SU1622347A1 - Method of producing ceramic material - Google Patents

Abstract

The invention relates to methods for producing high-temperature ceramic materials based on refractory compounds, in particular silicon nitride and silicon carbide, and can be used in the manufacture of heat-resistant parts, such as gas turbine engines. The purpose of the invention is to increase heat resistance. The method involves forming a billet from 60–90 wt.% Silicon powder and 10–40 wt.% Silicon-organic polymer from the class of polyphenylsiloxy and carrying out nitriding roasting by heating to 1973 K. The heat resistance of the resulting material containing ft –SiC, tf and 5 - JCH and SiaN20, 70-120 thermal cycles 1523 K (nitrogen) ± 283 K (water). 1 tab. Ј (C

Description

The invention relates to methods for producing high ceramics of ceramic materials based on tightly fusible compounds, in particular silicon nitride and silicon carbide, and can be used in the manufacture of heat-resistant parts operating under cyclic thermal loads.

The aim of the invention is to increase the heat resistance.

Production of ceramic composites based on nitride and silicon carbide with a high-temperature nitriding thermo-processing of compacts formed from powder mixtures of polysiloxane and silicon, is caused by the processes of polymer thermal decomposition and chemical interaction of silicon with thermal decomposition products and gaseous nitrogen:

Si02 + SiC-rC

Si04 + 2C- $ iOCO; Si + C 3Si + 2NU-Si, 3SiOi + 6C- -2N -. + 6CO; Si + SiO + + Yr - Si2NЈ0; 2Si02 + 3C + + 3CO;

SiO + Si ,.

The thermal decomposition of polysiloxane in a nitrogen atmosphere begins at a temperature of / 573 K with breaks in the energetically weakest bonds of Si – C. Heating to 773 K leads to the intensification of these processes.

High-temperature (1973 K) heat treatment of powder mixtures poly t g

with

organosiloxane (10–40 wt.%) with silicon (90: 60 wt.%) results in the formation of a SiC – Si N component on the oxynitride bond.

From the above scheme of possible chemical transformations in the silicon-polymer system, it can be seen that the origin of all components of the resulting composite can be different.

X-ray analysis shows that the formation of the SiC-Si.H composite on the oxynitride bond begins already at 1773 K, and at higher temperatures the concentration of the resulting phases is redistributed towards an increase in the SiaHaO content.

An increase in the nitriding calcination temperature (more than 1973 K) leads to an increase in the dissociation rate of silicon nitride and, consequently, to a decrease in the content of nitrogen-containing phases in the final product, which impairs the heat resistance of the resulting material.

A decrease in the firing temperature (less than 1973 K) leads to a sharp inhibition of the nitriding processes and, consequently, to a decrease in the concentration of nitride and silicon oxynitride in the final product, which leads to a decrease in the heat resistance of the resulting material.

Heat treatment of molded compacts containing less than 10 wt.% Or more than 40 wt.% Of polyorganosiloxane does not lead to an increase in the heat resistance of the synthesized material compared to the prototype.

A polysiloxane with a predominantly linear structure, the side chains of which are aryl radicals, is used as the polyorganosiloxane. Linear polysiloxane chains can be crosslinked with -Si-0-Si-, leading to the formation of a friable spatial structure.

In a simplified form, the structure of the polymer used can be represented as follows:

, G /

40-Si-f

P

where n Ј1РОО.

0

five

0

five

0

five

0

five

High level of heat resistance of SiC-SijN j composites. obtained by the proposed method is partly associated with the development of a compensation mechanism for thermal expansion coefficient of individual phases of the composite in intergrain hollow microvolumes, but mainly due to the structural features of the silicon oxynitride crystal lattice.

The lattice is of the frame type, in which, due to the differences in the Si-H and Si-N bonds, it is possible to distinguish layers (Si-Si) co, co, which are interconnected by Si-0-Si island bridges, which are the easiest deformable lattice elements. The tendency of the lattice to elastic deformation is the reason that leads to a significant (almost 6 times) increase in the heat resistance of the material obtained by the proposed method, compared with the known one.

Example. A 70% by weight silica powder (specific surface area of 4 m / g) is taken and mixed for 5 hours in a ball mill with a 30% by weight powder of silicone resin K-9, which is a polysiloxane with a predominantly linear structure with rare side branches - -Si-0-Si bridges interconnecting linear chains. In the side chains of the K-9 resin, aryl radicals are predominantly present. Therefore, the resin used can be classified as polyphenylsiloxane.

From the mixture obtained, the method of dry pressing (IPa) is used to form blanks f 5 and a height of 1 cm. The resulting compact is heated at a rate of l 120 degrees / h to a temperature of 1973 K in a stream of nitrogen (1.5 l / min) and maintained at this temperature for 10 hours. After the furnace has cooled to room temperature, the samples are removed and physicochemical studies of their properties are carried out.

The resistance of the specimens to thermal shots is determined as follows. From the sintered material, cut right angles ie bars with dimensions of 5x5 x x 30 mm. The samples are placed in an electric furnace in a stream of nitrogen and kept there for 1523 K. After that, the samples are transferred to inlet water and cooled therein for 10 minutes. After the sample has dried, these procedures are repeated until a crack appears on it. The heat resistance value is determined by the number of thermal cycles before the appearance of cracks.

The composition of the initial mixture and the resulting material, the mode of synthesis and the properties of the semi-armature material are given in the table.

A comparative analysis of the data in the table shows that the proposed method provides an almost sixfold increase in its heat resistance compared to the known. In proportion to the growth of the heat resistance of the material, the working life of the parts made from it grows.

Claims (1)

Invention Formula The method of obtaining a ceramic material based on nitride and silicon carbide, including the molding of a billet of 60-90 may. / Silicon powder and 10-40 wt.% Organosilicon Acid firing by heating to a temperature of 1973 K, about tl and h and t and so that, in order to increase the heat resistance, polyorganosiloxane of the following structure is used as silicon silicon 40 45 I where n Ј 1000.