental technology on SOFC, which was supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan.
