Measuring and testing – Hardness – By penetrator or indentor
Reexamination Certificate
2001-04-25
2001-11-13
Williams, Hezron (Department: 2856)
Measuring and testing
Hardness
By penetrator or indentor
C073S078000
Reexamination Certificate
active
06314798
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to cutting tools and wear-resistant materials, such as bearing balls, which are formed of silicon-nitride-based sintered bodies.
2. Description of the Related Art
Conventionally, silicon nitride sintered bodies, which contain as a main component silicon nitride and have excellent strength, are used for cutting tools and wear-resistant parts (wear-resistant materials) such as bearing balls.
Recently, there has been proposed a technique for improving the wear resistance of a silicon-nitride sintered body used as a cutting tool, through a reduction in the amount of a sintering aide (a certain type of oxide) added to the main component thereof (see Japanese Kohyo (PCT) Patent Publication No. 8-503664).
Separately, in order to realize a cutting tool having high wear resistance, the present inventors have studied a technique for controlling the hardness of a cutting tool in the vicinity of its cutting edge. However, conventionally, such control is not performed in practice.
Conventionally, when the Vickers hardness of a cutting tool is measured, an indentor is pressed against a substantially central portion of the rake face of the cutting tool in order to measure the hardness, and the hardness of the cutting tool in the vicinity of the cutting edge is not measured. This is because, since measurement of Vickers hardness requires some area, accurate measurement of the hardness of a cutting tool in the vicinity of the cutting edge has been impossible.
SUMMARY OF THE INVENTION
The present invention addresses the above-described problems, and an object of the present invention is to provide cutting tools and wear-resistant materials which are formed of a silicon-nitride-based sintered body having excellent characteristics, such as high wear resistance, and a quality control method therefor.
The silicon-nitride-based sintered body or ceramic preferably contains silicon nitride in an amount of 50-99 wt. % and one or more other ceramics such as alumina, magnesia and yttria in an amount of 1-50 wt. %.
a) In the present invention, universal hardnesses of cutting tools and wear-resistant materials are measured through a so-called universal hardness test.
As shown in
FIG. 1
, in the universal hardness test, a load (test load) is applied onto an indentor in order to indent a sample surface. While this state (i.e., a state in which the indentor press-intrudes into the sample surface) is maintained, the depth of a resultant depression (depth of intrusion) is measured. The hardness of the sample is determined on the basis of the test load and the depth of intrusion.
The term “hardness” is defined as “resistance of a certain object against penetration of another, harder object.” The universal hardness test enables accurate measurement of hardness in which elastic deformation is taken into consideration, even for a material which undergoes a relatively large degree of elastic deformation.
That is, the universal hardness test is less likely to involve human errors as compared with a conventional method in which an indentor is pressed into a sample surface and is then removed, after which a resultant depression is observed under a microscope so as to obtain a hardness.
Further, the universal hardness test enables measurement of hardness of a sample in a smaller region, as compared with a conventional method for measurement of Vickers hardness.
b) Values of microhardness used in relation to the present invention are those obtained through a universal hardness test which is prescribed in German standard DIN 50359-1.
That is, hardness HU in the present invention is the universal microhardness HU measured through the above-described universal hardness test, and the microhardness H. Plast is called a universal plastic hardness.
(1) Among the two kinds of hardness, the universal hardness HU, as represented by the following equation (1), is a value [N/mm
2
] obtained through division of a test force F [N] by an area (=indentor surface area) A(t) [mm
2
] calculated from an intrusion depth t [mm] under application of the test force F.
Universal hardness HU=Test load F/Indentor surface area A(t) (1)
Since the tip end of an indentor is formed into a quadrangular pyramidal shape having an inter-surface angle &agr; of 136°, the indentor surface area A(t) is calculated from the intrusion depth (t) by the following equation (2), in which the geometry of the indentor is taken into consideration.
A
(
t
)
=
4
·
{
sin
(
α
/
2
)
/
cos
2
(
α
/
2
)
}
·
t
2
=
26.43
·
t
2
(
2
)
Accordingly, once a depth (t) to which the sample surface is intruded by the indentor subjected to the test load F is measured, the universal hardness HU is calculated in accordance with the following equation (3) derived from equations (1) and (2).
HU=F/(26.43·t
2
) (3)
(2) In contrast, the universal plastic hardness H. Plast is a value [N/mm
2
] obtained by the following equation (4), which corresponds to equation (3) with the intrusion depth t in equation (3) replaced with hr [mm].
H. Plast=F/(26.43·hr
2
) (4)
where, as shown in
FIG. 1
, hr is the intersection between the horizontal axis representing intrusion depth and a line tangential to an intrusion depth curve in the case where the test force F is maximum, or Fmax (in a region where the test force is lowered).
(3) In the present invention, the universal hardness HU and the universal plastic hardness H. Plast are values for the case in which the maximum test force (test load) Fmax is 1000 mN. The Vickers hardness (Hv) is a value for the case in which the maximum test force (test load) Fmax is 30 kgf.
According to one aspect, the present invention provides a cutting tool formed of a silicon-nitride-based sintered body, wherein a microhardness H. Plast as measured in the vicinity of a cutting edge of the cutting tool is 21.2 GPa or greater. Since the microhardness H. Plast in the vicinity of a cutting edge of the cutting tool is 21.2 GPa or greater, the cutting tool is excellent in terms of wear resistance, as is apparent from a test example, which will be described later.
Here, the phrase “in the vicinity of a cutting edge” preferably means a region having a width of 0.2 mm or less and extending from cutting edges—which form the sides of a cutting tool—including nose portions at comers), as indicated by hatching in FIG.
2
. This preferred definition will apply to the descriptions hereinafter.
Preferably the microhardness H. Plast as measured in the vicinity of the cutting edge is 22.5 GPa or greater. Since the microhardness H. Plast in the vicinity of the cutting edge of the cutting tool is 22.5 GPa or greater, the cutting tool is more excellent in terms of wear resistance, as is apparent from the test example, which will be described later.
According to another aspect, the present invention provides a cutting tool formed of a silicon-nitride-based sintered body, wherein a microhardness HU as measured in the vicinity of a cutting edge of the cutting tool is 11.2 GPa or greater. Since the microhardness HU in the vicinity of a cutting edge of the cutting tool is 11.2 GPa or greater, the cutting tool is excellent in terms of wear resistance, as is apparent from the test example, which will be described later.
Preferably the microhardness HU as measured in the vicinity of the cutting edge is 11.7 GPa or greater. Since the microhardness HU in the vicinity of the cutting edge of the cutting tool is 11.7 GPa or greater, the cutting tool is more excellent in terms of wear resistance, as is apparent from the test example, which will be described later.
The present invention further provides a cutting tool formed of a silicon-nitride-based sintered body, wherein a microhardness H. Plast as measured in the vicinity of a cutting edge is 21.2 GPa or greater, and a microhardness HU as measured in the vicinity of the cutting edge is 11.2
Kato Hideki
Nomura Makoto
Yoshida Hiroshi
NGK Spark Plug Co. Ltd.
Sughrue Mion Zinn Macpeak & Seas, PLLC
Williams Hezron
Wilson Katina
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