Stock material or miscellaneous articles – Composite – Of silicon containing
Reexamination Certificate
2000-09-07
2002-10-29
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of silicon containing
C428S312600, C428S402000, C428S408000, C428S689000, C428S690000, C428S701000, C428S698000, C428S704000
Reexamination Certificate
active
06472075
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a sintered silicon nitride member, and to a sintered silicon nitride member in the form of a ceramic ball.
2. Description of the Related Art
Recently, silicon nitride ceramic has been applied to bearings of machine tools and hard disk drives of computers, because of its high strength and excellent wear resistance. Also, because of its corrosion resistance at high temperature, silicon nitride ceramic has been applied to bearings and sliding components used in high-temperature, corrosive environments, such as bearings and sliding components of driving units of semiconductor equipment and sliding components of automobiles, such as tappets.
In manufacture of the above-mentioned ceramic components, after precision polishing, the ceramic components are inspected for material-related defects, such as foreign matter and pores, and defects caused by polishing, such as scratches, cuts, and chips. This inspection may be carried out using a stereomicroscope or metallographic microscope. Recently, in order to cope with mass production and to reduce cost, an automatic appearance inspection machine has been employed, in some cases. Specifically, an image of a polished surface is captured, and the captured image is inspected for defects, such as foreign matter or pores on the polished surface. Such defects are identified by means of contrast of color tone or brightness between a defective portion and a background portion.
3. Problems to be Solved by the Invention
Generally, the appearance of a sintered silicon nitride body assumes a color tone of white or bright gray. Similarly, a defect such as a pore or a chip appears in a bright tone. Thus, contrast is hardly produced between such a defect and the background, resulting in poor accuracy in defect identification. Particularly, in the case of an automatic appearance inspection machine, which identifies a defect by processing data regarding the difference in color and/or lightness between background and a defective portion, low contrast therebetween is directly linked to an impairment in inspection accuracy. According to a method disclosed in Japanese Patent Application Laid-Open (kokai) Nos. 254471/1992 and 254473/1992, a sintered body is intentionally colored black or dark gray by sintering a shaped body containing a source of carbon or by impregnating a porous body with carbon, to thereby decrease nonuniform color tone appearing on the sintered body. Background in black or dark gray facilitates identification of, for example, a pore or a chip, which appears in a relatively bright tone, but encounters difficulty in identifying foreign matter, particularly metallic foreign matter, which appears in black or blackish gray.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a sintered silicon nitride member which facilitates identification of defects and foreign matter of different color tones and which is capable of enhancing inspection accuracy, as well as to provide a ceramic ball formed of the same.
The above objects of the present invention have been achieved by providing a sintered silicon nitride member formed predominantly of silicon nitride, wherein at least a surface of the sintered member has a lightness VS of 3.0 to 9.0. As used herein, the lightness VS is defined in a color specification as specified by JIS Z 8721 (published Feb. 19, 1993, Japanese Standards Association).
In the present invention, unless otherwise specified, the term “predominant” used in relation to content means that the content of a substance in question is contained in an amount of not less than 50% by weight (the terms “predominantly” and “mainly” have the same meaning as used herein).
In the present invention, by imparting a lightness VS of 3.0 to 9.0 to the surface of a sintered silicon nitride member, the surface, particularly when polished, of the member establishes a background color tone which facilitates identification of a defect, such as a pore, a chip, or a cut, as well as foreign matter. The background is toned differently from such a defect, thereby improving inspection accuracy of, for example, an automatic appearance inspection machine.
When the polished surface of a ceramic component is inspected for metallic foreign matter and defects, such as pores, cuts, and chips (hereinafter, these defects are generically called a “pore, etc.”), foreign matter or a pore, etc., is identified by means of difference in color or lightness between a base material serving as background and the foreign matter or pore, etc. Specifically, by observing with a stereomicroscope or by observing with a metallographic microscope using polarized light, metallic foreign matter appears in a color tone close to black, whereas an internal pore or a cut appears in a color tone close to white.
Herein, the meaning of color includes not only chromatic color but also achromatic color that has no chroma. Therefore, when it is acknowledged that colors are different from each other, it means that at least one of lightness, chroma and hue differs between the colors.
By imparting a lightness VS of 3.0 to 9.0 to a background portion, at a portion where metallic foreign matter or a pore, etc., is present, the background assumes a substantially neutral tint between metallic foreign matter and a pore, etc., thereby producing a distinct contrast with both metallic foreign matter and a pore, etc., which assume different color tones. Thus, metallic foreign matter or a pore, etc. can be readily identified by observing with a stereomicroscope or with a metallographic microscope using polarized light.
When the base material assumes a lightness of less than 3.0 (i.e., dark black), metallic foreign matter is hardly identified, since metallic foreign matter appears in black. When the base material assumes a lightness of greater than 9.0 (i.e., near white), a pore, etc., is hardly identified, since the pore, etc., appears in white. The above-mentioned lightness VS is preferably 4.0 to 8.5, more preferably 4.5 to 8.0. In order to enhance contrast with foreign matter or a pore, etc., the appearance of the sintered silicon nitride member assumes a chroma CS of not greater than 3.0 as defined by JIS Z 8721 (published Feb. 19, 1993, Japanese Standards Association), preferably not greater than 2.0, more preferably not greater than 1.0.
The lightness VS and the chroma CS are measured according to “4.3 Measuring method for reflecting objects” of “4. Spectrophotometric method” in JIS Z 8722 (published Mar. 4, 1994, Japanese Standards Association) “Methods of color measurement.” Among conditions a to d specified in the section 4.3, an optimum condition may be selected according to the shape of a surface to be measured. For example, when the polished surface of a ceramic ball, which will be described below, is to be measured, condition d (A specimen shall be illuminated with a bundle of rays whose beam axis angle forms an angle not exceeding 10° referring to the normal line to the specimen surface, and the light reflected in all directions shall be collected and received. The bundle of illuminating rays shall not include a ray deviating by 5° or more from its center line.) is preferred. As a simplified method, lightness and chroma may be determined by visual comparison with standard color chips prepared in accordance with JIS Z 8721 (1993).
Typically, ceramic balls formed of the sintered silicon nitride member of the present invention are applied to bearing balls used, for example, in machine tools and hard disk drives of computers, as well as to bearing balls used in high-temperature, corrosive environments, such as bearing balls of driving units of semiconductor equipment. This is because the ceramic balls have good corrosion resistance at high temperature. The inventive sintered silicon nitride member is also applicable to mechanical sliding components other than bearings, such as tappets used in automobiles. However, application of the present invention is not limited thereto. When the
Bahta Abraham
Jones Deborah
NGK Spark Plug Co. Ltd.
Sughrue & Mion, PLLC
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