SINTERED ALUMINA-BASED AND ZIRCONIA-BASED PRODUCT

Abstract

Sintered product having a chemical analysis such that, in percentages by mass based on the oxides, ZrO2 partially stabilised with CeO2 and Y2O3: remainder up to 100%, Al2O3: >10% and <19%of an additive selected from CaO, the oxides of manganese, ZnO, the oxides of praseodymium, SrO, the oxides of copper, Nd2O3, BaO, the oxides of iron, and mixtures thereof: 0.2-6%, impurities: <2%, CeO2 and Y2O3 being present in quantities such that, in molar percent based on the sum of ZrO2, CeO2 and Y2O3, CeO2: 2.5 mol % and <5.5 mol %, and Y2O3: 0.5-2 mol %.

Claims

1. A sintered product having a chemical analysis such that, as percentages by weight on the basis of the oxides, ZrO.sub.2 partially stabilized with CeO.sub.2 and Y.sub.2O.sub.3: balance to 100%, Al.sub.2O.sub.3: >10% and <19% additive chosen from CaO, manganese oxides, ZnO, praseodymium oxides, SrO, copper oxides, Nd.sub.2O.sub.3, BaO, iron oxides, and mixtures thereof: 0.2-6%, impurities: <2%, CeO.sub.2 and Y.sub.2O.sub.3 being present in amounts such that, as a molar percentage on the basis of the sum of ZrO.sub.2, CeO.sub.2 and Y.sub.2O.sub.3, CeO.sub.2: 2.5 mol % and <5.5 mol % Y.sub.2O.sub.3: 0.5-2 mol %.

2. The sintered product as claimed in claim 1, wherein the molar content of CeO.sub.2 is less than 5% and greater than 3%.

3. The sintered product as claimed in claim 2, wherein the molar content of CeO.sub.2 is less than 4%.

4. The sintered product as claimed in claim 1, wherein the molar content of Y.sub.2O.sub.3 is less than 1.7% and greater than 1%.

5. The sintered product as claimed in claim 1, wherein the content of alumina Al.sub.2O.sub.3 is greater than 11% and less than 16%, as a percentage by weight on the basis of the oxides.

6. The sintered product as claimed in claim 1, wherein the content of additive is greater than 0.5% and less than 4%, as a percentage by weight on the basis of the oxides.

7. The sintered product as claimed in claim 1, wherein the additive is a mixture of CaO on the one hand and of one or more manganese oxides on the other hand.

8. The sintered product as claimed in claim 7, wherein the content of CaO is greater than 0.2% and less than 1%, and wherein the content of manganese oxide(s) expressed in the form MnO is greater than 0.2% and less than 1%.

9. The sintered product as claimed in claim 1, having a particle size distribution such that the mean size of the grains having a shape factor of less than 2.5, or compact grains, is less than 2 m, and a mean length of the aluminous elongated nodules is less than 20 m, an aluminous elongated nodule being a structure having a shape factor greater than or equal to 2.5 and formed of an aluminous grain or of several adjacent aluminous grains, an aluminous grain being a grain formed, for more than 40% of its weight, of Al.sub.2O.sub.3 and of the additive.

10. The sintered product as claimed in claim 9, wherein the ratio of the surface covered by the aluminous elongated nodules to the surface covered by he aluminous elongated nodules and by the compact grains comprising more than 40% by weight of alumina, expressed as percentages, is greater than 5% and less than 95%.

11. The sintered product as claimed in claim 9, wherein the mean length of the aluminous elongated nodules is greater than 1 m.

12. A particulate mixture comprising ZrO.sub.2 particles, Al.sub.2O.sub.3 particles, CeO.sub.2 particles, Y.sub.2O.sub.3 particles and particles of an additive chosen from CaO, and/or particles of one or more manganese oxides and/or SrO particles and/or BaO particles and/or particles of precursors of these oxides, and/or particles of several of these oxides and/or precursors of these oxides, the particulate mixture having a chemical composition suitable for the manufacture of a sintered product as claimed in claim 1.

13. The particulate mixture as claimed in claim 12, comprising ZrO.sub.2 particles, Al.sub.2O.sub.3 particles, CeO.sub.2 particles, Y.sub.2O.sub.3 particles, CaO particles and particles of one or more manganese oxides, preferably MnO particles and/or Mn.sub.3O.sub.4 particles, and/or particles of precursors of these oxides, and/or particles of several of these oxides and/or precursors of these oxides.

14. The particulate mixture as claimed in claim 12, the median size of which is less than 1 m.

15. A device chosen from: a mechanical wearing part, a dental article, an optical fiber connector, a decorative article chosen from the group formed by a jewel, a watch, a bracelet, a necklace, a ring, a brooch, a tie pin, a handbag, a telephone, a piece of furniture, a household utensil, a handle, a button, a veneer, a visible part of a consumer goods item, a part of a spectacle frame, a piece of crockery and a frame, comprising a sintered product as claimed in claim 1 or manufactured from a particulate mixture comprising ZrO.sub.2 particles, Al.sub.2O.sub.3 particles, CeO.sub.2 particles, Y.sub.2O.sub.3 particles and particles of an additive chosen from CaO, and/or particles of one or more manganese oxides and/or SrO particles and/or BaO particles and/or particles of precursors of these oxides, and/or particles of several of these oxides and/or precursors of these oxides, the particulate mixture having a chemical composition suitable for the manufacture of the sintered product.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0101] Other features and advantages of the invention will become more apparent on reading the following detailed description and on examining the appended drawing in which FIGS. 1 and 2 represent photographs of a polished section of the sintered product of example 12, according to the invention, obtained after sintering at a temperature of 1450 C., the sintered product having undergone, after polishing, a thermal treatment at 1400 C. for 30 minutes in order to reveal the grain boundaries.

DETAILED DESCRIPTION

[0102] In order to manufacture a sintered product according to the invention, the steps a) to c) described above, and presented in detail below, may be followed.

[0103] In step a), a milling of the raw materials may be necessary in order to obtain a median size, after mixing, of less than 1.0 m.

[0104] In particular, the powders of raw materials providing the oxides may be milled individually or, preferably, co-milled, if they do not meet the desired particle size distribution, and in particular if they have a median size of greater than 1 m, greater than 0.6 m, greater than 0.5 m, greater than 0.3 m or greater than 0.2 m. The milling may be carried out in a wet environment, for example in an attrition mill. After wet milling, the milled particulate mixture is preferably dried.

[0105] Preferably, in step a), the powders used, in particular the powders of ZrO.sub.2, of alumina Al.sub.2O.sub.3, of Y.sub.2O.sub.3, of CeO.sub.2, and of additive each have a median size of less than 5 m, less than 3 m, less than 1 m, less than 0.7 m, preferably less than 0.6 m, preferably less than 0.5 m. Advantageously, when each of these powders has a median size of less than 1 m, preferably less than 0.8 m, preferably less than 0.6 m, preferably less than 0.5 m, or else less than 0.3 m, or else less than 0.2 m, the milling is optional.

[0106] The use of powders having a small median size also advantageously enables the sintering temperature to be reduced.

[0107] These powders may also be replaced, at least partially, by powders of precursors of these oxides, introduced in equivalent amounts.

[0108] Preferably, the zirconia powder used has a specific surface area, calculated by the BET method, of greater than 5 m.sup.2/g, preferably greater than 6 m.sup.2/g, preferably greater than 7 m.sup.2/g, and less than 20 m.sup.2/g, preferably less than 15 m.sup.2/g. Advantageously, the sintering temperature in step d) is reduced, and the milling, generally in suspension, and suspending operation are facilitated thereby.

[0109] The addition of CaO, and/or of a manganese oxide, and/or of ZnO, and/or of a praseodymium oxide, and/or of SrO, and/or of a copper oxide, and/or of Nd.sub.2O.sub.3, and/or of BaO, and/or of an iron oxide and/or of precursors of these oxides advantageously makes it possible to increase the amount of aluminous elongated nodules contained in the sintered product and to improve the mechanical performance.

[0110] The powders providing the oxides or the precursors are preferably chosen so that the total content of impurities is less than 2%, as a percentage by weight on the basis of the oxides.

[0111] In one embodiment, Y.sub.2O.sub.3 is introduced at least partly in the form of a zirconia partially stabilized with yttrium oxide.

[0112] In one embodiment, CeO.sub.2 is introduced at least partly in the form of a zirconia partially stabilized with cerium oxide, or else stabilized with cerium oxide.

[0113] As is well known to a person skilled in the art, the feedstock may comprise, in addition to the particulate mixture, a solvent and/or an organic shaping additive and/or a dispersant, the natures and the amounts of which are suitable for the shaping method of step b).

[0114] Preferably the solvent is water.

[0115] The organic shaping additive may be chosen from polyethylene glycols (or PEGs), polyvinyl alcohols (or PVAs), lattices, cellulose derivatives and mixtures thereof.

[0116] The dispersant may for example be a polyacrylate.

[0117] All these elements disappear during the subsequent manufacturing steps, possibly leaving however some traces thereof remaining.

[0118] In step b), the feedstock is shaped by any technique known to person skilled in the art, preferably by tape casting or by pressing, preferably by uniaxial pressing, by hot pressing or by isostatic pressing. In the case where the feedstock is shaped by pressing, a prior step of drying, for example by spray drying, may be carried out. The size of the spray-dried particles may for example be between 20 m and 250 m.

[0119] Optionally, the shaping comprises a drying of the preform.

[0120] In step c), the preform is sintered at a temperature above 1300 C., preferably above 1350 C., preferably above 1400 C., so as to obtain a sintered product according to the invention. Preferably, the sintering temperature is below 1600 C., preferably below 1550 C., preferably below 1500 C. The sintering is preferably carried out in air at atmospheric pressure.

[0121] Preferably, the sintering time is greater than 1 hour, greater than 2 hours, and/or less than 10 hours, less than 7 hours, or less than 5 hours. Preferably, the sintering time is between 2 and 5 hours.

[0122] The sintering temperature is preferably proportionally higher when the amount of alumina is substantial.

[0123] The inventors have noted the presence of a particular microstructure in the sintered products according to the invention.

[0124] As represented in FIG. 1, the microstructure is characterized by the presence of aluminous elongated nodules 3, which may be in the form of substantially rectilinear rods. FIG. 1 also shows inclusion grains 5, in particular grains of zirconia, within the aluminous elongated nodules. The mean length of the aluminous elongated nodules is typically greater than 1 m and/or less than 20 m. The microstructure specific to the products according to the invention also comprises compact grains. The compact grains typically have a mean size of less than 2 m and/or greater than 0.1 m.

[0125] An aluminous elongated nodule 3 may be formed by a grain, as in FIG. 1 or by a cluster of adjacent aluminous grains 11, as in FIG. 2. The aluminous grains 11 have coalesced during the sintering. An aluminous grain is preferably formed, for more than 50%, more than 60%, or else more than 70% of its weight, of Al.sub.2O.sub.3 and of said additive.

[0126] Typically, more than 90%, more than 95%, or else more than 98% or 100% of the weight of the zirconia is in the form of compact grains of zirconia 7. The inventors have noted that more than 60%, preferably more than 80%, more preferably more than 90% of the volume of the zirconia is in the tetragonal phase.

[0127] CeO.sub.2 and Y.sub.2O.sub.3 are used to stabilize the zirconia but may also be present outside thereof.

[0128] Preferably, more than 90%, more than 95%, more than 98%, or else substantially 100% of the other compact grains are grains formed, for more than 40% of their weight, of alumina 9.

[0129] An analysis has shown that the aluminous elongated nodules 3 comprise aluminum and the metal cations of the oxides added as additive (Ca and/or Mn and/or Zn and/or Pr and/or Sr and/or Cu and/or Nd and/or Ba and/or Fe). Said aluminous elongated nodules may also comprise the element cerium (Ce). Thus, if the additive comprises CaO and a manganese oxide, said aluminous elongated nodules comprise the elements Al, Ca, Mn and Ce.

[0130] The inventors have observed that the aluminous elongated nodules are substantially formed, depending on the additive, of a hibonite-type phase and/or of a magnetoplumbite-type phase.

[0131] The ratio of the mean surface area of the aluminous elongated nodules to the mean surface area of the compact grains is preferably greater than 5, greater than 10, greater than 20, greater than 30, and/or less than 200, less than 150, less than 100.

[0132] The ratio of the number of compact grains to the number of aluminous elongated nodules is preferably greater than 10, greater than 20, greater than 30, greater than 40, greater than 50, and/or less than 2000, less than 1500, less than 1000, less than 500.

Examples

[0133] The following nonlimiting examples are given for the purpose of illustrating the invention.

[0134] Sintered products were prepared from: [0135] an yttrium-stabilized zirconia powder containing a molar content of Y.sub.2O.sub.3 equal to 3%, having a specific surface area of the order of 10 m.sup.2/g and a median size of less than 0.3 m for example 1, [0136] a zirconia powder with a purity of greater than 99%, having a specific surface area of the order of 10 m.sup.2/g and a median size of less than 0.3 m for example 2, [0137] an yttrium-stabilized zirconia powder containing a molar content of Y.sub.2O.sub.3 equal to 1.2%, having a specific surface area of the order of 8 m.sup.2/g and a median size of less than 5 m for examples 3 to 16, [0138] a CeO.sub.2 powder with a purity of greater than 99% and having a median size of less than 10 m for examples 2 to 16, [0139] an aluminia powder with a purity of greater than 99% and having a median size of less than 0.5 m for examples 1 to 16, [0140] a powder of manganese oxides, mainly in the Mn.sub.3O.sub.4 form and also containing MnO, with a purity, expressed in the MnO form, of greater than 88%, and more than 90% by weight of the particles of which having a size of less than 44 m, for examples 3 to 16, [0141] calcium carbonate powder having a median size equal to 5 m for examples 3 to 16.

[0142] These powders were mixed then co-milled in a wet environment until a particulate mixture having a median particle size of less than 0.3 m was obtained. Polyvinyl alcohol was then added in an amount equal to 2% on the basis of the solids of the particulate mixture. The feedstock obtained was then spray dried in the form of a powder of spray-dried particles having a median size equal to 60 m, a relative density of between 30% and 60% and an index of sphericity of greater than 0.85 in a spray dryer, the relative density of a powder of spray-dried particles being the ratio equal to the true density divided by the absolute density, expressed as a percentage; the absolute density of a powder of spray-dried particles being the ratio equal to the weight of solids of said powder after milling to a fineness such that substantially no closed pore remains, divided by the volume of this weight after milling, measured by helium pycnometry, and the true density of a powder of spray-dried particles being the mean of the bulk densities of each spray-dried particle of the powder, the bulk density of a spray-dried particle being the ratio equal to the mass of said spray-dried particle divided by the volume that said spray-dried particle occupies.

[0143] In step b), each powder of spray-dried particles was then pressed on a uniaxial press at a pressure equal to 100 MPa.

[0144] In step c), the preforms obtained were then transferred to a sintering furnace where they were brought, at a rate of 100 C./h, up to 1450 C. The temperature of 1450 C. was maintained for 2 hours. The drop in temperature was carried out by natural cooling.

[0145] Measurement Protocols

[0146] The hardness of the sintered products is measured using Vickers indentations at 0.3 kg.

[0147] After measuring the length of the radial cracks, the toughness was calculated using the universal formula developed by Liang et al. (Evaluation by indentation of fracture toughness of ceramic materials, 1990).

[0148] The 3-point bending modulus of rupture is measured on the sintered products under the conditions of the standard ISO 6872.

[0149] The bulk density of the sintered products is measured by hydrostatic weighing.

[0150] The chemical analysis of the sintered products is measured by inductively coupled plasma or ICP for elements in an amount that does not exceed 0.5%. In order to determine the content of the other elements, a pearl of the product to be analyzed is manufactured by melting the product, then the chemical analysis is carried out by x-ray fluorescence.

[0151] The shape factor of the grains and of the nodules of the sintered products, the mean length of the aluminous elongated nodules and the ratio H equal to the ratio of the surface covered by the aluminous elongated nodules to the surface covered by said aluminous elongated nodules and the grains comprising more than 40% by weight of alumina, are measured on images obtained by backscattered electron scanning electron microscopy, of samples of sintered products, said sections having first been polished until a mirror quality is obtained then thermally treated to reveal the grain boundaries, in a cycle having a rate of temperature increase equal to 100 C./h, to a hold temperature 50 C. below the sintering temperature, maintained for 30 minutes, and a temperature drop by natural cooling. The magnification used for capturing the images is chosen so as to display between 2 and 4 aluminous elongated nodules on one image. 20 images per sintered product were acquired.

[0152] The mean size of the grains of the compact sintered products was measured by the mean linear intercept method. A method of this type is described in the standard ASTM E1382. According to this standard, analysis lines are plotted on images of the sintered products, then, along each analysis line, the lengths, referred to as intercepts, between two consecutive compact grain boundaries cutting said analysis line are measured. The analysis lines are determined so as not to cut the aluminous elongated nodule.

[0153] Next the mean length I of the intercepts I is determined.

[0154] For the test below, the intercepts were measured on images, obtained by scanning electron microscopy, of samples of sintered products, said sections having first been polished until a mirror quality is obtained then thermally treated, at a temperature 50 C. below the sintering temperature, to reveal the grain boundaries. The magnification used for capturing the images is chosen so as to display around 100 compact grains on one image. 5 images per sintered product were acquired.

[0155] The mean size d of the grains of a sintered product is given by the relationship: d=1.56.l. This formula is derived from the formula (13) from Average Grain Size in Polycrystalline Ceramics, M. I. Mendelson, J. Am. Cerm. Soc. Vol. 52, No. 8, pp. 443-446.

[0156] The specific area is measured by the BET (Brunauer Emmet Teller) method as described in Journal of American Chemical Society 60 (1938), pages 309 to 316.

[0157] Table 1 below summarizes the results obtained.

TABLE-US-00001 TABLE 1 Chemical analysis (weight %) Mean size base sum ZrO.sub.2 + Additives of the CeO.sub.2 + Y.sub.2O.sub.3 ZrO.sub.2 Manganese oxide compact (mol %) partially expressed in grains Ex ZrO.sub.2 CeO.sub.2 Y.sub.2O.sub.3 stabilized Al.sub.2O.sub.3 the form MnO CaO Impurities (m) 1 (*) 97 0 3 79.7 20.0 0 0 0.3 <1 2 (*) 88 12 0 97.6 2.0 0 0 0.4 <1 3 (*) 94.7 4.1 1.2 73.9 24.9 0.4 0.3 0.5 <1 4 94.9 4 1.1 83.8 14.9 0.6 0.3 0.4 0.33 5 (*) 93.1 5.8 1.1 98.7 0.3 0.5 0.3 0.2 <1 6 (*) 93.8 5.1 1.1 95.9 3.0 0.4 0.3 0.4 0.40 7 (*) 97 1.8 1.2 88.8 10.1 0.5 0.2 0.4 <1 8 (*) 94.9 4 1.1 94 5.0 0.5 0.3 0.2 <1 9 (*) 96.8 2 1.2 83.9 14.9 0.5 0.3 0.4 <1 10 94 4.9 1.1 81.1 16.9 0.6 0.3 1.1 0.36 11 94 4.9 1.1 88.4 10.1 0.5 0.3 0.7 0.35 12 93 5.9 1.1 88.1 10.5 0.4 0.3 0.7 0.44 13 (*) 91.4 7.5 1.1 88.7 10.2 0.4 0.4 0.3 <1 14 (*) 89.9 9 1.1 89.2 10.0 0.4 0.3 0.1 <1 15 95.5 3.4 1.1 84.7 14.1 0.5 0.3 0.4 <1 16 (*) 93.4 5.5 1.1 94 5.0 0.5 0.3 0.2 <1 Mean length of the 3-point aluminous bending elongated Ratio Bulk Relative modulus of nodules H density density Vickers Toughness rupture Ex (m) (%) (g/cm.sup.3) (%) hardness (MPa .Math. m.sup.1/2) (MPa) 1 (*) 0 5.39 98.1 1430 8.5 780 2 (*) 0 6.08 97.1 780 11 560 3 (*) 6.0 16 5.35 99.80 1390 8.7 4 7.6 37 5.63 99.87 1290 13.9 860 5 (*) 7.7 >95 6.11 99.78 1180 13.9 6 (*) 9.1 >95 6.02 99.94 1180 14.4 7 (*) 7.4 25 5.7 98.90 1170 15.1 8 (*) 6.8 84 5.92 99.59 1160 16.2 9 (*) 6.2 33 5.463 97.23 1130 8.2 10 6.4 27 5.58 99.81 1340 12.8 740 11 6.1 58 5.78 99.72 1220 12.9 780 12 6.5 58 5.76 99.47 1260 12.7 720 13 (*) 5.2 59 5.75 98.83 1180 11.1 14 (*) 6.5 62 5.79 99.16 1170 8.3 15 5.7 37 5.6 99.07 1280 14.1 860 16 (*) 6.8 92 5.94 99.64 1200 12.4 820 (*): examples outside of the invention

[0158] The inventors consider that there is a good compromise between the hardness, the toughness and the 3-point bending modulus of rupture when: [0159] the Vickers hardness is greater than or equal to 1210, and [0160] the toughness is greater than or equal to 10 MPa.m.sup.1/2, and [0161] the 3-point bending modulus of rupture is greater than or equal to 700 MPa.

[0162] Preferably, the hardness is greater than or equal to 1250, and/or the toughness is greater than or equal to 11 MPa.m.sup.1/2, preferably greater than or equal to 12 MPa.m.sup.1/2, preferably greater than or equal to 13 MPa.m.sup.1/2, preferably greater than or equal to 14 MPa.m.sup.1/2, and the 3-point bending modulus of rupture is greater than or equal to 750 MPa, preferably greater than 800 MPa.

[0163] Examples 1 and 2, outside of the invention, show that a sintered product comprising a zirconia partially stabilized with 3 mol % of Y.sub.2O.sub.3 and an alumina content equal to 20%, and that a sintered product comprising a zirconia stabilized with 12 mol % of CeO.sub.2 and an alumina content equal to 2% respectively do not satisfy the desired compromise.

[0164] A comparison of example 3, outside of the invention, and example 4 shows the need for an alumina content of less than 19%. This comparison also makes it possible to observe that for low cerium oxide contents, increasing the amount of alumina beyond 19% leads to an abrupt reduction in the toughness.

[0165] Examples 5 and 16 however show the need for a minimum alumina content of greater than 10%.

[0166] Examples 7 and 9, outside of the invention, show that a molar content of CeO.sub.2 equal to 1.8% and 2% respectively is too low and does not make it possible to achieve the desired compromise.

[0167] Examples 13 and 14, outside of the invention, show that a molar content of CeO.sub.2 equal to 7.5% and 9% respectively is too high and does not make it possible to achieve the desired compromise. Examples 13 and 14 also show that, for low alumina contents according to the invention, the presence of an amount of cerium oxide of greater than 6.5 mol % leads to an unsatisfactory hardness.

[0168] Of all the examples, example 15 is preferred. Example 15 shows that it is particularly advantageous to limit the content of cerium oxide to less than 5%, to less than 4%, and even to less than 3.5%.

[0169] As is now clearly apparent, the inventors have discovered that the simultaneous presence of a low content of alumina and a low content of cerium oxide advantageously makes it possible to obtain a sintered product based on alumina and on zirconia that has a good compromise between hardness, toughness and modulus of rupture.

[0170] Of course, the invention is not limited to the examples and embodiments described above.