JPH10331857A - Ceramic rolling element and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase compressive strength so as to improve a rolling life by forming a rolling element such as a bearing into a sphere, a cylinder, or a cone made of ceramic and performing the formation so that substantially no void exits in the vicinity of the surface in the cross section while voids with specific diameters exist in the vicinity of the center. SOLUTION: A rolling element such as a ball 1 and a cylindrical roller is used for a rolling bearing such as a bearing. The ball 1 is formed of ceramic so as to be constructed of a surface peripheral part 11 possessing no void substantially and a center peripheral part 12 possessing voids 13 with maximum diameter of 1 μm or more. In this way, a compression stress of 1 kg/mm<2> or more can remain in the surface peripheral part 11, so that compressive strength can be increased, and a rolling life can be prolonged. In the production of this kind of rolling body 1, the ceramic material is molded and burnt under a pressurization condition so that its surface periphery is compacted with the voids 13 left in the vicinity of the center, and subsequently, a hot static pressure pressurizing processing is applied.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ceramic rolling element, and more particularly to a ceramic rolling element having excellent resistance to a static compressive load and excellent rolling life under a high load.

[0002]

2. Description of the Related Art In recent years, ceramic members have been used as materials for machine parts by taking advantage of their characteristics such as wear resistance, corrosion resistance, heat resistance and light weight. In particular, silicon nitride ceramics are expected to be used as materials for structural parts for automobiles and as balls for rolling elements of bearings due to their excellent properties.

When used as a rolling element of a bearing,
In order to increase the pressure resistance, when producing silicon nitride ceramics, after firing under normal pressure or under atmospheric pressure, the HI
It has been practiced to reduce voids by using P (hot isostatic pressing) treatment in combination. For example, Japanese Patent No. 2549636 discloses a rolling element made of silicon nitride ceramics having a void of 3 μm or less by performing HIP processing.

According to Japanese Patent Application Laid-Open No. Hei 6-48813, the surface of a ceramic rolling element is polished by a special method to generate a residual stress consisting of a tensile stress on the surface, thereby rolling the rolling element. It has been proposed to improve the life.

[0005]

SUMMARY OF THE INVENTION However, Japanese Patent Laid-Open No.
In the ceramic rolling element disclosed in Japanese Patent No. 48813,
Residual stress can only be left on the polished surface and has little effect on static compressive loads. Also,
As a matter of fact, when a residual tensile stress was generated in the ceramic rolling element, the rolling life could not be improved, and the rolling element had no effect.

On the other hand, in a ceramic rolling element having a reduced void as disclosed in Japanese Patent No. 2549636, although the pressure resistance can be increased to some extent, a high pressure resistance cannot be obtained in a stable manner. Therefore, higher pressure resistance is desired.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a ceramic rolling element with increased pressure resistance, improved rolling life in a high-load region, reduced variation in characteristics, and improved reliability.

[0008]

The inventors of the present invention have conducted various studies. As a result, if a residual stress consisting of not a tensile stress but a compressive stress is generated in the vicinity of the surface of a ceramic rolling element, the above object is achieved. Can be achieved. In addition, it is difficult to stably generate the residual compressive stress, but it has been found that a predetermined residual compressive stress can be stably generated near the surface by positively providing a void near the center of the rolling element. Thus, the present invention has been accomplished.

First, the present invention relates to a spherical, cylindrical or conical rolling element made of ceramics, wherein substantially no void exists in the vicinity of the surface in cross section, and a void having a maximum diameter of 1 μm or more is present in the vicinity of the center. Is present.

[0010] That is, by manufacturing so that a void exists near the center, a residual compressive stress can be stably generated near the surface, as will be described in detail later. Can be improved.

Further, the present invention is a spherical, cylindrical or conical rolling element made of ceramics, characterized in that a residual compressive stress of 1 kg / mm ² or more exists near the surface.

That is, when a static compressive load is applied to the ceramic rolling element, the maximum tensile stress generated thereby is located near the surface immediately below the load contact. for that reason,
By leaving compressive stress in a layer having a certain thickness near the surface of the ceramic rolling element, the tensile stress is reduced, and the pressure resistance can be increased. Further, even at the time of high-load rotation, the tensile stress applied to the surface can be similarly reduced, and the rolling life can be improved.

Further, the present invention provides a method of forming a ceramic raw material into a spherical or cylindrical shape, and then firing under pressure conditions to densify the vicinity of the surface while leaving voids near the center, and then hot working. The present invention is characterized by a method for manufacturing a ceramic rolling element, which comprises a step of applying a hydrostatic pressure treatment.

That is, in the firing step of ceramics, by firing under pressure conditions before densification, an atmosphere gas is trapped in the vicinity of the center of the sintered body so that voids remain, and thereafter, HIP processing is performed. Is applied, a predetermined amount of compressive stress is left near the surface by the action of the internal voids.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings.

As the rolling element of the present invention, there are a ball 1 shown in FIG. 1A, a cylindrical roller 2 shown in FIG. 1B, or a conical shape (not shown). Used for rolling bearings. The ball 1 and the cylindrical roller 2 are entirely made of ceramics and have a void near the center thereof.

That is, as shown in FIG. 2 which shows a cross section passing through the center of the ball 1, the ball 1 is composed of a portion 11 near the surface where substantially no void exists and a portion 12 near the center where a void 13 exists. ing. Therefore, as will be described in detail later, a compressive stress can be left in the surface vicinity portion 11, and as a result, the pressure resistance can be increased and the rolling life can be improved.

The phrase "substantially free of voids" in the vicinity of the surface 11 means that no voids exist or the maximum diameter is less than 1 μm even if voids exist. This is because if a void having a size of 1 μm or more exists in the vicinity 11 of the surface, the void tends to be a starting point to easily generate a crack, and the pressure resistance decreases.

On the other hand, in the vicinity 12 of the center, one or more voids 13 are present, and the maximum diameter of the voids 13 is 1 μm or more, preferably 3 μm or more. This is because if the void 13 in the central portion 12 is less than 1 μm, the effect of leaving a compressive stress in the surface near portion 11 is poor. However, if the voids 13 are too large, the mechanical properties may be degraded. Therefore, the maximum diameter is preferably set to 20 μm or less.

The thickness T of the portion 11 near the surface where substantially no voids exist is expressed by T / T with respect to the radius R of the rolling element 1.
It is preferable to set R to 10 to 90%. this is,
If the T / R is less than 10%, the strength of the ball 1 may be reduced due to the influence of the voids 13. On the other hand, if the T / R exceeds 90%, the effect of improving the pressure resistance due to the residual compressive stress is poor. The T / R range is preferably from 15 to 80%, and particularly preferably from 15 to 50%.

Although the cross section of the ball 1 has been described with reference to FIG. 2, the cylindrical roller 2 and other shapes may have the same structure as that of FIG. 2 in a circular cross section perpendicular to the central axis. good.

Generally, it is common sense to reduce the number of voids in order to improve the mechanical properties of ceramics in general, not limited to rolling elements. On the other hand, in the present invention, by virtue of the presence of the voids 13 in the central portion 12, the compressive stress remains in the surface near portion 11, the pressure resistance as a rolling element can be increased, and the rolling life is improved. They found what they could do.

The reason why the compressive strength can be improved by leaving a compressive stress in the vicinity of the surface 11 of the rolling element is as follows.

That is, when a static compressive load is applied to a rolling element such as the ball 1, the maximum tensile stress generated by the static compressive load is located in the vicinity of the surface 11 immediately below the load contact. Therefore, it is considered that, by leaving the compressive stress in the vicinity 11 of the surface, the above-described tensile stress is reduced, and the pressure resistance can be increased. Further, it is considered that even at the time of high-load rotation, the tensile stress applied to the surface vicinity portion 11 can be similarly reduced, and the rolling life can be improved.

In order to obtain such an effect,
The residual compressive stress in the vicinity of the surface of the rolling element is 1 kg / m
m ² or more, preferably 2 kg / mm ² or more.

Further, the present invention has been found that it is sufficient that residual compressive stress is present in the vicinity 11 of the surface of the rolling element as described above.
Methods other than the presence of 3 can also be used.

For example, from the inside to the surface of the ceramics forming the rolling elements, crystals having a high melting point are gradually distributed at a high density as a grain boundary phase, so that residual compressive stress exists in the vicinity 11 of the surface, or By rapidly lowering the temperature after firing the ceramics, residual compressive stress can also be present in the vicinity 11 of the surface. Even if these methods are used, if a residual compressive stress of 1 kg / mm ² or more is present in the vicinity 11 of the surface, an excellent effect as a rolling element can be obtained.

In the present invention, the diameter of the void 13 is determined by processing the cross section of the rolling element by image analysis and calculating the equivalent circle diameter of the existing void 13.

Specifically, in the case of the ball 1, a cross section passing through the center, and in the case of the cylindrical roller 2, a circular cross section perpendicular to the central axis is mirror-polished, and a micrograph of about 1000 times is taken. Using this photograph, an image is analyzed by an image analysis device such as Luzex under the condition that a void having an equivalent circle diameter of 1 μm or more can be detected. Then, in the cross section as shown in FIG. 2, the distribution state of the voids 13 of 1 μm or more is examined.

Further, the existence and magnitude of the residual compressive stress in the vicinity of the surface of the rolling element can be determined by measuring the crystal distortion due to the stress by X-ray diffraction by the following method.

That is, when the lattice spacing d of the crystal changes by Δd, the X-ray diffraction angle θ changes by Δθ. Since the change Δθ of the diffraction angle cannot be directly known, the diffraction angle 2θ is obtained for some incident angles ψ, and is calculated as sin ²
plotted against 、 and its gradient {2θ / {sin ² }
(Tan α), the residual stress α (kg /
mm ² ) is given by α = -E / 2 (1 + ν) · cot θ · ∂2θ / ∂sin
² ψ where E is the elastic constant (kg / mm ² ) and ν can be calculated from Poisson's ratio.

The ceramics forming the rolling element of the present invention include alumina (Al ₂ O ₃ ), zirconia (Zr
O ₂ ), silicon carbide (SiC), silicon nitride (Si ₃ N)
₄ ) A material containing aluminum nitride (AlN) or the like as a main component and a known sintering aid or the like is used.

Of these, Si ₃ N ₄ is the main component, and Al ₂ O ₃ , Y ₂ O ₃ , and Yb _{2 are} used as sintering aids.
Silicon nitride ceramics containing 1 to 20% by weight of metal oxides such as O ₃ and MgO are preferred.

Next, a method of manufacturing a ceramic rolling element according to the present invention will be described.

First, the above-mentioned various ceramic raw material powders are mixed with a predetermined binder component, and the mixture is formed into a predetermined shape as shown in FIG. 1 by a known molding method such as press molding or extrusion molding.

Next, the molded body is fired.
It is important to fire under pressure conditions. In particular,
At the start of firing, or at least before the ceramic is densified, firing is performed while supplying a gas such as nitrogen or argon at 10 to 100 atm. By doing so, the densification proceeds in the vicinity of the surface of the ceramics, but the atmospheric gas is trapped in the vicinity of the center of the inside, the densification is hindered, and a void remains.

The sintering is completed in this state, and then HI
P processing is performed. The conditions at this time are preferably 1000 to 2000 atm and 1500 to 2000 ° C. Then, HIP (hot isostatic pressing) is applied near the surface of the rolling element.
During the process, pressure is applied from the surface and also from the inside by the pressure of the gas in the void existing near the center. is there.

By changing the conditions of the firing step and the HIP processing step, it is possible to adjust the diameter of the existing voids, the thickness T near the surface where the voids do not substantially exist, and the like.

The rolling element of the present invention is a general term for a member that performs a rolling motion while supporting another member. In addition to the above-described bearing ball 1 and cylindrical roller 2, for example, a ball screw ball is used. It can be used for various purposes.

[0040]

Embodiments of the present invention will be described below.

First, 10% by weight of Yb ₂ O ₃ powder and 3% by weight of Al ₂ O ₃ powder were added to Si ₃ N ₄ powder as sintering aids, respectively, and thoroughly mixed with water to obtain a raw material slurry. It was adjusted. Next, an organic binder was added to the slurry, and raw material granules were obtained by spray drying. next,
Using the raw material granules, a spherical molded body was produced by press molding, and the molded body was heated in a nitrogen atmosphere to decompose and degrease the organic binder.

Next, the compact was fired in a nitrogen gas atmosphere under pressure. At this time, as shown in Table 1, pressurized pressure,
The temperature at which pressurization was started and the firing temperature were each changed.

Thereafter, HIP treatment is performed at 1700 ° C. under a pressure of 1000 to 2000 atm, and the diameter is 9.5 m.
m ball 1 was obtained.

As a comparative example, a sample which was not pressurized at the time of sintering under an atmospheric pressure (No. 1) or a sample which was densified after reaching a sintering temperature and fired by applying a pressure (No. 1).
2) was subjected to HIP treatment to produce a similar ball.

The residual stress in the vicinity 11 of the surface of the obtained ball 1 was measured by X-ray diffraction at the lattice spacing of the crystal which changes in proportion to the magnitude of the residual stress as described above (sin ² ψ method). ).

The diameter of the void 13 was determined by polishing a cross section passing through the center of the ball 1 using a 4000-diameter diamond paste, taking a 1000-times enlarged photograph with a scanning microscope, and using an image analyzer (Luzex). And evaluated. Then, the maximum diameter of the void 13 existing in the vicinity 12 of the center and the thickness T of the surface vicinity 11 where no void 13 of 1 μm or more was present were measured.

Further, the crushing strength of the ball 1 was evaluated by a load when two balls 1 having the same size were stacked and subjected to a compressive load to break.

The results of these evaluations are as shown in Table 2. From these results, it was found that the sample that was not pressurized during firing (N
o. 1) and those that started pressurizing after densification (No.
In Comparative Examples 2) and the like, no void 13 exceeding 1 μm was present in the vicinity 12 of the center, and as a result, no residual compressive stress was present in the vicinity 11 of the surface. Therefore, the crushing strength was as low as 2.0 tonf.

On the other hand, in the examples of the present invention (Nos. 3 to 8) in which the pressing was started before densification during firing, voids 13 of 1 μm or more could be generated in the vicinity 12 of the center. . As a result, 1 kg / mm ²
It was found that the above residual compressive stress can be generated, and the crushing strength can be as high as 2.7 to 3.4 tonf.

No. In Examples 3 to 8 of the present invention,
By changing the firing temperature, it was possible to change the maximum diameter of the voids 13 and the thickness T of the portion 11 near the surface where no voids of 1 μm or more exist. And void 13
It can be seen that the larger the maximum diameter of the steel and the smaller the thickness T of the surface vicinity 11, the larger the residual compressive stress and the higher the crushing strength.

In particular, when the maximum diameter of the void 13 is 3 μm or more (Nos. 3 to 7), the residual compressive stress is 2 kg / mm.
² or more, and the ratio T / R between the thickness T and the radius R of the surface vicinity portion 11 where no voids exist is 15 to 50%.
(No. 3 to 6), the residual compressive stress is 3 k
g / mm ² or more, the crushing strength could be 3 tonf or more, which was preferable.

[0052]

[Table 1]

[0053]

[Table 2]

Next, the rolling life of the ball was evaluated using each of the samples shown in Table 1. Specifically, using a thrust tester, the Hertz stress applied to each ball is 5.88.
The ball was continuously tested for 400 hours under the condition of GPa, and the time (rolling life) until the surface was damaged and the test could not be continued was measured.

As shown in FIG. 3, the rolling life of the comparative example having no residual compressive stress was about 100 hours, whereas the rolling life was about 100 hours in the embodiment of the present invention having the residual compressive stress of 1 kg / mm ² or more. It can be seen that the life can be extended to 300 hours or more.

Although only the rolling elements made of silicon nitride ceramics have been described in the above embodiments, similar results were obtained with rolling elements made of other various ceramics.

[0057]

As described above, according to the present invention,
In a spherical, cylindrical or conical rolling element made of ceramics, substantially no voids exist near the surface in the cross section, and a void having a maximum diameter of 1 μm or more exists near the center, so that 1 kg / mm ² or more of compressive stress can be left, and as a result, the pressure resistance can be increased and the rolling life can be improved.

According to the present invention, the ceramic raw material is formed into a spherical, cylindrical or conical shape, and then fired under a pressurizing condition so as to densify the vicinity of the surface while leaving a void near the center. Then, by manufacturing the ceramic rolling element from the step of performing the hot isostatic pressing, a rolling element having a residual compressive stress can be stably manufactured by a simple process.

[Brief description of the drawings]

FIGS. 1A and 1B are perspective views showing an embodiment of a ceramic rolling element according to the present invention.

FIG. 2 is a sectional view taken along line XX in FIG.

FIG. 3 is a graph showing a relationship between a residual compressive stress of a ceramic rolling element and a rolling life.

[Explanation of symbols]

 1: ball 2: cylindrical roller 11: near surface 12: near center 13: void

Claims (3)

[Claims] A spherical, cylindrical or conical rolling element made of ceramics, wherein substantially no voids exist near the surface in the cross section, and voids having a maximum diameter of 1 μm or more exist near the center. A ceramic rolling element characterized in that: 2. A spherical, cylindrical or conical rolling element made of ceramics, wherein 1 kg / mm ²
A ceramic rolling element characterized by having the above residual compressive stress. 3. A ceramic raw material is formed into a spherical or cylindrical shape, and then fired under a pressurizing condition so as to densify the vicinity of the surface while leaving a void near the center. A method for manufacturing a ceramic rolling element comprising a step of performing a treatment.