A METHOD FOR PROCESSING ZIRCONIA

Abstract

A problem to be solved is to provide a method for processing zirconia without producing a monoclinic crystal. The solution is a method for processing zirconia, including the step of irradiating the zirconia with a laser with a pulse duration of 10.sup.−12 seconds to 10.sup.−15 seconds at an intensity of 10.sup.13 to 10.sup.15 W/cm.sup.2.

Claims

1. A method for processing zirconia, comprising the step of irradiating the zirconia with a laser with a pulse duration of 10.sup.−12 seconds to 10.sup.−15 seconds at an intensity of 10.sup.13 to 10.sup.15 W/cm.sup.2.

2. The method according to claim 1, wherein the irradiating is conducted to a monoclinic crystal produced on a surface of the zirconia.

3. The method according to claim 1, further comprising the step of joining the irradiated zirconia and another material.

4. The method according to claim 3, wherein the zirconia and the material to be joined are different in thermal expansion coefficient.

5. The method according to claim 1, wherein a wavelength of the laser is 810 nm.

6. The method according to claim 1, wherein the zirconia contains a solid solution with any of Y, Ca, Mg, and Ce ions or a combination thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1 is a schematic diagram of stress-induced transformation strengthening mechanism by which crystal phase transition inhibits the expansion of cracks.

[0021] FIG. 2 illustrates photomicrographs of the zirconia which was irradiated with a laser under various conditions.

[0022] FIG. 3 illustrates diagrams indicating the measurement results of the surfaces of the zirconia which was irradiated with a laser under various conditions.

[0023] FIG. 4 illustrates diagrams indicating the XRD measurement results of the surfaces of the zirconia which was irradiated with a laser under various conditions.

[0024] FIG. 5 illustrates schematic diagrams of the method for processing zirconia using a laser according to the present invention.

DESCRIPTION OF EMBODIMENTS

[0025] In the method for processing zirconia according to the present invention, zirconia is irradiated with a laser with a pulse duration of 10.sup.−12 seconds to 10.sup.−15 seconds at an intensity of 10.sup.13 to 10.sup.15 W/cm.sup.2.

[0026] Such laser irradiation under particular conditions achieves remelting and removal on only the surface of the processed material (zirconia), as illustrated in FIG. 2 (1). Thus, the processed material (zirconia) has little dimensional change.

[0027] Without wishing to be bound by any particular theory, the mechanism of the present invention may be considered as follows. Processing only the surface of a material to be processed is considered to increase the surface temperature, causing melt recrystallization or evaporation. Still, the temperature increase is not limited to the outermost surface only, but the vicinity of the surface is overheated, whereby melting and internal boiling may cause a large shape change. Now, the intensity of a laser is increased, and the laser having a raised electric field strength is irradiated. Assuming that from the laser-irradiated atom, nuclear electrons are removed through relativistic effect and that the coulomb repulsion of the ionized atom causes the surface of the processed material to be exfoliated and removed, thermal energy does not reach the inside of the processed material nor cause a dimensional change due to heat. In the case of mechanical processing such as grinding, sand blast, or the like, a large mechanical energy is loaded, and the inside of the processed material may transform from the tetragonal crystal to the monoclinic crystal, decreasing the strength, but according to the present invention, thermal energy does not reach the inside of the processed material, and accordingly there is no such case where the internal crystal structure is changed with the strength decreased. However, when the intensity is raised with the pulse duration still at a conventional level, for example, a level of nanometers, up to the electric field strength at which coulomb repulsion occurs, the incident energy becomes very large, and the excessive heat melts or evaporates the inside of the processed material, thereby causing a dimensional change.

[0028] The laser pulse duration is 10.sup.−12 seconds to 10.sup.−15 seconds. A laser pulse duration longer than this range will cause the surface of the zirconia to be melted and evaporated (ablation), so that the laser-irradiated portion results in having a hole, and the shape of the zirconia changes significantly.

[0029] The laser intensity is 10.sup.13 to 10.sup.15 W/cm.sup.2. A laser intensity larger than this range will cause the surface of the zirconia to be melted and evaporated (ablation), so that the laser-irradiated portion results in having a hole, and the shape of the zirconia changes significantly, as is the case with a longer pulse duration. At an intensity lower than 10.sup.13, the surface of the processed material (zirconia) fails to be exfoliated by coulomb explosion nor heated in an instant (at a pulse duration at a level of femtoseconds).

[0030] According to the present invention, the laser irradiation may be conducted to monoclinic crystals produced on the zirconia surface. This allows the monoclinic crystals on the zirconia surface to be removed or modified into the tetragonal crystals. The absence of monoclinic crystals can provide high strength. In addition, owing to the absence of monoclinic crystals, the tetragonal crystal is transformed to the monoclinic crystal when loaded with a stress (stress-induced transformation strengthening mechanism), through which cracks are expectedly inhibited. Stress-induced transformation strengthening mechanism will be described with reference to FIG. 1. When a large pressure is applied to zirconia, its crystal structure is instantly transformed from the tetragonal crystal to the monoclinic crystal (phase transition) without being heat-treated. This is called martensitic transformation. When a microcrack is created by coincidence on a zirconia material loaded with a bending stress, compressive stress is concentrated at the end of the crack, and the crack, if in a material to which no phase transition occurs, will expand because of the concentrated stress. A martensite transformation caused by a stress from the tetragonal crystal to the monoclinic crystal expands the crystal, causes a compressive stress on the cracked portions, and inhibits the expansion of the cracks, because the monoclinic crystal has a larger volume than the tetragonal crystal. FIG. 1 illustrates a schematic diagram of stress-induced transformation strengthening mechanism by which crystal phase transition inhibits the expansion of cracks.

[0031] According to the present invention, the laser-irradiated zirconia may further have another material joined.

[0032] The laser irradiation can form roughness on the surface of the zirconia. Because the zirconia has a rough surface, joining it and another material gives an increased joining area and then an enhanced joining ability. The laser irradiation may be conducted all over the joined face or, as in FIG. 5 (b), dispersed in spots over the joined face. This allows the surface roughness, or surface ruggedness, to be controlled and can achieve a desired joining ability.

[0033] The laser irradiation also causes the outermost surface of the zirconia to have cracks or unstuck shear faces (shear planes) formed thereon (see FIG. 2 (1)). When the joined materials are loaded with stress, only the stress-loaded cracks and shear faces are destroyed (exfoliated), and the exfoliation of the joined portions can be inhibited from expanding all over.

[0034] A material to be joined may have a different thermal expansion coefficient than zirconia.

[0035] In joining materials having different thermal expansion coefficients, the rough surface buffers the thermal expansion difference and enhances the joining ability between the zirconia and the other material.

[0036] In addition, even when the cracks and unstuck shear faces (shear planes) formed by the laser irradiation on the outermost surface of the zirconia are loaded with a stress due to a thermal expansion difference between the joined materials and the zirconia, only the stress-loaded cracks and shear faces are destroyed, and the exfoliation of the joined portions can be inhibited from expanding all over.

[0037] According to the present invention, the laser wavelength may be 810 nm.

[0038] The laser wavelength is not particularly limited but a wavelength in a visible region or near-infrared region may be used. Currently, however, a wavelength of about 810 nm is used in order to achieve a pulse duration at a femtosecond level. It is about 810 nm because of variations of several % (±10%).

[0039] The zirconia may contain a solid solution with any of Y, Ca, Mg, and Ce ions or a combination thereof. Among the crystal phases of pure zirconia at room temperature, the monoclinic crystal is the most stable. However, when zirconia contains a solid solution with ions such as Y, Ca, Mg, and Ce, which have a larger ion radius than Zr, oxygen vacancies are formed in the structure, and, at room temperature, the cubical crystal or the tetragonal crystal is stable or metastable. This inhibits a structural destruction which accompanies a phase transition due to heating and cooling. A sintered material composed of an approximately 100% tetragonal crystal zirconia is referred to as a tetragonal zirconia polycrystal, and Y.sub.2O.sub.3—ZrO.sub.2(Y-TZP), Ce.sub.2O.sub.3—ZrO.sub.2(Ce-TZP), or the like may be used.

EXAMPLES

[0040] The present invention will be described with reference to the following examples. The scope of the present invention is not limited to these examples.

[0041] The surface of the zirconia which was irradiated with a laser at different pulse durations and intensities was observed. As a reference for the present invention, a laser irradiation was conducted under the conditions out of the range of the present invention.

[0042] As zirconia for laser irradiation, a single crystal Y-stabilized zirconia (containing 11% Y) was prepared and irradiated with a laser under the conditions (pulse duration and intensity) indicated in FIG. 2. The laser wavelength was set to 810 nm. FIG. 2 indicates the photomicrographs of the surface of the laser-irradiated zirconia.

[0043] As indicated in FIG. 2 (1), only the surface of the zirconia irradiated with a laser with a pulse duration of 110 fsec at an intensity of 10.sup.15 W/cm.sup.2, which is within the range of the present invention, achieved remelting and removal. As the result, the zirconia had little dimensional change. In FIG. 2 (1), it was also observed that cracks and unstuck shear faces (shear planes) were formed on the laser-irradiated portion.

[0044] FIG. 2 (2) is a case where a laser with a pulse duration of 110 fsec was irradiated at an intensity of 10.sup.16 W/cm.sup.2, in which the intensity was larger than the range of the present invention, so that the surface melted and evaporated (ablation), resulting in having a hole at the laser-irradiated portion and in changing the size and shape of the zirconia significantly.

[0045] FIG. 2 (3) is a case where a laser with a pulse duration of 300 fsec was irradiated at an intensity of 10.sup.13 W/cm.sup.2, in which the pulse duration was larger than the range (femtosecond levels) of the present invention, so that the dimensional change of the zirconia was significant.

[0046] FIG. 2 (4) is a case where a laser with a pulse duration of 15 nsec was irradiated at an intensity of 10.sup.12 W/cm.sup.2, in which the intensity was made lower than the range of the present invention but the pulse duration was larger than the range (femtosecond levels) of the present invention, so that the dimensional change of the zirconia was significant.

[0047] The surface of laser-irradiated zirconia was measured for shape. The results are indicated in Table 3.

[0048] As seen in FIG. 3 (1), the surface of the zirconia irradiated with a laser with a pulse duration of 110 fsec at an intensity of 10.sup.15 W/cm.sup.2, which are within the range of the present invention, exhibited roughnesses about 4 μm high.

[0049] FIG. 3 (2) is a case where a laser with a pulse duration of 15 nsec was irradiated at an intensity of 10.sup.12 W/cm.sup.2, in which the intensity was made lower than the range of the present invention but the pulse duration was larger than the range (femtosecond levels) of the present invention, so that the laser-irradiated portion resulted in having a hole about 15 μm deep and the shape change of the zirconia was significant.

[0050] The surface of the laser-irradiated zirconia was measured by XRD, and the crystal structure of the surface was examined. The results are indicated in Table 4. The measurement conditions of XRD were an angle of incidence of 10° and a wavelength λ=1 Å (1×10.sup.−10 m).

[0051] FIG. 4 (1) is the result of XRD of the laser with a pulse duration of 110 fsec irradiated at an intensity of 10.sup.15 W/cm.sup.2 within the range of the present invention, and it indicated no peak of a monoclinic crystal.

[0052] FIG. 4 (2) is a case where a laser with a pulse duration of 300 psec was irradiated at an intensity of 10.sup.13 W/cm.sup.2, in which the intensity was made to fall within the range (the lower limit) of the present invention but the pulse duration was larger than the range (femtosecond levels) of the present invention, and a peak of a monoclinic crystal was confirmed, though minute.