JP2000313672A - Valve made of silicon nitride-based ceramic and production of the same valve - Google Patents

Abstract

PROBLEM TO BE SOLVED: To simply produce a valve made of silicon nitride-based ceramics at a low cost, having high dimensional accuracy in a burnt state and high transverse strength in a surface left as it was burnt. SOLUTION: This valve made of silicon nitride-based ceramics which has high dimensional accuracy in a burnt state is formed by the following steps: adding 80-92.5 wt.% silicon nitride, 2-10 wt.% rare earth element oxide, 2-5 wt.% aluminum oxide and 0.5-5 wt.% (in terms of silicon oxide) excess oxygen so as for the ratio of quantities of aluminum oxide and excess oxygen in terms of silicon oxide to the quantity of rare earth element oxide to be respectively 0.3-1; making a compact from the above mixture in which a valve head 12 is formed at the end of a pillar-like stem 13 as one body and; burning the compact in a non-oxidizing atmosphere after inserting the stem in a cylindrical jig made of silicon nitride-based ceramics and having a wider inside diameter than the outside diameter of the stem so as to hold it.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon nitride ceramic valve mainly used for an internal combustion engine such as an automobile and a generator, and a method of manufacturing the same.

[0002]

2. Description of the Related Art
As an intake / exhaust valve (hereinafter simply referred to as a valve) used in an internal combustion engine, as shown in FIG.
And a column-shaped stem portion 33 integrally formed at the center of the umbrella portion 32. The tapered surface 34 formed on the outer periphery of the umbrella portion 32 is called a face portion, and is a portion that comes into contact with a valve seat of an intake / exhaust port of the internal combustion engine.

Conventionally, the valve 31 is generally made of metal, for example, a steel material such as SUH-3 or SUH-11 or a heat-resistant alloy such as Inconel.

However, high output and high speed of the internal combustion engine have been demanded, and accordingly, the valve 31 has been required.
However, it has been desired to be able to withstand severe environments mechanically and thermally, and to be lighter.

Therefore, it has been proposed that such a valve 31 be formed of a silicon nitride ceramic having excellent heat resistance, thermal shock resistance, abrasion resistance and oxidation resistance and high bending strength.

As the silicon nitride ceramics forming such a valve 31, there is generally used one containing silicon nitride as a main component and a rare earth element oxide such as Y ₂ O ₃ and aluminum oxide as sintering aids. (See JP-A-7-133550 and JP-A-6-20652).

When a silicon nitride ceramic valve is manufactured, if the density of the compact is not uniform, the temperature in the firing furnace is uneven, and the heating is not uniform during firing, the stem portion 33 of the valve 31 is not suitable. Is deformed (mainly bent)
In some cases, the stem portion 33 may be deformed by 0.5 mm or more in terms of coaxiality, and in severe cases, may be deformed by 1 mm or more.

In order to suppress such deformation of the stem portion 33 as much as possible, Japanese Patent Application Laid-Open No. Hei 3-1370065 discloses FIG.
As shown in the figure, the weight 4 is attached to the end of the stem 43 of the molded body 41.
5 and inserted into the cylindrical jig 46, the umbrella portion 42 of the molded body 41 is hooked and held on the opening of the cylindrical jig 46, and the stem portion 43 is directly lowered by the tensile stress of the weight 45. It has been proposed to fire in a heated state.

However, when the silicon nitride ceramic valve 31 is manufactured by the method disclosed in Japanese Patent Application Laid-Open No. 3-1370065, the deformation of the stem portion 33 can be suppressed, but the tensile stress of the weight 45 causes the stem portion 33 to deform. There has been a problem that the density of the stem portion 33 is reduced and the mechanical strength is reduced.

Moreover, in this method, since the weight 45 must be formed at the end of the stem 43 of the molded body 41 and then removed after firing, waste of materials and work steps increase, and productivity is poor. There was also an inconvenience.

[0011]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a valve made of silicon nitride ceramics which uses silicon nitride ceramics with little deformation during firing, has a high accuracy by a simple manufacturing method, and has excellent bending strength. is there.

[0012]

In view of the above problems, the present invention provides a silicon nitride ceramic valve comprising an umbrella and a columnar stem integrally formed at the center of the umbrella. Its composition is 80-92.5% by weight silicon nitride,
The rare earth element contains 2 to 10% by weight of a rare earth element in terms of oxide, 2 to 5% by weight in terms of aluminum oxide, and 0.5 to 5% by weight of excess oxygen in terms of silicon oxide. The ratio of aluminum oxide equivalent to aluminum oxide equivalent is 0.3 to
1, and made of silicon nitride ceramics in which the ratio of the amount of excess oxygen to the amount of oxide of the rare earth element is 0.3 to 1 and the coaxiality of the stem is 0.3 mm or less. It is characterized by having done.

The present invention also provides a silicon nitride ceramic valve in which at least 30% of the surface of the stem portion is a burned-out surface, and a flexural strength of the burned-out surface is 700 MPa.
It is characterized by having the above.

Further, according to the present invention, in manufacturing the silicon nitride ceramic valve, the silicon nitride is used in an amount of 80 to 9%.
2.5% by weight, 2 to 10% by weight of rare earth element oxide, 2 to 5% by weight of aluminum oxide, 0.5 to 5% by weight of excess oxygen in terms of silicon oxide, and based on the amount of the rare earth element oxide. The ratio of the amount of aluminum oxide and the ratio of the amount of excess oxygen in terms of silicon oxide to the amount of the rare earth element oxide are adjusted to be 0.3 to 1, and this is then integrated with one end of a columnar stem portion with an umbrella portion. Then, the stem portion of this molded body is placed in a silicon nitride ceramic cylindrical jig having an inner diameter that is wider than the outer diameter of the stem portion by 0.5 to 10 mm. Insert and hold the compact in a cylindrical jig. Then, in a non-oxidizing atmosphere,
It is characterized by firing at a temperature of 0 to 1800 ° C.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.

As shown in FIG. 1, a valve made of a silicon nitride ceramic (hereinafter referred to as a valve) according to the present invention has
2 and a columnar stem portion 13 integrally formed at the center of the umbrella portion 12.
3 mm or less, preferably 0.2 mm or less, which is characterized by almost no warpage or bend.
% Or more, the coaxiality of 0.3 mm or less is obtained even if the as-baked surface remains.

Therefore, according to the valve 11 of the present invention, since the stem portion 13 hardly bends or bends, the grinding after sintering can be reduced, and the grinding after sintering becomes unnecessary depending on the application. Therefore, it can be manufactured at low cost.

Incidentally, in order to achieve such excellent coaxiality, it is necessary that the silicon nitride ceramics be less deformed at the time of sintering. Y, E
Rare earth elements such as r, Yb, Lu, Sm and Dy are 2 to 10% by weight, preferably 5 to 9% by weight in terms of oxides, and aluminum is 2 to 5% by weight, preferably 3 to 4% in terms of aluminum oxide. % Of excess oxygen, and 0.5 to 5% by weight, preferably 2 to 5% by weight of excess oxygen in terms of silicon oxide.
4% by weight, and the ratio of the aluminum oxide equivalent to the rare earth oxide equivalent is 0.3 to 1.0, preferably 0.5 to 0.8,
In addition, the ratio of excess oxygen equivalent to silicon oxide equivalent relative to the rare earth element equivalent to oxide is 0.3 to 1.0, preferably 0.1 to 1.0.
It is important that it is between 5 and 0.8. Here, when the contents and ratios of silicon nitride, rare earth elements, aluminum, and excess oxygen are out of the above-mentioned ranges, the sinterability and the effect of suppressing deformation during sintering are small, and the coaxiality of the stem 13 is set to 0.1. 3
mm or less.

Further, if each component of the sintering aid is less than the above range, the liquid phase component becomes insufficient at the time of firing, and firing at a high temperature is required for densification. When growth occurs, the transverse rupture strength decreases, and when each component is more than the above range, the liquid phase component becomes too large at the time of firing, so that the grain growth of silicon nitride particles progresses, and the short axis length in the surface layer is reduced. Alternatively, crystal grains having a large major axis length are likely to be formed. As a result, the coarse grains serve as a fracture source, and the bending strength is reduced.

Thus, by containing the contents and ratios of silicon nitride, rare earth elements, aluminum, and excess oxygen within the above ranges, the surface of the stem portion 13 can be burned 700 times even in the burned-out surface.
A bending strength of not less than MPa can be realized.

From the viewpoint of maintaining a transverse rupture strength of 700 MPa or more, the average crystal grain size of the silicon nitride particles in the sintered body is preferably 50 μm or less, more preferably 30 μm or less.

Further, in the silicon nitride ceramics,
Other components include metals of elements of Groups 4a, 5a, and 6a of the periodic table, and TiC, TiN, TaC, TaN, VC, and Nb.
C, WC, WSi ₂ , Mo ₂ C, etc.
It is also possible to improve the characteristics by incorporating at least one of carbides, nitrides, and silicides of elements a and 6a, or SiC or the like in the form of dispersed particles or whiskers. However, the total content of these is preferably 5% by weight or less.

Next, a method of manufacturing the silicon nitride ceramic valve 11 of the present invention will be described.

First, a silicon nitride powder is prepared. The silicon nitride powder may be in any state of α-Si ₃ N ₄ and β-Si ₃ N ₄ , and the particle size is 0.4 to 1.2 μm.
m and containing 0.5 to 1.5% by weight of oxygen.

Then, the silicon nitride powder was added to a powder of 80-92.
5% by weight of rare earth oxide as sintering aid
10 to 10% by weight, preferably 5 to 9% by weight, and aluminum oxide in the range of 2 to 5% by weight, preferably 3 to 4% by weight, and silicon oxide in the range of 0.5 to 5% by weight, preferably Is added in the range of 2 to 4% by weight, and the ratio of the added amount of aluminum oxide to the added amount of the rare earth element oxide is 0.3 to 1.0, preferably 0.5 to 1.0.
0.8, and the ratio of the amount of silicon oxide to the amount of the rare earth oxide added is from 0.3 to 1.0, preferably from 0.5 to 1.0.
Mix to give 0.8. However, the amount of silicon oxide is determined by a value obtained by adding the amount of excess oxygen contained as an impurity in the silicon nitride powder in terms of silicon oxide to the amount of silicon oxide powder to be added.

After adding an organic solvent such as ethanol or isopropyl alcohol and a binder to the raw material powder prepared in these ranges, the raw material powder is reduced by a known pulverizing method, for example, a ball mill, a vibration mill, a rotary mill, a barrel mill or the like. The uniformly mixed and pulverized product is press-formed, cast-molded, injection-molded, cold isostatically pressed by known ceramic molding means or the like, and an umbrella portion is formed at one end of a columnar stem portion 23 as shown in FIG. A molded body 21 integrally formed with 22 is manufactured.

Next, the stem portion 23 of the obtained molded body 21
The stem 23 is inserted into a cylindrical jig 1 made of silicon nitride ceramic having an inner diameter Q that is wider than the outer diameter W by 0.5 to 10 mm from the outer diameter W of the cylindrical jig 1. Hook and hold on the open end. Here, the cylindrical jig 1 is formed of silicon nitride ceramics. By forming the jig with silicon nitride of the same quality, the decomposition of the auxiliary component from the molded body 21, which is a major factor of deformation, can be suppressed. Because.

The inner diameter Q of the cylindrical jig 1 and the stem 23
The difference between the outer diameter W and the outer diameter W is 0.5 to 10 mm.
If the difference between the inner and outer diameters is smaller than 0.5 mm, there is a possibility that the stem portion 23 of the molded body 21 may be damaged when inserted into the cylindrical jig 1, and if the inner and outer diameter difference exceeds 10 mm, sintering may be performed. When the stem portion 23 is deformed sometimes, the concentricity after sintering becomes 0.3
This is because it is difficult to keep the thickness below mm. Preferably, the difference between the inner and outer diameters is 1 to 3 mm. In addition, by using the cylindrical jig 1 having a small difference between the inner and outer diameters, the displacement of the center of gravity during setting is reduced, so that the deformation can be further suppressed.

Thereafter, the molded body 21 held by using the cylindrical jig 1 is fired. The molded body 21 is fired at a temperature of 1700 to 1800 ° C., preferably 1750 to 1800 ° C. in a nitrogen atmosphere or an atmosphere containing SiO. It may be fired at normal pressure. If the sintering temperature is lower than 1700 ° C., the sinterability is insufficient and densification cannot be performed. Conversely, if the firing temperature exceeds 1800 ° C., the cylindrical jig 1 made of silicon nitride ceramics may be used. This is because it is difficult to promote the decomposition of the auxiliary component in the molded body 21 and to suppress the deformation of the stem 13.

Under these conditions, silicon nitride is added to Y, Er, Yb, L in an amount of 80 to 92.5% by weight.
rare earth elements such as u, Sm, Dy, etc.
% By weight, aluminum is 2 to 5 in terms of aluminum oxide
% By weight and excess oxygen in the range of 0.5 to 5% by weight in terms of silicon oxide, and the ratio of the aluminum oxide equivalent to the rare earth oxide equivalent is 0.3 to 1.0. And the ratio of the amount of excess oxygen converted to silicon oxide to the amount of rare earth element converted to oxide is 0.3 to 1.
Thus, a valve 11 made of silicon nitride-based ceramics, which is made of silicon nitride-based ceramics and has a coaxiality of the stem portion 13 of 0.3 mm or less can be obtained even when the surface is annealed.

According to the present invention, 10 to 70% by weight of the silicon nitride powder as a starting material can be replaced with silicon powder. Then, heat treatment is performed at a temperature of 1000 to 1400 ° C. to nitride the Si powder to generate silicon nitride, and the density of the molded body 21 is increased, followed by firing under the above firing conditions. According to this manufacturing method, shrinkage during firing is suppressed, and a dense and highly dimensional accurate silicon nitride ceramic valve 11 can be obtained.

[0032]

(Example 1) A powder of a rare earth element oxide (average) was used as a sintering aid for silicon nitride powder (BET specific surface area 9 m ² / g, α rate 98%, oxygen content 1.2% by weight). Particle size 1.5 μm) and aluminum oxide powder (purity 99.9)
%, Average particle size of 2 μm) and further powder of silicon oxide (purity 9
9.9%, average particle size of 2 μm) was prepared such that the composition after firing was as shown in Table 1, and after adding a binder and a solvent and kneading and drying, the mixture was mixed with the umbrella section 22 by cold isostatic pressing. The stem 23 has an outer diameter of 10 m.
m, a molded body 2 having a length of 100 mm as shown in FIG.
1 was formed.

Next, a cylindrical jig 1 made of a silicon nitride ceramic is prepared. The stem 23 of the molded body 21 is inserted into the cylindrical jig 1, and an umbrella 22 is attached to the open end of the cylindrical jig 1. Is held in a sagger made of silicon carbide-based ceramics, and held in a nitrogen atmosphere at 1700 ° C. to 1800 ° C.
By firing at a temperature of ° C., a silicon nitride ceramic valve 11 shown in FIG. 1 was manufactured.

After the obtained silicon nitride ceramic bulb 11 was confirmed to have the composition shown in Table 1 by ICP emission spectroscopy, the burned-out surface of the stem 13 was moved in the longitudinal direction. The contact length measuring device was scanned along the same, the coaxiality of the stem portion 13 was measured, and then the bending strength of the annealed surface was measured by a four-point bending tester.

The results are as shown in Table 1.

[0036]

[Table 1]

As a result, the difference between the inner and outer diameters of the inner diameter Q of the cylindrical jig 1 and the outer diameter W of the stem 23 during sintering is 0.5 mm.
Sample No. smaller than In Sample No. 5, when the stem portion 23 of the molded body 21 was inserted into the cylindrical jig 1, the sample No. 5 was broken, and the difference in inner and outer diameters was larger than 10 mm. In No. 6, the amount of deformation of the stem portion 13 after sintering was as large as 0.42 mm, and the transverse rupture strength was as low as less than 700 MPa.

Sample No. 16, 17, 20, 2
As in 1,24,25,28,29,32,33, the amount of rare earth element as oxide is 2 to 10% by weight, the amount of aluminum as aluminum oxide is 2 to 5% by weight, and excess oxygen as silicon oxide. 0.5 to 5% by weight, the ratio of aluminum oxide equivalent to aluminum oxide equivalent to rare earth oxide equivalent is 0.3 to 1, the ratio of excess oxygen equivalent to silicon oxide equivalent to rare earth oxide equivalent. In all cases, the coaxiality of the stem portion 13 could not be reduced to 0.3 mm or less, and the transverse rupture strength was as low as less than 700 MPa.

On the other hand, the sample No. 1-4, 7-1
5,18,19,22,23,26,27,30,31
Means that the composition of the silicon nitride ceramic is 80%
-92.5% by weight, 2-10
% By weight, aluminum is 2 to 5 in terms of aluminum oxide
% By weight, excess oxygen is 0.5 to 5% by weight in terms of silicon oxide, and the ratio of the aluminum oxide equivalent to the rare earth oxide equivalent is 0.3 to 1 and the oxidation of the rare earth element is The ratio of the amount of excess oxygen converted to silicon oxide to the amount converted to material is 0.3 to 1, and the valve 1
At the time of firing, the molded body 21 is inserted into a cylindrical jig 1 made of silicon nitride ceramic having an inner diameter that is wider than the outer diameter of the stem portion 23 of the molded body by 0.5 to 10 mm, and is sintered. Therefore, the deformation of the stem 13 after firing can be suppressed, the coaxiality of the stem 13 can be reduced to 0.3 mm or less even in the as-baked surface, and the transverse rupture strength in the as-baked surface can be as high as 700 MPa or more. Could be achieved.

[0040]

As described above, according to the present invention, a silicon nitride ceramic valve comprising a head portion and a columnar stem portion integrally formed at the center of the head portion has a composition. 80 to 92.5% by weight of silicon nitride, 2 to 10% by weight of rare earth element in terms of oxide, 2 to 5% by weight of aluminum in terms of aluminum oxide, and 0.5 to 5% by weight of excess oxygen in terms of silicon oxide In addition to containing in the range of
The ratio of the equivalent amount of aluminum oxide to the equivalent amount of oxide of the rare earth element and the ratio of the equivalent amount of excess oxygen to the equivalent amount of oxide of the rare earth element are equivalent to 0.
Since it is made of silicon nitride ceramics having a diameter of 3 to 1 and the coaxiality of the stem is 0.3 mm or less, grinding after sintering can be reduced, and the number of working steps can be reduced. It is possible to provide a highly accurate valve and to provide a valve which has a bending strength of a stem portion of 700 MPa or more and is hard to be broken even if it is an annealed surface.

Further, according to the present invention, in manufacturing the silicon nitride ceramic valve, silicon nitride is
To 92.5% by weight, 2 to 10% by weight of rare earth oxide, 2 to 5% by weight of aluminum oxide, and 0.5 to 5% by weight of excess oxygen in terms of silicon oxide. The ratio of the amount of aluminum oxide to the amount of the material and the ratio of the amount of excess oxygen in terms of silicon oxide to the amount of the rare earth element oxide are adjusted to be 0.3 to 1, and then this is provided with an umbrella at one end of the columnar stem. The molded body is formed into an integrally formed body, and then the stem of the formed body is made of a silicon nitride ceramic cylindrical jig having an inner diameter that is wider than the outer diameter of the stem by 0.5 to 10 mm. The molded body is held in a cylindrical jig by inserting it inside, and then fired at a temperature of 1700 to 1800 ° C. in a non-oxidizing atmosphere. Small, high-precision silicon nitride It is possible to manufacture the electrochromic valve made, it is possible to reduce the grinding after sintering, can also reduce the working steps, it can be manufactured at low cost.

[Brief description of the drawings]

FIG. 1 is a side view showing a silicon nitride ceramic valve of the present invention.

FIG. 2 is a cross-sectional view for explaining a method for firing a silicon nitride ceramic valve of the present invention.

FIG. 3 is a side view showing a conventional silicon nitride ceramic valve.

FIG. 4 is a cross-sectional view for explaining a conventional method of firing a silicon nitride ceramic valve.

[Explanation of symbols]

1: Cylindrical jig 11: Silicon nitride ceramic valve
12: umbrella 13: stem 21: molded body 22: umbrella of molded body 23: stem of molded body

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4G001 BA03 BA08 BA09 BA32 BA73 BB03 BB08 BB09 BB32 BB73 BC12 BC13 BC21 BC23 BC52 BC62 BD14 BE11 BE31 BE35

Claims (3)

[Claims] A valve comprising an umbrella portion and a columnar stem portion integrally formed in the center of the umbrella portion is made of silicon nitride having a thickness of 80 nm.
-92.5% by weight, rare earth element is 2-10
% By weight, aluminum is 2 to 5 in terms of aluminum oxide
% By weight, excess oxygen is contained in the range of 0.5 to 5% by weight in terms of silicon oxide, and the ratio of the aluminum oxide equivalent to the rare earth oxide equivalent and the oxide equivalent of the rare earth oxide A valve made of silicon nitride ceramics, wherein the ratio of the amount of excess oxygen in terms of silicon oxide to silicon oxide is 0.3 to 1, and the coaxiality of the stem portion is 0.3 mm or less. 2. The baked surface accounts for at least 30% of the surface of the stem portion, and the baked surface has a flexural strength of 700M.
2. The valve according to claim 1, wherein the pressure is equal to or higher than Pa. 3. A silicon nitride of 80 to 92.5% by weight, a rare earth element oxide of 2 to 10% by weight and an aluminum oxide of 2 to 10% by weight.
5% by weight, the excess oxygen is 0.5 to 5% by weight in terms of silicon oxide, and the ratio of the amount of aluminum oxide to the amount of rare earth oxide and the ratio of the amount of excess oxygen to the amount of rare earth oxide in terms of silicon oxide are: 0.3 to 1, and then molded into a molded body in which an umbrella portion is integrally formed at one end of a columnar stem portion, and then the stem portion of the molded body is replaced with the stem. Inserted into a cylindrical jig made of silicon nitride ceramic having an inner diameter that is wider than the outer diameter of the part in the range of 0.5 to 10 mm to hold the molded body in the cylindrical jig, and then
The molded body is placed in a non-oxidizing atmosphere at 1700 to 1800
A method for producing a valve made of silicon nitride ceramic, characterized by firing at a temperature of ° C.