Research Article |
Corresponding author: Nataliya Yu. Tabachkova ( ntabachkova@gmail.com ) © 2023 Mikhail A. Borik, Aleksej V. Kulebyakin, Elena E. Lomonova, Filipp O. Milovich, Valentina A. Myzina, Polina A. Ryabochkina, Natalya V. Sidorova, Nataliya Yu. Tabachkova, Artem S. Chislov.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Borik MA, Kulebyakin AV, Lomonova EE, Milovich FO, Myzina VA, Ryabochkina PA, Sidorova NV, Tabachkova NYu, Chislov AS (2023) Effect of heat treatment on the structure and mechanical properties of zirconia crystals partially stabilized with samarium oxide. Modern Electronic Materials 9(3): 123-131. https://doi.org/10.3897/j.moem.9.3.115614
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The effect of high-temperature treatment in different media on the phase composition, microhardness and fracture toughness of (ZrO2)1-х(Sm2O3)х crystals with x = 0.02÷0.06 has been studied. The crystals have been grown using direction melt crystallization in a cold skull. The crystals have been heat treated at 1600 °C for 2 h in air and in vacuum. The phase composition of the crystals has been studied using X-ray diffraction and Raman scattering. We show that samarium cations enter the ZrO2 lattice mainly in a trivalent charge state and do not change their charge after air or vacuum annealing. The as-annealed phase composition has changed in all the test crystals except for the (ZrO2)0.94(Sm2O3)0.06 composition. After air or vacuum annealing the (ZrO2)1-x(Sm2O3)x crystals with 0.002 ≤ x ≤ 0.05 contain a monoclinic phase. The (ZrO2)0.94(Sm2O3)0.06 crystals contain two tetragonal phases (t and t´) with different tetragonality degrees. After air or vacuum annealing of the (ZrO2)0.94(Sm2O3)0.06 crystals the lattice parameters of the t and t´ phases change in opposite manners, suggesting that the tetragonality degree of the t phase increases whereas the tetragonality degree of the t´ phase decreases. The microhardness and fracture toughness of the as-annealed crystals depend on the Sm2O3 concentration in the solid solutions. The formation of the monoclinic phase in the (ZrO2)1-х(Sm2O3)х crystals with 0.037 ≤ x ≤ 0.05 significantly reduces the microhardness and fracture toughness of the crystals. Annealing of the (ZrO2)0.94(Sm2O3)0.06 crystals triggers more efficient hardening mechanisms and thus increases the fracture toughness of the crystals. We show that air or vacuum annealing of the (ZrO2)0.94(Sm2O3)0.06 crystals increases the fracture toughness of the crystals by 1.5 times as compared with that of the as-grown crystals.
zirconia, ZrO2–Sm2O3, crystal growth, microhardness, fracture toughness, optical spectroscopy, Raman scattering
Zirconia based materials are distinguished by good mechanical and heat insulating properties combined with high chemical inertness that provide for their wide application in high-temperature engineering [
A large number of works have dealt with the dependence of the chemical, composition, structure and mechanical and thermophysical properties on the composition and synthesis and heat treatment conditions of the ZrO2–Y2O3 solid solutions [
An increase in the cation radius of stabilizing oxides leads to an increase in the temperature of the phase transition from the high-temperature cubic phase to the two-phase region, in accordance with the ZrO2 – stabilizing oxide phase diagram. From this viewpoint the synthesis of crystals with a predominant tetragonal phase that does not undergo a transition to the monoclinic phase can be implemented via the use of stabilizing oxides with greater ionic radii of cations than Y3+. The ionic radius of trivalent samarium cations is greater than those of gadolinium and yttrium. It has been shown [
The aim of this work is to study the effect of high-temperature annealing in air and in vacuum on the phase composition and mechanical properties of ZrO2 based solid solutions partially stabilized with Sm2O3.
(ZrO2)1-х(Sm 2O3)х solid solution crystals with x = 0.02÷0.06 were grown using directional melt crystallization in a 100 mm diam. water-cooled crucible with direct induction heating. The power source was a 63 kW high-frequency generator at 5.28 MHz. The raw powders (main material content at least 99.99 %) were preliminarily mechanically mixed before loading into the crucible. The charge weight was 4.5–5 kg. Melting was initiated using metallic zirconium. The melt was crystallized by removing the crucible from the heated zone at a 10 mmph speed. The cross-sections and lengths of the as-grown crystals were 5 to 20 mm and 30 to 40 mm, respectively.
The crystals were heat treated at 1600 °C in air and in vacuum. The density of the specimens was measured by hydrostatic weighing. The density measurement error was 0.1 %.
The phase composition of the crystals was studied using X-ray diffraction and Raman spectroscopy on a Bruker D8 diffractometer and a Renishaw in Via microscope and spectrometer, respectively. Wafers for the studies were cut from the middle parts of the crystals. Crystals grown using directional melt crystallization in a cold skull have no preferential growth directions. Therefore test crystal orientations were verified on the X-ray diffractometer and then wafers were cut perpendicular to the <100> direction.
The microhardness and cracking resistance of the crystals were compared via indentation on the {001} plane at different specimen rotation angles in their planes. The instruments used were a DM 8 В AUTO microhardness tester with a Vickers indenter (maximum load 20 N) and a Wolpert Hardness Tester 930 with a minimum load of 50 N. After cutting the specimens were subjected to chemomechanical polishing for damaged surface layer removal. The polishing agents were compositions of nanometer-sized amorphous silica particles. The roughness of the as-polished surfaces was 0.3–0.5 nm, and the surfaces contained no microscratches and were leveled and smooth.
The cracking resistance (K1с) was calculated using the formula [
K 1c = 0,035(L/a)-1/2(CE/H)2/5Ha1/2C-1, (1)
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 (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).
The high-temperature stability of the structure and mechanical properties of the (ZrO2)1-х(Sm2O3)х solid solution crystals with x = 0.02, 0.028, 0.032, 0.037, 0.04, 0.05 и 0.06 was studied in different media. Hereinafter te crystal compositions will be denoted as xSmSZ where x is the Sm2O3 stabilizing oxide concentration in mol.%. Figure
After air annealing the color of the crystal changed, whereas after vacuum annealing the crystals became black. The dark color of the crystals was caused by nonstoichiometric vacancies whose formation during crystal vacuum annealing leads to the formation of an absorption band in the visible region. Air and vacuum annealing of the (ZrO2)1-х(Sm2O3)х crystals of all the experimental compositions except 6SmSZ led to noticeable changes in their surface appearance.
Changes in the crystal surface morphology were examined under an optical microscope. Figure
The surface of the as-grown 3.7SmSZ crystals was smooth and uniform. Air and vacuum annealing of the 3.7SmSZ crystals led to the formation of a surface texture. The surface texture of the crystals after vacuum annealing is more pronounced and contains larger structural features. Similar surface morphology changes were observed for the 4SmSZ and 5SmSZ crystals. The surface of the (ZrO2)1-х(Sm2O3)х crystals with x < 0.037 appeared non-uniform before annealing. Annealing of those crystals increased their surface roughness and the number of microcracks. Only the as-annealed 6SmSZ crystals exhibited no surface morphology changes. The surface of the 6SmSZ crystals appeared smooth and uniform both before and after annealing.
Study of the phase composition of the (ZrO2)1-х(Sm2O3)х crystals showed that air and vacuum annealing led to the formation of the monoclinic phase in the 3.7SmSZ, 4SmSZ and 5SmSZ crystals and increased the content of the monoclinic phase in the 3.2SmSZ ones. Figure shows by way of example a diffraction pattern for the 3.7SmSZ crystals after air annealing.
The tetragonal structure of the 6SmSZ crystals persisted after air and vacuum annealing. However, annealing led to changes in the lattice parameters and hence changes in the tetragonality degree of the phases. The ratio of the phases did not change any appreciably. Table
The data in Table
Vacuum annealing of the crystals reduced the lattice parameters of the crystals, potentially indicating the formation of nonstoichiometric vacancies. However, the tetragonality degree of the t phase increased after vacuum annealing whereas the tetragonality degree of the t’ phase decreased as compared with those of the as-grown crystals.
The observed changes in the tetragonality degree of the crystals after air and vacuum annealing suggest that the content of Sm2O3 in the t phase decreases, while that in the t’ phase increases, i.e., annealing of two-phase crystals drives the system to a more equilibrium state. After vacuum annealing the tetragonality degree of the crystals is lower than that after air annealing, probably indicating an additional stabilizing effect of the nonstoichiometric vacancies forming as a result of vacuum annealing.
The charge state of the samarium cations in the as-annealed crystals was checked using optical spectroscopy. Figure
The luminescence spectra before and after annealing contain bands in the green, yellow and red regions corresponding to the 4G5/2 → 6H5/2, 4G5/2 → 6H7/2 and 4G5/2 → 6H9/2 transitions in the Sm3+ ions. No bands typical of the Sm2+ ions were observed in the luminescence spectra. In oxide crystals such bands can be present in the 675–775 nm region. Thus one can conclude that samarium cations mainly enter the ZrO2 lattice in the trivalent charge state and do not change their charge state after air or vacuum annealing.
Comparison of the densities of the crystals before and after annealing showed that the densities of different crystals behave in different manners as a result of annealing, depending on the content of stabilizing Sm2O3 (Table
The microhardness of the as-annealed 2SmSZ, 2.8SmSZ and 6SmSZ changed but slightly as compared with the as-grown ones (Table
Figure
Annealing of the 3.7SmSZ, 4SmSZ and 5SmSZ crystals reduced their fracture toughness by more than twofold as compared with the as-grown figures, the fracture toughness after air and vacuum annealing being close. Unlike those crystals, annealing of the 6SmSZ solid solutions increased K1с by ~ 1.5 times.
The possibility of a tetragonal-to-monoclinic transition (t → m) largely affects the strength of the materials. The formation of the monoclinic phase in the crystals bulk after annealing tenders the transformation hardening mechanism in the 3.7SmSZ, 4SmSZ and 5SmSZ crystals impossible and therefore dramatically reduces their fracture toughness. The phase composition of the 6SmSZ did not change after annealing. However, 1600 °C annealing causes a redistribution of Sm2O3 in the t and t’ tetragonal phases. The decrease in the Sm2O3 content in the transformable t phase after annealing can increase the efficiency of the transformation hardening mechanism and hence increase the fracture toughness of the material. One should also bear in mind the possibility of the ferroelastic hardening mechanism in the as-annealed crystals of this composition. In order to analyze the contributions of the transformation and ferroelastic hardening mechanisms to the mechanical properties of the crystals, we recorded Raman scattering spectra for the indentation areas and studied the effect of local phase composition inside and around the indentations for finding the monoclinic phase areas forming as a result of the transformation hardening mechanism. The intensity of the tetragonal-to-monoclinic phase transition (Rm) was calculated from the monoclinic and tetragonal phase band intensity ratios in the Raman spectra using the following formula [
(2)
The contribution of the ferroelastic hardening mechanism was evaluated from the orientation-dependent tetragonal phase band intensity ratios in the Raman spectra: I (146 cm-1)/I (260 cm-1). The Raman spectra were taken in local areas along indentation diagonals and laterally to indentation sides with a ~10 mm step.
Figure
Figure
Evaluation of the contribution from the ferroelastic hardening mechanism from the tetragonal phase band intensity ratios in the Raman spectra showed that the orientation-dependent band intensity ratios also increase as a result of vacuum annealing.
Thus, analysis of the experimental data suggests that the fracture toughness of the as-annealed 6SmSZ crystals increases due to both the transformation and ferroelastic hardening mechanisms.
Surface images of (а–c) 3.7SmSZ and (d–f) 6SmSZ crystals (a, d) before and after (b, e) air and (c, f) vacuum annealing
Phase compositions, lattice parameters and tetragonality degrees of the 6SmSZ crystals before and after annealing
Specimen | Phase | Content (wt.%) | a (nm) | c (nm) | c/√2a |
As-grown | t | 60 ± 5 | 0.36073 | 0.51767 | 0.10147 |
t´ | 40 ± 5 | 0.36438 | 0.51672 | 0.10028 | |
Air-annealed | t | 64 ± 5 | 0.36070 | 0.51769 | 0.10149 |
t´ | 36 ± 5 | 0.36443 | 0.51670 | 0.10026 | |
Vacuum-annealed | t | 62 ± 5 | 0.36068 | 0.51764 | 0.10148 |
t´ | 38 ± 5 | 0.36436 | 0.51659 | 0.10025 |
Specimen | Density (g/cm3) | ||
As-grown | Air-annealed | Vacuum-annealed | |
2SmSZ | 5.890 ± 0.034 | 5.890 ± 0.034 | 5.863±0.004 |
2.8SmSZ | 5.951 ± 0.011 | 5.951 ± 0.015 | 5.917±0.013 |
3.2SmSZ | 6.010 ± 0.012 | 6.005 ± 0.012 | 5.997±0.008 |
3.7SmSZ | 6.181 ± 0.008 | 6.012 ± 0.021 | 5.995±0.011 |
4SmSZ | 6.197 ± 0.005 | 6.041 ± 0.021 | 6.031±0.008 |
5SmSZ | 6.206 ± 0.011 | 6.017 ± 0.010 | 6.093±0.019 |
6SmSZ | 6.264 ± 0.017 | 6.260±0.012 | 6.253±0.006 |
Microhardness of the (ZrO2)1-x (Sm2O3)x crystals before and after air and vacuum annealing
Specimen | HV (GPa) | ||
As-grown | Air-annealed | Vacuum-annealed | |
2SmSZ | 8.65 ± 0.30 | 8.55 ± 0.30 | 8.50 ± 0.30 |
2.8SmSZ | 8.75 ± 0.30 | 8.65 ± 0.30 | 8.60 ± 0.30 |
3.2SmSZ | 10.75 ±0.30 | 8.75 ± 0.30 | 8.65 ± 0.30 |
3.7SmSZ | 11.30 ± 0.30 | 9.25 ± 0.30 | 8.70 ± 0.30 |
4SmSZ | 12.15 ± 0.30 | 9.60 ± 0.30 | 8.75 ± 0.30 |
5SmSZ | 12.30 ± 0.30 | 10.50 ± 0.30 | 8.90 ± 0.30 |
6SmSZ | 12.45 ± 0.30 | 12.40 ± 0.30 | 12.50 ± 0.30 |
Luminescence spectra of 6SmSZ crystals (1) before and after (2) air and (3) vacuum annealing
Anisotropy of crack resistance in {100} plane for different indenter diagonal orientation in specimen plane for (a) 3.7SmSZ, (b) 4SmSZ, (c) 5SmSZ and (d) 6SmSZ crystals before and after annealing
Study of the phase composition of (ZrO2)1-х(Sm2O3)х crystals showed that air and vacuum annealing leads to the formation monoclinic phase in all the test crystals except the 6SmSZ composition for which the lattice parameter and hence the tetragonality ratio changed as a result of annealing.
After air and vacuum annealing the Sm2O3 content in the t phase decreases and that in the t’ phase increases, i.e., annealing of two-phase crystals drives the system to a more equilibrium state. After vacuum annealing the tetragonality degree of the crystals is lower than after air annealing, which can be accounted for by an additional stabilizing effect of nonstoichiometric vacancies forming as a result of vacuum annealing.
We showed that samarium cations mainly enter the ZrO2 lattice in the trivalent charge state and do not change their charge state after air or vacuum annealing.
The observed changes in the microhardness and fracture toughness of the crystals are accounted for by changes in the phase composition of the crystals as a result of annealing and depend on the Sm2O3 concentration in the solid solutions. The formation of the monoclinic phase in the (ZrO2)1-х(Sm2O3)х crystals with 0.037 ≤ x ≤ 0.05 significantly reduces the microhardness and fracture toughness of the crystals. Annealing of the (ZrO2)0.94(Sm2O3)0.06 crystals increases the efficiency of the hardening mechanisms and hence increases their fracture toughness. Ferroelastic hardening provides additional contribution to the increase in the fracture toughness of the crystals. We showed that air and vacuum annealing of the (ZrO2)0.94(Sm2O3)0.06 crystals increases the fracture toughness of the crystals by 1.5 times in comparison with that of the as-grown crystals.
The work was financially supported by Russian Science Foundation Grant No. 22-29-01220.