FR2560593A1 - PROCESS FOR PRODUCING A SINTERED SILICON NITRIDE PRODUCT - Google Patents

Abstract

THE INVENTION RELATES TO A PROCESS FOR PRODUCING A SINTERED BODY OF SILICON NITRIDE. ACCORDING TO THE INVENTION A MOLDED BODY IS PREPARED BY ADDING TO PULVERULENT SILICON GRAINS A COMPOUND WHICH DECOMPOSES ON HEATING TO PRODUCE H OR HYDROCARBON COMPOUNDS LIKE CH AND CH OVER A TEMPERATURE RANGE OF 500 TO 1200C, ON SUBJECT THIS MOLDED BODY TO THERMAL TREATMENT IN A NITRANT GAS AND OR INERT GAS ATMOSPHERE, AND THEN THE SILICON IS CHEMICALLY CONVERTED TO SILICON NITRIDE IN A NITRANT GAS AT A TEMPERATURE GREATER THAN 1200C. THE INVENTION APPLIES IN PARTICULAR TO CERAMICS.

Description

The present invention relates to a method for producing ceramics

  fine silicon nitride type. The processes for producing sintered silicon nitride products can be divided into a reaction sintering method and an atmospheric sintering method. The present reaction sintering method

  the merit that sintered products of larger dimensions

  can be produced more easily compared to the atmospheric sintering method and the sintered products obtained by this method do not exhibit any

  resistance reduction even at a high temperature.

  However, since the reaction sintering method can not produce a very dense and strong structure, the products obtained by the process are not suitable as components, currently, for uses of

high resistance.

  The major reason why high density products can not be obtained by reaction sintering is that sintering shrinkages hardly occur when the structure of the

  Molded material is converted into sintered silicon nitride.

  A two-stage sintering method is known as a method for improving the above-mentioned defects of reaction sintering. This two-stage sintering method comprises the steps of preparing a porous silicon nitride sintered product by reaction sintering, impregnating the pores of the sintered product with a compound of Al, Mg, Y and the like as agent promoting the sintering of silicon nitride,

  then sinter it again to force the narrowing

  sintering to finally densify the structure of the product. However, this two-stage sintering method has a defect in that the sintered product thus obtained exhibits the same significant reduction in product strength as that in the atmospheric sintering method in a high temperature range above 1000 C. and therefore this defect causes a loss of merit of the reaction sintering itself. On the other hand, as another method for improving the density and strength of the sintered product, a method of using pulverized metallic silicon grains as a raw material and densifying the product by shrinking during sintering of the grains has been proposed. However, this method has not really been successful. This is because a silica (silica) oxide film formed on the surface of the powdered silicon grains causes sintering obstruction

  between the grains of silicum. However, although the use

  H2 gas can be considered to remove the silica film, the reaction between H2 gaseous and the silica film takes place at a high temperature above 1200 C, where the sintering between the silicon grains is passes rapidly, to cause sintering shrinkage so large that it removes the open pores necessary for the nitriding reaction in the molded product. So, like the nitriding reaction itself

  is inhibited, this method can not be used.

  Therefore, since the optimum temperature for controlling the sintering in the silicon powder is about 1100 C, it is necessary that the silica film on the surface of the powdered metallic silicon grains be removed before the temperature is increased to this

level.

  It is an object of the present invention to overcome the above problems by providing a reaction sintering method for obtaining a silicon nitride sintered product having high strength even at a high temperature, having better strength and better strength.

density of the structure.

2560.593

  This invention has been accomplished using the fact that hydrogen ions and carbon ions, which are released by compounds capable of producing H2 gaseous and hydrocarbons such as CH4 and C2H6 at a high temperature just after the decomposition of the compounds, are a very active state and have activity for an action

extremely powerful reducer.

  For the compound described above, it is advantageous to

  use high molecular weight organosilicon compounds with C and Si as the main components of the backbone, or compounds such as

  phenolic resins. These compounds are thermally de-

  compounds at a temperature above 500 C to clear

  H2 gas and hydrocarbon gas as CH4 and C2H6.

  The compounds usable in the invention are not limited simply to high molecular weight organosilicon compounds and phenolic resins, but any compound that emits H ions and activated C ions with the function of reducing the silica film nearby. of O11000C can be used if required. However,

  the use of such a compound that decomposes thermal-

  For example, polyvinyl butyral is not desired since the compound itself reacts to lose its reducing nature before reaching the reaction temperature range of about 1100 ° C. and therefore it becomes

  difficult to maintain the reducing atmosphere at the same

reduction of silica.

  A sintering shrinkage of about 6% can be attained in the sintered product by dispersing to incorporate the heat decomposed gas-releasing compound at a temperature of 500 to 1100 C in the vicinity of the powdered silicon particles in the starting molded body. , by applying a heat treatment in an inert gas and then maintaining it at 1100 C for a time

  preset, for example 20 hours.

2560593-

  The gas that can be used for the heat treatment in the first stage comprises the nitriding gases such as N 2, a mixture of N 2 and H 2 and NH 3 gases, or inert gases such as Ar and He. However, the use of the inert gas at a temperature above 1200 C is undesirable because the molded body rapidly causes significant sintering shrinkage at a temperature exceeding 12000C, making it difficult to maintain open pores in the molded body, required for the nitriding reaction to take place. But; Moreover, since the reaction takes place between Si and N2 in the nitriding gas at a temperature exceeding 1200 ° C., the sintering shrinkage between the Si grains is rapidly reduced.

  and is totally eliminated at 1300 C.

  According to the invention, the amount of sintering shrinkage in the molded body is controlled using

the properties of these gases.

  The effect of using an organic silicon polymer (hereinafter referred to as PCS) and a phenolic resin for the releasing compound H2 gaseous and the hydrocarbon gases such as CH4 and C2H6 at a temperature of 500 ° C. to 1100 ° C. for sintering pulverulent silicon grains according to the invention will now be described with reference to

examples.

Example 1

  A solution in hexane which contains, in dissolu-

  PCS comprising C and Si as main skeletal ingredients was mixed in powdered silicon particles of less than 44 p particle size, at a mixing ratio of 90 wt.% of powdered silicon particles and 10 wt. % by weight of PCS Then, after evaporation of the hexane under stirring and mixing, a body

  molded was prepared from the powder thus obtained.

  The molded body was nitrided at heat treatment conditions up to 1300 C indicated in the table

  1 and the results are shown on the board.

  As a comparison example, sintering was also performed using a solution in polyvinylbutyral alcohol (P.V.B) instead of P.C.S under the same conditions as above. The amount. addition of P.V.B was further decreased to 5% by weight in one case. The results of the comparison examples are also

shown in Table 1.

  The heat decomposition property of PCS in a non-oxidizing atmosphere was such that it evolved gases of the hydrogen and methane series at about% in a range between 250 C and 500 C and at about 30%. % in a range between 500 C and 1200 C. Moreover, polyvinylbutyral had completely decomposed

at 500 C.

Table 1.

  No. Type of gas type additive Treatment condition Thermal property (% by weight): up to 1300 C shrinkage Grade shrinkage Nitriding (.%) Sf 1 PCS (10% / ) Temperature increase Example of PVB (10% 6) 40 C / h, 1100 C x20h hand- 6.3 2.92 618 Ar comparison held, then temperature 0.9 2.36 66.7 increased, passed b N2 gaseous

2 P.C.S. (10%

  Example of P.V.B (5%) Ar Increase in temperature 6.3 2.92 618 Comparison 40 C / h, 110 ° C x 20h maintained, then temperature increased, increased to N2 gaseous

3 P.C.S (10%)

  Example of P.V.B (10%) N Temperature increase 5.9 2.88 628 comparison 2 40 C / h, 1200 C x 20 h hand-0.10 238 67.7 held, 1300 C x 30h maintained

4 P.C.S (10%

  Example of PVB (10.1%) 90: N2 Increase temperature 6.1 2.90 608 comparison + 40 C / h, 1200 C x 20 h hand-ut: H2 tens, 1300 C x 30 h maintained 0.13 2 , 42 91o w Table 1 (cont.) No. Type of additive Type of gas Treatment condition Property (by weight) thermal: up to 1300 C Shrinkage quality

to nitride

tion (, o)

P.C.S (10%)

  Example of P.V.B (10%) 50: N2 Increase in temperature 6.4 2.96 598.4 comparison + 40 C / h, 1200 Cx20h maintained, 0.16 2.48 98: Ar then passed to N2 gas alone

6 P.C.S (10%)

  Example of PVB (10%) 45: N2 Increase in temperature 6.5 2.97 608 comparison + 40 C / h, 1200 Cx20h maintained, 0.16 2.48 101 Ar then passed to N2 gaseous ___ __ __ _ + single: H Note: fs: Apparent density of the sintered product 6f: Flexural strength of the sintered product at 1400 C (MPa unit) PCS: Organic silicon polymer PVB: Polyvinylbutyral

Example 2

  A solution of phenolic resin (PHR) dissolved in the alcohol was mixed with powdered silicon particles having a particle size of less than 44pu at a mixing ratio of 90% by weight of the powdered silicon particles and 10% by weight of the phenolic resin, and after evaporation of the alcohol under mixing and stirring, a molded body was prepared in the conventional manner using the powder thus obtained and nitrided under the conditions indicated in Table 2. The results

  are also shown on the board. The property of decompo-

  The heat content of the phenolic resin in a non-oxidizing atmosphere was such that it began to release gases from about 450 ° C. and the evolution of gas ended at about 1200 ° C. Hydrogen and methane series gas released in this range was about 50%. In the same way, an experiment was also carried out for the samples prepared from P.C.S and P.V.B as described in Example 1 under the same conditions. The amount of addition of P.C.S

  and P.V.B was 10% by weight for each of the cases.

Table 2.

  No. Type of additive Type of gas Treatment condition Thermal property: up to 1300 C Shrinkage Quality Shrinkage Quality

to nitride

  (%) ps Sf 1 PHR 90: Ar Increase in temperature 6.8 2.90 638 + 40 C / h, 1100 Cx20h maintained,: H2 then switched to N2 gas 2 PHR 90: He Temperature increase 6.8 2 , 90 638 + 40 C / h 1100 Cx20h maintained,: H2 edsuite passed to N2 gas 3 PHR 45: Ar Increase of temperature 6.8 2.90 638 + 40 C / h, 1l00OCx20h maintained,: He then passed to N2 gaseous ntk] 4 P.H.R. 90: He Temperature increase 7.9 2.49 124.5 + 40 C / h 1350 Cx10H maintained, 10: H2 then passed to N2 gas only P.H.R. 50: He Temperature increase 6.6 2.89 608 + 40 C / h, 1200 C x 20 h maintained: N2 then passed to N2 gas 6 P.H.R. 90: He Temperature increase 7.1 2,94 618 P.c. + 40 C / h, 1150 Cx20h maintained, 7.0 2.95 598 P.V.B. 10: H2 then passed to N7 gas 0.18 2.49

Note from Table 2.

  s: Apparent density of the sintered product.

   f: Flexural strength of the sintered product (MPa unit)

P.H.R .: Phenolic resin.

  As shown in Examples 1, 2, the effect of P.C.S and the phenolic resin giving off reducing gases, i.e., H 2 and CH 4 gas, between about 10

  500 and 1200 C is extremely high.

  The heat shrinkage rate in the conventional molded body is about 0.1% under the same temperature condition and, therefore, the effect obtained by the present invention is much greater in

  compared to that with the conventional method.

  The sintered product of silicon nitride obtained according to the invention has, as physical properties, an apparent density of 2.92, a flexural strength at an atmospheric temperature of 589 MPa and a flexural strength at 14000 ° C. of 618 MPa. These product performances are at a high level that can not be reached by the conventional reaction sintering method and the strong resistance at 14000C is the highest among the quality

  ceramics currently known.

  The heat-resistant components obtained by the new reaction sintering method according to the invention are unexpectedly fulfilled and can be re-estimated in applications such as motors, turbines and blades made of ceramic that we considered

  until now as being difficult to manufacture.

Claims (4)

R E V E N D I C A T I 0 N S   1. A process for producing a silicon nitride sintered product, characterized in that it consists in preparing a molded body by adding, to grains of powdered silicon, a compound which decomposes on heating to produce H 2 or compounds of hydrocarbons such as CH4 and C2H6 over a temperature range of 500 C to 1200 C, subjecting said molded body to heat treatment in a gas atmosphere of a nitriding gas and / or an inert gas and then to chemically convert the silicon silicon nitride in a nitriding gas at a temperature above 12000C.   2. Production process according to claim 1, characterized in that the nitriding gas for the heat treatment comprises a gas mainly composed of N and NH3 and the temperature of the heat treatment is lower. at 13000C.   3. Production process according to claim 1, characterized in that the inert gas for the heat treatment comprises Ar or He and the temperature for the   heat treatment is less than 1200 C.   4. Production process according to claim 1, characterized in that the nitriding gas for the chemical conversion of silicon to silicon nitride comprises N2.   gas or a gaseous mixture of gaseous N 2 and H 2 gas.