IL42147A - Process for the preparation of ceramic material based on sinterable metal powders and the material obtained - Google Patents

Abstract

1430144 Ceramic material BABCOCK & WILCOX CO 19 April 1973 [12 May 1972] 19159/73 Heading C1A A ceramic material comprises grains of sintered aluminium oxide having grains with a size distribution observed on a fracture surface with a scanning electron microscope at 10,000 magnifications as follows: less than 0 3 = 0%, between 0 3 and 0 5 = 25%, between 0 3 and 0 7 = 54%, between 0 3 and 0 9 = 80% and between 0 3 and 1 5 = 100%. A suitable method of converting a sinterable powder into a fine grain ceramic metal oxide of the above type comprises a first step of heating the powder to a temperature that produces an onset of powder shrinkage, a step of applying a maximum process physical pressure to the powder with the onset of shrinkage, a step of raising the temperature of the powder, with the onset of shrinkage, at a lower rate than the first heating step, to a temperature of more than 800 C. and holding the pressure and attained temperature for more than two minutes. It is preferred to heat the powder in the first heating step to more than 400 C. at a rate of at least 400 C. per minute and to use a maximum hot process pressure of 2000-6000 lbs./sq. inch. The preparation of Al 2 O 3 and UO 2 ceramic materials are exemplified. [GB1430144A]

Description

ηχιοπη npas Ο·*Ό y »ar>pnain man1? T' nn peion TDinn piputV mn'jn nsnD Process for the preparation of ceramic material , based on sinterable metal powders and the material obtained THE BABCOCK & WILCOX COMPANY : 40324 BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This Invention relates to materials and manufacturing processes for these materials and, more particularly, to an improved uniformly fine-grain alumina and a technique for producing this material, and the like.

DESCRIPTION OF THE PRIOR ART Alumina and alumina compounds, have been used for high temperature and high strength purposes for many years. For example, in refractory applications and in metalworklng tools that are subjected to high speeds and great wear, these materials have found widespread industrial acceptance.

It appears, moreover, that the strength of this material is in some manner related to its density nd crystal size, the more dense, and smaller crystal structures providing stronger and more durable tools. Consequently, there is a great deal of emphasis on producing ceramic cutting tools with these characteristics". When used as a cutting edge, however, alumina occasionally fractures. In general, these fractures seem to be related to the presence of relatively large alumina crystals, or "grains", in an essentially small crystal or "fine" grain structure. Thus, much of the alumina research effort has been directed to the more specific development of techniques for large-scale production of a high density material with a uniformly fine grain structure .

The crystal growth that occurs when the raw powder an amount of 0.5# or less. This heating can be.accomplishfr in a vacuum furnace that raises the material temperature to a l00° to 1550°C range. Processes of this sort have been reported to provide a material that has a crystal size on the order of 2 to 3 microns. To attain this result, however, heatin times in excess of four hours during sintering are required.

U.S. Patent Specification No. 3,093,498 discloses a hot press prooess at a fixed pressure yielding material of an average crystal size of 1 to 10 microns. U.S. Patent Specification No.3,311, 82 relates to material of an average grain size of about 7 microns obtained by a sintering process not including pressurizing the material'f U.S. Patent specification No. 3,377,176 describes a cold forming process which leads to a crystal size of material of about 2 to 5 microns.

In the interest of ef icienc and production economy, , it is clear that a reduction in heating time is desirable, especially if the reduced heating time can be . coupled with the production of a more uniformly ine grain structure. Because of the tendency or alumina tools to. fracture, there also is -a need for a technique to produce the even smaller crystal sizes that lead to greater strength. 5T3MMARY OF THE INVENTION In accordance with the invention, reduced heating time and a fine crystal structure of significantly improved uniformity in size than that which heretofore has been avail able is achieved through a novel control of the physical pressure that is applied to the alumina powder and the rate at which the pressurized powder is heated. Some material . produced through this technique has compressive and modulus of rupture strength? that are signi icantly greater than the best available alumina.

The process characterizing, the invention is, essentially, a form of rate-controlled sintering in which a relatively low pressure is applied to the die while the contained alumina powder is bein heated. In the course of this heating the compacted powder at first expands In volume. There is a point, however, termed the "onset of shrinkage temperature", at which sintering commences and the volume of the powder begins to shrink. A maximum hot process pressure is applied to the powder when this condition is reached. Subsequently, the powder temperature also is increased to reach the maximum temperature attained in the process . Thus , it seems that the physical pressure applied to the sintering powder lends an additional driving force that not only reduces; production time, but also provides a demonstrably superior product.

Exam le: Alpha alumina powder of less than one micron particle size is worked or ball milled in a dry mill from four to eight hours. Preferably, alumina sold by W. R. Grace Company under the name "Grace-KA 210" should be used as a raw material for the practice of the invention. This alumina powder has a surface area on the order of 9 meters ^/gram. It is, moreover, of very high purity, although it does contain 0.1% addition of MgO, Other aluminas also can be used, although experimental data does seem to indicate that best results are achieved With the Grace-KA 210 material.

To maintain alumina powder purity, moreover, the ball mill also should be formed from very pure alumina.

Upon completion of the milling step, the powder is baked for another four to eight hours at 50° to 100°C. o Baking the powder at 72 C seems to be a preferred temperature for this step in the process. These ball milling and drying operations appear to have the effect of removing excess surface gases to produce a finer-grained end product. 42147/2 It is possible, however, that the surface gas behaves as an ' impurity phase that causes severe selective grain growth at high temperatures.

After outgassing, the powder is screened through a 200 meshAUnited States Standard sieve to break up any agglomerates that may have formed.- The sifted powder is placed in a high temperature, high strength die. Typically, a graphite die in an inert, vacuum or reducing atmosphere is suitable for. the purpose. A compacting pressure of 3,000 to 8,000 pounds per square inch (psi) is applied to the powder within the die. In most situations, it has been found ' that an initial compacting, or "pre-press" pressure of 5750 psi leads to the best end product reeults. This pre-press ·.'·■ force is then reduced to a range of 200 to 1,000 psl. Generally, a reduction in pressure to 1,000 psl will produce acceptable results.

The powder and the die are placed in a hot press : or other high temperature and high pressure sintering device. A protective atmosphere, moreover, is established i the system in order to preserve the die.. A vacuum, a helium atmosphere, or a mixed atmosphere of helium and 8$ by weight of hydrogen have been found suitable for this purpose.

Starting then with the reduced pressure on the j compacted powder, the powder and die is raised to a selected i temperature by means of an induction heater at a rate that is bounded by 400°C per minute up to 1000°C per minute. In most situations, raisin the temperature of the powder and die to a selected temperature of 800°C as measured with an optical . ' 42147/2 billet. The onset of shrinkage usually commences as the 800 C I temperature is reached. This shrinkage may be observed with the aid of a linear variable displacement transducer that is attached to the ram that applies the pressure to the sintering -powder . When this shrinkage commences, the pressure on the powder is increased to reach 3600 psi. Although in this connection 3600 psi is a preferred pressure, suitable results can be attained with applied pressures in the 2,000 psi to 6,000 psi range.

As the application of this pressure continues, 'the temperature also is increased, but at a lower rate than that which characterized the initial increase to 800°C. " Thus, within six to ten minutes, the maximum; process temperature Is reached in the range from 1200° to 1800°C. Best results seem to be achieved with a temperature of about l600°C that is reached about eigh,t minutes after 800°C was ·■ ' ' · attained. These higher temperatures also are observed with an optical pyrometer. This maximum temperature is sustained for two to six minutes, and preferably for three minutes, if a maximum process temperature of lo00°C is achieved.

After this period of curing or sustained heatin · . at maximum temperature, the induction heater, or other ! source of heat is turned off and the pressure on the alumina i within the die is reduced to zero. A cooling period of one ! to five minutes should be sufficient to enable the die and the now sintered alumina to cool to room temperature for removal from the press and separation from the die.

Samples of sintered alumina, produced in the fore- tests the following average characteristics: Number of Avg. Knoop Sam les Hardness Grace- A100 8 20 5 Grace- A210 21 2334 Commercial Sample A 10 2277 Commercial Sample B 10 ' 1952 Avg. Compressive Avg. Modulus of Strength psi Rupture, psi Grace-KA 100 326,700 44,100 Grace-KA210 543,200 82,600 Commercial Sample A 321,000 £9,500 Commercial Sample B 404,300 : 65,700n Modulus of Rupture Standard Deviation, psi Grace-KA100 16,500 Grace-KA210 23,200 Commercial Sample A 16,000 Commercial Sample B 11,300 Compressive Strength Standard Deviation,. psi Grace-KAIOO 115,000 Grace-KA210 122,300 Commercial Sample A 111,600 Commercial Sample B . 104,200 Average Grain Size P— Grace-KAIOO 2.6 In this connection it. should be noted that the term "standard deviation" as used herein is the square root of the arithmetic mean of the squares of the deviations of the physical test data from their arithmetic mean. The superior properties, on the average, of the sintered alumina that can be obtained if the Grace-KA210 powder is used as a basic raw material in the process characterizing the invention is apparent. It should be noted that the Grace KA100 powder does not have an added 0.1$ MgO crystal growth inhibitor. In developing the foregoing test data, sample preparation has been found to exert a significant influence. Chemical polishing of the samples, for Instance, provides more, realistic modulus of rupture test data. Mechanical polishing, however, seems to be detrimental to the actual strength of the sample that is undergoing testing.

Studies with a scanning electron microscope at 10, 000 magnifications of the fracture surfaces of a representative sample of alumina ceramic that was produced in the manner described above demonstrates that the material has a grain size distribution as follows: Grain Size Range Percent of Grain Structure Less than 0.3 micron Qfo Between 0.3 and 0.5 micron 5$ Between 0.3 and 0.? micron 5^ Between 0.3 and 0.9 micron 8 Between 0.3 and 1.5 micron 100$ Thus, alumina ceramics manufactured in accordance with the principles of the invention have a grain structure terized the prior art. Crystals of much larger average size, e.g. two or three microns, ordinarily were grown in these prior art aluminas. Accordingly, a new alumina ceramic with a fine grain size and better grain size distribution that heretofore was. unobtainable Is provided under the terms of the invention; The invention, moreover, is not limited in appli- : cation to alumina but also can be used in connection wit •other metal-oxides* For example, uranium dioxide (U02) pellet fabrication can be Improved through the practice of . the. invention. Typically, a pellet density that is within fo of the theoretical attainable maximum can be reached by means of thi3 pressure and temperature rate controlled sintering. Illustratively, to achieve 5/6 of the theoretical maximum density, the powder is subjected to maximum process temperatures that are on the order to 800° - 900°C in an eight to nine minute heating cycle. Within this time cycle, moreover, physical pressure also is applied to the powder that is being sintered. There is, of course, an Initial or preliminary heating period of about one minute, characterized by the onset of shrinkage, during which time the powder is raised rapidly to a higher temperature and subjected to Increasing. physical or mechanical pressure .

The resulting uranium; dioxide pellets do not require grinding or other finishing operations because they are fabricated in dies of correct diameter. The elimination of a machine finishing operation in the fabrication of uranium dioxide reactor fuel pellets is especially beneficial because it reduces processin costs and eliminates a major The' various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and. specific objects attained by its use, reference should be had to the foregoing descriptive matter in which there are described preferred embodiments of the Invention.' » 4 42147/3

Claims (1)

1. CLAIMS 1 A method for converting aXmetal oxide powder into a fine grain ceramic material comprising the steps of heating th power within one to a temperature that produces an onset of powder applying a maximum process physical pressure to the powder with said onset of raising the temperature of the powder with said onset of shrinkage at a lower rate than said one minute heating to a temperature of more than and discontinuing said pressure and heating at the end of more than eight A method according to 1 converting a powder a fine grain ceramic metal oxide comprising the steps of heating the powder to more than in approximately one applying an increasing physical pressure to the powder while the powder being heated to said to attain a maximum hot process increasing the temperature from said to more than in an interval of about seven maintaining physical pressure at said maximum pressure for the hot and holding said pressure and said temperature for two A method according to Claim 2 wherein said step of heating the powder to old further comprises the step of applying a physical pressure to the powder of about 1000 pounds per square inch before said step A according to Claim wherein said maximum process pressure is in the range o 2000 to 6000 pounds per square method according to Claim 1 solidifying a powder the powde to of a the powde to a at lower of said powde a for a Δ of sold grain having a size distribution on with a o c of a a of A ceramic according to physical of of with a of ceramic to Claim 9 attributes compris an average E of about A crystalline alumina according to inch at the end said increasing the temperature alumina to about in a of about minutes afte the end of said and maintaining per square inch pressure and 1600 temperature an three insufficientOCRQuality