Background of the Invention

Field; This invention relates to magnesium oxide compositions, and particularly to magnesium oxide compositions which have been modified with varying amounts of zirconia and/or hafnia.

State of the Art: The production of ceramic bodies from magnesium oxide has long been practiced. Generally, sintering of a magnesium oxide ceramic body has been done either by hot-pressing or by hot isostatic pressing.

Strengthening of magnesia bodies with zirconia has been disclosed in U.S. Patent No. 4,678,761 of Dr. Anil Virkar et al., and in an article published in 1985 entitled ^l! Zr0 -Toughened MgO and Critical Factors in

Toughening Ceramic Materials by Incorporating Zirconia," Journal of Material Science Letters 4 (1985) 63-66.

The primary thrust of the aforementioned U.S. patent of Dr. Virkar, which has the same assignee as this application, was to improve sinterability of magnesium oxide compositions containing zirconia or other materials. It disclosed, in particular, manganese oxide and iron oxide as exceptionally effective sintering aids.

Summary of the Invention The instant invention comprises magnesium oxide bodies containing zirconia and/or hafnia wherein magnesia is present from about 10% to about 80% by volume of the composition and wherein zirconia and/or hafnia are present from about 20% to about 90% by volume of the ceramic body. Other ingredients may be included, especially sintering aids such as manganese oxide and iron oxide according to the description pertaining to said sintering aids contained in U.S. Patent No. 4,678,761 of

Virkar et al. issued July 7, 1987, which patent is hereby incorporated by reference. Compositions falling within the above-stated ranges exhibit superplasticity when subjected to deformation loads at temperatures in excess of about 1100°C for compositions containing a sintering aid and above about 1200 ^β C for compositions containing no sintering aid.

The ceramic compositions of the instant invention are prepared in a conventional manner from ceramic powders and typical organic binders and formed into a body by pressing, slip-casting, tape-casting, or other conventional preparation of a green ceramic body from oxide powder mixes.

The green ceramic is generally processed by heating to a relatively low temperature, e.g. 300°C, to burn off any binders present and then heated at a much higher temperature, for example, at least about 1200°C and usually above about 1250 ^β C, and especially above about 1300°C, for a sufficient period to sinter the body into a dense ceramic body. Usually the sintered density achieved is in excess of 97% of theoretical density when the temperatures of sintering are in excess of 1350°C.

The zirconia and/or hafnia present in these magnesium oxide bodies may be of any particular crystal form. The ceramic bodies, after sintering, may be cooled to room temperature and surface finished by grinding, polishing or like procedures, then heated to a temperature at least of about 1100°C for forming into complex or other desirable shapes by application of a load equal to or in excess of that required to cause deformation.

Alternatively, the hot (approximately 1300°C) sintered body may be subjected to a deformation, shape- forming load without the sintered ceramic being cooled to a temperature below its superplastic deformation temperature.

Brief Description of the Drawings Fig. 1 graphically illustrates load deformation characteristics of a 60 MgO/40 Zr0 ₂ sintered ceramic body with MnO at 1100°C and at 1200°C. Fig. 2 contains a graph illustrating the determination of "m," the strain rate sensitivity index.

Fig. 3 graphically illustrates the various strain rates of a 60 MgO/40 Zr0 ₂ sintered ceramic body at 1200°C under deflection loads varying from 0.005 cm/minute to about 0.05 cm/minute.

Fig. 4 is a graph illustrating the proportionality of the flow stress to the deflection rate of the sample of Fig. 3.

Fig. 5 graphically illustrates stress deflection curves for various MgO/Zrθ2 compositions, including pure MgO and pure Zr0 ₂ .

Fig. 6 graphically illustrates the effect of various deformation temperatures at a constant deflection rate. Fig. 7 is a plot of flow stress versus reciprocal time to determine activation energy (Q) .

Fig. 8 is a tabular comparison of the activation energy (Q) from Fig. 7 with reported values of Q for pure MgO and pure Zr0 ₂ . Fig. 9 graphically presents data from a stress versus deflection test for pure MgO, 60 MgO/40 Zr0 ₂ and pure Zr0 ₂ .

Fig. 10 illustrates a non-linear Maxwell model test. Figs. 11 and 12 illustrate graphically and tabularally the results of the test of Fig. 10.

Description of the Invention

The instant invention comprises magnesium oxide ceramic bodies containing a wide range of zirconia and/or hafnia. The ceramic compositions, in a dense, sintered condition, exhibit superplasticity at temperatures in excess of 1100°C. Sintering occurs at temperatures in excess of preferably about 1350"C. The compositions comprise manganese at about 10% to about 80% by volume and zirconia and/or hafnia present from about 20% to about 90% by volume although usually not in excess of about 80%.

The compositions of the instant invention are particularly unique inasmuch as their processing may be done according to conventional ceramic processing techniques to obtain a densified ceramic body exhibiting superplastic properties.

Dense bodies of magnesium oxide may be prepared by admixing finely ground particles of magnesium oxide with a sintering aid such as manganese oxide, iron oxide, or the like for a sufficient period of time to obtain a substantially uniform admixture. This admixing may be done dry or in a wet state, e.g., in presence of deionized water containing 2% ammonium hydroxide. If done in a wet state, the admixture is then dried at an appropriate temperature, for example, about 200°C for a sufficient time to drive off the water.

Subsequent to the admixing step, the material may then be ground for a sufficient period of time to achieve an appropriate particle size which, for purposes of this invention, is generally less than about 50 mesh, and preferably less than about 100 mesh. The ground material is mixed in liquid such as acetone with a binder solution such as 2% polyvinyl butyral solution. Other known binders may also be used.

Again, the material is dried at a sufficient temperature and for a sufficient time to dry the material so that it may be further processed. Optionally, the

aterial may then be again ground so that it preferably passes through a 100 mesh screen. If the material is processed in its dry condition, it is then mechanically pressed to an appropriate shape. The material may be formed in a wet state by slip-casting, tape-casting and similar techniques. The shaped body is then sintered under conventional conditions, e.g., in the presence of pressure (hot pressing or hot isostatic pressing) if no sintering aid is present or in the absence of pressure if a sintering aid is present, at a temperature generally below about 1400°C for a sufficient period of time to achieve a desired density. Generally a time period of less than about three hours is sufficient at the sinterin temperature, depending upon the size of the ceramic body, and frequently two hours or less is sufficient to obtain dense, sintered ceramic bodies.

A particular advantage of the instant invention is that ceramic bodies may be formed by slip-casting, extrusion, mechanical pressing, and other conventional ceramic pressing techniques to achieve, a particular shape or configuration of ceramic body. Ceramic bodies with complex shapes generally cannot be readily sintered eithe by hot-pressing or hot isostatic pressing. While it is possible to hot isostatically press a complex shape, the shape must be relatively nonporous in order to achieve densification.

Sintered ceramic bodies of the instant inventio may be prepared in a simple, uncomplex shape initially an then formed into more complex shapes by applying deformation loads in certain directions and using certain confining means for the ceramic at temperatures in excess of about 1100°C.

While MgO materials containing, for example, 40% zirconia in a sintered condition retain their room temperature strength up to about 1000°C, such materials when tested under a deformation load at about 1100°C,

especially when doped with a small quantity of manganese oxide, show significant plasticity. The testing of various compositions of magnesia and zirconia both with and without manganese oxide showed significant plasticity as exhibited in Fig. 1. In order to characterize the deformation behavior of these materials as "superplastic," the material has to satisfy the requirements reported in Fig. 2, attached hereto, which are:

1. Absence of work hardening. 2. Strain rate sensitivity index "m" greater or equal to 0.5 as determined by the following formula: m = d(ln o-fτ. _ow ) dlnέ σ = k έ ^m

6 = A a ⁿ and m = 1. n

The formula and graph associated therewith are illustrated in Fig. 2. 3. Equiaxed cavitation free microstructure after large deformation.

The magnesia bodies of the instant invention exhibited these characteristics. Further description of the invention may be facilitated by reference to the attached figures.

EXAMPLE 1

The data illustrated in Fig. 3 was determined from four samples of magnesia containing 40% by volume of zirconia in the total ceramic body. This particular sample contained no sintering aids such as manganese oxide. These samples were prepared in a conventional manner and sintered at temperatures of about 1400°C. The samples were then cooled and then ground and polished. These samples were then tested at 1200°C in a 4 point bend test. The samples had been sintered at 1500°C for four hours to a density of 96.1% of theoretical. The samples

were surface ground to the same thickness and polished to a 6 micron finish.

The top span of the 4 point bend test is 10 millimeters and the bottom span is 40 millimeters. All the samples had the same thickness of 0.444 + about 0.002 cm. Each sample was tested at a different cross head speed, which varied from 0.005 cm/minute to about 0.050 cm/minute.

These cross head speeds may be converted to strain rates after finding the stress exponent. The samples were tested until a deflection of 0.304 cm was achieved. The samples were then unloaded, that is, removed from any application of force. The stress- deflection characteristics are shown in Fig. 3. In each case, a well developed flow stress was observed without any indication of work hardening. The log of flow stress l (σf^ow) -*- ^s plotted against log deflection rate (In <5) . Since all the samples had the same thickness and tested in the same span lengths, the deflection rate will be proportional to the strain rates. Such data are illustrated in Fig. 4. The slope of the curve provides strain rate sensitivity index m and is found to be equal to about 0.68, and the stress exponent from about 1.4 illustrated in Figs. 3 and 4 shows that two characteristic requirements of superplasticity are met in these materials, that is, an absence of work hardening and m > 0.5.

The maximum strain on the tension side is about 6%. When tested in some other bending set up, large strains are observed and the sample did not fail. A first sample was deformed in a creep experiment set up. (All these samples were doped with 3 Mol. Mn0 ₂ . ) The outer span in the creep experiment is 2.54 cm and the inner span is 1 cm. A maximum strain rate of 10 ^_4 /sec. was observed. A second sample was tested in 5 point bend set up (3 sharp supporting points for the bottom and 2 in the top) . The

sample was loaded at a cross head speed of 0.005 cm/minute at 1150°C. The sample was loaded for 100 minutes and then unloaded. The sample did not fail. A third sample was tested for determination of strain rate sensitivity index and the strain on the outer tension side is about 6%. This latter sample established the possibility of hot forming these sintered ceramics to desired, predetermined shapes by application of forces in predetermined directions and by confining the sintered ceramic.

SEM and TEM Studies

The tension side of the deformed samples was prepared for SEM and TEM investigations. An SEM examination showed a cavitation free microstructure. A TEM examination showed cleaner triple points and absence of cavitation or cracking and glassy phase. Also in both examinations, the grains appear equiaxed.

With these experiments, all the requirements were met to establish superplastic behavior. Hence, the material MgO + 40 v/o Zr0 ₂ is characterized as "superplastic."

Deformation of MαO -. ZrOo Ceramics as a Function of Volume Fraction of ZrQ

Fig. 5 shows stress deflection curves for several compositions. Pure MgO was hot-pressed and had a grain size of crystals of about 0.5 microns. The YTZP had 100% tetragonal single phase crystals and had a grain size of about 0.5 microns. Both these samples failed when tested at 1200"C under increasing load. However, samples with dual phases of MgO and Zr0 ₂ showed plasticity. MgO + 90 v/o Zr0 ₂ failed while all other samples had to be unloaded. All these materials had a grain size of about 1-2 microns.

Activation Energy Determination

MgO + 40 v/o Zr0 ₂ samples were deformed by bending at a constant deflection rate of 0.0127 cm/minute (which corresponds to a strain rate of 3.5 x 10~ ⁵ /sec.) at temperatures ranging from 1150°C to 1250°C. All samples were unloaded after a flow stress was established. The stress deflection plot is shown in Fig. 6. Fig. 7 shows a plot of log flow stress vs 1/time. The slope is Q/nr (from E x σ ⁿ exp [- Q/RT] , n is assumed constant between 1150°C and 1250°, and the change in shear modulus is considered negligible) . From the slope, Q is estimated to be 94 kcal/mole. Fig. 8 compares Q for MgO + 40 v/o Zr0 ₂ with values reported for pure MgO and YTZP in constant strain rate tests. Notice that the dual phase has a Q value that is lesser than pure MgO or YTZP.

Compression Tests

Fig. 9 shows a stress vs. deflection plots for samples pure MgO, MgO + 40 v/o Zr0 ₂ , and MgO + 90 v/o Zr0 ₂ . Pure MgO cavitated and showed visible vertical cracks after testing. MgO + 40 v/o Zr0 ₂ showed a well- established flow stress, while MgO + 90 v/o Zr0 ₂ sample was hard to deform and required very high stress. After a fixed deflection, the cross head was stopped, allowing the stress to relax. A Maxwell element with a nonlinear Newtonian dashpot was modeled and the stress exponents, relaxation time and viscosity were determined. These are illustrated in Figs. 10 and 11 and the results are summarized in Fig. 12. MgO + 40 v/o Zr0 ₂ has a diffusive and dislocation contribution towards its deformation, while MgO and MgO + 90 v/o Zr0 ₂ had only dislocation contribution. The relaxation time and viscosity of the MgO + 40 v/o Zr0 ₂ is lower than that of MgO and MgO + 90 v/o Zr0 ₂ .

Mechanism:

Mg ⁺² has an ionic radius of about 0.72 A for a coordination number of 6 (CN = 6) . Zr ⁺⁴ also has an ionic radius of about 0.72 A for CN = 6. Zr0 ₂ may very well dissolve in MgO perhaps in a small quantity. Solu¬ bility of Zr0 ₂ in MgO will create cation vacancies in Mg site as shown in the following equation: Zr0 ₂ — ^Zr Mg ⁺ 20 _o + V" _M g. This explains the diffusive contribution in MgO + 40 v/o Zr0 ₂ samples. Furthermore, the flow stress increases with increasing Zr0 ₂ content as seen from Fig. so the main contribution to deformation is from softer Mg phase.

A summary of conclusions regarding the above experiments is as follows: 1. MgO + 40 v/o Zr0 ₂ has a strain rate sensitivity index of 0.68.

2. MgO + 40 v/o Zr0 ₂ ceramics have an equiaxe and cavitation free microstructure after deformation.

3. These ceramics do not exhibit any work hardening.

4. There exists a possibility for hot forming

5. MgO + Zr0 ₂ composites are plastic whereas the individual constituent phases are not.

When hafnia is substituted for zirconia in the above examples, similar results are achievable.

As illustrated in Fig. 5, pure MgO and pure Zr0 do not exhibit any superplastic characteristics with an MgO 10%/Zr0 ₂ 90% by volume exhibiting limited superplastic properties. Those compositions in the range of 80% MgO/20% Zr0 ₂ to 20% MgO/80% Zr0 ₂ exhibit good superplastic characteristics while compositions in the range of MgO 80%/Zr0 ₂ 20% to MgO 40%/Zr02 60% exhibit excellent superplastic properties. The best compositions to demonstrate superior superplastic properties are those in which magnesia predominates.

Minor quantities of other ceramic materials

such as alumina, bismuth oxide, silicon carbide, silicon nitride, mullite and the like may be included in the ceramic compositions without unduly altering the superplastic property of the sintered ceramic at elevated temperatures.

Sintering aids such as manganese oxide, iron oxide, and the like may be advantageously included in the MgO-Zr0 ₂ /Hf0 ₂ compositions to facilitate pressureless sintering at relatively low temperatures, e.g., temperatures as low as 1200°C.

Hot-pressed and hot-isostatically pressed ceramics of various "superplastic" MgO-Zr0 ₂ /Hf0 ₂ compositions, with or without sintering aids, may be readily formed in simple shapes which may then be formed into complex shapes at elevated temperatures, i.e., greater than 1100°C, by application of deformation type forces.