Sintered ceramic material, powder composition for obtaining thereof, manufacturing process and ceramic pieces thereof

Abstract

The present application discloses a sintered ceramic material having high fracture toughness and bending strength, which is obtained from an yttria-stabilized Zirconia powder, the powder composition for obtaining said material, sintered ceramic pieces and manufacturing process thereof. One of the solutions of the present invention discloses a sintered ceramic material which is obtained from an yttria-stabilized zirconia powder, comprising between 1.8 and 2.1 mol % yttria, wherein the sintered ceramic material has a percentage of tetragonal phase greater than 90% at room temperature, a grain size between 0.1 to 0.25 m, the bending strength is between 1150-2100 MPa, and simultaneously a toughness greater than 10 MPa.Math.m.sup.1/2. This material may be applied in different sintered ceramic pieces, including pieces for the automotive sector, for diverse machinery, ornamental applications such as timepieces or pieces for biomedical applications, among others.

Claims

1. Sintered ceramic material obtained from a powder composition, the powder composition consisting essentially of: yttria-stabilized zirconia consisting essentially of 1.8 to 2.1% mol.sub.yttria, a crystallite size of less than 40 nm a surface area of 17-35 m.sup.2/g, 80% of the powder particles comprises a particle size of 0.2 -0.4 m, doped with alumina between 0.2 and 1.5% W.sub.alumina/W.sub.material, and a chemical purity degree of yttria-stabilized zirconia greater than 99.9% considering the sum of Zr+Hf+Y+Al+O, wherein the sintered ceramic material has a tetragonal phase greater than 90% at room temperature, and a grain size between 0.1 m and 0.25 m.

2. Sintered ceramic material according to claim 1 wherein toughness ranges from 10 to 25 MPa.Math.m.sup.1/2 and bending strength ranges from 1150 to 2100 MPa.

3. Sintered ceramic material according to claim 1 wherein the yttria-stabilized zirconia comprises 1.8 to 1.99% mol.sub.yttria.

4. Sintered ceramic material according to claim 1 wherein the yttria-stabilized zirconia comprises 1.85 to 1.95% mol.sub.yttria.

5. Sintered ceramic material according to claim 1, wherein the monoclinic phase after aging is less than 18%.

6. Sintered ceramic material according to claim 1, wherein the sintered ceramic material comprises a grain size between 0.10 and 0.20 m.

7. Sintered ceramic material according to claim 1, wherein the sintered ceramic material density is greater than 5.97 g/cm.sup.3.

8. Sintered ceramic material according to claim 1, with a porosity less than 2%.

9. Sintered ceramic material according to claim 1, wherein tetragonal phase is greater than 91%.

10. Sintered ceramic piece comprising the sintered ceramic material according to claim 1.

11. Sintered ceramic piece according to claim 10 wherein the piece is an extrusion die, or prosthesis, or a cutting tool, or a motor component, or a drawing component, or prostheses, or implants, or ornamental applications.

12. Manufacturing process of the sintered ceramic piece according to claim 11 comprising the following steps: feeding into a shaping die a powder composition consisting essentially of yttria-stabilized zirconia consisting essentially of 1.8 to 2.1% mol.sub.yttria, a crystallite size of the powder particle less than 40 nm, doped with alumina between 0.2 and 1.5% W.sub.alumina/W.sub.material, and a chemical purity degree of yttria-stabilized zirconia greater than 99.9% considering the sum of Zr+Hf+Y+Al+O; shaping and sintering the powder composition at a temperature between 1100 C. and 1400 C., wherein the sintered ceramic piece has a tetragonal phase greater than 90% and a grain size between 0.1 m and 0.25 m; and obtaining the sintered ceramic piece.

13. Sintered ceramic material obtained from a powder composition, the powder composition consisting essentially of: yttria-stabilized zirconia consisting essentially of 1.8 to 2.1% mol.sub.yttria, a crystallite size of less than 40 nm, doped with alumina between 0.2 and 1.5% W.sub.alumina/W.sub.material, and a chemical purity degree of yttria-stabilized zirconia greater than 99.9% considering the sum of Zr+Hf+Y+Al+O, wherein the sintered ceramic material has a tetragonal phase greater than 90% at room temperature, and a grain size between 0.1 m and 0.25 m.

Description

DESCRIPTION OF THE DRAWINGS

(1) For an easier understanding of the technique, drawings are herein attached, which represent preferred embodiments and which, however, are not intended to limit the scope of the present application.

(2) FIG. 1 shows the phase diagram ZrO.sub.2-Y.sub.2O.sub.3.

(3) FIG. 2 shows an electron scanning microscope image of a sample of the ceramic material.

(4) FIG. 3 shows a crystallite size distribution of the starting powder and micrograph thereof obtained by transmission electron microscopy.

(5) FIG. 4 shows the effect of yttria percentage on bending strength and fracture toughness of the ceramic material obtained by conventional sintering.

(6) FIG. 5 shows the effect of alumina percentage on monoclinic phase percentage before and after the aging test for samples obtained by HIP.

(7) FIG. 6 shows the effect of alumina percentage on bending strength before and after aging for samples obtained by HIP.

(8) FIG. 7 shows the effect of alumina percentage on fracture toughness before and after aging for samples obtained by HIP.

DESCRIPTION OF THE EMBODIMENTS

(9) The technology shall now be described in this subsection using some embodiments and figures, which are not intended to limit the scope of protection of the present application.

(10) The present application describes a sintered ceramic material with high fracture toughness and bending strength, as well as the manufacturing process and possible uses thereof.

(11) The sintered ceramic material now presented comprises a fracture toughness ranging between 10 and 25 MPa.Math.m.sup.1/2 and a bending strength (depending on the shaping/sintering method used) between 1150 and 2100 MPa. This sintered ceramic material is obtained from yttria-stabilized zirconia powder containing between 1.8 and 2.1 mol % yttrium.

(12) With such yttria doping percentage, associated with a number of other starting powder properties, namely: a molar percentage of yttria between 1.8 and 2.1; preferably an alumina content from 0.2 to 1%; a crystallite size of less than 40 nm, preferably between 5-40nm; a surface area preferably between 17-35 m.sup.2/g; an initial powder particle size between 0.2 and 0.4 m; preferably a chemical purity degree of yttria-stabilized Zirconia greater than 99.9%, considering (Zr+Hf+Y+Al+O)>99.9%; preferably a high homogeneity in the distribution of yttria in the zirconia;

(13) A high binomial of properties, fracture toughness and bending strength, in the sintered ceramic material is achieved.

(14) The sintered ceramic material shall have a well-defined final microstructure grain size, with good homogeneity in distribution, since the stability at room temperature of tetragonal structure is only achieved if after sintering the grain size is less than a determined critical size, otherwise the conversion into monoclinic phase occurs spontaneously. In the case of the present composition wherein yttria content is between 1.8% and 2.1 mol % yttria, the ceramic material may have a grain size between 0.1 m and 0.25 m, preferably between 0.15 and 0.20 m.

(15) The sintered ceramic material herein disclosed may be used in structural applications, such as prosthesis, cutting tools, extrusion dies, motor components, among others.

(16) In a preferred embodiment, the process for obtaining the ceramic material comprises the following steps: feeding into a shaping die the powder compositions of yttria-stabilized zirconia herein described; shaping and sintering process; obtaining the sintered ceramic material.

(17) In order to obtain the sintered ceramic material, the yttria-stabilized zirconia powder characteristics play a key role, since only a combination of some of them in particular: a molar percentage of yttria between 1.8 and 2.1; an alumina content between 0.2 and 1%; a crystallite size of less than 40 nm, preferably greater than 5 nm; a specific surface area between 17-35 m.sup.2/g; a particle size between 0.2 and 0.4 m; preferably a chemical purity degree of yttria-stabilized zirconia greater than 99.9%, considering (Zr+Hf+Y+Al+O)>99.9%; preferably a high homogeneity in the distribution of yttria in the zirconia.

(18) The present invention discloses that the combination of these seven characteristics of yttria-stabilized zirconia powder makes it possible to obtain a zirconia sintered ceramic piece with high density with a homogeneous grain size less than 0.25 m, preferably greater than 0.10 m, having a tetragonal phase greater than 90% at room temperature and having a toughness greater than 10 MPa.Math.m.sup.1/2 and a bending strength between 1150 and 2100 MPa, the latter depending on the shaping/sintering method used.

(19) Application Examples

(20) Table I shows several embodiments for which some characterization tests of the sintered ceramic material were performed, with regard to mechanical properties, specifically in terms of bending strength and fracture toughness.

(21) Tests 1-7 (Effect of the Yttria Content)

(22) For this series of tests samples of yttria-stabilized zirconia powder obtained by EDS (Emulsion Synthesis Detonation), with a chemical purity greater than 99.9% (expressed in terms of zirconia+hafnium+yttria+alumina) with a particle size with a d50 value of 250 nm and doped with 0.4% alumina were used, wherein different yttria molar percentages of 1.7%, 1.8%, 1.9%, 2%, 2.1%, 2.25%, 2.5% and 3 mol % have been studied. The remaining control parameters of the starting powder, including surface area, crystallite size. Powder samples with different yttria content were shaped within a dimensional die (20 mm of cavity diameter) by uniaxial pressing at 70 MPa for 30 s. Subsequently, all samples were subject to a sintering cycle with a heating rate of 2.0 C./min until 1350 C., remaining there for 2 hours, then being cooled at a rate of 5 C./min until room temperature, the results shown in Table I being obtained.

(23) TABLE-US-00001 TABLE I Variation on bending strength and fracture toughness with yttria percentage added to zirconia. Grain Bending Fracture Test % crystallite BET size Density Tetra strength toughness No. Y.sub.2O.sub.3 (nm) (m.sup.2/g) (m) (g/cm.sup.3) phase % (MPa) (MPa .Math. m.sup.1/2) 1 3 22 24 0.23 6.05 99 1250 5.4 2 2.5 23 25 0.25 6.03 98 1310 7.2 3 2.25 24 25 0.22 6.03 97 1254 8.9 4 2.10 25 26 0.21 6.02 96 1210 11.2 5 2 22 25 0.24 6.04 95 1285 15.1 6 1.8 25 26 0.21 6.01 95 1270 22.0 7 1.7 22 27 0.22 6.01 80 600 20.0

(24) As can be seen from Table I in all tests undertaken the sintered ceramic material had densification levels above 98% and a homogeneous grain size between 0.2 and 0.25 m.

(25) From the analysis of the above figures it can be seen that the reduction in yttria percentage leads to an increase in the fracture toughness values, which would be expected. However, the tetragonal phase percentages surprisingly remained above 95% and the bending strength was also maintained above 1200 MPa, virtually unchanged. Merely below 1.8% (with 1.7 mol %) does the tetragonal phase begins to decrease and bending decreases considerably. It has been found that within the range between 1.8% and 2.1 mol % yttria a binomial of bending strength and fracture toughness values greater than 1200 MPa and 10 MPa.Math.m.sup.1/2, respectively, can be obtained by uniaxial pressing at 70 MPa.

(26) Tests 8-9 (Shaping Method by Hot Isostatic Pressing (HIP))

(27) In tests 8 and 9, zirconia samples identical to those used IN test 6, that is with an yttria content of 1.8 mol %. In a first step (FIG. 4) these samples were subject to uniaxial pressing at 70 MPa, followed by sintering at 1300 C. for 2 hr, with a heating and cooling rate of 2 C./min and 5 C./min respectively, aiming at removing completely the open porosity of the sintered material. Afterwards, hot isostatic pressing at a temperature of 1250 C. for 1 h was applied. The characterization of the samples is shown in Table II.

(28) TABLE-US-00002 TABLE II Influence of the sintering method by HIP on bending strength and fracture toughness of samples prepared with 1.8 mol % yttria. Grain Bending Fracture Test mol % crystallite BET size Density Tetra strength toughness No. yttria (nm) (m.sup.2/g) (m) (g/cm.sup.3) phase % (MPa) (MPa .Math. m.sup.1/2) 8 1.8 22 24 0.19 6.06 95 1811 17.4 9 1.8 23 25 0.18 6.06 97 2020 15.2

(29) It is noted that with hot isostatic pressing (HIP) the final grain size is less than 0.15 to 0.20 and the bending strength values obtained are substantially higher, shortly, BY HIP is made possible to achieve a binomial of bending strength and fracture toughness in the range 1800-2100 MPa and 15-20 MPa.Math.m.sup.1/2, respectively.

(30) Tests 10-11 (Surface Area Effect)

(31) In tests 10 and 11, similar samples were used with a molar yttria content (1.8 mol %), but now with a lower (16 m.sup.2/g instead of 25 m.sup.2/g) BET (surface area measurement from Brunauer, Emmett, Teller). Zirconia sintered samples were prepared in the same manner as in tests 1-7, that is, were shaped by uniaxial pressing at 70 MPa for 30 s. Thereafter, sample 10 was subject to a sintering cycle at a heating rate of 2.0 C./min at 1350 C., where it remained for 2 hours, then being cooled at a rate of 5 C./min at room temperature. Sample 11, in turn, was sintered at 1450 C., with heating/cooling rates identical to sample 10. The results are shown in Table III.

(32) TABLE-US-00003 TABLE III Variation on bending strength and fracture toughness upon decrease of specific surface area of the starting powder. Grain Bending Fracture Test Mol % crystallite BET size Density Tetragonal strength toughness No. yttria (nm) (m.sup.2/g) (m) (g/cm.sup.3) Phase % (MPa) (MPa .Math. m.sup.1/2) 10 1.8 22 16 0.22 5.96 95 950 13.4 11 1.8 23 16 0.35 6.04 88 1010 15.2

(33) It has been found that upon decreasing BET, a temperature of 1350 C. is not enough to obtain a densification degree greater than 98% and hence sample 10 has a low bending strength (950 MPa). In order to achieve the required densification degree, the sintering temperature must be increased to 1450 C. (sample 11), but this temperature increase causes the microstructure grain to grow excessively (0.35 m) and the tetragonal phase and bending strength to decrease.

(34) Test 12-14 (Aging and the Alumina Content)

(35) In order to assess strength against aging, the sample from test 8 was subject to an aging test according to ISO 13356 Implants for surgery (autoclave at 134 C./5hr at 0.2 MPa), the following results being obtained:

(36) TABLE-US-00004 TABLE IV bending strength and fracture toughness of samples prepared by HIP after aging test Grain Bending Fracture Test Mol % crystallite BET size Density Mono strength toughness No. yttria (nm) (m.sup.2/g) (m) (g/cm.sup.3) phase % (MPa) (MPa .Math. m.sup.1/2) 12 1.8 22 24 0.19 6.06 18% 1650 14.4

(37) The appearance of the monoclinic phase (18%) was observed below the threshold provided in the standard (maximum 25%), and a decrease of about 150 MPa on bending strength and 3 MPa.Math.m.sup.1/2 in fracture toughness is observed, even though both decreases are within the limits provided for in the standard (<20%).

(38) Then a sample was prepared identically to that of test 8 was prepared, but with alumina doping content increased from 0.4 to 1%, thereby yielding the following results before and after the aging test.

(39) TABLE-US-00005 TABLE V Variation on bending strength and fracture toughness with yttria percentage added to zirconia (1 mol %). Grain Bending Fracture Test Mol % crystallite BET size Density Tetra strength toughness No. yttria (nm) (m.sup.2/g) (m) (g/cm.sup.3) phase % (MPa) (MPa .Math. m.sup.1/2) 13 1.8 22 24 0.19 6.06 96 1911 15.4 14 1.8 23 25 0.18 6.06 93 1854 12.2

(40) It has been found that upon increasing the alumina content, a slight decrease in toughness was obtained, however, the aging strength is markedly increased thus forming only 7% of monoclinic phase.

(41) The present embodiment is of course in no way restricted to the embodiments herein described and a person of ordinary skill in the art will be capable of providing many modification possibilities thereto without departing from the general idea of the invention as defined in the claims.

(42) The embodiments described above are obviously combinable with each other. The following claims define further preferred embodiments.