Method of manufacturing a ceramic mirror

Abstract: A ceramic mirror having a surface flatness of less than 1 µm and/or a centerline average roughness of less than 1 nm measured by a laser interference method. A method of manufacturing ceramic mirrors, comprising the steps of forming titanium oxide powder having an average grain size of less than 1 µm and a purity of more than 99 %, and thereafter sintering the powder in the atmospheric air, an inert atmosphere, a vacuum atmosphere or a reducing atmosphere at a temperature of 1000-1300 °C.

Description: This invention relates to a method of manufacturing titanium oxide ceramic mirrors for use in laser and X-ray apparatuses, cameras, and other precision optical instruments and the like. Mirrors for use with precision optical instruments and the like are predominantly made of glass, ruby, sapphire and the like, and also of single crystal ceramics. In recent years, aluminia and SiC have partly come in use as polycrystalline ceramics. K. Kurosawa et al described in Rev. Sci. Instrum. 61(2) (1990), pp 728-731, the manufacture of a super-polished high-reflecting SiC mirror for high power operation of excimer lasers in a vacuum UV spectral range with a void free SiC film of 400 µm thickness deposited onto a high-purity graphite substrate by chemical vapor deposition, the surface roughness was 0.2 nm rms and an energy density of 1 J/cm 2 on the mirror surface was tolerable. Surface flatness, centerline average roughness-height or irregular reflection are not specified. From U.S. patent specification 4,492,669, a method for making a beryllium mirror is known wherein a mirror body is made by hot isostatic pressing (HIP) of fine beryllium powder and grinding and polishing the thus obtained sintered body (blank). Glass-based mirrors are made relatively thick to make up for the inadequate strength and fragility, and therefore the heavy weight and large size of the mirrors as a whole have been a problem. Polycrystalline alumina, SiC, and other ceramic materials are used in the form of sintered bodies after grinding and lapping. None of them have proved fully satisfactory in respect to the properties such as surface flatness or smoothness and optical properties such as reflectivity and irregular reflection of light, because the pores inherent to the ceramic materials remain exposed on the lapped surfaces. The present inventor, in an effort to solve these problems, sintered a high-purity, fine titanium oxide powder at a relatively low temperature. This has been found to allow to achieve a mirror having excellent surface smoothness with a smaller number of pores than before and having superior optical properties due to less irregular reflection of light. The discovery has now led to the present invention. The invention provides, in essence, two methods of manufacturing a titanium oxide ceramic mirror. These methods are defined in claims 1 and 2. It has been found advantageous to additionally treat the body obtained in accordance with either of these methods by hot isostatic pressing at a temperature between 800 and 1500°C and under a pressure of no less than 500 kg/cm 2 . The invention will now be described in detail. Under the invention a fine titanium oxide powder of high purity is used as a material and sintered at a lower temperature than usual sintering temperatures to obtain a sintered body with excellent surface smoothness, inhibiting the particle growth and preventing the pore size increase while achieving an adequate sintered density. The reason why a ceramic mirror having outstanding characteristics can be obtained by the afore-said means is theoretically to be clarified yet. Possibly the low temperature sintering of a fine titanium oxide powder keeps the pores within the crystal grains (a phenomenon known as "ghosting"), suppressing the accumulation and growth to a grain boundary phase. The expression "a lower temperature than usual sintering temperatures" as used herein means a temperature range from 1000 to 1300°C. If the temperature is below 1000°C the sintering itself does not proceed. A temperature above 1300°C, on the other hand, causes rapid grain growth and therefore a decrease in mechanical strength and pore migration and growth to a grain boundary phase. Consequent reduction of surface smoothness and irregular reflection of light renders it impossible to obtain a mirror of satisfactory optical characteristics. If the material powder is not at least 99% pure as titanium oxide or has a larger average particle diameter than 1 µm, it is no longer easily sinterable at low temperatures. Sintering in the temperature range of 1000 to 1300°C will not give a product with an adequate sintered density. For this reason it is desirable that the titanium oxide powder have a purity of at least 99%, preferably at least 99.8%, and an average particle diameter of no more than 1 µm, preferably no more than 0.5 µm. The titanium oxide powder is compacted by CIP (cold isostatic pressing), mechanical pressing, doctor blade method, extrusion, or other technique depending on the configuration of the product. Whatever compacting technique is chosen, an organic binder or the like suited for the particular method is used as a compacting assistant. The compact thus obtained is sintered in air, vacuum, or an atmosphere of N 2 , Ar, H 2 or the like, at sintering temperatures in the optimum range of 1000 to 1300°C. Where greater surface smoothness is required, the compact is hot press sintered under a pressure of at least 50 kg/cm 2 . Such a sintered body is further treated by hot isostatic pressing (HIP) under a pressure of not less than 500 kg/cm 2 . The TiO 2 sintered body so obtained is machined, e.g., by surface grinding or lapping, to a mirror for use in a laser or X-ray apparatus, camera, or other precision optical instrument. When necessary, the mirror is coated with a metal of high reflectivity so as to be a mirror for a precision optical application. Such a ceramic mirror has a surface flatness of no more than 1 µm and/or a centerline average height of no more than 1 nm. Moreover, the amount of its irregular reflection of incident light measured with a colorimeter is 5% or less. If the characteristic values exceed the numerical values specified above, too much irregular reflection will result, making the optical characteristics inadequate for a mirror. Best Mode for Carrying Out the Invention Examples 1 to 4 of the present invention and Comparative Examples 5 to 8 are given below. (Example 1) To a titanium oxide powder having an average particle diameter of 0.25 µm and a purity of 99.8% ("CR-EL" (trade name), a product of Ishihara Sangyo Kaisha) was added 2% polyvinyl alcohol (PVA) as a compacting assistant. After mixing in a mortar and drying, the mixture was sieved for particle size regulation. The mixture was compacted by CIP at a pressure not lower than 1000 kg/cm 2 and sintered at ordinary pressure in air at 1200°C. The sintered body so obtained was lapped, and its surface flatness and amount of irregular reflection were measured. The results are given in Table 1. Sample No. TiO 2 powder Sintering condition Surface flatness (µm) Centerline aver. height (nm) Amount of irregular reflection (%) Purity (%) Aver. part. dia. (µm) Example (1) 99.8 0.25 Sintered at ordinary pressure in air at 1200°C 0.9 0.8 4 (2) " " Sintered at ordinary pressure in Ar at 1100°C 0.8 0.7 3 (3) " " Hot press sintered in N 2 at 1000°C 0.4 0.5 2 (4) " " After sintering in air at 1200°C, HIP'd in Ar at 1000°C and 1800 kg/cm 2 0.2 0.3 1 Comp.Ex (5) 96.0 0.25 Sintered at ordinary pressure in air at 1200°C 1.8 3.2 10 (6) " " Sample 5 sintered in air at 1000°C and HIP'd at 1800 kg/cm 2 1.4 2.7 8 (7) 99.5 1.5 Sintered at ordinary pressure in air at 1200°C 2.4 6.5 15 (8) " " Sample 7 sintered at 1000°C and HIP'd at 1800 kg/cm 2 2.0 4.1 13 (Example 2) The titanium oxide powder of Example 1 was used. Following compacting, it was sintered in an Ar atmosphere at 1100°C. The properties of the sintered body after lapping are shown at (2) in Table 1. (Example 3) Using the titanium oxide powder of Example 1, hot press sintering was performed in an N 2 atmosphere at 1000°C and at a pressure of 200 kg/cm 2 . The sintered body after lapping exhibited properties as shown in Table 1. (Example 4) The sintered body obtained in Example 1 was treated by HIP in an Ar atmosphere at 1000°C and at a pressure of 1800 kg/cm 2 . Table 1 gives the properties of the sintered body as lapped. (Comparative Example 5) Sample No. 5 was obtained by following the same procedure as described in Example 1 with the exception that a titanium oxide powder having an average particle diameter of 0.25 µm and a purity of 96.0% was employed instead. The properties of the sintered body after lapping are shown in Table 1. (Comparative Example 6) Sample No. 5 was compacted and sintered in air at 1000°C and at ordinary pressure and then treated by HIP under the conditions given in Example 4 to obtain Sample No. 6. The sintered body upon grinding showed properties as in Table 1. (Comparative Example 7) Except for the use of a titanium oxide powder having an average particle diameter of 1.5 µm and a purity of 99.5%, the procedure of Example 1 was repeated, when Sample No. 7 was obtained. The sintered body properties after grinding are given in Table 1. (Comparative Example 8) Sample No. 7 was sintered in air at 1000°C and at ordinary pressure and treated by HIP under the conditions of Example 4 to obtain Sample No. 8. Table 1 shows the sintered body properties after grinding. Industrial Applicability As described above, sintering a high-purity, minute titanium oxide powder at a low temperature below the usual sintering temperature range makes ghosting of pores in the resulting sintered body possible. In this way titanium oxide sintered parts have been obtained which exhibit high enough surface smoothness and optical characteristics for mirrors to be used in laser and X-ray apparatuses, cameras, and other precision optical instruments.

Claims: A method of manufacturing a ceramic mirror, in which a ceramic powder is compacted and sintered, and the thus obtained sintered body is then machined, characterized by compacting a titanium oxide powder having an average particle diameter of no more than 1 µm and a purity of at least 99 %, then sintering the compact in air or an inert atmosphere at a temperature between 1000 and 1300°C, and then machining the thus obtained sintered body to obtain a mirror having a surface flatness of no more than 1 µm and/or a centerline average height of no more than 1 nm as measured by the laser interference method. A method of manufacturing a ceramic mirror, in which a ceramic powder is sintered, and the thus obtained sintered body is then machined, characterized by hot-press sintering in air or an inert atmosphere a titanium oxide powder having an average particle diameter of no more than 1 µm and a purity of at least 99 % under a pressure of no less than 50 kg/cm 2 and at a temperature between 1000 and 1300°C, and then machining the thus obtained sintered body to obtain a mirror having a surface flatness of no more than 1 µm and/or a centerline average height of no more than 1 nm as measured by the laser interference method. The method of manufacturing a titanium oxide ceramic mirror set forth in claim 1 or 2 characterized by additionally treating by hot isostatic pressing the sintered body at a temperature between 800 and 1500°C and under a pressure of no less than 500 kg/cm 2 .

Inventors: KISHI YUKIO, KISHI, YUKIO, KISHI, YUKIO 302, KOKUO FUNABASHI HAITSU
Applicants: NIHON CEMENT, NIHON CEMENT CO., LTD.
Application year: 1992
Main classification: G02B5
IPC codes: G02B5