Rapid rate sintering of ceramics

High-strength, fine-grain multi-phase substantially crystalline sintered ceramic bodies are produced by a process comprising the steps of cold pressing, followed by sintering at a high temperature, the temperature increase to maximum sintering temperature being accomplished by the use of a heating rate in excess of about 100.degree. C./minute.

This invention relates to the provision of reliable, reproducible 
manufacturing methods for producing high-strength, fine-grain sintered 
ceramic bodies. Although the prime interest is in the preparation of 
alumina-base sintered bodies, the process of this invention appears to be 
broadly applicable to the sintering of ceramic powders. 
BACKGROUND OF THE INVENTION 
Ceramic cutting tools have been in use for a long time, but the relatively 
low strength of ceramic materials compared with other common tool 
materials, such as cemented carbides, and the resulting generally poor 
performance characteristics of ceramics has limited the application of 
tools prepared from these materials. 
The predominant manufacturing process for the preparation of dense 
polycrystalline ceramic bodies is hot pressing. In this process, ceramic 
particles are placed in a graphite mold and sintered under conditions of 
temperature ranging from 1500.degree. to 1800.degree. C. with a 
simultaneous pressure application ranging from about 100 to 400 kg./sq.cm. 
Increases in rupture strength have been obtained through refinement of the 
grain size of the sintered product. This has required increasingly 
stringent process conditions encompassing the use of very fine starting 
powder, utilizing as low a process temperature as would be effective in 
bringing about the sintering, and the addition of sintering additives. One 
pronounced advance in alumina-base ceramic cutting tool technology has 
been the use of alumina-titanium carbide (Al.sub.2 O.sub.3 -TiC) composite 
material. 
Some of the early sintered ceramic cutting tools contained small amounts of 
carbides, including TiC as sintering additives (Ceramics in Machining 
Processes., by A. G. King and W. M. Wheildon, Academic Press, New York, 
1966), but the first commercial dense polycrystalline ceramic cutting tool 
containing major amounts of TiC in addition to Al.sub.2 O.sub.3 is a 
relatively recent development. The production thereof is described in U.S. 
Pat. No. 3,580,708--Ogawa et al., and employs hot pressing. Experimental 
production of a similar composition material for hot pressing at higher 
temperatures (i.e., 1800.degree.-1850.degree. C.) was reported earlier 
("Preparation of Alumina-Titanium Carbide Bodies by Hot Pressing 
Techniques", H. N. Barr, G. D. Cremer and W. J. Koshuba; Powder Met. Bull, 
Vol. 5, No. 4, September 1950). 
Cutting tools containing a significant amount of metal in addition to 
Al.sub.2 O.sub.3 -TiC are described in U.S. Pat. No. 3,542,529--Bergna et 
al. The addition of titanium oxide to the Al.sub.2 O.sub.3 -TiC system is 
described in U.S. Pat. No. 4,063,908--Ogawa et al, such addition making it 
possible to reduce the hot pressing temperature. All of the above rely 
upon hot pressing to accomplish densification. Another hot pressing patent 
is U.S. Pat. No. 4,204,873--Yamamoto et al., in which a different 
alumina-base system is employed; namely, alumina-tungsten carbide with an 
addition of titanium nitride. 
The sintering of cold pressed powder compacts of aluminum oxide and 
refractory transition metal diborides is described in U.S. Pat. No. 
4,022,584--Rudy. It is also disclosed therein that grain growth stability 
of the alloy phases is significantly improved by the addition of carbides 
and nitrides, such additions necessitating higher sintering temperatures 
or pressure-sintering (i.e., hot pressing). U.S. Pat. No. 
4,383,957--Yamakawa et al., describes the sintering of a ceramic 
composition in an atmosphere of, or containing, carbon monoxide gas. The 
Yamakawa et al. patent describes hot pressing as having the disadvantage 
of being "very high priced and unsuitable for the production . . . of an 
article with a complicated shape" (col. 1, lines 59-62). In the Yamakawa 
et al. patent, certain sintered bodies were further subjected to hot 
isostatic pressing to increase the density thereof. 
Pending U.S. patent application Ser. No. 332,903--M. Lee and L. Szala, 
filed Dec. 21, 1981 and assigned to the assignee of the instant invention 
uses alumina, carbon, and titanium hydride as starting materials, the 
carbon to titanium ratio being somewhat less than the required ratio for 
stochiometric TiC. 
The use of high heating rates during multi-stage sintering of thoria powder 
compacts is disclosed in "Material Transport During Sintering of Materials 
With the Fluorite Structure" by Morgan and Yust [Journal of Nuclear 
Materials 10, 3 (1963) 182-190, North-Holland Publishing Co., Amsterdam]. 
Densification data therein for a range of heating rates (i.e. 1.6.degree. 
to 8.0.degree. C./sec.) shows that the density achieved in compacts of 
ThO.sub.2 powder heated to a particular temperature and then air quenched 
was almost independent of the time required to reach that temperature. 
Data are also reported for heating rates up to 150.degree. C./sec. The 
maximum theoretical density achieved by their reported techniques was less 
than 90%. 
The following definitions are applicable to an understanding of this 
invention and/or the prior art: 
SINTERING: development of strength and associated densification of a powder 
compact through the application of heat alone. 
HOT PRESSING: the combined application of heat and of pressure applied 
through the action of a mechanical piston on the powder-filled cavity of a 
die. Under such conditions the pressure on the powder compact is 
non-uniformally applied due to die wall friction and the axial application 
of the piston force. Under proper conditions of temperature and pressure 
densification of the compact can result. 
HOT ISOSTATIC PRESSING (HIP): The simultaneous application of isostatic 
pressure and heat to a sample body whose porosity is to be reduced. 
Pressure is applied uniformly to the sample body by an inert gas. The 
sample body may be (a) a powder compact encapsulated in a gas impermeable, 
but deformable, envelop such as a tantalum foil can or a glass coating or 
(b) any solid substantially devoid of open porosity. 
ROOM TEMPERATURE: 67.degree.-72.degree. F. 
The sintered product of this invention is considered to be "substantially 
crystalline", because it is not atypical to encounter minor amounts of 
non-crystalline material (e.g. glasses) in the grain boundary phases. 
This invention addresses a particularly troublesome problem encountered in 
the sintering of multiphase systems. Such systems frequently contain 
components, which will chemically interact at elevated temperatures. If 
such chemical reaction proceeds fast enough to inhibit the desired 
densification or, if the nature of the reaction is such that it results in 
degradation of the system (i.e. undesirable solid, liquid or gaseous 
phases are produced), manufacture of the desired product cannot be 
successfully accomplished by sintering. 
This invention is primarily described herein in respect to the Al.sub.2 
O.sub.3 -TiC system, because this particular material system presents the 
very problem in densification discussed herein above. However, the 
essential aspects of the sintering process disclosed herein are not 
dependent upon either the use of particular sintering additives, 
particular material proportions, or the nature of minor impurities. The 
process is expected to be broadly applicable to the sintering of powdered 
ceramic materials, that contain components which will chemically react at 
elevated temperatures to inhibit densification or degrade the system so 
that an undesirable sintered product results. 
DESCRIPTION OF THE INVENTION 
In the practice of the process of this invention in its most essential 
aspects, a plurality of powdered ceramic materials are consolidated under 
pressure to produce a cold pressed green compact of some preselected shape 
and volume, the compact is heated to a maximum sintering temperature with 
at least the final stage of heating in which the maximum sintering 
temperature is reached being accomplished at a heating rate in excess of 
about 100.degree. C. per minute, holding the compact at the maximum 
sintering temperature for at least the length of time needed to achieve 
uniform temperature distribution throughout the compact, and then 
permitting the sintered body so produced to cool. 
It has been found that--when the starting powdered ceramic materials 
comprise, or contain, components that will react together chemically at 
elevated temperatures and either inhibit densification of the green body 
or degrade the system such that any resulting sintered product is 
undesirable--by utilizing sufficiently rapid rate heating, bodies can now 
be satisfactorily sintered without reliance on sintering additives. The 
condition required to attain these results is the application of a heating 
rate during sintering that is fast enough to produce a rate of 
densification of the green body that exceeds the rate of the unwanted 
chemical reaction. Usually such a heating rate will exceed about 
100.degree. C. per minute. In fact, in sharp contrast to current 
commercial practice in which very slow heating rates are coupled with long 
periods of holding at the sintering temperature, sintering rates of 
400.degree. C. per minute and hold periods of one minute or less are 
routinely employed in the practice of this invention. 
In the preferred practice of this invention, the green compact is heated to 
an intermediate transition temperature (ITT) using slow heating (i.e. less 
than about 50.degree. C. per minute) with subsequent rapid rate heating 
(i.e., greater than about 100.degree. C. per minute) to the maximum 
sintering temperature. If desired, of course, more than one change in 
heating rate may be utilized. By applying the teachings set forth herein a 
program of multi-rate heating can be developed specific to the sintering 
of a particular ceramic system, which will combine the optimum sintering 
cycle with the most economic low temperature heating operation. 
In another modification of the process of this invention, after heating to 
the ITT, the compact can be permitted to cool and then this presintered 
body can subsequently be heated all the way to the maximum sintering 
temperature to accomplish the full sintering desired. With this 
modification, presintering can be accomplished at one location, the 
presintered compacts can be transferred to another site, and the final 
sintering can be done at the second location. 
Since most ceramic bodies sintered by the practice of this invention 
achieve a density exceeding 97% of theoretical and have very little open 
porosity, these sintered bodies can be subjected to HIP without the usual 
requirement of additional gas impermeable containment or encapsulation. 
It is preferable to conduct the sintering operation in a resistance-heated 
furnace (e.g., molybdenum element) in a high-purity helium atmosphere, the 
heating rate of the furnace being scheduled over the desired temperature 
range by a programmable controller. Maximum sintering temperatures 
employed are typically in excess of 1800.degree. C.

MANNER AND PROCESS OF MAKING AND USING THE INVENTION 
This invention is the outgrowth of studies of the sintering characteristics 
of Al.sub.2 O.sub.3 -TiC and other ceramic materials, focusing on the rate 
of heating of the unconfined (i.e., as contrasted to being confined as in 
hot pressing) green compact as the primary parameter variable. In sharp 
contrast to conventional sintering practice, in which green samples are 
heated at a typical rate of from about 2.degree. to about 5.degree. 
C./min. to an isothermal hold at the maximum sintering temperature, which 
is chosen as low as possible to minimize grain growth, both the heating 
rates and the sintering temperatures employed are much greater. 
Experimental Procedure and Equipment 
A high-temperature furnace equipped with a precision dilatometer was used 
for the conduct of most of the experiments. This furnace was of the 
resistance heater type, employing a molybdenum heating element with the 
temperature being controlled by a Data-trak (Research, Inc.) programmable 
controller using a tungsten-rhenium thermocouple. With this equipment 
temperatures of over 2000.degree. C. could be maintained in the furnace 
with an accuracy within a few degrees of the desired value. The capability 
was also available to enable the application of a very wide range of 
heating rates ranging from less than one degree per minute to several 
hundreds of degrees per minute. Provisions were available for evacuating 
the furnace to 0.1 torr, however, high purity helium or high purity gas 
such as hydrogen, argon, nitrogen or carbon monoxide were used as the 
sintering environment during most of the studies. 
The dilatometer was made with two molybdenum bars as the reference rod and 
the push rod in contact with the sample, respectively. The molybdenum bars 
were mounted horizontally with the push rod cantilevered in a pair of 
precise bearings. About 10 grams of bias was applied to the sample push 
rod to compensate for any frictional resistance in the bearing assemblies 
against sliding movement. This counterweight eliminated occasional 
problems of sticking of the push rod, but was insufficient to cause any 
deformation of the sample. The relative position of the push rod and the 
reference rod was detected by a linear voltage differential transformer 
(LVDT). The LVDT of the dilatometer at its maximum sensitivity range of 
322 volts/inch is linear in response over the range of .+-.13 volts. The 
digitized output of the LVDT as well as the furnace temperature and 
sintering times were recorded automatically by a North Star microcomputer. 
The unique advantage offered by the apparatus arrangement described was the 
capability for obtaining from the dilatometer traces an appreciation of 
the extremely rapid rate at which green compact shrinkage occurs during 
sintering runs. It was the understanding obtained from this information, 
which led to the conclusion that expending a long time to reach the 
sintering temperature and/or holding the body being sintered at high 
temperature for extended periods are detrimental to the achievement of 
maximum densification. 
A second somewhat larger molybdenum heating element furnace similar in 
design to the above-described furnace but without a dilatometer was used 
in the conduct of sintering studies of larger samples to be used as 
cutting tools. An ASEA Pressure Systems, Inc. Mini Hipper was used for the 
treatment by HIP of sintered samples. In HIP studies, pressure and 
temperature were varied but the pressure medium for most of these studies 
was nitrogen. 
Powders for the sintering studies were ball milled, usually employing 
cemented tungsten carbide milling media. The milling of Al.sub.2 O.sub.3 
-TiC powder for periods ranging from 24 to 48 hours in a carbide ball mill 
will leave as much as 4 wt % (w/o) of tungsten carbide and about 0.4 w/o 
of cobalt in the milled powder. Some controlled milling experiments were 
carried out using 99.5+ purity alumina milling media in a plastic 
container to eliminate residual cemented tungsten carbide contaminants. 
This work provided a comparison for determining the lack of effect of 
milling contaminants on sintering. 
Heating Rate 
One of the sintering parameters found most crucial in influencing the 
ultimate sintered density and fine grain microstructure of an Al.sub.2 
O.sub.3 -TiC composite at a given sintering temperature was the rate of 
heating of the green compact up to the maximum sintering temperature. The 
graphs in FIG. 1 indicate the percent change in compact dimension recorded 
by the dilatometer as a function of temperature for several sintering runs 
employing very different heating rates. The curves resulting from heating 
at the various heating rates are identified in the following Table. 
TABLE I 
______________________________________ 
Curve Heating Rate (.degree.C./min) 
______________________________________ 
a 20 
b 30 
c 100 
d 400 
______________________________________ 
The apparent temperature readings for the green compacts, or samples, were 
recorded by means of a thermocouple placed adjacent to each sample. For 
the runs reflected in curves a, b, c and d each sample was held at the 
sintering temperature of 1820.degree. C. for 1 hour. 
Certain features common to curves a, b, c and d of FIG. 1 prove to be 
particularly important. Except for the normal thermal expansion of the 
green body as temperature was increased, no other change in sample 
dimension was apparent until the temperature reached about 1000.degree. C. 
From that point on all of the samples densified very rapidly, the 
densification rate being so rapid that, when the expected difference 
between the actual sample temperature and the value recorded by the 
thermocouple is taken into account, the degree of densification can be 
seen to have been mainly a function of sample temperature. This condition 
prevailed for the broad spectrum of heating rates until a temperature 
slightly in excess of 1600.degree. C. was reached. The densification rate 
above 1600.degree. C., however, depended on the rate of heating of the 
samples reflecting the extent to which densification rate or rate of 
chemical reaction prevailed. Recorded dilatometer data indicated that at a 
given sample temperature above 1600.degree. C., the greater the rate of 
heating, the higher the final sintered sample density. In fact, that data 
indicated that the heating rate (HR) necessary to achieve a desired 
fraction of theoretical density at a maximum sintering temperature of 
1820.degree. C. is an exponential function of the fraction of theoretical 
density, which is 
##EQU1## 
A similar relationship is expected to hold at higher maximum sintering 
temperatures. Although impractical at present, an Al.sub.2 O.sub.3 -TiC 
composite is expected to fully densify at 1820.degree. C. at heating rates 
greater than 10,000.degree. C./min. 
Sintering Temperature 
Another important sintering parameter is sintering temperature. When heat 
is applied to an Al.sub.2 O.sub.3 -TiC green compact at a constant heating 
rate, the final sintered density thereof increases as the maximum 
sintering temperature is raised. The final densities obtained for sintered 
samples heated at 400.degree. C./min. to temperatures from 1820.degree. to 
1950.degree. C. are shown as a function of sintering temperature in FIG. 2 
(curve f). Each sample was held for two minutes at the preselected 
sintering temperature. As is manifest in curve f, the final density of the 
samples increased linearly with increasing sintering temperature within 
the range investigated. Sample densities greater than 99% of theoretical 
were obtained by sintering at 1950.degree. C. with a heating rate of 
greater than 200.degree. C./min. As is shown by curve g, samples sintered 
at the higher temperatures have essentially no open porosity. This is of 
particular importance, because samples so prepared can, in a subsequent 
step after cooling, be hot isostatically pressed without the need for 
encapsulation. 
Total Sintering Time 
The total sintering time at temperature (i.e., the isothermal hold) is 
still another important parameter. The degree of densification achieved by 
holding samples (i.e., consolidated powder ceramic materials as green 
compacts) at the sintering temperature for the same length of time is 
different depending upon the sample density at the beginning of the 
sintering hold. Samples heated at a slower rate underwent less shrinkage 
at the beginning of the holding period and somewhat more shrinkage during 
the holding period than did rapidly heated samples. However, as is shown 
in FIG. 1 by the total shrinkages displayed, the final densities achieved 
for the rapidly heated samples were still greater. At sintering 
temperatures approaching 1950.degree. C., runs conducted at heating rates 
greater than 200.degree. C./min., the sample density reached nearly 100% 
of the theoretical density using a very short isothermal hold time. In 
fact, if the isothermal hold is extended beyond the minimum time required 
to achieve a uniform temperature distribution throughout the sample, the 
result will be an actual decrease in the density of the sintered body 
produced. This was demonstrated in the densification of pressed powder 
samples of magnesia-doped alumina. 
Whereas it would be expected that the use of very high sintering 
temperatures would produce larger-than-acceptable grain size in the 
sintered body and thereby adversely affect hardness and strength, it has 
been found that the grain size of rapid-rate sintered Al.sub.2 O3-TiC 
parts actually is comparable to that of state-of-the-art commercial 
products of similar composition produced by HIP at much lower 
temperatures. FIG. 3 is an optical micrograph showing a microstructure 
produced by sintering at 1950.degree. C. In spite of the high sintering 
temperature, the average particle diameter is smaller than about 2 
micrometers. 
Use of Hot Isostatic Press 
The feasibility of closing residual porosity in sintered bodies by the use 
of HIP was also investigated. Most bodies sintered by the practice of this 
invention to a density exceeding 97% theoretical have very little 
remaining open porosity. Such open porosity as remained was concentrated 
mainly in a thin surface layer over the sintered body. Frequently, 
sintered samples having densities as low as 94% theoretical showed dense 
microstructures without any interconnected network of pores beneath this 
thin surface layer, or scale. A number of sintered samples having 
densities ranging from about 97.5% to 98.5% theoretical were subjected to 
HIP under 15,000 psi argon for 15 minutes at various temperatures. The 
final density and Rockwell A(RA) hardness of the sample after HIP are 
shown in FIG. 4. All samples subjected to HIP at temperatures above 
1400.degree. C. achieved densities greater than 99.5% and also exhibited 
excellent hardness. Although subjecting samples to HIP at 1350.degree. C. 
showed some gains in density and hardness, the gains were not considered 
adequate. These studies have shown that a minimum temperature of at least 
about 1450.degree. C. is required during HIP to produce a technologically 
useful sintered body using the Al.sub.2 O.sub.3 -TiC system. One HIP 
experiment was carried out at 1550.degree. C., which showed that no 
additional pore closure is achieved after about 2 minutes at temperature 
and pressure. Comparison of the microstructures before and after HIP at 
1650.degree. C. for 15 minutes showed that the TiC grains increased in 
size during the HIP cycle. 
Thermal Fracture Consideration 
It has been found that, when cold pressed Al.sub.2 O.sub.3 -TiC bodies are 
subjected to rapid rate sintering from room temperature to the maximum 
sintering temperature, the green body frequently cracks during the 
sintering process due to the severe thermal shock. This thermal shock 
occurs, because the green body has insufficient strength to offset the 
internal thermal gradients, and associated thermal stresses, imposed 
during overly rapid rate heating. In hot pressing the ceramic powders are 
placed in a mold, which, after assembly, is placed with its contents in 
the vacuum chamber of a vacuum hot press where the mold is subjected to 
the pressure of opposing pistons to consolidate the powder prior to moving 
the mold into the hot zone of the furnace for conduct of the hot pressing 
operation. Because of its containment within a mold, the cold pressed 
green body in this instance is supported against thermal stresses imposed 
during the sintering process and the rapidity of the rate of heating does 
not invoke the problem of thermal fracture encountered during sintering of 
an unsupported green body. 
According to the sintering characteristics monitored by the dilatometer, 
sintering begins about 1000.degree. C. (FIG. 1). As noted hereinabove, 
densification of Al.sub.2 O.sub.3 -TiC green bodies as a function of 
temperature is essentially independent of the heating rate up to a 
temperature of about 1600.degree. C. However, observable heating rate 
effects are apparent above about 1600.degree. C. 
In accordance with these findings and the teachings of this invention, a 
high material density for sintered ceramic specimens can be achieved 
advantageously by heating the green bodies slowly up to some given ITT and 
then increasing the heating rate to reach the desired maximum sintering 
temperature. A schematic description of a typical sintering cycle 
according to this invention for Al.sub.2 O.sub.3 -base ceramics is shown 
in FIG. 5. A series of experiments was conducted in helium in which the 
heating rate was changed abruptly from 20.degree. C./min. to 400.degree. 
C./min. The temperature (ITT) at which this change was made was increased 
by 100.degree. C. in each succeeding experiment. The effect that the ITT 
had on the final density in these experiments is shown in FIG. 6. Samples 
for these tests were cold pressed at 30,000 psi, heated to 320.degree. C. 
for a 15 minute hold and then heated, first to the selected ITT and then 
to a 30 second isothermal hold at the maximum sintering temperature of 
1950.degree. C. The heating regime in each instance was in general accord 
with that shown in FIG. 5. The modified heating cycle represented by FIG. 
5 successfully prevented the occurrence of thermal cracking. 
Modifications in the heating cycle as represented in FIG. 5 may be made to 
accommodate green bodies of various thicknesses. Thus, given a green body 
with a maximum thickness of about 3/4 inch, or thinner, the heating cycle 
of FIG. 5 (with perhaps a higher ITT) will be typical. For those instances 
in which the green body is thicker than about 3/4 inch, the heating rate 
of ITT will usually be lower than the 40.degree. C./min. shown, the proper 
value being routinely determinable. 
In those instances in which green bodies were presintered at one site for 
transport to a second site for sintering, a useful presinter sequence for 
Al.sub.2 O.sub.3 -TiC compacts was as follows: 
(1) the Al.sub.2 O.sub.3 -TiC compacts were prepared by pressing WC 
ball-milled powder to 50 Kpsi; 
(2) the compacts were presintered by heating in vacuum to 1400.degree. C. 
at a heating rate of about 5.degree. C./min and 
(3) the presintered compacts were permitted to cool to room temperature. 
Thereafter at the second site the heating cycle of FIG. 5 (using a 30 
second hold at 1950.degree. C.) was employed to achieve the desired 
densification. 
The hold at maximum sintering temperature can vary depending on the size of 
the body being sintered, but preferably the hold period will not exceed 
about 2 minutes. 
Selection of the ITT to be employed can be routinely accomplished with the 
understanding that selecting too high an ITT results in the samples being 
at high temperature for extended lengths of time and this contributes to 
the formation of surface scale in which there is a higher concentration of 
pores than in the body of the sample itself. In the case of Al.sub.2 
O.sub.3 -TiC not only the formation of porosity, but also loss of TiC from 
the surface layer due to chemical reaction can occur. As long as the ITT 
is selected high enough to provide the sample with sufficient internal 
strength to counteract the thermal stresses that will be induced by 
heating at the high rate (i.e., at greater than 100.degree. C./min.), the 
slow heating will have achieved its objective. The simplest approach to 
determining an optimum ITT is by the joint application of the teachings 
provided by FIGS. 1 and 6 and the description relative thereto. When the 
sintered product is to be used as a cutting tool, machining tests should 
also be included as a parameter in optimizing the heating sequence. 
Property Tests 
One engineering property that is quite sensitive to the presence of minor 
residual porosity in the sintered body as well as to the grain size of 
sintered material therein is macrohardness. Rockwell hardness of materials 
that have been subjected to HIP in FIG. 4 demonstrate that sintered 
material produced by the practice of this invention develops a hardness at 
least equal to, or harder than, the average hardness (a value of about 94 
Rockwell A) of the best commercial hot pressed material of similar 
composition. Rockwell A and Rockwell C hardness as used and determined 
herein are in accordance with ASTM designation: E18-74. 
Transverse rupture strength (TRS) of some of the as-sintered samples and of 
the sintered samples after being subjected to HIP, was determined by the 
use of the 3-point bend test. The results are shown in Table II. As the 
data set forth therein will indicate, some individual samples of 
as-sintered bars have very high strength and the ASTM B406-70 value of TRS 
for the limited number of as-sintered samples tested is lower than that of 
bars subjected to HIP. However, scatter in the data is much less for the 
samples subjected to HIP than for the as-sintered bars. 
TABLE II 
______________________________________ 
After 
As Sintered 
HIP 
Sample (psi) (psi) 
______________________________________ 
1 69,000 96,000 
2 148,000 104,000 
3 94,000 88,000 
4 83,000 89,000 
5 107,000 
6 107,000 
7 92,000 
Average 98,500 97,600 
ASTM B406-70 82,000 97,600 
______________________________________ 
The HIP TRS value of about 98,000 psi average is in good agreement with 
reported TRS values of Al.sub.2 O.sub.3 -TiC composites. 
Machining Tests 
A number of cutting tools were produced in the following manner: alumina 
having an average particle diameter of less than 1 micrometer was mixed 
with 30 wt % TiC powder having an average particle diameter of less than 2 
micrometers; the mixture was ball milled in a cemented tungsten carbide 
mill; isostatically pressed to the shape of a cylinder having a diameter 
of about 1/2"; discs about 3/8" thick were cut off the bar and these discs 
were sintered at 1950.degree. C. for about 1 minute as described herein. 
The heating rate was 40.degree. C./min. up to about 1100.degree. C. after 
which the heating rate was raised to 400.degree. C./min. to bring the 
discs to the maximum sintering temperature. All sintered discs were 
subsequently subjected to HIP for the times, temperatures and pressures 
shown in Table III. The resulting high density sintered discs were ground 
to 3/8" diameter with a thickness of 3/16". The edge of each tool was 
ground to give a 20.degree. chamfer. The hardness of the chilled cast iron 
workpiece machined for the tests was 57 to 58 Rockwell-C. The depth of cut 
was 0.04" and the feed per revolution was 0.008". The tool holder used had 
a 5 degree negative back and side rake angles. The cutting speed for all 
tests was 300 surface feet per minute. No coolant was used for the test 
and after 5 minutes of machining, the uniform flank wear land was measured 
along with any depth of cut line (DCL) notch. The test results indicate 
that the sintered discs prepared by the practice of this invention 
function as excellent cutting tools for the machining of hard cast iron. 
TABLE III 
______________________________________ 
Flank Depth-of- 
Tool HIP Conditions Wear Cut Notch 
No. T(.degree.C.) 
t(min) P(ksi) 
(in) (in) Comments 
______________________________________ 
50A 1650 30 15 .003 .007 
50B 1650 30 15 .0025 -- 
46 1550 10 15 .0021 .0035 Minor spall 
48 1525 30 15 .0028 .0035 
52A 1525 30 15 .0028 -- Minor edge 
chipping 
49 1450 60 15 .0028 -- 
53A 1450 60 15 .0028 -- 
53B 1450 60 15 .0028 -- 
51A 1350 60 15 .0035 .0056 Chips on 
cutting 
edge 
51B 1350 60 15 .0030 .0049 Chips on 
cutting 
edge 
** .0042 .007 Edge Spall 
______________________________________ 
**Commercial hot pressed Al.sub.2 O.sub.3 --TiC Tool of similar 
composition 
A limited number of machining tests were carried out on an IN718 alloy 
workpiece. The sintered cutting tools were prepared in the manner 
described hereinabove and the cutting tools exhibited performances 
equivalent to those of similar grade commercial tools. 
Effect of Additives 
It was decided to investigate the effect of the presence of additives or 
impurities, it being appreciated, however, that sintering aids are not 
required in the successful practice of this invention. 
For this evaluation a powder composition was prepared containing 59 w/O 
Al.sub.2 O.sub.3, 30 w/o TiC and 11 w/o ZrO.sub.2. Samples of this ternary 
composite powder were sintered in general accordance with the heating 
program of FIG. 5 after determining the temperature dependent sintering 
characteristics thereof by monitoring with the in-furnace dilatometer. 
Some of the samples were exposed to a maximum sintering temperature of 
1935.degree. C. These samples actually increased in dimension (i.e., 
swelled) after reaching maximum density. Optical micrographs of the 
cross-section of such a sintered sample revealed that fairly large random 
pores had resulted. By reducing the maximum sintering temperature to 
1900.degree. C., the swelling was eliminated. Another unique feature 
encountered with this ternary system in all samples sintered at a 
temperature in excess of 1880.degree. C., was the appearance on cooling of 
shrinkage at about 1875.degree. C. This shrinkage is probably associated 
with solidification at the ZrO.sub.2 -induced eutectic temperature. In 
initial trials, all sintered samples of this ternary system cracked. The 
cracking problem was resolved, however, by reducing the sintering 
temperature below 1875.degree. C. and using ZrO.sub.2 particles less than 
one micrometer average particle diameter. 
Sintered bodies were also prepared in which small quantities (about 0.5 w/o 
of cobalt) was added to the Al.sub.2 O.sub.3 -TiC. No apparent problem was 
encounted in either the rapid rate sintering of the Al.sub.2 O.sub.3 -TiC 
or in the subsequent HIP of the as-sintered bodies. Three point bend 
strength tests of the samples yielded values of 103 kpsi. These strengths 
are comparable to the values obtained from the Al.sub.2 O.sub.3 -TiC 
powder mixture substantially free of impurities and clearly free of 
cobalt. The sintering atmosphere used for most experiments having the 
cobalt addition was high purity helium. 
The ceramic powders can be mixed by the use of any of the conventional 
techniques such as, for example, ball milling, laboratory milling or jet 
milling to produce a substantially uniform or homogeneous dispersion or 
mixture. The more uniform the dispersion, the more uniform the 
microstructure and, consequently, the properties of the resulting sintered 
body. 
In the powder mixtures prepared for consolidation into a green compact, the 
average particle diameter ranges from 0.1 .mu.m to about 5 .mu.m. An 
average particle size less than about 0.1 .mu.m is not useful, since it is 
generally difficult or impractical to compact such powders to a density 
sufficient for handling purposes. Powders with an average particle size 
greater than about 5 .mu.m will not produce a useful end product. 
Preferably, the average particle size of the powder mixture ranges from 
about 0.3 .mu.m to about 1 .mu.m. 
A number of techniques can be used to shape the ceramic powder mixture into 
a green compact. For example, it can be extruded, injection molded, 
die-pressed, isostatically pressed or slip cast to produce a compact of 
desired shape. Any lubricants, binders or similar materials used in 
shaping the powder mixture should have no significant deteriorating effect 
on the resulting sintered body. Such materials are preferably of the type 
which evaporate on heating at relatively low temperatures preferably below 
about 350.degree. C., leaving no significant residue. The green compact 
should have a density at least sufficient to enable handling thereof and, 
preferably, its density will be as high as can be obtained to promote the 
overall densification occurring during sintering. 
The present invention makes it possible to reproducibly and economically 
fabricate complex shaped ceramic articles directly. The sintered product 
of this invention can be produced in the form of a useful, simple, complex 
or hollow shaped article without machining. The dimensions of the sintered 
product would differ from those of the green compact by the extent of 
dimensional change occurring during shrinkage. The Al.sub.2 O.sub.3 -TiC 
system as sintered in the practice of this invention has particular 
utility in the preparation of tool inserts for machining operations. 
The invention is further illustrated by the following examples. In each 
example the powders were prepressed isostatically to 50 Kpsi. The 
compacted plugs (.about.3/4" long by 1/4" diameter) so produced were 
processed in the dilatometer-equipped furnace. The furnace was evacuated 
to 50 .mu.m vacuum, helium was introduced and was allowed to purge the 
furnace at one atmosphere pressure during the rest of the cycle. In each 
example the sintering cycle used a heating rate of 40.degree. C./min to 
reach 1120.degree. C. (ITT) including a 15 minute hold at 320.degree. C. 
to degas and stabilize the furnace and controller. Upon reaching 
1120.degree. C. the heating rate was immediately increased to the rapid 
rate of 400.degree. C. (except in EXAMPLE 1) and ramped to the preselected 
maximum temperature. In EXAMPLE 1 the rapid rate heating was conducted at 
350.degree. C./min, because of an error in the programming of the 
controller. Natural furnace cooling was relied upon to reduce the 
temperature to room temperature. 
Where values are indicated, material density was determined by immersion 
density measurement. In all examples, the sintered bodies had essentially 
no open pores. Microstructural observations were made in most instances. 
EXAMPLE 1 
The starting powder mixture was 50 wt % Al.sub.2 O.sub.3 (0.3 .mu.m) and 50 
wt % TiC (1-5 .mu.m). This mixture was milled with WC balls for 24 hours 
using acetone solvent. The maximum sintering temperature was 1950.degree. 
C. and a sintered density of 4.5 g/cc was produced for the sintered body. 
HIP of the sintered body in argon followed with the temperature thereof 
being raised from room temperature to 1520.degree. C. at the rate of 
30.degree. C./min and then maintaining the temperature at 1520.degree. C., 
a pressure of 15 Kpsi was applied for 20 minutes. The HIP treatment 
increased the density to 4.53 g/cc. 
The sintered body was extremely dense and exhibited a hardness of RA=94.6. 
EXAMPLE 2 
Starting with a powder mix of 85 wt % Al.sub.2 O.sub.3 (0.3 .mu.m) and 15 
wt % TiC (0.14 .mu.m) ball milling with WC balls (acetone solvent) was 
conducted for 24 hours. The firing conditions for sintering were the same 
as in EXAMPLE 1 and the sintered body had a density of 3.65 g/cc. 
Thereafter HIP was conducted as described in EXAMPLE 1 resulting in a 
density of 3.67. 
The resulting body exhibited some residual porosity indicative of 
unsatisfactory sintering. It was concluded that the initial TiC grain size 
was too fine and was responsible for poor initial powder packing density. 
EXAMPLE 3 
Al.sub.2 O.sub.3 and ZrO.sub.2 powder (Al.sub.2 O.sub.3 -12.7 ZrO.sub.2 by 
weight) were attritor milled for 6 hours. The maximum sintering 
temperature was 1800.degree. C. resulting in a density of 4.244 g/cc 
(99.8% of theoretical). The body was well sintered as was evident from 
microstructural observation. 
EXAMPLE 4 
The powder mix (Al.sub.2 O.sub.3 -30 wt % TIC-7 wt % ZrO.sub.2) was ball 
milled (WC balls) for 12 hours. Maximum sintering temperature was 
1835.degree. C. and the sintered density was 4.24 g/cc. After HIP 
(1520.degree. C. and 15 Kpsi) for 20 minutes in argon the density was 
increased to 4.46 g/cc. Hardness was measured (RA=93.6) and 
microstructural observations established that the material was fully 
dense. 
EXAMPLE 5 
Starting powder was Al.sub.2 O.sub.3 (0.3 .mu.m)-30 wt % TiN (-325 mesh). 
This mix was milled for 172 hours using WC balls. During the sintering the 
maximum temperature of 1820.degree. C. was held for one minute. Sintered 
density was 4.396 g/cc. 
The sintered body showed residual closed pore porosity. Subsequent HIP 
should eliminate this condition. 
EXAMPLE 6 
The powder used was Al.sub.2 O.sub.3 doped with 500 ppm MgO. Sintering was 
carried to 1970.degree. C. with a hold of 30 seconds at this temperature. 
The measured density was 3.90 g/cc (98% of theoretical), which density 
increased to 99.5% of theoretical after HIP (1550.degree. C./15 Kpsi for 
15 minutes in argon). 
EXAMPLE 7 
Commercial grade Y.sub.2 O.sub.3 powder was sintered using a maximum firing 
temperature of 1960.degree. C. producing a nearly pore-free structure. 
Although the process of this invention was used in EXAMPLES 8, 9 and 11, 
selection of the particular ceramic systems was not the invention of the 
inventive entity involved herein. 
EXAMPLE 8 
The starting ceramic powder mix was 62 wt % Al.sub.2 O.sub.3 (0.3 .mu.m)-38 
wt % (50 wt % WC-50 wt % TiC as a solid solution). The WC/TiC powder was 
-325 mesh. The WC/TiC was milled for 100 hours using WC balls; the 
Al.sub.2 O.sub.3 was added and milling proceeded for an additional 72 
hours. During sintering the temperature was ramped to 1950.degree. C. and 
held for 30 seconds. Density of the sintered body was 4.97 g/cc; HIP 
produced further densification to 5.2 g/cc. 
Examination showed extremely fine well-sintered material with sub-micron 
size grain size for the carbide phase. Hardness was determined to be 93.8 
(RA). 
EXAMPLE 9 
Three powder compositions: 70.3 wt % Al.sub.2 O.sub.3 (0.3 .mu.m)-24.7 wt % 
TiC (-325 mesh)-5 wt % Cr.sub.3 C.sub.2 (-325 mesh) were processed in a 
carbide (i.e. WC balls) mill with the TiC and Cr.sub.3 C.sub.2 ball milled 
first for 120 hours after which the Al.sub.2 O.sub.3 powder was added and 
milling proceeded for a further 120 hours. Sintering employed a maximum 
temperature of 1950.degree. C. with a hold period of one minute. Sintered 
density was found to be 4.626 g/cc. HIP raised the density to 4.63 g/cc. 
The product exhibited very fine grain size. Some reaction between TiC and 
Cr.sub.3 C.sub.2 may have occurred. 
EXAMPLE 10 
In this example the ceramic powder mixture was Al.sub.2 O.sub.3 -25 wt % 
Si.sub.3 N.sub.4. Carbide milling was conducted for 16 hours. Maximum 
sintering temperature was 1775.degree. C. with no hold at temperature. 
The resulting body was well-sintered, dense and had a uniform 
microstructure. However, the body was very brittle and X-ray analysis 
indicated that some Al.sub.2 O.sub.3 reacted with the Si.sub.3 N.sub.4 
forming SiAlON compound. 
EXAMPLE 11 
Sialon powder was mixed with 10 wt % YAG (yttrium-aluminum-garnet) and 
carbide milled for 15 hours. The sialon composition (values expressed in 
molar percent) was Si=6-x; Al=x; O.sub.2 =x; N.sub.2 =8-x where x=0.8. The 
temperature ramp from 320.degree. C. to 1810.degree. C. during sintering 
was at a heating rate of 200.degree. C./min with a one minute hold at 
1810.degree. C. Sintered density was found to be 3.32 g/cc (98.8% of 
theoretical). 
As is the case with all sialon compositions, achieving full density 
requires HIP. Analysis by X-ray of the sintered body shows the presence of 
some residual alpha Si.sub.3 N.sub.4. This result is in contrast to the 
product produced by either hot pressing or by long-time sintering wherein 
all of the Si.sub.3 N.sub.4 is converted to the beta form. Although alpha 
Si.sub.3 N.sub.4 is harder than beta Si.sub.3 N.sub.4 it is not as strong.