CA1249609A - Ceramic composition - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Novel transformation toughened ceramic compositions comprising finely divided ZrO2-HfO2 solid solution particles dispersed in a AL2O3-Cr2O3 (alumina) or 3Al2O3.2SiO2-3Cr2O3 .2SiO2 (mullite) solid solution matrix phase. It is found that increased hardness and modulus associated with higher relative Cr2O3 content (e.g. 20 mole %) in the matrix phase and increased fracture toughness associated with higher re-lative HfO2 content (e.g. 10 to 20 mole %) in the disperse phase represent viable and improved ceramic compositions.
Such transformation toughened ceramic compositions are con-sidered useful in superhot light diesel engine applications, high temperature gas turbines and ultrahigh temperature ceramic heat exchangers.

Description

~Z496U9 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, lZ496~)9 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 ~;~496U~

- 3 - 72664-1 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-12496~)9

- 4 - 72664-1 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~ ~

1~4~6U9

- 5 - 72664-1 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 ~, 12~6~9

- 6 - 72664-1 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

lZ4~6(~9

- 7 - 72664-l 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 lZ4~61~9

- 8 - 72664-1 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 iZ496Q9

- 9 - 72664-1 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 12~6~9

- 10 - 72664-1 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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U, ~ l ~ l l l lZ4~6~9 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-...

lZ496~9 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 lZ~6~

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 12~96;~9 - 15 - 72664-l 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).

lZ4!~6~9 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 - 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 ~, J

lZ49609 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%

lZ~96~9 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 124!96~)~

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 12~96Ug 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 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.

:, .

Claims (18)

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

1. A ceramic composition comprising:
(a) a matrix phase selected from the group consisting of solid solutions characterized by the formula Al2O3.xCr2O3 and the formula 3Al2O3.2SiO2 + x[3Cr2O3.2SiO2] where x is the relative mole fraction of Cr2O3 or 3Cr2O3.2SiO2; and (b) a dispersed phase characterized by the formula ZrO2.yHfO2 where y is the relative mole fraction of HfO2.

2. A ceramic composition of Claim 1 wherein x is from about .1 to about .5 and y is from about .02 to about .3.

3. A ceramic composition of Claim 1 wherein said matrix phase is Al2O3.XCr2O3.

4. A ceramic composition of Claim 1 wherein said matrix phase is 3Al2O3.2SiO2 + x[3cr2O3.2SiO2]

5. A ceramic composition of Claim 3 wherein x is from about .1 to about .5 and y is from about .02 to about .3.

6. A ceramic composition of Claim 4 wherein x is from about .1 to about .5 and y is from about .02 to .3.

7. A ceramic composition of Claim 3 wherein x is from about .12 to about .2 and y is at least about .2.

8. A ceramic composition of Claim 1 wherein said matrix phase is present in about 85 volume percent and said dispersed phase is present in about 15 volume percent.

9. A ceramic composition of Claim 2 wherein said matrix phase is present in about 85 volume percent and said dispersed phase is present in about 15 volume percent.

10. A ceramic composition of Claim 3 wherein said matrix phase is present in about 85 volume percent and said dispersed phase is present in about 15 volume percent.

11. A ceramic composition of Claim 4 wherein said matrix phase is present in about 85 volume percent and said dispersed phase is present in about 15 volume percent.

12. A ceramic composition of Claim 5 wherein said matrix phase is present in about 85 volume percent and said dispersed phase is present in about 15 volume percent.

13. A ceramic composition of Claim 6 wherein said matrix phase is present in about 85 volume percent and said dispersed phase is present in about 15 volume percent.

14. A ceramic composition of Claim 7 wherein said matrix phase is present in about 85 volume percent and said dispersed phase is present in about 15 volume percent.

15. A ceramic composition comprising:
(a) a continuous matrix phase of Al2O3-Cr2O3 solid solution;
and (b) a dispersed phase within said continuous matrix phase of finely divided ZrO2-HfO2 solid solution particles.

16. A ceramic composition comprising:
(a) a continuous matrix phase of 3Al2O3.2SiO2-3Cr2O3.2SiO2 solid solution; and (b) a disperse phase within said continuous matrix phase of a finely divided ZrO2-HfO2 solid solution particles.

17. A ceramic composition comprising finely dispersed zirconium dioxide/hafnium dioxide solid solution particles in a matrix phase of aluminum oxide/chromium oxide solid solution.

18. A ceramic composition comprising finely dispersed zirconium dioxide/hafnium dioxide solid solution particles in a matrix phase of aluminum mullite/chromium mullite solid solution.