Study of the Relationship Between Microstructure, Phase Composition and Strength Characteristics in Composite Ceramics Based on ZrO₂-Al₂O₃ System

Abstract

ZrO₂-MgO-Al₂O₃ ceramics, despite a long history of research, still attract the attention of researchers due to the high potential of their applications as refractories and matrices for metal ceramics. A unique composition combining high strength and temperature stability is particularly in demand. In this paper, a comprehensive study of ceramics of the composition (90−x)·ZrO₂-10·MgO-x·Al₂O₃ (x = 10–80 wt.%) obtained by solid-phase sintering with preliminary annealing is carried out. Preliminary annealing was used for the possible formation of metastable phases with outstanding mechanical properties. Using the X-ray diffraction method, it was found that most of the samples consist of monoclinic zirconium oxide, magnesium–aluminum spinel, and corundum phases. The exception is the sample with x = 10 wt.%, in which the main phase was a cubic modification of zirconium oxide. By formation this type of ZrO₂ polymorph in the composition hardness and flexural strength significantly increased from 400 to 1380 and 50 to 210 MPa, respectively. The total porosity of ceramics under study lies in the range 6–28%. Using the scanning electron microscopy method, it was found that the phase composition significantly affects the morphology of the microstructure of the sintered bodies. Thus, for sintered ceramics with a high corundum content, the microstructure is characterized by high porosity and a large grain size. For the first time, by applying preliminary annealing, a new type of ternary ceramic ZrO₂-MgO-Al₂O₃ was sintered with potentially outstanding mechanical properties. The presence of a stabilized zirconium oxide phase, stresses in the crystal lattice of the matrix phase, and the formation of cracks in the microstructure are the main factors influencing shrinkage, porosity, microhardness, and biaxial flexural strength.

1. Introduction

Ceramic materials based on oxide systems are often used in structural applications involving exposure to high temperatures, aggressive chemical environments, and cyclic thermal loads [1,2,3,4]. The ZrO₂-MgO-Al₂O₃ composite oxide system is a promising material in which each phase plays a unique role in forming the necessary properties for use in functional and structural materials [5].

In particular, zirconium dioxide is often used due to its high melting point, chemical inertness, low thermal conductivity, and ability to be strengthened by phase transition of the tetragonal phase to the monoclinic phase [6]. The main disadvantage of ZrO₂-based ceramics, which limits their application, is the tendency to destruction during thermal cycling due to uncontrolled phase transitions, accompanied by volume expansion, accumulation of internal stresses, and formation of defects [7,8,9]. The addition of MgO promotes the stabilization of the tetragonal phase of ZrO₂ with the formation of partially stabilized zirconium dioxide with high thermal stability [10,11]. A number of studies have shown that sintering of ZrO₂-MgO composites allows for a reduction in the m-ZrO₂ content, improvement of the structure density, and an increase in phase stability [12,13,14].

When aluminum oxide is introduced into the composite system, the formation of spinel MgAl₂O₄ is observed, which significantly increases heat resistance, strength, and resistance to cracking [15]. In the ZrO₂-MgO-Al₂O₃ structure, this spinel can act as a barrier that dissipates stress and slows down the propagation of cracks during thermal cycling, and also improves the adhesion between MgO and ZrO₂ crystallites, thereby forming a strong and stable multicomponent matrix [16].

One of the mechanisms of strengthening of composites based on ZrO₂-MgO-Al₂O₃ is the formation of microcracks arising as a result of local stresses caused by the difference between the coefficients of thermal expansion between MgO and MgAl₂O₄ particles. In [17], it was shown that such stresses lead to the formation of microcracks, which increase the resistance to thermal shock. Additionally, Gu et al. [18] established that spinel nanograins are fixed at the boundaries of MgO grains and on their surface, realizing the pinning effect. This mechanism prevents grain growth, promotes crack deflection, and, in combination, improves the thermal shock resistance of the composite material.

Also, ZrO₂-MgO-Al₂O₃ ceramics have high potential as an alternative to traditional chromium-containing refractory materials. As noted in [19], the introduction of spinel and zirconium dioxide into MgO composites improves strength, Young’s modulus, and thermal shock resistance. An important factor is that such ceramics are significantly less toxic and more environmentally friendly than chromium compounds. A study by Yan et al. [20] showed that controlled cooling in the ZrO₂-MgO-Al₂O₃ system results in the formation of a continuous mesh microstructure in which the MgO, MgAl₂O₄, and ZrO₂ phases are uniformly dispersed and spatially intertwined. This configuration is similar to the structural characteristic of chromite-containing refractories and can contribute to an increase in the thermal stability of the material due to a more uniform distribution of phases and a reduction in the concentration of internal stresses.

Thus, the ternary system ZrO₂-MgO-Al₂O₃ combines the mechanisms of transformation strengthening, microcracking, crack deflection, and interphase pinning. These effects together provide the formation of materials with high thermal shock resistance, microstructural stability, and cracking resistance during operation. However, optimization of the phase composition, spinel synthesis, and sintering conditions remains an urgent task requiring further research.

In this paper, we investigated the influence of the component composition on the structure and properties of ceramics of the composition (90−x)·ZrO₂-10·MgO-x·Al₂O₃ (coefficients before oxides—initial weight concentration), obtained by solid-phase synthesis. This composition was chosen to study the mechanical characteristics of compositions similar to zirconia toughened alumina (ZTA) and alumina toughened zirconia (ATZ) with a fixed additive of periclase. At present, there is little data on the use of double annealing in the production of ceramic products from the ZrO₂-MgO-Al₂O₃ system. In addition, little attention has been paid to the formation of metastable states in ceramics that could improve mechanical properties. The study examined the effect of changes in chemical composition on the formation of microstructure, phase composition, and mechanical properties in sintered ceramics. The analysis is carried out using scanning electron microscopy, X-ray diffraction, and microhardness measurements, which together allow us to identify patterns between the morphology and strength properties of the material and determine the most effective directions for creating new technical ceramics. As a result of the work carried out, the effect of significantly improving the strength properties of ZTA ceramics due to the formation of a cubic phase when doped with MgO and Al₂O₃ oxides was investigated for the first time.

2. Materials and Methods

Ceramics (90−x)·ZrO₂-10·MgO-x·Al₂O₃ with x = 10 wt.% variation within 10–80 wt.% were synthesized by standard ceramic technology, including mixing, milling, manufacturing of green bodies, and sintering. In this work, zirconium, magnesium, and aluminum oxides were selected as initial components (all reagents are Sigma Aldrich (Darmstadt, Germany) with 99.99% purity).

For uniform mixing of the oxides, a Pulverisette 6 planetary ball mill (Fritsch, Germany) with balls and a grinding jar (ball size 10 mm) made of tungsten carbide was used. Mixing was carried out at 250 rpm for 30 min with the addition of ethyl alcohol. After mixing, the alcohol was removed by drying at a temperature of 45 °C for 5 h. The dried samples were pre-annealed at a temperature of 1300 ° C for 5 h in a Nabertherm LHT 08/18 furnace (Nabertherm, Lilienthal, Germany) in air. To determine the obtained phases, XRD phase analysis was performed on a Rigaku SmartLab diffractometer (Rigaku, Tokyo, Japan) using the PDF-2024 database. The fraction of a smaller size was achieved by wet grinding in a Pulverisette 7 premium) line planetary micromill (Fritsch, Idar-Oberstein, Germany) at 400 rpm for 30 min with 1.6 mm WC balls. The size of the resulting fraction was analyzed using an Analysette 22 MicroTec plus device (Fritsch, Idar-Oberstein, Germany with a water suspension dispersion unit. The green powders were manufactured with the addition of polyvinyl alcohol (PVA) as a plasticizer. The mass of PVA was selected so that the plasticizer constituted 2% of the mass of oxide powders. To obtain the green powder, PVA was poured into a glass with distilled water heated to 90 °C, and after the PVA was completely dissolved, the dried mixture of oxides was added to the solution. The green powders, dried at a temperature of 45° C for several hours, were ground in an agate mortar and kept in a heating cabinet at a temperature of 90 °C for 12 h. Using an automatic hydraulic press Paratus (Paratus, Yekaterinburg, Russia) with a force of 5 (156.2 MPa) and then 10 tons (312.3 MPa, action time of 5 s), green bodies in the form of disks with a diameter of 20 mm were produced. The obtained tablets were pre-annealed at a starting temperature of 100° C with an increasing step of 50 °C every half hour up to 500° C to obtain brown bodies. The final sintering was performed in a muffle furnace Nabertherm LHT 08/18 at a temperature of 1500° C for 5 h. The phase composition of the sintered disks was also studied using the XRD method. The microstructure and elemental composition were studied using scanning electron microscopy on a Thermo Fisher Phenom X microscope (Thermo Fisher Scientific, Eindhoven, The Netherlands) at an accelerating voltage of 15 kV (backscattered electron imaging).

The strength of the tablets was estimated by calculating the flexural strength when the disk was compressed on three ball supports with a pin [21]:

$${\mathrm{\sigma }}_{\mathrm{f}\mathrm{l}\mathrm{e}\mathrm{x}}={\displaystyle \frac{3\left(1+\mathrm{\nu }\right)\mathrm{P}}{4\mathrm{\pi }{\mathrm{t}}^2}}\left[1+2\ln {\displaystyle \frac{\mathrm{a}}{\mathrm{b}}}+{\displaystyle \frac{(1-\mathrm{\nu })}{(2+\mathrm{\nu })}}\left(1-{\displaystyle \frac{{\mathrm{b}}^2}{2{\mathrm{a}}^2}}\right){\displaystyle \frac{{\mathrm{a}}^2}{{\mathrm{R}}^2}}\right]$$

(1)

where σ_flex—biaxial flexural strength;

P—load in N;

υ—Poisson’s ratio (0.305);

t—disk thickness;

a—radius of the circle on which the spherical supports are located;

b—radius of the pin pressing on the disk;

R—radius of ceramic disk.

The load during compression test was measured by a universal electromechanical single-column WalterBai LFM-L 10 kN machine (WalterBai, Lohningen, Switzerland). Before the microhardness assessment, the surface of the experimental samples was polished on a Tegramin grinding and polishing machine (Struers, Ballerup, Denmark) for sample preparation. Microhardness was determined by the Vickers method with forces of 0.5 HV (0.5 kgf) on a Dupoline-M1 microhardness tester (Metkon Instruments Inc., Bursa, Turkey).

3. Results and Discussion

Figure 1a shows the XRD patterns of the sintered ceramic disks and the designations of the crystalline phases. For the phase identification, the PDF-2 2024 database was used, using the cards: No. 01-080-0964 for c-ZrO₂, No. 00-037-1484 for m-ZrO₂, No. 01-075-4396 for spinel MgAl₂O₄, as well as No. 01-073-6190 and No. 00-045-0946 for corundum (α-Al₂O₃) and periclase (MgO), respectively. The PDF data were used for Whole Powder Pattern Fitting (WPPF) analysis in Rigaku SmartLab Studio II using the Rietveld method [22]. An example of the fitting and related statistical parameters is shown in Figure S1 and Table S1 in the Supplementary Information. The phase relationships in the sintered ceramics were obtained from the WPPF analysis. The dependence of the phase composition on the concentration x is shown in Figure 1b. A high concentration of stabilized zirconium dioxide c-ZrO₂ and t-ZrO₂ (75 wt.%) was found only in the sample with a concentration of x = 10 wt.%. With an increase in the concentration of corundum in the initial mixture, a high content of the monoclinic phase ZrO₂ is observed. In addition, the content of the spinel phase MgAl₂O₄ increases to 52 wt.%. The lack of stabilization of zirconium dioxide in most samples occurs due to the fact that the MgO component of the mixture at temperatures below the m → t transition (1170 °C) enters into a solid-phase reaction with partial formation of magnesium–aluminum spinel [23,24]. However, when the concentration of aluminum oxide in the mixture is 10 wt.%, a metastable condition occurs in which MgO also interacts with zirconium oxide particles, which leads to stabilization of the c-ZrO₂ phase. It is worth noting that sintering at a temperature of 1500 °C for 5 h does not lead to an equilibrium state in the ZrO₂-MgO-Al₂O₃ system, and some solid-phase chemical reactions may be incomplete [25].

The XRD phase analysis data show that after preliminary annealing, the formation of the spinel phase is observed, and the amount of MgO in the mixture, which could have been used to stabilize the cubic phase of ZrO₂, was spent on the formation of spinel. The results confirming this are shown in Figure S2 in the Supplementary Information. When x = 40 wt.% is exceeded, the α-Al₂O₃ phase appears in the ceramics, which occurs due to the fact that there is an excess of aluminum oxide in the initial mixture, which is not spent on the reaction of spinel formation. According to the phase analysis results, the transition from ZTA to ATZ ceramics is attributed to concentrations of 50–70 wt.%, since a gradual change in the matrix phase ZrO₂-m → α-Al₂O₃ is observed.

Figure 2a shows the dependence of the apparent density and volume shrinkage on the concentration x of the initial mixture. As can be seen from the measured values, the concentration dependences of the apparent density and volume shrinkage completely repeat each other. With an increase in the concentration of aluminum oxide in the mixture, the density and shrinkage decrease, and there are two sections with different slopes of the curves. The section with a smaller slope is found in the concentrations of the ZTA-ATZ transition. This fact indicates different densification mechanisms in ceramic disks. Possible densification mechanisms will be given below. In general, it can be stated that a decrease in the sintering intensity is observed with an increase in the Al₂O₃ composition of the mixture. Figure 2b shows the calculated values of the apparent porosity of the ceramic disks. The porosity of the samples was calculated using the formulas:

$${\mathrm{\rho }}_{\mathrm{comp}}^{\mathrm{theo}}={\mathrm{\rho }}_1^{\mathrm{theo}}\cdot {\mathrm{C}}_1^{\mathrm{m}}+{\mathrm{\rho }}_2^{\mathrm{theo}}\cdot {\mathrm{C}}_2^{\mathrm{m}}+\cdots +{\mathrm{\rho }}_{\mathrm{n}}^{\mathrm{theo}}\cdot {\mathrm{C}}_{\mathrm{n}}^{\mathrm{m}}$$

(2)

$$\mathrm{Porosity}=(1-\frac{{\mathrm{\rho }}_{\exp }}{{\mathrm{\rho }}_{\mathrm{comp}}^{\mathrm{theo}}})$$

(3)

where ${\mathrm{\rho }}_{\mathrm{comp}}^{\mathrm{theo}}$—theoretical density of composite;

${\mathrm{\rho }}_{\mathrm{n}}^{\mathrm{theo}}$—theoretical density of n component;

${\mathrm{C}}_{\mathrm{n}}^{\mathrm{m}}$—weight concentration of n component.

The mass concentrations for porosity calculations were taken from Figure 1b. With increasing concentration x, the porosity in the sintered disks increases, which is also associated with a decrease in the intensity of the sintering process.

In many cases in sintering, porosity as well as density is correlated with the grain size by the formula [26,27,28]:

$$\mathrm{p}={\mathrm{p}}_0·{\left(\frac{{\mathrm{G}}_0}{\mathrm{G}}\right)}^2$$

(4)

where p₀—initial porosity;

p—final porosity;

G₀—initial grain size;

G—final grain size.

The results of the comparison of calculations by Equation (4) and experimental data are presented in Figure 2b. It is evident that the calculated data differ significantly from the experimental ones. In the course of interpretation of the obtained results, it was suggested that the significant difference is caused by the fact that the data on the average size obtained by means of a laser particle analyzer (Figure 2a and Figure S6) are distorted due to the formation of conglomerates in the aqueous suspension. By analyzing the SEM images of the press powders (Supplementary Figures S3, S4, and Table S2), the average particle sizes G₀ were calculated, which were also used for calculations by Formula (2). It was found that with the corrected G₀ values, Formula (4) gives good agreement in the concentration range of x = 10–30 wt.%. Above this concentration, the calculated values are significantly less (by 8–20%) than the values measured on the disks. This may be due to the fact that, during sintering from pre-annealed ZrO₂-MgO-Al₂O₃ powders, in addition to sintering, a chemical reaction of spinel formation may continue. Also, the sintering result may be affected by the fact that the size and shape of the particles of different phases may initially be such that no dense packing of particles was observed when obtaining a press blank. In this case, the sintering process is complicated by a large proportion of air pores. In other words, Formula (2) may poorly describe the sintering of multiphase materials.

Figure 3 shows SEM images of the surfaces of ceramic disks. Light grains in the images refer to the ZrO₂ phase, while dark grains refer to MgO/Al₂O₃/MgAl₂O₄. Analysis of SEM images shows that the microstructure varies significantly with increasing Al₂O₃ content in the initial mixture. At concentrations of x = 10, 20 wt.%, high grain cohesion is observed, which is associated with the sintering mechanism, in which the grain size increases due to the movement of grain boundaries. This is also indicated by the fact that the grains of the MgO/MgAl₂O₄ phase (dark grains) are inside the zirconium oxide grain (see SEM images for the sample x = 10 wt.%). The surface morphology of the sample x = 20 wt.% shows a metastable state, in which the ZrO₂ grains did not have time to grow into polygonal formations at 1500 °C. Their shape can be described as similar to a husk with a high aspect ratio. Apparently, the increase in the Al₂O₃ content led to a decrease in the intensity of magnesium diffusion into ZrO₂ particles, which resulted in a significant decrease in the amount of c-ZrO₂ phase and also affected the microstructure of the ceramics. It can be assumed that the intense diffusion of Mg²⁺ into the crystal lattice of zirconium oxide promotes the sintering process. There are data in the literature that MgO, Al₂O₃ are grain growth inhibitors for zirconium oxide ceramics [29]. For this reason, the microstructure of the sample x = 20 wt.% could have undergone significant changes compared to the sample x = 10 wt.%. It is also worth noting the developed morphology of ZrO₂ grains, which is characterized by a perforated surface. Previously, it was shown that such grain morphology is the result of the influence of eutectoidic decomposition, in which c-ZrO₂ is transformed into m-ZrO₂ and MgO [30,31,32]. Also, dry sintering during green body formation can be considered, because applied pressure (312 MPa) was enough to initiate that process in CaCO₃, silica, and other compounds [33,34,35]. However, in our case, the dwell time of pressing was no more than 10 s, and the effect of dry sintering is not pronounced.

In the case of the sample with x = 30 wt.%, the formation of longitudinal cracks is observed, caused by the reverse transition upon cooling t → m [36]. Since this transition is accompanied by a significant change in the volume of the crystal lattice, significant deformations occur in the grains, leading to the formation of cracks upon cooling. With a further increase in x, the formation of bipyramidal grains is observed, characteristic of the cubic and rhombohedral lattices, which are characteristic of spinel and corundum, respectively (see Figure S5 in the Supplementary). At the concentration range of x = 40–50%, a transition between ZTA-ATZ ceramics is visible since the matrix phase changes from ZrO₂ (white grains) to an Al₂O₃/MgAl₂O₄ matrix. At the same time, the grain size for the phases containing Mg, Al increases, and the porosity also increases. The absence of a large number of fusion necks suggests that the high pore concentration did not lead to stabilization of the tetragonal or cubic phase. Partial stabilization of the tetragonal phase was observed in ZTA ceramics previously obtained by other authors [37,38].

Analysis of SEM images allows us to draw conclusions about the possible sintering mechanisms. In the sample with x = 10 wt.%, compaction during sintering occurs due to the formation of necks during the diffusion of atoms along the particle surface, diffusion of atoms along initially stationary intergranular boundaries, and movement of intergranular boundaries with concomitant growth of ZrO₂ grains [39]. In the case of the sample with x = 20 wt.%, the latter mechanism did not take place during sintering of the ceramics, which created a dense fine-grained structure. With an increase in the Al₂O₃ concentration in the initial mixture, sintering was accompanied only by the diffusion of atoms along the surface, which led to the formation of a porous structure without pronounced cohesion of grains. This can also be seen if we consider the results of calculations using Formula (2) in Figure 2b. In the concentration range x = 40–80 wt.%, there is a significant discrepancy between the experimental and calculated data due to the fact that the grain size has increased without a significant decrease in porosity. This indicates the absence of compaction due to the movement of grain boundaries or the formation of a dense structure during grain coalescence.

In order to find out the reasons for the obtained morphology, the results of the estimation of the average particle size in the pre-annealed powders and the average grain size in the ceramics were plotted. For the pre-annealed powders, the particle size was determined using a laser particle analyzer, while the average grain size in the ceramics was determined using the ImageJ version 1.54 d program by measuring the maximum Feret diameter [40]. As can be seen from Figure 4a, the average particle size after pre-annealing passes through a minimum at the point x = 20 wt.% and then increases. The increase in the average particle size can be attributed to the fact that with an increase in the corundum content in the mixture for a fixed time of wet milling, the intensity of the grinding process decreases. This, in turn, is caused by the fact that the hardness of α-Al₂O₃ is higher than that of zirconium dioxide; therefore, more time is required to obtain powders with a small size.

Figure 4b shows the dependence of the average size of all grains (obtained from the histograms in Figure S7) and the two types of grains separately on the concentration in the sintered ceramics. As can be seen from the obtained graphs, the concentration dependences of the size of the annealed particles have common trends with a change in the average size of all grains in the section up to x = 50 wt.%. Thus, in ZTA ceramics, the small grain size contributed to significant compaction of ceramics of 30–45%. At the same time, after reaching the ZTA/ATZ transition (x = 50 wt.%), the grain size in Figure 4b Al₂O₃/MgAl₂O₄ continues to increase, which also correlates with the data in Figure 4a. In addition to the increase in the average size of the annealed particles, the appearance of tails of ~3–5 μm can be seen in the size distribution diagrams (Figure S6 in Supplementary Information). The presence of such tails can significantly affect the packing of particles during pressing. It can be concluded that it is the larger average grain size of MgO/Al₂O₃/MgAl₂O₄ in the annealed powders that leads to high porosity, lower shrinkage, and density after the sintering process in ATZ ceramics.

Figure 5a,b show the dependences of microhardness and flexural strength for sintered disks of (90−x)·ZrO₂-10·MgO-x·Al₂O₃ composition, respectively. It is evident that the maximum values of microhardness HV = 1380 and flexural strength σ_flex = 217 MPa were measured for the sample with x = 10 wt.%. As was shown earlier, these values are due to several reasons. Firstly, the samples with x = 10, 20 wt.% had a relatively low porosity of ~6%. The measured values are lower than the tabulated ones since even a small percentage of porosity can significantly reduce the values of such parameters as hardness and strength. Secondly, the sample with 10 wt.% has a high proportion of the stabilized c-ZrO₂ phase. This phase in the ceramic composition significantly strengthens its mechanical characteristics, since the cubic modification of zirconium dioxide is an oxide with outstanding strength properties [36,41,42,43]. In particular, this is a high fracture toughness, which complicates the propagation of cracks in the volume of composite ceramics. However, in some cases, fracture toughness is inversely correlated with the strength of ceramics [44,45,46]. It is also worth noting that the presence of Al₂O₃/MgO grains in the ZrO₂ grain also provides a reinforcing effect by deflecting the direction of crack propagation [47].

Despite the same porosity in the x = 10, 20 wt.% samples, the flexural strength in the x = 20 wt.% sample dropped to 130 MPa due to the absence of a stabilized zirconium dioxide phase. The microhardness decreased from 1380 to 1150, but the standard deviation increased. The smaller decrease in the HV0.5 values can be attributed to the fact that when measuring the microhardness, information about the mechanical properties of the material is obtained from a small area of about 100 by 100 μm. At this micro level, ceramics can retain their strength, but when measuring the bending strength, cracks are generated that spread throughout the entire thickness of the sample. In this case, the presence of pores and the absence of high-strength phases in the composition become critical factors.

More dramatic changes are observed for the sample with x = 30 wt.%. Due to the occurrence of cracks due to the m → t phase transition upon cooling, the flexural strength decreased to 25 MPa, and the microhardness HV to 970 with a standard deviation of ±230. Figure 5c shows the values of internal stresses ε obtained during fitting by the WPPF method. As can be seen from the obtained dependences of stress on the concentration x, no pronounced dependences were found over the entire concentration range, but in the region of 10–40 wt.%, an increase in stress values by more than 2 times for the m-ZrO₂ phase was found. These data show how much the crystal lattice is deformed in m-ZrO₂ grains. Since ZTA ceramics are located in this region, the stresses in the crystal lattice of zirconium oxide will determine the properties of the composite ceramics. With further increase in x, small values of microhardness (220–400) and biaxial bending strength (30–60 MPa) are mainly due to the high porosity of sintered ceramic disks. A small increase in the values of HV0.5 and σ_flex in the concentration range of 50–80 wt.% can be associated with lower stresses ε in the Al₂O₃ matrix.

Thus, ZTA ceramics with 10 wt.% MgO addition can be considered as robust technical ceramics for structural material applications, while ATZ are more suitable as porous refractory materials.

4. Conclusions

In this work, ceramics of the composition (90−x)·ZrO₂-10·MgO-x·Al₂O₃ were obtained by the method of standard ceramic route with preliminary annealing. This composition can be considered as ZTA-ATZ ceramics with the addition of MgO. The results of XRD phase analysis showed that a high content of the stabilized phase c-ZrO_2te is observed only in the sample x = 10 wt.%. In general, the study showed that stabilization of zirconium dioxide gives the highest shrinkage (~45%), the lowest porosity (6%), and maximum values of mechanical characteristics HV0.5 = 1380, σ_flex = 217 MPa. It was found that preliminary annealing makes it difficult to grind powders with a high content of Al₂O₃. Large particle size in such powders reduces the intensity of sintering processes, which causes high porosity of 24–28% and low shrinkage of 20–25%. A significant decrease in the values of mechanical properties in the range of x = 20–40 wt.% is due to increasing stresses in ZrO₂ grains, which lead to the appearance of cracks in the volume of ceramics. The experimental results show that the main factors of high values of mechanical properties in (90−x)·ZrO₂-10·MgO-x·Al₂O₃ ceramics are porosity and the content of the stabilized phase ZrO₂-c.