JP4693374B2 - Manufacturing method of sintered silicon nitride - Google Patents

Description

The present invention relates to a method for producing a silicon nitride body mainly composed of silicon nitride, and in particular, has a uniform crystal structure and a small variation in grain boundary strength, such as bearing rolling elements such as bearing balls, cutting tools, rolling jigs, The present invention relates to a method for producing a silicon nitride sintered body that can stably exhibit excellent mechanical strength, wear resistance, and rolling life characteristics when used as a constituent material for a sliding member or the like.

  Conventionally, a metal material such as bearing steel has been generally used as a bearing (bearing) member for supporting a rotating shaft, particularly as a component of a bearing ball. However, since metal materials such as bearing steel do not have sufficient wear resistance, variations in bearing life will increase in product fields that require high-speed rotation of 5,000 rpm or more, such as electronic equipment. There was a problem that a reliable high-speed rotation drive could not be obtained stably.

  As a means for solving the above-mentioned problems, recently, an attempt has been made to use a silicon nitride sintered body as a component of a bearing ball. Since the silicon nitride sintered body has excellent sliding characteristics among ceramics, the wear resistance is sufficient in some usage modes, and even when high-speed rotation is performed, highly reliable rotational driving is performed for a certain period of time. It has been confirmed that this can be realized.

Known sintered compositions of silicon nitride sintered bodies include silicon nitride-rare earth oxide-aluminum oxide system, silicon nitride-yttrium oxide-aluminum oxide-aluminum nitride-titanium system, and the like. Sintering aids such as rare earth oxides such as yttrium oxide (Y ₂ O ₃ ) in the above-mentioned sintering composition have been conventionally used as sintering aids, and the sintered body is improved by increasing the sinterability. It is added to increase the strength.

Further, as a conventional silicon nitride sintered body used for a rolling bearing member that requires wear resistance, particularly excellent sliding characteristics, for example, a predetermined amount of rare earth oxide, MgAl ₂ O ₄ spinel, silicon carbide, A silicon nitride ceramic containing an oxide such as Ti, Zr, Hf, having a porosity of 1% or less, a three-point bending strength of 900 MPa or more, and a fracture toughness value of 6.3 MPa · m ^1/2 or more. A wear-resistant member made of a bonded body has been proposed (see, for example, Patent Document 1).

In addition, in order to form a homogeneous sintered body with reduced generation of defects that impair the wear resistance by reducing impurities, the conventional sintered body is generally synthesized as a raw material powder by, for example, an imide pyrolysis method Manufactured using high-purity silicon nitride fine powder.
JP 2003-34581 A

  However, the present inventors have clarified that the sliding characteristics of the bearing ball are greatly influenced by the surface accuracy and homogeneity of the sintered body. In conventional silicon nitride bearing balls, it is necessary to use a high-purity silicon nitride raw material in order to suppress and control the segregation of the auxiliary component that affects the surface accuracy.

  In addition, conventional silicon nitride sintered bodies used for rolling bearing members are manufactured using expensive raw material powders synthesized by the imide pyrolysis method, and have mechanical strength and fracture toughness values. Since it is too high, there is a problem that workability after sintering is poor and an increase in manufacturing cost of the wear-resistant member product is unavoidable.

  In addition, the silicon nitride sintered body produced by the above-described conventional method has improved bending strength, fracture toughness value, and wear resistance, but is insufficient in terms of rolling characteristics and durability particularly required as a bearing member. Further improvements are required. In particular, the correlation between the lifetime of silicon nitride bearing balls and the effect of additive components has not been fully investigated, and measures to extend the lifetime are being sought.

  In recent years, the demand for ceramic materials as components for precision equipment has increased. In such applications, the characteristics of ceramics, which are high hardness, light weight, and excellent wear resistance, are combined with the properties of high corrosion resistance and low thermal expansion. It's being used. In particular, from the viewpoint of high hardness and wear resistance, the use as a wear resistant member constituting a sliding portion such as a bearing is rapidly expanding.

  However, when a rolling ball such as a bearing is made of a ceramic wear-resistant member, the rolling life of the wear-resistant member is still not sufficient when the rolling ball rolls while repeatedly contacting at a high stress level. Because the surface of the wear-resistant member peels off or cracks due to short-term operation, it is easy to cause an accident that causes vibration or damage to the device equipped with the bearing. There was a problem that durability and reliability as a component material were low.

The present invention has been made to cope with the above-described problems, and is formed by using low-purity and inexpensive silicon nitride raw material powder, such as silicon nitride powder produced by a metal nitriding method. Even in such a case, it is possible to control the dispersion state of the auxiliary component, and it is possible to reduce the variation in the grain boundary strength, which is uniform and mechanical strength equal to or higher than that of the conventional silicon nitride sintered body. Another object of the present invention is to provide a method for producing a silicon nitride sintered body suitable as a rolling bearing member having excellent workability in addition to wear resistance and rolling life characteristics.

  In order to achieve the above-mentioned object, the present inventor, when producing a conventional silicon nitride sintered body, the type of silicon nitride raw material powder, the amount of impurities, the sintering aids and additives generally used. The effect of these elements on the properties of the sintered body was confirmed by experiments by changing the type and amount of addition, the mixing conditions of the raw material powder, and the firing conditions.

  As a result, an inexpensive and fine silicon nitride raw material powder synthesized by a metal nitriding method or the like is added to a rare earth element, an aluminum component such as aluminum oxide or aluminum nitride, an oxygen element, and Ti, Hf, Zr, W as required. , Mo, Ta, Nb, Cr, when a raw material mixture to which at least one selected from the group consisting of Mo, Ta, Nb, and Cr is added in a predetermined amount is uniformly mixed and then molded and sintered, and further after sintering When processed under hot isostatic pressing (HIP) conditions, in addition to the same or better density, mechanical strength, wear resistance, and rolling life characteristics as conventional silicon nitride sintered bodies, it has excellent workability. The knowledge that a silicon nitride sintered body suitable as a rolling bearing member can be obtained was obtained.

  In particular, the raw material powder for one lot is divided into a plurality of parts in advance, and each divided raw material powder is sufficiently mixed individually, and then the raw material powders are combined to form a single raw material body. When molded and sintered, aggregation of sintering aid powders was suppressed, and a silicon nitride sintered body having a homogeneous structure was obtained. In particular, it was found that the diameter of the segregation and agglomeration part of the sintering aid component present in the silicon nitride sintered body can be made 30 μm or less, and a sintered body with few surface defects and excellent wear resistance can be obtained.

  In addition, after heating the silicon nitride molded body in a vacuum atmosphere to a temperature of about 1500 ° C. in the pre-stage of the sintering process, the silicon nitride compact is switched to a non-oxidizing atmosphere and pressure sintered at a predetermined temperature, so that silicon nitride is obtained. It was also found that the β conversion ratio can be 95% or more, and the sliding characteristics of the sintered body can be effectively improved.

  Furthermore, by adding a predetermined amount of Ti element to the sintered body and causing the Ti element to exist as compound particles such as TiN and TiCN, variation in the grain boundary strength of the sintered body is reduced, and the repeated stress of the sintered body is reduced. It has become possible to significantly increase the resistance to the occurrence of a bearing ball with excellent durability.

  The present invention has been completed based on the above findings.

That is, the silicon nitride sintered body obtained by the method of the present invention contains 3% by mass or less of a rare earth element, 3% by mass or less of Al element, and 5% by mass or less of oxygen element as a sintering aid component. A silicon nitride sintered body having a β conversion ratio of 95% or more, and a silicon nitride needle shape in which the ratio of the major axis to the minor axis of the silicon nitride crystal grains is 2 or more in the crystal structure of the silicon nitride sintered body The area ratio of crystal grains is 50% or more.

In the silicon nitride sintered body obtained by the method of the present invention , the β conversion ratio of silicon nitride needs to be 95% or more. Β-phase silicon nitride having a hexagonal crystal structure is superior to α-phase silicon nitride in terms of stability at high temperatures and structural strength, and improves the required properties of wear-resistant materials. It is suitable. Therefore, excellent wear resistance can be ensured by increasing the β conversion ratio of the silicon nitride to 95% or more.

The β conversion ratio of the silicon nitride sintered body obtained by the method of the present invention is determined by the X-ray diffraction method (XRD method) after polishing the surface of the sintered body, and the diffracted X-ray intensity from the α phase and β phase on the surface. Measured and calculated by the following formula (1).

[Equation 1]
Betaization rate (%)
= Maximum β-phase peak intensity / (maximum α-phase peak intensity + maximum β-phase peak intensity) × 100
...... (1)

As described above, in order to increase the β conversion ratio of silicon nitride to 95% or more, in the sintering stage of the silicon nitride molded body, the molded body is placed in a vacuum atmosphere with a vacuum degree of 1 × 10 ⁻³ Torr or lower. After heating up to about 1500 ° C., switching to a non-oxidizing atmosphere and sintering at a predetermined sintering temperature, and continuing pressurization at 5 atm or higher until the temperature reaches 1500 ° C. or lower, β-silicon nitride The conversion rate can be 95% or more.

Further, in the crystal structure of the silicon nitride sintered body obtained by the method of the present invention , the area ratio of elongated silicon nitride needle-like crystal particles in which the ratio of the major axis to the minor axis (aspect ratio) of the silicon nitride crystal particles is 2 or more By setting the content to 50% or more, a high-strength sintered body in which needle-like silicon nitride crystal particles having a high aspect ratio are complicated can be obtained. When the area ratio is less than 50%, the effect of improving the strength of the sintered body is small. Therefore, the area ratio of the silicon nitride needle crystal grains having an aspect ratio of 2 or more is set to 50% or more, preferably 70% or more, and more preferably 90% or more.

  The area ratio of the silicon nitride crystal particles having the predetermined aspect ratio is measured as follows. That is, after processing the arbitrary two processed surface structures and the two cross-sectional structures in the sintered body to elute the auxiliary component, an enlarged photograph of the structure (magnification of about 200 times) is taken, and the photograph The short diameter and the long diameter of each silicon nitride particle existing in a region corresponding to 100 × 100 μm are measured, and the area of the silicon nitride particles having a ratio of the long diameter to the short diameter (aspect ratio) of 2 or more with respect to the entire region Each rate is measured and calculated as an average value of the four locations.

The silicon nitride sintered body needs to have an oxygen content of 5% by mass or less. When the oxygen content of the sintered body is excessive so as to exceed 5% by mass, the ratio of the grain boundary phase increases, and defects such as pores and cracks due to gas components such as oxygen and SiO ₂ tend to occur.

  The amount of oxygen in the sintered body (total oxygen amount) is a value determined by an oxygen analyzer in accordance with the inert gas melting-infrared absorption method, and the total amount of oxygen constituting the silicon nitride sintered body is mass%. It is shown by. Therefore, when oxygen is present in the silicon nitride sintered body as a metal oxide or oxynitride, not the amount of the metal oxide (and oxynitride) but the metal oxide (and oxynitride) It focuses on the amount of oxygen.

  In order to adjust and reduce the amount of oxygen in the sintered body, a rare earth component or an aluminum component used as a sintering aid is not an oxide, but a nitride or the like, or the molded body is placed in a vacuum atmosphere during sintering. Or by degassing the gas component.

In the silicon nitride sintered body obtained by the method of the present invention, the silicon nitride sintered body preferably contains 5% by mass or less of Ti element, and this Ti element is present as at least one compound of TiN and TiCN. The presence of the Ti element as a compound such as TiN or TiCN increases resistance to repeated stress, and can exhibit excellent rolling characteristics and durability when a bearing ball is formed of a sintered body. .

  The presence form of the Ti element can be confirmed by X-ray diffraction (XRD) analysis of the processed surface of the silicon nitride sintered body. The amount of Ti compound is adjusted so that the ratio of the maximum X-ray peak intensity of the TiCN compound to the maximum X-ray peak intensity of silicon nitride is 50% or less when the processed surface of the sintered body is subjected to X-ray diffraction analysis. It is preferable to do. If the X-ray maximum peak intensity ratio of the Ti compound exceeds 50%, the reaction amount between the auxiliary component Ti and other components (C and N) varies, and the life as a wear resistant member Characteristics are not stable. Therefore, the X-ray maximum peak intensity ratio of the Ti compound is 50% or less, but a range of 5 to 35% is more preferable. By making the X-ray maximum peak intensity ratio of this Ti compound 50% or less, it is possible to greatly reduce the variation in the life time (rolling fatigue life) of the bearing ball as the wear resistant member made of this sintered body. And a silicon nitride sintered body having a Weibull coefficient of 10 or more is obtained.

  In order to make the production amount of the Ti compound within the above range, when the compact of the silicon nitride raw material mixed powder is accommodated in the firing container and sintered, the filling ratio of the compact to the firing container is 40 to 80. It can be achieved by adjusting the firing atmosphere pressure to about 3 to 10 atm while adjusting to about%.

In the silicon nitride sintered body obtained by the method of the present invention, it is preferable that the maximum pore diameter of the silicon nitride sintered body is 3 μm or less. Voids such as voids and pits remaining on the surface of the sintered body are likely to be the starting point of breakage and cracking. However, if the maximum pore diameter is 3 μm or less, the probability of occurrence of breakage and cracking is reduced and the operation reliability is high. A wear-resistant member is obtained.

Furthermore, in the silicon nitride sintered body obtained by the method of the present invention, there are pores remaining in the surface region of 30 × 30 μm after the silicon nitride sintered body is polished, and the number of pores having a diameter of 1 μm or more is 5 The following is preferable. Even if the maximum pore diameter is less than 3 μm, if a large number of them remain, the probability of occurrence of breakage or cracking naturally increases. However, by reducing the number of pores remaining per unit tissue area with a diameter of 1 μm or more as described above to five or less, failure due to breakage or cracking can be reduced and the operational reliability of the equipment can be improved. it can.

Furthermore, in the silicon nitride sintered body obtained by the method of the present invention , the Young's modulus of the silicon nitride sintered body is preferably 290 GPa or more. If the Young's modulus of this silicon nitride sintered body is 290 GPa or more, even if the maximum diameter of the segregated agglomerated portion described later is increased to about 20 to 40 μm, excellent rolling characteristics and durability are ensured as a wear resistant material. be able to. This fact is true even when a raw material powder containing a large amount of impurities that cause large segregation and agglomeration parts is used, or when a non-uniform raw material powder with a low mixing degree of the raw material powder is used. If the Young's modulus can be increased to 290 GPa or more, it means that a silicon nitride sintered body having excellent wear resistance can be obtained.

  Furthermore, it is preferable that the maximum length of the silicon nitride crystal particles constituting the silicon nitride sintered body is 40 μm or less.

  Causes of silicon nitride crystal particles grown abnormally so that the maximum length of the silicon nitride crystal particles exceeds 40 μm deteriorates the surface properties of the sintered body and reduces the wear resistance and sliding properties Therefore, the maximum length of the silicon nitride crystal particles is preferably 40 μm or less.

  The maximum length of the silicon nitride crystal particles is measured as an average value of the maximum lengths of crystal particles in each region selected from three arbitrary measurement regions (100 μm × 100 μm) on an enlarged photograph of the sintered body structure. Is done.

  The maximum length of the silicon nitride crystal grains can be adjusted by optimizing the rare earth element, the amount of aluminum components, and the sintering conditions that prevent abnormal grain growth.

  Furthermore, in the silicon nitride sintered body, it is preferable that the maximum diameter of the segregation aggregate portion of the auxiliary component in the crystal structure of the silicon nitride sintered body is 30 μm or less.

  Here, the maximum diameter of the segregated agglomerated part means the maximum length of the segregated part or agglomerated part of the sintering aid component formed in the grain boundary phase between adjacent silicon nitride crystal grains, and silicon nitride sintering. With respect to the surface or cross section of the body, it is defined as the longest diagonal line in the segregation part or the aggregation part in an enlarged photograph with a magnification of about 2000 times (50 μm is displayed in 10 cm) or more.

  The segregated and agglomerated part is fragile and tends to be a starting point of breakage, and lowers slidability and wear resistance. Therefore, in the present invention, the maximum diameter of the segregation aggregate portion of the auxiliary component is set to 30 μm or less, but is preferably 10 μm or less, more preferably 5 μm or less. Further, in order to constitute a highly functional wear-resistant member, it is more preferable that the maximum diameter of the segregated and agglomerated part is in the range of 0.1 to 1.5 μm.

  The enlarged photograph is not particularly limited, but metal microscopes, electron microscopes, XDS, EPMA (Electron Probe Microanalyzer), etc. are common, and it becomes easy to judge the auxiliary component when color mapping is performed. . In addition, when judging the structure state in the enlarged photograph, since the end portion of the photograph is curved and distorted when the spherical portion is photographed like a bearing ball, the auxiliary component of the sintered body surface Although it is conceivable that the presence state is not accurately indicated, when a minute range such as a unit area of 50 μm × 50 μm is photographed, the above-described photographed image has little distortion and substantially no problem occurs.

  In order to reduce the maximum diameter of the segregation and agglomeration part, it is important to prevent aggregation of the auxiliary component powder and achieve uniform mixing of the raw silicon nitride powder and the sintering auxiliary powder. When aggregation of the auxiliary component occurs, a segregated aggregate portion having a maximum diameter of 30 μm or more is easily formed. In order to prevent the formation of the segregation and agglomeration part, as described later, for example, when mixing the raw material powder for one lot (total amount of about 5 kg), the raw material powder is divided into two or more, preferably 3 to 5, respectively. It is effective to use a method in which relatively small amounts are mixed and finally homogenized.

  Note that there is no particular problem if a mixed powder with a small amount of agglomerated particles of the auxiliary component powder is obtained in one lot, but in such a case, an attempt to obtain a uniform mixed state with few agglomerated particles makes the mixing time longer than necessary. Therefore, it is not necessarily good for manufacturability. Further, when the raw material powders are mixed together in large quantities at once, a site where the size of the auxiliary component exceeds 30 μm tends to be generated when the final silicon nitride sintered body is obtained.

  As another mixing method, first, silicon nitride powder and a plurality of types of sintering aids are mixed. A method in which the sintering aid powder having particularly poor dispersibility is divided and added sequentially in the mixed powder is effective. For example, the addition amount of the sintering aid component powder having poor dispersibility is divided into two or more, preferably 3 to 5, and added for the first time, and after a predetermined time (preferably 30 minutes or more) has passed, the second and subsequent times. This is a method of adding minutes in order.

  If the raw material powder is uniformly mixed by such a method, the aggregation of the sintering aid powders can be effectively suppressed. Therefore, even if there is a segregation part or an aggregation part, In this case, the maximum diameter of the segregation and agglomeration part of the auxiliary component can be 30 μm or less, preferably 5 μm or less.

  In the silicon nitride sintered body, the silicon nitride sintered body preferably contains 10 to 3500 ppm of Fe as impurities and 10 to 1000 ppm of Ca. When an aluminum component is contained as a sintering aid, the sliding performance of the wear-resistant member made of silicon nitride can be improved by containing a small amount of iron and calcium components. If the content of iron as an impurity is 3500 ppm or less, the sliding performance of the wear-resistant member is good, but even if the iron content is 10 ppm or more, inconvenience such as a decrease in strength and sliding performance. Does not occur.

  Further, in the wear resistant member made of the silicon nitride sintered body, the sliding performance of the wear resistant member is good when the calcium content is 1000 ppm or less. On the other hand, even if the calcium content is 10 ppm or more, there is no inconvenience such as a decrease in strength and sliding performance. The Fe content and Ca content in the silicon nitride sintered body can be measured by pressure decomposition-ICP emission analysis.

  Further, in the silicon nitride sintered body, it is preferable that the silicon nitride sintered body has a Vickers hardness Hv of 1300 to 1600.

  Regarding the hardness required for the silicon nitride sintered body constituting the wear-resistant member, it is preferable that the Vickers hardness has a characteristic of Hv 1300 or more. When the hardness of the silicon nitride sintered body is less than 1300 in terms of Vickers hardness Hv, the wear resistance is significantly reduced. In particular, the sliding characteristics (rolling life characteristics) required for bearing balls and the like cannot be sufficiently satisfied. The Vickers hardness of the silicon nitride sintered body is more preferably Hv1400 or higher. In addition, when the Vickers hardness Hv becomes excessive so as to exceed 1600, the aggression against the counterpart material becomes remarkable. Therefore, the Vickers hardness Hv is preferably in the range of 1300 to 1600.

The fracture toughness value of the silicon nitride sintered body is preferably 6 MPa · m ^1/2 or more. Furthermore, it is preferable that the minimum bending strength in the three-point bending test of the silicon nitride sintered body is 700 MPa or more and the average bending strength is 900 MPa or more.

In the silicon nitride sintered body, the silicon nitride sintered body is preferably a cutting tool or a rolling element of a bearing. Further, it is possible to form a rolling jig with the above silicon nitride sintered body. Further, when the silicon nitride sintered body is a spherical rolling element of a bearing, the crushing strength of the silicon nitride sintered body is preferably 150 N / mm ² or more. When the crushing strength is less than 150 N / mm ² , the rolling life characteristics of the spherical rolling element of the bearing are deteriorated.

  In the silicon nitride sintered body, a rolling ball made of the silicon nitride sintered body is prepared, and the rolling ball is arranged on a track having a diameter of 40 mm set on the upper surface of the SUJ2 steel plate. When the load is applied so that the maximum contact stress of 5.9 GPa is applied to the ball under the condition of a rotation speed of 1200 rpm, the surface of the rolling ball made of the silicon nitride sintered body is peeled off. It is preferable that the rolling fatigue life defined by time is 100 hours or more. When the rolling fatigue life that allows continuous rolling under the above severe conditions is 100 hours or more, a highly reliable rotating part can be formed, and the reliability of the rotating device can be dramatically improved.

The Vickers hardness Hv was measured at room temperature (25 ° C.) with a test load of 198.1 N in accordance with the measurement method defined in JIS-R-1610. The fracture toughness value (K _1C ) is measured based on the IF method defined by JIS-R-1607, and is calculated by the Niihara equation. The crushing strength was measured by applying a compression load with an Instron type tester and measuring the load at the time of breaking by a measuring method according to the old JIS standard B1501. Furthermore, the bending strength was measured by a measuring method according to a three-point bending strength test defined by JIS-R-1601.

The method for producing a silicon nitride sintered body according to the present invention contains 1.5% by mass or less of oxygen, 80% by mass or more of α-phase type silicon nitride, has an average particle size of 1 μm or less , and is produced by a metal nitriding method. The raw material mixture obtained by adding 1.5 to 3 % by mass of rare earth elements, 1 to 3% by mass of Al element, and 5% by mass or less of oxygen element as sintering aid components to the silicon nitride powder thus obtained. Is divided into a plurality of divisions of 2 to 5, and each divided raw material mixture is individually mixed, then combined as one raw material body 5 kg , and further mixed to form the raw material powder obtained and molded the body was prepared, after performing the deaeration treatment by heating to a temperature 1500 ° C. at a vacuum degree of the molded body is less atmosphere 1 × 10 -3 ^torr, switching the atmosphere to a non-oxidizing gas atmosphere, the temperature is 1600 The pressure is 5 for 0.5-10 hours at -1900 ° C. The maximum diameter of the segregation and agglomeration part of the auxiliary component in the crystal structure of the silicon nitride sintered body obtained by performing pressure sintering at a pressure higher than the pressure is 30 μm or less .

In the sintering step of the silicon nitride molded body in the above manufacturing method, the molded body is heated to about 1500 ° C. in a vacuum atmosphere with a vacuum degree of 1 × 10 ⁻³ Torr or less, and then switched to a non-oxidizing atmosphere to be predetermined. The β-conversion rate of silicon nitride can be 95% or more by continuing the pressurization at 5 atm or more until the temperature reaches 1500 ° C. or less.

  Further, in the method for producing a silicon nitride sintered body, after sintering, the silicon nitride sintered body may be subjected to a hot isostatic pressing (HIP) treatment at a pressure of 30 MPa or more in a non-oxidizing atmosphere. preferable. By performing this HIP process, voids such as voids remaining during sintering can be reduced or the size thereof can be reduced. In conventional HIP processing, a high pressure gas having a pressure of less than 300 atm (about 30 MPa) is introduced into the HIP processing apparatus and heat treatment is performed at a temperature close to the sintering temperature. However, there is a problem that large voids tend to remain. It was. Therefore, in the present invention, the HIP treatment is performed at a pressure of 300 atm (about 30 MPa) or more. In particular, by introducing a high-pressure gas of 1000 atm or more, specifically 1000 to 2000 atm (about 100 to 200 MPa) and performing the HIP treatment, the size of pores such as voids remaining in the sintered body and The number can be greatly reduced.

  Specifically, the maximum pore diameter of the silicon nitride sintered body can be 3 μm or less, and further, the diameter remaining in the surface region of 30 × 30 μm after the silicon nitride sintered body is polished. It is also possible to reduce the number of pores of 1 μm or more to 5 or less.

  According to the above manufacturing method, rare earth elements, aluminum elements, oxygen elements and, if necessary, components such as Ti, Hf, Zr, and the like are added to an inexpensive silicon nitride raw material powder. Because these components are mixed uniformly enough, these components react with the rare earth element evenly with the silicon nitride raw material powder to generate a liquid phase and function as a sintering accelerator, enabling the densification of the sintered body. In addition to the mechanical strength, wear resistance, and rolling life characteristics of the silicon nitride sintered body, the silicon nitride sintered body is particularly excellent in uniformity and workability. Is obtained.

  As the silicon nitride powder that is used in the method of the present invention and is the main component of the silicon nitride sintered body constituting the wear-resistant member or the like, for example, an inexpensive silicon nitride raw material powder synthesized by a metal nitriding method is used. In consideration of the sinterability, bending strength and fracture toughness value, the α-phase type nitriding silicon having an oxygen content of 1.5% by mass or less, preferably 0.9 to 1.2% by mass. Silicon nitride powder containing 80% by mass or more, preferably 90 to 97% by mass, and having an average particle size of 0.2 to 3 μm, preferably 1 μm or less, more preferably about 0.2 to 0.9 μm is used. It is preferable to do. The α-phase type silicon nitride powder becomes a β-phase type silicon nitride sintered body depending on sintering conditions.

  As the silicon nitride raw material powder, α-phase type and β-phase type powders are known, but the α-phase type silicon nitride raw material powder tends to have insufficient strength when formed into a sintered body. On the other hand, the β-phase type silicon nitride raw material powder can provide a high-strength sintered body in which silicon nitride crystal particles having a high aspect ratio are complicated.

  In the method of the present invention, the reason why the blending amount of the α-phase type silicon nitride powder is limited to the range of 80% by mass or more is that the bending strength, fracture toughness value and rolling life of the sintered body are remarkably limited within the range of 80% by mass or more. This is because the excellent characteristics of silicon nitride become remarkable. On the other hand, considering the sinterability, the range is up to 97% by mass. Preferably it is set as the range of 90-95 mass%.

  As a result, the silicon nitride starting material powder has an oxygen content of 1.5% by mass or less, preferably 0.9 to 1.% in consideration of sinterability, bending strength, fracture toughness value, and rolling life. It is preferable to use silicon nitride powder which is 2% by mass, contains α-phase type silicon nitride of 80% by mass or more, and has an average particle size of 1 μm or less, preferably about 0.6 to 0.9 μm.

  In particular, by using fine raw material powder having an average particle size of 0.8 μm or less, it is possible to form a dense sintered body having a porosity of 1% or less even with a small amount of sintering aid. . The porosity of this sintered body can be easily measured by the Archimedes method.

  Examples of rare earth elements added as a sintering aid to the silicon nitride raw material powder include oxides such as Y, Ho, Er, Yb, La, Sc, Pr, Ce, Nd, Dy, Sm, and Gd, or sintering operations. Thus, these oxide substances may be used alone or in combination of two or more oxides. These sintering aids react with the silicon nitride raw material powder to form a liquid phase and function as a sintering accelerator. The average particle size of the sintering aid is preferably 3 μm or less.

  The amount of the sintering aid added is in the range of 3% by mass or less based on the raw material powder in terms of rare earth elements. When the amount added is less than 3% by mass, densification or high strength of the sintered body is insufficient, and particularly when the rare earth element is an element having a large atomic weight such as a lanthanoid element, it is relatively low. A strong sintered body is formed. On the other hand, when the added amount exceeds 3% by mass, an excessive amount of grain boundary phase is generated, and the amount of pores generated increases or the strength starts to decrease. In particular, it is desirable to set it as 1.5 to 2.5 mass% for the same reason.

The aluminum component is added as aluminum oxide (Al ₂ O ₃ ), aluminum nitride (AlN), or the like, and the addition amount is in the range of 3 mass% or less in terms of Al element. Specifically, Al ₂ O ₃ promotes the function of a rare earth element sintering accelerator, enables densification at a low temperature, and functions to control grain growth in the crystal structure, and thus Si ₃ N ₄ sintering. In order to improve the bending strength (bending strength) of the body and the mechanical strength such as fracture toughness value, it is added in the range of 3% by mass or less. However, when the addition amount is less than 1% by mass, the effect of addition is insufficient. On the other hand, when the addition amount exceeds 3% by mass, the oxygen content rises, resulting in components in the grain boundary phase. Since uneven distribution occurs and the rolling life is shortened, the amount of Al ₂ O ₃ added is in the range of 1 to 3% by mass, preferably in the range of 1.5 to 3% by mass.

  On the other hand, AlN serves to suppress the evaporation of silicon nitride components during the sintering process, and to further promote the function of rare earth elements as a sintering accelerator, and is 3% by mass or less in terms of nitride. It is desirable to add in a range. However, when the addition amount is less than 1% by mass, the above function becomes insufficient. On the other hand, when the addition amount exceeds 3% by mass, the mechanical strength of the sintered body and the wear resistance member are reduced. Since the rolling life characteristics are deteriorated, the amount added is in the range of 1 to 3% by mass in terms of nitride.

Incidentally, the nitride silicon powder, when both the addition of the Al ₂ O ₃ 2-4 wt% and 1-3 wt% of AlN, it is possible to increase the mechanical properties of the sintered body more effectively When the total amount of both is excessive, the rolling life characteristics as the wear-resistant member are deteriorated. Therefore, the total content of aluminum components in the raw material mixture is preferably 6% by mass or less in terms of oxide.

  On the other hand, at least one selected from the group consisting of oxides, carbides, nitrides, silicides and borides of Ti, Hf, Zr, W, Mo, Ta, Nb, and Cr as components to be selectively added. In order to improve the mechanical strength and rolling life of the silicon nitride sintered body by promoting the function as a sintering accelerator such as the above-mentioned rare earth oxides, and also the function of strengthening dispersion in the crystal structure. It is contained in the range of 5% by mass or less in terms of element. Particularly preferred are Ti, Mo and Hf compounds. If the addition amount of these compounds is less than 0.5% by mass in terms of element, the effect of addition is insufficient. On the other hand, if the addition amount exceeds 5% by mass, the strength and rolling life of the sintered body are reduced. The added amount is in the range of 5% by mass or less. In particular, the content is desirably 0.5 to 4% by mass.

In addition, the compounds such as Ti, Hf, Zr, W, Mo, Ta, Nb, and Cr perform the function of dispersion strengthening in the crystal structure similarly to the SiC, and improve the mechanical strength of the silicon nitride sintered body. Let As a result, a fine grain boundary phase containing rare earth elements or the like is formed in the silicon nitride crystal structure, and the maximum width of the aggregated segregation portion formed in the grain boundary phase is 30 μm or less, and the porosity Is 1% or less, the minimum bending strength is 700 MPa or more at room temperature, the average bending strength is 900 MPa or more, the fracture toughness value is 6 MPa · m ^1/2 or more, and the crushing strength is 150 MPa or more. A silicon nitride sintered body having excellent characteristics can be obtained. In addition, the said average bending strength is an average value of each bending strength about the three bending test pieces cut out from the same sintered compact.

  The compounds such as Ti, Zr, and Hf also function as a light-shielding agent that imparts opacity by coloring the silicon nitride sintered body black.

  The porosity of the sintered body is preferably 1% or less because it greatly affects the rolling life and bending strength of the wear-resistant member. When the porosity exceeds 1%, the number of pores that become the starting point of fatigue failure increases rapidly, and the rolling life of the wear-resistant member decreases, and the strength of the sintered body decreases. A more preferable porosity is 0.5% or less. This porosity can be adjusted by controlling the sinterability, molding pressure, firing conditions, HIP treatment conditions after sintering, and the like by selecting the various sintering aids.

The silicon nitride sintered body obtained by the method of the present invention is manufactured through the following process, for example. That is, a predetermined amount of sintering aid, an aluminum component such as Al ₂ O ₃ and AlN, an organic binder, etc. are necessary for the fine silicon nitride powder having the predetermined fine particle size and low oxygen content. A raw material mixture is prepared by adding a suitable additive and, if necessary, a compound such as Ti.

  Here, in the preparation process of the raw material mixture, in order to prevent the aggregation of the sintering aid component, each raw material powder, that is, the raw material of the sintering aid powder and the silicon nitride powder is previously divided and mixed at once. The powder weight is limited and the divided sintering aid powder and silicon nitride powder are mixed well. The other divided raw material powders are similarly mixed to prepare a plurality of mixed powders. A method for producing a silicon nitride sintered body by integrating these divided uniform mixed powders into one and further thoroughly mixing them to prepare raw material powder, followed by degreasing and sintering. It is characterized by adoption.

  For example, when mixing the raw material powder for one lot (total amount 5 kg), after mixing the raw material powders into two or more, preferably 3 to 5 and mixing them uniformly in relatively small amounts, finally one raw material Combine as a mixture and mix well.

  In this way, a more homogeneous raw material mixture is obtained by combining the mixture of the sintering aid and the silicon nitride raw material powder into at least two parts and then mixing them together and then integrating them into one. Can be obtained.

  Next, the obtained raw material mixture is molded to obtain a molded body having a predetermined shape. As a method for forming the raw material mixture, a general-purpose mold pressing method, a CIP (cold isostatic pressing) method, or the like can be applied. In particular, when producing a spherical bearing ball, a cold isostatic pressing (CIP) molding method is suitable.

  When forming a molded body by the above-described mold pressing method or CIP molding method, the molding pressure of the raw material mixture is set to 120 MPa or more in order to form a grain boundary phase in which pores are unlikely to occur particularly after sintering. It is necessary. When this molding pressure is less than 120 MPa, a portion where the rare earth element compound that mainly constitutes the grain boundary phase is aggregated is easily formed, and a sufficiently dense molded body cannot be formed, and cracks are generated. Only a large number of sintered bodies can be obtained. Since the portion (segregation aggregate portion) where the auxiliary component aggregates in the grain boundary phase is likely to be a starting point of fatigue failure, the life durability of the wear-resistant member is lowered.

  On the other hand, when the molding pressure is excessively increased to exceed 200 MPa, the durability of the molding die is lowered, so that the productivity is not necessarily good. Therefore, the molding pressure is preferably in the range of 120 to 200 MPa.

  Subsequent to the molding operation, the molded body is heated in a non-oxidizing atmosphere at a temperature of 600 to 800 ° C. or in air at a temperature of 400 to 500 ° C. for 1 to 2 hours. Remove and degrease.

  Next, the degreased molded product is sintered under normal pressure at a temperature of 1600 to 1900 ° C. for 0.5 to 10 hours in a non-oxidizing atmosphere filled with an inert gas such as nitrogen gas, hydrogen gas or argon gas. Perform pressure sintering. As the pressure sintering method, various pressure sintering methods such as atmospheric pressure sintering, hot pressing, and hot isostatic pressing (HIP) sintering are used. When manufacturing a bearing ball made of a silicon nitride sintered body, it is preferable to perform HIP sintering after performing atmospheric pressure sintering or pressure sintering.

  In addition, after the above normal sintering, the obtained silicon nitride sintered body is further subjected to hot isostatic pressing (HIP) treatment in a non-oxidizing atmosphere with a pressure of 30 MPa or more, preferably 100 MPa or more. As a result, the influence of the pores of the sintered body, which becomes the starting point of fatigue fracture, can be further reduced, so that a silicon nitride sintered body having further improved wear resistance characteristics and rolling life characteristics can be obtained.

  The silicon nitride sintered body produced by the above method has a total oxygen amount of 5% by mass or less, a porosity of 1% or less, a minimum bending strength of 700 MPa or more at room temperature, and an average value of 900 MPa or more. Excellent mechanical properties.

Also, a silicon nitride sintered body having a crushing strength of 150 N / mm ² or more and a fracture toughness value of 6 MPa · m ^1/2 or more can be obtained.

According to the method for manufacturing a silicon nitride sintered body according to the present invention, a predetermined amount of rare earth elements, aluminum components such as Al ₂ O ₃ and AlN, and Ti, Hf, and Zr as necessary, in an inexpensive silicon nitride raw material powder. Since the raw material mixture is prepared by adding elements such as,, etc., the sinterability is greatly improved. In addition to the compactness and high mechanical strength equal to or higher than the conventional silicon nitride sintered body, it is excellent A silicon nitride sintered body suitable as a rolling bearing member having excellent wear resistance, particularly rolling life characteristics and workability can be obtained.

  Therefore, when this silicon nitride sintered body is used as a rolling bearing member and a bearing portion is prepared, it is possible to maintain good rolling characteristics over a long period of time, and operational reliability and durability. Can be provided at low cost. In addition, as other applications, it can be applied to various fields that require wear resistance, such as cutting tools, rolling jigs, check valves for check valves, engine parts, various jigs, various rails, and various rollers. .

As described above, according to the method for manufacturing a silicon nitride sintered body according to the present invention, a predetermined amount of rare earth element, an aluminum component such as Al ₂ O ₃ , Ti Ti as required, etc. , Hf, Zr, and other elements are added to prepare the raw material mixture, so that the sinterability and workability are greatly improved, and the compactness and high mechanical strength equal to or higher than those of conventional silicon nitride sintered bodies In addition to mechanical strength, excellent wear resistance can be obtained, and in particular, a silicon nitride sintered body suitable as a rolling bearing member having excellent rolling life characteristics can be provided at low cost.

  Further, since the generation of pores is suppressed and uneven distribution of components in the grain boundary phase is eliminated, a silicon nitride sintered body having excellent rolling life characteristics and durability can be obtained. Therefore, when this silicon nitride sintered body is used as a rolling bearing member and a bearing portion is prepared, it is possible to maintain good rolling characteristics over a long period of time, and operational reliability and durability. It is possible to provide an excellent rotating device.

  Next, the embodiments of the present invention will be specifically described with reference to the following examples.

[Examples 1 to 12]
A silicon nitride raw material powder produced by a metal nitriding method, having an oxygen content of 1.3% by mass and containing an α-phase type silicon nitride of 85% and an average particle size of 0.7 μm Si ₃ N ₄ (nitriding Silicon) raw material powder, Y ₂ O ₃ (yttrium oxide) powder having an average particle size of 0.8 μm as a sintering aid, Al ₂ O ₃ powder having an average particle size of 0.9 μm as an aluminum component, and an average particle size of 0 .9 μm AlN powder, rare earth element oxide such as hafnium oxide (HfO ₂ ) having an average particle diameter of 0.8 μm, TiO ₂ (titanium oxide) powder having an average particle diameter of 1 μm, Mo ₂ C (average particle diameter of 1 μm) Refractory metal compound powder such as molybdenum carbide) powder was prepared.

  To the silicon nitride raw material powder, the sintering aid is added and blended in predetermined amounts so that the rare earth element amount, aluminum element amount and refractory metal element amount are the values shown in Table 1. One lot (5 kg) was prepared. Next, each raw material powder was divided by the number of divisions shown in Table 1, and wet mixed with a ball mill using ethyl nitride balls as a grinding medium in ethyl alcohol for 24 hours and then dried to prepare raw material bodies. A plurality of raw material bodies divided and mixed were combined and further mixed for 48 hours to produce each raw material powder in which the auxiliary component was uniformly dispersed without aggregation.

  Each raw material powder was molded by a cold isostatic pressing method (CIP method) at a molding pressure of 150 MPa, and the obtained compact was sintered in an inert gas atmosphere as shown in Table 1 (pressure, temperature, time). The silicon nitride according to each example as shown in Table 1 is obtained by performing hot isostatic pressing (HIP) treatment under conditions (pressure, temperature, time) shown in Table 1 as necessary. A sintered body was produced.

  In addition, the silicon nitride sintered body according to each of Examples and Comparative Examples has a square columnar sample (test piece) for strength measurement having dimensions of 3 × 3 × 10 mm and a diameter of 9.525 mm (3/8 inch). ) To be a bearing ball sample. Further, the bearing ball sample was subjected to a surface polishing process corresponding to the grade 3 of the bearing ball defined by the Japanese Industrial Standard (JIS).

[Comparative Examples 1 to 7]
As Comparative Example 1, it was molded, degreased and sintered in the same manner as in Example 1 except that one lot (5 Kg) of the raw material mixture was wet-mixed for 24 hours by a ball mill without being divided. Such rod-like and spherical silicon nitride sintered bodies were prepared.

  As Comparative Example 2, a rod-like and spherical silicon nitride sintered body according to Comparative Example 2 was prepared by treating under the same conditions as in Example 1 except that no aluminum component was added.

  As Comparative Example 3, a rod-like and spherical silicon nitride sintered body according to Comparative Example 3 was prepared by processing under the same conditions as in Example 1 except that an excessive amount of rare earth element was added.

  In addition, as a comparative example 4, the sintered body obtained in the comparative example 1 was subjected to HIP treatment in which a pressure of 100 MPa was applied in a nitrogen gas atmosphere at a temperature of 1700 ° C. for 1 hour. A spherical silicon nitride sintered body was prepared.

Further, as Comparative Example 5, a silicon nitride powder produced by a metal nitriding method, Si _{3 having} an average particle size of 1.5 μm, containing an oxygen content of 1.7% by mass and an α-phase type silicon nitride of 70%. A rod-like and spherical silicon nitride sintered body according to Comparative Example 5 was prepared by processing under the same conditions as in Example 2 except that N ₄ raw material powder was used.

Further, Comparative Example 6 was treated under the same conditions as Example 2 except that silicon nitride raw material powder synthesized by the imide pyrolysis method and hafnium oxide (HfO ₂ ) were used as Comparative Example 6. Rod-like and spherical silicon nitride sintered bodies according to the above were prepared.

  As Comparative Example 7, a rod-like and spherical silicon nitride sintered body according to Comparative Example 7 was prepared by processing under the same conditions as in Example 1 except that the aluminum component was added in an excessive amount.

About each silicon nitride sintered body according to each Example and Comparative Example thus obtained, the rare earth element content, the aluminum element content, the oxygen content, the Fe content, the Ca content, the segregation aggregation part of the auxiliary component Maximum diameter, maximum length of silicon nitride crystal particles, porosity, Vickers hardness Hv, fracture toughness value (K _1C ) by the new original method in the microindentation method, minimum and average bending strength at room temperature, crushing The results shown in Table 1 were obtained by measuring strength, thermal conductivity, rolling fatigue life as a bearing ball, and Weibull coefficient of the life.

  The content of the rare earth element, the content of aluminum element, and the content of other elements such as Ti are quantified by chemical analysis using ICP (inductively coupled plasma emission spectroscopy) after melting the silicon nitride sintered body. did. The oxygen content was measured by an inert gas melting-infrared absorption method. The Fe content and Ca content were measured by pressure decomposition-ICP emission analysis. In addition, the maximum diameter of the segregation and agglomeration part of the auxiliary component is arbitrarily set as an observation area (area corresponding to an area of 50 μm × 50 μm) for a total of four places of two surfaces and two cross sections of each sintered body. The maximum diameter of the segregated and agglomerated part was measured and indicated by the average value of the four places. The maximum length of the silicon nitride crystal particles is selected from three arbitrary measurement regions (100 μm × 100 μm) on the enlarged photograph of the sintered body structure, and is the average value of the maximum lengths of the crystal particles in each region. It was measured.

  When the sintered body structure is confirmed by an enlarged photograph such as SEM, the segregated aggregate portion appears darker than the normal grain boundary phase (for example, in the case of a black and white photograph, the silicon nitride crystal particles are black, the grain boundary phase is Is displayed in white, and in segregation aggregation, white is projected darkly). Further, if the presence of rare earth elements and refractory metals is confirmed by EPMA as necessary, the concentration of rare earth elements is projected to be darker than the normal grain boundary phase, so that this method can also be distinguished.

Further, the porosity of each sintered body was measured by Archimedes method. The Vickers hardness Hv was measured in accordance with JIS-R-1610, the fracture toughness value (K _1C ) was measured by the Nihara method in the microindentation method, and the thermal conductivity was measured by the laser flash method.

  In each measurement value, the sample shape is a quadrangular prism for the sake of convenience in this embodiment. However, for example, even when measuring each characteristic of a true spherical bearing ball, it can be handled by performing lapping similarly.

  Further, the rolling fatigue life as a bearing ball was evaluated as follows. That is, a bearing ball having a diameter of 9.525 mm was prepared using the silicon nitride sintered bodies according to the examples and comparative examples, and the surface of each bearing ball was polished to a grade 3 state. The rolling fatigue life was measured using the thrust type rolling wear test apparatus shown.

This test apparatus includes a flat bearing steel plate 9 disposed in the apparatus main body 1, a silicon nitride bearing ball as a plurality of rolling balls 8 disposed on the upper surface of the bearing steel plate 9, and the rolling ball. 8, a guide plate 4 disposed on the upper portion of the guide plate 8, a drive rotating shaft 5 connected to the guide plate 4, and a cage 6 that regulates the arrangement interval of the rolling balls 8. The apparatus main body 1 is filled with lubricating oil 7 for lubricating the rolling part. The bearing steel plate 9 and the guide plate 4 are made of high carbon chrome bearing steel (SUJ2) defined by Japanese Industrial Standard (JIS G 4805). As the lubricating oil 7, paraffinic lubricating oil (viscosity at 40 ° C .: 67.2 mm ² / S) or turbine oil is used.

The rolling fatigue life of the bearing balls made of the silicon nitride sintered bodies according to the respective examples and comparative examples is 3 pieces each having a diameter of 9.525 mm (3/8 inch) from each silicon nitride sintered body as described above. On the other hand, the above-mentioned three rolling balls 8 are arranged on a raceway having a diameter of 40 mm set on the upper surface of the SUJ2 bearing steel plate 9, and this rolling is performed under oil bath lubrication conditions of turbine oil. When the ball 8 is rotated under the condition of a rotational speed of 1200 rpm with a load applied so that the maximum contact stress of 5.9 GPa acts, the surface of the rolling ball 8 made of a sintered silicon nitride body is peeled off. Measured as the time until. Each said measurement result is put together in the following Tables 1-2, and is shown.

  As is apparent from the results shown in Tables 1 and 2, the silicon nitride sintered body according to each example and the bearing ball as the wear-resistant member formed from the sintered body have a homogeneous composition. Since the predetermined additive component is contained and formed, the generation of pores is suppressed, the maximum diameter of the segregation and aggregation portion of the auxiliary component in the grain boundary phase is small, and uneven distribution of the component is not observed, Although the strength characteristics were somewhat lower than those of the comparative examples, a silicon nitride wear-resistant member having excellent rolling life and durability was obtained. Specifically, it has been found that even when a bearing ball formed of the silicon nitride sintered body according to each example is operated at a high speed for a long time, no problem occurs. On the other hand, in Comparative Example 1 and the like where the maximum diameter of the segregated aggregated portion is large, it was confirmed that a failure occurred and the rolling life and durability were low.

  Further, when the maximum diameter of the segregation aggregate portion of the auxiliary component in the grain boundary phase is 30 μm or less, the minimum value of the bending strength of the silicon nitride sintered body is 700 MPa or more, and the average value is 900 MPa or more. It was found to have excellent structural strength. Further, the thermal conductivity was 40 W / m · K or more in all cases. On the other hand, when the maximum diameter of the segregated aggregated part is as large as about 50 μm, the minimum value of the bending strength is below 600 MPa.

  On the other hand, in Comparative Example 1 in which the raw material powder was mixed at one time without dividing the raw material powder in the raw material powder mixing stage, the liquid phase component agglomerates and segregates, the distribution of the component in the grain boundary phase is large, the strength characteristics and rolling. Lifespan has decreased.

  Further, in Comparative Example 2 containing no aluminum component, the aggregation and segregation of the liquid phase component was increased, the distribution of the component in the grain boundary phase was uneven, and the strength characteristics and rolling life decreased.

  Further, in Comparative Example 3 in which the rare earth component was added in an excessive amount, the aggregation and segregation of the liquid phase component was increased, the distribution of the component in the grain boundary phase was large, and the strength characteristics and the rolling life were reduced.

  On the other hand, even if the HIP treatment is performed on the sintered body formed of the raw material mixed at once without dividing the raw material powder as in Comparative Example 4, the three-point bending strength is high, but the component in the grain boundary phase The effect of reducing the distribution unevenness was not sufficient, and the rolling life decreased.

  Further, in Comparative Example 5 using the silicon nitride raw material powder having a low ratio of α-type silicon nitride (70%) even when using the raw material powder synthesized by the metal nitriding method, the components in the grain boundary phase It has been found that the rolling life is reduced because of the uneven distribution.

  Furthermore, in Comparative Example 6 using silicon nitride powder synthesized by the imide pyrolysis method, the porosity, the bending strength, the fracture toughness value, the maximum diameter of the segregation condensation part in the grain boundary phase, and the rolling life are all However, it was reconfirmed that the manufacturing cost greatly increased because of the difficulty in workability and the expensive raw material powder.

  Further, in Comparative Example 7 in which the aluminum component was added in an excessive amount, the strength characteristics and the rolling life decreased.

  As described above, in the silicon nitride sintered body according to the present embodiment, particularly when the wear resistant member such as a bearing ball is constituted by the silicon nitride sintered body in which the maximum diameter of the segregation aggregate portion of the auxiliary component is 30 μm or less. In addition, the excellent sliding characteristics of silicon nitride can be further improved, and it is possible to greatly reduce problems that are likely to occur when used in electronic devices such as hard disk drives.

  Next, the effects on rolling, sliding characteristics and life when Ti carbide or nitride (TiCN) is formed as a reactant in the sintered silicon nitride and the amount of the reactant is controlled are based on the following examples. I will explain.

[Example 13]
As prepared in Example 1, a raw material mixture containing a predetermined amount of Y, aluminum, oxygen and Ti as rare earth elements was mixed and crushed by adding a solvent and a binder, and granulated with a spray dryer. The granulated powder was CIP-molded under a pressure of 150 MPa to form a large number of spherical shaped bodies. This molded body was degreased in the same manner as in Example 1, and then filled in a carbon firing container at a predetermined filling rate shown in Table 3, and sintered at a temperature of 1850 ° C. for 5 hours. The atmosphere at the time of sintering was nitrogen gas, and the atmosphere gas pressure was changed within the range shown in Table 3, thereby controlling the firing atmosphere and performing the atmosphere pressure sintering (GPS) process.

  FIG. 2 is a cross-sectional view showing a state in which the rolling ball 8 of the sample 6 sintered in a firing atmosphere at 5 atm has a filling rate in the firing container 10 of 70%, and FIG. It is sectional drawing which shows the state which the filling rate in the container 10 is 30%, and the rolling ball 8 of the sample 1 sintered in the baking atmosphere of 1 atmosphere is baking-processed.

  Further, each spherical sintered body was subjected to HIP treatment for 1 hour at a pressure of 200 MPa and a temperature of 1850 ° C. Thereby, a bearing ball sample having a diameter of 9.525 mm made of each silicon nitride sintered body was produced. Further, the surface of each bearing ball sample was subjected to a surface polishing process corresponding to the grade 3 of the bearing ball defined by Japanese Industrial Standards (JIS).

With respect to the polished surface of each bearing ball thus prepared, the ratio of the maximum X-ray peak intensity of the TiCN compound to the maximum X-ray peak intensity of silicon nitride is measured by X-ray diffraction (XRD). Was mounted on a thrust type rolling wear test apparatus shown in FIG. 1 to measure the rolling fatigue life, and the Weibull coefficient of the rolling fatigue life was measured. The measurement results are shown in Table 3 below.

  As is clear from the results shown in Table 3 above, the sintering atmosphere pressure and the filling ratio of the molded body into the firing container are increased to appropriately set the atmosphere during sintering, and the X-ray intensity of TiCN against silicon nitride In bearing ball samples 2, 5 to 7, which are made of a sintered body whose ratio is controlled to 50% or less, variation in the grain boundary strength of silicon nitride is reduced, so that the resistance to repeated stress is increased and rolling. The sliding characteristics and life were stabilized, a Weibull coefficient of 9 to 10 or more was obtained, and a bearing ball with stable life characteristics was obtained. In particular, a bearing ball having stable life characteristics was obtained when the X-ray intensity ratio of TiCN to silicon nitride was in the range of 5 to 35%.

  On the other hand, in the bearing ball samples 1 and 3 made of a sintered body in which the X-ray intensity ratio of TiCN to silicon nitride exceeds 50%, the reaction of Ti, which is an auxiliary component of the silicon nitride sintered body, with C or N Since the amount varies, it has been confirmed that the variation in grain boundary strength becomes large and the rolling and sliding characteristics and the life become unstable.

  In the above embodiments, the case where the silicon nitride sintered body according to the present invention is applied to a rolling element (bearing ball) of a bearing has been described as an example. However, the silicon nitride sintered body according to the present invention is limited to the above application. In addition, it can exhibit excellent wear resistance and durability even when used as a constituent material for cutting tools, rolling jigs and the like. By the way, when the roll for rolling was formed using the silicon nitride sintered body according to Example 1, compared with the roll for roll of the same size formed of super hard material, the durability until a rolling mark or a molding defect was generated. It was possible to extend the life to 2.8 to 3.4 times. Moreover, compared with the cutting tool of the same dimension formed with the cemented carbide material, the duration of sharpness could be extended to 2.4 to 2.9 times, and the outstanding durability was confirmed.

Sectional drawing which shows the structure of the thrust type | mold rolling wear test apparatus for measuring the rolling fatigue life of the bearing ball formed with the silicon nitride sintered compact concerning this invention. Sectional drawing which shows the baking method of the bearing ball formed with the silicon nitride sintered compact concerning this invention. Sectional drawing which shows the other baking method of the bearing ball formed with the silicon nitride sintered compact concerning this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Apparatus main body 4 Guide plate 5 Drive rotating shaft 6 Cage 7 Lubricating oil 8 Rolling ball 9 Bearing steel plate 10 Firing container

Claims (3)

As a sintering aid component, silicon nitride powder containing 1.5% by mass or less of oxygen and 80% by mass or more of α-phase type silicon nitride and having an average particle size of 1 μm or less and manufactured by a metal nitriding method is used. Rare earth element 1.5-3 mass% , Al element 1-3 mass% , oxygen element 5 mass% or less are added, and the resulting raw material mixture is divided into a plurality of divisions of 2-5 and divided. After mixing each raw material mixture individually, they are united as one raw material body 5 kg , and further mixed to form a raw material powder to prepare a molded body. This molded body has a vacuum degree of 1 × After performing deaeration treatment by heating up to a temperature of 1500 ° C. in an atmosphere of 10 ⁻³ torr or less, the atmosphere is switched to a non-oxidizing gas atmosphere, and the temperature is 1600 to 1900 ° C. for 0.5 to 10 hours. Is obtained by performing pressure sintering at 5 atm or higher. A method for producing a silicon nitride sintered body, wherein the maximum diameter of the segregation and aggregation portion of the auxiliary component in the crystal structure of the silicon nitride sintered body is 30 μm or less . After sintering, the non in an oxidizing atmosphere with respect to the silicon nitride sintered body, silicon nitride sintered body of claim 1, wherein the performing the above hot isostatic pressing pressure 30 MPa (HIP) process Manufacturing method. The resulting silicon nitride sintered body The manufacturing method of the silicon nitride sintered body according to claim 1, characterized in that it Fe a 10~3 5 00ppm, the Ca contained 10~1000ppm as an impurity.

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