Structure formation and properties of corundum ceramics based on metastable aluminium oxide doped with stabilized zirconium dioxide

The work presents a successful example of the use of YSZ binary systems for production of composite ceramic materials based on θ -Al 2 O 3 with improved physical and mechanical characteristics which was previously considered an unpromising material. It was first obtained result of extreme nature of dependence of physical and mechanical properties of Al 2 O 3 + YSZ on the concentration of YSZ (ZrO 2 – 3mol% of Y 2 O 3 ) additive. The sintering temperature was decreased on 250 ◦ C (from 1800 to 1550 ◦ C). The phase composition of powders and the structure of ceramics of the Al 2 O 3 + YSZ system were investigated depending on the amount of YSZ dopant, the structure-properties relationship was established. It is shown that maximum of physical and mechanical char- acteristics achieved at YSZ concentration equal to 10% wt.


Introduction
Ceramic materials take a leading place among materials used in many spheres of technology, industry and medicine. Ceramics based on aluminum oxides have become the most widespread among all ceramic materials. This material is used in construction and industry as heat insulation material that is resistant to corrosive media and does not release harmful substances during operation [1,2]. Corundum ceramics is widely used in mechanical engineering for the manufacturing of refractory and high-temperature parts of machines, and it is used in medicine as a material for creating dental implants [3][4][5].
Corundum ceramics has become widespread due to such physical and mechanical properties as high strength, hardness, wear resistance, refractory resistance, thermal conductivity, chemical resistance and high electrical strength. Sintering temperature of pure aluminum α-oxide powders is 1700-1800 • С, and in the presence of eutectic additives -1550-1650 • С [6,7]. Relatively high sintering temperatures and polymorphism (there are eight metastable modifications of aluminum oxide) are a certain scientific and technological problems that initiate an interest in optimizing the producing technology of aluminum oxide ceramics while simultaneously increase in the level of their physical properties.
There are a number of independent hypotheses describing the causes of the appearance and nature of metastable polymorphic modifications of Al 2 O 3 existing in the temperature range up to 1200 • С. According to Refs. [8,9], the primary reason for the formation of metastable phases is the presence of impurity ions: OH − , SO 4 2− , CO 3 2− , etc. in the structure of the material, the removal of which during thermal treatment "destabilizes" the metastable state. In works [10,11,13,14] the influence on the process of emergence and existence of metastable phases of particle size (dimensional factor) was suggested. This assumption is supported by the fact of obtaining metastable phases when crushing large powder materials, as well as the existence of metastable phases in thin films [9]. However, the energy theory [10,11] does not describe enough the conditions for the formation of metastable phases. In particular [12], suggests a predominant role in the mechanism of the emergence of metastable phases of martensitic transition and micro domains.
Structural water plays an essential role as a structure-forming factor. Sufficiently large sizes of aluminum hydroxide crystals are explained by the presence in the structure of water in the form of coordinated hydro groups OH − . Thermal treatment leads to dihydroxylation of Al(OH) 3 , reduction of crystal size and formation of intermediate metastable phases of aluminum oxide [15]. Fig. 2 shows bar diagram of particle sizes (coherent scattering regions -CSR), in the process of thermal evolution. According to the X-ray structural analysis, the CSR value in the hydroxide Al(OH) 3  One of the ways to increase the competitiveness of corundum ceramics is to obtain binary and more complex systems of ceramic oxides from those previously considered unpromising.
Due to the polymorphism, low density and high porosity [16] in comparison to the thermodynamically stable α-Al 2 O 3 phase [17,18], the θ-Al 2 O 3 modification has been poorely studied as a raw material for the corundum ceramics production. Solid solutions based on YSZ due to the lower sintering temperature and higher density are of interest as an doping for Al 2 O 3 systems. The phase diagram of the YSZ -Al 2 O 3 system is not comprehensively studied at high concentrations of Al 2 O 3 however, the existing experimental background of the authors in the study of zirconium solid solution of the YSZ system with an admixture of Al 2 O 3 [19,20] suggests that the addition of YSZ in θ-Al 2 O 3 will reduce the sintering temperature and increase the density of corundum ceramics.
The main goal of the present research is confirmation of this assumption experimentally by examining corundum ceramics based on the metastable θ-Al 2 O 3 oxide doped with stabilized tetragonal zirconium dioxide (YSZ) additives structure and its physical and mechanical properties.

Materials and research methods
The powders for the study were prepared by chemical coprecipitation from a solution of aluminum chloride (AlCl 3 • 6H 2 O), zirconium chloride (ZrOCl 2 • 8H 2 O), and yttrium nitrate salts (Y(NO 3 ) 3 ) at room temperature. To obtain a homogeneous product, the method of reverse chemical precipitation was used: chemical mixing of salts was carried out before their introduction into the precipitator. The coprecipitation method is a more laborious process than mechanical mixing, but the degree of homogenization of the co-precipitated masses is incomparably higher than the degree of homogenization of the mixture obtained mechanically. The co-precipitation method provides a greater value of the dispersion and homogeneity of the nanopowder system, which plays an important role in obtaining ceramics with high physical and mechanical properties [21]. Crystallization and formation of powder particles were carried out in air by calcining hydroxides at temperatures of 1000, 1200 and 1300 • C for 2 h. The resulting oxides had the structure θ-Al2O3 + nYSZ, where n = 0, 1, 5, 10, 15 wt%.
The powders were compacted in steel molds by uniaxial pressing at a pressure of 20 MPa, followed by treatment of compacts with high hydrostatic pressure (400 MPa). Three series of samplescompacts in the form of beams with dimensions 4x4x40 mm were sintered in air at  temperatures, respectively, 1450 (series 1), 1500 (series 2) and 1550 • C (series 3) for 2h. Polymorphic modifications of aluminum oxide have been little studied in as a raw material for the production of corundum ceramics, and the choice of the sintering temperature with a step of 50 • C will allow a detailed study of this system.
The analysis of the phase composition of powders and ceramics was carried out on an Empyrean diffractometer (PANalytical), in filtered copper radiation. Density was measured by hydrostatic weighing on AXIS ANG 220 analytical weights. Vickers hardness was measured on a TP-7R-1 installation with a load of 5-20 kg, with a step of 2,5 kg in three times repetition. Strength was examined by 4-point bending on the T-Series Materials Testing Machine H50K-T (Tinius Olsen) five-fold repetition. The surface microstructure was examined on a Jeol scanning electron microscope JSM-6490LV.

Powder systems
Phase composition of the Al 2 O 3 powders and powder mixture Al 2 O 3 + nYSZ annealed at a temperature 1000 • C and containing min (1%) and max (15%) of concentration of YSZ are given in Fig. 3.
The presence of θ and γ phases Al 2 O 3 is explained by the fact that during thermal treatment of Al(OH) 3 , which is a mixture of bayerite (β-Al(OH) 3 ) and nordstrandite (γ-Al(OH) 3 ), the crystal lattices transform through two intermediate states. In the range 200-250 • С from Al(OH) 3 , 0,5 amount of water molecules are removed and boehmite is partially formed, and with complete dehydration corresponding to the temperature range 300-500 • С, a low-temperature cubic γ-Al 2 O 3 is formed (way 1).
In the range of 250-350 • С, 1.5 molecules of H 2 O are removed from Al(OH) 3 to form the Al 2 O 3 -phase and 1 molecule of H 2 O, and at complete dehydration corresponding to the temperature of 350-500 • С, a monoclinic θ-Al 2 O 3 is formed (path 2). The presence of peaks of tetragonal zirconium dioxide even at its minimum amount (1%) is explained by its strong scattering ability, which is directly proportional to Z 4 , where Z is an ordinal number in the periodic table of elements [22,23].
Phase composition of co-precipitated powder mixtures of Al 2 O 3 + nYSZ composition, where n = 0, 5, 15 wt%, obtained at temperature 1200 • С are given in Fig. 4. It can be seen from the x-ray diffraction patterns that for Al 2 O 3 + 0% YSZ powder the phase transition to stable α-Al 2 O 3 phase occurs at 1200 • С, and for powder Al 2 O 3 + nYSZ (n = 5, 15 wt%) the phase transition to stable α-Al 2 O 3 state does not occur at the same temperature ( Fig. 4). Powder mixtures of the composition Al 2 O 3 + 5% YSZ and Al 2 O 3 + 15% YSZ annealed at 1200 • C and 1300 • C (Fig. 5) were investigated to determine the temperature range of the phase transition from the metastable of θ-Al 2 O 3 to stable α-Al 2 O 3 state.
Thus, based on the results of XRD analysis we can conclude that the addition of zirconium dioxide to aluminum oxide powders impedes the phase conversion process and increases the transition temperature of alumina powders from metastable to stable α-Al 2 O 3 . Phase transition of aluminum oxide to a stable state for the system Al 2 O 3 + nYSZ, n = 1, 5, 10, 15% corresponds to the temperature 1300 • C, and in Al 2 O 3 + 0%YSZ system the phase transition occurred at a temperature 1200 • C. The reason for the delay in the phase transformation is the effect of mutual protection against crystallization, which is characteristic of powder mixtures obtained by the method of chemical deposition: two coprecipitated substances heated to a certain temperature can remain amorphous, while each of them separately, is heated to the same temperature, completely crystallizes [24].

Ceramics
Based on powder mixtures annealed at a temperature 1000 • C, three sets of ceramic samples were prepared at different sintering temperatures. Fig. 6 shows XRD patterns of ceramic materials of following composition Al 2 O 3 + 10% YSZ, obtained at 1450, 1500 and 1550 • C,. As can be seen typical ceramic features of the system Al 2 O 3 + nYSZ, where n = 1, 5, 10, 15 wt% are observed. Thus, is a composite system including matrix α-Al 2 O 3 (corundum) with YSZ filler.
Physical and mechanical characteristics of aluminum-zirconium ceramics. Fig. 7 shows the dependence of the density of aluminum-zirconium ceramics based on powders annealed at 1000 • C on the concentration of YSZ and sintering temperature.
The range of density change of obtained ceramics depending on concentration of the stabilized zirconium dioxide and agglomeration temperature varied from 3,42 to 4,04 g/cm 3 ). The theoretical density of  the aluminum-zirconium composite for the Al2O3 + 10% Z3Y system is 4.19 g/cm 3 . The maximum density value α max = 4.04 g/cm 3 was achieved at a sintering temperature of 1550 • C for 2 h and a zirconium dioxide concentration n = 10 wt%, which is 96.3% of the theoretical density of the aluminum-zirconium composite (Fig. 7). Comparison of physical parameters with similar control batches of two-phase powder based on α-Al 2 O 3 + nYSZ where n = 0, 1, 5, 10, 15 wt% received under the same conditions (ρ max = 2.05 ÷ 2.84 g/cm 3 , porosity ς = 35 ÷ 50%) testifies the advantage to use metastable phases of aluminum oxide to obtain dense corundum ceramics at agglomeration temperatures of ~1450-1550 • C.
Variation of hardness of the analyzed ceramic samples on zirconium dioxide concentration and sintering temperatures is given in Fig. 8. Depending on concentration of the YSZ and agglomeration temperature, the hardness (HV) changed in range 8 ÷ 19.5 GPa. Maximal hardness of ceramic samples of 19.5 GPa correspond to sintering temperature T max = 1550 • C and concentration of stabilized zirconium dioxide n max = 10 wt%. Fig. 9 shows the strength values of the studied ceramics versus additive amount wt%. Strength tested according to ASTM C1161 standard.
Depending on the concentration of zirconium dioxide the strength of the studied ceramics varied in the range from A = 240 ÷ 315 MPa. The maximum strength value corresponds to composites obtained at 1550 • C, containing 10 and 15% of YSZ and samples obtained at 1500 • C, containing 5% of YSZ 2 . As can be seen from Fig. 10, if the concentration of YSZ increases up to 5%, the strength of the ceramic samples increases significantly. Further increase of the doping element concentration, leads to the strength saturation and practically does not change. Fig. 10 shows the surface structure of the samples of compositions: a -Al 2 O 3 5% YSZ and b -Al 2 O 3 10% YSZ obtained in air at a temperature 1550 • C for 2h. SEM data represent the presence of zirconium dioxide at the grain boundaries of aluminum oxide (Fig. 10). Probably, at the stage of ceramic sintering, zirconium dioxide is distributed along the boundaries of aluminium grains, which led to the formation of a composite matrix with inclusions of zirconium dioxide in the intergranular space    ( Fig. 10).

Microstructure
Based on the behavior of the physical characteristics of the studied ceramics (Figs. 7-9), with low additions of the doping element (n = 1-10%), zirconium dioxide uniformly fills the internal grain space, forming a damping interlayer, leading to increase in the resistance of the composite ceramics to mechanical effects, increased density and reduced porosity. Here, a special property of zirconium ceramics is used -the effect of super-plasticity, due to this effect it is possible to obtain the so-called "ceramic steels" [25,26]. With a zirconium dioxide concentration of n ≥ 10% agglomeration occurs (aggregate size up to 1,5 μm) which leads to a softening of the ceramic structure.
Thus, in aluminum oxide ceramics the average grain size of 3,5-4 μm and when an YSZ is added, the average grain size is halved. At the same time, the effective grain size of YSZ increases with its concentration in the aluminum dioxide matrix. In ceramics of the composition Al 2 O 3 5% YSZ, the average grain size of zirconium dioxide is 0,2-0,3 μm, and at a zirconium dioxide concentration of 10%, the average grain size is 0,5-0,75 μm (Fig. 10).
The above data show that when YSZ of 10 wt% is added to aluminum dioxide ceramic, the diffuseness of the ceramic composite material decreases and, as a result, its physical and mechanical properties are improved. Fig. 11 shows the image of a surface of ceramics of structure Al 2 O 3 10%YSZ at increase is submitted x2500.
When YSZ concentration increases to 10 wt%, a small amount of zirconium dioxide grain agglomerates in the material is forming (Fig. 11). Based on the physic-mechanical properties of ceramics (Figs. [7][8][9] and SEM data, it can be concluded that in Al 2 O 3 + 10% YSZ ceramics, the maximum physical and mechanical properties are achieved, since this concentration of zirconia is optimal for the uniform distribution of YSZ over the entire volume of the alumina matrix, and a small amount of the formed agglomerates is insufficient to deteriorate the properties of the composite.
In alumocyrconium ceramics doped with 15% YSZ, a deterioration in physical and mechanical characteristics is observed. Fig. 12 shows the surface structure of ceramics of the composition Al 2 O 3 +15%YSZ.
When the concentration of the YSZ increases to 15%, a large number of agglomerates concentrated in the inter-grain space are formed which lead to softening of the material and, as a result, to deterioration of the physical and mechanical characteristics of the studied ceramics.
Samples containing 5% YSZ were examined for the effect of sintering temperature on the structure of composite ceramics. The effect of sintering temperature on the structure of composite ceramics of the   composition Al 2 O 3 5%3 is shown in Fig. 13.
At sintering temperature of 1500 • С there is observed a large number of surface pores of different size and shape (Fig. 13, a). The ceramic density of the composition Al 2 O 3 5% YSZ obtained at a sintering temperature of 1500 • C is 3,78 g/cm 3 . At a sintering temperature of 1550 • C, a significant decrease in the number of surface pores (Fig. 13, b) is observed. The density of the composite ceramic obtained at the sintering temperature of 1550 • C is 3,90 g/cm 3 .
These data indicate that the sintering temperature plays an important role in the production of dense and non-porous aluminum oxidebased ceramics. The optimum sintering temperature of the metastable θ-aluminum oxide based composite ceramics is 1550 • C.

Funding
This Project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowvska-Curie grant agreement 871284. Also, this study was performed in the scope of the RO-JINR Projects No. 267/2020 item 25, N • 268/2020 item 57 and N • 614 item 3.