PROCESS FOR DENSIFYING A CERAMIC PART

Abstract: A process for densifying a ceramic part includes creat­ ing an oxygen-containing sealed atmosphere, heating and pressurizing a ceramic part within the oxygen-containing sealed atmosphere to a high temperature and a high pres­ sure, soaking the part at the high temperature and high pressure, and then cooling the part to ambient temperature and ambient pressure.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a process for densifying a sintered ferrite part comprising the steps of placing the ferrite part in a sealed container, introducing an atmosphere containing an amount of oxygen into said container, the remainder of the atmosphere being substantially inert, raising the temperature to a constant level over 1100 o C, raising the pressure to a constant level, soaking the ferrite part at said constant temperature and constant pressure, cooling the ferrite part to ambient temperature and ambient pressure. 2. Description of the prior art A process as described hereinbefore is described in German Patent DE-B2-2261644, according to which process an inert atmosphere is preferred, an amount of oxygen being used to maintain the desired oxygen content of the ferrite. The term ferrite generally denotes a class of non-metallic, ceramic ferromagnetic materials (MFe₂O₄) having a spinel-crystal structure. The M in the formula popularly is MnZn or NiZn. Ferrites have desirable magnetic properties such as a high resistivity relative to ferro-magnetic metals, low hysteresis losses and good permeability making them particular useful in magnetic heads for video, audio, data storage and other related applications. Generally, ferrites are manufactured by mixing measured amounts of metal oxides or metal carbonates and then pulverizing the mixture. The pulverized mixture is calcined and then wet-milled. The resulting wet-slurry is then spray-dried with a binder to produce a ready to press powder. Ferrite parts are formed by pressing, molding or extruding. After forming, the parts are sintered at temperatures, for example, of about 1,000 o C before the sintered parts are further densified to approximately theoretical density in a hot isostatic pressing (hipping) procedure. Prior art densifying processes "hip" the sintered ferrite parts in a sealed totally-inert atmosphere, typically, or argon. See, for example U.S. Patent No. 4,440,713, issued April 3, 1984, entitled "Process for Making Fine Magnetic Ferrite Powder and Dense Ferrite Blocks", for a discussion of hipping nickel-zinc ferrite in a pure argon atmosphere. Generally, hipping a ferrite or other ceramic part includes the steps of disposing a sintered ferrite part in a pressure vessel at ambient temperature and at ambient pressure, creating a sealed atmosphere within the vessel, heating the part to a soaking temperature while pressurizing the part to a soaking pressure, soaking the part at the soaking temperature and pressure, and then cooling the part to ambient temperature and pressure. A major drawback encountered with prior art hipping procedures is that, during hot isostatic pressing, the ferrite undergoes chemical changes which adversely affect the magnetic properties of the resulting hipped ferrit part. The inventors believe that such changes are caused substantially by hipping in the totally-inert atmosphere. In a total or pure argon atmosphere at a high temperature (e.g. 1185 o C) and a high pressure (e.g. 105 MPa), the inventors believe that the Fe ions of the ferrite are reduced, and that the reduced ions then change their sites in the crystal lattice of the ferrite. Thus, the chemistry of the hipped ferrite part is changed from that before hipping. For a discussion of various mechanisms which may be involved in this chemical change, see Van Vlack, Elements of Materials Engineering , Third Edition, - (Addison-Wesley Publishing Company, 1975). An additional drawback of the prior art hot isostatic pressing procedure is the necessity for closely surrounding the sintered ferrite or other sintered ceramic with a protective matrix (or "frit"). The frit consists of sintered ceramic pieces having the same chemical composition as the sintered ceramic material to be hipped. The frit is utilized in a prior art attempt to minimize the previously discussed chemical changes. SUMMARY OF THE INVENTION It is the principal object of the present invention to provide a ferrite densifying process which substantially preserves the chemistry of the ferrite to that before densifying. It is a further object of the present invention to provide a ferrite densifying process which eliminates the necessity for a protective matrix surrounding the ferrite part to be densified. In accordance with the invention, a process according to the preamble for densifying a sintered ferrite part is characterized in that the constant pressure has a level over 90 MPa and in that the atmosphere contains a fixed amount of oxygen within a critical range, said fixed amount being that which, at the conclusion of the process, leaves a thin layer of oxidation on the ferrite part without substantially affecting the chemical composition of the ferrite part. The inventors have discovered that, by precisely determining (at a point in the process prior to the soaking step) the level of oxygen gas included in the sealed atmosphere, the chemistries of the ferrite part prior to and after hipping are substantially identical. Interestingly, an oxidation layer has been observed by the inventors at the outer surfaces of a ferrite part hipped according to the invention. The oxidation layer evidences that the hipped ferrite part has not been reduced to any significant degree. Also, ferrite hipped according to the invention has a greater than 99% theoretical density. Preferably, the level of oxygen gas is a preferred level which the inventors have discovered is dependent upon the specific chemical composition of the respective type of ferrite material which is to be processed. Thus, for a MnZn ferrite material having a specific chemical composition (discussed below), the preferred level is .05% by volume of the sealed atmosphere. Using the preferred level results in the least amount of chemical changes in the specific composition during densifying. However, the inventors have discovered further that, for each type of ferrite material (e.g. for MnZn ferrite materials), the level of oxygen gas must be within a respective critical range of levels. Processing a type of ferrite material by using an oxygen level outside of its respective critical range produces a densified part having undesirable magnetic and undesirable other properties. Further and still other objects of the present invention will be more readily apparent in light of the description when taken in conjunction with the accompanying drawing. BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is a block diagram showing the general steps of hot isostatically pressing a ferrite part according to the invention, Fig. 2 is a graph showing (temperature and pressure) versus time of a preferred densifying process according to the present invention, and Fig. 3 is a schematic diagram of a hot isostatic pressing system for practicing the invention. DETAILED DESCRIPTION OF THE INVENTION The general inventive steps for densifying a ferrite part or other ceramic part are shown in Fig. 1. Step 1 represents creating an oxygen containing sealed atmosphere, step 2 represents heating the ceramic, step 3 represents soaking the ceramic and step 4 represents cooling the ceramic. According to the invention, the part is densified in an oxygen-containing sealed atmosphere created within the pressure vessel of the hot isostatic pressing system. The oxygen-containing sealed atmosphere must be created within the pressure vessel before the soaking step at which the part is maintained at a constant high temperature (e.g. 1200 o C) and at a constant high pressure (e.g. 105 MPa) for a suitable time period (e.g. 2 hours). More specifically, oxygen gas must be introduced into the pressure vessel at a point of the process prior to the soaking step so that the sealed atmosphere includes a level of oxygen. The inventors have discovered that the oxygen level, within a range for each type of ferrite or other ceramic, is critical for purposes of the invention. For example, a too high oxygen level (i.e. a high level outside of the respective critical range) when processing a MnZn ferrite can ruin the MnZn ferrite's magnetic properties. A too low level when processing a NiZn ferrite (i.e. a low level outside of the respective critical range) can ruin that ferrite's magnetic properties. Preferably, each specific composition of a respective type of ferrite or other ceramic material is processed using a preferred level of O₂. Processing not at the preferred oxygen level but at another level within the respective critical range produces a satisfactorily although not a most desirably processed part. For example, when processing a specific composition of MnZn ferrite material, the preferred level of O₂ within the sealed atmosphere of the pressure vessel prior to soaking is .05% by volume of the sealed atmosphere. Processing most specific compositions of NiZn ferrite or other ceramics (listed below) preferably employs an O₂ level in a range of 1.00% ≦ O₂ ≦ 1.66% by volume. The remaining portion of the sealed atmosphere in any and all of these cases can consist essentially of an inert gas, a combination of inert gases (e.g. argon and/or helium), or even nitrogen whether or not in combination with an inert gas or gases. The present inventors have compiled critical ranges for the O₂ levels for various types of ceramic materials. The O₂ level of the sealed atmosphere must be within the respective critical range in order to provide satisfactory results -- i.e. substantially not affecting the chemistry of, for example, the ferrite. Analysis of the O₂ level can be accomplished by known oxygen analyzation techniques performed at a point of the process prior to soaking. Any oxidation layer resulting at the outer surfaces of the hipped part can be removed, for example, by known machining techniques. Apparatus for densifying (hipping) the part according to the invention is commercially available and may be a Model No.15Q 16-42 SCF 140 hot isostatic press system manufactured by Autoclave Engineers, Inc., Erie, PA. As shown schematically in Fig. 3, the system 10 generally comprises a pressure vessel 20 which includes a heater 30, a pedestal 40, and a controllable gas vent 50. An evacuation pump is fluid coupled to the internal atmosphere 21 of the vessel 20. The system 10 also comprises gas handling equipment which includes a pair of high pressure pumps 60a, 60b which are fluid coupled to the contents of gas storage tanks 70a, 70b. The heater 30 is coupled to a power source 31 while the pumps 60a, 60b are fluid coupled to the internal atmosphere 21 of the vessel 20. See also U.S. Patent No. 4,247,755, issued January 27, 1981. entitled "High-pressure Autoclave". Tank 70a contains, for example, a pure Argon gas at approximately 12 MPa. Tank 70b contains, for example, a gas blend consisting essentially of Argon 98.3% and Oxygen 1.7% by volume, at 12 MPa. A preferred process for densifying a sintered manganese-zinc ferrite part will be discussed with reference to the curves P, T of Fig.2 in combination with the schematic of Fig. 3. The preferred process requires approximately 16 hours, after which the ferrite part is returned to ambient temperature and ambient pressure in any suitable known additional steps. The additional steps can require, for example, approximately 1-3/4 hours. A sintered manganese-zinc ferrite part 80 of a specific composition (mole%: Fe₂O₃-52.4%, ZnO-18.1%, MnO-29.5%) is suitably positioned on the pedestal 40 inside the vessel 20 whose internal atmosphere is at ambient temperature and ambient pressure. The vessel is sealed. The sealed vessel is evacuated by an evacuation pump 60c to a suitable vacuum (e.g. 25 Pa). At a starting time 0, argon gas from the storage tank 70a is introduced into the vessel 20 (e.g. by opening valves 71) until the pressure within the vessel is approximately 12 MPa. Raising the pressure to 12 MPa by thic technique (called "pre-filling") requires approximately one hour. The tanks 70a, 70b contain a pure argon gas and a blend of argon/oxygen gases, respectively. The oxygen/argon blend, preferably, is such that when the gaseous blend is introduced into the vessel's sealed atmosphere 21 which is at a particular temperature and pressure, the blend creates the preferred level of O₂ (in this case, .05% by volume) within the sealed atmosphere. At 12 MPa, additional argon gas is high pressure pumped (e.g. 17 MPa/hour) into the vessel 20 until the pressure within the vessel is approximately 16.4 MPa. At 164 MPa, high pressure pump 60b pumps (e.g. 17 MPa/hour) a blended gas mixture of O₂/Ar into the vessel from the storage tank 70b whose gaseous content, consists essentially of approximately 98.3% of argon by volume and approximately 1.7% of O₂ by volume. Pumping from the tank 70b ceases when the pressure within the vessel is approximately 16.7 MPa at which pressure further pumping from argon tank 70a raises the pressure within the vessel to approximately a 27.2 MPa set point. Introducing the argon gas between 167 and 27.2 MPa thoroughly mixes the O₂ gas within the sealed atmosphere of the vessel. Reaching the 27.2 MPa set point from the starting time takes approximately two hours. At the set point, the sealed atmosphere within the vessel isanalyzed to confirm that the preferred level of O₂ (in this case, .05%) is present within the vessel 20. After confirming the presence of the preferred level of O₂, heat is applied to the sealed atmosphere and, thus, by convection, to the part 80 by the heater 20 coupled to the power source 31. Preferably, heat is applied so that the temperature T within the vessel increases at a rate of approximately 240 o C per hour. Rates greater than 300 o C per hour created thermal shock problems for the ferrite. The pressure P within the vessel, of course, increases with increasing temperature. When the temperature and pressure within the vessel reach a soak point (preferably 1200 o C at 105 MPa) the application of heat is adjusted and the ferrite part is held (soaked) at this high temperature and high pressure for a soak time of preferably approximately two hours. The soak time may be longer, but should not be less than one hour for properdensification. After soaking, heater 30 is shut off and the part is cooled and depressurized naturally to a first let down point of approximately 130 o C and 35 MPa as shown by the curves T, P of Fig. 2.Once the part has cooled to the let down point, complete densification has occurred. The natural cooling, of course, results in the concomitant pressure reduction as shown by the curve P in Fig. 2. Cooling to the first let down point requires approximately seven hours. Returning the part to ambient temperature and ambient pressure can be accomplished by any known techniques. For example, at the first let down point, gases are released (e.g. by venting to the ambient atmosphere) from the vessel for approximately one hour until the pressure drops to about 28 MPa and the temperature to about 50 o (second let down point). Further venting four about one-half hour decreases the pressure to about 3.5 MPa. A final exhaust to the ambient atmosphere for about fifteen minutes decreases the temperature and pressure to ambient temperature and ambient pressure. The specific composition of the manganese-zinc ferrite part densified according to the above process exhibited substantially no change in its chemistry. However, the outer surfaces of the densified part has an oxidation layer of approximately 2.5 µm thickness. Significantly, analyzation of the sealed atmosphere at various points during processing revealed that the initially created O₂ level (.05 %) decreased by approximately 20% during a 16 hour processing cycle. The inventors believe that such decrease may be attributable to the ferrite part utilizing the O₂ in the sealed atmosphere in order to create the protective oxidation layer on its outer surfaces. Additional oxygen may be utilized by certain heating elements (e.g. SiC) of the heater 30. While there has been shown and described what is at present considered the preferred embodiment of the process according to the present invention, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended Claims. For example, heat can be applied at a faster rate not to exceed, however, approximately 300 o C/hr dependent, of course, upon the mass of the ceramic workpiece. Further, the faster rate can be utilized to achieve a soak temperature of 1300 o C at the soak pressure of 105 MPa. Also, a soak temperature of 1100 o C and a soak pressure of 91 MPa have also provided satisfactory results for specific compositions of ceramics. Generally, soak temperatures can be within a range of constant temperatures of between approximately 1100 o C and 1300 o C, while soak pressures can be within a range of constant pressures of between approximately 90 MPa to 105 MPa. It should be noted that natural cooling of ferrite parts during hipping should not exceed approximately 500 o C per hour in order to avoid thermal shock problems. Finally, instead of natural cooling, a ferrite or other ceramic part can be controllably cooled to the first let down point by controllable venting the high temperature and high pressure gases of the sealed atmosphere to the ambient atmosphere. The controlled cooling can also be used, for example, to reduce the level of O₂ (more specifically the partial pressure of O₂; partial pressure = mole fraction x total pressure) within the sealed vessel during cooling. Thus, the partial pressure of O₂ can be varied with temperature and the ferrite equilibrium relations stated by P.I. Slick, can be followed during cooling. See the article by Slick, P .I., entitled "A Thermogravimetric Study of Equilibrium Relations between a MnZn Ferrite and and O₂-N₂ Atmosphere," FERRITES: Proceedings on the International Conference , July 1970, Japan, pags 81-83, and particularly Fig.3.

Claims: A process for densifying a sintered ferrite part comprising the steps of placing the ferrite part in a sealed container, introducing an atmosphere containing an amount of oxygen into said container, the remainder of the atmosphere being substantially inert, raising the temperature to a constant level over 1100 o C, raising the pressure to a constant level, soaking the ferrite part at said constant temperature and constant pressure, cooling the ferrite part to ambient temperature and ambient pressure, characterized in that the constant pressure has a level over 90 MPa and in that the atmosphere contains a fixed amount of oxygen within a critical range, said fixed amount being that which, at the conclusion of the process, leaves a thin layer of oxidation on the ferrite part without substantially affecting the chemical composition of the ferrite part. A process according to claim 1, characterized in that the sintered ferrite part is a manganese-zinc ferrite and in that the sealed atmosphere includes oxygen at a level in the range from 0.005% to 0.1% by volume. A process according to claim 1, characterized in that the sintered ferrite part is a nickel-zinc ferrite and in that the sealed atmosphere includes oxygen at a level in the range from 0.05% to 2.0% by volume.

Inventors: BRAITLING, JOSEPH GEORGE, KACZENSKI, MARIE, WANK, LARRY ALAN
Applicants: NORTH AMERICAN PHILIPS CORPORATION
Application year: 1986
Main classification: C04B35
IPC codes: C04B35, H01F1, H01B3, C04B41