RU2699641C1 - Method of making articles from ultra-fine grained silicon graphite - Google Patents

Abstract

FIELD: chemistry; metallurgy.

SUBSTANCE: invention is intended for chemical and metallurgical industry and can be used in manufacturing of bearings, seals and facing plates. First, press mass is prepared on the basis of graphite powder of 5–20 mcm fraction mixed with technical carbon, including nanosized, and polysiloxane or polysilazane binder. Then billet is formed by pressing and polymerisation of binder. Obtained workpiece is fired at 1000–1200 °C, respectively in vacuum or at atmospheric pressure in argon medium and at 1500–1550 °C and pressure in the reactor is not more than 36 mm Hg. After that silicizing is carried out by vapour-liquid-phase method at mass transfer of silicon into pores of material by mechanism of capillary condensation of its vapours at heating from 1300 to 1500 °C at reactor pressure of not more than 36 mm Hg. with subsequent exposure at temperature 1550–1600 °C for 1–2 hours and cooling in silicon vapour. Workpiece billet annealing and its sieving can be performed in single technological process. Billet annealing based on polysiloxane binder can be carried out at atmospheric pressure with subsequent heating to 1300 °C. Before mass transfer of silicon into pores of workpiece material based on polysilazane binder can be performed its heating to 1500–1550 °C and intermediate cooling up to 1300 °C at temperature higher than that of silicon vapour.

EFFECT: improving operational characteristics of items from siliconized graphite, as well as reproducibility of obtained results.

4 cl, 1 tbl, 6 ex

Description

The invention relates to the production of structural products based on carbon and graphite, in particular, siliconized graphite, intended for use in thrust and thrust bearings, plain bearings, mechanical seals of pumps pumping various liquids, including with abrasive particles, in the manufacture of cladding plates in the chemical and metallurgical industries, in the production of glass and mineral fibers, etc.

A known method of manufacturing siliconized graphite, including the preparation of the press mass on the basis of powders of graphite and silicon carbide and a polymeric binder, molding the workpiece by pressing the press mass and polymerization of the binder, firing the workpiece and siliconizing. In accordance with it, a graphite powder of a fraction of not more than 200 μm and a powder of silicon carbide of a finer fraction are used in the press composition [US Pat. RU No. 2370435, 2009].

The disadvantage of this method is that the resulting siliconized graphite contains relatively much free silicon, which leads to a decrease in its corrosion resistance in aggressive environments (alkalis and acids). In addition, graphite silicified by this method has an insufficiently homogeneous structure, relatively high open porosity, and insufficiently high mechanical strength. The consequence of this is the insufficiently high efficiency of products made of this type of siliconized graphite, primarily in chemically aggressive environments. Another disadvantage of this method is the use in the preparation of the press mass of more expensive than graphite, silicon carbide.

The closest to the claimed technical essence and the achieved effect is a method of manufacturing products from fine-grained siliconized graphite, including the preparation of a press mass based on graphite powder and a polymeric binder, molding a workpiece by pressing a press mass and polymerizing a binder, firing the obtained preform and siliconizing it [ O.Yu. Sorokin, I.A. Bubnenkov et al. "Development of fine-grained siliconized graphite with improved properties" / University Bulletin. ser. Chemistry and Chemical Technology, 2012, vol. 55, no. 6, p. 12-16]. In accordance with it, in the preparation of the press mass, a polymer binder and graphite powder with a degree of graphitization> 0.62 with a particle size of 30-100 μm are used, and the workpiece is silicified (after the firing operation) by the liquid-phase method by impregnation with silicon melt.

The method provides the possibility of manufacturing products from siliconized graphite with higher work efficiency, including in chemically aggressive environments by reducing the content of free silicon in it, increasing the uniformity of the material and its mechanical strength. In addition, silicon carbide powder is not used in its manufacture, which makes it possible to cheapen its production.

Despite the increase in the operational characteristics of products of siliconized graphite obtained in comparison with the analogue method, they could be even higher. In addition, the disadvantage of this method is its relatively low reproducibility and the dependence of the results of silicification on the type of graphite, which limits the possibilities of the method.

The objective of the invention is to increase the operational characteristics of products made of silicon graphite, increasing the reproducibility of the results and expanding the technological capabilities of the method.

The problem is solved due to the fact that in the method of manufacturing products from ultrafine-grained siliconized graphite, including the preparation of a press mass on the basis of graphite powder and a polymeric binder, molding a preform by pressing and polymerizing a binder, firing the obtained preform and siliconizing it, in accordance with the claimed technical the solution for the preparation of the press mass using graphite powder fractions 5-20 microns in a mixture with carbon black, including nanosized, and polysiloxane f or polysilazane binder, the firing of the obtained preform is carried out, respectively, at 1000-1200 ° C in vacuum or at atmospheric pressure in argon and at 1500-1500 ° C and a pressure in the reactor of not more than 36 mm Hg, and siliconization is carried out with steam by the liquid-phase method during the mass transfer of silicon into the pores of the material by the mechanism of capillary condensation of its vapor when heated from 1300 to 1550 ° C at a pressure in the reactor of not more than 36 mm Hg followed by exposure at a temperature of 1550-1600 ° C for 1-2 hours and cooling in silicon vapor.

The solution to the problem also contributes to the fact that:

a) firing the workpiece and its silicification is carried out in a single technological process.

b) firing a preform based on a polysiloxane binder is carried out at atmospheric pressure, followed by heating to 1300 ° C;

c) before mass transfer of silicon to the pores of the polysilazane-based billet material, it is heated to 1500-1550 ° C and intermediate cooled to 1300 ° C at a temperature higher than the temperature of silicon vapors;

The use of a fraction of 5-20 microns in a mixture with carbon black, including nanosized, for the preparation of a press mass of graphite powder creates the prerequisites for obtaining (after firing) a preform material (carbon base for siliconizing) of a finely porous structure.

Use for preparing a press mass of a polysiloxane or polysilazane binder in combination with the firing of the molded preform, respectively, at 1000-1200 ° C in vacuum (or at atmospheric pressure) and at 1500-1550 ° C and a pressure in the reactor of not more than 36 mm Hg It allows to obtain a workpiece material (carbon base for silicification) not only of a finely porous structure (the prerequisites for which are created by the aforementioned feature), but also mainly with open porosity.

In addition, this makes it possible to transfer carbon black, primarily nanosized, to silicon carbide, and on carbide particles of a fraction of 5-20 μm, to form a silicon carbide coating. This is due to the fact that with the claimed technical parameters of firing a workpiece based on a polysiloxane binder, SiC is formed by the reactions:

что можно выразить суммарной реакцией

what can be expressed by the total reaction

The formation of SiC by reaction (3) is accompanied by the shrinkage of the polymer matrix and the release of CO, as a result of which open porosity forms in the material.

At temperatures below 1000 ° C, the formation of SiC does not end.

Carrying out firing at a temperature of more than 1200 ° C leads to unreasonable complication of the method, because at 1200 ° С, the SiC formation process is already completed.

With the claimed technological parameters of firing a workpiece based on a polysilazane binder, the following occurs. The product of polysilazane pyrolysis in the presence of carbon is silicon carbide. The process proceeds in stages, namely: first, during the pyrolysis of polysilazane, silicon carbonitride is formed (Si _3-x C _{x + y} N ₄ ), which decomposes at a temperature of 1440 ° C by the reaction:

The resulting silicon nitride (Si ₃ N ₄ ) at a temperature of 1500-1550 ° C in vacuum, as established by us experimentally, is reduced by carbon by the reaction:

The formation of SiC from polysilazane is accompanied by shrinkage of the polymer matrix and the release of volatiles, to which nitrogen evolution is also added by reaction (5). As a result, a material with high open porosity is formed.

With increasing pressure in the reactor, the temperature of reduction of silicon nitride by carbon is shifted to the region of higher temperatures. So, at atmospheric pressure, the reduction of silicon nitride by carbon occurs even at temperatures above 1600 ° C.

This leads to unreasonable complication of the firing process.

The features considered above, taken together, make it possible to obtain a carbon base for silicification with an ultrafine porous structure (with its high open porosity). Another distinctive feature of the structure is that it is formed by ultrafine silicon carbide located in larger pores, which in turn are formed by particles of graphite fraction 5-20 microns, having a silicon carbide coating. Thus, conditions are created for limiting the amount of silicon entering into each individual pore and introducing it into the pores by the mechanism of capillary condensation of its vapor. Conditions are also being created to facilitate the filling of ultrafine pores with silicon, since they are formed and covered with silicon carbide, which eliminates the rapid reaction between silicon and carbon in the form of graphite, which may result in partial blocking of the mouths of the transport pores, as well as interruption of the condensation of silicon vapor due to a local increase in the temperature of the workpiece.

In addition, conditions are created for use in the fraction with a particle size of 5-20 μm graphite with both low and high chemical activity to silicon, because when they react with SiO ₂ or Si ₃ N ₄ , a silicon carbide coating is formed on them.

Siliconization by the vapor-liquid-phase method during mass transfer of silicon into the pores of the material by the mechanism of capillary condensation of its vapor in the range of 1300-1500 ° C at a pressure in the reactor of not more than 36 mm Hg even ultra-thin pores (less than 3-5 microns in size) can be filled with silicon, which is also facilitated by the fact that the pores are covered with silicon carbide.

At temperatures below 1300 ° C, there is a likelihood of the formation of silicon vapor on the surface of the solid condensate blank, the melt of which does not penetrate into pores smaller than 3-5 microns. The implementation of mass transfer of silicon into the pores of the material at temperatures above 1500 ° C, leads to unreasonable complication of the method. In addition, the temperature approaches the holding temperature of 1550-1600 ° C. Mass transfer of silicon into the pores of the material by the mechanism of capillary condensation of its vapor at a pressure in the reactor of more than 36 mm Hg not possible due to a significant decrease in the rate of evaporation of silicon.

Holding the workpiece at a temperature of 1550-1600 ° C for 1-2 hours allows you to complete the carbidization of the smallest graphite particles that were not completely carbidized during their chemical reaction with SiO ₂ or Si ₃ N ₄ , and create only a carbide shell silicon on the surface of larger particles of graphite, on which a thin silicon carbide coating was previously formed.

At temperatures below 1550 ° C and a holding time of less than 1 hour, graphite particles of a fraction of 5-20 μm, having silicon carbide coating after interaction with SiO ₂ or Si ₃ M ₄ , are insufficiently carbidized from the surface, as a result of which the sintering of the material decreases, which means Its mechanical strength. In addition, it increases the content of free silicon.

At temperatures above 1600 ° C, a holding time of more than 2 hours, graphite particles of a fraction of 5-20 μm, despite the presence of a silicon carbide coating on them, are carbidized from the surface to a considerable depth, up to complete carbidization, resulting in a significant decrease in graphite content and an increase in the unevenness of its distribution in ultrafine-grained siliconized graphite.

In addition, holding exposure at temperatures above 1600 ° C leads to unreasonable complication of the method.

Carrying out cooling of the preform (after holding it at 1500-1600 ° C) in silicon vapors in the absence of a difference between the temperature of silicon vapors and the preform allows, on the one hand, to fill the open pores of the material (obtained after this exposure) with free silicon and thereby reduce its permeability , on the other hand, allows to minimize the formation of condensate sag of silicon vapor on the surface of the workpiece.

Carrying out (in a preferred embodiment of the method) firing the workpiece and its silicification in a single technological process allows to simplify the method. In this case, firing a preform based on a polysiloxane binder at atmospheric pressure (at 1000-1200 ° C) with subsequent heating to 1300 ° C allows you to save mass transfer of silicon vapor to the siliconized preform in the range of 1300-1500 ° C. Otherwise (i.e., at a high diffusion rate of CO in vacuum), partial carbidization of silicon particles loaded into crucibles occurs, and, as a result, blocking of silicon vapor in SiC capsules, as a result of which mass transfer of silicon to the workpiece becomes impossible the range of 1300-1500 ° C.

Carrying out mass transfer of silicon into the pores of the workpiece material (during firing of the workpiece based on a polysilazane binder and silicification in a single technological process) heating it to 1500-1550 ° C and intermediate cooling to 1300 ° C at a temperature exceeding the temperature of silicon vapors allows you to save the beginning mass transfer of silicon into the pores of the material from 1300 ° C, and not from higher temperatures, and thereby ensure the filling of pores with silicon, starting with the smallest.

The method is as follows.

On the basis of graphite powder and a polymer binder, a press mass is prepared. At the same time, graphite powder of a fraction of 5–20 μm mixed with carbon black, including nanosized and polysiloxane or polysilazane binder, is used for its preparation.

Then, a preform is formed from the press mass by pressing and polymerizing the binder. After this, the resulting billet is fired. In this case, the firing of a preform based on a polysiloxane binder is carried out at 1000-1200 ° C in vacuum or at atmospheric pressure in an argon atmosphere, and the firing of a preform based on a polysiloxane binder is carried out at 1500-1550 ° C and a pressure in the reactor of not more than 36 mm Hg.

The billet obtained after firing is siliconized by the vapor-liquid-phase method during mass transfer of silicon into the pores of the billet material by the mechanism of capillary condensation of its vapor during heating from 1300 to 1550 ° C, followed by exposure at a temperature of 1550-1600 ° C for 1-2 hours and cooling in silicon vapors.

In a preferred embodiment of the method, the firing of the preform and its silicification are carried out in a single technological process.

In this case, the firing of the preform based on the polysilaxane binder is carried out at atmospheric pressure, followed by heating to 1300 ° C.

In this case, before mass transfer of silicon to the pores of the workpiece based on a polysilazane binder, heating is carried out to 1500-1550 ° C and intermediate cooling to 1300 ° C at a temperature exceeding the temperature of silicon vapors.

The following are examples of specific implementation of the method.

In all examples, the manufactured product was a ∅100 × ∅80 × h 20 mm ring.

Example 1

The press mass was prepared on the basis of a mixture of graphite powder of a fraction of 5-20 μm with carbon black (soot) of the P-514 grade in the form of granules 1-5 μm in size, consisting of particles of ~ 50 nm in size.

The carbon black content was 15% of the particle volume of graphite powder fraction 5-20 microns.

A solution of organosilicon varnish of the brand KO-916 (belonging to the type of siloxane binders) in toluene with a nominal viscosity of 20 seconds was used as a polymer binder.

A press mass was prepared by mixing a mixture of these powders and a binder in a paddle mixer.

Then the resulting mass was unloaded and dried in air in pallets to reduce the solvent content in it. The dried mass was pressed through a sieve with a mesh size of 3 × 3 mm in order to obtain granules on its basis. The estimated number of granules was loaded into the mold. The preform was formed under a pressure of 20 MPa at a final temperature of 160 ° C. Thus, the blank was formed by the so-called semi-dry method.

Then, the molded billet was fired by heat treatment at atmospheric pressure in an argon atmosphere at a temperature of 1200 ° C.

The resulting preform was siliconized by the vapor-liquid-phase method during mass transfer of silicon into the pores of the material by the mechanism of capillary condensation of its vapor (carried out by heating from 1300 to 1500 ° C at a pressure in the reactor of no more than 36 mm Hg) followed by exposure at a temperature of 1550-1600 ° C for 1 hour, after which cooling was performed in silicon vapors. Moreover, for the process of capillary condensation of silicon vapor to occur, the preform was heated from 1300 to 1500 ° C at a temperature of silicon vapor exceeding the preform temperature by 120-30 degrees, respectively.

The properties of the resulting siliconized graphite are shown in the table.

Example 2

The product was made analogously to example 1 with the significant difference that the content of carbon black was 8.5% of the volume by particles of graphite powder fractions 5-20 microns, and the billet was fired at 1100 ° C.

The properties of the resulting siliconized graphite are shown in the table.

Example 3

The product was made analogously to example 1 with the significant difference that for the preparation of the press mass used carbon black brand T-900 with a particle size of 50-400 nm (0.05 ÷ 0.4 μm), and the billet was fired at 1000 ° C.

The properties of the resulting siliconized graphite are shown in the table.

Example 4

The product was made analogously to example 1 with the significant difference that in the preparation of the press composition, a solution of polydimethylsilazane in toluene with a nominal viscosity of 25 seconds was used as polymer binder, polymer was polymerized at 250 ° C, and fired at 1500-1550 ° C and pressure in a 27 mmHg reactor

The properties of the material obtained with this are shown in the table.

Example 5

The product was made analogously to example 1 with the significant difference that the firing of the workpiece and its silicification was carried out in a single technological process, for which the firing was carried out in a tool for siliconizing. In this case, the billet was fired at atmospheric pressure in an argon atmosphere at 1000 ° C, after which, to realize the process of capillary condensation of silicon vapor (during siliconization), it was continued to be heated to a temperature of 1300 ° C, after which a pressure of 27 mm was created in the reactor Hg

The properties of the resulting siliconized graphite are shown in the table.

Example 6

The product was manufactured analogously to example 4 with the significant difference that the firing of the workpiece and its silicification was carried out in a single technological process, for which the firing was carried out in a tool for siliconizing. The billet was fired at 1500-1550 ° C and a pressure in the reactor of 27 mm Hg. In this case, the preform was heated to the indicated temperature at a temperature exceeding the temperature of silicon vapors (crucible with silicon). After that (before the mass transfer of silicon to the pores of the workpiece material), intermediate cooling to 1300 ° C was carried out at a workpiece temperature exceeding the temperature of silicon vapors (crucible with silicon).

The properties of the resulting siliconized graphite are shown in the table.

The remaining examples 7-10 of a specific implementation of the method, including the above (1-6), indicating the properties of the resulting siliconized graphite are shown in the table where examples 1-7 correspond to the claimed method, and examples 8-10 with deviation from it.

It also shows the properties of siliconized graphite obtained in accordance with the prototype method (example 11).

From the analysis of the table data follows:

1. The manufacture of parts from ultrafine silicified graphite by the claimed method (examples 1-7) allows to obtain it with a significantly lower content of free silicon and higher strength characteristics than in fine-grained siliconized trafit obtained in accordance with the prototype method (example 11 );

2. The implementation of the method in accordance with the preferred variants of the method (allowing to simplify it) allows to obtain ultrafine graphite no worse than the main version of the method (compare among themselves example 1 with example 5, and example 4 with example 6);

3. If the conditions of the combined firing and silicification regime are not observed, it is impossible to obtain a high-quality product (example 10);

4. Conducting isothermal exposure at temperatures below 1550 ° C (example 8) leads to an increase in siliconized graphite free silicon and graphite, and most importantly to a decrease in the strength characteristics of the material, which is apparently due to insufficient sintering between the components of the material;

5. Holding isothermal exposure at temperatures above 1600 ° C (example 9) leads to a significant decrease in graphite in siliconized graphite, the consequence of this, as is known, is a decrease in its frictional characteristics.

Claims (4)

1. A method of manufacturing products from ultrafine-grained siliconized graphite, including the preparation of a press mass based on graphite powder and a polymeric binder, molding a workpiece by pressing and polymerizing a binder, calcining the resulting workpiece and siliconizing it, characterized in that graphite powder is used to prepare the press mass fractions of 5-20 microns in a mixture with carbon black, including nanosized, and polysiloxane or polysilazane binder, the firing of the preform is carried out respectively -retarded at 1000-1200 ° C in vacuum or at atmospheric pressure under argon and at 1500-1550 ° C and a reactor pressure of no more than 36 mm Hg. century, and silicification - by the vapor-liquid-phase method during mass transfer of silicon into the pores of the material by the mechanism of capillary condensation of its vapor during heating from 1300 to 1500 ° C at a pressure in the reactor of not more than 36 mm RT. Art. followed by exposure at a temperature of 1550-1600 ° C for 1-2 hours and cooling in silicon vapor. 2. The method according to p. 1, characterized in that the firing of the workpiece and its silicification is carried out in a single technological process. 3. The method according to p. 2, characterized in that the firing of the workpiece based on a polysiloxane binder is carried out at atmospheric pressure, followed by heating to 1300 ° C. 4. The method according to p. 2, characterized in that before mass transfer of silicon into the pores of the workpiece material based on a polysilazane binder, it is heated to 1500-1550 ° C and intermediate cooling to 1300 ° C at a temperature exceeding the temperature of silicon vapor.