Patent Application
20220169573
The present disclosure relates to a ceramic sintered body and ceramic powder containing zirconia as a main component.
Conventionally, in various technical fields, there has been a demand for a ceramic sintered body (the one made by forming and sintering ceramic powder) having both relatively high strength and relatively high toughness. As an example of such a ceramic sintered body, Patent Document 1 below describes a ceramic sintered body suitable for artificial joints or the like.
For example, in uses of knives or the like, a ceramic sintered body is required to have both higher strength and toughness.
A ceramic sintered body of the present disclosure is a ceramic sintered body containing zirconia as a main component, including a plurality of sintered body crystals and a grain boundary part located between the plurality of sintered body crystals, containing at least CeO2 (ceria), and having a content of Ce higher in the grain boundary part than in a central region of the sintered body crystal.
The ceramic sintered body and a ceramic powder of the present disclosure includes zirconia-based compound oxide containing 75 to 95% by mass of zirconia, 5 to 25% by mass of alumina, and 0.2 to 0.4% by mass of zinc oxide when the sum of the zirconia and the alumina is 100% by mass, and partially stabilized zirconia containing Y2O3 (yttria) in which the zirconia is 1.5 to 2.8 mol % and partially stabilized zirconia containing 8 to 12 mol % of CeO2, and the ratio of the partially stabilized zirconia containing Y2O3 to the total amount of the zirconia is 15 to 50%.
A knife according to the present disclosure includes a blade body including the above-described ceramic sintered body. Further, a jig tool according to the present disclosure includes a cutting part or a wear resistant part including the above-described ceramic sintered body.
The ceramic sintered body of the present disclosure has high bending strength and high fracture toughness.
Hereinafter, an embodiment of the ceramic sintered body of the present disclosure will be described with reference to the drawings.
A ceramic sintered body 1 shown in
Specifically, the content ratio of the elements other than zirconia is such that the content of Ce of the grain boundary part 12 is 110% by mass or more, preferably 120% by mass or more, and further 150% by mass or less, preferably 140% by mass or less with respect to the content of Ce of the central region A of the sintered body crystal 10 (100% by mass).
As can be seen from the results shown in
That is, since the content of Ce in the grain boundary part 12 is larger than that in the central region A of the sintered body crystal 10, the stabilization of zirconia in the grain boundary part 12 proceeds. As a result, the critical stress required for the stress induced transformation becomes high, and it brings an effect of increasing the strength. On the other hand, the content of Ce required for stabilizing zirconia becomes small in the central region A. Therefore, the critical stress required for the stress induced transformation becomes low, and the toughness increases. The central region A is a region of a circle having a radius of 50% of the radius of a circumscribed circle of the sintered body crystal 10 from the center of the circumscribed circle in the electron microscope photograph.
As is clear from the results shown in
This results in a continuous change in the stabilized state of zirconia from the grain boundary part 12 to the central region A of the sintered body crystal 10. As a result, the part where the critical stress required for the stress induced transformation is high and the part where the critical stress required for the stress induced transformation is low coexist in harmony, and there is an effect of achieving both the strength and the toughness of the entire ceramic sintered body.
In
The content of Ce in the grain boundary part 12 is 8% by mass or more and 15% by mass or less. When the content of Ce of the grain boundary part 12 is in such a range, a sintered body having relatively high strength and toughness can be obtained.
The alumina nanoparticle 14 preferably has a particle size of 0.8 μm or less, more preferably 0.5 μm or less, and 0.01 μm or more. When the particle size of the nano-level alumina present in the zirconia is 0.8 μm or less, the particle size is different from the crystal particle size of other alumina. Therefore, it becomes easier to be involved in complicating a concentration mechanism of Ce in the zirconia, which is a feature of the present disclosure, and further contributes to the improvement of the strength and the toughness of the ceramic sintered body.
The sintered body 1 of the present embodiment includes zirconia-based composite oxide containing zirconia of 75 to 95% by mass, alumina of 5 to 25% by mass, and zinc oxide of 0.2 to 0.4% by mass when the sum of the zirconia and the alumina is 100% by mass. The zirconia includes a mixed phase of partially stabilized zirconia containing Y2O3 of 1.5 to 2.8 mol % and partially stabilized zirconia containing CeO2 of 8 to 12 mol %, and the ratio of the partially stabilized zirconia containing Y2O3 is 15 to 50% of the total amount of the zirconia, that is, the entire mixed phase of the partially stabilized zirconia.
In the sintered body 1 having such a composition, a dispersion enhancing mechanism is effectively generated in the entire ceramic sintered body since the main compositions of zirconia and alumina are present at the respective concentrations. Further, the presence of zinc oxide at the above-mentioned appropriate concentration has an effect of increasing sintering density by acting as a sintering aid in the grain boundary parts of the alumina or the zirconia at the time of sintering. Since the zirconia is a mixed phase of the partially stabilized zirconia containing the predetermined Y2O3 and CeO2 concentrations as described above, the partially stabilized zirconia of Y2O3 and the partially stabilized zirconia of CeO2 with different critical stresses required for a stress induced transformation enhancing mechanism are present in a dispersed manner in the zirconia throughout the ceramic sintered body in a macroscopic range. As a result, it has an effect of increasing resistance response of the zirconia to fracture resistance from external stress. Zinc oxide not only acts as a sintering aid in the sintering process, but also it forms a compound of alumina and CeO2. The stabilizers Y2O3 and CeO2 of the partially stabilized zirconia are subjected to an action of balancing the concentration distribution by mutually diffusing between the zirconia phases at the time of sintering. However, when zinc oxide forms a liquid phase at the grain boundary part of the zirconia in the sintering process, chemical equilibrium occurs in the direction of forming the above compound of alumina and CeO2, and it results in an effect of forming a region where the concentration of Ce is concentrated in the grain boundary part of the zirconia after sintering.
The sintered body 1 includes zirconia-based composite oxide (zirconia-alumina composite oxide) and zinc oxide as a sintering aid, and has a sintered density of 97.6% or higher with respect to the theoretical density when sintered at 1450° C. under an atmospheric pressure. Such a sintered body 1 has a high sintering density after sintering, and has an effect of reducing structural defects in the ceramic sintered body due to voids or the like, and reducing fracture sources. As a result, such a sintered body 1 can be expected to be used as a ceramic sintered body with high reliability in terms of strength.
A fracture toughness value of the sintered body 1 is preferably 12.0 MPa·m0.5 or more by the IF method and 10.0 MPa·m0.5 or more by the SEVNB method. Such a sintered body 1 has an effect of showing high resistance to progress of internal cracks at the time of fracture, and exhibits behavior closer to that of a metal as compared with conventional ceramic sintered bodies. Therefore, it can be applied to the field of metal substitute where conventional ceramic sintered bodies could not be used due to insufficient fracture toughness values.
The sintered body 1 has the Vickers hardness HV50 of 1100 or more. Such a sintered body 1 is harder than general metal materials and has an effect of being excellent in wear resistance. Therefore, it can be applied to the field of metal substitute as described above where the wear resistance is required more than that of metal. The sintered body 1 has a bending strength value of 980 MPa or more at three-point bending fracture. Due to the effects of the various enhancing mechanisms described so far, such a sintered body 1 shows that a high strength value is maintained as compared with other ceramic sintered bodies, which is a feature that a zirconia-based ceramic sintered body originally has even after sintering in composite with each composition. Therefore, such a sintered body 1 is suitable for application to a structure of a ceramic sintered body.
Thus, since the sintered body 1 has a high fracture toughness value and a high bending strength value, when it is used for a knife or the like, sharpness lasts for a relatively long time, and it is difficult to crack. A knife including a blade body including the sintered body 1, which is an embodiment of the present disclosure, has a relatively long-lasting sharpness and is difficult to crack.
A jig tool including a cutting part or a wear resistant part including the sintered body 1, which is another embodiment of the present disclosure, is effective in improving reliability with respect to functions of jig tools that involve cutting, rotation, sliding, friction or the like, such as sliding parts of various processing devices, industrial cutters or the like.
The content of CeO2 in the partially stabilized zirconia of CeO2 in the zirconia is preferably 8 to 12 mol % to stabilize the zirconia as a tetragonal crystal and to suppress the precipitation of monoclinic crystals and cubic crystals. The content of Y2O3 in the partially stabilized zirconia of Y2O3 in the zirconia is preferably 1.5 to 2.8 mol % for the same reason as above.
If the content of CeO2 in the partially stabilized zirconia of CeO2 in the zirconia, which is the main component of the zirconia-based composite oxide in the present disclosure, is less than 8 mol %, a monoclinic crystal, which is a metastable phase, tends to precipitate. This may cause the ceramic sintered body to fracture due to volume expansion before and after the sintering process. On the contrary, if the content of CeO2 is more than 12 mol %, cubic crystals tend to increase and the stress induced transformation mechanism of the zirconia, which is a feature of the sintered body, is not demonstrated. As a result, the bending strength, the toughness, and the hardness may decrease. The content of CeO2 in the partially stabilized zirconia of CeO2 in the zirconia may be 9 mol % or more and 11 mol % or less.
On the other hand, if the content of Y2O3 in the partially stabilized zirconia of Y2O3 in the zirconia is less than 1.5 mol %, the monoclinic crystal, which is the metastable phase, tends to precipitate, as with CeO2. This may cause the ceramic sintered body to fracture due to the volume expansion before and after the sintering process. On the contrary, if the content of Y2O3 is more than 2.8 mol %, cubic crystals increase and the occurrence of the stress induced transformation mechanism of the zirconia, which is a feature of the sintered body, tends to be impaired in the same manner. As a result, the bending strength, the toughness, and the hardness may decrease. The content of Y2O3 in the partially stabilized zirconia of Y2O3 in the zirconia may be 1.7 mol % or more and 2.6 mol % or less.
If the proportion of the zirconia constituting the zirconia-based composite oxide in the present disclosure is less than 75% by mass and the proportion of the alumina is more than 25% by mass, the toughness of the mechanical properties may be particularly reduced. On the contrary, if the proportion of the zirconia is more than 95% by mass and the proportion of the alumina is less than 5% by mass, the concentration mechanism of the concentration of Ce, which is a feature of the ceramic sintered body, may be relatively reduced due to the reduction of alumina, and the bending strength and the Vickers hardness of the mechanical properties may be particularly reduced.
In the zirconia-based composite oxide of the present disclosure, alumina may be present at the grain boundary of the zirconia in that the alumina is involved in the concentration mechanism of Ce. Furthermore, the nano-level alumina particles generated in the zirconia at a constant rate in the sintering process may be present, as they have an effect of complicating the regions of different critical stresses of the stress induced transformation by being involved in the concentration mechanism of Ce in the zirconia.
In the zirconia-based composite oxide of the present disclosure, the content of zinc oxide is preferably in the range of 0.2 to 0.4% by weight if the sum of the zirconia and the alumina is set to be 100% by mass. If the content of zinc oxide is less than 0.2% by weight, the effect as a sintering aid is lost, and the residual pores in the ceramic sintered body tend to increase and the sintered density tends to decrease. As a result, there is a risk that reliability in terms of strength is lowered. On the other hand, if the content of zinc oxide is more than 0.4% by mass, the zinc oxide tends to promote the formation of a compound of alumina and CeO2, resulting in the formation of a large amount of needle-like crystals in the ceramic sintered body. As a result, the presence state of the zirconia and the alumina may become heterogeneous, and the improvement of strength and toughness, which are the features of the sintered body, may be impaired.
The mean particle sizes of both zirconia and alumina of the present disclosure may be 1 μm or less, and the mean particle sizes are more preferably 0.3 to 0.8 μm in terms of increasing the bending strength and the toughness.
If the mean particle sizes of zirconia and alumina are 1 μm or less, the zirconia crystal is less likely to change from a tetragonal system to a monoclinic system, and the toughness tends to be less likely to decrease. The occasional presence of large crystals in the composite ceramic tends to cause missing of zirconia or alumina particles during the wear resistance test, and the wear resistance decreases. For this reason, the maximum crystal diameters of the zirconia and the alumina are desirable to be 2 μm or less, particularly 1.5 μm or less.
Next, a method for manufacturing the sintered body of the present disclosure will be described.
In the present disclosure, ceramic powder is first prepared, and then the powder is molded into a desired shape. The ceramic powder includes zirconia-based mixed powder in which first zirconia powder containing 8 to 12 mol % of CeO2, second zirconia powder containing 1.5 to 2.8 mol % of Y2O3, alumina powder, and zinc compound are mixed. The mean particle sizes of the first and second zirconia powders and the alumina powder are preferably 1 μm or less, and particularly 0.8 μm or less. The first and second zirconia powders are mixed and used in the proportions of 50 to 85% by mass of the first zirconia powder and 15 to 50% by mass of the second zirconia powder. In particular, it is preferable to contain 50 to 75% by mass of the first zirconia powder and 25 to 50% by mass of the second zirconia powder for the purpose of increasing the toughness and the mechanical strength of the composite ceramic obtained by sintering.
Furthermore, in the present disclosure, 75 to 95% by mass of the zirconia powder containing the first and second zirconia powders, and 5 to 25% of the alumina powder are mixed and used. If the ratio of the alumina powder exceeds it, the fracture toughness value of the composite ceramic after sintering will decrease and the strength value will also decrease. On the contrary, if the ratio of the alumina powder is lower than it, the composite enhancing mechanism of alumina tends to be lost.
In this case, the mean particle sizes of the first and second zirconia powders and the alumina powder used are preferably 1 μm or less. If those having the mean particle size of 1 μm or less are used, the mean particle sizes of zirconia and alumina constituting the sintered composite ceramic are unlikely to be large. Preferably, each powder is made finer by wet grinding or other means, or when each powder is mixed, the powder particles are made finer by wet grinding or other means to set the mean particle sizes of the first and second zirconia powders and the alumina powder in the range of an appropriate mean diameter of 0.1 to 0.6 μm.
The purity of the zirconia powder such as the first and second zirconia powders and the alumina powder used in the present disclosure is preferably 99.9% or more.
Furthermore, in the present disclosure, it is desirable to add a zinc compound in the amount of 0.2 to 0.4% by mass expressed in terms of zinc oxide when mixing with zirconia and alumina. The zinc compound may be added in the form of carbonates, nitrates, sulfates, organic compounds or the like. For example, it is desirable to add zinc oxide with a small ignition loss such that pores are not generated in the ceramic sintered body during sintering.
In this case, the particle size of the zinc compound is desirable to be 0.1 to 1.0 μm, preferably 0.1 to 0.5 μm. When the particle size of the zinc compound is 0.1 to 1.0 μm, zinc oxide is less likely to remain as a lump at the grain boundary part of zirconia or alumina during sintering, and the effect as a sintering aid is less likely to be impaired. The purity of the zinc compound used in the present disclosure is preferably 99.9% or more.
In the present disclosure, the molded body obtained in this way is sintered to obtain a ceramic sintered body. It does not require hot isostatic press sintering and can be sintered in an air atmosphere of 1450° C. or less. This is because zinc oxide acts as a sintering aid when present in an appropriate amount, and it is desirable that sintering is conducted at a temperature of 1350 to 1450° C. When sintering at a temperature of 1450° C. or less, grain growth of zirconia and alumina becomes gentler, and the mean crystal size is less likely to become larger than necessary. As a result, the zirconia is less likely to transition from tetragonal to monoclinic, and the toughness and the wear resistance are less likely to decrease. The lower limit of the sintering temperature is 1300° C. or more, more preferably 1350° C. or more, which is a temperature range in which zinc oxide enhances the sinterability as a sintering aid.
In the present disclosure, the zirconia powder used for adjusting the ceramic powder may be either obtained by mixing CeO2, Y2O3, and zirconia powder in powder form and then calcining, or by mixing metallic salts or alkoxides of Ce, Y, and zirconia in a pH-adjusted aqueous solution (hereinafter, it may be referred to as a hydrolysis method). Powder synthesized by a hydrolysis method is preferable because it has a uniform particle size and more stable zirconia can be obtained.
Hereinafter, the ceramic sintered body of the present disclosure will be described in detail with examples, but the present disclosure is not limited to the following examples.
First, zirconia powder prepared by a hydrolysis method, in which partially stabilized zirconia powder containing 10 mol % of CeO2 (99.9% of purity, 0.5 μm of particle size) and partially stabilized zirconia powder containing 2.0 mol % of Y2O3 (99.9% of purity, 0.5 μm of particle size) were mixed to have the predetermined ratio shown in Table 1, alumina powder (99.9% of purity, 0.5 μm of particle size) and zinc oxide (99.9% of purity, 0.2 μm of particle size) were mixed to have the composition shown in Table 1. The mixing was carried out by using a wet ball mill for 24 hours with IPA (isopropyl alcohol) as a solvent, and using a high purity wear resistant alumina ball and a polyethylene container. Subsequently, ceramic sintered bodies (Samples Nos. 1 to 10) were manufactured by pressing and molding the ceramic powder obtained by drying, and sintering at 1350 to 1450° C. for 2 hours in the air.
The crystal structure of the obtained ceramic sintered body was observed by using a transmission electron microscope. Specifically, Sample No. 1 was prepared by cutting the ceramic sintered body into a size which can be set in a sample chamber of the transmission electron microscope and suitable for transmission observation of an electron beam. This sample was set in the sample chamber of the transmission electron microscope, and the image formed by the transmission electron beam was observed. The results are shown in
Next, the obtained ceramic sintered body was ground to prepare a sample of 4×3×35 mm. The density of the sintered body was measured by the Archimedes method. The bending strength was evaluated by three-point bending strength at room temperature according to JIS-R1601.
The Vickers hardness of the ceramic sintered body was evaluated by the Vickers hardness test method of JIS-R1610, and the fracture toughness value was evaluated by the IF method of JIS-R1607. Since the sintered body in the present disclosure has high strength and toughness, the indenter press-in pressure in each test was set to be 50 kgf (490 N). The results are shown in Table 1.
From Table 1, it can be seen that the ceramic sintered body of the present disclosure has high bending strength and high fracture toughness.
For the ceramic sintered bodies of Samples Nos. 1 to 10, composition concentrations of the grain boundary part and the central region part on the ceramic sintered body were measured by using an energy dispersive X-ray analysis of a scanning transmission electron microscope. For the measurements, NSS300E: an EDS analyzer manufactured by Thermo Scientific, was used, and as measurement conditions, an acceleration voltage was set to be 200 kV, and a measurement Livetime was set to be 50 sec.
From the values in Tables 1 and 2, it can be seen that the ceramic sintered body having a larger content of Ce at the grain boundary part than in the central region of the sintered body crystal has high bending strength and high fracture toughness.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2019-044207 | Mar 2019 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2020/009982 | 3/9/2020 | WO | 00 |
