JPH1143372A - Silicon nitride-based ceramic and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a silicon nitride-based ceramic contrived in the lowering of frictional coefficient under a non-lubricating condition and in the improvement of mechanical characteristics, chemical stability and self lubricity by molding a mixture of silicon nitride powder with carbon and/or a compound capable of thermally producing carbon and subsequently sintering the molded product at a specific temperature in a non-oxidizing atmosphere. SOLUTION: This silicon nitride-based ceramic is obtained by mixing carbon and/or a compound capable of thermally producing carbon with silicon nitride powder or silicon powder and a sintering auxiliary, molding the mixture, and subsequently sintering the molded product at a temperature of 1,300-1,900 deg.C for <=30 min in a non-oxidizing atmosphere, if necessary, containing nitrogen gas. The produced silicon nitride-based ceramic has an average-particle diameter of <=1 μm, a free carbon content of 0.5-50 wt.%, a silicon nitride-based crystal particle average minor axis diameter of <=0.2 μm, a frictional coefficient of <=0.2 μm under a non-lubricating condition and a flexural strength of >=750 Mpa. It is possible to use a compound capable of producing carbon with heat on the sintering treatment, such as a phenol resin, etc. instead of the carbon powder.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a silicon nitride ceramic having excellent mechanical properties and chemical stability, a low coefficient of friction, and self-lubricating properties, and a method for producing the same.

[0002]

2. Description of the Related Art Silicon nitride ceramics are widely used as engine parts, cutting tools, sliding members, etc. because of their excellent mechanical strength and high corrosion resistance in a wide temperature range from low to high temperatures. In particular, as a sliding member, it is possible to reduce the coefficient of friction under lubrication by improving the surface accuracy, and it is possible to obtain performance excellent in wear resistance.

However, silicon nitride-based ceramics, like other ceramic materials, have a high coefficient of friction under non-lubricating or boundary lubricating conditions, and cannot be said to be particularly excellent in sliding characteristics. Thus, various attempts have been made to impart self-lubricating properties to silicon nitride-based ceramics and improve frictional sliding characteristics.

[0004] For example, Japanese Patent Application Laid-Open No. 221352/1990 discloses a sliding method in which flaky or spherical graphite having an average size of 1 to 50 µm is uniformly deposited in a hard member at a rate of 0.3 to 15 vol%. -The material for the friction part is described. Japanese Patent Application Laid-Open No. 4-254471 proposes a composite sintered body mainly composed of silicon nitride and containing more carbon on the surface than on the inside. Further, JP-A-59-307
No. 69 discloses a solid lubricant selected from the group consisting of carbon having a size of 1 μm or less, boron nitride and silicon carbide containing free carbon.
A silicon nitride sintered body containing volume% has been proposed.

[0005]

However, the techniques disclosed in the above-mentioned JP-A-2-221352, JP-A-4-254471 and JP-A-59-30769 disclose the following.
Even if carbon is added to silicon nitride ceramics, the friction coefficient does not sufficiently decrease, but rather, the strength of the silicon nitride ceramic decreases due to the addition of carbon, and the reliability as a structural material is greatly reduced. there were.

In addition, the addition of carbon makes it easier for silicon nitride-based particles to fall off from silicon nitride-based ceramics,
There was also a problem that the amount of wear was increased due to the falling off. Further, as described in JP-A-4-254471, in order to change the distribution of carbon between the surface portion and the inside, there is a disadvantage that the manufacturing method becomes very complicated and the manufacturing cost increases. .

The present invention has been made in view of such a conventional situation,
An object of the present invention is to provide a silicon nitride ceramic having excellent mechanical properties and chemical stability, a low coefficient of friction even under non-lubricated conditions, excellent self-lubricating properties, and excellent practicality, and a method for producing the same. Aim.

[0008]

Means for Solving the Problems To achieve the above object, the silicon nitride-based ceramic provided by the present invention contains free carbon in an amount of from 0.5% by weight to less than 50% by weight, and has an average shortness of silicon nitride-based crystal grains. It is characterized in that the shaft diameter is 0.5 μm or less and the friction coefficient under non-lubricated conditions is 0.2 or less.

In the silicon nitride ceramics of the present invention, by reducing the average particle size of the free carbon contained to 1 μm or less, the strength reduction of the silicon nitride ceramics caused by the addition of carbon as a solid lubricant is overcome. it can. A preferable result is obtained by using graphite as the free carbon.

Further, the silicon nitride-based ceramic of the present invention has excellent self-lubricating properties by adding free carbon and also has excellent mechanical properties, and particularly has a bending strength of 750M.
Pa or more, and high mechanical reliability can be secured.

The silicon nitride-based ceramic of the present invention is obtained by mixing carbon and / or a compound capable of forming carbon by heating with silicon nitride powder, molding the mixture to form a molded body, and then oxidizing the molded body. 1300 in neutral atmosphere
And sintering by heating at 1900 ° C. for 30 minutes or less.

The silicon nitride-based ceramic of the present invention is obtained by mixing carbon and / or a compound capable of generating carbon by heating with silicon powder, molding the mixture to form a compact, and then removing the compact with nitrogen. In a non-oxidizing atmosphere containing
It can also be obtained by a reaction sintering method of heating and nitriding at 450 ° C. or lower. Further, after nitriding as described above, sintering may be performed by heating at 1900 ° C. or lower for 30 minutes or less to further densify.

In the present invention, "silicon nitride-based" is used as a term meaning Si ₃ N ₄ and / or Sialon.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The silicon nitride-based ceramics of the present invention differs from the above-mentioned known techniques in that, first, the average minor axis diameter of silicon nitride-based crystal grains is as fine as 0.5 μm or less. is necessary. That is, when the average short axis diameter of the silicon nitride crystal grains exceeds 0.5 μm, the bending strength of the sintered body when mixed with carbon is reduced, and the friction coefficient under non-lubricated conditions also exceeds 0.2. It is because.

If the amount of free carbon contained in the silicon nitride-based ceramics is less than 0.5% by weight, the solid lubricating effect is reduced, and a desired friction coefficient cannot be obtained. Since the bending strength of the sintered body is lowered and the wear amount is rapidly increased, the sintered body is more than 0.5% by weight.
The range is less than weight%.

According to the present invention, excellent lubricating action is exhibited by the addition of free carbon while maintaining excellent mechanical properties and chemical stability of the silicon nitride-based ceramics.
A silicon nitride ceramic having a friction coefficient of 0.2 or less under non-lubricated conditions is obtained.

It is particularly preferable that the free carbon contained in the silicon nitride ceramic has an average particle size of 1 μm or less. This is because if the average particle size of the free carbon exceeds 1 μm, the strength of the sintered body rapidly decreases. In addition, as the carbon to be added, there are graphite powder, carbon powder, and the like, and particularly, preferable characteristics can be obtained by using graphite powder.

Further, the silicon nitride-based ceramic of the present invention has excellent self-lubricating properties as described above, and at the same time can achieve excellent bending strength of 750 MPa or more. If the flexural strength is less than 750 MPa, the bonding strength of the silicon nitride-based crystal grains will be low, the amount of wear will be large due to the falling off, and the reliability will be significantly low, such as breaking during use as a sliding part. Become. The bending strength in the present invention refers to a three-point bending strength according to JIS standards.

Next, a method for producing the silicon nitride-based ceramic of the present invention will be described. In the first method, a silicon nitride powder and a carbon powder are mixed, and the compact is heated and sintered at 1300 to 1900 ° C. in a non-oxidizing atmosphere within 30 minutes. A known oxide powder as a sintering aid can be added to the mixed powder in order to promote densification of the silicon nitride-based ceramic. Further, instead of the carbon powder, a compound that generates carbon by heating during sintering, such as a phenol resin, can be used.

If the sintering temperature is less than 1300 ° C., the densification becomes insufficient, the bending strength of the sintered body becomes insufficient, and the abrasion resistance decreases. If the sintering temperature exceeds 1900 ° C., the added carbon reacts with silicon nitride or a sintering aid and does not remain as free carbon in the sintered body. In addition, the silicon nitride-based crystal grows and coarsens, and the average minor axis diameter is 0.1.
It exceeds 5 μm. Shorter sintering time is preferable,
When the time exceeds 30 minutes, the residual free carbon is reduced, and the crystals grow to coarse grains.

Further, as a second method, a metal silicon powder is used in place of part or all of the silicon nitride powder, and a mixed powder of the metal silicon powder and the carbon powder is heated in a non-oxidizing atmosphere containing nitrogen. Alternatively, reaction sintering in which silicon is nitrided and converted into a silicon nitride-based main component can be used. Furthermore, after conversion to the silicon nitride-based main component by this reaction sintering, it is possible to further densify by raising the heating temperature and further sintering.

In this case, if the reaction sintering for nitridation is performed at a temperature exceeding 1450 ° C., the metal silicon is melted and the shape of the formed body is changed. The reaction sintering time for nitriding is

Further, sintering for densification after the above reaction sintering must be performed at a temperature of 1900 ° C. or less within 30 minutes. When the sintering temperature for densification exceeds 1900 ° C. or the sintering time exceeds 30 minutes, free carbon remaining in the sintered body is reduced as in the case of the sintering of the first method. This is because the silicon nitride-based crystal becomes coarse and the average minor axis diameter exceeds 0.5 μm.

[0024]

EXAMPLES Example 1 Si ₃ N ₄ powder (average particle size 0.3 [mu] m), was added 5 wt% of Y ₂ 0 ₃ powder and 2 wt% of A1 ₂ 0 ₃ powder as a sintering aid, Except for Sample 1, graphite or phenolic resin (abbreviated as FR) shown in Table 1 was further added and mixed as a carbon component. Thereafter, each mixed powder was molded, and the obtained molded bodies were subjected to hot press sintering under the conditions shown in Table 1 below.

Table 1 shows the average short-axis diameter of the silicon nitride-based crystal, the amount of free carbon contained, the average particle size of free carbon, and the bending strength of each of the obtained sintered bodies. The friction coefficient of each sintered body was measured by a pin-on-disk test under a non-lubricated condition at a sliding speed of 2 m / sec and a load of 5 kgf / mm ² , and the results are shown in Table 1.

[0026]

[Table 1] Carbon componentsCharacteristics of Si ₃ N ₄ sintered body Addition amount Sintering condition Short axis diameter Free carbon Carbon particle size Strength Frictionsample (wt%) (℃ × hr) (μm) (wt%) (μm) (MPa) coefficient  1 * None 1750 × 0.2 0.3> 0.1 − 1000 0.7 2 * Graphite 0.2 1750 × 0.2 0.2 0.2 0.8 900 0.5 3 Graphite 1.0 1750 × 0.2 0.2 0.8 0.8 850 0.15 4 Graphite 5.0 1750 × 0.2 0.2 4.5 0.8 800 0.1 5 Graphite 20 1750 × 0.2 0.2 18.5 0.8 800 0.1 6 Graphite 40 1750 × 0.2 0.2 16.0 0.8 750 0.07 7 Graphite 60 1750 × 0.2 0.2 54 0.8 400 0.07 8 Graphite 5.0 1750 × 0.2 0.2 4.0 0.2 850 0.12 9 FR 5.0 1750 × 0.2 0.2 4.0 0.2 750 0.12 10 * graphite 5.0 1950 × 0.1 0.4 0.2 0.8 600 0.5 11 * graphite 5.0 1750 × 2.0 0.8 0.4 0.8 650 0.3 12 * graphite 5.0 1250 × 0.5 0.2 5.0 0.8 150 0.1 13 * graphite 5.0 1750 × 0.5 0.2 5.0 1.5 500 0.2 ( Note) Samples marked with * in the table are comparative examples.

Example 2 To a metal Si powder (average particle size: 0.8 μm), 3% by weight of Y ₂ O ₃ powder and 2% by weight of Al ₂ O ₃ powder were added as sintering aids. Except for this, graphite was further added and mixed as a carbon component at a ratio shown in Table 2 below. Thereafter, each mixed powder was molded, and each compact was nitrided by performing reaction sintering at a temperature shown in Table 2 for 2 hours in a non-oxidizing atmosphere containing nitrogen. Further, except for the sample 17, the sample was sintered and densified in a non-oxidizing atmosphere under the conditions shown in Table 2.

Table 2 shows the average minor axis diameter of the silicon nitride-based crystal, the amount of free carbon contained, the average particle size of free carbon, and the bending strength of each of the obtained sintered bodies. Further, the friction coefficient of each sintered body was measured by a pin-on-disk test under a non-lubricated condition at a sliding speed of 2 m / sec and a load of 5 kgf / mm ² , and the results are shown in Table 2.

[0029]

[Table 2] Nitriding reaction Characteristics of densified Si ₃ N ₄ sintered graphite Graphite sintering temperature Sintering condition Short axis diameter Free carbon Carbon particle size Strength Friction sample (wt%) (° C) (° C × hr) (μm ) (wt%) (μm) (MPa) Coefficient 14 * None 1400 1750 × 0.2 0.4> 0.1 − 900 0.7 15 * 0.2 1400 1750 × 0.2 0.4 0.2 0.8 800 0.5 16 1.0 1400 1750 × 0.2 0.3 0.8 0.8 800 0.15 17 1.0 1400 None 0.3 0.8 0.8 750 0.15 18 20 1400 1750 × 0.2 0.3 18.5 0.8 750 0.1 19 * 5.0 1500 1750 × 0.2 0.3 1.5 0.8 350 0.3

[0030]

According to the present invention, a silicon nitride ceramic having excellent mechanical properties and chemical stability, a low friction coefficient even under non-lubricating conditions, and excellent self-lubricating properties is provided at a low cost. be able to.

Therefore, the silicon nitride-based ceramics of the present invention can be widely used as mechanical structural parts such as sliding members and friction members that require lubricating properties, reduce friction loss of mechanical parts and reduce energy loss. Can be reduced.

Claims (7)

[Claims] (1) free carbon in an amount of not less than 0.5% by weight and not more than 50% by weight;
And the average minor axis diameter of silicon nitride crystal grains is 0.5μ
m or less and a coefficient of friction under no-lubrication conditions of 0.2 or less. 2. The silicon nitride-based ceramic according to claim 1, wherein the average particle size of the free carbon is 1 μm or less. 3. The silicon nitride-based ceramic according to claim 1, wherein the free carbon is graphite. 4. The silicon nitride-based ceramic according to claim 1, wherein the flexural strength is 750 MPa or more. 5. A method of mixing carbon and / or a compound capable of generating carbon by heating with silicon nitride powder, molding the mixture into a molded body, and then heating the molded body in a non-oxidizing atmosphere at 1300 to 1900 ° C. Sintering by heating for 30 minutes or less under the above conditions. 6. A mixture of carbon and / or a compound capable of producing carbon by heating and silicon powder, and molding the mixture to form a molded body. The molded body is placed in a non-oxidizing atmosphere containing nitrogen at 1450 ° C. A method for producing a silicon nitride-based ceramic, comprising heating and nitriding the following. 7. The sintered body obtained by the method according to claim 1 is further heated to 1900 ° C. or lower in a non-oxidizing atmosphere.
A method for producing a silicon nitride-based ceramic, comprising sintering by heating within 0 minutes.