Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
1999-07-21
2001-11-13
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S403000, C428S404000, C428S332000, C428S697000, C428S698000, C428S699000, C428S704000
Reexamination Certificate
active
06316094
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a cubic boron nitride (cBN) sintered body, particularly to a cBN sintered body for use in cutting tools that are improved in wear resistance and chipping resistance.
2. Description of the Background Art
Because cBN is the substance next to diamond in hardness, cBN-base sintered bodies have been used in cutting tools, wear-resistant parts, impact-resistance parts, etc. This type of sintered body has difficulty in achieving both high hardness and high strength. Published Japanese patent applications Tokukosho 62-25630, Tokukosho 62-25631, and Tokukaihei 5-186272 have disclosed techniques aiming at the achievement of the compatibility of hardness and strength. The disclosed techniques, however, have not necessarily been sufficient in the compatibility of hardness and strength. For instance, single point tools comprising the foregoing sintered body, when used in high-speed cutting, allow the flank wear and crater wear to sharpen the cutting edge, resulting in easy chipping of the cutting edge. Even with ordinary cutting speeds, the tools tend to suffer chipping of the cutting edge from an impact when used in applications where strong impacts are applied as in interrupted cutting. Those drawbacks have lead to unstable tool life.
SUMMARY OF THE INVENTION
An object of the present invention is to offer a cBN sintered body superior in chipping resistance resulting from the optimization of the crater-wear resistance and mechanical strength. Another object of the present invention is to offer a cBN sintered body superior in chipping resistance resulting from the optimization of the impact resistance and mechanical strength.
The first sintered body that the present invention offers is a sintered body in which cBN particles are bonded through a bonding phase. The bonding phase has a two-dimensionally continuous structure. The bonding phase comprises at least one material selected from the group consisting of (a) carbide, nitride, carbonitride, or boride of a IVB-, VB-, or VIB group transition metal in the periodic table; (b) nitride, boride, or oxide of Al; (c) at least one kind of carbide, nitride, carbonitride, and boride of Fe, Co, or Ni; and (d) a mutual solid solution of those. The bonding phase has the thickness of which the average value is 1.5 &mgr;m or less and the standard deviation is 0.9 &mgr;m or less. In the above description, the thickness of the bonding phase means the distance between cBN particles on an arbitrarily drawn straight line in a sintered body. The percentage of the cBN content is 45 to 70% in volume. The cBN particles have an average particle size of 2 to 6 &mgr;m inclusive. The average particle size means the particle diameter at which the cumulative volume percentage reaches 50%.
The second sintered body that the present invention offers is a sintered body in which cBN particles are bonded through a bonding phase. The bonding phase has a two-dimensionally continuous structure. The bonding phase comprises at least one material selected from the group consisting of (a) carbide, nitride, carbonitride, or boride of a IVB-, VB-, or VIB group transition metal in the periodic table; (b) nitride, boride, or oxide of Al; (c) at least one kind of carbide, nitride, carbonitride, and boride of Fe, Co, or Ni; and (d) a mutual solid solution of those. The bonding phase has the thickness of which the average value is 1.0 &mgr;m or less and the standard deviation is 0.7 &mgr;m or less. In the above description, the thickness of the bonding phase means the distance between cBN particles on an arbitrary straight line in a sintered body. The percentage of the cBN content is 45 to 70% in volume. The cBN particles have an average particle size of not less than 0.01 &mgr;m and less than 2.0 &mgr;m. The average particle size means the particle diameter at which the cumulative volume percentage reaches 50%.
Conventional cBN sintered bodies have had a bonding phase with the thickness that varies considerably, producing portions in which the bonding phase alone constitutes a large volume. Because those portions are mechanically weak portions (defects) in a sintered body, cracks are prone to develop from those portions, resulting in an insufficient chipping resistance of the tool.
High-speed cutting in particular reduces the material strength because of the high temperature in a cutting edge. High-speed cutting also develops crater wear, sharpening the cutting edge and thus reducing the strength of the cutting edge. Such a condition allows at the crater-wear portion the generation of cracks, parallel to the cutting edge, caused by impacts applied to the cutting edge. The cracks grow by the interrupted applications of impacts, resulting in chipping.
Even with ordinary cutting speeds, an impact concentrates stresses at the foregoing defective portion when a tool is used in applications where impacts are applied as in intermittent cutting. This stress concentration generates a fracture at this mechanically-weak defective portion, resulting in the chipping of the cutting edge.
Considering the foregoing failure mechanism, the sintered body of the present invention is intended to improve the chipping resistance by reducing the thickness variations of the bonding phase in comparison with the conventional sintered bodies so that defective portions are diminished. When the average value and standard deviation of the thickness of a bonding phase exceed the foregoing specified values, the portions of which a large volume is constituted only by the bonding phase increase, decreasing the improving effectiveness on the chipping resistance. It is desirable that the bonding phase have a lower limit of about 0.2 &mgr;m in the average thickness in order to exert its functions.
If cBN particles have an excessively small diameter, the particles lose the heat resistance to allow easy development of wear; if an excessively large diameter, the cBN particles themselves cleave from an impact to allow chipping of the cutting edge, permitting the tool to lose the chipping resistance. Consequently, the first sintered body of the present invention should have the cBN particles with a particle size of 2 to 6 &mgr;m in order to be superior in heat resistance and suitable for high-speed cutting. Similarly, the second sintered body of the present invention should have the cBN particles with a particle size of not less than 0.01 &mgr;m and less than 2.0 &mgr;m in order to be superior in impact resistance.
The sintering material of the present invention can be obtained by coating cBN with a bonding phase material or by mixing materials with a special method. The coating of a bonding phase material is performed by one of the following methods:
(a) chemical vapor deposition (CVD) method, physical vapor deposition (PVD) method, or electroless plating before the sintering or
(b) utilization of mechanochemical reaction induced by compressive shearing force, frictional force, and impact force at the time of mechanical mixing.
The special mixing method is most suitably provided by the ultrasonic mixing method or by a dispersant-aided ball-mill (BM) method.
The sintering process for the sintering material of the present invention is carried out by plasma sintering equipment, hot-press equipment, or ultrahigh-pressure sintering equipment, for example.
BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the present invention are described below.
REFERENCES:
patent: 5639285 (1997-06-01), Yao et al.
patent: 0 712 941 (1996-05-01), None
patent: 0 879 806 (1998-11-01), None
patent: 2 320 725 (1998-07-01), None
patent: 58-060679 (1983-04-01), None
patent: 58-060678 (1983-04-01), None
patent: 62-25631 (1987-06-01), None
patent: 62-25630 (1987-06-01), None
patent: 5-186272 (1993-07-01), None
Fukaya Tomohiro
Kukino Satoru
Shiraishi Junichi
Jones Deborah
McDermott & Will & Emery
McNeil Jennifer
Sumitomo Electric Industries Ltd.
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