PRODUCTION OF SILICON NITRIDE SINTERED BODY

Abstract: A process for manufacturing a silicon nitride sintered body comprises molding a mixed powder of a silicon nitride starting material and a sintering aid and firing the molding. The firing is carried out in an N₂ atmosphere or a mixed atmosphere of N₂ and an inert gas, to which CO₂ or a mixed gas of CO₂ and CO is added. Thereby, the O₂ partial pressure is increased to restrain the evaporation of SiO₂ and nitriding of SiO₂ during the firing. The silicon nitride sintered body suffers almost no deterioration of the fired surface due to the evaporation of SiO₂ and the nitriding of SiO₂, and exhibits more uniform four point bending strength and oxidation resistance with respect to the fired surface and the inside.

Description: The present invention relates to a process for manufacturing a silicon nitride sintered body. According to a conventional process for producing a silicon nitride sintered body, a sintering aid which forms a liquid phase in grain boundaries, such as Y₂0₃, Al₂0₃, Mg0, etc. is added to a raw material powder of silicon nitride, and a molding obtained from the resulting mixture is fired in an N₂ atmosphere or a mixed atmosphere of N₂ and an inert gas. Such an atmosphere may be under pressure. For instance, Japanese Patent Publication No. 58-49,509 discloses a process for performing firing in an N₂ atmosphere under pressure or in a mixed atmosphere of N₂ and an inert gas under pressure. In these cases, a carbonaceous heater or a firing jig is ordinarily used, and the atmosphere is an N₂ atmosphere or a mixed atmosphere with a low 0₂ partial pressure in which the amount of 0₂ present as an impurity in the N₂ gas is reduced. Si0₂, which is inherently present in the oxide additive and the silicon nitride raw material and serves as a sintering aid, forms a glass in grain boundaries through reaction and effectively aids densification of a structure and formation of a fine structure. However, in the above-mentioned conventional processes, when the silicon nitride molding is fired in an N₂ atmosphere or an N₂ atmosphere under pressure, with a low 0₂ partial pressure, as shown in the following expressions (1) and (2), the oxide additive and Si0₂ evaporate from the glass phase, or are nitrided. Thereby, the ratio between 0 and N in the glass phase varies so that the composition of the glass phase at the grain boundary changes. Evaporation reaction of Si0₂:Si0₂⇆Si0₂+½0₂ (1) Nitriding reaction of Si0₂:3Si0₂+2N₂⇆Si₃N₄+30₂ (2) For this reason, the conventional process has the drawback that a sufficiently densified silicon nitride body cannot be obtained or a difference occurs in fine structure between the surface at which evaporation easily takes place and the inside portion at which the evaporation is difficult. Inferior properties of the fired surface result. It is therefore an object of the present invention to eliminate or reduce the above-mentioned problem and to provide a process for producing a silicon nitride sintered body which suffers almost no deterioration of a fired surface resulting from the evaporation and nitriding reaction of Si0₂. According to the present invention, there is provided a process for producing a silicon nitride sintered body, which comprises molding a mixed powder of a starting material and a sintering aid, and firing the resulting molding in an N₂ atmosphere or a mixed atmosphere of N₂ and an inert gas in which C0₂ or a mixed gas of C0₂ and C0 is present. The amount of CO₂ present is not less than 0.001%. By this measure, the 0₂ partial pressure is increased to restrain the evaporation of Si0₂ and the nitriding of Si0₂ when the molding is fired. A desired 0₂ partial pressure can be selectively determined depending upon an equilibrium 0₂ partial pressure, the evaporation speed and the nitriding speed in the evaporation and nitriding reactions of Si0₂, and the firing time at the firing temperature. To increase this 0₂ partial pressure, C0₂ or a mixed gas of C0₂ and C0 is in the invention mixed into the N₂ atmosphere or the mixed atmosphere of N₂ and inert gas. Thereby, the 0₂ partial pressure is increased by utilizing the equilibrium O₂ partial pressure resulting from the C0₂ dissociation reaction shown by the following expressions (3) and (4). Equilibrium reactions of C0₂: CO₂ ⇆ CO + ½O₂ (3) CO₂ ⇆ C + O₂ (4) The reason why CO₂ or the mixed gas of CO₂ or CO is selected as an addition gas to enhance the O₂ partial pressure in the present invention is that by controlling the O₂ partial pressure through the CO₂ dissociation reactions, damage to a carbonaceous heater, a jig, etc. frequently used for firing the silicon nitride sinterable body are reduced and that a range of the O₂ partial pressure can be appropriately selected and can be easily controlled. The reason why 0.001% or more of CO₂ is favourably mixed into the N₂ atmosphere or the mixed atmosphere of N₂ and the inert gas is that no effect is recognized when less than 0.001% is present because less than 0.001% is lower than the impurity content usually present in N₂ gas. More preferably, the CO₂ content of the firing atmosphere is at least 0.1%. On the other hand, if the added amount of CO₂ is too great, the O₂ partial pressure may become too high so that the oxidation reaction of Si₃N₄ unfavourably becomes noticeable. The maximum CO₂ content is preferably 50%, more preferably 25%. The reason why the total pressure of the N₂ atmosphere or the mixed atmosphere is preferably set at not less than 1 atm., is that the oxidation reaction of Si₃N₄ can be restrained and firing can be performed by increasing the N₂ partial pressure even when the O₂ partial pressure is high. Thus, the evaporation of the SiO₂ can be effectively restrained. In summary, the O₂ partial pressure is raised to restrain the evaporation of SiO₂. The O₂ partial pressure is increased and balanced with the N₂ partial pressure to restrain the nitriding of SiO₂. Further, the N₂ partial pressure is increased and balanced with the O₂ partial pressure to restrain the oxidation of Si₃N₄. Furthermore, when CO is added to the firing atmosphere together with CO₂, the CO formation reaction shown in the following formula (5) is inhibited and the amount of carbon (C) consumed is decreased, thereby reducing damage of the carbonaceous heater, jig, etc. frequently used in the firing of the silicon nitride sinterable body. CO formation reaction: CO₂ + C → 2CO (5) When CO is present the CO content is preferably at least 0.001%, more preferably at least 0.1%. It is preferable that the added amount of CO is greater than that of CO₂. However, if CO has a partial pressure higher than the equilibrium partial pressure of CO in the reaction shown by the formula (5), a reaction to form CO₂ and C reverse to the reaction shown in the formula (5) takes place so that carbon is produced and deposits on the silicon nitride sintered body or reacts with SiO₂ in the silicon nitride sinterable body. Thus, a CO partial pressure higher than the CO equilibrium partial pressure is unfavorable. In addition, in order to reduce damage to the carbonaceous heater, jig, etc. in the firing furnace for the silicon nitride sinterable body and control the O₂ partial pressure in the firing furnace to an appropriate range, it may be that an N₂ gas or a mixed gas of N₂ and an inert gas which contains CO and CO₂ is prepared by passing N₂ gas or the mixed gas of N₂ and the inert gas which in either case contains one or more of O₂, H₂O, air, CO₂, etc. through a heater having a consumable carbon source therein, and is then introduced into the firing furnace for the silicon nitride sinterable body. Alternatively a gas containing CO and CO₂ is prepared by passing the gas comprising O₂, H₂O, air, CO₂, etc. through the heater having the carbon source and is then mixed into an N₂ gas or a mixed gas of N₂ and an inert gas, which is then introduced into the silicon nitride sinterable body-firing furnace. Moreover, also in order to reduce damage to the carbonaceous heater, jig, etc. in the firing furnace for the silicon nitride sinterable body and control the O₂ partial pressure in the firing furnace to the appropriate range, it is possible that an N₂ gas or a mixed gas of N₂ and an inert gas containing CO and CO₂ is prepared by reacting an N₂ gas or a mixed gas of N₂ and the inert gas which in either case contains one or more of O₂, H₂O, air, CO₂, etc. with a consumable carbon source at an initial stage in the path of gas introduction to the firing furnace, and then introduced into a certain location of the heater of the jig in the firing furnace. One typical process for manufacturing the silicon nitride sintered body according to the present invention will now be explained in more detail. First, the silicon nitride powdery starting material is prepared. The silicon nitride powdery starting material is composed of a formulated powder of a silicon nitride raw material powder and a sintering aid. Y₂O₃, MgO, Al₂O₃ etc. is added as a sintering aid as it is or in the form of an aqueous solution. If appropriate, other raw material may be present, provided that silicon nitride predominates. Next, the above silicon nitride starting material powder is crushed and mixed by means of a mill using media. Mills of a wet type and a dry type may be both used. For instance, a ball mill, an attrition mill, a vibration mill, etc. may be used. Then, the molding powder obtained is molded by a dry type press, injection molding, slip casting, etc., to obtain the desired molding. The molding is fired in an N₂ atmosphere or a mixed atmosphere of N₂ and an inert gas into which CO₂ or a mixed gas of CO₂ and CO has been added. The firing temperature is preferably in the temperature range from 1,600°C to 2,000°C. The added amount of CO₂ to N₂ is not less than 0.001%. It is preferable that the total pressure of the N₂ atmosphere or the mixed atmosphere is not less than 1 atm. (1 x 10⁵ Pa). The present invention will be illustrated below in more detail by the following non-limitative examples. Example 1 Various powders were formulated by adding a sintering aid to a silicon nitride powdery starting material of 97.1% by weight in purity with a content of oxygen being 1.5% by weight in the amounts shown in Table 1. After mixing and crushing by means of a water wet type ball mill, each powder was dried and granulated, thereby providing a molding powder. The molding powder was preliminarily molded, and then molded by a hydrostatic press under a pressure of 300 MPa (3 ton/cm²,) to prepare a planar molding of 60x60x6 mm. Some moldings were fired in the atmosphere and at the temperature given in Table 1, thereby forming silicon nitride sintered bodies Nos. 1-12 by the process of the present invention. Further such moldings were fired in an atmosphere outside that of the process according to the present invention as given in Table 1, to provide silicon nitride sintered bodies Nos. 13-24 as comparative examples. The firing atmospheres given in Table 1 were controlled by supplying N₂, CO₂ and CO raw gases to the firing furnace in the amounts given in Table 1. For the silicon nitride sintered bodies Nos. 1-24, Table 1 also shows the bulk density, the oxygen content, the four point bending strength when a fired face or an inside worked face was employed as a tensile face, and the amount of increased oxidation occurring per unit area in the fired face and the inside worked face when heated at 1,200°C in air for 100 hours. The bulk density and the four point bending strength were measured according to an Archimedes method and the fine ceramics bending strength testing method of JIS R-1601, respectively, for both the fired face and the inside worked face which was at a depth greater than 1 mm from the fired surface. The amount of of oxidation occurring on heating was determined from weight increase and surface area, for a sample whose whole surface was a fired surface and for a sample whose whole surface was an inside worked face at a depth not less than 1 mm from the fired surface. It is clear from Table 1 that, as compared with the products of the conventional process, the sintered bodies of the invention having the same composition are denser and exhibit more uniform four point bending strength and oxidation resistance at the fired surface and at the interior. With the process of the present invention, the silicon nitride sintered body suffers less or almost no deterioration of the fired surface resulting from the evaporation and nitriding reaction of SiO₂. Moldings having the same composition as that of Sample No. 5 of the present invention in Example 1 were fired at 1900°C in respective atmospheres containing CO and CO₂ as shown in Table 2. As a result, sintered bodies each having approximately the same four point bending strength and increased amount of oxidation as in Sample No. 5 of the present invention with respect to the fired face and the inside worked face were obtained. At that time, a carbon pellet of 20 mm in diameter and 10 mm in height was placed and fired together with the silicon nitride sinterable body in the firing furnace. The resulting reduction of weight of carbon pellets due to the firing are also shown in Table 2. As understood from Table 2, the weight reduction of the carbon pellet in the firing is reduced by mixing CO to the firing atmosphere. That is, damage of the carbonaceous heater or jig in the firing furnace can be reduced by addition of CO.

Claims: A process for manufacturing a silicon nitride sintered body, which comprises molding a mixed powder of a starting material and a sintering aid, and firing the resulting molding in an N₂ atmosphere or a mixed atmosphere of N₂ and an inert gas characterised in that said atmosphere contains added CO₂ or an added mixture of CO₂ and CO and the amount of CO₂ in the firing atmosphere is not less than 0.001%. A process according to claim 1 wherein the total pressure of the firing atmosphere is not less than 1 x 10⁵ Pa (1 atm.) A process according to claim 1 or 2 wherein the said atmosphere contains a mixture of CO₂ and CO in which the partial pressure of CO is greater than that of CO₂. A process according to any of claims 1 to 3, wherein the partial pressure of the mixed CO in the firing atmosphere is lower than the equilibrium CO partial pressure in the reaction of the following formula: CO₂ + C ⇄ 2CO

Inventors: SOMA, TAKAO, TAKAHASHI, TOMONORI
Applicants: NGK INSULATORS, LTD.
Application year: 1986
Main classification: C04B35
IPC codes: C04B35