WO2014092021A1

WO2014092021A1 - Silicon nitride sintered body and sliding member using same

Info

Publication number: WO2014092021A1
Application number: PCT/JP2013/082858
Authority: WO
Inventors: 青木　克之; 小松　通泰; 開船木; 山口　晴彦
Original assignee: 株式会社東芝; 東芝マテリアル株式会社
Priority date: 2012-12-14
Filing date: 2013-12-06
Publication date: 2014-06-19
Also published as: CN104768900A; JPWO2014092021A1; JP6334413B2; CN104768900B

Abstract

This silicon nitride sintered body is characterized in that, during XRD analysis, when the strongest peak intensities detected at 29.6±0.3° and 31.0±0.3° corresponding to a hexogonal α-SiAlON crystal are I_29.6° and I_31.0° and the strongest peak intensities detected at 33.6±0.3° and 36.1±0.3° corresponding to a β-Si₃N₄ crystal are I_33.6° and I_36.1°, these strongest peak intensities fulfill the relation (I_29.6° + I_31.0°) / (I_33.6° + I_36.1°) = 0.10 to 0.30 ... (1), the area ratio of the grain boundary phase per 100μm×100μm unit area in an arbitrary cross section of the silicon nitride sintered body is 25-40%, and the machinable coefficient is 0.100-0.120. By means of the present invention, it is possible to provide a silicon nitride sintered body optimal for sliding members having sliding characteristics that remain stable for a long time, and to provide a sliding member using the same.

Description

Silicon nitride sintered body and sliding member using the same

Embodiments of the present invention relate to a silicon nitride sintered body and a sliding member using the same.

Silicon nitride sintered bodies are widely used as constituent materials for sliding members such as bearing balls and rollers. For example, 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, there has been a problem that high-speed rotation drive with high reliability cannot be stably obtained due to large variations in bearing life.

As a means for solving the above problems, recently, a silicon nitride sintered body has been used as a component of a bearing ball. Since the silicon nitride sintered body is superior in sliding characteristics among ceramics, it has sufficient wear resistance in some usage modes, and even during high-speed rotation, a highly reliable rotational drive 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. For example, it is disclosed in JP 2006-36554 A (Patent Document 1).

In addition, Japanese Patent Laid-Open No. 2002-326875 (Patent Document 2) discloses a silicon nitride sintered body that exhibits a long life of 400 hours or longer when a predetermined wear resistance test is performed.

On the other hand, sliding members such as bearing balls using a silicon nitride sintered body are required to have a long life of 400 hours or more, and further about 800 hours. By extending the life of the bearing ball, it is possible to realize maintenance-free sliding parts such as bearings.

Sliding members such as bearings are used in products in various fields such as machine tools, electronic devices, automobiles, aircraft, and even wind power generation. When the service life of the sliding member is increased, the service life of various products can be increased and further maintenance-free can be achieved.

JP 2006-36554 A JP 2002-326875 A

The problem to be solved by the present invention is to provide a silicon nitride sintered body that can have further long-term reliability of sliding characteristics and a sliding member using the same.

According to one embodiment of the present invention, when the silicon nitride sintered body is subjected to XRD analysis, the temperature is 29.6 ± 0.3 ° and 31.0 ± 0.3 ° corresponding to the hexagonal α-SiAlON crystal. The detected strongest peak intensities are I _{29.6 °} and I _{31.0 °} , while 33.6 ± 0.3 ° and 36.1 ± 0.3 ° corresponding to β-Si ₃ N ₄ crystal. When the detected strongest peak intensity is I _{33.6 °} and I _{36.1 °} , each strongest peak intensity is represented by the following relational expression:
(I _{29.6 °} + I _{31.0 °} ) / (I _{33.6 °} + I _{36.1 °} ) = 0.10 to 0.30 (1)
The area ratio of the grain boundary phase per unit area 100 μm × 100 μm in an arbitrary cross section of the silicon nitride sintered body is 25 to 40%, and the machinable coefficient is 0.100 to 0.120. A silicon nitride sintered body is obtained.

Further, when the silicon nitride sintered body was subjected to XRD analysis, the strongest peak intensity I _{39.5 °} detected at 39.5 ± 0.3 ° corresponding to Y ₄ Si ₂ O ₇ N ₂ (J phase) was The following relational expression:
(I _{39.5 °} ) / (I _{33.6 °} + I _{36.1 °} ) = 0.03 to 0.10 (2)
It is preferable to satisfy. Also, when XRD analysis is performed on a silicon nitride sintered body, it corresponds to any one or more of Y ₂ Si ₃ O ₁₂ N (H phase), YSiO ₂ N (K phase), or Y ₂ Si ₃ O ₃ N ₄ The strongest peak intensity I _{31.9 °} detected at 31.9 ± 0.3 ° is _expressed by the following relational expression:
(I _{31.9 °} ) / (I _{33.6 °} + I _{36.1 °} ) = 0.05 to 0.15 (3)
It is preferable to satisfy. The XRD analysis is preferably performed on an arbitrary cross section of the silicon nitride sintered body.

In addition, it is preferable to have a grain boundary phase including an amorphous phase containing an Hf—Y—O-based compound crystal and Y—Al—O. Moreover, it is preferable to comprise particles of carbide, oxide, nitride or the like having an average particle diameter of 2 μm or less. The particles are preferably molybdenum compound particles. Also, Al is 5 to 10% by mass in terms of oxide, and any one or more of rare earth elements is 1 to 10% by mass in terms of oxide, and any one or more of 4A, 5A, and 6A elements is oxide. The content is preferably 1 to 5% by mass in terms of a converted value, and the molar ratio of Al to the rare earth element is preferably Al (mol): rare earth element (mol) = 1: 1 to 8: 1. Further, it is preferable that the Vickers hardness (Hv) is 1500 or more, the fracture toughness value (K _1C ) is 6.0 MPa · m ^1/2 or more, and the three-point bending strength is 900 MPa or more.

Further, the sliding member according to the embodiment is characterized by using the silicon nitride sintered body of the embodiment. The sliding member is preferably a bearing ball. The sliding surface is preferably a polished surface having a surface roughness (Ra) of 0.5 μm or less.

This embodiment can provide a silicon nitride sintered body that can provide long-term reliability with respect to sliding characteristics. In addition, since the machinable coefficient is adjusted, even if the surface is polished, it is easy to obtain a flat sliding surface because degranulation can be reduced. Therefore, long-term reliability can be obtained also with respect to the sliding member using the sintered body.

It is a perspective view which shows one Embodiment of the sliding member formed using the silicon nitride sintered compact concerning this invention.

Hereinafter, embodiments will be described.

(First embodiment)
In the first embodiment, when the silicon nitride sintered body is subjected to XRD analysis, it is detected at 29.6 ± 0.3 ° and 31.0 ± 0.3 ° corresponding to the hexagonal α-SiAlON crystal. While the strongest peak intensities are I _{29.6 °} and I _{31.0 °} , they are detected at 33.6 ± 0.3 ° and 36.1 ± 0.3 ° corresponding to β-Si ₃ N ₄ crystal. When the strongest peak intensity is I _{33.6 °} and I _{36.1 °} , the strongest peak intensities are related to each other:
(I _{29.6 °} + I _{31.0 °} ) / (I _{33.6 °} + I _{36.1 °} ) = 0.10 to 0.30 (1)
The area ratio of the grain boundary phase per unit area 100 μm × 100 μm in an arbitrary cross section of the silicon nitride sintered body is 25 to 40%, and the machinable coefficient is 0.100 to 0.120. This is a silicon nitride sintered body.

First, the conditions for performing XRD analysis will be described. The measurement surface is an arbitrary surface or an arbitrary cross section of the sintered body, and is a polished surface having a surface roughness Ra of 1 μm or less. XRD analysis is performed with a Cu target (Cu-Kα), tube voltage 40 kV, tube current 40 mA, scan speed 2.0 ° / min, slit (RS) 0.15 mm, scan range (2θ) 10 ° -60 °. Shall. The scanning range (2θ) may be a wide range as long as it includes 10 ° to 60 °. In particular, the cross section of the sintered body is preferable. If it analyzes in a cross section, it can also be used as a surface for obtaining the area ratio of the grain boundary phase.

In the first embodiment, when XRD analysis is performed, the strongest peak intensities detected at 29.6 ± 0.3 ° and 31.0 ± 0.3 ° corresponding to the hexagonal α-SiAlON crystal are represented by I _{29.6. °} , I _{31.0 °} , while the strongest peak intensities detected at 33.6 ± 0.3 ° and 36.1 ± 0.3 ° corresponding to β-Si ₃ N ₄ crystals are I _{33.6 °} , I _{36.1 °} , each strongest peak intensity is the following relational expression: (I _{29.6 °} + I _{31.0 °} ) / (I _{33.6 °} + I _{36.1 °} ) = 0.10˜ It shall satisfy 0.30. The peak position by XRD analysis is determined by the lattice constant of each crystal.

If hexagonal α-SiAlON crystals are present, peaks are detected at 29.6 ± 0.3 ° and 31.0 ± 0.3 °. In other words, when peaks are detected at 29.6 ± 0.3 ° and 31.0 ± 0.3 °, it means that hexagonal α-SiAlON crystals are present. The reason why both peak intensities of I _{29.6 °} and I _{31.0 °} are used is to mitigate the influence of changes in peak intensity due to crystal orientation.

If β-Si ₃ N ₄ crystals are present, peaks are detected at 33.6 ± 0.3 ° and 36.1 ± 0.3 °. In other words, β-Si ₃ N ₄ crystals are present when peaks are detected at 33.6 ± 0.3 ° and 36.1 ± 0.3 °. The reason why both peak intensities of I _{33.6 °} and I _{36.1 °} are used is to mitigate the influence of changes in peak intensity due to crystal orientation.

In the first embodiment, (I _{29.6 °} + I _{31.0 °} ) / (I _{33.6 °} + I _{36.1 °} ) = 0.10 to 0.30 is satisfied. The magnitude of the peak intensity is determined according to the abundance of each crystal. (Hexagonal α-SiAlON crystal / β-Si ₃ N ₄ crystal) The peak intensity ratio (I _{29.6 °} + I _{31.0 °} ) / (I _{33.6 °} + I _{36.1 °} ) is 0.10 A value of ˜0.30 means that a predetermined amount of hexagonal α-SiAlON crystal is present with respect to β-Si ₃ N ₄ crystal.

If hexagonal α-SiAlON crystal is present, peaks are also detected in the vicinity of 34.4 ° and 35.1 °. If β-Si ₃ N ₄ crystals are present, peaks are also detected in the vicinity of 23.4 ° and 27.1 °. The presence / absence of the detection of the peak at this side may be used to grasp the presence / absence of the hexagonal α-SiAlON crystal and β-Si ₃ N ₄ crystal. Further, when it is difficult to discriminate due to overlapping with other crystal peaks described later, various qualitative analyzes may be combined.

The presence of the hexagonal α-SiAlON crystal results in a structure in which a hexagonal α-SiAlON crystal and a β-Si ₃ N ₄ crystal are mixed, and the presence of grain boundary phases can be reduced. The phase can be strengthened. Therefore, hardness, toughness, etc. as a sintered body can be improved, and wear resistance can be improved. The hexagonal α-SiAlON crystal may have both a spherical shape and a columnar shape.

In particular, the hexagonal α-SiAlON crystal preferably has an aspect ratio of 2 or less. The β-Si ₃ N ₄ crystal has a long columnar shape with an aspect ratio of 2 or more. In the silicon nitride sintered bodies of Patent Document 1 and Patent Document 2, β-Si ₃ N ₄ crystals are intertwined in a complex manner to form a silicon nitride sintered body having high hardness and toughness. On the other hand, since β-Si ₃ N ₄ crystals have a long columnar shape, there is a large variation in grain boundary size between β-Si ₃ N ₄ crystals, and there are formed portions where there are some grain boundary phases and where there are few. It was. For this reason, the long-term life of sliding characteristics was reduced.

In the first embodiment, since the long columnar β-Si ₃ N ₄ crystal and the spherical or columnar hexagonal α-SiAlON crystal are mixed, hexagonal is formed in the gap between the β-Si ₃ N ₄ crystals. A structure in which crystal α-SiAlON crystal enters can stabilize the existence ratio of the grain boundary phase.

If the peak intensity ratio of (hexagonal α-SiAlON crystal / β-Si ₃ N ₄ crystal) is less than 0.10, the amount of hexagonal α-SiAlON crystal is too small and the variation in the presence ratio of the grain boundary phase becomes large. On the other hand, when the peak intensity ratio of (hexagonal α-SiAlON crystal / β-Si ₃ N ₄ crystal) is larger than 0.30, the proportion of β-Si ₃ N ₄ crystal decreases, and β-Si ₃ N _The structure in which the _four crystals are intertwined in a complicated manner is reduced, and the sliding characteristics are lowered.

Further, by setting the area ratio of the grain boundary phase per unit area of 100 μm × 100 μm in an arbitrary cross section of the silicon nitride sintered body to 25 to 40%, it becomes easy to control the variation in the presence of the grain boundary phase. In the first embodiment, grains other than β-Si ₃ N ₄ crystal and hexagonal α-SiAlON crystal are used as the grain boundary phase. In the silicon nitride sintered body according to the first embodiment, the area ratio of the grain boundary phase per unit area of 100 μm × 100 μm is in the range of 25 to 40% regardless of the cross section of any section, that is, any cross section. . Since the ratio of the grain boundary phase is controlled in a small region of unit area 100 μm × 100 μm, not only the hardness and fracture toughness of the sintered body can be improved, but also long-term reliability of sliding characteristics can be obtained.

In addition, the measuring method of the area ratio of a grain boundary phase is as follows. First, an arbitrary cross section of the silicon nitride sintered body is obtained. This cross section is polished so that the surface roughness Ra is 1 μm or less. In order to clarify the region of the grain boundary phase with the β-Si ₃ N ₄ crystal and the hexagonal α-SiAlON crystal, a plasma etching process is performed on the obtained polished surface.

When the plasma etching process is performed, the etching rate of the grain boundary phase is different from that of the β-Si ₃ N ₄ crystal and the hexagonal α-SiAlON crystal. For example, in plasma etching using CF4, β-Si ₃ N ₄ crystal and hexagonal α-SiAlON crystal have higher etching rates (easily etched), so β-Si ₃ N ₄ crystal and hexagonal α- The SiAlON crystal becomes a concave portion and the grain boundary phase becomes a convex portion.

The etching process can also be chemical etching using acid and alkali. The mirror surface after the etching process is taken with an SEM image (magnification of 1000 times or more). In the SEM photograph, β-Si ₃ N ₄ crystal and hexagonal α-SiAlON crystal and the grain boundary phase can be distinguished by the difference in contrast. Usually, the grain boundary phase appears white. By performing the etching process, the difference in contrast can be made clearer.

The area ratio of the grain boundary phase per unit area can be measured by image analysis of the SEM photograph. For image analysis, it is effective to perform image analysis by color mapping the grain boundary phase portion. In addition, when the unit area does not become 100 μm × 100 μm in one field of view, the image may be taken a plurality of times, and the unit area may be 100 μm × 100 μm in total.

The silicon nitride sintered body of the first embodiment has a machinable coefficient of 0.100 to 0.120.

The machinable coefficient Mc is a value calculated from the following equation (4).

Mc = Fn ^9/8 / (K _1c ^1/2 · Hv ^5/8 ) (4)
In Formula (4), Fn is an indentation load, and is 20 kgf here. The indentation load Fn of 20 kgf is a suitable value for measuring the hardness and toughness of the silicon nitride sintered body. Vickers hardness (Hv) shall be measured according to JIS-R-1610. The fracture toughness value (K _1C ) is measured according to the indenter press-in method (IF method) of JIS-R-1607. For the calculation of fracture toughness value, Niihara's formula shall be used. The bearing ball described later is measured using its cross section.

The machinable coefficient Mc is a coefficient indicating workability using the indentation load (Fn), the Vickers hardness (Hv), and the fracture toughness value (K _1C ). This is a relational expression of the lateral crack fracture model, and Mc indicates the amount of material removed by one abrasive grain. This means that the larger the machinable coefficient Mc, the larger the amount that can be processed at one time.

The lateral crack fracture model is a model proposed by Evans and Marshall as a material removal mechanism during grinding. In this model, the amount of material (Delta V) that is removed when one abrasive grain passes through the material surface is determined by the force Fn, Vickers hardness (Hv), and fracture toughness value that push the abrasive grain vertically into the material. In relation to (K _1C ), it is shown that the value is proportional to the value of [Fn ^9/8 / (K _1c ^1/2 · Hv ^5/8 )]. Here, delta V is replaced with a machinable coefficient Mc.

Processing is roughly divided into brittle mode and ductile mode. The brittle mode corresponds to so-called roughing, and the ductility mode corresponds to so-called finishing. Since wear is considered to correspond to ductility mode, in order to satisfy the required performance of wear resistant members, it is important to improve the workability of the brittle mode without reducing the workability of the ductile mode. . Further, as one of the wear models, a mechanism is considered in which minute precracks are generated at the grain boundaries and the propagation of the cracks leads to the destruction of the material surface, thereby causing wear.

A parameter Sc. Representing the severity of mechanical contact of the wear model. m is expressed by the following equation from the friction coefficient μ, the maximum Hertz stress Pmax, the crystal grain size d of the material, and the fracture toughness value K _1c .

Sc. m = [(1 + 10 · μ) · Pmax · (d ^1/2 )] / K _1c
Parameter Sc. When m is large, wear is large, and the parameter Sc. If m is small, it means that wear is small. It can be seen that wear can be suppressed by reducing the crystal grain size d of the material or increasing the fracture toughness value _K1c .

In consideration of these points, the machinable coefficient Mc is preferably in the range of 0.100 to 0.120. When the machinable coefficient Mc is less than 0.100, the polishing time of the silicon nitride sintered body increases because the amount of polishing with abrasive grains is small. When the machinable coefficient Mc exceeds 0.120, the polishing amount of the silicon nitride sintered body by the abrasive grains becomes too large. When the polishing amount is large, the workability is improved, but the durability as a sliding member is lowered.

A silicon nitride sintered body having a machinable coefficient Mc in the range of 0.100 to 0.120 can improve sliding properties while improving hardness and fracture toughness as a sintered body. Further, since the grain drop marks when polishing the sliding surface can be reduced, it is easy to obtain a flat surface with a surface roughness Ra of 0.5 μm or less, and further 0.1 μm or less. In consideration of workability, the machinable coefficient Mc is preferably in the range of 0.110 to 0.120.

Further, when the silicon nitride sintered body was subjected to XRD analysis, the strongest peak intensity I _{39.5 °} detected at 39.5 ± 0.3 ° corresponding to Y ₄ Si ₂ O ₇ N ₂ (J phase) is The relational expression: (I _{39.5 °} ) / (I _{33.6 °} + I _{36.1 °} ) = 0.03 to 0.10 is preferably satisfied.

Y ₄ Si ₂ O ₇ N ₂ (J phase) is a crystal phase present in the grain boundary phase. The fact that a peak is detected at 39.5 ± 0.3 ° means that Y ₄ Si ₂ O ₇ N ₂ (J phase) exists. By setting the peak intensity ratio (I _{39.5 °} ) / (I _{33.6 °} + I _{36.1 °} ) of Y ₄ Si ₂ O ₇ N ₂ (J phase) in the range of 0.03 to 0.10 , The grain boundary phase can be strengthened. By strengthening the grain boundary phase, the long-term reliability of the sliding characteristics can be further improved.

If Y ₄ Si ₂ O ₇ N ₂ (J phase) is present, the vicinity of 31.0 °, 34.4 °, 36.1 °, 39.5 °, 44.5 ° (± 0.3 ° ) Is also detected. The reason why I _{39.5 °} is selected is that there is a high possibility of showing the strongest peak among the peaks corresponding to the J phase.

Also, when XRD analysis is performed on a silicon nitride sintered body, it corresponds to any one or more of Y ₂ Si ₃ O ₁₂ N (H phase), YSiO ₂ N (K phase), or Y ₂ Si ₃ O ₃ N ₄ The strongest peak intensity I _{31.9 °} detected at 31.9 ± 0.3 ° is (I _{31.9 °} ) / (I _{33.6 °} + I _{36.1 °} ) = 0.05 to 0.00. 15 is preferably satisfied.

Any one of Y ₂ Si ₃ O ₁₂ N (H phase), YSiO ₂ N (K phase), and Y ₂ Si ₃ O ₃ N ₄ is a crystal phase present in the grain boundary phase. The fact that a peak is detected at 31.9 ± 0.3 ° means that any one of Y ₂ Si ₃ O ₁₂ N (H phase), YSiO ₂ N (K phase), and Y ₂ Si ₃ O ₃ N ₄ is detected. This means that there are more than one crystalline phase.

By setting the peak intensity ratio (I _{31.9 °} ) / (I _{33.6 °} + I _{36.1 °} ) in the range of 0.05 to 0.15, the grain boundary phase can be strengthened. By strengthening the grain boundary phase, the long-term reliability of the sliding characteristics can be further improved. The reason why I _{31.9 °} is selected is the peak corresponding to one or more of Y ₂ Si ₃ O ₁₂ N (H phase), YSiO ₂ N (K phase), or Y ₂ Si ₃ O ₃ N _4. This is due to the fact that it is one of the strongest peaks.

Further, it is preferable to have a grain boundary phase including an amorphous phase containing Hf—YO compound crystal and Y—Al—O. The Hf—Y—O-based compound crystal is a compound crystal containing hafnium, yttrium, and oxygen. The amorphous phase containing Y—Al—O is an amorphous phase containing at least yttrium, aluminum, and oxygen.

Hafnium is an active component, reacts with yttrium and oxygen to form Hf—Y—O-based compound crystals, and promotes the growth reaction of Si ₃ N ₄ . At this time, formation of one or more of Y ₄ Si ₂ O ₇ N ₂ (J phase), Y ₂ Si ₃ O ₁₂ N (H phase), YSiO ₂ N (K phase), or Y ₂ Si ₃ O ₃ N ₄ It is thought that it contributes to.

Further, by making the phase containing Y—Al—O an amorphous phase (glass phase), it becomes easy to control the homogeneous distribution of the grain boundary phase. Note that the presence or absence of an amorphous phase containing Hf—Y—O-based compound crystals and Y—Al—O can be confirmed by TEM analysis.

Further, it is preferable to have particles of carbide, oxide, nitride or the like having an average particle diameter of 2 μm or less. The particles are selected from silicon (Si), 5A group vanadium (V), niobium (Nb), tantalum (Ta), 6A group chromium (Cr), molybdenum (Mo), tungsten (W). It is preferable that the particles are at least one kind of particles.

The above particles contribute to strengthening of the grain boundary phase by being present in the grain boundary phase. Further, in order to realize strengthening of the grain boundary phase, the average particle diameter is preferably 2 μm or less, more preferably 1.5 μm or less. If the average particle size is larger than 2 μm, the continuous distribution of grain boundaries is hindered, which may cause structural defects.

The particles are preferably molybdenum compound particles. Since the molybdenum compound particles have lubricity, the presence of the molybdenum compound particles on the sliding surface (the surface of the silicon nitride sintered body) can improve the sliding characteristics of the sliding surface. Further, molybdenum compound particles (Mo ₂ C) particles are excellent in lubricity.

Further, the β-Si ₃ N ₄ crystal preferably has a major axis of 2 μm or more and a maximum aspect ratio of 7 or less. The hexagonal α-SiAlON crystal preferably has an average particle diameter of 2 μm or less in both spherical and columnar shapes. In addition, the average particle diameter of the spherical shape and the columnar shape (aspect ratio of 2 or less) is obtained from the average value of 100 grains, with the diameter of the equivalent circle using the major axis as the particle diameter.

Also, Al is 5 to 10 mass (wt)% in terms of oxide, and any one or more of rare earth elements is 1 to 10 mass% in terms of oxide, and one or more of 4A, 5A, and 6A elements Is preferably 1 to 5% by mass in terms of oxide, and the molar ratio of Al to the rare earth element is preferably Al (mol): rare earth element (mol) = 1: 1 to 8: 1.

Al (aluminum) is a component for improving the sinterability and is a component necessary for forming a hexagonal α-SiAlON crystal and a Y—Al—O-based compound amorphous phase. Further, if added as a sintering aid, _Al 2 _{O 3} (aluminum oxide) is preferably AlN (aluminum nitride). If the Al content is less than 5% by mass in terms of oxide, the proportion of the grain boundary phase may decrease, and the formation of various crystal components and amorphous components may be insufficient. On the other hand, if it exceeds 10 mass%, the grain boundary phase may be excessive.

The rare earth elements are Y (yttrium), La (lanthanum), Ce (cerium), Pr (praseodymium), Nd (neodymium), Pm (promethium), Sm (samarium), Eu (europium), Gd (gadolinium), Tb. At least one of (terbium), Dy (dysprosium), Ho (holmium), Er (erbium), Tm (thulium), Yb (ytterbium), and Lu (lutetium) is preferable. Moreover, when adding as a sintering auxiliary agent, adding as a rare earth oxide is preferable. In addition, when one or more of rare earth elements is less than 1% by mass and more than 10% by mass in terms of oxide, the proportion of the grain boundary phase may be out of the range of the embodiment, It becomes difficult to adjust the molar ratio.

Of the rare earth elements, yttrium is preferable. In the case of yttrium, Y ₄ Si ₂ O ₇ N ₂ (J phase), Y ₂ Si ₃ O ₁₂ N (H phase), YSiO ₂ N (K phase), Y ₂ Si ₃ O ₃ N ₄ , Hf—Y It functions as a component that forms an amorphous phase containing —O-based compound crystals and Y—Al—O. It also promotes the formation of α-SiAlON crystals.

Further, it is preferable to contain 1 to 5% by mass of any one of 4A, 5A and 6A elements in terms of oxide. The 4A group elements are Ti (titanium), Zr (zirconium), and Hf (hafnium). The group 5A elements are V (vanadium), Nb (niobium), and Ta (tantalum). The 6A group elements are Cr (chromium), Mo (molybdenum), and W (tungsten). The oxide conversion of the group 4A element is converted with TiO ₂ , ZrO ₂ , and HfO ₂ . Also, in terms of oxide of the Group 5A element _shall be converted by _{_{_{V 2 O 5, Nb 2 O}}} 5, Ta 2 O 5. Also, in terms of oxide of the Group 6A elements _shall be converted in _{_{Cr 2 O 3, MoO 3,}} WO 3.

Moreover, when adding as a sintering auxiliary agent, it is preferable to add at least 1 type of an oxide, a carbide | carbonized_material, and nitride. Further, the Group 4A element is preferably added as an oxide, and the Group 5A element and the Group 6A element are preferably added as carbides. The 4A group element is preferably Hf, and the 6A group element is preferably Mo. As described above, Hf functions as a component that forms Hf—YO compound crystals. Moreover, it is preferable to add Mo as molybdenum carbide (Mo ₂ C) particles. As described above, the molybdenum compound particles serve as a grain boundary phase strengthening component and function as a component for improving the lubricity of the sliding surface. The particles of the 5A group element and the 6A group element have lubricity, and among them, the molybdenum compound has the most excellent lubricity.

Moreover, as a component other than the above, silicon carbide (SiC) is exemplified as the carbide particles. When silicon carbide is added, it is preferably in the range of 1 to 5 wt%.

Further, the molar ratio of Al to the rare earth element is preferably Al (mol): rare earth element (mol) = 1: 1 to 8: 1. By making Al content 1 to 8 times the amount of rare earth element, hexagonal α-SiAlON crystals are easily formed. Further, Al (mol): rare earth element (mol) is preferably in the range of 1.4: 1 to 7.0: 1.

Further, when each of the above-mentioned components is added as a sintering aid, the total oxygen content of the sintering aid is in the range of 1.20 to 2.50 parts by mass when the amount of silicon nitride powder is 100 parts by mass. It is preferable to become. The amount added as an oxide can be made the said range by adjusting the ratio added as nitride or carbide as an addition form of various sintering aids. For example, a method of adding an Al component with AlN and a Mo component with Mo ₂ C or the like can be mentioned. A hexagonal α-SiAlON crystal can be easily formed by reducing the total amount of oxygen in the sintering aid and increasing the number of nitrides. The oxide sintering aid contributes to the formation of a grain boundary phase such as an amorphous phase containing Y—Al—O. Therefore, the area ratio of the grain boundary phase can also be controlled by setting the total oxygen content of the sintering aid within the above range.

In the case of the silicon nitride sintered body as described above, the Vickers hardness (Hv) is 1500 or more, the fracture toughness value (K _1c ) is 6.0 MPa · m ^1/2 or more, and the three-point bending strength is 900 MPa or more. Excellent properties can be shown.

The Vickers hardness (Hv) is measured according to JIS-R-1610. The fracture toughness value (K _1C ) is measured according to the indenter press-in method (IF method) of JIS-R-1607. In addition, Niihara's formula is used to calculate the fracture toughness value. The three-point bending strength is measured according to JIS-R-1601.

(Second embodiment)
The second embodiment is a sliding member using the silicon nitride sintered body of the first embodiment. Examples of the sliding member include a bearing ball, a roller, a check ball, a wear pad, a plunger, and a roller. These sliding members slide with a mating member made of a metal member or ceramics. In order to increase the durability of the sliding surface, it is preferable to use a polished surface having a surface roughness (Ra) of 0.5 μm or less, further 0.1 μm or less, more preferably 0.05 μm or less. By flattening the sliding surface, it is possible to improve the durability of the silicon nitride sintered body and reduce the aggressiveness to the mating member. By reducing the aggression of the mating member, the wear of the mating member can be reduced, so that the durability of the device incorporating the sliding member can be improved.

As the device incorporating the sliding member, there are products in various fields such as machine tools, electronic devices, automobiles, airplanes, and even wind power generation.

FIG. 1 shows a bearing ball as an example of a sliding member and a rolling member. In FIG. 1, reference numeral 1 denotes a bearing ball. The bearing ball 1 is a true sphere. In general, a bearing is configured by arranging a plurality of bearing balls. The entire surface of the bearing ball is a sliding surface. In addition, since a plurality of bearing balls are used in configuring the bearing, shape uniformity is also required. In the silicon nitride sintered body according to the first embodiment, the area ratio of the grain boundary phase and the machinable coefficient Mc are adjusted to 0.100 to 0.120, so that the surface roughness (Ra) is 0.5 μm or less. Even if it is processed, there is little degranulation and the degranulation trace can be reduced. Therefore, when a polishing process using diamond abrasive grains is performed, a clean flat surface with few degranulation traces can be obtained.

Also, the sliding characteristics can be stabilized over a long period of time by controlling the area ratio of the grain boundary phase per unit area 100 μm × 100 μm, the crystal component, and the like. For example, in the case of a conventional bearing ball, which has a durability of 400 to 500 hours in continuous operation, a durability of 700 hours or more, further 800 hours or more can be obtained. For this reason, not only the long-term reliability of the sliding member is maintained, but also the effects such as the long-term reliability and maintenance-free of the apparatus incorporating the sliding member can be obtained.

(Method for producing a silicon nitride sintered body according to the first embodiment)
Next, a manufacturing method will be described. As long as the silicon nitride sintered body of the first embodiment has the above-described configuration, the manufacturing method is not particularly limited, but the following methods can be cited as methods for obtaining them efficiently.

First, silicon nitride powder is prepared. The silicon nitride powder has an oxygen content of 1.7% by mass or less, contains α-phase type silicon nitride (α-Si ₃ N ₄ ) of 85% by mass or more, has an average particle size of 1.0 μm or less, and further 0.8 μm. The following is preferable. By growing grains of α-Si ₃ N ₄ powder into β-Si ₃ N ₄ crystals in the sintering step, a silicon nitride sintered body having excellent sliding characteristics can be obtained.

Next, a sintering aid powder is prepared. As a sintering aid, an Al component and a rare earth element component are essential components. Moreover, at least 1 sort (s) chosen from a 4A group element component, a 5A group element component, and a 6A group element component and silicon carbide shall be added as needed.

Further, as described above, it is preferable to add such that the molar ratio of Al to the rare earth element is Al (mol): rare earth element (mol) = 1: 1 to 8: 1. Further, when adding the sintering aid, when the amount of silicon nitride powder is 100 parts by mass, the total oxygen content of the sintering aid is preferably in the range of 1.20 to 2.50 parts by mass. In order to control the amount of oxygen of the component added as a sintering aid, it is preferable to add the Al component as AlN, the 5A group element component, and the 6A group element component as carbides. In particular, AlN is a sintering aid effective for reacting with α-Si ₃ N ₄ together with rare earth elements to form a hexagonal α-SiAlON crystal.

Next, the silicon nitride powder and the sintering aid powder are mixed. In the mixing step, mixing is performed using a ball mill mixer or the like so as to obtain a uniform mixed state. In particular, when the dispersion state of the α-Si ₃ N ₄ powder, the AlN powder, and the rare earth compound is uniformly mixed, hexagonal α-SiAlON crystals are easily formed uniformly. For uniform mixing, it is preferable to carry out a mixing process using a ball mill mixer or the like for 50 hours or more. It is also effective for uniform mixing to use a wet mixing method in which the mixing step is performed in a solution.

Further, in the step of preparing the raw material mixture in a wet process, it is preferable to prepare the Al-based compound and the rare earth compound in advance in a slurry having good dispersibility, and then mix it with the slurry of silicon nitride powder as the main raw material. The index of dispersibility at this time is preferably managed using the following thixotropy index (TI value). When the shear speeds a and b are continuously increased in a rotational viscometer, the viscosity of a fluid having cohesive properties generally decreases. At this time, the ratio of the viscosity value η at the shear rates a and b becomes the TI value.

TI value = ηb / ηa
Although the values of the shear rates a and b are not particularly determined, it is preferable to set the TI value to be 1 or more. The closer the TI value is to 1, the closer to the behavior of the Newtonian fluid, suggesting a highly dispersed slurry with no or very weak aggregation. In this case, adjustment was made so that the TI value was 1.0 to 2.0 when a = 6 (1 / s) and b = 60 (1 / s). Here, s indicates seconds. By using such a mixing method, it is possible to effectively perform uniform grain growth of hexagonal α-SiAlON crystals during sintering.

Next, a binder is added to the raw material mixture obtained by mixing the silicon nitride powder and the sintering aid powder. The mixing of the raw material mixture and the binder is performed using a ball mill or the like while performing pulverization and granulation as necessary. The raw material mixture is formed into a desired shape. The molding process is performed by a die press, a cold isostatic press (CIP), or the like. The molding pressure is preferably 100 MPa or more. Next, the molded body obtained in the molding process is degreased. The degreasing step is preferably performed at a temperature in the range of 300 to 600 ° C. The degreasing step is performed in the air or in a non-oxidizing atmosphere, and the atmosphere is not particularly limited.

Next, the degreased body obtained in the degreasing step is sintered at a temperature in the range of 1600 to 1900 ° C. If the sintering temperature is less than 1600 ° C., the crystal growth of silicon nitride crystal particles may be insufficient. That is, the reaction from α-Si ₃ N ₄ to β-Si ₃ N ₄ is insufficient, and a dense sintered body structure may not be obtained. In this case, the reliability as a material of the silicon nitride sintered body is lowered. If the sintering temperature exceeds 1900 ° C., the silicon nitride crystal grains grow too much, which may cause a decrease in strength and the ratio of the grain boundary phase may be out of the range.

The above sintering step may be performed by either normal pressure sintering or pressure sintering. The sintering step is preferably performed in a non-oxidizing atmosphere. Examples of the non-oxidizing atmosphere include a nitrogen atmosphere and an argon atmosphere.

It is preferable to perform a hot isostatic pressing (HIP) treatment of 30 MPa or more in a non-oxidizing atmosphere after the sintering step. Examples of the non-oxidizing atmosphere include a nitrogen atmosphere and an argon atmosphere. The HIP treatment temperature is preferably in the range of 1500 to 1900 ° C. By performing the HIP process, the pores in the silicon nitride sintered body can be eliminated. If the HIP processing pressure is less than 30 MPa, such an effect cannot be sufficiently obtained.

The silicon nitride sintered body thus manufactured is subjected to polishing processing at a necessary portion to produce a sliding member. The polishing process is preferably performed using diamond abrasive grains. Since the silicon nitride sintered body according to the embodiment has good workability, it is possible to reduce the processing cost when manufacturing the sliding member from the silicon nitride sintered body. Further, a flat surface having a surface roughness (Ra) of 0.5 μm or less, further 0.1 μm or less, and 0.05 μm or less can be obtained.

(Example 1)
(Examples 1 to 13 and Comparative Examples 1 and 2)
As the silicon nitride powder, one having an oxygen content of 1.2% by mass, an average particle diameter of 0.7 μm, and a ratio of α-Si ₃ N ₄ of 99% by mass was prepared. Next, those shown in Table 1 were prepared as sintering aids. As the sintering aid powder, those having an average particle size of 1.3 μm or less were used.

Next, the silicon nitride powder and the sintering aid powder were blended and wet mixed for 50 hours by a ball mill. Mixing was performed so that the TI value at this time was 1.0 to 2.0. Next, after taking out from the solution and drying, it mixed with the binder and performed the mixing process for 20 hours with the ball mill, and mixed raw material powder was prepared, respectively.

Next, each raw material mixture was molded by a die press and degreased at 460 ° C. Next, the degreased body was sintered in a nitrogen atmosphere at 1700 to 1800 ° C. for 4 to 6 hours.

Next, the obtained sintered body was subjected to HIP treatment. The HIP treatment was performed under the conditions of 1600 ° C. × 1 to 2 hours under a pressure of 100 MPa. Thus, silicon nitride sintered bodies according to Examples 1 to 13 and Comparative Examples 1 and 2 were produced. A sample for measuring three-point bending strength (silicon nitride sintered body) was used after being processed into a size of 3 mm × 4 mm × 50 mm.

The samples of Examples 1 to 13 and Comparative Examples 1 and 2 (silicon nitride sintered bodies) were polished to a surface roughness (Ra) of 0.1 μm or less. Thereafter, Vickers hardness (Hv), fracture toughness value (K _1C ), three-point bending strength, and machinable coefficient Mc were measured. Vickers hardness (Hv) was measured by a method according to JIS-R-1610 with an indentation load of 20 kgf. The fracture toughness value was measured according to the indenter press-fitting method (IF method) of JIS-R-1607 with an indentation load of 20 kgf and determined by the Niihara equation. The three-point bending strength was measured by a method according to JIS-R-1601. The results are shown in Table 2.

As is clear from the results shown in Tables 1-2 above, the silicon nitride sintered body according to each example has a Vickers hardness (Hv) of 1500 or more and a fracture toughness value (K _1C ) of 6.0 MPa · m ^1/2 or more, the three-point bending strength was 900 MPa or more, and the machinable coefficient Mc was in the range of 0.100 to 0.120. As an example of the calculation of Mc, the machinable coefficient of Example 1 is that indentation load Fn = 20 kgf, Vickers hardness Hv = 1599, fracture toughness value K _1c = 6.6 MPa · m ^1/2 , Mc = 20 ^9/8 / (6. 6 ^1/2 · 1592 ^5/8 ).

Moreover, the peak intensity ratio of each crystal was determined by XRD analysis using the polished surface of the cross section. The results are shown in Table 3.

Next, the area ratio of the grain boundary phase per unit area of 100 μm × 100 μm was determined using an SEM photograph of the polished surface of the cross section. As for the area ratio of the grain boundary phase, the unit area of 100 μm × 100 μm was measured at five locations, and the upper limit and lower limit were described. Further, the presence or absence of “Hf—Y—O-based compound crystal” and “Y—Al—O-based compound amorphous phase” in the grain boundary phase was examined by TEM analysis. The results are shown in Table 4 below.

As is apparent from the results shown in Table 4, the silicon nitride sintered body according to each example had an area ratio of the grain boundary phase in the range of 25 to 40%.

(Examples 1B to 13B and Comparative Examples 1B to 2B)
Using the same manufacturing method as in Examples 1 to 13 and Comparative Examples 1 and 2, bearing balls as sliding members were produced. The bearing ball was polished to have a diameter of 9.525 mm and a surface roughness (Ra) of 0.01 μm.

Regarding the polishing process, the surface roughness (Ra) is prepared as a sample before polishing to 0.01 μm, and the surface roughness when the polishing process is performed using a diamond grindstone (# 120) is compared. did. The polishing conditions were as follows: the surface roughness after processing until the time when there was no change in the surface roughness (Ra), with the processing area of the sample being constant, the load being 40 N, the rotational speed of the grinding machine being 300 rpm, The thickness (Ra) was measured. The degranulation state can be measured by this polishing process. The degranulation state has a correlation with the surface roughness, meaning that the larger the value, the easier the degranulation occurs, and it is assumed that the reliability in the rolling life test tends to decrease.

Also, the rolling life and the change in crushing strength before and after the rolling life were measured. In measuring the rolling life and crushing strength, a bearing ball having a finished surface that was polished so that the surface roughness (Ra) was 0.01 μm was used.

Also, for the rolling life, three bearing balls according to each embodiment are prepared, and the three bearing balls are arranged at equal intervals on a track having a diameter of 40 mm set on the upper surface of the bearing steel SUJ2. This is the rolling life as the time until the surface of the bearing ball peels off at a rotational speed of 1200 rpm with a load applied so that the maximum contact stress of 5.9 GPa acts on the bearing ball under the oil bath lubrication condition of turbine oil. Was measured. Note that the rolling life was measured with an upper limit of 800 hours continuous.

Further, regarding the crushing strength before and after the rolling test, the load at which the bearing ball was broken was determined by the two-ball crushing method. The results are shown in Table 5.

As is clear from the results shown in Table 5 above, the bearing balls according to the respective examples exhibited excellent sliding characteristics for 700 hours or more. In particular, it was found that Examples 1 to 6 having Hf—Y—O-based compound crystals and adding Mo ₂ C as a sintering aid maintained excellent characteristics even after 800 hours.

The bearing ball is a sliding member that uses the entire surface of the sintered body as a sliding surface. Therefore, if the characteristic as a bearing ball is excellent, even if it is used for other sliding members, the excellent characteristic is exhibited. Therefore, the silicon nitride sintered body of the embodiment can be applied to various sliding members.

In addition, although several embodiment of this invention was described, these embodiment is shown as an example and is not intending limiting the range of invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

According to the silicon nitride sintered body and the sliding member using the same according to the present invention, it is possible to provide a silicon nitride sintered body that can provide long-term reliability with respect to sliding characteristics. In addition, since the machinable coefficient is adjusted, even if the surface is polished, it is easy to obtain a flat sliding surface because degranulation can be reduced. Therefore, long-term reliability can be obtained also with respect to the sliding member using the sintered body.

1. Bearing ball <Silicon nitride sintered body, sliding member>

Claims

When the silicon nitride sintered body was subjected to XRD analysis, the strongest peak intensity detected at 29.6 ± 0.3 ° and 31.0 ± 0.3 ° corresponding to the hexagonal α-SiAlON crystal was I 29. 6 ° and I 31.0 ° , while the strongest peak intensities detected at 33.6 ± 0.3 ° and 36.1 ± 0.3 ° corresponding to β-Si 3 N 4 crystals are I 33. When 6 ° and I 36.1 ° , each strongest peak intensity is expressed by the following relational expression:
I 29.6 ° + I 31.0 ° ) / (I 33.6 ° + I 36.1 ° ) = 0.10 to 0.30 (1)
The filling,
Nitriding characterized in that the area ratio of the grain boundary phase per unit area of 100 μm × 100 μm in an arbitrary cross section of the silicon nitride sintered body is 25 to 40% and the machinable coefficient is 0.100 to 0.120. Silicon sintered body.
When the silicon nitride sintered body was subjected to XRD analysis, the strongest peak intensity I 39.5 ° detected at 39.5 ± 0.3 ° corresponding to Y 4 Si 2 O 7 N 2 (J phase) was as follows: Relational expression:
(I 39.5 ° ) / (I 33.6 ° + I 36.1 ° ) = 0.03 to 0.10 (2)
The silicon nitride sintered body according to claim 1, wherein:
When the silicon nitride sintered body is subjected to XRD analysis, it corresponds to any one or more of Y 2 Si 3 O 12 N (H phase), YSiO 2 N (K phase), or Y 2 Si 3 O 3 N 4 31 The strongest peak intensity I 31.9 ° detected at .9 ± 0.3 ° is the following relational expression:
(I 31.9 ° ) / (I 33.6 ° + I 36.1 ° ) = 0.05 to 0.15 (3)
The silicon nitride sintered body according to any one of claims 1 to 2, wherein:
The silicon nitride sintered body according to any one of claims 1 to 3, wherein the XRD analysis is performed on an arbitrary cross section of the silicon nitride sintered body.
5. The grain boundary phase including an amorphous phase containing a Hf—Y—O-based compound crystal and Y—Al—O, according to any one of claims 1 to 4. Silicon nitride sintered body.
The silicon nitride sintered body according to any one of claims 1 to 5, comprising at least one of oxide, carbide, and nitride particles having an average particle diameter of 2 µm or less.
The silicon nitride sintered body according to claim 6, wherein the particles are molybdenum compound particles.
5 to 10% by mass in terms of oxide of Al, 1 to 10% by mass in terms of oxide of any one or more of rare earth elements One or more of 4A, 5A, and 6A elements in terms of oxide 1 to 5% by mass, and the molar ratio of Al to the rare earth element is Al (mol): rare earth element (mol) = 1: 1 to 8: 1. Item 8. The silicon nitride sintered body according to any one of items 7 to 9.
The Vickers hardness (Hv) is 1500 or more, the fracture toughness value (K 1C ) is 6.0 MPa · m 1/2 or more, and the three-point bending strength is 900 MPa or more. The silicon nitride sintered body according to any one of claims 8 to 9.
A sliding member comprising the silicon nitride sintered body according to any one of claims 1 to 9.
The sliding member according to claim 10, wherein the sliding member is a bearing ball.
The sliding member according to any one of claims 10 to 11, wherein the sliding surface is a polished surface having a surface roughness (Ra) of 0.5 µm or less.