Shaped sintered ceramic bodies composed of Y2O3-stabilized zirconium oxide and process for producing a shaped sintered ceramic body composed of Y2O3-stabilized zirconium oxide

Abstract

Disclosed is a ceramic sintered shaped body containing Y.sub.2O.sub.3-stabilized zirconia with a sintered density of at least 99% of the theoretical sintered density and having a mean grain size of <180 nm. The zirconia fraction of the sintered shaped body comprises tetragonal and cubic phases. Also disclosed is a process for the production of a ceramic sintered shaped body containing Y.sub.2O.sub.3-stabilized zirconia, which process comprises dispersion of a submicron powder and comminution of the dispersed submicron powder by means of grinding media having a diameter of less than or equal to 100 μm to a particle size d.sub.95 of <0.42 μm; shaping of the dispersion to form a body, and sintering of the body to form the sintered shaped body.

Claims

1. A ceramic sintered shaped body wherein the sintered shaped body consists of Y.sub.2O.sub.3-stabilized zirconia consisting of 97 mol % zirconia and 3 mol % Y.sub.2O.sub.3 and having a sintered density of at least 99% of a theoretical sintered density; a mean grain size of smaller than 180 nm; a zirconia fraction comprising tetragonal and cubic phases in a concentration of at least 98 mass %; and a monoclinic fraction of less than 3 mass % after 120 hours at 134° C. and 2 bar in a water vapor atmosphere.

2. A ceramic sintered shaped body, wherein the sintered shaped body comprises Y.sub.2O.sub.3-stabilized zirconia and 2-15 mass % of unstabilized zirconia and has a sintered density of at least 99% of a theoretical sintered density; a mean grain size of smaller than 180 nm; a zirconia fraction comprising tetragonal and cubic phases in a concentration of at least 98 mass %; and a monoclinic fraction of less than 3 mass % after 120 hours at 134° C. and 2 bar in a water vapor atmosphere.

3. The ceramic sintered shaped body of claim 2, wherein the Y.sub.2O.sub.3-stabilized zirconia consists of 97 mol % zirconia and 3 mol % Y.sub.2O.sub.3.

4. The ceramic sintered shaped body of claim 2, wherein the unstabilized zirconia is present in the tetragonal phase.

5. The ceramic sintered shaped body of claim 2, wherein the shaped body consists of Y.sub.2O.sub.3-stabilized zirconia and 2-15 mass % of unstabilized zirconia.

6. The ceramic sintered shaped body of claim 2, wherein 75-95 mass % of the zirconia fraction is present in the tetragonal phase and 5-25 mass % of the zirconia fraction is present in the cubic phase, and wherein a monoclinic fraction of less than 2 mass % is present.

7. A ceramic sintered shaped body, wherein the sintered shaped body comprises Y.sub.2O.sub.3-stabilized zirconia and 0.2-20 mass % of α-Al.sub.2O.sub.3 and has a sintered density of at least 99% of a theoretical sintered density; a mean grain size of smaller than 180 nm; a zirconia fraction comprising tetragonal and cubic phases in a concentration of at least 98 mass %; and a monoclinic fraction of less than 3 mass % after 120 hours at 134° C. and 2 bar in a water vapor atmosphere.

8. The ceramic sintered shaped body of claim 7, wherein the Y.sub.2O.sub.3-stabilized zirconia consists of 97 mol % zirconia and 3 mol % Y.sub.2O.sub.3.

9. The ceramic sintered shaped body of claim 7, wherein the shaped body consists of Y.sub.2O.sub.3-stabilized zirconia and 0.2-20 mass % of α-Al.sub.2O.sub.3.

10. The ceramic sintered shaped body of claim 7, wherein 75-95 mass % of the zirconia fraction is present in the tetragonal phase and 5-25 mass % of the zirconia fraction is present in the cubic phase, and wherein a monoclinic fraction of less than 2 mass % is present.

11. The ceramic sintered shaped body of claim 7, wherein the shaped body has a four point bending strength of at least 1000 MPa.

12. A process for the production of the ceramic sintered shaped body of claim 2, wherein the process comprises: dispersing a submicron powder comprising at least 65 mass % of Y.sub.2O.sub.3-stabilized zirconia and from 2 to 15 mass % of unstabilized zirconia, comminuting dispersed submicron powder by grinding media having a diameter of less than or equal to 100 μm to a particle size d.sub.95 of less than 0.42 μm, shaping a dispersion of comminuted submicron powder to form a body, and sintering the shaped body to form the ceramic sintered shaped body.

13. The process of claim 12, wherein the submicron powder has a specific surface area of less than 20 m.sup.2/g.

14. The process of claim 12, wherein shaping is carried out by slip casting.

15. The process of claim 12, wherein the body is sintered at a sintering temperature of from 1200° C. to 1350° C.

16. The process of claim 15, wherein the process further comprises subjecting the sintered body to hot isostatic pressing at a temperature of from 1200° C. to 1350° C.

17. A process for the production of the ceramic sintered shaped body of claim 7, wherein the process comprises: dispersing a submicron powder comprising at least 65 mass % of Y.sub.2O.sub.3-stabilized zirconia and from 0.2 to 20 mass % of α-Al.sub.2O.sub.3, comminuting dispersed submicron powder by grinding media having a diameter of less than or equal to 100 μm to a particle size d.sub.95 of less than 0.42 μm, shaping a dispersion of comminuted submicron powder to form a body, and sintering the shaped body to form the ceramic sintered shaped body.

18. The process of claim 17, wherein the submicron powder has a specific surface area of less than 20 m.sup.2/g.

19. The process of claim 17, wherein shaping is carried out by slip casting.

20. The process of claim 17, wherein the body is sintered at a sintering temperature of from 1200° C. to 1350° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is described more fully in the following with reference to embodiment examples, drawings and tables. The drawings show:

(2) FIG. 1 a first table containing compositions and grinding parameters of investigated sintered bodies;

(3) FIG. 2 particle sizes d.sub.50 and d.sub.95 which were measured in the slurry after grinding a submicron powder according to the process according to the invention;

(4) FIG. 3 sintering curves of cast sample bodies with indicated HIP temperature (arrows) and relative density after HIP (hot isostatic pressing) (gray-shaded symbols);

(5) FIG. 4 FESEM pictures of portions of conventional sintered bodies (top, sample Z-1) and sintered bodies according to the invention (center and bottom, samples Z-2 and ZA-10, respectively);

(6) FIG. 5 grain sizes of the sintered bodies;

(7) FIG. 6 aging stability of the sintered bodies determined by accelerated aging tests in autoclaves at 134° C. and 2 bar in water vapor;

(8) FIG. 7 a second table containing structure characteristics of examined sintered bodies;

(9) FIG. 8 a third table containing mechanical properties of examined sintered bodies after treating by HIP;

(10) FIG. 9 four point bending strengths and Weibull moduli of examined sintered bodies.

DETAILED DESCRIPTION OF THE INVENTION

(11) Example 1: In a stirring unit, 270 ml of water are placed and a suitable dispersant is added. Next, 500 g of 3Y-TZP powder with a specific surface area of 6 m.sup.2/g are stirred in. The slurry is put in an agitator ball mill. The mill is filled up to 85% by grinding balls having a diameter of 100 μm. The slurry is ground for two hours at 3500 R.P.M., which corresponds to a circumferential speed of 11 m/s, to a particle size distribution characterized in that the d.sub.95 value is <0.42 μm and the d.sub.50 value is <0.3 μm.

(12) The milling time is approximately two hours for this amount of solids. The slurry is cast in plaster molds which are dimensioned so as already to make allowances for shrinkage. The standing time is determined by the size of the component. The green bodies are dried in air at 50° C. and then sintered for two hours at 1250° C. The sintered shaped bodies have approximately 95% of the theoretical density. The shaped bodies are then subjected to hot isostatic pressing at 1250° C. and finally have a density of 6.07 g/cm.sup.3. This corresponds to 99.5% of the theoretical density.

(13) The mean grain size is determined by means of linear intercept methods in accordance with DIN EN 623-3 and is 160 nm for this material. The strength is 1063 MPa, and the microhardness is 18 GPa. The sintered shaped bodies are aged in a water vapor atmosphere for 120 hours at 134° C. The monoclinic fraction after aging is determined by XRD (X-Ray Diffraction). The samples have a monoclinic phase content of between 0.5% and 3%.

(14) Example 2: In a stirring unit, 270 ml of water are placed and a suitable dispersant is added. Next, 450 g of 3Y-TZP powder and 50 g of α-Al.sub.2O.sub.3 powder with a specific surface area of 6 m.sup.2/g and 12 m.sup.2/g, respectively, are stirred in. The slurry is put in an agitator ball mill.

(15) The mill is filled up to 85% with grinding balls having a diameter of 100 μm. The slurry is ground for two hours at 3500 R.P.M., which corresponds to a circumferential speed of 11 m/s, to a particle size distribution characterized in that the d.sub.95 value is <0.42 μm and the d.sub.50 value is <0.30 μm. The milling time is approximately two hours for this amount of solids. The slurry is cast in plaster molds which are dimensioned so as already to make allowances for shrinkage. The standing time is determined by the size of the component. The green bodies are dried in air at 50° C. and then sintered for two hours at 1300° C. The sintered shaped bodies have approximately 95% of the theoretical density. The shaped bodies are then subjected to hot isostatic pressing at 1300° C. and finally have a density of 5.76 g/cm.sup.3. This corresponds to 99.5% of the theoretical density of 5.79 g/cm.sup.3.

(16) The mean grain size is determined by means of linear intercept methods in accordance with DIN EN 623-3 and is 143 nm for this material. The strength is 1700 MPa. The sintered shaped bodies are aged in a water vapor atmosphere for 120 hours at 134° C. The monoclinic fraction after aging is determined by XRD. The samples have a monoclinic phase content of between 0 and 1%.

(17) EXAMPLE 3: Two 3Y-TZP charges and one ATZ charge (90 wt. % 3Y-TZP/10 wt. % Al.sub.2O.sub.3) are prepared. TZ3Y-SE (Tosoh, Japan) and TM-DAR (Taimei Chemicals, Japan) with particle sizes of 70 nm and 100 nm, respectively, were used as raw materials. The powders were dispersed in water using 0.5% ammonium polyacrylate (Zschimmer & Schwarz, Germany) and then ground and dispersed in an agitator ball mill (Mini Cer, Netzsch FMT, Germany) using different grinding ball diameters. Table 1 shows the compositions and the grinding parameters.

(18) The particle size distribution in the slurry was analyzed with an Ultrafine particle analyzer (UPA, Microtrac, USA). Disks were shaped to the dimensions of 3×20×30 mm by slip casting in plaster molds. Subsequently, sintering curves were prepared and the HIP temperatures were derived therefrom. The density of the HIPed sample bodies was determined by the Archimedes principle. The surfaces of the disks were polished with diamond paste. The roughness R.sub.a of the surfaces was between 8.5 and 16 nm. The structures were examined with a FESEM (Zeiss Ultra 55+; Carl Zeiss NTS Germany). The mean grain size was determined by the linear intercept method.

(19) The samples were aged in an autoclave at 134° C. and 2 bar in water vapor for up to 200 h. The phase composition of the aged samples was measured by XRD (D8 Advance, Bruker, Germany) and quantified by Rietveld refinement (AutoQuan, GE-Sensing Technology, Ahrensburg, Germany).

(20) Four point bending strength was tested on 2×2.5×25 mm bending rods in accordance with EN 843-1. Fifteen bending rods were tested and the mean bending strength σ.sub.0 and Weibull modulus m were then determined.

(21) Sample Z-1 was prepared by a standard procedure and samples Z-2 and ZA-10 were prepared in an optimized process (Table 1). Although the grinding energy in charges Z-2 and ZA-10 is eight times greater than in charge Z-1, smaller particle sizes were measured in the slurry, which is shown in FIG. 1.

(22) It was possible to appreciably reduce the d.sub.95 value and, therefore, the particle size distribution in the slurry by means of the optimized preparation. This results in a considerable increase in sinter activity, which is illustrated by the sintering curves in FIG. 2.

(23) Sample bodies Z-1 were sintered at 1450° C. without subsequent HIP. Sample bodies Z-2 and ZA-10 were sintered at 1250° C. and 1300° C., respectively, and HIPed at the same temperature. The relative densities of all of the samples were greater than 99.5% of the theoretical densities of 6.1 g/cm.sup.3 for 3Y-TZP and 5.79 g/cm.sup.3 for ATZ 90/10. The microstructure of the sintered and HIPed sample bodies is shown in FIG. 2.

(24) The effect of the optimized preparation can be clearly seen from FIG. 2. The grain sizes can be appreciably reduced by an optimized preparation and the accompanying increase in sinter activity. The grain sizes of the sample bodies are shown in FIG. 3.

(25) The grain sizes in the dense-sintered ceramics are proportional to the d.sub.95 value of the particle size distribution in the slurry (compare FIG. 1). The particle size distribution in the slurry is accordingly a key value for the optimization of very fine-grained structures. The aging stability of the ceramics was tested in accelerated aging tests in the autoclave. FIG. 4 shows the monoclinic phase content in the samples as a function of the aging time at 134° C. in water vapor.

(26) Sample Z-1 showed a very rapid rise of the monoclinic phase after short aging times. Samples Z-2 and ZA-10 show no rise in the monoclinic phase over the period of study. Accordingly, the ceramics may be designated as aging-stable. It is assumed that the reason for this is the stabilization of the tetragonal phase through the small grain size. This could also affect the strength characteristics of the very fine-grained 3Y-TZP ceramics. The four point bending strength and the Weibull modulus of the 3Y-TZP and ATZ ceramics are shown in FIG. 5.

(27) The strength of the Z-2 ceramic is reduced compared to the Z-1 ceramic. It is assumed that this effect also stems from the stabilization of the tetragonal phase so that the transformation toughening and hydrothermally induced phase transformation are inhibited.

(28) In contrast, the ATZ ceramic ZA-10 has a very high strength of 1700 MPa and a Weibull modulus of 14.3. The addition of Al.sub.2O.sub.3 to the Y-TZP matrix compensates for the negative influence of the small grain size and even leads to higher strengths compared with conventionally produced 3Y-TZP ceramic. The reason for this is assumed to be a mechanical stressing of the grains due to thermal mismatch. When cooled after sintering, a local ring tensile stress forms in the Y-TZP matrix around the Al.sub.2O.sub.3 grains so that the driving force for the phase transformation is increased locally and can provide for a higher strength of the ceramic. This results in a high-strength, aging-stable dispersion ceramic which is excellently suitable for use as bio-inert implant material.

(29) FIG. 1 contains a table in which are indicated the submicron powder used as starting material for a process according to the invention, the composition of the submicron powder and the diameter of the grinding media (grinding balls) used for the comminution of the respective submicron powder.

(30) FIG. 2 shows the particle sizes contained in the slurry after grinding and measured in d.sub.50 and d.sub.95 standards. These size distributions of the particles were achieved through the use of 500-μm grinding media (sample Z-1) and 100-μm grinding media (samples Z-2 and ZA-10).

(31) FIG. 3 shows the resulting sintering curves of cast sample bodies of the submicron powders ground according to the above specifications. The relative density of the obtained sintered bodies is indicated in percent (relative density [%]) and plotted over the sintering temperature in ° C. The temperatures of the hot isostatic pressing (arrows) and the relative densities (gray-shaded symbols) achieved in this way are shown.

(32) FIG. 4 shows FESEM illustrations of the structure of the obtained sintered shaped bodies of samples Z-1 (top), Z-2 (center) and ZA-10 (bottom). The bright grains are zirconium, the dark grains are aluminum.

(33) FIG. 5 shows the grain sizes of the obtained sintered shaped bodies. The grain sizes of the submicron powders comminuted by means of grinding balls having a diameter of 100 μm are substantially lower than the grain size of the submicron powder comminuted by grinding balls having a diameter of 500 μm. The grain size (μm) is plotted over the samples (Sample Name).

(34) FIG. 6 shows the monoclinic phase content in percent by weight (wt. %) over time (Aging Time [h]). The sintered shaped bodies were tested under a pressure of 2 bar in water vapor at 134° C. It can be clearly seen that sintered bodies Z-2 and ZA-10 show no increase in monoclinic phase content far in excess of 120 hours (up to at least 192 hours). This means that sintered shaped bodies Z-2 and ZA-10 are aging-stable for over at least 192 hours under water vapor atmosphere. In contrast, Z-1 shows a massive and steep rise in monoclinic phase content of more than 60 mass % and is not stable with respect to hydrothermal aging.

(35) FIG. 7 shows a second table listing material compositions of the dense sintered shaped bodies. The error is indicated as three times the standard deviation (as given by the Rietveld program).

(36) FIG. 8 shows a third table listing the mechanical properties of the sintered shaped bodies after HIP treatment. The properties are: bending strength, Weibull modulus m, microhardness HV0.1, microhardness HV10, fracture toughness (SEVNB), fracture toughness (Anstis), fracture toughness (Niihara) and transformability m-ZrO.sub.2 on the fracture surface.

(37) Finally, FIG. 9 shows the four point bending strength [Mpa] of the sintered shaped bodies Z1, Z-2 and ZA-10 and the respective associated Weibull modulus (indicated in the box on each column).