CA1078642A - Identifying densification rate of sinterable material - Google Patents

Abstract

ABSTRACT
To identify approximately the natural rate of densifi-cation of a sinterable material, a first physical pressure and heating rate are applied to a first batch of the material to begin the sintering of said first batch, an increased second physical pressure and a second heating rate are applied to said first batch, the density of the sintered first batch is recorded, a physical pressure and heating rate are applied to a second batch of the material to begin the sintering of said second batch, an increased third physical pressure and a third heating rate are applied to said second batch, the density of said sintered second batch is recorded to identify the density of said first and second batches of the material that is closest to the theoretical maximum density of said material.

Description

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-titanium carbide material and a technique for producing this material, and the like.
DESCRIP ION OF THE PRIOR ART
Alumina (A12O3) and alumina compounds have been used for hi~h temperature and high strength purposes for many years.
For example, in refractory applications and in metalworking tools that are subjected to hlgh speeds and great wear, these .~
materials have found widespread industrial acceptance. ~ :
In appears, mor~over, that the strength of this material is in some manner related to its density and crystal size, the more dense and smaller crystal structures providing stronger and more durable tools. Conse~uently, there is a great deal of emphasis on producing ceramic cutting tools with these characteristics. When used as a cutting ed~e, however, alumina occasionally fractures. In general, these fractures seem to be related to the presence of relatively large alumina crystals, or "grains", in an essentiall~ small crystal or "fine" grain structure. Thus, much of the alumina research effort has be~n --.
directed to the more specific development of techniques ~or large-s~ale production of a high density material ~ith a .~
uniformly fine grain structure. .
The crystal gro~th that occurs when the raw powder material is hea~ed to coalesce (:or is "sintered"~ often is retarded through.the addi.tion of ma~nesium oxide (M~O~ in an amount of 0.5% or less. This heating can be accomplished in a vacuum furnace that rai.ses the material temperature to a 1400 to 1550C. range. Processes o~ this sort have been ~ ;

reported to provide a material that has a crystal size on th.e order of 2 to 3 microns. To attain this result, however, :, ' ' ' ' :
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heating times in excess of four hours during sintering are required.
In the interest of efficiency 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 fine grain structure. Because of the tendency for alumina tools to fracture, there also is -a need for a technique to produce the even smaller crystal sizes that lead to greater strength.
SUMMARY OF THE INVENTION
In accordance with the invention, reduced heating time and a fine crystal structure of significantly improved unifor-mity in size than that which heretofore has been available is achieved through a novel control of the physical pressure that is applied to powder to be sintered and the rate at which the pressurized powder is heated. Some material produced through this technique has compressive and modulus of rupture strengths that are signi~icantly greater than t:he best available alumina.
The process characterizing the invention is, essentiall~, a form of rate-controlled sintering in which a relatively low pressure is applied to the die while the contained powder is being heated. In the course o~ this heating the compacted powder at first expands in volume. There is a point, however, termed the "onset of shrinkage temperature" or "onset of powder shrinkage" also called the "break away point", at which sintering commences and the volume o~ the powder ~egins to shrink. A maximum hot process pressure is applied to the powder when this condition is reached. Subsequently, the powder temperature also is i~creased to reach the maxiumum 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.

2 - -i36~2 According to the invention, powders are densified into fine grained sintered ceramics by applying a first compressing force to the powder and heating it at a predetermined first temperature rate during said force application to an onset of powder shrinkage and then densifying the powder by applying an ~ -increased second physical pressure and a second, lower heating rate thereto. To identify approximately the natural rate of densification, the density of a sintered first batch is recorded, and then a second batch is similarly treated, by applying a physical pressure and heating rate thereto to begin the sin-tering of the second batch, and then applying an increased third physical pressure and a third heating rate to the second batch and recording the density o the second sintered batch, to identify the density of the first and second batches that is closest to the theoretical maximum density of the material.
Conditions for approximating the natural densification rate, for the material under consideration, can thus be identified.
After the onset of powder shrinkage the densification rate is substantially constant but i* ~ecomes non-linear as the condition of maximum densification is approached.
B~IEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic graph of ram displacement versus time illustrate the "break away point", and Fig. 2 is an array of graphs that show pressure, temperature, density and breakaway point as function of time for a num~er of materials.
DETAILED DESCRIPTION OF THE PREFERR$D EMBODIMENTS
Fig. 1 graphically illustrates features of the invention expressed in terms of the movement or displacement of the ram ;
that is used to compress the powdered material which is being ~ `~
sintered as a function oE time. The ram displacement lQ is necessary to "prepress" the powdered mixture in order to enhance ~;
sintering and to remove any entrapped gases in the powder between

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time o and tl. After time tl and before time t2, the application of heat to the precompressed powder leads to a thermal expansion displacement 12 of the ram. This step in the process is terminated by a "break away point" 13 at the time t2. This "break away point" is characterized by a change from the expansion of the precompressed powder to a contraction 14 that commences as sintering begins. The contraction culminates at time t3. The time t3 is a time of ma~imum densification and coalescence of the sintered po~der. The further application of heat after time t3 produces excessive grain growth or "bloating" 16 as indicated ~y the increase in ram displacement. It is at this time t3, before the material starts bloating, that the process is terminated.
Fig. 2 is a graphic representation of sintered product temperature, density and "break away points" as a function of ~ -time for the following materials:
Billet Diameter Material 1/4" U2 1" A123 5" A123 1" A12O3-TiC

For purposes of orientation between Figs. 1 and 2 the initial time, zero, of Fig. 2 corresponds to the time tl in Fig. 1.
The pressure "histor~" 20 for all of these materials is bounded by straight line segments that identify a pressure increase, a step function from the initial pressure to the maximum hot process pressure that is maintained throughout the remainder of the process.
The temperature history 22 is ~ounded by straight line segments. These temperature bounds indicate an increasing :

temperature i~ response to the initial heating, follo~ed by a ;
minimum and maximum process temperature range for the remainder ;~

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of the process.
The theoretical maximum density "history" 24 follow paths to maximum values which are represented by a generalized graph 24. The theoretical maximum density is defined as the closest possible packlng of atoms into the crystalline structure of the compound, exclusive of any and all impurities, that will produce a minimum interstitial volume between the packed atoms.
The break away points as a function time 30 vary, moreover with the material and billet size under consideration.
Examples: -Alpha alumina powder of less than one microm, preferably less than one tenth 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-K~ 210"
(trade mark~ 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 meters2/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 (trade mark) material.
To maintain powder purity, moreover, the ~all mill also should be formed from very pure alumina.
Upon completion of the milling step, the powder is baked for another four to e.ight hours at 50 to 100C. Baking the powder at 72C. seems to be a preferred temperature for this -~
step in the process. These ball miliing and drying operations appear to have the effect of removing excess sur~ace gases to ~;
produce a finer-grained end product. The relation bet~een the surface gas and the grain size of the fully processed material has not been definitely established. It is possible, ho~ever, that the surface gas behaves as an impurity phase that causes severe selective grain growth at high temperatures.

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After outgassing, to produce a one-inch diameter billet of A12O3 in accordance with the invention, the powder is screened through a 200 mesh United 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 4000 to 8000 pounds per square inch (pse) is applied to the powder within the die. This pressure is applied to initially compact the powder to 30% to 50% of its maximum theoretical density.
For this sample, it has been found that an initial compacting or "pre-pressing" pressure of 5750 psi leads to the best end -results. This prepress force is then reduced to a range of 500 to 1000 psi. Generally, a reduction in pressure to 1000 psi 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 in this system in order to preserve the die. A vacuum, a helium or other inert atmosphere, or a mixed atmosphere of inert gas and 8% by welght o~ hydrogen have been found suitable for this purpose.
~Furthermore, relatively less~expensive nitrogen gas may be used for~proces~ economy.
Starting then with the reduced pressure on the compacted -powder, the temperature o~ the powder and die is raised by ~-means~of an induction heater at a rate that is ~ounded by 400 to lOQ0 per minute. By pxoper positioning and sizing of the .: .
induction heater and the billet generally uniform heating throughout the powder can be established. ~ithin the a~ove range it appears that the rate of temperature change can be varied in an almost random manner until the onset o$ shrinkage of "break away point" 13 ~Fig. 1~ is reached without degrading the quality of the final product.

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7~2 With respect to the sample under consideration, numerous tests indicate that raising the temperature, within the above rate boundaries, of the powder and the die to 760 to 815C. as measuredwith an optical pyrometer will produce the desired result. That is, the onset of shrinkage or "break away point" usually commences as the temperature reaches about 800C. In accordance with a feature of the invention, while the temperature is being raised to the illustrative 800C., to commence shrinkage, the reduced pressure of lO00 psi also is applied to the powder billet. 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.
After the "break away point" is ~eached, both temperature and pressure are increased in order to promote the rate of densification that is inherent or natural to the particular material and billet size. Both pressure and temperature can be monitored and adjusted to approximate this natural rate.
This natural densification rate is îdentified through a series of tests conducted with sample powders. In each of these tests, pressure and temperature increase rates are varied to identify the ranges of pressures~ 20 ~Fig. 2~ and temperatures 22 that provide the closest approach to the theoretical maximum density 24. It should be noted in Fig. l that the natural rate of densification changes as the powder is sintered into its maximum densification as indicated ~y the minimum billet volume at time t3.~ ~
With respect to the above alumina examplel the onset of powder shrinkage is accompanied by an application to the now sintering billet of a physical or ram pressure o~ 3600 psi.
Although this a preferred maximum process pressure, suitable results are obtained with pressures in the range of 200Q to 6000 psi. This rapid increase in pressure is reflected in the step-function pressure change that characterizes the pressure .

graph 20.
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 800C. Best r~sults seem to be achieved with a temperature of about 1600C. that is reached about eight minutes after the earlier 800C. temperature was attained. These higher temperatures also are observed through an optical pyrometer. This maximum temperature and pressure are sustained ~or two to six minutes, and preferably for three minutes, if a maximum process temperature of 1600C~
is achieved. During this time, the alumina is sintering at its "natural" or inherent rate of densification.

The linear change in ram displacement between the times t2 and t3 shown in Fig. 1 is a characteristic ~eature of a billet that is sintering at this natural rate. Other natural densification rate indices are possible, although ram displace-ment is a most con~enient technique. ~-In accordance with the invention, from a broad viewpoint the pressure and temperature that arè applied to the sintering billet after the "break away point" 13 has been reached are justed to establish and mai~tain this natural densification rate. The natural densification rate will, of course, vary according to the material that is being processed. Thls natural rate, moreover, also ma~ vary for different batches of the same material~ Consequently, the precise temperature and pressures that should be applied to the sintering bill~t for any particular material can be determined through a nu~ber of tests each performed on a different batch of the material.
~ These tests will identi~y those conditlons that produce the linea~ ram displacement 1~ (Fig. 1), or other indications of the natural densification rate, for the material under con~

sideration. Once these sintering conditions are identified, subsequent billets can be processed ~ithout ram displacement .

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observations and the like.
Thus, the natural rate of densification can be approxi-mately identified by applying a first physical pressure and heating rate to a first batch of the material to begin the sintering of said first batch, applying an increased second physical pressure and a second heating rate to said first batch, recording the density of the sintered first batch, applying a physical pressure and heating rate to a second batch of the material to begin the sintering of said second batch, and applying an increased third physical pressure and a third heating rate to said second batch, recording the density of said sintered second batch to identify the density of said first and second batches of the material that is ~-closest to the theoretical maximum density of said material.
A more detailed consideration of Fig. l indicates that the ram displacement is not entirely linear toward the completion of sintering 17. Thus, as shown in the drawing, the rate of ram displacement as a function of time decreases as the sintering billet approaches a condition of maximum i;-densification. As this terminal portion of the sintering process is approached, the pressure and temperature applied to the billet is stabilized for t~o to six minut~s to "cure" the now sintered billet.
Thus, in the process of Fig. l to density a metal oxide powder into a fine grained ~intered ceramic, during stage 12, a compressing force is applied to the powder and the powder is heated at a predetermined temperature rate during said force application to an onset of powder shrinkage at 13, and the powder is subsequently densified (sinteredl through at least two sequential decreasing densification ~sintering) rates, during the sintering and curing stage 14, by variation of said force. The densification rate is constant, or linear, until the terminal portion at 17 is approached when the densi~ication _ 9 _ -" ~ .
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rate is non-linear, the rate of sintering being controlled by adjustment of the applied pressure.
Care must be exercised to terminate production conditions at this point`in order to prevent the development of a "bloated" billet. This "bloating" 16 is characterized by a reduced density billet, as indicated through the greater billet volume which the increasing ram displacement registers.
Turning once more to the completion of sintering 17, it is possible to more precisely promote the natural rate of sintering, which apparently changes as maximum densification is approached, by adjusting the temperature and pressure that is applied to the sintering billet in a manner that will ~-enable the ram displacement to more nearly approximate the preferred curve illustrated in Fig. 1.
After the period of curing, or sustained heating at the maximum process temperature and pressure, the induction heater, or other source of heat, i.9, ~turned off and the pressure on the alumina within the die is reduced to zero. A cooling period of one to five minutes is sufficient to enable th~e die (and the now sintered alumina~ to cool to room temperature ~or removal from the press and SeparatiQn ~rom the die.
Samples of sintered alumina, produced in t~e foregoing manner, have shown in care~ully executed laboratory tests the Eollowing characteristics: ~

Num~er-of Ayg.~Knoopll~ a *
Samples Hardness Grace-K~ 100 (trade mark~ 8 2045 Grace-KA 21Q (trade mark) 21 2334 Commercial Sample A10 2277 Commercial Sample Bl ln 1952 -- ~
* Knoop hardness is a measure of the microhardness of a material by means of a long, narro~, diamond shaped impression.
The hardness number is calculated as the ratio of the indenting . ' .' ~ ' ' '. , ~>71~

load to the projected area of the indentation: THE MAKING, SHAPING AND TREATING OF STEEL, UNITED STATES STEEL 8th EDITION.
In this connection it should be noted that the term "standard deviation" as used herein is the s~uare root of the arithmetic mean of the squares of the deviations of the physical test data ' from their arithmetic mean.

Avg. Compressive Avg. Modulus of Strength psi Ru~ture, psi Grace-KA 100 (trade mark) 326,700 44,100 Grace-KA 210 (trade mark) 543,200 82,600 ~' Commercial Sample A 321,000 59,500 ' -Commercial Sample B 404,300 65,700 Modulus of Rupture Standard Deviation, ~si ~ . _ :
Grace-KA 100 (trade mark) 16, 5ao Grace-KA 210 (,trade mark) 23,200 Commercial Sample A16,000 Commercial Sample B11/300 ' ,, Compressive Strength Standard Deviation, '' _ psi -Grace-KA 100 (trade~mark) 115,000 '~

Grace-KA 21Q (trade mark~, 122,300 ':

Commercial Sample A111,600 Commercial Sample B104,200 Average-Grain Size Grace-KA 100 (trade mark~ 2.6 '~
Grace-KA 210 (,trade mark) 0.72 Commercial Sample A 1.3 Commercial Sample B 1.7 The superior properti~es, on the average, of the sintered alumina that can be obtained i~ the Grace-KA 21a (trade mark) powder is used as a basic raw material in the process character-izing the invention is apparent. It should be noted that the : .- . . . , , ,: .: .~ . ,, : : . :

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Grace-KA 100 (trade mark) powder does not have an added 0.1%
MgO crystal growth inhibitor. In developing the foregoing test data, moreover, 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 a ' magnification of 10,000~ of the fracture surfaces of represen-tative samples of alumina ceramic billets in the 1" to 5"
diameter range that were produced in the manner described above demonstrate that the material has a grain size distribution as '~
follows:
Grain Size Range'Pe'rcent'of Grain Struc'ture Less than 0.3 micron 0%
Between 0.3 and 0.5 micron 25~
Between 0.3 and 0.7 micron 54%
Between 0.3 and 0.9 micron 80%
Between 0.3 and 1.5 micron 10Q%
The "break away point" graph 30 in Fig. 2 illustrates the relation between the diameter o~ the end product billet and the process conditions. Thus, to manufacture a ~ive inch diameter ~illet o~ the ~12O3 in accordance with the principles of the invention, some~hat higher tamperatures and pressures should be applied during processing than these conditi~ns which are mentioned above with respect to the` one inch diameter billet. It shbuld be kept in mind, hbwever, that a basic ~ -feature of the invention for all of the'materials and billet ~'~sizes described herein is ~Ae-appll~at~on of ~n ~increased process pressure, within descri~ed boundaries throughout the sintering process, i.e. after the 'l~reak away point" ~Fig. 1~.

Moreover, a maximum process pressure, an observed optimum, is identified within the described bounds obtained by comparing the pressure "history" of the sintering billet with the density of the billet, and may be more conveniently applied to the billet to provice the desired closest approach to the theoretical ma~imum density.
Thus, alumina ceramics manufactured in accordance with the principles of the invention have a grain structure that is different from those grain sizes that have characterized the prior art. Crystals of much larger average size, e.g. two or three microns, ordinarily were grown in these prior art alumina.
Accordingly, a new alumina ceramic with a fine grain size and better grain size distribution that heretofore was unobtainable is provided through the invention.
The invention, moreover, i5 not limited in application to alumina but also can be used in connection with other metal- `
oxides. For exarnplet uranium dioxide ~UO2) pellet fabrication can be improved through the practice of the invention. Typically, -a pellet density that is within 1/2% of the theoretical attain- -able maximum can be reached by means of this pressure and temperature rate controlled sinterinc~. Illustratively, to achieve 95% of the theoretical maximum density, the po~der is subjected to maximum process temperatures that are on the order to 8Q0 to 900C in an eight to nine minute heating cycle.
Within this time cycle, morebver, physical pressure also is applied to the powder that is being sintered. There is, o~
course, an initial or preliminary heating period of about one minute, characterized by the` onset of powder 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 grïnding or other finishing operations because they are fahricated 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 ' .:, ~

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~71~ L2 processing C05tS and eliminates a major source o~ fissionable material manufacturing waste.
A further example of the invention comprises the sintering of alumina with other carbides~ nitrides or oxides to improve further the physical properties of the resulting product. As a specific example five inch diameter billets of alumina-titanium carbide (.A12O3-TiC) were made from 70% alumina powder Grace-KA 210, trade mark and 30~ titanium carbide powder.
The original particle size of the titanium carbide powder is 2 to 4 microns. The particle size is reduced by ball milling for 16 hours in alcohol, to an average particle size of 1 micron.
The ball-milled powder is mechanically mixed with t.he alumina powder for uniform distribution o~ the two materials in the resulting powder. Illustrati.vely, the alumina and the ball milled TiC are blended together .in an alcohol mixture in a ball mill ~or ~our hours. These mixed materials are removed from the ball mill, the alcohol i.s evaporated and the resulting powder, having thus been ~orked to r~emo~e surface gases and to reduce agglomerates in the powder, is prepressed or compacted Z0 with a pressure in the ra.n~e of 4QQ0 psi to 80Q0 psi to achieve a prepressed ~illet that has a density that is 30% to 50~ of ; the maximum theoretical density. For the example under con- -sideration, the 630Q psi pre-pressing pressure affords a sui.table . balance between powder pack.ing and the eliminat;on of entrapped gases. The applied ram pressure is then reduced to a range of 500 to lQQQ psi. While this lower pressure is ~eing applied, the material is heated at a rate that is not less than 400.C per minute nor more than lQG0C per minute until the onset of shrinkage commences., usually at a~out 800~C. ~hile the materi.al is being heated to this 80QC temperature the aforementioned i~
reduced pressure is maintained constant to provide ~illet ..
integrity, a.s noted abo~e. With the onset of powder shrinkage that occurs at point 13 on the "b.reak away" graph 30 in Fig 2, . :
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the ram pressure on the now sintering billet is increased to 5000 psi, the preferred maximum hot process pressure. Suitable results, however, can be obtained with applied ram pressures in the 3000 to 9500 psi range.
As the application of this pressure continues, the temperature is increased, but at a lower rate than that which characterized the initial increase to 800C. Thus, within six to ten minutes, the maximum process temperature is reached in the range from 1200C to 1800C. Based on available experimen- :-tal data, best results are achieved with a temperature of about 1500C. For curing this maximum temperature and 5000 psi pressure are sustained for two to six minutes. Thus, the powder ..
is sintered at a rate of densi.fication that approaches the theoretical maximum density of the powder until material . .
densification is complete~ -. :.. :
As shown in Table I ~elow, t~.e resulting materi.al is supe~ior to chemically similar materi.als that are produced . -through.prior art processes. - -T~lenty, five-inch diameter bi.llets of alumina-tï.tani.um carbide ~ere fabricated i.n accordance with the principles of the invention to demonstrate process reproducibility and the , euperi~or physical characteristics o~ the product.
The resulting densi.ty data ~or all 2Q ~illets, is shown~
in Table I. The average billet density ~as 4.257 g/cc -0.07%, whereas the prior art density for this material is 4.21 g/cc.
The term average, as used herein, is the quotient of the arithmetic sum o~ the data divided hy the number of data values used in calculating the sum. .:
TABLE I. BILLET DENSITIES . . .

BILLET NUMBER ~ LLYL8~
. . .
:. ,.
63 4.249 ....

64 4.259 ~ .

4.254 ~
.

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, ~ , . . .. . , . . . ~ -. :. .. .. : .

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BI LLET NUMBERDENSITIES (b/cc ) 66 4.257 67 4.256 68 4.260 69 4.260 4.256 71 4.254 72 4.258 :
73 4.258 74 4.2~0 4.262 76 4.257 77 4.258 -78 4.258 79 ~ .256 4.260 81 4.254 82 4.260 Average 4.257 gm/ccStandard Deviation 0.00 3 gm/cc (Q.Q7~
~ The billets were ground top and bottom on a Blanchard model No. 11 grinder and diced into 21 blank~, each 3/4" square and 5/16" thick. From each of the 20 ~illets, two of the 21 blanks ~ere randomly selected ~or transverse rupture strength tests. ~TRS). The two ~elected rupture test blanks were each ;sliced into three 1~4'l by 3~4" by 5/16" parallele~ipeds to provide a total of six rupture specimens for each hillet. ~-The specimens were surace ground on all sides ~or edge sharpness ~-.: .
and size uniformity.
The individual specimens were tested for transverse rupture 6trength by a three-point loadin~. Thè transverse rupture strength (TRS) results of these tests are ta~ulated in Table II.
In Table II, the average TRS of the six rupture specimens taken from each billet is tabulated below along with the standard :. .

~7~ 12 deviation for this data. The overall average (the average of the average of each group of six samples) and -the standard deviation of this overall average was found to be 124,333 - 11,542 psi.
TABLE II
BILLET TRANSVERSE RUPTURE STRENGTHS
BILLET NO. SAMPLE NO. TRS (PSI) AVE. TRS (PSI) 63 1 114,379 2 145,899 ~ -3 89,388 124,557 ..
4 133,232 ~18,750 139,993 .
6 124,453 . . .
64 1 139,002 2 150,663 - 3 146,399 146,187 ~ .
4 137,252 ~ 5,967 ~ :
5 149,353 6 154,456 ~ .
, 1 136,167 20 : 2 150,363 :` 3 120,494 113,799 ; 4 I04,129 ~28,545 ~; 5 59,861 . .:
.
6 111,783 66 1 89,962 --.~;
~.~ . . .
2 127,815 ~ ~.
~ 3 78,107 117,92~ : -; 4 132,966 -24,591 , . .
142,334 6 136,392 - 17 ~
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.. ~
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BILLET NO. SAMPLE NO. TRS (PSI) AVE. TRS (PSI) ~ . .
67 1 119,570 2 12~,8~8 3 87,309 97,015 4 70,860 ~20,937 104,966 6 74,539 68 1 135,543 2 139,812 -3 125,472 132,003 4 118,659 ~ 7,904 131,558 6 140,972 69 1 136,554 2 106,444 3 8g,163 117,195 4 148,363 ~21,286 : :
125,49~8 ~ -6 97,147 -:
. .-: 20 70 1 161,221 , 2 147,558 ~ -3 84,116 130,23~
4 108,674 ~26,180 ~ `
:~ 5 145,953 ~ : -6 ~133,870 ,' ":
71 1 141,234 .~:
2 128,75Q : ......... .

3 ~ 135~,508 137,381 4 131,122 ~ 6,832 14g ,525 :
6 138,146 , - 18 - ~

, . . . .

- ~37~6~2 BILLET NO. SAMPLE_NO. TRS (PSI? AVE. TRS (PSI) 72 1 119,675 2 126,684 3 68,657 108,767 4 142,746 ~27,271 86,072 6 No Test 73 1 144,735 2 146,156 3 132,57~ 137,011 4 151,390 +12,447 133,613 6 113,596 74 1 116,096 2 136,419 - ~ .
3 99,93s 120,030 . , .
4 118,341 -14,520 14a,984 .:
6 108,405 , "', ' . '. ' :~ 20 7S 1 149,731 ~ :
2 132,475 . .
3 143,868 143,487 :
: : 4 151,635 - 6,244 ~ ;
:: -141,792 6 141,422 ~
,, 76 1 121,548 :
:: 2 : 133~975 :`
3 79,607 121 j824 4 139,120 +19,871 123,1Q4 : .
6 133,589 BILLET NO. SAMPLE NO. I'RS (PSI) AVE. TRS (PSI) 77 1 132,863 2 141,150 3 107,048 130,709 4 111,856 +15,711 147,159 6 144,178 78 1 137,132 2 118,460 3 113,052 120, ~46 4 108,541 ~18,356 96,384 ~ .
6 151,509 : ~
.. ': ,, "
79 1 103,270 :-2 118,557 ::
3 110,070 120,667 ~ .
4 124,957 -11,877 13~,715 6 127,435 ~: .

~: ~0 : 8Q 1 6~,338 -2 gO,804 :-, . .
:`~ 3 152,226 . 119,563 4 110,48~ +.31~824 . .
151,916 :
~ 6 142,611 :
: 81 1 14Q,372 2 141,461 ;
: 3 136,273 130,407 ~ 4 138,052 ~16,9a8 g3,083 6 133,200 ~: -., . . . : .. , , . :.... . ~ . .

BILLET NO. SAMPLE NO. TRS (PSI) AVE. TRS (PSI) _ 82 1 141,398 2 78,340 3 139,6~2 117,042 4 88,848 +25,~37 115,036 6 138,986 The broken transverse rupture specimens were mounted and polished for hardness testing. Rockwell A hardness tests were discontinued when three of the Rockwell indentors were ruined after application to only five billets. In passing, it should ~e noted that a Rockwell test is a measure of hardness as manifested by the materials resistance to the penetration of an indentor in response to the application of a known load.
The subscript, A in this test, indicates the load and indentor type used in the test or this material (THE MAKING, SHAPING
AND TREATING OF STEEL, UNITED STATES STEEL 8th EDITION, 1964).
Knoop hardness tests, however, were performed on all twenty billets. The hardness data are tabulated in Table III.
Although thR RockwellA tests are not conclusive due to the above mentioned ~reakage problem, the average of ~ive data points indicates a 0.8 increase in the RockwellA hardness over ~ -the prior art. This ~.8 increase is a significant improvement over the prior art because increases of 0.1 are of practical ; `~
importance in the industry, e.g., tools are graded ~y increases of 0.1 in RockwellA hardness.
TABLE III. BILLET ~RDNESS
BILLET NO. - R. HARDNESS KNOOP HARDNESS
63 g3.75 3557 6~ 93.78 3557 98.83 3557 66 g3.78 3557 67 93.95 3557 . . .

BILLET NO. R. HARDNESS KNOOP HARDNESS
68 N.D. 3557 69 N.D. 3557 N.D. 3557 71 N. D . 3227 72 N~D. 3557 73 N.D. 3557 74 N,D. 3557 N.D. 3557 10 76 N.D. 3557 77 N.D. 3227 -78 N.D. 3557 79 N,D. 2940 N,D. 3227 81 N.D. 3557 -82 N.D. 3557 Av~rage ~3.82 N.D. = Not Determined Average 3477 Two of the six ~roken transverse rupture specimens from each hillet were photographed at a ten power ~10X) magnification, Sample A, for macro-homogeneity, i.e. visible differences in the color of the sample material under inspection. Only one specimen from all of the samples studîed showed an inhomogeneity (a 0.4 mm equivalent diameter titanium carbide particle) as enumerated in Tahle IV belo~. The equivalent size o~ the inhomogeneities listed in Tahle IV, moreover, are defined as the average of the major and minor axis of the inhomogeneity.
TABLE IV. BILLET MACRO-HOMOGENEITY
- NUMBER OE VISIBLE EQUIVALENT
DIFFERENCES IN SIZE OF
30 BILLET NO. SAMPLE A NO., _OLOR _ _ INHOMOGENEITY

64 1 a -- .. --.':

... .
.. : . . . . . .. .. .

3L~71~ Z:

NUMBER OF VISIBLE EQUIVALENT
DIFFERENCES IN SIZE OF
BILLET NO. SAMPLE A NO. COLOR INHOMOGENEITY

2 0 __ 2 0 __ 2 0 __ 2 0 __ 2 0 __ 2 ~ __ 2 0 __ -2 0 __ 2 0 __ 1 1 0.4 mm~ ~-2 0 __ :

2 0 __ 2 0 __ 7~ 1 0 __ 79 1 o __ 2 0 _ 1 0 -- ' 2 0 _~

2 0 __ Two of the broken transverse rupture specimens from the , :
30 remaining samples of each billet, Sample B, were randomly seIected for micro-homogeneity. These micro-homogeneity samples were polished and photomicrographed at 900X magnification. The results tabulated in Ta~le V below indicate that the average largest titanium carbide agglomerate was 12 microns, and the average titanium carbide grain was 4.82 microns. It should be noted that agglomerates are combinations of two or more grain~

into one mass.

,-' ~."
;'.'' " ~
.,:

'' ' ' .. . .

.

1~7~2 TAsLE V. BILLET MICRO-HOMOGENEITY

LARGEST TiC TiC AGGLOMERATES
AGGLOMERATE OVER 10 ~
BILLET SAMPLE EQUIVALENT EQUIVALENT LAROEST TiC
NO. B NO. DIAMETER DIAMETER GRAIN

2 15 2 5.5~
6~ 1 9 0 4 ~ -2 16 3 5 ~`

2 20.5 2 4 69 1 12 1 5.5 2 12 1 5.5 1 8 0 5.5 2 15 2 5.54 73 1 12 1 3.3~ -2 15.5 1 6 2 10.5 1 5 2 19 3 5.5 76 1 7; 0 6 77 1 10 0 5.5 2 10.5 1 7 2 10 0 6 ~ -79 1 12.2 1 3.3 2 7.8 0 5 1 13 ~. 3 81 1 10.5 1 4 2 12 2 5.5 82 1 15.5 2 5 AVERAGE 12.0 1.08 4.8 A ,~canning electron microscope indicates that the alumina grain size of this material is of the same order as the grain size (0.3 1~5 ~) of the sintered alumina alone. The TiC, however, is on the order of 1 ~, which was the size of the ;~

ball milled titanium carbide powder.
As described, the process produces a ~igni~icantly ;
improved product in comparlson with the prior art~ The increased . . . . . . .. . .

~ ~3786~ :

density of alumina-titanium carbide indicates that the applied rate controlled sintering technique immediately following the "break away" point maximizes the densification of the material relative to that which was heretofore obtainable. This process, moreover, is applicable to other powdered materials once the "break away" point is determined and the inherent or natural ~ :
densification rate for the material in question is established.

' ":

, . ' ,,

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of approximately identifying the natural rate of densification of a sinterable material comprising the steps of subjecting each of a plurality of identical batches of the material to a physical pressure and heating rate to begin sintering of the batch of material, then when the volume of each batch of material commences to diminish subjecting each batch of material to an increased further physical pressure and further heating rate which is dis-continued when the batch of material attains a minimum volume, said increased further physical pressure and further heating rate differing for the different batches of material, and recording the density of each batch of material when each batch of material attains the minimum volume thereof, said increased further physical pressure and further heating rate to which the batch of material having the maximum recorded density was subjected constituting an approximate identifi-cation of the natural rate of densification of said material.