High temperature heat-treating jig

Abstract: This specification discloses a high temperature heat-treating jig comprising a heat-resistant base, characterised by a refractory metal or refractory metal alloy layer formed on the surface of the heat-resistant base. The surface layer, preferably of tungsten or tungsten alloy, and preferably at least 0.2 micrometer thick, may be formed by thermal diffusion or vapour deposition and dispersion.

Description: Background of the Invention This invention relates to a jig for high temperature heat treatment, and particularly a jig which is used for sintering various ceramics, and more particularly to a high temperature heat-treating jig which has excellent high-temperature strength, to which ceramics hardly adheres, and whose discoloration and color shading hardly occur. Heretofore, as a high temperature heat-treating jig, a plate material of molybdenum or molybdenum alloy which is a heat-resisting material has been generally used. This plate material has been generally produced as follows. First, an ingot prepared by sintering molybdenum powder is subjected to hot working such as forging or rolling at high temperature into a plate material. This plate material is put to practical use as a jig as it is, or subjected to annealing to remove distortion caused during processing distortion at a secondary recrystallization temperature or below, generally at a temperature range of 800 to 1200 degrees C, then to fabrication before being put to practical use. However, the inventors of the present invention found that the aforesaid conventional high temperature heat-treating molybdenum jig sometimes caused discoloration and color shading of a sintering part and the molybdenum jig during sintering of ceramics (for example at sintering temperatures 1500 to 2000 degrees C), and sometimes caused the sintering part to adhere to the jig. This invention has been completed to solve the above problems and aims to provide a high temperature heat-treating jig which has been solved the aforementioned disadvantages of a conventional high temperature heat-treating jig, which never causes discoloration or color shading during the heat treatment at a high temperature and which causes hardly any adhesion between a member to be heat-treated and the jig. By way of background, it is known from US-A-3072983 to use tungsten-coated molybdenum as a rocket nozzle insert. Transition metal coatings are known in other fields also, and for example in US-A-4812372 a chromium coating protects a chromium-molybdenum alloy refractory metal substrate from deterioration at high temperatures in the atmosphere. The invention provides a jig according to Claim 1; the use of a jig, as defined in Claims 5 and 6; a method of manufacture of a jig according to Claim 10; and a method according to Claim 18 of sintering a ceramic substrate. The inventors made various examination and found that adhesion of the jig with ceramics and discoloration or color shading during heat treatment take place by the dispersion of the element of a member to be heat-treated into a floor plate when treating at a high temperature. And it was found that providing a barrier through which dispersion is hardly made is very effective to prevent the dispersion. Then, various elements were examined and it was found that dispersion into tungsten is about 1/1000 as compared with the dispersion into molybdenum for example, although this dispersion varies with different elements. Since tungsten has sufficient heat resistance, providing a tungsten layer on the surface of a heat-resisting base has been found to be a very effective way of achieving the object of this invention. In other words, the high temperature heat-treating jig of this invention has a tungsten layer or tungsten alloy layer formed on the surface of a heat-resisting base, which consists of molybdenum or an alloy thereof. One example of the method for producing the high temperature heat-treating molybdenum jig is characterized in that tungsten powder or tungsten oxide (W-Blue-Oxide) powder is placed on a molybdenum base and annealed at 1700 degrees C or above, thereby forming a tungsten layer on the molybdenum base. Another production method of the high temperature heat-treating molybdenum jig has characteristics that tungsten powder or tungsten oxide (W-Blue-Oxide) powder is dissolved in a solvent to prepare paste, which is then applied to a molybdenum base, and annealed at 1700 degrees C or above, thereby forming a tungsten layer on the molybdenum base. And, still another production method of the high temperature heat-treating molybdenum jig is characterized in that a salt solution of tungsten is applied on a molybdenum base and annealed at 1700 degrees C or above to form a tungsten layer on the molybdenum base. Further a production of the high temperature heat-treating molybdenum jig is characterized in that a tungsten plate or tungsten alloy plate is placed on a molybdenum base and annealed at 1700 C or above, thus forming a tungsten layer on the molybdenum base. An even further production method of the high temperature heat-treating molybdenum jig is characterized in that coating of tungsten is formed on a molybdenum base by CVD or PVD method. As the heat-resisting base, those made of molybdenum, ceramics such as alumina or thermet can be used. And in view of resistance to deformation, processabilility and costs, one made of molybdenum is preferable. For example, as a structural material of the molybdenum base, a conventionally used high temperature heat-treating molybdenum material such as a doped molybdenum material containing one or more of Al, Si and K can be used. And pure molybdenum can be also used. When using the doped molybdenum material, a sintered doped molybdenum is hot-worked, then it is used as processed for fabrication or it is annealed at recrystallization temperature or below, generally at 800 to 1200 degrees C, to remove distortion before fabricating, or further heat-treated at a temperature higher than the recrystallization temperature (for example, 100 degrees C higher than the recrystallization temperature to 2200 degrees C) before being used as the molybdenum base. On the surface of the above heat-resisting base, a tungsten layer or tungsten alloy layer is formed, so that the tungsten layer or tungsten alloy layer works to prevent the dispersion of the elements of a member to be heat-treated from being dispersed into the heat-resisting base during heat treatment. For example, when the dispersion coefficient of each element to Mo and W base materials is compared, the dispersion coefficient of Fe at 1700 degrees C for example is 1.33x10⁻¹⁴ m²/s to the Mo base material and 5.37x10⁻¹⁹ m²/s to the W base material, the dispersion coefficient of Nb is 2.09x10⁻¹⁵ m²/s to the Mo base material and 2.41x10⁻¹⁹ m²/s to the W base material, the dispersion coefficient of Re is 4.23x10⁻¹⁶ m²/s to the Mo base material and 7.15x10⁻¹⁹ m²/s to the W base material, and the dispersion coefficient of U is 3.23x10⁻¹⁵m²/s to the Mo base material and 9.39x10⁻¹⁹ m²/s to the W base material. Dispersion into W is quite small as compared to that into Mo, though different depending on kinds of dispersion elements. This is almost the same to other heat-resisting bases (such as Ta). Therefore, forming the tungsten or tungsten alloy layer on the heat-resisting base surface prevents the dispersion of the elements of a member to be heat-treated into the heat-resisting base. As a result, discoloration and color shading of the jig and the member to be heat-treated can be prevented from occurring and also the jig and the member to be heat-treated can be prevented from adhering to each other. Further, tungsten has sufficient heat resistance and excellent strength at high temperatures, so that the jig's service life can be kept long. An example of the tungsten alloy layer includes rhenium-tungsten alloy. The tungsten layer or tungsten alloy layer to be formed on the heat-resisting base surface has a thickness of 0.2 micrometer or above, and preferably 0.5 micrometer or above. When it is less than 0.2 micrometer, providing the layer does not result in sufficient barrier effect. The upper limit of the layer thickness is not particularly restricted but making the layer very thick takes a long time for heat treating. Therefore, it is preferably up to about 20 micrometers. Six examples of production methods embodying the invention will now be described, by way of example. (1) A forming method of the tungsten layer on the molybdenum base by the present invention is that tungsten powder or tungsten oxide powder is placed on a molybdenum base and annealing is effected at 1700 degrees C or above. The tungsten powder or tungsten oxide powder used here has an average particle diameter of about 0.4 to 5 micrometers, and the heat-treating temperature is 1700 degrees C or above and up to 2200 degrees C, and preferably 1800 degrees C or above and up to 2000 degrees C. When the heat-treating temperature is less than 1700 degrees C, sintering takes a long time, so that such a temperature must be retained for a long time. On the other hand, when the temperature exceeds 2200 degrees C, a furnace service life is shortened very much and it is not economical. The heat-treating time is about one to ten hours. The heat treatment is preferably effected in reducing atmosphere such as hydrogen or wet hydrogen atmosphere. The thickness of the tungsten layer formed by the heat treatment varies depending on conditions such as heat-treating temperature and heat-treating time. For example, by the heat treatment effected at 1800 degrees C for 8 hours, a tungsten layer having a thickness of about 1 micrometer is formed. (2) Another forming method of the tungsten layer on the molybdenum base by the present invention is that tungsten powder or tungsten oxide powder is dissolved in a solvent to prepare paste, which is then applied on the molybdenum base, then annealing is effected at a temperature over 1700 degrees C. The tungsten powder or tungsten oxide powder used here has the same average particle diameter as above. The solvent used to form the paste includes for example methyl cellulose-based binder, ethanol, acetone and water. Application of the paste onto the molybdenum base is done by using a brush or by spraying. Thus, the paste is applied on the molybdenum and the solvent is thermally decomposed at about 400 degrees C, then annealing is made at a temperature of 1700 degrees C or above. The heat-treating conditions (temperature, time and atmosphere) for annealing are the same as above. The thickness of the tungsten layer formed by the heat treatment varies depending on conditions such as heat-treating temperatures and heat-treating time. For example, by the heat treatment effected at 1800 degrees C for 8 hours, a tungsten layer having a thickness of about 0.8 micrometer is formed. (3) Another forming method of the tungsten layer on the molybdenum base by the present invention is that a salt solution of tungsten is applied on the molybdenum base and annealing is effected at a temperature of 1700 degrees C or above. The salt solution of tungsten used here includes for example tungsten acid ammonia solution, tungsten acid sodium solution and tungsten acid solution. The salt solution of tungsten is applied on the molybdenum base and the solvent is thermally decomposed at about 400 degrees C, then annealing is effected at a temperature of 1700 degrees C or above. The heat treating conditions (temperature, time and atmosphere) for annealing are the same as above. The thickness of the tungsten layer formed by the heat treatment varies depending on the conditions such as a heat-treating temperature and heat-treating time. For example, by the heat treatment effected at 1800 degrees C for 3 hours, a tungsten layer having a thickness of about 1.1 micrometer is formed. (4) Another forming method of the tungsten layer on the molybdenum base by the present invention is that a tungsten plate or tungsten alloy plate is placed on the molybdenum base and annealing is effected at a temperature of 1700 degrees C or above. A tungsten plate or tungsten alloy plate having a thickness of about 0.1 to 10mm is placed on the molybdenum base or sandwiched between the molybdenum bases and heat treatment is effected for dispersion, thereby forming a tungsten or tungsten alloy layer on the molybdenum base surface. The tungsten alloy used here includes rhenium-tungsten alloy. The heat treating conditions (temperature, time and atmosphere) for annealing are the same as above. The thickness of the tungsten layer formed by the heat treatment varies depending on the conditions such as a heat-treating temperature and heat-treating time. For example, by the heat treatment effected at 1800 degrees C for 3 hours, a tungsten layer having a thickness of about 0.3 to 0.5 micrometer is formed. (5) Another forming method of the tungsten layer on the molybdenum base by the present invention is that a tungsten coating is formed on the molybdenum base by CVD or PVD method. In the CVD method, a reactive gas is flown over a molybdenum base of a high temperature to deposit a solid layer of tungsten on the base. The treating conditions include a base temperature of about 900 to 1100 degrees C, and reactive gas includes tungsten hexafluoride, H₂ or H₂+N₂ gas. The PVD method is a method that tungsten is vapor deposited or sputtered on the molybdenum base in vacuum or low-pressure gas and includes vacuum vapor deposition, sputtering and ion plating methods. Any of these methods can be used but the ion plating method is desirable. The CVD or PVD method forms a tungsten coating of about 0.2 to 20 micrometer thick. (6) Another method for forming a tungsten layer on the molybdenum substrate according to this invention calcines (i.e. sinters) the ceramics substrate (e.g. Al₂0₃,AlN, etc) having conductive layer W (at 1100 o C to 1800°C for example) to form the conductive layer W on the molybdenum substrate by vaporizing and depositing and dispersing. Examples of the conductive layer on the ceramics substrate include many such as molybdenum, tantalum and tungsten. Calcining the ceramics substrate possessing tungsten can form a tungsten layer on the molybdenum substrate. The thickness of the tungsten layer formed by this thermal treatment varies depending on a thermal treating temperature, thermal treating time and ceramics substrate's size and numbers. For example, when a 130 x 130-mm Al₂0₃ substrate possessing conductive layer W is thermally treated at 1800°C for 3 hours, there is formed a 0.3 to 0.5-micrometer tungsten layer. This method does not require a user who used to employ a molybdenum plate to use a special device and is very useful. That is to say, when a molybdenum jig is used, it is sufficient by calcining the ceramics substrate possessing tungsten conductive layer to intentionally form layer W. As to examples of the production method of this invention, they are mainly related to the forming of the tungsten layer. But it is obvious that such methods can be applied for forming a tungsten alloy layer. The following further examples are to illustrate the invention more specifically. It shall be noted that the invention is not limited to these examples. Example 1 On a molybdenum base, tungsten oxide powder (average particle diameter: 5 micrometers) was evenly placed. Sintering was made by heating in hydrogen or wet hydrogen atmosphere at 1700 to 2000 degrees C for 8 hours (in which the tungsten oxide powder was reduced). From the sintered product obtained, excess W powder was removed. W was dispersed into a molybdenum plate during the high-temperature treatment and formed a W layer to a thickness of about 1 micrometer. On the molybdenum floor plate thus obtained, an alumina base is placed, and sintering was effected at 1700 degrees C for 5 hours. The same sinterings were performed 50 times. As a result, the molybdenum floor plate did not adhere to the alumina base. And either of the alumina base or molybdenum floor plate was not suffered from discoloration or color shading. Example 2 At the final annealing of a molybdenum base, a 0.2-mm W plate having the same size as the molybdenum base was sandwiched one to another.The heat-treating conditions include 1800 degree C x three hours in hydrogen atmosphere. As a result, it was confirmed that W layer having a thickness of about 0.3 to 0.5 micrometer was formed on the molybdenum plate surface. An alumina plate was placed on the molybdenum floor plate and sintered at 1700 degrees C for 5 hours. The same sinterings were performed 50 times. As a result, the molybdenum floor plate did not adhere to the alumina plate obtained at all. And the alumina plate and the molybdenum floor plate were not gone discoloration or color shading. Example 3 To remove the oxides and adhered substance from the surface of a molybdenum base, it was washed with nitric acid, hydrochloric acid and hot water and dried. Then it was placed in a CVD furnace and kept at 1100 degrees C. Tungsten hexafluoride and hydrogen gas were sent in to form a tungsten CVD coating to a thickness of about 1 micrometer. On the molybdenum floor plate obtained, an alumina base was placed. Sintering was effected at 1700 degrees C for 5 hours. The same sinterings were performed 50 times. As a result, the molybdenum floor plate did not adhere to the alumina base at all. And either of the alumina base or the molybdenum floor plate was not suffered from discoloration or color shading. Example 4 As a molybdenum plate material, molybdenum powder having a purity of 99.9% or above and an average particle diameter of 3 to 5 micrometers was press-molded under a pressure of 2 tons/cm² by a hydraulic press according to a powder metallurgy method and sintered at 1900°C for 5 hours to form a pure molybdenum ingot having a thickness of about 30mm. This ingot was heated to the maximum temperature of 1300°C and rolled while gradually lowering the heating temperature according to the ordinary hot processing method. This procedure was repeated. Through the hot roll processing and cold roll processing, a molybdenum plate having a thickness of 2mm was obtained. This molybdenum plate was subjected to the crystal grain control method in a current of hydrogen at 2250°C for about 2 to 3 hours to obtain a molybdenum plate in which the disc shaped crystals in the circular part has a disc diameter of 20mm in average. The multilayer ceramics substrate having layer W which is first calcined from the above molybdenum plate will be described. A raw material of green sheet was prepared by adding a sintering aid of 1.2µm mean dia. Y₂O₃ or 3 wt.% to 1.5µm mean particle size AlN powder including 1.4 wt.% oxygen as impurity and by wet-blending the two for 24 hours with a ball mill. An organic binder was dispersed into this prepared raw material together with an organic solvent for form a slurry. The slurry was formed into a green sheet with a uniform thickness of 100 to 400µm in accordance with doctor blade method. The green sheet was cut into an about 130 x 130 mm square insulating body, and a 300µm dia. hole was formed to connect electric circuits formed on the insulating layers. To adjust tungsten paste to 97 wt.%, 1.29 wt.% of A1₂O₃ having an average particle diameter of 1 micrometer and 1.71 wt.% of Y₂O₃ having an average particle diameter of 1.2 micrometers were added. The resulting tungsten paste was printed on a green sheet by the screen printing method. Naturally, the holes in the green sheet are filled with the tungsten paste. This green sheet was piled to one to another and hot-pressed to prepare a laminate green sheet. This laminate green sheet was placed on the molybdenum plate obtained above and subjected to the next heating treatment. To evaporate the binder, the sheet was heated in N₂ atmosphere, then sintered in N₂ atmosphere at 1800°C for 5 hours. There was obtained a multilayer AlN substrate. At the same time, a tungsten layer having a thickness of about 0.7 micrometer was obtained on the molybdenum plate. To make it sure, the same molybdenum plate was calcined and sintered. Specifically, the laminate green sheet was differently positioned from the above and treated by the same procedure as above except that sintering was effected for 3 hours. As a result, a tungsten layer having a thickness of about 1 micrometer was formed on the molybdenum plate. On the obtained molybdenum floor plate was placed an alumina substrate then sintered at 1700°C for 5 hours Even after repeating this procedure 50 times, the molybdenum floor plate did not adhere to the alumina substrate. And the alumina substrate and the molybdenum floor plate did not undergone discoloration or color shading. Effects of the Invention With the high temperature heat-treating jig of this invention, a tungsten layer or tungsten alloy layer was formed on the surface of a heat-resisting base. As compared with a conventional high temperature heat-treating jig, a member to be heat-treated and the jig during the high temperature treating hardly adhere, and the occurrence of discoloration and color shading can be prevented. Particularly, when the heat-resisting base consists of molybdenum, since tungsten is very similar to molybdenum in properties such as heat resistance and strength at high temperature, the high temperature heat-treating jig of this invention can be used for high temperature heat treatment under the same conditions as those for a conventional molybdenum jig. The foregoing description and examples have been set forth merely to illustrate the invention and are not intended to be limiting. Modifications of the described embodiments may occur to persons skilled in the art, within the scope of the appended claims.

Claims: A high temperature-resistant product comprising a tungsten or tungsten alloy layer formed on one surface of a molybdenum or molybdenum alloy base, characterized in that the product is a jig suitable for sintering ceramics, in the form of a plate with a flat working surface. A jig according to Claim 1, wherein the crystal grains in the molybdenum or molybdenum alloy plate are disc-shaped. A jig according to Claim 1 or 2, wherein the tungsten or tungsten alloy layer has a thickness of about 0.2 micrometre or above. A jig according to Claim 3, wherein the tungsten or tungsten alloy layer has a thickness of 0.5 micrometre or above. Use of a jig according to any preceding claim as a heat-treating jig in the sintering of ceramics. Use of a high-temperature-resistant heat-treating jig with a heat-resistant base in the sintering of ceramics, characterized in that the base has on one face a layer of a transition element metal or metal alloy which has the effect of avoiding adherence and discoloration of objects to be sintered thereon. Use of a jig according to Claim 6, in which the materials of the said base and of the said layer thereon have a similar thermal resistivity and high-temperature strength. Use of a jig according to Claim 6 or 7, in which the said transition metal is tungsten. Use of a jig according to Claim 6, 7 or 8, in which the base is a molybdenum or molybdenum alloy. A method of manufacture of a jig according to Claim 1, comprising applying the said layer as a coating to the said surface of the base. A method according to Claim 10, wherein said applying step comprises placing tungsten powder or tungsten oxide powder on said molybdenum or molybdenum alloy base and annealing at about 1700°C or above. A method according to Claim 10, wherein said applying step comprises dissolving tungsten powder or tungsten oxide powder in a solvent to prepare a paste, which is then applied on the base, and then annealing the coated base at about 1700°C or above. A method according to Claim 10, wherein said applying step comprises applying a salt solution of tungsten onto the base and annealing at a temperature of 1700°C or above. A method according to Claim 13, wherein the salt solution of tungsten is selected from tungsten acid ammonia solution, tungsten acid sodium solution, and tungsten acid solution. A method according to Claim 10, wherein said applying step comprises placing a tungsten plate or tungsten alloy plate on the molybdenum base and annealing at about 1700°C or above. A method according to Claim 10, wherein said applying step comprises providing a tungsten coating on the base by a CVD or PVD method. A method according to Claim 10, wherein said applying step comprises placing a layer comprising tungsten and a binder over the base, and evaporating the binder. A method of sintering a ceramic substrate, comprising the steps of: providing a sintering vessel comprising a heat-treating jig having a heat-resistant base; placing on said jig a ceramic substrate to be sintered; and subjecting said ceramic substrate to sintering conditions while positioned on said layer; characterized in that the base has a layer of tungsten or tungsten alloy thereon, and the ceramic substrate is sintered whilst resting on that layer.

Inventors: ISHIHARA HIDEO C O INTELLECTUA, KAJIMA SHIGEKI C O INTELLECTUA, KAWAI MITUO C O INTELLECTUAL P, KIBATA MASANORI C O INTELLECTU, KITAMORI NOBORU C O INTELLECTU, YAGI NORIAKI C O INTELLECTUAL, YOKOSU KAZUNORI C O INTELLECTU, ISHIHARA, HIDEO, C/O INTELLECTUAL PROP. DIV., KAJIMA, SHIGEKI, C/O INTELLECTUAL PROP. DIV., KAWAI, MITUO, C/O INTELLECTUAL PROP. DIV., KIBATA, MASANORI, C/O INTELLECTUAL PROP. DIV., KITAMORI, NOBORU, C/O INTELLECTUAL PROP. DIV., YAGI, NORIAKI, C/O INTELLECTUAL PROP. DIV., YOKOSU, KAZUNORI, C/O INTELLECTUAL PROP. DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1991
Main classification: C23C30
IPC codes: C23C30