RU2584992C1 - Method of producing alumina structural ceramic - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to technology of high-temperature structural ceramic materials with improved thermomechanical properties and can be used as thermal generating units lining, heat-resistant ammunition, shockproof protection elements. For making structural alumina ceramic initial aluminium alloy with silicon (10-14 WT%) is treated with aqueous solution of lithium hydroxide with concentration of 12-20 % at heat sink from reaction volume by coolant with temperature of 25-45 °C. Then sediment is separated from mother solution of aluminium hydroxide with inclusions of lithium metasilicate, its washing is carried out to pH value of medium of 8.2-8.6. Sediment is dried, heat treated in air at 1,250-1,350 °C for 30-60 minutes. Produced cake is milled and charge is prepared. Articles are pressed under pressure of 150-200 MPa and sintered in air at 1,450-1,550 °C for 40-60 minutes.

EFFECT: increased strength and density of material.

1 cl, 3 dwg, 3 ex, 1 tbl

Description

The invention relates to ceramic materials for structural purposes and can be used for the manufacture of durable products used as bearings, thread guides, ball valves in devices for pumping suspensions, as well as parts of paper machines.

A known method of producing structural alumina ceramics [1], including the molding of a raw billet, removing organic binder from it, followed by installation in a graphite mold, hot pressing (at a temperature of 0.5-0.7 of the melting point of Al ₂ O ₃ ), removal of the product from the mold and its diamond processing.

The disadvantage of this method is the unsatisfactory wear resistance of the resulting material due to chipping of the largest grains (formed due to recrystallization) from the surface of the working layer when interacting with a solid counterbody as a result of shear stresses.

The basis for the development of ceramic material with improved service properties was taken as a method for producing structural alumina ceramics [2] (adopted as a prototype), including processing an aluminum alloy (Al-Si _{10-14% wt.} ) With an aqueous solution of caustic soda during heat removal from the reaction volume with refrigerant , isolation of the precipitate formed from the mother liquor (aluminum hydroxide with sodium metasilicate inclusions), washing it with water (up to pH 8-9), drying, heat treatment in air (1280-1350 ° С, 1-3 hours), preparation from This product is a mixture, pressing and sintering in air of pressed blanks (1450-1500 ° C, 1-2 hours).

According to this method, an alumina material is obtained containing nepheline (25-27% v / v) uniformly distributed over the surface of α-Al ₂ O ₃ grains in the form of thin interlayers, the thickness of which relates to the micron size range.

The strength of such a material increases due to the presence of thin layers of nepheline, which are evenly distributed over the surface of α-Al ₂ O ₃ grains, forming a strong frame in the form of a grid.

However, the strength of the material obtained by the prototype method is insufficient due to its relatively low density and porosity, which correlate with each other and accordingly affect the strength.

The technical task of this invention is to increase the density and strength of the resulting material.

To solve the technical problem of the invention in a method for producing structural alumina ceramics, an aluminum alloy with silicon, containing in the amount of 10-14% wt., Taken in the form of chips from machining, with an aqueous solution of lithium hydroxide with a concentration of 12-20% at a heat sink is first processed. from the reaction volume with a refrigerant with a temperature of 25-45 ° C. Then, a precipitate of aluminum hydroxide with inclusions of lithium metasilicate is isolated from the mother liquor. After that, the precipitate is washed to a pH of 8.2-8.6. Then the precipitate is dried in air at a temperature of 80-100 ° C. Then conduct the heat treatment of the obtained precipitate in air at a temperature of 1200-1300 ° C for 30-60 minutes. To prepare the mixture in the crushed product obtained after heat treatment of the precipitate, add a 7-10% solution of polyvinyl alcohol in an amount of 5-7%, calculated on the dry residue of the substance. Then the blanks are molded (pressed) under a pressure of 150-200 MPa, and the sintering of pressed blanks is carried out at a temperature of 1450-1550 ° C for 40-60 minutes.

The invention is illustrated by the following figures:

FIG. 1. The surface view of the ceramic material after impact bending. The length of the tag line is 5 microns.

FIG. 2. The surface view of the ceramic material after impact bending. The length of the tag line is 5 microns.

FIG. 3. The surface view of the ceramic material after impact bending. The length of the tag line is 2 microns.

To obtain the material according to the claimed method, an alloy of aluminum with silicon was used as a feedstock (the ionic radius of Si is 20% less than the ionic radius of Al). As a result of its treatment with an aqueous solution of lithium hydroxide, according to reaction 1, a mixture of aluminum hydroxide with lithium metasilicate is formed, which precipitates as a precipitate, a solution of lithium aluminate and hydrogen:

The reaction products are: precipitate - a mixture of complex aluminum-lithium hydroxide of variable stoichiometric composition (x = 0.5-1; y = 0.5-1; n = 2.5-3) with lithium metasilicate, a solution of lithium aluminate and hydrogen (sediment composition was established by x-ray phase and petrographic analysis).

The precipitate was isolated from the mother liquor, washed, dried, heat treated in air (1200-1300 ° C) to form α-Al ₂ O ₃ , aluminum-lithium spinel cubic syngony:

The precipitate obtained was isolated from the mother liquor, washed, dried, and heat treated in air (1200–1300 ° C) to form the dominant alumina phase (Al ₂ O ₃ ), according to reaction 2, while the lithium metasilicate (Li ₂ SiO ₃ ) was melted (T _pl = 1201 ° C), according to reaction 3, uniformly covering the grains of aluminum oxide, and was saturated with aluminum ions with the formation of a melt of eucryptite LiAlSiO ₄ (T _pl = 1423 ° C).

After cooling, the resulting product was a cake. A mixture for pressing products was obtained from crushed cake. During the sintering of the compacts (1450–1550 ° C), the formation of a melt of eucryptite also took place, the melting point of which is lower than the sintering temperature (eucryptite was recorded in the sintered material by the XRD method - 27% vol). Thus, there was a sintering involving the liquid phase. When the sintered products are cooled, the eucriptite melt crystallizes in the trigonal syngony into a regular triangle, and α-Al ₂ O ₃ crystallizes in the hexagonal syngony with the corresponding crystal structure.

In accordance with the claimed method used an alloy of aluminum with silicon, taken in the form of chips. Alloy shavings were obtained during machining (turning and planing, drilling, milling). In this case, the amount of silicon (C ₁ ) in the alloy with aluminum should not exceed 14% wt. and be less than 10 wt.%, since it is in this range of values of C _{1 that the} desired phase composition of the sintered material is achieved. If C _{1 is} more than 14 wt.%, Then silicon dioxide and mullite are fixed in the sintered material with excellent thermal expansion coefficients (from alumina), which led to its cracking. The decrease With ₁ less than 10% wt. did not provide the formation of eucryptitis.

The alloy was treated with an aqueous solution of lithium hydroxide with a given concentration (C ₂ ). A decrease in C _{2 of} less than 12% is not advisable, since the reaction is almost completely stopped according to reaction (1). An increase in C _{2 of} more than 20% is also not advisable, since a sharp heating occurred with increasing concentration in the reaction volume, since the reaction is exothermic and the solution began to boil, which led to the ejection of reaction products 1 (in this case, there was no improvement in the physicomechanical properties of sintered material).

Since reaction (1) is exothermic, it is necessary to remove heat from the reaction volume in order to avoid sharp boiling and evaporation of water, leading to the release of reaction products. Reduction of the coolant temperature (T ₁₎ less than 25 ° C led to a decrease in the chemical reaction rate and productivity of the process. An increase in T ₁ above 45 ° C led to an increase in the size of the crystals of aluminum hydroxide in the precipitate, which negatively affected the mechanical properties of ceramics.

The washing of the precipitate of aluminum hydroxide with inclusions of lithium metasilicate was carried out to a pH of the medium in the range of 8.2-8.6. In this range of the pH indicator (weakly alkaline medium), a certain additional amount of Li ⁺ ions participating in the synthesis of eucryptite remains in the composition of the precipitate. An increase in pH of more than 8.6 is impossible, since it leads to the synthesis of lithium aluminate in the sintered material with a loss of strength properties. A decrease in pH of less than 8.2 led to the polymerization of the precipitate with the formation of a gel, the washing of which by vacuum filtration becomes impossible.

The precipitate is dried in air at a temperature of 80-100 ° C. At the same time, it is not advisable to dry the precipitate at a temperature (T ₂ ) below 80 ° C due to a significant increase in the time required to achieve zero humidity. The increase in T ₂ more than 100 ° C is also not advisable, since at T _{2 is} achieved quick drying without thermal decomposition of the hydroxide.

Heat treatment of the precipitate is carried out in air at a temperature (T ₃ ) and isothermal exposure time (τ ₁ ) of at least 1200 ° C and 30 minutes, also not more than 1300 ° C and 60 minutes, respectively. In this case, a decrease in T ₃ and τ ₁ below the indicated values leads to an extremely high shrinkage of the ceramic during sintering and, as a consequence, to its possible deformation and destruction. An increase in T ₃ and τ ₁ above the indicated values is not advisable due to the loss of activity of the powder to sintering and the inability to achieve a high level of mechanical properties of ceramics.

A concentration of an aqueous solution of polyvinyl alcohol (C ₃ ) of less than 7% is not advisable, since otherwise a larger volume of the introduced binder solution is required and the drying time of the charge increases. An increase in C _{3 of} more than 10% leads to an increase in the viscosity of the solution and the deterioration of the conditions of mixing with the powder during the preparation of the charge.

The amount of polyvinyl alcohol (C ₄ ) in the charge of less than 5% does not provide high-quality pressing: cracking is observed due to insufficient plasticity of the powder mixture with the organic binder. An increase in C _{4 of} more than 7% is not advisable, since there is a decrease in the mechanical properties of the sintered material.

Pressing pressure (P) less than 150 MPa led to a decrease in the mechanical properties of ceramics, an increase in P, more than 200 MPa, leads to the effect of the formation of “re-pressing” cracks.

Sintering of the material is carried out at a temperature (T ₄ ) and an isothermal holding time (τ ₂ ) of at least 1450 ° C and 40 minutes, as well as no more than 1550 ° C and 60, respectively. Reduction T ₄ and τ ₂ less than 1450 ° C and 40 minutes resulted in a drop in the mechanical properties of the material due to partial completion of the sintering process, and the increase in these parameters over 1,550 ° C, and 60 minutes also led to a decrease in strength properties due to the large recrystallization grains of corundum.

In this technical solution, the presence of a positive effect is explained by the formation of a special material structure (Fig. 1) due to the zonal sintering of a highly dispersed powder billet with the participation of the liquid phase - the eucriptite melt. The result of zonal sintering is the bimodal character of the size distribution of pores in sintered ceramics (submicron intragranular pores and intergranular pores of 1-10 μm are observed, the grain size is comparable to the sizes of agglomerates obtained after heat treatment of aluminum hydroxide (Fig. 2). Alumina grains are sintered during sintering (lamellar form) are covered with a melt of eucryptit (Fig. 3), which spreads over their surface and forms triangular grains of the correct form. It also helps to reduce the growth of alumina oxide grains I am due to collective recrystallization (the grain-boundary phase lowers the surface energy of grains). When the sintered ceramics are cooled, crystallization of the eucriptite melt occurs, closing the pores inside the grains and from the surface of the product (closed porosity - 11% with a total porosity of 12-13% and open porosity - 1-2 %, see table.) The resulting closed pore space significantly contributes to the dissipation of impact energy during mechanical and thermal loading (in contrast to the material obtained by the prototype method, with total porosity of 25-27%). Moreover, the presence of layers of eucryptite between alumina grains (the adhesive type of bond between α-Al ₂ O ₃ and LiAlSiO ₄ ) provides a sufficiently high strength with a porosity of 12-13%.

Thus, the technical task of the present invention has been achieved - an increase in density and strength under impact impact of the load is achieved, at a sufficiently low sintering temperature.

Examples of the implementation of the claimed method.

Example 1. 100 grams of an alloy of aluminum with silicon (amount of silicon C ₁ = 10% wt.) In the form of chips was loaded into a glass flask made of heat-resistant glass, placed in water, acting as a refrigerant. The temperature of the refrigerant was kept constant (T ₁ = 25 ° C) using a thermostat with an accuracy of ± 2 ° C. 2000 cm ^{3 of a} 12% aqueous solution of alkali LiOH with a concentration of C ₂ = 12% was poured into a flask with shavings of an aluminum alloy to completely dissolve the alloy. Dissolution of the alloy, or, in other words, its chemical dispersion, was carried out with continuous stirring with a glass propeller stirrer. After chemical dispersion was completed, a precipitate was isolated from the mother liquor by vacuum filtration — a mixture of aluminum hydroxide and lithium metasilicate. This precipitate was repeatedly washed with distilled water and pumping liquid (also by vacuum filtration). The final pH recorded for the pumped liquid was 8.2. The resulting precipitate was dried in air to zero humidity at a temperature of T ₂ equal to 80 ° C. After that, it was placed in a corundum container and heat treated in air at a temperature of T ₃ equal to 1200 ° C for a time τ ₁ equal to 30 minutes. The obtained cake was crushed and an aqueous solution of polyvinyl alcohol (PVA) was introduced into it with a concentration of C ₃ equal to 10%, in terms of the PVA content on a dry residue - C ₄ equal to 5% wt. The dried mixture was a mixture from which samples were pressed (raw), applying a pressure P equal to 150 MPa. After burning a temporary organic binder - PVA (300 ° C, 1 hour, in air) from raw, it was sintered in air at a temperature of T ₄ and an isothermal exposure time of τ ₂ equal to 1450 ° C and 40 minutes, respectively.

According to the XRD data, the phase composition of the ceramic is represented by α-Al ₂ O ₃ (76% vol.) And LiAlSiO ₄ (24% vol.).

Example 2. All technological operations coincide with those described in example 1.

100 grams of an alloy of aluminum with silicon in the form of chips (C ₁ = 12% wt.) Was treated with an aqueous solution of sodium hydroxide (C ₂ = 16%) at a refrigerant temperature T ₁ = 35 ° C. After the chemical dispersion of the alloy was completed, a precipitate was filtered out of the mother liquor — a mixture of aluminum hydroxide and lithium metasilicate, which was washed with distilled water to a final pH of 8.4.

The obtained precipitate was dried in air to zero humidity (T ₂ = 90 ° C) and heat treated in air at the following temperature-time parameters: T ₃ = 1250 ° C, τ ₁ = 45 minutes.

To prepare the mixture used an aqueous solution of PVA (C ₃ = 8.5%, C ₄ = 6% wt).

The raw material was pressed at P = 175 MPa after burning an organic binder from it, sintering was carried out in air at the following temperature and time parameters: T ₄ = 1500 ° C, τ ₂ = 50 minutes.

According to XRD data, the phase composition of the ceramic is represented by α-Al ₂ O ₃ (74% vol.) And LiAlSiO ₄ (26% vol.).

Example 3. All technological operations coincide with those described in example 1.

100 grams of an alloy of aluminum with silicon (C ₁ = 14% wt.) In the form of chips was treated with an aqueous solution of lithium hydroxide (C ₂ = 20%) at a refrigerant temperature T ₁ = 45 ° C. After the chemical dispersion of the alloy was completed, a precipitate was filtered out of the mother liquor — a mixture of aluminum hydroxide and lithium metasilicate, which was washed with distilled water to a final pH of 8.6.

The obtained precipitate was dried in air to zero humidity (T ₂ = 100 ° C) and heat treated in air at the following temperature-time parameters: T ₃ = 1300 ° C, τ ₁ = 60 minutes.

To prepare the mixture used an aqueous solution of PVA (C ₃ = 7%, C ₄ = 7% wt.).

Compressing raw produced at P = 200 MPa, after burning out the organic binder therefrom sintering was performed in air at the following time-temperature parameters: T = ₄ 1 550 ° C, τ ₂ = 60 minutes.

According to the XRD data, the phase composition of the ceramic is represented by α-Al ₂ O ₃ (72% vol.) And LiAlSiO ₄ (28% vol.).

The test results of the material obtained in accordance with the claimed method, in comparison with the material manufactured by the prototype method, are shown in the table.

Bending strength was determined on prismatic samples (7 × 8 × 50, mm) using a three-point loading scheme (TIRATEST-2300 testing machine), and impact bending strength was determined on prismatic samples (7 × 8 × 50, mm) using a pendulum ram [3].

From the above data it is seen that the material obtained by the proposed method has a higher density and impact bending strength compared to the material made by the prototype method. I would like to note a significant reduction in sintering time (40-60 minutes - according to the claimed method, 60-180 - according to the prototype).

Information sources

1. RF patent No. 2461530, publ. 09/20/2012, bull. No. 26. A method of obtaining a composite material Al ₂ O ₃ -Al; C04B 35/65, 35/117, B22F 3/23.

2. RF patent No. 2453517, publ. 06/20/2012, A method for producing structural alumina ceramics; C04B 35/111, C04B 35/626, C01F 7/42, (prototype).

3. Workshop on the technology of ceramics and refractories / Ed. Poluboyarinova D.N. and Popilsky R.Ya. M .: Publishing house of literature on construction, 1972, 352 p.

Claims (1)

A method for producing structural alumina ceramics, including treating an aluminum alloy with a water-containing reagent, isolating the precipitate formed from the mother liquor, washing, drying, heat-treating it, preparing the mixture from the resulting product, molding and sintering the molded billets, characterized in that the chips are made of an aluminum alloy with silicon, contained in an amount of 10-14 wt.%, which is treated with an aqueous solution of lithium hydroxide with a concentration of 12-20% with heat removal from the reaction volume of the refrigerant volume with a temperature of 25-45 ° C, aluminum hydroxide precipitate with lithium metasilicate inclusions is isolated from the mother liquor, the precipitate is washed to a pH of 8.2-8.6, the precipitate is dried in air at a temperature of 80-100 ° C, heat treatment the precipitate is carried out in air at a temperature of 1250-1350 ° C for 30-60 minutes, to prepare the mixture in the crushed product obtained after heat treatment of the precipitate, add 7-10% solution of polyvinyl alcohol in an amount of 5-7% in terms of dry the remainder of the substance, the molding of blanks is carried out by means of presses Nia pressure of 150-200 MPa and sintering in air at 1450 -1550 ° C for 40-60 minutes.

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