JPH02124771A - Silicon nitride-base sintered body for rolling bearing material and its production - Google Patents

Abstract

PURPOSE:To obtain the title sintered body providing bearing material excellent in rolling contact fatigue life by mixing prescribed amounts of powdered sintering auxiliary with powdered silicon nitride, applying precompacting and presintering to the resulting powder mixtuer, and then subjecting the presintered compact to HIP treatment under specific pressure and temp. conditions. CONSTITUTION:A powder mixture is prepared by mixing >=70wt.% powdered silicon nitride and <=30wt.% powdered sintering auxiliary (e.g., Y2O3), and the above powder mixture is precompacted and then presintered under a nitrogen- containing nonoxidizing atmosphere of <=300atm. Subsequently, HIP(hot isostatic pressing) treatment is applied to the above under a nitrogen-containing nonoxidizing atmosphere of >=300atm at a temp. higher by >=200 deg.C than the temp. at which liquid phase occurs in the precompacted body, by which the silicon nitride-base sintered body for rolling bearing material can be obtained. Since the resulting sintered body is free from pores and a segregation zone of the sintering auxiliary components in which maximum length exceeds 10mum, the occurrence of breakage due to the above can be prevented.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The first invention relates to a silicon nitride-based sintered body for rolling bearing materials, and the second invention relates to a method for manufacturing the same. The present invention relates to a silicon nitride-based sintered body for bearing materials and a method for manufacturing the same.

[Conventional technology]

Conventionally, bearing steel has mostly been used as a bearing material, but since bearing steel has a high specific gravity, there are problems such as large centrifugal force and shortened life, especially when used in high-speed rotation bearings. Ta. Therefore, in recent years, silicon nitride-based sintered bodies, which have low specific gravity, high mechanical strength, and excellent heat resistance and wear resistance, have come into use.

[Problem to be solved by the invention]

However, conventional silicon nitride-based sintered bodies have the drawback of having greater variation in rolling fatigue life and lack of reliability than bearing steel.

If there are pores in the sintered body, it becomes a starting point for failure due to rolling fatigue. Therefore, as the silicon nitride-based sintered body, a sintered body subjected to HIP treatment to collapse and eliminate the pores in the sintered body has come to be used as a bearing material.

However, the rolling fatigue life of sintered bodies subjected to HIP treatment is improved to some extent compared to sintered bodies with many remaining pores, but it is still not sufficient, and the variation in life is also not sufficient. A sintered body with excellent fatigue life and reliability and a method for manufacturing the same have been desired.

The present invention has been made in view of the above points, and as a result of various experiments, it was possible to identify the defects that caused the failure of samples that had a short life in the rolling fatigue life test of HIP sintered bodies, and to identify the defects. It was completed by newly elucidating the generation mechanism.

That is, an object of the present invention is to provide a silicon nitride-based sintered body for a rolling bearing material, which has a long rolling fatigue life and little variation in its life, and a method for manufacturing the same.

[Failure to solve the problem]

The silicon nitride-based sintered body for rolling bearing materials of the present invention is composed of 70% by weight (hereinafter simply referred to as %) or more of silicon nitride and 30% or less of a sintering aid, and this sintered body contains: It is characterized in that it is substantially free of pores and that the maximum length of the segregated portion consisting mainly of the sintering aid does not exceed 10 μm.

This sintered body is a preformed body made of a mixed powder of 70% or more silicon nitride powder and 30% or less sintering aid powder.
The first step is pre-sintering in a non-oxidizing atmosphere containing nitrogen at 00 atm or less, and then 200"C or more above the temperature at which a liquid phase occurs in the preform under a non-oxidizing atmosphere containing nitrogen at 300 atm or more. It can be manufactured by performing the second step of performing HIP treatment under high temperature.

In the above, if the silicon nitride content is less than 70%, the strength decreases and the material cannot be used as a bearing material. The sintering aid may be selected from various types depending on the purpose and use, but usually alumina, ittria, magnesia, zirconia, rare earth element oxides, chromium oxide, etc. are used.

The segregated portion is composed of not only the sintering aid but also impurities normally contained in silicon nitride powder (for example, silica, etc., which is usually mentioned). The maximum length of the segregated portion does not exceed 10 μm. Here, "length" means the maximum length of the segregated part, for example, if it is an elliptical shape, its major axis, other rod shapes,
In the case of a rectangular parallelepiped shape, it means its maximum diagonal length.

If this maximum length exceeds 108 m, damage will occur at this point and its life will be shortened. The specific explanation is as follows. That is, in a bearing, for example, the rolling elements rotate while contacting the inner and outer rings, and if a load is applied at this time, contact stress acts on the contact portion. The repetition of this stress causes cracks to occur in the material, eventually leading to partial break-off. At this time, if there are defects such as pores or segregated areas in the material, stress tends to concentrate, and it is thought that the rolling fatigue life will be shortened. Since there are virtually no pores in the HIP sintered body, the HIP
Segregated areas of green sintered P are recognized as defects that cause short life. At this time, although the segregated area is a defect, it is filled with substances, so the concentration of stress is slower than in the case of pores, and the effect is not as acute as in the case of pores, so the maximum length of the segregated area is 10 μm or less. In this case, in practice, the influence cannot be tolerated in the life test. In addition to the sintering aid, even when additives that do not participate in the formation of the liquid phase are added, or when a part of the sintering aid reacts or precipitates during the sintering process to form a third phase, The finer the particle size, the less influence it has on rolling life.

The atmosphere in both of the above steps may be a non-oxidizing atmosphere that contains nitrogen and does not contain oxygen, and may also contain an inert gas such as helium or argon. The reason for "containing nitrogen" is to suppress thermal decomposition of silicon nitride. Usually, a nitrogen atmosphere is used in both steps. The atmospheric pressure for preliminary sintering may be 300 atmospheres or less. This is because if the pressure exceeds 300 atm, the pores become difficult to collapse during the HIP treatment, making it easier for the pores to remain in the sintered viscous material. This pressure is preferably 1 to 10 atmospheres. This is because the pressure during pre-sintering affects the density unevenness of the pre-sintered body, and the higher the pressure, the worse the densification of the central part of the sintered body, which increases the amount of pores. The following degree is preferable. If the pressure is less than 1 atm, thermal decomposition of silicon nitride tends to occur, which is not preferable.

In addition, if the ambient air pressure during the HIP treatment is less than 300 atm, the pores will be difficult to collapse and will tend to remain. The treatment temperature is 200° below the temperature at which the liquid phase occurs.
℃ or higher, preferably about 250 to 300℃ higher. The temperature at which a liquid phase occurs is the temperature at which the shrinkage rate of the molded body suddenly increases or the amount of transition from the α-type to the β-type of silicon nitride in the molded body rapidly increases during the heating process of the molded body. be. This treatment temperature varies depending on the composition of the liquid phase, but
When alumina and ittria are used as sintering aids, the temperature is about 1650°C or higher, and when magnesia is used as a sintering aid, the temperature is about 1550°C or higher.

[Effect]

In a bearing, the rolling elements rotate while contacting the inner and outer rings, and if a load is applied at this time, contact stress is exerted at the contact portion. At this time, if there are defects such as pores or segregation in the material, stress tends to concentrate. The inventors have discovered that H.I.P.
After conducting various studies on processed silicon nitride-based sintered bodies that are substantially free of pores, we found that 10
It has been newly discovered that the lifespan is shortened because damage originates from segregated parts larger than μm. At this time, although the segregated portion is a defect, the concentration of stress is less severe than in the case of pores, which are filled with substances, and it does not affect the stress as sharply as in the case of pores. Therefore, if the maximum diameter of the segregated portion is 10 μm or less, no effect is actually observed in the life test.

Therefore, from the above point of view, the sintered body of the first invention is substantially free of pores and does not have segregated portions with a maximum length exceeding 10 μm.

The HIP sintering method uses 300 atmospheres or more, usually 1000 to 20
This is a method in which a high gas pressure of about 1,000 atmospheres is applied to the object to be treated to eliminate pores in the object. - Since it is difficult to sinter the silicon nitride by itself, oxides such as alumina are usually added as sintering aids, and these sintering aids and impurities contained in the silicon nitride powder are combined. Sintering occurs through a liquid phase generated by reaction with silica. Therefore,
A sintered body made of such silicon nitride and a liquid phase component is HI
When performing P treatment, pressure also acts on the fluid liquid phase, so if there are pores in the material to be treated, the liquid phase is forced into the pores, changing the liquid phase composition. A segregated part will be generated. If the HIP temperature is low and the sinterability is poor, the pores will disappear, but their influence will remain as segregated areas.

On the other hand, if the HIP treatment temperature is high and the sinterability of silicon nitride through the liquid phase is high, silicon nitride will precipitate into the pores,
Because growth occurs, no segregation occurs.

From the above, in the second invention, since preliminary sintering is performed in an atmosphere of 300 atmospheres or less, the pores are easily crushed by the subsequent HIP treatment. In the subsequent HIP treatment, the treatment is carried out at a pressure of 300 atm or higher and at a temperature 200°C or higher higher than the temperature at which the liquid phase is generated, so that the liquid phase is forced into the pores and the segregated parts are generated. The maximum length is as small as 10 μm or less.

〔Effect of the invention〕

The silicon nitride-based sintered body of the first invention has no pores and no segregation of auxiliary components with a maximum length of more than 10 μm, so no breakage occurs due to pores, and therefore it has excellent rolling fatigue life and This is a rolling bearing material with less variation in service life.

According to the manufacturing method of the second aspect of the present invention, as described above in [Operation], a silicon nitride-based sintered body for a rolling bearing material having this effect can be manufactured.

〔Example〕

Hereinafter, the present invention will be specifically explained with reference to Examples.

Example 1 900 parts by weight of silicon nitride powder with an average particle size of 0.6 μm Added 5 parts by weight each of alumina and ittria as sintering aids,
After mixing in a ball mill for 24 hours, an organic binder (
Microcrystalline wax) 6 stacked parts were added, and the mixture was dried and granulated using a spray drying method. Trichloroethane was used during mixing and as a solvent for the binder. After pre-sintering this powder into a spherical shape in a mold, the pressure was 1 t.
Rubber press under on/cm2, diameter approximately 11
A spherical molded body was obtained.

After degreasing this molded body, it was heated to 1700 m
Pre-sintering was performed at ℃ for 4 hours, and the relative density was 95%.
A preliminary sintered body was obtained. This preliminary sintered body was subjected to HIP treatment under the conditions shown in Table 1 to produce dense sintered bodies (Nα1 to 4). In addition, in this composition, the temperature at which a liquid phase occurs is about 1450°C, and the processing temperature in the example of the present invention is 200°C.
In the comparative example, the temperature is 150° C. higher or lower.

The sintered body after HIP treatment is polished and processed to have a diameter of 9.52 mm.
mm, surface roughness 0. A ball sample of OIAlmRa was manufactured. The surface roughness was evaluated as the centerline average roughness obtained by measuring the ball on its equator using a stylus-type surface roughness measuring device. Regarding this sample, the density of the sintered body after the IP treatment, the rolling fatigue life, and the defects that caused the breakage were observed, and the results are shown in Table 1. This life test was conducted using a thrust type bearing life tester by rotating a flat plate made of bearing steel while applying a load to three balls. The load was 150 kg per ball, the rotation speed was 2000 rpm, and the lubrication conditions were a turbine oil bath, and the test was carried out for a maximum of 300 hours.

In addition, observation of the above defects is performed using E PMA (Elect
ron Pr.

It was carried out using an electron microscope equipped with a micro-analysis. A normal part and an abnormal part which is a defect can be distinguished from each other because they look different under electron microscope observation, and elemental analysis of this abnormal part was performed using EPMΔ.

According to the results, the ball of the invention example (NCL1, 2) did not break even after 300 hours of testing, and its lifespan was significantly extended. On the other hand, a comparative example with a low HIP treatment temperature (Nα3
．． In 4), damage (peeling) occurred in one ball in each case, and the defect that became the starting point was segregation of Ai' and Y elements, which are auxiliary components. The maximum lengths of the segregated portions that caused the damage were 20 μm and 15 μm, respectively, and both were 15 μm or more. Especially when the segregation part is 20 μm (
For Nα3), peeling occurred in just 58 hours, clearly indicating that the larger the segregation area, the shorter the life.

Example 2 In this example, experiments were conducted by changing the type of sintering aid, the amount added, HIP treatment conditions, life test conditions, etc.

A mixed powder consisting of 92 parts by weight of silicon nitride, 2762 parts by weight of alumina and magnetite, and 4 parts by weight of ittria,
Obtained in the same manner as in Example 1. Using this mixed powder, ball-shaped and disc-shaped molded bodies are manufactured, which are degreased.
Thereafter, preliminary sintering was performed at 1650° C. for 2 hours in a nitrogen atmosphere of 1 atm, followed by HIP treatment under the conditions shown in Table 1 to produce sintered bodies (Nα5 to 8). In this composition, the temperature at which a liquid phase occurs is about 1350°C, and the processing temperature in the present invention example is 200°C or higher, and in the L comparative example, it is only 150°C higher.

This sintered body was processed to create a ball with a diameter of 9.52 mm and a disk-shaped sample with a diameter of 60 mm, and life evaluation was performed in the same manner as in Example 1. The results are shown in Table 1. The test was conducted at a maximum contact stress of 600 kg/mm' and a rotation speed of 110.
The test was carried out for up to 1000 hours under the condition of 00 rpm.

According to the results, none of the samples of the present invention examples (k5 to k7) were damaged even during the long test of 1000 hours. On the other hand, in Comparative Example Nα8 in which the HIP treatment temperature was low, ball peeling occurred in 305 hours, and the starting point was a segregated part consisting mainly of the sintering aid component and having a maximum length of 30 μm.

Example 3 In this example, experiments were conducted by varying the type of sintering aid, the amount added, the preliminary sintering conditions, and the IIP treatment conditions.

Silicon nitride powder with an average particle size of 0.6 μm and a sintering aid in the proportions shown in Table 2 were mixed in the same manner as in the previous example, dried, and molded into test pieces of 6 x 10 x 30 mm using a mold press. A molded body was obtained. This molded body was subjected to preliminary sintering and HIP treatment under the conditions shown in Table 2 to obtain a sintered body sample. The density and bending strength of the obtained sintered body were measured, and the results are shown in Table 2. Incidentally, in all of these invention examples Nα9 to Nα12, no segregation was observed, and the sintered body structure was homogeneous. This bending strength measurement was performed using a #160 diamond grindstone with a sample of 8 mm width and 4 m thickness.
After grinding to a diameter of m, three-point bending was performed with a span of 20 mm and a crosshead speed of 0.5 mm/min.

According to this result, invention examples Nos. 9 to 12 all have a relative density of 100% (and a bending strength of 850 to 100%).
It is large at 1350MPa. Further, good results were obtained when the pre-sintering pressure was 1 atm or 10 atm.

On the other hand, Comparative Example N was subjected to IP treatment (under low pressure).

Both Nos. 13 and 14 have low relative densities, are not sufficiently densified, and pinholes remain. Therefore, these are considered to have a short lifespan.

In addition, Comparative Examples Nα13 to Nα15 all have low bending strength, especially Comparative Example Nl1 with a low silicon nitride ratio of 65%.
Although 15 is densified, its strength is very low at 600 MPa. From the above, this comparative example Nα13 to 15 (
In particular, 15) is considered to be easily damaged and is considered undesirable as a bearing material.Patent applicant: NGK Spark Plug Co., Ltd. Agent

Claims (2)

[Claims] (1) A silicon nitride-based sintered body composed of 70% by weight or more of silicon nitride and 30% by weight or less of a sintering aid, the sintered body having substantially no pores; A silicon nitride-based sintered body for a rolling bearing material, characterized in that the silicon nitride-based sintered body comprises at least the sintering aid of the impurities and sintering aid in the silicon nitride and does not have a segregated part having a maximum length exceeding 10 μm. (2) A preformed body made of a mixed powder of 70% by weight or more of silicon nitride powder and 30% by weight or less of sintering aid powder,
A first step of pre-sintering in a non-oxidizing atmosphere containing nitrogen at a pressure of 00 atm or less, followed by a step of pre-sintering in a non-oxidizing atmosphere containing nitrogen at a pressure of at least 300 atm and at a temperature at least 200°C higher than the temperature at which a liquid phase occurs in the preform. HIP (Hot Isostatic Press) under temperature
A method for producing a silicon nitride-based sintered body for a rolling bearing material, the method comprising: a second step of performing a treatment.