Research Article |
Corresponding author: Artem S. Chislov ( chislov.artem@bk.ru ) © 2024 Artem S. Chislov, Mikhail A. Borik, Aleksey V. Kulebyakin, Elena E. Lomonova, Filipp O. Milovich, Valentina A. Myzina, Aleksey A. Reu, Polina A. Ryabochkina, Natalya V. Sidorova, Nataliya Yu. Tabachkova.
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Citation:
Chislov AS, Borik MA, Kulebyakin AV, Lomonova EE, Milovich FO, Myzina VA, Reu AA, Ryabochkina PA, Sidorova NV, Tabachkova NYu (2024) Comparison of the structure and physicochemical properties of ZrO2 based crystals partially stabilized with Y2O3, Gd2O3 and Sm2O3. Modern Electronic Materials 10(1): 3-10. https://doi.org/10.3897/j.moem.10.1.122043
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The phase composition, density, microhardness and fracture toughness of (ZrO2)1-x(R2O3)х crystals (where R = Y, Sm and Gd) for x = 0.02–0.04 have been compared. The crystals have been grown using directional melt crystallization in a cold crucible. The phase composition of the crystals has been studied using X-ray diffraction and Raman spectroscopy. The microhardness and fracture toughness of the crystals have been evaluated by means of indentation. At stabilizing oxide concentrations of ≥ 2.8 mol.% for Y2O3 and Gd2O3 and ≥ 3.7 mol.% for Sm2O3 the crystals have densities close to the theoretical ones and contain two tetragonal phases. At lower stabilizing oxide concentrations the crystals contain the monoclinic phase. The fracture toughness of the tetragonal crystals increases with the ionic radius of the stabilizer. The highest fracture toughness values achieved when stabilized by a specific oxide are 11.0, 13.0 and 14.3 MPa·m1/2 for the 2.8YSZ, 2.8GdSZ and 3.7SmSZ crystals, respectively. The fracture toughness proves to depend on the crystallographic orientation of the crystals. The results of this work can be used in the design and fabrication of various structural components and devices.
directional crystallization, growth from melt, partially stabilized zirconia, toughness
Zirconia based solid solutions deliver a unique combination of chemical, optical, mechanical, thermophysical and electrical properties which determine their widespread applications in biomedical, structural, heat-insulating, optical and tribotechnical materials [
Pure ZrO2 has three polymorphic modifications at normal pressure: monoclinic, tetragonal and cubic, with only the former one being stable at room temperature. The high-temperature cubic and tetragonal modifications are stabilized by zirconia doping with alkaline-earth or rare-earth element oxides [
These tetragonal solid solutions are distinguished by a combination of good mechanical and tribological parameters with chemical and biological inertness, low heat conductivity and high thermal expansion coefficient and therefore attract great attention of researchers [
A distinctive feature of the tetragonal solid solutions is a high fracture toughness combined with a high mechanical strength. The high fracture toughness of these materials originates from the so-called transformation hardening [
The metastability of the tetragonal phase which largely determines its transformability depends on the concentration and type of the stabilizing oxide used. Stabilizing oxides can be, e.g., yttria and oxides of alkaline-earth and rare-earth elements. The most studied ZrO2 solid solutions containing 3 mol.% Y2O3 exhibit high bending strengths (800–1200 MPa) and microhardness (11–13 GPa), but only moderate fracture toughness (~6 MPa∙m1/2) [
The use of directional melt crystallization allows growing crystals without grain structures and grain boundaries thus eliminating the effect of microstructure-related factors on the properties of the materials. Moreover, studies of single crystals reveal the anisotropy of their mechanical properties which is a difficult task for isotropic ceramic specimens. Studies of the anisotropy of mechanical properties allow fabricating single crystal products with preset crystallographic orientations for which the fracture toughness and/or strength is the highest. ZrO2 based single crystal solid solutions stabilized with various oxides were obtained using this method earlier [
This work reports a comparative study of the phase composition and mechanical properties of partially stabilized zirconia synthesized using directional melt crystallization. Gd2O3 and Sm2O3 were chosen as rare-earth element oxides since the ionic radii of Gd3+ and Sm3+ are greater than that of Y3+. The ionic radii of Y, Gd and Sm oxides change in the following sequence: RY3+ = 0.1019 nm < RGd3+ = 0.1053 nm < RSm3+ = 0.1074 nm. This work is a continuation of our earlier study [
The crystals were grown using directional melt crystallization in a water-cooled 100 mm diam. crucible by direct induction heating. This growth method was described in detail elsewhere [
The density of the specimens was measured by hydrostatic weighing on a Sartorius hydrostatic weighing device; the measurement error being 0.1%.
The phase composition of the crystals was studied using X-ray diffraction with a Bruker D8 instrument in CuΚα radiation. The diffractometer operation mode was 40 кV @ 40 mA. The study was conducted using the conventional method for single crystals. The as-grown crystals had no predominant crystallographic orientations. Therefore, each crystal was preliminarily oriented along specific crystallographic directions in the diffractometer. Wafers for the studies were cut from the middle parts of the crystals. The phase composition of the crystals was studied for wafers cut from the crystals perpendicular to the <100> direction. The 2θ/ω-mode scanning range was 20 to 140 arc deg with 0.02 arc deg steps. The {100} planes of the multiphase composition crystals exhibited several simultaneous reflections from a single cut that were produced by different phases, these reflections being split at high 2θ (~130 arc deg). The phase fractions were determined from the diffraction peak intensities normalized to the integral reflection coefficients of the phases.
Local phase analysis in the vicinity of indentations was carried out using Raman spectroscopy in the 50–1400 cm-1 wavenumber range under a Renishaw inVia Raman confocal microscope. A 532 nm laser was used as an excitation source. The laser focus point was chosen using a built-in optical microscope (×20). The focused beam diameter on the sample was ~1 μm. For Raman spectra recording, the laser radiation power was set to 100 mW, the signal accumulation time being 5 seconds. The rate of the tetragonal-to-monoclinic phase transition (Rm) was calculated from the Raman band intensity ratio for the monoclinic and tetragonal phases using the following formula [
(1)
For mechanical studies, 5 mm thick plates were cut from the crystals and then grinded and polished. The specimens were grinded with α-SiC (М10) suspension. АСМ3/2 or АСМ2/1 diamond pastes were used for polishing. Chemomechanical polishing was used at the final stage for damaged surface layer removal. The polishing agents were compositions of nanometer-sized amorphous silica particles. The chemomechanical polishing time was 30–60 min. The roughness of the as-polished surfaces was 0.3–0.5, and the surfaces contained no microscratches and were leveled and smooth.
The microhardness and fracture toughness of the crystals were measured via indentation on the {001} plane at different specimen rotation angles in their planes. Anisotropy was measured in the 0–90 arc deg specimen rotation angular range with 22.5 arc deg steps, the 0 arc deg position corresponding to the <100> direction. The instruments used were a DM 8ВAUTO microhardness tester with a Vickers indenter (maximum load 20 N) and a Wolpert Hardness Tester 930 with a minimum load of 50 N. The microhardness and fracture toughness were measured at 5 and 100 N loads, respectively, with 10 s dwell times. The indentation spacing was 400 mm. A total of 25 indentations were made for each crystallographic direction.
The fracture toughness (K1C) was calculated using the Niihara formula for the Palmqvist Crack system as reported earlier [
K 1C = 0.035(L⁄a)-1⁄2(CE⁄H)2⁄5 Ha1⁄2C-1, (2)
where K1C is the stress intensity coefficient (MPa∙m1/2); L is the radial crack length (m); a is the indentation half-width (m); C is the constraint factor (= 3); E is Young’s modulus (= 250 Pa); H is the microhardness (Pa).
The K1C parameter was calculated for radial cracks around indentations if the crack length met the criterion (0.25 ≤ l/a ≤ 2.5) for Palmqvist cracks.
The test materials were (ZrO2)1-x(R2O3)x crystals (where R = Y, Sm and Gd) for x = 0.02; 0.028; 0.032; 0.037 and 0.04, synthesized using directional melt crystallization under similar temperature and time conditions. The use of similar synthesis conditions justifies comparative analyses of crystal parameters for similar stabilizing oxideconcentrations. Hereinafter, the crystals will be denoted as xYSZ, xGdSZ and xSmSZ where x is the concentration of Y2O3, Gd2O3 and Sm2O3 stabilizing oxides in mol.%, respectively. Figure
Appearance of crystals. Inserts: optical images in transmitted polarized light of the samples microstructure
The densities of the crystals partially stabilized with Y2O3, Gd2O3 and Sm2O3 were measured. The densities of the tetragonal crystals were close to their theoretical values. This indicates the absence of defects in the form of pores and microcracks. Figure
With an increase in the concentration of stabilizing oxide from 2.0 to 2.8 mol.% for YSZ and GdSZ and from 2.0 to 3.7 mol.% for SmSZ the experimentally measured densities of the crystals increase, this being mainly accounted for by a decrease in the content of the monoclinic phase the density of which is lower than that of the tetragonal phase. With further increases in the stabilizing oxide concentration the densities of the GdSZ and SmSZ crystals increase whereas that of the YSZ crystal decreases. This is caused by the fact that, unlike the Y atoms which are lighter than the Zr ones, the Sm and Gd atoms are heavier than the Zr ones. The density of the tetragonal crystals (at stabilizing oxide concentrations of ≥ 3.7 mol.%) for comparable concentrations increases in the sequence Y → Sm → Gd, in agreement with the atomic weights of the respective elements.
A disruption in the monotonic pattern of the crystal density vs concentration function can serve as an indicator of changes in the phase composition of the crystals and hence can be used for finding a lower concentration limit of tetragonal structure stabilization in the crystals.
Table
Phase composition, phase weight fractions and tetragonality degrees of (ZrO2)1-x (R2O3)х solid solutions (where R = Y, Sm and Gd) for x = 0.02; 0.028; 0.032; 0.037; 0.04
x (mol.%) | YSZ | GdSZ | SmSZ | ||||||
Phase | wt.% | c/√2a | Phase | wt.% | c/√2a | Phase | wt.% | c/√2a | |
2.0 | m | 75(5) | 1.0164 | m | 85(5) | 1.0170 | m | 100(5) | – |
t | 25(5) | t | 15(5) | ||||||
2.8 | t | 87(5) | 1.0152 | t | 90(5) | 1.0162 | t | 40(5) | 1.0720 |
t' | 13(5) | 1.0054 | t` | 10(5) | 1.0053 | m | 60(5) | ||
3.2 | t | 78(5) | 1.0147 | t | 84(5) | 1.0159 | t | 65(5) | 1.0710 |
t' | 22(5) | 1.0052 | t` | 16(5) | 1.0047 | t' | 10(5) | 1.0036 | |
m | 25(5) | ||||||||
3.7 | t | 70(5) | 1.0145 | t | 77(5) | 1.0154 | t | 85(5) | 1.0167 |
t' | 30(5) | 1.0050 | t` | 23(5) | 1.0040 | t' | 15(5) | 1.0035 | |
4.0 | t | 62(5) | 1.0143 | t | 72(5) | 1.0151 | t | 76(5) | 1.0165 |
t' | 38(5) | 1.0049 | t` | 28(5) | 1.0037 | t' | 24(5) | 1.0034 |
At the lowest stabilizing oxide concentration which is 2.0 mol.% all the test crystals contained the monoclinic phase. The concentration limits at which the monoclinic phase was not observed were 2.8, 2.8 and 3.7 mol.% for Y2O3, Gd2O3 and Sm2O3 stabilizing oxides, respectively.
At stabilizing oxide concentrations of ≥ 2.8 mol.% for Y2O3 and Gd2O3 and ≥ 3.7 mol.% for Sm2O3 the crystals contain two tetragonal phases having different tetragonality degrees. Thus, for Sm2O3 stabilizing oxide the tetragonal phaseis stabilized in the whole crystal bulk at a higher stabilizing oxide concentration.
An increase in the stabilizing oxide concentration in the tetragonal crystals causes a decline in the quantity of the transformable tetragonal phase (t) and an increase in the quantity of the non-transformable tetragonal phase (t'), the tetragonality degrees (c/√2a) of these phases decreasing with an increase in the stabilizing oxide concentration.
To discuss the phase composition data for the crystals we analyze the ZrO2–Y2O3 binary system phase diagram fragment for the 2–4 mol.% Y2O3 concentration range [
Table
Concentration (mol.%) | Y2O3 | Gd2O3 | Sm2O3 |
HV (GPa) | |||
2.0 | 10.4 ± 0.4 | 9.0 ± 0.4 | 8.6 ± 0.4 |
2.8 | 12.9 ± 0.4 | 12.5 ± 0.4 | 8.8 ± 0.4 |
3.2 | 13.0 ± 0.4 | 12.6 ± 0.4 | 10.8 ± 0.4 |
3.7 | 13.6 ± 0.4 | 12.8 ± 0.4 | 11.3 ± 0.4 |
4.0 | 13.9 ± 0.4 | 13.4 ± 0.4 | 12.2 ± 0.4 |
For all the test compositions, an increase in the stabilizing oxide concentration leads to an increase in the microhardness. The observed microhardness behavior regularities depending on stabilizing oxide ionic radius agree with earlier data [
Recommended fracture toughness test methods for ceramic and brittle materials are single edge v-notched beam (SEVNB), chevron-notched beam (CNB), single-edge pre-cracked beam (SEPB) and surface crack in flexure (SCF) [
Figure
The fracture toughness is the highest among the Y2O3, Gd2O3 and Sm2O3 stabilized crystals for the 2.8YSZ, 2.8GdSZ and 3.7SmSZ solid solutions, respectively. The crystals containing 3.7 mol.% Sm2O3 have the highest K1C (14.3 MPa·m1/2) among all the test crystals. The fracture toughness values for the 2.8YSZ and 2.8GdSZ crystals were 11.0 and 13.0 MPa·m1/2, respectively. All these compositions exhibited clear K1C anisotropy, with K1C being the lowest for the <110> indenter diagonal orientation. K1C of the YSZ and GdSZ crystals decreased with an increase in the stabilizing oxide concentration, and their anisotropy became less expressed. Of the Sm2O3 stabilized crystals studied, those containing 2.8 and 3.2 mol.% Sm2O3, in which the monoclinic phase was found, had the lowest K1C.
Fracture toughness measured in the {100} plane for different angles of indenter diagonal relative to the <100> direction in specimen plane for YSZ, GdSZ and SmSZ crystals
Note that the highest K1C regardless of stabilizing oxide type were observed at the lowest concentrations that are required for tetragonal phase stabilization and complete monoclinic phase suppression. These boundary concentrations are controlled not only by the type of the stabilizing oxide but also by the methods and conditions of solid solution synthesis. It should also be noted that the increase of K1C in the sequence 2.8YSZ → 2.8GdSZ → 3.7SmSZ can be attributed to an increase in the transformability of the material due to an increase in the tetragonality degree of the transformable phase (c/√2a = 1.0152, 1.0162 and 1.0167 for 2.8YSZ, 2.8GdSZ and 3.7SmSZ, respectively). An increase in the fracture toughness with an increase in the rare-earth cation radius was also observed for 3.5 mol.% RE2O3 (RE = Dy, Y, Er, Yb) stabilized ZrO2 [
The contribution of the transformation hardening mechanism to the increase in the fracture toughness was theoretically estimated on the basis of micromechanical models in accordance with the following equation [
(3)
where f is the volume fraction of the tetragonal phase that is transformable in the transformation zone, E is the elastic modulus of the material, ε is the volume deformation involved in the transformation, h is the width of the transformation zone and ν is Poisson’s ratio.
Equation (3) suggests that an increase in the content of the transformable phase and enlargement of the transformation zone should increase the fracture toughness of the material.
One can hypothesize that the width of the transformation zone is proportional to the width of the monoclinic phase zone around the indentation. Figure
Tetragonal-to-monoclinic phase transition rate for 4.0YSZ, 4.0GdSZ and 4.0SmSZ crystals in local areas near indentations. Inset: indentation images with Raman spectra recording points marked
As follows from Fig.
Thus, analysis of the experimental data presented above suggests that the ionic radii of stabilizing oxides affect the mechanical parameters of the crystals in an indirect manner, more specifically, via the specific features of phase formation and changes of phase ratios in the test solid solutions.
The phase composition, density, microhardness and fracture toughness of (ZrO2)1-x(R2O3)х (R = Y, Gd, Sm) solid solution crystals for x = 0.02–0.04 were compared. The highest fracture toughness figures were 11.0, 13.0 and 14.3 MPa·m1/2 for the 2.8YSZ, 2.8GdSZ and 3.7SmSZ crystals, respectively. All the high-K1C crystals contained two tetragonal phases differing in the chemical compositions. The fracture toughness of the tetragonal crystals increased with the trivalent cation ionic radius due to an increase in the transformability of the metastable t phase. The crystals having fracture toughness values of above ~10.0 MPa·m1/2 exhibited clear anisotropy. K1C for the <100> direction were ~20% higher than those for the <110> direction.
Analysis of the results obtained suggests that the ionic radii of stabilizing oxide cations affect the mechanical parameters of the crystals in an indirect manner, more specifically, via the specific features of phase formation and changes in the phase ratios of the test solid solutions.
This research was funded by the Russian Science Foundation, Grant No. 22-29-01220.