GB2157668A - Producing silicon nitride sintered products - Google Patents

Abstract

A moulded body is prepared by adding, to powdery silicon grains, a compound that decomposes under heating to produce H2 or hydrocarbon compounds such as CH4 and C2H6 in a temperature region from 500 DEG C to 1200 DEG C. The moulded body is heat treated in an atmosphere of a nitriding gas and/or an inert gas. Subsequently the silicon is chemically converted into silicon nitride in a nitriding gas at a temperature higher than 1200 DEG C.

Description

SPECIFICATION Producing silicon nitride sintered products This invention relates to a process for producing silicon nitride sintered products. particularly fine ceramics.

Production processes for silicon nitride sintered products can be classified into a reaction sintering method and an atmospheric sintering method. The reaction sintering method has merits that sintered products of larger size can be produced more easily as compared with the atmospheric sintering method and that the sintered products obtained by this method show no reduction in strength even under a high temperature. However, since the reaction sintering method cannot produce a highly dense and strong structure, the products obtained by the process are not suitable as components for high strength uses at present.

The major reason that high density products cannot be obtained by reaction sintering is that sintering shrinkage scarcely occurs when the structure of the moulded material transforms into sintered silicon nitride.

A two-stage sintering method has been known as a process to improve the above-mentioned defects in 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 a sintering promoting agent of silicon nitride, and then sintering the same again to cause sintering shrinkage to finally densify the structure of the product. However, this two-stage sintering method has the defect that the sintered product thus obtained shows the same significant reduction in product strength as that in the atmospheric sintering method, in a high temperature range above 1 000 C, and therefore this defect causes loss of the merit of reaction sintering itself.

Further, as another method for improving the density and strength of the sintered product, a method of using powdery elemental silicon grains as the starting material and densifying the product through shrinkage during sintering of the grains has been proposed. However, this method has not yet actually been successful, because a film of silicon oxide (silica) formed on the surface of the powdery silicon grains causes obstruction of the sintering between silicon grains. However, although the use of H2 gas may be considered for removing the silica film, the reaction between H2 gas and the silica film takes place only at a high temperature, above 1 2000C, where the sintering between silicon grains proceeds rapidly, to cause such a significant sintering shrinkage as to eliminate the open pores necessary for the nitriding reaction in the moulded product.Thus, since the nitriding reaction itself is inhibited, this method cannot be utilized.

Therefore, since the optimum temperature for controlling the sintering between silicon powder is about 11 00 C. it is necessary that the silica film on the surface of the powdery elemental silicon grains be eliminated before the temperature is increased to that level.

It would therefore be desirable to be able to overcome the foregoing problems and provide a reaction sintering method for obtaining a silicon nitride sintered product having high strength even under high temperature, improved in the strength and the density of the structure.

The present invention provides process for producing a silicon nitride sintered product, which comprises preparing a moulded body by adding, to powdery silicon grains, a compound that decomposes under heating to product H2 or hydrocarbon compounds such as CH4 and C2H6 over a temperature region from 500"C to 1 200 C, subjecting said moulded body to a heat treatment in a gas atmosphere of a nitriding gas and/or an inert gas, thereafter, chemically converting silicon into silicon nitride in a nitriding gas at a temperature hig her than 1200"C.

This invention has been accomplished by utilizing the fact that hydrogen ions and carbon ions, which are released from those compounds capable of producing H2 gas and gaseous hydrocarbons such as CH4 and C2H6 at a high temperature just after the decomposition of the compounds, are in highly activated state and have an activity for extremely powerful reducing action.

As the said compound, high molecular organic silicon compounds having C and Si as the main skelton component or those compounds such as phenolic resins can suitably be used. These compounds are thermally decomposed at a temperature higher than 500"C to evolve H2 gas and gaseous hydrocarbons such as CH4 and C2H6. The compounds usable in this invention are not limited only to the high molecular organic silicon compounds and the phenol resins but any compound that evolves activated H ions and C ions having the function of reducing a silica film near 1100"C can be used as required.However, the use of such a compound that is thermally decomposed already at a temperature lower than 500"C to evolve such gases, for example, polyvinylbutyral is not desired, since the compound itself reacts to lose its reducing nature before reaching the reaction temperature range of about 110000, and therefore it becomes difficult to maintain the reducing atmosphere at the reducing temperature of silica.

Sintering shrinkage of about 6% can be attained in the sintered product by dispersing to incorporate the compound evolving heat decomposed gases at a temperature from 50000 to 110000 in the vicinity of powdery silicon particles in the starting moulded body, applying heat treatment in an inert gas and, thereafter, maintaining the same at 110000 for a predetermined time, for example, 20 hours.

The gas usable for the heat treatment at the first step can include nitriding gases such as N2 or mixtures of N2 and H2 and NH3, or inert gas such as Ar and He. However, the use of the inert gas at a temperature higher than 120000 is not desired, since the moulded body rapidly causes a significant sintering shrinkage at a temperature in excess of 1200 C, making it difficult to keep open pores in the moulded body required for the nitriding reaction to take place. On the other hand, since the reaction proceeds between Si and N2 in the nitriding gas at a temperature in excess of 1200 C,the sintering shrinkage between Si grains is rapidly reduced, and is completely eliminated at 130000.

Accordingly, the amount of sintering shrinkage in the moulded body can be controlled by utilizing the properties of these gases.

The effect of using organic silicon polymer (hereinafter referred to as PCS) and phenol resin as compounds for evolving H2 gas and hydrocarbons such as CH4 and C2H6 at a temperature from 50000 to 110000 in the sintering of the powdery silicon grains will now be described by way of example.

Example 1 A hexane solution containing, dissolved therein, PCS comprising C and Si as the main skelton components was admixed into powdery silicon particles less than 44 m in particle size, at a mixing ratio of 90% by weight of the powdery silicon particles and 10% by weight of the PCS. Then, after evaporating hexane under mixing and agitation, a moulded body was prepared from the thus obtained powder.

The moulded body was nitrided under the conditions of heat treatment up to 13000Cshown in Table 1 below, and the results are also shown in the table.

As a comparative example, sintering was also carried out by using an alcoholic solution of polyvinylbutyral (PVB) instead of PCS under the same conditions as above. The addition amount of PVB was further decreased to 5% by weight in one of the cases. The results of the comparative examples are also shown in Table 1.

Heat decomposition of PCS under non-oxidative atmosphere was such that it evolved hydrogen and methane series gases by about 15% in a range between 250 C and 500 C and by about 30% in a range between 500 C and 120000. On the other hand, polyvinylbutyral had completely decomposed up to 500 C.

Table 1 Property Type of Type Heat treatment and condition: Shrin- Quality No. additive ofgas up to 1300 C kate in (wt%) nitri- ps #f dation (%) 1 P C S (10%) temperature rise 40001h, 6.3 2.92 63 Ar 110000 x 20 h kept, thereafter Comparative P V 8 (10%) temperature increased, switched 0.09 2.33 6.8 Example to N2 gas 2 PCS(10%) temperature rise 40"Cih, 6.3 2.92 63 Ar 110000 x 20 h kept, thereafter Comparative PVB( 5%) temperature increased, switched 0.12 2.41 9.1 Example to N2 gas 3 P CS (10%) temperature rise 4000Ih, 5.9 2.88 64 N2 1200 C x 20 h kept, Comparative PVB(10%) 1300 Cx30hkept 0.10 2.38 6.9 Example 4 P CS(10%) 90:N2 temperature rise 40"C/h , 6.1 2.90 62 + 1200 C x 20 h kept, Comparative PVB(10%) 10:H2 1300 Cx30hkept 0.13 2.42 9.3 Example 5 P CS (10%) 50:N2 temperature rise 40 C/h, 6.4 2.96 61 + 120000 x 20 h kept, thereafter Comparative PVB(10%) 50:Ar switched to only N2 gas 0.16 2.48 10 Example 6 PCS(10%) 45: :N2 temperature rise 4000/h, 6.5 2.97 62 + 1200 C x 20 h kept, Comparative PVB(10%) 45:Ar thereafter switched to 0.16 2.48 10.3 Example + only N2 gas 10:H2 Note: ps : Bulk density of sintered product 3f: Bending strength of sintered product at 1400 C (unit:kg/mm2) P C S : Organic silicon polymer PVB: Polyvinylbutyral Example 2 A solution of phenolic resin (PHR) dissolved in alcohol was admixed with powdery silicon particles less than 441lm im particle size at a mixing ratio of 90% by weight of powdery silicon particles and 10% by weight of phenol resin and, after evaporating allcohol under mixing and stirring, a moulded body was prepared in the conventional manner by using the thus obtained powder and nitrided under the conditions shown in Table 2 below.The results are also shown in the table.

The heat decomposition of the phenol resin under a non-oxidative atmosphere was such that it started to evolve gases from about 450 C and completed the gas evolution about at 1200 C, the amount of hydrogen and methane series gases evolved within this range being about 50%. in the same manner, an experiment was also carried out for the specimens prepared from PCS and PVB as described in Example 1 under the same conditions. The addition amount of PCS and PVB was 10% by weight for each of the cases.

Table 2 Property Type of Type Heat treatment condition: Shrinkage Quality No. additive of gas up to 1300"C in nitri dation (%) ps 5f 90:Ar temperature rise 40 C/h, 1 P H R + 1100 C x 20 h kept, thereafter 6.8 2.90 65 10:H2 switched to N2 gas 90:He temperature rise 40 C/h, 2 P H P + 110000 x 20 h kept, thereafter 6.8 2.90 65 10:H2 switched to N2 gas 45:Ar + temperature rise 40 C/h, 3 PHR 45:He 11000C > c20hkept,thereafter 6.8 2.90 65 + switched to N2 gas 10:H2 90:He temperature rise 40"Clh , 4 PHR + 1350"C x 10 h kept, thereafter 7.9 2.49 13 10:H2 switched to only N2 gas 50::He temperature rise 400C/h, 5 P H P + 120000 x 20 h kept, thereafter 6.6 2.89 62 50:N2 switched to only N2 gas P H P 90:He temperature rise 400O/h, 7.1 2.94 63 6 P C S + 115000 x 20 h kept, thereafter 7.0 2.95 61 PVB 10:H2 switchedto N2 gas 0.18 2.49 10.3 Note: ps : Bulk density of sintered product bf: Bending strength of sintered product (unit: kg/mm2) P H R: Phenol resin As shown in Examples 1 and 2, the effect of PCS and phenolic resin evolving reducing gases, that is, H2 and CH4 gas, within the range 500 C to 1 200CC is extremely high.

The heat shrinkage rate in the conventional moulded body is about 0.1% under the same temperature conditions and, accordingly, the effect obtained by this invention is much more significant as compared with the conventional method.

A silicon nitride sintered produce can thus be obtained having, as physical properties, bulk density of 2.92, bending strength at ambient temperature of 60 kg/mm2, and bending strength at 1400"C of 63 kg/mm2. Such a high level of performance could not be attained by the conventional reaction sintering method, and the great strength at 14000C is the highest among the quality of ceramics known at present.

Hot strength components obtained by the process according to this invention can be considered for such applications as engines, turbines, and blades made of ceramics, that have been considered difficult hitherto.

Claims (5)

1. A process for producing a silicon nitride sintered product, which comprises preparing a moulded body by adding, to powdery silicon grains, a compound that decomposes under heating to produce H2 or a hydrocarbon in a temperature region from 500"C to 1200 C, subjecting the moulded body to a heat treatment in a gas atmosphereofa nitriding gas and/or an inert gas, and thereafter chemically converting the silicon into silicon nitride in a nitriding gas at a temperature higher than 120000.

2. A process as claimed in claim 1, wherein the nitriding gas for the heat treatment comprises a gas mainly composed of N2 and NH3 and the temperature for the heat treatment is lower than 130000.

3. A process as claimed in claim 1, wherein the inert gas for the heat treatment comprises Ar or He and the temperature for the heat treatment is lower than 1200to.

4. A process as claimed in any of claims 1 to 3, wherein the nitriding gas for chemically converting the silicon into silicon nitride comprises N2 gas or a gas mixture of N2 gas and H2 gas.

5. A process as claimed in claim 1, substantially as described in Example 1 or Example 2.