JP2000247750A - Valve for engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive valve for an engine having excellent abrasion resistance and strength and excellent in durability. SOLUTION: This valve for an engine has 0.1-10 peak intensity ratio represented by H1/H2 when the peak intensity of Si appearing at 521 cm-1 is H1 and the peak intensity of silicon nitride appearing at 206 cm-1 is H2 in an analytical chart in a position at 200 μm depth from the surface of a neck part according to a Raman spectroscopic analytical method in the valve for the engine comprising a silicon nitride-based sintered compact consisting essentially of a β-silicon nitride crystal phase and having >=3.2 g/cm3 density and <=2% porosity and equipped with a head part, a stem part and the neck part. The sintered compact especially contains Y and/or a rare earth element in an amount of 1-15 wt.% expressed in terms of an oxide, aluminum in an amount of 0.01-5 wt.% expressed in terms of an oxide and impure oxygen in an amount of <=10 wt.% expressed in terms of silicon oxide.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an engine valve made of a silicon nitride sintered body and having excellent durability and excellent wear resistance and strength.

[0002]

2. Description of the Related Art Conventionally, silicon nitride sintered bodies have been developed as engineering ceramics, particularly as parts for heat engines such as turbo rotors and engine valves, because of their excellent heat resistance and thermal shock resistance. Department has been put to practical use.

[0003] Such a silicon nitride sintered body is usually
Al ₂ O ₃ or Y ₂ as a sintering aid for silicon nitride
It is manufactured by adding a rare earth element oxide such as O ₃ , forming, and firing at a temperature of 1700 to 2000 ° C. in a nitrogen atmosphere. Examples of the firing method include a normal pressure firing method, a nitrogen gas pressure firing method of firing in high pressure nitrogen and firing at a high temperature, a hot press firing method of firing by applying a mechanical pressure with a press mold, and a denser method. A hot isostatic sintering method (HIP method) in which sintering is performed while applying a hydrostatic pressure of 1000 atm or more in order to achieve the formation is known.

[0004]

However, when a conventional silicon nitride sintered body is used to manufacture an engine valve, and when the valve is driven, it is a connecting portion between the stem portion and the umbrella portion of the valve. As a result of applying a high stress to the neck portion, cracks are generated in the neck portion, and the cracks gradually develop and eventually break. The same tendency was observed in the cotter portion formed to support the valve. Further, in order to increase the densification and the high-temperature strength, a nitrogen pressure firing method or HI
When the P method is used, there is also a problem that the cost of the product is high because the equipment is very expensive.

As described above, a conventional engine valve using a silicon nitride sintered body has sufficient characteristics as an engine valve even though a silicon nitride sintered body having excellent characteristics is used. As a result, the durability of the valve is not sufficient due to the burnt skin strength and processing damage, and the production cost is high.

Accordingly, an object of the present invention is to provide an inexpensive engine valve having excellent wear resistance and strength and excellent durability.

[0007]

Means for Solving the Problems The present inventors have conducted intensive studies on the above-mentioned problems, and as a result, in an engine valve part made of a silicon nitride sintered body, at least a neck portion of the engine valve has Si. It has been found that the presence of such a valve enhances the strength and wear resistance of the neck portion, and provides a valve with excellent durability.

That is, the engine valve of the present invention has a β-
Density of 3.20 g / cm mainly composed of silicon nitride crystal phase
^{It is made of a} silicon nitride sintered body having a porosity of ³ or more and a porosity of 2% or less, and includes an umbrella portion, a stem portion, and a neck portion.
When the peak intensity of Si shown in 521 cm ^-1 in an analysis chart by Raman spectroscopy and H _1, the peak intensity of 206cm ^-1 silicon nitride with H ₂ at a depth position of the [mu] m, tables in H ₁ / H ₂ Peak intensity ratio of 0.1 to 1
It is characterized by being 0.

Preferably, the silicon nitride sintered body has an average void diameter of 5 μm or less, a room temperature strength of 900 MPa or more, and a 1000 ° C. strength of 700 MPa or more. The sintered body contains 1 to 15% by weight of a rare earth element in terms of oxide, 0.01 to 5% by weight in terms of oxide, and 10% by weight or less of impurity oxygen in terms of silicon oxide. It is particularly desirable that the silicon nitride-based sintered body is produced by a normal-pressure firing method.

Further, M is added to the silicon nitride sintered body.
At least one selected from the group consisting of g, W, Mo, Mn, Cu and Fe may be contained as an oxide, nitride, oxynitride or silicide in a proportion of 8% by weight or less.

Further, the surface roughness of the neck portion is
The ten-point average roughness (Rz) is desirably 10 μm or less.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The engine valve of the present invention
-It is constituted by a sintered body having a silicon nitride crystal phase as a main component. Further, the β-silicon nitride crystal phase has an average minor axis of 0.5 to 2 μm and an average aspect ratio of 3 or more, and has a structure in which they are entangled, thereby improving fracture toughness and strength. Also, it contributes to improvement of strength and toughness characteristics from room temperature to high temperature.

Further, in terms of composition, silicon nitride is used in an amount of 75 to 9%.
It contains 5% by weight, preferably 80 to 90% by weight, and contains 1 to 15% by weight, preferably 3 to 15% by weight of a rare earth element in terms of oxide.
8% by weight, aluminum in an amount of 0.01 to 5% by weight, preferably 1 to 3% by weight in terms of oxide, and furthermore, impurity oxygen in an amount of 10% by weight or less, preferably 8% by weight or less in terms of silicon oxide. It is desirable to include them in proportions, and within these ranges, excellent wear resistance is obtained, and further, as described later, sintering at normal pressure becomes possible. The rare earth elements include Y, Er, Yb,
Lu, Sm, and the like.

Here, the impurity oxygen refers to the rare earth element (RE) and A in the sintered body based on the total amount of oxygen in the sintered body.
1 to the stoichiometric composition (RE ₂ O ₃ and Al
₂ O ₃ ), which is the amount of oxygen remaining after subtracting the amount of oxygen assumed to be bound by the oxygen, and most of the amount is composed of unavoidable oxygen in the silicon nitride powder or SiO ₂ component intentionally added. .

The rare earth element, aluminum, forms a glass phase in the grain boundary phase, or forms a rare earth element-
It may exist as a Si ₃ N ₄ —SiO ₂ system crystal phase. Note that aluminum may be partially dissolved in the β-silicon nitride crystal phase.

The silicon nitride sintered body contains at least one of Mg, W, Mo, Mn, Cu and Fe as an auxiliary component, and contains at least one of oxide, nitride, oxynitride or silicide as an auxiliary component. By containing at a ratio of not more than% by weight, sinterability can be enhanced, densification can be promoted, and characteristics can be further improved.

The sintered body has a density of 3.2.
g / cm ³ or more, preferably 3.23 g / cm ³ or more, and porosity of 2% or less, preferably 1% or less. This is because if the density is lower than 3.2 g / cm ³ , the valve is liable to be damaged or the members are worn out. If the porosity is higher than 2%, minute shedding from the pores is considered. This is because abnormal wear such as chipping or chipping may occur from the occurrence of wear.

Further, the sintered body preferably has an average void diameter of 5 μm or less, particularly 3 μm or less, a room temperature strength of 900 MPa or more, and a 1000 ° C. strength of 700 MPa or more in order to enhance the durability as a valve.

In general, as shown in FIG. 1, the engine valve has a substantially cylindrical stem portion 1, an umbrella portion 2, a neck portion 3 forming a connecting portion between the stem portion 1 and the umbrella portion 2, and The stem 1 is formed at an end of the stem 1 opposite to the umbrella 2 and is constituted by a cotter 4 for holding the cotter 4 on a holding member. The stem 1 of this valve is generally of a size having a cross-sectional diameter of about 5 to 15 mm.

In the engine valve of the present invention, when at least a position 200 μm deep from the surface of the neck portion 3 is analyzed by Raman spectroscopy, minute Si is detected. H ₁ peak intensity of Si shown in 521 cm ^-1 in an analysis chart by, when the peak intensity of 206cm ^-1 silicon nitride was H _2, the peak intensity ratio 0.1 represented by H ₁ / H ₂ 1
It is a great feature that it is 0, especially 0.2 to 2.5.

This Si is used for scanning electron microscope (SEM)
Is a level that cannot be observed even in the case of, and is detected only by Raman spectroscopy. Although this Si cannot be detected by SEM observation, it is presumed that it is probably dispersed in the grain boundaries or in the silicon nitride grains.

As described above, the presence of a predetermined amount of Si at a depth of 200 μm from the surface of the neck causes propagation of cracks generated from the surface of the neck to this S.
As a result, it is possible to improve the strength of the neck portion and the abrasion resistance, thereby improving the impact resistance of the valve and causing chipping or abnormal wear. Defects and peel wear can be suppressed.

However, it is important that the amount of Si particles present in the sintered body is very small in order to exhibit the above-mentioned characteristics. For example, Si particles are present at a level detected by X-ray diffraction measurement. When present, it induces wear and degrades the wear resistance of the neck.

Therefore, if the peak ratio is lower than 0.1, the effect of suppressing abnormal wear is low, and desired characteristics cannot be obtained. If the peak ratio exceeds 10, the precipitated Si itself becomes a source of wear or breakage and becomes resistant to wear. This is because abrasion and strength are deteriorated.

In the engine valve of the present invention, the surface of the valve is formed of a machined surface or a burnt surface. For example, since the neck portion is formed by a curved surface,
If this is mirror-polished, the processing cost will be high. Therefore, it is desirable that at least a part be constituted by a burnt skin surface.

In this case, it is desirable that the peak intensity (H ₁ / H ₂ ) in the analysis chart by the Raman spectroscopic analysis on the surface of the burnt surface is 0.1 or less. This is because if Si is present on the surface, the abrasion resistance may be degraded. The amount of Si on this surface can be controlled by controlling the atmosphere and temperature during firing and nitriding Si on the surface of the burnt skin.

Further, according to the present invention, the peak intensity ratio based on Raman spectroscopy at the neck of the valve is limited as described above. Most preferably have similar characteristics for the entire valve.

Further, according to the engine valve of the present invention, the surface roughness of the surface of the neck portion is equal to the ten-point average roughness (Rz).
Is preferably 10 μm or less. This surface roughness is 1
If the thickness exceeds 0 μm, the groove on the surface acts as a defect, which lowers the fatigue strength of the component, abnormally wears the sliding partner, and induces chipping of the valve itself. In particular, the abnormal wear and chipping pose a problem when used in a corrosive gas atmosphere. In particular, according to the present invention, it is desirable that the neck portion is constituted by a burnt surface of 10 μm or less.

The engine valve of the present invention has the following features.
For example, it is manufactured by the following manufacturing method. First, as silicon nitride powder, α-Si itself is used.
Either ₃ N ₄ or β-Si ₃ N ₄ may be used, but in order to enhance sinterability, the α ratio is desirably 90% or more. The average particle size of the silicon nitride powder is 0.4 to 1.
2 μm, and the amount of impurity oxygen is suitably 0.5 to 1.5% by weight.

Also, a substantial amount of 80% by weight or less of the silicon nitride raw material may be replaced with silicon powder. When silicon powder is contained and nitridation is performed at a low temperature, α-Si ₃ N ₄ is easily generated, and the content of α-Si ₃ N ₄ in the compact after nitriding can be increased. By firing such a compact having a large content of α-Si ₃ N ₄ , the generation of needle-like β-silicon nitride crystal phase can be increased, and the strength and toughness of the sintered compact can be increased. It is advantageous to be able to.

Next, a rare earth element oxide powder, an Al ₂ O ₃ powder, and in some cases, a SiO ₂ powder are fired with the silicon nitride powder or the mixed powder of the silicon nitride powder and the silicon powder. The composition of the former compact has a rare earth element oxide equivalent of 1 to 15% by weight, particularly 3 to 8% by weight, aluminum has an oxide equivalent of 0.01 to 5% by mole,
In particular, it is 1 to 3% by weight, and the remaining oxygen amount obtained by subtracting the oxygen content in the rare earth element oxide powder and the Al ₂ O ₃ powder from the total oxygen content in the compact is 1% in terms of SiO _2.
It is added so as to be 0% by weight or less, particularly 8% by weight or less.

The reason why the content of these sintering aid components is limited as described above is that if each component is lower than the above-mentioned value, the liquid phase becomes insufficient in the firing process, a dense body cannot be obtained, and the strength decreases. As a result, if each component is more than the above value, the liquid phase during firing increases,
This is because silicon nitride tends to cause abnormal grain growth, and the abnormal grains serve as a fracture source to lower the strength. In addition, silicon nitride is strongly decomposed in the surface layer and the strength is reduced.

Further, as other components, Mg, W, Mo,
At least one oxide of the group of Mn, Cu and Fe;
Nitride, oxynitride or silicide may be added at a ratio of 8% by weight or less.

Thereafter, the mixed powder prepared in the above ratio is mixed by mesh pass granulation, spray granulation, dry granulation or the like.
After forming a granulated body having a size of 0 to 300 μm, the granulated body is used to form a valve for an engine. Examples of the molding method include rubber press molding, extrusion molding, sludge molding, cast molding, and press molding. In particular, rubber press molding is preferable in that a molded article having a shape close to a product shape can be produced.

Next, the obtained molded body is placed in a nitrogen atmosphere containing SiO at 1700 to 1800 ° C., particularly 1750 to 18 ° C.
By firing at normal pressure at a temperature of 00 ° C. and, in some cases, appropriately polishing the surface of the sintered body to a mirror surface, an engine valve as a product can be manufactured.

The above-mentioned SiO atmosphere is SiO ₂ + S
i or a mixed powder of SiO ₂ + Si ₃ N ₄ can be formed by putting together and firing in a firing bowl in which the molded body is stored.

Further, according to the present invention, in order to deposit a predetermined amount of Si in the sintered body, the sintering temperature, Si ₃ N ₄ equilibrium decomposes into silicon and nitrogen gas silicon nitride at atmospheric pressure Firing in a temperature range lower by about 30 ° C. than the temperature decomposes a very small amount of Si ₃ N ₄ . This decomposition causes the generated Si to exist as particles in the grain boundaries. The amount of Si can be arbitrarily controlled by, for example, the holding time in the above temperature range.

When SiO is not contained in the sintering atmosphere or at a sintering temperature exceeding 1800 ° C., silicon nitride is decomposed violently, and it is difficult to perform detailed control for decomposing only a small amount of silicon nitride. On the other hand, when the temperature is lower than 1700 ° C., the sinterability is reduced, and precipitation of Si cannot be expected.
Improvement of strength and toughness cannot be expected.

[0039]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Silicon nitride (Si ₃ N ₄ ) powder having an average particle size of 1 μm, α ratio of 92% and oxygen content of 1% by weight, rare earth oxide (RE ₂ O ₃ ), aluminum oxide (Al ₂ O ₃ ) Using powders of silicon oxide (SiO ₂ ) and the respective components, the components were blended so as to have the composition shown in Table 1, and a granulated product having a particle size of 40 to 200 μm was prepared by spray drying.
Thereafter, rubber press (isostatic press) molding was performed under a pressure of 1 to 3 ton / cm ² to produce a valve-shaped molded body and a molded body for evaluating mechanical properties by cutting. The amount of SiO ₂ in the table is Si ₃ N
₄ Includes SiO ₂ converted to impurity oxygen in powder.

Next, the molded body produced as described above was placed in a silicon carbide sagger, and
SiO ₂ + Si at 5% of the weight of the compact (1: 1 by weight)
The mixed powder was placed and baked at normal pressure for 5 hours under the conditions shown in Table 1. Thereafter, the cotter portion of the valve was processed to produce an engine valve having a stem portion having a diameter of 7 mm and an umbrella portion having a diameter of 36 mm. For the valve surface, the stem, umbrella and cotter are mirror-finished to match the dimensions,
The neck was left on the baked surface.

The density, porosity, strength, and average void diameter of each of the thus obtained sintered bodies for evaluation were measured by the following methods, and the results are shown in Table 2. The density and porosity are
It was processed to a shape under the conditions specified by JISR1601, and determined by specific gravity measurement based on the Archimedes method. The strength was determined by performing a four-point bending strength test at room temperature and 1000 ° C. based on JISR1601. Further, the average diameter of voids was examined from a scanning electron micrograph.

Further, with respect to the engine valve thus obtained, the burnt surface of the neck portion and the polished surface of which the surface was polished by 200 μm were subjected to Raman spectroscopic analysis to obtain a silicon nitride 20 μm.
A peak intensity of H ₂ 6 cm ^-1, was determined peak intensity ratio H ₁ / H ₂ ratio of the peak intensity H ₁ of Si 521 cm ^-1.
FIGS. 1 and 2 show Raman spectroscopic analysis charts of Sample No. 8 at the surface and at a depth of 200 μm.
Further, the ten-point average surface roughness (Rz) of the neck portion was measured,
The results are shown in Table 2.

As a valve durability test, a commercially available displacement of 2
Mounted as a 000cc 6-cylinder direct-drive exhaust valve, performing a practical durability fatigue test while giving a 10mm stroke at 10,000 to 15000rpm using natural gas, and endurance until the exhaust valve causes abnormal wear such as destruction and chipping. The time was determined from the accumulated time. In addition,
The integration time was set to a cutoff time of 500 hours, and for a sample that did not undergo abnormal wear, the thickness of the thinned portion of the sliding portion was taken as the amount of wear.

[0044]

[Table 1]

[0045]

[Table 2]

According to the results shown in Tables 1 and 2, in Sample No. 1 in which no Si was detected inside the neck, abnormal wear occurred due to chipping after 400 hours in the durability test.
Further, in Sample No. 14 in which the amount of Si detected, that is, the peak intensity ratio was larger than 10, the breakage occurred in 450 hours from the neck portion where the wear was usually greatly reduced. Further, in Sample Nos. 15 to 17 in which the density of the bulb was lower than 3.20 g / cm ³ , the abrasion was large and the bulb was broken in a shorter time than the neck and the cotter. Further, in Sample No. 18 having a porosity of more than 2%, chipping occurred due to abnormal wear from the void portion.

For these comparative examples, the sample N of the present invention was used.
o.2 to 13 have a room temperature strength of 900 MPa or more,
1000 ° C strength of 700MPa or more, endurance time of 500
For more than an hour, the thinning wear amount was 1 mm or less, and the surface roughness Rz of the neck portion had a good surface of 10 μm or less even on the baked surface.

[0048]

As described above, according to the engine valve of the present invention, a valve component containing a β-silicon nitride crystal phase as a main component is controlled to a specific composition and at least comprises a silicon nitride sintered body. Specific amount of S on neck
By precipitating i, the wear resistance and strength of the valve can be increased, and the durability of the valve can be improved.

[Brief description of the drawings]

FIG. 1 is a schematic view of an engine valve according to the present invention.

FIG. 2 is a Raman spectroscopic analysis chart of a neck surface of an engine valve (sample No. 8) of the present invention.

FIG. 3 is a Raman spectroscopic analysis chart at a depth of 200 μm from the neck surface of the engine valve (sample No. 8) of the present invention.

[Description of Signs] 1 stem part 2 umbrella part 3 neck part 4 cotter part

 ──────────────────────────────────────────────────続 き Continued on front page F term (reference) 4G001 BA01 BA03 BA06 BA08 BA09 BA11 BA48 BA61 BA73 BB01 BB03 BB06 BB08 BB09 BB11 BB24 BB31 BB32 BB48 BB62 BB73 BC12 BC13 BC35 BC54 BC56 BC62 BD12 BD14 BD15 BE03 BE11 BE31 BE33 BE33

Claims (6)

[Claims] 1. A silicon nitride sintered body having a β-silicon nitride crystal phase as a main component and a density of 3.2 g / cm ³ or more and a porosity of 2% or less, comprising an umbrella portion, a stem portion, and a neck portion. In an engine valve comprising: a peak intensity of Si at 521 cm ⁻¹ in an analysis chart by Raman spectroscopy at a depth of 200 μm from the surface of the neck portion is H ₁ , and a peak intensity at 206 cm ⁻¹ of silicon nitride is silicon nitride; when was the H _2, an engine valve, wherein the peak intensity ratio represented by H ₁ / H ₂ is 0.1 to 10. 2. The silicon nitride sintered body has an average void diameter of 5 μm or less and a room temperature strength of 900 MPa or more.
The engine valve according to claim 1, wherein the 000 ° C strength is 700 MPa or more. 3. The silicon nitride sintered body is characterized in that the rare earth element is 1 to 15% by weight in terms of oxide, aluminum is 0.01 to 5% by weight in terms of oxide, and impurity oxygen is in terms of silicon oxide. The engine valve according to claim 1 or 2, wherein the content is 10% by weight or less. 4. The method according to claim 1, wherein the silicon nitride sintered body includes Mg, W, M
4. The method according to claim 1, wherein at least one of the group consisting of o, Mn, Cu and Fe is contained as an oxide, nitride, oxynitride or silicide in a proportion of 8% by weight or less. Engine valve according to any of the preceding claims. 5. The engine valve according to claim 1, wherein the silicon nitride sintered body is manufactured by a normal pressure firing method. 6. The engine valve according to claim 1, wherein a surface roughness of the neck portion is 10 μm or less in ten-point average roughness (Rz).