Note: Descriptions are shown in the official language in which they were submitted.
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CERAMIC COMPOSITIONS
BACKGROUND OF THE I~VENTIO~
1. Field of the Invention:
This invention relates to a high strength, high toughness
and low thermal conductivity ceramic composition. More specifi-
cally, this invention relates to a ceramic composite of finely
divided ZrO2-HfO2 solid solution particles dispersed in a mixture
of A12O3-Cr2O3 solid solution.
2. Brief Description of the Prior Art:
It is well known in the internal combustion engine art
that an increase in temperature in the combustion chamber and a
minimization of the associated heat loss during combustion will
theoretically result in increased efficiency of the engine. This
in turn will lead to improved performance and economy. Thus, the
so-called adiabatic (no heat loss) ceramic engine, high tempera-
ture ceramic-based turbine and ceramic recuperators (heat exchan-
gers) are well known and publicly acknowledged as contemporary
research and development objectives (for example, see "The Coming
Age of Ceramic Engines", March 1982, Popular Science, p 64).
However, at these higher temperatures, the conventional ceramic
materials employed in constructing such devices are inadequate in
one or more critical properties.
Thus, in the case of ceramic lined diesel engines and
similar applications, the ideal ceramic composition used as a
lining material should possess and exhibit high strength, high
toughness and very low thermal conductivity at ultrahigh combus-
tion temperatures as well as high resistance to thermal shock,
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wear and corrosion. Although ceramics are generally known for
their high temperature strength, heat resistance and high tempera-
ture thermal insulation characteristics, they are also known as
being extremely brittle. In general, to overstress a ceramic part
leads to disintegration of the ceramic composition.
Although contemporary applications involving calcium and
yttrium stabilized zirconia are reported to result in improved
strength, toughness and thermal conductivity, it has also been
reported that such partially stabilized zirconia (PSZ)
deteriorates rapidly at temperatures below engine operating
temperatures. Furthermore, the use of titanium alloys as found in
U.S. patent 3,152,523; the silicon nitride, lithium aluminium
silicate, fused silica, silicon carbide, sintered silicon carbide,
reaction sintered silicon carbide and reaction bonded silicon
nitride ceramics as proposed in U.S. patent 4,242,948; and the
cordierite, beta spodumene-mullite and fused silica-clay of U.S.
patent 4,245,611 are felt to be deficient as ceramic compositions
for the adiabatic engine in one or more of the above critical
properties.
More specifically, it is generally known that zirconium
dioxide (zirconia) exists in three allotropic forms; monoclinic,
tetragonal and cubic and that there is a large volume expansion
during the transition from monoclinic to tetragonal. Further, it
has been historically accepted that because of this disruptive
phase transition, the refractory properties of zirconia cannot be
used. However, recent developments relating to suppressing or
disrupting the deleterious effects of the phase transition have
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been discovered. For example, in the so-called partially stabi-
lized zirconia (PSZ) the addition of metal oxide (e.g. lime stabi-
lized zirconia) is viewed as creating a multiphase material having
a fine-scale precipitate of monoclinic zirconia in a stabilized
cubic matrix which in turn results in enhanced strength. More
recently, an even more powerful strengthening mechanism viewed as
involving a dispersion of a metastable tetragonal zirconia in
cubic zirconia has been suggested. In this more recent develop-
ment, the martensitic transformation (fast and diffusionless)
between monoclinic and tetragonal phases is partially alleviated
by inducing and creating tetragonal zirconia in sintered bodies or
domains of a resulting time-stabilized zirconia. Although these
transformation stabilizing ceramic mechanisms and their underlying
rationales may be questionable and although their respective
effects on high temperature properties of the resulting trans-
formation toughened ceramics are encouraging, the breadth of
applicability of these general principles to the field of ceramics
generally and the extent to which the properties can be improved
is still not well defined or understood. Thus, the use of
transformation toughened ceramics and ceramic coatings in specific
pragmatic applications (e.g. light diesel engines and/or ceramic
engines) still remains uncertain.
This invention was made with Government support under
DAAG-460-82-C-0080 awarded by the Department of Army. The Govern-
ment has certain rights in this invention.
SUMMARY OF THE I~VENTIO~
In view of the deficiencies associated with known cera-
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mic compositions particularly relative to the high temperature
properties required in such applications as -the ceramic engine, I
have discovered an improved ceramic composition comprising:
(a) a matrix phase selected from the g~oup consisting of
solid solutions characterized by the formula A1203 xCr203 and
the formula 3A1203 2SiO2 + x[3Cr203 2SiO2] where x is the
relative mole fraction of Cr203 or 3Cr203 2SiO2, and
(b) a disperse phase characterized by the formula
ZrO2 yHfO2 where y is the relative mole fraction of HfO2.
The improved ceramic compositions according to the
present invention are viewed as transformation toughened ceramics
wherein a fine dispersed ZrO2-HfO2 solid solution is present in
either a chromium alumina or chromium mullite solid solution
matrix wherein the HfO2 and Cr2o3 content of the respective solid
solutions can be selected to optimize the balance of the high
temperature properties. According to the present invention, the
mole fraction of Cr2o3 is preferably from about .02 (2 mole %) to
about .3 (30 mole %) and the mole fraction of HfO2 is preferably
from about 0 to about .5(50 mole %) with 20 mole percent Cr203 and
10 to 20 mole percent HfO2 representing a particularly preferred
combination.
It is an object of the present invention to provide
material suitable for application as light diesel engine cylinder
and head liner and piston cap. It is a further object to provide
transformation toughened ceramics useful in the super hot adi-
abatic engine, gas-turbine engine and recuperator heat exchanger
applications. Fulfilment of these objects and the presence and
i~ ~
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fulfilment of additional objects will be apparent on complete
reading of the specificatio~ and attached claims when taken in
conjunction with the attached drawing.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a typical plot of the indentation diagonal
squared, a2, versus the applied load, P, used to evaluate the
hardness, Hv, of a ceramic composition according to the present
invention.
FIGURE 2 is a typical plot of the crack length of the
three halves power, c3/2, versus the applied load, P, used to
valuate the fracture toughness KIC, of a ceramic composition
according to the present invention.
FIGURE 3 is a typical linear plot of the crack length to
the three halves power c3/2, versus the indentation diagonal
squared, a2, characteristic of a ceramic composition according to
the present invention.
FIGURE 4 is a cross sectional view of a thermal conduct-
ivity specimen cell used to measure high temperature thermal con-
ductivity of ceramic composition according to the present inven-
tion.
FIGURES 5 through 12 illustrate experimental data plotsusing indentation test data measured for the (A12O3 xCr2O3)-15
vol. % (CrO2 yHfO2) ceramic compositions according to the
present invention.
FIGURES 13 through 18 illustrate experimental data plots
using indentation test data measured for the (3A12O3 2SiO2 +
x[3Cr2O3 2SiO2])-(ZrO2 yHfO2) ceramic compositions according
~,
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to the present invention.
FIGURES 19 through 23 illustrate experimental data plots
using thermal conductivity data measured for compositions of
Figures 5 through 12.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The novel transformation toughened ceramic compositions
according to the present invention, how they are prepared and the
pragmatic significance of their high temperature properties can
perhaps be best explained and understood by reference to a series
of compositions characteristic of the alumina system and the mul-
lite system. The particular alumina system of interest is the
ceramic compositions having the continuous matrix phase of
A1203 xCr203 solid solution and a dispersed phase within this
continuous matrix phase of finely divided ZrO2 yHfO2 solid solu-
tion particles. The x and y represent the mole fraction or mole
percent of Cr203 relative to A1203 and mole fraction or mole
percent of HfO2 relative to ZrO2 in the respective solid solu-
tions. Similarly, the mullite system of interest is the ceramic
compositions having a continuous matrix phase of 3A1203 2SiO2
20 + x[3Cr203 2SiO2] and a finely dispersed phase of ZrO2 yHfO2
where x and y again represent the relative mole fraction or mole
percent of the 3Cr203 2SiO2 and HfO2, respectively.
For purposes of this invention, the mole fraction or the
mole percent, whether designated by x or y, refers to the relative
mole fraction of the second component of a two component system
wherein it is to be understood that the mole fraction (mole ~) of
the first component plus the x or y sums to unity, e.g.,
ZrO2 yHfO2 means (l-y) ZrO2 yHfO2.
``X
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In order to study and evaluate the high temperature
properties of the above ceramic compositions, twenty-two composi-
tions in the alumina system and ten compositions in the mullite
system were prepared. In the alumina system the continuous matrix
phase, A12O3-xCr2O3, was present in 85 percent by volume and the
dispersed phase, ZrO2 yHfO2, was present in 15 percent by
volume; i.e. (A1203 xCr2O3) plus 15 vol. % (ZrO2 yHfO2). In
the mullite system, the continuous matrix phase, 3Al203 2SiO2 +
x[3Cr2O3-SiO2], was present as the major phase and the dispersed
phase, ZrO2 yHfO2, was present as the minor phase. In both sys-
tems, the relative mole fractions of both the chromium oxide com-
ponent and the hafnium oxide component were varied such as to
charactèrize a broad relative range of concentrations of each
respective component. Thus, in the case of the alumina system,
the twenty-two specimens were distributed as illustrated in TABLE
I wherein two of the samples involved either no HfO2 or no ZrO2
(controls) and the remaining twenty involved compositions with all
four oxides. Similarly, the x and y (relative mole fractions) of
the mullite samples were intentionally distributed over a broad
range of compositions with four additional specimens less one
component being included as controls (as explained later in TABLE
II).
s
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TABLE I
COMPOSITIONS PREPARED IN THE ALUMINA SYSTEM:
(Al203 xCr203) + 15 vol. % (ZrO2 yHfO2)
x, mole fraction of Cr203 y mole fraction of HfO2
in A1203-Xcr203 in ZrO2 YHf02
00.1 0.2 0.30.5 1.0
O * *
.02 * * * *
.05 * * * *
.1 * * * *
.2 * * * *
.3 * * * *
** A1203 - Alcoa# XA- 139
Cr203 - Reagent grade, J.T. Baker Chemical Company
Zr2 - zircoa# A
HfO2 - 99.9% Apache Chemicals, Inc.
Specimens representative of both the alumina and mullite
composition were prepared by hot pressing of mixtures of pre-
solutionized powders. Hardness and fracture toughness were deter-
mined by the microhardness indentation method. Thermal conducti-
vity of the respective specimens was determined by comparison with
known standards. The following examples illustrate the prepara-
tion and composition of the specimens employed in the high temper-
ature property evaluation.
EXAMPLE I
Five separate solid solutions of Al203-Cr203 having the
# Trade Mark
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mole fraction of Cr203 specified in the ordinate of TABLE I and
the four separate solid solutions of ZrO2-HfO2 having the speci-
fied mole fraction of HfO2 as found in the abscissa of TABLE I
were prepared by mixing appropriate amounts of metal oxides in a
ball mill and then reacting the mixture at 1350C, for 24 hours.
The metal oxides employed were from commercial sources as follows:
A1203 was Alcoa XA-139: the Cr203 was Reagent Grade from J. T.
Baker Chemical Company, ZrO2 was zircoa A and the HfO2 was 99.9~
HfO2 from Apache Chemical Inc. The twenty-two compositions speci-
fied in TABLE I were prepared by adding 85 parts by volume of the
A1203 xCr203 solutionized powder and 15 parts by volume of the
ZrO2 yHfO2 solutionized powder and then ball milling the mixture
for 43 hours. Specimens for microindentation tests were hot
pressed at 1600C for one hour in boron nitride coated graphite
dyes under a pressure of 30 MN/m2. After hot pressing, the sam-
ples were then oxidized in air at 1350C for two hours. Specimens
for thermal conductivity measurements were hot pressed at 1600C
for one hour under a pressure of 15 MN/m2. In both cases, full
density was achieved.
EXAMPLE II
In order to evaluate the mullite system, (3A1203 2SiO2
+ x[3Cr203 SiO2])-(ZrO2 yHfO2), a series of ten compositions
and four controls as specified in TABLE II was prepared. Half of
the compositions (type B in TABLE II) were prepared by a physical
mixing or blending technique analogous to the process described in
EXAMPLE I. The other half of the compositions were prepared by a
co-precipitation technique (type C in TABLE II). Appropriate
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stoichiometric amounts of aluminum hydroxide, silicic acid and
chromic acid were used as starting materials for the acid/base
neutralization reaction used to prepare the mullite solid solu-
tions. Oxide powders were used to prepare the ZrO2-HfO2 solid
solutions, again in a manner described in EXAMPLE I. Weighed
powders were mixed in proportions corresponding to TABLE II
and ball milled for seventy-two hours. In the co-precipitated
compositions, the slurries corresponding to the compositional
properties of TABLE II were dried and hot pressed at 1550C for 30
minutes at a pressure of 30 MN/m2. The hot pressing (HP) and the
annealing (Ann.) of the specimens was otherwise as specified in
TABLE II.
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With respect to measuring and evaluating the high
temperature properties of the ceramic compositions of EXAMPLES I
and II, the microindentation technique was used to determine rela-
tive fracture toughness and absolute hardness. A Tukon microhard-
ness testing machine was used for microhardness and indentation
fracture toughening studies. A minimum of five indentations were
made at each of the five or more different loads for each sample.
The loads varied from 2 kg to 15.9 kg depending on specimen
composition. A Vickers diamond indentor (136C) was used in all
studies. The following equation (1) developed by Anstis et al
(for further explanation see "A Critical Evaluation of Indentation
Techniques for Measuring Fracture Toughness: I Direct Crack
Measurements", J. Am. Ceram. Soc~ 64 (9) 33-8 (1981)) was used to
calculate these two material properties.
constant = KIC (H/E)1/2/(P/c3/2) (1)
The above equation relates a material independent constant to the
fracture toughness, hardness, elastic modulus, crack size and
applied load. According to this equation, the C3/2 versus P plot
where c is the crack length and P is the applied load should yield
a straight line with a slope equal to:
slope = constant (E/H)1/2/KIC (2)
By rearranging the above equation and solving for fracture tough-
ness, the following is calculated:
KIC = constant (E/H)1/2/slope (3)
The hardness was determined from the a2 versus P curve
where a is one half of the diagonal of the indentation. From
Hucke's work ("Process Development for Silicon Carbide Base Struc-
...
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tural Ceramics", Report DAAG 46-80-C-0056-P0004, June, 1982,
AMMRC, Watertown, Massachusetts), this hardness, Hv, is indepen-
dent of load and is the hardness at large loads. The equation
used to calculate this value is:
Hv = k/s' (4)
where k is a proportionality cons~ant for a Vickers diamond inden-
tor (136) and is equal in this case to 4,636. The sl~pe, s', is
determined from the a2 versus P curve. The value of the constant
in equation (4) is of little significance since a standard (NC203)
was used to make all the calculations.
To determine KIC, the value of the elastic modulus,
E, for the composition being measured must be available or deter-
mined. For the alumina system, the literature value of the elas-
tic modulus for A1203 of 60 x 106 psi was used (see Engineering
Property Data on Selected Ceramics, Vol. III, Single Oxides, MCIC-
HB-07, Metal and Ceramics Information Center, Battelle, Columbus,
Ohio). It was assumed that this value was constant and character-
istic of all alumina compositions employed. For the mullite sys-
tem a value of 25 x 106 psi was used. This value is intermediate
20 between two values, 21 and 32 x 106 psi, reported in literature.
~See Van Vlack, L.H., Elements of Material Science, Addison-
Wesley, Reading, Massachusetts, pf. 5.4.1-22 and Matdigasmi, K.
S~, and Brown, L. M., "Synthesis in Mechanical Properties of Stoi-
chiometric Alumin Silicate (Mullite), J. Am. Ceram. Soc. 55 (11)
548-552 (1972)).
Plots of a2 versus P and C3/2 versus P curves were cons-
tructed for all indentation test data generated. The degree of
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linear fit was excellent, r2 = .99,for a 11 a2 versus P curves.
The linear fit was good, for most tests r2 was at least .97, for
the C3/2 versus P. Curves. Figures 1 through 3 of the drawings
illustrate typical a2 versus p, C3/2 versus P, and C3/2 versus a2
curves generated by plotting the respective data characteristic of
compositions according to the present invention. As clearly indi-
cated in Figures 1 through 3, the experimentally measured data is
essentially linear across the entire range of interest consistent
with the nature of the above equations.
The thermal conductivity measurements of compositions
according to the present invention were performed by a comparative
method. As illustrated in Figure 4, the method involved the use
of a commercially available comparative thermal conductivity
instrument manufactured by Dynatech Corporation (Model TCFCM)~
The comparative measurement involved placing test specimen 10 to
be measured within the Model TCFCM 12 such that it is stacked
between a top reference standard specimen 14 and a bottom
reference standard 16. As further illustrated in Figure 4, the
stack of specimens is sandwiched between an upper surface plate 18
and a lower surface plate 20 which in turn rest on auxiliary
heater 22 and is capped by a main heater 24. The top heater 24 in
turn is covered by insulation, while the auxiliary heater 22 rests
on a spacer 28 and heat sink 30. A plurality of thermocouples 32
are strategically positioned within the sample cavity 34 at
critical interfaces, such as to make temperature measurements
while pressure pad 36 compresses the stack of specimens.
The actual measurement and computation of the thermal
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conductivity is defined by the following equation:
q = kA(dT/dx) (5)
where q is heat flux, A is the specimen cross section area, T is
temperature, x is distance between two pints in the sample and k
is the desired thermal conductivity. When the specimens with the
same A of two different materials are arranged as shown in Figure
4 then:
T(k/~x) = ~T(k/~x) (6)
sample reference
Thus, by measuring the temperature difference between two thermo-
couples at a distance x apart in both reference materials and te
test sample, the thermal conductivity of the test material can be
evaluated.
Figures 5 through 12 graphically present and summarize
the results of the indentation tests on the alumina system
compositions of EXAMPLE I as specifically set out in TABLE III.
The twenty-two alumina compositions measured contained about 2 to
about 30 mole percent Cr203 in the alumina/chromium solid solution
matrix and from 0 to about 50 mole percent HfO2 in the
zirconia/hafnium dioxide dispersed solid solution particles. As
illustrated in Figures 5 through 8 and as presented in TABLE III,
the measured fracture toughness generally decreased with increas-
ing HfO2 content at constant Cr2O3 content. This may be attri-
buted to the decrease in the critical particle size of the tetra-
gonal-monoclinic phase transition with increasing HfO2 content.
However, it has not been verified that the particle size of the
dispersed ZrO2-HfO2 solid solution phase in any of the samples was
small enough to retain the tetragonal phase (as further discussed
below).
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TABLE I I I
SAMPLE THERMAL CONDUCTIVITY
(A1203 ~ xCr 23) ~ k = q/A ( dt /dx )
15 vol % (Zro2 YHf2) (cal/cm C sec)
x y 70 C 250 C 400 C
O - 100 0.0399 0.219 0.0156
O - O 0.0273 0.0177 0.0137
2 - 10 0.0378 0.0240 0.182
2 - 20 0.0332 0.0180 0.0132
2 - 30 0.0348 0.0179 0.0120
2 - 50 0.0292 0.0197 0.0147
5 - 10
- 20 0.0274 0.0174 0.3133
- 30 0.0277 0.0164 0.0121
- 50 0.0274 0.0165 0.0127
- 10 0.0165 0.0125 0.0108
- 20 0.0241 0.0170 0.0138
- 30 0.0229 0.0144 0.0108
- 50 0.0233 0.0152 0.0118
- 10 0.0153 0.0105 0.0082
- 20 0.0179 0.0121 0.0094
- 30 0.0188 0.0137 0.0111
- 50 0.0191 0.0138 0.0110
- 10 0.0168- 0.0122 0.0099
- 20 0.0156 0.0113 0.0093
- 30 0.0178 0.0139 0.0119
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Figures 9 through 12 and TA~LE III illustrate that for
constant HfO2 contents of 10, 20 and 50 mole percent there appears
to be a shift in the peak of the fracture toughness, KIC,
versus Cr203 content curve. The peaks appear at 10, 20 and 30
mole percent Cr203 content, respectively. The fracture toughness
continually decreased with increasing Cr2O3 content with the
effect of the HfO2 content on the hardness being less discernible.
Generally, the hardness stayed fairly constant with increasing
HfO2 content and constant Cr203 content.
The average particle size for the dispersed ZrO2-HfO2
phase in most of the above specimens was of the order of 5;~m.
This is felt to be too large to effectively retain the metastable
tetragonal phase which, as previously indicated, mechanistically
is thought to produce an increase in fracture toughness due to a
stress-induced phase transformation of even greater significance
than the originally observed phenomena associated with PSZ. In
this respect, the time-stabilized zirconia and the associated high
energy absorbing tetragonal to monoclinic transition beneficial
affects may not be fully realized (a feature which from a particle
side analysis should be kept in mind when evaluating the co-
precipitation versus ball milled specimens of EXAMPLE II). In
interpreting the above experimental data and property measure-
ments, it should also kept in mind that using a constant modulus
value, E, for all compositions will introduce some error in the
calculated fracture toughness value. Although present, this error
is felt to be small.
In view of th~ above data, the transformation toughened
ceramic alloys involving twenty percent Cr203-10% HfO2 and/or 20%
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Cr2O3-20% HfO2 having a relatively high Cr203 content should ex-
hibit significantly increased hardness and modulus. Further, high
HfO2 content (e.g. 50 mole %) should significantly increase the
transformation temperature and thus, increase the potential for
transformation toughening. For the same reason, the critical
particle size for transformation is preferably decreased. To
achieve a reasonable increase in the toughening and keep the
critical particle size within pragmatic limits, intermediate
values of HfO2 (e.g., about 10 to about 20 mole ~) in the
dispersed solid solution phase are preferred.
Figures 13 through 18 graphically display and summarize
the results of the indentation tests on the mullite system of
EXAMPLE II as specifically found in the far right columns of data
of TABLE II. Again, the figures illustrate and suggest certain
general trends in the data. For example, Figures 13 through 15
suggest that for the co-precipitated samples, keeping HfO2 cons-
tant and increasing the Cr mullite content, results in an increase
in the fracture toughness; while, the effect of a change in the
HfO2 content on fracture toughness is more obscure. Similarly,
for mechanically mixed samples, an increase in HfO2 content
generally results in a decrease in fracture toughness at constant
Cr mullite content. With no HfO2 the fracture toughness remains
constant while increasing the Cr mullite content. In all cases
except one, for the same composition, the fracture toughness of
the co-precipitated samples was greater than the fracture tough-
ness of the mechanically mixed samples. This may be due to the
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difference in hot pressing conditions and/or subsequent particle
size and distribution.
Figures 16 through 18 indicate that the hardness of the
co-precipitated samples increased with increasing HfO2 content
with 0 percent and 5 percent Cr mullite. The hardness stayed
relatively constant with increasing HfO2 content with 10 percent
mullite. For the mechanically mixed specimens, the hardness
generally decreased with increasing HfO2 content. For seven of
the ten compositions the hardness of the co-precipitated samples
were greater than the corresponding values of the mechanically
mixed samples. Thus, the co-precipitation method of sample prepa-
ration appears to enhance both fracture toughness and hardness
simultaneously.
The thermal conductivity of the twenty-two compositions
in the alumina system,(A1203 xCr203)-(ZrO2 yHfO2), prepared in
EXAMPLE I were measured at three different temperatures in the
previously described thermal conductivity instrument illustrated
in Figure 4. The specimens for these measurements were hot
pressed cylinders 3 centimeters in diameter and 2 centimeters in
height. Both top and bottom surfaces were lapped with 15~ m
diamond disk. The results of the thermal conductivity measure-
ments expressed in cal/cm C sec at the three respective temper-
atures are given in TABLE III. The sample notations are expressed
in terms of the x and y of the above alumina formula and represent
the relative mole percent of the Cr2o3 in the matrix phase and the
HfO2 in the dispersed phase. The data are also plotted as thermal
conductivity versus Cr203 content at various HfO2 contents in
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Figures 19 through 22. As indicated in these figures, the curves
can be extrapolated back to zero percent CrO2 and no HfO2. From
the extrapolation, it can be concluded that pure ZrO2 dispersed in
a pure A12O3 matrix should have a thermal conductivity value of
about 0.038 cal/cm C.sec. Greve et al (see "Thermal
Diffusivity/Conductivity of Alumina with Zirconia ~ispersed Phase"
Am. Ceram. Soc. Bull. 56 (5) 514-5 (1977)) reported that an A1203
matrix containing fifteen volume percent ZrO2 had a thermal
conductivity value of 0.018 cal/cm C sec. Thus, the 70C
thermal conductivity curves in Figures 19 through 22 can be
normalized by multiplying a factor of 0.018/0.038 to become the
normalized curve shown in Figure 23.
The compositions of the present invention are advanta-
geously employed as transformation toughened ceramics at high
temperatures in application that require a balance of physical,
chemical and mechanical properties. In particular, the composi-
tions are useful in that the minor constituents can be altered to
take advantage of the specific enhancement of critical properties
such as hardness, fracture toughness and low thermal conductivity
(as demonstrated in the presented test data) without significantly
sacrificing the known corrosion, wear and high temperature stabil-
ity of the major ceramic constituent. As such, the present inven-
tion is to be viewed as being directed broadly to a class of novel
ceramic compositions wherein the Cr203 content in the matrix phase
and the HfO2 in the zirconia dispersed phase can varied to achieve
the improvement in the critical properties. Similarly, this novel
concept is envisioned as being applicable and beneficial at
12496~9
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various relative proportions of disperse phase to host matrix
phase.
Having thus described and exemplified the preferred
embodiments with a certain degree of particularity, it is to be
understood that the invention is not limited to the embodiments
set forth herein for purposes of exemplification, but is to be
limited only by the scope of the attached claims, including a full
range of equivalents to which each element thereof is entitled.
:, .
