Removal method of glass adhered to sintered object during isostatic pressing process

Abstract: Method of removing a layer of glass formed on a sintered body manufactured by hot isostatic pressing of a preformed body of inorganic powder such as silicon nitride powder, silicon carbide powder or the like with a pressure medium at a sintering temperature, wherein the sintered body of inorganic powder is retained in alkali solution at a high temperature for a predetermined time, preferably under a predetermined pressure.

Description: The present invention relates to a method of removing glass adhered to sintered objects of inorganic powder such as silicon nitride powder, silicon carbide powder or the like during hot isostatic pressing process. In the manufacturing of sintered object of silicon nitride, a preformed body of silicon nitride powder coated with high-melting glass is subjected to hot isostatic pressing during which the preformed powder body is isotropically pressed with a pressure medium such as inert gas under pressure at a sintering temperature. The hot isostatic pressing is advantageous to manufacture sintered objects of complicated shapes having the same strength in all directions at a high sintering temperature. During the hot isostatic pressing process, however, the glass used as a sealing material is inevitably adhered to recesses of the sintered powder body. To remove the adhered glass from the sintered powder body, there have been proposed a hammering method of removing the glass from the sintered powder body by a mechanical external force applied thereto, a method of crystallizing the glass adhered to the sintered powder body under control of its cooling temperature after sintering to facilitate removal of the glass, and a method of removing the glass from the sintered powder body by vaporizing the pressure medium melted in the layer of glass for foaming the glass and cooling the foamed glass. In the hammering method, it is required to mechanically apply a great external force to the sintered powder body for removal of the glass adhered thereto. This results in an increase of the manufacturing cost of this kind of sintered object. The second crystallizing method is effective only in use of special glass such as silica glass. In the third method, foaming of the glass is greatly influenced by a condition of the hot isostatic pressing and the kind of the glass. For the foregoing reasons, it is very difficult to remove the glass from the sintered powder body at a low cost. EP-A-118702 and EP-A-320927 briefly mention the removal of glass containing 48-52% B 2 O 3 or 90% B 2 O 3 by weight, using an alkali solution. It is, therefore, a primary object of the present invention to provide a method capable of easily removing the glass from the sintered powder body at a low cost. According to the present invention, there is provided a method of removing a layer of glass formed on a sintered body manufactured by hot isostatic pressing of a preformed body of inorganic powder such as silicon nitride powder or silicon carbide powder, ceramic powder or metallic powder, with a pressure medium at a sintering temperature, as set out in claim 1. Preferably, the removal method of glass is actually practiced as follows: (1) To retain the sintered body in alkali solution stored in a closed container under a predetermined pressure. (2) To use potassium hydroxide or sodium hydroxide as the alkali solution. In the hot isostatic pressing, nitrogen gas may be used as the pressure medium. With the removal method of glass described above, the glass adhered to the sintered body can be effectively partially removed in a condition where the sintered body is retained at a high temperature under presence of alkali solution. In this case, it is desirable that potassium hydroxide or sodium hydroxide is used as the alkali solution, and it is also desirable that the sintered body is retained in the closed container at a high temperature for more than one hour. Since in the removal method a brittle reaction layer is formed in the layer of glass adhered to the sintered body, the finishing time of the sintered body can be shortened. Hereinafter, a preferred embodiment of the present invention will be described in detail. Prepared first in this embodiment were 3000-1200 preformed powder bodies each in the form of a silicon nitride ball of 13mm in diameter containing 93 percent by weight Si 3 N 4 , 4 percent by weight Al 2 O 3 and 3 percent by weight Y 2 O 3 . The preformed powder bodies were place in a pot filled with borosilicate glass such as Pyrex glass containing 80.8 percent by weight SiO 2 , 2.3 percent by weight Al 2 O 3 , 3.9 percent by weight Na 2 O and 12.5 percent by weight B 2 O 3 the volume of which was approximately the same as the preformed powder bodies, and the pot was placed in a hot isostatic press device so that the preformed powder bodies are subjected to hot isostatic pressing at the sintering temperature of 1700°C under 700 atmospheric pressure. The preformed powder bodies thus sintered were cracked into blocks of about 4 cm in diameter, and the blocks were placed in a HU-100 type decomposition container made by Sanai Kagaku Kabushiki Kaisha. To remove the glass from the sintered powder bodies, alkali solution of 40 ml was added into the decomposition container at various concentration shown in an attached Table 1. In this test for removal of the glass, the pressure in the container was determined as shown in a attached Table 2 and calculated by Clapeyron-Clausius' equation. In Table 1, double circle marks in the column of heating temperature and time each represent a condition where almost all the glass was removed from the sintered powder blocks, single circle marks each represent a condition where the sintered powder blocks were still covered with the glass which can be removed by the conventional hammering method, and "X" marks each represent a condition where the glass can not be removed from the sintered powder blocks. As is understood from Table 1, it has been found that the glass adhered to the sintered powder body can be effectively removed in a condition where the sintered powder body is retained at a high temperature under presence of alkali solution. In this case, it is desirable that potassium hydroxide or sodium hydroxide is used as the alkali solution, and it is also desirable that the sintered powder body is retained in the decomposition container at a high temperature for a long time. With the test results represented by the double circle marks, it has been confirmed that a potassium hydroxide reaction layer of about 100 microns is formed in the layer of glass adhered to the sintered powder body. Since the potassium hydroxide reaction layer is brittle, the layer of glass adhered to the sintered powder body can be easily removed by abrasive processing. This is useful to shorten the finishing time of the sintered object. In another embodiment of the present invention, sintered powder bodies each in the form of a SiC block of 5 x 5 x 10 mm containing 98.5 percent by weight SiC, 0.8 percent by weight B 4 C and 0.5 percent by weight Free-carbon were manufactured by hot isostatic pressing in the same manner as described above and cracked into blocks of about 4 cm in diameter. The blocks of the sintered powder bodies were placed in the decomposition container used in the above embodiment. To remove the glass from the sintered powder bodies, alkali solution of 40 ml was added into the decomposition container at various concentration shown in an attached Table 3. In Table 3, double circle marks in the column of heating temperature and time each represent a condition where almost all the glass was removed from the sintered powder blocks, single circle marks each represent a condition where the sintered powder blocks were still covered with the glass which can be removed by the conventional hammering method, and "X" marks each represent a condition where the glass can not be removed from the sintered powder blocks. AMOUNT OF BLOCKS (g) CONCENTRATION OF REAGENT (WT/V)% AMOUNT OF REAGENT (ml) Heating Temperature & Time 160°C 230°C 1hr 5hr 10hr 16hr 1hr 10hr 16hr Potassium hydroxide 30 10 40 X X ○ ○ X ○ ○ 30 20 40 X X ○ ○ X ○ o ○ 30 40 40 X ○ ○ o ○ ○ o ○ o ○ Sodium hydroxide 30 10 40 X ○ ○ ○ ○ ○ ○ 30 20 40 X ○ ○ ○ ○ ○ o ○ 30 40 40 X ○ ○ o ○ ○ o ○ o ○ Lithium hydroxide 30 20 40 X X X X X X ○ 30 40 40 X X X X X X ○ Potassium carbonate 30 40 40 X X X X X X ○ Sodium carbonate 30 40 40 X X X X X X ○ Hydrofluoric acid 30 10 40 X X X X X X ○ Water 30 40 X X X X X X X Reagent Temp (°C) Concentration (wt/v)% Pressure (kg/cm 2 ) Potassium hydroxide 160 22 6.9 40 5.7 230 22 38.4 40 32.2 Sodium hydroxide 160 20 6.7 40 5.1 230 20 37.4 40 30.7 AMOUNT OF BLOCKS (g) CONCENTRATION OF REAGENT (WT/V)% AMOUNT OF REAGENT (ml) Heating Temperature & Time 160°C 230°C 1hr 5hr 10hr 16hr 1hr 10hr 16hr Potassium hydroxide 30 10 40 X X ○ ○ X ○ ○ 30 20 40 X X ○ ○ X ○ o ○ 30 40 40 X ○ ○ o ○ ○ o ○ o ○ Sodium hydroxide 30 10 40 X ○ ○ ○ ○ ○ ○ 30 20 40 X ○ ○ ○ ○ ○ ○ 30 40 40 X ○ ○ o ○ ○ o ○ o ○ Lithium hydroxide 30 20 40 X X X X X X ○ 30 40 40 X X X X X X ○ Potassium carbonate 30 40 40 X X X X X X ○

Claims: Method of removing a layer of glass formed on a sintered body manufactured by hot isostatic pressing of a preformed body of inorganic powder, ceramic powder or metallic powder having a glass covering using a pressure medium at a sintering temperature, wherein the sintered body with the glass covering is treated in alkali solution, characterised by the following steps (a) and (b), wherein in step (a) the sintered body with the glass covering is retained in an alkali solution having a concentration of alkali of at least 10% wt/v at a temperature of at least 160°C for at least one hour, the alkali concentration, the temperature and the retention time being selected to form a brittle reaction layer, and wherein after step (a) in step (b) the remaining glass is removed by abrasive processing or hammering, and excluding the case where the glass is a water-soluble boron-containing glass. Method according to claim 1, wherein potassium hydroxide at a concentration of at least 40% wt/v or sodium hydroxide at a concentration of at least 10% wt/v is used as the alkali.

Inventors: MATSUDA HIROTO, NARUKAWA AKIRA, WATANABE MITSUYOSHI, MATSUDA, HIROTO, NARUKAWA, AKIRA, WATANABE, MITSUYOSHI
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1992
Main classification: C04B35
IPC codes: B22F3, C04B41, C04B35