Microstructural evolution and mechanical properties of nano-ATZ ceramics by solid solution precipitation

Alumina toughened zirconia (ATZ) nanoceramics with high-strength, high-toughness and high-hardness were prepared by in-situ nanoprecipitation from solid solution micro-powders. The submicron Al 2 O 3 (~450 nm) and ZrO 2 (~350 nm) grains contained low-density precipitated nano-ZrO 2 (~40 nm) and nano-Y 4 Al 2 O 9 (YAM, ~90 nm) particles, respectively, making high-performance nano-ATZ ceramics with ultrafine intracrystalline nanostructure yet achieved. There was a parallel or eutectic lattice orientation relationship between the submicrocrystals and its internal nanoparticles of their crystal planes, which is very conducive to the improvement of the mechanical properties of nano-ATZ ceramics. The fracture toughness and hardness of 30wt%Al 2 O 3 /70wt%ZrO 2 (3mol%Y 2 O 3 ) can be as high as 5.68 ± 0.17 MPa·m 1/2 (single-edge V-notched beam method, SEVNB) and 16.32 ± 0.45 GPa, respectively, which are improved by ~25 % and ~20 % compared with those of 3Y-TZP ceramics. Therefore, this method can be used to prepare nano-ATZ ceramics contained ultrafine nanoparticles and uniform distribution of Al 2 O 3 phases.

Moreover, these nano-ATZ ceramics are mainly obtained by nanopowders, which have many problems such as easy agglomeration of powders, easy abnormal growth of grains during sintering and uneven distribution of Al2O3 phase. Hence, it is necessary to find a novel way to prepare nano-ATZ ceramics with high-strength, high-toughness and high-hardness.
In this paper, we prepared nano-ATZ ceramics containing 30 wt% Al2O3 by a new method of solid solution precipitation from micro-powders. This kind of powder with high-energy state and high supersaturation was obtained by combustion synthesis assisted spray atomization at 10~50 MPa and rapid water cooling [20][21]. In this method, the melt holding time is long (several minutes), so that the phases are fully fused to each other, and because of the ultra-high cooling rate, the powder has a very high degree of supersaturation [21], making it no longer dependent on the phase diagram [22]. The nano-ATZ ceramics sintered by this method had higher mechanical properties than those sintered by nanopowders, due to its ultrafine nanostructure and uniform distribution of Al2O3 phase by solid solution precipitation. Moreover, there are a few nanoparticles in Al2O3 and ZrO2 grains, which exerts the effect of intracrystalline nano-toughening. This kind of microstructure is difficult to be realized by the previous preparation methods of nano-ATZ ceramics [6,[11][12][13][14][15][16][17][18][19]. More importantly, ZrO2 contained unprecipitated Al2O3, which may be very beneficial to improve the aging resistance of ZrO2 ceramics. Therefore, this method has far-reaching guiding significance for nano-ATZ ceramics.

Raw materials and preparation of solid solution powders
The supersaturated 30 wt% Al2O3 + 70 wt% ZrO2 (3 mol% Y2O3) solid solution powders (named as AZSSP, as shown in Fig. S1) with an average particle size of about 3 μm were obtained via combustion synthesis (Equation 1) assisted rapid water cooling (Fig. S2). The specific preparation method was detailed in our previous work [20][21].
This non-equilibrium solidification greatly improves the solid solubility of the two phases, making it no longer dependent on the phase diagram of Al2O3/ZrO2 [22] (Fig.   S3). The adiabatic temperature and the spray pressure of powder preparation were set to 4000 K and 20 MPa, respectively. The AZSSP was sieved through 200 mesh to obtain coarse and fine powders.

Characterization of AZSSP and ATZ
where, f is the flexural strength, F and P are the bending load, a is the bending arm length, w is the width of the sample, h is the thickness of the sample, KIc is the fracture toughness, L is the span, l is the notched depth and Y(l/h) is the form factor (as shown in Equation 4).
The Archimedes drainage method was used to test the relative density of ATZ.

The analysis of AZSSP
Though the X-ray diffraction (XRD) analysis ( Fig. 1a), most of the Al2O3 phase in the AZSSP was dissolved into ZrO2. Therefore, the lattice constant of 3Y-ZrO2 became smaller because the radius of Al 3+ is smaller than that of Zr 4+ . The a = b = 3.6008 Å and c = 5.1793 Å (PDF #83-0113) of the fine powder became a = b = 3.5989 Å and c = 5.1475 Å (see Table 1 for details), respectively, and these values of the coarse powder became a = b = 3.5987 Å and c = 5.1519 Å. The fine powder (6.05 wt%) contained more Y4Al2O9 (YAM) phase than the coarse powder (1.80 wt%), and the former (18.86 wt%) had more m-ZrO2 phase than the latter (4.54 wt%) in Table 2 (semiquantitative calculation from XRD), because Y2O3 in the former mainly formed YAM instead of stabilizing the ZrO2 phase. The cooling rate of coarse powder is lower than that of fine powder, so α-Al2O3 phase appeared in the former, while only γ-Al2O3 existed in the latter (Fig. 1a). The fine powder contained a small amount of precipitated nanospheres, while the coarse powder contained more spheres and a lot of pores, as shown in Fig. 1. This is because large particles of Al2O3/ZrO2 melt are ejected from the nozzle, leaving a small amount of gas remaining in the melt to form pores. In addition, the rapidly cooling water enters the melt and vaporizes into water vapor, which also forms holes. These nanospheres are composed of ZrO2-rich solid solution, from the BSE image of Fig. 1d, and they are surrounded by Al2O3-rich solid solution.

The microstructure and mechanical properties of nano-ATZ
ATZ-1400, ATZ-1450 and ATZ-1500 were successfully obtained by AZSSP after HP. From the BSE images of their surfaces (Fig. 2), the Al2O3 phase was evenly distributed among the ZrO2 grains. As the temperature increased, the grain size of Al2O3 gradually became larger. ATZ-1500 had abnormally grown Al2O3 grains with a size of 1-2 μm. The average grain sizes of ZrO2 and Al2O3 are 350 nm and 450 nm, respectively, from the TEM and STEM images of ATZ-1450 (Fig. 3, Fig. 4 and Fig. S5). The Al2O3 crystal contains a small amount of nano-ZrO2 particles with diameter of ~40 nm, and the ZrO2 crystal also contains nano-YAM particles (see Fig. 7b for detailed analysis) with diameter of ~90 nm. This ultrafine structure will be very beneficial to improve the properties of nano-ATZ ceramics.
The STEM-EDX mapping and line scan analysis of ATZ-1450 in Fig. 5 and Fig.   6 show that a small amount of Al2O3 was not precipitated from ZrO2. The unprecipitated Al2O3 acts as a stabilizer and inhibits the t-m phase transformation of ZrO2, which may be very beneficial to the hydrothermal aging resistance of Y-TZP. The precipitated Al2O3 and the γ-Al2O3 in the AZSSP finally exist in the ceramic as the α-phase at hightemperature [21]. This was also confirmed by the selected area electron diffraction (SAED) of Al2O3 in area A, as shown in Fig. 5a, 5b. The ZrO2 existed in the form of m-and t-phase from the SAED of ZrO2 grains in area B and C (Fig. 5c, 5d). MPa and 16.32 ± 0.45 GPa, respectively, as shown in Table 3. The flexural strength is similar to that of 3Y-TZP and ATZ ceramics [2,12] but the hardness is increased by about 20 %. In order to accurately measure the fracture toughness of nano-ATZ ceramics, IM, SENB and SEVNB are used respectively. The U-notched root radius of the SENB specimen is about 100 μm (Fig. 8a), and the fracture toughness is greatly overestimated [25]. Many studies [25][26][27] have shown that using a femtosecond laser to cut an ultra-sharp V-notch at the root of the U-notch (the SEVNB method) can be used to characterize the toughness of ceramics. When the V-notched tip radius is less than 1.5 to 3 times of the grain size [27][28][29], the fracture toughness of the ceramic can be accurately measured. The V-notched tip radius of nano-ATZ ceramics is ~0.6 μm ( Fig. 8b, 8c), which is much smaller than three times of their grain size, so the SEVNB method can accurately measure their fracture toughness. In addition, many studies [30][31] have shown that when the U-notched root radius is less than the depth of the Vnotch, and the equivalent notch angle is less than 60 ° (as shown in Fig. 9a), the SEVNB method can truly measure the fracture toughness accurately. The U-notched root radius  Table 4. The test results of the ATZ-1400 and ATZ-1500 were also the same as that of ATZ-1450. Compared with SEVNB, the fracture toughness of IM and SENB were overestimated by ~26.99 % and ~138.40 %, respectively, as shown in Table 5. The difference between SENB and SEVNB methods can be seen from their typical stress-displacement curves (Fig. 10a). For the SEVNB specimen (Fig. 10b), the V-notched tip provides the source of cracks. When the external loading reaches F0, the crack tip expands steadily and the stress begins to release at the crack tip, so the stressdisplacement curve shows a smooth nonlinear change. Because ZrO2 ceramics contains t-m phase transition, it has strong resistance to crack extension, so crack propagation can be seen in the stress-displacement curve of SEVNB. The crack expands steadily up to the maximum load (Fmax) and then grows instability, and eventually the specimen fractures at the load of F * . For SENB specimens (Fig. 10c), the bottom of the smooth U-notch cannot provide the source of cracks. When the external load increases to a certain value, crack induction, crack propagation and fracture occur simultaneously.
Because the load required to induce a microcrack is much greater than that for crack propagation and specimen fracture, the fracture toughness measured by the SENB method is overestimated. The SENB sample has no crack propagation, leading to direct fracture of the sample, so the stress-displacement curve has no deflection (Fig. 10c).
Therefore, the SEVNB method can accurately measure the ceramic toughness of resistance to crack propagation.
The fracture surface morphology of nano-ATZ ceramics, as shown in Fig. 11, can further show the reasons for the high strength and high toughness of ceramics prepared by this method. The fracture surfaces of the ATZ-1400, ATZ-1450 and ATZ-1500 specimens are uneven (Fig. 11a, 11b, 11c), and there are a lot of crack deflection and crack bifurcation (Fig. 11d, 11e, 11f), which greatly increases the crack growth path and improves the strength and toughness of ceramics. The high-magnification BSE and SEM images (Fig. 12) of the fracture surface for ATZ-1400, ATZ-1450 and ATZ-1500 show that nano-ATZ ceramics mainly occur intergranular fracture, and Al2O3 phases are evenly distributed in the ZrO2 matrix. The ATZ-1400 fracture surface (Fig. 12a,   12b) contains a lot of microcrack toughening, especially the evenly distributed Al2O3 phase deflects the cracks (pointed by red arrows in Fig. 12b), which is very beneficial to improve the toughness of ceramics. In addition, some ZrO2 phases undergo transgranular fracture (pointed by green arrows in Fig. 12b), which makes the t-m phase transition as high as 9.72 % (SENB, as shown in Fig. 13 and Table 4) and 6.04 % (SEVNB), so its fracture toughness is as high as 5.53 ± 0.15 MPa·m 1/2 (SEVNB), which is higher than that of 3Y-TZP (4.4 MPa·m 1/2 , SEVNB) and increased by about 25 % [27]. The fracture morphology of ATZ-1450 is similar to that of ATZ-1400, and there are also microcrack propagation and crack deflection toughening (pointed by red arrows in Fig. 12d and Fig. 14). The difference is that there are some particles pulled out (pointed by blue arrows in Fig. 12d), which greatly increases the toughness of ceramics.
The cross-sectional STEM image (Fig. 14) of microcrack growth in ATZ-1450 provides a more visual representation of the toughening mechanism. The uniformly distributed Al2O3 phase causes intergranular or transgranular fracture to promote crack deflection, as shown in red and green arrows, respectively. Similarly, some ZrO2 phases undergo transgranular fracture (pointed by blue arrows in Fig. 14), which makes the t-m phase transition as high as 10.58 % (SENB, as shown in Fig. 13 and Table 4) and 4.47 % (SEVNB). At the same time, the YAM in the ZrO2 crystal makes the crack deflection again, as shown in the dashed area in Fig. 14. Furthermore, YAM and ZrO2 have a unique orientation relationship (Fig. 7a), so transgranular fracture of ZrO2 consumes a lot of energy, thereby greatly improving the fracture toughness of ceramics. ATZ-1500 also mainly occurs intergranular fracture (Fig. 12e, 12f), and the grain size is significantly larger than that of ATZ-1400 and ATZ-1450. Like ATZ-1450, ATZ-1500 also has particle pull-out toughening, as shown in blue arrows in Fig. 12f. In addition, the large-size grains of ATZ-1500 contain a lot of small particles on the grain boundary ( Fig. 12f), which greatly increases its crack propagation path, thereby improving the toughness of ceramics. The t-m phase transition of ATZ-1500 is as high as 13.45 % (SENB) and 3.15 % (SEVNB), as shown in Fig. 13 and Table 4. Therefore, nano-ATZ ceramics prepared by solid solution nanoprecipitation have ultra-fine microstructure and high mechanical properties.

Conclusion
In summary, nano-ATZ nanoceramics were successfully prepared via the solid

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.  Fig. (c). The light phase represents the ZrO2 phase, and the dark phase represents the Al2O3 phase.  Fig. (b) represent the ZrO2 phase and the Al2O3 phase, respectively. Fig. 4 The size distribution of nano-ZrO2 grains in the Al2O3 grains (a), nano-Y4Al2O9 grains in the ZrO2 grains (b), submicro-ZrO2 grains (c) and submicro-Al2O3 grains (d) in Fig. 3(b). The average grain size of these four grains is about 40 nm, 90 nm, 350 nm and 450 nm, respectively.   Fig. (a) and Fig. (b), respectively.
Insets in I and II are the corresponding FFT patterns.    Fig. (a).  Fig. (c). The light phase represents the ZrO 2 phase, and the dark phase represents the Al 2 O 3 phase.    Fig. 3(b). The average grain size of these four grains is about 40 nm, 90 nm, 350 nm and 450 nm, respectively.   The SEM image of SEVNB sample showing that a femtosecond laser is used to cut an ultra-sharp Vnotch based on the U-notch. (c) An enlarged view of the V-notch with the tip radius of ~0.6 μm (much less than three times the grain size of ceramics [27][28][29]) indicating that this method can accurately measure the fracture toughness of ceramics.   Fig. (a).      Table 1 The lattice parameters and the unit cell volume of t-ZrO2 measured by XRD. Table 2 The phase content (Semi-quantitative calculation), crystallinity and average grain size obtained from the XRD pattern. Table 3 Comparison of flexural strength (), Vickers hardness (Hv) and relative density of ATZ.      The cross-sectional BSE pictures of ATZ-1400, ATZ-1450 and ATZ-1500. The bright phase and the dark phase represent the ZrO2 phase and the Al2O3 phase, respectively.  The size distribution of nano-ZrO2 grains in the Al2O3 grains (a), nano-Y4Al2O9 grains in the ZrO2 grains (b), submicro-ZrO2 grains (c) and submicro-Al2O3 grains (d) in Fig. 3(b). The average grain size of these four grains is about 40 nm, 90 nm, 350 nm and 450 nm, respectively.   HRTEM images of nanoparticles inside ZrO2 crystal (a) and Al2O3 crystal (b). I, II and III are enlarged images of different regions in Fig. (a) and Fig. (b), respectively. Insets in I and II are the corresponding FFT patterns.  SEM image of SEVNB sample showing that a femtosecond laser is used to cut an ultra-sharp V-notch based on the U-notch. (c) An enlarged view of the V-notch with the tip radius of ~0.6 μm (much less than three times the grain size of ceramics [27][28][29]) indicating that this method can accurately measure the fracture toughness of ceramics.

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