RU2701765C1 - Method of producing nanostructured composite ceramics based on zirconium, aluminum and silicon oxides - Google Patents

Abstract

FIELD: manufacturing technology.

SUBSTANCE: invention relates to methods of producing high-strength materials, specifically composite ceramics based on stabilized zirconium dioxide and corundum with the addition of silicon dioxide. Invention can be used in production of strong and wear-resistant parts for various industries, including those operating under conditions of high temperatures and corrosive media (draw plates, drawing dies, bearings, rollers, grinding balls, pistons of brake discs, etc.), and products for medicine (cutting tools for surgery, implants for prosthetics, etc.). To obtain nanostructured composite ceramics based on oxides of zirconium, aluminum and silicon by mechanical method, mixing nanodispersed powders of synthetic zirconium dioxide (89.68–90.17 wt%), calcium oxide (2.84–2.85 wt%), corundum (4.87–4.9 wt%) and silicon dioxide (2.08–2.61 wt%). Obtained mixture is dispersed and homogenized in distilled water, grinding is performed in a planetary mill, the mixture is dried in a furnace at temperature of 80 °C. Sample is molded by uniaxial pressing at pressure of 500 MPa. Sintering is performed in two-step mode: first, at 1,300 °C, then reducing temperature to 1,200 °C and maintained for 4 hours.

EFFECT: obtaining composite ceramics with high ratio of hardness and fracture toughness.

1 cl, 2 dwg, 2 tbl, 2 ex

Description

The invention relates to methods for producing high-strength materials, namely, composite ceramics based on stabilized zirconia and corundum with the addition of silicon dioxide. Composite ceramics based on zirconia (ZrO ₂ ) and alumina (Al ₂ O ₃ ) have a unique combination of mechanical properties, chemical, thermal and radiation resistance, as well as bioinertness. This leads to its widespread demand in metallurgy, mechanical engineering, nuclear, mining, processing, electrical and light industries and medicine.

A known method of producing a ceramic composite material based on oxides of aluminum and zirconium [RU 2549945 C2, publ. 05/10/2015 Bull. No. 13]. In this method, the stabilization of zirconium dioxide in the tetragonal phase is carried out mechanically: the zirconium salt and the stabilizer (rare earth salt) are mixed in the activator. Then the mixture is heat treated at a temperature of 500-600 ° C for 1-3 hours. The content of rare earth oxide is 3-10 mol. % of the content of zirconium dioxide in terms of oxides. In the activator, the obtained stabilized zirconia and alumina with the addition of magnesium carbonate are crushed separately, then they are mixed. The products are molded by axial pressing at a pressure of 190-300 MPa, and firing is carried out at a temperature of 1550-1600 ° C for 1-3 hours. Grinding and mixing of all components is performed in a high-speed activator with acceleration of grinding media of at least 10 g. Wet grinding of a mixture of aluminum oxide and magnesium carbonate is carried out to a particle size of less than 100 nm.

The disadvantage of this method is the use of high-temperature sintering, which is uneconomical and causes an increase in the grain size of zirconium dioxide, as well as in the low fracture toughness of the resulting composite ceramic (5.4 MPa⋅ ^1/2 ≤ K _1C ≤ 8 MPa⋅ ^1/2 ).

A known method of producing ceramics by hot pressing a powder consisting of alpha-alumina with a particle size of 0.1-2.0 (0.6) microns and zirconium oxide stabilized (doped) with yttrium with a particle size of ≤ 0.1 microns [JP H04 -002613, 01/07/1992]. To obtain a powder with a uniform distribution of the components of the mixture, a ball mill was used in which the previously synthesized coarse alpha-alumina and doped with yttrium zirconium coarse powders were ground. In this case, alpha alumina was taken in the form of a finished powder, and yttrium doped zirconia powder was obtained by precipitation from a mixed solution of zirconium oxychloride and yttrium chloride. Finished products were obtained by isostatic hot pressing at temperatures of 1200-1600 ° C.

The main disadvantage of this method is the use of chlorides, and, therefore, the presence in the ceramics of residual chlorine, which negatively affects the performance of the products. In addition, the high sintering temperatures used contribute to grain growth [Edelstein A.S., Cammarata R.C. Nanomaterials, synthesis, properties, applications. Institute of Physics Publishing, Bristol, 1998, 593 p.], Which also negatively affects the mechanical properties of the resulting ceramics.

The closest analogue to the invention is the invention "A method of obtaining products from high strength ceramics" [RU 2636336 C2, publ. 11/22/2017 Bull. No. 33]. In this invention, high-strength ceramics are obtained from a powdery plasma-chemical mixture consisting of nanodispersed powder of cubic and tetragonal zirconium oxide 70-80 wt. %, alumina 15-25 wt. %, yttrium oxide 4-7 wt. % and plasticizer. The mixture is formed by hydrodynamic pressing at a pressure of (0.4-0.6) GPa, a loading duration of 10 ^-2 s, and calcined at a temperature of 900-1100 ° C in a reducing medium.

The disadvantages of this method of obtaining a ceramic composite material are the technical complexity of preparing a plasma-chemical nanosized powder, the need for sintering in a hydrogen medium, a significant variation in the relative density of the samples (67-99.8%), and insufficient compressive strength (not more than 2200 GPa).

The objective of the invention is to obtain a high-strength nanostructured composite ceramic based on oxides of zirconium, aluminum, calcium and silicon, with a high ratio of microhardness and fracture toughness, high density and compressive strength.

The problem is solved due to the fact that in the claimed method for producing composite ceramics, less expensive calcium oxide is used as a stabilizer of the tetragonal phase ZrO ₂ ; sintering of ceramics is carried out at relatively low temperatures (≤1300 ° C), which, firstly, allows to reduce thermostimulated grain growth, and, secondly, reduces the cost of equipment (high-temperature furnaces) and energy consumption; and also an additional component is introduced into the composite - silicon dioxide (at a concentration of 2.08-2.61 wt.%), which provides a reduction in mechanical stresses at the triple joints of ZrO ₂ and Al ₂ O ₃ grains, and, accordingly, increases the ratio of microhardness and viscosity fracture of the material and significantly increasing compressive strength.

By combining zirconia, which has record breaking fracture toughness values for oxide ceramics, and corundum having high hardness, high ratios of hardness and crack resistance of composite ceramics are achieved. This is ensured by the simultaneous action of transformational and dispersion hardening mechanisms.

The transformation mechanism of hardening inherent in zirconia is due to the phase transition of the tetragonal phase t-ZrO ₂ (metastable at room temperature) to the thermodynamically stable monoclinic phase m-ZrO ₂ . Such a transition is accompanied by a change in the specific volume of these phases and the occurrence of compressive mechanical stresses that inhibit the propagation of cracks. To stabilize the tetragonal phase at room temperature, additives (stabilizers) Y ₂ O ₃ , CeO ₂ , MgO, etc. are usually introduced into zirconia. The proposed stabilization of the tetragonal phase t-ZrO _{2 by the} introduction of CaO is less studied, but has significant potential for practical use due to its low cost and the availability of calcium oxide.

The dispersion mechanism of hardening is based on the dissipation of the energy of a propagating crack as a result of its deviation from the initial direction during a “collision” with harder inclusions (particles) of foreign material. An increase in the content of finely dispersed corundum particles in the zirconia matrix enhances the role of the dispersion hardening mechanism of the composite. However, an increase in the content of corundum is equivalent to a decrease in the concentration of zirconia in the composite, which leads to a decrease in the contribution of the transformation mechanism of hardening of the material. The introduction of minor amounts of other additives filling the intergranular space can also contribute to improving the performance of ceramics.

Thus, varying the composition (the ratio of the concentrations of ZrO ₂ and Al ₂ O ₃ , as well as the type and concentration of the stabilizer and other additives) and the structure (due to the technological parameters of grinding, molding and sintering) of the composite ceramic can control its properties (optimize the ratio hardness / crack resistance, increase compressive strength, etc.).

The proposed method for producing high-strength composite ceramics is based on the optimization (in terms of performance and production costs) of the ratio of the concentrations of the main components (ZrO ₂ , Al ₂ O ₃ , CaO) and the added SiO ₂ additive, as well as grinding, molding and sintering parameters . The features of the proposed method are, firstly, the introduction of fine particles of SiO ₂ in the composition of the mixture to obtain ceramics, secondly, the use of CaO as a stabilizer of the tetragonal phase of zirconia, and thirdly, a decrease in the sintering temperature of ceramics.

Nanostructured composite ceramics based on zirconium, aluminum and silicon oxides are prepared as follows. According to Table 1, the components of the composite are mixed in the form of fine powders of synthetic zirconia (89.68-90.17 wt.%), Calcium oxide (2.84-2.85 wt.%), Corundum (4.87 - 4, 9 wt.%) And silicon dioxide (2.08-2.61 wt.%). The resulting mixture of powders is dispersed in distilled water (in a mass ratio of 1: 3) and homogenized using ultrasound. Milling is performed in a planetary mill for 5 hours. Then, the mixture is dried in an oven (in air) at a temperature of T ₀ = 80 ° C for 24 hours. Samples are formed by uniaxial dry pressing at a pressure of 500 MPa for 30 s. Sintering of composite ceramics is carried out in air in a two-stage mode. At the first stage, the samples are heated to a temperature of T ₁ = 1300 ° C. Then, the temperature is reduced to T ₂ = 1200 ° C and maintained for 4 hours. The inventive method allows sintering in air (at a temperature of several hundred degrees lower than traditionally used for sintering composite ceramics based on ZrO ₂ -Al ₂ O ₃ [ J. Fan, T. Lin, F. Hu, Yi Yu, M. Ibrahim, R. Zheng, Sh. Huang, J. Ma, Effect of sintering temperature on microstructure and mechanical properties of zirconia-toughened alumina machinable dental ceramics // Ceramics International. 2017. V. 43. No. 4. P. 3647-3653.]), Which simplifies the technology and reduces the cost of ceramic production.

Example 1

To prepare the initial mixture of finely divided powders in the ratio shown in Table 1 (Example 1), zirconia having an average crystallite size (monoclinic phase) of 50-70 nm was used; corundum having an average crystallite size of 50-70 nm; calcium oxide having an average crystallite size of 50-100 nm and silicon dioxide having an average crystallite size of 20 nm. A mixture of powders was subjected to mixing, grinding and nanostructuring in a Pulverisetter 7 Premium Line planetary mill (Fritsch, Germany) using ZrO ₂ ceramic balls stabilized with magnesium oxide having a diameter of 1.5 mm. In accordance with previous experiments to optimize grinding conditions, the planetary disk rotation speed was 900 rpm.

After grinding, the suspension (together with grinding balls) was dried at the above parameters. The dried powder was separated from the grinding balls using an Analysette 3 Spartan vibration analyzer (Fritsch, Germany) using a sieve with a mesh size of 1 mm.

For molding samples in the form of cylinders, the uniaxial dry pressing method was used according to the method described above. Sintering of composite ceramics was carried out in an air atmosphere in a two-stage mode, with the parameters described above.

To study the mechanical characteristics of the synthesized composite ceramic, samples made in the form of cylinders were subjected to mechanical grinding and polishing. The samples thus prepared were subjected to micro- and nanoindentation using a Duramin-300 automatic math hardness tester (EmcoTest, Austria) and a Nanolndenter G200 nanoindentometer (MTS Nanolnstruments, USA), respectively. To determine the fracture toughness, the expression [Anstis G.R., Chantikul P., Lawn B.R., Marshall D.V. // J. Am. Ceram. Soc. 1981. V. 64. N. 9. P. 533-538.]:

where E is Young's modulus (determined using a Nanolndenter G200 nanoindentometer), H is microhardness (determined using a Duramin-A300 automatic hardness tester), L is the average length of radial cracks near the indent (determined using an inverted metallographic microscope Axio Observer Aim, Carl Zeiss , Germany with an image analyzer Structure 5.0, Videotest, Russia), k is the empirical calibration coefficient (k = 0.016 ± 0.004 [Anstis GR, Chantikul P., Lawn BR, Marshall DB // J. Am. Ceram. Soc. 1981. V. 64. N. 9. P. 533-538.])

To determine the compressive strength, parallelepipeds with a cross section of 2 × 2 mm were cut from cylinders. Loading of the samples (recording of the σ (ε) diagram) was carried out on the basis of the two-column servo-hydraulic testing machine MTS 870 Landmark (USA). Information on the phase composition (fraction of the monoclinic, tetragonal, and cubic phases of zirconia) was obtained using a D2 Phaser powder X-ray diffractometer (Bruker AXS, Germany). The structure was studied and elemental mapping was performed using a Merlin high-resolution scanning electron microscope (Carl Zeiss, Germany). In addition, for each type of sample, the porosity of P was determined by the standard formula:

Here ρ _t is the theoretical density of the composite (taking into account the content of corundum), ρ _v is the density of the composite, measured by hydrostatic weighing.

The main test characteristics of the obtained composite ceramics are presented in Table 2. A typical σ (ε) diagram recorded during the compression test is presented in Fig. 1. It is seen that with the introduction of silicon dioxide (2.08 wt.%) In the composition of composite ceramics (CaO) ZrO ₂ - Al ₂ O ₃ , the compressive strength reaches σ _m = 2.44 GPa, which exceeds the data obtained on ceramics synthesized according to the method selected for the prototype.

Example 2. For the manufacture of composite ceramics used a mixture of components in the concentration ratio shown in Table 1 (Example 2). The parameters of grinding, drying, compacting and sintering, as well as the tested characteristics and research equipment were identical to Example 1.

As follows from those presented in Table 2 and Fig. 1 data, an increase in the concentration of silicon dioxide in the mixture of components to 2.61 weight. %, leads to a decrease in porosity and an increase in the performance of composite ceramics. In this case, a simultaneous increase in microhardness (H = 10.9 GPa) and fracture toughness (K _C = 12.43 GPa m ^1/2 ) deserves special attention. The compressive strength at this ratio of components reaches σ _m = 2.73 GPa.

The discovered improvement in the mechanical properties of ceramics (sintered at temperatures T≤1300 ° C) when combining zirconia stabilized with calcium oxide, corundum and zirconia is the result of several hardening mechanisms:

1) the transformation hardening inherent in zirconia stabilized at room temperature in the tetragonal phase;

2) dispersion hardening associated with the distribution of fine particles of a harder material (corundum) over the matrix of the base material (zirconium dioxide);

3) hardening associated with a decrease in grain size, which is achieved due to ultrafine grinding and low sintering temperatures;

4) reduction of mechanical stresses at the joints of ZrO ₂ and Al ₂ O ₃ grains, achieved by “enveloping” ZrO ₂ and Al ₂ O ₃ crystallites with an ultrathin silicon dioxide film.

The action of these mechanisms is indirectly confirmed by X-ray diffractometry, scanning electron microscopy and elemental mapping. So, when the content of silicon dioxide in the composite is from 2.08 weight. % to 2.61 weight. % the proportion of the tetragonal phase of t-ZrO ₂ is at the level of 71-73%, which provides the transformation mechanism of hardening. The SEM image of the cleaved composite ceramic (with the ratio of components corresponding to Example 2) and elemental mapping (Fig. 2) clearly demonstrate the uniform distribution of corundum crystallites over the zirconia matrix, which confirms the effect of the dispersion hardening mechanism. According to the analysis of the SEM image (Fig. 2, a ), the average crystallite size of ZrO ₂ does not exceed 120 nm, which is in favor of the third of the indicated strengthening mechanisms. Finally, elemental mapping (Fig. 2f) indicates a uniform distribution of SiO ₂ over the cleaved surface, which confirms the possibility of the fourth hardening mechanism.

An increase in the content of silicon dioxide in composite ceramics (up to 7% or more) leads to a sharp decrease in fracture toughness and compressive strength. Thus, the optimized concentration ratios of the components (Example 1 and Example 2), as well as grinding, drying, pressing and sintering parameters, provide conditions for producing composite ceramics with higher density and compressive strength than the analogue, as well as a high ratio microhardness and fracture toughness.

The technical result of the claimed development is an economical method for producing a ceramic composite material based on zirconium, aluminum, calcium, and silicon oxides, which makes it possible to obtain nanostructured, high-density ceramics with an increased compressive strength with a high ratio of microhardness and fracture toughness. Due to the high strength characteristics obtained by the claimed method, composite ceramics can be used for the manufacture of products subjected to intensive mechanical loads during operation (dies, dies, bearings, rollers, grinding balls, etc.).

Claims (1)

A method of producing a nanostructured composite ceramic, characterized in that the mixture of nanosized powders of synthetic zirconia (89.68-90.17 wt.%), Calcium oxide (2.84-2.85 wt.%), Corundum (4.87 -4.9 wt.%) And silicon dioxide (2.08-2.61 wt.%) Are subjected to wet grinding in a planetary mill (1: 3 ratio of mixture to distilled water) and drying in an oven (in air) at a temperature T ₀ = 80 ° C for 24 hours; the samples are molded by uniaxial dry pressing at a pressure of 500 MPa for 30 s; sintering is carried out in air in a two-stage mode: first, the samples are heated to a temperature of T ₁ = 1300 ° C, then the temperature is reduced to T ₂ = 1200 ° C and held for 4 hours.

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