JP2001151576A - Sliding member - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve a problem that when a sliding member made of a ceramic material, such as a conventional ceramic bearing is used at a part highly loaded and rotating with a high speed, the bearing is seized due to the generation of the frictional heat and becomes impossible to be used. SOLUTION: A sliding member is made of a silicon nitride sintered compact which contains silicon nitride as a main component, 1 to 30 wt.%, expressed in terms of the oxide, of a compound of a rare earth element and 2 to 20 wt.% SiC having the aspect ratio of <=3 and an average longitudinal diameter of <=6 μm and which has a bulk density of >=98% of the theoretical density.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sliding member made of a silicon nitride sintered body having excellent seizure resistance and used for, for example, a bearing of a mechanical part.

[0002]

2. Description of the Related Art Silicon nitride sintered bodies are expected to be used as structural mechanical component materials because of their high strength and excellent wear resistance and rigidity. Ball, roller, roller pin for high pressure pump, etc.

In firing the above silicon nitride sintered body,
Since silicon nitride as a raw material of the silicon nitride-based sintered body has no self-sintering property, it is fired by adding a sintering aid. As the sintering aid, a rare earth element oxide such as Y ₂ O ₃ or an oxide such as Al ₂ O ₃ , MgO or CaO is generally used in combination. Then, these sintering aids are mixed with silicon nitride powder, molded, and fired to obtain a silicon nitride sintered body. Examples of the firing method include normal-pressure firing performed under normal pressure and atmospheric pressure firing using nitrogen or argon gas. Further, in the sintered body obtained by the normal-pressure firing, the residual In order to eliminate pores and improve mechanical strength, HIP (Hot Isostatic Pressing) treatment is performed.

[0004] In particular, when used as a sliding part, defects inherent in the material (pores, inclusions, abnormalities in the structure, etc.) cause peeling on the surface due to fatigue on the sliding surface. After baking at a relatively low temperature to suppress crystal grain growth, HIP
Techniques such as performing a treatment and firing under atmospheric pressure are used. The sintered body thus obtained is used as a sliding component after precision processing as a product.

[0005] Further, sliding parts such as bearing parts tend to be heated to a high temperature due to friction when rotating at a high load and at a high speed, and may locally reach a temperature at which oil or grease in the sliding part is carbonized. It is also important to efficiently disperse the heat generated from the sliding portion. For this reason, ceramic sliding parts that are lightweight and resistant to wear and heat have attracted attention.

In recent years, with the increase in speed of machine tools, which are the main applications of ceramic bearings, and the market development in the aviation and space industries, the need for high-load ceramic bearings in high-temperature environments, and high-speed HDDs that dislike noise have been increasingly developed. There is a growing need for ceramic bearings for bearings. Roller pins used in high-pressure small pumps for fuel injection of automobiles are also used in high-temperature and high-load environments, and thus the need for ceramic roller pins is increasing.

As described above, the environment in which ceramic bearings are used is a part which is likely to be heated to a high temperature due to frictional heat or the like. However, in recent trends, ceramic bearings are used for cooling and lubricating the bearings for the purpose of improving the work environment in the workplace. The use of oil is abolished, and applications where lubrication and cooling are difficult, such as HDD bearings and the space industry, are increasing, and a harsher and non-cooling environment is becoming more severe. Then, problems such as breakage of the sliding portion due to seizure of the ceramic bearing and generation of dust due to carbonization of the grease are caused. This is true not only for ceramic bearings but also for sliding parts such as roller pins for high-pressure pumps.

That is, in order for a sliding component such as a ceramic bearing to maintain sufficient reliability and good sliding characteristics for a long period of time, the thermal conductivity of a sliding member such as a rolling element of the ceramic bearing must be increased. In addition, it is necessary to disperse the temperature of the sliding portion so as not to be locally heated to prevent seizure of the sliding portion and carbonization of lubricating oil or grease.

[0009]

As described above, ceramic sliding parts typified by conventional ceramic bearings cannot be used in a high-load, high-speed rotating part because the bearing is seized due to heat generation due to friction. There was a problem of disappearing.

In general, as a method for increasing the thermal conductivity of a silicon nitride sintered body, a method of mixing and sintering a substance having a higher thermal conductivity than silicon nitride, a method of mixing a silicon nitride sintered body, There is a method in which the amorphous phase at the crystal grain boundaries is crystallized by adjusting the composition or further heat-treating the silicon nitride sintered body.

However, in the method of adding a substance having a high thermal conductivity as described above or the method of crystallizing an amorphous phase at a crystal grain boundary, the additive may adversely affect sintering or may be broken. However, there is a problem that the properties required for the rolling elements and sliding parts of the ceramic bearing cannot be sufficiently satisfied, because the strength is likely to decrease due to the growth of silicon nitride grains and the crystal grain boundaries are easily embrittled. Was.

[0012]

The inventors of the present invention have conducted intensive studies and as a result, have found that silicon nitride is the main component and 1 to 30 in terms of oxide.
A silicon nitride-based sintered body containing 2% to 20% by weight of a rare earth element compound of 2% by weight, 2% to 20% by weight of SiC having an aspect ratio of 3 or less and an average major axis of 6 μm or less, and a bulk density of 98% or more of the theoretical density. It has been found that the above problem can be solved by using the characteristic sliding member.

A rare earth element compound containing silicon nitride as a main component, 1 to 30% by weight in terms of oxide, and an average particle diameter of 5 μm.
It has been found that a similar result can be obtained in a silicon nitride based sintered body containing 1 to 30% by weight of HfO ₂ , TiN or the like of m or less.

[0014]

Embodiments of the present invention will be described below.

The sliding member of the present invention comprises silicon nitride as a main component, 1 to 30% by weight of a rare earth element compound in terms of oxide, and Si having an aspect ratio of 3 or less and an average long diameter of 6 μm or less.
Contains 2 to 20% by weight of C and has a bulk specific gravity of 98% of the theoretical density
It is characterized by being composed of the silicon nitride sintered body described above.

The reason why a rare earth element compound is used as a sintering aid for silicon nitride is that it has a higher fracture toughness than a case where magnesium oxide, calcium oxide, or the like is used as a sintering aid, and is preferable as a wear-resistant part. Although the reason for this is not clear, it is considered that the properties of the grain boundary phase are different, and the amount is preferably 1 to 30% by weight in terms of oxide. The reason for selecting this range is that if it is less than 1% by weight, it is necessary to raise the firing temperature for densification, so that the mechanical properties of the obtained nitrided sintered body tend to decrease. ,Also,
If the content exceeds 30% by weight, the mechanical properties of silicon nitride tend to deteriorate. That is, in order to obtain a firing temperature that does not impair the characteristics of silicon nitride, the content of the rare earth element compound is desirably 1 to 30% by weight in terms of oxide.

Incidentally, as the kind of the rare earth element used in the present invention, Y, Er, Yb and Lu can be used.

Next, the contained SiC will be described. The amount is preferably 2 to 20% by weight. S
When the iC is less than 2% by weight, the crystallization of the grain boundary phase is insufficient, and the ratio of the grain boundary crystal phase to the entire crystal phase does not become 50% or more. Can not be obtained. On the other hand, if it exceeds 20% by weight, a large amount of hardly sinterable SiC will be contained in the sintering aid, and the sintering during sintering will not proceed and the densification will not be sufficient. The bulk density tends to be 98% or less of the theoretical density,
Deterioration of mechanical properties is seen. Further, the aspect ratio of SiC is 3 or more (for example, needle-like particles such as whiskers).
When the major axis is 6 μm or more, the SiC particles become large with respect to the crystal grain boundary structure having a fine and three-dimensionally complicated shape, so that the dispersion at the crystal grain boundaries becomes non-uniform, and the sintering resistance becomes poor. This is because the SiC particles tend to agglomerate and the mechanical properties of the sintered body tend to decrease.

In order to obtain good seizure resistance, it is important that the ratio of the grain boundary crystal phase to the entire crystal phase is 50% or more and the thermal conductivity is 22 W / m · K or more. A sliding portion using ceramics is a portion that slides at a high speed while receiving a high load, or a portion where cooling and lubrication cannot be performed, and the sliding portion is likely to be heated to a high temperature. That is, in order to maintain excellent seizure resistance in such an environment, it is important to efficiently transmit heat generated by friction of the sliding portion to the surrounding members to release the heat. For that purpose, the thermal conductivity of the sliding member moving at high speed is 22 W
/ m · K or more is desirable. In general, when improving the thermal conductivity of a silicon nitride based sintered body, for example, the amount of aluminum oxide that acts to suppress crystallization is reduced, and the composition of the grain boundary phase is adjusted to a composition that facilitates crystallization. . However, such a composition greatly deteriorates the original mechanical properties of the sintered body, so that the sintered body may be damaged or chipped during high-speed motion or high-speed rotation while receiving a high load, and the surface of the sintered body may be damaged. Peeling or uneven wear occurs, causing noise or reducing reliability.

Here, when the above-described SiC is contained, SiC is dispersed as crystals in the grain boundary phase.
It has been found that it promotes the precipitation of a crystalline phase such as ₁₀ Al ₂ Si ₃ O ₁₈ N ₄ . Although the reason for promoting the precipitation of such a crystal phase is not clear, this effect reduces the SiC content to 2%.
Even if the amount is as small as 4% by weight, the ratio of the grain boundary crystal phase to the entire grain boundary phase becomes 50% or more without impairing the intrinsic characteristics of the silicon nitride sintered body, and becomes 22 W / m · K or more. Is obtained.

The ratio of the grain boundary crystal phase at this time (A)
Was calculated as follows.

A = V2 / V1 * 100 (%) V1: Volume ratio of the whole grain boundary phase V2: Volume ratio of the grain boundary crystal phase The volume ratio (V1) of the whole grain boundary phase is obtained by etching the polished surface of the sintered body. Then, after performing processing so that the grain boundary phase and the silicon nitride particles can be distinguished, the area ratio (%) of the grain boundary phase is obtained from a photograph obtained by an electron microscope, and the value is calculated as the volume ratio of the whole grain boundary phase. And

To determine the volume ratio (V2) of the grain boundary crystal phase, the obtained sintered body is first crushed, and the peak intensity of the silicon nitride crystal phase and the peak of the grain boundary crystal phase are determined by X-ray diffraction. The strength was determined and calculated by the following equation.

V2 = H1 / H2 * 5 * β * (100−V1) / 100 H1: Main peak height of grain boundary crystal phase H2: Si ₃ N ₄ β (110) peak height β: Si ₃ N ₄ At this time, when the main peak of the grain boundary crystal phase does not overlap with the peak of Si ₃ N ₄ and cannot be determined, Ix / I0 = K (Ix:
Survey peak height, I0: Main peak height)
Is used, and a value obtained by multiplying the peak intensity by 100 / K is used.

Further, the above-mentioned effect is achieved by reducing the content of a rare earth element oxide containing 1 to 30% by weight of oxide and HfO _{2 and} TiN having an average particle diameter of 5 μm or less to 0.5 to 100% by weight.
It was also confirmed when it contained 30% by weight. This is because the HfO ₂ , ZrB ₂ , and TiN particles of the high-melting point material do not dissolve in the grain boundary phase but remain in the grain boundary phase. It is thought that it is reprecipitated in the initial state in the grain boundary phase, but it is not clear. When this sintered body was pulverized and analyzed by fluorescent X-ray, HfO ₂ , T
An X-ray peak unique to each iN was obtained. When the content is 0.5% by weight or less, the proportion of the grain boundary crystal phase is so small that a thermal conductivity of 22 W / m · K or more cannot be obtained.
When the content is 30% by weight or more, HfO ₂ and TiN have a high melting point, which tends to hinder the sintering of the silicon nitride sintered body, and the mechanical properties of the sintered body are deteriorated.

The silicon nitride-based sintered body of the present invention has a four-point bending strength (JIS R1601) of 700 MPa or more and a fracture toughness (JIS R1607) of 4 without impairing the original characteristics of the silicon nitride-based sintered body. It has high strength and high toughness mechanical properties of 0 MPam ^1/2 or more.

Next, a manufacturing method for obtaining the silicon nitride sintered body of the present invention will be described.

A predetermined amount of the raw material powder is weighed, and mixed by a known mixing method, for example, a rotary mill, a vibration mill, or a barrel mill.
PA, methanol, water and the like are mixed as a solvent. In some cases, dry mixing without using a solvent may be used.

The resulting mixed powder is molded into a desired molding means,
For example, an arbitrary shape is formed by a die press, a cold isostatic press, an extrusion molding, an injection molding, a casting molding, or the like.
Depending on the molding method, granulation by spray drying or the like,
Preparations such as preparation of a clay having a certain viscosity together with water and an organic binder are also required, but a normal ceramics molding procedure may be used.

If drying and degreasing are required after molding, heat treatment is performed at a temperature of 50 ° C. to 1400 ° C. in nitrogen, vacuum, or air.

The sintering is performed at 1600 ° C. to 2000 ° C. in a nitrogen-containing non-oxide atmosphere. If the sintering temperature is too high, silicon nitride crystals, which are the main phase, grow and the strength decreases. It is desirable to carry out at a temperature of ° C. In the case of firing at 1800 ° C. or more, it is necessary to fire at a partial pressure of nitrogen equal to or higher than the equilibrium pressure since silicon nitride is decomposed. Furthermore, after these calcinations, a more dense sintered body can be obtained by performing HIP (Hot Isostatic Pressing) treatment. The HIP temperature is 1500 to 17
50 ° C. is preferred.

As a result of the above calcination, the raw material of the silicon nitride is α,
In any case of β, a sintered body containing a β-silicon nitride main crystal phase and a grain boundary phase is finally obtained. When a rare earth element oxide or aluminum oxide is used as a sintering aid, the grain boundary phase is composed of a grain boundary phase containing a rare earth element, aluminum, oxygen and nitrogen. SiC or HfO ₂ , TiN is distributed as crystals, and furthermore, Y ₁₀
The crystallization of Al ₂ Si ₃ O ₁₈ N _{4 and the} like is also promoted, and the ratio of the grain boundary crystal phase to the entire crystal phase becomes 50% or more, and 22 W
A silicon nitride-based sintered body having a thermal conductivity of / mK or more can be obtained.

As described above, the silicon nitride sintered body of the present invention can be used for various ceramic parts, in particular, for balls such as rolling elements of bearings, piston pins, roller pins, rocker arm chips, roller bushes, cam rollers, valves and the like. It can be widely used for parts exposed to high temperatures such as sliding parts and cutting tools.

[0034]

Embodiments of the present invention will be described below.

First, silicon nitride powder (BET specific surface area 9 m^Two
/ G) Yb as a sintering aid_TwoO_ThreePowder, Er_TwoO_Three
Powder, Y_TwoO_Three0.5 to 35% by weight of one of the powders, A
l_TwoO _Three3% by weight of powder and 1-22 weight of SiC powder
% Or HfO_TwoPowder, one of TiN powders is 0.5
４０40% by weight in each proportion, and use water.
The slurry was prepared by mixing well. Then the above
Add an organic binder to the slurry and spray dry.
Raw material granules were obtained. Next, using the raw material granules,
Then, a columnar molded body was produced by molding, and this molded body was used for 1.
Temperature range of 800 ° C to 1400 ° C under reduced pressure of 33 kPa or less
To remove the binder.

Next, the compact was fired in a nitrogen gas atmosphere under pressure. The atmosphere was pressurized before the sintering of the silicon nitride was started, and the pressure was increased to 1600 to 200 ° C. under the pressure to fire. Thereafter, HIP treatment was further performed under a nitrogen pressure of 1000 to 2000 atm to obtain a sintered body of a cylindrical roller pin. The obtained sintered body of the cylindrical roller pin was processed to a predetermined outer diameter by a centerless polishing machine, and a surface to be a sliding surface was mirror-polished.

The columnar roller pin sintered body was cut, the cross section thereof was mirror-polished, etched, and the area ratio of the grain boundary phase was determined from the photograph of the 3,000-fold reflected electron image taken with an electron microscope, that is, The volume ratio of the whole grain boundary phase was determined.

Further, the sintered body was pulverized, and the peak height of the silicon nitride crystal phase and the peak height of the grain boundary crystal phase were measured by X-ray. Thereby, the contained SiC crystal peak and Y ₁₀ Al ₂ Si ₃ O larger than this SiC crystal peak were obtained.
A crystal phase of ₁₈ N ₄ was observed. In addition, this Y ₁₀ Al ₂ Si
It was found that the peak intensity of the ₃ O ₁₈ N ₄ crystal phase was proportional to the amount of SiC added. The ratio of the crystal phase in the grain boundary was determined by substituting the respective peak heights into the above-mentioned equation.

The bending strength and fracture toughness were evaluated as follows:
A test piece was prepared by press molding using the above granules, fired under the same firing conditions, polished with a surface grinder, and determined to have a bending strength based on JIS R1601.
Based on R1607, the fracture toughness was measured with an Instron universal testing machine at a crosshead speed of 5 mm / min.

The evaluation of seizure resistance was performed by setting a roller pin machined to a ／ inch diameter in an 8-pin type bearing (grease: lubricating oil 5W-30 base oil) under a load condition of 2100 rpm with a load of 1000 N on the bearing. A durability evaluation test was performed to evaluate seizure resistance. The material of the inner ring and the outer ring of the bearing was SUK-2.

Next, how to determine the ratio of the bulk density to the theoretical density will be described. The theoretical density (T) determines the structure and component ratio of silicon nitride and each component according to the structural analysis and composition analysis values by X-ray diffraction, and assumes that each component is blended at each theoretical density and component ratio. Calculated. The bulk density (B) was measured according to JIS R-2205. The ratio of the bulk density to the theoretical density was determined by calculating B / T.

Table 1 summarizes the evaluation results.

[0043]

[Table 1]

As is apparent from Table 1, the rare earth element compound as the sintering aid was less than 1% by weight in terms of oxide. In the case of 1, the sintering is insufficient and the bulk density is 7
It becomes 9% or less, so that there are many voids on the surface and inside of the sintered body, so that it cannot be used as a sliding part. Further, when the rare earth element oxide contains more than 30% by weight of N
o. 8 shows that the strength of the sintered body is deteriorated because the volume ratio of the fragile grain boundary phase is increased. On the other hand, NO. Nos. 2 to 7 and No. 2 using Er, Yb, and Y as rare earth elements. Nos. 9 to 12 have a bulk density of 98% or more of the theoretical density and a strength of 700 MPa or more, and can be used as sliding parts.

Next, regarding the amount of SiC contained,
o. From the results of Nos. 14 to 19, No. 14 containing less than 2% by weight of SiC. In No. 14, the crystallization of the grain boundary phase is insufficient, and the ratio of the grain boundary crystal phase to the entire crystal phase does not become 50% or more, so that the thermal conductivity is low and good seizure resistance cannot be obtained.
In the durability evaluation, the composition having a thermal conductivity of 22 W / m · K or more did not burn for 400 hours or more. In No. 14, burning occurred in 120 hours. In the case of No. 3 having SiC exceeding 20% by weight. In No. 19, a large amount of hardly sinterable SiC was contained in the sintering aid, and the result was that the obtained sintered body had a low density and a large deterioration in strength. On the other hand, No. 1 within the scope of the present invention. In Nos. 15 to 18, the ratio of the grain boundary crystal phase in the grain boundaries was calculated to be in the range of 52 to 112%, and no significant strength deterioration was observed. The thermal conductivity was 22 W / when the amount of SiC was 2% by weight.
m · K, 10 Wt%, 30 W / m · K, SiC
A good correlation was found between the addition amount and the ratio of the grain boundary crystal phase, and between the SiC addition amount and the thermal conductivity.

Regarding the aspect ratio and shape of SiC, 4 and no. According to Nos. 20 to 24, the aspect ratio of SiC exceeds 3 (for example, needle-like particles such as whiskers). No. 24 and No. 24 whose major axis is 6 μm or more. No. 22, large SiC particles hinder sintering of silicon nitride crystal grains having a fine and three-dimensionally complex shape, and the SiC particles cannot be uniformly dispersed at crystal grain boundaries, resulting in non-uniform dispersion. Thus, aggregation of the hardly sinterable SiC particles was observed, and the mechanical properties of the sintered body tended to decrease.
On the other hand, according to the claims of the present invention, No. 1 having an aspect ratio of SiC of 3 or less and a major axis of 6 μm or less was used.
Nos. 4, 20, 21, and 23 have no strength deterioration because the bulk density of the sintered body is 98% or more of the theoretical density.

Further, Hf having an average particle size of 5 μm or less
No. was evaluated for those containing 0.5 to 30% by weight of O ₂ and TiN. 25 to 34. HfO ₂ , TiN
No. was added in an amount of less than 1% by weight. 25 and no. 30
No thermal conductivity of 22 W / m · K or more was obtained due to the low proportion of the grain boundary crystal phase. In addition, the content of No. 29 and No. 34 is HfO ₂ , T
Since iN has a high melting point, it tends to inhibit the sintering of the silicon nitride-based sintered body, and the mechanical properties of the sintered body have been deteriorated.

On the other hand, when the amounts of HfO ₂ and TiN added were within the scope of the present invention, 26 to 28 and No. 26; Three
Samples Nos. 1 to 33 were found to have a thermal conductivity of 22 W / m · K or more and exhibit good seizure resistance.

[0049]

According to the present invention, the ratio of the crystal phase in the grain boundary of the silicon nitride based sintered body is 50% or more, and the thermal conductivity is 22 W /
By setting it to m · K or more, it becomes possible to provide a sliding member made of a silicon nitride sintered body having improved seizure resistance.

Claims (4)

[Claims] 1. A composition comprising silicon nitride as a main component and an oxide equivalent of 1 to 1.
30% by weight of a rare earth element compound and 2 to 20% by weight of SiC having an aspect ratio of 3 or less and an average long diameter of 6 μm or less
A sliding member comprising: a silicon nitride sintered body having a bulk density of 98% or more of a theoretical density. 2. A composition containing silicon nitride as a main component and having an oxide equivalent of 1 to 2.
A sliding member having an average particle size of 30 wt% of the rare earth compound is characterized by containing the following HfO _2, TiN 5μm 1~30 wt%. 3. The ratio of the grain boundary crystal phase to the entire crystal phase is 5%.
The sliding member according to claim 1, wherein the sliding member has a thermal conductivity of 22 W / m · K or more at 0% or more. 4. A four-point bending strength of 700 MPa or more, a fracture toughness
4.0MPam ^1/2It is characterized by the above
The sliding member according to claim 1 or 2.